3178
Orientation in Pyridine-Jodine Complexes
NOTES
1.01
by P. L. Kronick The Franklin Institute Research Laboratories, Chemistry Division, Philadelphia, Pennsylvania 19109 (Received March 16, 1966)
In solution, pyridine and iodine form a molecular complex with a blue-shifted iodine band and an unusually high heat of formation (-8.0 kcal.).l On the basis mostly of spectroscopic and thermodynamic properties, it is believed that the interaction between the donor and acceptor components in this complex involves the unpaired electrons on the nitrogen atom in pyridine. The simple 1: 1 complex pyridine-iodine is believed to be not a 7r-complex but an n-complex. When iodine is absorbed by solid poly(4-vinylpyridine) in fairly small amounts, an adduct is formed which resembles the pyridine-iodine solutions. Films of the adduct, for example, have the blueshifted iodine bands. Also, the system is reversible,2 desorbing iodine under high vacuum, but much more slowly. than polystyrene-iodine. Because of similarities in chemical composition and in physical behavior between iodine-poly(Pviny1pyridine) on one hand, and iodine-pyridine and 4-picoline solutions in aliphatic hydrocarbon solvents on the other, it seems likely that both should have similar structures. The actual configuration in this type of complex, however, has been determined directly only for the 4picoline-bromine crystal. As expected, the conformation is not the same as that in benzene-hdogen complexes in which the halogen molecules interact with the a-electrons of the benzene ringe4 In the complex with picoline, the halogen molecules interact specifically with the nitrogen atoms, forming linear arrays, the Br-Br-N directions being inclined to the planes of the rings by 13’. This angle of inclination may mean that crystal-ordering forces are operating in defining the complex in the crystal and raises the question whether the solid state configuration is at all comparable to that in amorphous phases. Some information about this question can be obtained from a measurement of the dichroism of the shifted iodine band in oriented poly(4-~inylpyridine)-iodinefilms.
Experimental Poly(4-vinylpyridine) was prepared by peroxideinitiated polymerization of Pvinylpyridine. The polymer was dissolved in pyridine and cast as films. When peeled from the substrate and stretched, the fdms became negatively birefringent, the direction of
\
0 650
600
5 00 WAVELENGTH ( m k )
400
2 5 0
Figure 1. Dichroism of the shifted iodine absorption band in oriented polyvinylpyridine-iodine film.
greatest molecular polarizability being perpendicular to the direction of stretch. This behavior is to be expected from the simplest model of deformation of high polymers, the aromatic ring planes assuming orientations perpendicular to the direction of stretch; the main chains, ~ a r a l l e l . ~ Films of polyvinylpyridine are brittle and difficult to stretch. To facilitate uniform drawing for the dichroism measurement, they were cast for support on a 5-mil polyethylene substrate, which itself showed no absorption in the near-ultraviolet region and did not absorb iodine. The thickness of the polyvinylpyridine layer was 1 to 10 p . The double films were then oriented by stretching. The complex was formed by exposing the oriented films to iodine vapor in the presence of air. (1) C. Reid and R. S. Mulliken, J. Am. Chem. SOC.,76, 3869 (1954). (2) S. B. Mainthia, P. L. Kronick, and M. M. Labes, J . Chem. Phys., 41,2206 (1964). (3) 0.Hassel, “Investigation of Molecular Structures,” U. 5. Department of Commerce Office of Technical Services, Publication Board Report 155,783(1960). (4) 0. Hassel and K. 0. Stromme, A d a Chem. S c a d . , 12, 1146 (1958); 13, 1781 (1959). (5) R. D.Andrewa, J . Appl. Phys., 25, 1223 (1954).
NOTES
The dichroism of the shifted iodine band was observed by vertically polarizing the two beams of a Bausch and Lomb 505 spectrophotometer with Clam polarizers and measuring the spectrum of the stretched film with the direction of stretch first parallel and then perpendicular to the polarization direction. The spectra are shown in Figure 1. It is seen that the absorption intensity is strongest for light polarized in the direction of th.e stretch. The electronic transition of the iodine is TI0++ 'Z+, polarized perpendicular to
3179
Acknowledgment. The assistance of Mr. P. J. Hackett in determining the spectra is gratefully acknowledged. (6) J. Ham, J. Am. Chem. SOC.,76, 3886 (1954). (7) R. S. Stein and F. H. Norris, J . Polymer Sci., 21, 381 (1956). (8) B. Baule, 0. Kratky, and R. Treer, 2. physik. Chem., SOB, 255 (1941). (9) M. C. Tobin and M. J. Carrano, J. Polymer Sci., 24, 93 (1967).
molecules are preferentially oriented perpendicular to Free Energy of Formation of Fe(CN)P(aq) the direction of stretch. Since the pyridine rings are and Fe(CN)a4-(aq)' also perpendicular to the molecular chains, the iodine molecules are parallel to the planes of the aromatic rings. by R. H. Busey We were unable to measure directly the degree of Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, orientation of the chains in the polyvinylpyridine Tennessee (Received March 26,1966) part of the film because of interference from the polyethylene substrate both in the quartz-ultraviolet absorption spectrum and in the birefringence effect. Hepler, et U Z . , ~ give 63.4 1.0 cal. de@;.-' mole-' The polyethylene film itself was stretched to a birefor the standard partial molal entropy of the ferricyfringence of 0.005, corresponding to 0.41 for (cos2+).v, anide ion, Fe(CN)P(aq). This result was derived in the average orientation cosine, calculated from the the usual manner from their measurements of the heat expression (cos2 $)Bv = 2(An/An,) -I- '/a using known of solution of KaFe(CN)6combined with other thermoparameter^.^ This value in turn corresponds to a dynamic data available from the literature. For the stretch ratio of 1.15, both experimentally and theoentropy of KsFe(CN)a required in the calculation, they retically.7,* The dichroic ratio of the CHZ rocking used 100.4 cal. deg.-' mole-' and referred to heat frequency in polyethylene with this stretch ratio71B capacity measurements of Stephenson and Morrow. a is the same as that for our shifted iodine band. PolyThey apparently do not include, however, in the vinylpyridine being essentially amorphous has a difestimated uncertainty of 11.0 cal. deg.-' mole-' in ferent internal texture than polyethylene; yet it would the entropy of Fe(CN)P(aq) the uncertainty attached seem that the polyvinylpyridine chains "follow" by Stephenson and Morrow to their value for the enthe orientation of the polyethylene substrate, at least tropy of KaFe(CN)s. The latter uncertainty is conat low degrees of stretch. cerned with what portion of the magnetic entropy ( R These observations show that the iodine molecules In 2 for the spin-paired 3d6complex) exists below 15OK. interact strongly with the pyridine rings, taking up More recently Watt, et u Z . , ~ determined calorimetrifixed orientations with their long axes parallel to the cally the standard heat changes, AHo,for the reactions planes of the rings. The stability of the complex Fea+(aq) 4- 6CN-(aq) = Fe(cN)~~-(aq) (1) over that of the polystyrene complex is independent evidence of interaction with the nitrogen atom. Thus Fe2+(aq) 6CN-(aq) = Fe(CN)a"(aq) (2) the iodine molecules may lie folded over the These data were combined with appropriate entropy pyridine rings but parallel to the C4-N direction; they data from the literature, which included the above may lie in arrays tending toward linear C4-N-1-1 value for ,%o(Fe(CN)~a-(aq)), to obtain AFo for each arrangements; or they may be perpendicular to C4-N. The symmetrical perpendicular arrangement gives, (1) Research sponsored by the U. 8. Atomic Energy Commission to a first approximation, zero overlap, not to be exunder contract with Union Carbide
*
+
tions and the results for the picoline-bromine crystalsa are more consistent with the linear model. None of these structures is consistent with the "oblique model."l
82* 304 (lg60).
(3) C. C. Stephenson and J. C. Morrow, ibid., 78, 275 (1956). (4) G.D.Watt, J. J. Christensen, and R. M. Izatt, Inorg. C h a . , 4, 220 (1965).
Volume 69, Number 9 September 1966
