Exploring excited-state hydrogen atom transfer along an ammonia wire cluster:Competitive reaction paths and vibrational mode selectivity

摘要

The excited-state hydrogen-atom transfer (ESHAT) reaction of the 7-hydroxyquinoline-(NH_3)_3 cluster involves a crossing from the initially excited ~1pipi to a ~1pisigma state.The nonadiabatic coupling between these states induces homolytic dissociation of the O-H bond and H-atom transfer to the closest NH_3 molecule,forming a biradical structure denoted HT1,followed by two more Grotthus-type translocation steps along the ammonia wire.We investigate this reaction at the configuration interaction singles level,using a basis set with diffuse orbitals.Intrinsic reaction coordinate calculations of the enol->HT1 step predict that the H-atom transfer is preceded and followed by extensive twisting and bending of the ammonia wire,as well as large O-H...NH_3 hydrogen bond contraction and expansion.The calculations also predict an excited-state proton transfer path involving synchronous proton motions;however,it lies 20-25 kcal/mol above the ESHAT path.Higher singlet and triplet potential curves are calculated along the ESHAT reaction coordinate:Two singlet-triplet curve crossings occur within the HT1 product well and intersystem crossing to these T_n states branches the reaction back to the enol reactant side,decreasing the ESHAT yield.In fact,a product yield of approx=40% 7-ketoquinoline (NH_3)_3 is experimentally observed.The vibrational mode selectivity of the enol->HT1 reaction step [C.Manca,C.Tanner,S.Coussan,A.Bach,and S.Leutwyler,J.Chem.Phys.121,2578 (2004)] is shown to be due to the large sensitivity of the diffuse pisigma state to vibrational displacements along the intermolecular coordinates.