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  • 1
    Electronic Resource
    Electronic Resource
    The unimolecular dissociation of H2CO on the lowest triplet potential-energy surface (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 5281-5288 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The unimolecular dissociation reaction of H2CO on the triplet potential-energy surface has been studied via ab initio electronic structure theory. The stationary point geometries for the equilibrium and transition state are determined employing the configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory with large basis sets up to the correlation consistent (cc)-pVQZ basis. With the best method, cc-pVQZ CCSD(T), the first excited triplet (a˜ 3A″) state lies 72.2 kcal/mol (25 260 cm−1) above the ground (X˜ 1A1) state of H2CO, which is in excellent agreement with the experimental observation of 72.03 kcal/mol (25 194 cm−1). The dissociation limit (H⋅+HCO⋅) is located at 86.3 kcal/mol (30 170 cm−1) above the ground state of H2CO, which is again in good agreement with the two experimentally determined values of 86.57 kcal/mol (30 280 cm−1) and 86.71 kcal/mol (30 328.5 cm−1). With the same method the triplet dissociation transition state lies 92.4 kcal/mol (32 300 cm−1) above the ground state. Consequently, the activation energy for the dissociation reaction of H2CO on the triplet surface is determined ab initio to be 18.9–20.1 kcal/mol (6620–7040 cm−1) (including an estimated error bar of 1.2 kcal/mol or 420 cm−1). The zero-point vibrationally corrected exit barrier height is predicted to be 4.9–6.1 kcal/mol (1710–2130 cm−1). These newly predicted energies are consistent with the recent experimental observations by the Moore group at University of California-Berkeley (1987) and by the Wittig group at University of Southern California (1997). © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476315
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  • 2
    Electronic Resource
    Electronic Resource
    Oxirene: To Be or Not To Be? (1994)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 98 (1994), S. 8660-8665 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100086a013
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  • 3
    Electronic Resource
    Electronic Resource
    The 1-silaketenyl radical (HSiCO): Ground and first excited electronic states (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 2168-2175 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The two lowest-lying (X˜ 2A″ and A˜ 2A′) electronic states and lowest linear stationary point (1 2Π) of the 1-silaketenyl radical (HSiCO) have been investigated systematically using ab initio electronic structure theory. The lowest linear stationary point possesses two distinct imaginary vibrational frequencies along the HSiC bending coordinates, indicating a strong Renner–Teller interaction. The ground and first excited states of HSiCO are found to have trans-planar bent structures and they are more distorted from linearity but less polar than the corresponding states of HCCO. Specifically, the X˜ 2A″ structure features a small HSiC bond angle of 84°. With our most reliable method, cc-pVQZ CCSD(T), the classical X˜−A˜ splitting has been predicted to be 35.7 kcal/mol (1.55 eV, 12 500 cm−1). The barriers to linearity were determined to be 53.5 kcal/mol (2.32 eV, 18 700 cm−1) for the X˜ 2A″ state and 17.8 kcal/mol (0.77 eV, 6240 cm−1) for the A˜ 2A′ state. The ground state of HSiCO was found to be relatively stable thermodynamically against the two dissociation reactions HSiCO(X˜ 2A″)→H(2S)+SiCO(X˜ 3Σ−) and HSiCO(X˜ 2A″)→SiH(X˜ 2Π)+CO(X˜ 1Σ+). Due to the large infrared (IR) intensities of some of the vibrational modes, IR spectroscopic investigation of the HSiCO radical may be feasible. HSiCO is the global minimum for these four atoms, lying energetically below SiCOH (38.5 kcal/mol), HCSiO (40.7 kcal/mol), and CSiOH (76.3 kcal/mol) at the TZ2P(f,d) configuration interaction with single and double excitations (CISD) level of theory. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481591
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  • 4
    Electronic Resource
    Electronic Resource
    A high level theoretical investigation of the cyclic hydrogen fluoride trimer (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 9627-9633 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A high level ab initio theoretical investigation of the cyclic hydrogen fluoride trimer was carried out. The structures of the hydrogen fluoride monomer, dimer, and trimer were fully optimized at the coupled-cluster level of theory including single, double, and perturbatively applied connected triple excitations [CCSD(T)] using three large basis sets. Geometrical parameters, dipole moments, harmonic vibrational frequencies, infrared intensities, and total energies are reported for each equilibrium structure. Changes in bond lengths and shifts in HF stretching frequencies relative to the monomer, as well as the dissociation energies corresponding to various fragmentation pathways, are given for the dimer, trimer, and their deuterated isotopomers. The theoretical results presented here are compared to the available experimental data and to those obtained from empirically refined potential energy surfaces. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473861
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  • 5
    Electronic Resource
    Electronic Resource
    The disilaketenyl radical (HSiSiO) in its ground and first excited electronic states (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 227-234 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The disilaketenyl (HSiSiO) radical, an isovalent isomer of the ketenyl (HCCO) radical, has been investigated theoretically using ab initio electronic structure theory. For the two lowest-lying electronic states (X˜ 2A″ and A˜ 2A′) of HSiSiO, total energies and physical properties including equilibrium geometries, dipole moments, harmonic vibrational frequencies, and associated infrared (IR) intensities were predicted at the self-consistent-field (SCF) and configuration interaction with single and double excitations (CISD) levels of theory with a wide range of basis sets. At the CISD optimized geometries coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitations [CCSD(T)] energies were also determined. The ground and first excited electronic states of HSiSiO were predicted to be transplanar bent structures, while the linear 1 2Π state was found to be a saddle point with two imaginary vibrational frequencies. The X˜ 2A″ and A˜ 2A′ states of HSiSiO are more distorted from linearity and more polar than the corresponding states of HCCO. In particular the HSiSiO ground state is predicted to have a peculiarly acute HSiSi bond angle of only 75°, almost suggesting an Si–Si bridging hydrogen. At the CCSD(T) level of theory with the largest basis set, Dunning's cc-pVQZ, the first excited state was predicted to lie 36.3 kcal/mol (1.57 eV, 12 700 cm−1) classically above the ground state. With the same method the barriers to linearity were determined to be 45.2 kcal/mol (1.96 eV, 15 800 cm−1) for the ground state and 8.9 kcal/mol (0.39 eV, 3100 cm−1) for the first excited state, respectively. Due to their large dipole moments and relatively large vibrational infrared (IR) intensities, the two lowest-lying electronic states of HSiSiO may be suitable for IR spectroscopic studies, and the ground state for microwave spectroscopic investigations. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479286
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  • 6
    Electronic Resource
    Electronic Resource
    The 3d Rydberg (3A2) electronic state observed by Herzberg and Shoosmith for methylene (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 8753-8759 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In 1959 and 1961 Herzberg and Shoosmith reported the vacuum ultraviolet spectrum of the triplet state of CH2. The present study focuses on a characterization of the upper state, the 3d Rydberg (3A2) state, observed at 1415 Å. The theoretical interpretation of these experiments is greatly complicated by the presence of a lower-lying 3A2 valence state with a very small equilibrium bond angle. Ab initio electronic structure methods involving self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), complete active space (CAS) SCF, state-averaged (SA) CASSCF, coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], CASSCF second-order (SO) CI, and SACASSCF-SOCI have been employed with six distinct basis sets. With the largest basis set, triple zeta plus triple polarization with two sets of higher angular momentum functions and three sets of diffuse functions TZ3P(2 f,2d)+3diff, the CISD level of theory predicts the equilibrium geometry of the 3d Rydberg (3A2) state to be re=1.093 Å and θe=141.3 deg. With the same basis set the energy (Te value) of the 3d Rydberg state relative to the ground (X˜ 3B1) state has been determined to be 201.6 kcal mol−1 (70 500 cm−1) at the CCSD (T) level, 200.92 kcal mol−1 (70 270 cm−1) at the CASSCF-SOCI level, and 200.89 kcal mol−1 (70 260 cm−1) at the SACASSCF-SOCI level of theory. These predictions are in excellent agreement with the experimental T0 value of 201.95 kcal mol−1 (70 634 cm−1) reported by Herzberg. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473935
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  • 7
    Electronic Resource
    Electronic Resource
    The 3A2, 1A2, 3B2, and 1B2 electronic states of CH2: Small bond angle states (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 1819-1826 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Molecular structures with very small bond angles are a curiosity in chemistry. The two triplet (3A2 and 3B2) and two singlet (1A2 and 1B2) excited states of CH2 have been investigated systematically using ab initio electronic structure theory. For these four states total energies and physical properties including geometries, dipole moments, harmonic vibrational frequencies, and associated infrared intensities were determined with the single and double excitation configuration interaction (CISD) method using four different basis sets. It is confirmed in this study that the four states of CH2 all have bent structures with longer CH bond lengths and smaller bond angles than the four lower-lying (X˜, a˜, b˜, and c˜) states of CH2. At the CISD optimized geometries single point energies were determined with complete active space self-consistent-field (CASSCF) and CASSCF second-order configuration interaction (SOCI) levels of theory. For the triplet excited states single point energies were also determined employing coupled cluster with single and double excitations (CCSD) and CCSD with perturbative triple excitations methods. At the CISD level with the largest basis set, the triple zeta plus triple polarizations with two sets of higher angular momentum and two sets of diffuse functions basis set [TZ3P(2 f,2d)+2diff], the bond angles were predicted to be 40.6° (3A2), 46.1° (1A2), 76.3° (3B2), and 81.3° (1B2), while the dipole moments were determined to be 2.35 (3A2), 2.26 (1A2), 1.69 (3B2), and 1.60 debye (1B2), respectively. With the most accurate method in this study, the CASSCF-SOCI level with the TZ3P(2 f,2d)+2diff basis set, the energy separations (Te value) between the ground state (X˜ 3B1) and the four excited states were predicted to be 73.7 kcal/mol (3.20 eV, 25 800 cm−1) for the 3A2 state, 96.8 kcal/mol (4.20 eV, 33 800 cm−1) for the 1A2 state, 151.0 kcal/mol (6.55 eV, 52 800 cm−1) for the 3B2 state, and 182.5 kcal/mol (7.91 eV, 63 800 cm−1) for the 1B2 state, respectively. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473323
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  • 8
    Electronic Resource
    Electronic Resource
    The silaketenylidene (SiCO) molecule: Characterization of the X˜ 3Σ− and A˜ 3Π states (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 3201-3207 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground (X˜ 3Σ−) and first excited triplet (A˜ 3Π) electronic states of carbonylsilene or silaketenylidene, SiCO, have been investigated systematically using ab initio electronic structure theory. The total energies and physical properties including equilibrium geometries, dipole moments, harmonic vibrational frequencies, and associated infrared (IR) intensities were predicted using self-consistent-field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), equation-of-motion (EOM) CCSD, CCSD with perturbative triple excitations [CCSD(T)] methods with a wide range of basis sets. The linear X˜ 3Σ− ground state of SiCO has a real degenerate bending vibrational frequency, whereas the A˜ 3Π state of SiCO is subject to the Renner–Teller effect and presents two distinct real vibrational frequencies along the bending coordinate. The bending vibrational frequency of the A˜ 3Π state was evaluated via the EOM-CCSD technique. At the highest level of theory with the largest basis set, cc-pVQZ CCSD(T), the adiabatic X˜–A˜ splitting without the zero-point vibrational energy (ZPVE) correction (Te value) was determined to be 68.5 kcal/mol (2.97 eV, 23 900 cm−1) and the adiabatic splitting with the ZPVE energy correction (T0 value) to be 69.0 kcal/mol (2.99 eV, 24 100 cm−1), which are in excellent agreement with the experimental T0 value of 68.78 kcal/mol (2.983 eV, 24 056 cm−1). The theoretical ground state harmonic Si–C stretching frequency ω3=564 cm−1 is much less than the experimental estimate of 800 cm−1. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481593
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  • 9
    Electronic Resource
    Electronic Resource
    Scratching the surface of the water dication (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 11856-11864 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The X˜ 3Σg−, a˜ 1Δg, and b˜ 1Σg+ states of the water dication, H2O2+, have been investigated using several high-level ab initio methods and a range of basis sets. With Dunning's augmented correlation consistent polarized valence quadruple-ζ (aug-cc-pVQZ) basis set at the complete active space self-consistent field second-order configuration interaction (CAS-SOCI) level, it is confirmed that the ground and first two excited states of H2O2+ are all of D∞h symmetry, in violation of Walsh's rules for 6 valence electron AH2 systems. The singlet–triplet splitting (X˜ 3Σg−—a˜ 1Δg) is predicted to be 53.6 kcal/mol (2.32 eV, 18 700 cm−1), while the X˜ 3Σg−—b˜ 1Σg+ separation is predicted to be 91.1 kcal/mol (3.95 eV, 31 900 cm−1). The vertical double ionization potentials (IPs) from X˜ 1A1 H2O to the X˜ 3B1, 1 1A1, b˜ 1B1, and 2 1A1 states of H2O2+ are predicted within the cc-pVQZ basis to be 40.1, 41.2, 42.6, and 46.1 eV, respectively, in good agreement with recent double-charge-transfer spectroscopic results. The corresponding adiabatic double IPs are 37.0, 39.3, and 41.0 eV to the X˜ 3Σg−, a˜ 1Δg, and b˜ 1Σg+ states of H2O2+, respectively. The activation barrier to fragmentation of H2O2+ (X˜ 3Σg− H2O2+→3Σ− OH++H+) at the cc-pVQZ CAS-SOCI level is predicted to be 2.1 kcal/mol (0.10 eV, 738 cm−1), and the reaction is exothermic by 126.4 kcal/mol (5.48 eV, 44 210 cm−1), providing a challenge for direct experimental detection of this elusive molecule. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479127
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  • 10
    Electronic Resource
    Electronic Resource
    The GaOH–HGaO potential energy hypersurface and the necessity of correlating the 3d electrons (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 8516-8523 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground state potential energy hypersurface of the GaOH–HGaO system has been investigated using high level ab initio molecular electronic structure theory. The geometries and physical properties of two equilibrium structures, one isomerization transition state and one inversion transition state were determined at the self-consistent field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory with four sets of basis functions. It has been found that freezing the 3d electrons of the Ga atom in the correlation procedures is not appropriate for this system. For the energy difference ΔE (GaOH–HGaO) the freezing of the 3d electrons results in an error of 25 kcal/mol! The dipole moments, harmonic vibrational frequencies, and infrared (IR) intensities are predicted for the four stationary points. At the highest level of theory employed in this study, CCSD(T) using triple zeta plus double polarization with higher angular momentum and diffuse functions [TZ2P(f,d)+diff] basis set, the bent GaOH was found to be 41.9 kcal/mol more stable than the linear HGaO species; with the zero-point vibrational energy (ZPVE) correction, the energy separation becomes 40.4 kcal/mol. The classical barrier height for the exothermic isomerization (1,2 hydrogen shift) reaction HGaO→GaOH is determined to be 44.5 kcal/mol and the barrier height with the ZPVE correction 42.3 kcal/mol. The classical barrier to linearity for the bent GaOH molecule is determined to be 1.7 kcal/mol and the barrier height with the ZPVE correction to be 1.2 kcal/mol. The predicted dipole moments of GaOH and HGaO are 1.41 and 4.45 Debye, respectively. The effects of electron correlation reduce the dipole moment of HGaO by the sizable amount of 1.2 Debye. The two equilibrium species may be suitable for microwave spectroscopic investigation. Furthermore, they may also be detectable by IR techniques due to the relatively large intensities of their vibrational modes. The geometrical and energetic features are compared with those of the valence isoelectronic HXO–XOH systems, where X is a group IIIA atom and the HXO+–XOH+ systems, where X is a group IVA atom. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471763
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