Crecca Christina R, Roitberg Adrian E
Department of Chemistry and Quantum Theory Project, University of Florida, Gainesville, FL 32611, USA.
J Phys Chem A. 2006 Jul 6;110(26):8188-203. doi: 10.1021/jp057413c.
A series of azobenzenes was studied using ab initio methods to determine the substituent effects on the isomerization pathways. Energy barriers were determined from three-dimensional potential energy surfaces of the ground and electronically excited states. In the ground state (S(0)), the inversion pathway was found to be preferred. Our results show that electron donating substituents increase the isomerization barrier along the inversion pathway, whereas electron withdrawing substituents decrease it. The inversion pathway of the first excited state (S(1)) showed trans --> cis barriers with no curve crossing between S(0) and S(1). In contrast, a conical intersection was found between the ground and first excited states along the rotation pathway for each of the azobenzenes studied. No barriers were found in this pathway, and we therefore postulate that after n --> pi (S(1) <-- S(0)) excitation, the rotation mechanism dominates. Upon pi --> pi (S(2) <-- S(0)) excitation, there may be sufficient energy to open an additional pathway (concerted-inversion) as proposed by Diau. Our potential energy surface explains the experimentally observed difference in trans-to-cis quantum yields between S(1) and S(2) excitations. The concerted inversion channel is not available to the remaining azobenzenes, and so they must employ the rotation pathway for both n --> pi and pi --> pi excitations.
使用从头算方法研究了一系列偶氮苯，以确定取代基对异构化途径的影响。从基态和电子激发态的三维势能面确定了能垒。在基态（S(0)）中，发现反转途径是首选。我们的结果表明，供电子取代基会增加沿反转途径的异构化能垒，而吸电子取代基则会降低该能垒。第一激发态（S(1)）的反转途径显示出反式→顺式能垒，且S(0)和S(1)之间没有曲线交叉。相比之下，在所研究的每种偶氮苯中，沿着旋转途径在基态和第一激发态之间发现了一个锥形交叉点。在该途径中未发现能垒，因此我们推测在n→π（S(1)←S(0)）激发后，旋转机制占主导。在π→π（S(2)←S(0)）激发时，可能有足够的能量打开如Diau所提出的另一条途径（协同反转）。我们的势能面解释了实验观察到的S(1)和S(2)激发之间反式到顺式量子产率的差异。其余的偶氮苯无法利用协同反转通道，因此它们在n→π和π→π激发时都必须采用旋转途径。
