Showing papers on "Nuclear quadrupole resonance published in 1968"
TL;DR: In this article, a nuclear double-resonance method was used to measure the magnetic order of abundant nuclei which are dipolar-coupled to the low-abundance nuclear species.
Abstract: The pure nuclear quadrupole resonance of a low-abundance spin species in zero field is observed for molar concentrations as low as one part in ${10}^{7}$ by the application of a nuclear-double-resonance method. The quadrupole resonance is measured in terms of a decrease in the magnetic order of abundant nuclei which are dipolar-coupled to the low-abundance nuclear species. The double-resonance process is analyzed from the point of view of two energy reservoirs coupled by a dipolar perturbation, and spin diffusion among abundant nuclei is included phenomenologically. Radio-frequency power, which causes the main quadrupole transitions of rare spins, is frequency-modulated to provide saturation in the rotating frame and to enable identification of unknown spin transitions. The pure nuclear quadrupole resonance of Na and Cl nuclei near impurities and imperfections in NaCl is measured. Particular attention is given to the resonances associated with ${\mathrm{K}}^{+}$ and ${\mathrm{Br}}^{\ensuremath{-}}$ ions injected as impurities into the NaCl lattice. An unsuccessful search in zero field has been made for the naturally abundant 0.0156% deuterium quadrupole resonance in CaS${\mathrm{O}}_{4}$\ifmmode\cdot\else\textperiodcentered\fi{}2${\mathrm{H}}_{2}$O and ${\mathrm{C}}_{10}$${\mathrm{H}}_{8}$, where the proton resonance is monitored as the abundant nuclear species. The proton dipolar absorption in zero magnetic field is anomalously broad and overlaps too much with the deuterium quadrupole resonance to permit independent observation of the latter. A narrowing of proton dipolar absorption in zero field is observed for strong radio-frequency fields larger than internal dipole fields.
128 citations
TL;DR: In this paper, the 35Cl NQR frequencies of a number of chlorobenzene derivatives have been measured and the frequencies have been correlated with characteristic parameters, such as Taft's σI and σR.
Abstract: The 35Cl NQR frequencies (at 77°K) of a number of chlorobenzene derivatives have been measured. The frequencies (including additional data from the literature) have been correlated with characteristic parameters, such as Taft's σI and σR. It is found that the Bray–Barnes relation can be somewhat improved by introducing a parameter κ: ν = ν0 + ∑ iκi. The following relations are found: (a) the effects of more than one substituent on the benzene are additive; (b) steric effects (o‐substituents) are not important for the NQR frequencies; (c) a correlation between the NQR frequency ν and Taft's σI and σR (correlation coefficient r = 0.976) shows the importance of the inductive effect on NQR. The mesomeric effect plays only a minor role in 35Cl NQR in benzene derivatives. Unknown σI values may be gained from NQR measurements; for the evaluation of σR, the determination of the chemical shift of 19F NMR seems to be the better method.
50 citations
TL;DR: The semi-empirical SCF-MO-CNDO theory was 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 as discussed by the authors.
Abstract: 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.
43 citations
TL;DR: In this article, the σ and π charge densities at the ring nitrogen are discussed in terms of the relative electrophilic properties of the substituents and excellent qualitative agreement with the classical ideas of chemistry is obtained.
Abstract: Nitrogen‐14 nuclear quadrupole resonance has been observed in 15 substituted pyridines, including cyano, acetyl, and chloro derivatives and the methyl esters of the pyridine monocarboxylic acids. The substituents produce variations of 5%–10% in the field gradient at the nitrogen. The σ and π charge densities at the ring nitrogen are discussed in terms of the relative electrophilic properties of the substituents and excellent qualitative agreement with the classical ideas of chemistry is obtained. Correlation of the nitrogen NQR data with the Hammett σ values and with chlorine‐35 NQR results is also considered. The charge distribution of the cyano group is analyzed and discussed in connection with the ring nitrogen. The ionic character of the cyano C–N bond is found to be approximately 17%.
37 citations
TL;DR: In this paper, a detailed study of the pure nuclear quadrupole resonance of 14N in solid triethylene diamine is described, and the resonance frequency, spin-lattice relaxation time, and free-induction decay time are measured as a function of temperature from 80°K to the plastic transition at 351°K.
Abstract: A detailed study of the pure nuclear quadrupole resonance of 14N in solid triethylene diamine is described. The resonance frequency, spin–lattice relaxation time (T1), and the free‐induction decay time (T2) were measured as a function of temperature from 80°K to the plastic transition at 351°K. From 200°–300°K T1 is dominated by hindered molecular rotations about the trigonal axis of the molecule modulating the dipolar interaction with the protons. The activation for this reorientation was found to be 8.17 kcal/mole, in reasonable agreement with NMR results of Smith. The linewidth is discussed in detail and indicates an acentric structure of the molecule. Effects of the approach to the plastic transition are evident in all the three quantities measured but could not be interpreted in detail.
36 citations
TL;DR: In this article, the crystallographic phases and phase transitions of molecular crystals of malononitrile were investigated using 14N pure nuclear quadrupole resonance, and the resonance frequencies, linewidths, and spin-lattice relaxation times were measured using pulse techniques.
Abstract: The crystallographic phases and phase transitions of molecular crystals of malononitrile were investigated using 14N pure nuclear quadrupole resonance. The resonance frequencies, linewidths, and spin–lattice relaxation times were measured using pulse techniques. The relation between quadrupole resonance results and the order parameter of a phase transition are analyzed. Detailed measurements were carried out near one of the transitions (at 294.7°K) and the results are interpreted in terms of critical exponents. The resonance frequencies give β = 0.5 ± 0.02 and the linewidths γ = 0.3 ± 0.07 and γ′ < 0.1 ± 0.1. Our quadrupole resonance results are compared with specific‐heat measurement of Girdhar, Westrum, and Wulff. As two of the phase transitions show very small transition entropies, it is suggested that these are displacive phase transitions associated with instabilities in the collective modes of the crystal. The meaning of such transitions is discussed both from a thermodynamic and a microscopic point...
32 citations
TL;DR: In this paper, the NQR spectra of the three isomeric aminopyrides and the five aminomethylpyrides have been analyzed in terms of the electronic populations of the ring-nitrogen pπ and σ orbitals.
Abstract: The nitrogen‐14 nuclear quadrupole resonance (NQR) spectra of the three isomeric aminopyrides and the five isomeric 2‐aminomethylpyrides have been observed. The data is interpreted in terms of the electronic populations of the ring‐nitrogen pπ and σ orbitals. The results show that the amino group is a strong π‐electron donor and, in addition, behaves as a weak σ‐electron donor. A model for the amino group which is consistent with the NQR data is presented.
30 citations
TL;DR: In this article, the nuclear quadrupole resonance frequencies of the halogen atoms in the trihalide ions 13 and Ibr2 in some of their crystalline salts have been measured.
Abstract: The nuclear quadrupole resonance frequencies of the halogen atoms in the trihalide ions 13 and Ibr2 in some of their crystalline salts have been measured. The results have been correlated with a simple LOAD-MO bonding description and the charge distributions in the ions have been calculated. These ions can be regarded as charge-transfer complexes as described by Mulliken. The effect of the crystal field on the charge distribution in the I3 ion has been investigated.
29 citations
29 citations
24 citations
TL;DR: In this article, the Pople theory of chemical shift has been employed to present a theoretical interpretation for observed nitrogen chemical shifts in heteroaromatic molecules, and it was found that the chemical shifts of the diazines depend on both the pi and sigma-electronic distributions on the nitrogen atoms.
Abstract: The Pople theory of chemical shift has been employed to present a theoretical interpretation for observed nitrogen chemical shifts. In the absence of low‐lying n→π* transitions which give rise to a large paramagnetic shift, the shielding of nitrogen, as predicted by theory, arises primarily from: (1) the electron density on nitrogen; (2) the mean‐excitation energy; and (3) the effect of multiple bonds of nitrogen. A theoretical expression was also derived for calculations of nitrogen chemical shifts in heteroaromatic molecules. It was found that the chemical shifts of the diazines depend on both the pi‐ and sigma‐electronic distributions on the nitrogen atoms. The variations in the nitrogen sigma‐electronic densities estimated from chemical shift data are in reasonable agreement with those observed in nuclear quadrupole resonance studies.
TL;DR: The 14N nuclear quadrupole resonance frequencies of aniline, o- and pphenylene diamine, and pchloro-, p-bromo-, and p-iodo-aniline were measured with a super-regenerative oscillator over a temper...
Abstract: The 14N nuclear quadrupole resonance frequencies of aniline, o- and p-phenylene diamine, and p-chloro-, p-bromo-, and p-iodo-aniline were measured with a super-regenerative oscillator over a temper...
TL;DR: In this article, the 14N nuclear quadrupole resonance spectrum of ferroelectric sodium nitrite is interpreted to yield the charge distribution of the nitrite radical, and contributions to the electric field gradient due to lattice changes are estimated by performing a lattice sum, while the local electrons are treated using the approximation of the Townes and Dailey theory.
Abstract: The 14N nuclear quadrupole resonance spectrum of ferroelectric sodium nitrite is interpreted to yield the charge distribution of the nitrite radical. Contributions to the electric field gradient due to lattice changes are estimated by performing a lattice sum, while the local electrons are treated using the approximations of the Townes and Dailey theory. Occupation numbers for the N–O sigma‐ and pi‐bond wavefunctions are obtained, and the effective charge on the nitrogen is deduced. The latter quantity is found to be very close to zero.
TL;DR: In this paper, the authors examined the 81Br nuclear quadrupole resonance spectra for a series of complexes of CBr4 with methyl-substituted benzenes and pyridines.
Abstract: The 81Br nuclear quadrupole resonance spectra have been examined for a series of complexes of CBr4 with methyl‐substituted benzenes and pyridines. The p‐xylene−CBr4 complex gives two lines as expected from the known structure, and the benzene–CBr4 spectrum is the same, with a lower splitting. The spectra of a number of other complexes of unknown structure do not correlate simply with these. Estimates of the direct crystal‐field gradients due to the charge distribution in the aromatics suggest that the direct field gradient effects are much too low to account for observed shifts in quadrupole frequencies. The temperature dependencies of the resonances in the p‐xylene and durene complexes have been studied. The p‐xylene complex follows the Bayer equation, but the durene complex requires the inclusion of a temperature dependence of the torsional frequency. Both complexes show a fading of the resonances at about 50° below the melting points.
TL;DR: In this article, a detailed analysis of the cross-relaxation experiment and of the anisotropies in spin-lattice relaxation leads to a determination of the relative orientation of the principal axes system of the field-gradient tensors at the two sites.
Abstract: Linewidths and spin–lattice relaxation times of the six nuclear quadrupole resonance lines of 14N in hydrazine were measured from 200° to 260°K using pulse technique. Cross‐relaxation (Overhauser) experiments between corresponding transitions at the two sites were also performed. It is shown that in this temperature range the dominant relaxation mechanism is a molecular motion which interchanges nuclei between the two inequivalent crystallographic sites. A detailed analysis of the cross‐relaxation experiment and of the anisotropies in spin–lattice relaxation leads to a determination of the relative orientation of the principal axes system of the field‐gradient tensors at the two sites. The molecular motion has an activation energy of 14.3 ± 0.4 kcal/mole and a rate ∼ 2500 sec−1 at 250°K.
TL;DR: In this paper, the nuclear quadrupole resonance spectra of substituted tricarbonylmanganese cyclopentadienyls were investigated and the influence transmitted from the substituent in the ring to the metal according to a conjugation mechanism to a greater degree than according to an induction mechanism.
TL;DR: In this paper, the nuclear quadrupole resonance frequencies of antimony trichloride were measured between 20°K and about 150°K to learn molecular motions and intermolecular forces in the solid.
Abstract: Nuclear quadrupole resonance frequencies of 121 Sb, 123 Sb, and 35 Cl in solid antimony trichloride were measured between 20°K and about 150°K to learn molecular motions and intermolecular forces in the solid. By use of a newly computed eigenvalue table, the quadrupole coupling constant and the asymmetry parameter of 121 Sb and 123 Sb were calculated. The asymmetry parameter decreases from 0.192 at 20°K to 0.180 at 140°K. The results were interpreted in terms of librational motion and weak intermolecular covalent bonds between Sb and Cl atoms.
TL;DR: In this paper, the mean-squared angular amplitudes of vibration are calculated for a one-dimensional crystal model with torsional nearest-neighbor interaction, and the effect is shown to be amplification of amplitude of magnitude dependent on the extent of interaction.
Abstract: Mean‐squared angular amplitudes of vibration are calculated for a one‐dimensional crystal model with torsional nearest‐neighbor interaction. Vibrational modes with nonzero wave vector are all included in the averaging, and the effect is shown to be amplification of amplitude of magnitude dependent on the extent of interaction. Raman spectra for the 0–100‐cm−1 region of crystalline p‐dichlorobenzene and p‐dichlorobenzene‐d4 are reported and an assignment suggested for these librational fundamentals. Raman frequencies so assigned are used in an approximate calculation of the temperature dependence of nuclear quadrupole resonance frequencies of p‐dichlorobenzene and the results compared with experiment. An interpretation is given of the result of Dinesh and Narasimhan for the nuclear quadrupole resonance frequency of HgCl2.
TL;DR: The 35Cl pure quadrupole resonance spectrum of the chlorine atoms in trichlorocyclopropenium ion has been measured and the asymmetry parameter determined by the effect of a weak magnetic field on the spectrum of polycrystalline sample as mentioned in this paper.
Abstract: The 35Cl pure quadrupole resonance spectrum of the chlorine atoms in the trichlorocyclopropenium ion has been measured and the asymmetry parameter determined by the effect of a weak magnetic field on the spectrum of the polycrystalline sample. These data indicate that the carbon–chlorine bond in this compound has 16%π-character and that somewhere between one half and three quarters of the excess of positive charge on the carbon atoms has been transferred to the chlorine substituents.
TL;DR: In this paper, it was shown that the quadrupole coupling constant e2Qqmol/h = 508·296 Mc/s can be rationalized on the basis of the theory of Townes and Dailey if 50% s2-hybridisation and 14 per cent ionicity are attributed to the gold-iodine σ-bond.
TL;DR: In this paper, a logarithmic super-regenerative spectrometer was used to obtain the sideband responses to a nuclear quadrupole resonance line. But the results were not compared with those obtained with a new type of spectrometers in which a normally incoherent super-generative oscillator was made coherent by injecting an external r.f. voltage into its tank circuit.
Abstract: The shapes of the sideband responses to a nuclear quadrupole resonance line obtained with a logarithmic super-regenerative spectrometer are discussed and compared with those obtained with a new type of spectrometer in which a normally incoherent super-regenerative oscillator is made coherent by injecting an external r.f. voltage into its tank circuit. The phases and amplitudes of the Fourier components of the r.f. output of the locked super-regenerative oscillator are derived and its line shape response is shown to depend on the frequency difference between the locking signal and the r.f. pulses in the super-regenerative oscillator. By controlling this difference, any desired mixture of absorption and dispersion line shapes is obtainable. Other advantages are that the frequency stability is improved, the coherence is independent of quench parameters, and sideband suppression and frequency measurement are both simplified. A practical locked super-regenerative spectrometer is described in which mechanical ganging between the tank circuits of the two oscillators is supplemented by an automatic phase control negative feedback loop. An output signal can also be obtained from the automatic phase control system and it is shown that this enables separate absorption and dispersion line shape responses to be obtained simultaneously. The actual super-regenerative oscillator circuit requires no adjustments in order to maintain full sensitivity over wide tuning ranges. The performance of the spectrometer is illustrated by recordings of 35Cl nuclear quadrupole resonance in KClO3 and in the K form of (PNCl2)4.
TL;DR: Nuclear quadrupole resonance studies as mentioned in this paper confirm the existence of α-PdCl2, β-Cl2 and β-PtCl2 structures, and show that whilst the βCl2 structure is distorted, the α-Cl 2 structure is regular.
Abstract: Nuclear quadrupole resonance studies confirm the existence of α-PdCl2, β-PdCl2, and β-PtCl2, and show that whilst the β-PtCl2 structure is distorted the β-PdCl2 structure is regular. The magnitudes of the coupling constants show that the α- and β-structures have nearly identical electron distributions, all differences being due, presumably, to changes in the metal–chlorine–metal bond angle.
TL;DR: In this paper, the nitrogen-14 nuclear quadrupole resonance spectrum of simple nitrites has been observed and found to be relatively insensitive to the particular crystal lattice, and the results are interpreted in terms of the electronic structure of the NO − 2 ion.
TL;DR: In this paper, a Zeeman study of the nuclear quadrupole resonance spectra of 35Cl using frozen samples of chlorobenzene has been carried out at 77°K.
Abstract: A Zeeman study of the nuclear quadrupole resonance spectra of 35Cl using frozen samples of chlorobenzene has been carried out at 77°K. The experimental lower limit obtained for the asymmetry parameter (η) at the chlorine site is 0.10 ± 0.01. A theoretical estimate of the η value using Bersohn's analysis yields a value of 0.092 in agreement with experiment.