FT NQR Research Articles

Structural and magnetic phase transitions in (Chloroanilinium)2CuX4 (X = Cl, Br)

Successive structural phase transitions of (4-ClC6H4NH3)2CuCl4, which occur in a very narrow temperature range were reinvestigated by Fourier transform nuclear quadrupole resonance (FT NQR) measurements. The phase transitions at 275.5 and 277.0 K were confirmed. The effect of the deuteration of the ammonium end on these transitions was studied. The35Cl NQR frequencies of organic cation were observed to decrease by about 4 kHz and the phase transition temperatures to decrease by about 2 K by the deuteration, suggesting that the −NH3+ … Cl hydrogen bond is weakened by the deuteration. The magnetic phase transition temperature of 8.6 K showed no remarkable change within experimental error by the deuteration. It was found that the magnetically ordered state is broken by the radio-frequency magnetic field of about 15–35 Oe usually employed in pulsed NQR. However, in the deuterated compound (4-ClC6H4ND3)2CuCl4, the ordered state was found to be stabler for the usual radiofrequency power. By combining with the NQR data of (4-ClC6H4NH3)2CuBr4 and (3,5-Cl2C6H3NH3)2CuCl4, the possibility is discussed of tuning the interlayer interaction between the organic cation layer and the inorganic complex anion layer by the halogen substitution in the organic cation as well as by the halogen replacement in the inorganic complex anion.

Zeeman-Perturbed Spin-Echo FT NQR Spectroscopy

An approach for the evaluation of the asymmetry parameter, η, of the electric field gradient tensor for spin I = 3 2 systems from Zeeman NQR spectroscopy of polycrystalline samples is presented. The method relies on Fourier transformation of the second half of the Zeeman-perturbed Hahn spin-echo signal, generated via a two-pulse excitation sequence. Suitable phase cycling of the RF pulses of the spin-echo sequence has been derived and implemented, for the first time in NQR spectroscopy, to retain only the echo component in the detected signal. The RF pulse phase-cycling scheme employed in this work permits the extraction of the parameters of interest with improved sensitivity. A numerical procedure for simulating the powder Zeeman spin-echo FT NQR spectra is also given. A comparison of the theoretically simulated spin-echo spectrum with that recorded experimentally is shown to permit the evaluation of η. The method has been successfully demonstrated for the two 35Cl sites in 2,4,6-trichloro-1,3,5-triazine ("cyanuric chloride").

Single Crystal 35Cl NQR of 2,3,4,4-Tetrachloro-1-oxo-1,4-dihydronaphthaline and of 2,2,3,4-Tetrachloro-1 -oxo-1,2-dihy dronaphthaline

Abstract 2,3,4,4-Tetrachloro-1-oxo-1,4-dihydronaphthaline, ß-TKN, crystallizes in the orthorhombic space group D1 6 2 h - Pnma with Z = 4, and the 35Cl NQR spectrum consists of three lines with an intensity ratio of 2:1:1. The crystal structure of 2,2,3,4-tetrachloro-1-oxo-1,2-dihydronaphthaline, α-TKN, is described in literature as belonging to the polar space group C2 2 - P 21 with Z = 2. In contradiction to the reported space group and Z, we observed a three line 35Cl NQR spectrum with an intensity ratio of 2:1:1. Therefore, the crystal structure of α-TKN was redetermined at 7 = 296 K: space group C2 2h-P21/m, Z = 2, a = 888.4 (2) pm, b = 692.3 (2) pm, c = 869.6 (2) pm, β = 91.483 (5)°. The nuclear quadrupole coupling tensors have been investigated using a 4 π Zeeman NQR goniometer and FT NQR for tracing the zero splitting cones. According to the crystal structure of β-TKN, two symmetry related zero splitting cones have been found for Cl(2), two for Cl(3), and four for Cl(4) ( T = 297 K). From the single crystal Zeeman split NQR measurements of α-TKN one zero splitting cone has been found for Cl(3), one for Cl(4), and two for CI(2) (T=290 K). The asymmetry parameters η and the direction cosines of the principal axes Φxx, and Φyy, and Φzz of the nuclear quadrupole coupling tensors have been determined. The results are: Cl(j), v (35Cl(j)), e2 Q q h -1 (35Cl(j)), η(35Cl(j)) for β-TKN (at T = 297 K): Cl(2), 37.442 MHz, 74.469 MHz, 0.184; Cl(3), 36.854 MHz, 73.083 MHz, 0.160; Cl(4) , Cl(4'), 38.685 MHz, 77.307 MHz, 0.070 and for α-TKN (at T = 290 K): Cl(2), CI(2'), 38.297 MHz, 76.501 MHz, 0.086; Cl(3), 36.943 MHz, 73.700 MHz, 0.123; Cl(4), 36.311 MHz, 72.314 MHz, 0.160. By comparing the crystal structures and the 35Cl NQR results, the C-Cl bonds in the title compounds are discussed.

Phase transitions and anionic dynamics of dimethylammonium hexachlorotellurate(IV) as studied by pulsed nuclear quadrupole resonance of chlorine

The temperature dependence of 35Cl NQR frequencies has been reinvestigated for dimethylammonium hexachlorotellurate(IV) using an FT NQR spectrometer. A new line is observed just above the lowest-frequency line already reported for the intermediate-temperature phase. In the lowest-temperature phase, the temperature dependence is definitely determined by finding new lines. The NQR frequencies can be precisely measured even in the vicinity of the phase transitions. The temperature dependence of the nuclear quadrupole relaxation time, T 1Q of 35Cl and 37Cl nuclei has also been observed. In a temperature range 64–160 K, the isotope ratio, T 1Q( 37Cl)/ T 1Q( 35Cl) is 1.5 for each line, suggesting that the quadrupolar relaxation arises mainly from the libration of the complex anion. Above 160 K, T 1Q decreases very rapidly with increasing temperature for each line. This can be interpreted in terms of the onset of the reorientation of the anion as a whole, which is responsible for the fade-out phenomenon of the two lines occurring near 220 K. The nature of the phase transitions and the anionic dynamics are discussed in detail. Especially, the phase transition at 96 K which is rather unusual and is explained through softening of the librations and the rotational displacement of the anions successively operated.

Phase transition and anionic motion in solid [(CH3)2NH2]2SnCl6 as studied by the temperature dependence of 35Cl NQR frequencies and chlorine nuclear relaxation times determined with a pulsed NQR spectrometer

AbstractThe temperature dependence of 35Cl NQR frequencies of [(CH3)2NH2]2SnCl6 was precisely determined between 78 and ca. 250 K. by means of an FT NQR spectrometer. The high‐frequency line of the two 35Cl NQR signals having the intensity ratio of 2:1 already reported was observed to be split below 102 K. into sharp and broad lines with approximately equal integrated intensify. This clearly indicates the occurrence of a structural phase transition of a second‐order nature at this temperature. The presence of phase transition around this temperature has been anticipated from the IR and NQR studies previously done. The temperature dependence of chlorine nuclear spin‐lattice relaxation time, T1Q shows a dip with a clear depression at 102 K. The mechanism of the phase transition is discussed from the T1Q results and the fact that the frequency separation of the doublet increases with decreasing temperature. Above 180 K, T1Q of both lines decreased rapidly and the log T1Q vs. T−1 curves were almost linear. This decrease of T1Q was explained through the onset of the C4 reorientation of the anions with the activation energy of ca. 35 kJ mol−1. The fade‐out phenomenon of the two NQR lines occurring far below room temperature can be well explained by the onset of the above motion with the very low activation energy.

35Cl NQR zeeman study of perchloro-bicyclopentadienyl by a pulsed FT nor spectrometer

Zeeman measurements were performed on 35Cl NQR lines in a single crystal of perchloro-bicyclopentadienyl at room temperature by a pulsed FT NQR system. The compound gives ten NQR lines : ν 1 = 36347.6, ν 2 = 36361.2, ν 3 = 36634.2, ν 4 = 36684.8, ν 5 = 36715.5, ν 6 = 37008.0, ν 7 = 37194.4, ν 8 = 37266.9, ν 9 = 38871.1, and ν 0 = 39323.8(kHz) at 291.5 K. Each line yields two electric field gradients (efgt's) in a magnetic field. The asymmetry parameters (η) of ν 1, ν 2, ν 3, ν 4, ν 5, ν 6, ν 7, ν 8, ν 9 and ν 0 are 14.1, 14.0, 14.2, 13.9, 14.5, 9.6, 10.3, 12.1, 4.2 and 4.8%, respectively. From the relative orientations between the principal axes of the efgt's, the molecular structure has been deduced. The molecule has a cisoid conformation rotated by 51 o from the pure s -cis form. The rotation angle agrees well with the value calculated using Urey-Bradley force field constants.

ChemInform Abstract: ZEEMAN NQR STUDY OF A PERCHLOROPENTAFULVALENE SINGLE CRYSTAL

Abstract The 35Cl NQR Zeeman effect in a single crystal of perchloropentafulvalene has been studied for the four lines ν1 = 37245.5, ν2 = 37080.0, ν3 = 36095.0 and ν4 = 36019.1 kHz at 291.5 K by a pulsed FT NQR system. The asymmetry parameters of ν1, ν2, ν3 and ν4 are 0.10, 0.11, 0.15 and 0.17, respectively. The twist angle between the two five-membered rings is estimated to be 40°, obtained from the directions of the principal X axes for ν3 and ν4. The results of this work are compared with those of an X-ray three-dimensional analysis.

Zeeman NQR study of a perchloropentafulvalene single crystal

The 35Cl NQR Zeeman effect in a single crystal of perchloropentafulvalene has been studied for the four lines ν 1 = 37245.5, ν 2 = 37080.0, ν 3 = 36095.0 and ν 4 = 36019.1 kHz at 291.5 K by a pulsed FT NQR system. The asymmetry parameters of ν 1, ν 2, ν 3 and ν 4 are 0.10, 0.11, 0.15 and 0.17, respectively. The twist angle between the two five-membered rings is estimated to be 40°, obtained from the directions of the principal X axes for ν 3 and ν 4. The results of this work are compared with those of an X-ray three-dimensional analysis.

35Cl NQR Zeeman study of perchloro-2-cyclopentenone by a pulsed FT NQR spectrometer

Zeeman measurements have been performed on the 35Cl NQR lines of a single crystal of perchloro-2-cyclopentenone at room temperature using a pulsed FT NQR system. The compound exhibits six NQR lines, ν(1) = 36 589.4, ν(2) = 37 383.7, ν(3) = 38 631.7, ν(4) = 38 812.0, ν(5) = 39 429.7 and ν(6) = 39 750.0, at 288 K. Each line yields two electric field gradient tensors in a magnetic field. The asymmetry parameters (η) of ν(1), ν(2), ν(3), ν(4), ν(5) and ν(6) are 18.3, 12.8, 1.9, 0.8, 4.9 and 5.6%, respectively. From the relative orientations of the principal axes of the field gradient tensors, the molecular structure has been deduced. The five-membered ring is puckered by ca. 30°. The η values and the nature of the bonding are discussed in connection with the molecular structure.

Zeeman study of 35Cl NQR in tetrachlorocyclopentene-1,3-dione

The 35Cl NQR Zeeman spectrum in a single crystal of tetrachlorocyclopentene-1,3-dione has been studied for the two lines, 38734.1 (H) and 35667.0 (L) KHz at 21°C by a pulsed FT NQR spectrometer equipped for automatic measurements. Each line yields 12 efgt's in a magnetic field. Their directional symmetry (three two-fold and four three-fold axes) indicates that the crystal system is cubic. This is confirmed by its X-ray powder pattern and inspection with the polarization microscope; space group Fd3 or Fm3, a = 21.910 A ̊ , Z = 48, D c = 1.78, D o = 1.75 . The molecular structure and the molecular orientations in the unit cell are determined from the results. The η-values are 5.8 and 21.6% for H and L, respectively.

Zeeman study of a boron trichloride single crystal by pulsed FT NQR spectroscopy

A 35Cl Zeeman NQR study of a single crystal of boron trichloride yields the value of the field-gradient asymmetry parameter of 0.557±0.011, in agreement with previous values of 0.54 and 0.50 obtained from studies of polycrystalline samples. Apart from the well known doublet due to an isotope effect, further fine structure whose details depend on the orientation of the Zeeman field is observed. It is shown that this fine structure arises from 11B–35Cl dipole–dipole interaction.

Zeeman nuclear quadrupole resonance study of a germanium tetrachloride single crystal

Zeeman studies of a single crystal of GeCl 4 have been carried out on a previously described pulsed FT NQR spectrometer. The technique for growing single crystals from liquid samples is described. The Zeeman-split spectrum for each of the resonances at μ 1 = 25.4493, μ 2 = 25.7133, μ 3 = 25.7354, and μ 4 = 25.7457 MHz reveals four quadruplets. The asymmetry parameters found lie in two groups of average values η = 0.035 and η = 0.078, much lower than the previously reported value of η = 0.35 obtained from a polycrystalline sample. The crystal of GeCl 4 is found to be orthorhombic with mmm or D 2 h symmetry. From the directions of the field gradient axes the interbond angles have been calculated and show that the GeCl 4 molecule is a slightly distorted tetrahedron.

35Cl Nqr zeeman study of perchloro-3-cyclopentenone by a pulsed ft nqr spectrometer

The 35Cl NQR Zeeman spectrum in a single crystal of perchloro-3-cyclopentenone has been studied for the three lines, v 1=36969.3, v 2=37992.2, and v 3=38365.0 KHz, at 294 K by a pulsed FT NQR spectrometer equipped for automatic measurements. The results of the analyses revealed that the molecule has a C 2 -symmetry in its crystal and that the five-membered ring is almost planar. The asymmetry parameters for v 1, v 2, and v 3 are determined to be 0.144, 0.027, and 0.035, respectively. The frequency split for v 2 and v 3 was explained by the difference in the hyperconjugation between the CO and the C-Cl σ-bonds for the geminal chlorines. The molecular structure was derived from the results.

Automation of a pulsed FT NQR spectrometer with increased s/n ratio and resolution for zeeman studies of single crystals

With a view to determine the parameters of the electric field gradient tensor from Zeeman measurements in a simple and precise way, we have adopted the system in which a Bruker pulse spectrometer is coupled to a NicoLet 1074 signal averager and a PDP 8/E mini-computer. The automation of the measurements and the precision of the experiment have been achieved by the development of a series of programs in the Assembler language and in FORTRAM and by the fruitful utilisation of the software instructions associated with the PDP-NICOLET INTERFACE, OS/8 Monitor system, and USR (User Service Routine) to replace the Keyboard Monitor. In most cases, the complete EFGT parameters can be derived from the rotation of the crystal around the r.f. axis alone, - for which the requirement of the goniometer is simple. In other cases, a second perpendicular rotation is needed to select the correct orientation of the Z-principal axis. For this, a two-circle goniometer has been constructed. Both these goniometers can be thermostatically stabilised at different preselected temperatures. The result of the experiment with a single crystal of CH 2ClCOOH is presented.