Wang Jin, Burdzinski Gotard, Gustafson Terry L, Platz Matthew S
Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, OH 43210, USA.
J Am Chem Soc. 2007 Mar 7;129(9):2597-606. doi: 10.1021/ja067213u. Epub 2007 Feb 10.
Ultrafast photolysis of p-biphenylyldiazoethane (BDE) produces an excited state of the diazo compound in acetonitrile, cyclohexane, and methanol with lambdamax = 490 nm and lifetimes of less than 300 fs. The decay of the diazo excited state correlates with the growth of singlet carbene absorption at 360 nm. The optical yields of diazo excited states produced by photolysis of p-biphenylyldiazomethane (BDM) and BDE are the same; however, the optical yield of singlet p-biphenylylmethylcarbene (1BpCMe) is 30-40% less than that of p-biphenylylcarbene (1BpCH) in all three solvents. The results are explained by rearrangement in the excited state (RIES) of BDE to form p-vinylbiphenyl (VB) in parallel with extrusion of nitrogen to form 1BpCMe in reduced yield. This interpretation is consistent with product studies (ethanol-OD in cyclohexane) which indicate that there is an approximately 25% yield of VB that is formed by a mechanism that bypasses the relaxed singlet carbene. The decay of 1BpCMe is biexponential, and that of 1BpCH is monoexponential. This is attributed either to efficient relaxation of vibrationally excited 1BpCMe by 1,2 migration of hydrogen to form VB (minor) or to the increased number of low-frequency vibrational modes provided by the methyl group (major). A methyl group retards the rate of intersystem crossing (ISC), relative to a hydrogen atom, and ISC is more rapid in nonpolar solvents. Reaction of 1BpCMe with methanol is much faster than spin equilibration. Both the lifetime of 1BpCMe and 1BpCH are the same in cyclohexane and in cyclohexane-d12. This demonstrates that spin equilibration is faster than reaction of either carbene with the solvent. The lifetimes of 1BpCMe and 1BpCMe-d3 are the same in cyclohexane. This indicates that 1,2 hydrogen migration of 1BpCMe to form VB is slower than spin equilibration in cyclohexane. In acetonitrile, however, the lifetime of 1BpCMe-d3 is 1.5 times longer than that of 1BpCMe in the same solvent. Thus, in acetonitrile, where ISC is slow, the rate of 1,2 hydrogen shift of 1BpCMe is competitive with ISC. In cyclohexene, the lifetime of 1BpCH is shortened relative to that in cyclohexane. The lifetime of 1BpCMe is the same in cyclohexene and cyclohexane. The data indicate that spin relaxation is slow relative to reaction of 1BpCH with neat alkene but that spin relaxation is fast for 1BpCMe relative to reaction with neat cyclohexene.
对 - 联苯二氮乙烷（BDE）的超快光解在乙腈、环己烷和甲醇中产生重氮化合物的激发态，其λmax = 490 nm，寿命小于300飞秒。重氮激发态的衰减与360 nm处单线态卡宾吸收的增长相关。对 - 联苯重氮甲烷（BDM）和BDE光解产生的重氮激发态的光学产率相同；然而，在所有三种溶剂中，单线态对 - 联苯甲基卡宾（1BpCMe）的光学产率比对 - 联苯卡宾（1BpCH）低30 - 40%。结果可以通过BDE的激发态重排（RIES）来解释，即BDE在激发态重排形成对 - 乙烯基联苯（VB），同时挤出氮气形成产率降低的1BpCMe。这种解释与产物研究（环己烷中的乙醇 - OD）一致，该研究表明通过绕过弛豫单线态卡宾的机制形成的VB产率约为25%。1BpCMe的衰减是双指数的，而1BpCH的衰减是单指数的。这要么归因于通过氢的1,2迁移形成VB（次要）对振动激发的1BpCMe的有效弛豫，要么归因于甲基提供的低频振动模式数量增加（主要）。相对于氢原子，甲基会延迟系间窜越（ISC）的速率，并且ISC在非极性溶剂中更快。1BpCMe与甲醇的反应比自旋平衡快得多。1BpCMe和1BpCH在环己烷和环己烷 - d12中的寿命相同。这表明自旋平衡比任何一种卡宾与溶剂的反应都快。1BpCMe和1BpCMe - d3在环己烷中的寿命相同。这表明1BpCMe的1,2氢迁移形成VB的速度比环己烷中的自旋平衡慢。然而，在乙腈中，1BpCMe - d3的寿命是同一溶剂中1BpCMe寿命的1.5倍。因此，在ISC较慢的乙腈中，1BpCMe的1,2氢转移速率与ISC具有竞争力。在环己烯中，1BpCH的寿命相对于环己烷缩短。1BpCMe在环己烯和环己烷中的寿命相同。数据表明，相对于1BpCH与纯烯烃的反应，自旋弛豫较慢，但相对于与纯环己烯的反应，1BpCMe的自旋弛豫较快。
