Conformational Isomerization of 5-Phenyl-1-pentene Probed by SEP-Population Transfer Spectroscopy

Stimulated emission pumping-population transfer (SEP-PT) spectroscopy is used to experimentally determine upper and lower bounds on the energy thresholds to conformational isomerization between 14 X→Y reactant−product conformer pairs of isolated 5-phenyl-1-pentene (5PPene). This work builds directly on the spectroscopic assignments of the five observed conformers of 5PPene in the preceding paper. The observed thresholds fall into two energy ranges: near 600 cm−1 for isomerization processes that involve only reorientation of the terminal vinyl group, and in the 1200−1374 cm−1 range for barriers that involve hindered rotation about the alkyl chain carbon−carbon bonds. As a result, this latter threshold opens up much of the conformational phase space to exploration, with multiple isomerization pathways connecting any two of the conformational minima.