J. GRUNDNES, M. TAMRES, AND S.N. BHAT
3682
Vapor-Phase Charge-Transfer Complexes. VI.
Diethyl Ether-Iodine
by Just Grundnes, Milton Tamres,* and S. N. Bhat Chemistry Department, University of Michigan, Ann Arbor, Michigan 48104 (Received J u n e 23?1971) Publication costs assisted by the National Science Foundation
The spectral and thermodynamic properties of the diethyl ether-iodine complex in the vapor phase have been reinvestigated. Analysis of the charge-transfer spectral data shows there is considerable uncertainty in separating the equilibrium constant ( K O )and the extinction coefficient (emax) from the Kcemaxproduct. The change in internal energy on complexation (AE), obtained from the temperature dependence of Kcem,,, is -4.5 & 0.2 kcal mol-I. This value is larger than previously reported. I t is shown also that a blue-shifted iodine band is present, although the characterization of this band is not established.
Introduction Several spectrophotometric studies have been made of the diethyl ether-iodine complex in solution.‘-5 However, only one such study of this complex has been made in the vapor phase,6 with rather different spectral and thermodynamic results, It is reported that the charge-transfer (CT) band of diethyl ether-iodine is blue-shifted and the equilibrium constant is increased in going from solution to the vapor phase.6 Such changes seem to be typical for complexes that are weak to only moderately strong’ but not for the strongest complexes.8 There are, however, other aspects of the reported vapor-phase results that warrant further consideration. First, the shape of the vapor CT band is far more skewed, with a steep slope on the high-energy side, compared to the same band in solution or compared to the vapor CT band for diethyl s u l f i d e - i ~ d i n e . ~Second, ~~~ the conditions for separating the equilibrium constant ( K c ) and the extinction coefficient (E) from the K c € product are a t about the limit of the criteria that have been recommended.11p12 The results are rendered more uncertain by the fact that the total absorbances are rather low (I. Tamres and S.K . Bhat, ibid., 75, 1057 (1971). (15) C. N. R . Rao, G. C. Chaturvedi, and S. N. Bhat, J . Mol. Spectrosc., 33, 554 (1970). (16) M. Tamres and J . Grundnes, J . A m e r . Chem. Soc., 93, 801 (1971). (17)
M.Kroll, ibid., 90,
1097 (1968).
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VAPOR-PHASE CHARGE-TRANSFER COMPLEXES Table I : Concentration and Spectral Data for Diethyl Ether-Iodine in the Vapor Phase in a 100.0-Cm Cell at the Wavelength 234 mp 106 [I21
Run
M
4.49 4.58 4.42 4.85 4.24 3.11 2.78c 2.7lC
102[ether],
u
69.8'
67.0'
1.277 1.389 I.476 2.004 2.836 3.821 5.523 5.625
0.351 0.378 0.396 0.599 0.736 0.698 0.906 0,904
0.299 0.326 0,335 0.510 0.626 0.612 0.780 0.773
Corrected absorbancea------80.00
0.232 0.256 0.265 0.404 0.491 0.490 0,610 0.611
f12(X 480 mp),b
89.80
97.8'
0.195 0.216 0.220 0.339 0.414
0.175 0.188 0.193 0.298 0.364
...
...
0.514 0,513
0.451 0.439
M-1
cm-1
350 348 350 348 348 352 364 358
b Calculated from the absorbance of the mixture a t 100' in the visible region and the concentration of a Error limit -0.005 unit. iodine determined by weight. Iodine vapor alone or in the presence of hydrocarbons shows little temperature dependence in the region of 480 mp,l6 and €1, at 480 mp has been determined as 350 =t2 M - l cm-1. c If the concentrations were to be calculated from the absorbances, assuming no effect of the ether on the iodine absorbance, then the concentration in runs 7 and 8 would be increased to 2.89 X and 2.7 X 10-6 M , respectively, based upon 01, equal to 350 M-1 cm-1. This corresponds to a change of 4.1 and 2.4%, respectively.
to a concentration from 2.71 X to 4.85 X M . An approximate check of the concentration was made at the end of a run by taking the system to 100" and recording the absorbance at 480 mp,la as will be mentioned in the Discussion. This check by the two determinations, weight and spectral, indicated an error in iodine concentration of the order of 1%, although it may have been as high as 2% in a few runs. The much larger weights of ether, 1.628-7.212 g, make the errors in ether concentration negligible. The concentration range was 1.28 X l o + to 5.62 X M ; the highest concentration corresponds to a pressure of 1.71 atm at 97.8'. Because of the donor volatility, the ether break-seal tube was opened first. Then the iodine was introduced into the main body of the cell either by gently heating the opened iodine break-seal tube or by condensing the ether in with the iodine and washing it out into the cell. The cells used had a path length of 100.0 ern as determined with a Wild cathetometer. Their volumes ranged from 1720 to 1740 ml; the small variation was due to differing lengths and bulb sizes of the breakseal tubes. In a few experiments, a slow irreversible reaction occurred as evidenced by an increase in absorbance at 260 mp and a decrease at 480 mp, as had been observed by Lang and Strongs above 90". For the data reported in this paper, the diethyl ether-iodine system was stable for the duration of the run (-8-10 hr). Temperature dependence was studied going down from 97 to 60" and back up again. Reproducibility was excellent, of the order of 0.005 absorbancy unit. The source and purification of iodine has been de~ c r i b e d . ~Mallinckrodt " diethyl ether from a freshly opened container was used without purification. In several runs, freshly distilled ether was used with no noticeable difference in results.
All calculations were programmed for the IBM 360-67 computer.
Results The experimental data for eight ether-iodine mixtures are presented in Table I. Special care was taken to correct for the small contribution of ether and of iodine to the total absorbance by studying their spectra separately from different samples over the temperature range of interest. Figure 1 shows the spectrum of the diethyl etheriodine CT band in the vapor phase, with a maximum a t 234 mp, as found by Lang and Strong6 The absorbance scale here is about 10 times that of Lang and Strong, which corresponds to the factor difference in cell paths in the two studies. The temperature dependence of the absorbance is shown in Figure 2. The study of the blue-shifted iodine band is shown in Figure 3. There is a small but distinct enhancement in the iodine absorbartce at wavelengths shorter than 480 mp. Further, this enhancement shows a small decrease with an increase in temperature, which is opposite to the effect due to normal temperature broadening but is in accordwith the effect due to dissociation of a complex. A similar but much more pronounced effect has been reported recently for the stronger diethyl sulfide-iodine complex.14
Discussion The CT band of diethyl ethcr-iodine, shown in Figure 1, is more symmetrical than that reported by Lang and Strong6 The reason for this difference apparently is in the reported spectrum of free iodine in the ultraviolet region. The higher absorbance in the figure of Lang and Strong results in a larger free iodine correction on the short-wavelength side of the total absorbance curve of the mixture. The half-width a t The Journal of Physical Chemistry, Vol. 76, N o . 94, 1971
3684
J. GRUNDNES, M. TAMRES, AND S. N. BHAT
l\
0.8
A (mp)
Figure 1. Diethyl ether-iodine spectrum in the vapor phase a t 59.8" in a 100.0-cm cell: (4) iodine, 4.24 X M; (3) diethyl ether, 2.830 X lov2M; (1) mixture of 3 4; (2) complex.
+
0.8
0.8 W
8
8 04
Figure 3. Diethyl ether-iodine absorption in the vapor phase in the visible region; 75.0-cm cell; iodine, 3.97 X M; diethyl ether, 4.914 X 10-2 M; (1)iodine alone at 45", (2) iodine ether a t 80°, (3) iodine ether a t 60".
2
+
0.2
I
I
'250
280
I
310
1hfi Figure 2. Temperature dependence of the diethyl ether-iodine absorption in the vapor phase; iodine, 4.24 X 10-6 M ; diethyl ether, 2.836 X 10-2 M; cell, 100.0 cm: (1)59.8', (2) 67.0", (3) 80.0°,(4) 97.8".
half-maximum of the CT band, taken as pmax - vl/, (long-wavelength side), has an average value of -3150 cm-l over the temperature range 59.8-97.8". This is perhaps only a little larger than that for the iodine complex with mesitylene'* or with diethyl sulfide.9 17,19 The data in Table I were analyzed in the usual fashion to determine the constants K Oand e using the quadratic equation t
where DOand AOare the initial concentrations of donor and acceptor, respectively, OD' is the corrected optical density,12 and e' is the corrected extinction coThe Journal of Physical Chem$stry,Vol. 76, No. 94, 1971
+
Actually, two sets of efficient (= ECT - E D values are given; those in parentheses correspond to slightly different iodine concentrations for runs 7 and 8 as will be mentioned. It has been discussed1lalZthat meaningful separation of K , and E depends upon the relative magnitudes of the terms l/Kc and (Do Ao). (The last term within the brackets in eq 1 corresponds to the concentration of the complex, which is small in this case and need not be considered.) Because of the limitations in a concentration of 10-1 pressure within the cell (q., M at 100" corresponds to a pressure of -3 atm) the concentration in the present study did not exceed ~ 5 . 6X 10-2 M.20 If K , is of the order of ~ 1 - 2 M-l, then the ratio of 1 / K , to (DO Ao) is just about at the condition where separation of the Koe product into the individual values of K O and e is difficult to achieve within the desired confidence level. The
+
+
(18) W. K.Duerksen and M. Tamres, J. Amer. Chew. SOC.,90,1379 (1968). (19) The value in ref 17 is for the full width at half-maximum, i.e., ih/r(short-wavelength side) - Fl,a(long-wavelength side). (20) I n one study a t a concentration of -7 X lo-, M in Do,which corresponds t o ~2 atm pressure at 80°,a quartz window in the particular cell used was blown out.
VAPOR-PHASE CHARQE-TRANSFER COMPLEXES
3685
Table 11: Analysis of K Oand emax for the Diethyl Ether-Iodine Complex"9b Temp,
Kormax,
O C
M-2 om-1
M-1 cm-1
KO, M -1
6140 f 180 (6230 rt 209) 5230 i:96 (5300 rt 156) 4100 k 107 (4160 f 169) 3440 rt 89 (3490 rt 126) 3050 rt 110 (3100 rt 127)
8,720 k 10,460 (4,160 f 2,690) 11,150 f 12,650 (4,230 rt 2,880) 11,32Ort223,530 (3,630 k 3,730) 9,470 rt 20,070 (2,890 rt 2,570) 3,770 f 4,980 (1,850 k 1,340)
0.705 rt 0.864 (1.50 rt 1.02) 0.469 f 0.539 (1.25 f:0.884) 0.363 k 0.762 ( 1 . 1 5 1 1.22) 0.363 f 0.777 (1.21 rt 1.11) 0.810k1.09 (1.68rt 1.27)
59.8 67.0 80.0
89.8 97.8
emax,
0.639 (0.838) 0.660 (0.827) 0.434 (0.697) 0.478 (0.792) 0.657 (0.846)
0.741 (1.94) 0.630 (1.65) 0.495 (1.30) 0.415 (1.08) 0.368 (0.964)
Upper values based on data in Table I ; lower values (in parentheses) obtained by increasing the iodine concentration in runs 7 and 8 by 4.1 and 2.4%, respectively. b Error limits for 95% confidence level. 0 Correlation constant. KO,, = Koemax/eav. 5
K,E product itself is quite good, however. These aspects are reflected in the error limits associated with the values reported in Table 11. The relative contribution of (Do Ao)t o 1/K, can be readily noted from the data in Table I by making a plot of the linear form of eq 1. The change in the y coordinate (DoAo/OD)is not more than 5% over the entire range of the x coordinate (Do AO). Contributing factors to this change are the errors in absorbance (estimated to be 0.005 unit) and in the iodine concentration (estimated to be 1% and perhaps 2% in some cases). The error limits for the latter are based on the following consideration. A check on the iodine concentration was made after each analysis of the CT region by taking the visible spectrum a t 100". The value a t 480 mp of E I ~350 3t 2 M-l cm-' had been established previously. Analysis of a slow-scan spectrum of the mixture in Figure 3 showed that, for the concentrations used, the absorbance of iodine at 480 mp in the presence of diethyl ether is enhanced
