Correlation-consistent Triple-zeta Basis Research Articles

Ab initio study of the mechanism of the reaction ClO + O --> Cl + O2

Abstract. Ab initio investigation on the reaction mechanism of ClO + O --> Cl + O2 reaction has been performed using correlation consistent triple zeta basis set. The geometry and frequency of the reactants, products, minimum energy geometries and transition states are obtained using MP2 method and energetics are obtained at the QCISD(T)//MP2 level of theory. Primarily, a possible reaction mechanism is obtained on the basis on IRC calculations using MP2 level of theory. To obtain true picture of the reaction path, we performed IRC calculations using CASSCF method with a minimal basis set 6-31G**. Some new equilibrium geometries and transition states have been identified at the CASSCF level. Energetics are also obtained at the QCISD(T)//CASSCF method. Possible reaction paths have been discussed, which are new in literature. Heat of reaction is found to be consistent with the experimental data. Bond dissociation energies to various dissociation paths are also reported.

On the influence of water on fragmentation of the amino acid L-threonine

Abstract The present study describes the fragmentation of the amino acid L-threonine (C4H9NO3) with and without the inclusion of water influence. The fragmentation of the L-threonine molecule was theoretically studied using the Becke’s three-parameter hybrid functional method by applying the non-local correlation provided by Lee, Yang and Parr (B3LYP) with the correlation consistent triple zeta basis (cc-pVTZ). The polarizable continuum model (PCM) was used to evaluate the influence of water. Fragments were selected based on data from mass spectrometry experiments. The chemical compositions of fragments were identified, and the appearance energy was calculated. Based on the obtained results, we can conclude that water affects the appearance energy, the fragment structure and the fragment formation processes.

A new ab initio potential energy surface for the collisional excitation of N2H(+) by H2.

We compute a new potential energy surface (PES) for the study of the inelastic collisions between N2H(+) and H2 molecules. A preliminary study of the reactivity of N2H(+) with H2 shows that neglecting reactive channels in collisional excitation studies is certainly valid at low temperatures. The four dimensional (4D) N2H(+)-H2 PES is obtained from electronic structure calculations using the coupled cluster with single, double, and perturbative triple excitation level of theory. The atoms are described by the augmented correlation consistent triple zeta basis set. Both molecules were treated as rigid rotors. The potential energy surface exhibits a well depth of ≃2530 cm(-1). Considering this very deep well, it appears that converged scattering calculations that take into account the rotational structure of both N2H(+) and H2 should be very difficult to carry out. To overcome this difficulty, the "adiabatic-hindered-rotor" treatment, which allows para-H2(j = 0) to be treated as if it were spherical, was used in order to reduce the scattering calculations to a 2D problem. The validity of this approach is checked and we find that cross sections and rate coefficients computed from the adiabatic reduced surface are in very good agreement with the full 4D calculations.

Can two T-shaped isomers of OCS–C2H2 van der Waals complex exist?

Molecular properties of the OCS–C2H2 van der Waals complex are studied in detail in order to explore the possibility of formation of three isomers of the complex and to verify the experimental findings. Both dispersion-corrected and -uncorrected double hybrid density functional theories with correlation consistent triple zeta basis set are used to investigate the complex. The results obtained by the dispersion corrected density functional method, mPW2PLYP-D, agree very well with the experimental values. The Symmetry Adapted Perturbation Theory (SAPT) is applied to study the interaction.

Large-amplitude dynamics in vinyl radical: The role of quantum tunneling as an isomerization mechanism

We report tunneling splittings associated with the large amplitude 1,2 H-atom migration to the global minima in the vinyl radical. These are obtained using a recent full-dimensional ab initio potential energy surface (PES) [A. R. Sharma, B. J. Braams, S. Carter, B. C. Shepler, and J. M. Bowman, J. Chem. Phys. 130(17), 174301 (2009)] and independently, directly calculated "reaction paths." The PES is a multidimensional fit to coupled cluster single and double and perturbative treatment of triple excitations coupled-cluster single double triple (CCSD(T)) with the augmented correlation consistent triple zeta basis set (aug-cc-pVTZ). The reaction path potentials are obtained from a series of CCSD(T)/aug-cc-pVnTZ calculations extrapolated to the complete basis set limit. Approximate 1D calculations of the tunneling splitting for these 1,2-H atom migrations are obtained using each of these potentials as well as quite different 1D Hamiltonians. The splittings are calculated over a large energy ranges, with results from the two sets of calculations in excellent agreement. Though negligibly slow (>1 s) for the vibrational ground state, this work predicts tunneling-promoted 1,2 hydride shift dynamics in vinyl to exhibit exponential growth with internal vibrational excitation, specifically achieving rates on the sub-μs time scale at energies above E ≈ 7500 cm(-1). Most importantly, these results begin to elucidate the possible role of quantum isomerization through barriers without dissociation, in competition with the more conventional picture of classical roaming permitted over a much narrower window of energies immediately below the bond dissociation limit. Furthermore, when integrated over a Boltzmann distribution of thermal energies, these microcanonical tunneling rates are consistent with sub-μs time scales for 1,2 hydride shift dynamics at T > 1400 K. These results have potential relevance for combustion modeling of low-pressure flames, as well as recent observations of nuclear spin statistical mixing from high-resolution IR/microwave spectroscopy on vinyl radical.

Is HO3 minimum cis or trans? An analytic full-dimensional ab initio isomerization path

The minimum energy path for isomerization of HO(3) has been explored in detail using accurate high-level ab initio methods and techniques for extrapolation to the complete basis set limit. In agreement with other reports, the best estimates from both valence-only and all-electron single-reference methods here utilized predict the minimum of the cis-HO(3) isomer to be deeper than the trans-HO(3) one. They also show that the energy varies by less than 1 kcal mol(-1) or so over the full isomerization path. A similar result is found from valence-only multireference configuration interaction calculations with the size-extensive Davidson correction and a correlation consistent triple-zeta basis, which predict the energy difference between the two isomers to be of only Δ = -0.1 kcal mol(-1). However, single-point multireference calculations carried out at the optimum triple-zeta geometry with basis sets of the correlation consistent family but cardinal numbers up to X = 6 lead upon a dual-level extrapolation to the complete basis set limit of Δ = (0.12 ± 0.05) kcal mol(-1). In turn, extrapolations with the all-electron single-reference coupled-cluster method including the perturbative triples correction yield values of Δ = -0.19 and -0.03 kcal mol(-1) when done from triple-quadruple and quadruple-quintuple zeta pairs with two basis sets of increasing quality, namely cc-cpVXZ and aug-cc-pVXZ. Yet, if added a value of 0.25 kcal mol(-1) that accounts for the effect of triple and perturbative quadruple excitations with the VTZ basis set, one obtains a coupled cluster estimate of Δ = (0.14 ± 0.08) kcal mol(-1). It is then shown for the first time from systematic ab initio calculations that the trans-HO(3) isomer is more stable than the cis one, in agreement with the available experimental evidence. Inclusion of the best reported zero-point energy difference (0.382 kcal mol(-1)) from multireference configuration interaction calculations enhances further the relative stability to ΔE(ZPE) = (0.51 ± 0.08) kcal mol(-1). A scheme is also suggested to model the full-dimensional isomerization potential-energy surface using a quadratic expansion that is parametrically represented by a Fourier analysis in the torsion angle. The method illustrated at the raw and complete basis-set limit coupled-cluster levels can provide a valuable tool for a future analysis of the available (incomplete thus far) experimental rovibrational data.

Calculations of rovibrational energies and dipole transition intensities for polyatomic molecules using MULTIMODE

We report rigorous calculations of rovibrational energies and dipole transition intensities for three molecules using a new version of the code MULTIMODE. The key features of this code which permit, for the first time, such calculations for moderately sized but otherwise general polyatomic molecules are briefly described. Calculations for the triatomic molecule BF(2) are done to validate the code. New calculations for H(2)CO and H(2)CS are reported; these make use of semiempirical potentials but ab initio dipole moment surfaces. The new dipole surface for H(2)CO is a full-dimensional fit to the dipole moment obtained with the coupled-cluster with single and double excitations and a perturbative treatment of triple excitations method with the augmented correlation consistent triple zeta basis set. Detailed comparisons are made with experimental results from a fit to relative data for H(2)CS and absolute intensities from the HITRAN database for H(2)CO.

CCSD(T) Complete Basis Set Limit Relative Energies for Low-Lying Water Hexamer Structures

MP2 and CCSD(T) complete basis set (CBS) limit relative electronic energies (DeltaE(e)) have been determined for eight low-lying structures of the water hexamer by combining explicitly correlated MP2-R12 computations with higher-order correlation corrections from CCSD(T) calculations. Higher-order correlation effects are quite substantial and increase DeltaE(e) by at least +0.19 kcal mol(-1) and as much as +0.59 kcal mol(-1). The effects from zero-point vibrational energy (ZPVE) have been assessed from unscaled harmonic vibrational frequencies computed at the MP2 level with a correlation consistent triple-zeta basis set (cc-pVTZ for H and aug-cc-pVTZ for O). ZPVE effects are even more significant than higher-order correlation effects and are uniformly negative, decreasing the relative energies by -0.16 kcal mol(-1) to -1.61 kcal mol(-1). Although it has been widely accepted that the cage becomes the lowest-energy structure after ZPVE effects are included [Nature 1996, 381, 501-503], the prism is consistently the most stable structure in this work, lying 0.06 kcal mol(-1) below the nearly isoenergetic cage isomer at the electronic MP2 CBS limit, 0.25 kcal mol(-1) below at the electronic CCSD(T) CBS limit, and 0.09 kcal mol(-1) below at the harmonic ZPVE corrected CCSD(T) CBS limit. Moreover, application of any uniform scaling factor less than unity to correct for anharmonicity further stabilizes the prism and increases the relative energies of the other structures.

The photodissociation mechanisms of acrylonitrile: Ab initio calculations on reaction channels and surface intersections

The dissociations of CH(2)CHCN into CH(2)CH+CN and CH(2)C+HCN in the S(0), T(1), and (1)pi(2)pi(C[triple bond]N) ( *) (definitions of pi orbitals can refer to computational details) states, have been explored at the complete active space self-consistent field level of theory employing the Dunning correlation consistent triple-zeta basis set. The lowest energy points of the surface crossing seams have been searched. Two conical intersections, from (1)pi(C[triple bond]N)pi(1) ( *) to (1)pi(2)pi(1) ( *) (CI(1)) and from (1)pi(2)pi(1) ( *) to S(0) (CI(2)), and one intersystem crossing point (T(1)/S(0)) have been located. The energies of all critical points have been recomputed with the multiconfigurational second-order perturbation method. At each conical intersection, derivative coupling and unscaled gradient difference vectors have been analyzed to determine the relaxation channels that the molecule may evolve in after nonradiative decay. Once the molecule is photoexcited to the (1)pi(2)pi(1) ( *) or (1)pi(C[triple bond]N)pi(1) ( *) state, it would relax along the similar pathway: funneling through CI(1) and then CI(2), and finally populate the ground state. Our results show that upon 193 nm photoexcitation, the most probable reaction channel is the ground-state HCN elimination following radiationless decays from excited states through surface crossings, which consists with experimental results J. Chem. Phys. 108, 5784 (1998). The investigated dissociation channels on the (1)pi(2)pi(C[triple bond]N) ( *) surface, which are inaccessible upon 193 nm photoexcitation, may provide information for reactions induced by higher energy excitations.

MS-CASPT2 analysis of the UV thermochromism of octamethyltrisilane§

We interpret the reversal of the direction of the thermochromic shift of the first absorption band of peralkylated oligosilanes as the silicon chain is extended, based on multistate complete active space second-order perturbation theory (MS-CASPT2) calculations for octamethyltrisilane, Si3Me8. The observed shift is attributed to the effect of b1 distortions from ground state equilibrium geometry on vertical excited state energies and intensities. A generally contracted basis set of atomic natural orbitals (ANOs) at a ground state geometry optimized in the second-order Møller–Plesset perturbation theory (MP2) approximation with Dunning's correlation consistent triple-zeta basis set (cc-pVTZ) for the silicon atoms, the 6-31G* set for the carbon atoms and the 6-31G set for the hydrogen atoms was used. §This paper is dedicated to Professor Michael A. Robb on the occasion of his 60th birthday.

Experimental and theoretical studies of the vibrational spectra of cis-1-bromo-2-fluoroethene

The gas-phase infrared spectrum of cis-1-bromo-2-fluoroethene has been studied at low resolution in the range 200–6500 cm −1, leading to a complete assignment of the fundamentals, except the lowest vibrational mode ν 9 predicted at 167 cm −1. The remaining vibrational structure has been mainly interpreted in terms of first overtone or two quanta combination bands. Isotopic 79/81Br shift has been observed only in the ν 8 fundamental. The equilibrium structure and the quadratic force field have been investigated theoretically at CCSD(T) level of theory employing Dunning’s correlation consistent triple-zeta basis set. Cubic and semidiagonal quartic force field have been calculated using second-order Møller–Plesset perturbation theory and Ahlrich’ split valence (SV) contracted basis set. After a minor scaled quantum mechanical (SQM) adjustment of the quadratic force constants, the vibrational analysis, based on the second-order perturbation theory, has been carried out with the calculated force constants.

Dehydrophenylnitrenes: quartet versus doublet states.

The geometries and energies of 4-, 3-, and 2-dehydrophenylnitrenes (3, 4, and 5) are investigated using complete active space self-consistent field (CASSCF), multiconfiguration quasi-degenerate second-order perturbation (MCQDPT), and internally contracted multiconfiguration-reference configuration interaction (MRCI) theories in conjunction with a correlation consistent triple-zeta basis set. 4-Dehydrophenylnitrene 3 has a quartet ground state ((4)A(2)). The adiabatic excitation energies to the (2)A(2), (2)B(2), (2)A(1), and (2)B(1) states are 5, 21, 34, and 62 kcal mol(-1), respectively. The (2)B(2) state has pronounced closed-shell carbene/iminyl radical character, while the lowest-energy (2)B(1) state is a combination of a planar allene and a 2-iminylpropa-1,3-diyl. The MCQDPT treatment overestimates the excitation energy to (2)B(2) significantly as compared to CASSCF and MRCI+Q. Among quartet states, (4)A(2)-3 is the most stable one, while those of 4 and 5 (both (4)A'') are 3 and 1 kcal mol(-1) higher in energy. 5 also has a quartet ground state and a (2)A' ' state 7 kcal mol(-1) higher in energy. On the other hand, the doublet-quartet energy splitting is -6 kcal mol(-1) for 4 in favor of the doublet state ((2)A''). Hence, (2)A''-4 is the most stable dehydrophenylnitrene, 3.5 kcal mol(-1) below (4)A(2) of 3. The geometry of (2)A''-4 shows the characteristic features of through-bond interaction between the in-plane molecular orbitals at N and at C3. The (2)A' state of 4 resembles the (2)A(1) state of 3 and lies 32 kcal mol(-1) above (4)A''-4. The lowest-energy (2)A' state of 5, on the other hand, resembles the (2)B(2) state of 3 and lies 22 kcal mol(-1) above (4)A''-5.

An ab initio anharmonic force field of SiHCI3

An ab initio quartic force field of SiHCl3 is derived using the second-order Møller-Plesset perturbation theory and Dunning's correlation consistent triple-zeta basis set. After a minor empirical adjustment for the six diagonal quadratic force constants, most fundamentals of SiHCl3 and SiDCl3 agree with the experimental values within 1 cm−1. Additionally the observed overtones, combinations and hot band centres can also be well reproduced. Vibrational analysis based on the second-order perturbation theory is carried out with the calculated force constants. Two sets of spectroscopic constants are predicted for 28SiH35Cl3 and 28SiD35Cl3, respectively.

Multiconfigurational second-order perturbation calculation of the electronic absorption spectrum of trisilane, Si3H8

The low-lying singlet excited states of trisilane have been computed using the complete active space self-consistent field (CASSCF) method, second-order perturbation theory (CASPT2), and the extended multistate CASPT2 (MS-CASPT2) approach, and a generally contracted basis set of atomic natural orbitals (ANOs) including Rydberg functions. The ground state structure was obtained from an ab initio optimization at the second-order Møller-Plesset perturbation theory (MP2) level using Dunning's correlation-consistent triple-zeta basis set (cc-pVTZ) and agrees well with experiment. The calculation of the electronic transitions included the lower valence excited states and two Rydberg series converging to the first two ionization potentials. Only the MS-CASPT2 method gave an accurate description of the observed electronic excitations since it provided an effective separation of the valence and Rydberg states. In contrast with previous assignments, three states of valence character were found below the first Rydberg state, located at 7.20eV. As a consequence, the first broad band observed in the spectrum is entirely of valence character. Vertical dipole-allowed valence transition energies are calculated at 6.55, 7.09, 7.32, 7.60, 8.07 and 8.18eV (excitations to 11B2, 21A1, 31A1, 11B1, 51B2 and 21B1, states, respectively). Two vertical dipole-forbidden valence transitions assigned to the 11A2 and 41A2 states are found at 6.89 and 8.52eV, respectively. The 4s, 4p and 3d members of the Rydberg series converging to the first and second ionization potentials are found at 7.20–8.62 eV and 7.82–9.61 eV, respectively. A detailed theoretical interpretation of the absorption spectrum of trisilane based on the new assignments has been proposed, and comments on the ability of the CASSCF, CASPT2 and MS-CASPT2 approaches to describe accurately the absorption spectrum of oligosilanes are made.

Heat of formation of the CF 2++ dication: a theoretical estimate

Relative energies and ionization potentials of neutral and ionic fragments of CF 4 were calculated using the coupled cluster single double (triple)/correlation consistent triple zeta basis set ab initio method. The individual values were compared with available experimental and theoretical data to assess the accuracy of this approach for these particular systems. In addition, reaction enthalpies for possible fragmentation reactions were calculated. Maximum errors found were in the order of 5 kcal/mol (0.2 eV). The estimation of the heat of formation of the CF 2 ++ dication, derived from the calculations based on four different reactions, is 687 ± 5 kcal/mol (29.8 ± 0.2 eV). This value is considerably smaller than the one which follows from the appearance potential measurements (732 ± 7 kcal/mol).

Comparison of ab initio and density functional methods for TeF 6

The molecular geometry and vibrational frequencies of TeF 6 have been calculated using different quantum chemical (Hartree–Fock, MP2, DFT) theories in conjunction with various basis set combinations including relativistic effective core potentials supplemented with polarised double- and triple-zeta valence basis sets for Te and with a 6-31+G*, 6-311+G* and Dunning's correlation consistent bases for F. The results demonstrate that the errors of the individual theories and basis sets are fairly systematic. Investigating the basis set convergence for the molecular geometry, an increase of the valence basis of tellurium resulted in negligible alteration of the calculated bond length. On the other hand, the results were sensitive to the basis set of fluorine. An augmented correlation consistent triple-zeta basis was required to provide theoretical results in agreement with experiment.

Ab initio potential energy surfaces for He–Cl2, Ne–Cl2, and Ar–Cl2

The three-dimensional ground state potential energy surfaces for He–Cl2, Ne–Cl2, and Ar–Cl2 have been calculated using the single and double excitation coupled-cluster approach with noniterative perturbational treatment of triple excitations [CCSD(T)]. Calculations have been performed with the augmented correlation consistent triple zeta basis sets supplemented with an additional set of bond functions. Single point calculations for approximate minima have also been performed with several other basis sets including the quadruple zeta basis set (aug-cc-pVQZ) with bond functions. For He–Cl2 and Ar–Cl2 the CCSD(T) results show that the linear configuration is lower in energy than the T-shaped one. For Ne–Cl2 the CCSD(T) approach predicts the T-shaped configuration to be lower in energy. The linear configuration has been found to be more sensitive than the T-shaped one to the changes of the Cl–Cl bond length with the interaction becoming weaker when the Cl–Cl bond length is shortened from its equilibrium value and stronger when it is lengthened. More detailed analysis shows that sensitivity of component energies such as exchange, dispersion, and induction is much greater than that of supermolecule results. The interaction in the T-shaped configuration becomes slightly stronger for shorter Cl–Cl bonds. For He–Cl2 and Ar–Cl2 the larger zero-point vibrational energy of the linear configuration is responsible for making the T-shaped configuration the ground vibrational state. Vibrational effects further increase the difference in energy between the ground state T-shaped configuration of Ne–Cl2 and its linear counterpart.

An ab initio study of He–F2, Ne–F2, and Ar–F2 van der Waals complexes

Single and double excitation coupled-cluster approach with noniterative perturbational treatment of triple excitations [CCSD(T)] has been used to calculate the ground state potential energy surfaces for He–F2, Ne–F2, and Ar–F2 van der Waals complexes. Calculations have been performed with the augmented correlation consistent triple zeta basis sets supplemented with an additional set of bond functions (aug-cc-pVTZ+bf). Single point calculations for approximate minima have also been performed with a larger quadruple zeta basis set (aug-cc-pVQZ+bf). For He–F2 and Ar–F2 the CCSD(T) results show that the linear configuration is lower in energy than the T-shaped one. For Ne–F2 the CCSD(T) interaction energies of the two configurations are virtually the same. The linear configuration of each complex has been found to be much more sensitive than the T-shaped one to the changes of the F–F bond length with the interaction becoming weaker when the F–F bond length is shortened from its equilibrium value and stronger when it is lengthened. More detailed analysis shows that sensitivity of component energies such as exchange, dispersion, and induction is much greater than that of supermolecule results. High-order correlation corrections have been found to play an important role in determining the relative stability of the linear and T-shaped configurations. The harmonic approximation zero-point vibrational energy for He–F2 exceeds the depth of both wells. For Ne–F2 the zero-point vibrational energy is greater for the linear configuration and, because of that, the complex has a T-shaped ground vibrational state. When the zero-point vibrational energy is taken into account for the Ar–F2 complex the linear and the T-shaped configurations are found to have nearly identical energies.

Static and dynamic polarisabilities, Cauchy coefficients and their anisotropies: a comparison of standard methods

The static and dynamic polarisabilities of a range of small molecules have been calculated and compared with earlier experimental and computational work. The polarisabilities have been calculated using SCF and density functional theory (DFT) methods, the latter using local and non-local, hybrid functionals. Calculations at several low frequencies enable the Cauchy moment S(−4) to be found for both the isotropic and anisotropic polarisabilities. Results have been computed using two basis sets, Sadlej's medium-size polarized basis set and Dunning's correlation consistent triple-zeta basis set augmented with additional polarisation functions. The DFT values obtained with the hybrid functional are in good agreement with both experimental values and coupled-cluster results.