Measurement of the electric quadrupole moment of CO

Measurements of the temperature dependence of the Buckingham effect (electric-field-gradient-induced birefringence, EFGIB) for gaseous carbon monoxide are presented. The measurements span the temperature range 301.2-473.9 K, which allows for separation of the temperature-independent hyperpolarizability contribution from the temperature-dependent quadrupole contribution. It is demonstrated that in the case of carbon monoxide, quantization of the rotational motion of the molecules needs to be considered, the analysis yielding a quadrupole moment of Θ = (-8.77 ± 0.31) × 10(-40) C m(2) and a hyperpolarizability term of b' = (-0.1243 ± 0.0078) × 10(-60) C(3) m(4) J(-2). For dipolar molecules, the quadrupole moment is origin dependent, and the value reported here is referred to an origin called the effective quadrupole center. Comparison of this value with the center-of-mass quadrupole moment obtained from other experiments yields information about the dynamic dipole-quadrupole and dipole-magnetic dipole polarizabilities. The temperature-independent term, which contributes (7.0 ± 0.6)% to the EFGIB at room temperature, is by no means insignificant, and must necessarily be accounted for if the quadrupole moment is to be definitively established. The measured Θ and b' are compared with the best available ab initio calculated values.     

Measurement of the electric quadrupole moment of N2O

Measurements of the temperature dependence of the Buckingham effect (electric-field-gradient-induced birefringence, EFGIB) for gaseous nitrous oxide are presented. Measurements span the temperature range 298.5-473.9 K, which allows for separation of the temperature-independent hyperpolarizability term from the temperature-dependent quadrupole term, yielding a quadrupole moment of Θ = (-11.03 ± 0.41) × 10(-40) C m(2), and a hyperpolarizability term of b = (-0.638 ± 0.063) × 10(-60) C(3) m(4) J(-2). For dipolar molecules, the quadrupole moment is origin dependent, and the value reported here is referred to an origin called the effective quadrupole center (EQC). Comparison of this value with the center of mass (CM) quadrupole moment obtained from MBER experiments yields information about the dynamic dipole-quadrupole and dipole-magnetic dipole polarizabilities. The temperature-independent term, previously assumed to contribute negligibly to the EFGIB, is found to contribute some (5.2 ± 0.6)% to the effect at room temperature and clearly needs to be accounted for if the quadrupole moment is to be definitively established.     

Nuclear quadrupole moment of 197Au from high-accuracy atomic calculations

The electric field gradient (EFG) at the gold nucleus is calculated using a finite field approach, to make the extraction of the nuclear quadrupole moment Q from experimental nuclear quadrupole coupling constants possible. The four-component Dirac-Coulomb Hamiltonian serves as the framework, 51 of the 79 electrons are correlated by the relativistic Fock-space coupled cluster method with single and double excitations, and the contribution of the Gaunt term, the main part of the Breit interaction, is evaluated. Large basis sets (up to 26s22p18d12f8g5h uncontracted Gaussians) are employed. Energy splittings of the 2D5/2 and 2D3/2 levels, rather than level shifts, are used to extract the EFG constants, as the former remain linear with Q up to 10(-5) a.u., whereas the latter display significant nonlinearity even at Q=10(-8) a.u. Larger Q values lead to larger energy changes and better precision. Excellent agreement (0.1%) is obtained between Q values derived from 2D5/2 and 2D3/2 data. Systematic errors connected with neglecting triple and higher excitations, truncating the basis and orbital active space, and approximating the Gaunt contribution are evaluated. The final value of Q(197Au) is 521(7) mb. It is lower than the muonic 547(16) mb and agrees within error bounds with the recent value of 510(15) mb obtained from molecular calculations.     

The nuclear electric quadrupole moment of antimony from the molecular method

Relativistic Dirac-Coulomb (DC) Hartree-Fock calculations are employed to obtain the analytic electric field gradient (EFG) on the antimony nucleus in the SbN, SbP, SbF, and SbCl molecules. The electronic correlation contribution to the EFGs is included with the DC-CCSD(T) and DC-CCSD-T approaches, also in the four-component framework, using a finite-difference method. The total EFG results, along with the experimental nuclear quadrupole coupling constants from microwave spectroscopy, allow to derive the nuclear quadrupole moments of (121)Sb and (123)Sb, respectively, as -543(11) and -692(14) mb.     

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.     

Arene-cation interactions of positive quadrupole moment aromatics and arene-anion interactions of negative quadrupole moment aromatics

Intermolecular interactions involving aromatic pi-electron density are widely believed to be governed by the aromatic molecular quadrupole moment, Theta(zz). Arene-cation binding is believed to occur primarily with negative Theta(zz) aromatics, and arene-anion binding is believed to occur largely with positive Theta(zz) aromatics. We have performed quantum mechanical computations that show the cation binding of positive Theta(zz) aromatics and the anion binding of negative Theta(zz) aromatics is quite common in the gas phase. The pi-electron density of hexafluorobenzene, the prototypical positive Theta(zz ) aromatic (experimental Theta(zz) = 9.5 +/- 0.5 DA), has a Li+ binding enthalpy of -4.37 kcal/mol at the MP2(full)/6-311G**level of theory. The RHF/6-311G** calculated Theta(zz) value of 1,4-dicyanobenzene is +11.81 DA, yet it has an MP2(full)/6-311G** Li+ binding enthalpy of -12.65 kcal/mol and a Na+ binding enthalpy of -3.72 kcal/mol. The pi-electron density of benzene, the prototypical negative Theta(zz) aromatic (experimental Theta(zz) = -8.7 +/- 0.5 DA), has a F- binding enthalpy of -5.51 kcal/mol. The RHF/6-311G** calculated Theta(zz) of C6H2I4 is -10.45 DA, yet it has an MP2(full)/6-311++G** calculated F- binding enthalpy of -20.13 kcal/mol. Our results show that as the aromatic Theta(zz) value increases the cation binding enthalpy decreases; a plot of cation binding enthalpies versus aromatic Theta(zz) gives a line of best of fit with R2 = 0.778. No such correlation exists between the aromatic Theta(zz) value and the anion binding enthalpy; the line of best fit has R2 = 0.297. These results are discussed in terms of electrostatic and polarizability contributions to the overall binding enthalpies.     

Nuclear quadrupole moment of 139La from relativistic electronic structure calculations of the electric field gradients in LaF, LaCl, LaBr, and LaI

Relativistic coupled cluster theory is used to determine accurate electric field gradients in order to provide a theoretical value for the nuclear quadrupole moment of (139)La. Here we used the diatomic lanthanum monohalides LaF, LaCl, LaBr, and LaI as accurate nuclear quadrupole coupling constants are available from rotational spectroscopy by Rubinoff et al. [J. Mol. Spectrosc. 218, 169 (2003)]. The resulting nuclear quadrupole moment for (139)La (0.200+/-0.006 barn) is in excellent agreement with earlier work using atomic hyperfine spectroscopy [0.20(1) barn].     

The quadrupole moment of substituted cyclopentadienyl anions

Previous work in our group on the cation binding of substituted cyclopentadienyl anions (Cp) showed the curious result that Cp traceless electric quadrupole moments (Θ(zz)) are almost all positive. Probing this issue further here we show that substituted Cp Θ(zz) values are always significantly more positive than the analogous substituted benzenes. Given the nature of aromatic Θ(zz) values, this is the opposite of what would be predicted. Furthermore, we show that the quadrupole moments of Cp anions do not behave as one would expect based on Cp substitutions. Unlike the quadrupole moments of substituted benzenes, which generally become more negative with the addition of electron-donating groups and more positive with the addition of electron-withdrawing groups, Cp quadrupole moments become more positive when any substituent is added, regardless of the electron-donating/withdrawing nature of the substituent. To explain these results we propose a model where the anionic Cp π-electron density repels the substituent electron density toward the molecular periphery and AIM calculations support this view.     

Nuclear quadrupole resonance of bromothiophenes

Br⁷⁹ NQR frequencies of a number of bromothiophene derivatives have been measured at 77°K. The frequencies correlate with Hammett δ values and this correlation demonstrates the importance of the inductive effect. The influences of more than one substituent on the thiophene ring are additive.     

Nuclear quadrupole resonance of amidinium tetrachlorophosphorates

The³⁵C1 NQR spectra and relaxation parameters of the cyclic amidinium tetrachlorophos-phorates PCl₄(NCH₃)₂CX, where X=C₆H₅ (1), CCl₃ (2), and CF₃ (3), have been studied at 77 °K. The crystal of (3) has been found to contain two coexisting phases of the compound, which consist, respectively, of zwitterionic molecules of a hexacoordinated phosphorus atom with square-pyramidal symmetry of the PCl₄ group and molecules of chlorotropic isomers with a trigonal-bipyramidal environment for the phosphorus atom. The structural features of the crystalline state and the nature of the phosphorus—ligand bonds have been discussed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 6, pp. 727–730, November–December, 1985.     

Far infrared absorption and quadrupole moment of nitrous oxide

The effects of quadrupole interaction energy and use of consistent Lennard-Jones parameters are shown to give a quadrupole moment Q = 4.05 –4.6 (in units 10^?26 esu) for N₂O from measured integrated infrared intensity due to pair interactions. The figure are considerably smaller than Baise's original Q ≈ 8 and in much better agreement with Q = 3.5 from birefringence.     

Nuclear quadrupole resonance in 2,4-dinitrochlorobenzene

A study of the spin lattice relaxation (T₁) and the nuclear quadrupole resonance frequency (ν_Q) gives an important information about the dynamics of molecular groups in molecular solids. In the present paper, we analyze the contributions of the reorientational motion of nitro groups of 2,4-dinitrochlorobenzene to the NQR parameters of the chlorine nucleus in the molecule.

We found two contributions to T₁ and ν_Q due to the onset of the reorientation of nitro groups in the molecule; one of these contributions is mostly due to intermolecular effects in the crystal. For the chlorine nuclei, the efficiency of the modulation mechanism is usually provided by the change of the electric field gradient due to the moving molecular group; this gives us a way of how to assign each contribution to T₁ from the ortho and para positions of the NO₂ groups in the molecule. It is observed that there are two different potential barriers depending on the position of the nitro groups in the molecule. The behavior in the temperature dependence of the line width shows a thermal history dependence of the molecular crystal.