Resonance structures rotations about

Some fundamental structure-stability relationships can be employed to illustrate the use of resonance concepts. The allyl cation is known to be a particularly stable carbocation. This stability can be understood by recognizing that the positive charge is delocalized between two carbon atoms, as represented by the two equivalent resonance structures. The delocalization imposes a structural requirement. The p orbitals on the three contiguous carbon atoms must all be aligned in the same direction to permit electron delocalization. As a result, there is an energy barrier to rotation about the carbon-carbon... 

Dynamic structural characteristics can also be interpreted in terms of resonance. There is a substantial barrier to rotation about the C—N single bonds in carboxamides. A frequently observed consequence is the nonidentity of NMR peaks due to the syn and anti... 

Aldiough diese structures have a positive charge on a more electronegative atom, diey benefit from an additional bond which satisfies file octet requirement of the tricoordinate carbon. These carbocations are well represented by file doubly bonded resonance structures. One indication of file participation of adjacent oxygen substituents is file existence of a barrier to rotation about the C—O bonds in this type of carbocation. 

In allylic systems, favorable overlap of the p orbitals of the n system should require a coplanar arrangement of the three sp2 carbons and their five substituent atoms evidence that such a structure is indeed preferred comes, for example, from proton magnetic resonance observations that demonstrate barriers to bond rotation in the isomeric dimethylallyl ions 21, 22, and 23. These ions form stereo-specifically from the three dimethylcyclopropyl chlorides (Section 12.2), and barriers to rotation about the partial double bonds are sufficiently high to prevent interconversion at low temperature. At — 10°C, 21, the least stable isomer,... 

Rotation about the carbon-nitrogen bond is required to average the environments of the two methyl groups, but this rotation is relatively slow in amides as the result of the double-bond character imparted to the carbon-nitrogen bond, as shown by these two resonance structures. 

An understanding of the internal rotation about the amide bond is important because of its relevance to protein structure. Formamide is the simplest amide. The coplanarity and the remarkable rotational barrier about the C-N bond in formamide can be rationalized by resonance between the n electrons of the carbonyl group and the lone pair of the nitrogen atom [1, 50]. According to VB theory, the Jt electronic structure of formamide may be described by six resonance structures. 

Contribution from resonance structure 3, which contains a formal double bond between carbon and nitrogen, is considered to be primarily responsible for the coplanarity and the high rotational barrier about the amide bond [58], The introduction of resonance structure 3 also implies that there is significant charge-delocalization from the nitrogen lone pair to the carbonyl oxygen. 

Resonance structures 26 and 27 must be considered for the ground state of enamines. Hindrance of free rotation about the C—N bond is dependent upon the contribution of 27. The kinetics of this process have been studied by dynamic NMR spectroscopy. With the aim of simplifying the equilibrium system, many investigators have studied the compounds where X1 = X2 and R1 = R2. For such a case, the two types of equilibria, 26 < 28 and 29 < 30, involve equivalent structures. In any of the equivalent conformers, the two constitutionally equivalent X groups are diastereomerically related in the minimum energy conformation of the molecule (vide infra). They should therefore,... 

Another application of solid-state NMR is in the study of solid-state conformation. In solution molecules tumble rapidly, and rotations about single bonds are fast recall that in solution spectra the three methyl protons are normally equivalent (Section 4.2). In the solid state many of these motions are restricted or suppressed entirely, and many of the chemical shift degeneracies to which we have become accustomed are now split. For example, pnra-dimethoxybenzene, structure 15-1, exhibits a solution l3C spectrum in which the two methoxy carbons are equivalent to each other, as are the two substituted aromatic carbons, and each set gives rise to a single resonance ... 

The effect of organic solvents on the rate constant for amide rotation in Af,A -dimethylacetamide (DMA) has also been investigated (Drakenberg et ai, 1972). As the solvent is changed from water to acetone to cyclohexane, first-order rate constants for rotation increase from 0.025 to 0.33 to 1.5 sec . This observation that nonpolar solvents increase reaction rates indicates that the transition state for amide rotation is nonpolar relative to the reactant state and, thus, is stabilized in nonpolar solvents. This transition state is presumably characterized by partial rotation about the amide bond. In this transition state, polar resonance structures for the amide bond no longer exist and, thus, the transition state is less polar than the reactant state. The 60-fold rate acceleration that accompanies transfer of DMA from water to cyclohexane will provide an important clue in understanding enzymatic prolyl isomerization (see below). 

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