Carbon-13 chemical shifts of methanolic carbon and electron reorganization induced by hydrogen bonding for methanol–base complexes

Abstract

Hydrogen-bonding effects in seven complexes between methanol and n or π bases have been studied by ¹³C n.m.r. spectroscopy and CNDO/2 calculations. Methanol–pyridine associations involve only a transfer of σ electrons from aromatic bases to the hydroxy compound, but in methanol an increase of electron population on the carbon atom is observed along an axis perpendicular to the plane of complex. This suggests a reorganization of in-plane and out-of-plane complex electrons in the methanolic moiety. Linear relationships are established, between i.r. frequency shifts (Δν_OH) and semi-empirical quantum parameters, but with different slopes depending on the nature of the substituent. This behaviour is attributed to the method of calculation since a unique linear correlation is obtained when Δν_OH is compared with an experimental parameter such as the ionization potentials of pyridines. Associations induce a shielding effect in the methanol carbon atom whereas the corresponding electron population decreases. A good linear relationship with a negative slope is established between ¹³C chemical shifts and Δν_OH values. Therefore, the sensitivity of the sp³ carbon chemical shift to electron population is reversed relative to sp² carbon behaviour in neutral molecules or in phenol complexed with bases.