Halogen Nuclear Quadrupole Coupling Constants Research Articles

Systematic behaviour of electron redistribution on formation of halogen-bonded complexes BXY, as determined via XY halogen nuclear quadrupole coupling constants.

Equilibrium nuclear quadrupole coupling constants associated with the di-halogen molecule XY in each of 60 complexes BXY (where B is one of the Lewis bases N2, CO, HCN, H2O, H2S, HCCH, C2H4, PH3, NH3 or (CH3)3N and XY is one of the di-halogens Cl2, BrCl, Br2, ICl, IBr or I2) have been calculated ab initio. The Townes-Dailey model for interpreting the changes in the coupling constants when XY enters the complex was used to describe the electron redistribution in the di-halogen molecule in terms of the fraction δi of an electron transferred from the Lewis base B to atom X and the fraction δp of an electron transferred simultaneously from atom X to atom Y. Systematic relationships between the δi values for the six series are established. It is shown that, in reasonable approximation, δi decays exponentially as the first ionisation energy IB of the Lewis base B increases, that is δi = A exp(-bIB). It is concluded from the results for the series BBrCl, BBr2, BICl, BIBr and BI2 that the coefficients A and b in regression fits to the corresponding logarithmic version ln(δi) = ln(A) -b(IB) of the equation are not strongly dependent on either the halogen atom X directly involved in the halogen bond in BXY or, for a given X, on the nature of Y. The behaviour of PH3 as a Lewis base appears to be anomalous. Values of δi and δp calculated by the quantum theory of atoms-in-molecules and natural bond orbital methodologies are very close to those from application of the Townes-Dailey approach described.

Quadrupole coupling constants and isomeric Mössbauer shifts for halogen-containing gold, platinum, niobium, tantalum and antimony compounds

We have analyzed by means of Density functional theory calculations the nuclear quadrupole coupling constants of a range of gold, antimony, platinum, niobium and tantalum compounds. The geometrical parameters and halogen nuclear quadrupole coupling constants obtained by these calculations substantially corresponded to the data of microwave and nuclear quadrupole resonance spectroscopy. An analysis of the quality of the calculations that employ pseudo-potentials and all-electron basis sets for the halogen compounds was carried out. The zero order regular approximation (ZORA) method is shown to be a viable alternative for the calculation of halogen coupling constants in molecules. In addition, the ZORA model, in contrast to the pseudo-potential model, leads to realistic values of all metal nuclear quadrupole coupling constants. From Klopman’s approach, it follows that the relationship between the electrostatic bonding and covalent depends on the nature of the central atom. The results on Mossbauer chemical shifts are also in a good agreement with the coordination number of the central atom.

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 XY⋯B 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 ICl⋯B complexes the electrostatic bonding is predominant relative to covalent bonding.

The pure rotational spectra of the lanthanum monohalides, LaF, LaCl, LaBr, LaI

Pure rotational spectra have been measured for all the major isotopomers of the lanthanum monohalides, LaF, LaCl, LaBr, and LaI, in their ground and (except for La 81 Br ) excited vibrational states. The spectra were observed with a cavity pulsed jet Fourier transform microwave spectrometer in the frequency range 5–24 GHz. The molecules were prepared by laser ablation of La metal and allowing the resulting plasma to react with SF 6, Cl 2, Br 2, or CH 3I precursor in an Ar carrier gas of the pulsed jet. For LaBr this is the first reported spectrum of any kind. Rotational constants, centrifugal distortion constants, nuclear quadrupole coupling constants, and nuclear spin–rotation constants have been determined for all the molecules. Accurate equilibrium ( r e ) internuclear distances have given an indication of where the Born–Oppenheimer approximation is beginning to fail. From the centrifugal distortion constants and vibration–rotation ( α e ) constants good estimates of the harmonic vibration frequencies and bond dissociation energies have been obtained. The halogen nuclear quadrupole coupling constants indicate the molecules to be highly ionic.

Microwave spectra, geometries, and hyperfine constants of OCAgX (X = F, Cl, Br).

A pulsed jet cavity Fourier transform microwave spectrometer has been used to measure the rotational spectra of OCAgX (X = F, Cl, Br) in the frequency range 5-22 GHz. Metal atoms were generated via laser ablation and were allowed to react with CO and a halide precursor, prior to stabilization of the products within a supersonic jet of argon. These are the first experimental observations of OCAgF and OCAgBr, and the first high resolution spectroscopic study of OCAgCl. All three molecules are linear. Accurately determined rotational constants have been used to evaluate the various internuclear distances, which are found to be consistent with trends established for OCAuX and OCCuX species. The C-O distances are short, and the M-C distances are significantly longer than those in other molecules containing a metal-carbonyl bond. Precise values of centrifugal distortion constants and halogen nuclear quadrupole coupling constants have also been determined. The coupling constants are compared with the results of previous studies of OCCuX and OCAuX and are used to infer trends in the electron distributions of the molecules. Ab initio calculations have been performed and employed to predict the geometries, vibrational frequencies, and Mulliken valence orbital populations of the various species.

Rotational spectroscopy of the gas phase complex of water and bromine monochloride in the microwave region: Geometry, binding strength and charge transfer

Pulsed-nozzle Fourier transform microwave spectroscopy has been used to measure the ground-state rotational spectra of eight isotopomers H2O···79Br35Cl, H2O···81Br35Cl, H2O···79Br37Cl, H2O···81Br37Cl, D2O···79Br35Cl, D2O···81Br35Cl, HDO···79Br35Cl and HDO···81Br35Cl of a complex formed by water and bromine monochloride. Fitting of measured frequencies yields the spectroscopic constants B0, C0, ΔJ, ΔJK, χaa(X), [χbb(X) − χcc(X)] and Mbb(X) for each of the first six species. For HDO containing isotopomers only K−1 = 0 transitions were observed and hence only (B0 + C0)/2, ΔJ, χaa(X) and Mbb(X) were determinable. The geometry of the complex is shown to have the arrangement O···Br–Cl of the heavy nuclei, with a weak bond formed between O and Br. Analysis shows that the zero-point state of the complex is effectively planar (C2v) with a low potential energy barrier to the motion that interconverts the two equivalent equilibrium conformers of Cs symmetry. The distance r(O···Br) is determined to be 2.7809(3) A and the out-of-plane angle, ϕ = 48.0(2)° in the ground state. The Townes–Dailey model has been used to interpret the halogen nuclear quadrupole coupling constants. This indicates that a fraction of an electron, δi = 0.013, is transferred from O to Br and a fraction, δp = 0.043, is transferred from Br to Cl on complex formation. Comparison between the intermolecular force constants, kσ, of the complexes H2O···BrCl and H2O···HBr indicates that the interaction in the H2O···BrCl complex is of similar strength to that of the hydrogen bond in H2O···HBr.

Rotational spectroscopy of H3P⋯BrCl and the systematics of intermolecular electron transfer in the series B⋯BrCl, where B=CO, HCN, H2O, C2H2, C2H4, H2S, NH3, and PH3

Ground-state rotational spectra of the isotopomers H3P⋯79Br35Cl, H3P⋯81Br35Cl, H3P⋯79Br37Cl, H3P⋯81Br37Cl, D3P⋯79Br35Cl, and D3P⋯81Br35Cl, of the phosphine–bromine monochloride complex were observed by the pulsed-jet, Fourier-transform method, incorporating a mixing nozzle to preclude reaction among the component gases. Each isotopomer exhibited a symmetric-top-type spectrum which yielded accurate values of the spectroscopic constants B0, DJ, DJK, χaa (Br), χaa (Cl), Maa (Br), and Mbb (Br) on analysis. Interpretations of the changes in the B0 values with isotopomer showed that the intermolecular bond involves P and Br, with r(P⋯Br)=2.869(1) Å and that the BrCl bond increases in length by ∼0.04 Å on complex formation. Changes in the halogen nuclear quadrupole coupling constants when H3P⋯BrCl is formed lead, with the aid of the Townes–Dailey model, to the conclusion that a fraction δi=0.100(5) of an electron is transferred from P to Br on complex formation, while the polarization of BrCl by PH3 can be viewed as the transfer of 0.128(2)e from Br to Cl, leading to a net change of −0.028(5)e in the population of the 4pz orbital of Br. The complex is only of moderate strength, with an intermolecular stretching force constant kσ=11.5 Nm−1. Values of δi, similarly determined, for the series B⋯BrCl, where B=CO, HCN, H2O, C2H2, C2H4, H2S, NH3, or PH3, are presented. It is shown that the variation of δi with the ionization energy IB of the Lewis base B can be described by an expression δi=Aexp(−bIB). This behavior is compared with that for the corresponding series B⋯ICl.

The microwave spectra and structures of Ar–AgX (X=F,Cl,Br)

The rotational spectra of the complexes Ar–AgF, Ar–AgCl, and Ar–AgBr have been observed in the frequency range 6–20 GHz using a pulsed jet cavity Fourier transform microwave spectrometer. All the complexes are linear and rather rigid in the ground vibrational state, with the Ar–Ag stretching frequency estimated as ∼140 cm−1. Isotopic data have been used to calculate an r0 structure for Ar–AgF, while for Ar–AgCl and Ar–AgBr partial substitution structures have also been obtained. To reduce zero-point vibrational effects a double substitution method (rd) was employed to calculate the structures of Ar–AgCl and Ar–AgBr. The Ar–Ag bond distance has been found to be rather short and to range from 2.56 Å in Ar–AgF to 2.64 Å in Ar–AgBr. Ab initio MP2 and density functional theory calculations for Ar–AgF and Ar–AgCl model the geometries and stretching frequency well, and predict an Ar–Ag bond energy in Ar–AgF of ∼23 kJ mol−1. These results indicate that the Ar–AgX complexes are more strongly bound than typical van der Waals complexes. Analysis of the halogen nuclear quadrupole coupling constants was unable to confirm whether extensive electron rearrangement occurs upon formation of the complexes.

Inter- and intramolecular electronic transfer on formation of H3P···ICl as determined by rotational spectroscopy

Ground-state rotational spectra of the three isotopomers H3P···I35Cl, H3P···I 37Cl and D3P···I35Cl of a symmetric-top complex formed between phosphine and iodine monochloride were observed by pulsed-nozzle, Fourier transform microwave spectroscopy. The spectroscopic constants B0, DJ, DJK, χaa(I), χaa(Cl), Maa(I) and Mbb(I) were determined in each case. Changes in B0 on isotopic substitution established that the nuclei lie in the order H3P···ICl, with the distance r(P···I)=2.963(1) Å. The changes in the halogen nuclear quadrupole coupling constants χaa(I) and χaa(Cl) from their counterparts in the free ICl molecule led to the conclusion that a fraction δi =0.144(7) of an electron is transferred from P to I on complex formation and that the polarization of ICl by PH3 is equivalent to the intramolecular transfer of a fraction δp(Cl)=0.157(2) from I to Cl. The complex is about as strongly bound as H3N···HCl, when the intermolecular stretching force constant kσ is used as a criterion. It is found that the distance r(P···I) contracts by 0.017 Å between H3P···I35Cl and D3P···I35Cl. The systematic variation of δi within a series of complexes B···ICl is discussed.

Angular geometry, binding strength and charge transfer for the complex H 2S⋯ICl determined by rotational spectroscopy

Ground-state rotational spectra of the isotopomers H 2 S⋯ I 35 Cl , H 2 S⋯ I 37 Cl , HDS⋯ I 35 Cl and D 2 S⋯ I 35 Cl of a complex of hydrogen sulphide and iodine monochloride were measured. Rotational constants, centrifugal distortion constants, halogen nuclear quadrupole coupling constants, and I spin–rotation coupling constants were determined. Interpretation of various spectroscopic constants showed that the complex has a geometry in which the S⋯I–Cl nuclei are collinear, with r(S⋯I)=3.154(3) Å and the ICl subunit almost perpendicular to the plane of the H 2S nuclei [ φ=91.9(12)°]. Observed I and Cl nuclear quadrupole coupling constants are used with a simple model to deduce that intramolecular charge transfer from S to I is ∼0.05 e −. The intermolecular stretching force constant k σ =16.6(3) N m −1.

A π-electron donor–acceptor complex of ethene and iodine monochloride: geometry, binding strength and charge redistribution determined by rotational spectroscopy

An ethene/ICl complex was characterised by rotational spectroscopy. Rotational constants, centrifugal distortion constants and halogen nuclear quadrupole coupling constants of C2H4⋯I35Cl and C2H4⋯I37Cl were interpreted to establish several conclusions. The ICl molecule lies along the C2 axis perpendicular to the nuclear plane of ethene, with I nearer to the π bond than Cl and r(*⋯I)=3.033(2) Å (* is the centre of the π bond). An estimated 0.054 e− is transferred from ethene to I on complexation, while the ICl polarisation is represented as a transfer 0.080 e− from I to Cl. Hence the net change at I is −0.026 e−. The binding of C2H4⋯ICl is stronger than for other C2H4⋯XY.

Rotational spectroscopy of mixtures of ethyne and iodine monochloride: isolation and characterisation of the π-type complex C2H2···ICl

The ground-state rotational spectrum of a complex formed by ethyne and iodine monochloride was observed by using pulsed-nozzle, Fourier-transform microwave spectroscopy. A fast-mixing nozzle was utilised to avoid chemical reaction of the component gases prior to their supersonic expansion. Rotational constants A0, B0 and C0, quartic centrifugal distortion constants ΔJ, ΔJK and δJ, and halogen nuclear quadrupole coupling constants χaa(X) and {χbb(X)-χcc(X)}, where X=Cl or I, were determined for the three isotopomers C2H2···I35Cl, C2H2···I37Cl and C2D2···I35Cl. Detailed interpretation of the rotational constants established that the equilibrium geometry of the complex has a planar, T-shape of C2v symmetry in which ethyne acts as the bar of the T. This geometry, with the zero-point distance r(*···I)=3.115(1) A between the centre of the π-bond of ethyne and the nearest halogen atom I, establishes that the interaction in this complex is between ethyne as a π electron donor and I of ICl as the electron acceptor. The halogen nuclear quadrupole coupling constants χaa(X) were interpreted on the basis of a simple model to show that, on complex formation, fractions δ1=0.026 and δ2=0.056 of an electronic charge are transferred from the ethyne π bond to I and from I to Cl, respectively, leading to a net decrease of 0.030e at I. The complex is weakly bound, according to the intermolecular stretching force constant kσ=12.2(1) Nm-1 determined for the isotopomers C2H2···I35Cl and C2H2···I37Cl from ΔJ values. The opportunity is taken to compare the properties of C2H2···ICl established here with those similarly determined for the series C2H2···XY, where XY=Cl2, ClF, or BrCl.

Halogen Nuclear Quadrupole Coupling Constants in Non-axially symmetric Molecules; Ab initio Calculations, which Include Correlation, Compared with Experiment

Abstract Ab initio determination of the electric field gradient (EFG) tensors at halogen and other centres ena-bled determination of the nuclear quadrupole coupling constants (NQCC) for a diverse set of C2v , C3v and other symmetry molecules of general formula MH2X2 and MHX3 , where the halogen atoms (X) are Cl, Br and I, and the heavy central atoms (M) are C and Si. The study presents results at a standardised level of calculation, triple-zeta in the valence space plus polarisation functions (TZVP) for the equilib-rium geometry stage; all-electron MP2 correlation is included in all these studies. For the bromo and iodo compounds, especially the latter, it is essential to allow core polarisation, by decontraction of the p,d-functions. This is conveniently done by initial optimization of the structure with a partly contract-ed basis, followed by reestablishment of the equilibrium structure with the decontracted basis. The NQCCs, derived from the EFGs, using the 'best' values for the atomic quadruple moments Cl, Br and I, lead to good agreement with the inertial axis (IA) data obtained from microwave spectroscopy. When the data from the present study is plotted against the values derived from the IA data, obtained by whatever approximations chosen by the MW authors, we obtain a linear regression for the data (85 points) with the slope 1.0365 and intercept -0.1737, with standard errors of 0.0042 and 0.2042, respectively; these are statistically identical results irrespective of whether the data is restricted to IA or EFG principal axis (PA) data. Since as in the C3v MH3X compounds studied previously, a close correlation of the microwave spectral data with the calculations was observed using the 'best' current values for Qz , there seems no need to postulate that the values of QBr for both 79Br and 81Br are seriously in error. A scaling downwards of Qz by about 5% for Br and I increases the agreement with experiment, but the contributions of relativistic effects are unknown, and could lead to further reassessment. Of the two common assumptions used in MW spectroscopy, to convert from IA to EFG-PA data, either (a) cylindrical symmetry of the NQCC along the bond direction, or (b) coincidence of the tensor principal element with the bond axis, the latter is found to be a much more realistic approximation.

Halogen Nuclear Quadrupole Coupling Constants: Comparison of ab initio Calculations which include Correlation, with Experiment

Abstract Ab initio determination of the electric field gradient (EFG) tensors at halogen and other centres ena-bled determination of the nuclear quadrupole coupling constants (NQCC) for a diverse set of axially symmetric (C3v , C∞v, D∞h and other symmetries) inorganic and organic molecules, where the heavy ele-ments are Cl, Br, and I with C, Si, Ge, and Sn hydrides. The latter elements are in an approximately tetrahedral environment. The study presents results at a standardised level of calculation, triple-zeta in the valence space (TZV) plus polarisation functions (TZVP) for the equilibrium geometry stage; all-electron MP2 correlation is included in all these studies. f-Orbital exponents were optimised for both Br and I centres in the methanes; the atomic populations of the f-orbital components are very small for the Br-and I-atoms, confirming their role as polarisation functions rather than having any bonding character. The EFG are determined at equilibrium with the TZVP basis set, except Sn and I centres where the basis set is TZV + MP2. For the bromo and iodo compounds, especially the latter, it is essential to allow for core polarisation, by decontraction of the p,d-functions. This is conveniently done by initial optimization of the structure with a partly contracted basis, followed by reestablishment of the equilibrium structure with the decontracted basis. A close correlation of the observed (microwave spectral) data with the calculations was observed, using the 'best' values for the atomic quadrupole moments for Cl, Br, and I; thus there seems no need to postulate that the value of QBr for 79Br and 81Br are in error. The SCF and MP2 wave-functions were converted into localised molecular orbitals by the Boys Method. This allowed a study of the differing s/p/d-hybridisation ratios, and the centroid positions, to be compared with the quadrupole coupling constants. The charge distributions for the atoms were converted into local bond dipoles, which in turn are correlated with the electronegativity differences of the bonded atoms.

‘Charge-transfer’ complexes of ammonia with halogens. Nature of the binding in H3N⋯BrCl from its rotational spectrum

The ground-state rotational spectra of the four isotopomers H315N⋯79Br35Cl, H315N⋯81Br35Cl, H315N⋯79Br37Cl and H315N⋯81Br37Cl of a complex formed by ammonia and bromine monochloride have been observed by pulsed-nozzle, FT microwave spectroscopy. A fast-mixing nozzle was used to prevent a chemical reaction between the components and allow the encounter complex to be isolated. The nature of the spectra establishes that the observed isotopomers are symmetric-top molecules. Assignment and analysis of the spectrum in each case lead to the rotational constant Bo, the centrifugal distortion constants DJ and DJK, the halogen nuclear quadrupole coupling constants χ(Br) and χ(Cl), and the component Mbb(Br) of the bromine spin–rotation coupling tensor. The rotational constants allowed an ro-type value of the distance N⋯Br of 2.627 A to be established. An rs-type method led to the distances r(N⋯Br)= 2.59(1)A and r(Br—Cl)= 2.186 A, the former requiring a value of Bo for H314N⋯79Br35Cl obtained from unperturbed transition centres estimated without a full hyperfine structure analysis. A consideration of the intermolecular stretching force constant kσ, determined from DJ, provides evidence of a relatively strong interaction between the subunits. However, an analysis of the χ(X)(X = Br, Cl) values reveals that the molecular interaction is mainly electrostatic in origin, with probably only a small extent of intermolecular electric charge redistribution on complex formation.

Rotational spectrum of the gas-phase dimer OC⋯BrCl

The ground-state rotational spectra of the six isotopomers 16O12C⋯79Br35Cl, 16O12C⋯81Br35Cl, 16O12C⋯79Br37Cl, 16O12C⋯81Br37Cl, 16O13C⋯79Br35Cl, and 16O13C⋯81Br35Cl, of a dimer formed by carbon monoxide and bromine monochloride have been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. The rotational constant B0, the centrifugal distortion constant DJ and the halogen nuclear quadrupole coupling constants χaa(X), where X = Cl or Br, have been interpreted to give the properties of the weakly bound complex. It was found that the molecule is linear and has the arrangement OC⋯BrCl, with the Br atom forming the intermolecular bond to carbon. Values of the rs type for the distances r(C⋯Br) and r(BrCl) have been derived and the limitations imposed on these by the small coordinate aBr are considered. The effect of bond shrinkage resulting from isotopic substitution at Br indicates that the rs value of aBr is underestimated and it is concluded that there is probably only a small increase in r(BrCl) on formation of the complex. An interpretation of the χaa(X) values indicates that the electric charge distribution in BrCl is also only slightly perturbed.

An SCV-MS- Xα study of the bonding and nuclear quadrupole coupling in oxygen compounds of the halogens

SCF-MS- Xα calculations of the electronic structure of the oxy-halogen compounds Cl 2O, ClO, ClO 2, ClO 2 −, ClO 3 −, ClO 4 −, BrO, BrO 2, BrO 3 −, IO 3 −, and IO 4 − have been used to investigate the bonding and nuclear quadrupole coupling in these molecules and ions. The halogen d orbital participation in the bonding increases with increasing oxidation number of the halogen atom in the molecule or ion. The inclusion of the halogen d orbitais results in an increase in the principal component of the halogen electric field gradient tensor, but only in some cases this result in improved agreement between the calculated and observed halogen nuclear quadrupole coupling constants. In other cases better agreement is obtained for calculations involving the halogen p orbitais only in the valence shell. Thus the results do not provide unambiguous evidence for the involvement of halogen d orbitais in the bonding. In the case of the ClO 3 − and BrO 3 − ions for which 17O quadrupole coupling constants have been reported, the Xα values have been calculated. Agreement between the calculated and observed values is much poorer than for the halogen coupling constants. The results of the calculations have also been used to investigate some of the approximations made in the Townes and Dailey method of analysis of nuclear quadrupole coupling data.

The extent of proton transfer from X to N in the gas-phase dimers (CH 3) 3- nH nN...HX. Evidence from rotational spectroscopy

Ground-state spectroscopic constants have been determined for the hydrogen-bonded dimers (CH 3) 3 14N...H 81Br and CH 3 15NH 2...H 35Cl from their rotational spectra. Comparison of halogen nuclear quadrupole coupling constants and intermolecular stretching force constants along the series, HX, H 3N...HX, CH 3NH 2...HX, (CH 3) 3N...HX and Na +X − indicates that for the species (CH 3) 3- n H n N...HX the extent of proton transfer from X to N increases along the series X=F, CN, Cl and Br for n=0 and increases as n decreases for X=Cl. Estimated values Δ H m for the process (CH 3) 3N...HX=(CH 3) 3N +H...X − are consistent with the experimental conclusions.

ChemInform Abstract: An SCF‐MS‐Xα Study of the Bonding and Nuclear Quadrupole Coupling in 1:1 Complexes of Amines with Diatomic Halogens and Interhalogens

SCF-MS-Xα calculations of the electronic structure of the 1 : 1 complexes of pyridine with I2, IBr and ICl, and of the 1 : 1 trimethylamine–I2 complex have been carried out to investigate the bonding and nuclear quadrupole coupling in these molecules. Good agreement (to within 10% in most cases) is obtained between the calculated and observed halogen nuclear quadrupole coupling constants for these molecules. The agreement in the case of the nitrogen quadrupole coupling parameters is not as good, but the calculations do support some of the previously reported conclusions concerning the changes in the nitrogen orbital populations which occur upon complex formation. The results essentially support previous conclusions, based on Townes and Dailey analyses of the experimental quadrupole coupling data, concerning the extent of charge transfer and the distribution of the transferred charge in these molecules. They are, however, at variance with some of the results obtained from chemical shifts in the photoelectron spectra. The electric dipole moments of the molecules have also been calculated. These are ca. 30% higher than the highest reported experimental values, but show the expected trends from one compound to another.

An SCF-MS-X? study of the bonding and nuclear quadrupole coupling in 1 : 1 complexes of amines with diatomic halogens and interhalogens

SCF-MS-Xα calculations of the electronic structure of the 1 : 1 complexes of pyridine with I2, IBr and ICl, and of the 1 : 1 trimethylamine–I2 complex have been carried out to investigate the bonding and nuclear quadrupole coupling in these molecules. Good agreement (to within 10% in most cases) is obtained between the calculated and observed halogen nuclear quadrupole coupling constants for these molecules. The agreement in the case of the nitrogen quadrupole coupling parameters is not as good, but the calculations do support some of the previously reported conclusions concerning the changes in the nitrogen orbital populations which occur upon complex formation. The results essentially support previous conclusions, based on Townes and Dailey analyses of the experimental quadrupole coupling data, concerning the extent of charge transfer and the distribution of the transferred charge in these molecules. They are, however, at variance with some of the results obtained from chemical shifts in the photoelectron spectra. The electric dipole moments of the molecules have also been calculated. These are ca. 30% higher than the highest reported experimental values, but show the expected trends from one compound to another.