Raman excitation of (+)-(R)-methyloxirane and its origin of optical activity via bond polarizabilities



We studied the bond polarizabilities of chiral (+)-(R)-methyloxirane from its Raman intensities. The bond polarizabilities provide much information concerning the electronic structure of its nonresonant Raman-excited virtual state. At the initial moment of Raman excitation by the 514.5 nm laser, the tendency of the excited charges (mapped out by the bond polarizabilities) is to spread to the methine bond near the stereogenic center and its triangular oxirane skeleton. Thereby, the coupling of the electric dipole induced by the excited charges in the methine bond and the magnetic moment vibrationally induced by the electric current in the triangular oxirane skeleton as the molecule vibrates is shown to be the key factor leading to its significant Raman chirality. When the final stage of Raman relaxation is approached, the relative magnitudes of the bond polarizabilities are congruent to the bond electronic densities of the ground state, which are otherwise by the theoretical quantities via the quantum chemical calculation. During Raman relaxation, we found that the polarizabilities of the peripheral C[BOND]H bonds relax faster than the rest, as indicated by their relaxation characteristic times. Copyright © 2010 John Wiley & Sons, Ltd.