TIME-RESOLVED RESONANCE RAMAN AND MOLECULAR-ORBITAL STUDIES OF THE STRUCTURES OF THE TRANSIENT SPECIES INVOLVED IN THE PHOTOCHROMIC REACTION OF 2,2'-SPIROBI[2H-1-BENZOPYRAN]

Resonance Raman spectra of the transient species of 2,2'-spirobi [2H-1-benzopyran] in various solvents reveal that at least two isomers exist in solution, the relative abundance of which depends on the polarity and hydrogen-bond donor ability of the solvent. Vibrational assignment of the transient species based on C-13 substitutions shows that the Raman bands mainly attributable to the vibrations of the open benzopyran part are enhanced by the Raman excitation wavelengths longer than 460 nm, whereas those assignable to the vibrations of the closed benzopyran part are observed only by the Raman excitation wavelengths shorter than 460 nm. Semi-empirical molecular orbital calculations show that the contribution of the ortho-quinoidal form to the resonance hybrid structure of the transient species is much larger than that of the zwitterionic form. Also, it is shown that the trans-trans-trans (TTT) configuration about the three C-C partial double bonds of the transient species is most stable.