The (n, 3s) Rydberg State in Deuterated Aldehydes: Jet-Cooled MPI Spectra of the sym and asym Rotamers of CHD2CHO and CDH2CHO

Abstract

The jet-cooled, 2 + 1 resonance enhanced multiphoton ionization (REMPI) spectrum of the 3s ← n Rydberg transition has been measured for several isotopically substituted acetaldehydes utilizing a time-of-flight detection system. The 000, 1010, 1020, and 910 bands have been assigned for the isotopomeric series CH3CHO, CD3CHO, CH3CDO, and CD3CDO. The same bands of the partially methyl deuterated acetaldehydes, CH2DCHO and CHD2CHO, show a characteristic splitting due to the possible sym and asym rotamers of the methyl group. Furthermore, the nontotally symmetric vibrational mode, ν14, appears in the spectra of the asym rotamers which is due to the loss of the plane of symmetry in these isotopomers. The zero-point energy level of the sym rotamer lies 60.0 cm−1 lower than that of the asym rotamer in the (n, 3s) Rydberg state of CH2DCHO, compared with a corresponding splitting of 15.5 cm−1 in the ground state. The zero-point energy level of the asym rotamer is 61.3 cm−1 less than that of the sym rotamer in the (n, 3s) Rydberg state of CHD2CHO (assuming 15.5 cm−1 in the ground state). For comparison, vibrational frequencies and zero-point energies were determined from ab initio molecular orbital calculations of the ground state, the lowest ionic state, and the (n, 3s) Rydberg state.