Rotational barriers solvent effects

Foresman et al.175 applied the DFT(B3LYP)/SCRF calculations to obtain the polar solvent effect on conformational equilibria in furfuraldehyde and on the C-C rotational barrier of (2-nitrovinyl)amine. The authors demonstrated that the poor results obtained using either spherical or ellipsoidal cavities can be significantly improved upon performing the SCRF calculations for the cavity of molecular shape. 

The apparent lowering of the rotational barrier in triafulvenes is open to interpretation either by substituent or solvent stabilization of ground-state polarity leading to a decrease of C3/C4 double bond character or by stabilization of a more polar - probably perpendicularly orientated184 — transition state by substituent or solvent effects. 

On substitution of allyllithium with methyl groups, the structures are distorted tt complexes becoming more jj -like. The previously described allyllithiums are contact ion pairs (CIP) whose dissociation is too low to permit study of the free carbanion. However, this is not the case for a more delocalized system such as 1,3-diphenylallyl whose lithium salts can exist as solvent separated ion pairs (SSIP) in ethereal solutions for which the organic moiety could be treated essentially as a free carbanion55 Boche and coworkers studied the effect of substitution at C(2) in their 1,3-diphenylallyl lithiums on the rotational barriers... 

Most of the data in Table 12 come from the work of Shvo et al. (78). Careful band-shape analysis and solvent-effect studies permitted evaluation of the rate constants and AG values at 298 K, which renders the discussion of substituent effects more meaningful than usual. The authors obtained reasonably linear Hammett plots when correlating log km with Or (79) for X and Y, holding one of these substituents constant. They also found that the dihydropyridine system may act as an unusually efficient donor, giving a AG of 17.6 kcal/mol with X, Y = H, CN, the only barrier below 25 kcal/mol reported for any donor-substituted cyanoethylene. However, with other acceptor combinations the dihydropyridine moiety is not so outstanding, and this illustrates the difficulty of measuring donor and/or acceptor effects by rotational barriers alone (vide infra). 

The second chapter, by Jan Sandstrom, deals with stereochemical features of push-pull ethylenes. The focus is on rotational barriers, which span a large range of values. The ease of twisting is partly a matter of electron delocalization and partly a matter of steric and solvent effects. Electronic structure and such related items as dipole moments and photoelectron spectra for these systems are discussed. The chapter also deals with the structure and chiroptical properties of twisted ethylenes that do not have push-pull effects, such as frans-cyclooctene. 

All of the molecules examined show low barriers (of torsional type) for isomer interconversion according to theoretical results. These barriers are lower than in unsubstituted double bond derivatives and in imines. For 3c and 3d, the values for the rotational barrier are similar to open-chain derivatives. The calculated values, including solvent effects, are close to the experimental values. 

Semiempirical molecular orbital computations have been used in numerous studies to establish various properties of heteroaromatics, such as conformations and rotational barriers. The value of such calculations is more difficult to estimate sometimes good agreement with experiment is obtained, sometimes not, making their predictional power questionable. The benefits of semiempirical MO computations are, of course, that large systems may be studied at much lower expense in terms of computer time. Thus, MINDO/3 calculations, which also take solvent effects into account, have been used to rationalize the syn-anti preference in the 2-formyl derivatives of furan, pyrrole, and thiophene (81JHC1055). 

Table XIII gives typical examples of 7t-barriers for planar heterocycles comparing the electron-attracting or electron-donating moiety to push-pull ethylenes. Steric effects destabilize the ground state and thus greatly reduce the 71-barriers to rotation. In these push-pull ethylenes the large entropy of activation and solvent effects hampered easy comparison of the barriers without high-quality determination of the activation parameters.

Ion-pair effects, as in the case of the unsubstituted allyl alkali metal compounds 23a-d, do not markedly influence the rotational barriers of the 1,3-diphenylallyl-lithium species 27a-f, although the reason is different 27a-f are solvent-separated ion pairs 36). Addition of HMPT to the THF solution of 27a raises the AG3 c value by 0.9 kcal/mol which corresponds to a rate retardation of 5-6 times. In the case of the methyl-substituted allyl anion 27b HMPT slows down the rate by a factor of only 2-3. With the 2-cyano anion 27c the AG values of the Li+ compound in THF and the Li +, Na+ andK+ species in dimethyl sulfoxide (DMSO) are the same18). In the case of 27e and 27f the rate of the rearrangement is not affected if HMPT or TMEDA are added to the THF solutions. The observations of some solvent dependence in the case of the sterically less hindered 27a and b, but of no effect with the more crowded 27e and f are in line with the general observation that solvent-separated ion pairs are favored with respect to contact ions pairs by increasing steric hindrance 25). Hence, these experimental results could be interpreted to mean that in the case of 27a and b contact ion pairs participate in the allyl anion rearrangement reaction. 

Shchavlev, A.E., Pankratov, A.N., and Shalabay, A.V., DFT computational studies on rotation barriers, tautomerism, intramolecular hydrogen bond and solvent effects in 8-hydroxyquinoline, Int. J. Quantum Chem., 106, 876-886 (2006). 

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