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  • Author: Ibon Alkorta x
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Abstract

The acidities of a large number of carbon acids have been theoretically calculated for the gas-phase and for DMSO solution. The gas-phase values, both ΔH and ΔG, are very well correlated with the available experimental data. From the calculated ΔG values in DMSO and the pKas in the same solvent, a homogeneous set of pK a (DMSO) values was devised that was used to generate pK a (water). These last pK as were used to establish the limits of the acidity of carbon acids for reactions under PTC conditions both alkylations and H/D exchange. A step further led to the pK as in liquid ammonia and from them to the virtual use of PTC using liquid ammonia instead of water.

Abstract

A theoretical study of the complexation of cyclo[8]pyrrole dication, 2, and the corresponding system in neutral form, 3, with six anionic molecules has been carried out up to the B3LYP/6–311++G(2d,2p) computational level. The effect of the water solvation has been taken into account by means of the PCM method. The gas phase results correspond to the very large interaction energies expected for the interaction of molecules of opposite charge. In all the complexes, the analysis of the electron density by means of the Atoms In Molecules (AIM) methodology shows the presence of eight intermolecular interactions between the individual molecules. The results, using the water solvent model, indicate that the 2:SO42− complex is more stable than the 2:NO3−, in agreement with experimental results.

Abstract

Two-, three-, and four-bond 19F-19F spin-spin coupling constants (nJ FF) for a set of small fluoro-substituted organic molecules have been computed using ab initio equation-of-motion coupled cluster singles and doubles (EOM-CCSD) theory. The computed values reproduce the experimental signs and magnitudes of nJ FF. The straight line that relates the experimental and computed coupling constants has a slope of approximately 1 and passes through the point (0,0 Hz) within the uncertainties of the fit. Hence, EOM-CCSD values of nJ FF should be excellent predictors of experimental values when these are not available. All of the components of nJ FF except for the diamagnetic spin-orbit term may be large and must be evaluated if agreement between theory and experiment is to be obtained.

Summary

Selective procedures for the synthesis of 6- and of 7-monosubstituted derivatives of pyrazinoI2,3-c̱|-1 ,2,6- thiadiazine 2,2-dioxide and their reactions with N-bromosuccinimide are reported. The structures of the compounds are discussed on the basis of 1 H -, 13C_ NMR and UV spectroscopic data. The pKa values of the compounds have also been determined.

Abstract

The experimental chemical shifts and coupling constants of five borates of general formula BHnPz4–n [from the borohydride to tetrakis(pyrazol-1-yl)borate] anions were compared with calculations carried out at the B3LYP/6-311++G(d,p) level (GIAO for absolute shieldings), in general with satisfying results. The most stable conformations of pyrazolylborate anions are similar to those of neutral pyrazolylmethanes.

Abstract

The X-ray molecular structure of two benzimidazoles unsubstituted on the nitrogen atoms (2-ethyl-1H-benzimidazole and 2-propyl-1H-benzimidazole) has been determined and analysed in the context of crystallographic and solid-state NMR data of seven substituted benzimidazoles. The first compound presents polymorphism, and two polymorphs [forms 3a and 3c] were obtained from different solutions of dichloromethane/hexane. Polymorph (3a) contains two independent molecules differing in the conformation of the ethyl group. Polymorph (3c) contains only one molecule in the asymmetric unit, with similar configuration as the ordered molecule of polymorph (3a). Another polymorphic form (3b) is obtained by cooling form (3a) below 179 K. The transition is related to an order-disorder process and involves a change in the hydrogen-bonding scheme. The second compound (4) presents enantiomorphism and its asymmetric unit contains four independent molecules that differ in the conformation of the 2-propyl group. For form (3a), the splitting of NMR signals is almost perfectly consistent with the features of the crystal structure, whereas not splitting is observed for compound (4). Agreements between NMR and X-ray crystallography techniques are also observed in other simple 2-R-1H-benzimidazoles (R = C4H9, CH2C6H5 and C6H5), but disagreements are found for R = H, CH3.

Abstract

A systematic ab initio investigation has been carried out to determine the structures, binding energies, and spin-spin coupling constants of ternary complexes X:HBO:Z for X, Z= LiH, HNC, HF, HCN, HCl, ClF, and HBO. All complexes X:HBO:Z are linear with C∞ v symmetry, except for HCl:HBO:Z and ClF:HBO:Z which have Cs symmetry, thereby reflecting the structures of the corresponding X:HBO and HBO:Z complexes. Cooperative effects on energies are synergistic in all ternary complexes. The enhanced binding energies of complexes X:HBO:Z correlate with the binding energies of the X:HBO and HBO:Z complexes. Coupling constants 1J(B-H) and 2hJ(B-A) across B-H···sA hydrogen bonds correlate with the B-A distance, and exhibit synergistic effects due to the presence of Z. 1hJ(H-A) indicates that these bonds have little proton-shared character. Coupling constants across D-H···sO hydrogen bonds, H-Li···sO lithium bonds, and F-Cl···sO halogen bonds are also sensitive to the synergistic effects arising from the presence of X. D-H···sO hydrogen bonds in ternary complexes are traditional (normal) hydrogen bonds.

The term “hydrogen bond” has been used in the literature for nearly a century now. While its importance has been realized by physicists, chemists, biologists, and material scientists, there has been a continual debate about what this term means. This debate has intensified following some important experimental results, especially in the last decade, which questioned the basis of the traditional view on hydrogen bonding. Most important among them are the direct experimental evidence for a partial covalent nature and the observation of a blue-shift in stretching frequency following XH···Y hydrogen bond formation (XH being the hydrogen bond donor and Y being the hydrogen bond acceptor). Considering the recent experimental and theoretical advances, we have proposed a new definition of the hydrogen bond, which emphasizes the need for evidence. A list of criteria has been provided, and these can be used as evidence for the hydrogen bond formation. This list is followed by some characteristics that are observed in typical hydrogen-bonding environments.

A novel definition for the hydrogen bond is recommended here. It takes into account the theoretical and experimental knowledge acquired over the past century. This definition insists on some evidence. Six criteria are listed that could be used as evidence for the presence of a hydrogen bond.