Rotations within molecules

Tom Morton was born in Los Angeles, where his forebears (both Heilman and Morton) were employed by the motion picture industry. He received his A.B. from Harvard University in classics (Greek) and fine arts, and his Ph.D. from Caltech in chemistry under the joint direction of R. G. Bergman and J. L. Beauchamp. Since 1972, he has served on the chemistry faculties of Brown and Brandeis Universities and, from 1981, the University of Cahfornia, Riverside, where he is professor of chemistry. His research centers on the chemical consequences of internal rotation within molecules, including recognition of conformationaUy mobile structures by chemosensory systems. He is also affiliated with UCR s graduate programs in biochemistry and neuroscience. 

Molecular Nature of Steam. The molecular stmcture of steam is not as weU known as that of ice or water. During the water—steam phase change, rotation of molecules and vibration of atoms within the water molecules do not change considerably, but translation movement increases, accounting for the volume increase when water is evaporated at subcritical pressures. There are indications that even in the steam phase some H2O molecules are associated in small clusters of two or more molecules (4). Values for the dimerization enthalpy and entropy of water have been deterrnined from measurements of the pressure dependence of the thermal conductivity of water vapor at 358—386 K (85—112°C) and 13.3—133.3 kPa (100—1000 torr). These measurements yield the estimated upper limits of equiUbrium constants, for cluster formation in steam, where n is the number of molecules in a cluster. 

Methyl rotors pose relatively simple, fundamental questions about the nature of noncovalent interactions within molecules. The discovery in the late 1930s1 of the 1025 cm-1 potential energy barrier to internal rotation in ethane was surprising, since no covalent chemical bonds are formed or broken as methyl rotates. By now it is clear that the methyl torsional potential depends sensitively on the local chemical environment. The barrier is 690 cm-1 in propene,2 comparable to ethane,... 

The non-collective motions include the rotational and translational self-diffusion of molecules as in normal liquids. Molecular reorientations under the influence of a potential of mean torque set up by the neighbours have been described by the small step rotational diffusion model.118 124 The roto-translational diffusion of molecules in uniaxial smectic phases has also been theoretically treated.125,126 This theory has only been tested by a spin relaxation study of a solute in a smectic phase.127 Translational self-diffusion (TD)29 is an intermolecular relaxation mechanism, and is important when proton is used to probe spin relaxation in LC. TD also enters indirectly in the treatment of spin relaxation by DF. Theories for TD in isotropic liquids and cubic solids128 130 have been extended to LC in the nematic (N),131 smectic A (SmA),132 and smectic B (SmB)133 phases. In addition to the overall motion of the molecule, internal bond rotations within the flexible chain(s) of a meso-genic molecule can also cause spin relaxation. The conformational transitions in the side chain are usually much faster than the rotational diffusive motion of the molecular core. 

We simulated [38] the orientation TCF for sub-ensembles of molecules that have different OH stretch frequencies at t 0. We found that within 100 fs there was an initial drop that was frequency-dependent, with a larger amplitude of this drop for molecules on the blue side of the line. For times longer than about 1 ps the decay times for all frequencies were the same. We argued that since molecules on the red side of the line have stronger H bonds, they are less free to rotate than molecules on the blue side, leading to a smaller initial decay. For times... 

The energy gap between electronic states is much greater than that between vibrational states, which in turn is much greater than that between rotational states. As a result, we are able to adequately describe the effects of electronic transitions within molecules by considering quantised electronic and vibrational states. 

Under favourable conditions these excited molecules eventually return to the ground state by emission of fluorescence radiation. The electric vector of emitted radiation will be parallel to the transition moment of emission oscillator. If the excited molecules do not rotate within their lifetimes, the angular relationship between absorption oscillator and emission oscillator will be maintained. Therefore, the electric vector of... 

Hydrate formation is physical rather than chemical in nature. Apparently, no strong chemical bonds are formed between the hydrocarbon and water molecules. Actually, the hydrocarbon molecules are free to rotate within the void spaces. 

Infrared Spectrophotometry (IR). Atoms are in constant motion within molecules, and associated with these motions are molecular energy levels that correspond to the energies of quanta of IR radiation. These motions can be resolved into rotation of the whole molecule in space and into motions corresponding to the vibration of atoms with... 

If only the presence of the surface itself destroys the mirror symmetry of the free species, i.e., even without any distortion of the molecular backbone, the two resulting enantiomers cannot be superimposed by translation and rotation within the plane. The resulting absolute configuration depends on which enantiotopic face of the molecule is turned towards the substrate. Interconversion is only possible by reflection with the mirror plane perpen-... 

Table 4. Nature of the half-way point, depending on the values of S° and rot when = 63 kcal/mole. The blank terms of the array correspond to non reactive trajectories. Several rotations within the diradical lead either to the starting molecule (1) or to the geometrical isomer (2)... 

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