Halogen Nuclei Research Articles

The molecular Zeeman effect in CH3Cl, CH3Br and CH3I

A microwave rotational Zeeman study is performed on various halogen isotopes of CH3Cl, CH3Br and CH3I. The rotational spectra of these molecules is complicated by nuclear quadrupolar interactions. A method of data analysis is outlined which avoids the complication of performing a complete matrix diagonalization and allows one to extract information about the molecular g values, the anisotropy in the magnetic susceptibility, as well as chemical shift anisotropies using first-order perturbation theory. This method utilizes data from transitions defined by F = I + J → F′ = I + J + 1 and |MF | = F → |MF′| = F′. These transitions were heavily relied upon in the data analysis and the following information was obtained: for CH3 127I, and For CH3 79Br, and For CH3 35Cl, and (1 - σ)gI is the shielded nuclear g value, g ⊥, and g ∥ are the molecular g values perpendicular and parallel to the molecular symmetry axis; x ⊥, x ∥, σ⊥ and σ∥ are similarly defined elements of the magnetic susceptibility and the chemical shift tensors. Values of the molecular quadrupole moments, all the elements of the paramagnetic, diamagnetic and total magnetic susceptibility tensors and second moments of the charge distributions are calculated using these results and previously determined values of the total susceptibility. Trends in the methyl halide series are found and discussed with respect to the molecular quadrupole moments, with respect to the excited state summation and with respect to c 2, the ground state expectation value of the square of the electronic coordinates perpendicular to the symmetry axis. The constant <c 2> suggests that the radius of the halogen nucleus in the methyl halides is very close to its neutral atom radius and quite far from its negative ion radius. The expected sign for the electric dipole moment of +CH3Cl- is found by Zeeman studies of the 35Cl and 37Cl isotopic species.

Direct nuclear quadrupole resonance evidence for vanadium–halogen bonding in vanadium trihalide hexahydrates

The nuclear quadrupole resonance spectra of VCl3,6H2O and VBr3,6H2O show frequencies characteristic of metal-bonded halogen nuclei.

11B Quadrupole Coupling Constants in Solid Boron Halides

The 11B nuclear quadrupole coupling constants have been measured in the solid boron halides and found to be 2.64 MHz in BF3, 2.54 MHz in BCl3, 2.46 MHz in BBr3, and 2.40 MHz in BI3. These values do not agree with calculated values based on descriptions of the boron–halogen bonds obtained from quadrupole coupling studies of the halogen nuclei. The discrepancy is attributed to the inadequacy of some assumptions used in the Townes and Dailey method for the case of boron. A fine structure was observed on the B11 nuclear magnetic resonance in BF3 and is completely explained in terms of a dipole–dipole interaction between the boron nucleus and the three neighboring fluorine nuclei.

Nuclear magnetic shielding of hydrated alkali and halide ions in aqueous solution

Various methods for the determination of the paramagnetic term of the shielding constant of alkali and halide ions in dilute aqueous solution are discussed. Values of the chemical shift of the hydrated ions, σp Maq+ and σp Xaq− , are obtained from solvent isotope chemical shifts of halide ions; spin relaxation rates of quadrupolar nuclei in dilute aqueous solution and from the concentration dependence of the chemical shift and relaxation rates of alkali and halogen nuclei in aqueous solutions of alkali halides.

Nuclear quadrupole resonance of charge-transfer complexes. II. The aminehalogen complexes

Nuclear quadrupole resonance frequencies have been measured for some of the halogen nuclei in a number of crystalline charge-transfer complexes of amines with bromide, iodine, iodine monobromide, and iodine monochloride. From these measurements it has been possible to calculate approximate charge distributions for the bromine, iodine monobromide, and iodine monochloride complexes of 3,5-di-bromopyridine. The results correlate well with a three-centre delocalized LCAO-MO bonding theory. The correlation of this MO description with Mulliken's charge-transfer bonding scheme for n-σ complexes is discussed. This approach indicates the importance of considering an ?ionic? structure (which does not involve transfer of charge from the donor to the acceptor) in the electronic ground state of these complexes.

Chemical shift measurements in the presence of strong nuclear quadrupole couplings

A method is presented for measuring chemical shifts of halogen nuclei (except 19F) in solids in the presence of strong nuclear quadrupole couplings. This method employs the measurement of the frequency versus magnetic field dependence of a resonance which arises from those nuclei for which the z-direction of the electric field gradient tensor is oriented at 90° to the applied magnetic field. Measurements of chemical shifts of 35Cl and 81Br in NaClO 3. NaBrO 3. anf KBrO 3 are reported.

Semi-empirical all valence electrons SCF-MO-CNDO theory

The semi-empirical SCF-MO-CNDO theory is used to calculate nuclear quadrupole coupling constants, considering only contributions to the electric field gradient from valence-shell p Orbitals of the same atom as the nucleus of interest, as suggested originally by Townes and Dailey. For halogen nuclei, fairly accurate results are obtained, but for nitrogen nuclei the results are much poorer.

Electron spin resonance studies. Part XII. Characteristics of the iminoxy-radicals from the 1-halogenofluorenone oximes

The hyperfine splittings of the halogen nuclei in the 1-halogenofluorenone iminoxy-radicals are remarkably large; they are discussed in terms of an interaction between the orbital of the unpaired electron on the iminoxy-function and one on the halogen which, though four bonds removed, is suitably sited by virtue of the fixed geometry of the system. The lines in the spectrum of the iodine-containing radical are markedly broadened, and their widths vary with the temperature and viscosity of the medium in a manner which indicates the operation of nuclear-quadrupole relaxation.

Elektronen-Kern-Doppelresonanz-Untersuchung vonU 2-Zentren in Kaliumchlorid

U 2-centers in alkali halides are neutral hydrogen atoms in interstitial lattice sites, as has been shown by EPR measurements. The hyperfine interactions with the proton and with the four nearest halogen nuclei are resolved in the EPR spectrum. In order to resolve hyperfine interactions with further nuclei of the surrounding lattice ENDOR measurements have been performed onU 2-centers in KCl at 77 °K. The analysis of the ENDOR spectra gave precise values for the hyperfine and quadrupole interaction constants of the nearest neighbour chlorine and potassium nuclei. The isotropic hyperfine constant of the chlorine neighbours is 24 times larger than that of the potassium neighbours although both nuclei are on equivalent first shell lattice positions. The hyperfine interactions of second shell potassium nuclei [(1/2, 1/2, 3/2)-position] show an unexpectedly large isotropic hyperfine constant. One expects a pure magnetic dipole-dipole interaction for the outer shell nuclei because of the concentrated hydrogen wave function. Two further chlorine shells could be approximately analysed. A theoretical estimate of the hyperfine and quadrupole interaction constants was made by orthogonalizing the 1s hydrogen wave function to the cores of the surrounding ions. If one takes into account the mutual overlap of neighbouring potassium and chlorine ions, one gets the right order of magnitude of the measured constants and a value of 10.4∶1 for the ratio of the isotropic hyperfine constants of the first shell chlorine and potassium nuclei. The relatively large isotropic constant of the second shell potassium nuclei can also be explained on this basis.

Dependence of Nuclear Quadrupole Relaxation Times in Ionic Crystals on the Phonon Spectrum

Nuclear quadrupole relaxation times are calculated for the halogen and alkali nuclei in a number of alkali halides using lattice phonon density curves computed by Karo. The calculated values of T/sub 1/ at room temperature are generally lower than those obtained using the Debye model and is in better agreement with experiment. The variation of T/sub 1/ with temperature was also calculated over the range 20 deg K to room temperature for /sup 23/Na and /sup 27/ I nuclei in NaI. Excellent agreement is obtained with the experimental data on the temperature variation of T/sub 1/ when use is made of the Karo distribution function. The Debye model gives a temperature variation comparable with experimental above 70 deg K but a relatively slower variation below 70 deg K. (auth)

Nuclear Quadrupole Resonance and Bonding in Solid Layer Type Metal Halides

Pure nuclear quadrupole resonance spectra have been reported for halogen nuclei in a number of metal halides crystallizing in layer structures of the CdI2 or related types. Because of the extreme smallness of the quadrupole coupling constants, particularly in the case of the dihalides, the possible appropriateness of a purely ionic model to account for the electric field gradients has been investigated. Detailed calculations of the ion-lattice field gradient have been made for a number of compounds representing several types of infinite-layer structure. In no case is the coupling due to the ion-lattice alone sufficient to explain the observed interactions. In fact, comparison with the experimental data suggests that empirical values of the antishielding factor γ∞ about 3–5 times smaller than the theoretical values are appropriate to account for the ion-lattice contribution to the coupling. Covalent bonding between the metal and halogen layers involving only small departures from pure p-bond configurations at the halogens can account for the experimental results. Taking the ion-lattice gradient into account causes in general a small increase in the values of s hybridization needed in the covalent bond model.

Nuclear quadrupolar relaxation in ionic crystals

The nuclear quadrupolar relaxation time of alkali halide crystals is investigated by the Heitler-London approximation. We use the Slater determinant which is constructed by the Hartree-Fock wave functions of the free ions. The charge distribution around an ion is not spherically symmetrical because of mutual overlap of the atomic wave functions of nearest-neighbor ions. Moreover, the charge distribution around an ion changes its shape during thermal vibrations of the ions, because the degree of the overlap depends upon the distance apart of the ions. Therefore, the quadrupolar interaction in alkali halide lattices is the resultant of a combination of the thermal vibration with the charge overlap. As illustrations of the theory, we have computed the quadrupolar relaxation time of the 39K and the 35Cl nuclei in the KCl crystal, and the 23Na nucleus in the NaCl crystal. The theoretical results are in good agreement with those of experiments. We have also computed the ratio of the quadrupolar relaxation time of the metal nucleus to that of the halogen nucleus for some alkali halide crystals. After computing the same ratio by the covalent model and the deformation model, we have compared these results with the available experimental data. Finally, by using the same overlap model as that mentioned previously, we have developed a formula which gives the chemical shift of ionic crystals.

The electronic structure of an H-center

The electronic structure of a new type of color center, which is produced in KCl, KBr, and LiF crystals by X-irradiation at low temperatures, has been determined by electron-spin-resonance experiments. Crudely, this center can be described as a halogene − 2 molecule-ion located in a halide-ion vacancy. The unpaired electron interacts strongly with the two nuclei of the moleculeion and weakly with two more halogen nuclei. All four nuclei lie in a straight row along a [110] axis. This paramagnetic center may be identical with the center that gives rise to the optical H-band, since in KCl it has the same production, bleaching, and symmetry characteristics.

Complex hyperfine structures in microwave spectra of covalent iridium compounds

Magnetic resonance spectra are described for two compounds containing covalently bound [IrCl 6 ] 2- complexes and one containing [IrBr 6 ] 2- , and the results are analyzed using the π-bonding theory developed by Stevens. The spectra show hyperfine structure lines arising from interaction of the magnetic electrons both with the Ir nucleus ( I = 3/2) and w ith the Cl ( I = 3/2) or Br ( I = 3/2) nuclei, and different examples are discussed where the numbers of resolved lines are 4, 8, 13, 16 and 22. It is shown how these structures, the ( g -values and the difference between the spectra in [IrCl 6 ] 2- and [IrBr 6 ] 2- are all consistent with the hypothesis that the magnetic electrons move partly in d -orbits round the central Ir nucleus and partly in p π-orbits round the outer halogen nuclei of the complex.

Electronic Structure and Stability of Hydrogen Halides and of Complex Ions XO4

(1) It is shown that in the hydrogen halide molecules (internuclear distance r0) the proton penetrates the electronic shell of the anion to a depth which for the simplified case of spherical symmetry can be characterized by the condition: The amount of negative charge beyond the sphere of radius r0 equals −1e. (2) From the dipole moments μ=xer0 of the hydrogen halide molecules it can be concluded: The wave mechanical distribution of the negative charge of the free halide ions is changed by the introduction of the proton in such a way that the center of gravity of an amount of charge equal to —(1−x)e is shifted from the halogen nucleus to the proton. The fraction (1—x) increases with the electronic polarizability of the anion, and would be equal to 1 for an ion of infinitely large polarizability, leading to a completely unpolar type of binding in this case. (3) It is shown that for the complex ions SiO44—, PO43—, SO4=, and ClO4−, the gradation of the X–O distances and of the molar dispersion can be easily understood from the point of view used in 1924 for the case of the molar refraction: These ions represent the result of the polarization of O= by Si4+, P5+, S6+, and Cl7+, and the X–O binding in them shows gradual changes toward the unpolar type. (4) It is pointed out that the relatively unstable HI and ClO4− approach the unpolar type of binding more closely than any other of the compounds considered here. The generalization of this connection between instability and the degree of deformation of electronic shells explains why compounds like FO4− and BrO4− are unknown.