The crystal structure of 2 , 6-dimethyl-4-nitro-phenol , C 8 H 9 NO 3

C8H9NO3, monoclinic, P21/n (no. 14), a = 6.9121(4) Å, b = 26.9700(15) Å, c = 8.7204(5) Å, β = 106.152(3)°, V= 1561.48(16) Å, Z=8,Rgt(F ) = 0.0486,wRref(F ) = 0.1399, T = 200 K.


Experimental details
Carbon-bound H atoms were placed in calculated positions (C-H 0.95 Å for aromatic carbon atoms) and were included in the refinement in the riding model approximation, with U(H) set to 1.2U eq (C).  The H atoms of the methyl groups were allowed to rotate with a fixed angle around the C-C bond to best fit the experimental electron density (HFIX 137 in the SHELX program suite [3]), with U(H) set to 1.5U eq (C).
The H atoms of the hydroxyl groups were allowed to rotate with a fixed angle around the C-O bond to best fit the experimental electron density (HFIX 147 in the SHELX program suite [3]), with U(H) set to 1.5U eq (O).

Comment
Phenols are interesting bonding partners for a variety of transition metals and elements from the p-block of the periodic system. They can act as neutral orupon deprotonationas anionic monodentate ligands. Upon variation of the substituents on the aromatic system, a seemingly endless series of symmetric as well as asymmetric phenol derivatives featuring different steric pretenses and acidities of the hydroxyl-group are available. For phenol and phenolate, all bonding modes from monodentate (e.g. towards beryllium [7]) via bidentate (e.g. towards tin [8]) and tridentate (e.g. towards zinc [9,10]) to tetradentate (e.g. towards sodium [11]) have been observed and secured based on structural studies conducted on single crystals. In continuation of our interest in structural features of derivatives of phenol [12][13][14][15][16] the molecular and crystal structure of the title compound was elucidated.
The title structure proves the presence of a phenol derivative bearing two methyl groups in ortho position to the hydroxyl group as well as nitro group in para position to the protic substituent. The asymmetric unit contains two complete molecules (see the figure). C-N bond lengths of 1.4537(18) and 1.4546(18) Å as well as C-O bond distances of 1.3542(17) and 1.3578 (17) Å are in good agreement with other nitro-and hydroxy-substituted compounds whose structures have been deposited with the Cambridge Structural Database [17]. Both molecules are essentially flat as the least-squares planes defined by their respective nonhydrogen atoms show maximum deviations of only 0.022(2) and 0.033(2) Å respectively (invariably for one of the exocyclic carbon atoms)a finding that can be rationalized on grounds of extensive resonance stabilization that engulfs the substituents able to partake in it. The two molecules are further almost co-planar as the least-squares planes as just defined enclose an angle of 5.40(4)°only.
In the crystal, classical hydrogen bonds of the O-H/O type are observed next to C-H/O contacts whose range falls by more than 0.1 Å below the sum of van-der-Waals radii of the atoms participating in them. The classical hydrogen bonds employ one of the oxygen atoms of the nitro group in a symmetry-generated equivalent of the atom bearing the donating O-H group as acceptor. In contrast to this,  the C-H/O contactsinvariably supported by one hydrogen atom on one of the methyl substituents eachseek the remaining oxygen atom of the nitro group in the other molecule present in the asymmetric unit as acceptor. In terms of graph-set analysis [18,19], the descriptor for the classical hydrogen bonds is C 1 1 (8)C 1 1 (8) on the unary level while the C-H/O contacts necessitate a R 2 2 (14) R 2 2 (14) on the binary level. In total, the two molecules present in the asymmetric unit are connected to strands along the crystallographic c axis. The crystal structure is further characterized by π stacking interactions with the shortest distance between two centers of gravity measured at only 3.7608(8) Å.
Author contribution: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.