THE (N, 3S) RYDBERG STATE IN DEUTERATED ALDEHYDES - JET-COOLED MPI SPECTRA OF THE SYM AND ASYM ROTAMERS OF CHD2CHO AND CDH2CHO

The jet-cooled, 2 + 1 resonance enhanced multiphoton ionization (REMPI ) spectrum of the 3s <-- n Rydberg transition has been measured for se veral isotopically substituted acetaldehydes utilizing a time-of-fligh t detection system. The 0(0)0, 10(0)1, 10(0)2, and 9(0)1 bands have be en assigned for the isotopomeric series CH3CHO, CD3CHO, CH3CDO, and CD 3CDO. The same bands of the partially methyl deuterated acetaldehydes, CH2DCHO and CHD2CHO, show a characteristic splitting due to the possi ble sym and asym rotamers of the methyl group. Furthermore, the nontot ally symmetric vibrational mode, nu14, appears in the spectra of the a sym rotamers which is due to the loss of the plane of symmetry in thes e 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 stat e 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 6 1.3 cm-1 less than that of the sym rotamer in the (n, 3s) Rydberg stat e 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 lowe st ionic state, and the (n, 3s) Rydberg state. (C) 1994 Academic Press , Inc.