NOTES 941
VoZ.1,No. 4, November, 1962 four runs, -8420 f 400 &./mole, has been taken to represent the heat of reaction of a-N%NzOa in its standard macrocrystalhe state.
Discussion From the measured value of the heat of formation of ar-Na2N20ain 0.1 m NaOH solution and the standard heats of formation of sodium ion (-57,279) and N ~ O Z(-2,590) -~ in dilute aqueous solution given by Rossini, Wagman, Evans, Levine, and JafTe,5 AH has been calculated for the process a-NazNzOdaq) = NazNz04aq)
+ ‘/20dg); AH = +20.000 cat
The heat associated with removal of 1/2 0 2 from NaONg(aq) to form NaN02(aq) is calculated from the heat of this reaction for crystalhe NaN03 and NaNOt [(+24,900)9], and the stand(9) J. D. Ray, Abstracts of Papers, 16th Annual Calorimetry Conference, Ottawa, Aug. 14-17,1961.
ards heats of solution of NaN08(c) (+4,889)5 and NaNOz(c) (+3,320)1° N a O g (as) = XaNOs(aq) + I/nOp; AH = +23,331 cal. Since the heat of rearrangement of NaONOO(as) to NaONOp(aq) is -34,000 cal.,l’ it follows that NaONOO(aq) = NaN%(aq)
+ ‘/20~; AH = - 10,969 cal.
By analogy, removal of 1/2 O2 from an N e ONN: structure would give a AH of approximately +23,000, and removal of 1/2 0 2 from Na20NNO-O should give AH of -11,000. Since the actual value is +20,000, the Na20NNg structure is suggested. Acknowledgments.-This research was supported in part by a grant from the Research Corporation and in part by a grant (NSF-G15560) from the National Science Foundation. (IO) C.-EL W u and L. G.Hepler, J . Chem. Eng. Duta, in press. (11) J. D. Ray, “Kinetics and HeaL of Isomerization of Peroxynitrite to Nitrate,” Abstracts of Papers, Southeastern Renional Meeting, American Chemical Society, Birmingham, Ala., Nov. 3-5, 1960.
Notes CONTRI~UTION FROM THE DEPARTMENT OF CEJXMISTRY,The great increase in viscosity2 with increasing UNIVERSITY OF ARKANSAS, FAYETTEVILLE, ARKANSAS concentration cannot be accounted for in terms of
An X-Ray Diffraction S t u d y of Aqueous Zinc Chloride Solutions’ BY R.F. KRUHAND C. L. STANDLEY
Received December 8,1961
The solubility of zinc chloride in water is so great that the mole ratio of water to zinc chloride can easily be made smaller than 2 :1 even at room temperature. At such high concentration it becomes a semantic question as to whether the system is called a solution or a molten salt, and unusual physical properties suggest interesting structural features whose nature might be deduced from diffraction data. High viscosity, low electrical conductivity, aud behavior in extraction experiments all suggest polymerization in concentrated solution. (1) The support of the U. S. Atomic Energy Commission is gratefully acknowledged.
simple species, and extraction into 2-octano13 shows less than one water molecule per zinc ion. One therefore suspects, if he assumes at least four-coijrdination for zinc, the existence of some polynuclear species in which chlorines are shared. Indeed, four-coordination of zinc by chlorine occurs in crystalline zinc chloride4 and in solutions containing zinc chloride together with an added but it remains to test the assumption in the system ZnCI2-H20 and to establish the identity of the ligands. (2) L. R. l)awwit, H. K.Zitnmerman, W. P;,Sweeney, aiicl (>.1’. Uinaga, J . Am. C.‘hrm. Suc., 78, 4326 (1951); 1.. K. I)awsrin, A. Tockman, H. K . Zimmerman. and 0. K. 1,ender. i b i d . , 7 8 , .4327 (19511. (3) P. C . Yates, K. Laran, K. 1s. Williams, and T. IS. Moore, ibid., 76, 2212 (1953). (4) 33. Brehler, n’alwwisse?zschu.f/rn,46, 106, 554 (1959); Fur/.*,-kr. Mkcral.. S8, 198 (1960); H. R. Oswald and H. Jdggi, l f r l s . Chini. A r k , 48, 72 (1960). (5) M. Bredig and E. R. van Artsddlen, J. Chem. Phys., 24, 178 (1956). (6) B. M. McCarroU and T. F. Young, Technical Report No. 5 ONR Contract N6 ori 02045, January 1, 1959. (7) M. Delwaulle, Bull. SOC. c h i n . France, 24, 1204 (1955).
942 NOTES
Inorganic Chemistry
To find out about the coordination of zinc we measureds the X-ray intensity diffracted (smsx = 14.00 A.-l) by the following solutions at room temperature (m = molality) 27.5 m 8.5 m 5.0 m
Zno.d!b.dHzO)o.a Z~O.IIC~.ZZ(HZO)O.W Z~.orCb.lr(HzO)o.~e
Intensity was corrected for background, polarization, absorption, and incoherent scattering. Radial distribution curves are shown in Fig. 1.
P
2.28 A., which agrees with zinc-chlorine distances iu crystalline zinc chloride4and in the tetrachlorozincate For the 27.5 and 8.5 m solutions this peak is fairly well resolved, and its area can be reliably assigned. The area of 700 el2 found for the 27.5 m solution lies between the values expected'O for three chlorines and four chlorines per zinc, namely 607 and 810 el2. In fact, this area (700 el2)is exactly what is expected for three chlorine and one oxygen (water) per zinc. Figure 2 shows that a first maximum calculatedlo
k 400
c
27.5 m
Fig. 2.-A comparison of calculated and observed first maxima in the radial distribution functions. For each solutip two component peaks, a zinc-cFlorine peak a t 2.28 A. and a zincoxygen peak a t 2.05 A , , are added to give the calculated peak, and these components (see text) are shown in the lower diagrams. In the upper pictures the sums of these components (represented by circles) are compared with the observed maxima.
/
0 n
4001
u
I \/
n
I.o
2.0 I,
3.0
4.0
A
Fig. 1.-Radial distribution functions for zinc chloride solutions. Molality is given a t left of each curve. The smooth curves are 4rf*po, the function corresponding to a uniform distribution of X-ray scatterers within the solutions. The contribution of hydrogen atoms to po is omitted.
Results For all three solutions the first maximum in the radial distribution function (RDF) occurs a t (8) See P. C . Sharrah. J. I. Petz, and R. F. Kruh, J . Chem. Phyr., 32, 241 (1960),for description of apparatus.
on this basis agrees with that observed. Moreover this assignment agrees roughly with extraction e~perirnents,~ which show that for aqueous molalities above 7 there is 0.8 water molecule per zinc extracted into 2-octanol. We conclude that, on the average, each zinc has one water and three chlorine neighbors in 27.3 nz solution. Figure 2 shows that the sanie assignnierit satisfactorily accounts for the 8.5 m solution as well. The results for these solutions support the assumption of four-coordination. The interpretation of the RDF for the 5.0 m solution depends on a judgment as to how much area should be associated with the first peak, which is not a t all well resolved. If we arbitrarily extrapolate the left hand side of the second peak to the abscissa there remains an area of (9) B. Brehler. Z. Krisf., 109, 68 (1957); Nuluruissenschujlen, 46, 69 (1959); B. Morosin and E. C. Lingafelter, Acts Crysl., 12, 611
(1959).
(IO) J. Waser and V. Schomaker, Res. M o d . Phys., 26, 671 (1953).
VoZ. 1,No. 4, November, 1962 230 el2 under the first peak. Even though this estimate may be in error by 10 to 15%, its magnitude and the position of the peak indicate that dilution to 5.0 m does not bring about extensive replacement of chlorines by water, but it must be admitted that the results for this solution are ambiguous and do not provide a unique determination of the coordination number for zinc. An assignment of area based on four-coordination leads to a reasonable picture, however. On this assumption, we find that the area of the first peak lies between the values expected for two water-two chlorine neighbors and for one water-three chlorine neighbors per zinc, namely 216 and 250 el2. The average coordination of zinc therefore would consist of about 2.4 chlorine and 1.6 water neighbors in 5.0 m solution. For this concentration, extraction measurementsSindicate 2.2 waters per zinc.
Discussion The results show that in very concentrated solution zinc has a coordination number of four and that its coordination is very much like that in crystalline zinc chloride, with water substituted, however, for part of the chlorine. There is a suggestion that the coordination is tetrahedral, as it is in the crystal, since there is a maximum in the RDF a t about d83 X 2.8 = 3.72 A. On the other hand, it must be admitted that, at least for the 27.5 m solution, chlorinechlorine contacts would occur in this range regardless of the coordination of the zinc. For the 27.5 and 8.5 m solutions, consideration of stoichiometry (two chlorines per zinc) and diffraction data (one water and three chlorine neighbors per zinc) makes inescapable the conclusion that half the chlorines, on the average, are shared between zincs. Assuming that all zincs are equivalent we therefore may describe the average coordination of zinc in solution in terms of Zn(H20)C12 units which are coupled through one of the two chlorines per zinc. Any other description requires non-equivalence of zincs or the sharing of more than one chlorine per zinc. Although the RDF does not furnish enough detail about second nearest neighbors to permit a discussion of extended or long range structure, we believe that the results show convincingly the existence of shared ligands and that they provide a basis for understanding some of the unusual properties of the solutions.
NOTES 943 CONTRIBUTION FROM THE DEPARTMENT OF CHEMISTRY, DUKEUNIVERSITY, DURHAM, NORTHCAROLINA
Boron-Nitrogen Compounds. VII.1s2 B-Trifluororobazine BY KURTNIEDENZU
Received March 26, 1962
B-Trichloro- and B-tribromoborazine (-BX"-)a are well known compounds and can be prepared by several methods.a B-Trifluoroborazine, however, has not been described in the literature. As a matter of fact, the knowledge of Bfluorinated borazines is extremely limited : Btrilluoro-N-trimethylborazine first was obtained on thermal decomposition of the methylamine adduct of dimethylflu~roborane~;B-trifluoro-Ntrisilylborazine has been mentioned in the literaturej6but no supporting data or analysis were presented; a vapor pressure of 5.9 mm a t 24.1' was recorded for a material believed to be B-difluoroN-trimethylborazine.6 A more recent attempt to prepare fluorinated borazines resides in the interaction of diborane and tetrafluorohydrazine,' but has yet to be demonstrated as a preparative method. All this available information makes it appear that B-fluorinated borazines are either difficult to prepare or relatively unstable materials. I t now has been found that B-trifluoroborazine can be prepared easily through transhalogenation of B-trichloroborazine. The latter is commercially availables and a number of its reactions have been s t ~ d i e d . ~Halogen exchange a t the boron atom, however, has not been reported. Several fluorinating agents have been found suitable for the reaction (-BCl-"-)a
f 3MF
-t
(-Bl?-NII-)a
+ 3MC1
(1)
where M is a metal ion. Potassium fluoride yielded small amounts of the desired product after extensive refluxing in high boiling solvents; isolation of the B-trifluoroborazine from the reaction mixture proved to be a difficult and tedious procedure.
.
(1) Part VI: K.Niedenzu, H. Beyet, and J. W. Dawson, I n w g Chcm., 1, 738 (1962). (2) Supported by the U. S. Army Research Office (Durham). (3) J. C. Sheldon and B. C. Smith, Quari. Rm. (London), 200 (1960). (4) E. Wiberg and G. Horeld, Z. Nawrfwsch., bb, 338 (1951). (5) S. Sujishi and S. Wirtz, J . Am. C k m . SOC.,79,2447 (1957). (6) H.I. Schlesinger, A. Finch, J. Kerrigan, and J. Murib, O N R Technical Report Project N356-255,July, 1956, p. 29. (7) R. K. Pearson and J. W. Brazer, J . Inorg. Nucl. Chrm., 41, 188 (1961). (8) U. S. Borax Research Corp., Anaheim, California.
