Koichi Mano

35Cl-NQR of some chlorocarbons and oxochlorocarbons with a small ring.

35Cl-NQR of two chlorocarbons and six oxochlorocarbons, all having a small ring, were measured in the temperature range 77–300 K. The NQR line groups for the allylic and vinylic chlorine atoms appear separately in the five-membered ring compounds, whereas they are concentrated in a rather narrow region near 38 MHz in the four-membered ones. Perchlorocyclopropene possesses at least three crystal modifications at 77 K, the most stable one of which has only two lines in the temperature range examined. The line assignments, and the hyperconjugative effect in some of them, are discussed with reference to the CNDO/2 MO calculation data.

Zeeman effect of 35Cl NQR in perchloro-5-methylcyclopentadiene

Abstract Perchloro-5-methylcyclopentadiene gives five NQR lines at 77 K: v 1 = 40.769, v 2 = 39.761, v 3 = 39.433, v 4 = 37.905, and v 5 = 37.086 MHz, all without fading out below its mp, 366 K. The Zeeman spectrum of this compound revealed that the crystal exhibits an orthorhombic lattice with the mm2 or mmm symmetry and that the principal Z axes of efg tensors for v 2 and v 3 are parallel to each other. The signal assignments for v 2 and v 3 were made by comparison of their S N ratios at 365 K. The lines v 1 and v 3 were assigned to the CCl 3 group. The crystal and molecular structures were discussed on the basis of the results of the Zeeman-effect analyses. The frequency difference (ca. 1.3 MHz) between v 1 and v 3 was interpreted in terms of “homohyperconjugation” and a transannular interaction of the chlorine atom corresponding to v 3 with the diene system.

Polymorphism of octachloropropane as shown by 35Cl NQR and thermal analysis

Abstract It was found that the room-temperature phase (Crystal I) of octachloropropane (OCP) is a plastic crystal and that polymorphism exists in a low-temperature region. A lowtemperature stable phase (Crystal II) is transformed into Crystal I at 243 K, and two other metastable phases, Crystal III and Crystal IV, are transformed to a supercooled state of Crystal I at 239 and 231 K, respectively. In addition, there are possibly two other metastable phases (Crystal V and Crystal VI). The solid phase obtained by quenching Crystal I into a liquid nitrogen bath (Crystal V) is likely to contain random orientations of the molecules, as judged by the absence of a 35 CI NQR signal. It is worth noting that the presence of impurity affects the transition temperatures as well as the melting point, and that recrystallization at ca. 200 K is an effective method for the purification of OCP. The NQR signals observed for Crystal II were found to fade out at the transition temperature from Crystal II to Crystal I; the fade-out phenomenon is presumably due to overall rotations of the molecules.

Application of Maximum Entropy Method (MEM) for Precise Determination of NQR Frequencies

Abstract The maximum entropy method (MEM) is applied to the analysis of the closely spaced nuclear quadrupole resonance spectra of 35Cl in hexachlorobenzene and 14N in trarts-azobenzene. The superiority of the MEM to usual FFT spectra is discussed with respect to the precision of frequency measurements and the noise characteristics in broad and noisy NQR spectra.

Phase Transition and Crystal Dynamics of 4-Bromobenzyl Alcohol

Abstract For the title compound a phase transition from Phase II to Phase I (low and room temperature phases, respectively) was found at ca. 217 K. The temperature dependence of the 81Br NQR frequency and that of the dielectric constant showed anomalies at ca. 195 K that were tentatively attributed to a higher order phase transition. A similar anomaly was found at ca. 218 K for 4-chlorobenzyl alcohol which showed a II-I transition at 236 K. The dielectric dispersion observed for both compounds at low temperatures indicates an excitation of a molecular motion with the dielectric relaxation rate of ca. 1 kHz. The temperature dependence of the 81Br NQR frequencies of 2-and 3-bromobenzyl alcohol, measured at T > 77 K, gave no evidence of phase transition in their crystals.

NQR and Phase Transitions in Hexachlorocyclopropane Crystal

Abstract 35Cl NQR and DSC studies on phase transitions in hexachlorocyclopropane (HCCP), C3Cl6 , are reported. It is found that HCCP has three solid phases: A high temperature disordered phase (Phase I) above 301 K (no NQR spectrum was observed); a metastable phase (Phase II), which exhibited 6 NQR lines from 77 to 270 K; a low temperature phase (Phase III) in which a 24-multiplet of 35Cl NQR lines at 77 K, the most complex multiplet spectrum ever reported was observed. DSC measurement shows a A-type transition at 301 K and a broad transition of very slow rate at 285 K. The structure and mechanism of phase transitions in HCCP crystal are discussed.

Computer Enhanced SRO NQR-Spectrometer

An automatic computer supported SRO NQR spectrometer system was constructed for the measurement of time dependent NQR signal intensities. The system has several functions: fast scanning (500 kH z/25 s), averaging, smoothing, automatic noise level estimation, automatic peak detection, etc. The process of the ß → α phase transition of p-dichlorobenzene is illustrated by the 3-dimensional spectrum .

Recent Development of Nuclear Quadrupole Resonance Spectra Database

Abstract The Nuclear Quadrupole Resonance Spectra (NQRS) Database contains all the numerical data of resonance frequencies that have been published since 1950, the year NQR was discovered, through the end of 1989. The total number of records compiled is 10,155 as of May, 1991. One record usually covers data of a particular polymorphic modification of a crystalline substance. About 3/4 of all the records deal with organic or organometallic substances. Recent changes in the data content and format, development of online and stand-alone search software, statistics of the database, and updating problems are described. A personal computer version of the database with load/search software was prepared and is described in the Appendix

Narrowing of the 35Cl NQR lines of trichloroacetamide caused by its ferroelectric phase transition

Abstract Dielectric measurements on single crystals and DSC of the title compound (monoclinic, space group P21) revealed a ferroelectric transition at ca. 355 K (ΔH ≈ 0.5 kJ mol-1) Powder x-ray diffraction indicated that the high temperature phase is also monoclinic and the change in volume at the transition is very small. A displacement of hydrogen atoms is considered as responsible for the appearance of ferroelectricity. The 35Cl NQR signals at ca. 77 K were considerably broad, but a drastic narrowing took place after the compound had once experienced the phase transition. A tentative assignment of the NQR spectrum was made from a calculation of the NQR frequencies based on the CNDO/2 method. Another phase transition was evidenced by a small drift of the DSC curve and a slight anomaly of the dielectric behavior around 358 K. The present 1H NMR experiments gave no indication of the onset of torsional motion of the NH2 group reported to occur at ca. 210 K.

Relaxation of Lattice Imperfections as Studied by Chlorine NQR

The intensities, linewidths, and frequencies of 35Cl NQR signals in 6-nitro-2,4-bis(trichlorom ethyl)-benzo[1,3]dioxine were found to vary remarkably depending on the crystallization methods and annealing. This finding was correlated to the degree of crystal disorder. For a sample obtained by slow crystallization from an ethanolic solution of the compound, the growth of the height of the NQR absorption signal due to annealing was measured as a function of the isothermal annealing time. A kinetic analysis of the growth process gave an activation energy of approx. 110 kJ/mole for the relaxation process of the imperfection dominating the NQR signals. The dimorphism of the compound and the magnitude of the activation energy suggest the presence of misoriented molecules accompanied by vacancies in the crystal lattice of the stable phase.