Nuclear Spin-spin Coupling Constants Research Articles

Ab initio calculation of the NMR shielding and indirect spin-spin coupling constants of fluoroethylene

The spin-Hamiltonian parameters that characterize the high-resolution NMR spectrum of C2H3F (i.e., the full set of six nuclear shielding constants and 15 indirect nuclear spin-spin coupling constants) have been calculated using multiconfigurational self-consistent field (MCSCF) wavefunctions. The dependence of the parameters on the basis set and on the correlation treatment is investigated. Although convergence is difficult to achieve, for most parameters a systematic improvement is found with extension of the atomic orbital basis and the MCSCF configuration space. All the final results are in satisfactory agreement with available experimental data.

Vibrational and thermal averaging of the indirect nuclear spin-spin coupling constants of CH4, SiH4, GeH4 and SnH4

The effects of nuclear motion on the indirect nuclear spin-spin coupling constants of CH4, SiH4, GeH4 and SnH4 were calculated up to first order in the normal coordinates. We report the random-phase approximation, multiconfigurational linear response and second-order polarization propagator calculations of the zero-point rovibrational corrections, as well as the temperature dependence of both one-bond and two-bond coupling constants. We find that the random-phase approximation overestimates the zero-point corrections. The results demonstrate that rovibrational corrections are as important as the non-contact terms and must be included in accurate determinations of indirect nuclear spin-spin coupling constants of these molecules.

Partitioned equation-of-motion coupled cluster approach to indirect nuclear spin-spin coupling constants

Two approximations to equation-of-motion coupled cluster theory for second-order properties are introduced that increase the computational efficiency of the approach without forsaking the accuracy of the method. Using partitioning techniques the doubles-doubles block in the EOM Hamiltonian is replaced by a diagonal approximation, and in addition the CCSD ground state may be replaced by its analog from first-order perturbation theory. It is shown that partitioning hardly affects the accuracy of the approach. The Fermi contact contribution to the coupling constant is found to be most sensitive to the inclusion of ground state correlation.

Spin-orbit corrections to the indirect nuclear spin-spin coupling constants in XH4 (X = C, Si, Ge, and Sn)

Using the quadratic response function at theab initio SCF level of approximation we have calculated the relativistic corrections from the spin-orbit Hamiltonian,H SO, to the indirect nuclear spin-spin coupling constants of XH4 (X = C, Si, Ge, and Sn). We find that the spin-orbit contributions toJ X-H are small, amounting only to about 1% forJ Sn-H. For the geminal H-H coupling constants the relativistic corrections are numerically smaller than forJ X-H, but in some cases relatively larger compared to the actual magnitude ofJ H-H. We also investigate the use of an effective one-electron spin-orbit Hamiltonian rather than the fullH SO in the calculation of these corrections.

Theoretical study of spectroscopic properties of bicyclobutane, tricyclopentane, tricyclohexane, and octabisvalene

The outer-valence ionization potentials, low-lying electronically excited states, and 1J(CC) nuclear spin-spin coupling constants of bicyclobutane, tricyclopentane, tricyclohexane, and octabisvalene have been investigated at the ab initio level by using the outer-valence Green function (OVGF), random-phase approximation (RPA), and equations-of-motions (EOM) approaches, which incorporate the main dynamical correlation effects. A comprehensive theoretical prediction of these spectroscopic observables is presented, which shows several interesting aspects stemming from the unique electronic structure of the bridge bond. © 1996 John Wiley & Sons, Inc.

On the calculation of nuclear spin-spin coupling constants. The bond length dependence of the Fermi contact term in H 2 and HD

A theoretical study of the Fermi contact contribution to the HH and HD spin-spin coupling constant is reported, with special emphasis on its calculation using quantum chemical techniques over a wide range of internuclear distances, that has necessitated an extension of the existing methodology so the effects of near-degeneracy are properly treated. A detailed configuration interaction calculation on H 2 shows that as the molecule is stretched the coupling constant displays a sharp increase before decaying to zero as the molecule dissociates. Such distance dependence is reflected in the calculated vibrational averages of the coupling constant for HD that show a rapid increase with vibrational excitation.

A study of the correlativity between the ab initio maximum bond order hybrid orbital calculation results and the nuclear spin-spin coupling constants

The ab initio maximum bond order hybrid orbital (MBOHO) procedure has been performed with an STO-3G basis set in order to study the correlativity between the ab initio MBOHO calculation results and the nuclear spin-spin coupling constants. The generalized relationship which involves the contributions of not only the hybrid orbitals, but also the net atomic charges has been applied with the calculated s-character of the MBOHOs including only the valence atomic orbitals and the net atomic charges to obtain concrete relationships for calculation of the nuclear spin-spin coupling constants of various CH, CC, CN and CF bonds. The coupling constants evaluated by use of these concrete relationships are in good agreement with the experimental data, which shows that the ab initio MBOHO procedure and the generalized relationship including also the net atomic charges are feasible for elucidating the nuclear spin-spin coupling constants between the directly bonded atoms.

MBOHO calculations of phosphorus-carbon nuclear spin-spin coupling constants

The novel generalized correlation of the nuclear spin-spin coupling constants with the atomic hybrids and net charges is employed to give a new relationship for calculating the directly bonded phosphorus-carbon coupling constants by use of the maximum bond order hybrid orbital procedure together with the extended Huckel molecular orbital calculation. The calculated coupling constants of phosphorus-carbon are all in good agreement with the experimental data, which shows that the new relationship obtained in the present paper is quite satisfactory for calculation of the phosphorus-carbon coupling constants.

A density functional theory of the Fermi contact contribution to the nuclear spin-spin coupling constant

The authors' magnetic field density functional theory is extended to include electron spin-dependent interactions. Coupling the new theory with traditional spin density functional theory in the local limit yields a linear differential equation for the net spin density. The coefficients in the equation are functions of the electron density in the absence of a nuclear spin.

Theoretical study of the 1J( 13C 13C) indirect nuclear spin-spin coupling constant of the bridgehead bond in [1.1.1]propellane and related systems

Ab initio calculations using the equations of motion approach are reported for 1 J ( 13C 13C) of the bridgehead bond in [1.1.1]-, [2.1.1]-, [2.2.1]-, [2.2.2]-, [3.2.1]-propellane, bicyclobutane, bicyclopentane, bicyclohexane and tetracyclopentane. The results give an overall picture of the influences exerted on sign and magnitude of 1 J( 13C 13C) by progressive condensation of the cyclobutane frame and by enlargement of the fused rings. Extraordinarily small values are predicted for [1.1.1]- propellane and [2.1.1]-propellane.

Ab initio MO studies of nuclear spin-spin coupling constants in CH4, SiH4, AlH 4 ? and GeH4 systems

Ab initio molecular orbital calculations of electron coupled nuclear spin-spin coupling constants are performed for CH4, SiH4, AlH 4 − and GeH4 systems using the SCF perturbation theory. Basis set dependence of the major contributing terms such as orbital diamagnetic, orbital paramagnetic, spin dipolar and Fermi contact terms are studied. The study also illustrates the relative importance of bond centred functions and nuclear centred polarization functions in predicting the directly bonded and geminal couplings in the systems selected. Basis sets having uncontracted cores functions and augmented with bond functions seem to predict most of these couplings fairly satisfactorily when compared to the experimental values.

Correlated calculations of indirect nuclear spin-spin coupling constants for XH4 (X = Si, Ge, and Sn)

We report multiconfigurational linear response calculations of the one-bond and geminal indirect nuclear spin-spin couplings constants in SiH4, GeH4, and SnH4. Results of a basis set study on SnH4 using 13 different basis sets on Sn are given, showing the effects on the individual contributions to the coupling constant. In addition to the more conventional choice of active orbital space (one correlating orbital for each strongly occupied valence orbital), calculations employing extended active spaces were carried out. The results are compared with available experimental data showing that correlation effects are very important. For the one-bond couplings good agreement with experiment, within 1–5%, is obtained, whereas the geminai proton-proton couplings show larger deviations. The calculations reported here are, to our knowledge, the first that take into consideration correlation effects in all four contributions to the coupling constants for molecules as large as GeH4 and SnH4. All X-H couplings are dominated by the Fermi contact term while the picture is more diverse for the geminal couplings.

Ab initio calculation of natural hybrid orbitals

The natural hybrid orbital (NHO) procedure is tested at the ab initio level by use of the density matrix in a Löwdin orthogonalized atomic orbital basis. The direct NHO calculation based on the whole density matrix also includes the hybridization of the inner atomic orbitals, and the NHO calculation employing the valence orbital part of the density matrix considers only the hybridization of the valence atomic orbitals. The numerical results obtained by using the NHO calculation based on the ab initio calculation with an STO-3G basis show that the components of the s atomic orbitals in the NHOs obtained from the two kinds of NHO calculations are very close to each other, and that the two kinds of NHOs have excellent correlations with the nuclear spin-spin coupling constants.

An ab initio nuclear magnetic resonance spectrum of vinyllithium

We present a complete ab initio nuclear magnetic resonance spectrum. All parameters in the spin Hamiltonian have been determined from a single restricted active space (RAS) multiconfigurational self-consistent field (MCSCF) wavefunction. All contributions to the indirect nuclear spin-spin coupling constants have been determined. The nuclear shieldings have been calculated using London atomic orbitals. We show that a modest sized RAS MCSCF wavefunction is capable of predicting the qualitative, and to some extent also the quantitative, features of the proton spectrum of vinyllithium. Simulated spectra are presented both for 60 and 200 MHz NMR instruments.

Ab initio calculation of maximum bond order hybrid orbitals

The maximum bond order hybrid orbital (MBOHO) procedure is tested onab initio level by use of the density matrix in Lowdin orthogonalized atomic orbital basis. The direct MBOHO calculation based on the whole density matrix includes also the hybridization of the inner atomic orbitals, and the MBOHO calculation based on the valence orbital part of the density matrix considers only the hybridization of the valence atomic orbitals. The concrete MBOHO calculations based on theab initio calculation with STO-3G basis show that the components of the s atomic orbitals in MBOHOs and the maximum bond orders obtained from the two kinds of MBOHO calculations are very close to each other, and that the two kinds of MBOHOs all have the excellent correlativity with the nuclear spin-spin coupling constants.

Theoretical study of spectroscopic properties of cubane

The outer valence ionization potentials, low-lying electronically excited states and nuclear spin-spin coupling constants of cubane have been investigated at ab initio level by using the outer valence Green function, random phase approximation and equations of motions approaches, respectively, which incorporate the main portion of the electron correlation effects. The theoretical results give an overall picture of these spectroscopic observables, which bear peculiar aspects stemming from the unique electronic structure of the cubic cage.

Calculation of nuclear spin-spin coupling constants with ab initio molecular orbital wave functions

We investigate the use of ab initio molecular wave functions to evaluate the Fermi contact contribution to nuclear spin-spin coupling constants n J AB (A, B=C, H) for a data set of 19 molecules having well-defined experimental geometry and coupling values. We critically examine the theoretical and numerical problems associated with the Dirac δ function operator and its perturbation-theoretic treatment. We show that the common approximation of neglecting basis orbitals not centered on the coupled nuclei is a major source of error in previous calculations

Maximum bond order hybrid orbitals II. Correlativity with C-H and C-C spin-coupling constants

The excellent correlativity between the maximum bond order hybrid orbitals and the nuclear spin-spin coupling constants of directly bonded C-H and C-C is shown in the present paper. The maximum bond order hybrid orbital procedure is performed by use of the first-order density matrices obtained from CNDO/2 calculation to get the bonding hybrid orbitals and the corresponding maximum bond orders for a number of hydrocarbons and hetero-substituted hydrocarbons. The relations between the obtained calculation results and the experimental coupling constants are examined by using the basic relationships proposed by Muller and Pritchard, by Maksic et al. and by Gil, and summarized in the concrete relationships which are the most suitable for the maximum bond order hybrid orbital calculation. The obtained relationships combined with the maximum bond order hybrid orbital calculation is quite successful in predicting substituent effects on the C-H and C-C coupling constants in molecules which contain no substituents of the −I− type.

Synthetic, nuclear magnetic resonance and X-ray crystallographic studies of conformations of polyorganophosphorus compounds

Addition reactions between ERH2(E = P or As; R = Ph, CH2CH2CN, Bu or C6H2But3-2,4,6) and (Ph2P)2CCH2 or (Ph2P)2CCHR′(R′= H or Me) yield the triphosphines (Ph2P)2CHCHR′ERH and the pentaphosphines [(Ph2P)2CHCH2]2ER and (Ph2P)2CHCH(Me)P(Ph)CH2CH(PPh2)2. The two- and three-bond 31P–31P nuclear spin-spin coupling constants in these vary substantially with the bulk of substituents on or near phosphorus, probably as a result of conformation differences. The structures of (Ph2P)2CHCH2PHPh, (Ph2P)2PCHCH2PH(C6H2But3-2,4,6) and [(Ph2P)2CHCH2]2PPh have been determined by X-ray diffraction and suggest that each of these species adopts a conformation in solution that is very similar to that found in the solid state.

Quantum chemical analysis of the orientational lone-pair effect on spin-spin coupling constants

The quantum-chemical analysis of factors defining the stereospecific behaviour of nuclear spin-spin coupling constants is shown to be an important aid in obtaining experimental information on molecular structures from high-resolution NMR spectroscopy. The analyses presented in this paper are based on the contributions from localized molecular orbitais within the polarization propagator approach (CLOPPA) method and examples are given where factors defining heteroatom lone-pair orientational effects on spin-spin coupling constants are discussed.