STRUCTURE OF METHYLAMINE MOLECULE IN AN EXCITED ELECTRONIC STATE

Ultraviolet absorption spectra of four isotopic methylamines, CH3NH2, CH3ND2, CD3NH2, and CD3ND2, have been observed in their gaseous states in the spectral region of 2000-2500 Å. There is an electronic band whose 0-0 peaks are observed respectively at 2397, 2376, 2392, and 2372 Å. The vibrational structure of the band was analyzed in terms of two frequencies of the ground state and two frequencies of the excited state. The two frequencies of each state were assigned to the amino-wagging and methyl-rocking vibrations. On the basis of the observed energy levels of the amino-wagging vibration in the CH3NH2 and CH3ND2 molecules, the potential function of the excited molecule along this coordinate was determined: it has a broad minimum at the position of the planar amino group, and it is nearly harmonic up to the position where the HNH plane inclines 90° from the CN bond. On the basis of the observed vibrational frequencies, a normal coordinate treatment was made of the four methylamine molecules in the excited state. By assuming the values of two force constants (fw and fr), all of the seven normal frequencies observed of the four excited methylamines were explained. It was shown that the vibrational coupling of the amino-wagging and methyl-rocking modes becomes greater in the order of CH3ND2, CH3NH2, CD3ND2, and CD3NH2. © 1969.