VAPORPHASE CHARGE-TRANSFER COMPLEXES Acknowledgments. The authors wish to thank Dr. F. S. Stone for valuable discussions and for critically reading the manuscript and Dr. L. Piselli for the help
1057 given in performing the reduction experiments. The financial support of SNAM-PROGETTI is also gratefully acknowledged.
Vapor Phase Charge-Transfer Complexes. V. The Blue-Shifted Iodine Band by Milton Tamres* and S. N. Bhat Chemhtry Department, University of Michigan, Ann Arbor, Michigan 4.8104
(Received August 10, 1970)
Publication costs assisted by the National Science Foundation
A search for the blue-shifted iodine band in the vapor phase was made on the system diethyl sulfide-iodine using more favorable conditions to observe complexation than those previously attempted. There is found a pronounced enhancement in absorbance of the iodine visible band toward the blue region, and the absorbance is markedly temperature dependent. The first characterination of a blue-shifted iodine band in the vapor phase is given.
Introduction Charge-transfer (CT) complexes of iodine in solution show a characteristic blue shift of the iodine visible band. This shifted band often has been used to determine the thermodynamic properties of the complexes, e.g., with ethers’ and with sulfides.2 I n Rhlliken’s view,a because the antibonding orbital in iodine has a larger effective size than the ground-state orbital, the blue shift is attributed to the larger exchange repulsion between the iodine molecule in the excited state and the adjacent donor. On this basis, one would expect to see the blue shift in the vapor phase as well as in solution. I n vapor studies to date, evidence for the presence of the blue-shifted iodine band has been mostly negative or not conclusive. For mixtures of ether-iodine, an effect has been reported on the long wavelength portion of the iodine band where there is band structure4 (ie., above 4 0 0 mp), but not in the continuum part of the s p e c t r ~ m . ~For ! ~ the stronger sulfide-iodine complexes, there is indication of a small effect but the existence of a blue-shifted band has not been definitely e ~ t a b l i s h e d . ~ -A~ slight, rather uniform enhancement in absorbance at wavelengths shorter than 480 mp has been noted for thiacyclopentane-iodine.8 I n a brief, more recent report, some enhancement in absorbance has been stated for iodine in the presence of several donors (diethyl sulfide, diethyl ether, triethylamine, and ammonia), with a comment that the shifted band must be quite broad.9 Several explanations have been offered for the lack of confirmation and identification of the blue-shifted band in the vapor phase: (I) the extinction coefficient of the
complexed band ( e ~ in~ solution ~ ~ ~compared ~ ) to vapor may be greatly enhanceds by a cage effect of solvent molecule^^^^ which would decrease the intermolecular distance between donor and acceptor, lo,ll and hence increase orbital overlap; (2) freer rotation in the vapor phase allows for less favorable donor-acceptor ~ ~ ~ ~ diminishing , orientation with smaller B I ~ - thereby the average value of E I ~ - (3) ~ the ~ ~geometry ~ ; ~ of ~ the complexes may be different in the two p h a s e ~ ; and ~,~~ (4) cofnplexes with electronically excited iodine should not be observed in the vapor. l4 This problem has been reexamined for the system diethyl sulfide-iodine by going to more favorable conditions to observe complexation. I n this paper, positive evidence is presented for the existence of the blue(1) M . Brandon, M. Tamres, and 82, 2129 (1960).
s. Searles, J . Amer.
Chem. SOC.,
(2) M . Tamres and S. Searles, J . Phys. Chem., 66, 1099 (1962). (3) R. S. Mulliken, Recl. Trav. Chim. Pays-Bas, 75, 845 (1956). (4) C. A . Goy and H. 0. Pritchard, J . Mol. Spectrosc., 12, 38 (1964). (5) F. T. Lang and R. L. Strong, J. Amer. Chem. SOC.,87, 2345 (1965). (6) E. I. Ginns and R. L.Strong, J . Phys. Chem., 71, 3059 (1967). (7) M. Tamres and J. M. Goodenow, {bid., 71, 1982 (1967). (8) M . Kroll, J . Amer. Chem. Soc., 90, 1097 (1968). (9) C. N. R . Rao, G. C. Chaturvedi, and 9. N . Bhat, J. Mol. Spectrosc., 33, 554 (1970). (10) J . Prochorow and A . Tramer, J . Chem. Phys., 44, 4545 (1966). (11) P. J. Trotter, J. Amer. Chem. Soc., 88, 5721 (1966). (12) 0 . K. Rice, Int. J. Quantum Chem., Symp., 2, 219 (1968). (13) R. 8 . Mulliken and W. E. Person, “Molecular Complexes,” Wilev-Interscience, New York, N. Y., 1969, p 161. (14) E. M. Voigt, J . Phys. Chern., 72, 3300 (1968).
The Journal of Physical Chemistry, Vol. 76,No. 8,1071
MILTONTAMRES AND 5. N. BHAT
1058
shifted band and the first characterization of such a band is given.
Experimental Section Procedure. The experimental procedure for filling the gas cell with weighed amounts of reagent sealed in break-seal tubes has been described p r e v i o ~ s l y . ~ ~ ~ ~ The concentration of iodine was determined from its absorption in the visible region (at 480 mp, E = 350 f 2 1. mol-l cm-l). This determination agreed with that calculated from the known volume of the cell and the weighed sample of iodine. The weight of iodine used was 12.3 0.1 mg. The volume of the cell was -900 ml, varying only slightly from one run to another because of the small differences in the sizes of the breakseal tubes. The cell path was 50.0 cm, determined with a Wild cathetometer. Materials. The source and purification of iodine and of n-heptane has been described previously.' The diethyl sulfide (Eastman Organic) was distilled and the middle cut was taken. Vapor phase chromatography showed the sample to be better than 99.9% pure.
Results and Discussion I n Figure 1 there are shown the spectra of iodine vapor alone (curve 1: [I2]= 5.33 X M at 85") and of the same iodine in the presence of n-heptane ([n-C7H16] = 3.93 X M ; curve 2 at 120", and curve 3 at 135"). The variation of absorbance at the higher temperatures is attributable to a temperature broadening and is not due to an effect of the n-heptane. Broadening of the iodine band over a wide temperature range has been reported by Sulzer and Wieland.lB It was studied in this work in order to determine the extinction coefficient of free iodine, tIz, over a limited temperature range. The region near 480 mp is fairly insensitive to temperature variation over this limited range15 or to the presence of foreign gases at this and shorter wavelength~.~Jj,'~ Therefore, this region is useful in determining iodine concentration^.^^'^ Spectra of iodine vapor alone and in the presence of varying concentrations of diethyl sulfide (6.55 X M , and 3.88 X Al) are shown in M , 1.91 X Figure 2. The conditions are much more favorable for observation of complex formation than had been tried previously. The iodine concentration is -3 times as large and the sulfide concentration -23 times as large as used by Kroll,8although his path cell was about twice as long. The concentrations are closer to those of Tamres and C o ~ d e n o wbut , ~ the path cell in the present study was five times as long. The enhancement in absorbance (Figure 2) on the low wavelength side of the iodine band in the presence of diethyl sulfide is quite pronounced and increases with increasing sulfide concentration. Considering that n-heptane at these pressures does not affect the iodine band in this region, it is apparent that the enThe Journal of Physical Chemietry, Vol. 76,No. 8,1971
3
Figure 1. Absorption spectra of iodine (5.33 X M) and n-heptane (3.93 X 10-2 M ) mixtures: (1)iodine alone a t 85'; (2) iodine n-heptane at 120"; (3) iodine n-heptane a t 135'; 50.0-cm cell.
+
+
hancement is due to complexed iodine. No maximum is observed in the absorbance curves for the mixtures, but inspection of the difference in absorbance of the iodine band in the presence and absence of diethyl sulfide reveals that the difference passes through a maximum. Taking the difference between curve 4 (run at 115") and curve 1 (after correction for temperature broadening from 85" to 115") results in the dashed curve with a maximum at -445450 mp. Further evidence that the blue-shifted enhancement is due to complexation comes from the temperature dependence study shown in Figure 3. The decrease in absorbance with increase in temperature is opposite in trend to that of temperature broadening, but is in accord with having an equilibrium between complexed and free iodine. I n solution, increasing the concentration of diethyl sulfide for a fixed concentration of iodine gives a series of curves that go through an isosbestic point at 493 mp.17 An increase in absorbance of the complexed (15) M. Tamres and J. Grundnes, J. Amer. Chem, Soc., 93, 801 (1971). (16) P. Sulzer and K. Wieland, Helc. Phys. Acta, 25, 653 (1952). (17) J. M. Goodenow, Ph.D. Thesis, University of Michigan, Dee 1965.
VAPORPHASE CHARGE-TRANSFER COMPLEXES
o r 1 5 ; 0 1
'
480
440 1
?,
(my'
1
" I
400
1059 band is accompanied by a decrease in the free iodine band. This is not observed in the present vapor phase data, as is seen in Figure 2, and therefore does not lead to an identifiable isosbestic point. There is very little change in absorbance near the band maximum of iodine when sulfide is added, and at the highest sulfide concentration the absorbance of the mixture appears to be very slightly greater than that of the original iodine vapor at all wavelengths including the band maximum, in spite of the fact that part of the iodine now is complexed. Resolution of the total absorbance into the component bands of free and complexed iodine depends on a knowledge of the equilibrium constant for complex formation, K , and of 61%. The latter is readily calculated from the data such as those in Figure 1. Values at three temperatures are listed in Table I. They are slightly lower around the maximum compared to the data of Sulzer and Wieland. l6
360
Figure 2. Donor concentration dependence of absorbance for diethyl sulfide-iodine (5.33 X M) a t 115": (1) iodine alone (at 85'); (2) iodine sulfide (6.55 X IO+ M); (3) iodine sulfide (1.91 X l o F 2M ) ; (4) iodine sulfide (3.88 X 10-2 M ) ; 50.0-cm cell. Dashed curve is the difference between curve 4 and curve 1 (corrected for temperature broadening to 115').
+
+
+
Table I : Extinction Coefficient'tb of Iodine in the Vapor Phase in the Visible Region at Several Temperatures
A,
mB
400 405 410 415 420 425 430 435 440 445 450 455 460 465 470 475 480 485 490 495 500
f800,
fW0,
flZO~,
1. mol-1 om-]
1. mol-] om-'
1. mol-1 om-'
(1) 4 7 13 22 32 48 66 90 118 151 192 238 290 349 406 470 531 570
1 2 2 6 9 16 24 36 52 71 95 125 158 199 247 298 351 405 464 522 563
1 3 3
7 12 20 28 42 57 76 101 131 164 204 253 304 352 404 461 516 555
a B is the average value of several determinations for iodine in the concentration range 5.08-8.85 X 10+ mol 1.-'. * Error limit is &2 1. mol-' cm-l.
0
520
480
440
400
360
1 (my) Figure 3. Temperature dependence of absorbance for diethyl sulfide (3.88 X 10-2 M)-iodine (5.33 X M ) : (5) 115'; (4) 120"; (3) 127"; (2) 135"; (1) iodine alone a t 85", 50.0-cm cell.
It is more difficult to establish a reliable value for K . Generally in the study of CT complexes, it is the K E product that is well determined, but error in the separation of the terms can be a p p r e ~ i a b 1 e . l ~For ~ ~ ~the (18) W. B. Person, J. Amer. Chem. SOC.,87, 167 (1965). (19) R. A. LaBudde and M. Tamres, J . Phys. Chem., 74, 4009 (1970).
The Journal of Physical Chemistry, Vol. 76, No. 8,1971
1060
MILTONTAMRES AND S. N. BHAT
7
1200
'4W. I
1 (my) Figure 4. Resolved blue-shifted iodine band based on data of Kroll.8
two vapor phase studies on the diethyl sulfide-iodine complex that have been reported by Krol18 and by Tamres and go ode no^,^ there is agreement within the limits of error in the values for the KE product, and these results lead to agreement also in the determination of the change in internal energy on complexation, AEO (-7.6 and -7.5 kcal mol-', respectively). However, the values reported for the separate K and E terms differ by a factor of -3.5 (Kroll,8 K37a = 58 1. mol-'; and Tamres and Goodenow,' Ka,a = 16.5 1. mol-'). This difference in K has a pronounced effect in characterizing the blue-shifted iodine band. From the data of Kroll,s resolution of the total absorbance curves in Figure 3 produces the curves for the blueshifted iodine shown in Figure 4. It is assumed in resolving the bands that €Ia is independent of the presence of added gases at the pressures used because no effect was observed in the presence of n-heptane. The curves in Figure 4 show no apparent band maximum, and extrapolation mould give a maximum at a longer wavelength than for free iodine. This is not a normal characteristic of a blue-shifted band. Thus, if Kroll's data were valid, a modification in theory would be required. Similar analysis at two temperatures based on the data of Tamres and Goodenom7 produces the B set of curves in Figure 5. The difference from Figure 4 is quite marked and shows how dependent the shapes of the curves are on the value of K . Probably the best thermodynamic data presently available20 are Kam N 11.5 1. mol-' and BE" N -8.3 kcal mol-'. These data support the earlier study of Tamres and GoodeThe Journal
of
Physical Chemiatry, Vol. 76,N o . 8,1071
425 I
450 I
'x (my)
475 I
5'
Figure 5 . Resolved blue-shifted iodine band: set A, based on data of Tamres and Bhat;20 set B, based on data of Tamres and Goodenow.?
cuuu
;/Ti, .^^^I
:
I
I,'
E 1200
I
\ \ \'
3
7 h j . 2
Figure 6. Extinction coefficients in the visible region for the system diethyl sulfide-iodine: vapor phase (solid line), (1) free iodine, (2) complexed iodine; n-heptane solution (dashed line), (3) free iodine, ( 4 ) complexed iodine.
OW^ which employed a quite different experimental procedure. Their use gives the set of curves marked A in Figure 5 . This set will be discussed in characterizing the blue-shifted iodine band. The general conclusions would apply also to the B set, however, because the features of the two sets of curves are similar. The blue-shifted iodine in Figure 5 is more in line with theoretical expectation. There is a maximum at -450455 mp, which is close to that of the difference curve in Figure 2. Only the region between 400 and 500 (20) This laboratory, study in progress on several sulfide-iodine complexes; t o be published.
VAPORPHASE CHARGE-TRANSFER COMPLEXES mp was taken. At wavelengths longer than 500 mp band structure appears. At wavelengths shorter than 400 mp the absorbance rises again as a result of the contribution from the intense GT band with its maximum at 290 rnp.'l* The tail of the CT band probably contributes a little to the absorbance near 400 mp, but not at 450 mp. The variation of e ~ with ~ temperature is too small to be considered outside the range of experimental error. The curves in Figure 5 have some interesting features. The band maximum is at a longer wavelength than that observed for the complex in n-heptane solu437 mp),2 This would seem reasonable. tion (A, A solvent cage which reduces the intermolecular distance between donor and should give an additional blue shift as a result of the greater interaction between the repulsive excited state of iodine and the ground state of the adjacent donor. I n addition, a t their respective vapor phase maxima, eIa--oomp > B I ~ ,which also is observed in solution. I n fact the , ~ much / E I different ~ in the two phases, ratio ~ I ~ - ~ ~is~not i.e., -1.9 in the vapor phase at 115' and -2.1 in nheptane solution at 25". This may be seen in Figure 6 , which gives the extinction coefficients as a function
1061 of wavelength for free and complexed iodine in both phases. Figure 6 also makes clear why the isosbestic point is prominent in solution but is not observed in the vapor phase. Although the solvent enhances the intensity of both the free and complexed iodine bands, it affects -their ~absorbance ~ ~maxima ~ in different ways. The band maximum of free iodine in the vapor phase is redshifted in solution, whereas the band maximum of complexed iodine in the vapor phase is blue-shifted in solution. The greater overlap of the bands in the vapor phase, coupled with the increase in intensity of the complexed iodine relative to the free iodine, results in the extinction coefficient of the complexed iodine being larger than that of free iodine over the entire continuum range of the spectrum for free iodine, including the band maximum (A, 500 mp). I n solution, the free and complexed bands are separated to a sufficient extent to result in a crossing of the bands. Acknowledgment. This research was supported by the National Science Foundation in part through Grant GP-9216, and in part through Grant GP-10367 to this department for the purchase of the Gary-14 spectrophotometer used in this study.
The Journal of Physical Chemistry, Vol. 76,No. 8,1071
