Ab initio calculations on SnCl2 and Franck-Condon factor simulations of its ã-X and B-X absorption and single-vibronic-level emission spectra

Minimum-energy geometries, harmonic vibrational frequencies, and relative electronic energies of some low-lying singlet and triplet electronic states of stannous dichloride, SnCl(2), have been computed employing the complete-active-space self-consistent-field/multireference configuration interaction (CASSCF/MRCI) and/or restricted-spin coupled-cluster single-double plus perturbative triple excitations [RCCSD(T)] methods. The small core relativistic effective core potential, ECP28MDF, was used for Sn in these calculations, together with valence basis sets of up to augmented correlation-consistent polarized-valence quintuple-zeta (aug-cc-pV5Z) quality. Effects of outer core electron correlation on computed geometrical parameters have been investigated, and contributions of off-diagonal spin-orbit interaction to relative electronic energies have been calculated. In addition, RCCSD(T) or CASSCF/MRCI potential energy functions of the X(1)A(1), ?(3)B(1), and B(1)B(1) states of SnCl(2) have been computed and used to calculate anharmonic vibrational wave functions of these three electronic states. Franck-Condon factors between the X (1)A(1) state, and the ? (3)B(1) and B (1)B(1) states of SnCl(2), which include anharmonicity and Duschinsky rotation, were then computed, and used to simulate the ?-X and B-X absorption and corresponding single-vibronic-level emission spectra of SnCl(2) which are yet to be recorded. It is anticipated that these simulated spectra will assist spectroscopic identification of gaseous SnCl(2) in the laboratory and/or will be valuable in in situ monitoring of SnCl(2) in the chemical vapor deposition of SnO(2) thin films in the semiconductor gas sensor industry by laser induced fluorescence and/or ultraviolet absorption spectroscopy, when a chloride-containing tin compound, such as tin dichloride or dimethyldichlorotin, is used as the tin precursor.     

Ab initio calculations on SF2 and its low-lying cationic states: anharmonic Franck-Condon simulation of the UV photoelectron spectrum of SF2

Geometry optimization calculations were carried out on the (approximate)X(1)A(1) state of SF2 and the (approximate)X(2)B(1), (approximate)A(2)A(1), (approximate)B(2)B(2), (approximate)C(2)B(2), (approximate)D(2)A(1), and (approximate)E(2)A(2) states of SF2(+) employing the restricted-spin coupled-cluster single-double plus perturbative triple excitation [RCCSD(T)] method and basis sets of up to the augmented correlation-consistent polarized quintuple-zeta [aug-cc-pV(5+d)Z] quality. Effects of core electron (S 2s(2)2p(6) and F 1s(2) electrons) correlation and basis set extension to the complete basis set limit on the computed minimum-energy geometries and relative electronic energies (adiabatic and vertical ionization energies) were investigated. RCCSD(T) potential energy functions (PEFs) were calculated for the (approximate)X(1)A(1) state of SF2 and the low-lying states of SF2(+) listed above employing the aug-cc-pV(5+d)Z and aug-cc-pV5Z basis sets for S and F, respectively. Anharmonic vibrational wave functions of these neutral and cationic states of SF2, and Franck-Condon (FC) factors of the lowest four one-electron allowed neutral photoionizations were computed employing the RCCSD(T) PEFs. Calculated FC factors with allowance for Duschinsky rotation and anharmonicity were used to simulate the first four photoelectron bands of SF2. The agreement between the simulated and observed first bands in the He I photoelectron spectrum reported by de Leeuw et al. [Chem. Phys. 34, 287 (1978)] is excellent. Our calculations largely support assignments made by de Leeuw et al. on the higher ionization energy bands of SF2.     

Ab initio calculations on low-lying electronic states of SnCl(2)- and Franck-Condon simulation of its photodetachment spectrum

Geometry optimization and harmonic vibrational frequency calculations have been carried out on low-lying doublet and quartet electronic states of stannous (tin(II)) dichloride anion (SnCl(2)(-)) employing the CASSCF and RCCSD(T) methods. The small-core fully-relativistic effective core potential, ECP28MDF, was used for Sn in these calculations, together with valence basis sets of up to augmented correlation-consistent polarized-valence quintuple-zeta (aug-cc-pV5Z) quality. The ground electronic state of SnCl(2)(-) is determined to be the X(2)B(1) state, with the A(2)B(2) and ?(4)Sigma state, calculated to be ca. 1.50 and 2.72 eV higher in energy respectively. The electron affinities of the X(1)A(1) and ?(3)B(1) states of SnCl(2) have been computed to be 1.568+/-0.007 and 4.458+/-0.002 eV respectively, including contributions of core correlation and extrapolation to the complete basis set limit. The SnCl(2) (X(1)A(1)) + e <-- SnCl(2)(-) (X(2)B(1)) and SnCl(2) (?(3)B(1)) + e <-- SnCl(2)(-) (X(2)B(1)) photodetachment bands have been simulated with computed Franck-Condon factors, which include an allowance for anharmonicity and Duschinsky rotation.     

Ab initio calculations on SCl2 and low-lying cationic states of SCl2+: Franck-Condon simulation of the UV photoelectron spectrum of SCl2

Geometry optimization calculations were carried out on the (approximate)X (1)A(1) state of SCl(2) and the (approximate)X(2)B(1), (approximate)A(2)B(2), (approximate)B(2)A(1), (approximate)C(2)A(1), (approximate)D(2)A(2), and (approximate)E (2)B(2) states of SCl(2) (+) at the restricted-spin coupled-cluster single-double plus perturbative triple excitation [RCCSD(T)] level with basis sets of up to the augmented correlation-consistent polarized quintuple-zeta [aug-cc-pV(5+d)Z] quality. Effects of core electron correlation, basis set extension to the complete basis set limit, and relativistic contributions on computed minimum-energy geometrical parameters and/or relative electronic energies were also investigated. RCCSD(T) potential energy functions (PEFs) were calculated for the (approximate)X (1)A(1) state of SCl(2) and the low-lying states of SCl(2)(+) listed above employing the aug-cc-pV(5+d)Z basis set. Anharmonic vibrational wave functions of these neutral and cationic states of SCl(2), and Franck-Condon (FC) factors of the lowest four one-electron allowed neutral photoionizations were computed employing the RCCSD(T)aug-cc-pV(5+d)Z PEFs. Calculated FC factors with allowance for the Duschinsky rotation and anharmonicity were used to simulate the first four photoelectron (PE) bands of SCl(2). The agreement between simulated and observed He I PE spectra reported by Colton et al. [J. Electron Spectrosc. Relat. Phenom. 3, 345 (1974)] and Solouki et al. [Chem. Phys. Lett. 26, 20 (1974)] is excellent. However, our FC spectral simulations indicate that the first observed vibrational component in the first PE band of SCl(2) is a "hot" band arising from the SCl(2)(+)(approximate)X(2)B(1)(0,0,0)<--SCl(2)(approximate)X (1)A(1)(1,0,0) ionization. Consequently, the experimental adiabatic ionization energy of SCl(2) is revised to 9.55+/-0.01 eV, in excellent agreement with results obtained from state-of-the-art ab initio calculations in this work.     

Franck-Condon simulation, including anharmonicity, of the photodetachment spectrum of P2H(-): restricted-spin coupled-cluster single-double plus perturbative triple and unrestricted-spin coupled-cluster single-double plus perturbative triple -F12x potential energy functions of P2H and P2H(-)

Geometry optimization and harmonic vibrational frequency calculations have been carried out on the X?(2)A(') state of P(2)H and the X?(1)A(') state of P(2)H(-) using the restricted-spin coupled-cluster single-double plus perturbative triple excitation [RCCSD(T)] and explicitly correlated unrestricted-spin coupled-cluster single-double plus perturbative triple excitation [UCCSD(T)-F12x] methods. For RCCSD(T) calculations, basis sets of up to the augmented correlation-consistent polarized valence quintuple-zeta (aug-cc-pV5Z) quality were employed, and contributions from extrapolation to the complete basis set limit and from core correlation of the P 2s(2)2p(6) electrons were also included. For UCCSD(T)-F12x calculations, different atomic orbital basis sets of triple-zeta quality with different associated complementary auxiliary basis sets and different geminal Slater exponents were used. When the P 2s(2)2p(6) core electrons were correlated in these F12x calculations, appropriate core-valence basis sets were employed. In addition, potential energy functions (PEFs) of the X?(2)A(') state of P(2)H and the X?(1)A(') state of P(2)H(-) were computed at different RCCSD(T) and UCCSD(T)-F12x levels, and were used in variational calculations of anharmonic vibrational wavefunctions, which were then utilized to calculate Franck-Condon factors (FCFs) between these two states, employing a method which includes allowance for anharmonicity and Duschinsky rotation. The photodetachment spectrum of P(2)H(-) was then simulated using the computed FCFs. Simulated spectra obtained using the RCCSD(T)/aug-cc-pV5Z and UCCSD(T)-F12x(x = a or b)/aug-cc-pCVTZ PEFs are compared and found to be essentially identical. Based on the computed FCFs, a more detailed assignment of the observed vibrational structure than previously reported, which includes "hot bands," has been proposed. Comparison between simulated and available experimental spectra has been made, and the currently most reliable sets of equilibrium geometrical parameters for P(2)H and its anion have been derived. The photodetachment spectrum of P(2)D, yet to be recorded, has also been simulated.     

An ab initio study on the ground and low-lying doublet electronic states of SbO2

Geometry optimization and harmonic vibrational frequency calculations have been carried out on the low-lying doublet electronic states of antimony dioxide (SbO(2)) employing a variety of ab initio methods, including the complete active space self-consistent field/multireference configuration interaction and the RCCSD(T) methods. Both large and small core relativistic effective core potentials were used for Sb in these calculations, together with valence basis sets of up to aug-cc-pV5Z quality. Contributions from outer core correlation and off-diagonal spin-orbit interaction to relative electronic energies have been calculated. The ground electronic state of SbO(2) is determined to be the X (2)A(1) state, as is the case for dioxides of other lighter group 15 p-block (or group VA) elements. However, the A (2)B(2) and B (2)A(2) states are estimated to be only 4.1 and 10.7 kcalmole above the X (2)A(1) state, respectively, at the complete basis set limit. Reliable vertical excitation energies from the X (2)A(1) state to low-lying excited states of SbO(2) have been computed with a view to assist future spectral assignments of the absorption and/or laser-induced fluorescence spectra of SbO(2), when they become available.     

The enthalpies of formation of AsX(n) molecules, where X=H, F or Cl, and n=1, 2 or 3, by RCCSD(T) and UCCSD(T)-F12x calculations

RCCSD(T) and UCCSD(T)-F12x calculations were performed on AsX(n) molecules, where X = H, F or Cl, and n = 1, 2 or 3, and related species, in order to evaluate their enthalpies of formation (ΔH(f)(?)). The recommended ΔH(f)(?) values obtained from the present investigation are AsH, 57.7(2); AsF, -7.9(3); AsCl, 27.2(4); AsH(2), 39.8(4); AsF(2), -96.6(9); AsCl(2), -17.8(10); AsH(3), 17.1(4); AsF(3)-196.0(5) and AsCl(3), -59.1(27) kcal mole(-1). These values are anchored only on one thermodynamic quantity, namely, ΔH(f)(?)(As) (= 70.3 kcal mole(-1)). In the calculations, the fully-relativistic small-core effective core potential (ECP10MDF) was used for As. Contributions from outer core correlation of As 3d(10) electrons were computed explicitly in both RCCSD(T) and UCCSD(T)-F12 calculations with additional tight basis functions designed for As 3d(10) electrons. Basis sets of up to augmented correlation-consistent polarized valence quintuple-zeta (aug-cc-pV5Z) quality were used in RCCSD(T) calculations and computed relative electronic energies were extrapolated to the complete basis set (CBS) limit. For the simplified, explicitly correlated UCCSD(T)-F12x calculations, basis sets of up to quadruple-zeta (QZ) quality were employed. Based on the RCCSD(T)/CBS benchmark values, the reliability of available theoretical and experimental values have been assessed.     

Theoretical characterization of the SiC3H- anion

Highly correlated ab initio methods are used to predict the equilibrium structures and spectroscopic parameters of the SiC(3)H(-) anion. The total energies and physical properties are reported using CASSCF/MRCI, RCCSD(T), and RCCSD(T)-F12 approaches and extended basis sets. The search of stable geometries leads to a total of 12 isomers (4 linear and 8 cyclic), for which electronic ground states have close-shell configurations. The stability of the linear form, l-SiC(3)H(-), is prominent. For the most stable linear isomer, the B(e) equilibrium rotational constant has been calculated with RCCSD(T) and a complete basis set. Core-correlation and vibrational effects have been taken into account to predict a B(0) of 2621.68 MHz for l-SiC(3)H(-) and 2460.48 MHz for l-SiC(3)D(-). The dipole moment of l-SiC(3)H(-) was found to be 2.9707 D with CASSCF/aug-cc-pV5Z and the electron affinity to be 2.7 eV with RCCSD(T)-F12A/aug-cc-pVTZ. Anharmonic spectroscopic parameters are derived from a quadratic, cubic, and quartic RCCSD(T)-F12A force field and second order perturbation theory. CASSCF/MRCI vertical excitations supply three metastable electronic states, (1)Σ(+) (3)Σ(+) and (3)Δ. Electron affinities calculated for a series of chains type SiC(n)H and SiC(n) (n=1-5) allow us to discuss the anion formation probabilities.     

Ab initio calculations on the X (1)A(') and A (1)A(") states of HPO and Franck-Condon simulation of the single vibronic level emission spectra of HPO and DPO

Minimum-energy geometries and relative electronic energies of the X (1)A(') and A (1)A(") states of HPO have been computed employing the coupled-cluster single-double plus perturbative triple excitations {RCCSD(T)} and/or complete-active-space self-consistent-field (CASSCF) multireference internally contracted configuration interaction (MRCI) methods with basis sets of up to the augmented correlation-consistent polarized-valence quintuple-zeta (aug-cc-pV5Z) quality. In addition, RCCSD(T)/aug-cc-pVQZ and CASSCF/MRCI/aug-cc-pVQZ potential energy functions, anharmonic vibrational wave functions, and energies involving all three vibrational modes for both electronic states of HPO and DPO, and Franck-Condon factors between the two electronic states, which allow for Duschinsky rotation and anharmonicity, were computed. Computed Franck-Condon factors were then used to simulate single vibronic level (SVL) emission spectra recently reported by Tackett and Clouthier [J. Chem. Phys. 117, 10604 (2002)]. Excellent agreement between the simulated and observed spectra was obtained for the A (1)A(")(1,0,0)-->X (1)A(') SVL emission of HPO and DPO, when the best estimated ab initio geometries of the two states, which include contributions from core correlation and extrapolation to the complete basis set limit, were used in the simulation, suggesting that the best estimated ab initio geometry of the A (1)A(") state of HPO, particularly the bond angle of 94.5 degrees , is more reliable than the available experimentally derived geometry. A discussion on the geometrical parameters derived from rotational constants obtained from the rotational analysis of a high-resolution spectrum and from Franck-Condon simulation of the vibrational structure of an electronic spectrum is given.     

Low-lying quartet electronic states of nitrogen dioxide

The environmentally active molecule nitrogen dioxide (NO2) has been systematically studied using high level theoretical methods. The electronic ground state and the low-lying quartet states of NO2 have been investigated. Single reference restricted open-shell self-consistent field (SCF), complete active space SCF (CASSCF), spin-restricted (R) and spin-unrestricted (U) configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], and internally contracted multireference configuration interaction (ICMRCI) methods along with Dunning's correlation consistent polarized valence cc-pVXZ and augmented cc-pVXZ (where X=T,Q,5) basis sets were used in this research. At the aug-cc-pV5Z/UCCSD(T) level the classical adiabatic excitation energies (Te values) of the three lowest-lying quartet excited states were predicted to be 83.3 kcalmol (3.61 eV, 29 200 cm(-1)) for the ? 4A2 state, 93.3 kcalmol (4.05 eV, 32 600 cm(-1)) for the b 4B2 state, and 100.8 kcalmol (4.37 eV, 35 300 cm(-1)) for the c 4A1 state. The quantum mechanical excitation energies (T 0 values) were determined to be 81.6 kcalmol (3.54 eV, 28 500 cm(-1)) for the a 4A2 state and 90.7 kcalmol (3.93 eV, 31 700 cm(-1)) for the b 4B2 state. The lowest quartet linear Renner-Teller 4Pi state gives rise to the a 4A2 state with 112.8 degrees and the b 4B2 state with 124.4 degrees <(ONO) bond angles upon bending. The b state shows some peculiar behavior. Although CASSCF, RCISD, UCISD, RCCSD, UCCSD, and RCCSD(T) methods predicted the presence of a Cs equilibrium geometry (a double minimum 4A' state), SCF, UCCSD(T), and ICMRCI wave functions predicted the C2v structure for the b 4B2 state. The importance of both dynamical and nondynamical correlation treatments for the energy difference between C2v and Cs structures of b state is highlighted in this context. The c 4A1 state is predicted to have a very small bond angle of 85.8 degrees . Potential energy diagrams with respect to the bond angles of the ground state and four quartet states are presented.     

Ab initio potential energy surfaces, bound states, and electronic spectrum of the Ar-SH complex

New ab initio potential energy surfaces for the (2)Pi ground electronic state of the Ar-SH complex are presented, calculated at the RCCSD(T)/aug-cc-pV5Z level. Weakly bound rotation-vibration levels are calculated using coupled-channel methods that properly account for the coupling between the two electronic states. The resulting wave functions are analyzed and a new adiabatic approximation including spin-orbit coupling is proposed. The ground-state wave functions are combined with those obtained for the excited (2)Sigma(+) state [D. M. Hirst, R. J. Doyle, and S. R. Mackenzie, Phys. Chem. Chem. Phys. 6, 5463 (2004)] to produce transition dipole moments. Modeling the transition intensities as a combination of these dipole moments and calculated lifetime values [A. B. McCoy, J. Chem. Phys. 109, 170 (1998)] leads to a good representation of the experimental fluorescence excitation spectrum [M.-C. Yang, A. P. Salzberg, B.-C. Chang, C. C. Carter, and T. A. Miller, J. Chem. Phys. 98, 4301 (1993)].     

An ab initio benchmark study of the H + CO --> HCO reaction

The H + CO --> HCO reaction has been characterized with correlation consistent basis sets at five levels of theory in order to benchmark the sensitivities of the barrier height and reaction ergicity to the one-electron and n-electron expansions of the electronic wave function. Single and multireference methods are compared and contrasted. The coupled cluster method RCCSD(T) was found to be in very good agreement with Davidson-corrected internally-contracted multireference configuration interaction (MRCI+Q). Second-order Moller-Plesset perturbation theory (MP2) was also employed. The estimated complete basis set (CBS) limits for the barrier height (in kcal/mol) for the five methods, including harmonic zero-point energy corrections, are MP2, 4.66; RCCSD, 4.78; RCCSD(T), 4.15; MRCI, 5.10; and MRCI+Q, 4.07. Similarly, the estimated CBS limits for the ergicity of the reaction are: MP2, -17.99; RCCSD, -13.34; RCCSD(T), -13.79; MRCI, -11.46; and MRCI+Q, -13.70. Additional basis set explorations for the RCCSD(T) method demonstrate that aug-cc-pVTZ sets, even with some functions removed, are sufficient to reproduce the CBS limits to within 0.1-0.3 kcal/mol.     

Mobility of O+ in He and interaction potential of HeO+

New experimental measurements are reported for the mobility of O(+) ions in He gas at 300 K. The accuracy of these new values is estimated as +/-2.5%, which allows them to serve as a stringent test of a new ab initio potential that we have calculated using the RCCSD(T) method. We employed the aug-cc-pV5Z basis set with counterpoise corrections and took spin-orbit coupling into account. The present experimental values lie below the calculated ones, but the difference becomes statistically significant only at moderate and high values of the ratio of the electric field strength to the gas number density; even there they are only marginally significant.     

Comparison of experimental and computationally predicted sulfoxide bond dissociation enthalpies

The accurate estimation of S-O bond dissociation enthalpies (BDE) of sulfoxides by computational chemistry methods has been a significant challenge. One of the primary causes for this challenge is the well-established requirement of including high-exponent d functions in the sulfur basis set for accurate energies. Unfortunately, even when high-exponent d functions were included in Pople-style basis sets, the relative strength of experimentally determined S-O BDE was incorrectly predicted. The aug-cc-pV(n+d)Z basis sets developed by Dunning include an additional high-exponent d function on sulfur. Thus, it was expected that the aug-cc-pV(n+d)Z basis sets would improve the prediction of sulfoxide S-O BDE. This study presents the S-O BDE predicted by B3LYP, CCSD, CCSD(T), M05-2X, M06-2X, and MP2 combined with aug-cc-pV(n+d)Z, aug-cc-pVnZ, and Pople-style basis sets. The accuracy of these predictions was determined by comparing the computationally predicted values to the experimentally determined S-O BDE. Values within experimental error were obtained for dialkyl sulfoxides when the S-O BDEs were estimated using an isodesmic oxygen transfer reaction at the M06-2X/aug-cc-pV(T+d)Z level of theory. However, the S-O BDE of divinyl sulfoxide was overestimated by this method.     

Franck-Condon simulation of the single-vibronic-level emission spectra of HPCl/DPCl and the chemiluminescence spectrum of HPCl, including anharmonicity

Restricted-spin coupled-cluster single-double plus perturbative triple excitation [RCCSD(T)] potential energy functions (PEFs) were calculated for the X (2)A" and A (2)A' states of HPCl employing the augmented correlation-consistent polarized-valence-quadruple-zeta (aug-cc-pVQZ) basis set. Further geometry optimization calculations were carried out on both electronic states of HPCl at the RCCSD(T) level with all electron and quasirelativistic effective core potential basis sets of better than the aug-cc-pVQZ quality, and also including some core electrons, in order to obtain more reliable geometrical parameters and relative electronic energy of the two states. Anharmonic vibrational wave functions of the two states of HPCl and DPCl, and Franck-Condon (FC) factors of the A (2)A'-X (2)A" transition were computed employing the RCCSD(T)/aug-cc-pVQZ PEFs. Calculated FC factors with allowance for Duschinsky rotation and anharmonicity were used to simulate the single-vibronic-level (SVL) emission spectra of HPCl and DPCl reported by Brandon et al. [J. Chem. Phys. 119, 2037 (2003)] and the chemiluminescence spectrum reported by Bramwell et al. [Chem. Phys. Lett. 331, 483 (2000)]. Comparison between simulated and observed SVL emission spectra gives the experimentally derived equilibrium geometry of the A (2)A' state of HPCl of r(e)(PCl) = 2.0035 +/- 0.0015 A, theta(e) = 116.08 +/- 0.60 degrees, and r(e)(HP) = 1.4063+/-0.0015 A via the iterative Franck-Condon analysis procedure. Comparison between simulated and observed chemiluminescence spectra confirms that the vibrational population distribution of the A (2)A' state of HPCl is non-Boltzmann, as proposed by Baraille et al. [Chem. Phys. 289, 263 (2003)].     

反应体系F+H2→HF+H的一个全域从头算势能面

A global three dimensional potential energy surface for the F+H2→HF+H reaction has been developed by spline interpolation of about 15,000 symmetry-unique ab initio points, obtained from the multi-reference configuration interaction level with Davidson correction using the aug-cc-pV5Z basis set. In the entrance channel the spin-orbit coupling energy is also included.     

An ab initio study of the low-lying electronic states of YO2 and Franck-Condon simulation of the first photodetachment band of YO2(-)

A variety of density functional theory and ab initio methods, including B3LYP, B98, BP86, CASSCF, CASSCF/RS2, CASSCF/MRCI, BD, BD(T), and CCSD(T), with ECP basis sets of up to the quintuple-zeta quality for Y, have been employed to study the X(2)B2 state of YO2 and the X(1)A1 state of YO2(-). Providing that the Y 4s(2)4p(6) outer-core electrons are included in the correlation treatment, the RCCSD(T) method gives the most consistent results and is concluded to be the most reliable and practical computational method for YO2 and YO2(-). In addition, RCCSD(T) potential energy functions (PEFs) of the X(2)B2 state of YO2 and the X(1)A1 state of YO2(-) were computed, employing the ECP28MDF_aug-cc-pwCVTZ and aug-cc-pVTZ basis sets for Y and O, respectively. Franck-Condon factors, which include allowance for Duschinsky rotation and anharmonicity, were calculated using the computed RCCSD(T) PEFs and were used to simulate the first photodetachment band of YO2(-). The simulated spectrum matches very well with the corresponding experimental 355 nm photodetachment spectrum of Wu, H.; Wang, L.-S. J. Phys. Chem. A 1998, 102, 9129, confirming the reliability of the RCCSD(T) PEFs used. Further calculations on low-lying electronic states of YO2 gave T(e)'s and T(vert)'s of the A(2)A1, B(2)B1, and C(2)A2 states of YO2, as well as EAs and VDEs to these states from the X(1)A1 state of YO2(-). On the basis of the ab initio VDEs obtained in the present study, previous assignments of the second and third photodetachment bands of YO2(-) have been revised.     

A quantum chemistry study of the Cl atom reaction with formaldehyde

The elementary vapor-phase reaction between Cl atoms and HCHO has been studied by ab initio methods. Calculations at the MP2, MP3, MP4(SDTQ), CCSD, CCSD(T), and MRD-CI levels of theory show that the reaction is characterized by a low electronic barrier; excluding the effects of spin-orbit splitting in Cl, our best estimate at the MRD-CI/aug-cc-pVTZ//RHF-RCCSD(T)/aug-cc-pVTZ level of theory predicts a Born-Oppenheimer barrier height of 0.7 kJ mol-1. The energies of the lowest six electronic states as resulting from MRD-CI calculations are presented at discrete points along the reaction path, and two avoided crossings are found in the transition state region. The spin-orbit splitting in Cl is also calculated along the reaction path; it is not negligible in the transition state region and is found to increase the barrier by only 1.4 kJ mol-1 at the RCCSD(T)/aug-cc-pVTZ transition state geometry. The minimum energy path of the reaction connects an energetically weakly stabilized adduct on the flat potential surface on the reactant side and an energetically strongly stabilized postreaction adduct. The reaction rate coefficient and the kinetic isotope effects were calculated using improved canonical variational theory with small curvature tunneling (ICVT/SCT), and the results were compared to experimental data. The experimental reaction rate coefficient is reproduced within its uncertainty limits by variational transition state theory with interpolated single-point energy corrections (ISPE) at the MP4(SDTQ) level of theory and by conventional transition state theory with interpolated optimized energies (IOE) at the MRD-CI//RCCSD(T) level of theory and interpolated optimized geometries at the RCCSD(T) level of theory on an MP2/aug-cc-pVTZ potential energy surface when employing scaled vibrational frequencies.     

Connection between the upper and lower energy regions of the potential energy surface of the ground electronic state of the HSO2 system

The importance of the HSO(2) system in atmospheric and combustion chemistry has motivated several works dedicated to the study of associated structures and chemical reactions. Nevertheless, controversy still exists about a possible connection between the upper and lower energy regions of the potential energy surface (PES) for the ground electronic state of the system. Very recently, a path to connect these regions was proposed based on studies at the CASPT2/aug-cc-pV(T+d)Z level of calculation but the small energy difference between some of the transitions states along that path suggested the necessity of calculations at a higher level of theory. In the present work, we report a CCSD(T)/aug-cc-pV(T+d)Z study of the stationary states associated to the proposed connection path, including assessment of the most reliable complete basis set (CBS) extrapolation scheme for the system. Among the new features, the present calculations show that there are no structures corresponding to the HSO(2)(b) minimum and the TS3 saddle point obtained at the CASPT2 level and that the connection path between the upper and lower energy regions of the PES for the ground electronic state involves only one transition state and most probably more than one electronic state.     

Predicted thermochemistry and unimolecular kinetics of nitrous sulfide

The geometry of N(2)S was obtained at the CCSD(T)/aug-cc-pV(T + d)Z level of theory and energies with coupled-cluster single double triple (CCSD(T)) and basis sets up to aug-cc-pV(6 + d)Z. After correction for anharmonic zero-point energy, core-valence correlation, correlation up to CCSDT(Q) and relativistic effects, D(0) for the N-S bond is estimated as 71.9 kJ mol(-1), and the corresponding thermochemistry for N(2)S is Δ(f)H(0)(°)=205.4 kJ mol(-1) and Δ(f)H(298)(°)=202.6 kJ mol(-1) with an uncertainty of ±2.5 kJ mol(-1). Using CCSD(T)/aug-cc-pV(T + d) theory the minimum energy crossing point between singlet and triplet potential energy curves is found at r(N-N) ≈ 1.105 ? and r(N-S) ≈ 2.232 ?, with an energy 72 kJ mol(-1) above N(2) + S((3)P). Application of Troe's unimolecular formalism yields the low-pressure-limiting rate constant for dissociation of N(2)S k(0) = 7.6 × 10(-10) exp(-126 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1) over 700-2000 K. The estimated uncertainty is a factor of 4 arising from unknown parameters for energy transfer between N(2)S and Ar or N(2) bath gas. The thermochemistry and kinetics were included in a mechanism for CO/H(2)/H(2)S oxidation and the conclusion is that little NO is produced via subsequent chemistry of NNS.