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, invo lving 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 hydro gen bonds involving >C=O accepters in ketones, carboxylic acids and am ides. 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 ap proach between the two nitro-O atoms, rather than between the C and O substituents. However, hydrogen approach between the two O accepters i s usually strongly asymmetric, the H atom being more closely associate d 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 p utative O-atom lone-pair directions is clearly observed. Ab-initio-bas ed molecular orbital calculations (6-31G* basis set level), using int ermolecular perturbation theory (IMPT) applied to the nitromethane-met hanol model dimer, agree with the experimental observations. IMPT calc ulations yield an attractive hydrogen-bond energy of ca -15 kJ mol(-1) , about half as strong as the >C=O ... H bonds noted above.