Comparison of IR and Raman forms of vibrational optical activity

Natural vibrational optical activity consists of two principal forms. The IR form is known as vibrational circular dichroism (VCD) and is simply the extension of electronic circular dichroism into the IR vibrational region of the spectrum. The Raman form, known as Raman optical activity (ROA), is a new form of optical activity that has no counterpart in the classical forms of optical activity. In this paper, the similarities and differences of the IR and Raman forms of vibrational optical activity will be examined. Although both VCD and ROA were discovered and confirmed in the period from 1973 to 1975, each field has evolved independently with key advances in theoretical description, instrumentation and application coming at different times over the past 20 years. The current relative strengths and weaknesses of VCD and ROA will be discussed, and specific examples of VOA spectra of (-)-alpha-pinene and the amino acid L-alanine, for which overlapping VCD and ROA data are available, will be presented.