The 330-nm S1- S0 electronic spectrum of 1-pyrazoline: Probable hindered pseudorotation in S1 and confirmation of the ring-puckering potential function in S0 by single vibronic level fluorescence spectroscopy

Abstract

The S 1- S 0 absorption spectrum of 1-pyrazoline is rotationally sharp but vibrationally extremely irregular, and other techniques are necessary to aid its assignment. The relaxed fluorescence spectrum shows a very long progression in the NN twisting vibration, suggesting that the ring is twisted in S 1 whereas, in S 0, this part of the ring is planar but the CH 2 group in position 4 is puckered. With a twisted ring in S 1 it seems likely that the NN twisting and CH 2(4) puckering modes in S 0 should be combined and newly described as radial and hindered pseudorotational modes in S 1. The vibronic transitions accompanying such an S 1- S 0 electronic transition are derived. Single vibronic level fluorescence spectra from many vibronic levels of S 1 show progressions in both the NN twisting and CH 2(4) puckering vibrations in S 0, but only with Δ v even. This strongly supports the suggestion that these two modes are heavily mixed in S 1, and indicates that the fluorescing states are either above the barrier to pseudorotation or not far below it, so that there is appreciable tunnelling through the barrier. The progressions in the CH 2(4) puckering vibration allow us to assign uniquely the puckering quantum number, in S 0, of the band in which excitation took place. In addition, the spacings in these progressions further confirm the preferred potential function derived from the far-infrared spectrum and confirmed previously from the microwave spectrum.