Consistent Polarized Valence Double Zeta Research Articles

Mechanistic and kinetic study of the CH3CO+O2 reaction

Potential-energy surface of the CH3CO + O2 reaction has been calculated by ab initio quantum chemistry methods. The geometries were optimized using the second-order Moller-Plesset theory (MP2) with the 6-311G(d,p) basis set and the coupled-cluster theory with single and double excitations (CCSD) with the correlation consistent polarized valence double zeta (cc-pVDZ) basis set. The relative energies were calculated using the Gaussian-3 second-order Moller-Plesset theory with the CCSD/cc-pVDZ geometries. Multireference self-consistent-field and MP2 methods were also employed using the 6-311G(d,p) and 6-311++G(3df,2p) basis sets. Both addition/elimination and direct abstraction mechanisms have been investigated. It was revealed that acetylperoxy radical [CH3C(O)OO] is the initial adduct and the formation of OH and alpha-lactone [CH2CO2(1A')] is the only energetically accessible decomposition channel. The other channels, e.g., abstraction, HO2 + CH2CO, O + CH3CO2, CO + CH3O2, and CO2 + CH3O, are negligible. Multichannel Rice-Ramsperger-Kassel-Marcus theory and transition state theory (E-resolved) were employed to calculate the overall and individual rate coefficients and the temperature and pressure dependences. Fairly good agreement between theory and experiments has been obtained without any adjustable parameters. It was concluded that at pressures below 3 Torr, OH and CH2CO2(1A') are the major nascent products of the oxidation of acetyl radicals, although CH2CO2(1A') might either undergo unimolecular decomposition to form the final products of CH2O + CO or react with OH and Cl to generate H2O and HCl. The acetylperoxy radicals formed by collisional stabilization are the major products at the elevated pressures. In atmosphere, the yield of acetylperoxy is nearly unity and the contribution of OH is only marginal.

Ab initio calculation of bowl, cage, and ring isomers of C20 and C20−

High-level ab initio calculations have been carried out to reexamine relative stability of bowl, cage, and ring isomers of C(20) and C(20)(-). The total electronic energies of the three isomers show different energy orderings, strongly depending on the hybrid functionals selected. It is found that among three popular hybrid density-functional (DF) methods B3LYP, B3PW91, PBE1PBE, and a new hybrid-meta-DF method TPSSKCIS, only the PBE1PBE method (with cc-pVTZ basis set) gives qualitatively correct energy ordering as that predicted from ab initio CCSD(T)/cc-pVDZ [CCSD(T)-coupled-cluster method including singles, doubles, and noniterative perturbative triples; cc-pVDZ-correlation consistent polarized valence double zeta] as well as from MP4(SDQ)/cc-pVTZ [MP4-fourth-order Moller-Plesset; cc-pVTZ-correlation consistent polarized valence triple zeta] calculations. Both CCSD(T) and MP4 calculations indicate that the bowl is most likely the global minimum of neutral C(20) isomers, followed by the fullerene cage and ring. For the anionic counterparts, the PBE1PBE calculation also agrees with MP4/cc-pVTZ calculation, both predicting that the bowl is still the lowest-energy structure of C(20)(-) at T=0 K, followed by the ring and the cage. In contrast, both B3LYP/cc-pVTZ and B3PW91/cc-pVTZ calculations predict that the ring is the lowest-energy structure of C(20)(-). Apparently, this good reliability in predicting the energy ordering renders the hybrid PBE method a leading choice for predicting relative stability among large-sized carbon clusters and other carbon nanostructures (e.g., finite-size carbon nanotubes, nano-onions, or nanohorns). The relative stabilities derived from total energy with Gibbs free-energy corrections demonstrate a changing ordering in which ring becomes more favorable for both C(20) and C(20)(-) at high temperatures. Finally, photoelectron spectra (PES) for the anionic C(20)(-) isomers have been computed. With binding energies up to 7 eV, the simulated PES show ample spectral features to distinguish the three competitive C(20)(-) isomers.

Basis set and electron correlation effects on the polarizability and second hyperpolarizability of model open-shell π-conjugated systems

The basis set and electron correlation effects on the static polarizability (alpha) and second hyperpolarizability (gamma) are investigated ab initio for two model open-shell pi-conjugated systems, the C(5)H(7) radical and the C(6)H(8) radical cation in their doublet state. Basis set investigations evidence that the linear and nonlinear responses of the radical cation necessitate the use of a less extended basis set than its neutral analog. Indeed, double-zeta-type basis sets supplemented by a set of d polarization functions but no diffuse functions already provide accurate (hyper)polarizabilities for C(6)H(8) whereas diffuse functions are compulsory for C(5)H(7), in particular, p diffuse functions. In addition to the 6-31G(*)+pd basis set, basis sets resulting from removing not necessary diffuse functions from the augmented correlation consistent polarized valence double zeta basis set have been shown to provide (hyper)polarizability values of similar quality as more extended basis sets such as augmented correlation consistent polarized valence triple zeta and doubly augmented correlation consistent polarized valence double zeta. Using the selected atomic basis sets, the (hyper)polarizabilities of these two model compounds are calculated at different levels of approximation in order to assess the impact of including electron correlation. As a function of the method of calculation antiparallel and parallel variations have been demonstrated for alpha and gamma of the two model compounds, respectively. For the polarizability, the unrestricted Hartree-Fock and unrestricted second-order Møller-Plesset methods bracket the reference value obtained at the unrestricted coupled cluster singles and doubles with a perturbative inclusion of the triples level whereas the projected unrestricted second-order Møller-Plesset results are in much closer agreement with the unrestricted coupled cluster singles and doubles with a perturbative inclusion of the triples values than the projected unrestricted Hartree-Fock results. Moreover, the differences between the restricted open-shell Hartree-Fock and restricted open-shell second-order Møller-Plesset methods are small. In what concerns the second hyperpolarizability, the unrestricted Hartree-Fock and unrestricted second-order Møller-Plesset values remain of similar quality while using spin-projected schemes fails for the charged system but performs nicely for the neutral one. The restricted open-shell schemes, and especially the restricted open-shell second-order Møller-Plesset method, provide for both compounds gamma values close to the results obtained at the unrestricted coupled cluster level including singles and doubles with a perturbative inclusion of the triples. Thus, to obtain well-converged alpha and gamma values at low-order electron correlation levels, the removal of spin contamination is a necessary but not a sufficient condition. Density-functional theory calculations of alpha and gamma have also been carried out using several exchange-correlation functionals. Those employing hybrid exchange-correlation functionals have been shown to reproduce fairly well the reference coupled cluster polarizability and second hyperpolarizability values. In addition, inclusion of Hartree-Fock exchange is of major importance for determining accurate polarizability whereas for the second hyperpolarizability the gradient corrections are large.

Ab initio potential energy surfaces of the ion-molecule reaction: C2H2 + O+.

High level ab initio calculations using complete active space self-consistent field and multi reference single and double excitation configuration interaction methods with cc-pVDZ (correlation consistent polarized valence double zeta) and cc-pVTZ (triple zeta) basis sets have been performed to elucidate the reaction mechanism of the ion-molecule reaction, C2H2(1Sigmag+) + O+(4S), for which collision experiment has been performed by Chiu et al. [J. Chem. Phys. 109, 5300 (1998)]. The minor low-energy process leading to the weak spin-forbidden product C2H2+ (2Piu) + O(1D) has been studied previously and will not be discussed here. The major pathways to form charge-transfer (CT) products, C2H2+ (2Piu) + O(3P) (CT1) and C2H2+ (4A2) + O(3P) (CT2), and the covalently bound intermediates are investigated. The approach of the oxygen atom cation to acetylene goes over an energy barrier TS1 of 29 kcal/mol (relative to the reactant) and adiabatically leads the CT2 product or a weakly bound intermediate Int1 between CT2 products. This transition state TS1 is caused by the avoided crossing between the reactant and CT2 electronic states. As the C-O distance becomes shorter beyond the above intermediate, the C1 reaction pathway is energetically more favorable than the Cs pathway and goes over the second transition state TS2 of a relative energy of 39 kcal/mol. Although this TS connects diabatically to the covalent intermediate Int2, there are many states that interact adiabatically with this diabatic state and these lead to the other charge-transfer product CT1 via either of several nonadiabatic transitions. These findings are consistent with the experiment, in which charge transfer and chemical reaction products are detected above 35 and 39 kcal/mol collision energies, respectively.

The chlorobenzene-argon ground state intermolecular potential energy surface.

Using the coupled cluster singles and doubles including connected triple excitations model with the augmented correlation consistent polarized valence double zeta basis set extended with a set of 3s3p2d1f1g midbond functions, we evaluate the ground state intermolecular potential energy surface of the chlorobenzene-argon van der Waals complex. The minima of 420 cm(-1) are characterized by Ar atom position vectors of the length 3.583 A, forming an angle of 9.87 degrees with respect to the axis perpendicular to the chlorobenzene plane. These results are compared to those obtained for similar complexes and to the experimental data available. From the potential the three-dimensional vibrational eigenfunctions and eigenvalues are calculated and the results allow to correct and complete the experimental assignment.

The helium–, neon–, and argon–cyclopropane van der Waals complexes: Ab initio ground state intermolecular potential energy surfaces and intermolecular dynamics

Using the coupled cluster singles and doubles including connected triples model and the augmented correlation consistent polarized valence double zeta basis set extended with a set of 3s3p2d1f1g midbond functions, ab initio helium–, neon–, and argon–cyclopropane ground state intermolecular potential energies are evaluated and fitted to an analytic function including up to four-body interactions. These are the first ab initio potential energy surfaces available for these complexes and are characterized by an absolute minimum of −73.3 cm−1 at a distance on the cyclopropane C3-axis of 3.291 Å, −125.3 cm−1 at 3.435 Å, and −301.1 cm−1 at 3.696 Å for helium, neon, and argon, respectively. The bound van der Waals states are calculated. Two types of tunneling motion cause splittings of these levels: a C3 tunneling between the three equivalent local minima placed in the cyclopropane plane, and a C2 tunneling motion of the rare gas atom between the global minima above and below the cyclopropane plane.

Theoretical study of structures, energetics and vibrational properties of BC 2H 5 species

Hybrid density functional theory (B3LYP) and quadratic configuration interaction including single and double excitations (QCISD) methods with the Dunning correlation consistent polarized valence double zeta (cc-pVDZ) basis set have been used to examine the structures, vibrational spectra and stabilities of the singlet spin multiplicity molecular species with BC 2H 5 stoichiometry. Six isomers have been located. Total energies, refined at the CCSD(T)/cc-pVTZ//B3LYP/cc-pVDZ level, predict vinylborane molecule (H 2BCHCH 2, C s) to be the absolute minimum followed by the H 2CBCH 3 molecule (C s), 2.3 kcal/mol less stable. Borirane (HB(CH 2) 2, C 2v with BC 2 ring) and HBCHCH 3 (C s) molecules are calculated, respectively, to be 13.5 and 18.5 kcal/mol higher than vinylborane. The BH 3:C 2H 2 donor–acceptor complex is found about 42 kcal/mol higher than vinylborane. Ethylborine molecule BCH 2CH 3, the highest energy isomer, is predicted 45 kcal/mol above the absolute minimum. The calculated vibrational spectra for H 2BCHCH 2 and H 2CBCH 3 agree with the recent matrix isolation experiments and confirm their argon matrix IR characterization.

Ground state benzene–argon intermolecular potential energy surface

A highly accurate ab initio intermolecular potential energy surface for the benzene–argon van der Waals complex is evaluated using the coupled cluster singles and doubles model including connected triple excitations [CCSD(T)] model with an augmented correlation consistent polarized valence double zeta basis set extended with midbond functions. The vibrational energy levels obtained by full three-dimensional dynamical calculations are in excellent agreement with the available experimental data.

Theoretical study of structures, energetics and vibrational properties of BC 2H 3 species

Theoretical study using self-consistent field (SCF), hybrid density functional theory (B3LYP) and quadratic configuration interaction including single and double excitations (QCISD) with the Dunning correlation consistent polarized valence double zeta (cc-pVDZ) basis set have been used to examine the structures and vibrational properties of the singlet species with BC 2H 3 stoichiometry. Relative stabilities are estimated at the CCSD(T)/cc-pVTZ level using QCISD/cc-pVDZ optimized geometries. Five species corresponding to different nuclear arrangements have been studied. The absolute minimum corresponds to the 2 π aromatic borirene molecule (HBC 2H 2 with a BC 2 ring). Ethynylborane (H 2BCCH, C 2 v ) and borallene (H 2CCBH, C 2 v ) are respectively 6.4 and 24.3 kcal/mol higher. Vinylborine (C 2H 3B, C s ) and H 2CBCH (C 2 v ) are much less stable, 46.2 and 49.1 kcal/mol respectively higher than borirene. The calculated vibrational spectra agree with experiment and confirm the infrared matrix characterization of the three most stable species.

A combined density functional theory/molecular mechanics formalism and its application to small water clusters

We present a study of the conformations of the water dimer, trimer and tetramer using a combined density functional theory/molecular mechanics method (DFT/MM). Both local density approximation (LDA) and non-local density functional (NLDF) calculations are reported, employing two basis sets (polarized valence double zeta (DZVP) and augmented correlation consistent polarized valence double zeta (aug-cc-pVDZ)). In comparing the optimized geometries and the binding energy from the DFT/MM calculations with those derived from pure DFT and previous MP2 treatments, we find that an NLDF calculation is required to derive energetics which match the MP2 results. In all the cases studied, LDA is found to overestimate the binding energy and underestimate the oxygen–oxygen bond lengths ( R(O–O)) between two hydrogen-bonded water molecules. We attribute this to the importance of an NL description of hydrogen bonds, as previously found by other researchers. For the combined DFT/MM calculations, the situation is more complicated. While the MM term is, of course, not sensitive to NLDF, the DFT component of a partitioned DFT/MM system exhibits the same sensitivity as the pure DFT to NLDF, while the DFT/MM coupling term is less sensitive. The sensitivities of the DFT and DFT/MM calculations to the basis set are similar to the NLDF case. In all cases, the aug-cc-DZVP gives longer R(O–O) values and higher (less negative) binding energies than DZVP, but underestimates the binding energy when the H bond(s) exist(s) in the DFT region. The DFT/MM coupling term is found to be dependent on the initial geometry to some extent. In summary, the DFT/MM calculations with properly selected basis sets and NLDF give energetics and geometric information which compare favorably with MP2 results. © 1997 Elsevier Science B.V.

Characterization of the minimum energy paths for the ring closure reactions of C4H3 with acetylene

The ring closure reaction of C4H3 with acetylene to give phenyl radical is one proposed mechanism for the formation of the first aromatic ring in hydrocarbon combustion. There are two low-lying isomers of C4H3; 1-dehydro-buta-1-ene-3-yne (n-C4H3) and 2-dehydro-buta-1-ene-3-yne (iso-C4H3). It has been proposed that only n-C4H3 reacts with acetylene to give phenyl radical, and since iso-C4H3 is more stable than n-C4H3, formation of phenyl radical by this mechanism is unlikely. We report restricted Hartree–Fock (RHF) plus singles and doubles configuration interaction calculations with a Davidson’s correction (RHF+1+2+Q) using the Dunning correlation consistent polarized valence double zeta basis set (cc-pVDZ) for stationary point structures along the reaction pathway for the reactions of n-C4H3 and iso-C4H3 with acetylene. n-C4H3 plus acetylene (9.4) has a small entrance channel barrier (17.7) (all energetics in parentheses are in kcal/mol with respect to iso-C4H3 plus acetylene) and the subsequent closure steps leading to phenyl radical (−91.9) are downhill with respect to the entrance channel barrier. Iso-C4H3 plus acetylene also has an entrance channel barrier (14.9) and there is a downhill pathway to 1-dehydro-fulvene (−55.0). 1-dehydro-fulvene can rearrange to 6-dehydro-fulvene (−60.3) by a 1,3-hydrogen shift over a barrier (4.0), which is still below the entrance channel barrier, from which rearrangement to phenyl radical can occur by a downhill pathway. Thus, both n-C4H3 and iso-C4H3 can react with acetylene to give phenyl radical with small barriers.

An evaluation of correlation-consistent basis sets in calculating the structure and energetics of (H3Si)2O, H3SiOH, and H3SiO−

We studied the structure of disiloxane (H3Si–O–SiH3), silanol (H3Si–OH), and the silanol anion (H3Si–O−) with ab initio molecular orbital theory and the correlation consistent polarized basis sets of Dunning and co-workers. We present results for the correlation consistent polarized valence double zeta (cc-pVDZ), triple zeta (cc-pVTZ), and quadruple zeta [cc-pVQZ(-g)] basis sets. Optimized geometries and energies are given at both the restricted Hartree–Fock (RHF) level and with the inclusion of electron correlation by second order Mo/ller–Plesset perturbation theory (MP2). The correlation consistent basis sets provide a systematic expansion of the orbital basis set, with each set of additional functions adding a similar contribution to the correlation energy. We find that the calculated molecular properties show exponential convergence with increasing basis set size. These calculations answer long-standing questions regarding the structure and barrier to linearization of disiloxane. Results at the highest level of theory [MP2/cc-pVQZ(-g)] for disiloxane gave a Si–O–Si bond angle of 147.0°, a Si–O bond length of 1.641 Å, and a barrier to linearization of 0.4 kcal/mol. All of these values are in excellent agreement with experimental results. Similar calculations for silanol gave a Si–O bond length of 1.655 Å, an O–H bond of 0.955 Å, and a Si–O–H angle of 117.9°. The MP2/cc-pVQZ(-g) deprotonation energy (ΔE0) for silanol, calculated as the energy difference between silanol and its anion, is −366.6 kcal/mol. The effects of electron correlation at the MP4 level are studied in all three molecules using the cc-pVDZ and cc-pVTZ basis sets.

Ab initio variational transition state theory calculations for the O+NH2 hydrogen abstraction reaction on the 4A′ and 4A″ potential energy surfaces

We report high level ab initio calculations that characterize the quartet potential energy surfaces 4A″ and 4A′ for the O+NH2 direct hydrogen abstraction reaction. Minimum energy reaction pathways have been computed on both surfaces at the full-valence complete active space self-consistent field level using a correlation consistent polarized valence double zeta basis set. Energies along these reaction pathways have been further refined by multireference configuration interaction calculations with a correlation-consistent polarized valence triple zeta basis set. Canonical variational transition state theory calculations using the ab initio potential energy surface information as input and incorporating tunneling through the ground state vibrationally adiabatic potential energy curves indicate that below about 2000 K, the O+NH2 reaction is dominated by addition/(isomerization)/dissociation pathways, and for temperatures below about 1000 K, even the OH+NH product channel is dominated by the addition/isomerization/dissociation route.

The protonation of N2O reexamined: A case study on the reliability of various electron correlation methods for minima and transition states

The protonation of N2O and the intramolecular proton transfer in N2OH+ have been studied using large basis sets in conjunction with second-order many-body perturbation theory (MP2), singles and doubles coupled cluster (CCSD), the augmented coupled cluster method [CCSD(T)], and complete active space self-consistent field (CASSCF) methods. It is shown that MP2 is inadequate even for HNNO+, which has a minor nondynamical correlation effect; for the transition state only CCSD(T) produces a reliable geometry due to serious nondynamical correlation effects. Harmonic frequencies accurate to 50 cm−1 or better are predicted for both protonated species. The proton affinity at 298.15 K is found to be 137.6 kcal/mol, in excellent agreement with the recent experimental redetermination of 137.3±1 kcal/mol; the HNNO+ isomer is found to be 4.4 kcal/mol above the HONN+ isomer, with an interconversion barrier of ∼89 kcal/mol, herewith confirming recent experimental evidence that both species occur together with an energy difference of 6±1.5 kcal/mol. Comparison of the traditional double-zeta plus polarization (DZP) basis and the newer correlation consistent polarized valence double zeta (cc-pVDZ) basis set appears to indicate that the latter might lead to more accurate geometries and harmonic frequencies, although a more detailed investigation would be needed before any definitive conclusions.