Methyl vibrational relaxation time

For the DTO model we must have an estimate of the torsional vibration frequency and the barrier to internal rotation of the constituent monomers. The DTO model fits the experimental data for bulk polymer if H = 5.4 kcal/mole, vt — 1012 c.p.s., and Zt = 30 which are not unreasonable values. One would expect the barrier height to decrease upon dilution (if it changes at all) as the chain environment loosens up. Assuming that rotation about C—O—C bonds is predominate, we take the experimental values of H = 2.63 kcal/mole, vt = 7.26 x 1012 c.p.s. of Fateley and Miller (14) for dimethyl ether. Eq. (2.8) predicts rSJ° = 0.47 X 10-8 sec at 253° K with Zt = 30. We shall use this as our dilute solution result. [The methyl pendant in polypropylene) oxide will act to increase the barrier height due to steric effects, making this calculated relaxation time somewhat low for this choice of a monomer analog.] Tmax is seen to change only by a factor of 102—103 upon dilution in the DTO model. 

Using time-resolved Raman spectroscopy, Kitagawa and coworkers investigated the vibrational relaxation of the V4 and v-, modes of the heme following photoexcitation in wild-type sperm whale Mb and its modifications, in which one or the other isopropionate side chain is selectively replaced by a methyl group [64]. Both modified hemes have similar relaxation rate (1.5 + 0.1 ps for the V4 mode and 4.9 0.3 ps for the... 

Figure 23 A proposal for dephasing in ethanol by solvent-assisted intramolecular vibrational redistribution (IVR). The yym-methyl stretch is initially excited, but rapidly equilibrates with one or more modes within kT (the ayym-methyl stretch and/or CH bend overtones). Dephasing occurs with this rapid equilibration time Tivr- However, significant population remains in the sym-methyl stretch after equilibration. Relaxation from this group of state to lower states causes the final relaxation of the population to zero, which is measured as Tj in energy relaxation experiments. (Adapted from Ref. 7.)...

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