Kemnitz Carl R, Loewen Mark J
Department of Chemistry, California State University-Bakersfield, 9001 Stockdale Highway, Bakersfield, CA 93311, USA.
J Am Chem Soc. 2007 Mar 7;129(9):2521-8. doi: 10.1021/ja0663024. Epub 2007 Feb 13.
Complete basis set calculations (CBS-QB3) were used to compute the CN rotation barriers for acetamide and eight related compounds, including acetamide enolate and O-protonated acetamide. Natural resonance theory analysis was employed to quantify the "amide resonance" contribution to ground-state electronic structures. A range of rotation barriers, spanning nearly 50 kcal/mol, correlates well to the ground-state resonance weights without the need to account for transition-state effects. Use of appropriate model compounds is crucial to gain an understanding of the structural and electronic changes taking place during rotation of the CN bond in acetamide. The disparate changes in bond length (DeltarCO << DeltarCN) are found to be consonant with the resonance model. Similarly, charge differences are consistent with donation from the nitrogen lone pair electrons into the carbonyl pi* orbital. Despite recent attacks on the resonance model, these findings demonstrate it to be a sophisticated and highly predictive tool in the chemist's arsenal.
使用完备基组计算（CBS-QB3）来计算乙酰胺和八种相关化合物（包括乙酰胺烯醇盐和O-质子化乙酰胺）的C-N旋转势垒。采用自然共振理论分析来量化“酰胺共振”对基态电子结构的贡献。一系列跨越近50千卡/摩尔的旋转势垒与基态共振权重具有良好的相关性，而无需考虑过渡态效应。使用合适的模型化合物对于理解乙酰胺中C-N键旋转过程中发生的结构和电子变化至关重要。发现键长的不同变化（ΔrCO << ΔrCN）与共振模型一致。同样，电荷差异与氮孤对电子向羰基π*轨道的捐赠一致。尽管最近对共振模型进行了抨击，但这些发现表明它是化学家武器库中一种复杂且具有高度预测性的工具。
