Understanding The Coupling Rules Of Stretching C-H Modes Of Glycerol Molecule: A DFT Study

The vibrational coupling of stretching C-H modes extensively exists in organicmolecules，it is worthy of further study that how stretching C-H modes vibration couplingis dependent on the molecular conformations. This study focuses on the coupling rules ofC-H stretching modes of glycerol molecule (CH2OH-CHOH-CH2OH) within the13stableconformations optimized at M06/6-311++G(d,p) level of density functional theory (DFT).At first, each uncoupling frequency was obtained with deuterium instead of the otherfour hydrogen atoms. It is found that there is a well linear relationship between theuncoupling C-H frequency and the C-H bond length, i.e. The C-H frequency is increasingby1.1332cm-1with the decrease of bond length of10-4.Secondly, coupling C-H modes for the13conformations were obtained with the linearcombination of the five uncoupling C-H modes. Combined with the method of PED, thepotential energy distributions of the five uncoupling C-H modes to each coupling C-Hmode were obtained.The main coupling way of C-H modes occurred on the common carbon atom for themost conformers. While, is found as a special conformer whose coupling effect of C-Hmodes mainly occurred on the non-common carbon atom and it existed the stronger and theweaker absorption intensity among the thirteen conformations. In addition, in-conformation, the five uncoupling C-H modes make greater contributions to the couplingC-H modes(＞90%) which means that each absorption peak in the C-H stretching region iscorresponding to the independent C-H stretching mode.Our results may be helpful for the assignment of spectrum in the C-H stretching region.