Vibrational spectra and assignments of 2-phenylethanol and 2-phenoxyethanol

The structural stability of 2-phenyl- and 2-phenoxyethanols were investigated at the DFT-B3LYP/6-311G**, MP2 and MP4(SDQ) levels of theory. From the calculations at the three levels of theory 2-phenylethanol and 2-phenoxyethanol were predicted to exist predominantly in non-planar gauche conformations. For 2-phenylethanol the lowest energy Gg1 structure was predicted to be stabilized by an interaction between the hydroxyl H atom and the phenyl ring. For 2-phenoxyethanol the Ggg1 structure was predicted to be strongly stabilized by dipolar interactions between the hydroxyl H atom and the phenoxy O atom of the alcohol. For both alcohols the planar trans structure with minimum steric interactions between the CH2 groups was predicted to be significantly higher in energy than the ground state gauche structure of the alcohols. The dipolar interactions are reported to play more important role than steric ones in stabilizing the molecules. The vibrational frequencies of each of the two alcohols in its lowest energy gauche structure were computed at the B3LYP level and tentative vibrational assignments were made for their normal modes on the basis of the calculated and experimental data.

Atom numbering of the Gg1 form of 2-phenylethanol (upper) and the Ggg1 form of 2-phenoxyethanol (lower).Figure optionsDownload as PowerPoint slideHighlights
► The study represents the analysis of the structure and vibrational spectra of two important H-bonded alcohols.
► The possible structures of 2-phenyl- and 2-phenoxyethanlols were optimized at the DFT-B3LYP, MP2 and MP4(SDQ) levels of theory and their complex multi-rotor conformational profiles were analyzed.
► The vibrational wavenumbers of the lowest energy conformations of each of the two alcohols were calculated at the DFT-B3LYP level of theory.
► Vibrational assignments of the normal modes were made on the basis of combined experimental and theoretical data.