IR spectral and structural changes caused by the conversion of 3-methoxy-4-hydroxybenzaldehyde (vanillin) into the oxyanion

The spectral and structural changes, caused by the conversion of the vanillin molecule into the corresponding oxyanion have been studied by IR spectra and normal coordinate calculations within the Onsager self-consistent reaction field (SCRF) model, using a density functional theory (DFT) method at the Becke3LYP/6-31+G** level. Structures of all conformational isomers of vanillin and of its anion have been located, as well as their total and relative energies have been determined. The conversion leads to geometry changes in the whole species, due to the strong O−/acceptor polar resonance through the para phenylene ring. The conversion causes a 41 cm−1 decrease in the frequency of the carbonyl stretching band νCO, strong intensity increases (1.6 – 7.2-fold) of the aromatic skeletal ν8 and ν19 as well as formyl νCH bands. According to the calculations the oxyanionic charge is delocalized over aldehyde group (0.25 e−), phenylene ring (0.13 e−), methoxy group (0.07 e−) and oxyanyonic center (0.55 e−).