The torsional modes that contribute to the change in nuclear quadrupole resonance (NQR) frequencies with temperature in 2,6-dichlorophenol are identified by correlating the observed variation of 35Cl NQR frequencies with temperature with the theoretical expectations using the low frequency vibrational modes obtained from the far infrared spectrum.

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Nuclear quadrupole resonance (NQR) Zeeman effect studies were carried out on cylindrical single crystals of 2,5-dichlorophenol, using the two 35Cl-NQR frequencies. A self-quenched superregenerative NQR spectrometer was used, and the spectra were analysed ot obtain information on the nature of the crystalline unit cell. An analysis of the experimental data reveals that: (1) the results are in good agreement with the structural reports of Bavoux and Perrin; (2) the crystal unequivocally belongs to the monoclinic system; (3) there are two crystallographically equivalent but physically inequivalent directions for the principal field gradient axes for both the low- and high-frequency resonance lines; (4) as the number of physically inequivalent directions for each of the two resonance lines is two, the minimum number of molecules per unit cell is two; (5) the b axis (90°,90°) is identified as the symmetry axis; (6) the growth axis is slightly inclined to the c axis; (7) the asymmetry parameters obtained for the loci corresponding to the low-frequency line, which is hydrogen bonded, are greater than those for the high-frequency line, which is nonhydrogen bonded; (8) the double-bond character is greater for the hydrogen-bonded chlorine than for the non-hydrogen-bonded chlorine; (9) the ratios of the various bond characters estimated for both the low- and high-frequency resonance lines are 69:24:7 and 74:24:2.

Zeeman effect studies on the two 35Cl NQR lines in cylindrical single crystals of 2,6-dichlorophenol were carried out using a self-quenched super-regenerative NQR spectrometer to obtain information on the nature of the crystalline unit cell and the effect of hydrogen bonding on the electric field gradient tensor. Analysis of the experimental data reveals: (1) the results are in good agreement with those reported from X-ray studies; (2) the crystal is unequivocally identified as belonging to the orthorhombic system; (3) there are two crystallographically equivalent and four physically nonequivalent directions for the principal field gradients for both the low and high frequency resonance lines; (4) the directions of the crystalline a, b, c axes are uniquely identified as (90°, 0°), (0°, −), and (90°, 90°); (5) the b-axis is identified as the growth axis; (6) there are a minimum of four molecules per unit cell, the four molecules lie in different planes, which are, however, connected by symmetry operations; (7)_there exists a weak intramolecular hydrogen bonding in the crystal; (8) the asymmetry parameters for the loci corresponding to the low frequency resonance line, which is affected by hydrogen bonding, are less than the asymmetry parameters of the loci corresponding to the high frequency resonance line, which is not affected by hydrogen bonding; (9) the single bond and ionic bond characters for the hish frequency line are less than that of the low frequency line, while the double bond character for the low frequency line is less than that of the high frequency line and (10) the small deviation between the single bond and double bond characters of the two resonance lines is attributed to the existence of weak hydrogen bonding in the crystal.

The Zeeman spectrum of the 35Cl NQR line in a cylindrical single crystal of α,α′‐dichloro‐o‐xylene was recorded at room temperature and analysed to obtain information regarding the nature of the crystalline unit cell. The crystal system is determined from the analysis. The two observed loci are due to the two chlorines in a single molecule, and all the benzene rings are parallel to one another whereas the two CCl bonds are inclined to one another at an angle of 102°. The absence of a cleavage plane may be due to the out‐of‐plane CCl and CH bonds in the chloromethyl groups. The ionic, single bond and double bond characters are in the ratio 28.54:70.64:0.82.

AbstractA temperature variation study of the nuclear quadrupole resonance (NQR) frequencies for the two resonance lines in 1,2,4,5‐tetrabromobenzene was carried out in the range 77–300 K. The far‐infrared spectrum of 1,2,4,5‐tetrabromobenzene was recorded and the low‐frequency vibrational modes noted. From the nine observed frequencies, the torsional modes contributing to the change in NQR frequencies with temperature are identified by correlating the calculated values of the NQR frequencies at different temperatures with the observed values.

Etude sur monocristal cylindrique a temperature ambiante: cristal orthorhombique ou monoclinique; existence de deux directions non equivalentes pour les axes Z principaux de gradient de champ avec un angle de 69° entre eux. Rapport du caractere de liaison ionique/simple/double=25/72/3