Correlated MO Study of the Low-Barrier Intramolecular Motions in Donor−Acceptor Ethenes

Bakalova, Snezhana M.; Frutos, Luis Manuel; Kaneti, José; Castaño, Obis

doi:10.1021/jp051864f

Cited by 2 publications

(2 citation statements)

References 28 publications

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“…The data in the tables suggest that the electronic structure of the transition states is complicated. ,, There is a significant change in the calculated transition state energies on going from the 6-311+G(d,p) basis set to higher basis sets, but further calculations with better models indicate that the 6-311++G(2df,2p) basis is sufficient (see Table S1, Supporting Information). There are also significant thermal and zero-point corrections (Table ), but they are similar for all processes, especially the two dimethylamino rotations, so they do not change the conclusions.…”

Section: Resultsmentioning

confidence: 99%

“…Electronic structure calculations play a key role in the study of these systems. ,,− ,, The molecules are small enough that high-quality calculations can be done and the details of the electronic structure, such as the full chemical shielding tensors, can be probed. Furthermore, techniques are now available to calculate solvent effects − in a reliable fashion.…”

Section: Introductionmentioning

confidence: 99%

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Solvent Effects on Chemical Exchange in a Push–Pull Ethylene as Studied by NMR and Electronic Structure Calculations

et al. 2011

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NMR measurements of chemical exchange in a push-pull ethylene, dissolved in a number of different solvents, are presented. These are complemented by high-level electronic structure calculations, using both gas-phase conditions and those which simulate solvents. The results show that it is essential to include entropy effects in order to understand the observed trends. For instance, the equilibrium state in this case represents the state with lowest Gibbs free energy, as it must, but not the lowest enthalpy. The particular molecule is methyl 3-dimethylamino-2-cyanocrotonate (MDACC). The geometry at the carbon-carbon double bond can be either E or Z with roughly equal populations at ambient temperature. We have measured the equilibrium constant and the rates for the exchange between these states in a number of solvents: methanol, chloroform, acetonitrile, toluene, dichloromethane, acetone, and tetrahydrofuran. Furthermore, the N,N-dimethylamino group attached to the double bond also shows restricted rotation, and this has been measured in both the E and Z conformations. The equilibrium constant and the three rotational barriers provide excellent probes of the solvent effects. Electronic structure calculations with a number of basis sets up to the 6-311++G(2df,2p) level, using both Hartree-Fock and density functional (B3LYP) methods were used to predict the E and Z ground states, and the three transition states. The calculations were done for an isolated molecule and also for solvent models representing toluene, acetone, and ethanol. The E conformation is more stable in solution, is the structure in the crystal, and is also the prediction for the gas phase from the calculations. However, the dependence of the equilibrium constant on temperature shows that the Z conformation actually has lower enthalpy. The stability of the E conformation in solution must be due to entropic effects. Similarly, the solvent effect on the E-Z barrier is primarily due to entropy. The measured enthalpy of activation is similar in all the solvents, but the entropy of activation increases with the solvent polarity. The barrier to rotation of the N,N-dimethylamino group shows a combination of entropy and enthalpy effects. This combination of experiments and theory gives an extraordinarily detailed picture of solvent-solute interactions.

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Section: Resultsmentioning

confidence: 99%