Density functional theory investigations on the conformational stability and vibrational spectra of the model compound of vitamin K

Becke 3-Lee-Yang-Parr density functional theory (DFT/B3LYP) using 6-31G(d) and 6-311G(d) basis sets and the scaled quantum mechanics (SQM) force field method are used to study molecular conformations and vibrational spectra of a model compound of vitamin K (VK). In this molecule, there are six conformers on the torsional potential energy map of the dihedral angles C8C14C15C16 (α) and C7C8C14C15 (β). It is shown that the VK_1 conformer (α = 237.7° and β = 274.2°) is the most stable form. For this lowest energy conformer, the harmonic force fields calculated by B3LYP/6-311G(d) and B3LYP/6-31G(d) methods are scaled with one scale factor of 0.9623 and a set of different scale factors transferred from the previous studies for the other similar molecules, respectively. The vibrational frequencies, IR intensities, and Raman activities are obtained for the lowest energy conformer. On the basis of B3LYP calculations, the normal mode analysis is performed to assign the vibrational fundamental frequencies according to the potential energy distributions. The computational frequencies are in good agreement with the observed results.