Hydrogen Transfer vs Proton Transfer in 7-Hydroxy-quinoline‚(NH 3 ) 3 : A CASSCF/ CASPT2 Study Antonio Ferna ´ ndez-Ramos, ² Emilio Martı ´nez-Nu ´ n ˜ ez, ² Saulo A. Va ´ zquez,* ,² Miguel A. Rı ´os, ² Carlos M. Este ´ vez, ‡ Manuela Mercha ´ n, § and Luis Serrano-Andre ´ s* ,§ Departamento de Quı ´mica Fı ´sica, Facultade de Quı ´mica, UniVersidade de Santiago de Compostela, Santiago de Compostela, Spain, Departamento de Quı ´mica Fı ´sica, Facultade de Quı ´mica, UniVersidade de Vigo, Vigo, Spain, and Unidad de Quı ´mica Teo ´ rica, Instituto de Ciencia Molecular, UniVersitat de Vale ` ncia, Vale ` ncia, Spain ReceiVed: April 2, 2007; In Final Form: April 24, 2007 Multiconfigurational CASSCF and CASPT2 calculations were performed to investigate the enol f keto tautomerization in the lowest singlet excited state of the 7-hydroxyquinoline‚(NH 3 ) 3 cluster. Two different reaction mechanisms were explored. The first one corresponds to that proposed previously by Tanner et al. (Science 2003, 302, 1736) on the basis of experimental observations and CASSCF optimizations under C s - symmetry constraints. This mechanism comprises four consecutive steps and involves nonadiabatic transitions between the valence 1 ππ* state and a πσ* Rydberg-type state, resulting in hydrogen-atom transfer. Single- point CASPT2 calculations corroborate that for C s -symmetry pathways hydrogen-atom transfer is clearly preferred over proton transfer. The second mechanism, predicted by CASSCF optimizations without constraints, implies proton transfer along a pathway on the 1 ππ* surface in which one or more ammonia molecules depart significantly from the molecular plane defined by the hydroxyquinoline ring. The results suggest that both mechanisms may be competitive with proton transfer being somewhat favorable over hydrogen-atom transfer. I. Introduction Proton transfer (PT) and hydrogen-atom transfer (HAT) reactions play a fundamental role in a variety of chemical and biological processes. 1-13 A comprehensive understanding of the mechanisms and dynamics of these reactions is, therefore, of great importance. However, research on PT and HAT reactions at the molecular level is in general very difficult because of the structural complexity, very short time scales, and solvent fluctuations involved in these processes. Some of the difficulties associated with bulk systems can be avoided by preparing small clusters of reactants and solvent molecules. 14-30 One example is that of 7-hydroxyquinoline‚(NH 3 ) n clusters 26-30 in which hydrogen-bonded molecular wires formed by n ammonia molecules are attached to 7-hydroxyquinoline (7HQ). The latter compound is a heteroaromatic scaffold molecule containing an O-H donor group and an N acceptor site located far enough to accommodate a small solvent wire. Leutwyler and co-workers 26-34 extensively investigated HAT or PT along the ammonia wire in 7HQ‚(NH 3 ) 3 using spectro- scopic methods and ab initio calculations. In the electronic ground state (S 0 ) the tautomeric 7-ketoquinoline‚(NH 3 ) 3 [7KQ‚ (NH 3 ) 3 ] form is predicted to be 8.6 kcal mol -1 less stable than 7HQ‚(NH 3 ) 3 . Neither PT nor HAT has actually been observed for 7HQ‚(NH 3 ) 3 in the electronic ground state. In the lowest singlet excited state (S 1 ) this energetic ordering is reversed; in other words, the O-H group has a more pronounced acidic character, whereas the basicity of the N atom becomes enhanced. When 7HQ‚(NH 3 ) 3 is excited to the S 1 origin (one π electron is promoted to a π* molecular orbital), no reaction takes place but additional excitation of ammonia-wire vibrations triggers a fast enol ( 1 ππ*) f keto ( 1 ππ*) tautomerization with a reaction threshold of about 200 cm -1 . 26 In order to interpret these observations, Leutwyler and co-workers 26 performed molecular structure calculations for the S 1 and S 2 states using the configuration interaction singles (CIS) 35 and complete active space self-consistent field (CASSCF) 36,37 methods, concluding that after the S 1 r S 0 excitation the enol f keto tautomerization proceeds via HAT (or coupled electron-proton transfer) rather than through PT. The whole (excited state) acid-base process comprises four consecutive H-atom translocations along the ammonia wire (see Figures 1 and 2) 26 in a way similar to the Grotthuss proton conduction mechanism in water. 38 The interpretation of Leutwyler and co-workers 26 follows the mechanism of Domcke and Sobolewski proposed as a new paradigm of excited-state proton-transfer reactivity. 39 In this mechanism, inferred from CASSCF and CASPT2 calculations on phenol, pyrrol, indole, and their clusters with water and ammonia, 40-42 the key role is played by excited singlet states of πσ* nature (where σ* is a Rydberg-type orbital), which have repulsive potential-energy surfaces with respect to the stretching of O-H or N-H bonds. For the particular case of 7HQ‚(NH 3 ) 3 , as the O-H bond stretches, the energy of the 1 ππ* state increases and soon this state crosses the 1 πσ* state, which becomes more stable as the reaction progresses. 26 After a nonadiabatic transition to the 1 πσ* state, the structure of 7HQ‚ (NH 3 ) 3 should be viewed as a radical pair (σ* is a diffuse orbital centered on one of the N atoms of the ammonia wire) rather than as an ion pair, which would be the case if only the proton were transferred to the ammonia wire. In the final step of the * To whom correspondence should be addressed. E-mail: qfsaulo@usc.es, Luis.Serrano@uv.es. ² Universidade de Santiago de Compostela. ‡ Universidade de Vigo. § Universitat de Vale `ncia. 5907 J. Phys. Chem. A 2007, 111, 5907-5912 10.1021/jp072575p CCC: $37.00 © 2007 American Chemical Society Published on Web 06/13/2007