Snels Marcel, Hollenstein Hans, Quack Martin
Istituto di Scienze dell'Atmosfera e del Clima (ISAC), Sezione di Roma, Consiglio Nazionale delle Ricerche (CNR), Via del Fosso del Cavaliere 100, I-00133 Roma, Italy.
J Chem Phys. 2006 Nov 21;125(19):194319. doi: 10.1063/1.2366704.
High resolution (0.004 and 0.01 cm(-1) instrumental bandwidth) interferometric Fourier transform infrared spectra of (14)NH2D and (14)ND2H 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 (14)NH2D with term values Tv(s)=1389.9063(2) cm(-1) and Tv(a)=1390.4953(2) cm(-1) for the nu(4b) fundamental and Tv(s)=1605.6404(7) cm(-1) and Tv(a)=1591.0019(7) cm(-1) for the nu(4a) fundamental, and of (14)ND2H with term values of Tv(s)=1233.3740(2) cm(-1) and Tv(a)=1235.8904(2) cm(-1) for the nu(4a) fundamental and Tv(s)=1461.7941(9) cm(-1) and Tv(a)=1461.9918(19) cm(-1) for the nu(4b) fundamental. In all cases Tv(s) gives the position of the symmetric inversion sublevel (with positive parity) and Tv(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 Cs symmetry and a indicates the symmetric, b the antisymmetric normal mode with respect to the Cs 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 2nu(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 2nu(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.
在超声速射流膨胀条件下以及室温下，使用Bomem DA002光谱仪测量了(¹⁴)NH₂D和(¹⁴)ND₂H的高分辨率（仪器带宽分别为0.004和0.01 cm⁻¹）干涉傅里叶变换红外光谱。我们报告了对(¹⁴)NH₂D弯曲基频的分析，对于ν(4b)基频，其项值Tv(s)=1389.9063(2) cm⁻¹和Tv(a)=1390.4953(2) cm⁻¹；对于ν(4a)基频，Tv(s)=1605.6404(7) cm⁻¹和Tv(a)=1591.0019(7) cm⁻¹。对于(¹⁴)ND₂H，对于ν(4a)基频，项值为Tv(s)=1233.3740(2) cm⁻¹和Tv(a)=1235.8904(2) cm⁻¹；对于ν(4b)基频，Tv(s)=1461.7941(9) cm⁻¹和Tv(a)=1461.9918(19) cm⁻¹。在所有情况下，Tv(s)给出对称反转子能级（正宇称）的位置，Tv(a)给出反对称反转子能级（负宇称）的位置。基频ν(4a)和ν(4b)的符号是通过与C₃v对称分子NH₃和ND₃中的简并ν(4)模式相关联来选择的。在Cs对称中简并解除，a表示相对于NH₂D和ND₂H中Cs对称平面的对称正常模式，b表示反对称正常模式。通过基态组合差确定了明确的归属。对于每个基频，可以准确确定有效S约化哈密顿量的约20个分子参数。特别地，观察到2ν(2)泛音与ν(4a)弯曲模式的费米共振效应，导致ND₂H情况下反转分裂增加，以及NH₂D中反转分裂大幅增加且两个反转能级顺序颠倒。用ν(4b)弯曲基频观察到的转动微扰可能是由于与反转泛音2ν(2)的科里奥利相互作用。这些结果对于理解氨反转中的同位素效应以及通过该分子高维势能超曲面上的量子动力学处理的不同振动模式激发对其的选择性催化和抑制具有重要意义。
