Department of Chemistry, Temple University , Philadelphia, Pennsylvania 19122, United States.
J Phys Chem A. 2013 Nov 21;117(46):12165-74. doi: 10.1021/jp407758w. Epub 2013 Oct 4.
In order to investigate experimentally observed phototautomerization of gas-phase cytosine, several excited-state tautomerization mechanisms were characterized at the EOM-CCSD and TDDFT levels. All pathways that took place exclusively on the S1 surface were found to have significant barriers that were much higher than the barriers involved in radiationless decay of cytosine tautomers through conical intersections back to the ground state; tautomerization in this fashion cannot compete with radiationless relaxation. However, an alternative possibility is that the conical intersections that facilitate radiationless decay could also facilitate tautomerization. Barrierless pathways indicate that it is energetically possible that bifurcation at the conical intersections can lead to a subset of the population reaching different tautomers. This could be an explanation for the observed tautomerization of keto cytosine after exposure to low-energy UV light.
为了实验研究气相胞嘧啶的光互变异构现象，我们在 EOM-CCSD 和 TDDFT 水平上对几种激发态互变异构机制进行了表征。研究发现，所有只发生在 S1 表面的途径都存在显著的势垒，其高度远高于通过锥形交叉点回到基态的无辐射衰减过程中涉及的势垒；因此，这种互变异构不能与无辐射弛豫竞争。然而，另一种可能性是，促进无辐射衰减的锥形交叉点也可以促进互变异构。无势垒途径表明，在锥形交叉点发生分叉可能会导致一部分种群达到不同的互变异构体，这在能量上是可能的。这可能是低能紫外光照射后酮式胞嘧啶发生互变异构的原因。
