Hydrogen-bond acceptor properties of nitro-O atoms: A combined crystallographic database and ab initio molecular orbital study

Crystallographic data for 620 C-nitro-O ... H-N,O hydrogen bonds, involving 560 unique H atoms, have been investigated to the van der Waals limit of 2.62 Angstrom. The overall mean nitro-O ... H bond length is 2.30 (1) Angstrom, which is much longer (weaker) than comparable hydrogen bonds involving >C=O accepters in ketones, carboxylic acids and amides. The donor hydrogen prefers to approach the nitro-O atoms in the C-NO2 plane and there is an approximate 3:2 preference for hydrogen approach between the two nitro-O atoms, rather than between the C and O substituents. However, hydrogen approach between the two O accepters is usually strongly asymmetric, the H atom being more closely associated with one of the O atoms: only 60 H atoms have both O ... H distances less than or equal to 2.62 Angstrom. The approach of hydrogen along putative O-atom lone-pair directions is clearly observed. Ab-initio-based molecular orbital calculations (6-31G** basis set level), using intermolecular perturbation theory (IMPT) applied to the nitromethane-methanol model dimer, agree with the experimental observations. IMPT calculations yield an attractive hydrogen-bond energy of ca -15 kJ mol(-1), about half as strong as the >C=O ... H bonds noted above.