Rotating configuration

Positional changes of atoms in a molecule or supermolecule correspond on the molecular scale to mechanical processes at the macroscopic level. One may therefore imagine the engineering of molecular machines that would be thermally, photochem-ically or electrochemically activated [1.7,1.9,8.3,8.109,8.278]. Mechanical switching processes consist of the reversible conversion of a bistable (or multistable) entity between two (or more) structurally or conformationally different states. Hindered internal rotation, configurational changes (for instance, cis-trans isomerization in azobenzene derivatives), intercomponent reorientations in supramolecular species (see Section 4.5) embody mechanical aspects of molecular behaviour. 

The calculated gas-phase barriers to internal rotation225 around the C1=C2 bond decrease by 56.60, 47.01 and 41.59 kcal mol-1 in the sequence CN > CHO > N02, which parallels the increase in electron-attracting power of these three substituents. The transition state of this rotation is best described by a zwitterionic rotated configuration of the type indicated in 98b to 98d. The barrier to internal rotation around the C1—N1 bond varies in the reversed order from 21.82, 17.56 to 12.77 kcal mol-1 for 129, 130, and 131, respectively. Both trends are consistent with the variations of the electron-acceptor properties of the substituents which decrease in the sequence N02 > CHO > CN. The rotational barriers of / -CH3 and / -F substituted vinylamines 127 and 128 are calculated in the 6-31G basis set to be much lower 3.13 kcal mol-1 for the E-CH3 substituent 127a, (in parentheses, the value for the higher barrier to type 120 4.64 kcal mol-x), 2.80 (4.64 kcal mol-1) for Z-CH3 (127b) for E-F 0.33 (2.89 kcal -mol-1), and for 128b a reversal of the two barriers. That barrier to type 121 is now 4.55 kcal mol-1, and that to 120 is 3.89 kcal mol-1. 

Applying this projector on each of the terms of a rotational configuration interaction expansion, built up on the basis of the double free rotor solutions, the symmetry eigenvectors are obtained [21,22,34]. 

The usefulness of the RDE stimulated the development of several other rotating configurations that are worth mentioning. Some of these have evolved in response to specific experimental needs, whereas others serve as possible extensions of the technique. 

The temperature dependence of the end-to-end distance of the isolated chain is related to the energy barriers between the rotational configurations of the chain (see Appendix 1). Thus the energy effects in rubber elasticity can, in principle, be related directly to structural features of the molecular chains. 

CH3SCH2CH2SCH3 (DTH) yields Re2Cl5(DTH)2, which has the structure57 shown in Fig. 26-D-10. Here a reduction has occurred and the. electron distribution is such that there is only a triple bond between the rhenium atoms. The rotational configuration is then staggered instead of... 

Hsieh CH, Erdem OF et al (2012) Structural and spectroscopic features of mixed valent Fe(II) Fe(I) complexes and factors related to the rotated configuration of diiron hydrogenase. J Am Chem Soc 134 (31) 13089-13102... 

The success of the RDE method has stimulated the development of several other rotating configurations. The RRDE is perhaps the most useful extension of the idea of the RDE. The RRDE was first developed by Frumkin and Nekrasov to detect unstable intermediates in electrode reactions. The RRDE, Fig. 4, consists of a central disk electrode surrounded by... 

Our two final chapters are devoted to a discussion of proton-transfer, electron-transfer, and group-transfer reactions. These reactions illustrate some major themes that we noted in the foreword the natural affinity of kinetic investigations with energetics and hence with spectroscopic studies of very fast molecular processes (vibration, rotation, configurational change) the use of simple classical models along with quantum-mechanical calculations and the multiple roles of the solvent, from inert diluent to reactant. 

Rotational configurations chloroalkanes Primary chloroalkanes 730-720 13.70-13.89 s s Cl atom trans to C atom... 

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