14:B
Ian ilrr Wanly arid J. C. Platteauu, “.4drances in Chernrcal Pliysics,” Vol 11, Interscience Yubl~ahars, New York, N Y , 1959, p. 4. (17) J. W. hlellor, “A Co~nprelio~lsive Treatise on Inorganic and Theoretical Chemistrj ,” Lorigmans. Green and Co., London. 1922.
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heat capacity (Table 111). The position of hydrogen selenide is apparently anomalous since it is less polar than hydrogen sulfide, forms a clathrate type gas hydrate,’s and yet has a very low change in heat capacity between 15 and 35’. This apparent anomaly will probably disappear when more accurate data become available. I n summary, the evidence indicates that for hydrogen bonding solutes only a minimum distortion in the structure of the water occurs and that the solute is hydrogen bonded to the water. However, the nonpolar solutes act to promote in the surrounding water a more ordered (ice-like) structure in which the water is hydrogen bonded to itself. (18) AI. V. Staekelberg and 11. R. Muller, Z. Eleelroehem., 68, 25 (1054).
S‘l’UDY OF THIS EIZCTROS SPIN RESONASCE OF CHROnilIUM(II1) COJWLESES I S AQCEOCS SOLUTION 1 3 Isperirnentsin wvhic*hhigh concaentrations of hlg(ClO& and Ca(NOa)2were added. The optical spectra of the strong nitric ticaid solutions suggest that a dinuclear species is present to account for the decrease of spin intensity. ISvidcnre of outer-splierc complex formation is provided by line width measurements. The relaxation mechanisms w1iic.h account for the observed line widths are discussed.
Introduction Electron spin rcsoiiance (e,s.r.) measuremcnts of aqueous solutions of chromic ions, Cr(II1), have been included as part of studies of transition metal ions in ~ o l u t i o n . ~ -The ~ principal emphasis in these studies has been-on the dependence of the line width of the resonance on such parameters as type of ligand, concentration, and temperature, and on comparison with the resonance of the d i d . We observed a strong dependence of the observable spin intensity upoii the acid concentration of solution of Cr(1IJ) nitrate atid perchlorate, and this dependence \vas stuJieC1 iii turnis of various possible complexes. Experimental The e.5.r. spectra of various solutions a t 15-20’ were measured in a 0.5-mm. i.d. quartz capillary with a Varian V-4502 X-b:tnd spectrometer with a multipurpose cavity whose Q is approximately 7000, and employing 100-Itc. field modulation and a modulation amplitude of about 7 g a i i ~ . The e.s.r. spectrum was taken the same day,the solution was prepared, except where noted. The (1) R. G. Hayes, Lawrence Radiation Laboratory Report UCRL-9873, Srpt. 29, 1961. (2) R. R. hIcCarvey, J . I’hus. Chem., 61, 1232 (1957).
(a) V. I. Avvakirmov, N. S. Garifyanov, U. >I. I 3 for which he observed reduction of magnetic moment. There appears to be a discrepancy between Mulay’s observations and ours. We observe similar e.s.r. properties for the dinuclear species and for the specics presciit in weakly acidic solutions, e.g., pH 4.73, and differences are attributed principally to dissociation of the latter upon aging. A comparison of the magnetic susceptibility and e.s.r. properties of these solutions, taking into consideration possible changes upon aging, mill perhaps clarify the question of whether morc than one dinuclear spccies is involved. It is unnecessary to postulate spin pairing for sonic Cr(II1) complcxes and we have observed no spin resonance for aqueous solutions of Cr(II1) acetate, oxalate, and ethylenediaminetetraacctic acid in which spin pairing probably does not occur. Two relaxation processes may contribute to the substantial line broadening and hence to thc undetectability of the resonance of the polynuclear Cr(II1) complex. The first is due to the anisotropy of the ligand field, resulting in zero-field splitting of the ground statc, as discussed by 1LIcGarvey.2 Also, Hayebl has attributcd thc broad resonances in two Cr(II1) complcxes (unspecified) to distortion of the octalicdral configuration. A second relaxation proccss may arise from orbital interactions betwecn thc Cr(II1) ions via thc oxygcn bridge. The latter mechanism has been proposcd to explain spin pairing in crystalline basic rhodo complexes.16l7 The relative contributions of each of the two relaxation processes will depend upon thc molecular configuration of thc complex, the prcscnce and number of -01 and -oxo bridges affixcd to a Cr(II1) ion, and thc extent of polynuclcation. Thcre is evidence of second co6rdination interaction from the rcsults of line width. For exaniplc, upon incrcasing the concentration of nitric acid, the line width passcd through a minimum of about 140 gauss and then iiicrcased to 310 gauss; for perchloric acid solutions the line width dccrcased in the same acid oonccntration as nitric acid but rcmaincd a t a low value of 113 gauss (Table I and Fig. 3). This bchavior is interprcted as cvidencc of a second coordiiiation intcraction (outcr-sphcre) with the licxaaquo ion. Thc aiiions KO3- or c104-prcsciit in thesc solutions arc belicved to orient about the Cr(II1) at, positiolis corresponding to the centcr of the faces of the octahedron. As the anion concentration is increascd, thc electric field about Cr(II1) due to this outer-sphere coordination becomes more symmetrical and results in a (16) A. Earnshaw a n d J. Lewis, J. Chem. Sac., 396 (1961). (17) W. K. Wilmarth, €1. Graff, a n d S. T. Gustin, J . A m . Chem. Sac., 78, 2083 (1956).
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reduction in line width. However, as the nitric acid concentration exceeds 3 ill (-log [H903] = -0.5), the line width of the observable rcsonance increases because protons in solution tend to interact with tlic outer-sphere coordinated S O 3 - to form undissociated HN03. This results in a fluctuation of the electric field about the Cr(II1) a t a freyuencylMapproaching thc microwave frequency, and provides a relaxation process adequate to broaden the line to 310 gauss. In perchloric acid solutions tlie concentration of undissociated HClOl is not adequate to providc this relaxation process; however, it is adequate to reduce spin iiitcnsity slightly, presumably by formation of a polynuclear complex. The linc width also depends upon the type of anion associatcd with chroniium in the salt used to prepare the solutions. With solutions of pH 2 0.5 and of comparable Cr(II1) concentrations, the line width was grcater for KO3- than C104- [Table I and Fig. 3), and in preliminary work the line width was even greater for solutions, obtained from Cr2(S04)3.1CiH20. Typical values of line width obscrved for 0.04 J f chromic salt solutions a t pH 1 are 140, 133, and 180 gauss for C104-, NO3-, and S04-2,respectively. This broadening suggests a second coordination cffect which depends upon the clcctric field about Cr(II1) as affected by the charge distribution on the anion. Evidence of another type of second coordinatioii intcraction appears from a pi-eliminary study made of thc effect of non-paramagnetic salts on the line width of Cr(II1). The results (Fig. 3 ) show that for a given molar concciitration of the two salts, the line width was iiicrcased significantly morc by Ca(X03)2 than by hIg(ClOJ2. Thc additional line broadening. in tlic presence of these salts is due principally to the cffcct of the nictal catioii, since comparablc coiiccntrations of S O 3 - and c104-providcd by thc correspoudiiig acids did not provide such broad liiics. Thc meclianisni of line broadcning here is postulatcd to be duc to ionpair formation bctwcen tlic cations Ca+2 and XIg+2 and the anious in the second coordination sphere. Acknowledgment.-The authors arc indebt c>d i o I’rofcssor Hcnry ‘l’auh of Stanford University for valuablc discussions and suggcstions and to I>r. Burtoll Zabin, also of Stanford Uliiversity, for thc chromous prrchlorate solution prcparcd by clcctrolytic reduction. They arc also indcbted to their colleagues, Dr. A. 1’. Brady and Dr. A. H. Samuel, for helpful suggestions concerning the interpretation of the results, and to Charles Gill for the preparation of solutions. (18) hl. Eigen, Discussions ParadaU SOC.,17, 19.1 (1954).
