Symmetry molecular rotational resonance

However, the NMR properties of solid-phase methane are very complex, due to subtle effects associated with the permutation symmetry of the nuclear spin set and molecular rotational tunnelling.55 Nuclear spin states ltotai = 0 (irred. repr. E), 1 (T) and 2 (A) are observed. The situation is made more complicated since, as the solids are cooled and the individual molecules go from rotation to oscillation, several crystal phases become available, and slow transitions between them take place. Much work has been done in the last century on this problem, including use of deuterated versions of methane for example see Refs. 56-59. Much detail has emerged from NMR lineshape analysis and relaxation time measurements, and kinetic studies. For example, the second moment of the 13C resonance is found to be caused by intermolecular proton-carbon spin-spin interaction.60 Thus proton inequivalence within the methane molecules is created. 

In addition the reader may find tables with selection rules for the Resonance Raman and Hyper Raman Effect in the book of Weidlein et al. (1982). Special discussions about the basics of the application of group theory to molecular vibrations are given in the books of Herzberg (1945), Michl and Thulstrup (1986), Colthup et al. (1990) and Ferraro and Nakamoto (1994). Herzberg (1945) and Brandmiiller and Moser (1962) describe the calculation of thermodynamical functions (see also textbooks of physical chemistry). For the calculation of the rotational contribution of the partition function a symmetry number has to be taken into account. The following tables give this number in Q-... 

Nuclear magnetic resonance (NMR) spectra can yield information on magnetic properties, rotational states and of the symmetry of both the molecules and then-environment. Mostly, is used as a probe, but in alkali salts, alkali atoms as Na or Li have also been applied. The effect of molecular dynamics, including pseudorotations, on the NMR line shape is thoroughly discussed in [20,28],... 

The H NMR room temperature spectrum of 37b reflects an absence of symmetry, where all three rings of the triptycene are nonequivalent. At 160 °C, but not at lower temperatures, however, some peak broadening of triptycene (but not helicene) resonances is observed, indicating somewhat less retarded rotation. Extrapolation, based on analogy to our earlier work with the molecular brake, suggests a coalescence temperature of about 220 °C, which corresponds to a AG of approximately 25 kcal mol-1, a value that is corroborated by the studies described below and which is reassuringly close to the calculated barrier (AH, not AG ) of 22 kcal mol-1. 

The effects of molecular structure on the rates of energy transfer are manifested in many ways. Additional pathways are available in collisions with molecules, such as the influence of ionic potential surfaces [35], the availability of near resonant electronic-to-vibrational and rotational energy transfer pathways [36], and the introduction of nonadiabatic transitions due to the breaking of the molecular orbital symmetry [37]. For the studies considered here, we might also add the competition between reaction and the desired energy transfer process, the possibility of energy transfer processes in the entrance or exit channels, selective changes in... 

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