743
NOTES
I
1
I
1
1.2
2.3
3.4
4.5
I
Reaction (n-1, n )
Figure 1. Enthalpy changes, “l+(NHs)n-~ -t NHa = “(+(“a)” proceeding in the gas phase.
for reactions
An earlier study of the competitive solvation of NH4+ by water and ammonia vapor4 had shown that NH3 molecules are taken up preferentially in NHd+(NHa), clusters with 7~ 5 4,while in larger clusters the ligands had to be divided into two groups: one of four molecules with preference for ammonia and one of n - 4 molecules with preference for water. These results were interpreted to mean that the NH4+ forms an inner shell of four MH3 molecules. The buildup of a distinct outer shell might be expected to lead to a drop off in the - AH4,5value. This expected drop-off is confirmed by the present data. It is interesting to note that the value of -AHo,l is considerably larger than -AH1,2, - AH2,3, and AH3,4. A possible explanation of this behavior is the assumption that the reaction 0,l leads to the species (“3)2H+, in which the proton is equally shared between the two ammonia molecules, while reaction 1,2 leads to a reorganization, in which the “normal” species NH4+(NH3)2is formed. Further additions of one and two ammonia molecules then continue the buildup of the inner four shell. Turning to the -ASon--l,n values we find that - A S O I , ~and - AS04,5are lower than the rest. This behavior is in line with the greater freedom found in a transition from (NH&H+ to NH4+(NH3)2,and in the transition from the inner four shell to the singly occupied outer shell. Unfortunately, the A S values are obtained from the subtraction of two large terms, AG and AH, and are, therefore, prone to show up the combined experimental error. The trends of the A S values should, therefore, be considered with more caution. It is interesting to compare the present results with recently reported work3J on the system H+(H20),. The results for the water system also suggested that the proton is evenly shared by the water molecules. I n the water system, however, the indication was that this sharing continues beyond the two molecule proton
-
complex. Considering the existence of an outer shell in the water system, it was found that the AHwwl,, values decreased quite continuously. No distinct transition to an outer shell was indicated by the AHW-1,, values up to w = 8. In the discussion, it was concluded that in some systems crowding (or gradual expansion) of an inner shell might occur with consequent gradual decrease of the inner-shell AH values. In such cases, the transition to an outer shell is not marked by a sharp drop-off in the enthalpy values. The ammonia system represents an example where a marked change occurs in the transition. This behavior must be due to the tetrahedral shape of the NH4+ ion and the pyramidal structure of the ammonia molecules, which in the tetraammoniate form a compact but uncrowded structure. On addition of another molecule, no reorganization of this structure occurs so that the new molecule is forced into an outer position. The relative concentrations of the clusters NH4+(NH& are of interest in the radiation chemistry of ammonia vapor. We wish to point out that the data in Table I allow the calculation of the relative concentrations for a wide range of temperatures and pressures. A simple method for such calculations is d e scribed in ref 3. (4) A. M. Hogg and P. Kebarle, J. Chem. Phys., 43, 449 (1965). (5) P. Kebarle, R. M. Haynes,, and J . G. Collins, to be published.
Reaction of Oxygen Atoms with ICN
by Q. J. F. Grady, C. G. Freeman, and L. F. Phillips Chemistry Department, University of Canterbury, Christchurch, New Zealand (Received August 22, 1967)
In a previous study,’ the reaction of oxygen atoms with I2was found to yield mainly solid 1 2 0 s on the wall of the reaction vessel. The present work was undertaken to find whether, in the reaction with ICN, iodine oxide could be made to deposit on the wall and leave CN radicals behind in the gas phase. If this happened, the reaction might provide a convenient source of CN radicals in a flow system. Oxygen atoms were produced either by discharging 0 2 or, in the absence of 0 2 , by titrating nitrogen atoms Matheson prepurified nitrogen and ultrawith high-purity oxygen were used. Nitric oxide was purified by distillation from soda-asbestos. ICN was prepared by the method of Goy, Shaw, and Pritchard.3 (1) D.I. Walton and L.F. Phillips, J . Phys. Chem., 70, 1317 (1966). (2) J. E.Morgan, L. Elias, and H. I. Schiff, J . Chem. Phys., 33, 930 (1960). (3) C.A. Goy, D. H. Shaw, and H. 0. Pritchard, J . Phys. Chem., 69, 1504 (1965). Volume 72,Number 2 February 1868
NOTES
744
The reactions were carried out in a 25-mm 0.d. Pyrex tube at room temperature (25') and pressures near 3 torr. A 1P21 photomultiplier in combination with a filter transmitting at 5350 A was used to monitor the NO titrations. ICN, carried in a stream of argon, was introduced into an excess of atomic oxygen through one inlet jet, and excess I 2 was similarly introduced through a second jet, ca. 30 msec downstream from the first. The products of reaction at the second jet were trapped at 77°K and later analyzed for the presence of cyanide. No cyanide was ever detected in the trapped products. Instead, a brown film of CN polymer formed on the walls of the reaction tube, near to and on the ICN inlet and extending a short distance along the tube. The white iodine oxide film which formed at the same time extended for a much greater distance. It was therefore concluded that the reaction of 0 atoms with ICN is not a very convenient source of CN radicals because the wall reaction which removes CN is faster than that which removes iodine. Poisoning the walls with phosphoric acid did not affect this conclusion. The primary reaction in this system is given by
0
+ ICN
+ CN
(1)
+ ICN--.,I + CNO
(2)
--3
IO
The alternative reaction
0
can be ruled out on the basis of the evidence which follows. The reaction with discharged oxygen was accompanied by a bright, yellow-white luminescence which extended the whole length of the reaction tube (35 cm). When this luminescence was photographed, using a Hilger medium quartz, spectrograph and Ilford HP3 plates, it was found to be indistinguishable from the well-known NO-0 continuum. We therefore conclude that CN radicals reacted with 02,close to the ICN inlet, to produce NO according to the scheme4 CN
+0 2 4 C N O+0
CNO+OACO+NO
(3)
(4)
(The alternative four-center reaction4 CN
+
CO
0 2 4
+ NO
(5)
is expected to be too slow to produce appreciable amounts of NO in the short time available before the CN radicals polymerize on the wall.) No luminescence was observed when the oxygen atoms were prepared, in the absence of 0 2 , by the NO titration, but if reaction 2 did occur, some nitric oxide would be formed by the subsequent reaction 4, and the NO-0 continuum would be produced. Hence this reaction can be eliminated in favor of (1). A very small increase in photomultiplier current was noted when ICN was introduced at the null point of the NO titration; this was tentatively attributed to the presThe Journal
of
Physical Chemistry
ence of a trace of cyanogen resulting from the equilibrium 21CN
I2
+ C2Nz
The termolecular reaction
+ +
+
CN 0 M ----+ CNO M (7) which would also lead to NO formation by reaction 4, is expected to be much too slow to compete with CN polymer formation. The possibility of producing NO in absence of 0 2 was also investigated mass spectrometrically, using apparatus similar to that described by Phillips and Schiff.s When ICN was introduced into a stream of discharged O2 the peak a t mass 30 showed a large increase, corresponding to the formation of NO. But when ICN was introduced into a stream of discharged N2 to which had been added a very small excess of NO, the peak at mass 30 showed only a small decrease, due to the increased pressure in the flow system. No attempt was made to measure the rate of the primary reaction mass spectrometrically because of the need to avoid coating the mass spectrometer sampling leak with CN polymer. From the length of the polymer deposits on the walls of our reaction tubes we estimate that the rate constant of reaction 1 is 10'0 cma mole-' sec-l or greater. Acknowledgments. This work was supported by the New Zealand Universities Research Committee and by Grant AF-AFOSR-1265-67 from the U. S. Air Force Office of Scientific Research. (4) N. Basco, Proc. Roy. SOC.(London), A283, 302 (1965). (5) L. F. Phillips and H. I. Schiff, J. Chem. Phys., 36, 1609 (1962).
Crystal-Field Splitting of Fundamentals in the Raman Spectrum of Rhombic Sulfur by A. T. Ward Research Laboratories, Xerox Corporation Rochester, New York 14680 (Received Auguet 22, 1067)
The fundamental vibrational spectrum of the SS molecule in crystalline rhombic sulfur has been investigated previously by far-infrared methods1p2 and by Raman spectroscopy.a-Q Only in the far-infrared (1) R. B. Barnes, Phya. Rev., 39, 570 (1932). (2) G. W.Chantry, A. Anderson, and H. A. Gebbie, Speotrochim. Acta, 20, 1223 (1964). (3) D. W.Scott, A. B. Guthrie, J. F. Messerly, 8. 9. Todd, W. T. Berg, I. A. Hosaenlopp, and J. P. McCullough, J. Phys. Chem., 66, 911 (1962). (4) A. Caron and J. Donohue, Acta Cryst., 14, 648 (1961). (6) P. Krishnamurti, Indian J. Phys., 5 , 106 (1930).
