Ground-state compounds, dynamics

In the ground state, aminomethylenemalonates possess an essentially planar geometry, which maximizes the electron delocalization in the molecules. In the heteropolar transition state, the plane of the groups R3 and NR R2 and the plane of the two carbonyl groups occupy orthogonal positions. More details of the dynamic and static stereochemistry of push-pull ethylenes, as in compounds 1 and 2, are discussed in two excellent reviews (73TS295 83TS83). 

The reader is also referred to the innovative nonphotochemical electron transfer studies of Weaver et al. [147], These authors have been exploring dynamical solvent effects on ground state self-exchange kinetics for or-ganometallic compounds. This work has explored many aspects of solvent control on intermediate barrier electron transfer reactions, including the effect on a distribution of solvation times. The experimental C(t) data on various solvents have been incorporated into the theoretical modeling of the ground state electron transfer reactions studied by Weaver et al. [147]. 

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,... 

Compound 220 in solution gives rise to a dynamic equilibrium between the enaminoketone (i )-220 and iV-acyl forms (Z)-221 in the ground state (Scheme 14). In nonpolar solvents, such as hexane, benzene, and toluene, the equilibrium is displaced toward isomer ( )-220, which is stabilized by intramolecular hydrogen bond it absorbs in the region of 470 nm. In polar solvents like DMSO, the equilibrium shifts almost completely toward the N-acylated form (Z)-221. 

This conclusion was further supported by EPR results. A large mixture of x -y character would be necessary to explain the large gy shift in this compound if it were compressed with a predominately ground state. However, such a large mixture would only arise from a soft angular potential that would also result in a large temperature dependence of the EPR. This is not observed, as the EPR is almost temperature independent down to 10 K. Below 10 K, orthorhombic g values result, indicating that the dynamic disorder has been frozen out at this temperature. 

Ru complexes much lower values of r were found, implying that a time-dependent process other than rotational reorientation is operating. Modulation of the ground state potential energy surface via a dynamic Jahn-Teller effect is suggested as the process controlling the electron spin relaxation in these compounds. 

Many bloactlve molecules are flexible, and their minimum energy conformation need not correspond to the receptor-bound conformation.7 Furthermore, conformational dynamics may be Important, with receptor molecule and substrate both changing conformation in order to elicit the biological response.5,30 In some cases, then, a system could show different pharmacophoric patterns for compounds which preferentially bind to the receptor s ground state, its activated state, and an intermediate state (transition state analogs).4... 

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