Excited-State Structure and Dynamics of Isoprene from Absolute Resonance Raman Intensities.

Absolut resonance Raman excitation profiles have been measured for isoprene (2-methylbutadiene) in resonance with lowest allowed 1 B u excited electronic state at 220 nm. The absolute cross sections and the absorption spectrum have been modeled by using a time-dependent, wavepacket propagation technique to quantitate the ground-to-excited-state geometric distortions for ten totally symmetric modes. Strong methylene wagging overtone intensities indicate a substantial reduction of C=C torsional force constants in the 1 B u state; the frequencies of the a u and b g wags decrease from ground-state values of 622 and 755 cm -1 to 0±200 and 300±100 cm -1 , respectively