DISCOVERY OF 2 ROTATIONAL ISOMERS OF 4-METHOXYSTYRENE BY LASER-INDUCED FLUORESCENCE IN A SUPERSONIC JET

The S-1-S-0 fluorescence excitation spectrum of 4-methoxystyrene seede d into a supersonic free jet shows two 0(0)(0) bands, corresponding to the cis and trans rotational isomers. Dispersed fluorescence, with ex citation in each of these bands, show that some of the resulting groun d-state vibration wavenumbers are appreciably different. This confirms the 0(0)(0) band assignments to different isomers. One prominent vibr ation has a wavenumber of 373 or 400 cm(-1), depending on the isomer i nvolved. These two bands are also observed in the Raman spectrum of th e liquid and the fact that the 373 cm(-1) band disappears in the solid -state Raman spectrum at 269 K identifies the 0(0)(0) band of the more stable isomer. Ab initio calculations at the 6-31G level indicate th at the trans isomer is the more stable but by only 77 cm(-1). This com pares with a possible value of 174 cm(-1) derived from experimental da ta.