Time-Resolved IR Spectroscopy of N-Methylthioacetamide: Trans → Cis Isomerization upon n−π* and π−π* Excitation and Cis → Trans Photoreaction

Time-resolved infrared spectroscopy was used to study the photoisomerization of N-Methylthioacetamide (NMTAA) in D2O in both the cis → trans and the trans → cis direction upon selective excitation of the n−π* (S1) and π−π* (S2) electronic transitions. While isomerization and the return to the ground state takes place on two distinct time scales (≤8 ps, ∼250 ps) upon π−π* excitation of both cis- and trans-NMTAA in D2O, ground state recovery is only observed on the slower time scale upon n−π* excitation. The quantum efficiency for trans → cis isomerization is 30−40%, independent of the electronic state excited, while the cis → trans isomerization proceeds with a 60−70% quantum efficiency. These results support a mechanism by which isomerization takes place via one common intermediate state independent of electronic excitation energy and initial conformation.