Ab initio calculations of oxygen nuclear quadrupole coupling constants and oxygen and silicon NMR shielding constants in molecules containing SiO bonds

Ab initio coupled Hartree—Fock perturbation theory (CHFPT) calculations employing large gaussian basis sets have been used to evaluate the electric field gradient at O, q^o, and the NMR shieldings at O and Si, σ^o and σ^Si, in the molecules SiO, SiO₂, Si₂O₂, H₂SiO, SiO⁻⁴₄, Si(OH)₄ and (H₃Si)₂O. Species containing Si bonded to three or fewer atoms have small NMR shieldings at both O and Si while those with four-coordinate Si have systematically larger O and Si shieldings. A significant positive correlation is observed between calculated O and Si NMR shieldings. Reduction of the SiO distance in SiO⁴⁻₄ and H₃SiOSiH₃ gives a significant reduction in the magnitude of q^o and a small increase in σ. Anisotropies in σ^Si are large for two- and three-coordinate Si (200–900 ppm) but for (H₃Si)₂O the σ^Si anisotropy is only ≈60 ppm. Anisotropies in σ^o are generally larger than those in σ^Si, with values larger than 200 ppm for both SiO⁻⁴₄ and (H₃Si)₂O. Values of q^o for SiO⁴⁻₄ and (H₃Si)₂O are in qualitative agreement with experimentally determined values for nesosilicates and SiO₂ polymorphs, respectively, but all the q^o values appear to be exaggerated at the Hartree—Fock level. Also, q^o values are not greatly different for the exotic species SiO, Si₂O₂, etc. compared to the typical silicate models SiO⁴⁻₄ and (H₃Si)₂O. Calculated isotropic chemical shifts yield good values for the Si chemical shift differences of SiF₄, SiO⁴⁻₄ and (H₃Si)₂O and for the O chemical shift difference of SiO⁴⁻₄ and H₂O. For SiO⁴⁻₄ the paramagnetic contribution to the Si shift, σ^pSi is dominated by contributions from the t₂ symmetry SiO bonding orbital and σ^pO is dominated by contributions from the t₂ symmetry O 2p non-bonding orbital.     

Calculations of the indirect nuclear spin–spin coupling constants of PbH4

We report ab initio calculations of the indirect nuclear spin–spin coupling constants of PbH₄ using a basis set which was specially optimized for correlated calculations of spin–spin coupling constants. All nonrelativistic contributions and the most important part of the spin–orbit correction were evaluated at the level of the random phase approximation. Electron correlation corrections to the coupling constants were calculated with the multiconfigurational linear-response method using extended complete and restricted active space wavefunctions as well as with the second-order polarization propagator approximation and the second-order polarization propagator approximation with coupled-cluster singles and doubles amplitudes. The effects of nuclear motion were investigated by calculating the coupling constants as a function of the totally symmetric stretching coordinate. We find that the Fermi contact term dominates the Pb‐H coupling, whereas for the H‐H coupling it is not more important than the orbital paramagnetic and diamagnetic contributions. Correlation affects mainly the Fermi contact term. Its contribution to the one-bond coupling constant is reduced by correlation, independent of the method used; however, the different correlated methods give ambiguous results for the Fermi contact contribution to the H‐H couplings. The dependence of both coupling constants on the Pb‐H bond length is dominated by the change in the Fermi contact term. The geometry dependence is, however, overestimated in the random phase approximation. Received: 16 November 1998 / Accepted: 30 March 1999 / Published online: 14 July 1999     

Fluorine spin–spin coupling constants of pentafluorobenzene revisited at the ab initio correlated levels

All possible spin–spin coupling constants, ¹⁹F–¹⁹F, ¹⁹F–¹³C, and ¹⁹F–¹H, of pentafluorobenzene were calculated at five different levels of theory, HF, DFT, SOPPA (CCSD), CCSD, and the SOPPA (CCSD)-based composite scheme with taking into account solvent, vibrational, relativistic, and correlation corrections. Most corrections were next to negligible for the long-range couplings but quite essential for the one-bond carbon–fluorine coupling constants. Hartree–Fock calculations were found to be entirely unreliable, while DFT results were comparable in accuracy with the data obtained using the wave function-based methods.     

Ab initio calculations for the potential curves and spin–orbit coupling of Mg2

 The ground state and several low-lying excited states of the Mg₂ dimer have been studied by means of a combination of the complete-active-space multiconfiguration self-consistent-field (CASSCF)/CAS multireference second-order perturbation theory (CASPT2) method and coupled-cluster with single and double excitations and perturbative contribution of connected triple excitations [CCSD(T)] scheme. Reasonably good agreement with experiment has been obtained for the CCSD(T) ground-state potential curve but the dissociation energy of the only experimentally known A¹Σ _u ⁺ excited state of Mg₂ is somewhat overestimated at the CASSCF/CASPT2 level. The spectroscopic constants D _e, R _e and ω_e deduced from the calculated potential curves for other states are also reported. In addition, some spin–orbit matrix elements between the excited singlet and triplet states of Mg₂ have been evaluated as a function of internuclear separation. Received: 10 May 2001 / Accepted: 15 August 2001 / Published online: 30 October 2001     

NMR chemical shielding and spin–spin coupling constants across hydrogen bonds in uracil–α-hydroxy-N-nitrosamine complexes

One- and two bond spin–spin coupling constants, ¹ J, ^1h J , and ^2h J across X–H?O hydrogen bonds and shielding constants of bridging hydrogens have been computed for complexes formed from interaction between the α-hydroxy-N-nitrosamine (NP) and four preferential binding sites of the uracil (U) at B3LYP/6-311++G(2d,2p)//MP2/6-311++G(2d,2p) level of theory. All complexes are stabilized by two H_U?O_NP and H_NP?O_U hydrogen bonds. Very good correlations were found between NMR spin–spin coupling constant as well as isotropic shielding constant and the binding energy, H-bond distance, red-shift of vibration frequency, charge transfer energy, and electron density at H-bond critical point.     

31P and 17O nuclear spin—lattice relaxation in some phosphorylated molecules. Determination of 31P spin—rotation coupon constants, 17O quadrupolar coupling constants and chemical shielding anisotropy of the 31P nucleus

The spin—lattice relaxation time of the ³¹P nucleus was measured for 11 phosphorylated molecules (phosphine oxides, trialkylphosphates and phosphoramides) dissolved in nitromethane at three different frequencies and as a function of the temperature for three compounds. The different contributions to the relaxation rate due to dipolar, chemical shift anisotropy and spin—rotation interactions were determined and the reorientational correlation times of the molecules were deduced when the anisotropy of the chemical shift tensor of the ³¹P nucleus could be (re)determined. The quadrupolar coupling constant of the ¹⁷O nucleus was also determined from the linewidth of the nuclear magnetic resonance signals, for phosphine oxides and triphenylphosphate, giving some information on the electronic distribution into the phosphoryl bond. The spin—rotation coupling constants for trimethylphosphine oxide and triphenylphosphine oxide were deduced and the chemical shift anisotropy Δσ of trialkylphosphates estimated.     

Correlation of the 13C13C spin—spin coupling constants with the stretching force constants of single and double carbon—carbon bonds

Linear correlations between the spin—spin carbon—carbon coupling constants and the carbon—carbon stretching force constants for single and double bonds have been found through analysis of the available literature data. The corresponding equations are K^s_CC = 0.0344 J^s_CC+3.25 and K^d_CC = 0.180 J^d_CC−3.25 for single and double bonds respectively.     

One-bond CC spin—spin coupling constants in derivatives of benzene. Non-linearity of 1J(CC) vs substituent electronegativity

A set of one-bond CC coupling constants has been determined for mono- and disubstituted benzenes. Large ¹J(CC) values have been found within the benzene rings bearing highly electronegative substituents such as halogens, methoxy and nitro groups and small values for those with electropositive substituents. The total range of ¹J(CC) couplings observed in our work is larger than 50 Hz. These large variations of CC spin—spin couplings are interpreted in terms of Fermi-contact contributions and the redistribution of s-electrons within a CC bond under influence of substituents. Contrary to some previous findings the data obtained in the present work indicate that the relationship between ¹J(CC) and the substituent electronegativity is not linear.     

Ab initio study of the transition-metal carbene cations

The geometries and bonding characteristics of the first-row transition-metal carbene cations MCH_2~ were investigated by ab initio molecular orbital theory (HF/LANL2DZ). All of MCH_2~ are coplanar. In the closed shell structures the C bonds to M with double bonds; while in the open shell structures the partial double bonds are formed, because one of the σ and π orbitals is singly occupied. It is mainly the π-type overlap between the 2p_x orbital of C and 4p_x, 3d_(xz), orbitals of M~ that forms the π orbitals. The dissociation energies of C—M bond appear in periodic trend from Sc to Cu. Most of the calculated bond dissociation energies are close to the experimental ones.     

LCAO Xα calculations of nuclear magnetic shielding in molecules

On the basis of the four-current density functional formalism, a method to calculate chemical shifts is developed, allowing the use of Xα wavefunctions in computations of magnetic properties of molecules. First results of calculated nuclear magnetic shielding constants and diamagnetic susceptibilities in small molecules using a LCAO Xα scheme within a minimal basis set are presented and compared with values obtained by CHF, UCHF, and FPT methods and with experiment. The CHF results show better agreement with experiment than our calculations, but our results are of the quality of UCHF data.     

Ab initio study of tautomerism and hydrogen bonding of β-carbonylamine in the gas phase and in water solution

All the possible conformations of the three tautomeric isomers of simple β-carbonylamine were fully optimized at ab initio MP2/6-31G** and B3LYP/6-31G** levels in order to determine the conformational equilibrium and the energies of the O—H···N and O···H—N hydrogen bridges. For the most interesting conformations, further calculations in water solution were also carried out. It was found that carbonylamine is the most stable tautomer, followed by enolimine and carbonylimine. This order of stability does not change in solution. O—H···N is the strongest hydrogen bridge, but in solution its energy as well as that of the O···H—N one are dramatically lowered. The deprotonation energy was also calculated and discussed. Received: 16 September 1999 / Accepted: 3 February 2000 / Published online: 5 June 2000     

The nuclear magnetic shielding constants of formamide: the contribution of the non-tightly bound water molecules of the “first hydration shell”

The magnetic shielding constants of the nuclei of formamide are calculated for the isolated molecule, the molecule surrounded by its “first hydration shell” obtained from a Monte Carlo treatment, and the molecule surrounded only by the four water molecules engaged in the short solute—solvent hydrogen bonds. The results show that the six “remote” waters produce important chemical shift variations which in the case of the NH protons, are qualitatively determining. They are due mainly to a variation of the polarization of the wavefunction of the solute.     

Vibronic spin—orbit coupling in the phosphorescence of 4-hydroxypyridine

A phosphorescence emission and polarization study of 4-hydroxypyridine has been performed in EPA and in alkaline EPA. In contrast to the lack of luminescence from pyridine, 4-hydroxypyridine phosphoresces with φ_p = 0.40 (77°K) in EPA and τ_p = 0.27 sec. The 00 band of phosphorescence in EPA is clearly resolved at 29,940 cm⁻¹ and is out of plane polarized. An out of plane vibronic perturbation is evident in the polarization curve and is believed to arise from second order effects involving vibronic spin—orbit coupling. The out of plane vibration contributes in plane polarization outside the 00 band. The positive polarization of the 00 band in alkaline EPA is attributed to the inversion of two close lying perpendicularly polarized singlet states. No phosphorescence was observed in methylcyclohexane. The lowest triplet is assigned as π,π*³.     

Hydrogen bonding of the hydroxyl group and spin—spin coupling of the ring protons of substituted phenols

The chemical shifts and coupling constants of the hydroxyl and ring protons of the following substituted phenols and anisoles have been obtained in basic and nonbasic solvents: 2-adamant-1-yl-4-tert-butylphenol, 6-bromo-2-adamant-1-yl-4-tert-butylphenol, 3,5-di-tert-butylphenol, 3,5-dichlorophenol, 6-bromo-2-adamant-1-yl-4-tert-butyl anisole and 6-bromo-2,4-di-tert-butyl anisole. Hydrogen bonding of the hydroxyl group of the substituted phenols to the solvent does not cause any detectable change in the spin—spin coupling between the meta protons, but causes a very small increase (<0.10 Hz) in the coupling between the ortho and para protons.     

Ab initio study of the reaction path of the reaction HNCO+CN

HNCO is a convenient photolytic source of NCO and NH radicals for laboratory kinetics studies of elementary reaction[1] and plays an important role in the combustion and atmosphere chemistry. It can re- move deleterious compounds rapidly from exhausted ga…     

Ab initio configuration interaction study of the structure and magnetic properties of radicals and radical ions derived from group 13–15 trihydrides

The structures, characteristic vibrations and magnetic properties of two isoelectronic series of radicals and radical ions derived from group 13–15 trihydrides have been investigated by post-Hartree-Fock theoretical techniques. Møller-Plesset perturbation theory based on an unrestricted Hartree-Fock determinant has been employed to determine the structures and vibrational frequencies in the 9-electron series, BH⁻₃, CH₃, and NH⁺₃. These species are found to be planar. Spin density distributions and ionization energetics have been estimated using a variational configuration interaction procedure. A positive electron affinity for BH₃ has not been demonstrated. The effect of out-of-plane vibrations on the hyperfine coupling constants is determined at a similar level of theory. In the 17-electron series AlH⁻₃, SiH₃, and PH⁺₃, pyramidal structures are found by using and extended split-valence basis at the SCF level. The computed harmonic force field suggests that a tentative assignment of a matrix isolated infrared spectrum to SiH₃ is incorrect. This conclusion is reinforced by calculation of the vibrational intensity patterns. Hyperfine interaction tensors computed at the optimized geometries from the UHF wavefunction with a more complete polarized double-zeta basis set are in accord with experiment. Vibrational effects are estimated by averaging the UHF spin density over an energy surface determined by second-order perturbation theory. Corrections due to vibrations are smaller than in the carbon series and single-point configuration interaction calculations confirm the reliability of the UHF spin densities.     

Ab initio study of dynamical E × e Jahn-Teller and spin-orbit coupling effects in the transition-metal trifluorides TiF3, CrF3, and NiF3

Multiconfiguration ab initio methods have been employed to study the effects of Jahn-Teller (JT) and spin-orbit (SO) coupling in the transition-metal trifluorides TiF(3), CrF(3), and NiF(3), which possess spatially doubly degenerate excited states ((M)E) of even spin multiplicities (M = 2 or 4). The ground states of TiF(3), CrF(3), and NiF(3) are nondegenerate and exhibit minima of D(3h) symmetry. Potential-energy surfaces of spatially degenerate excited states have been calculated using the state-averaged complete-active-space self-consistent-field method. SO coupling is described by the matrix elements of the Breit-Pauli operator. Linear and higher order JT coupling constants for the JT-active bending and stretching modes as well as SO-coupling constants have been determined. Vibronic spectra of JT-active excited electronic states have been calculated, using JT Hamiltonians for trigonal systems with inclusion of SO coupling. The effect of higher order (up to sixth order) JT couplings on the vibronic spectra has been investigated for selected electronic states and vibrational modes with particularly strong JT couplings. While the weak SO couplings in TiF(3) and CrF(3) are almost completely quenched by the strong JT couplings, the stronger SO coupling in NiF(3) is only partially quenched by JT coupling.     

Ring-closing metathesis as a new strategy for the C–C coupling of monosaccharide derivatives via silicon tethering

Suitable carbohydrate-derived terminal olefins have been coupled by ring-closing metathesis reaction following a silicon tethering strategy, generating very long contiguous chains of carbon atoms each equipped with specific stereochemistry and functionality.