NOTES
2750
The structure of the uranyl ion appears to be a collinear arrangement of 0-U-0 with ligands positioned in an equatorial belt around the ~ r a n i u m . ~The U-0 bond distance in uranyl has been found to vary with the type of equatorial ligand,sp10but the generalization can be made that this distance is considerably shorter than the uranium-ligand bond distance. The source of the asymmetry in the ''0 nmr signal is not known; apparently it is not to be found in the water molecules which are in the coordination sphere of the uranium since the chemical shift of these waters would be expected to be much less than that for the uranyl oxygens. The resonance asymmetry can be constructed by the superposition of two absorption signals with a peak-to-peak separation of about 5 Hz. The relative areas of the downfield and upfield peaks a t 8 MHz are 1 : 1.4. Many publications have been concerned with the various polynuclear species of uranyl which are formed in weakly acidic or basic solution. Some of these species could give rise to different chemical environments for the oxygen with intensities in the ratio of about 2: 3, as for example a dimer of the formula (U02. U03)2+with the assumption of bonding involving two bridging oxygens. However, it would not be expected that! appreciable concentrations of polynuclear species, even if formed during the solution preparation, would exist as such in the highly acidic solutions used in this work. Acknowledgment. The author wishes to express his appreciation to B. B. McInteer and R . Potter of this laboratory for supplying the enriched NO for this study. (9) For a discussion of this structure see S. P. McGlynn, J. K. Smith, and W. C. Neely, J. Chem. Phys., 35,105 (1961). (10) W.H.Zachariasen, Acta Cryst., 8 , 847 (1954).
Spectroscopic Studies of the Pyridine-Iodine Complex
by Hari D. Bist' Department of Chemistry, University of Iowa, Iowa City, Iowa
and Willis B. Person2 Department of Chemistry, University of Florida, Gainesville, Florida (Received January 27,1967)
Because of the importance of the pyridine-iodine complex as a relatively stable example of an n-u complex between the lone-pair (n) donor, pyridine, and the The Journal of Physical Chemistry
antibonding u acceptor Iz, there have been many studies of its properties and especially of its formation constant K.3-7 These studies have special significance in that pyridine is the simplest member of an interesting series of electron-donor molecules which contain both n and a electrons. The pyridine complex with Iz seems to be significantly less stable than Iz complexes with other amines. The intensity of the charge-transfer band, on the other hand, is apparently anomalously high.8 There is considerable disagreement among the values reported for K3-' at 25". Part of this disagree ment may be due to real differences in K because of solvent effects; however, the system is somewhat reactive, especially a t higher concentrations of pyridine,3 so that the measured values of K may be in error because of further side reactions complicating the results. Reid and Mulliken3 reported the only really careful study of the intensity of the charge-transfer band. Because their value of K seemed significantly different
Table I: Formation Constants for the Pyridine-Iodine Complexes a t 25" K
Solvent
RemarkI
140 & 1 . 3 From the shifted visible Iz band a t 422 mp 138 =k 5 . 1 From the absorption around 305 mp 132 =k 4 . 2 From the charge-transfer region Interpolated from Figure 4 n-Heptaneb 185 of ref 3 Carbon tetrachloride" 101 From shifted visible IZband Carbon tetrachloride) n-Heptane From infrared bands Cyclohexaned lo7* 25 Chloroforme 43.7 From shifted visible band
n-Heptane"
1
'
This work. From Reid and Mulliken.3 a From Popov and From Maki and Plyler.6 e From Chaudhuri and Basu.' Rygg.4
(1) Fulbright Exchange Fellow. (2) Reprint requests and correspondence should be addressed t o Dr. Person a t this address. (3) C. Reid and R. S. Mulliken, J . Am. Chem. Soc., 76, 3869 (1954). See this reference for references to the earlier literature. (4) A. I. Popov and R. H. Rygg, ibid., 79, 4622 (1957). (5) J. N. Chaudhuri and S. Basu, Trans. Faraday SOC.,55, 898 (1959). (6) A. G.Maki and E. K. Plyler, J. Phys. Chem., 66, 766 (1962). (7) For a convenient summary of the literature, see G. Briegleb, "Elektronen-Donator-Acceptor-Komplexe," Springer-Verlag, Berlin, 1961. (8) R. S. Mulliken, J . Chim. Phys., 61, 20 (1964).
NOTES
2751
Table 11: Some Spectroscopic Properties of the Pyridine-Iodine Complex in n-Heptane Solution at 25" Complex (free), cm-1
vm
Band
*IIl aII0J
+
I&+
+
12,+
Reference
This work This work a
Charge-transfer band
This work a
14,300 19,225 19,225
vm
(complex), cm-1
18,500 23,700 23 ,690 42,300 42,540
Aul/*,
Free
Complex
om -1
em
am
1000 4400 4300 5250 5400
40 900 900
... ...
45 1540 1320 51 ,730 50,000
--Complex---f
4 . 6 x 10-4 0.033 0.024 1.40 1.12
ClVN
0.23 1.71 8.75
From Reid and Mul1iken.a
from other values4-? and because the values of K and E are linked,?*gwe thought that it was desirable to repeat the work of Reid and Mulliken, with special emphasis on the spectroscopic properties of both the visible Iz band and the charge-transfer band, making every attempt to control experimental conditions to avoid side reactions. Pyridine,lo n-heptane," and IZ1l were purified by standard techniques. Stock solutions of pyridine in n-heptane and of I2 in n-heptane were prepared and then mixed just, before study. Precautions were taken to keep the solutions dry and free of oxygen. A Beckman far-ultraviolet DK-2A double-beam spectrometer was used with 1- and 10-cm matched quartz cells and also with UV-0-2 variable short path length cells obtained from Limit Research Corp. The path lengths of the latter cells were measured by the method of interference fringes. In obtaining the spectrum of the complex, a matching solution of pyridine in n-heptane was used in the reference beam in order to cancel the pyridine absorption in the region of the charge-transfer band. At a path length of about 0.1 mm the pyridine absorption in this region could be cancelled satisfactorily. Our studies were made with pyridine concentrations less than 0.1 M . These solutions appeared to be quite stable, with only a 2% decrease in the absorbance of the complexed IZ a t 422 mp in the most concentrated pyridine solution even after 15 days.12 Furthermore, a plot of the absorbance a t 295 mp (where 13- is expected to absorb strongly) vs. the mole ratio [Py]/[Iz] showed no anomalies when compared to similar plots of absorbances a t 525,425, and 235 mp, where 1 3 - absorption is of differing importance. Hence, we conclude that there was no significant formation of 1 3 - in the solutions which we studied. Absorbances were read a t 14 wavelengths (375, 400, 425, and 450; 295, 300, 305, 310, and 315; and 225, 230, 235, 240, and 245 mp) for six different solutions ranging in pyridine concentration from 0 to 0.074
M and with [I2I0= 5.444 X M. The formation constant K and the molar absorptivities ~i were obtained separately from the three groups of data above using the Liptay method of a n a l y s i ~ . ~ , ~ ~ J ~ The formation constants which we find are listed in We Table I together with values reported consider that the values we find in the three different spectral regions agree within the experimental error. Our value of Kzss = 137 f 5 I./mole for the formation reaction in n-heptane is somewhat lower than the value reported by Reid and Mulliken, but we believe the agreement is within the experimental error. The results found for formation of the complex in CC144 and in CHCLSare probably significantly different from our results and probably provide some estimate of the solvent effect on K . Some spectroscopic properties of the complex are summarized in Table 11. Perhaps the most significant result is our essential verification of the high intensity for the charge-transfer band reported by Reid and M ~ l l i k e n . ~The values we report for the oscillator strength f and for the transition dipole ~ V Nare obtained by plotting e for the charge-transfer band, obtained by the Liptay method, against v and integrating numerically with the aid of a planimeter; from this14 f = (2.303mcz/ae2No)~~dv
(1)
= 4.318 X 10-gfedv (9) See also W. B. Person, J . Am. Chem. Soc., 87, 167 (1965),for example. (10) D. G. Leis and B. C. Currans, ibid., 67, 79 (1945). (11) L. Julien, Ph.D. Thesis, University of Iowa, 1966, following purification techniques similar to those described.*-' (12) This observation may be consistent with the ionic dissociation constant ([PyI +][I-])/[PyIz] reported by G. Kortnm and H. Wilski. 2. Physik. Chem. (Leipng), 202, 35 (1953), which predicts that about 2% of the Iz should be dissociated at these concentrations. However, we did not observe even this small change in studies made on freshly prepared solutions. (13) W. Liptay, 2. Elektrochem., 65, 375 (1961). (14) See R. S. Mulliken and W. B. Person, Ann. Rev. Phys. Chem., 13, 107 (1962),for example.
Volume 7 1 , Number 8 July 1967
COMMUNICATIONS TO THE EDITOR
2752
and
Our results agree quite well with the earlier values and seem to us to remove the possibility that the intensity anomaly for the charge-transfer band in pyridine could be due to experimental error. Finally, we should like to report that we have reproduced the changes reported by Reid and Mulliken
as more pyridine is added. We cannot add significantly to their discussion of these puzzling phenomena, except to note our support for their idea that the complex reacts to form 11- as pyridine is first added and then the Ia- reacts to form I- as more pyridine is added. We should stress that the reaction to form 13- is an equilibrium reaction and that the system is in stable equilibrium. Acknowledgments. We are grateful to Public Health Service Research Grants No. GM-10168 and GM14648 for financial support of this work. Discussions and help with the computing from Dr. Larry Julien together with support from the University of Iowa Computing Center are gratefully acknowledged.
COMMUNICATIONS TO THE EDITOR
The Retention of Optical Configuration during Energetic Chlorine Atom Exchange i n Gaseous Alkyl Halides
Sir: The stereochemistry of the substitution at asymmetric carbon atoms of energetic halogen atoms, activated by nuclear recoil, has been previously studied only in condensed phase systems, in which radicalradical cage combination reactions can play an important role.’V2 We have now studied this substitution reaction CP*
+ R C 1 4 R C P + C1
(1)
a t asymmetric positions in gas-phase experiments and have observed that the exchange proceeds with almost complete retention of optical configuration with either DL- or meso-2,3-dichlorobutane. I n the condensed phase, in contrast, both radioactive isomers of 2,3-dichlorobutane are observed in amounts varying with temperature and phase. The total yield of all organic radioactive products is normally much higher in condensed phases than in gas-phase experiments with haloalkanes consistent with the importance of cage combination effect^.^ Table I contains a summary of the observed results for the reactions of C13s formed by the (7,y) reaction on the natural C13’ of the parent 2,3-dichlorobutane. The organic The Journal of Physical Chemistry
products eluting from the gas chromatographic column through the 2,3-dichlorobutane peaks represent approximately 3% of the total C P formed (in experiments without moderators) and the yield of C13a-2,3-dichlorobutanes is about one-tenth of this total-an over-all yield of 0.3% for the substitution of C13*for C1 in these molecules. The other observed organic products include those expected from the replacement of CH3 by C138,H by C138,etcS3 The observed stereospecificity of the substitution reaction is unaffected by the presence of large excesses of argon or xenon. We have also studied the same reactions with energetic C13@ from a nuclear reaction with entirely different characteristics, Ar40(y ,p) C13g. These experiments, carried out in the presence of large quantities of argon, are in complete agreement with those of Table I, as shown in Table 11. No absolute yield measurements have yet been carried out in our system for reactions with Ar40(y,p)C139. The agreement between the results obtained with (1) C. M.Wai, C. T. Ting, and F. S. Rowland, J . Am. Chem. Soc., 86,2525 (1964).
(2) F.5.Rowland, C. M. Wai, C. T. Ting, and G. Miller, “Chemical Effects of Nuclear Transformations,” Vol. 1, International Atomic Energy Agency, Vienna, 1965,p 333. (3) J. E. Willard, “Chemical Effects of Nuclear Transformations,” Vol. 1, International Atomic Energy Agency, Vienna, 1965, p 221.
