Validation of potential energy distribution by VEDA in vibrational assignment some of β-diketones; comparison of theoretical predictions and experimental vibration shifts upon deutration

Abstract

The harmonic vibrational frequencies of the cis-enol forms of some of β-diketones with different substitution in beta position, vis. H, CH3, and Ph ring, as the symmetric and asymmetric molecules, were calculated using density functional theory (DFT) at the B3LYP/6–311++G(d,p) level of theory. The results of DFT calculations were used to obtain the potential energy distribution (PED) by VEDA software. The PED results compared with the Gauss View animation, as our reassignments, and the experimental IR shifts upon deuteration of hydrogen in the OH and CHα. According to our study, the PED contributions, Gauss View animation and observed shifts show similar results for most of the bands which are not coupled with the OH and/or CHα bending, such as asymmetric and symmetric CH3 stretching and in-plane deformations, CH3 rocking vibrations and 8a, 19b, 9a, 15, 18a, and 12 motions of the phenyl ring. The largest discrepancies were observed in the 1700-1000 cm−1 region, likely due to the coupling with the OH and CHα in-plane bending vibrations, such as νaC = C–C = Ο, νsC = C–C = Ο and δOH. Furthermore, the calculated PED contributions by VEDA software do not well define the vibrational contributions to those groups in the molecule that are directly involved in the intramolecular hydrogen bond and the observed failure of the VEDA procedure is possibly due to inappropriateness of the default options.