Hydroxyl rotational relaxation

The importance of vibration-vibration exchange is now apparent in the results of the early experiments on HC1 and on hydroxyl (from H+03 - OHp+02) at pressures above 0.1 torr. These showed vibrational distributions that were nearly Boltzmann, but with shape parameters corresponding to temperatures of several thousand degrees, far greater than the translational temperatures. These distributions were samples taken after rotational relaxation and after the efficient vibration-vibration energy exchange... 

Fig. 14. Relaxation times in a rotating frame T p for protons attached to different carbons in pure PS(OH) (A), PS(OH) in the blends (A), and PMMA in the blends (O) as functions of the hydroxyl content in PS(OH). Thin horizontal line for pure PMMA as a reference [111]...

With the backbone of one strand cleaved, the DNA can now rotate around the remaining strand, driven by the release of the energy stored because of the supercoiling. The rotation of the DNA unwinds supercoils. The enzyme controls the rotation so that the unwinding is not rapid. The free hydroxyl group of the DNA attacks the phosphotyrosine residue to reseal the backbone and release tyrosine. The DNA is then free to dissociate from the enzyme. Thus, reversible cleavage of one strand of the DNA allows controlled rotation to partly relax supercoiled DNA. 

Relaxation phenomena (TSDC), molecular mobility (NMR, TPDMS), and chemical reactions (TPDMS of associative desorption of water) are observed for adsorbed water/LiChrolut EN adsorbent over a wide temperature range. These phenomena are characterized by very different activation energies from 10 kJ/mol (rotational mobility of hydroxyls in WAW molecules), 20-40 kJ/mol (rotational mobility of the molecules in SAW), 40-80 kJ/mol (rotational and translational mobility of the water molecules in pores of different sizes), and 60-200 kJ/mol (molecular and associative desorption of water) (Figure 5.34). As a whole, all the distribution functions of activation energy fiJS) obtained using different methods are well concordant. This is caused by the nature of activated processes whereas all the processes are caused by the molecular mobility of water dependent on the topological and chemical characteristics of confined space in nano- and mesopores in LiChrolut EN adsorbent. 

The EPR spectrum of hydroxyl radicals cannot be observed in liquids, which is believed to result from very rapid relaxation due to inefficient quenching of the orbital angular momentum (which gives rise to anisotropy in the g-tensor). In the gas phase the ground state is known to be 113/2, which is further split due to interaction of the orbital motion of the electron and the rotation of the nuclei (A-doubling). 

Spin rotation interaction can also provide another mechanism by which the hydroxyl radical can undergo spin relaxation due to the anisotropy of the g-tensor, in which relaxation could occur via the Zeeman interaction. The rate of spin relaxation via this mechanism has been found to be [19]... 

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