1,721,054 research outputs found
The hydrogen-bonded cluster anions Br-center dot center dot center dot HCCH and I-center dot center dot center dot HCCH: results of coupled cluster calculations
No full textLarge-scale coupled cluster calculations at the CCSD(T) level of approximation have been carried out for the anionic complexes Br-. . . HCCH and I-. . . HCCH, both of which have a linear equilibrium structure. The equilibrium dissociation energies (D-e) are predicted to be 3126 and 2550 cm(-1), respectively. Agreement with available experimental data, obtained through predissociation infrared spectroscopy, is very good. In addition, many predictions are made for spectroscopic properties of various isotopomers of the two species. Particular emphasis is given to the effects of vibrational anharmonicity, conveniently expressed by anharmonicity constants X-ij and vibration-rotation coupling constants alpha (r). Transition dipole moments were calculated for various stretching vibrational transitions
An incremental correlation approach to excited state energies based on natural transition/localized orbitals
No full textA new incremental approach to the computation of vertical excitation energies is presented. The method works based on the definition of a dominant occupied orbital where the excitation takes place (natural transition orbital) and by localizing the remaining occupied space. The use of a reduced two-body expansion leads to a linear number of terms to be computed. A series of benchmark calculations have been carried out on small to medium sized photoactive systems. The results compare well to the full calculations, with maximum deviations of 0.3 eV, and an average absolute deviation of about 0.08 eV. In addition, a energy decomposition analysis is made on the basis of orbital distances to the chromophore region. First results indicate that orbitals beyond a relatively small radius can be safely neglected, leading to further drastic savings in the calculations. (C) 2011 American Institute of Physics. [doi:10.1063/1.3522881]German Excellence Initiative of the University of Gottinge
Structure and rearrangement reactions of bis(organosilyl) (organostannyl)hydroxylamines: A joint theoretical/experimental study
No full textO-Lithium-N,N-bis(tent-butyldimethylsilyl)hydroxylamide reacts with chlorotrimethyl-stannane to give N,O-bis(tert-butyldimethylsilyl)-N-(trimethylstannyl)hydroxylamine (1), the crystal structure of which is presented. The primarily formed N,N-bis(tert-butyldimethylsilyl)O-(trimethylstannyl)hydroxylamine undergoes a dyotropic rearrangement. This reaction mechanism is corroborated by quantum-chemical calculations (B3LYP), partly employing an effective core potential for tin. The possibility for insertion of a stannyl group has been studied by quantum-chemical calculations. The transition states in the various rearrangement reactions of the bis(organylsilyl)stannylhydroxylamine system are discussed in detail
A theoretical investigation of the silicon-carbon chain molecule SiC8
No full textLarge-scale coupled cluster calculations have been carried out for linear SiC8, a molecule of interest to astrochemistry. An accurate equilibrium structure has been established and a large equilibrium dipole moment of mu(e) = -9.96 D is predicted, where the positive end of the dipole is located at the silicon site. Most promising for future detection by infrared spectroscopy are the stretching vibrations at 1973 and 2103 cm(-1) with absolute intensities of 6662 and 3699 km mol(-1), respectively. The lowest bending vibration has a harmonic wavenumber of 38 cm(-1). The collinear fragmentation process SiC8 --> SiC6 + C-2 has been investigated and the corresponding dissociation energy is predicted to be D-0 = 568 kJ mol(-1)
Coupled cluster calculations for the SN2 reaction Cl− + CH3Br → ClCH3 + Br−
No full textCoupled cluster calculations including single and double excitation operators plus a perturbative treatment of connected triples have been carried out for five stationary points of the potential energy surface of the S(N)2 reaction Cl- + CH3Br --> ClCH3 + Br-. The ion-dipole complexes have well depths of 11.2 kcal mol(-1) in the reactant and 18.2 kcal mol(-1) in the product channel, respectively, and are separated by a barrier which is lower than the asymptotic reactant energy by 2.2 kcal mol(-1). The enthalpy of reaction at 298 K is calculated to be -8.15 kcal mol(-1). (C) 2000 Elsevier Science B.V
A Simple One-Body Approach to the Calculation of the First Electronic Absorption Band of Water
No full textA one-body decomposition approach for investigating the electronic absorption spectra of molecular systems was proposed and applied to water clusters (H2O)N including up to N = 80 water molecules. Two specific aspects of the present implementation are the inclusion of the coupling between excited states and a simplified representation for the N-body Coulombic effects. For smaller clusters, the results based on the one-body decomposition scheme are in good agreement with full EOM-CCSD calculations. Two different regimes can be identified in the electronic absorption profile of larger water clusters. The first low-energy regime is dominated by local excitonic states on the cluster surface, whereas the higher-energy excitations associated with the second one are of delocalized nature
Accurate bond dissociation energies (D-0) for FHF- isotopologues
No full textAccurate bond dissociation energies (D-0) are determined for three isotopologues of the bifluoride ion (FHF-). While the zero-point vibrational contributions are taken from our previous work (P. Sebald, A. Bargholz, R. Oswald, C. Stein, P. Botschwina, J. Phys. Chem. A, DOI: 10.1021/jp3123677), the equilibrium dissociation energy (D-e) of the reaction FHF (-) -> F- + HF was obtained by a composite method including frozen-core (fc) CCSD(T) calculations with basis sets up to cardinal number n = 7 followed by extrapolation to the complete basis set limit. Smaller terms beyond fc-CCSD(T) cancel each other almost completely. The D-0 values of FHF-, FDF-, and FTF- are predicted to be 15,176, 15,191, and 15,198 cm(-1), respectively, with an uncertainty of ca. 15 cm(-1)
Theoretical investigations of proton-bound cluster ions
No full textSeveral proton-bound cluster ions have been studied by means of coupled cluster calculations with large basis sets. Among these are complexes of a krypton or xenon atom with the cations HCO+, HN2+ and HNCH+. Various spectroscopic properties have been calculated in all cases. Effects of vibrational anharmonicity are particularly pronounced for the intramolecular stretching vibrations of and Kr . . . HN2+ and Xe . . . HN2+. The proton stretching vibration of (N-2)H+(N-2) is predicted around 800 cm(-1), with a large transition dipole moment of 1.15 D. Both(N-2)H+(N-2) and (HCN)H+ (NCH) have linear centrosymmetric equilibrium structures. Those of (OC)H+ (CO) and (HCC-)H+(CCH-) are asymmetric with barrier heights to the centrosymmetric saddle points of 382 and 2323 cm(-1), respectively. The dissociation energy of the anionic complex Cl-. . . HCCH is calculated to be D-o = 3665 cm(-1), 650 cm(-1) larger than the corresponding value for Br-. . . HCCH. The complex between a fluoride ion and acetylene is more strongly bound and shows strongly anharmonic behaviour, similar to the bihalides FHF- or ClHCl-. Strong Fermi resonance interaction is predicted between v(3) (similar to proton stretch) and 2v(4) (Drst overtone of intermolecular stretch)
Accurate Potential Energy Surface and Calculated Spectroscopic Properties for CdH2 Isotopomers
No full textAb initio calculations employing the coupled cluster method CCSD(T), in conjunction with a small-core pseudopotential for the cadmium atom, have been employed to construct a near-equilibrium potential energy function (PEF) and an electric dipole moment function (EDMF) for CdH2. The significance of the spin-orbit interaction was checked and found to be of minor importance. Making use of two pieces of experimental information for the most abundant isotopomer (CdH2)-Cd-114, we obtained a refined PEF, which, within variational calculations of rovibrational states and wave functions, reproduces all available experimental data (S. Yu, A. Shayesteh, and P. F. Bernath, J. Chem. Phys 2005, 122, 194301) very well. In addition, numerous predictions are made. In particular, the nu(2) band origins for (CdH2)-Cd-114 and (CdD2)-Cd-114 are predicted at 605.9 and 436.9 cm(-1), respectively, and the state perturbing the e parity levels of the (0,0(0),1) state of (CdH2)-Cd-114 at J = 12-17 is identified as the (0,3(3),0) state. Assignments for further perturbations found in the emission spectra are given as well.Deutsche Forschungsgemeinschaft (DFG); Fonds der Chemischen Industri
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