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  • American Institute of Physics (AIP)  (10)
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  • 1
    Digitale Medien
    Digitale Medien
    The decarboxylation and dehydration reactions of monomeric formic acid (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 1158-1166 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Two unimolecular dissociation reactions of formic acid have been investigated theoretically. The decarboxylation reaction yields molecular hydrogen and carbon dioxide, while the dehydration reaction produces water and carbon monoxide. The 1,2-hydrogen shift rearrangement from formic acid to dihydroxymethylene has also been considered. Methods ranged from double zeta plus polarization self-consistent field to triple zeta plus double polarization coupled cluster singles and doubles. For certain key structures, the coupled cluster method including single, double, and linearized triple excitations (CCSDT-1) was applied as well with the double zeta plus polarization basis set. A barrier height of ∼71 kcal mol−1 with zero point vibrational energy correction is predicted for the dissociation to molecular hydrogen and carbon dioxide. A rather comparable value of ∼68 kcal mol−1 is predicted for the barrier to the dehydration reaction. The 1,2-hydrogen shift transition state is somewhat higher in energy at ∼79 kcal mol−1. These predicted energy barriers are discussed with reference to the existing experimental results on the thermal decomposition of formic acid.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.462203
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  • 2
    Digitale Medien
    Digitale Medien
    The reaction of sulfur atoms with carbon disulfide: Potential energy surface features (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7449-7457 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The lowest singlet and triplet potential energy surfaces of the reaction, S+CS2→CS3→CS+S2, were investigated by the 6-31G* ab initio self-consistent-field (SCF) method with the inclusion of electron correlation by Møller–Plesset perturbation theory. The triplet reactants and products [S(3P)+CS2(1Σ+g),S2(3Σ−g)+CS(1Σ+)] are predicted to be more stable than their singlet counterparts [S(1D)+CS2(1Σ+g),S2(1Δg)+CS(1Σ+)] in agreement with experiment. However, the CS3 complex is more stable in its singlet as opposed to triplet state, leading to interesting surface crossings in the intermediate regions of the surfaces. The triplet surface contains shallow wells corresponding to cis- and trans-CS3 chain isomers. A low-lying singlet C2v ring structure, carbon trisulfide, was connected to a chain molecule, carbon disulfide S-sulfide, by a relatively low-lying transition state. Another transition state was found with a modest barrier, which joined the C2v ring structure to another relatively low-lying minimum, a symmetric D3h structure. Relative to the singlet ring compound, the singlet chain isomer and the D3h structure are 14.3 and 4.1 kcal/mol higher in energy, and the triplet cis and trans minima 31.2 and 34.1 kcal/mol higher in energy. At the unrestricted Hartree–Fock level (UHF or UMP2), transition states join the chain structure to both reactants and products on the singlet surface. From the experimental evidence and the predictions of this work, an intersystem crossing from the triplet potential energy surface to the singlet one is expected in the region of those transition states leading to the products, CS+S2.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.462881
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  • 3
    Digitale Medien
    Digitale Medien
    Calculated intensity in the local mode overtone spectra of hydrogen peroxide (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 5556-5564 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Calculations of OH stretching vibrational intensities for hydrogen peroxide are performed with a harmonically coupled anharmonic local mode model for the OH stretching vibrational wave functions and ab initio calculations to obtain the dipole moment function. The ab initio calculations are carried out at three different levels: Self-consistent field (SCF), second-order Møller–Plesset perturbation theory (MP2), and configuration interaction including all single and double excitations (CISD), all with a 6–31G* basis set, to yield the dipole moment function. From these results, total oscillator strengths are calculated for the three different types of rotational bands in a given OH stretching transition. The calculated overtone intensities for ΔvOH=2–5 are not sensitive to the particular dipole moment function. However, a significant decrease in intensity occurs for the fundamental transition in going from the molecular orbital to the correlated level calculations. Our calculated results are compared to near infrared, photoacoustic, and molecular beam experiments by other workers. The calculations can explain many of the experimental results, although the interpretation is not always straightforward. Our calculations suggest that transitions to the symmetric OH stretching mode, should be included in the analysis and assignment of the OH stretching overtone regions.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.461629
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  • 4
    Digitale Medien
    Digitale Medien
    Formyl fluoride photodissociation: Potential energy surface features of singlet HFCO (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4907-4915 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The unimolecular dissociation of formyl fluoride to hydrogen fluoride and carbon monoxide and its unimolecular rearrangement to fluorohydroxymethylene have been examined theoretically. Methods ranged from double zeta plus polarization self-consistent-field to triple zeta plus double polarization coupled cluster singles and doubles. The coupled cluster method including single, double, and linearized triple excitations (CCSDT-1) was applied as well with the double zeta plus polarization basis. The present studies predict a barrier height of ∼47 kcal mol−1 to the molecular dissociation which is ∼11 kcal mol−1 lower than previous theoretical work and in reasonable agreement with experiments that indicate this barrier lies in the range 40 to 43 kcal mol−1. The existence of singlet trans- and cis-fluorohydroxymethylenes at ∼43 kcal mol−1 above formyl fluoride is further confirmed theoretically. The 1,2-hydrogen shift transition state connecting formyl fluoride to the trans-fluorohydroxymethylene lies at ∼77 kcal mol−1. The trans- and cis-fluorohydroxymethylenes are separated by an internal rotation barrier of ∼17 kcal mol−1.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.458679
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  • 5
    Digitale Medien
    Digitale Medien
    The COS2 potential energy surface: Aspects of the lowest singlet and triplet potential energy surfaces for the reaction of oxygen atoms with carbon disulfide (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 5566-5578 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Portions of the lowest singlet and triplet potential energy surfaces for the reaction of oxygen atoms with carbon disulfide O(3P)+CS2(1Σg+)→CS(1Σ+)+SO(3Σ−), →OCS(1Σ+)+S(3P),→CO(1Σ+)+S2(3Σg−) were investigated by 6-31G* ab initio self-consistent-field methods and with the inclusion of electron correlation by Møller–Plesset perturbation theory. Earlier experiments and the present ab initio results indicate that the dominant reaction pathway is the direct stripping mechanism leading to the CS and SO diatomic products. The ab initio results support the experimentally observed branching ratios as a relatively simple mechanism was located for the formation of CS and SO, while more complex paths with higher barriers lead to the OCS and S or CO and S2 products. The most stable structure on either the singlet or triplet surface is singlet dithiiranone, a three-membered CSS ring with a carbonyl group. As the predominant reaction forms CS(1Σ+)+SO(3Σ−), the triplet cis or trans carbon disulfide S-oxide molecules are probable intermediates.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.464903
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  • 6
    Digitale Medien
    Digitale Medien
    Ab initio predictions of the structures and spectra of some simple thiosulfeno (XS2) free radicals (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 2924-2931 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Ab initio theory with split-valence plus polarization function and larger basis sets at the self-consistent field (SCF) and second order Møller–Plesset (UMP2) levels was used to predict the geometries and spectroscopic parameters for the ground and first excited electronic states of the HS2, FS2, and ClS2 free radicals. The ground-state species are predicted to be bent and may be described as having a sulfur–sulfur double bond, a sulfur-substituent single bond, and an unpaired electron which is delocalized principally over the sulfur centers. The first electronic transition involves an (n,π*) electron promotion which leads to a longer S–S bond, smaller bond angle, and greater localization of the unpaired electron on the terminal sulfur atom. The adiabatic transition energies are predicted to be at ∼6700, 13 700, and 10 600 cm−1 for HS2, FS2, and ClS2, respectively. The theoretical results are in good agreement with the rather limited amount of experimental data available. These ab initio predictions provide critical information to aid in future experimental studies of the matrix isolation or gas-phase spectra of the thiosulfeno radicals.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.466434
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  • 7
    Digitale Medien
    Digitale Medien
    Singularities in the behavior of density functionals in predictions of singlet biradicals: The 1,2-dichalcogenins (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 10085-10094 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The performance of conventional exchange-correlation functionals for the description of singlet biradicals which possess a small admixture of an auxiliary electronic state was examined using a broken symmetry spin unrestricted density functional approach. A pure density functional, BLYP, and the hybrid B3LYP and BHandHLYP exchange-correlation functionals were employed to study the thermal ring opening of 1,2-dichalcogenins and the subsequent formation of the bicyclic products. A stepwise mechanism was predicted for the reaction of the 1,2-dithiin and 2-selenathiin rings and no biradical structures were located. For 1,2-diselenin, the biradical stepwise mechanism competes energetically with a concerted reaction which is slightly favored. For 1,2-ditellurin, a high-energy open intermediate with considerable biradical character was located. The hybrid functionals show an increase in biradical character with an increase in the percentage of Hartree–Fock exchange incorporated into the exchange functional. The instability of the restricted Kohn–Sham solutions was used to indicate the existence of biradical structures with lower energy. Interesting incorrect behavior of certain of the hybrid functionals was noted. The instability of the restricted BHandHLYP solution results from too large a contribution of Hartree–Fock exchange. An unexpected stability of the restricted B3LYP solution was found and attributed to cancellation of two different sources of instability: one from the pure density functional and the other from the inclusion of Hartree–Fock exchange. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.481650
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  • 8
    Digitale Medien
    Digitale Medien
    Density functional theory with fractionally occupied frontier orbitals and the instabilities of the Kohn–Sham solutions for defining diradical transition states: Ring-opening reactions (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 7705-7712 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Density functional theory with fractionally occupied frontier orbitals was combined with an analysis of the instability of the Kohn–Sham solutions and applied to the study of the ring-opening reactions of 1,2-dioxetene, 1,2-dithiete, 1,2-diselenete, and 1,2-ditellurete. These molecules possess transition structures and reactive intermediates for the ring-opening reactions which exhibit strong nondynamical electron correlation. All restricted density functional theory (DFT) solutions for the closed shell transition state structures for all these species are unstable. The solutions with the hybrid Hartree–Fock DFT functionals, B3LYP and B3PW91, are triplet unstable, while for the pure DFT functional BLYP the instability is due to a violation of the Aufbau principle. The same types of instabilities were found for the 1,2-diselenete and 1,2-ditellurete intermediates. Lower energy stable solutions for the diradical transition structures were found with unrestricted DFT methods allowing fractionally occupied orbitals. This DFT approach indicates a decrease in the active orbital space from four fractionally occupied natural orbitals in earlier multireference predictions to two fractionally occupied Kohn–Sham orbitals. © 1999 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.480108
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  • 9
    Digitale Medien
    Digitale Medien
    Methoxycarbene and methylhydroxycarbene: Energies, structures, vibrational frequencies, and unimolecular reactivities (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 3975-3984 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: In the light of recent experimental interest in the spectroscopy and photochemistry of matrix isolated oxycarbenes, ab initio calculations have been employed to investigate two prototypical oxycarbenes, CH3COH and CH3OCH, derived from acetaldehyde. Singlet state methylhydroxymethylene, CH3COH, exists as trans and cis isomers lying at 61.8 and 64.2 kcal mol−1 above ground state acetaldehyde. Partial CO double bond character as evidenced by a barrier to internal rotation from trans to cis of 27.3 kcal mol−1 accounts for the existence of geometric isomers. CH3COH (S0) faces a barrier of 27.7 kcal mol−1 to the Woodward–Hoffmann "forbidden'' 1,2-hydrogen shift leading to ground state acetaldehyde. Trans and cis singlet methoxycarbenes, CH3OCH, are 73.6 and 77.3 kcal mol−1 less stable than S0 acetaldehyde. The large trans to cis rotational barrier of 30.4 kcal mol−1 is again evidence for a partial CO double bond. The barrier to the 1,2-methyl shift from trans-methoxycarbene to CH3CHO is 28.3 kcal mol−1. These carbenes, substituted with electron donating groups, OH or OCH3, are singlet stable. The singlet–triplet gaps are 23.0 kcal mol−1 for CH3COH and 21.3 kcal mol−1 for CH3OCH. Triplet CH3COH lies at 84.8 kcal mol−1 relative to ground state acetaldehyde which is 8.0 kcal mol−1 greater than the triplet acetaldehyde origin. The barrier for the triplet state 1,2-hydrogen shift from CH3COH to CH3CHO is calculated to be 33.3 kcal mol−1. Although all these methylene isomers lie high in energy in comparison with acetaldehyde, the transition state for the rearrangement to singlet CH3COH is lower than the energies of ∼95 kcal mol−1 employed in many gas phase studies of acetaldehyde photochemistry. It should be possible toisolate the trans- and cis-CH3COH species in an inert matrix following acetaldehyde photolysis and to characterize these methylenes by their vibrational spectra as predicted herein. Ion cyclotron double resonance experiments are suggested that could afford an experimental measure of the relative thermochemical stability of CH3COH with respect to CH3CHO which is calculated to be 61.8 kcal mol−1.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.450919
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  • 10
    Digitale Medien
    Digitale Medien
    Acetaldehyde photochemistry: The radical and molecular dissociations (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 2682-2690 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The dissociation of acetaldehyde to radical products, CH3+HCO and H+CH3CO, and to the molecular limit of CH4+CO has been investigated by ab initio SCF calculations. Effects of zero point energy corrections and of electron correlation on energy differences have also been considered. The computed T1 origin of acetaldehyde is in good agreement with recent experimental determinations. On the T1 surface, CH3CHO faces activation barriers of 12–15 kcal mol−1 for dissociation into radicals. The existence of and the quantitative determination of the barrier heights have been accurately computed for the first time. Vibrational excitation of T1 acetaldehyde formed with energy equal to that of its spectroscopic origin is anticipated to play an important role in acetaldehyde photochemistry as has been inferred experimentally. The barrier to the unimolecular dissociation to molecular products on the S0 surface is slightly lower than the T1 radical barriers. If hot ground state acetaldehyde can be formed with energies of ∼85 kcal mol−1 by some process then the molecular dissociation channel should also be accessible.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.450341
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