High Rotational Barrier Research Articles

Dynamic stereochemistry of imines and derivatives. Part V. Acid catalysis of E–Z imine interconversion

The rate of E–Z isomerization in N-(α-phenyl-4-nitrobenzylidene)methylamine at 35° is shown to be markedly increased by trace amounts of benzoic acid. Dynamic n.m.r. experiments at higher temperature show that the isomerization rate is proportional to the acid concentration and enable activation parameters to be determined. CNDO/2 Calculations performed on the benzylideneammonium ion suggest that the C[graphic omitted] rotational barrier is ca.46 kcal mol–1, and high temperature n.m.r. studies on a related immonium perchlorate also indicate a high rotational barrier. It is suggested that benzoic acid promotes imine isomerization by nucleophilic addition to the C[graphic omitted] bond in the conjugate acid. The relevance of the rate of imine isomerization to the mechanism of the imine–peroxyacid reaction is discussed.

The origin of rotational barriers in amides and esters

We discuss in this article the origin and magnitude of the single bond rotational barrier in amides and esters. The high rotational barrier of amides is biochemically manifested in the limited conformational freedom of proteins, Since there are only two instead of three bonds to rotate about per arnino acid residue. On the basis of thermochemical estimates with model compounds, we find that the resonance energy of esters is somewhat higher than that of amides. However, the experimental rotational barrier for the former is considerably lower than the latter. We suggest esters have lower rotational barriers than the corresponding amides because they retain a large fraction of the resonance energy in the transition state. Justification is offerred using an orbital delocalization argument.

High rotational barriers about CN bonds in aryl substituted heterocyclic compounds lacking bulky [formula omitted] substituents

High rotational barriers about CN bonds in aryl substituted heterocyclic compounds lacking bulky [formula omitted] substituents

Entropies and heat capacities of free radicals

AbstractMethods are presented for rapidly estimating the entropies and heat capacities of free radicals from the known S0 and C of structurally similar compounds. The methods consist of estimating the differences due to changes in mass, vibration frequencies, spin, symmetry, and changes in rotational barriers. Tables of contributions to S0 and C by different frequencies over the temperature range 300–1500°K are presented to facilitate the tabulation of the above differences. Conjugated radicals, such as benzyl and allyl, are included. It is shown that the greatest uncertainties in the estimates arise from uncertainties in the barriers to rotation in the radicals.The results are applied to kinetic data on the pyrolysis of branched hydrocarbons and the reverse reactions of radical recombination. Major discrepancies exist in these data which can be nearly reconciled by postulating improbably high rotational barriers of 8 kcal for CH3 rotation in isopropyl and t‐butyl radicals.It is shown that radical thermochemistry can be fitted into group schemes and tables of groups values are given for the rapid estimation of ΔH, S0, and C for different organic radicals, including those containing sulfur, oxygen, and nitrogen.