May 5, 1WO [CQNTRIBUTION FROM
I’YRIDINIUhl. THE
CORZPLEXES : CHARGE-TRAXSFER BANDS
2195
DEPARTMENT OF CHEMISTRY, THE UNIVERSITY OF WISCONSIN, MADISON 6, WISCONSIN]
Pyridinium Complexes. 111. Charge-Transfer Bands of Polyalkylpyridinium Iodides BY EDWARD M. KOSOWER AND JOSEPH A. SKORCZ’ RECEIVED AUGUST10, 1959
4 study of the first (long wave length) charge-transfer band of twenty different alkylpyridinium iodides under carefully standardized conditions has established that (1) increasing the number of methyl groups raises the transition energy, ( 2 ) changing a methyl group from the 3- to the 2- to the 4-position increases the transition energy, (3) varying the nature of the alkyl group from methyl to t-butyl on the 4-position has virtually no effect upon the transition energy and (4) the solvent sensitivity of 1-methylpyridinium iodide is appreciably greater than t h a t of 1-ethyl-4-carbomethoxypyridiniumiodide, as might have been expected on the basis of a previously proposed model. The maxima observed varied from 3738 A . for 1methylpyridinium iodide to 3253 A . for 1,2,3,4,5,6-hexamet~ylpyridinium iodide. Surprisingly, replacement of the 1methyl by hydrogen moved the charge-transfer band t o 3463 A. Two points favor the interpretation of the effect of alkyl groups as being due to the bond dipole generated by the combination of a n sp3 bonding orbital with an sp2 orbital: (1) the effect of the 3-methyl group can be accounted for on the basis of electrostatic stabilization of the ground state and ( 2 ) a change in the nature of the alkyl group on the 4-position does not elicit electron-supply dependent on the number of ahydrogens, as would have been expected for hyperconjugation.
densities in the spectroscopic measurements. It was assumed that the activities of the pyridinium iodide ion-pair complexes were all a constant fraction of the concentrations, but it was not possible to extrapolate the data to zero concentration because oi experimental difficulties in working with extremely low concentrations of iodide salts. Although temperature certainly affects the position of charge-transfer bands,4 it was considered more expedient to allow convenient ambient temperatures to control operating conditions ( 1 for almost every measurement) rather than to sacrifice speed in obtaining the maxima. Oxidation could be detected in the rapid procedure by small changes in consecutive curves. The most serious difficulty in measurement of pyridinium iodide charge-transfer maxima is the ever present possibility that the spectrum may inResults clude some absorption by triiodide ion, which Alkylpyridinium iodide charge-transfer band possesges two intense maxim? in chloroform a t positions are sensitive to solvent, salt concentra- 3625 A. ( E 21,600) and 2950 A. ( E 33,000). The tion and temperature. A non-polar solvent was long wave length band is about eighteen times as chosen so as to provide completely associated intense as the first charge-transfer maximum of pyridinium iodide ion-pairs (complex), 6 , 9 thus many alkylpyridinium iodides. Fortunately, the simplifying the comparison of absorption intensi- ratio of the intensities of the two maxima of trities. Chloroform containing 0.90% ethanol was iodide ion differs considerably from that for 1used in this study because it possessed the requisite methylpyridinium iodide, and i t was therefore solvent power and had a solvent polarity (de- possible to control the experimental techniques pendent upon ethanol content5) controllable within used. No analytical method of the accuracy certain limits. Ethylene chloride6 was not a needed to correct for triiodide ion was available; sufficiently good solvent for some of the salts. it was necessary to measure the spectra under conSalt concentrations (ie., alkylpyridinium iodide ditions where triiodide was not formed. molarity) were held within very narrow limits, Wet pyridinium iodides seem far more sensitive 0.91 =t 0.01 X M being selected as the concentration which gave adequately high optical to light and oxygen than the dry salts. To prevent oxidation in the pyridinium iodides, all salts (1) Based in part on a portion of a thesis submitted by Joseph A. were stored and weighed in a good dry box under a Skorcz in partial fulfillment of t h e requirements for the h1.S.degree, dry nitrogen atmosphere. All solutions were preJune, 1959. (2) E. hZ. Kosower, THIS J O U R N A L , 77, 3883 (1955). pared in the same dry box and all cells for spectro(3) A. Hantzsch, B e y . , 44, 1783 (1911). scopic measurements were filled with the same (4) E. M. Kosower, THISJ O U R N A L , 80, 3253 (1958). protection. Dim light was used as an additional (5) E. ll. Kosower, J. A. Skorcz, W. M. Schwarz, Jr.. and J . W. precaution. Reproducibility of maxima and abPatton, i b i d . , 88, 2188 (1960). (6) E. M. Kosower and W. M. Schwarz, Jr., unpublished results. sorption intensities served as the prime criteria of (7) E. M. Kosower and J. A. Skorcz, Paper presented a t Fourth valid spectroscopic data. The intensity ratio of the Meeting of European Molecular Spectroscopy Group a t Bologna, Italy, two bands of I-methylpyridinium iodide served as a September 7-12, 1959. check on the procedure and indicated that, in (8) This study was supported b y funds granted b y the Air Force Office of Scientific Research through contract A F 49(838)-282 and by general, the triiudide concentration did not exceed t h e h-ational Institute of Allergy and Infectious Diseases under grant x 10-8 M. 2 E-1608. The alkylpyridinium iodides used were rigor(9) E. h l . Kosower and J. C. Burbach, THISJ O U R N A L , 78, 5838 (1956). ously purified. All of the maxima listed in the
The special light absorption of alkylpyridinium iodides2 first discovered by Hantzsch3 is now well established as a transition involving charge-transfer from the iodide ion to the alkylpyridinium ion on the basis of solvent effect^,^ A TE value^,^ conductivity measurements6 and a few substituent effects.’ In order to define the substituent effects on these absorption bands in a more precise manner, a series of different alkylpyridinium iodides was investigated carefully. It was hoped that the relationships found would also cast some light upon the means by which alkyl groups supply electronic charge to electron-deficient systems. By rigid control of the many variables which affect the environment-sensitive pyridinium iodide transition, it was possible to obtain data applicable to both of these purposes.8
EDWARD M. KOSOWER AND JOSEPH A.SKORCZ
2196
Vol. 52
I CHLOROFORM
c = 0 095 x
IO-^ M
z
W
I
a:$,
2800
,
,
,
,
3000
3200
,
,
3400
,
I
,
3WO
1
38M
4000
1
,A Fig. 1.-The
spectrum of 1-niethylpyridiniuini iodide in "standard" chloroform.
various tables represent the average of the best five of six separate determinations of absorption
curves; each determination included three to five measurements of the absorption curve in the region of the maximum. A complete description of the technique is included in the Experimental section. All of the maxima are measured under standard M conditions, which we shall define as 0.91 X alkylpyridinium iodide in chloroform containing 0.90% ethanol a t 26 =t 1'. Changes in concentration or solvent are specifically noted. Although real ambiguity exists only for 1-methylpyridinium iodide for which two bands are observed under standard conditions, it should be stated explicitly that all maxima belong to the first charge-transfer band @e., the longest wave length band). The first maximuq for 1-methylpyridinium iodide is found a t 3738 A,, as seen in the absorption spectrum shown in Fig. 1. Replacement of a pyridinium ring hydrogen by another methyl group shifts the maximum but the extent of the change depends markedly upon the positiafz of the additional methyl group. At the 3-position, the methyl group moves the maximum to 3700 A., while a %methyl group shifts the band to 3640 A. and a $-methyl group to 3590 A. These data are summarized in Table I. TAULE I DIMETHYLPYRIDIXIUM IODIDES Substituent
1-Me 1,2-hZez 1,3-;LIe~ 1,4-Me*
Amax,
A.
3738 3640 3700 3590
ET,
Error,
A. 1 5 1 5 *5 i5
c,,~:~~
1200 860 1310 1330
kcal./mule
76.5 78.5 77,3 79.6
Additional methyl groups cause further increases in transition energies, i e . , shifts of the maximum to shorter wave lengths. The positions for the 1,2,5-trimethyl-,1,3,5-triniethyl- and 1,2,4,6-tetrainethylpyridinium iodides are those predicted on the naive basis that the effects of methyl groups as measured with the band positions for the dimethyl pyridiniuni iodides are additive. Three other polyrnethylpyridiniuin iodides possess absorption bands
-\ 0
1
'
.
Fig. Z.-The
'
I
m
'
lbm
k,
am
.ID)
spectrum of 1,2,3,~,5,6-1iesamethylDyridiliiuin iodide in "standard" chloroforni.
which deviate somewhat from the locations expected, but only hexamethylpyridinium iodide has a maximum a t a wave length much shorter than calculated as mentioned above. The data for the polymethylpyridinium iodides are listed in Table I1 and the curve for hexamethylpyridinium iodide is shown in Fig. 2. TABLE I1 POLYMETHYLPYRIDINUM IODIDES Substituent
1,2,3-Mea 1,2,4-?&3 1,2,5-Me3 1,2,6-Me3 1,3,5-Me3 1,2,4,6-Mei 1,2,3,4,5,G-Me~
-4. 3569 3530 3607 3580 3660 3418 3253
Xrn.i*,
Error,
A.
cmap
15
820 940 1030 440 1370 480 560
5 =t 5 A10 1 5 110 A10 &
ET kcal./inole
80.1 81.0 79.3 79.9 78.1 83.7 87.9
X change in the alkyl group on the 4position of the pyridinium ring causes only a negligible variation in band position or in absorption intensity, as shown by the values listed in Table 111. TABLE I11 1-1\.IETHYL-~-ALKYLPYRIDINIUMI O D I D E S Error, 81., A. tmlx kcal./mole Substituent A,,,,,, A.
4-Me 3590 1 5 1230 79.6 3586 1 5 1200 79.7 4-Eta 4-n-Pr 3599 =t5 1210 79.4 *5 1210 79.5 4-i-Pr 3594 4-t-Bu 3590 1 5 1170 79.6 6 1-Methyl-4-ethylpyridiniurn iodide could not be obtained as a completely colorless salt, b u t only a s pale yellow crystals, in spite of many repeated crystallizations and charcoal treatments. The close agreement of the maximum absorption intensity with the c values found for other compounds listed in Table I11 suggests that t h e yrllow color is not due t o triiodide (which would have increased t h e e considerably) but t o a trace of dihydropyridine formed by loss of a proton from the a-position of t h e ethyl group. T h e postulated impurity is I==\
C ' F I L C H q S?H; \-::I It is also true t h a t the sulution for 1-methyl-4-cthylpyridinium iodide was measured at a teniperature 2" lower than most of the other solutioiis; it is easily possible t h a t the maximum listed should be increased 4-6 -4.
A surprising result is noted when the effect of alkyl group variation on the 1-position, ;.e., a group
May 5 , 1960
PYRIDINIUM
COMPLEXES: CHARGE-TRANSFER BANDS
"t
Fig. 3.-Half-curves for the charge-transfer bands of the various iodides: ( A ) 1,3-dimethylpyridinium iodide; ( B ) l-methyl-4-n-propyl-, 1-methyl-4-;-propyl- and l-methyl4-t-butylpyridinium; ( C ) 1-i-propylpyridinium; (D) 1,2,3trimethylpyridinium; ( E ) 1,2,4,6-tetramethylpyridinium.
directly attached to the nitrogen, is examined. Pyridinium hydriodide has a charge-transfer maximum a t a wave length considerably shorter than that of 1-methylpyridinium iodide, although the opposite shift might have been expected from the general effect of methyl groups in moving the pyridinium iodide charge-iransfer maxima to shorter wave lengths. Other variations in the alkyl group have only a small effect, as can be noted in Table ISr.
o
SOLVENT, CHCI,
(+E+OH)
CONC 0906 X LO.'
o2
01
VOLUME
2197
M
0.8
on
ID
1.2
PERCENT 'ETHANOL
Fig. 4.-Plot of transition energies (charge-transfer band) of 1-methylpyridinium iodide against volume 70ethanol in chloroform.
A simple demonstration that the electronic transitions observed in each case are all very similar in character in spite of the considerable variation in position and intensity is provided by Fig. 3, in which half-curves for seven of the alkylpyridinium iodides are plotted so that their maxima coincide. All of the curves are quite characteristic for charge-transfer absorption bands of pyridinium iodides and, except for the variation in intensity, are similar in shape and breadth.lO
One of the most characteristic properties of the charge-transfer bands of pyridinium iodides is their extreme sensitivity to solvent, and, in fact, the nature of the variation observed on solvent change may be used to identify the maximum as a chargetransfer transition.ll Variation of ethanol content in chloroform solvent causes a change in the position of the maximum, although, as Fig. 4 makes clear, the shift is not linear over the whole range of ethanol concentrations. The curve shown is a plot of transition energies against ethanol content, based on data reported in an accompanying paper.s More extensive increases in the polarity of the solvent cause correspondingly greater changes in the position of the maximum; these data are given in Table VI. Previous studies of 1-ethyl-4-cyanopyridinium iodide4 led to a plot of charge-transfer transition energies (corresponding to the maxima) ET against 2, a standard of solvent polarity based on the position of the maxima for 1-ethyl-4-carbomethoxypyridinium i ~ d i d e with , ~ a slope of 0.97. In contrast, the slope of the plot of ET for l-methylpyridinium iodide against Z is 1.12. The linear correlation with Z permits the estimation of the maximum for the charge-transfer band of l-methylpyridinium iodide in water as 2561 -k., or 111.6 kcal./mole (ET). As pointed out elsewhere, the 1methylpyridinium ion is thus shown to be a much more effective electron acceptor than water (A,, 2259 .$.) for the electron excited by light away from the iodide ion.5 Two points for pyridinium iodide also are shown in Fig. 5. The large shift in the position of the absorption band observed for pyridinium iodide under standard conditions to much shorter wave lengths in acetonitrile provides evidence that the transition is like that of 1-1nethylpyridinium iodide, or, in other words, that the long wave length absorption band is a charge-transfer traiisition (Table VI).
(10) Figure 3 is a plot of optical density against wave length but the comparison is still valid for optical density against wave number.
(11) E. RI. Kosower, "The Enzymes." Vol. 111, Academic Press, Inc., liew York, N. Y., ISGO, Chapter 13.
TABLE IT' 1-ALKYLPYRIDINIUM IODIDES Substituent
hmax,A.
1-H 1-Me 1-Et 1-i-Pr
3463 3738 3727 3710
cmax
ET, kcal, 1 mole
390 1200 1140 1040
82.5 76.5 76.7 77.1
Error,
A.
=t 10
* 5 =t5 f 5
The three 1-methyl-x-t-butylpyridinium iodides also were examined. Unlike the pair of iodides with a methyl and t-butyl group in the 4-position (Table 111),change from a 2-methyl to a 2-t-butylpyridinium iodide had a considerable effect on both the position and intensity of the charge-transfer maximum (Table V). TABLE V 1-hfETHYL-X-I-BUTYLPYRIDINIUM IODIDES Substituent
hmsi,
2-t-Bu 3-t-Bu 4-t-Bu
3694 3678 3590
a.
ET,
Error,
A.
*5 *5 &5
cmax
kcal.jmole
1240
77.4 77.7 79.6
1150 1170
EDWARD 11. KUSOWER AND JOSEPH A. SKORCZ
2188
1
I
Vol. 82
TABLEV I SOLVEXT YARIATIOK:~-METHYLPYRIDISIUM AXD PTRIDIKIUM IODIDES ET,
A. fnppmii kcal./niole l-,l.etliylpyridinium iodide 1200 i6.5 CHC13 (63.2) 3738 DMF” (68.5) 3480 1; 82.2 3391 31 84.3 CHaCK (71.3) i-PrOH (75,9)* 3145 394 90.9 E t O H (79.7)’ 3015 142 94.8 Pyridinium iodide CHCh ( 6 3 . 2 ) 3464 390 82.5 CHaCN (71.3) 3150 48 90.8 Dirnethylformairiide. Iierneasured for the solvent used for t h e determination. Solvent (2) 4
Amax,
methyl group would decrease the electron affinity of the ground state pyridinium ring, that is, stabilize the ground state, without making a corresponding change in the energy of the excited state -_ IOP 04 H 4 ring, 11. € 1 Methyl substitution does, in fact, always lead Fig. 5.-Plot of transition energies (charge-transfer band) to an increase in transition energy (;.e., a decrease for the charge-transfer band). The of 1-methylpyridinium iodide against 2: ET = 1.1205 2 in A,, 4- 5 6190 (acetonitrile neglected). The dashed line joins changes, however, are not the same for the three positions of the pyridinium ring; the data in t l t o points for pyridinium iodide. Table I show that the change in transition energy Discussion for monomethyl substitution is, AET, a t 2-, 2.0, The transformation for the charge-transfer ex- a t 3-, 0.8 and a t 4-, 3.1 kcal./niole, which represent cited by light may be expressed as in equations 1 the change from the transition energy for 1and 2 . Since the contribution of charge-trans- methylpyridinium iodide. Until recently the effects of alkyl groups have ferred form to the ground state is relatively small,6l 2 the results can be considered by an evalua- been commonly interpreted in terms of inductive tion of the effect of alkyl groups on the pyridinium effect or hyperconjugation.l5Vl6 However, armed ion (I) in the ground state and on the pyridinyl with new and accurate information on bond distances, l7 Petro,lYFischer-Hjalmars,lg Costain and radical (11) in the excited state. Stoicheff ,*O BernsteinZ1 and, especially, Dewar,z2 hv have reinterpreted the effects usually attributed to M P y + I - +hTPy. I. (1) hyperconjugation in terms of the hybridization of the orbitals composing the bonds under discussion. Mulliken,2 3 in acknowledging the validity of some of the criticism of hyperconjugation, has warned against the unconsidered use of “whole-number combinations” in forming hybrid orbitals. Clementz4has concluded from a comparison of p rnethylbenzyl chloride and p-t-butylbenzyl chloride that differential solvation could account for the Baker-Nathan effect (“hyperconjugative order”) 2 3 observed in hydrolysis rates. Stabilization of carbonium ion species by electron(15) R. S. llulliken, C. A. Rieke and W. G. Brown, THISJOURNAL, supplying groups is iar more effective than sta- 63, 45 (1941). (lfi) J. W. Baker, “Hyperconjugation,” Oxford University Press, bilization of free-radical species by the same groups. Thus, chloromethyl ether is only ca. l o 3 more reac- London, 1952. (17) G. Herzberg, Xalure. 1’75, 79 (1955). tive than methyl chloride toward sodium vapor (18) A. J. Petro, THISJ O U R N A L , 80, 4230 (1958). a t 2 i S 0 , l 3 while it is about 10” faster in the car(19) I. Fischer-Hjalmars, private communication. ( 2 0 ) C. C. Costain and B. P. Stoicheff, J. Chenr. Phqs.. SO, 7 7 7 bonium ion forming reaction of hydrolysis.l 4 Thus regardless of the precise mechanism by (1959). (21) H. J. Bernstein, J . Phys. Chem., 63, 565 (1959). which the methyl group supplied electronic charge, (22) M. J. S. Dewar and H. N. Schmeising, Tetrahedron, 6 , 166 it can be predicted that substitution of a methyl (1959). (23) R . S. hlulliken,ibid.,6, 68 (1959), has pointed o u t t h a t thelatest group into the pyridinium ring would raise the transition energy. The argument implies that the calculations of bond order in conjugated molecules (and estimates for
‘1
~~
~
I10
JOURNAL, 74,811 (1952); J. Phys. Chem., (12) R . S. Rlulliken, THIS 66, 801 (19.52). (19) J . Hitie, “Physical Organic Chrrnintry,” XlcGraw-Hill Buuk Cu., Kew Yurk, N. Y . , 19.56, p. 395. (14) C . K. lngold, “Structure and hfechanism in Organic Chemistry,” Cornell University Press, Ithaca, N. Y . , 1953, p. 334.
hyperconjugated molecules) suggest t h a t a combination of bond hybridization and conjugation can satisfactorily account for t h e bond distances. On p. 78, Mulliken states t h a t t h e fundamental premise of Baker and Kathan [the predominant role of a-hydrogen] is not necessarily correct and has thus made this lapin agile a difficult one t o catch. (24) K. A. Clement and J. N. Naghizadeh, THISJOURNAL, $1, 3154 (l95Y).
May 5 , 1960
PYRIDINIUM COMPLEXES : CHARGE-TRANSFER BANDS
2199
With the assumption that polarization of the u- (2 60.1) replacing chloroform (Z 63.2).*s3l The bond framework can be ignored and utilizing the third possibility is that steric strain in the pyribond moments estimated by Petrol8 for Cspr-Csp~, dinium iodide complex generated by interaction of Hls-CJp2 and Hl,-C,Z bonds, the gain in interaction the 2-alkyl group and the iodide ion reduces the energy upon substitution of a methyl group for a transition energy. We have pointed out elsewhere5 hydrogen on the pyridinium ring can be calculated that a more precise description of the excited state from E = qlqz/Dr. The results, expressed a s in- than previously given4 suggests that the aforecreased stabilization energy in kcal./mole, are for mentioned strain would be considerably lessened 2-, 1.2, for 3-, 0.8 and for 4-, 0.8.26To a first upon excitation, because electron transfer leads to approximation, these stabilization energies should a reduction in the size of the iodide ion (2.15 b. be reflected directly as increased transition energies ca. 1.33 b.)without, probably, a corresponding inin the corresponding pyridinium iodides. Even crease in the size of the pyridinium ring. From allowing for the nature of the approximations in- models, steric strain does not seem very important volved, the agreement between the value calcu- for the 2-methyl group but of consequence for the 2lated for electrostatic stabilization of the ground t-butyl group. Steric strain alone or a combinastate by a 3-methyl (0.8 kcal./mole) and the ob- tion of steric strain with “local solvent effect” served increase in the transition energy (0.8 could explain the appreciable reduction in transiiodide kcal./mole) is excellent and suggests that this tion energy for 1-meJhyl-2-t-butylpyridinium factor is sufficient to explain the effect of the 3- (1.1 kcal./mole, 54 A., cf. Table V) . A series of 1-methyl-4alkylpyridinium iodides methyl In contrast, it seems certain that the model used (Table 111) exhibits very little change in chargefor the 3-methyl group does not account for the transfer band position. I t seems simpler to exeffects obtained with 2- and 4-methyl groups. The plain the identity of the maxima for 4-methyl and discrepancy for the latter position is particularly 4-t-butyl with the proposal that the electrical striking. The simplest manner of rationalizing effects of saturated alkyl groups are the same and the increases is to consider the contributions of Ia, largely dependent upon the nature of the bond by I b and ICto the structure of I. Longuet-Higgins which they are attached to a given molecule.z7 and Coulsonzshave calculated the n-electron densi- Deviations from equality of effect would be exties a t the 2-, 3- and 4-positions in pyridine as plained in other terms, such as steric hindrance to ~~~~~ the methyl group 0.849, 0.947 and 0.822, respectively. More re- s o l v a t i ~ n .Alternatively, cently, Barnesz9has revised these figures to 0.943, would be required to stabilize the excited state of 0.991 and 0.950. The increased positive charge a t the pyridinium ion (pyridinium ion plus an electhe 2- and 4-positions would increase the inter- tron) more effectively than the t-butyl group to exactly the same extent as the ground state. action with the methyl substituent. The maximum absorption intensities of the Three plausible explanations may be advanced for the difference between AET for the 2- and 4- pyridinium iodide charge-transfer band can be compositions. A greater contribution of IC to the pared with one another since the band shapes are structure of the hybrid might lead to greater similar (Fig. 3) and calculation of the oscillator “effectiveness” for the 4-methyL30 Second, the strengths, a number proportional to the integrated absorption intensity, demonstrates that Emax is approximately parallel to fo, the oscillator strength. As compared with 1-methylpyridinium iodide, substitution a t the 4-position has little effect upon emsx, while 3-substitution increases Emax by a small amount. (The highest absorption intensity a t the maximum was found for 1,3,5-trimethylpyridinium exclusion of solvent by the 2-alkyl group from the iodide.) In general, substituents a t the 2-position region near the ion-pair dipole would lead to a de- reduce emax by a large factor but the 2-t-butyl group creased transition energy. The alkyl group can be is a noteworthy exception. Polymethylpyridinium regarded as a region of hydrocarbon solvent iodides conform reasonably well to the general behavior outlined here. (25) T h e parameters used are: ring C-C (C-N) distance, 1.38 A,,% The position of the charge-transfer band for 1ring C-H distance, -1.07 A . , 2 6 ring C-methyl C, 1.50 A . , 2 1 dielectric methylpyridinium iodide is about 12% more sensiconstant, D = 2.0. T h e bond dipoles were considered a s point charges tive to solvent than the band for the salt used to centered on t h e atoms of t h e bond and the interaction energies were obtained separately for hydrogen and methyl. T h e difference is t h e measure solvent 2-values, that is, l-ethyl-4“gain” referred t o in the text and can be regarded a s an electrostatic carbomethoxypyridinium iodide. According to the
-
stabilization. (26) A. Almenningen, 0. Bastiansen and L. Hansen, A d a Chem. Scond., 9, 1306 (195.5). (27) T h e further increase (0.4 kcal.) in transition energy observed for a 3-1-butyl group may be due t o slightly greater interaction of the same kind, although calculation in t h e manner described for 4-f-butyl predicts an increase smaller than the experimental error in the measurement. (28) H. C . Longuet-Higgins and C . A . Coulson, T r m s . Faraday Soc., 43, 87 (1947). (29) R. A. Barnes. T H I S J O U R N A L , 81, 1Y35 (1959) (30) Preliminary examination of a n ion with both 2- and 4-methyl groups by n.m.r. gives results not in accord with this suggestion. J. W. Patton, unpublished results.
(31) Since the standard solvent contained ethanol and the latter was probably segregated in p a r t from t h e bulk solvent (see Fig. 4), the exclusion effect might well be more serious than indicated by a comparison of Z-values. (32) Cf.,for example, B. hI. Wepster, Rec. Lrav. chim., 1 5 , 1473 (1956), in which t h e decrease in basiclty between 2-methylpyridine and 2-L-butylpyridine (no change for t h e same two groups in the 4position) is explained a s steric hindrance t o solvation in t h e 2-f-butylpyridinium ion. This point of view seems more satisfactory than t h a t of Brown88 which depends upon hyperconjugation. inductive effect and the difference between o-quinoid and p-quinoid resonance forms. (33) H. C. Brown and X. R. hlihm, THISJ O U R N A L , 11, 1723 (195.5).
2200
EDWARD A I . KOSOWER AND
JOSEPH
A. SKORCZ
Vol. 82
model previously discussed for the transition] the dipole of the ground state ion pair “flips” into the ring, removing electrostatic interaction between the pyridinium iodide ion-pair dipole and combination depends upon the nature of the acthe cybotactic region4 solvent molecules and, in ceptor.4J Rather than estimate a “corrected” fact, producing a n unstable state, with solvent position for hydrogen in pyridinium iodide (cf. molecules organized for a dipole which is no longer previous paragraph), a comparison between the there.4 The 4-carbomethoxy group causes the ion- aromatic ring with one methyi and that with six pair dipole to deviate somewhat from a perpen- methyl groups can be made. The transition dicular orientation to the pyridinium ring and re- energy differences are listed in Table 171; they duces thereby the energy requirements for the represent the change in transition energy, for “flip.” It is reasonable that the unsubstituted example, on changing the donor from toluene to salt has a greater solvent sensitivity than the hexaniethylbenzene in an aromatic hydrocarboncarbomethoxy derivative; even the size of the iodine complex. increase in sensitivity is in accord with this inTABLE VI1 terpretation (cf. ref. 4, footnote 19, p. 3259). EFFECTOF METHYLGROUPSO N TXANSITION ENERGIES nonor :acceptor I,kcal.lmole The change in the position of the maximum for 1methylpyridinium iodide when the ethanol content Iodide-1 -methylp)-1.iditiium +11.4 of the chloroform is varied (Fig. 4) suggests an Sl5.5“ interesting competition between ion-pairs and Toluene-iodine -18.3 chloroform molecules for ethanol. At the concenToluenc-iodine monochloride -13.5 tration levels used (ca. 0.91 X llf) and for Toluetie-tetracyanoeth~lene -17.9 other reasons, it is less likely that the curve found Based on effect of 4-lIlCthy~group, i.e., 5 X 3.1 on the reflects initial dissociation of quadrupoles such as assumption that the irinxitnurn effect is found only a t this found by Stern34for tetrabutylammonium bromide position (see text). in benzene plus methanol. If the figure of 15.3 kcal.imole is considered, it can It was anticipated that replacement of the 1- be seen that “maximurn electron donation” by methyl group by hydrogen would lead to a further methyl groups is roughly of the same magnitude displacement of the charge-transfer maximum in in acceptors as in donors. pyridinium iodide to longer wave lengths. 35 The Conclusions.-hIeasureiiient of the position of a maximum for pyridicium iocide in chloroform charge-transfer band for a substituted pyridinium proved to be a t 3464 A., 274 9. shorter than for 1- iodide offers a new and potentially valuable method methylpyridinium iodide. The shift observed in of evaluating substituent effects. While electrical the maximum onochange to acetonitrile as solvent effects alone appear to determine the results for 3is so great (314 -4.) as to leave little doubt that a and 4-substituents, a steric factor modifies the charge transfer transition of a type similar to that results for a 2-substituent. Scattered data for 4found for alkylpyridinium iodides was not in- substituted pyridinium iodides, reported in another volved (Table VI). Further study of the charge- paper,’ demonstrate that the pyridinium iodide transfer spectra of pyridinium iodides is required. charge-transfer transition is extremely sensitive to Extensive studies have been made o€ the spectra the nature of the substituent, with a p of 13.4 reof complexes of methylated benzenes and iodine,56 sulting from a plot of transition energies against iodine m o n ~ c h l o r i d e and ~ ~ tetra~yanoethylene.~~ the Hammett a-constant. I t is of some interest to compare the effectiveness of Experimental methyl groups in promoting electron transfer Materials. 1. A1kylpyridines.-Most of t h e alkyl(equation 3) from aromatic rings to electron ac- pyridines used were commercially available. All were ceptors with their role in Preoeizting electron transfer carefully redistilled and their refractive indices checked to the pyridinizim ring in the alkylpyridiniuln against the literature values. T h e 4-t-butylpyridine was ~~ 4 s in the case of the pyridinium iodides, prepared by the method of Brown and M ~ r p h e y .Samples of 2-t-butylpyridine and 3-t-butylpyridine were kindly prothe arrangement of methyl groups around the ring vided b y Professor Brown. Pentamethylpyridine was affects the position of the charge-transfer band, and synthesized according to the procedure of Karrer and the change in transition energy for a particular iVainoni.42 The phrsical oroperties of the pyridines are iL
(34) E . A. Richardson and K. H. Stern, American Chemical Society Abstracts, 135, 50 R (1959), Boston, Mass. (3.5) Hantzsch3 reported on both 1-methylpyridiniurn iodide and pyridinium iodide many years ago. However, there are considerable difficulties attached t o obtaining good spectra from pyridinium iodides. T h e results with 1-nlethylpyridinium iodide indicate the possible presence of some triiodide; the result for pyrldinium iodide, though, does in f a c t show t h e same direction of shift for the maximum as found in the present work. I n addition, the method used by Hantzsch (variation in cell length, photography) was not suitable for accurate spectra (6. XV. I
