Pure Quadrupole Resonance Frequency Research Articles

Estimates of Hammett's Sigma Values from Quadrupole Resonance Studies

A linear correlation exists between the Cl35 pure quadrupole resonance frequency in substituted chlorobenzenes and the Hammett parameter sigma for the substituent. The relation is expressed as f=1.024σ+34.826 Mcps.For the 52 frequencies used in determining this relation, the correlation coefficient is r = 0.96 and the root-mean-square deviation of the experimentally determined f values from those predicted by the equation is 360 kcps. A proper ortho sigma parameter for use in the f vs σ correlation is apparently the value derived from the Hammett relation σ = lnK/K0 in which K and K0 are the equilibrium constants for ionization of the substituted and unsubstituted benzoic acid in water at 25°C. This parameter is linearly proportional to the polar substituent σ* of Taft for the ortho substituent. In the many multiply substituted chlorobenzenes investigated, the proper total sigma value has been taken as the sum of the individual parameters. Few cases are apparent in which the linear relation of f to σ is seriously violated. Additivity of the electron displacement properties of substitution, suggested by Jaffe and indicated by studies of the magnetic shielding in fluorobenzenes, is apparently confirmed. Sigma values for 29 substituents have been estimated from the Cl35 pure quadrupole resonance frequency measured in the substituted chlorobenzene.

Precise Nuclear Resonance Thermometer

The properties of a thermometer based upon the temperature variation of the pure quadrupole resonance frequency (ν0) of the Cl35 nucleus in granular KClO3 have been investigated. In the range ∼10°K<T <300°K, the thermometer has a very high sensitivity, being better than ±0.002° at 273°K and ±0.004° at 77°K. The accuracy at 20°K is estimated as at least ±0.02°. The thermometer is exactly reproducible if care is taken that the KClO3 is of high chemical purity. No hysteresis effects were observed. Establishment of the thermometer requires a single precise determination of the ν0 versus temperature curve using the fundamental temperature standards to measure the temperature.

Dependence of the Pure Quadrupole Resonance Frequency on Pressure and Temperature

Measurements of the pure quadrupole resonance frequency ($\ensuremath{\nu}$) of the ${\mathrm{Cu}}^{63}$ nucleus in cuprous oxide, and the ${\mathrm{Cl}}^{35}$ nucleus in potassium chlorate and paradichlorobenzene have been made as a function of pressure in the range 1 to 10 000 kg/${\mathrm{cm}}^{2}$ for temperatures between -77\ifmmode^\circ\else\textdegree\fi{}C and 100\ifmmode^\circ\else\textdegree\fi{}C. A theory is presented in which the static value of the electric field gradient tensor (${{\ensuremath{\varphi}}_{\mathrm{ij}}}^{0}$) and the amplitudes ${{\ensuremath{\xi}}_{i}}^{0}$ of the normal modes of the lattice vibrations play a central role. The volume dependence of these quantities can be deduced from $\ensuremath{\nu}$-versus-volume isotherms which are constructed from the experimental data by using the equation of state. Information on the latter is augmented by theory. It is found that the temperature variation of the resonance frequency at atmospheric pressure can be understood by considering not only the explicit temperature dependence of the vibration amplitudes, but also by including the appreciable effects of the volume expansion on ${q}_{0}$ and ${{\ensuremath{\xi}}_{i}}^{0}$. Furthermore, the pressure dependence is found to be explicable in terms of the volume dependence of ${q}_{0}$ and ${{\ensuremath{\xi}}_{i}}^{0}$. In ${\mathrm{Cu}}_{2}$O, it is found that ${q}_{0}$ depends on volume roughly as ${V}^{\ensuremath{-}1}$, thereby indicating an ionic character for this crystal. On the other hand, in paradichlorobenzene ${q}_{0}\ensuremath{\propto}{V}^{n}$, with $0<n<0.04$, thus exhibiting the effect of an increase in the intermolecular hybridization of the C-Cl covalent bond.

NUCLEAR QUADRUPOLE RESONANCE SPECTRUM OF B11 IN KERNITE

The pure quadrupole resonance frequencies of B11 nuclei at two different sites (denoted by E and F) in a single crystal of kernite (Na2B4O7.4H2O) have been determined both by observing the B11 resonances in zero external magnetic field, and also by investigating their Zeeman splitting in magnetic fields up to 30 gauss. The values obtained are 1281.1 ± 2 and 1287.0 ± 1 kc./sec. for the E and F sites respectively. These frequencies agree within experimental error with the values 1286 ± 4 and 1287 ± 3 kc./sec. predicted on the basis of studies in a magnetic field of 7000 gauss. The agreement for the F site is much better than for the E site. These zero field resonances have also been observed in a polycrystalline sample. The observed weak field Zeeman splitting for the single crystal agrees very well with that calculated from high field data for F sites, but at present disagrees with the corresponding calculation for E sites. A Na23 pure quadrupole resonance in kernite has also been measured at 1560.0 ± 1 kc./sec. The second Na23 resonance expected from high field work has not been observed to date.

Nuclear Quadrupole Resonances in Bromobenzene Derivatives

The pure nuclear quadrupole resonance frequencies of the Br79 isotope are given for a number of previously unreported bromobenzene derivatives. The resonance frequencies of the Br81 isotope in these and other previously reported compounds are compared with the electron density in the vicinity of the carbon atom to which the bromine is bonded.