Introductory lecture. The theoretical background to vibrational optical activity
The difference in the response of a chiral molecule to left and right circularly polarized light is proportional to d/λ times the total response, where d is a typical internuclear distance and λ the wavelength of the light. Hence, while circular dichroism in electronic transitions in chiral molecules has been studied for many years, vibrational circular dichrosim (VCD) and differential Raman optical activity (ROA) have only been observed in the last 20 years. Major advances in experimental techniques have brought VCD and ROA to the point where differential vibrational spectra of large molecules can be recorded in a reasonable time. The basic theory of the rotational strength for vibrational transitions and of ROA is reviewed. The sum of the rotational strengths of all vibrational transitions from the ground state is shown to be zero. The differential absorption of left and right circularly polarized IR radiation by chiral molecules can also be attributed to the different forces acting on the nuclei that are proportional to the nuclear charge times the local electric field. The prospects for future applications of VCD and ROA are briefly discussed.