Lowest Triplet Potential Energy Surfaces Research Articles

Theoretical Study of the Full Photosolvolysis Mechanism of [Ru(bpy)3]2+: Providing a General Mechanistic Roadmap for the Photochemistry of [Ru(N^N)3]2+-Type Complexes toward Both Cis and Trans Photoproducts.

A complete mechanistic picture for the photochemical release of bipyridine (bpy) from the archetypal complex [Ru(bpy)3]2+ is presented for the first time following the description of the ground and lowest triplet potential energy surfaces, as well as their key crossing points, involved in successive elementary steps along pathways toward cis- and trans-[Ru(bpy)2(NCMe)2]2+. This work accounts for two main pathways that are identified involving (a) two successive photochemical reactions for photodechelation, followed by the photorelease of a monodentate bpy ligand, and (b) a novel one-photon mechanism in which the initial photoexcitation is followed by dechelation, solvent coordination, and bpy release processes, all of which occur sequentially within the triplet excited-state manifold before the final relaxation to the singlet state and formation of the final photoproducts. For the reaction between photoexcited [Ru(bpy)3]2+ and acetonitrile, which is taken as a model reaction, pathways toward cis and trans photoproducts are uphill processes, in line with the comparative inertness of the complex in this solvent. Factors involving the nature of the departing ligand and retained "spectator" ligands are considered, and their role in the selection of mechanistic pathways involving overall two sequential photon absorptions versus one photon absorption for the formation of both cis or trans photoproducts is discussed in relation to notable examples from the literature. This study ultimately provides a generalized roadmap of accessible photoproductive pathways for light-induced reactivity mechanisms of photolabile [Ru(N^N)(N^N')(N^N″)]2+-type complexes.

Accurate characterization of the lowest triplet potential energy surface of SO2 with a coupled cluster method.

The near-equilibrium potential energy surface (PES) of the ã 3B1 state of SO2 is developed from explicitly correlated spin-unrestricted coupled cluster calculations with single, double, and perturbative triple excitations with an augmented triple-zeta correlation-consistent basis set. The lowest-lying ro-vibrational energy levels of several sulfur isotopologues have been determined using this PES. It is shown that the new ab initio PES provides a much better description of the low-lying vibrational states than a previous PES determined at the multi-reference configuration interaction level. In particular, the theory-experiment agreement for the three lowest-lying vibrational transitions is within 1-3 cm-1.

A Key Factor Dominating the Competition between Photolysis and Photoracemization of [Ru(bipy)3]2+ and [Ru(phen)3]2+ Complexes.

Photolysis and photoracemization are two important photochemical phenomena of the prototype complexes [Ru(bipy)3]2+ and [Ru(phen)3]2+ (bipy = 2,2'-bipyridine, phen = 1,10-phenanthroline), but little is known about their relations. To solve this issue, the photoinduced chiral inversion Δ⇌Λ of the complexes was analyzed theoretically. The results indicated that the photoracemization reaction proceeds on the lowest triplet potential energy surface in three steps 3CTΔ↔3MCΔ, 3MCΔ↔3MCΛ, and 3MCΛ↔3CTΛ (CT = charge transfer state; MC = metal-centered state). Where the first and third steps are fast processes of picoseconds, the second is the rate-determining step (RDS) of microseconds. Such a slow step for the racemization leads to the excited molecule lingering around the bottom of 3MC state after the first step and, therefore, greatly enhances the possibility of deexcitation and photolysis mostly at the triplet-singlet crossing point. In other words, the photoracemization and photolysis of the complexes have a competition relation, not a slave relation as assumed by the photoracemization model suggested in literature. They are dominated by the RDS. This conclusion is also consistent with the Δ(δ S)⇌Λ(δ S) chiral inversion of the [Ru(bipy)2(L-ser)]+ series complexes, which is reversible with no detectable photolysis, as its second step is a fast one. Note that, although the photoracemization of the prototype complexes is very slow, it passes through the three steps reversibly and ends with a photon emitting, which could be detected with the time-resolved circularly polarized luminescence and related techniques. These findings are helpful to understand and control the photochemical behavior of the complexes in practice.

A Theoretical Study on the Dynamics of O(3P) + H2S(1A1) Reaction on an Interpolated Potential Energy Surface

Quasi-classical trajectory calculations at the DFT level and CVT calculations at the CCSD(T) level are performed to study the dynamics of O(3P) + H2S(1A1) reaction on the lowest triplet potential energy surface. In the dynamics part the potential energy surface has been constructed by interpolation technique following the method introduced by Collins and his coworkers. Total and individual classical reactive cross sections are calculated at collision energies from 13.1 to 126.0 kJ mol−1. The rate constants from QCT calculation are compared with those calculated from canonical variational transition state theory at the G3(MP2)B3 and CCSD(T)/Aug-cc-pVTZ levels. The energy partitioning in reactive collisions for the formation of main products (OH + SH and H + HSO) and in non-reactive collisions for the reactants is investigated. At 52.5 kJ mol−1 initial collision energy about 42% and 49% of the total available energy goes into the translational energy and internal motions of H + HSO products, respectively, w...

Computational study of two-body and three-body dissociation of CH3I2+

As one of the simplest alkyl halides, methyl iodide is extensively investigated in the research fields of the photodissociation and photoionization. In the experimental investigations of ionization and dissociation, many molecular fragments, such as Iq+(q3), CHn+(n3), H+, etc., are observed in the mass spectrum of CH3I. While the mechanisms for dissociation and ionization are not completely understood. As the doubly-ionized product, CH3I2+ exhibits different isomer structures and isomerization reactions. The dissociation channels of different isomers in combination with the corresponding transition states of CH3I2+ are helpful for better understanding the dissociation and ionization dynamics of CH3I in an intense laser field. In our present work, the dissociation channels of CH3I2+ are investigated by the density functional and couple cluster theory. The geometries and energies corresponding to the local isomers and the transition states of CH3I, CH3I+ and CH3I2+ are computed. The first and second ionization energies we measured are in good agreement with experimental values. Our computational results show that the ground state of the CH3I2+ is a triplet one with 3A2 symmetry. Totally 11 two-body and 15 three-body dissociation channels of the CH3I2+ on both the lowest singlet and the lowest triplet potential energy surfaces are computed and analyzed in detail. Our computations indicate that seven two-body dissociations channels, i.e., six singlet and one triplet ones, are exergonic, in which CH3I2+(1A')CH2++HI+(4A1) is the easiest process to achieve; four exergonic three-body dissociation channels with three on singlet potential energy surface and one on triplet potential energy surface are found. The possible mechanisms for producing the dissociative ionized fragments observed in experiments, CH3+, H+, and I+, are presented; furthermore, the dissociation channels generating other ions not observed in experiments, such as H3+ et al, are also given for further experimental study. The detailed information about dissociation channels and fragments is summarized for further experimental comparisons. In the computations, we find that the density functional theory and CCSD(T) methods give different energy orders for a few dissociation potential energy surfaces; and in this work, the analysis and discussion are performed based on the CCSD(T) results. Our computations indicate that the dissociation channels on singlet and triplet potential energy surface exhibit different behaviors.

Is photoisomerization required for NO photorelease in ruthenium nitrosyl complexes?

The factors that explain the competition between intramolecular NO linkage photoisomerization and NO photorelease in five ruthenium nitrosyl complexes were investigated. By applying DFT-based methods, it was possible to characterize the ground states and lowest triplet potential energy surfaces of these species, and to establish that both photoisomerization and photorelease processes can occur in the lowest triplet state of each species. This work highlights the crucial role of the sideways-bonded isomer, a metastable state also known as the MS2 isomer, in the photochemical loss of NO, while the results obtained also indicate that the population of the triplet state of this isomer is compulsory for both processes and show how photoisomerization and photorelease interfere. Graphical Abstract Illustration of the crucial role of the 3MS2 state in the photoreactivities of ruthenium nitrosyl complexes.

Pivotal Role of a Pentacoordinate (3)MC State on the Photocleavage Efficiency of a Thioether Ligand in Ruthenium(II) Complexes: A Theoretical Mechanistic Study.

A mechanistic study of the photocleavage of the methylthioethanol ligand (Hmte) in the series of ruthenium complexes [Ru(tpy)(N-N)(Hmte)](2+) (tpy = 2,2':6',2″-terpyridine, N-N = bpy (2,2'-bipyridine), biq (2,2'-biquinoline), dcbpy (6,6'-dichloro-2,2'-bipyridine), dmbpy (6,6'-dimethyl-2,2'-bipyridine)) was performed using density functional theory. These studies reveal the decisive role of two quasi-degenerate triplet metal-centered states, denoted (3)MChexa and (3)MCpenta, on the lowest triplet potential energy surface. It also shows how the population of the specific pentacoordinate (3)MCpenta state, characterized by a geometry more accessible for the attack of a solvent molecule, is a key step for the efficiency of the photosubstitution reaction. The difference in the photosubstitution quantum yields experimentally observed for this series of complexes (from φ = 0.022 for N-N = bpy up to φ = 0.30 for N-N = dmbpy) is rationalized by the existence of this (3)MCpenta photoreactive state and by the different topologies of the triplet excited-state potential energy surfaces, rather than by the sole steric properties of these polypyridinyl ligands.

Establishing the Two-Photon Linkage Isomerization Mechanism in the Nitrosyl Complex trans-[RuCl(NO)(py)4](2+) by DFT and TDDFT.

The density functional theory calculations presented in this work allow the first rationalization of the full linkage photoisomerization mechanism of trans-[RuCl(NO)(py)4](2+), in both the forward and reverse directions. These mechanisms are consistent with the experimental data establishing that blue-light irradiation triggers the forward process, while red or IR photons trigger the reverse process. Characterization of the singlet and lowest triplet potential energy surfaces shows that, despite the unfavorable thermodynamic character of the forward process, the topologies of the surfaces and particularly some crucial surface crossings enable the isomerization. In the forward Ru-NO → Ru-ON direction, a sequential two-photon absorption mechanism is unraveled that involves a sideways-bonded metastable state. In contrast, in the reverse reaction, two mechanisms are proposed involving either one or two photons.

O(3P) + CO2 Collisions at Hyperthermal Energies: Dynamics of Nonreactive Scattering, Oxygen Isotope Exchange, and Oxygen-Atom Abstraction

The dynamics of O((3)P) + CO(2) collisions at hyperthermal energies were investigated experimentally and theoretically. Crossed-molecular-beams experiments at <E(coll)> = 98.8 kcal mol(-1) were performed with isotopically labeled (12)C(18)O(2) to distinguish products of nonreactive scattering from those of reactive scattering. The following product channels were observed: elastic and inelastic scattering ((16)O((3)P) + (12)C(18)O(2)), isotope exchange ((18)O + (16)O(12)C(18)O), and oxygen-atom abstraction ((18)O(16)O + (12)C(18)O). Stationary points on the two lowest triplet potential energy surfaces of the O((3)P) + CO(2) system were characterized at the CCSD(T)/aug-cc-pVTZ level of theory and by means of W4 theory, which represents an approximation to the relativistic basis set limit, full-configuration-interaction (FCI) energy. The calculations predict a planar CO(3)(C(2v), (3)A'') intermediate that lies 16.3 kcal mol(-1) (W4 FCI excluding zero point energy) above reactants and is approached by a C(2v) transition state with energy 24.08 kcal mol(-1). Quasi-classical trajectory (QCT) calculations with collision energies in the range 23-150 kcal mol(-1) were performed at the B3LYP/6-311G(d) and BMK/6-311G(d) levels. Both reactive channels observed in the experiment were predicted by these calculations. In the isotope exchange reaction, the experimental center-of-mass (c.m.) angular distribution, T(θ(c.m.)), of the (16)O(12)C(18)O products peaked along the initial CO(2) direction (backward relative to the direction of the reagent O atoms), with a smaller isotropic component. The product translational energy distribution, P(E(T)), had a relatively low average of <E(T)> = 35 kcal mol(-1), indicating that the (16)O(12)C(18)O products were formed with substantial internal energy. The QCT calculations give c.m. P(E(T)) and T(θ(c.m.)) distributions and a relative product yield that agree qualitatively with the experimental results, and the trajectories indicate that exchange occurs through a short-lived CO(3)* intermediate. A low yield for the abstraction reaction was seen in both the experiment and the theory. Experimentally, a fast and weak (16)O(18)O product signal from an abstraction reaction was observed, which could only be detected in the forward direction. A small number of QCT trajectories leading to abstraction were observed to occur primarily via a transient CO(3) intermediate, albeit only at high collision energies (149 kcal mol(-1)). The oxygen isotope exchange mechanism for CO(2) in collisions with ground state O atoms is a newly discovered pathway through which oxygen isotopes may be cycled in the upper atmosphere, where O((3)P) atoms with hyperthermal translational energies can be generated by photodissociation of O(3) and O(2).

Direct dynamics simulations of photoexcited charge-transfer-to-solvent states of the I −(H 2O) n ( n = 4, 5 and 6) clusters

The dynamics of photoexcited charge-transfer-to-solvent (CTTS) states of the I −(H 2O) n ( n = 4, 5 and 6) clusters has been studied using the on-the-fly direct dynamics technique in order to understand recent femtosecond pump-probe experiments from a theoretical viewpoint. The lowest triplet potential energy surface at the hybrid B3LYP density-functional electronic structure level was employed to model the CTTS singlet excited-state potential energy surface due to small singlet–triplet splittings. A total of 13 structures of the I −(H 2O) n cluster were vertically excited with the initial kinetic energy being zero and subsequent trajectory simulations were performed up to 1.5–2.0 ps. It was found that features of time-evolution of vertical detachment energy of an excess electron and dipole moment along the trajectory is strongly dependent of the initial cluster structure employed. The simulations revealed that the structural change in the water network configuration due to breaking of hydrogen-bonds plays an important role in the dynamics. I-atom detachment from the cluster was not observed during simulation time but this result is presumably due to the too strong attractive interaction between I and H 2O on the B3LYP potential energy surface.

Direct dynamics simulations of photoexcited charge-transfer-to-solvent states of the I −(H 2O) 6 cluster

Direct molecular dynamics simulations have been carried out to understand the relaxation dynamics of photoexcited charge-transfer-to-solvent (CTTS) states for the I −(H 2O) 6 cluster and the subsequent excess electron stabilization dynamics by solvent molecules. Due to a small singlet–triplet splitting, the lowest triplet potential energy surface at the B3LYP-level calculations was used to model the CTTS singlet excited-state surface. Two book-type structures, which correspond to the lowest ground-state minimum-energy geometries, were vertically excited with the initial kinetic energy being zero. Although these two structures have a very similar geometry, it was found that the excess electron localization dynamics was totally different.

Theoretical study on HNC( 1Σ) production from the [formula omitted] reaction

The mechanism and dynamics of the reaction of C 2 ( X 1 Σ g + ) with NH(X 3Σ −) have been investigated using electronic structure methods. The CASPT2 calculations show that C 2 can add to NH without a barrier. Several intermediates involved in the reaction on the lowest triplet potential energy surface were optimized at the B3LYP level and then the potential energy diagram was refined at more accurate levels of theory. In order to understand the reaction dynamics more quantitatively, direct dynamics calculations have been performed at the B3LYP level. It has been found that the HNC molecule is efficiently produced from the C 2 + NH reaction.

Dynamical Calculations of Charge-Transfer-to-Solvent Excited States of Small I-(CH3CN)n Clusters

Relaxation dynamics of photoexcited charge-transfer-to-solvent (CTTS) states for the I(-)(CH(3)CN)(n) (n = 2 and 3) clusters has been theoretically studied using electronic structure methods. First, we have calculated several lowest singlet and triplet potential energy surfaces using the multireference configuration interaction method. It was found that the character of the singlet CTTS excited-state potential surfaces is very similar to that of the triplet CTTS states. Due to a small singlet-triplet splitting, the lowest triplet potential energy surface was used as a good model to understand the dynamics of the photoexcited singlet CTTS states. We have carried out direct molecular dynamics simulations on the lowest triplet surface at the B3LYP level. When an I(-) anion is exteriorly solvated by CH(3)CN molecules, we found that the (CH(3)CN)(n)(-) anion cluster is effectively produced. In addition, when the I(-) anion is placed in the interior in I(-)(CH(3)CN)(n) clusters, photoexcitation gives an acetonitrile monomer anion plus neutral monomers. However, if the initial geometric configuration is distorted from the minimum structure, we also found that the (CH(3)CN)(2)(-) anion cluster, where an excess electron is internally trapped, is formed via I(-)(CH(3)CN)(2) + hnu --> I + (CH(3)CN)(2)(-) process.

Quantum Dynamics Study on the Product Branching for the C(3P) + C2H2 Reaction: cyclic-C3H versus linear-C3H

Reduced-dimensionality quantum reactive scattering calculations for the C(3P) + C2H2 reaction have been carried out in order to understand the product branching dynamics of cyclic-C3H + H and linear-C3H + H. Our model treats only two degrees of freedom but can explicitly describe both of the C3H isomer product channels. The lowest triplet potential energy surface has been obtained by the hybrid density-functional method at the B3LYP/6-31G(d,p) level of theory. The calculated reaction probabilities were found to be dominated by resonance consistent with the complex-formation potential, and the results show that cyclic-C3H is preferentially formed via the cyclic-C3H2 intermediate produced by insertion of C(3P) into the CC bond. We have found that the isomerization from the cyclic-C3H2 to linear-C3H2 intermediate is suppressed by a barrier separating potential wells corresponding to these two intermediates. It has also been found that the energy dependence of the calculated total reaction cross section is in good agreement with the result of crossed molecular beam experiments.

Ab initio derived analytical fits of the two lowest triplet potential energy surfaces and theoretical rate constants for the N(4S)+NO(X 2Π) system

This work presents two new analytical fits of the ground potential energy surface (PES) (3A″) and the first excited PES (3A′) involved in the title reaction, considering the N-abstraction (1) and the O-abstraction (2) reaction channels, and the reverse reaction (−1). The PESs are derived from ab initio electronic structure calculations by means of second-order perturbation theory on a complete active-space self-consistent-field wave function (CASPT2 method). Stationary points and extensive grids of ab initio points (about 5600 points for the A″3 PES and 4900 points for the A′3 PES) were fitted along with some diatomic spectroscopic data to better account for the experimental exoergicity. Thermal rate constants were calculated (200–5000 K) for all mentioned reaction processes by means of the variational transition-state theory with the inclusion of a semiclassical tunneling correction. Excellent agreement with the experimental data was observed for reaction (1) and its reverse, within all the temperature range, substantially improving the results derived from previous analytical PESs. The contribution of the A′3 PES to the reaction rate constant (k1) was small even at high temperatures (e.g., only 10.8% at 2500 K). Moreover, the main contribution to reaction rate constant (k2) was due to the A′3 PES, differing from what happens for reaction (1). The O-abstraction reaction channel accounts for a 3.0% of the total reaction (k=k1+k2) at 5000 K, consistent with the very limited experimental information available.

Ab initio study of the two lowest triplet potential energy surfaces involved in the N(4S)+NO (X 2Π) reaction

This work presents ab initio electronic structure calculations of the two possible N(4S)+NO(X 2Π ) abstraction reaction channels on the lowest A″3 and A'3 potential energy surfaces (PESs). Complete active space self-consistent-field (CASSCF) calculations, second-order perturbation calculations (CASPT2), and multireference configuration interaction calculations (MR-CI) based on CASSCF wave functions, along with some coupled cluster (CC) calculations were carried out by using the standard correlation-consistent (cc-pVnZ and aug-cc-pVnZ, n=D,T,Q,5) Dunning’s basis sets. It was shown that there was no energy barrier along the minimum energy path in the A″3 PES for the N-abstraction reaction channel. However, an energy barrier (6.74 kcal/mol) was located in the A'3 PES. This energy barrier was considerably smaller than the previously reported MR-CCI value (14.4 kcal/mol). It was established that the N and O 2s electron correlation, neglected in previous studies of these authors, was the main source of this energy decrease. As a result, the present ab initio data will produce larger values of the thermal rate constants at high temperatures. High-energy barriers were found for the O-abstraction reaction channel in both PESs (41.13 and 30.77 kcal/mol for A″3 and A'3 , respectively), which agree with the accepted idea that this channel will be only important at high collision energies. Nonetheless, current ab initio results show that this channel will be open at not very high collision energies (e.g., over 30 kcal/mol could take place). Experimental studies on the O-abstraction reaction channel are missing and would be useful to confirm its ab initio expected importance.

Sterically Induced Conformational Relaxation and Structure of meso-Diaryloctaalkyl Porphyrins in the Excited Triplet State: Experimental and DFT Studies

The excited triplet state conformations of zinc and free base 5,15-diaryloctaalkylporhyrins are studied by experimental and computational (density functional theory, DFT) methods. From the observations of an unusual triplet state dynamics, i.e., fast nonradiative and biexponential decay, it has been suggested that these porphyrins exist in two distinguishable conformers in the lowest excited triplet state. X-ray crystallography and DFT (B3LYP/6-31G(d)) optimization of the ground state show that the porphyrins are planar prior to excitation. However, in the excited triplet state, the planar structure relaxes to an out-of-plane distorted saddle-shaped conformer. This distorted conformer and the lowest triplet potential energy surface are characterized by DFT calculations. It is suggested that the conformational relaxation explains the unusual triplet dynamics of this class of porphyrins.

A Quantum Study on the Reaction between C(3P) and Acetylene

The reaction between carbon atoms (C(3P)) and acetylene has been studied by a reduced dimensionality approach, restricted to the initial addition channels, in the energy range between 5 and 70 kJ mol-1. Coupled cluster calculations with single and double substitution and a non iterative estimate of the triple excitation (CCSD(T)) have been used to generate the lowest triplet potential energy surface. The flux into two different reaction channels, leading to linear and cyclic isomers of C3H, has been calculated by solving the time dependent Schrodinger equation. Results show that linear C3H is preferentially formed, while cyclic C3H is formed only at the highest energy of the calculations. Furthermore, the adiabatic capture-coupled states approximation (ACCSA) has been employed to generate rate constants at low temperatures, using the CCSD(T) potential, giving an improvement on the results obtained by using the long-range part of the potential.

Favorable performance of the DFT methods in predicting the minimum-energy structure of the lowest triplet state of WF4

The tetrahedral structure of the lowest triplet state of the WF4 complex was examined using different variants of the density functional theory (DFT) and conventional ab initio methods. The low-level, conventional, ab initio methods, such as SCF, MP2, MP3, and CISD, predict the tetrahedral structure to be a minimum, whereas the DFT schemes predict an imaginary frequency for the e vibrational mode. Only after recovering electron correlation effects at the MP4 and higher levels, the conventional electronic structure methods also predict the Td structure to be a second-order stationary point. This is not the correlation but the exchange part of the DFT functionals which is responsible for the discrepancy between the DFT and low-level, conventional, ab initio predictions. The lowering of symmetry to C2v leads to a minimum on the lowest triplet potential energy surface and the electronic energy difference between the Td and C2v stationary points amounts to 0.85 and 0.96 kcal/mol at the B3LYP and CCSD(T) levels, respectively. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 369–375, 1999

Theoretical Study of the Reaction CH(Χ2Π) + NO(Χ2Π). 3. Determination of the Branching Ratios

In this paper, which is the third of a series devoted to the title reaction, we present theoretical calculations of branching ratios for the product channels involved in the reaction. In the first paper of this series (Marchand, N.; Jimeno, P.; Rayez, J. C.; Liotard, D. J. Phys. Chem. 1997, 101, 6077.), we explored the topology of the lowest triplet potential energy surface determined with sophisticated ab initio methods and proposed several reaction paths connecting the reactants to the products. We have used these results to determine the branching ratios using two methods based on multichannel Rice-Ramsperger-Kassel-Marcus (RRKM) calculations: a μVTST/RRKM (μVTST = microcanonical variational transition state theory) method developed by one of us and an ACIOSA/RRKM (ACIOSA = adiabatic capture model using the infinite order sudden approximation) method dealing with a capture rate constant calculation (Marchand, N.; Stoecklin, T.; Rayez, J. C. To be submitted, of this series). Our present results reveal that, at 300 K, HCN + O is the major product channel involved in the reaction (72.0%), the other branching ratios being 13.9% for NCO + H, 8.2% for CO + NH, 3.3% for CNO + H, and 1.4% for CN + OH. All the others channels contribute for less than 1% each. These theoretical results are in agreement with the results of several experimental studies, especially those very recently obtained in our laboratory by Bergeat et al. Moreover, we observe no significant temperature dependence of the branching ratios.