742
cal reduction, an electron is added to the lowest antibonding orbital of the parent molecule and the anion is a typical nitro-type radical. The nature of the two orbitals is similarly characterized in Figure 3. This difference is then mirrored in the epr spectra. The pertinent coupling constants are given in parentheses, and they graphically demonstrate the nature of the orbitals involved. Note that in the cation spectrum rather large splittings are found for the amine nitrogen, methyl protons, and the ring protons ortho to the dimethylamino group. The spectrum of the anion is that of a typical substituted nitrobenzene. This is a direct result of the unpaired electrons being in different orbitals, and because of the substituents present on the benzene nucleus the orbitals are radically dissimilar. The symmetry of the hydrocarbon frontier orbitals is destroyed by the substituents and the lowest unfilled and highest filled MO's take on the characteristics of the substituents present. This molecule represents the extreme, in that the two opposing functional groups are highly dissimilar electronically. However, it best illustrates the fact that frontier orbitals in substituted aromatic hydrocarbons are not a t all alike, and the corresponding epr spectra of the anion and cation radicals are not comparable.
Experimental Section The epr spectra were recorded with a Varian V-4500 spectrometer with 6-in. magnet, 100-kc field modulation, and Fieldial attachment. The N,N-dimethyl-pnitroaniline was purified from a commercial product by column chromatography and recrystallization. Purification of the acetonitrile and TEAP has been previously described. HMO parameters were standard114J6as was the computer program used for HMO calculations.16 An attempt was made to correlate experimental coupling constants with those derived from HMO calculations and the McConnell relationship," U H = Q p , for ring protons, and that of Fraenkel, et ul.,lsJOfor the nitro group nitrogen, but the results were unsatisfactory. This may be due to shortcomings of the Huckel treatment or to noninclusion of nearestneighbor interactions in the McConnell relationship. Acknowledgments. This work was supported in part by the National Science Foundation through Grant GP-5079X. This support is gratefully acknowledged. We also wish to thank the University of Kansas Computation Center for its help and assistance. (13) E. T. Seo, R. F. Nelson, J. M. Fritsch, L. 8. Marooux, D. W. Leedy, and R. N. Adams, J . Am. Chem. SOC.,88, 3498 (1966). (14) P. H. Rieger and G. K. Fraenkel, J. Chem. Phys., 37, 2811 (1962). (15) A. Zweig, J. E. Lanoaster, M. T . Neglia, and W. H. Jura, J . Am. Chem. SOC.,86, 4130 (1964). (16) J. M. Fritsch, Ph.D. Thesis, Kansas University, 1965. (17) H. M. MoConnell, J . Chem. Phys., 24, 764 (1956). (18) M. Karplus and G . K. Fraenkel, ibid., 35, 1312 (1961). (19) P.H.Rieger and G. K. Fraenkel, ibid., 39, 609 (1963). The Journal of Physical Chemistry
NOTES Ion-Solvent-Molecule Interactions in the Gas Phase. Enthalpies and Entropies for the Reactions NH~+(NH&-I
+ NH3 = NHd+(NHa)n
by S. K. Searles and P. Kebarle Chemistry Department, University of Alberta, Edmonton, Alberta, Canada (Received August 16, 1967)
The principal positive ions in ammonia vapor irradiated by ionizing radiation are NHd+(NH3),. Study of the relative concentrations of these species in function of pressure and temperature allows the determination of the temperature dependence of the equilibrium constants & - I , ~ for the reactions (n - 1, n) proceeding in the gas phase. Van't Hoff plots lead then to the detel-
+ NH3 = NH4+(NHJn
(n - 1 , n ) mination of AH,+, and also ASon-l,n. I n previous work,l we reported values for AH, AGO, and ASo for the reactions 2,3 and 3,4. For the reactions 4,5, only the AGO value could be obtained (parenthesized data in the table). The range of reactions covered was limited since the temperature of the a-particle ionization chamber, used in that work, could be varied only between the limits of 20-100°. Recently, using the proton-beam mass spectrometer,21awe could extend the temperature range from -50 to +500°, and we were able to determine the thermodynamic quantities for the reactions 0,l-4,5. These data are summarized in Table I. The values are shown in Figure 1. The apparatus, experimental procedures, and quality of the data were very similar to those reported in ref 3. The ammonia-ion source pressure was varied from 0.2 to 4 torr and was measured directly with a McLeod gauge and a differential mercury manometer. NHl+(NH3).-1
Table I : Thermodynamic Data" on Gas-Phase Reactions. NHr+(NHs)n-l
+ NHs = NHd+(NHa)n AGO,
k cal/mole (298'K)
n-1,n
AH,
ASa,eu (298'K)
koal/mole
-27
-B
- 17
- 32
1, 2 2, 3 3, 4 4, 5
- 6 . 4 (-6.4)b -3.8(-3.8) -0.2 (-0.5)
-16.5 (-17.8)b -14.5(--15.9) -7.5
-26.8 -34 (-38)* -36(-40) 25
0, 1
-17.5
a Standard state 1 atm. from previous work, ref 1.
b
-
Data in parentheses are results
(1) A. M.Hogg, R. iM.Haynes, and P. Kebarle, J. Am. Chem. SOC., 88, 28 (1966).
(2) J. G.Collins and P. Kebarle, J . Chem. Phys., 46, 1082 (1967). (3) P. Kebarle, S. K. Searles, A. Zolla, J. Soarborough, and M. Arshadi, to be published.
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 C N 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
