Temperature dependent 1H-NMR was used to probe rotation around the C–N bond in N,N-dimethylbenzamide and its derivatives 3-bromo-N,N-dimethylbenzamide, 4-bromo-N,N-dimethylbenzamide, and 3-hydroxy-N,N-dimethylbenzamide (each dissolved in DMSO). Increasing bond rotation was observed as the temperature was increased over the 25 – 65 °C range, resulting in significant broadening and eventual coalescence of the methyl proton peaks. Total line-shape analysis was performed within the framework of a density-matrix quantum mechanical chemical exchange model to obtain the rate constant for rotation at each temperature. Subsequent analysis of the rate constant data yielded ΔG, ΔH, and ΔS of activation for bond rotation in each system. Experimental results were compared to theoretical values calculated using density functional theory [B3LYP/6-31G(d,p)]. In general, the substituted derivatives were found to have lower activation energies for bond rotation than the parent N,N-dimethylbenzamide compound.

Barriers to bond rotation in N,N-dimethylbenzamides probed using temperature dependent 1H-NMR spectroscopy

253rd National Meeting (2017)

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