STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS—VI. AB INITIO CONFIGURATION INTERACTION CALCULATIONS ON THE GROUND AND LOWER EXCITED SINGLET AND TRIPLET STATES OF ETHYL BACTERIOCHLOROPHYLLIDE‐a AND ETHYL BACTERIOPHEOPHORBIDE‐a‡

Ab initio quantum mechanical calculations on ethyl bacteriochlorophyllide‐a (Et‐BChl‐a) and ethyl bacteriopheophorbide‐a (Et‐BPheo‐a) are presented, including self‐consistent‐field (SCF) molecular orbital studies on the ground states using the molecular fragment procedure, and configuration interaction (CI) calculations on the low‐lying singlet and triplet states and absorption spectra. A characterization and comparison of many of the higher‐lying molecular orbitals obtained from the SCF studies is presented. The estimated first ionization potentials are 5.66 and 5.97 eV for Et‐BChl‐a and Et‐BPheo‐a, respectively. Excited state calculations show that the visible spectrum of both molecules consists of an intense, y‐polarized S1← S0 transition and a weakly‐allowed, x‐polarized S2← S0 transition. Both S1 and S2 states are 1(π, π*) in character, and are described by a four‐orbital model. Transitions to the remaining calculated states, S3‐S12, appear in the Soret region of the spectrum of both molecules. However, only transitions to S9(‘x’), S10(‘x’) and S11(‘y’) of Et‐BChl‐a, and S7(‘x’) and S10(‘y’) of Et‐BPheo‐a are of high intensity. The composition of the high intensity Soret states is 1(π, π*) and strongly “four‐orbital” in nature. The lowest triplet state, T1, is predicted to lie 9752 cm‐1 and 7880 cm‐1 above S0 for Et‐BPheo‐a and Et‐BChl‐a, respectively. In each molecule T2 and S1 are nearly degenerate, suggesting a favorable pathway for intersystem crossing. Calculated Tn← T1 transitions indicate that the y‐polarized T12← T1 transition in Et‐BChl‐a corresponds to the observed intense 24,400 cm‐1 absorption in the triplet‐triplet spectrum of BChl‐a. A similar type spectrum is also predicted for BPheo‐a.