Rovibrational effects

S. Rossano, F. Mauri, C. J. Pickard and 1. Farnan, First-principles calculation of and Mg NMR shieldings in MgO at finite temperature rovibrational effect in solids. /. Phys. Chem. B, 2005,109,7245-7250. 

We show in this subsection some calculational results for the H2/Cu(lll) system which has become the prototype for studying dissociative chemisorption on surfaces. The results presented in this subsection are obtained from calculations at the 3D flat-surface level using a LEPS PES (117). We are interested in rovibrational effect of hydrogen on dissociation. In particular, we will present some interesting results showing the effect of molecular orientation and homonuclear symmetry on dissociative chemisorption of hydrogen on Cu(lll). 

Absolute Shielding Scales. - For lighter elements, the way to the absolute shielding involves a non-relativistically valid relation between the spin-rotation constant, C, and shielding a, applied to at least one primary reference molecule. A review of how spin-rotation constants relate to shielding constants was provided by Bryce and Wasylishen. The C and a tensors for the primary reference are corrected for rovibrational effects and then connected to a commonly used secondary reference (usually a neat liquid substance) via an NMR measurement in the gas phase at the limit of zero density and in the liquid... 

On top of the basic and complicated theoretical problem, real practical problems exist in actually performing a shielding calculation. As discussed earlier, one must select a suitable basis set and a molecular geometry, decide how or indeed whether to deal with rovibrational effects, and devise a strategy for combating the gauge problem. We discuss these problems in the examples presented later in this chapter. 

Vaara, J., Lounila, J., Ruud, K., and Helgaker, T. (1998). Rovibrational effects, temperatme dependence, and isotope effects on the nuclear shielding tensors of water a new absolute shielding scale. J. Chem. Phys., 109, 8388 397. 

The approach is ideally suited to the study of IVR on fast timescales, which is the most important primary process in imimolecular reactions. The application of high-resolution rovibrational overtone spectroscopy to this problem has been extensively demonstrated. Effective Hamiltonian analyses alone are insufficient, as has been demonstrated by explicit quantum dynamical models based on ab initio theory [95]. The fast IVR characteristic of the CH cliromophore in various molecular environments is probably the most comprehensively studied example of the kind [96] (see chapter A3.13). The importance of this question to chemical kinetics can perhaps best be illustrated with the following examples. The atom recombination reaction... 

Molecules vibrate at fundamental frequencies that are usually in the mid-infrared. Some overtone and combination transitions occur at shorter wavelengths. Because infrared photons have enough energy to excite rotational motions also, the ir spectmm of a gas consists of rovibrational bands in which each vibrational transition is accompanied by numerous simultaneous rotational transitions. In condensed phases the rotational stmcture is suppressed, but the vibrational frequencies remain highly specific, and information on the molecular environment can often be deduced from hnewidths, frequency shifts, and additional spectral stmcture owing to phonon (thermal acoustic mode) and lattice effects. 

The electronic predissociation from different rovibrational levels of the A and B electronic states has been evaluated, to study the effect of the initial excitation on the process. The bounds states chosen are 1,2,3 and 6 for A, and fc=l,2,3,6 and 12 for B, which correspond to the bending progression of states appearing in the... 

We have also learned that VMP is an effective tool in molecular spectroscopy and molecular dynamics studies. It is effective, in particular, for determination of IVR lifetimes and for studying the vibrational spectroscopy of states that are difficult to study applying other methods. The above-mentioned limit of the size of the molecule is irrelevant here. For observing the mode selectivity in VMP, the vibrational excitation has to survive IVR in order to retain the selectivity since the subsequent electronic excitation has to be from the excited vibrational state. In contrast, monitoring vibrational molecular dynamics relies only on the efficacy of the excitation of the specific rovibrational state. When IVR is fast and rovibrational distribution reaches equilibrium, the subsequent electronic excitation will still reflect the efficacy of the initial rovibrational excitation. In other words, whereas fast IVR precludes mode selectivity, it facilitates the unraveling of the vibrational molecular dynamics. 

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