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E.11.KOSOWER, J. -4. SKORCZ, IT.AI. SCHWARZ, JR.,
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Pyridinium Complexes. I. The Significance of the Second Charge-Transfer Band of Pyridinium Iodides € 3 EDWARD ~ 11.KOSOWER, JOSEPH -4. SKORCZ,~ ~VILLIAM A I . SCIIWARZ, JR., AKD JAMES Tv. PATTON RECEIVED AUGUST10, 1959 A second charge-transfer absorption band appears in the spectra of many pyridinium iodide complexes in non-polar solvents. I t s relationship t o the first charge-transfer absorption band2 is shown by parallel behavior with respect to solvent variation and within limits, to change in the substituents on the pyridinium ring. The difference, ATB, between the transition energies for the two bands varies between 15.9 and 28.6 kcal./mole. However, ATE for 1-methylpyridinium iodide in chloroforni is 21.8 kcal./mole. Franck and Scheibe4 interpret the ultraviolet spectrum of iodide ion as a photoionization process, producingan iodineatomand a "solvated electron." Two bands are observed with ATE 21.2 kcal./mole, corresponding closely to the expected difference for electronic transitions leading to an iodine atom, which has two low-lying energy states, and *Pa/*, separated by 21.74 kcal./mole (0.943 ev.) as determined by analysis of iodine spectra. The close correspondence of the ATE value for 1-methylpyridinium iodide in chloroform to the ATE value expected for production of a n iodine atom is direct evidence for the charge (electron)-transfer nature of the excitation in the pyridinium iodide. I n addition, the position for the charge-transfer band of 1-methylpyridinium iodide in water cansbe derived from a plot of transition energies against Z, a standard of solvent polarity.z The position thus derived (2561 A , ) demonstrates t h a t the charge-transfer process is greatly facilitated by the 1-methylpyridinium ion, for iodide ion in water has a maximum
hv
a t 2269 -A. A complete description for the transition is thus: MPy+I-+MPy.I.. Systematic variations in ATE values are found for different types of iodide charge-transfer spectra, for which no rationalization is readily apparent.
In a previous paper, the existence of a second sorption by iodide i o n which exhibits two bands pyridiniurn iodide charge-transfer absorption band separated by a characteristic energy diference, dewas m e n t i ~ n e d . ~The , ~ purpose of the present fined a s ATE, leads to an excited state containing an paper is to show how the existence of the second iodine atom. An important qualification of the band is in accord with the current model2 for rule is that some binding, presumably small, exists pyridinium iodide charge-transfer transitions. between the iodine atom and the system to which Halide ions are of particular spectroscopic in- the electron has been transferred. It should be terest because of their monatomic nature. Iodide noted that Katzin'O recently made an attempt to ion in water has two absorption bands in the utilize the above rule for light absorption for any accessible ultraviolet region, a t 2259 and 1937 8. iodine compound, including poly-iodide metal ion Franck and Scheibe proposed many years ago that complexes and covalently bound iodo compounds. the separation between the two band maxima cor- An experimental study of a few of the latter, to be responded to the separation between the two low- reported elsewhere," does not bear out such a lying states of the iodine atom, the 2P3,2and generous extension of the rule. We shall regard ?PI,?states4 The inference was clear that light the rule as applying only to iodide ion; even with absorption by iodide ion was equivalent to a photo- this limitation, it seems likely that a narrow ionization. The Franck-Platzman theory of a definition of the magnitude of ATE is not possible. potential well formed by solvent molecules serves Results as an adequate description of the location of the electron after excitation,j although some conIn the course of an investigation of the chargetroversy still exists on some of the details.6 Eyua- transfer bands of alkyl-substituted pyridiniurn tion 1 illustrates the process for iodide. iodides,12 it was found that the second band for 1methylpyridinium iodide (I) readily could be observed under carefully controlled experimental conditions. In pure chloroform, for example, the The difference between the two lowest states of the iodine atom was estimated originally by Franck from data on neon7 and was quickly confirmed by Turner.* The most recent value is 0.943 ev. or 21.74 kcal./mole.9 The proposal of Franck and Scheibe may be generalized as follows: Light ab(1) Based in part 011 a portion of a thesis submitted by Joseph A. long wave length maximum (first charge-transfer Skorcz in partial iulfillment of the requirements for the M.S. degree, band) was a t 3796 ( E 1210) and the second June, 1!l39. maximum (secpnd charge-transfer band) was ( 2 ) E. 31. Kosower, TKISJ O U R X A L , 8 0 , 3253 (1958). located at 2945 A. (E 1550). The transition energy ( 3 ) T h i s research was supported in part b y funds granted b y t h e Air Force Ofice of Scientific Research through Contract 49(638)-282, difference, ATE, between these two maxima was Grant S o . E-1608 of t h e National Institute of Allergy and Infectious 21.8 kcal./mole, identical to that predicted for the Diseases and the Wisconsin Alumni Research Foundation through the formation of an iodine atom in the excited stateg liegearch Committee of the Graduate School. within the experimental error. The spectrum of 1(4) J . Franck and G. Scheibe, 2. p h y s i k . Chem., 8 1 3 9 , 22 (1928). ( 3 ) R. Platzman and J. Franck, "L. Farkas Memorial Volume," methylpyridinium iodide is shown in Fig. 1.
x.
Jerusalem, 1952, p . 21; 2 . P h y s i k . , 138, 411 (1954). (6) 31. Smith and 11.C. R . Symons, Trans. F a r a d a y S o c . , 54, 338, 346 (1958). (7) J. Franck, i b i d . , 21, 536 (192.5). (8) L A . Turner, Phys. Rev., 2 7 , 397 (1926). (0) K Mirakawa, 2. P h y s i k , 109, 16'2 (1938).
(10) L. I . Katzin, J. Chem. P h y s , 23, 2055 (1955). (11) E. M. Kosower, Pih-kuci C . Huang, and W. 31. Schwarz, Jr., unpublished results. (12) E. M. Kosower and J . A. Skorcz, THISJOURNAL, 82, 2195
(1960).
May 5, 1960
PYRIDINIURZ COMPLEXES : SECOND CHARGE-TRANSFER BAND
2189
A change in the solvent composition, effected by adding ethanol in increasing amounts to the chloroform, leads to a shift of the first maximum. In chloroform containing 0.97y0 ethanol, the first maximum for 1-methylpyridinium iodide is 3730 A., wbile the second maximum is now found a t 2890 A . The difference, ATE, is 22.2 kcal./mole, somewhat higher than that for pure chloroform; however, examination of the data in Table I for 1methylpyridinium iodide spectra taken in chloroTABLE I VARIATION OF MAXIMAWITH ETHANOL CONTENTFOR 1METHYLPYRIDINIUM IODIDEIN CHLOROFORM Vol. % E t O H
Xmhr
ET^
€mal
ATE
0.0
3796 1210 75.3 21.8 2945 1550 97.1 .27 3796 1250 75.3 21.5 2952 1570 96.8 .54 3785 1220 75.5 21.7 2942 1480 97.2 .62 3778 1200 75,7 21.6 2938 1480 97.3 .;2 3756 1190 76.1 21.9 2918 1480 98.0 .88 3738 1200 76.5 22.4‘ 2892h 1560 98.9 .97 3730 1190 76.7 22.21 2890’ 1500 98.9 a Transition energy 2.859 X Y (in cm.-l). The proximity of this maximum to the sharply rising absorption band for the 1-methylpyridinium ion probably moves it t o somewhat shorter wave lengths, thereby increasing A T E .
form solutions containing various amounts of ethanol suggests that ATE is substantially constant over the limited range available for consideration. Not only does ATE remain constant (or almost so) but the intensity relationship of the two bands is unaltered over the range of ethanol concentrations used. The remarkable solvent sensitivity of pyridinium iodide charge-transfer bands2 requires that only a closely related band could maintain a constant energy separation. The second band for all other alkylpyridinium iodides is only partially visible, due to occultation by the absorption of the pyridinium ring. For those cases in which a marked change in slope clearly indicated two types of absorption in the 28003100 A. region, it was possible to obtain some idea TABLEI1 POLYALKYLPYRIDINIUM IODIDES Alllax
ernax
El-
Xa
E5
1’E” c
3738 1200 7 6 . 5 3060 9 3 . 4 1-CH: 17 1,4-(CII3)n3587 1210 7 9 . 7 2990 9 5 . 6 16 16 1,WC & ) r 3637 8 i O 7 8 . 6 3010 9 5 . 0 l-i-CH(CH3)23705 1020 7 7 . 1 3010 9 5 0 18 l - C H 3 - 4 - n G H 7 3598 1150 7 9 . 5 3020 9 4 . 7 15 l-CH3-4-i-C3Hr 3593 1210 79.6 3015 9 4 . 8 15 l-CHa-4-t-CaHg3590 1180 7 9 . 6 2980 9 5 . 8 16 1-CzHa3726 1150 7 6 . 7 3060 9 3 . 4 17 l-CHa-4-CzHs3587 1190 7 9 . 7 2980 9 5 . 8 16 a Wave length a t which optical density was equal to that a t Amax. Transition energy corresponding to the wave length found according to a. Estimated as zkl kcal./mole. Solvent: chloroform containing 0.90yoethanol by volume.12
spectrum of 1-rnethylpyridiniuni iodide in pure chloroform.
of the behavior of the second band by finding the wave length a t which the absorption intensity became equal to that of the first charge-transfer band maximum. The differences thus found are listed as “ATE” in Table I1 and are certainly less accurate ( 1 kcal./mole) because the data for the second band were oobtained from curves run a t ordinary speed ( 5 A./sec.)12 The “ATE” values are 16 1 kcal./mole with only one exception and represent agreement with the proposition that the second band is a charge-transfer absorption like the first. For a change in substitution a t a particular position on the pyridinium ring, e.g., from 4-carbomethoxy to 4-cyano, the second band moves in the same direction as the first and roughly to the same extent (Table 111).
*
*
TABLE I11 A T E VALUESAND IODIDE SPECTRA Iodide Potassium
Solvent HOH
Sodium12
HOH
Sodium
(CH2)zCO
Pyridinium-1-CHi
Pure CHCk
Xmar 2259 1937
1-CHI-2-CN
CHzClz
l-CH3-2-COOCiHs-
CHC18
3796
4775 3528 4270 3243
l-CH3-3-COOCH3
CHC1iC
4070 3320
CHsCN CHLXz
emax
12300 14300 2262 13500 1935 13500 2551 12000b 2161 14700 2945
3595 4905 3361 ~ - C H ~ C H ~ - ~ - C O O CClCHiCHzCl HB 4508 3108 Triiodide, (CtHa)aN CHCN 3605 2918 1-CH3-4-CN
