Experimental studies of rotational relaxation

Schlemmer S, Kuhn T, Lescop E, Gerhch D. (1999) Laser excited Nj in a 22-pole trap, experimental studies of rotational relaxation processes. Int. J. Mass Spectrom. Ion Proc. 185 589-602. 

The factors which influence the rate of rotational relaxation are briefly discussed in the next two sections. These are then considered in relation to experimental studies of chemical reaction in which rotational effects may be of importance. 

Samson and Deutch [258] and Hess [259a] have also discussed the reaction of anisotropic molecules, though only Hess considered rotational relaxation effects. No studies have used the experimentally measured values of rotational relaxation times, which may be 1.5—10 times faster than the Debye equation, eqn. (108), predicts. The theory of Sole and Stockmayer [256] will underestimate the rate of chemical reactions when rotational relaxation is faster than they assumed. 

Finally, it is also interesting to compare the result (9.65) to the result (8.106) ofthe very different semiclassical formalism presented in Section (8.3.3). If we identify y of the present treatment with the factor Zgoi/< Eq. (8.96) the two results are identical for e = hco ksT. The rotating wave approximation used in the model (9.44) cannot reproduce the correct result in the opposite, classical, limit. Most studies of vibrational relaxation in molecular systems are done at temperatures considerably lower than s/ks, where both approaches predict temperature-independent relaxation. We will see in Chapter 13 that temperature-dependent rates that are often observed experimentally are associated with anhannonic interactions that often dominate molecular vibrational relaxation. 

Cl(jj) (w 25 ps ) and not two. We note that the initial decay of Cy(t) corresponds to an apparent solvent drag which is much smaller than hydrodynamic estimates that assume stick boundary conditions. For example, treating the methyl group as a sphere of radius a = 2.5 X (the van der Waals radius) to obtain the frictional coefficient, f, f = Sirria (ri is the shear viscosity, 0.01 P) we find 2I/f equal to 0.0003 ps, i.e., about 150 times smaller than that observed. In this sense, the observed drag is nearer to the hydrodynamic slip boundary condition limit the exact slip limit for a sphere corresponds to f = 0 and an infinite relaxation time. The relatively long relaxation time is consistent with the results of experimental studies of the rotational motion of small nonassoclated molecules ( ). 

Experimental measurements of dielectric relaxation confirm qualitatively the predictions of the rotational diffusion model, in that g has a low and high frequency relaxation, while g only shows relaxations at higher frequencies. Unfortunately there are few liquid crystal systems that have been studied over wide frequency ranges, and measurements at high frequencies >50 MHz on aligned samples are difficult. Some typical results are shown in Fig. 19. 

Despite the fact that relaxation of rotational energy in nitrogen has already been experimentally studied for nearly 30 years, a reliable value of the cross-section is still not well established. Experiments on absorption of ultrasonic sound give different values in the interval 7.7-12.2 A2 [242], As we have seen already, data obtained in supersonic jets are smaller by a factor two but should be rather carefully compared with bulk data as the velocity distribution in a jet differs from the Maxwellian one. In the contrast, the NMR estimation of a3 = 30 A2 in [81] brought the authors to the conclusion that o E = 40 A in the frame of classical /-diffusion. As the latter is purely nonadiabatic it is natural that the authors of [237] obtained a somewhat lower value by taking into account adiabaticity of collisions by non-zero parameter b in the fitting law. 

The value of the jump distance in the )0-relaxation of PIB found from the study of the self-motion of protons (2.7 A) is much larger than that obtained from the NSE study on the pair correlation function (0.5-0.9 A). This apparent paradox can also be reconciled by interpreting the motion in the j8-regime as a combined methyl rotation and some translation. Rotational motions aroimd an axis of internal symmetry, do not contribute to the decay of the pair correlation fimction. Therefore, the interpretation of quasi-elastic coherent scattering appears to lead to shorter length scales than those revealed from a measurement of the self-correlation function [195]. A combined motion as proposed above would be consistent with all the experimental observations so far and also with the MD simulation results [198]. 

Howartht17b)has used the theory of Internal Librational Motion to successfully predict the field dependent relaxation behavior of the 1,2-decanediol (DD), PBMA, and PHMA systems (using our published experimental data). We have utilized together multiple internal rotations (MIR) and distributions of correlation times. These methods individually have been successful in predicting relaxation behavior at one field. However, only the distribution theory predicts the observed field dependence for the carbons at or near sites of motional restriction, yet still having apparent correlation times <10 <-)sec. Our interest in the study of concerted motions along these alkyl chains has led us to combine the two approaches in the treatment of 13C relaxation parameters. 

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