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BY 4.0 license Open Access Published by De Gruyter Open Access December 31, 2022

Structural and spectroscopic properties of voriconazole and fluconazole – Experimental and theoretical studies

  • Beata Drabińska , Katarzyna Dettlaff , Kacper Kossakowski , Tomasz Ratajczak , Radosław Kujawski , Agnieszka Mikołajczyk and Jacek Kujawski EMAIL logo
From the journal Open Chemistry

Abstract

The article compares the experimental Fourier transform-infrared, Ultraviolet-visible (UV-vis), and proton nuclear magnetic resonance (1H NMR) spectra of voriconazole and fluconazole with the density functional theory (DFT) calculations using five different functionals. The results were compared with previously reported data related to its analogue – posaconazole. The analysis of calculated infrared (IR) spectra with the use of PBE1PBE (voriconazole) or APF (fluconazole) functionals shows good accordance with the experimental IR spectrum. The best compatibility between the experimental and theoretical UV spectra was observed with the use of PBE1PBE or B3LYP functionals for voriconazole or fluconazole, respectively. The reason for the difference in the UV-vis spectra of voriconazole and fluconazole was discussed based on linear response time-dependent DFT and natural bond orbital methods. The calculated 1H NMR spectrum shows that the DFT formalism, particularly the M06L or B3LYP functionals, gives an accurate description of the voriconazole and fluconazole chemical shifts.

1 Introduction

Voriconazole and fluconazole are azolyl-propanol-2 analogues of miconazole with proven antifungal activity. Fluconazole is used in the treatment of infections caused by C. albicans and C. neoformans. Voriconazole is a next generation agent with a broader spectrum of antifungal activity which is against C. albicans, C. krusei, C. glabrata, C. neoformans, and A. fumigatus [1].

Fluconazole was discovered in 1978 and licensed by Food and Drug Agency and European Agency in 1990 and there are multiple approved formulations: tablets, capsules, oral solution, and intravenous formulation. Apart from treating fungal infections, fluconazole is used as drug for treating and preventing recurring infections in patients with suppressed immunity, for example, patients with HIV, those undergoing chemotherapy, and to patients after bone marrow transplant [2].

Voriconazole was introduced in 2002 as a second-generation azole antifungal agent [3] and is mainly used in invasive aspergillosis as a first line treatment and in Candida spp. infections as a second-choice drug with fluconazole-resistant infections [4]. Due to its lowest toxicity, it is commonly used to treat fungal infections in children. Even though it is approved to be used in patients older than 12 years, it is also used in therapy of younger children aged 2–11 [5].

To the best of our knowledge, there are limited studies regarding spectral analysis of fluconazole and voriconazole. The reference data are limited to nuclear magnetic resonance (NMR) and differential scanning calorimeter (DSC) [6,7]. In continuation of the computational chemistry investigations on active azoles with antifungal properties [8,9,10], we have concentrated our studies on experimental and theoretical analysis of spectroscopic properties of fluconazole and voriconazole. Ultraviolet-visible (UV-vis) and Fourier transform-infrared (FT-IR) spectra have been calculated using different basis sets and methods and compared with the experimental results.

To achieve that we have compared the results of experimental FT-IR, UV-vis spectra, and 1H NMR of voriconazole and fluconazole with the calculated data with the use of different basis sets and methods. The purpose of our investigation was also to demonstrate which basis sets or methods applied for spectra predictions would produce the most consistent results with the experimental data. Albeit compounds have nitrogen atoms within their structure, the low abundance of the 15N isotope in comparison with the 1H isotope as well as the significant signals broadening due to the large quadrupole moment of N renders the nitrogen NMR spectroscopy impractical [11]. 1H NMR spectroscopy is broadly used in confirming the identity and purity of small molecule organic compounds. 13C NMR methodology, on the other hand, finds application, i.e., low-permeability investigations [12], analysis of modern materials like coal samples [13,14], understanding reactive behaviors and mechanisms of oxygen carriers [15], and hydrodeoxygenation process, which is considered as an efficient method to remove the oxygenated groups for further improving the quality of bio-oils [16]. Considering the low utility of 13C NMR spectroscopy for the investigations of interactions of small molecule organic compounds as well as the low natural abundance of 13C, we decided to use 1H NMR for the studies depicted in our article [11,17]. Furthermore, we expected to analyze which conformation of the compounds is preferred in terms of quantum chemistry. Such a study might help to improve the understanding of structural and spectral attributes responsible for biological function of the analyzed compounds.

2 Experimental methods

2.1 Chemicals

Voriconazole (1, VOR): (2R,3S)-2-(2,4-difluorophenyl)-3-(5-fluoropyrimidin-4-yl)-1-(1H-1,2,4-triazol-1-yl)butan-2-ol, was purchased from Shouguang Fukang Pharmaceutical Co., Ltd, China, purity ≥99% (in compliance with European Pharmacopoeia 8.0).

Fluconazole (2, FLU): 2-(2,4-difluorophenyl)-1,3-bis(1H-1,2,4-triazol-1-yl)propan-2-ol, was purchased from Shouguang Fukang Pharmaceutical Co., Ltd, China, purity ≥99% (in compliance with European Pharmacopoeia 8.0).

2.2 Spectroscopy

The IR spectra were recorded in KBr (1.00 mg of compound 1 or 2 per 300 mg of KBr) on a Shimadzu IRAffinity-1 spectrometer.

The UV spectra were run on a Perkin Elmer UV-Vis Lambda 20 spectrophotometer in 1 cm quartz cuvettes using 0.004, 0.01, 0.02, 0.05, and 0.1 mg/mL solutions of compound 1 or 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1.0 mg/mL solutions of compound 2 in methanol.

The NMR spectra were recorded at 298 K on a 500 MHz spectrometer operating at 500 MHz (1H) and 126 MHz (13C). Voriconazole (10 mg) or fluconazole were dissolved in 500 μL of dimethyl sulfoxide (DMSO) (Aldrich) or deuterated chloroform (CDCl3) (Aldrich). Trimethylsilane (TMS) was used as an internal standard.

The 1H, 13C, H–H COSY, and H–C HSQC spectra of 1 or 2 are given in the supplementary material (Figures S4a−S7d).

2.3 Theoretical calculations

The initial structures of the optimized rotamers of voriconazole 1 and fluconazole 2 were taken from the *.cif files given in the crystallographic base CCDC, and were named as follows: CEXMAU [18] and IVUQOF [19] for 1 and 2, respectively. They were initially optimized (Gaussian 16 A.03 program [20]) using density functional theory (DFT) formalism [21], namely, (a) B3LYP/6-31G(d,p) [22], (b) CAM-B3LYP/6-31G(d,p) [23], (c) B3LYP/6-311+G(d,p) [24,25], (d) PBE1PBE/6-31G(d,p) [26,27], (e) M06L/6-31G(d,p) [28], (f) M062X/6-31G(d,p) [29], and (g) APF/6-31G(d,p) [30] approaches in the gaseous phase (IR and UV spectrum calculations) or by applying the CPCM model [31] (UV and NMR spectrum calculations). For UV-vis calculations, using TD-DFT method [32], we applied CPCM solvation model, the linear response (LR) approach, and 14 solvents, namely, n-hexane, carbon tetrachloride, toluene, chloroform, chlorobenzene, tetrahydrofuran, n-octanol, acetone, methanol, ethanol, acetonitrile, N,N-dimethylformamide, dimethylsulfoxide, and water. The NMR shift for the TMS reference proton (Href) was calculated by the af approaches in DMSO or CHCl3 at 293 K using the gauge, including atomic orbital (GIAO) method [33] implemented in Gaussian G16 A.03 program and the protocol described in our previous reports [9,10,34]. The ChemCraft 1.7 software was utilized for visualization of all optimized rotamers [35].The highest occupied molecular orbital (HOMO)– lowest unoccupied molecular orbital (LUMO) orbitals for compounds were generated based on checkpoint files using GaussView 5.0 program [36]. The calculations were carried out using resources provided by Wrocław Center for Networking and Supercomputing (Bem clusters).

3 Results and discussion

3.1 Geometry optimization

Considering the initial geometries of the rotamers 1 and 2 (formulas are given in Figure 1), the magnitude of the RMSD error for the geometry of remaining optimized rotamers was 0.3221 and 0.5151 Å (for rotamers optimized at the B3LYP/6-31G(d,p) level of theory), 0.2897 and 0.5340 Å (for rotamers optimized at the B3LYP/6-311+G(d,p) level of theory), 0.3201 and 0.5122 Å (CAM-B3LYP/6-31G(d,p)), 0.3153 and 0.5073 Å (PBE1PBE/6-31G(d,p)), 0.3110 and 0.3913 Å (M06L/6-31G(d,p)), 0.3340 and 0.4151 Å (M062X/6-31G(d,p)), and 0.3166 and 0.5151 Å (APF/6-31G(d,p)), respectively. It is therefore shown that the smallest RMSD value for voriconazole 1 was obtained with respect to the B3LYP/6-311+G(d,p) approximation. In contrast, the smallest RMSD value concerning the optimized rotamer geometry of fluconazole 2 was obtained using the M06L functional. Next the molecular electrostatic potential (MEP) was determined by the B3LYP/6-311++G(2d,3p) or M06L/6-31G(d,p) approaches for the selected rotamer of azole 1 or 2, respectively, with the geometry previously optimized in the gaseous phase.

Figure 1 
                  Formulas for voriconazole (1) and fluconazole (2) and their optimized geometry at the B3LYP/6-311+G(d,p) (left) or M06L/6-31G(d,p) levels of theory (right).
Figure 1

Formulas for voriconazole (1) and fluconazole (2) and their optimized geometry at the B3LYP/6-311+G(d,p) (left) or M06L/6-31G(d,p) levels of theory (right).

To the best of our knowledge, the studies regarding the charge analysis for voriconazole 1 only involved protonation sites and was limited to the use of the approach B3LYP/ 6-31G** [37] or concerned accurate prediction of the site of metabolism [38] or involved the assessment of electrostatic potential (ESP) range with the use of Hartree-Fock (HF) methodology [39]. MEP analysis of fluconazole 2 have been limited only to the calculations involving solely the B3LYP/6-311++G(d,p) level of theory [40].

In our investigations, involving the multilevel approach to the conformational rotamers search, the results were refined using a basis set enriched with the higher-level polarization functions.

Regarding the analysis of MEP (Figure 2) for the charge distribution in the 1 and 2 optimized rotamers, we employed the key-words, “pop=full,” as well as the CHelpG (Charges from Electrostatic Potential) procedure with the calculation scheme developed by Breneman and Wiberg [41] (key-word “pop=chelpg”). In the latter scheme, atomic charges are fitted to reproduce the MEP at several points around the molecule.

Figure 2 
                  ESP map of compounds 1 (up) and 2 (down) calculated at the B3LYP/6-311++G(2d,3p)//B3LYP–6311++G(d,p) or M06L/6-311++G(2d,3p)//M06L–6311++G(d,p) levels of theory (2) levels of theory; gaseous phase; isovalue = 0.0004 a.u.; scale: red–blue from −6.243 × 10−2 to +6.243 × 10−2.
Figure 2

ESP map of compounds 1 (up) and 2 (down) calculated at the B3LYP/6-311++G(2d,3p)//B3LYP–6311++G(d,p) or M06L/6-311++G(2d,3p)//M06L–6311++G(d,p) levels of theory (2) levels of theory; gaseous phase; isovalue = 0.0004 a.u.; scale: red–blue from −6.243 × 10−2 to +6.243 × 10−2.

The list of atomic charges estimated according to the Mulliken and CHelpG methodologies is given in Table 1.

Table 1

Atomic charges [e] for nitrogen and oxygen atoms within the structure of voriconazole (geometry optimized at the B3LYP/6-311+G(d,p) or M06L/6-31G(d,p) levels of theory in gaseous phase, the Mulliken and CHelpG charge computations at the B3LYP/6-311++G(2d,3p) or M06L/6-311++G(2d,3p) levels of theory) estimated according to the Mulliken or CHelpG methodology (atoms numbering as in Figure 1)

Heteroatom Calculated charges
Mulliken CHelpG
B3LYP M06L B3LYP M06L
N1 0.190888 0.344732 0.415631 0.331346
N2 −0.253849 −0.240497 −0.638518 −0.567095
N3 −0.387118 −0.435279 −0.636094 −0.602799
N4 −0.349672 −0.477972 −0.693364 −0.643522
N5 −0.32645 −0.391797 −0.677053 −0.650402
O1 −0.598018 −0.746222 −0.666852 −0.60793
F1 −0.30939 −0.331395 −0.171092 −0.147631
F2 −0.314889 −0.319138 −0.242783 −0.21156
F3 −0.324502 −0.324655 −0.228264 −0.208324

From the performed analysis, the results (Table 1) show that the nitrogen atom N1 has the most positive charge value, while the remaining nitrogen atoms are characterized by a negative charge value, with the application of the Mulliken charges methodology, it indicates that nitrogen atoms N3 and N4 has the most negative values among all nitrogen atoms. In the case of the CHelpG charges methodology, this pattern is observed for nitrogen atoms N4 and N5 of voriconazole. On the other hand, in the case of fluconazole 2 (Table 2), it is shown that pyrrolic nitrogen atoms N1 and N4 are characterized by positive charge. Of all the nitrogen atoms within the structure of 2, the N6 atom was found to have the most negative charge value when the B3LYP functional and the Mulliken charges methodology were applied, as well as when the CHelpG charges were considered. Only the application of the Mulliken charges methodology and the M06L functionalization indicated the N3 atom as the one having the most negative charge value among all nitrogen atoms.

Table 2

Atomic charges [e] for nitrogen and oxygen atoms within the structure of fluconazole (geometry optimized at the B3LYP/6-311+G(d,p) or M06L/6-31G(d,p) levels of theory in gaseous phase, the Mulliken and CHelpG charge computations at the B3LYP/6-311++G(2d,3p) or M06L/6-311++G(2d,3p) levels of theory) estimated according to the Mulliken or CHelpG methodology (atoms numbering as in Figure 1)

Heteroatom Calculated charges
Mulliken CHelpG
B3LYP M06L B3LYP M06L
N1 0.083216 0.240459 0.38451 0.286563
N2 −0.34321 −0.29485 −0.62149 −0.56463
N3 −0.38084 −0.43462 −0.63669 −0.60543
N4 0.177336 0.155616 0.429703 0.386105
N5 −0.32032 −0.22171 −0.63373 −0.61775
N6 −0.38595 −0.41871 −0.63913 −0.61896
O1 −0.51262 −0.6559 −0.70729 −0.65768
F1 −0.3234 −0.33734 −0.23354 −0.20857
F2 −0.31633 −0.3203 −0.22778 −0.20771

3.1.1 IR analysis

The theoretical analysis of voriconazole (1) and fluconazole (2) IR spectrum had been limited to the abovementioned af approaches without correction term. We carried out the computations of 1 and 2 vibrational frequencies using the same level of theory that was used for the SCF optimization procedure and Grimme's D3 empirical (GD3) dispersion model [42] (for rotamers optimized using B3LYP, CAM-B3LYP, PB0, M06L, and M062X functionals; Table 2, rotamer of 2 was previously optimized at the B3LYP/6-311+G(d,p) level of theory in gaseous phase) [40] and the Petersson-Frisch dispersion model from the APFD functional (for rotamer optimized using the APF functional) [30], and the resulted IR spectra are shown in Figure 3. Small differences between the experimental and calculated vibrational modes can be observed because the experimental results were obtained in solid phase whereas the theoretical calculations were carried out in gaseous phase (Tables 3 and 4).

Figure 3 
                     Expended experimental (EXP) and theoretical IR spectra (DFT formalism, gaseous phase) of voriconazole (1) or fluconazole (2); B3LYP – B3LYP/6-31G(d,p)//B3LYP/6-31G(d,p) approach, APF – APF/6-31G(d,p)//APF/6-31G(d,p) approach, B3LYP_6311Gdp – B3LYP/6-311++G(d,p)//B3LYP/6-311++G(d,p) approach, CAM – CAM-B3LYP/6-31G(d,p)//CAM-B3LYP/6-31G(d,p) approach, M06L – M06L/6-31G(d,p)//M06L/6-31G(d,p) approach, M062X – M062X/6-31G(d,p)//M062X/6-31G(d,p) approach, and PBE1PBE – PBE1PBE/6-31G(d,p)//PBE1PBE/6-31G(d,p) approach.
Figure 3

Expended experimental (EXP) and theoretical IR spectra (DFT formalism, gaseous phase) of voriconazole (1) or fluconazole (2); B3LYP – B3LYP/6-31G(d,p)//B3LYP/6-31G(d,p) approach, APF – APF/6-31G(d,p)//APF/6-31G(d,p) approach, B3LYP_6311Gdp – B3LYP/6-311++G(d,p)//B3LYP/6-311++G(d,p) approach, CAM – CAM-B3LYP/6-31G(d,p)//CAM-B3LYP/6-31G(d,p) approach, M06L – M06L/6-31G(d,p)//M06L/6-31G(d,p) approach, M062X – M062X/6-31G(d,p)//M062X/6-31G(d,p) approach, and PBE1PBE – PBE1PBE/6-31G(d,p)//PBE1PBE/6-31G(d,p) approach.

Table 3

IR spectrum of compound 1; B3LYP/6-311++G(d,p)//B3LYP/6-31G(d,p)/gas level of theory

IR spectrum of voriconazole 1
Experimental wavenumber (cm−1) Vibrational assignments Calculated wavenumber (cm−1)
B3LYP/6-311++G(d,p)
ca. 3,203 νOH 3,619
3,121, 3,068 νC–H arom 3,265, 3,247, 3,224, 3,217, 321, 3,179, 3,167
2,969, 2,878 νC–H alkyl 3,104, 3,070, 3,042
1,615, 1,555, 1,509 νC–C arom 1,657, 1,631
1,451, 1,389, 1,332 νN═N and νC–N triazole 1,540, 1,476, 1,457
1,271, 1,183 δC–H arom 1,271, 1,261, 1,226
1,229 νC–O–C asym 1,238
1,120 νC–F arom 1,111
1,047, 1,020, 947 νC–O–C sym (alkyl-aryl ether and cyclic ether) and νC–O–C asym (cyclic ether) 1,066, 1,023
849, 823, 795, 732, 666 γC–H benzene and triazole 987, 978, 896, 691

Note: ν: stretching; δ: in-plane bending; γ: out-of-plane bending.

Table 4

IR spectrum of compound 2; M06L/6-31G(d,p)//M06L/6-31G(d,p)/gas level of theory

IR spectrum of fluconazole 2
Experimental wavenumber (cm−1) Vibrational assignments Calculated wavenumber (cm−1)
M06L/6-31G(d,p)
ca. 3,318 νOH 3,796
3,121, 3,063 νC–H arom 3,288, 3,279, 3,274, 3,265, 3,264, 3,251, 3,230
3,004, 2,985 νC–H alkyl 3,218, 3,203, 3,132, 3,115
1,602, 1,575 νC–C arom 1,714, 1,673, 1,587
1,504, 1,502 νN═N and νC–N triazole 1,565, 1,560
1,277 δO–H 1,296
1,147, 1,143 δC–H arom 1,338, 1,346
1,016, 1,010 νC–F arom 1,020
971, 966 δC–N triazole 979, 948
908, 885 νC–O–C sym (alkyl-aryl ether and cyclic ether) and νC–O–C asym (cyclic ether) 947, 925, 819
819, 792, 760, 736, 679 γC–H benzene and triazole 881, 871, 796, 728, 686

Note: ν: stretching; δ: in-plane bending; γ: out-of-plane bending.

From the spectra given in Figure 3, we can conclude that the use of the B3LYP/6-31G(d,p) and PBE1PBE/6-31G(d,p) (in the case of 1) or APF/6-31G(d,p) and M06L/6-31G(d,p) (in the case of 2) approaches for the rotamers optimization gives the highest conformity of the theoretical IR bands with the experimental spectrum, particularly in the ca. 3,500–2,800 cm−1 and ca. 1,750–500 cm−1 range, even though the rotamer optimized at the B3LYP/6-311++G(d,p) (1) or M06L/6-31G(d,p) (2) levels of theory has the lowest total energy. The utilization of the Grimme's D3 empirical dispersion model leads to better consistency between the theoretical and experimental IR spectral values. Moreover, including the diffuse functions in B3LYP/6-311++G(d,p) approach did not lead to the theoretical IR spectrum compared with the experimental one (as it is relatively more time-consuming in comparison with the B3LYP/6-31G(d,p) functional), particularly in the 3,100‒2,800 and 1,750–500 cm−1 region. The OH stretching bands were visible at 3,619 [B3LYP/6-311++G(d,p)] and 3,796 cm−1 [M06L/6-31G(d,p), 2], at the highest wavenumbers than for the experimental spectrum. In the computed spectra of 1 and 2, the estimated υ and δC–F, N═N, C–N (within the triazole moiety) and γC–H (related with benzene and triazole rings) absorptions were in excellent accordance with the experimental and literature data [40]. To the best of our knowledge, in the literature, there is no data regarding IR spectrum simulation using DFT formalism for voriconazole 1. It turned out that the use of the PBE1PBE or APF functional seems to be comparatively more effective because these approaches generally afford results without significant errors. A similar conclusion cannot be drawn from the method involving other functionals used in our investigations.

3.2 UV-vis analysis

The UV spectrum for 2 is in accordance with the literature data [40]. The spectra (Figures 4 and 5) display an absorption band at 256 nm (for 1) or 261 nm (for 2), which did not change with the concentration used. However, another absorption maximum (λ max) of analyte 1 or 2 is observed at 201–207 nm or 203–213 nm for 1 or 2, respectively, which migrated as a function of concentration.

Figure 4 
                  Experimental UV-vis spectrum of voriconazole 1 registered in methanol at various concentrations.
Figure 4

Experimental UV-vis spectrum of voriconazole 1 registered in methanol at various concentrations.

Figure 5 
                  Experimental UV-vis spectrum of fluconazole 2 registered in methanol at various concentrations.
Figure 5

Experimental UV-vis spectrum of fluconazole 2 registered in methanol at various concentrations.

To match the experimental (Figures 4 and 5 and Figure S1) and theoretical UV-vis spectra of analytes 1 and 2 (Figures S2 and S3), we optimized the molecules geometry and applied LR time-dependent DFT (TDDFT) method for the calculations. The vertical excited states were calculated for each optimized rotamer of compounds 1 and 2 at the functional/6-311++G(2d,3p) level of theory in gas phase, as well as in methanol (CPCM solvation model).

In the case of voriconazole 1 (Figure 6), the highest correspondence to the experimental data, especially with reference to the 256 nm band, was obtained using the PBE1PBE functional (absolute value of Δ = 5.63 nm), APF (absolute value of Δ = 11.22), and B3LYP (absolute value of Δ = 275.64 nm). Whereas in the case of fluconazole 2 (Figure 7), with a reference to the experimental band 261 nm, the highest agreement was possible using the B3LYP function (absolute value of Δ = 22.08 nm) and additionally the B3LYP/6311++G(d,p) approximation (absolute value of Δ = 23.41 nm) or the APF function (absolute value of Δ = 25.89 nm). It can also be concluded that the implementation of the CAM-B3LYP and M06L functions does not seem to be a favorable approach for correct prediction of UV-vis spectra of the analytes investigated.

Figure 6 
                  Experimental (EXP) and theoretical UV-vis spectra of voriconazole 1 registered in methanol and computed using CPCM solvation model (methanol as solvent); approximations: APF – APF/6-311++G(2d,3p)//APF/6-31G(d,p), B3LYP – B3LYP/6-311++G(2d,3p)//B3LYP/6-31G(d,p), CAM – CAM-B3LYP/6-311++G(2d,3p)//CAM-B3LYP/6-31G(d,p), M06L – M06L/6-311++G(2d,3p)//M06L/6-31G(d,p), M062X – M062X/6-311++G(2d,3p)//M062X/6-31G(d,p), PBE1PBE – PBE1PBE/6-311++G(2d,3p)//B3LYP/6-31G(d,p), and B3LYP_6311_Gdp – B3LYP/6-311++G(2d,3p)//B3LYP/6-311++G(d,p).
Figure 6

Experimental (EXP) and theoretical UV-vis spectra of voriconazole 1 registered in methanol and computed using CPCM solvation model (methanol as solvent); approximations: APF – APF/6-311++G(2d,3p)//APF/6-31G(d,p), B3LYP – B3LYP/6-311++G(2d,3p)//B3LYP/6-31G(d,p), CAM – CAM-B3LYP/6-311++G(2d,3p)//CAM-B3LYP/6-31G(d,p), M06L – M06L/6-311++G(2d,3p)//M06L/6-31G(d,p), M062X – M062X/6-311++G(2d,3p)//M062X/6-31G(d,p), PBE1PBE – PBE1PBE/6-311++G(2d,3p)//B3LYP/6-31G(d,p), and B3LYP_6311_Gdp – B3LYP/6-311++G(2d,3p)//B3LYP/6-311++G(d,p).

Figure 7 
                  Experimental (EXP) and theoretical UV-vis spectra of fluconazole 2 registered in methanol and computed using CPCM solvation model (methanol as solvent); approximations: APF – APF/6-311++G(2d,3p)//APF/6-31G(d,p), B3LYP – B3LYP/6-311++G(2d,3p)//B3LYP/6-31G(d,p), CAM – CAM-B3LYP/6-311++G(2d,3p)//CAM-B3LYP/6-31G(d,p), M06L – M06L/6-311++G(2d,3p)//M06L/6-31G(d,p), PBE1PBE – PBE1PBE/6-311++G(2d,3p)//B3LYP/6-31G(d,p), and M062X – M062X/6-311++G(2d,3p)//M062X/6-31G(d,p), B3LYP_6311_Gdp – B3LYP/6-311++G(2d,3p)//B3LYP/6-311++G(d,p).
Figure 7

Experimental (EXP) and theoretical UV-vis spectra of fluconazole 2 registered in methanol and computed using CPCM solvation model (methanol as solvent); approximations: APF – APF/6-311++G(2d,3p)//APF/6-31G(d,p), B3LYP – B3LYP/6-311++G(2d,3p)//B3LYP/6-31G(d,p), CAM – CAM-B3LYP/6-311++G(2d,3p)//CAM-B3LYP/6-31G(d,p), M06L – M06L/6-311++G(2d,3p)//M06L/6-31G(d,p), PBE1PBE – PBE1PBE/6-311++G(2d,3p)//B3LYP/6-31G(d,p), and M062X – M062X/6-311++G(2d,3p)//M062X/6-31G(d,p), B3LYP_6311_Gdp – B3LYP/6-311++G(2d,3p)//B3LYP/6-311++G(d,p).

The results of calculations involving the first excited states of 1 and 2 and using five different functionals are displayed in Tables 5 and 6, Figures 6 and 7, as well as in Tables S1 and S2 (supplementary material).

Table 5

First excited states of the voriconazole 1 computed using LR TDDFT approach in vacuum or in methanol; PBE1PBE/6-311++G(2d,3p)//PBE1PBE/6-31G(d,p) level of theory

Compound 1
Environment Energy (eV) Wavelength (nm) Oscillator strength Ground state−first excited state orbital transition
Vacuum 4.2967 288.55 0.0214 87 → 91
Methanol 4.4164 280.78 0.0117
Table 6

First excited states of the fluconazole 2 computed using LR TDDFT approach in vacuum or in methanol; B3LYP/6-311++G(2d,3p)//B3LYP/6-31G(d,p) level of theory

Compound 2
Environment Energy [eV] Wavelength [nm] Oscillator strength Ground state−first excited state orbital transition
Vacuum 5.1669 239.96 0.0196 79 → 80
Methanol 5.1894 238.92 0.0244

The contours of LUMO and HOMO orbitals for 1 and 2 are presented in Figures 8 and 9, respectively. The HOMO was visualized based on the checkpoint file (.chk) generated during the TDDFT computations. This orbital is located mainly over all aromatic ring, except for the 1,3-diazine (2) moieties, and the –CH2–CH(OH)– linker. It turned out that the voriconazole and fluconazole HOMO orbitals are similar to the HOMO orbitals of itraconazole [10] and posaconazole [34]. Similarity between posaconazole and itraconazole is also related with the LUMO of 1 and 2 – they cover the dihalogenophenyl residue as well.

Figure 8 
                  The HOMO orbitals generated for compound 1 (up; rotamer optimized at the PBE1BE/6-31G(d,p) level of theory in methanol) and 2 (down; rotamer optimized at the B3LYP/6-31G(d,p) level of theory in methanol); vertical excited states calculated at the functional/6-311++G(2d,3p) level of theory (functional: PBE1BE or B3LYP for 1 or 2, respectively).
Figure 8

The HOMO orbitals generated for compound 1 (up; rotamer optimized at the PBE1BE/6-31G(d,p) level of theory in methanol) and 2 (down; rotamer optimized at the B3LYP/6-31G(d,p) level of theory in methanol); vertical excited states calculated at the functional/6-311++G(2d,3p) level of theory (functional: PBE1BE or B3LYP for 1 or 2, respectively).

Figure 9 
                  The LUMO orbitals generated for compound 1 (up; rotamer optimized at the PBE1BE /6-31G(d,p) level of theory in methanol) and 2 (down; rotamer optimized at the B3LYP/6-31G(d,p) level of theory in methanol); vertical excited states calculated at the functional/6-311++G(2d,3p) level of theory (functional: PBE1BE or B3LYP for 1 or 2, respectively).
Figure 9

The LUMO orbitals generated for compound 1 (up; rotamer optimized at the PBE1BE /6-31G(d,p) level of theory in methanol) and 2 (down; rotamer optimized at the B3LYP/6-31G(d,p) level of theory in methanol); vertical excited states calculated at the functional/6-311++G(2d,3p) level of theory (functional: PBE1BE or B3LYP for 1 or 2, respectively).

The HOMO–LUMO gap calculated for 1 at the PBE1PBE/6-311++G(2d,3p) level is 5.3993 eV corresponding to an electron transition from spin-orbital 90 to spin-orbital 91. It can be assigned to the calculated second excitation state at 261.63 nm and is lower than that of fluconazole 2 where that gap was estimated at 6.4968 eV (PBE1PBE/6-311++G(2d,3p)//PBE1PBE/6-31G(d,p) approach). On the other hand, the HOMO–LUMO gap calculated for 2 at the B3LYP/6-311++G(2d,3p) level is 6.0880 eV is related to an electron transition from spin-orbital 79 to spin-orbital 80 and the first excitation state at 238.92 nm and is higher than that of voriconazole 1 where that gap was estimated at 4.9980 eV (B3LYP/6-311++G(2d,3p)//B3LP/6-31G(d,p) approach).

The first excited state for compound 1 (PBE1PBE/6-311++G(2d,3p)//PBE1PBE/6-31G(d,p) approach) relates mainly to the 280.74 nm band corresponding to an electron excitation from spin-orbital 87 to spin-orbital 91 (Table 5) and a HOMO-3 → LUMO transition (oscillator strength f = 0.0117, coefficient is 0.62200, and calculated energy is 4.4164 eV; data taken from the output file). In this case, the HOMO−LUMO contribution relative to the first excited state, calculated as duplicated coefficient square, is 77%, i.e., higher than the same contribution for fluconazole 2 [21%, HOMO–1 → LUMO+1 transition, oscillator strength f = 0.0244, coefficient is 0.57531, and calculated energy is 5.1894 eV, PBE1PBE/6-311++G(2d,3p)//PBE1PBE/6-31G(d,p) approach]. In the case of the UV spectrum of fluconazole 2 computed using the B3LYP/6-311++G(2d,3p)//B3LYP/6-31G(d,p) approximation, the first excited state is described by the following parameters: electron excitation from spin-orbital 79 to spin-orbital 80 (Table 6) and a HOMO → LUMO transition (oscillator strength f = 0.0244, coefficient is 0.57531, and calculated energy is 5.1894 eV; data taken from the output file) at the 238.92 nm, for which the first excited state, calculated as duplicated coefficient square, is 66%, i.e., lower than the same contribution for fluconazole 2 (77%, 287.07 nm, HOMO-3 → LUMO transition, oscillator strength f = 0.0154, coefficient is 0.59124, and calculated energy is 4.3189 eV, B3LYP/6-311++G(2d,3p)//B3LYP/6-31G(d,p) approach).

It is worth to mention that in the theoretical UV-vis spectrum of 1 (PBE1PBE/6-311++G(2d,3p)//PBE1PBE/6-31G(d,p) approach), the highest oscillator strength (f = 0.0863) can be assigned to the third excitation state at 231.57 nm. This absorbance relates to an electron transition from spin-orbital 90 to spin-orbital 93 that, in turn, corresponds to a HOMO → LUMO+2 transition (coefficient is 0.45623, transition energy = 5.3540 eV; data taken from output file; HOMO−LUMO contribution is 42%). However, in the case of 2 (B3LYP/6-311++G(2d,3p)//B3LYP/6-31G(d,p) approach), the highest oscillator strength (f = 0.1255) can be assigned to the third excitation state at 215.11 nm, an electron transition from spin-orbital 79 to spin-orbital 81 that, in turn, corresponds to a HOMO → LUMO+1 transition (coefficient is 0.56649, transition energy = 5.7637 eV; data taken from output file; HOMO−LUMO contribution is 64%). The above discussion shows that the DFT method can satisfactorily explain the observations taken from the experimental UV-vis spectra of the analyzed conazoles.

Next for 1 and 2, we computed several descriptors related to HOMO–LUMO electron transition, i.e., electronegativity (χ), chemical hardness (η), and electronic potential using the orbital energy of the HOMO and the orbital energy of the LUMO based on the DFT formalism, as well as the chemical potential (μ) of the molecule using Koopman’s theorem [43,44]. They are characterized by equations: μ = −(I + A)/2 and η = (IA)/2, and electronegativity χ = (I + A)/2, where I is the first ionization potential (I = −EHOMO) and A – electron affinity (A = −ELUMO). Regarding the abovementioned data, these descriptors are as follows [eV]: I = 7.2511 or 7.3499, A = 1.8517 or 0.8531, μ = −4.5514 or −4.1015, η = 2.6997 or 3.2484, and χ = 4.5514 or 4.1015 for 1 or 2, respectively, using the PBE1PBE/6-311++G(2d,3p)//PBE1PBE/6-31G(d,p) approach. In case of approximation of B3LYP/6-311++G(2d,3p)//B3LYP/6-31G(d,p), we obtained the following values for the listed descriptors I = 7.0718 or 7.1686, A = 2.0738 or 1.0806, μ = −4.5728 or −4.1246, η = 2.4990 or 3.0440, and χ = 4.5728 or 4.1246 for 1 or 2, respectively. With regard to analogues 1 and 2, i.e., itraconazole [10] and posaconazole [34], and the above descriptors, we used in previous studies CAM-B3LYP/6-311++G(2d,3p)//CAM-B3LYP/6-31G(d,p) approach. Regarding this approximation, these descriptors are as follows [eV]: I = 8.5123 or 8.6038, A = 0.7418 or −0.1891, μ = −4.6271 or −4.2073, η = 3.8853 or 4.3964, and χ = 4.6271 or 4.2073 for 1 or 2, respectively. On the other hand, these parameters for itraconazole and posaconazole had the following values: I = 6.5011 or 6.6260, A = 0.0199 or −0.2931, μ = −3.2605 or −3.1664, η = 3.2406 or 3.4595, and χ = 3.2605 or 3.1664, respectively (CAM-B3LYP/6-311++G(2d,3p)//CAM-B3LYP/6-31G(d,p) approach). The above data confirm notable similarity of the above UV spectra parameters for voriconazole 1 and fluconazole 2 that are determined by the HOMO–LUMO electron transitions. Differences were noted principally in relation to parameters: A and η (functionals B3LYP, PBE1PBE, and CAM-B3LYP). Furthermore, in comparison to itraconazole and posaconazole, somewhat higher absolute values of such parameters like first ionization potential (I), chemical potential (μ), and chemical hardness (η) were obtained.

Subsequently, using the TFDFT method, theoretical UV-vis spectra were determined for 1 and 2 including, in addition to methanol (Figures 6 and 7), the following solvents: n-hexane, carbon tetrachloride, toluene, chloroform, chlorobenzene, tetrahydrofuran, n-octanol, acetone, ethanol, acetonitrile, N,N-dimethylformamide, dimethylsulfoxide, and water (Tables S1 and S2 in the supplementary material). Analysis of the λ max bands of the theoretical UV-vis spectra of 1 in methanol indicates that the largest bathochromic shift occurs with M06L and the largest hypochromic shift with APF functionals. Whereas for 2, the largest bathochromic shift occurs with M06L method, but the largest hypochromic shift with M062X functional. However, analysis of the first excited state (λ1) bands of the theoretical UV-vis spectra of 1 and 2 in methanol indicates that the largest bathochromic shift occurs with M06L functional and the largest hypochromic shift with CAM-B3LYP (1) or M062X (2) functionals. From the standpoint of the solvatochromism phenomenon and the influence of the other solvents included in the calculations on the theoretically computed UV-vis voriconazole spectra, we noticed that the lowest wavelength values for the first excited state (λ1) occur when using water as solvent and the highest values for n-hexane. Whereas this trend is slightly altered when the results for λ max are compared. Calculations for all functions, except PBE1PBE, indicate that the smallest λ max values relate to the use of water as solvent and the largest relate to n-hexane. The use of PBE1PBE functional led to observations pointing out that the lowest λ max value was associated with the use of tetrahydrofuran as solvent and the highest for n-octanol. The discussion presented above provides important data relating to, for e.g., the effect of reaction field and solvent polarity on the values of λ max and first excited state (λ1) of theoretical UV-vis spectra of the azoles studied, depending on the functionals used for calculations. In addition, they significantly enrich the previous knowledge in this aspect, so far limited in literature to considerations of fluconazole 2, B3LYP functionals and DMSO, methanol, and water used therein as solvents [40].

3.3 Natural bond orbitals (NBOs) analysis

Considering the conclusions drawn from the UV-vis analysis, we carried out NBO studies (CPCM solvation model and methanol used as solvent). The NBO analysis was performed at the wB97XD/6-311++G(2d,3p) level of theory using the NBO 3.0 approach as implemented in Gaussian G16 A.03 software for rotamers previously optimized in the wB97XD/6-31G(d,p) approximation. Our attention was focused on the oxygen and nitrogen atoms, as well as aromatic rings, for which electrons were important for the distribution of HOMO and LUMO orbitals (Figure 1). The second-order perturbation theory, that involves Fock matrix in NBO basis, shows intramolecular hyper-conjugative interactions.

The fundamental structural differences between voriconazole 1 and fluconazole 2 are due to the presence of the 1,3-fluorodiazine system in the structure 1 compared to the other azole tested which has a triazole ring. However, Nitrogen atoms N4 and N5 of fluorodiazine (1) or N4, N5, and N6 within the triazole moiety (2) are characterized by fully filled 1-center bonding hybrid CR s orbitals.

The C1–N1 bond in voriconazole (Figure 1) can be depicted by an almost completely filled (1.98770e) 2-center bonding hybrid BD orbital (polarization coefficient 0.8002) formed by interaction between s (36.11% s) and p (63.82% p1.77) orbitals. The nitrogen atom has a greater contribution (64.04%) to this σC–N bonding orbital. The above bond in this rotamer is an NBO density donor to the following bonds formed by the antibonding orbitals BD*: C2–C3, N1–C9, N3–C9, N2–C10, and N1–N2. The C1–N1 bond interacts also with the antibonding Rydberg orbitals RY* of atoms: C2, N2, and C9. In comparison, the NBO characteristics concerning the analogous C–N bond in posaconazole and itraconazole are similar [9,10,34].

The O1–C2 bond in compound 1 (Figure 1) can be characterized by an almost completely filled (1.98995e) 2-center hybrid bonding orbital (polarization coefficient 0.8160) formed by the overlap of s (32.61% s) and p (67.18% p2.06) orbitals. The oxygen atom has a greater contribution (66.58%) in the formation of this σO–N bonding orbital. This bond is also an NBO density donor to the following bonds formed by the antibonding orbital BD*: C1–H1, C3–H3, and C11–C16, as well as antibonding Rydberg orbitals RY* centered on atoms C2, C11, and H14. The analogous O–C bond in itraconazole [10] and posaconazole [34] can be characterized similarly.

The N4–C5 bond in compound 1 (Figure 1) can be characterized by two almost completely filled (1.98848e) 2-center bonding hybrid BD orbitals (polarization coefficients 0.7756 and 0.7792) formed by the overlap of s (31.17% s) and p (68.71% p2.20) orbitals. In this bond the carbon atom has a greater contribution (60.16%) to the formation of this bonding orbital. This bond is also an NBO density donor to the following bonds formed by the antibonding orbitals BD*: C3–H3, C3–C5, C5–C8, C6–H7, N4–C6, and F1–C8. For comparison, the HOMO–LUMO distribution in fluconazole 2 does not cover the diazine ring.

The N5–C7 bond within the diazine moiety can be characterized by an almost completely filled (1.98395e) 2-center bonding hybrid BD orbital (polarization coefficient 0.7868) formed by the overlap of s (35.95% s) and p (63.96% p1.78) orbitals. The nitrogen atom has also a greater contribution (61.91%) in the formation of the σN–C bonding orbital. This bond is also an NBO density donor to the following bonds formed by the antibonding orbitals BD*: C6–H7, N5–C6, C7–H8, C7–C8, and F1–C8.

Similarly to fluconazole 2, the C1 and C2 atoms (Figure 1) within the –CH2–C(OH)< bond of the analyzed rotamers of 1 and 2 (the corresponding C7 and C8 atoms) can be characterized by the following NBOs:

(i) an almost completely filled core CR Lewis orbital (1.99900e–1.99930e, 100% s);

(ii) 22 antibonding Rydberg orbitals RY*.

The N1–C8 bond in 2 (Figure 1), analogous to the N1–C1 bond in voriconazole 1, can be characterized by an almost completely filled (1.98778e) 2-center hybrid BD (polarization coefficient 0.8006) formed by the overlap of s (36.06% s) and p (63.87% p 1.77) orbitals. The nitrogen atom has a greater contribution (64.09%) in the formation of the σN–C bonding orbital. This bond is also an NBO density donor to the following bonds formed by the antibonding orbitals BD*: C7–C11, N1–N2, N1–C10, N2–C9, and N3–C10, as well as the antibonding Rydberg orbitals RY* of atoms: N2, C7, and C10.

The N4–C11 bond in the fluconazole 2 can be characterized by an almost filled (1.98775e) 2-center hybrid BD (polarization coefficient 0.7996) formed by the overlap of s (35.96% s) and p (63.97% p 1.78) orbitals. The nitrogen atom has a greater contribution (63.93%) in the formation of the σN–C bonding orbital. This bond is also an NBO density donor to the following bonds formed by the antibonding orbitals BD*: C1–C7, N4–N5, N4–C13, N5–C12, and N6–C13.

Considering the presented data, we can conclude that the distribution of the NBOs for rotamers of voriconazole and fluconazole is almost identical and covers especially the triazole nitrogen and atoms as well as the –CH2–C(OH)< bridge. The sole difference, discussed above, is connected with the NBO donor–acceptor interaction, including the hyper-conjugate interaction energy (Figure 1). The differences come down to the fact that a fluoropyrimidine ring is present in the voriconazole structure as opposed to the fluconazole structure with additional triazole system instead. The presence of triazole or pyrimidine rings does not influence the NBOs population within the above specified molecules fragments.

3.4 NMR analysis

The signals in the 1H NMR spectra of voriconazole 1 and fluconazole 2 were registered in DMSO-d 6 and CDCl3 (Tables 710 and S3−S6 and Figures S4a−S7d). Noteworthy is the fact that the hydroxy proton is often revealed as a doublet in both the solvents. This is typical for a spectrum registered in DMSO-d 6 because this aprotic polar solvent can form a hydrogen bond with hydroxy group. However, such splitting is unusual for non-polar solvents like CDCl3. A rare exception occurs if hydroxy proton is involved in an intramolecular hydrogen bond [45]. The absence of such intramolecular bond does not lead to splitting of the OH signal into doublet in our experimental 1H NMR spectra of 1 or 2.

Table 7

Experimental (δ exp) and calculated chemical shifts (I) for compound 1

Atoms numbering δ exp I Δ Δδ
H1 4.35 4.85 −0.50 10
H2 4.82 4.27 0.55 13
H3 3.95 3.98 −0.03 1
H4 1.13 1.54 −0.41 27
H5 1.13 0.96 0.17 18
H6 1.13 1.02 0.11 11
H7 9.06 9.39 −0.33 4
H8 8.87 8.73 0.14 2
H9 8.25 8.69 −0.44 5
H10 7.63 7.69 −0.06 1
H11 7.28 7.59 −0.31 4
H12 6.94 6.73 0.21 3
H13 7.20 6.68 0.52 8
H14 6.00 6.36 −0.36 6

Note: errors (Δ): relative percentage errors (Δδ); calculated NMR shielding (M06L/631G(d,p)//M06L/6-31G(d,p)/DMSO) for proton H ref = 31.9537 ppm for TMS; median absolute deviation (MAD) = 0.30 (atoms numbering is as in Figure 1).

The theoretical 1H NMR spectra of 1 using the M06L (mean absolute error (MAE) = 0.30 for DMSO as solvent) or B3LYP functionals (MAE = 0.25 for CHCl3 as solvent) show the highest conformity of the chemical shifts with the experimental data (Tables 7 and 8). The largest values of percentage error (Δδ) were found to be more than 10% for the labile hydroxy proton, methylene, and methyl protons. These errors are due to steric reasons, namely, proximity of the fluoropyrimidine (closest distance CH3⋯C-5 is 2.75 or 2.77 Å for rotamers optimized using M06L functional in DMSO or B3LYP functional in chloroform, respectively), difluorophenyl rings (distance CH3⋯C-11 is 2.66 or 2.68 Å for rotamers optimized using M06L functional in DMSO or B3LYP functional in chloroform, respectively), and triazole functionality (closest distance CH2⋯N-1 is 2.07 Å for rotamers optimized using M06L functional in DMSO or B3LYP functional in chloroform, respectively). The large error for the hydroxy group is caused not only by the proton lability but also by the neighboring fluoropyrimidine ring (distance OH⋯N-4 is 1.94 or 1.90 Å for rotamers optimized using M06L functional in DMSO or B3LYP functional in chloroform, respectively). Fundamentally, it should be emphasized that in the case of voriconazole 1, the calculated values of chemical shifts in the 1H NMR spectrum gave significant correspondence with the experimental data (MAE error ranged from 0.30 to 0.36 or from 0.25 to 0.33 for rotamers optimized using DMSO or CHCl3 as solvents, respectively).

Table 8

Experimental (δ exp) and calculated chemical shifts (I) for compound 1

Atoms numbering δ exp I Δ Δδ
H1 4.34 4.85 −0.51 10
H2 4.74 3.84 0.90 23
H3 4.16 4.22 −0.06 1
H4 1.13 1.57 −0.44 28
H5 1.13 0.96 0.17 18
H6 1.13 0.89 0.24 27
H7 8.95 9.29 −0.34 4
H8 8.64 8.74 −0.10 1
H9 7.99 8.34 −0.35 4
H10 7.57 7.48 0.09 1
H11 7.64 7.76 −0.12 2
H12 6.88 6.83 0.05 1
H13 6.83 6.83 0.00 0
H14 6.50 6.67 −0.17 3

Note: Δ: errors; Δδ: relative percentage errors; calculated NMR shielding (B3LYP/6-31G(d,p)//B3LYP/6-31G(d,p)/CHCl3) for proton Href = 31.7489 ppm for TMS; MAD = 0.25 (atoms numbering is as in Figure 1).

For fluconazole 2, the compliance of the estimated values of chemical shifts with the experimental data of 1H NMR spectrum was expressed in the following ranges of MAE error values: 0.70−1.82 or 1.61−1.77 for rotamers optimized using DMSO or CHCl3 as solvents, respectively. In case of 2, the theoretical 1H NMR spectra using the B3LYP functional (MAE = 0.70 or 1.61 for DMSO or CHCl3 as solvent, respectively) show the highest conformity of the chemical shifts with the experimental data (Tables 9 and 10). The highest values of percentage error (Δδ) exceeding ca. 40% were related to the protons of the methylene groups, which was caused, as in the case of voriconazole, by steric reasons. We noticed the proximity of the rotatable triazole rings (closest distance CH2⋯N-1 or N-4 is ca. 2.06 Å for rotamers optimized using B3LYP functional in DMSO or chloroform), and hydroxyl functionality (closest distance CH2⋯OH is ca. 2.70 Å for rotamers optimized using B3LYP functional in DMSO or chloroform). The cause for the highest value of percentage error (Δδ) related to hydroxyl group is seen in proton lability and its significant shielding by rotating aromatic rings and proximity of methylene groups.

Table 9

Experimental (δ exp) and calculated chemical shifts (II) for compound 2

Atoms numbering δ exp II Δ Δδ
H1 7.26 7.00 0.26 4
H2 6.89 7.03 −0.14 2
H3 7.13 7.56 −0.43 6
H4 4.74 4.13 0.61 15
H5 4.57 5.17 −0.60 12
H8 7.81 7.69 0.12 2
H9 8.33 7.90 0.43 5
H7 4.74 4.26 0.48 11
H6 4.57 4.83 −0.26 5
H10 7.81 8.02 −0.21 3
H11 8.33 8.40 −0.07 1
H12 6.37 1.62 4.75 292

Note: Δ: errors; Δδ: relative percentage errors; calculated NMR shielding (B3LYP/631G(d,p)//B3LYP/6-31G(d,p)/DMSO) for proton Href = 31.7468 ppm for TMS; MAD = 0.70 (atoms numbering is as in Figure 1).

Table 10

Experimental (δ exp) and calculated chemical shifts (II) for compound 2

Atoms numbering δ exp II Δ Δδ
H1 6.76 6.97 −0.21 3
H2 6.86 6.99 −0.13 2
H3 7.45 7.51 −0.06 1
H4 4.77 4.11 0.66 16
H5 4.48 5.18 −0.70 13
H8 4.77 7.69 −2.92 38
H9 4.48 7.83 −3.35 43
H7 7.88 4.20 3.68 88
H6 8.09 4.82 3.27 68
H10 7.88 8.00 −0.12 2
H11 8.09 8.35 −0.26 3
H12 5.50 1.49 4.01 268

Note: Δ: errors; Δδ: relative percentage errors; calculated NMR shielding (B3LYP/631G(d,p)//B3LYP/6-31G(d,p)/CHCl3) for proton H ref = 31.7469 ppm for TMS; MAD = 1.61 (atoms numbering is as in Figure 1).

In consideration of the data presented, it should be emphasized that these data show that the DFT formalism, particularly the M06L or B3LYP functionals, results in a correct description of the voriconazole and fluconazole 1H NMR chemical shifts.

4 Conclusion

Our computations proved that the use of the PBE1PBE or APF functional seems to be comparatively more effective in the IR spectra predictions of voriconazole 1 and fluconazole 2 because these approaches generally afford results without significant errors. The best conformity with the experimental UV spectra was obtained with the use of B3LYP/6-31G(d,p) and PBE1PBE/6-31G(d,p) (for voriconazole 1) or APF/6-31G(d,p) and M06L/6-31G(d,p) (for fluconazole 2) methods. The first excited state is connected with an electron excitation corresponding to a HOMO-3 → LUMO (PBE1PBE functional) or HOMO → LUMO transitions (B3LYP functional) for conazole 1 or 2, respectively. The obtained data confirm similarity of the UV spectra parameters for voriconazole 1 and fluconazole 2. Differences were noted principally in relation to parameters: A and η (functionals B3LYP, PBE1PBE, and CAM-B3LYP). Furthermore, in comparison to itraconazole and posaconazole, we obtained higher absolute values of such parameters like first ionization potential (I), chemical potential (μ), and chemical hardness (η). Regarding the abovementioned data, these descriptors are as follows [eV]: I = 7.2511 or 7.3499, A = 1.8517 or 0.8531, μ = −4.5514 or −4.1015, η = 2.6997 or 3.2484, and χ = 4.5514 or 4.1015 for 1 or 2, respectively, using the PBE1PBE/6-311++G(2d,3p)//PBE1PBE/6-31G(d,p) approach. In case of approximation of B3LYP/6-311++G(2d,3p)//B3LYP/6-31G(d,p), we obtained the following values for the listed descriptors I = 7.0718 or 7.1686, A = 2.0738 or 1.0806, μ = −4.5728 or −4.1246, η = 2.4990 or 3.0440, and χ = 4.5728 or 4.1246 for 1 or 2, respectively. Moreover, analysis of the λmax bands of the theoretical UV-vis spectra of 1 in methanol indicates that the largest bathochromic shift occurs with M06L approach and the largest hypochromic shift occurs with APF functional. Whereas for 2, the largest bathochromic shift occurs with M06L method, but the largest hypochromic shift occurs with M062X functional. The analysis of the first excited state (λ1) bands of the theoretical UV-vis spectra of 1 and 2 in methanol indicates that the largest bathochromic shift occurs with M06L functional and the largest hypochromic shift occurs with CAM-B3LYP (1) or M062X (2) functionals. The distribution of the NBOs orbitals for rotamers 1 and 2 is almost identical and covers the triazole nitrogen and atoms as well as the –CH2–C(OH)< bridge. The presence of triazole or pyrimidine rings however does not influence the NBOs population within the above specified molecules’ structural fragments.

In our work, we compared 1H NMR experimental and theoretical spectra of 1 and 2. The calculated data show that the DFT formalism, particularly M06L or B3LYP functionals, results in a correct description of the voriconazole and fluconazole 1H NMR chemical shifts. The theoretical 1H NMR spectra of 1 using the M06L (MAE = 0.30 for DMSO as solvent) or B3LYP functionals (MAE = 0.25 for CHCl3 as solvent) show the highest conformity of the chemical shifts with the experimental data. The largest values of percentage error (Δδ) were found to be more than 10% for the labile hydroxy proton, methylene, and methyl protons. For fluconazole 2, the compliance of the estimated values of chemical shifts with the experimental data of 1H NMR spectrum was expressed in the following ranges of MAE error values: 0.70−1.82 or 1.61−1.77 for rotamers optimized using DMSO or CHCl3 as solvents, respectively.

The above conclusions show that our proposed methodology seems to be a potentially useful tool for prediction of IR and UV-vis properties of biologically active conazoles. We wish to investigate this standpoint further in the near future.


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Acknowledgments

This research was funded by the National Science Center (NCN, Poland) − Miniatura 6 grant (number 2022/06/X/NZ7/00227), as well as the NCN grant No. UMO-2016/20/S/ST5/00362. The calculations were carried out using resources provided by the Polish Grid Infrastructure (PL-Grid) and Wrocław Center for Networking and Supercomputing (WCSS Grant no. 327/2014). The authors are indebted to Dr. Dariusz Kędziera (Faculty of Chemistry, Nicolaus Copernicus University in Toruń, Poland) and Dr. hab. Jarosław J. Panek (Faculty of Chemistry, University of Wrocław, Poland) for his help in the TDDFT calculations.

  1. Funding information: This research was funded by the National Science Centre (NCN, Poland) − Miniatura 6 grant (number 2022/06/X/NZ7/00227), as well as the NCN grant No. UMO-2016/20/S/ST5/00362. The calculations were carried out using resources provided by the Polish Grid Infrastructure (PL-Grid) and Wrocław Center for Networking and Supercomputing (WCSS Grant No. 327/2014).

  2. Conflict of interest: The authors declare no conflict of interest.

  3. Ethical approval: The conducted research is not related to either human or animals use.

  4. Data availability statement: The supplement contains Cartesian coordinates of the rotamers and complete results of the UV-vis, and 1H NMR calculations (with additional experimental 1H NMR of 1 and 2). These supplementary data can be found online at doi: 10.1515/chem-2022-0253.

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Received: 2022-10-01
Revised: 2022-10-28
Accepted: 2022-11-12
Published Online: 2022-12-31

© 2022 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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