The microwave spectrum of bromine isocyanate,Brnco: Determination of rotational constants from quadrupole hyperfine structure

A novel method has been developed to evaluate accurate rotational constants from the microwave spectrum of the unstable molecule bromine isocyanate, using perturbations in nuclear quadrupole hyperfine structure. It has been applied to this prolate near-symmetric rotor to determine A_v and x_ab accurately, entirely from a-type R branches. The method has been made possible by the development of a special computer program for global léast-squares fitting to rotational and centrifugal distortion constants, along with all components of the Br nuclear quadrupole coupling tensor.     

The microwave spectrum of chloroacetyl chloride

The microwave spectrum, rotational constants and centrifugal distortion parameters for CH₂³⁵ClCO³⁵Cl are reported. The nuclear quadrupole coupling constants of the two non-equivalent Cl atoms were determined from partially resolved quadrupole splittings. The molecule is planar in the conformation studied here and both Cl atoms occupy the trans position as shown from their substitution coordinates.     

HFI and DFT study of the bonding in complexes of halogen and interhalogen diatomics with Lewis base

We have analyzed by means of DFT calculations with use of the pseudo-potential the nuclear quadrupole coupling constants of a range of XYB complexes (n aσ type in Mulliken notation) formed between diatomic interhalogen molecules XY and Lewis bases B. The geometrical parameters, rotational and halogen nuclear quadrupole coupling constants obtained by these calculations substantially corresponded to the data of microwave spectroscopy in the gas phase. An analysis of the quality of the calculations that employ the pseudo-potential and the expanded basis set for the halogen compounds was carried out. The ZORA model is shown to be a viable alternative to the computationally demanding BH and HLYP model for the calculation of halogen and nitrogen coupling constants in molecules. In addition, the ZORA model, in contrast to the pseudo-potential model, leads to realistic values of iodine nuclear quadrupole coupling constants. From electron partitioning analyses and Klopman's approach it follows that for the IClB complexes the electrostatic bonding is predominant relative to covalent bonding.     

An indo investigation of magnetic resonance constants of azaaromatic systems

INDO–MO calculations have been performed on the ESR hyperfine coupling constants of the pyridinyl radical and the radical anions of pyrimidine, quinoline, isoquinoline, acridine and benzcinnoline. The nuclear spin coupling constants and the ¹⁴N nuclear quadrupole coupling constant of pyridine have also been calculated. In general, calculated values are in satisfactory agreement with the experimental data.     

The microwave spectrum,chlorine nuclear quadrupole resonance spectrum and structure of propiolyl chloride

The microwave rotational spectra of the two most abundant isotopic species of propiolyl chloride have yielded rotational constants and nuclear quadrupole coupling constants. These, in combination with the chloride nuclear quadrupole resonance spectra, have yielded the molecular structure.     

Microwave spectra and structures of KrAuF, KrAgF, and KrAgBr; 83Kr nuclear quadrupole coupling and the nature of noble gas-noble metal halide bonding

Microwave spectra of the complexes KrAuF and KrAgBr have been measured for the first time using a cavity pulsed jet Fourier transform microwave spectrometer. The samples were prepared by laser ablation of the metal from its solid and allowing the resulting plasma to react with an appropriate precursor (Kr, plus SF6 or Br2) contained in the backing gas of the jet (usually Ar). Rotational constants; geometries; centrifugal distortion constants; vibration frequencies; and 197Au, 79Br, and 81Br nuclear quadrupole coupling constants have all been evaluated. The complexes are unusually rigid and have short Kr-Au and Kr-Ag bonds. The 197Au nuclear quadrupole coupling constant differs radically from its value in an AuF monomer. In addition 83Kr hyperfine structure has been measured for KrAuF and the previously reported complex KrAgF. The geometry of the latter has been reevaluated. Large values for the 83Kr nuclear quadrupole coupling constants have been found for both complexes. Both the 197Au and 83Kr hyperfine constants indicate a large reorganization of the electron distribution on complex formation. A thorough assessment of the nature of the noble gas-noble metal bonding in these and related complexes (NgMX; Ng is a noble gas, M is a noble metal, and X is a halogen) has been carried out. The bond lengths are compared with sums of standard atomic and ionic radii. Ab initio calculations have produced dissociation energies along with Mulliken populations and other data on the electron distributions in the complexes. The origins of the rigidity, dissociation energies, and nuclear quadrupole coupling constants are considered. It is concluded that there is strong evidence for weak noble gas-noble metal chemical bonding in the complexes.     

Nuclear quadrupole hyperfine structure in the microwave spectrum of HCl-N2O: electric field gradient perturbation of N2O by HCl

The microwave spectra of six isotopomers of HCl-N(2)O have been obtained in the 7-19 GHz region with a pulsed molecular beam, Fourier transform microwave spectrometer. The nuclear quadrupole hyperfine structure due to all quadrupolar nuclei is resolved and the spectra are analyzed using the Watson S-reduced Hamiltonian with the inclusion of nuclear quadrupole coupling interactions. The spectroscopic constants determined include rotational constants, quartic and sextic centrifugal distortion constants, and nuclear quadrupole coupling constants for each quadrupolar nucleus. Due to correlations of the structural parameters, the effective structure of the complex cannot be obtained by fitting to the spectroscopic constants of the six isotopomers. Instead, the parameters for each isotopomer are calculated from the A and C rotational constants and the chlorine nuclear quadrupole coupling constant along the a-axis, chi(aa). There are two possible structures; the one in which hydrogen of HCl interacts with the more electronegative oxygen of N(2)O is taken to represent the complex. The two subunits are approximately slipped parallel. For H (35)Cl-(14)N(2)O, the distance between the central nitrogen and chlorine is 3.5153 A and the N(2)O and HCl subunits form angles of 72.30 degrees and 119.44 degrees with this N-Cl axis, respectively. The chlorine and oxygen atoms occupy the opposite, obtuse vertices of the quadrilateral formed by O, central N, Cl, and H. Nuclear quadrupole coupling constants show that while the electric field gradient of the HCl subunit remains essentially unchanged upon complexation, there is electronic rearrangement about the two nitrogen nuclei in N(2)O.     

Nuclear hyperfine splitting in the BX electronic band system of 127I2

The spacings between seven hyperfine components in the R(127) line of the 11-6 band of the B-X electronic transition of ¹²⁷I₂ are fitted, with a standard deviation of 17.3 kHz, by varying the nuclear quadrupole coupling constants eqQ′ and egQ′', the magnetic spin—rotation constant C_I and the tensor and scalar nuclear spin—spin coupling constants d′ and a′ in the hyperfine hamiltonian. The P(13) line of the 430 band is also analysed using an identical hamiltonian and a standard deviation of 6.25 kHz is obtained. No evidence for a magnetic octupole coupling is found to the precision of the data although this effect was invoked by Hackel et al. for the P(13) line.     

Electric field gradients in the ionic crystals NaNO2, NaBF4, NaNO3, and Ba(NO3)2

The electric field gradients (EFG) at the sites of the cations and the “central” atoms of the anions in the ionic crystals NaNO₂, NaBF₄, NaNO₃ and Ba(NO₃)₂ are calculated by a method based on a combination of the semi-empirical INDO method for the charge distribution and the intramolecular EFG with a lattice summation in the framework of the extended multipole model. At some lattice sites the contribution of the induced dipole and quadrupole moments to the EFG is comparable with the contribution of the point charges. The charge distribution within the molecular ions is found by adjusting either the calculated asymmetry parameter η or the z-component of the EFG to the experimental value deduced from nuclear quadrupole coupling constants. These charge distributions are in good agreement with those gained from INDO calculations. The calculated and experimental quadrupole coupling constants of nuclei in anions and cations are compared.     

A study on the intermolecular hydrogen bonds of alpha-glycylglycine in its actual crystalline phase using ab initio calculated 14N and 2H nuclear quadrupole coupling constants

alpha-Glycylglycine in its actual crystalline phase is studied by ab initio calculated nuclear quadrupole coupling constants. These physical quantities are computed for 2H and 14N in the hydrogen bonds. The type of hydrogen bond is the N-H...O type. The computations are performed with the RHF and B3LYP methods and 6-31++G** and 6-311++G** basis sets using the Gaussian 98 program. Values of the calculated nuclear quadrupole coupling constants are shown in Tables 1-3. The aim of this work is the study of 2H and 14N quadrupole coupling constants which contribute in the CON2H...O=CN2H type of hydrogen bond. The computed nuclear quadrupole coupling constants of 2H nuclei meet the related experimental values. In addition, the computed chi value of 14N belonging to the -CO-14NH- group agrees well with values obtained experimentally. However, there are some discrepancies between calculated 14N chi values of the N+H3 residue and experiments. Also, the values of these physical parameters are calculated for >C2H2 of alpha-glycylglycine in its crystalline phase. Calculations for these parameters are carried out in a single molecule using X-ray diffraction coordinates, too.     

The rotational spectrum,nuclear spin—spin coupling,nuclear quadrupole coupling,and molecular structure of KrHF

Rotational spectra have been assigned for the ⁸²Kr, ⁸³Kr, ⁸⁴Kr, and ⁸⁶Kr isotopic species of the KrHF and KrDF van der Waals molecules by using pulsed microwave Fourier transform spectroscopy in a Fabry—Perot cavity with a pulsed supersonic nozzle molecular source. The rotational, centrifugal distortion, nuclear spin—spin, and nuclear quadrupole coupling constants are used to determine the structure and obtain intramolecular potential binding information. The ⁸³Kr nuclear quadrupole coupling constants are 10.28 ± 0.08 MHz and 13.83 ± 0.13 MHz for KrHF and KrDF respectively. The electric field gradient at the krypton nucleus is calculated from the coupling constant and the known nuclear quadrupole moment and explained by Sternheimer shielding and formation of the van der Waals bond. There is a negligible charge transfer in the KrHF bond.     

Nuclear quadrupole coupling constants of thionyl chloride

The nuclear quadrupole structure of some low J transitions with large splittings has been measured for thionyl chloride (SO³⁵Cl₂) and analysed in terms of a first order perturbation. The following nuclear quadrupole coupling constants were obtained: x_aa = ?25.01 ± 0.07 MHz, x_bb = ?0.03 ± 0.45 MHz and x_cc = 25.04 ± 0.45 MHz.     

14N quadrupole coupling in COANP

The ¹⁴N electric quadrupole coupling constants and asymmetry parameters have been determined for the three chemically non-equivalent nitrogen sites in 2-cyclo-octyl-amino-5-nitropyridine (COANP) with the help of proton-nitrogen nuclear quadrupole double resonance.     

An ab initio calculation of 14n quadrupole coupling constants

Ab initio SCF-MO calculations of ¹⁴N quadrupole coupling constants are reported for HCN, HNC, CH₃CN, CH₃NC, NH₃, NH₂NH₂, FCN, N₂O, (CN)₂, BrCN, pyridine and pyrazine. There is excellent correlation between calculation and experiment yielding Q = 1.503 ± 0.159 × 10^?26 cm² for the ¹⁴N nuclear quadrupole moment. Dunning sp basis sets are more than adequate for such calculations, STO/4G basis sets yielding almost identical results for pyridine and pyrazine. Unsuccessful attempts were made to correlate coupling constants with electronic population analysis indices.     

Theoretical description of nuclear quadrupole coupling in light diatomic molecules

A practical procedure for calculation of nuclear quadrupole coupling constants for light diatomic molecules is discussed. The procedure is based on a molecular wave function that explicitly describes nuclear motion. The approach is capable of yielding quadrupole coupling constants for excited rovibrational levels of diatomic molecules in their ground and excited electronic states. An application of the procedure to the X¹Σ⁺_g and B¹Σ⁺_u states of HD and D₂ is presented.     

Hydrogen bond studies : 79. Correlation curves for properties of hydrogen bonded water molecules based on ab initio calculations

Correlation curves have been derived from previous ab initio MO-LCAO-SCF calculations made on water molecules in hydrates. The OH-stretching frequency shifts have been correlated with: (a) ratios of the intensity of v?v_OH for bonded water to that for free water, (b) shifts in the OH-distances and (c) quadrupole coupling constants for deuterium in D₂O. Shifts in quadrupole coupling constants are also correlated with shifts in OH-distances. Comparisons have been made with experimental data and the agreement is found to be satisfactory.     

Hydrogen bond geometries from electron paramagnetic resonance and electron-nuclear double resonance parameters: density functional study of quinone radical anion-solvent interactions

Density functional theory was used to study the impact of hydrogen bonding on the p-benzosemiquinone radical anion BQ(*-) in coordination with water or alcohol molecules. After complete geometry optimizations, (1)H, (13)C, and (17)O hyperfine as well as (2)H nuclear quadrupole coupling constants and the g-tensor were computed. The suitability of different model systems with one, two, four, and 20 water molecules was tested; best agreement between theory and experiment could be obtained for the largest model system. Q-band pulse (2)H electron-nuclear double resonance (ENDOR) experiments were performed on BQ(*-) in D(2)O. They compare very well with the spectra simulated by use of the theoretical values from density functional theory. For BQ(*-) in coordination with four water or alcohol molecules, rather similar hydrogen-bond lengths between 1.75 and 1.78 A were calculated. Thus, the computed electron paramagnetic resonance (EPR) parameters are hardly distinguishable for the different solvents, in agreement with experimental findings. Furthermore, the distance dependence of the EPR parameters on the hydrogen-bond length was studied. The nuclear quadrupole and the dipolar hyperfine coupling constants of the bridging hydrogens show the expected dependencies on the H-bond length R(O.H). A correlation was obtained for the g-tensor. It is shown that the point-dipole model is suitable for the estimation of hydrogen-bond lengths from anisotropic hyperfine coupling constants of the bridging (1)H nuclei for H-bond lengths larger than approximately 1.7 A. Furthermore, the estimation of H-bond lengths from (2)H nuclear quadrupole coupling constants of bridging deuterium nuclei by empirical relations is discussed.     

Geometry and nature of the binding of the pre-reactive complex C2H4…ClF from its rotational spectrum

The encounter complex C₂H₄…ClF was isolated by using a fast-mixing nozzle before chemical reaction could occur between the components and was characterised by Fourier-transform microwave spectroscopy. Rotational constants, centrifugal distortion constants, Cl nuclear quadrupole constants and Cl spin-rotation constants were determined for the isotopomers C₂H₄…³⁵ClF and C₂H₄…³⁷ClF. The complex has C_2v symmetry with the ClF subunit perpendicular to the plane of C₂H₄ and oriented so that Cl is closer to C₂H₄. Both the centrifugal distortion constant Δ_J and the Cl nuclear quadrupole coupling constants indicate that the complex is relatively weakly bound and it is concluded that the interaction between the subunits is largely electrostatic in origin.     

Synthesis, high-resolution millimeter-wave spectroscopy, and ab initio calculations of ethylmercury hydride

The millimeter-wave rotational spectrum of an organomercury compound, ethylmercury hydride, has been recorded and assigned for the first time. The spectroscopic study is complemented by quantum chemical calculations taking into account relativistic effects on the mercury atom. The very good agreement between theoretical and experimental molecular parameters validates the chosen ab initio method, in particular its capability to predict accurate quartic centrifugal distortion constants related to this type of compound. Estimations of the nuclear quadrupole coupling constants have less predictive power than those of the structural parameters, but are good enough to satisfy the spectroscopic needs. In addition, the orientation of the axis of the H-Hg-C bonds deduced from the experimental nuclear quadrupole coupling constants compares well with the corresponding ab initio value. From the good agreement between experimental and theoretical results, together with the observation of the six most abundant isotopes of mercury, ethylmercury hydride is unambiguously identified as the product of the chemical reaction described here, and its calculated equilibrium geometry is confirmed.     

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].