ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
Mass resolved excitation spectroscopy (MRES) and high level ab initio calculations are employed to explore the low lying excited states of methyl amine, CH3NH2. Both (1+1) and (2+2) MRES of CH3NH2 produce well resolved vibronic features in the energy region around 39 770 to 46 000 cm−1. A complete data set in this region for (2+2) MRES is presented for the isotopic series CH3NH2, CD3NH2, CH3ND2, and CD3ND2. Two apparent Franck–Condon progressions can be qualitatively characterized in these spectra. In order to identify the excited state vibrations active in these spectra and to identify the nature of the excited electronic state(s) accessed, a rather extensive set of ab initio calculations are undertaken. An open shell Hartree–Fock force constant calculation proves central to assigning the observed vibrations. Agreement between the predicted and observed vibrational frequencies provide the strongest evidence to date for a planar excited state C–NH2 geometry. Using combinations and overtones of only two vibrations, the amine wag and scissors modes, all the major features of the low energy region of the spectra can be assigned for all the isotopically substituted methyl amines. Ab initio calculations indicate that the lowest A′ excited state is an A′ 3s Rydberg and the lowest A″ excited state is a valence electronic state. An additional A′ 3s Rydberg state is also found in this region, which because of its geometry, can be implicated in the methyl hydrogen elimination photodissociation reaction of methyl amine. Complete active space self-consistent field (CASSCF) calculations alone, and augmented by many body perturbation theory (MBPT), are also performed. The spectra are consistent with two excited electronic states in the 40 000 cm−1 region. This new characterization of the low energy absorption spectra, and the interpretation of the high energy region in terms of an addition electronic state, challenge the long held view of the nature of the methyl amine excited states. © 1995 American Institute of Physics.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.469895
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