Experimental observations and CNDO/2 calculations for hydroxy stretching frequency shifts, intensities, and hydrogen bond energies of intramolecular hydrogen bonds in ortho-substituted phenols

Abstract

I.r. spectral measurements were made to obtain frequency shifts (Δν_OH) and integrated intensities (A_OH) of the hydroxy stretching vibration for intramolecular hydrogen bonding in ortho-substituted phenols in dilute carbon tetrachloride solution. ¹H N.m.r. spectra were also measured to observe the chemical shift of the hydroxy proton. Two kinds of correlations were found between Δν_OH and A_OH: one was a large increase of A_OH with increasing Δν_OH, and the other a small increase of A_OH. The former was found in phenols having a proton acceptor unconjugated with the benzene ring and the latter in those having a conjugated acceptor. The latter correlation was interpreted as resulting from a large delocalization effect of the OH bond electrons through the hydrogen bond system; this was supported by model calculations using the CNDO/2 method to estimate A_OH. A linear correlation was also found between Δν_OH and Δδ_OH. An empirical equation linearly relating the hydrogen bond energy to Δν_OH is proposed. The calculated force constants, frequency shifts, and energies for the hydrogen bond systems were compared with the experimental data giving a fairly good result.