Nuclear quadrupole resonance in barium chloride dihydrate

Abstract

The pure nqr resonances of the la7Ba and lS5Ba nuclei in BaClz.2Hz0 were observed and measured using a noise-controlled superregenerative spectrometer.2 The resonances were observed and measured using a 25-g sample of BaClz*2Hz0 with the two isotopes present in their natural abundance, 11.32% for la7Ba and 6.59% for la6Ba. Frequency measurements were made a t 300°K. The resonances could not be detected at 77°K. Frequency measurements were made by modulating the quench frequency at a rate of 5-10 Hz, thereby allowing the carrier component of the superregenerative oscillator to be audibly identified on a communications receiver. The spectrometer carrier frequency, fs, which is identified with the resonance by virtue of the spectrometer drive being stopped when the recorder pen is on the center peak of the recorder display, mixes with the variable-frequency oscillator (VFO) signal, f l , of the receiver to produce an intermediate frequency (IF) signal of frequency fs -11. This IF in turn combines with the beat frequency oscillator (BFO) signal, fb, in the receiver to produce an audible output. The VFO is then tuned to produce a null in the audible signal. At the null point (fs -f l ) -f b = 0 and the spectrometer frequency is found by measurement of both the BFO and VFO signals. Frequency measurements were made using a Hewlett-Packard 524C electronic counter. The audible null could be ascertained to an accuracy of f100 Hz. The over-all accuracy of the frequency measurements is limited by the procedure of stopping the spectrometer sweep exactly on an absorption-peak maximum.The pure nqr frequencies of la7Ba and la5Ba in BaClz-2Hz0 were found to be 16.303 f 0.005 MHz (signa1:noise ratio 10) and 10.67 f 0.02 MHZ (signal: noise ratio 5), respectively. The spectrum observed for each isotope was clearly indicative of a single resonance. The observation of a single resonance is in agreement with the known crystal structure of BaC12. 2Hz0, where each barium atom in the unit cell is surrounded simultaneously by equivalent distorted tetrahedra of chlorine atoms and distorted tetrahedra of water molecules.~~4 A search down to 5 MHz revealed no additional resonances. The identification of the resonances as being those of barium rather than chlorine was established from the ratio of the frequencies of the two observed resonances, 1.528 f 0.005. This compares favorably with the reported nuclear quadrupole moment ratio, Q(137Ba)/Q(1s6Ba) = 1.537.5 There is Table I: Nqr Parameters for BaClz. 2H20" -Calod valuesBa*+ Ba1.8+ c E x p t l valuesc--Parameter c1-Cl0.0-7 = 0 7 = 0.78

Solid-State 137Ba NMR Spectroscopy: An Experimental and Theoretical Investigation of 137Ba Electric Field Gradient Tensors and Their Relation to Structure and Symmetry

“…Simulations of the spectra (Figure ) reveal a single barium site, in agreement with the crystal structure . The C Q value is similar to that obtained from NQR; however, the η Q value of 0.78(14) is different. , The η Q value obtained from our NMR experiments is likely more accurate, since the previously reported η Q value was estimated from calculations and quadrupolar parameters of analogous systems, and not directly measured in the NQR experiments (NQR experiments on spin-3/2 nuclei do not readily permit determination of η Q , unless special techniques are applied). , The barium atom is coordinated to four oxygen and five chlorine atoms (Scheme d), with Ba−O bond distances varying from 2.844 to 2.887 Å and Ba−Cl distances varying from 3.131 to 3.340 Å . The chlorine and oxygen atoms are arranged such that no simple geometrical assignments of shape can be made; in fact, the arrangement of atoms has been described “approximately as a square antiprism with one square face enlarged to accommodate the fifth Cl − ion” .…”

“…The 137 Ba quadrupolar parameters of barium chloride dihydrate were previously obtained from NQR experiments on single crystal 66 and polycrystalline samples. 67 In the former case, the extraction of the quadrupolar parameters was complicated by crystal twinning, while, in the latter case, a very large amount of crystalline sample was used (i.e., 25 g) in order to obtain the signal (again, this can be inconvenient and impractical in many situations where sample sizes are limited). In contrast, our 137 Ba SSNMR spectra were obtained at 9.4 and 21.1 T using a small amount of a powdered sample (i.e., 0.2 g).…”

“…56 The C Q value is similar to that obtained from NQR; however, the η Q value of 0.78 (14) is different. 66,67 The η Q value obtained from our NMR experiments is likely more accurate, since the previously reported η Q value was estimated from calculations and quadrupolar parameters of analogous systems, and not directly measured in the NQR experiments (NQR experiments on spin-3/2 nuclei do not readily permit determination of η Q , 68 unless special techniques are applied). 69,70 The barium atom is coordinated to four oxygen and five chlorine atoms (Scheme 1d), with Ba-O bond distances varying from 2.844 to 2.887 Å and Ba-Cl distances varying from 3.131 to 3.340 Å.…”

“…Ba quadrupolar parameters of barium chloride dihydrate were previously obtained from NQR experiments on single crystal 66 and polycrystalline samples. 67 In the former case, the extraction 137 Ba static SSNMR spectra of barium carbonate at two different magnetic field strengths.…”

135Ba and137Ba Fourier transform NMR and NQR studies

“…[16][17][18][19] and NQR (Ref. [19][20][21][22] results in solids and solutions measurements and calculations of the following quantities are important: ratio of the Larmor frequencies of the barium nuclei to that of a standard nucleus, nuclear magnetic moments, chemical shifts, shielding constants, linewidths and relaxation rates in aqueous and non-aqueous solutions, ratios of the quadrupole frequencies and quadrupole moments, NMR signals at low concentrations and natural abundance. In the present work our results on some of the mentioned aspects are described.…”