Solvent Effects On The Barrier To C-N Bond Rotation In N,N-Dimethylaminoacrylonitrile

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Journal Of The American Chemical Society


The barrier to rotation about the conjugated C-N bond of N,N-dimethylaminoacrylonitrile (DMAAN) was determined by dynamic NMR spectroscopy in the solvents methylcyclohexane, dibutyl ether, toluene, dichloromethane, chloroform, acetone, acetonitrile, nitromethane, methanol, and water. The barrier was found to increase with solvent polarity, as is the case for amides. In striking contrast to amides, however, the barrier was found not to depend on solvent hydrogen bond donor ability. For aprotic solvents, the variation of the DMAAN barrier with solvent correlated closely with the solvent dependence previously observed for the rotational barriers of dimethylformamide (DMF) and dimethylacetamide (DMA). Comparison of the solvent dependence of the barriers in these two compounds was used to estimate the gas-phase barrier in DMAAN at 9.3 kcal/mol. High-level ab initio calculations yielded good agreement with the gas-phase barrier extrapolated from the experimental data. For aprotic solvents, the solvent effects were linearly related to Brownstein's empirical solvent polarity parameter S. For a subset of solvents that were nonaromatic and non-chlorinated as well as aprotic, the barrier correlated closely with the Onsager dielectric function, (epsilon - 1)/(2 epsilon + 1). The estimate for the gas-phase barrier of DMAAN obtained from this correlation agreed closely with that derived earlier on the basis of the relationship with the DMA barriers. A calculated difference density plot showed that little electronic reorganization occurs at the nitrile functional group of DMAAN during rotation about the C-N bond.

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