Department of Chemistry and Biochemistry, University of California, Los Angeles, California, 90095-1569, USA.
J Am Chem Soc. 2010 Mar 10;132(9):3029-37. doi: 10.1021/ja909372f.
The dynamics of 1,3-dipolar cycloadditions of nine 1,3-dipoles with ethylene and acetylene have been explored by quasiclassical trajectory and single trajectory calculations in the retro-cycloaddition direction to compute energy partitioning of reactants among relative translation, vibration, and rotation. The results are interpreted with an expanded version of Polanyi's Rules for bimolecular reactions, and three trends are evident. (1) Relative translation of reactants is the main contributor to surmounting the barrier, since all transition states (TSs) are early with respect to sigma bond formation. (2) Vibrational excitation in the 1,3-dipole bending modes required for reaction is related to the lateness of the TS with respect to dipole bending: diazonium betaines (late TS, dipole bending required) > nitrilium betaines > azomethine betaines (early TS, dipole bending least important). This is also the order of the activation barriers (high --> low). (3) The previously reported linear correlation between activation barriers and the energy required to distort reactants to their TS geometries are understandable in terms of the requirements for vibrational excitation computed here. For the 1,3-dipolar cycloadditions, single trajectory calculations, which contain no zero point vibrational energy, give reasonable estimates of the mean energy partitioning of reactants derived from potential energy barrier release. The timing of bond formation and relative reactivities of different 1,3-dipoles are discussed.
通过在逆环加成方向上进行准经典轨迹和单个轨迹计算，研究了九个 1,3-偶极子与乙烯和乙炔的 1,3-偶极环加成的动力学，以计算反应物在相对平移、振动和旋转之间的能量分配。结果用双分子反应的 Polanyi 规则的扩展版本进行了解释，有三个明显的趋势。(1) 反应物的相对平移是克服势垒的主要贡献者，因为所有过渡态 (TS) 相对于 sigma 键形成都很早。(2) 反应所需的 1,3-偶极弯曲模式的振动激发与 TS 相对于偶极弯曲的滞后性有关：重氮𬭩叶立德（TS 晚，偶极弯曲要求高）>腈叶立德>亚胺叶立德（TS 早，偶极弯曲不重要）。这也是活化势垒（高-低）的顺序。(3) 先前报道的活化势垒与将反应物扭曲到其 TS 几何形状所需的能量之间的线性相关关系，可以根据这里计算的振动激发要求来理解。对于 1,3-偶极环加成，不包含零点振动能的单个轨迹计算给出了反应物的平均能量分配的合理估计，这些能量分配来自势能势垒释放。讨论了键形成的时间和不同 1,3-偶极子的相对反应性。
