Ag vibrational levels of cyclobutadiene on a new potential energy surface

GVB /[5s3p1d/3s1p] energies were calculated for 31 geometries of cyclobutadiene in the D_2h point group. These geometries differed in the values of the symmetrized internal coordinates for two CC stretching and one CCH bending modes. The data points were fitted to the expansions of in powers of . Variational calculations provided the following energies of the lowest A_g vibrational levels (with respect to the vibrational ground state): 4.4; 1161.2; 1162.3; 1304.0; 1322.8; 1920.3; and 1991.0 cm^?1.     

MNDO study of the electronic structure and potential energy surface of fluorinated singlet cyclopentadienyl and methylcyclopentadienyl cations

The electronic structure and geometry of some symmetric fluorinated singlet cyclopentadienyl and methylcyclopentadienyl cations are studied by the MNDO method. The structure of the potential energy surface (PES), which is a pseudorotation surface, is investigated. Extreme points of the PES, determining the PES barrier, correspond to structures with inverted frontier molecular orbitals. (Anti)aromaticity of fluorinated methylcyclopentadienyl cations is estimated using the Dewar-Breslow criterion. Novosibirsk Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 6, pp. 1023–1030, November–December, 1996. Translated by L. Smolina     

A multi-reference coupled-cluster study on the potential energy surface of N2 including ground and excited states: spin projections and wavefunction overlaps

This article reports on the calulation of 12 low lying states of the nitrogen molecule along its dissociation using the multi-reference exponential wavefunction ansatz (Hanrath in J Chem Phys 123:84102, 2005), the single-reference formalism multi-reference coupled cluster (Oliphant and Adamowicz in J Chem Phys 94:1229, 1991), and MRCI methods. Spin projection errors and state overlap errors are calculated and allow an analysis of the wavefunction with respect to properties different from correlation energies. Both criteria are very sensitive to errors in the wavefunction. Due to its lack of Fermi vacuum invariance the errors are more significant for the single-reference formalism based approach.     

Equation-of-motion spin-flip coupled-cluster model with single and double substitutions: Theory and application to cyclobutadiene

While the equation-of-motion coupled-cluster (EOM-CC) method is capable of describing certain multiconfigurational wave functions within a single-reference framework (e.g., open-shell type excited states, doublet radicals, etc.), it may fail in cases of more extensive degeneracy, e.g., bond breaking and polyradicals. This work presents an extension of the EOM-CC approach to these chemically important situations. In our approach, target multiconfigurational wave functions are described as spin-flipping excitations from the high-spin reference state. This enables a balanced treatment of nearly degenerate electronic configurations present in the target low-spin wave functions. The relations between the traditional spin-conserving EOM models and the EOM spin-flip method is discussed. The presentation of the formalism emphasizes the variational properties of the theory and shows that the killer condition is rigorously satisfied in single-reference EOM-CC theories. The capabilities and advantages of the new approach are demonstrated by its application to cyclobutadiene.     

The ground-state potential energy function of a beryllium dimer determined using the single-reference coupled-cluster approach

The accurate ground-state potential energy function of the beryllium dimer, Be(2), has been determined from large-scale ab initio calculations using the single-reference coupled-cluster approach in conjunction with the correlation-consistent core-valence basis sets up to septuple-zeta quality. Results obtained with the conventional and explicitly-correlated coupled-cluster methods were compared. The scalar relativistic and adiabatic (the diagonal correction) effects were also discussed. The vibration-rotation energy levels of Be(2) were predicted and found to be as accurate as those determined from the empirical potential energy function [J. M. Merritt et al., Science, 2009, 324, 1548]. The potential energy function of Be(2) was determined in this study to have a minimum at 2.444 ? and the well depth of 935 cm(-1).     

Ground state potential energy surface between cyclobutadiene and tetrahedrane looked down from S1/S0 conical intersections

We have explored the singlet ground state potential energy surface (S₀ PES) between cyclobutadiene (CBD) and tetrahedrane (THD) looked down from S₁/S₀ conical intersections through multi-configuration self-consistent field theory. On the basis of the obtained S₀ PES, we propose the revised process of the THD to CBD symmetry-forbidden reaction. According to the present result, the THD to CBD rearrangement occurs via plural steps similarly to previous suggestions, but via a tetra-radical species (instead of an endo-species), which is considered for the first time in this paper. Since the endo-species is significantly destabilized when hydrogen atoms are replaced by bulky substituents (such as tert-butyl group), the present one, where endo-species are not involved, would be realized in actual systems having bulky substituents.     

A multireference coupled-cluster potential energy surface of diazomethane, CH(2)N(2)

The intrinsically multireference dissociation of the C-N bond in ground-state diazomethane (CH(2)N(2)) at different angles has been studied with the multireference Brillouin-Wigner coupled-cluster singles and doubles (MRBWCCSD) method. The morphology of the calculated potential energy surface (PES) in C(s)() symmetry is similar to a multireference perturbational (CASPT3) PES. The MRBWCCSD/cc-pVTZ H(2)C-N(2) dissociation energy with respect to the asymptotic CH(2)(?(1)A(1)) + N(2)(X(1)Sigma(g)(+)) products is D(e) = 35.9 kcal/mol, or a zero-point corrected D(0) = 21.4 kcal/mol with respect to the ground-state CH(2)(X(3)B(1)) + N(2)(X(1)Sigma(g)(+)) fragments.     

Ab initio study of hydrazinyl radical: toward a DMBE potential energy surface

A series of stationary structures of the hydrazinyl radical have been characterized by optimization at the CCSD(T)/cc-pVTZ level of theory. CCSD(T)/aug-cc-pVXZ single-point calculations have also been carried out at the optimized geometries with basis sets of different cardinal numbers (X = T, Q), which were used to obtain accurate energies via extrapolation to the complete basis set limit. A discussion on the analytical modeling of the potential energy surface of hydrazinyl is also presented.     

A new analytical potential energy surface for the singlet state of He2H+

The analytic potential energy surface (APES) for the exchange reaction of HeH(+) (X(1)Σ(+)) + He at the lowest singlet state 1(1)A(∕) has been built. The APES is expressed as Aguado-Paniagua function based on the many-body expansion. Using the adaptive non-linear least-squares algorithm, the APES is fitted from 15 682 ab initio energy points calculated with the multireference configuration interaction calculation with a large d-aug-cc-pV5Z basis set. To testify the new APES, we calculate the integral cross sections for He + H(+)He (v = 0, 1, 2, j = 0) → HeH(+) + He by means of quasi-classical trajectory and compare them with the previous result in literature.     

Stabilization of polyheteroatomic aza and phospha analogs of cyclobutadiene and benzene

A method is proposed for the stabilization of polyheteroatomic rings consisting only of nitrogen or phosphorus atoms by introduction of acceptors of the unshared electron pair of the heteroatom, particularly oxygen atoms, with the formation of oxide forms. The positions of introduction of the acceptor groups into four- and six-membered rings were determined. The conclusions drawn were confirmed by calculations by the MNDO method of tetrazete 1,3-dioxide and 1,3-ylid and hexaphosphorine 1,3,5-trioxide and 1,3,5-ylid.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 250–255, February, 1988.     

Theoretical study on the potential energy surface and vibrational energy levels of HXeI

The potential energy surface for the electronic ground state of the HXeI molecule is constructed by using the internally contracted multi-reference configuration interaction with the Davidson correction（icMRCI＋Q）method and large basis sets. The stabilities and dissociation barriers are identified from the potential energy surfaces.The three-body dissociation channel is found to be the dominate dissociation channel for HXeI.Based on the obtained potentials,vibrational energy levels of HXeI are calculated using the Lanczos algorithm.Our theoretical results are in excellent agreement with the available observed values.     

Dissociation of toluene cation: a new potential energy surface

The potential energy surface (PES) for the formation of tropylium and benzylium ions from toluene cation (1) has been explored theoretically. Quantum chemical calculations at the B3LYP/6-311++G and G3//B3LYP levels were performed. A pathway to form o-isotoluene (5-methylene-1,3-cyclohexadiene) cation (5) from 1 was found. The isomerization occurs by two consecutive 1,2-H shifts from CH(3) to the ortho position of the aromatic ring via a distonic benzenium cation (2), which is also an intermediate in the well-known isomerization of 1 to cycloheptatriene cation (4). Since the barrier for the formation of 2 is the highest in the two isomerization pathways, 1, 4, and 5 are interconvertible energetically prior to dissociation. The benzylium ion can be produced via 5 as well as from 1 and the tropylium ion via 4. Rice-Ramsperger-Kassel-Marcus model calculations were carried out based on the obtained PES. The result agrees with previous experimental observations. From a theoretical analysis of kinetics of the isomerizations and dissociations, we suggest that 5 plays an important role in the formation of C(7)H(7)(+) from 1.     

The He-LiH potential energy surface revisited. II. Rovibrational energy transfer on a three-dimensional surface

We use our rigid rotor He-LiH potential energy surface [B. K. Taylor and R. J. Hinde, J. Phys. Chem. 111, 973 (1999)] as a starting point to develop a three-dimensional potential surface that describes the interaction between He and a rotating and vibrating LiH molecule. We use a fully quantum treatment of the collision dynamics on the current potential surface to compute rovibrational state-to-state cross sections. We compute excitation and relaxation vibrational rate constants as a function of temperature by integrating these cross sections over a Maxwell-Boltzmann translational energy distribution and summing over Boltzmann-weighted initial rotational levels. The rate constants for vibrational excitation of LiH are very small for temperatures below 300 K. Rate constants for vibrational relaxation of excited LiH molecules, however, are several orders of magnitude larger and show very little temperature dependence, suggesting that the collisions that result in vibrational relaxation are governed by long-range attractive interactions.     

Surveying a potential energy surface by eigenvector-following

We have developed a method to search potential energy surfaces which avoids some of the difficulties associated with trapping in local minima. Steps are directly taken between minima using eigenvector-following. Exploration of this space by low temperature Metropolis Monte Carlo is a useful global optimisation tool. This method successfully finds the lowest energy icosahedral minima of Lennard- Jones clusters from random starting configurations, but cannot find the global minimum in a reasonable time for difficult cases such as the 38-atom Lennard-Jones cluster where the face-centred-cubic truncated octahedron is lowest in energy. However, by performing searches at higher temperatures, we have found a pathway between the truncated octahedron and the lowest energy icosahedral minima. Such a pathway may be illustrative of some of the structural transformations that are observed for supported metal clusters by electron microscopy.     

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.     

The reaction of phenyl radical with molecular oxygen: a G2M study of the potential energy surface

Ab initio G2M calculations have been performed to investigate the potential energy surface for the reaction of C6H5 with O2. The reaction is shown to start with an exothermic barrierless addition of O2 to the radical site of C6H5 to produce phenylperoxy (1) and, possibly, 1,2-dioxaspiro[2.5]octadienyl (dioxiranyl, 8) radicals. Next, 1 loses the terminal oxygen atom to yield the phenoxy + O products (3) or rearranges to 8. The dioxiranyl can further isomerize to a seven-member ring 2-oxepinyloxy radical (10), which can give rise to various products including C5H5 + CO2, pyranyl + CO, o-benzoquinone + H, and 2-oxo-2,3-dihydrofuran-4-yl + C2H2. Once 10 is produced, it is unlikely to go back to 8 and 1, because the barriers separating 10 from the products are much lower than the reverse barrier from 10 to 8. Thus, the branching ratio of C6H5O + O against the other products is mostly controlled by the critical transition states between 1 and 3, 1 and 8, and 8 and 10. According to the calculated barriers, the most favorable product channel for the decomposition of 10 is C5H5 + CO2, followed by pyranyl + CO and o-benzoquinone + H. Since C6H5O + O and C5H5 + CO2 are expected to be the major primary products of the C6H5 + O2 reaction and thermal decomposition of C6H5O leads to C5H5 + CO, cyclopentadienyl radicals are likely to be the major product of phenyl radical oxidation, and so it results in degradation of the six-member aromatic ring to the five-member cyclopentadienyl ring. Future multichannel RRKM calculations of reaction rate constants are required to support these conclusions and to quantify the product branching ratios at various combustion conditions.     

Ab initio Calculations of the potential energy surface of the reaction of singlet methylene with the hydrogen molecule

The potential energy surface for the insertion of singlet methylene into H₂ has been computed on theab initio SCF level as well as with inclusion of electron correlation by means of the CEPA method. The results are compared with those of previous semiempirical,ab initio SCF and CI calculations. The system is a prototype of a reaction where an allowed and a symmetry-forbidden path can compete. The electron correlation energy was found to be very different for different regions of the surface, but did not have much influence on the optimum reaction path. From the computed heat of the reaction, the heat of formation of singlet methylene was estimated to be 101.5 kcal/mol. According to the calculations the reaction does not need any activation energy.     

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.     

The anharmonic potential energy surface of methyl fluoride

A large experimental spectroscopic data set sensitive to the cubic anharmonic potential energy surface (PES) of methyl fluoride has been compiled from the literature for six symmetric and asymmetric top isotopomers of methyl fluoride: 12CH3F, 13CH3FF, 12CD3F, 13CD3F, 12CHD2F and 12CH2DF. This empirical data set has been used to critically assess the best available literature ab initio cubic force field and various 'improved' theoretical force fields. A perturbation-resonance approach to the calculation of the observables from the force constants has been utilized and existing PESs were found to reproduce the data poorly. The careful treatment required for the correct theoretical reproduction of empirical spectroscopic constants arising from highly correlated least-squares fits to the original data is discussed. A new fit to the data has been performed (optimising 19 of the 38 cubic force constants) resulting in a much improved PES. The latter has been used to predict currently unknown spectroscopic constants for the least well-characterised isotopomer 12CH2DF. The prospects for a future empirical determination of the complete cubic force field of methyl fluoride are discussed and new data most likely to yield new information on the PES identified.     

The potential energy surface of the triazadiphosphole formation

The formal GaCl₃-assisted [3+2] cycloaddition of two (Me₃Si)₂N–N(SiMe₃)–PCl₂ molecules resulting in the formation of a triazadiphosphole has been studied by means of B3LYP/6-31G(d,p) computations. These calculations revealed a stepwise reaction mechanism starting from the disguised 1,3-dipole and dipolarophile (Me₃Si)₂N–N(SiMe₃)–PCl₂. Comparison of the potential energy surface for the formation of a triazadiphosphole in the presence and without a Lewis acid indicate, that addition of a Lewis acid such as GaCl₃ decreases the activation barriers to Me₃Si–Cl elimination, in accord with experiment. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.