Molecular rotational resonance

The determination of accurate molecular structure from molecular rotational resonance (MRR) spectra has always been a great challenge to this branch of spectroscopy [/]. There are three basic facts which make this task feasible (1) the free rotation of a rigid body is described in classical as well as in quantum mechanics by only three parameters, the principal inertial moments of the body, Ig, g = x, v, z ... 

Magnetic resonance Magnetic resonance imaging Mobile Raman microscopy Molecular rotational resonance (microwave spectroscopy)... 

Microwave 0.1-30 cm Molecular rotations Electron spin resonance... 

Evidence of molecular rotation may be given by non-crystallographic evidence the transition from a rotating to a non-rotating state is accompanied by sudden changes in specific heat, in dielectric constant, and in width of nuclear magnetic resonance bands (see Chapter VIII). 

An interesting development in molecular rotational relaxation has been the microwave double-resonance method176-178. The technique permits the exploration of the fine detail of the processes which occur in collisions of polyatomic molecules, and results for a number of symmetric tops have been reported. For example, Oka has described experiments on NH3 in which inversion doublets for selected J values were pumped by high microwave power. Pumping disturbs the population of the inversion doublet, and also that of other doublets which are populated from the original pair by collision processes. By absorption measurements of other inversion doublets with steady state irradiation, Oka has shown that in NH3/NH3 collisions, transitions which are allowed by the electric dipole selection rules (A/ = 0, 1, + - —) are preferred. Oka s analysis indicates that relaxation is most favourable in collision with molecules having similar J values, which are termed rotational resonances (R-R transfer). For example the process... 

Due to such small value of this parameter, the loss curve bonded molecule is so damped (mvib 1 /3) that resonance properties are not actually revealed for this type of molecular rotation. The absorption spectra obtained above in terms of the HC-EB model (Fig. 66b,c) evidently are close to spectra (Fig. 63a,b) calculated for the composite HC-SD model. The absorption/loss spectra resemble those found in Section VII. B. 

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. 

Spectroscopic methods, such as FT-infrared (FTIR) and Raman spectroscopy detect changes in molecular vibrational characteristics in noncrystalline solid and supercooled liquid states. Various nuclear magnetic resonance (NMR) techniques and electron spin resonance (ESR) spectroscopy, however, are more commonly used, detecting transition-related changes in molecular rotation and diffusion (Champion et al. 2000). These methods have been used for studies of the amorphous state of a number of sugars in dehydrated and freeze-concentrated systems (Roudaut et al. 2004). 

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