Lowest singlet and triplet potential energy surfaces of S2N2

Forty four stationary points have been located on the lowest singlet and triplet potential energy surfaces of S(2)N(2). Ten minima and ten saddle points on the lowest singlet surface and eleven minima and thirteen saddle points on the lowest triplet surface were found. All saddle points were connected to minima or lower-order saddle points by following the intrinsic reaction coordinate. Renner-Teller effects in the linear isomers were studied by examining their bending curves. The S(2)N(2) polymerization mechanism was investigated by first locating the transition state corresponding to ring opening and then considering all species connected to it that are close in energy. The commonly accepted mechanism is problematic due to the number of species that would lead to dissociation to SN + SN. Other possible isomers that are consistent with the experimental evidence but do not connect to SN radicals in the dissociation limit were examined. A mechanism of polymerization to (SN)(x)() is proposed that involves excitation of the square planar singlet molecule to the triplet surface. The triplet species then undergoes a puckering, and polymerization occurs in a direction approximately perpendicular to the S(2)N(2) plane. Consideration of the predicted vibrational frequencies suggests the structure of the second isomer of S(2)N(2). This isomer has a trans-NSSN structure with a long SS bond. The energetics of trans-NSSN are consistent with the observed temperature effects in the dimerization of SN. Analysis of the bending curves of linear NSSN and NSNS indicates that trans-NSSN is the only isomer which has a small yet significant barrier to that dimerization.     

Theoretical characterization of photoisomerization channels of dimethylpyridines on the singlet and triplet potential energy surfaces

Photoexcitations and photoisomerizations due to low-lying n pi* and pi pi* excited states of dimethylpyridines are investigated by density functional theory, CASSCF, CASPT2 and MRCI methodologies. Mechanistic details for the formation of Dewar dimethylpyridines and the interconversions of dimethylpyridines are rationalized through the characterization of minima and transition states on the singlet and triplet potential energy surfaces of relevant intermediates. Our present theoretical schemes suggest that Mobius dimethylpyridine intermediate 14 and azabenzvalene intermediate 10 can serve as possible precursors to Dewar dimethylpyridines and singlet phototransposition products, respectively. The calculations suggest that an S1(pi pi*)/S0 conical intersection in dimethylpyridines 2 is involved in the formation of 14. An azabenzvalene 10 might be formed through S2(pi pi*)/S1(n pi*) interaction followed by an S1/S0 decay in dimethylpyridine 6. Calculated barriers of isomerizations from 14 to Dewar dimethylpyridine 7 and from 10 to 4 are 8.4 and 28.5 kcal mol(-1) at the B3LYP/6-311 G** level, respectively. In the suggested triplet multistage transposition mechanism, an out-of-plane distorted geometry 19 due to vibrational relaxation of the T1(3B1) excited state of 3,5-dimethylpyridine 6 is a precursor of the interconversion of 6 to 2.4-dimethylpyridine 4. The formation of a triplet azaprefulvene 21 with a barrier of 20.7 kcal mol(-1) is a key step during the triplet migration process leading to another out-of-plane distorted structure 27. Subsequent rearomatization of 27 completes the interconversion of 6 with 4. Present calculations provide some insight into the photochemistry of dimethylpyridines at 254 nm.     

Homogeneous photochemical oxidation via singlet O2 in supercritical CO2

This communication describes the reaction chemistry of singlet oxygen in supercritical carbon dioxide, demonstrating rapid and quantitative conversion of alpha-terpinene to ascaridole.     

Ab initio calculations on the singlet and triplet energy surfaces for CsiH2

Singlet and triplet calculations, including configuration interaction, are reported for H₂CSi, HCSiH and CSiH₂, and for the transition state on both surfaces.     

The lowest-lying electronic singlet and triplet potential energy surfaces for the HNO-NOH system: energetics, unimolecular rate constants, tunneling and kinetic isotope effects for the isomerization and dissociation reactions

The lowest-lying electronic singlet and triplet potential energy surfaces (PES) for the HNO-NOH system have been investigated employing high level ab initio quantum chemical methods. The reaction energies and barriers have been predicted for two isomerization and four dissociation reactions. Total energies are extrapolated to the complete basis set limit applying focal point analyses. Anharmonic zero-point vibrational energies, diagonal Born-Oppenheimer corrections, relativistic effects, and core correlation corrections are also taken into account. On the singlet PES, the (1)HNO → (1)NOH endothermicity including all corrections is predicted to be 42.23 ± 0.2 kcal mol(-1). For the barrierless decomposition of (1)HNO to H + NO, the dissociation energy is estimated to be 47.48 ± 0.2 kcal mol(-1). For (1)NOH → H + NO, the reaction endothermicity and barrier are 5.25 ± 0.2 and 7.88 ± 0.2 kcal mol(-1). On the triplet PES the reaction energy and barrier including all corrections are predicted to be 7.73 ± 0.2 and 39.31 ± 0.2 kcal mol(-1) for the isomerization reaction (3)HNO → (3)NOH. For the triplet dissociation reaction (to H + NO) the corresponding results are 29.03 ± 0.2 and 32.41 ± 0.2 kcal mol(-1). Analogous results are 21.30 ± 0.2 and 33.67 ± 0.2 kcal mol(-1) for the dissociation reaction of (3)NOH (to H + NO). Unimolecular rate constants for the isomerization and dissociation reactions were obtained utilizing kinetic modeling methods. The tunneling and kinetic isotope effects are also investigated for these reactions. The adiabatic singlet-triplet energy splittings are predicted to be 18.45 ± 0.2 and 16.05 ± 0.2 kcal mol(-1) for HNO and NOH, respectively. Kinetic analyses based on solution of simultaneous first-order ordinary-differential rate equations demonstrate that the singlet NOH molecule will be difficult to prepare at room temperature, while the triplet NOH molecule is viable with respect to isomerization and dissociation reactions up to 400 K. Hence, our theoretical findings clearly explain why (1)NOH has not yet been observed experimentally.     

Computational mechanistic study of methanol and molecular oxygen reaction on the triplet and singlet potential energy surfaces

The reaction of CH₃OH with the O₂ on the triplet and singlet potential energy surfaces (PES) was carried out using the B3LYP, MP2, and CCSD(T)//B3LYP theoretical approaches in connection with the 6-311++G(3df–3pd) basis set. Three pre-reactive complexes, ¹C1, ¹C2, and ³C1, on the singlet and triplet PES were formed between methanol and molecular oxygen. From a variety of the complexes, seven types of products are obtained, of which four types are found to be thermodynamically stable. Results reveal that there exists one intersystem crossing between triplet and singlet PES. For P4 adduct that is the main and kinetically the most favorable product, the rate constants are calculated in the temperature range of 200–1,000 K in the reliable pathway.     

Mechanism and rate constants of the CH2 + CH2CO reactions in triplet and singlet states: A theoretical study

Ab initio and density functional CCSD(T)-F12/cc-pVQZ-f12//B2PLYPD3/6-311G** calculations have been performed to unravel the reaction mechanism of triplet and singlet methylene CH₂ with ketene CH₂CO. The computed potential energy diagrams and molecular properties have been then utilized in Rice–Ramsperger–Kassel–Marcus-Master Equation (RRKM-ME) calculations of the reaction rate constants and product branching ratios combined with the use of nonadiabatic transition state theory for spin-forbidden triplet-singlet isomerization. The results indicate that the most important channels of the reaction of ketene with triplet methylene lead to the formation of the HCCO + CH₃ and C₂H₄ + CO products, where the former channel is preferable at higher temperatures from 1000 K and above. In the C₂H₄ + CO product pair, the ethylene molecule can be formed either adiabatically in the triplet electronic state or via triplet-singlet intersystem crossing in the singlet electronic state occurring in the vicinity of the CH₂COCH₂ intermediate or along the pathway of CO elimination from the initial CH₂CH₂CO complex. The predominant products of the reaction of ketene with singlet methylene have been shown to be C₂H₄ + CO. The formation of these products mostly proceeds via a well-skipping mechanism but at high pressures may to some extent involve collisional stabilization of the CH₃CHCO and cyclic CH₂COCH₂ intermediates followed by their thermal unimolecular decomposition. The calculated rate constants at different pressures from 0.01 to 100 atm have been fitted by the modified Arrhenius expressions in the temperature range of 300–3000 K, which are proposed for kinetic modeling of ketene reactions in combustion. © 2018 Wiley Periodicals, Inc.     

Computational mechanistic investigation of the gas phase C2H4 + CO reaction on the singlet and triplet potential energy surfaces

Reaction pathways of ethylene and carbon monoxide on the singlet and triplet potential energy surfaces (PESs) have been calculated at B3LYP/6-311++G (3df, 3dp), G3B3 and CCSD(T)//B3LYP levels. Reaction mechanisms have been investigated by analysis of various structures. Suggested reaction mechanisms reveal that ³P3(CH₂CHCHO) and ³P4(CH₃CCHO) are thermodynamically stable adducts with the negative value in Gibbs free energies on the triplet PES. In addition, results show that one intersystem crossing exists between triplet and singlet PESs, which are obtained by scanning of the C–C bond length in ¹IN3 and ³IN7 species.     

Singlet and triplet potential surfaces for the O2+C2H4 reaction

Electronic structure calculations at the CASSCF and UB3LYP levels of theory with the aug-cc-pVDZ basis set were used to characterize structures, vibrational frequencies, and energies for stationary points on the ground state triplet and singlet O(2)+C(2)H(4) potential energy surfaces (PESs). Spin-orbit couplings between the PESs were calculated using state averaged CASSCF wave functions. More accurate energies were obtained for the CASSCF structures with the MRMP2/aug-cc-pVDZ method. An important and necessary aspect of the calculations was the need to use different CASSCF active spaces for the different reaction paths on the investigated PESs. The CASSCF calculations focused on O(2)+C(2)H(4) addition to form the C(2)H(4)O(2) biradical on the triplet and singlet surfaces, and isomerization reaction paths ensuing from this biradical. The triplet and singlet C(2)H(4)O(2) biradicals are very similar in structure, primarily differing in their C-C-O-O dihedral angles. The MRMP2 values for the O(2)+C(2)H(4)→C(2)H(4)O(2) barrier to form the biradical are 33.8 and 6.1 kcal/mol, respectively, for the triplet and singlet surfaces. On the singlet surface, C(2)H(4)O(2) isomerizes to dioxetane and ethane-peroxide with MRMP2 barriers of 7.8 and 21.3 kcal/mol. A more exhaustive search of reaction paths was made for the singlet surface using the UB3LYP/aug-cc-pVDZ theory. The triplet and singlet surfaces cross between the structures for the O(2)+C(2)H(4) addition transition states and the biradical intermediates. Trapping in the triplet biradical intermediate, following (3)O(2)+C(2)H(4) addition, is expected to enhance triplet→singlet intersystem crossing.     

Cross sections and rate constants for OH + H2 reaction on three different potential energy surfaces for ro-vibrationally excited reagents

A systematic study of the reagent ro-vibrational excitations in H(2) + OH reaction is presented on three different potential energy surfaces using the multiconfiguration time-dependent Hartree method. An exact form of the kinetic energy operator including Coriolis coupling has been used. Coupled channel results on WDSE surface for vibrational excitation of H(2) produce very large cross sections in accordance with the previous approximate results. The rate constant obtained for H(2)(v = 1) at 300 K on the YZCL2 surface shows an excellent agreement with the most recent experimental result. Quantum dynamical results for ro-vibrational excitation of reagents obtained on the WSLFH surface show similar behavior to previous quasiclassical trajectory studies. The integral cross sections obtained for excited reagent rotations exhibit contrasting trends on the three surfaces. The effects are explained considering the different orientations of the transition state structure and the individual surface characteristics.     

The singlet and triplet states of phenyl cation. A hybrid approach for locating minimum energy crossing points between non-interacting potential energy surfaces

The phenyl cation is known to have two low-energy minima, corresponding to ¹ A ₁ and ³ B ₁ states, the first of which is more stable by ca. 25 kcal/mol. The minimum energy crossing point between these two surfaces, located at various levels including a hybrid method first described here, lies just above the minimum of the triplet, 0.12 kcal/mol at the CCSD(T)/cc-pVDZ//B3LYP/SV level, and there is significant spin-orbit coupling between the surfaces at this point. On the basis of these results, the lifetime of the triplet is expected to be very short. Received: 24 October 1997 / Accepted: 25 November 1997     

Reaction of HOD+ with NO2: effects of OD and OH stretching, bending, and collision energy on reactions on the singlet and triplet potential surfaces

Integral cross sections and product recoil velocity distributions were measured for the reaction of HOD(+) with NO(2), in which the HOD(+) reactant was prepared in its ground state and with mode-selective excitation in the 001 (OH stretch), 100 (OD stretch), and 010 (bend) modes. In addition, we measured the 300 K thermal kinetics in a selected ion flow tube reactor and report product branching ratios different from previous measurements. Reaction is found to occur on both the singlet and triplet surfaces with near-unit efficiency. At 300 K, the product branching indicates that triplet → singlet transitions occur in about 60% of triplet-coupled collisions, which we attribute to long interaction times mediated by complexes on the triplet surface. Because the collision times are much shorter in the beam experiments, the product distributions show no signs of such transitions. The dominant product on the singlet surface is charge transfer. Reactions on the triplet surface lead to NO(+), NO(2)H(+), and NO(2)D(+). There is also charge transfer, producing NO(2)(+) (a(3)B(2)); however, this triplet NO(2)(+) mostly predissociates. The NO(2)H(+)/NO(2)D(+) cross sections peak at low collision energies and are insignificant above ~1 eV due to OH/OD loss from the nascent product ions. The effects of HOD(+) vibration are mode-specific. Vibration inhibits charge transfer, with the largest effect from the bend. The NO(2)H(+)/NO(2)D(+) channels are also vibrationally inhibited, and the mode dependence reveals how energy in different reactant modes couples to the internal energy of the product ions.     

Symmetry analysis of the potential energy surfaces for the photochemical decomposition of formaldehyde

Orbital Correspondence Analysis in Maximum Symmetry (OCAMS) is applied to the decomposition pathways of formaldehyde to H₂ + CO and to H + HCO. The symmetry adapted nuclear motions, which are preferentially incorporated in the energetically favoured fragmentation pathways on both the ground and excited state surfaces are singled out. The results of this analysis are in full agreement with those of published potential energy surfaces and consistent with the results of experimental investigations reported in the literature. The nuclear motions favouring the various processes thus appear to be deducible from considerations of orbital symmetry.This work was carried out during tenure of a Minerva Foundation grant to one of us. (E.A.H.)     

Improved direct diabatization and coupled potential energy surfaces for the photodissociation of ammonia

Diabatic potential energy surfaces are a convenient starting point for dynamics calculations of photochemical processes, and they can be calculated by the fourfold way direct diabatization scheme. Here we present an improved definition of the reference orbital for applying the fourfold way direct diabatization scheme to ammonia. The improved reference orbital is a geometry-dependent hybrid orbital that allows one to define consistent dominant configuration lists at all geometries important for photodissociation. Using diabatic energies calculated with the new reference orbital and consistent dominant configuration lists, we have refitted the analytical representations of the ground and the first electronically excited singlet-state potential energy surfaces and the diabatic coupling surface. Improved functional forms were used to reproduce the experimental dissociation energies and excitation energies, which will be important for subsequent simulations of photochemical dynamics. We find that the lowest-energy conical intersection point is at 5.16 eV, with C _2v symmetry. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users. This article is part of the special issue dedicated to the memory of the late Professor Fernando Bernardi.     

Singlet and triplet potential energy surfaces of C3H2

The detailed singlet and triplet potential energy surfaces of C₃H₂ involving nine isomers and 13 transition structures are studied at the G3 level of theory. The rearrangement mechanisms and the electronic properties of various isomers in a broad energy range have been studied in both singlet and triplet states. Cyclopropenylidene and propargylene are found to be the most stable isomers in the singlet and triplet states, respectively. The singlet isomers are found to be more kinetically stable species as a result of high conversion barriers through which they pass. The calculations indicate that cyclopropyne in its triplet state is the least kinetically stable isomer. It is realized that the G3 method comprises both computational cost and accuracy and thus can be applied to investigation of potential energy surface of small molecules.     

Dynamics of radiationless transitions: effects of displacement-distortion-rotation of potential energy surfaces on internal conversion decay rate constants

General expressions for calculating the internal conversion decay rate constants between two adiabatic electronic states and between two diabatic electronic states are derived. The expressions include the displacements, distortions, and rotations of potential energy surfaces as well as the temperature. For illustration, internal conversion rate constants between various singlet electronic states of ethylene and between the first excited S1 and the ground S0 singlet electronic states of azulene are calculated.     

Ab initio potential energy and dipole moment surfaces for H5O2 +

Full-dimensional ab initio potential energy surface (PES) and dipole moment surface (DMS) are reported for H(5)O(2) (+). Tens of thousands of coupled-cluster [CCSD(T)] and second-order Moller-Plesset (MP2) calculations of electronic energies, using aug-cc-pVTZ basis, were done. The energies were fit very precisely in terms of all the internuclear distances, using standard least-square procedures, however, with a fitting basis that satisfies permutational symmetry with respect to like atoms. The H(5)O(2) (+) PES is a fit to 48 189 CCSD(T) energies, containing 7962 polynomial coefficients. The PES has a rms fitting error of 34.9 cm(-1) for the entire data set up to 110 000 cm(-1). This surface can describe various internal floppy motions, including the H atom exchanges, monomer inversions, and monomer torsions. First- and higher-order saddle points have been located on the surface and compared with available previous theoretical work. In addition, the PES dissociates correctly (and symmetrically) to H(2)O+H(3)O(+), with D(e)=11 923.8 cm(-1). Geometrical and vibrational properties of the monomer fragments are presented. The corresponding global DMS fit (MP2 based) involves 3844 polynomial coefficients and also dissociates correctly.     

Photodissociation of N2O: potential energy surfaces and absorption spectrum

The ultraviolet photodissociation of N(2)O is studied by wave packet calculations using global three-dimensional potential energy surfaces for the two lowest (1)A' states. The incorporation of all internal degrees of freedom in the dynamics calculations is essential for a realistic treatment. The room-temperature absorption cross section is well reproduced, including the weak vibrational structures. Classical periodic orbits show that the latter can be attributed to large-amplitude NN stretch motion combined with strong excitation of the bend. Weakening of the NN bond toward the N + NO channel is the necessary prerequisite. The temperature dependence of the calculated cross section is significant, as expected for a dipole-forbidden transition of a linear molecule; but it is not as strong as observed experimentally [G. S. Selwyn and H. S. Johnston, J. Chem. Phys. 74, 3791 (1981)]. This shortcoming is due to an apparent underestimation of the (0,1,0) hot band absorption. On the other hand, the calculations yield reasonable predictions of the ratios of bending-state resolved absorption cross sections, σ(0, 1, 0)∕σ(0, 0, 0) and σ(0, 2, 0)∕σ(0, 0, 0), measured at 204 nm [H. Kawamata et al. J. Chem. Phys. 125, 133312 (2006)].     

Photoionization-induced dynamics of ammonia: ab initio potential energy surfaces and time-dependent wave packet calculations for the ammonia cation

An analytical anharmonic six-dimensional three-sheeted potential energy surface for the ground and first excited states of the ammonia cation has been developed which is tailored to model the ultrafast photoinduced dynamics. Selected ab initio cuts, obtained by multireference configuration interaction calculations, have been used to determine the parameters of a diabatic representation for this Jahn-Teller and pseudo-Jahn-Teller system. The model includes higher-order coupling terms both for the Jahn-Teller and for the pseudo-Jahn-Teller matrix elements. The relaxation to the ground state is possible via dynamical pseudo-Jahn-Teller couplings involving the asymmetric bending and stretching coordinates. The photoelectron spectrum of NH3 and the internal conversion dynamics of NH3+ have been determined by wave packet propagation calculations employing the multiconfigurational time-dependent Hartree method. Three different time scales are found in the dynamics calculations for the second absorption band. The ultrafast Jahn-Teller dynamics of the two excited states occurs on a 5 fs time scale. The major part of the internal conversion to the ground state takes place within a short time scale of 20 fs. This fast internal conversion is, however, incomplete and the remaining excited state population does not decay completely even within 100 fs.     

Ab initio-based double many-body expansion potential energy surface for the first excited triplet state of the ammonia molecule

A global single-sheeted double many-body expansion potential energy surface is reported for the first excited triplet state of NH(3). It employs an approximate cluster expansion of the molecular potential that utilizes previously reported functions of the same family for the triatomic fragments. Four-body energy terms have been calibrated from extensive accurate ab initio data so as to reproduce the main features of the title system. A new switching function formalism has been reported to approximate the true multisheeted nature of NH(3)((3)A(2) (')) potential energy surface, thus allowing the correct behavior at the NH(2)((2)A(")) + H((2)S) and NH(2)((4)A(")) + H((2)S) dissociation limits. The resulting fully six-dimensional potential energy function reproduces the correct symmetry under the permutation of identical atoms, and predicts the correct behavior at all dissociation channels while providing a realistic representation at all interatomic separations. The major attributes of the NH(3) double many-body expansion potential energy surface have also been characterized, and found to be in good agreement, both with the calculated ones from the raw ab initio energies and the theoretical results available in the literature.