Nitrogen electron nuclear double resonance and proton triple resonance experiments on the bacteriochlorophyll cation in solution.

Electron nuclear double resonance signals of the pyrrole nitrogens of bacteriochlorophyll alpha cation in a solution of CH2Cl2/CH3OH (6:1) have been observed. The nitrogens are inequivalent: the hyperfine coupling constants were determined to be 2.36 and 3.18 MHz (0.84 and 1.14 G, respectively). Triple electron nuclear double resonance experiments have yielded the signs and values of nine proton hyperfine coupling constants. The assignment based on these data agrees well with previous work (Borg, D.C., Forman, A. & Fajer, J. (1976) J. Amer. Chem. Soc. 98, 6889-6893); in addition, two methine protons have been identified. Spin densities calculated from the hyperfine couplings are compared to reported molecular orbital calculations.