Study of the 1s and 2s shell contributions to the isotropic hyperfine coupling constant in nitrogen

The istropic part of the hyperfine coupling constant is investigated by means of multireference configuration interaction calculations employing Gaussian basis sets. A detailed study of the 1s and 2s spin polarisation in the nitrogen atom and the NH molecule shows that the structure of the lower-energy space of the unoccupied orbitals is essential for the results. A contraction of the Gaussian basis is possible without loss of accuracy if enough flexibility is retained to describe the main features of the original space of unoccupied functions. Higher than double excitations are found to be non-negligible for the description of α$_{iso}$

Calculation of hyperfine coupling constants : An ab initio MRD-CI study for nitrogen to analyse the effects of the basis sets and CI parameter

The hyperfine coupling constant for the nitrogen atom is evaluated by large-scale MRD-CI calculations. A detailed analysis of the charge density at the nucleus and the spin polarization in the ls and 2s shell as a function of various technical parameters is undertaken. Various (s, p) AO basis sets and the inftuence of correlation orbitals is investigated as weil as selection threshold and other properlies in CI calculations. The best value, obtained for the isotropic hyperfine coupling constant in an s, p, d basis, based on theoretical judgment of' best' quantities, is 9·9 MHz compared to 10·4509 MHz

An MRD-CI study of low-lying electronic states in CaF

Dipole moments and various spectroscopic constants of some low-lying electronic states of the CaF molecule have been calculated using the multireference single· and double-excitation configuration-interaction (MRD-CI) method. The electronic structure of the highly ionic molecule in various excited states can be explained in tenns of different polarisations of the mainly Cacentered valence electron in the field of the F$^-$ anion. Plots of natural orbitals occupied by the valence electron in the different states give a qualitative picture of the charge distribution and provide a visualisation of the different polarisations of the valence electron in the various states. Comparisons with the electrostatic polarisation model ofTörring, Ernstand Kändler (TEK model) are made. The unknown A' $^2 \Delta$ state is predicted to lie about 21200 cm$^{-1}$ above the ground state

Study of orbital transformations in configurational interaction calculation of hyperfine coupling in nitrogen and CH molecule

Multi-reference configuration interaction calculations employing various orbital transformations are undertaken to obtain the isotropic hyperfine coupling constant a_{iso in nitrogen and a_{iso (H) in the CH molecule. The natural orbital (NO) basis is found to be more effective than the simple RHF-MO basis; the most obvious is a basis of spin natural orbitals (SNO). It is found that a_{iso is approached from opposite sides in the NO and 2s shell SNO basis if the CI expansion is increased. Both results are within a few percent of the full CI Iimit for the nitrogen atorn (in the given AO basis) and the experimental value for Hin the CH radical. Various features ofthe SNO are discussed

Theoretical study of FC2_2H4_4

Large-scale multireference configuration interaction (MRD-CI) calculations in a quite flexible AO basis are employed to study the energy hypersurface for the reaction intermediate FC$_2$H$_4$ • The reaction F + C$_2$H$_4$ -> FC$_2$H$_4$ as weil as the 1,2 migration of the fluorine atom in FC$_2$H$_4$ is investigated. In addition the rotation around the CC bond in the optimum conformation is studied. The absolute minimum in the potential energy is found for the asymmetric structure but the symmetric structure is also found to be stable with respect to the dissociation, so that a shuttling of the fluorine atom is in principle possible but highly unlikely because ( l) the activation energy is high ( II 5-130 kJ fmol) and the saddle point lies only 4(}-50 kJ jmol below the dissociation Iimit of F + C$_2$H$_4$ and (2) the competitive motion, i.e., rotation around the CC axis, is nearly free (I 1-17 kJ/mol)

The hyperfine coupling constants of the X3^3Σ−^- state of NH : Influence of polarization functions and configuration space on the description of spin polarization

The hyperfine coupling constants for the $^3$Σ$-$ ground state of the NH molecule are determined by configuration interaction calculations whereby the infl.uence of polarization functions as weil as of the configuration space on the spin polarization mechanism is analysed. The dipolar part Au(N) and Au(H) can be obtained very reliably without much computational effort (A .. (N) == -45·3 MHz and A"(H) = -62·3 MHz). The value for the isotropic contribution a1.., in the best AO basis and MRD-CI treatment is - 64·5 MHz for H and 16·6 MHz for nitrogen compared to the corresponding experimental quantities of -66 MHz and 19 MHz respectively. Their determination depends on a subtle balance of the lu, 2u and 3u shell correlation description, whereby the dominant contribution to a1..,(H) results from the 2u shell. It is shown that the often good agreement of a110 values with experiment in a small basis singledouble configuration interaction treatment results from a cancellation of two errors

The Reaction of Singlet and Triplet Methylen with Ethene - A Multi-Reference CI Study

Large-scale multireference configuration interaction (MRD-CI) calculations in a flexible atomic orbital (AO) basis are employed to study the reaction of C$_2$H$_4$ with CH$_2$ in its firSt triplet and singlet state. The minimum energy path (MEP) of both reactions is calculated, and different mechanisms are discussed. To examine the possible participation of the singlet state in the overall reaction starting from the triplet channel and terminating in the singlet-state c-C$_3$H$_6$, various cuts through both hypersurfaces are calculated. lt is found that favorable interconversion from the trip1et to the singlet surface can only occur at !arge separations of the two fragments of CH2 and C$_2$H$_4$. Experimental data considering the vibrational motion of CH$_2$ in connection with the relative position of both surfaces are used to obtain an estimate for the overall barrier of the reaction. The height of the barrier is about 6 kcal/mol, while the barrier of the pure triplet reaction is calculated to be 7-9 kcal/mol

Ab initio investigation of the vibronic structure of the C2_2H spectrum Computation of the vibronically-averaged values for the Hyperfine Coupling Constants

The vibronically averaged values for tbe hyperfine coupling constants in the X$^2 \sum$-A$^2 \Pi$ system of the ethynyl radical are computed by means of tbe ab initio metbod calculations. The results point at tbe importance of taking into account the coupling of a1l tbree electronic states in question ( I$^2$A', 2$^2$A', and 1$^2$A") for a reliable explanation of the available experimental findings. The mean values of the hfcc's for K = 0 and 1 levels in $^{13}$C$_2$H and $^{13}$C$_2$D in the energy range up to 6000 cm$^{-1}$ are predicted

On the chemi-ionization reaction O + CH ----> HCO+^++ e−^-. Coollinear O-CH Approach

We have investigated theoretically the importance of the O($^3$P)+CH(a$^4\sum^-$) and the O($^3$P)+CH(X$^2\Pi$) channels in the collinear chemi-ionization reaction O+CH->HCO$^+$ +e$^-$. We have found that both channels may lead to chemi-ionization via favorable Franck-Condon overlaps with the states ofthe ionic species

Theoretical study of the potential energy surface governing the stereochemistry in ClC2_2H4_4 reactions

Large-scale multireference configuration interaction calculations in a double·t·type AO basis including polarization functions are carried out for the potential surface of the ClC$_2$H_4\9 system. The charge distribution for various extreme points of the surface is discussed. The absolute minimum is found for an asymmetric ClC2H4 structure. The symmetrical bridged nuclear conformation is also found to be stable with respect to dissociation into Cl + C\(_2H$_4$• The activation energy for rotation about the C-C axis is calculated tobe around 18 kJ/mol, which is comparable tothat for the 1,2 migration {around 26 kJ/mol). The stereochemistry is governed by the fact that addition of CI to C$_2$H$_4$ (or dissociation) is a two-step reaction proceeding through a symmetrica1 intermediate. The direct addition pathway possesses a small barrier of about 8 kJ jmol