Nuclear electric quadrupole moments of Rb from the hyperfine spectrum of RbF

The molecular beam electric resonance technique has been used to examine the hyperfine spectrum of RbF. The Rb nuclear electric quadrupole interaction, the spin-rotation interactions, and tensor and scalar spin-spin interactions have been measured for both Rb isotopes, including their dependence on vibrational and rotational states. Transition frequencies have been determined to a precision of better than 1 Hz in many cases. The magnetic interactions in the two isotopomers are consistent with what is expected from the known masses and magnetic dipole moments. In the case of the Rb nuclear electric quadrupole interaction, adjustments have been made for a small isotopomer shift, and for the ratio of the effective nuclear electric quadrupole moments, Q(87Rb)Q(85Rb) = 0.483 830 1+/-0.000 001 8. The effective quadrupole interaction includes a pseudoquadrupole interaction that may be significant at this level of precision, but cannot be distinguished experimentally.     

Full configuration interaction calculation of singlet excited states of Be3

The full configuration interaction (FCI) study of the singlets vertical spectrum of the neutral beryllium trimer has been performed using atomic natural orbitals [3s2p1d] basis set. The FCI triangular equilibrium structure of the ground state has been used to calculate the FCI vertical excitation energies up to 4.8 eV. The FCI vertical ionization potential for the same geometry and basis set amounts to 7.6292 eV. The FCI dipole and quadrupole transition moments from the ground state are reported as well. The FCI electric quadrupole moment of the X (3)A(1) (') ground state has been also calculated with the same basis set (Theta(zz)=-2.6461 a.u., Theta(xx)=Theta(yy)=-1/2Theta(zz)). Twelve of the 19 calculated excited singlets are doubly excited states. Most of the states have large multiconfigurational character. These results provide benchmark values for electronic correlation multireference methods. (4ex6MO)CAS-SDCI values for the same energies and properties are also reported.     

Density-functional theory study of electric and magnetic properties of hexafluorobenzene in the vapor phase

A series of electric and magnetic properties of hexafluorobenzene have been calculated, including the electric dipole polarizability, magnetizability, electric quadrupole moment, and nonlinear mixed electric dipole-magnetic dipole-electric quadrupole hyperpolarizabilities needed to obtain estimates of the Kerr, Cotton-Mouton, Buckingham, Jones, and magnetoelectric birefringences in the vapor phase. Time-dependent density-functional theory was employed for the calculation of linear-, quadratic, and cubic response functions. A number of density functionals have been considered, along with Sadlej's triple-zeta basis set and the augmented correlation-consistent polarized valence double zeta and augmented correlation-consistent polarized valence triple zeta basis sets. Comparisons have been made with experiment where possible. The analysis of results allows for an assessment of the capability of time-dependent density-functional theory for high-order electromagnetic properties of an electron-rich system such as hexafluorobenzene.     

Electric dipole polarity of diatomic molecules

The restricted SCF (single-configuration SCF) and MCSCF (multiconfiguration SCF) calculations are performed to compute the ground-state electric dipole moments of four pairs of diatomic molecules—(1) CO and BF; (2) SiO and AlF; (3) CS and BCl; and (4) SiS and AlCl—at a number of internuclear distances on both sides of the equilibrium position. Near Hartree–Fock accuracy is obtained in the SCF calculations. All eight molecules have a range of internuclear distance in which electric dipole moments are of the polarity of A^?B⁺. The shapes of computed electric dipole moment functions are discussed in the language of the molecular orbital method and in relationship to electronegativities of atoms. The present study gives us deeper understanding of electron transfer inside molecules and consequently of the apparent contradiction between electronegativity and the dipole polarity of some molecules. © John Wiley & Sons, Inc.     

The quadrupole moment of the 3/2+ nuclear ground state of 197Au from electric field gradient relativistic coupled cluster and density-functional theory of small molecules and the solid state

An attempt is made to improve the currently accepted muonic value for the 197Au nuclear quadrupole moment [+0.547(16)x10(-28) m2] for the 3/2+ nuclear ground state obtained by Powers et al. [Nucl. Phys. A230, 413 (1974)]. From both measured Mossbauer electric quadrupole splittings and solid-state density-functional calculations for a large number of gold compounds a nuclear quadrupole moment of +0.60x10(-28) m2 is obtained. Recent Fourier transform microwave measurements for gas-phase AuF, AuCl, AuBr, and AuI give accurate bond distances and nuclear quadrupole coupling constants for the 197Au isotope. However, four-component relativistic density-functional calculations for these molecules yield unreliable results for the 197Au nuclear quadrupole moment. Relativistic singles-doubles coupled cluster calculations including perturbative triples [CCSD(T) level of theory] for these diatomic systems are also inaccurate because of large cancellation effects between different field gradient contributions subsequently leading to very small field gradients. Here one needs very large basis sets and has to go beyond the standard CCSD(T) procedure to obtain any reliable field gradients for gold. From recent microwave experiments by Gerry and co-workers [Inorg. Chem. 40, 6123 (2001)] a significantly enhanced (197)Au nuclear quadrupole coupling constant in (CO)AuF compared to free AuF is observed. Here, these cancellation effects are less important, and relativistic CCSD(T) calculations finally give a nuclear quadrupole moment of +0.64x10(-28) m2 for 197Au. It is argued that it is currently very difficult to improve on the already published muonic value for the 197Au nuclear quadrupole moment.     

Angular Distribution of the Multipole and Multiphoton Photofragmentation of Molecules

The effect of higher-order photo-excitation on the angular distribution of photo-dissociated molecular fragments is studied. A general formalism is derived that will cover non-linear as well as linear polyatomic molecules and will permit comparison of the processes of different orders. Some of the higher-order processes, e.g., magnetic dipole and electric quadrupole, will give rise to azimuthal-angle (Φ) besides polar-angle (θ) dependence of the fragments. Using a molecule of D_3h, Symmetry as an example, a comprehensive study is made of all possible excitations allowed by symmetry in this point group for one-photon electric/magnetic dipole and three-photon electric-dipole transitions. Plane wave approximation is used for the relative motion of the random and non-rotating molecules which dissociate instantaneously with a velocity large compared with thermal motion. These basic angular factors may be used in future extensions to rotating and vibrating molecules and to time-delayed dissociations and to vibronically excited fragment states, etc.     

Structure and dynamics of ND3BF3 in the solid and gas phases: a combined NMR,neutron diffraction,and Ab initio study

The decrease in D-->A bond lengths, previously reported for some Lewis acid/base complexes, in going from the gas to the solid phases is investigated by obtaining an accurate crystal structure of solid ND(3)BF(3) by powder neutron diffraction. The B-N internuclear distance is 1.554(3) A, 0.118 A shorter than the most recent gas-phase microwave value and 0.121 A shorter than the single molecule geometry optimized (1.672 A, CISD/6-311++G(d,p)) bond length. The crystal structure also shows N-D.F-B hydrogen bonds. The effects of this change in structure and of intermolecular hydrogen-bonding on nuclear magnetic shielding (i.e., chemical shifts) and the nuclear quadrupolar coupling constants (QCC) are investigated by ab initio molecular orbital and density functional theory calculations. These calculations show that the nitrogen ((15)N and (14)N) and boron ((11)B and (10)B) chemical shifts should be rather insensitive to changes in r(BN) and that the concomitant changes in molecular structure. Calculations on hydrogen-bonded clusters, based on the crystal structure, indicate that H-bonding should also have very little effect on the chemical shifts. On the other hand, the (11)B and (14)N QCCs show large changes because of both effects. An analysis of the (10)B[(19)F] line shape in solid ND(3)(10)BF(3) yields a (11)B QCC of +/-0.130 MHz. This is reasonably close an earlier value of +/-0.080 MHz and the value of +/-0.050 MHz calculated for a [NH(3)BF(3)](4) cluster. The gas-phase value is 1.20 MHz. Temperature-dependent deuterium T(1) measurements yield an activation energy for rotation of the ND(3) group in solid ND(3)BF(3) of 9.5 +/- 0.1 kJ/mol. Simulations of the temperature-dependent T(1) anisotropy gave an E(a) of 9.5 +/- 0.2 kJ/mol and a preexponential factor, A, of 3.0 +/- 0.1 x 10(12) s(-)(1). Our calculated value for a [NH(3)BF(3)](4) cluster is 16.4 kJ/mol. Both are much higher than the previous value of 3.9 kJ/mol, from solid-state proton T(1) measurements.     

Synthesis, structure, and magnetic properties of [(CH3CN)5V-O-V(CH3CN)5][BF4]4

The reaction of vanadium(III) acetylacetonate with HBF4 in acetonitrile yields [(CH3CN)5V-O-V(CH3CN)5][BF4]4, a material that serves as a convenient precursor to other [V-O-V]4+ species such as [(bipy)2(CH3CN)V-O-V(CH3CN)(bipy)2][BF4]4 (bipy=2,2'-bipyridine). Single-crystal X-ray diffraction shows that the V-O-V linkage of [(CH3CN)5V-O-V(CH3CN)5]4+ is linear. An Evans method measurement of the solution-phase magnetic susceptibility indicates strong ferromagnetic coupling between the vanadium centers. Magnetic susceptibility (chi) and magnetization (M(H)) data for a powdered sample and for a single crystal oriented with its V-O-V axis parallel to the applied field were measured over 1.8-300 K. The results suggest that the V(III) centers are ferromagnetically coupled with J approximately 72 K (approximately 50 cm(-1)) yielding a ground state with a total spin Stotal=2. Theoretical fit to the M(H) plot for the single crystal yielded g||=2.01+/-0.01 and the zero-field splitting parameter D=0.60+/-0.04 K (0.42+/-0.03 cm(-1)). EPR measurements at 34 and 101.6 GHz are consistent with the Stotal=2 ground state and yield g||=1.9825, g perpendicular=1.9725 and D=0.57+/-0.03 K.     

Permanent moment contributions to chiral discrimination

The discrimination energy, giving the difference in the interaction of two leavo molecules and one laevo with one dextro, is calculated where the coupling involves permanent electric and magnetic moments. Fixed electric dipoles arranged helically on a lattice (simulating the electric effects of a helical polymer for example) give chiral fields producing large (≈kT) discriminations. For the pairwise interaction of fixed molecules realistic electric dipole and electric quadrupole moments can lead to discriminations of about 100 cal mole^?1 at 0.5 nm separation. In the hypothetical case of molecules possessing permanent electric and magnetic moments it is four or more orders of magnitude less. In a freely rotating molecule pair the sequence is reversed and electric and magnetic dipoles give much the bigger averaged discrimination energy.     

Schwingungsspektren isotopensubstituierter B-Trifluor-und - Trichlor-Borazole

Vibrational Spectra of Isotope-labelled B-Trifluoro and B-Trichloro Borazines The infrared and Raman spectra of (BF? NH)₃, (¹⁰BF? NH)₃, (BF? ND)₃, (BCl? NH)₃, (¹⁰BCl? NH)₃, (BCl—¹⁵NH)₃, (¹⁰BCl—¹⁵NH)₃, (BCl? ND)₃, (¹⁰BCl? ND)₃, (BCl—¹⁵ND)₃ and (¹⁰BCl—¹⁵ND)₃ have been recorded. Most of the recent assignments are confirmed and supplemented by heretofore missing Raman frequencies of species A′₁ and E″. For six of the seven vibrations of species E′ of (BF—NH)₃, the Coriolis coupling constants have been computed from the band envelopes of the gas phase infrared spectra.     

Chiroptical properties of an optically pure dicopper(I) trefoil knot and its enantioselectivity in luminescence quenching reactions

Chiroptical spectroscopy is used to investigate the properties of an optically pure dinuclear copper(I) trefoil knot. For the metal-to-ligand charge tranfer (MLCT) transition in the visible region (520 nm), the electric and magnetic transition dipole moments are determined from absorption and circular dichroism spectra: 2.8 Debye and 0.5 Bohr magneton (muB). Circular polarization in the luminescence (CPL) of the knot is determined and this allows the electric and magnetic transition dipole moments in emission to be calculated: 0.02 Debye and 0.003 muB. The large difference between the moments in absorption and emission shows that the emission observed does not originate directly from the 1MLCT state. Given the low probability for radiative decay we assign the long-lived emitting excited state to a 3MLCT state. The copper(I) trefoil knot is found to quench the emission from TbIII and EuIII(dpa)3(3)-(dpa = pyridine-2,6-dicarboxylate) with a bimolecular rate constant of 3.2 and 3.3 x 10(7)M(-1)S(-1), respectively, at room temperature in water-acetonitrile (1:1 by volume). Experimental results indicate that the (lambda)-knot preferentially quenches the lambda enantiomer of the lanthanide complex with an enantioselectivity (ratio of quenching rate constants for lambda and lambda: kqlambda/kqdelta) of 1.012+/-0.002 for EuIII and 1.0180+/-0.003 for TbIII.     

Electronic structure of the organic half-metallic magnet: 2-(5-pyrimidinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-3-oxoimidazol-1-oxyl

An accurate full-potential density-functional method is used to study the magnetic and half-metallic properties in the pure organic materials: 2-(5-pyrimidinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-3-oxoimidazol-1-oxyl. The total and partial density of states and atomic spin magnetic moments are calculated and discussed. It is found that the unpaired electrons in this compound are localized in a molecular orbital constituted primarily of π^*(NO) orbital, and the main contribution of the spin magnetic moment comes from the NO free radicals. It is predicted that this compound is half-metallic magnet. It is also found that there exists ferromagnetic intermolecular interaction in the compound.     

An 17O NMR and quantum chemical study of monoclinic and orthorhombic polymorphs of triphenylphosphine oxide

Solid-state (17)O NMR spectroscopy is employed to characterize powdered samples of known monoclinic and orthorhombic modifications of (17)O-enriched triphenylphosphine oxide, Ph(3)PO. Precise data on the orientation-dependent (17)O electric field gradient (EFG) and chemical shift (CS) tensors are obtained for both polymorphs. While the (17)O nuclear quadrupolar coupling constants (C(Q)) are essentially identical for the two polymorphs (C(Q) = -4.59 +/- 0.01 MHz (orthorhombic); C(Q) = -4.57 +/- 0.01 MHz (monoclinic)), the spans (Omega) of the CS tensors are distinctly different (Omega = 135 +/- 3 ppm (orthorhombic); Omega = 155 +/- 5 ppm (monoclinic)). The oxygen CS tensor is discussed in terms of Ramsey's theory and the electronic structure of the phosphorus-oxygen bond. The NMR results favor the hemipolar sigma-bonded R(3)P(+)-O(-) end of the resonance structure continuum over the multiple bond representation. Indirect nuclear spin-spin (J) coupling between (31)P and (17)O is observed directly in (17)O magic-angle-spinning (MAS) NMR spectra as well as in (31)P MAS NMR spectra. Ab initio and density-functional theory calculations of the (17)O EFG, CS, and (1)J((31)P,(17)O) tensors have been performed with a variety of basis sets to complement the experimental data. This work describes an interesting spin system for which the CS, quadrupolar, J, and direct dipolar interactions all contribute significantly to the observed (17)O NMR spectra and demonstrates the wealth of information which is available from NMR studies of solid materials.     

Ab initio characterization of XH3 (X = N,P). Part II. Electric, magnetic and spectroscopic properties of ammonia and phosphine

The coupled cluster theory in conjunction with core valence triple and quadruple zeta basis sets has been employed for investigating electric, magnetic and spectroscopic properties of ammonia and phosphine. Namely molecular dipole and quadrupole moments, NMR shielding and spin-rotation constants, as well as spectroscopic properties such as rotational and centrifugal distortion constants as well as harmonic and anharmonic frequencies of NH₃ and PH₃ have been determined at a high level of accuracy. To obtain parameters directly comparable to experiment, vibrational effects have also been taken into account. In addition, the basis set convergence has been investigated for the molecular dipole moment.     

Comment on a sum rule for vibrational raman optical activity

The expressions for the sum of vibrational Raman optical activity (ROA) intensities indicate that the ROA intensity sum for chiral molecules is non-zero for those with an anisotropic electric dipole polarizability. The non-zero sum depends upon the electric dipole, magnetic dipole and electric quadrupole polarizability components and moments of inertia at equilibrium geometry.     

Resonance hyper-Raman scattering of fullerene C60 microcrystals

The hyper-Raman spectrum of buckminsterfullerene C60 was observed at room temperature. The spectrum clearly showed infrared-active modes with t(1u) symmetry and the silent modes with t(2u), g(u), and h(u) as a result of electronic resonances via the Herzberg-Teller mechanism. Moreover, Raman-active modes with a(g) and h(g) were also detected, although these were hyper-Raman-forbidden in the electric dipole approximation, suggesting a contribution of higher-order nonlinear processes such as magnetic dipole transitions. These results suggest that hyper-Raman spectroscopy and microscopy are indeed widely applicable in the field of molecular science.     

First principles study of the stability and electronic structure of the icosahedral La13, La(-1) (13), and La(+1) (13) clusters

The structural stability and electronic-structure of icosahedral La(13), La(-1) (13), and La(+1) (13) clusters have been studied by DMOL cluster method based on density-functional theory. The ground state of all-electron with relativity results is shown to be a distorted D(2h) icosahedron by the Jahn-Teller effect. However, the binding energies of D(3d) and D(5d) are very close to that of the D(2h) structure for La(13), La(-1) (13), and La(+1) (13) clusters. The effective core potential results show that the true ground state is D(5d) structure. The clusters have small magnetic moments and the symmetry of cluster is an important factor in determining the magnetic moments of the clusters. The effects of interatomic spacing and coordination on atomic magnetic moment are discussed. Further, 5d electrons dominate the hybrid orbitals below the Fermi level in the neutral cluster and contribute the main spin of clusters.     

An SCF-MS-Xα study of the bonding and nuclear quadrupole coupling in diatomic halogens and interhalogens in the ground X Σ and B Π 0 excited states

SCF-MS-Xα calculations of the electronic structure of diatomic halogens and interhalogens XY (X = I, Br, Cl; Y = I, Br, Cl, F) have been used to investigate the bonding and nuclear quadrupole coupling in these molecules. Calculations have been carried out for the ground X ¹ Σ electronic state, and for the excited B ³ Π₀ state in the case of I₂, Br₂, ICl and IBr. Good agreement (to within 10% in most cases) is obtained between the calculated and observed nuclear quadrupole coupling constants for the molecules in the ground state. For the excited state the agreement is not as good, but the calculation does reproduce the observed decrease in the coupling constants to less than one quarter of their ground state values, and analysis of the contributions to the field gradients clearly shows the reasons for this. The electric dipole moments and electric quadrupole moments of the molecules have also been calculated. However, these prove to be much more strongly dependent on the variables used in the calculation (atomic sphere radii, inclusion of d orbitals). The results of the calculations have also been used to test some of the assumptions made in the Townes and Dailey method of analysis of nuclear quadrupole coupling data.     

Molecular photoionization cross sections in electron propagator theory: angular distributions beyond the dipole approximation

Corrections to dipole approximation results for angular distributions in photoionization of first-row hydrides have determined by using Dyson orbitals calculated with ab initio electron propagator theory and by considering the full multipole expansion for the incident photon representation. The relative importance of first-order corrections which consist of electric quadrupole and magnetic dipole terms and of higher-order terms has been estimated as a function of photon energy. Multipole corrections to the dipole approximation depend on photon energy and on the characteristics of the Dyson orbitals.     

BF3-activated oxidation of alkanes by MnO4 -

The oxidation of alkanes and arylalkanes by KMnO(4) in CH(3)CN is greatly accelerated by the presence of just a few equivalents of BF(3), the reaction occurring readily at room temperature. Carbonyl compounds are the predominant products in the oxidation of secondary C-H bonds. Spectrophotometric and kinetics studies show that BF(3) forms an adduct with KMnO(4) in CH(3)CN, [BF(3).MnO(4)](-), which is the active species responsible for the oxidation of C-H bonds. The rate constant for the oxidation of toluene by [BF(3).MnO(4)](-) is over 7 orders of magnitude faster than by MnO(4)(-) alone. The kinetic isotope effects for the oxidation of cyclohexane, toluene, and ethylbenzene at 25.0 degrees C are as follows: k(C6H12)/k(C6D12) = 5.3 +/- 0.6, k(C7H8)/k(C7D8) = 6.8 +/- 0.5, k(C8H10)/k(C8D10) = 7.1 +/- 0.5. The rate-limiting step for all of these reactions is most likely hydrogen-atom transfer from the substrate to an oxo group of the adduct. A good linear correlation between log(rate constant) and C-H bond energies of the hydrocarbons is found. The accelerating effect of BF(3) on the oxidation of methane by MnO(4)(-) has been studied computationally by the Density Functional Theory (DFT) method. A significant decrease in the reaction barrier results from BF(3) coordination to MnO(4)(-). The BF(3) coordination increases the ability of the Mn metal center to achieve a d(1) Mn(VI) electron configuration in the transition state. Calculations also indicate that the species [2BF(3).MnO(4)](-) is more reactive than [BF(3).MnO(4)](-).