Vibronic Coupling through the In-Phase, CC Stretching Mode Plays a Major Role in the 2Ag- to 1Ag- Internal Conversion of all-trans-β-Carotene

Abstract

The major role of vibronic coupling through the in-phase, CC stretching (ν1) mode in the 2Ag- to 1Ag- internal conversion of all-trans-β-carotene has been shown by the use of isotopic effects on the rate of internal conversion and on the strength of vibronic coupling as follows: (1) The rates of internal conversion for all-trans-β-carotene having natural abundance isotope composition [NA], along with 2H-labeled [2H], 13C-labeled [13C], and 2H,13C-doubly labeled [2H,13C] all-trans-β-carotenes, were determined, by subpicosecond time-resolved absorption spectroscopy, to be in the ratio [NA]/[2H]/[13C]/[2H,13C] = 1:0.92:0.70:0.64. (2) The strength of vibronic coupling was estimated for each isotope species by using the frequency difference between the 2Ag- and 1Ag- states, which was determined, by picosecond Raman spectroscopy, to be in the ratio, [NA]/[2H]/[13C]/[2H,13C] = 1:1.21:0.89:1.07. On the other hand, a theory was presented to show that the nonadiabatic vibronic-coupling constant that determines the...