NQR Experiments Research Articles

Optically detected nuclear spin alignment in halogenated organic compounds

Optically detected NQR experiments in ground and excited electronic states of the model compound p-dichlorobenzene in p-dibromobenzene can be described by a time-dependent chlorine nuclear spin alignment. The influence of light intensity, microwave and radio frequency power are investigated. The comparison of the experimental results with model calculations for the optical pumping cycle is in quantitative agreement with the theoretical prediction. The largest obtained alignment for 35Cl is 0.54 in the case of a limited spin reservoir.

Temperature and Pressure Variation of the Cl-NQR in A2PbCl6-Crystals

The 35Cl NQR frequency and spin-lattice relaxation rate in the compounds A2PbCl6 (A = Cs, Rb, NH4, K) have been investigated in the range 4.2 K to 500 K, and as a function of pressure at room temperature. NQR experiments conducted on (K: NH4)2PbCl6 mixed crystals have been used to complete the NQR-frequency versus temperature diagram of K2PbCl6, revealing two structural transitions at Tc1 ≅ 358 K and at TC2 ≅ 333 K.

Electric field gradients in imidazole at 103 K from x-ray diffraction

The electric field gradients at the two distinct nitrogen nuclei of imidazole have been calculated from the charge density derived from Bragg diffraction data for crystalline inidazole at 103 K. The resulting 14N coupling constants and asymmetry parameters were N1: e2qQ/h = −2.6(0.3) MHz, η = 0.4(0.2); N3: e2qQ/h = −2.2(0.3) MHz, η = 0.4(0.2). In contrast with the corresponding results obtained from NQR experiments, these calculations could not distinguish between the two nitrogen sites. At each site the parameters were approximately equal to the mean of the NQR values for N1 and N3. Model calculations for the diatomic molecules N2 and BF suggest that the accuracy with which field gradients are obtained directly from the Bragg structure factors depends critically on the elimination of quadrupole components induced by vibrational averaging. Estimates of the coupling constants and asymmetry parameters for imidazole obtained directly from the Bragg structure factors were N1: e2qQ/h = −1.4(0.3) MHz, η = 0.2(0.3); N3: e2qQ/h = −0.9(0.3) MHz, η = 0.7(0.5). Detailed analysis showed that the magnitudes of the coupling constants estimated in this way are too small due to residual vibrationally induced quadrupole components.

Electric field gradients and charge density in lithium nitride

The magnitudes of the electric field gradients at all atomic positions in Li3N are known from NQR experiments. High-temperature NQR data indicate the gradients at Li(1) and Li(2) to be of opposite sign. In this paper, we calculate the field gradients from X-ray data and refine the parameters of the multipole deformation functions together with scale, extinction and temperature factors against X-ray and NQR data simultaneously, assuming various sign combinations for the gradients at Li(1), Li(2) and N. Despite an ill-defined scale factor of the X-ray intensities, negative signs at both Li(2) and N are obtained, and the sign at Li(1) is thus positive. The same signs are obtained from an ionic point-charge model. The extinction correction is important and dependent on the assumed signs. The model deformation map for the signs +, -, - at Li(1), Li(2) and N shows nearly spherical Li ions, whereas N appears to be strongly polarized perpendicular to c. With the assumption of the incorrect negative sign at Li(1), a strongly polarized Li(1) ion but approximately the same reliability indices for the X-ray data are obtained. Inclusion of NQR data in the charge-density refinement results in a better definition of the quadrupolar components of the deformation density close to the atomic centres.

35Cl NQR studies of bromine-doped hexachlorometallates

Chlorine-35 NQR in K 2(ReCl 6) 1− x (ReBr 6) x has been investigated as a function of concentration x in the temperature range 77 to 350 K. The NQR experiments have been complemented by far-infrared and X-ray diffraction measurements. Substitution by (ReBr 6) 2− raises the phase transition temperatures. The influence on 35C1 NQR depends on the temperature range. The NQR frequency v increases linearly with x at temperatures T > T c whereas v is reduced in the region T < T c. The observed concentration dependence of the NQR signal is explained by the change in the electric field gradient due to a volume effect.

Molecular Compounds of HgCl2 with Organic Donors. A 35Cl NQR Study

Molecular compounds of HgCl2 with organic donor molecules (dioxane, 1,3,5-trioxane, 1,3,5-trithiane, tetrahydrothiophene, 1,4-cyclohexandione, collidine, benzaldehyde, orthochlorobenzaldehyde, metachlorobenzaldehyde, parachlorobenzaldehyde) and the double salts K2HgCl4 • H2O and Rb2HgCl4 • H2O were studied by 35Cl NQR spectroscopy in the temperature range 77 K ≦ T ⪅ 300 K. The results are discussed with respect to the coordination around the Hg atom within the complex. The molecular compound, except those with sulfur-containing donors, seem to adapt an approximately octahedral coordination at the Hg atom. According to the 35Cl NQR results, HgCl2 may form dimers and trimers with orthochlorobenzaldehyde and parachlorobenzaldehyde, respectively. Isotypy of K2HgCl4 • H2O and Rb2HgCl4 • H2O is in accordance with the NQR experiments.

Temperature dependence of the electric field gradient in noncubic metals

The quadrupole hyperfine interaction of111Cd in Cd, In, Sn, and of67Zn,69Ge in Zn was measured with high accuracy using TDPAC and TDPAD methods; in addition the systems117In in Cd and In, and113Sn in Cd were investigated. The temperature dependence of the electric field gradienteq(T) can be reproduced by the simple relationeq(T)/eq(0)=1—B⋅T 3/2 over a range of temperatures from a few K up to the melting point. A comparison with other experiments was carried out to test the universality of this empirical relation. In particular, high precision NQR experiments on Ga, In and Sb strongly support this relation. It is discussed whether lattice vibrations can explain the observed temperature variation or whether the experimental data indicate the need for a new description. Some new ideas suggesting a basically different model are given.

Toroidal sample coil for nuclear quadrupole resonance experiments at high pressure

Use of toroidal sample coils for 14N NQR experiments at high pressure was found to improve the sensitivity of detection of the NQR signal in pressure cells constructed of steel. NQR measurements, in particular spin-lattice relaxation time T1 in solids, can therefore be performed in steel cells with the same sensitivity but to higher pressures than are possible in beryllium–copper pressure vessels.

Charge Distribution and Donor Properties in Complexes L·MeIIIX3 (L  ArCOAr, ArCOCl, ArOH; MeIII  Al, Ga; X  Cl, Br) and (ArOAlBr2)x. An NQR (35Cl, 81Br, 69Ga) and IR (ṽ(C  O), ṽ(CCl)) Investigation

AbstractMolecular compounds of the type L · MeX3 (L  benzophenones, benzoylchlorides, 4‐chlorophenol; MeX3  AlBr3, AlCl3, GaCl3) were studied by NQR and IR spectroscopy. Also, in two compounds of the type chlorophenoxy aluminumdibromide the halogen NQR was investigated.On the basis of the 35Cl and 81Br NQR experiments, benzoylchlorides appear to be weaker donors than benzophenones are. Yet, the shift of the carbonyl stretching frequency due to the formation of the complexes is less for benzophenones.In adducts of benzoylchlorides with AlCl3 the shift of the 35Cl NQR frequency (assigned to Cl linked to the carbonyl C atom), Δv(35Cl), and the shift of the CCl stretching frequency, Δṽ(CCl), were measured. A positive correlation of Δv(35Cl) with Δṽ(CCl) was found. On the basis of the 69Ga and 35Cl NQR frequencies measured for some molecular compounds L · GaCl3 the charge transfer was estimated with the aid of the Townes and Dailey Theory. A comparison with results obtained analogously for inorganic complexes incorporating GaCl3 is given. From the NQR experiments conclusions concerning the structure of the compounds investigated were drawn.