Isopropyl group, rotation

Other rate processes studied include isopropyl group rotation, (529) restricted rotations in phosphine derivatives of cyclopentadienyl complexes of iron and nickel, (530) and ring and nitrogen inversion in (en) complexes of praseodymium. (531) Molecular geometries of molecules in solution can be accurately determined from NMR spectra of the molecules oriented in a nematic phase of a liquid crystalline solvent. The effect of Eu(dpm), on the nematic phase spectrum of pyridine has been examined. The pyridine geometry is unaffected by the LSR. The observed LIS values can be separated into isotropic and anisotropic components. (532)... 

After finding that rotation of the rm-butyl group in dimethyl 9-rm-butyl-9,10-dihydro-9,10-ethenoanthracene-ll,12-dicarboxylate (95) was locked on the laboratory time scale, Oki and Suda introduced an isopropyl group in place of the rm-butyl group and found that the barrier was so much lowered ( , = 15.4 kcal/mol) that the attempt to isolate atropisomers had to be abandoned (140). Since then, triptycene systems have been found to give higher barriers to rotation than the ethenoanthracene system. Thus it became attractive to examine die barriers to rotation about a rec-alkyl-to-triptycyl bond. 

Compounds of type 111 should, in principle, be resolvable into stable enantiomers if the barrier is high enough. However, these compounds do not carry a functional group convenient for resolution. Oki and associates (162) modified one of the methyls in the isopropyl group to make the rotational isomers dia-stereomeric. They prepared 9-(2-methoxy-l-methylethyl)triptycenes (114) and... 

Furthermore, the estimation of the rotational barriers of ortho-substituted 6-aryl-1,1,5-trimethylindanes (Fig. 1a) [24] by NMR as well as conformational NMR-studies of (9-anthryl)carbinol derivatives (Fig. 1b) [25] have shown the CF3 substituent to have steric effects that are comparable to those of an isopropyl group (Fig. 1). 

Another remarkable feature of these results (Hirota and Weissman, 1960) is that hyperfine coupling to the /3-proton in the isopropyl group was only about 2-4 G. This is very small compared with the results for /3-protons in comparable alcohol radicals (Table 2), and is thought to arise because of restricted rotation about the C—C bond (Symons, 1962). Indeed, molecular models show that only the conformation of this ketyl in which the /3-proton lies in the radical plane is not severely strained. Under these conditions hyper conjugation is at a minimum, and the splitting detected may be a measure of residual coupling in the absence of hyperconjugation (Symons, 1962). 

Inaccuracies also arise in melting point estimations for the alkyl substituted aromatic compounds when these molecules display some degree of symmetry. If, for example, the isopropyl groups of p-diisopropylbenzene are considered freely rotating in the crystal,... 

Divinylmethanes, in which a methylene moiety has bulky substituents such as isopropyl groups, undergo DPM rearrangement by the triplet-sensitization, because free rotation of the vinyl moiety is inhibited (Scheme 4.13) [18]. 

The authors suggested244 that proton loss [Eq. (185)] is controlled by stereo-electronic considerations and not by the thermodynamic stability of the product. In the preferred conformation, the tertiary hydrogen on the isopropyl group of the p-cymene radical cation is located in the nodal plane of the benzene ring where its interaction with the n system in the transition state is minimized. The methyl group, on the other hand, rotates rather freely, and the loss of any one of the hydrogens is, therefore, not conformationally restricted. 

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