OUT-OF-PLANE VIBRATIONS OF NH2 IN 2-AMINOPYRIMIDINE AND FORMAMIDE

The out-of-plane vibrations of the amino group in primary amines invol ve its inversion and rotation relative to the rest of the molecule. Th ese two vibrations in 2-aminopyrimidine (see Fig. 1) were investigated with the combined use of matrix-isolation IR spectroscopy and ab init io quantum mechanical methodology. The ab initio methodology was also applied to another primary amine, formamide, for which a gas-phase IR spectra have been previously assigned. Ab initio potential energy surf aces were calculated in internal coordinates, omega and tau, whose dis placements mimicked the inversion and internal rotation normal-mode di stortions of the amino group, respectively. Vibrations along these two coordinates were considered uncoupled from all other nuclear motions. Total energy was calculated at the second-order Moller-Plesset pertur bation theory level at selected values of w and tau to allow a least-s quares fitting of an analytical function depicting the potential energ y curves and surface. A numerical procedure for determining the values of the kinetic energy operator in internal coordinates was also imple mented to which an analytical function was fit. Vibrational energy exp ectation values were variationally determined by utilizing products of Gaussian and sinusoidal functions as the basis set. The resultant cal culated fundamental transition energies for the coupled inversion and internal rotation vibrations of 2-aminopyrimidine are upsilon(inv) = 1 40.6 cm(-1) and upsilon(rot) = 440.3 cm(-1), respectively. These theor etical values reasonably match the experimental quantities of upsilon approximate to 200 cm(-1) and upsilon approximate to 500 cm(-1), and a llow firm assignment of these two experimental infrared spectral bands to the inversion and internal rotation vibrations of the amino group in 2-aminopyrimidine, respectively. For formamide (see Fig. 2), the ca lculated transition energies for the inversion and internal rotation v ibrations, upsilon(inv) = 249 cm(-1) and nu(rot) = 602, match the expe rimental frequencies of approximate to 289 cm(-1) and approximate to 6 02 cm(-1), and confirm the accuracy of the theoretical method. (C) 199 8 American Institute of Physics.