Contrasting ring-opening propensities in UV-excited α-pyrone and coumarin

Ring-opening quantum yields following UV-photoexcitation of coumarin and α-pyrone are influenced by the dynamics through, rather than just the geometries of, conical intersections.</p

A significant role of the totally symmetric valley-ridge inflection point in the bifurcating reaction pathway

Appearance of the valley-ridge inflection (VRI) point on the intrinsic reaction path (IRP) introduces geometrical instability of the reaction coordinate, and sometimes leads to two different product minima on the potential energy surface (PES). A significant role of the totally-symmetric VRI point on the IRP is discussed from the viewpoint of branching of the reaction pathway. As illustrative examples, ab initio calculations were performed to determine the IRP for XCHO^[-] + CH3Cl (X = H, CH3) at the Møller-Plesset second-order perturbation theory (MP2) level with 6-31+G(d) basis sets and geometric features of the PES around the IRP have been analyzed

Trifurcation of the reaction pathway

A concept of trifurcation of a reaction pathway is introduced to analyze the case where a downhill path from the first-order saddle point accompanies three branches via the valley-ridge inflection region, leading to three different product minima on the potential energy surface. We provide a detailed analysis on the reaction path for an electron transfer reaction, HCHO- + CH3Cl → OH2C-CH3⋅⋅⋅Cl-, as an illustrative example of the trifurcating reaction path

Combined gradient projection/single component artificial force induced reaction (GP/SC-AFIR) method for an efficient search of minimum energy conical intersection (MECI) geometries

We report a new approach to search for structures of minimum energy conical intersection (MECIs) automatically. Gradient projection (GP) method and single component artificial force induced reaction (SC-AFIR) method were combined in the present approach. As case studies, MECIs of benzene and naphthalene between their ground and first excited singlet electronic states (S-0/S-1-MECIs) were explored. All S-0/S-1-MECIs reported previously were obtained automatically. Furthermore, the number of force calculations was reduced compared to the one required in the previous search. Improved convergence in a step in which various geometrical displacements are induced by SC-AFIR would contribute to the cost reduction. (C) 2017 Elsevier B.V. All rights reserved

Photophysics of cytosine tautomers : new insights into the nonradiative decay mechanisms from MS-CASPT2 potential energy calculations and excited-state molecular dynamics simulations

A comprehensive picture of the ultrafast nonradiative decay mechanisms of three cytosine tautomers (amino-keto, imino-keto, and amino-enol forms) is revealed by high-level ab initio potential energy calculations using the multistate (MS) CASPT2 method and also by on-the-fly excited-state molecular dynamics simulations employing the CASSCF method. To obtain a reliable potential energy profile along the deactivation pathways, the MS-CASPT2 method is employed even for the optimization of minimum energy structures in the excited state and conical intersection (CI) structures between the ground and excited states. In the imino (imino-keto) form, we locate a new CI structure involving the twisting of the imino group, and the decay pathway leading to this CI is found to be barrierless, suggesting a remarkably efficient deactivation of imino cytosine. In the keto (amino-keto) form, the MS-CASPT2 calculations exhibit an efficient decay path to the ethylene-like CI involving the twisting of the C–C double bond in the six-membered ring, with a barrier of [similar]0.08 eV from the minimum of the 1ππ* state. In the enol (amino-enol) form, three types of CIs are identified for the first time. Among them, the ethylene-like CI with a similar molecular structure to the keto form provides the most preferred deactivation pathway in enol cytosine. This pathway exhibits a higher barrier of [similar]0.22 eV and a higher energy of CI than those of keto cytosine. Nonadiabatic molecular dynamics simulations provide a time-dependent picture of the deactivation processes, including the excited-state lifetime of each tautomer. In particular, the decay time of the imino tautomer is predicted to be only [similar]100 fs. Our computational results are in remarkably good agreement with the experimental findings of recent femtosecond pump–probe photoionization spectroscopy [J. Am. Chem. Soc., 2009, 131, 16939; J. Phys. Chem. A, 2011, 115, 8406], supporting the coexistence of more than one tautomer in the photophysics of isolated cytosine and that each tautomer exhibits a different excited-state lifetime

Structural dynamics of photochemical reactions probed by time-resolved photoelectron spectroscopy using high harmonic pulses

Femtosecond ring-opening dynamics of 1,3-cyclohexadiene (CHD) in gas phase upon two-photon excitation at 400 nm (=3.1 eV) was investigated by time-resolved photoelectron spectroscopy using 42 nm (=29.5 eV) high harmonic photons probing the dynamics of the lower-lying occupied molecular orbitals (MOs), which are the fingerprints of the molecular structure. After 500 fs, the photoelectron intensity of the MO constituting the C=C sigma bond (sigma(C=C)) of CHD was enhanced, while that of the MO forming the C-C sigma bond (sigma(CC)) of CHD was decreased. The changes in the photoelectron spectra suggest that the ring of CHD opens to form a 1,3,5-hexatriene (HT) after 500 fs. The dynamics of the sigma(C=C) and sigma(CC) bands between 200 and 500 fs reflects the ring deformation to a conical intersection between the 2(1)A and 1(1)A potential energy surfaces prior to the ring-opening reaction

Theoretical chemical reaction database construction based on quantum chemistry-aided retrosynthetic analysis

A theoretical database comprising experimentally accessible and inaccessible chemical reactions could complement the existing experimental databases and contribute significantly to data-driven chemical reaction discovery. Quantum chemistry-aided retrosynthetic analysis (QCaRA) can generate a network of elementary steps called a reaction-path network and predict hundreds or more of chemical reactions along with their theoretical yields. In contrast to ordinary simulations, QCaRA traces back the reaction paths from the target product to various reactant candidates while solving the kinetic equations. In this study, we propose theoretical reaction database construction based on QCaRA. Seven reaction-path networks containing 13,190 reactions, 108,754 reaction paths, and 2,552,652 geometries have been identified and discussed as examples. In addition to well-known reactions (i.e., synthesis of fluoroglycine, Wöhler’s urea synthesis, base-catalysed aldol reaction, Lewis-acid-catalysed ene reaction, cobalt-catalysed hydroformylation, Strecker reaction, and Passerini reaction), numerous unexplored reactions with high, medium, low, near-zero, or zero yields have been identified. We anticipate that such a QCaRA-based theoretical reaction database will provide information on hitherto unexplored reactivities, especially those that are experimentally inaccessible

Analyses of bifurcation of reaction pathways on a global reaction route map: A case study of gold cluster Au-5

A global reaction route map is generated for Au-5 by the anharmonic downward distortion following method in which 5 minima and 14 transition states (TSs) are located. Through vibrational analyses in the 3N - 7 (N = 5) dimensional space orthogonal to the intrinsic reaction coordinate (IRC), along all the IRCs, four IRCs are found to have valley-ridge transition (VRT) points on the way where a potential curvature changes its sign from positive to negative in a direction orthogonal to the IRC. The detailed mechanisms of bifurcations related to the VRTs are discussed by surveying a landscape of the global reaction route map, and the connectivity of VRT points and minima is clarified. Branching of the products through bifurcations is confirmed by ab initio molecular dynamics simulations starting from the TSs. A new feature of the reaction pathways, unification, is found and discussed. (C) 2015 AIP Publishing LLC

Orbital Energy-Based Reaction Analysis of S(N)2 Reactions

An orbital energy-based reaction analysis theory is presented as an extension of the orbital-based conceptual density functional theory. In the orbital energy-based theory, the orbitals contributing to reactions are interpreted to be valence orbitals giving the largest orbital energy variation from reactants to products. Reactions are taken to be electron transfer-driven when they provide small variations for the gaps between the contributing occupied and unoccupied orbital energies on the intrinsic reaction coordinates in the initial processes. The orbital energy-based theory is then applied to the calculations of several S(N)2 reactions. Using a reaction path search method, the Cl- + CH3I -> ClCH3 + I- reaction, for which another reaction path called "roundabout path" is proposed, is found to have a precursor process similar to the roundabout path just before this S(N)2 reaction process. The orbital energy-based theory indicates that this precursor process is obviously driven by structural change, while the successor S(N)2 reaction proceeds through electron transfer between the contributing orbitals. Comparing the calculated results of the S(N)2 reactions in gas phase and in aqueous solution shows that the contributing orbitals significantly depend on solvent effects and these orbitals can be correctly determined by this theory