PUREQUADRUPOLE RESONANCE OF IODINE IN TRIIODIDES
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Pure Quadrupole Resonance of Iodine in Ammonium, Rubidium, and Cesium Triiodides by Akinobu Sasane, Daiyu Nakamura, and Masaji Kubo Department of Chemistry, Nagoya Universily, Chikusa, Nagoya, Japan
(Received March 14, 196Y)
Pure quadrupole resonance studies have been made on iodine in ammonium, rubidium, and cesium triiodides at various temperatures. Three vl and two v2 frequencies were observed indicating the existence of three kinds of nonequivalent iodine atoms in crystals. The observed frequencies were assigned to three kinds of nonequivalent iodine atoms as revealed by X-ray analysis. The charges on different iodine atoms in 13- were evaluated and interpreted in terms of resonance among various electronic structures. Slight anomalies found for ammonium triiodide are attributable to the formation of hydrogen bonds between ammonium ions and terminal iodine atoms carrying a negative fractional charge.
Introduction Although it had been known for a long time that iodine dissolves in an aqueous solution of alkali iodides to form 13- ions, the geometric structure of the ions was determined for the first time in 1935 by Mooney, who carried out X-ray analysis on ammonium triiodide.’ The 13- ion was found to be almost linear, the two 1-1 distances being different from each other by about 0.3 A. The electronic state of this ion has been a subject of considerable dispute. Since both a molecular iodine and an iodine ion have completed octets, the formation of an additional covalent bond requires a t least one orbital of higher energy. Kimball2 considered the 5d or the 6s orbital for the bivalency of the central iodine atom (sp or pd hybridization) , while Pauling3 and others4 interpreted the linear structure of this ion in terms of the trigonal-bipyramidal orbitals of the central atom (sp3d hybridization). On the other hand, Pimente15 carried out a simple molecular orbital calculation and explained the bonding in the triiodide ion in terms of the pg orbitals without introducing higher atomic orbitals. The present investigation has been undertaken in order to determine the charge distribution in an 13-ion by the observation of pure quadrupole resonance frequencies and to discuss the electronic structure of this ion.
Experimental Section Apparatus. Two quadrupole resonance spectrometers already describeds were employed for the deter-
mination of the pure quadrupole resonance frequencies of iodine. A self-quenching parallel-line superregenerative spectrometer was used for recording frequencies ranging over 70-300 Rlclsec, while the frequency range of 300-600 Mc/sec was covered by means of an externally quenched superregenerative spectrometer equipped with a Lecher line resonator. Resonance frequencies were determined a t room, Dry Ice, and liquid nitrogen temperatures. Additional measurements were performed a t intervals of a few degrees between - 110 and 120’ for ammonium triiodide showing an anomalous temperature dependence of resonance frequencies. Materials. Equimolar amount,$of ammonium iodide and iodine were dissolved in a small quantity of water. The solution was left to stand over phosphorus pentoxide in a desiccator. The crystals of ammonium triiodide were filtered off, washed with dilute hydriodic acid, and dried. Rubidium and cesium triiodides were prepared in a similar manner from rubidium and cesium iodides, respectively. For the identification of the samples, they were decomposed with (1) R. C. L. Mooney, 2. Krist., 90, 143 (1935). (2) G.E.Kimbdl, J . Chem. Phys., 8 , 188 (1940). ( 3 ) .L. Pauling, “The Nature of the Chemical Bond,” 2nd ed, Cornel1 University Press, Ithaca, N. Y., 1948,p 111. (4) E. Cartmell and G. W. A. Fowles, “Valency and Molecular Structure,” Butterworth and Co. Ltd., London, 1956, p 177. (5) G. C. Pimentel, J. Chem. Phys., 19, 446 (1951). (6) D.Nakamura, Y.Kurita, K. Ito, and M. Kubo, J . A m . Chem. Soc., 8 2 , 5783 (1960).
Volume Y l , Number IO September 1967
A. SASANE, D. NAKAMURA, AND 11.KUBO
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sulfurous acid. The resulting solutions were neutralized with sodium hydroxide solution and subjected to the determination of iodine by the Volhard method. Anal. Calcd for NH413: I, 96.5. Found: I, 95.9. Calcd for RbIB: I, 81.7. Found: I, 81.5. Calcd for cs13:I, 74.1. Found: I, 72.5.
Results As shown in Table I, each compound gives rise to five absorption lines in the frequency range of 70-600 Mc/ sec at all temperatures studied. The resonance frequencies of rubidium triiodide agree fairly well with the corresponding frequencies of cesium triiodide, whereas the observed frequencies of ammonium triiodide do not show even approximate agreement with those of the foregoing two compounds except for a line at 366-369 Mc/sec. This is discussed below in connection with the anomalous temperature variation of resonance frequencies of ammonium triiodide.
Discussion Resonance Frequencies and Crystal StructuTe. Because lz7I has a nuclear spin equal to 5/z and yields Table I : Pure Quadrupole Resonance Frequencies of lZ7Iin R13 ( R = NH4, Rb, Cs) ----Frequency, Y1
‘23 a‘ -80 Liquid Nz 21 bq -79 Liquid Nz 29 ,Liquid Nz 29 a -6
