Ivana Paidarová

Ab initio calculations for Ar2+, He2+ and He3+, of interest for the modelling of ionic rare-gas clusters

Abstract Large CI ab initio calculations are performed for Ar2+, He2+ and He3+. For both ionic dimers a complete set of accurate data is determined, including the various electronic states and the transition moments to be used for spectroscopical studies and for modelling larger ionic rare-gas clusters. Spectroscopic constants for Ar2+ are in remarkable agreement with the experimental results. The study for He3+ confirms the failure of the minimal DIM model for helium ions.

The vibrational and temperature dependence of the indirect nuclear spin–spin coupling constants of the oxonium (H3O+) and hydroxyl (OH−) ions

Abstract The indirect nuclear spin–spin coupling constants of the gas phase oxonium (H 3 O + ) and hydroxyl (OH − ) ions, their temperature dependence and isotope shifts are predicted by ab initio calculations. The coupling constants are calculated as a function of the symmetric stretching and the inversional coordinates of H 3 O + and as a function of the bond length of OH − at the uncorrelated level of the random phase approximation (RPA), at the correlated levels of the second order polarization propagator approximation with coupled cluster singles and doubles amplitudes – SOPPA(CCSD) – and of the multiconfigurational random phase approximation (MCRPA) with a large complete active space wavefunction. Effective ro-vibrational state dependent coupling constants are obtained from these functions and the corresponding ro-vibrational wavefunctions. The effective coupling constants for several states are then used to determine the temperature dependence of the coupling constants. The results are compared with the coupling constants of H 2 O and the nuclear magnetic shielding constants of H 3 O + and OH − .

Theory of Fano profiles

The theory of Fano profiles currently presented in the framework of scattering theory can also be investigated from model Hamiltonians projected in the basis of discrete states. It is shown that the wave operator approach of quantum dynamics applied to these models simultaneously provides the lineshapes and the dynamics of the quasi-bound states of interest. An analytical expression which generalizes Fano profiles is presented.

Diatomics‐in‐molecules models for H2O and H2O−. I. Valence bond diatomic fragment matrices

Valence bond wave functions for the ground states OH(X 2Π) and OH−(X 2Σ+) were analyzed in order to arrive at DIM models for those states of H2O and H2O− correlating with the ground states of the separated atoms. The models are meant to provide a framework for studying the negative ion reaction O−+H2 → OH−+H and to test the utility of DIM in describing at a simple level the bonding in the water molecule. Projection analysis of the ab initio wave functions shows that in addition to the VB structures arising from the coupling of ground state atoms, ionic structures are important, especially for H2O, but that structures with excited states of H can be neglected. Within the space of the DIM model basis functions, diatomic fragment matrices were computed by the VB method to produce library files for eight state manifolds of OH−, six manifolds of OH, two manifolds of OH−−, and four manifolds of H−2. These will be useful as input for future DIM calculations on molecules containing O–H bonds.

Accurate ab initio calculation of potential energy curves and transition dipole moments of the Xe2+ molecular ion

Abstract A complete set of accurate ab initio data is produced for the low-lying electronic states of Xe 2 + , including potential energy curves and transition dipole moments, using a coupled cluster approach (RHF-RCCSD-T), accurate relativistic effective core potentials and an extended basis set. This set of data can be useful for spectroscopical studies as well as for modeling of the dynamics of Xe n + clusters. The spin–orbit coupling is included through a semiempirical treatment. The spectroscopic constants for Xe 2 + are in very good agreement with the experimental results.

Diatomics‐in‐molecules models for H2O and H2O−. II. A self‐consistent description of the 1A′, 1A″, 3A′, and 3A″ states of H2O

This paper describes a small (6 to 9 basis functions) model for the potential energy surfaces relevant to the chemical reaction O(1D)+H2→OH(X 2Π)+H. The model is optimized with respect to the 1A′ and 1A″ states correlating with the reagents and products of this reaction; this is done in such a way as to simultaneously provide a qualitatively correct description of the 3A′ and 3A″ states of H2O. In this sense the model is self‐consistent. In agreement with other semiempirical work, the results indicate that two 1A′ surfaces and one 1A″ surface are pertinent for a dynamical study of this reaction. The model adequately represents the most important features of H2O potential energy surfaces and is at the same time small enough to be used directly in a trajectory calculation of the reaction cross section.

Thermal averaging of the indirect nuclear spin-spin coupling constants of ammonia: The importance of the large amplitude inversion mode

Analytic internal-coordinate representations are reported for two accurate ab initio spin-spin coupling surfaces of the ammonia molecule, (1)J ((15)N,H) and (2)J(H,H). Calculations were carried out at the level of the second-order polarization propagator approximation involving coupled-cluster singles and doubles amplitudes (CCSD) and using a large specialized basis set, for a total of 841 different geometries corresponding to 2523 distinct points on the (1)J ((15)N,H) and (2)J(H,H) surfaces. The results were fitted to power series expansions truncated after the fourth-order terms. While the one-bond nitrogen-hydrogen coupling depends more on the internuclear distance, the geminal hydrogen-hydrogen coupling exhibits a pronounced dependence on the bond angle. The spin-spin parameters are first vibrationally averaged, using vibrational wave functions obtained variationally from the TROVE computer program with a CCSD(T) based potential energy surface, for ammonia and its various deuterated isotopologues. The vibrationally averaged quantities are then thermally averaged to give values of the couplings at absolute temperatures of 300 and 600 K. We find that the nuclear-motion corrections are rather small. The computed one-bond couplings and their minute isotope effects are in excellent agreement with the experimental values.

The vibrational and temperature dependence of the magnetic properties of the oxonium ion (H3O

Abstract First-and second-order polarization propagator calculations of magnetizability, rotational g -factor, spin-rotation constant, and hydrogen and oxygen shielding propterty surfaces are reported for the gas phase oxomium ion. Using these surfaces and the nonrigid inverter ro-vibrational eigenfunctions, effective magnetic molecular constants are obtained for the lowest ro-inversional states of H 3 17 O + and D 3 17 O + . The predicted constants exhibit sizable and non-monotonic dependence on the vibrational and rotational quantum numbers. We find nearly temperature independence of both the 1 H and 17 O shieldings, mainly due to cancellation effects between contributions from the inversional and the symmetric stretching modes. We compute a downfield shift in 1 H as well as 17 O shieldings of H 3 O + relative to pure water in agreement with earlier observations. The influence of hydrogen bonding on these shifts, as well as on the deuterium-induced isotope shifts on the 17 O shielding, is discussed.

Calculations of magnetic hyperfine structure constants for the low-lying rovibrational levels of LiH, HF, CH+, and BH

Abstract Spin-rotation constants for the six lowest vibrational states and the first excited end-over-end rotational state were calculated for several isotopomers of the X 1 Σ + diatomic molecules: LiH, HF, CH + and BH. For the ground vibrational state, the rotational dependence of the spin-rotation constant was also investigated and was found to be very weak. The electronic calculations were done at various levels of the polarization propagator approximation using atomic basis sets adapted for calculations of magnetic properties. The calculated spin-rotation constants for the isotopomers of LiH and HF, namely their rovibrational dependence, are in good agreement with the available experimental data. The extremely large values of the spin-rotation constants of C and B in CH + and BH previously predicted by coupled perturbed Hartree-Fock calculations at the equilibrium internuclear distances were confirm by the present correlated and rovibrational averaged calculations. For the ground vibrational and first rotational state the spin-rotation constant of B in 11 BH in 11 CH + are predicted to be −459.70 and −1186.15 kHz, respectively. The results are discussed in the light of recent calculations of the nuclear quadrupole coupling constants, since the relative strengths of both the magnetic and electric parameters determined the resulting pattern of the hyperfine structure of molecular rovibrational spectra. Improved experimental values for the nuclear magnetic shielding constants of HF ( σ F = 418.0 ± 2.0, σ H = 28.6 ± 2.0) at the equilibrium geometry were derived from recent measurements and calculated diamagnetic contributions.

The vibrational dependence of the magnetic hyperfine interaction constants of ammonia

Abstract Using the nonrigid invertor method and the first (RPA) and second order (SOPPA) polarization propagator approximations, we have evaluated the magnetic hyperfine structure constants (the direct nuclear spin-spin and the spin-rotation constants) of ammonia. We find a strong and non-monotonic v2-dependence of the pure inversional and centrifugal distortion hyperfine structure constants. We discuss how such a dependence can be used to provide useful information about intramolecular potential energy functions of NH3 and most likely also for other floppy molecules.