Computational analysis of the solvent effect on the barrier to rotation about the conjugated C-N bond in methyl N,N-dimethylcarbamate

It is known experimentally that, in contrast to the case of amides, barriers to rotation about the conjugated C-N bonds of carbamates show very little solvent dependence. Calculations of the relative solvation energies of the equilibrium and transition state structures of methyl N,N-dimethylcarbamate (MDMC) and N,N-dimethylacetamide (DMA) were carried out using a continuum reaction field model in order to investigate the reason that bulk solvent polarity raises the barrier for DMA but leaves the barrier for MDMC unchanged. The results confirmed that MDMC is insensitive to bulk solvent polarity, probably as a result of the relatively small molecular dipole moment. Calculations of proton affinities and of the strength of association with a single water molecule were then performed in order to investigate why hydrogen-bond-donating solvents affect DMA but not MDMC. These calculations showed that MDMC is a less capable hydrogen-bond acceptor than DMA, and that the rotational barrier of MDMC does not increase in response to protonation or hydrogen-bonding nearly as much as the barrier of DMA does. Both of these factors contribute to making the rotational barrier of MDMC insensitive to solvent hydrogen-bond donor ability.