Electronic excitation spectra of jet-cooled phenyl isocyanate, m-tolyl isocyanate, and p-phenylene diisocyanate: Assignment of the cis and trans rotational isomers.

Abstract

To assign cis and trans isomers of m-tolyl isocyanate (mTI) and p-phenylene diisocyanate (pPDI) in the electronic excitation transition, we measured the time-of-flight mass-selected resonant ionization spectra of jet-cooled phenyl isocyanate (PI), mTI, and pPDI in the region of the 275nm first ππ* absorption system. In the excitation spectra of jet-cooled mTI and pPDI, cis- and trans-rotational isomers appeared as doublets. Isomers were assigned by analyzing the methyl-group internal rotation for mTI and by applying low-frequency bending vibrations to the mutual exclusion rule between the one- and two-photon spectra for pPDI. The electronic spectra of the three molecules observed in the jet were assigned to the transition to the first ππ* and third singlet excited states with the aid of time-dependent (TD)-B3LYP/aug-cc-pVDZ and TD-CAM-B3LYP/aug-cc-pVDZ calculations. The 0 - 0 band of PI was observed at 36 354cm-1, those of the cis and trans isomers of mTI at 36 018 and 35 853cm-1, respectively, and those of the cis and trans isomers of pPDI at 34 437 and 34 383cm-1, respectively. All vibronic bands were diffuse, probably because of internal conversion to two singlet nπ* states. For mTI, based on changes in the barrier height of methyl-group internal rotation upon excitation, the Hammett-σm of PI was determined to be -0.12.