The large 1¹A_g^--2¹A_g^-C=C and C-C stretch vibronic interaction in all-trans polyenes

An ab initio theoretical approach at several levels of theory is employed to calculate the 11Ag--21Ag- (non)adiabatic vibronic couplings induced by the completely in-phase C=C and C-C bond stretching vibrations of five all-trans polyenes (from butadiene to dodecahexaene). The nonadiabatic couplings are shown to fit a linear relation with the natural logarithm of the number of double bonds, enabling a general law to be developed for the variation of the adiabatic coupling with chain elongation. Lower-level calculations demonstrate that higher-order excitations are important for the accurate calculation of the couplings.