Conical intersections in thymine

The mechanisms which are responsible for the radiationless deactivation of the npi* and pipi* excited singlet states of thymine have been investigated with multireference ab initio methods (the complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory with respect to the CASSCF reference (CASPT2)) as well as with the CC2 (approximated singles and doubles coupled-cluster) method. The vertical excitation energies, the equilibrium geometries of the 1npi*and 1pipi* states, as well as their adiabatic excitation energies have been determined. Three conical intersections of the S1 and S0 energy surfaces have been located. The energy profiles of the excited states and the ground state have been calculated with the CASSCF method along straight-line reaction paths leading from the ground-state equilibrium geometry to the conical intersections. All three conical intersections are characterized by strongly out-of-plane distorted geometries. The lowest-energy conical intersection (CI1) arises from a crossing of the lowest 1pipi* state with the electronic ground state. It is found to be accessible in a barrierless manner from the minimum of the 1pipi* state, providing a direct and fast pathway for the quenching of the population of the lowest optically allowed excited states of thymine. This result explains the complete diffuseness of the absorption spectrum of thymine in supersonic jets. The lowest vibronic levels of the optically nearly dark 1npi* state are predicted to lie below CI1, explaining the experimental observation of a long-lived population of dark excited states in gas-phase thymine.     

Potential energy surface intersections in the C(1D)H2 reactive system

Potential energy surface (PES) intersection seams of two or more electronic states from the 1 1A', 2 1A', 3 1A', 1 1A", and 2 1A" states in the C(1D)H2 reactive system are investigated using the internally contracted multireference configuration interaction method and the aug-cc-pVQZ basis set. Intersection seams with energies less than 20 kcal/mol relative to the C(1D) + H2 asymptote are searched systematically, and finally several seam lines (at the linear H-C-H, linear C-H-H, and C(2v), geometries, respectively) and a seam surface (at Cs geometries) are discovered and determined. The minimum energy crossing points on these seams are reported and the influences of the PES intersections, in particular, conical intersections, on the CH2 spectroscopy and the C(1D) + H2 reaction dynamics are discussed. In addition, geometries and energies of the 1 1A2 and 1 1B2 states of methylene biradical CH2 are reported in detail for the first time.     

Solvent effect on conical intersections in excited-state 9H-adenine: radiationless decay mechanism in polar solvent

The ground- and excited-state free energy minima and the conical intersections among these states of 9H-adenine in aqueous and acetonitrile solutions are studied theoretically to elucidate the mechanism of radiationless decay. We employ the recently proposed linear-response free energy (LRFE) to locate the energy minima and conical intersections in solution. The LRFE is calculated by using the reference interaction site model self-consistent field method. The geometry optimizations are carried out at the complete active space self-consistent field level, and the dynamic electron correlation energies are estimated by the multireference M?ller-Plesset method. We find that the conical intersection between the (1)L(a) and (1)L(b) states in aqueous solution occurs at a wide area of the free energy surface, indicating a strong vibronic coupling between them. On the other hand, the (1)npi(*) state is largely blue-shifted at planar geometries in solution, which suggests that the nonadiabatic transition to this state is suppressed. The importance of the (1)pisigma(*) channel is also examined in both the gas phase and solution. Based on the free energy characteristics obtained by the calculations, we intend to explain the experimental observations that the excited state of 9H-adenine decays monoexponentially with shorter lifetimes in polar solvents than that in the gas phase.     

Can a single-reference approach provide a balanced description of ground and excited states? A comparison of the completely renormalized equation-of-motion coupled-cluster method with multireference quasidegenerate perturbation theory near a conical intersection and along a photodissociation coordinate in ammonia

We calculated the two lowest electronically adiabatic potential energy surfaces of ammonia in the region of the conical intersection and at a sequence of geometries along which one of the N-H bonds is broken. We employed both a multireference (MR) method and a single-reference (SR) method. The MR calculations are based on multiconfiguration quasidegenerate perturbation theory (MC-QDPT) with a 6-311+G(3df,3pd) basis set. The SR calculations, carried out with the same basis, employ the completely renormalized equation-of-motion coupled-cluster method with singles and doubles, and a noniterative treatment of triples, denoted CR-EOMCCSD(T). At 91 geometries used for comparison, including geometries near a conical intersection, the surfaces agree to 7% on average.     

The fluorescence mechanism of 5-methyl-2-pyrimidinone: an ab initio study of a fluorescent pyrimidine analog

The photophysically important potential energy surfaces of the fluorescent pyrimidine analog 5-methyl-2-pyrimidinone have been explored using multireference configuration-interaction ab initio methods at three levels of dynamical correlation, all of which support a fluorescence mechanism. At vertical excitation S1 (dark, n(N)pi*) and S2 (bright, pipi*) are almost degenerate at 4.4 eV, with S3 (dark, n(O)pi*) at 5.1 eV. The excited system can follow the S1-S2 seam of conical intersections, accessible from the Franck-Condon region, to its minimum and then evolve from this conical intersection on the S1 (pipi*) surface to a global minimum. At lower levels of correlation, the S1 surface shows two minima separated by a barrier of up to 0.18 eV. The secondary minimum found at the lower levels of correlation becomes the global minimum with higher correlation. The S1 population at this minimum can be trapped from accessing the lowest energy S0-S1 (pipi*/gs) conical intersection by an energy gap at least 0.3-0.4 eV higher than the S1 minimum. The calculated emission energy from this minimum is 2.80 eV. Gradient pathways connecting important S1 geometries are presented, as well as other excited state conical intersections.     

Three-state conical intersections in nucleic acid bases

The involvement of three-state conical intersections in the photophysics and radiationless decay processes of the nucleobases has been investigated using multireference configuration interaction methods. Three-state conical intersections have been located for the pyrimidine base, uracil, and the purine base, adenine. In uracil, a three-state degeneracy between the S(0), S(1), and S(2) states has been located at 6.2 eV above the ground-state minimum energy. This energy is 0.4 eV higher than vertical excitation to S(2) and at least 1.3 eV higher than the two-state conical intersections found previously. In adenine, two different three-state degeneracies between the S(1), S(2), and S(3) states have been located at energies close to the vertical excitation energies. The energetics of these three-state conical intersections suggest they can play a role in a radiationless decay pathway present in adenine. The existence of two different seams of three-state conical intersections indicates that these features are common and complicate the potential energy surfaces of adenine and possibly many other aromatic molecules.     

On the accessibility to conical intersections in purines: hypoxanthine and its singly protonated and deprotonated forms

The dynamics following electronic excitation of hypoxanthine and its nucleoside inosine were studied by femtosecond fluorescence up-conversion. Our objective was to explore variants of the purinic DNA bases in order to determine the molecular parameters that increase or reduce the accessibility to ground state conical intersections. From experiments in water and methanol solution we conclude that both dominant neutral tautomers of hypoxanthine exhibit ultrashort excited state lifetimes (τ < 0.2 ps), which are significantly shorter than in the related nucleobase guanine. This points to a more accessible conical intersection for the fluorescent state upon removal of the amino group, present in guanine but absent in hypoxanthine. The excited state dynamics of singly protonated hypoxanthine were also studied, showing biexponential decays with a 1.1 ps component (5%) besides a sub-0.2 ps ultrafast component. On the other hand, the S(1) lifetimes of the singly deprotonated forms of hypoxanthine and inosine show drastic differences, where the latter remains ultrafast but the singly deprotonated hypoxanthine shows a much longer lifetime of 19 ps. This significant variation is related to the different deprotonation sites in hypoxanthine versus inosine, which gives rise to significantly different resonance structures. In our study we also include multireference perturbation theory (MRMP2) excited state calculations in order to determine the nature of the initial electronic excitation in our experiments and clarify the ordering of the states in the singlet manifold at the ground state geometry. In addition, we performed multireference configuration interaction calculations (MR-CIS) that identify the presence of low-lying conical intersections for both prominent neutral tautomers of hypoxanthine. In both cases, the surface crossings occur at geometries reached by out of plane opposite motions of C2 and N3. The study of this simpler purine gives several insights into how small structural modifications, including amino substitution and protonation site and state, determine the accessibility to conical intersections in this kind of heterocycles.     

Three-state conical intersections in cytosine and pyrimidinone bases

Three-state conical intersections have been located and characterized for cytosine and its analog 5-methyl-2-pyrimidinone using multireference configuration-interaction ab initio methods. The potential energy surfaces for each base contain three different three-state intersections: two different S(0)-S(1)-S(2) intersections (gs/pi pi(*)/n(N)pi(*) and gs/pi pi(*)/n(O)pi(*)) and an S(1)-S(2)-S(3) intersection (pi pi(*)/n(N)pi(*)/n(O)pi(*)). Two-state seam paths from these intersections are shown to be connected to previously reported two-state conical intersections. Nonadiabatic coupling terms have been calculated, and the effects of the proximal third state on these quantities are detailed. In particular, it is shown that when one of these loops incorporates more than one seam point, there is a profound and predictable effect on the phase of the nonadiabatic coupling terms, and as such provides a diagnostic for the presence and location of additional seams. In addition, it is shown that each of the three three-state conical intersections located on cytosine and 5-methyl-2-pyrimidinone is qualitatively similar between the two bases in terms of energies and character, implying that, like with the stationary points and two-state conical intersections previously reported for these two bases, there is an underlying pattern of energy surfaces for 2-pyrimidinone bases, in general, and this pattern also includes three-state conical intersections.     

HN2(2A') electronic manifold. I. A global ab initio study of first two states

A detailed ab initio multireference configuration interaction calculation with a standard aug-cc-pVTZ basis set is reported for the 1(2)A' and 2(2)A' states of the title system. The aim is to establish the dissociation scheme of all channels, while revealing the 2(2)A'/3(2)A' seam of conical intersections consistent with the crossings in the diatomic fragments. An ab initio mapping of linear NNH and T-shaped and linear NHN loci of conical intersections is also reported, jointly with a discussion of the topological features associated to a newly reported 2(2)A'/3(2)A' crossing seam.     

Explicitly correlated multireference configuration interaction with multiple reference functions: avoided crossings and conical intersections

We develop an explicitly correlated multireference configuration interaction method (MRCI-F12) with multiple reference functions. It can be routinely applied to nearly degenerate molecular electronic structures near conical intersections and avoided crossings, where the reference functions are strongly mixed in the correlated wave function. This work is a generalization of the MRCI-F12 method for electronic ground states, reported earlier by Shiozaki et al. [J. Chem. Phys. 134, 034113 (2011)]. The so-called F12b approximation is used to arrive at computationally efficient formulas. The doubly external part of the wave function is expanded in terms of internally contracted configurations generated from all the reference functions. In addition, we introduce a singles correction to the CASSCF reference energies, which is applicable to multi-state calculations. As examples, we present numerical results for the avoided crossing of LiF, excited states of ozone, and the H(2)?+?OH (A(2)Σ(+)) reaction.     

An ab initio study of the excited States, isomerization energy profiles and conical intersections of a chiral cyclohexylidene derivative

The excited valence and Rydberg states of the chiral (4-methylcyclohexylidene) fluoromethane (4MCF) have been investigated using multiconfigurational CASSCF and CASPT2, and coupled-cluster methods (RI-CC2). A 3s Rydberg state is predicted below the valence (1)pipi* state. To gain insight into the photophysics of the cis-trans isomerization of this olefin, potential energy profiles for the valence (10pipi* state along the twisting and pyramidalization reaction coordinates have been computed using variational methods (CASSCF and multireference configuration interaction with singles and doubles (MR-CISD)). Starting from geometries with energies close to degeneracy in the valence and ground-state curves, three minima on the crossing seam that can be correlated with the conical intersections known for fluoroethylene, have been found. On the basis of these features, the photochemistry of 4MCF is discussed.     

Beyond two-state conical intersections. Three-state conical intersections in low symmetry molecules: the allyl radical

Using multireference configuration interaction expansions comprised of over 7 million configuration state functions, three-state conical intersections are reported for the closely spaced, spectroscopically observed (tilde)B(2A1), (tilde)C(2B1), and (tilde)D(2B2) states (in C(2v) symmetry) of the allyl radical. These conical intersections of states which were previously assigned as the 3,4,5(2)A states and are here reassigned as the 4,5,6(2)A states, are expected to be accessible using optical probes. This conclusion is obtained from the structure of the minimum energy point on the 4,5,6(2)A three-state conical intersection seam which is similar to the equilibrium structure of the ground (tilde)X(2A2) state and only 1.1 eV above the (tilde)D(2B2) state at its equilibrium geometry. The seam of three-state degeneracies joins two two-state seams of conical intersection, the 4,5(2)A and 5,6(2)A conical intersection seams. The energy of the minimum energy point on the 4,5(2)A two-state seam is only 0.15 eV above that of the (tilde)D(2B2) state at its equilibrium structure. Three-state intersections are also reported for the 3,4,5(2)A states.     

Multireference configuration interaction calculation of the B(2)a"-X(2)a" transition of halogen- and methyl-substituted vinoxy radicals

Ground and second excited electronic states of halogen and monomethyl substituted vinoxy radicals were studied by multireference configuration interaction (MRCI) calculation. Optimized geometries, rotational constants and vibrational frequencies of vinoxy and 1-fluorovinoxy showed good agreement with experimental values. Differences in calculated and observed B-X electronic transition energies were less than 0.1 eV and observed trends of blue shift upon increasing the number of substituted halogen atoms were reproduced by MRCI calculation. Observed fluorescence lifetimes of the vibrationless level in B state were in good agreement with calculated values. Rotational profiles of the 0-0 vibronic bands were successfully simulated with calculated rotational constants and transition dipole moments. Energy differences between planar and nonplanar optimized geometries in B state showed good correlation with the onset of fast nonradiative decay in B state, supporting the proposed mechanism of nonradiative decay via avoided crossings from B to A state which is followed by the decay to the ground state via conical intersections.     

Ab initio studies on the radiationless decay mechanisms of the lowest excited singlet states of 9H-adenine

The mechanisms that are responsible for the rapid deactivation of the (1)npi and( 1)pipi excited singlet states of the 9H isomer of adenine have been investigated with multireference ab initio methods (complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory based on the CASSCF reference (CASPT2)). Two novel photochemical pathways, which lead to conical intersections of the S(1) excited potential-energy surface with the electronic ground-state surface, have been identified. They involve out-of-plane deformations of the six-membered aromatic ring via the twisting of the N(3)C(2) and N(1)C(6) bonds. These low-lying conical intersections are separated from the minimum energy of the lowest ((1)npi) excited state in the Franck-Condon region by very low energy barriers (of the order of 0.1 eV). These properties of the S(1) and S(0) potential-energy surfaces explain the unusual laser-induced fluorescence spectrum of jet-cooled 9H-adenine, showing sharp structures only in a narrow energy interval near the origin, as well as the extreme excess-energy dependence of the lifetime of the singlet excited states. It is suggested that internal-conversion processes via conical intersections, which are accessed by out-of-plane deformation of the six-membered ring, dominate the photophysics of the lowest vibronic levels of adenine in the gas phase, while hydrogen-abstraction photochemistry driven by repulsive (1)pisigma states may become competitive at higher excitation energies. These ultrafast excited-state deactivation processes provide adenine with a high degree of intrinsic photostability.     

Conical intersections involving the dissociative 1pisigma* state in 9H-adenine: a quantum chemical ab initio study

The conical intersections of the dissociative 1pisigma* excited state with the lowest 1pipi* excited state and the electronic ground state of 9H-adenine have been investigated with multireference electronic structure calculations. Adiabatic and quasidiabatic potential energy surfaces and coupling elements were calculated as a function of the NH stretch coordinate of the azine group and the out-of-plane angle of the hydrogen atom, employing MultiReference Configuration-Interaction (MRCI) as well as Complete-Active-Space Self-Consistent-Field (CASSCF) methods. Characteristic properties of the 1pipi*-1pisigma* and 1pisigma*-S0 conical intersections, such as the diabatic-to-adiabatic mixing angle, the geometric phase of the adiabatic electronic wavefunctions, the derivative coupling, as well as adiabatic and diabatic transition dipole moment surfaces were investigated in detail. These data are a prerequisite for future quantum wavepacket simulations of the photodissociation and internal-conversion dynamics of adenine.     

H-vinyl conical intersections for dienes: a mechanism for the photochemical hula twists

The potential role of "H-vinyl" conical intersections in a photochemical process known as the hula twist has been evaluated. The H-vinyl conical intersections of butadiene are explored with complete active space self-consistent field calculations, and proposals about the geometries involved in the hula-twist mechanism are discussed and contrasted with the conventional one-bond flip mechanism of cis-trans isomerization.     

Photoexcited tautomerization of vinyl alcohol to acetylaldehyde via a conical intersection from contracted Schrödinger theory

The photoexcited tautomerization of vinyl alcohol to acetylaldehyde via a conical intersection is analyzed through the direct calculation of two-electron reduced density matrices (2-RDMs) from solutions to the anti-Hermitian contracted Schr?dinger equation (ACSE). The study utilizes the recent generalization of the ACSE method for the treatment of excited states [G. Gidofalvi and D. A. Mazziotti, Phys. Rev. A, 2009, 80, 022507]. We computed absolute energies of the critical points as well as various intermediate points along the ground- and excited-state potential energy surface of vinyl alcohol and acetylaldehyde. The ACSE, MCSCF, second-order multireference many-body perturbation theory (MRMP2), and various coupled cluster methods all demonstrate the existence of a family of pathways from vinyl alcohol to acetylaldehyde via a conical intersection that are monotonically decreasing in energy. The conical intersection, proposed for the first time in this paper, is both structurally and energetically similar to the ground-state transition state. We observe a relationship between conical intersections and transition states both in this paper and in our previous work on bicyclobutane's ring conical intersection [J. W. Snyder, Jr. and D. A. Mazziotti, J. Chem. Phys., 2011, 135, 024107]. To treat multireference correlation, we seeded the ACSE with an initial 2-RDM guess from a multiconfiguration self-consistent field (MCSCF) calculation. The ACSE recovers more single-reference correlation energy than MRMP2 and more multireference correlation energy than comparable single-reference wave function methods. The 2-RDMs from the ACSE nearly satisfy all necessary N-representability conditions.     

Electronic quenching of OH A 2Sigma+ radicals in single collision events with molecular hydrogen: quantum state distribution of the OH X 2Pi products

We report a combined experimental and theoretical investigation of the nonreactive quenching channel resulting from electronic quenching of OH A 2Sigma+ by molecular hydrogen. The experiments utilize a pump-probe scheme to determine the OH X 2Pi population distribution following collisional quenching in a pulsed supersonic expansion. The pump laser excites OH A 2Sigma+ (nu'=0, N'=0), which has a significantly reduced fluorescence lifetime due to quenching by H2. The probe laser monitors the OH X 2Pi (nu", N") population via laser-induced fluorescence on various A-X transitions under single collision conditions. The experiments reveal a high degree of rotational excitation (N") of the quenched OH X 2Pi products observed in nu"=1 and 2 as well as a pronounced propensity for quenching into the Pi(A') Lambda-doublet level. These experiments have been supplemented by extensive multireference, configuration-interaction calculations aimed at exploring the topology of the relevant potential energy surfaces. Electronic quenching of OH A 2Sigma+ by H2 proceeds through conical intersections between two potentials of A' reflection symmetry (in planar geometry) that correlate with the electronically excited A 2Sigma+ and ground X 2Pi states of OH. The conical intersections occur in high-symmetry geometries, in which the O side of OH points toward H2. Corroborating and extending earlier work of Hoffman and Yarkony [J. Chem. Phys. 113, 10091 (2000)], these calculations reveal a steep gradient away from the OH-H2 conical intersection as a function of both the OH orientation and interfragment distance. The former will give rise to a high degree of OH rotational excitation, as observed for the quenched OH X 2Pi products.     

Photodissociation of methane: exploring potential energy surfaces

The potential energy surface for the first excited singlet state (S(1)) of methane is explored using multireference singles and doubles configuration interaction calculations, employing a valence triple zeta basis set. A larger valence quadruple zeta basis is used to calculate the vertical excitation energy and dissociation energies. All stationary points found on the S(1) surface are saddle points and have imaginary frequencies for symmetry-breaking vibrations. By studying several two-dimensional cuts through the potential energy surfaces, it is argued that CH(4) in the S(1) state will distort to planar structures. Several conical intersection seams between the ground state surface S(0) and the S(1) surface have been identified at planar geometries. The conical intersections provide electronically nonadiabatic pathways towards products CH(3)((approximately)X (2)A"(2))+H, CH(2)((approximately)a (1)A(1))+H(2), or CH(2)((approximately)X (3)B(1))+H+H. The present results thereby make it plausible that the CH(3)((approximately)X (2)A"(2))+H and CH(2)((approximately)a (1)A(1))+H(2) channels are major dissociation channels, as has been observed experimentally.