Dipole moment of the intramolecular hydrogen bond. 2-Nitrophenol and its derivatives

Dipole moments of substituted 2-nitrophenols VIa-VIf and substituted 2-nitroanisoles VIIa to VIIf were measured in benzene and dioxan solutions. Infrared spectroscopy confirmed that nitrophenols VI exist either solvent and at different concentrations as non-associated molecules with an intramolecular hydrogen bond. Therefore, the difference between the experimental dipole moment and that calculated from group moments can be attributed to charge redistribution raised by the hydrogen bond. Only a minute part of it may be due to electron transfer through the ring (conjugation of the functional groups) as follows particularly from he comparison with nitroanisoles VII. Nevertheless, the charge transfer, expressed as the vector μH, amounts only 1.7 . 10-30 Cm (at an angle of 138° to the H-O bond), i.e. several times less than observed previously in compounds with more powerful hydrogen acceptors. CNDO/2 and MNDO calculations agree fairly with the gross dipole moments of the compounds investigated but are unable to predict μH, not even as far as its direction is concerned.