Raman properties of chlorophyll d, the major pigment of Acaryochloris marina: Studies using both Raman spectroscopy and density functional theory

The Raman spectroscopy of purified chlorophyll (Chl) d extracted from Acaryochloris marina has been measured over the wide region of 250-3200 cm ^-1 at 77 K following excitation of its Soret band at 488 nm and analyzed with the aid of hybrid density-functional vibrational analyses. A Raman peak specific to Chl d, which arises from the formyl group 3¹ C = O stretching, was clearly observed at 1659 cm^-1 with medium intensity. Peaks due to other C = O stretching vibrations of the 13¹ keto-, 13³ ester- and 17³ groups were also observed. Four very strong peaks were observed in the range of 1000-1600 cm^-1, assigned to the CC stretching and mixtures of the CH₃ bend and CN stretching. CCC and NCC bending contribute to medium intensity peaks at 986 and 915 cm^-1. Out-of-plane CH bending at Chl d methine sites 10, 5 and 20 contribute to observed peaks at 885, 864 and 853 cm^-1, respectively. A few modes involving the MgN stretching and MgNC bending motions were observed in the very low frequency range. Density functional theory (DFT) calculations have been used to make assignments on the observed Raman spectrum and the DFT results have been found to be in good agreement with the experimental results.