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  • 1
    Electronic Resource
    Electronic Resource
    Isomerization pathway of the aluminum monocarbonyl/isocarbonyl pair, AlCO/AlOC: Evidence of a cyclic minimum (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 9398-9403 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The isomerization pathway between AlOC and AlCO has been explored at the self-consistent field, configuration interaction, and coupled-cluster levels of theory. Five stationary points on the Al+CO potential energy surface were located and show that the path of Al migration from the isocarbonyl to the monocarbonyl involves a very small barrier to a perhaps unexpected cyclic minimum structure followed by a second barrier to the AlCO isomer. A quantitative analysis of the relative stabilities of the isomers as well as the ZPVE-corrected isomerization barriers are presented and compared to the boron carbonyl analogs. At the coupled-cluster level with single, double, and perturbatively applied connected triple substitutions [CCSD(T)] using a TZ2P+f basis set, the cyclic minimum is 9.4 kcal/mol higher in energy than AlCO but is 11.4 kcal/mol more stable than AlOC. The barriers from AlOC to the cyclic isomer and to the dissociation products 2P Al and X 1Σ+ CO are only 3.5 and 1.0 kcal/mol, respectively, and leave the tentative experimental observation of AlOC in doubt. On the other hand, the cyclic structure lies in a substantial well with barriers of 19.4 and 14.9 kcal/mol to AlCO and AlOC, respectively. The barrier to Al+CO from the cyclic isomer is estimated to be near 2.5 kcal/mol. The C–O harmonic stretching frequency of the cyclic isomer at this level is predicted to be 1605 cm−1 and provides a guide for the possible experimental observation of this species. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476390
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  • 2
    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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  • 3
    Electronic Resource
    Electronic Resource
    Coupled-cluster studies of the hyperfine splitting constants of the thioformyl radical (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 6245-6254 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Hyperfine splitting constants (hfs) of the X˜ 2A′ electronic ground state of the thioformyl radical (HCS) have been determined at the coupled-cluster level with single, double, and perturbatively applied connected triple excitations [CCSD(T)] using 39 basis sets. Variation of the CCSD(T) hyperfine splittings with basis set was ascertained using a fixed geometry, optimized at the CCSD(T) level with Dunning's correlation-consistent polarized valence quadruple-ζ basis set (cc-pVQZ). Pople basis sets, 6-311G++(2d,2p) and 6-311G++(3df,3pd), give 1H isotropic coupling constants (1H Aiso) in good agreement with the experimental vibrationally averaged value of 127.4 MHz, deviating by 5.5 and 9.3 MHz, respectively. Dunning's valence correlation-consistent basis sets (cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, aug-cc-pVTZ, cc-pVQZ, aug-cc-pVQZ) deviate 6.4 MHz (aug-cc-pVQZ) to 14.9 MHz (cc-pVDZ) from the experimental value. The correlation-consistent core valence analogues of these sets give very similar values with deviations from experiment of 7.4 MHz (cc-pCVQZ) to 14.2 MHz (cc-pCVDZ). A direct comparison with the vibrationally averaged experimental value is not precisely possible since the hyperfine splittings are strongly geometry dependent and all theoretical predictions refer to the equilibrium geometry. Small Pople basis sets (3-12G, 6-31G, and 6-311G) give the worst results, deviating by 49.5, 34.1, and 31.8 MHz, respectively. All CCSD(T) 1H Aiso values fall below the experimental value. The 13C and 33S hyperfine splittings are not known experimentally, but the equilibrium values are predicted here to be 274.7 MHz (13C) and 21.7 MHz (33S) at the cc-pCVQZ CCSD(T) level of theory. Significantly different values are predicted by density functional theory (DFT) for the 13C and 33S hyperfine splittings. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481599
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  • 4
    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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  • 5
    Electronic Resource
    Electronic Resource
    Definitive ab initio structure for the X˜ 2A′H2PO radical and resolution of the P–O stretching mode assignment (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 2694-2699 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Previous ab initio studies of the X˜ 2A′H2PO radical have reported dramatically differing P–O bond distances when using spin-restricted wave functions predicting two artifactual isomers of H2PO: a singly bonded oxygen-centered radical and a doubly bonded phosphorus-centered radical. We show that large basis sets coupled with high levels of dynamical electron correlation are required to correctly describe the P–O bond in H2PO as well as the unpaired electron density as evidenced by the Fermi contact terms and anisotropic components of the 31P, 1H, and 17O hyperfine splitting (hfs) constants. The optimized geometry, harmonic vibrational frequencies, and hfs constants of H2PO were determined at several coupled-cluster levels of theory using both spin-restricted (ROHF) and spin-unrestricted (UHF) Hartree–Fock reference wave functions. The geometrical parameters at the coupled-cluster level with single, double, and perturbatively applied triple substitutions [CCSD(T)] using Dunning's correlation consistent polarized valence quadruple-ζ basis set (cc-pVQZ) are r(P–O)=1.492 Å; r(P–H)=1.410 Å; (angle)(HPH)=102.63°; (angle)(HPO)=114.92°. These are in excellent agreement with those derived from recent gas phase microwave data, with the surprising exception of the P–H distance which deviates 0.02 Å from experiment. The value of the P–O harmonic stretching frequency at the CCSD(T) level within the cc-pVQZ basis set is 1190 cm−1, in good agreement with the experimental fundamental frequency of 1147 cm−1 obtained by Withnall and Andrews and in constrast to previous speculation that this experimental band may have been misassigned. Hyperfine splitting constants determined at the TZ2P(f,d)/UHF-CCSD(T) level are in very good agreement with experimental values with an average deviation of 23 MHz. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476869
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  • 6
    Electronic Resource
    Electronic Resource
    Three- versus four-coordinate phosphorus in the gas phase and in solution: Treacherous relative energies for phosphine oxide and phosphinous acid (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 112-122 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Previous ab initio studies have consistently predicted phosphine oxide (H3PO) to be less stable than its nearly isoenergetic cis- and trans-phosphinous acid isomers (H2POH). However, complete basis set extrapolations employing the coupled-cluster series show that phosphine oxide is actually ca. 1.0 kcal/mol more stable than its acid forms in the gas phase. Incorporation of tight d functions via Dunning's core-valence (cc-pCVXZ) or newly constructed "plus d" [cc-pV(X+d)Z] basis sets is essential for rapid convergence of core polarization effects which are evident even at the SCF level. The precision to which the phosphorus hybridization is described in the three- and four-coordinate environments ultimately determines the predicted gas-phase relative energy orderings. Focal-point analyses demonstrate that this system represents a disturbing case where use of a conventional valence quadruple-ζ quality basis set (cc-pVQZ)—even at the CCSD(T) level—fails to provide the correct relative energy ordering for simple closed-shell species which do not exhibit appreciable multireference character. Thus, we underscore the importance of using phosphorus basis sets which have the flexibility to describe core polarization adequately. In addition, Monte Carlo (MC) free-energy perturbation simulations in solution clearly demonstrate that the small energy gap significantly increases in favor of the oxide (10.0 kcal/mol) upon solvation due to stronger hydrogen bonding with the highly polar Pδ+→Oδ− bond. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1418440
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  • 7
    Electronic Resource
    Electronic Resource
    What is the true electronic ground state of the disilaethynyl radical (SiSiH): 2B1 or 2A1? (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 2157-2164 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The two lowest-lying (H-bridged, cyclic) electronic states (2B1 and 2A1) of the disilaethynyl (SiSiH) radical have been investigated using ab initio electronic structure theory. Theoretical methods through the full coupled cluster with all triple excitations (CCSDT) have been used, and basis sets as large as Dunning's correlation consistent pentuple set adopted. While the SCF, MP2, CISD, and CCSD levels of theory predict the 2B1 state to be lower in energy, the CCSD(T) and CCSDT methods show that the 2A1 state is the true electronic ground state. With our most reliable method, the energy difference is predicted to be Te(2B1)=0.60 kcal/mol (0.026 eV,210 cm−1) and T0(2B1)=0.37 kcal/mol (0.016 eV,128 cm−1). This theoretical finding confirms the experimental assignment by Xu et al. [J. Chem. Phys. 108, 7645 (1998)] in 1998 that the ground state of SiSiH is the 2A1 state and it is 0.020±0.005 eV lower in energy than the 2B1 state. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1384418
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  • 8
    Electronic Resource
    Electronic Resource
    Coupled-cluster characterization of the ground and excited states of the CH2N and CH2P radicals (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 3055-3064 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High-level coupled-cluster theory with large basis sets was used to determine the optimized geometries and harmonic vibrational frequencies for the ground and low-lying excited electronic states of the CH2N and CH2P radicals. Additionally, isotropic hyperfine splitting constants were determined for the C2v CH2X, trans-HCXH and cis-HCXH (where X=N and P) isomers as a gauge of the delocalization of the unpaired electron. The geometrical parameters of X˜ 2B2 CH2X, the trans-HCXH and cis-HCXH conformers, and the first three excited states are reported at the coupled-cluster level with single, double, and perturbatively applied triple excitations [CCSD(T)] using Dunning's correlation consistent polarized valence quadruple-ζ basis set (cc-pVQZ). The C2v structures on the ground state surface are predicted to lie 9.3 and 13.5 kcal/mol lower than the trans- and cis-isomers, respectively, for CH2N and 28.1 and 30.0 kcal/mol, respectively, for CH2P. There are many other important properties of CH2N and CH2P which are not known from experiment. The geometrical parameters of the CH2N ground state [r(C–N)=1.2462 Å, r(C–H)=1.0921 Å, and θ(HCH)=119.4°] and the CH2P ground state [r(C–P)=1.6583 Å, r(C–H)=1.0842 Å, and θ(HCH)=118.9°] agree well with the C–N and C–P bond distances of the r0 structures derived from microwave data, although notable differences were observed in the C–H bond distance and HCH bond angle. This research resolves an earlier discrepancy between theory and experiment for the ground state C–P distance in CH2P. The Fermi contact terms for 1H, 13C, 14N, and 31P were determined at CCSD(T) level of theory with the cc-pVTZ and cc-pVQZ basis sets and are in reasonable agreement with the experimental values with a maximum deviation of 26 MHz for CH2N and 11 MHz for CH2P. The excited states of CH2N are predicted to lie 33 000 cm−1 (A˜ 2B1), 36 000 cm−1 (B˜ 2A′), and 38 000 cm−1 (C˜ 2A1) above the ground state, and the excited states of CH2P to lie approximately 21 000 cm−1 (A˜ 2A′), 26 000 cm−1 (B˜ 2B1), and 33 000 cm−1 (C˜ 2A1) above the ground state. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1337062
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  • 9
    Electronic Resource
    Electronic Resource
    Coupled-cluster electronic spectra for the Ca+–acetylene π complex and comparisons to its alkaline earth analogs (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 701-706 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Equation of motion coupled-cluster (EOM-CCSD) predictions of structures and electronic excitation energies for the recently detected Ca+–acetylene π-complex confirm three experimental state assignments, but suggest reinterpretation of the signals associated with the (2) 2B1 and (2) 2B2 states that correlate to the 2P←2S Ca+ atomic transition. The originally assigned 000 band for the (2) 2B1 state corresponds to the computed excitation energy to the (2) 2B2 state and simple reassignment is proposed. The true (2) 2B1 state was not assigned in the original spectrum. However, the computed oscillator strength is large and its optimized geometry is similar to that of the ground state. Furthermore, the experimental band tentatively attributed to the onset of the symmetric C–H stretching progression of the assigned state has a relative energy conspicuously close to the computed electronic energy for the unassigned (2) 2B1 state. Based on the computed energy separations of the optimized EOM-CCSD structures, reassignment of this vibronic band to the 000 line of the (2) 2B1 state is proposed. The newly assigned bands are also compared to the analogous transitions in the beryllium–and magnesium–acetylene π complexes. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481845
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