COMMUNICATIONS TO THE EDITOR
ratio is much larger than unity as it is for NO2. Thus, these classes of NO2-containingcompounds are virtually eliminated from consideration. The significant feature of the mass spectra of nitrate esters is the mass 46 peak. It is consistently more intense than the 30 peak, viz., 30/46 ratio less than unity. In addition, no peak is observed at mass 62 (NOo+). Therefore, the mass spectroscopic data indicate that NO3 or a nitrate is being observed. The mass spectra of all the alkyl nitrates including nitroglycerin (nine compounds) show either a significant mass 76 peak (CH20N02+)or a peak corresponding to the ion RCHON02+. Since these peaks and peaks corresponding to aliphatic fragments which vary in intensity as the 46 peak are not observed, the evidence indicates that NO3 is the species given off during the low pressure combustion of a typical double-base propellant. The finding of NOa as an intermediate in the combustion of a double-base propellant is also unexpected relative to previously reported mechanisms of the thermal decomposition of simple alkyl nitrates and of pure nitroglycerin. For example, Levy4 concludes that NO2 is formed in the initial step of the decomposition of ethyl nitrate. However, the major product over a temperature range of 161-201° is ethyl nitrite; at 181O a 75% yield of ethyl nitrite is obtained with an 80% conversiori of ethyl nitrate. Of greater immedi-
3669
ate relevance is the study by Krastins6 of the thermal decomposition of nitroglycerin. He finds that at 150°, vapor phase, the initial product is NOz followed almost immediately by formaldehyde. Within a short period, CO, GO2, and NO appear simultaneously. N20 is found late in the course of the reaction. Both Levy and Krastins used static reactors and analyzed samples by infrared spectrometry. Krastins in one series actually ran the pyrolysis in an infrared gas cell and followed the course of reaction by periodically recording infrared spectra. The data from this series appeared to substantiate his findings using the sampled static reactor technique. A n interesting point is Krastins' inability to find or detect water although it must be a major reaction product. It appears that the last word has not yet been written on the combustion of nitrate ester propellants.
Acknowledgment. This investigation was supported in part by the Army Materials Command under Contract No. DA-30-069-ATVIC-l22(A). The mass spectrometer was constructed in part by support provided by the Propulsion Research Division, Air Force OEce of Scientific Research, under Contract No. AF 49(638)173. (4) J. B. Levy, J. Am. Chem. SOC.,76, 3254 (1954). (5) G.Krastins, Ph.D. Thesis, University of Connecticut, 1957.
C O M M U N I C A T I O N S TO THE E D I T O R
Polymorphism in Palladium(I1) Chloride Sir: A structure of PdC12has been established' which is often given as an example of a linear, infinite-chain, inorganic molecule. We present evidence that this structure is of a high temperature polymorph which can exist metastably at room temperature for many months and that a third modification of PdC12 exists at still higher temperatures. Structural details of the latter, as well as of the form thermodynamically stable at room temperature, are as yet not known. Figure 1 shows the differential thermal analysis (d.t.a.) curve of PdC123which clearly indicates three
basis of previous studies4 and our confirmation of no significant weight loss below 600" by thermogravimetric analysis. The endotherms centering near 400 and 500" are thus due to crystalline transitions. The d.t.a. curves when heating was stopped at 575", prior to onset of decomposition, had exotherms during cooling at about 440" caused by reversal of the 500" (1)A. F.Wells, 2.Krist., 100, 189 (1939). (2) See, for example, L. Pauling, "The Nature of the Chemical Bond," 3rd Ed., Cornell University Press, Ithaca, N. Y., 1960,p. 157. (3) Obtained from J. Bishop & Co., Malvern, Pa., which kindly
furnished information that the maximum temperature attained during production of this material is 166'.
Volume 69, Number 10
Odober 1966
COMMUNICATIONS TO THE EDITOR
3670
Table I : Comparison of PdClz X-Ray Data -Web' hkl
010 020 100 030 110 001 011 120 021 040 130 101 031
d-spacings calculated from lattice constantsd,
A.
11.000 5.500 3.810 3.667 3 600 3.340 3.196 3.132 2.885 2.750 2.642 2.512 2.469 I
Fcsloda
-intermediate d , A!
Powder pattern of temperature form-IC
Diff.d
r
d,
A?
Powder pattern of low temperature form-? IC DiKd
A 80
5.5
S
-0.0
3.61
S
-0.01
3.21 3.15
m
-0.01 -0.02
5.3
S
0.2
3.28 3.14 3.07'
W
0.06 0.06 0.06
A A 96
A 42
-44 A 28 57
2.76 2.66 2.48"
W
vw W
S
108
W
vw
-0.01 -0.02 -0.01 (from average)
Numerical values are Ealculated structure factors from Wells' paper; A indicates plane giving no reflection in his study. Cu Kor radiation taken as 1.5418 A. Visual estimates of intensities. d-spacing calculated from Wells' lattice constants minus our measured d-spacing. ' Remainder of intermediate temperature form pattern had 18 lines in region from 2.29 to 1.24 A. Remainder of low temperature form pattern had 15 lines in region from 2.38 to 1.12 A.
'
transition, but during the remainder of cooling to room temperature no exotherm corresponding to the transition near 400" occurred with pure PdCL. Accordingly, the intermediate temperature form could be recovered for leisurely study. The X-ray diffraction powder pattern of our starting material agreed exactly with one published earlier,6 whereas that of PdCL heated above 425" and cooled was distinctly different. However, after a month or two at room temperature, the latter samples began to show the strongest X-ray diffraction lines of the low temperature form. This pattern increased gradually in intensity, but complete reversion required at least 5 months. Inasmuch as the single crystal of PdClz used in the previous structural study' was prepared a t 600", it is now of interest to determine which form was studied. Using a Control Data Corporation G-15 computer we calculated from the lattice constants determined by Wells all possible d-spacings in the range of those of our patterns. The first 13 are listed in the second column of Table I, together with their associated hkl values6J and calculated structure factors from Wells' paper.' The powder pattern of the intermediate temperature form of PdClz is seen to agree almost exactly both in d-spacings and intensities, whereas that of the low temperature form shows considerably greater deviations in lines that can be matched, lacks several important diffraction lines to be expected from Wells' structure, and requires one plane (001) whose reflection was absent in the single crystal study. On the basis of these comparisons, the differences in preparative The J O U Tof~Physical Chemistry
Temperature, "C.
Figure 1. Differential thermal analysis curve of PdClz heated 5'/min. in a nitrogen atmosphere. Deviations downward from base line show endothermic changes.
conditions of the samples, and our findings on the behavior of PdClzpolymorphs, we must conclude that the structure Wells determined is that of the intermediate temperature form. We have investigated the densities and] infrared absorption spectra of the low and intermediate temperature forms of PdC12. These findings plus more exten( 5 ) J. D. Hanawalt, H. W. Rinn, and L. K. Frevel, I n d . Eng. Chem., Anal. Ed., 10, 497 (1938). (6) In accordance with recent data compilations,' we have reversed the values of lattice constants b and c from those originally ascribed by Wells, and therefore also of indexes k and 1. (7) (a) R. W. G. Wyckoff, "Crystal Structures," 2nd Ed., Vol. 1, John Wiley and Sons, New York, N. Y . , p. 343; (b) ASTM X-ray Powder Data File, Card No. 1-0228.
COMMUNICATIONS TO THE EDITOR
3671
sive X-ray work will be reported on in full, together with implications regarding the structures of the several forms and revisions necessary in interpretations of previous work reported on this substance.
I
I
vs j I i
1
I
I
1
I
I
1
I
I
I
Acknowledgments. Partial support of this work by the Office of Naval Research is gratefully acknowledged. PENNSALT CHEMICALS CORPORATION J. R. SOULEN TECHNOLOQICAL CENTER WILLIAM H. CHAPPELL, JR. KINGOF PRUSSIA, PENNSYLVANIA RECEIVED JULY 30, 1965
Near-Infrared Spectrum of Liquid Water from 30 to 374"
Sir: Some years ago, the authors obtained the nearinfrared spectrum of liquid water over a temperature range of 30 to 374". Some of the spectra were published as an illustration of the operation of the high temperature cell1 and as a chapter in a doctoral thesis.2 Recently, much attention has been given to the structure of liquid water.3 Questions have been raised regarding the fraction of monomeric water present in the liquid. Certain facts clearly emerge from the spectra shown in Figure 1. (1) There is a very strong effect of temperature, since the single band becomes two bands and the frequencies shift. (2) However the structure changes, no totally free water molecules are present since no band is observed at 3750 cm.-l. (3) If the higher frequency band appearing at 200" is assumed to result from nonhydrogen-bonded species, then these results are consistent with the theory of Marchi and Eyring4giving the fraction of monomeric molecules as a function of temperature. (4) On the other hand, these two peaks can also reasonably be explained as v3 and ul, since their frequency difference is the same as found for v3 and v1 in the vapor phases and in dilute solutions of water in inert organic solvent^.^^^ This is also consistent with the apparent intensity reversal between v3 and y1 which takes place on liquefaction. In the gas phase, v3 is very much more intense; in the liquid phase, v1 is more intense. As the environment of the liquid water becomes more and more like that in the vapor, there is apparently a continuous change in relative intensity, both in pure water, as seen here, and in organic solvents. Acknowledgment. This research was sponsored in part by the Air Force Office of Scientific Research under Contract, No, A F 49(638)-3 and the French Atomic Energy Commission.
3800
0,i
i
I
3600 3400 Frequency, cm.-I.
3200
Figure 1. Infrared absorption of liquid water in the hydroxyl stretching region a t different temperatures. vg and v l , indicated in the figure, are the vapor phase values of these frequencies shown for comparison. (1) E. Fishman, Appl. Opt., 1,493 (1962). (2) P. Saumagne, Thesis, University of Bordeaux, 1961. (3) D. P.Stevenson, J . Phy8. Chem., 69,2146 (1965),with references to previous literature. (4) R. P.Marchi and H. Eyring, ibid., 68,221 (1964). Their eq. 10 for the fraction of monomeric molecules yields 7% at 200°, where the high-frequency shoulder just appears, to 41% at 374O,where the two peaks have nearly equal intensity. (6) G . H y b e r g , "Infrared and Raman Spectra of Polyatomic Molecules, D. Van Nostrand Co., Inc., New York, N. Y., 1946, p. 281. The antisymmetric and symmetric stretching vibrations of water, Fa and VI, are given as 3756 and 3652 cm.-l, respectively. The difference of 104 cm.-l is within experimental error of the differences between the two peaks at each temperature shown in Figure 1.
ERWINFISHMAN
DEPARTMENT OF CHEMISTRY SYRACUSE UNIVERSITY SYRACUSE, NEWYORK UNIVERSITY OF BREST BREST,FRANCE
PIERRE SAUMAGNE
RECEIVED AUQIJST2, 1965
Gas Phase Charge-Transfer Complexes
Sir: One major difliculty in comparing the experimental data for charge-transfer (c.t.) molecular complexes with theoretical predictions has been that of properly assessing the role of solvent. Existing spectroVohme 69,NurnbeT 10 October 1966
