Rotational-vibrational interconversion

Figure 9 Probabilities for rotational-vibrational interconversion in the scattering of H2 as a function of the molecular orientation at the classical turning point [71].

Rotational Raman spectra of N2 and O2 Resonance fluorescence spectrum of Br2 Vibrational-rotational spectra of CD3H and CH3D Spectrophotometric study of stabihty of metal ion-EDTA complexes Kinetics of the H2 + I2 = 2HI reaction in the gas phase Weak-acid catalysis of BH4 decomposition Photochemistry of the cis-trans azobenzene interconversion Isotope effect on reaction-rate corrstants... 

The dipolar structure (rdipolar interactions without correction for vibrations, but in which slower motions, if present (e.g., the rotation of methyl groups and the interconversion of conformers such as ring inversion or pseudorotation), have been accounted for. 

Water clusters have been studied extensively both experimentally and theoretically over the past two decades. Among the issues that have been addressed are the geometrical structures of the low-energy isomers [1-19], the role of cooperative effects in the bonding [20-26], the sensitivity of the vibrational and rotational spectra on the H-bonding arrangements [27-36], the dynamics of isomer interconversion [37-39], and the finite temperature behavior of the clusters [39-47]. 

Ab initio calculations of rotational barriers of single bonds between elements of group 14 predict a linear relationship between the bond lengths and the barrier, for instance for the series H3SiXH3 with X = C, Si, Ge, Sn and Pb [1]. The barriers decrease from about 6.7kJ/mol for Si-C to about 3.8 for Si-Si and 1.7 kJ/mol for Si-Pb bonds. For the spectroscopist interested in molecular conformations, barriers that lie well above RT at room temperature (2.48 kJ/mol) support the expectation that the interconversion of rotamers will be slow on the time scale typical for Raman vibrational spectroscopy (= lo sec) and that the rotamers can be distinguished by their individual vibrational spectra. It is our objective to compare conformational stabilities of carbosilanes (Si-C bonds) with those of disilanes (Si-Si bonds) bearing identical substituents. Fore this purpose, we have prepared the title compounds 1-4 and investigated their conformational compositions by variable temperature Raman spectroscopy. 

In anticipation of the discussion of photochemical reactions in the next chapter, it might be noted that spin-orbit coupling alone cannot promote intersystem crossing between the first excited, open shell, singlet and the triplet. Both have the same space symmetry, so their interconversion would require the presence of a totally symmetric rotation - non-existent in C2v to produce a triplet component with an Ai spin-function. Direct intersystem crossing in the parent methylene therefore has to rely on spin-vibronic coupling with the antisymmetric stretching vibration, of Fig. 4.3, its only non-totally symmetric... 

Such processes, in which kinetic energy is conserved, are known as elastic processes. In general, when the effect of molecular structure is considered, collisions are inelastic i.e., there is interconversion of translational and rotational or vibrational energy. Only inelastic collision processes can lead to chemical reaction. 

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