Mode selective tunneling dynamics observed by high resolution spectroscopy of the bending fundamentals of 14NH2D and 14ND2H

High resolution (0.004 and 0.01 cm(-1) instrumental bandwidth) interferometric Fourier transform infrared spectra of (NH2D)-N-14 and (ND2H)-N-14 were measured on a Bomem DA002 spectrometer in a supersonic jet expansion and at room temperature. We report the analysis of the bending fundamentals of (NH2D)-N-14 with term values T-v(s)=1389.9063(2) cm(-1) and T-v(a)=1390.4953(2) cm(-1) for the nu(4b) fundamental and T-v(s)=1605.6404(7) cm(-1) and T-v(a)=1591.0019(7) cm(-1) for the nu(4a) fundamental, and of (ND2H)-N-14 with term values of T-v(s)=1233.3740(2) cm(-1) and T-v(a)=1235.8904(2) cm(-1) for the nu(4a) fundamental and T-v(s)=1461.7941(9) cm(-1) and T-v(a)=1461.9918(19) cm(-1) for the nu(4b) fundamental. In all cases T-v(s) gives the position of the symmetric inversion sublevel (with positive parity) and T-v(a) the position of the antisymmetric inversion sublevel (with negative parity). The notation for the fundamentals nu(4a) and nu(4b) is chosen by correlation with the degenerate nu(4) mode in the C-3v symmetric molecules NH3 and ND3. The degeneracy is lifted in C-s symmetry and a indicates the symmetric, b the antisymmetric normal mode with respect to the C-s symmetry plane in NH2D and ND2H. Assignments were established with certainty by means of ground state combination differences. About 20 molecular parameters of the effective S-reduced Hamiltonian could be determined accurately for each fundamental. In particular, the effect of Fermi resonances of the 2 nu(2) overtone with the nu(4a) bending mode was observed, leading to an increased inversion splitting in the case of ND2H and to a strongly increased inversion splitting and an inverted order of the two inversion levels in NH2D. Rotational perturbations observed with the nu(4b) bending fundamentals are probably due to Coriolis interactions with the inversion overtone 2 nu(2). The results are important for understanding isotope effects on the inversion in ammonia as well as its selective catalysis and inhibition by excitation of different vibrational modes, as treated by quantum dynamics on high dimensional potential hypersurfaces of this molecule. (c) 2006 American Institute of Physics.