CHANGES IN THE CARBON-CARBON AND CARBON-NITROGEN BOND ORDERS IN THE MACROCYCLE OF CHLOROPHYLL A UPON SINGLET AND TRIPLET EXCITATION AS PROBED BY RESON ANCE RAMAN SPECTROSCOPY OF NATURAL-ABUNDANCE AND SINGLY AND DOUBLY LABELED SPECIES WITH 15N, 13C, AND 2H ISOTOPES

The Raman spectra of chlorophyll a (1700−1000 cm-1) in the S0, S1, and T1 states were recorded for the species of natural-abundance (NA) isotopic composition and those species that were totally labeled with the 15N, 13C, 13C + 15N, 2H, and 2H + 15N isotopes. An empirical normal-coordinate analysis using the S0 Raman lines of all the isotope species was performed to establish their assignments and to determine a set of force constants including the stretching, bending, nonbonded repulsive force constants and their cross-terms. The Ca−Cm and Ca−N stretching force constants in the S1 and T1 states were also determined, on the basis of the assumption that other force constants were the same as those in the S0 state, to achieve the classification of the observed Raman lines of the NA-, 13C-, and 2H-Chl a into two groups:  those which shift largely upon the 15N-substitution and are associated with the Ca−N stretching vibrations and those which do not and are associated with the carbon−oxygen and carbon−carbon s...