Browsing by Author "Sebald, Peter"

The metastable linear ZnH2 molecule in its X-1 Sigma(+)(g) electronic ground state has been investigated by the coupled cluster method CCSD(T) in conjunction with a small-core pseudopotential for the zinc atom. Using three pieces of spectroscopic information for the most abundant isotopomer (ZnH2)-Zn-64, an accurate near-equilibrium potential energy function (PEF) has been constructed and used in variational calculations of rovibrational energies and wave functions. The nu(1) and nu(2) band origins (in cm(-1)) for (ZnH2)-Zn-64 and (ZnD2)-Zn-64 (in parentheses) are predicted at 1886.4(1349.7) and 635.1(459.8), respectively. (ZnH2)-Zn-64 is characterised by strong Darling-Dennison and l-type rotational resonances. Various perturbations are analysed in detail, partly making use of calculated expectation values of the vibrational quantum number vertical bar l vertical bar. Einstein coefficients of spontaneous emission are predicted for several transitions.

Accurate 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).

Ab 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.

Ab initio calculations of the coupled cluster and spin-orbit configuration type, in conjunction with a small-core pseudopotential for the mercury atom, have been employed to construct near-equilibrium potential energy and electric dipole moment functions for HgH2. On that basis, rovibrational term energies and wavefunctions as well as transition dipole moments, absolute IR intensities and Einstein coefficients of spontaneous emission have been calculated variationally. Throughout, excellent agreement is obtained with recent experimental data from Fourier transform infrared emission spectroscopy. The gas-phase wavenumbers of the symmetric stretching and the bending vibrations of (HgH2)-Hg-202 and (HgD2)-Hg-202 ( in parentheses) are predicted to be 2012.3 (1442.8) cm(-1) and 784.3 (564.1) cm(-1), respectively. Various predictions are made for (HgHD)-Hg-202, for which no high-resolution spectra have yet been published.

The equilibrium structure and rovibrational energies of nitrous oxide (N2O) in its electronic ground state (X-1 Sigma(+)) are derived from a high-level ab initio potential energy function (PEF). This PEF is based on a composite approach with the basic contribution given by explicitly correlated coupled-cluster (CC) calculations. Smaller contributions include corrections due to inner-shell correlation, scalar-relativistic effects and higher-order correlation up to iterative pentuple excitations (CCSDTQP in CC nomenclature). The high importance of higher-order correlation in order to reach the desired accuracy led to the use of an extrapolation scheme to approximately account for the effect of hextuple and some pentuple excitations. A reasoning for the soundness of the method is given in this work. The results of the rovibrational calculations are compared to those of two multi-reference (MR) based composite PEFs, where the basic contribution is given by MR configuration interaction and MR average coupled-pair functional calculations. A highly accurate electric dipole moment function is also computed by the three composite methods in excellent agreement with the experimental values available. Subtle irregularities in the intensity pattern are reproduced in great detail and several kinds of resonances are analyzed without the need to empirically adjust our best ab initio PEF. The equilibrium bond lengths were determined by a mixed experimental/theoretical approach yielding R-e (NN) = 1.12695 (10) angstrom and R-e(NO) = 1.18539(5) angstrom.

Explicitly correlated coupled cluster theory at the CCSD(T )-F12b level in conjunction with the aug-cc-pV5Z basis set has been used in the calculation of three-dimensional potential energy and dipole moment surfaces for the bifluoride ion (FHF-). An empirically corrected analytical potential energy function (PEF) was obtained by fit to four pieces of accurate spectroscopic information. That PEF was used in variational calculations of energies and wave functions for a variety of rovibrational states of the isotopologues FHF-, FDF-, and FTF-. Excellent agreement with available data from IR laser diode spectroscopy is observed and many predictions, are being made. Unusual isotope effects among the spectroscopic constants and unusual features of the calculated line spectra are discussed.

An accurate local (near-equilibrium) potential energy surface (PES) is reported for the C-3 molecule in its electronic ground state ((X) over tilde (1)Sigma(+)(g)). Special care has been taken in the convergence of the potential relative to high-order correlation effects, core-valence correlation, basis set size, and scalar relativity. Based on the aforementioned PES, several rovibrational states of all C-12 and C-13 substituted isotopologues have been investigated, and spectroscopic parameters based on term energies up to J = 30 have been calculated. Available experimental vibrational term energies are reproduced to better than 1 cm(-1) and rotational constants show relative errors of not more than 0.01%. The equilibrium bond length has been determined in a mixed experimental/theoretical approach to be 1.294 07(10) A in excellent agreement with the ab initio composite value of 1.293 97 A. Theoretical band intensities based on a newly developed electric dipole moment function also suggest that the infrared active (1,1(1),0) (0,0(0),0) combination band might be observable by high-resolution spectroscopy. (C) 2016 AIP Publishing LLC.

In this work the rovibrational spectrum of the HCS+ molecular cation is revisited through high-level electronic structure and variational rovibrational calculations. A local potential energy function is built from explicitly correlated coupled-cluster results, incorporating corrections for core-valence, scalar relativistic and higher-order excitation effects. The computed spectroscopic parameters, based on variational calculations with Watson's isomorphic Hamiltonian for linear molecules lead to a nearly perfect agreement with experimentally reported values (Rosenbaum et al., 1989). Furthermore, the documented Fermi resonance within the (0, 0 degrees,1)/(0, 2 degrees, 0) and (1, 0 degrees,1)/(1, 2 degrees, 0) pairs of states is clarified. Based on a newly developed electric dipole moment function transition dipole moments of fundamental transitions are predicted for the most important isotopologues. (C) 2016 Elsevier Inc. All rights reserved.

Explicitly correlated coupled cluster theory at the CCSD(T )-F12b level (T. B. Adler, G. Knizia, and H.-J. Werner, J. Chem. Phys., 2007, 127, 221106) and two precise spectroscopic parameters (K. Kawaguchi, J. Chem. Phys., 1988, 88, 4186) were used to construct an accurate near-equilibrium analytical potential energy function (PEF) for the highly anharmonic centrosymmetric hydrogen-bonded complex ClHCl- (R-e = 3.1153 angstrom). From variational calculations with that PEF, a large number of rovibrational energies of different isotopologues up to high values of the rotational quantum number J was obtained. Theory helped with the assignment of lines observed by IR diode laser spectroscopy in the nu(1) + nu(3) combination band of (ClHCl-)-Cl-35-Cl-35 and (ClHCl-)-Cl-37-Cl-35 and enabled us to elucidate rather subtle patterns of rovibrational interactions. Furthermore, transition dipole moments were predicted and analysed as well as unusual isotopic effects.

Near-equilibrium potential energy surfaces for HBF+ isotopologues, obtained from high-level calculations beyond fc-CCSD (T), are employed in variational calculations for many rovibrational states. Calculated effective spectroscopic parameters are in excellent agreement with available experimental data and many predictions are being made, also for line intensities. The band origin of the bending vibration of (HBF+)-B-11 is predicted at 730.7 cm(-1). Combining a difference-frequency system with glow discharge and a discharge modulation scheme, six and seven lines of the v(1) bands for (HBF+)-B-11 and (HBF+)-B-10, respectively, were observed. Together with data obtained from microwave spectroscopy the spectroscopic constants of v(1) could be derived through least-squares fitting. (C) 2014 Elsevier Inc. All rights reserved.

An accurate near-equilibrium potential energy surface (PES) has been constructed for the azide ion (N-3(-)) on the basis of coupled cluster calculations up to CCSDTQ (Kallay, M.; Surjan, P. R. J. Chem. Phys. 2001, 115, 2945.), with contributions from inner-shell correlation and special relativity being taken into account as well. A larger number of rovibrational states has been investigated by variational calculations with Watson's isomorphic Hamiltonian for linear molecules. Analogous calculations for CO2 demonstrate the high quality of this type of calculations. The G(v) values of the symmetric stretching and bending vibration of N-14(3)- are predicted to be v(1) = 1307.9 cm(-1) and v(2) = 629.3 cm(-1), with an uncertainty of ca. 1 cm(-1). Fermi resonance is less pronounced for the lower polyads of N-14(3)- compared with (CO2)-C-12-O-16 but is as strong as in CO2 for the lowest diad of isotopologue 15-14-15. The band origin of the antisymmetric stretching vibration of N-14(3)- is calculated to be v(3) = 1986.4 cm(-1), only 0.1 cm(-1) lower than the experimental value. The corresponding vibrational transition dipole moment is predicted to be as large as mu = 0.476 D, 46% higher than calculated for CO2. The perturbed combination tone (01(1)1), which was accessible through diode laser IR spectroscopy, undergoes anharmonic interaction with at least two other vibrational states.

Highly accurate quantum chemical calculations beyond CCSD(T) have been used to study the molecular cation l-C3H+ which is the carrier of harmonically related radio lines observed in the Horsehead photodissociation region and toward Sgr B2(N). Excellent agreement with spectroscopic and radioastronomical measurements is obtained for the rotational constant, with the calculated value of B-0 = 11246.4 MHz only 1.5 MHz or 0.01% above the experimental value. The unusually large ratio of centrifugal distortion constants D-0(exp.)/D-e(theor.) = 1.80 is attributed to the shallow CCC bending potential of l-C3H+ and is quantitatively reproduced by variational calculations within a pseudo-triatomic model. A comparative study of centrifugal distortion constants in a series of four linear interstellar molecules (C3N-, C3O, l-C3H+, and C-3) is made and some general conclusions are drawn.

An accurate near-equilibrium potential energy surface (PES) for CNC+ is constructed based on a high-level composite ab initio method. By combining explicitly correlated all-electron CCSD(T)-F12b with scalar relativistic effects and higher order correlation up to coupled cluster theory with singles, doubles, triples and quadruples (CCSDTQ) we achieve convergence in the wavenumbers of the fundamentals to ca. 1 cm(-1). Rovibrational energies are calculated in a variational approach and vibrational term energies and rotational constants are in excellent agreement with available experimental data. Accurate values for centrifugal distortion constants of CNC+ in different vibrational states are predicted. Especially the centrifugal distortion constants in the vibrational ground state of D-0 = 0.563 center dot 10(-6) cm(-1) and H-0 = 0.188 center dot 10(-10) cm(-1) should be superior to experimentally derived values. Reassignments of some experimentally observed transitions are suggested based on a comparison of experimental and calculated term differences. The bending part of the PES appears to be almost quartic and the band origin of the bending vibration is predicted at 94.2 cm(-1). Absolute line intensities are calculated for various transitions in CNC+. For the bending vibration, an intensity is predicted that is three orders of magnitude smaller than for the antisymmetric stretching vibration.

Accurate near-equilibrium potential energy functions (PEFs) have been constructed for the nitronium ion (NO) by composite methods using either CCSD(T)-F12b or explicitly correlated multi-reference methods (MRCI-F12+Q or MRACPF-F12) as dominant contributions. Up to pentuple substitutions are required in the coupled-cluster based approach to reach convergence in the wavenumbers of the fundamentals to ca. 1 cm(-1). These are predicted to be nu(1) = 1386.0 cm(-1), nu(2) = 621.1 cm(-1) and nu(3) = 2342.8 cm(-1). All values differ significantly from the results of previous studies by zero-kinetic energy (ZEKE) spectroscopy and reanalysis or remeasurement is suggested. Compared to neon-matrix IR spectroscopic work of Jacox and coworkers the present calculations yield smaller wavenumbers of Delta nu(3) = -5.4 cm(-1) and Delta(nu(1) + nu(3)) = -7.9 cm(-1) so that blueshifting is predicted for those absorptions. The calculated isotopic shifts for both bands are in excellent agreement with the corresponding experimental values. Accurate values for rotational and centrifugal distortion constants of NO2+ in different vibrational states are predicted which should be of help in the search for forthcoming high-resolution spectra of that cation. (C) 2014 Elsevier Inc. All rights reserved.