M.
3468 transition would be of the “normal” helix-to-coil variety, and ‘hence increasing inert diluent concentration would increase Tc.} Whether such a maximum can actually be observed within the accessible temperature and solute solubility range, however, would depend on the thermodynamic parameters for the particular system under investigation. I n terms of the overall transition parameters, AHVH and u, also, the present system appears to be quite similar to the PBG-DCA system. The stoichiometry of the peptide-active solvent binding reaction which is,
DE
SORGO, B. WASSERMAN, AND M. SZWARC
of course, a key factor in determining the transition temperature and indeed the direction of the thermal transition, unfortunately, cannot yet be assessed from the available data. It would be reasonable to assume that the different potential binding sites in the gemdiol and the strong carboxylic acid would be reflected here. Acknowledgment. The work reported above was supported by the National Science Foundation, Grant GB 8080 (F. E. K.).
Aggregation of Salts of Thianthrene Radical Cations by Made Sorgo, B. Wasserman, and M. Szwarc” S U N Y Polymer Research Center, College of Forestry at Syracuse University, Syracuse, New York 13.210 (Received May 4, 1972) Publication costs assisted by the National Science Foundation
A partial association of the paramagnetic perchlorate salt of thianthrene radical cation (T. +, Clod-) into a diamagnetic dimer takes place in propionitrile and in trifluoroacetic anhydride containing 10% trifluoroacetic M solution in trifluoroacetic acid. Quantitative association occurs trifluoroacetic anacid, but not in hydride. The optical spectra of the monomer (T.+ or T.+, C 1 0 ~ and ) of the dimer (T. +, ClOa-)2 are reported. In propionitrile the equilibrium of association seems to be given by the equation, 2T.+ 2c104Le,, ion pairs T -+, C104- are dissociated in propionitrile but the dimer is bot. The equilibrium (T. constants and the relevant AH and AX are reported. The equilibrium of association in (CF&O),O 10% CF&OOH probably is described by the equation 2(T*+,c104-) (T. +,C104-)2.
+
+
+
*)
Investigations of the spectra and chemical behavior of thianthrene radical cation, T-+,led to some conflicting
T.+
r e ~ u l t s l - (see ~ e.g., ref 2a). The system seems to be complex and indeed Lucken’ reported that the spectra of solutions of 1[‘~+ vary with dilution, temperature, and the nature of the Bolvent. His attempts to follow quant)itatively these changes were unsuccessful. I n the course of our studies of various electron-transfer processes WF‘ remvestigated the optical and esr spectra of thianthrene perchlorate, T * +, ClOk-, in propionitrile and less extensively in trifluoroacetic acid and trifluoroacetic anhydride. The results clarify, a t least to some extent, the pumling features of this system and might be helpful in future investigations. Lucken suggested that thianthrene radical cations may dimerize, and the spectrum of the dimer may subThe Journal
0.f
€’hg&aE Chemistry, ‘Vol. 76, No. 28, 1972
stantially differ from that of the monomer. Our observations confirm this idea. At room temperature the dilute propionitrile solutions of T -+,C104- are paramagnetic, their esr spectra give the previously reported quintuplet, and the visible spectrum, shown in Figure 1, has a single peak a t Am,, 543 nm (e 1.2 X 10”. However, a t higher concentrations and a t lower temperatures the spectrum broadens and eventually the spectrum shown in Figure 2 is produced. This spec470 ( E 0.35 X trum is characterized by two bands A,, lo4) and 594 nm ( E 0.56 >( lo4) and the extinction coefficient at 543 nm (Am,, in dilute solution) is 0.31 X lo4. Furthermore, the concentrated solution is diamagnetic a t low temperature. The ratios of the intensities of the two bands, corrected for the absorption (1) E. A. C. Lucken, J . Chem. Soc., 4963 (1962).
(2) (a) H. J. Shine and L. Piette, J . Amer. Chem. &e., 84, 4798 (1962); (b) Y. Murata and H. J. Shine, J . Qrg. Chem., 34, 3368 (1969). (3) Y. Sato, M. Kinoshita, M. Sano, and €3. Akamatu, BulE. C h a . SOC.Jap., 40, 2539 (1967) ; 42,548 (1969).
re~pectively.~These results indicate that we deal here with only two distinct species in ~ q with each~ other, and thus we reconfirm the c o ~ cthat ~ the ~ two ~ ~ bands a t 470 and 494 nm, respectively, species. We attribute the spectrum shown in Figure II to tho paramagnetic, monomeric thianthrene radical cation, T +,whereas the spectrum shown in Figure 2 seems to be ~ tco ~ due to a diamagnetic aggregate. The ~ cientcj of the ~ ~ a r a m a ~ nspecies e t ~ c a1 410 arid 594 n m 0.25 X IO*and 0.15 X lo4,r e s p e ~ ~ ~Rernce, ~ r ~ a~i l ~ the" to ~ e ~ ~ ~ the m i~n e o ~ oi' the ~ soh~ ctrophoto~etr~cmethods w e le. The composition was c a ~ c u ~ afrim ~ e ~.&heratios of optical densities a t 543 and 594 nan. The results me summarhed in Table I and were s ~ ~ ~ pby ~eme analysis, calculat~ngthe ~ o n ~ ~of ~n ~a , ~ r ~~ species from the ~ v e ~ ~ o d ~ esr i l aeignd. te~~ Plots of log IrP*+-]us. log species] at various constant t ~ m p e r a t u r eare ~ shown in Figure 3. A serie8 of parallel straight lines with. slope 4 fit reasonably we11 the experimental points obtained at She lowest temprzlture, ~ ~ substan=k,iai ~ ~ o ~ dwiatio -20 and 0". This surprising result co preted as evidence for t e ~ r a of ~T -I-~ salts r in ~ p r ~ p ~ ~uix. ~ i ~ ~ ~ ~ ~ e ,
~ ~ ) ~ ~
a
~
~
~
~
s
~ e ~ ~ ~~ ~
~~~~~~~~~~~~~
Figure 1. Visible speebrum of thianthrinium perchlorate (monomeric rzldi::al ctiition) in propionitrile a t 21' ([T. +,ClOd-] = 1.5 x 1 0 - 4 ~ ) .
(T-+, ClO,->*
C104paramagnetic 4P.e
+j
diamagnetic
However, an alternative i n t e r ~ r e ~ ~ tis~ opossible. n Dielectric constant of propionitrile i s relatively high, -28 at 20" and still higher at lower ~ e ~ p e r a t ~ a r eThe s.
-2.0
- 3.0
.5
X nm 456
500
550
600
650
.4.0
Figure 2. Visible spectrum of thianthrinium perchlorate aggregate (the diamagnetic species) in propionitrile a t -80' (total salt concentration - 4 . 9 X IO+ M). -5.01
due to the paramagnetic species, are independent of concentration, Hence, it is plausible to assume that these two bands arise from one species only. The changes in the optical and esr spectra are reversible, Cooling and warming the solution or diluting it and then Concentrating restores the properties of the original solution. Moreover, the normalized optical spectra obtained at different concentrations show two isosbestra points at about 490 and 570 nm,
-5.0
-4.0
-3.0
Figure 3. Plot of log [T. +] us. log [diamagnetic species]. At 21" the data were obtained spectrophotometrically, a t other temperatures by esr determination.
(4) Similar changes in the optical spectra of acetonitrile solutions
T.+ and the appearance of two isosbestic points were noted by H. J. Shine (private communication).
The Journal of Physical Chemistry, Val. 76, N o . $3, 1978
~ ~
h ~
~
3470
M.
DE
SORGO,B. WASSERMAN, AND M. SZWARC
Table I: Equilibrium Concentrations of the Paramagnetic T. and Its Diamagnetic Aggregate in Propionitrile as Functions of Dklutiori and Temperature” +
T,oc
-
Concentrations
22.7 26.1
16.9 7.0
10.2 1.49
10.1 37.9
9.2
14.7
6.4 5.3
5.2 43.6
3.8 20.1
2.9 8.9
2.1 46.7
1.28 22.5
10.7
0.58 48.2
0.37 23.5
0.30 11.4
0.18 48.6
0.09 23.8
0.07 11.6
1.00
5.45 0.14
2.65 0.02 K = 1 X 1OSM-3 3.9 2.1 1.7 0.5 K = 2.6 X 1OSM-8 2.0 1.4 3.56 1.31 K = 2.2 X 1011 M-8 0.78 0.69 4.81 1.98 R = 1.0 x 10‘sM-a 0.25 0.24 5.34 2.43 K = 4 x 1014 M-3 0.06 0.06 5.53 2.61 K = 3 x 1017 M-3
1.38
1.1 0.25
0.76 0.62 0.39 0.99 0.14 1.24 0.04 1.34
All the concentratioiis given in 10-4 M units; Aggr. is counted as 2 equivalents.
equilibrium between the paramagnetic and diamagnetic species could, therefore, be described by the equation 2”.
Jr 2C104-
(T, +, c104-)2 diamagnetic
implying that under our experimental conditions the thianthrene perchlorate is virtually dissociated into free ions, whereas .the diamagnetic dimer is associated with two CIOA-, forming perhaps a sandwich-like structure. This simple rdcttion is probably valid a t the lowest temperatures, aince the ion pair dissociation is then favored. At, higher temperatures some association of T . + and C104- ions may take place leading to deviations reflected in Figure 3. To verify tha above ideas we investigated in a semiquantitative fashion the conductance of thianthrene perchlorate i n propionitrile a t room temperature. The results led to ili = 130 and 160 cm2/ohm equiv a t the and 6 X total concentration of the salt of 25 X M , respectively (A’s were calculated on the basis of the tota2 salt concentration). This indicates a high degree of didsociation of the salt in the investigated concentration yange.. We may add that the conductance of H30+,Clod- (a wet perchloric acid) in propionitrile is similstr, the respective A0 being about 200 cm2/ohm equiv. The results grven in Table I lead to the approximate equilibrium constants of the “tetramerization” which are listed in the .cable. The respective van? Hoff plot is shown in Figure 4 and leads to the values of A H = -23 kcal/mol and Ais = -42 eu. It is interesting t o compare the above AH and A S values with those found for the association of free ions of alkali salts i n t o ion pairs in tetrahydrofuran. The Journal of Phyfical Chemistry, Vol. 76, No. %?, 1978
Figure 4, Van’t Hoff plot of log K vs. 1/T.
AH and A S of such associations are often positive, indicating that desolvation of the ions takes place on their pairing. Apparently, propionitrile is not a strongly solvating agent; the dissociation of ion pairs is facilitated by its relatively high dielectric constant and not by a powerful solvation of the free ions. Further evidence for the ionic type of aggregation is provided by the following observations, The spectrum M solution of T -+,C104- in trifluoroacetic of 1 X at 540 nm; acid revealed only one peak with A,, apparently the concentration of the aggregate was negligible a t that concentration. The spectrum was broader a t the salt concentration of 3 X M , and a shoulder at about 610 nm was clearly discerned, imply-
In conclusion, we showed the i m ~ o r t ~of ~ionic ~ c ~ ing an increase in the proportion of the aggregate. aggregation in the investigated system and hope that ~ ~ ~ t e r ~ s t 1,he ~ n s~~ectrum g ~ y , of T. +, C104- in txifluorothese Endings might help in clarifyirig some ~ e ~ ~ ~ ~ a ~ i t i e acetic a n ~ i ~~~ ~~ ir t a~10% i ~n ~~ i ~ ~ trifluoroacetic acid ic of the aggregate at of this complex system.8 M and even a t Experimental Section a, c o ~ ~ ~ ~ eor~1t %: r a1W4 ~ ~ M~ the ~ i spectrum ~ clearly reveals the pwaenoe of an appreciable fraction of the Thianthrinium perchlorate was prepared by reacting ~ ~ ~ ~ ~ ~ ~ a t e s ~ equimolar amounts of t~hianthrene and KC104 in The dielectrk constant of trifluoroacetic acid6is only acetic anhydride.2b The resulting salt was precipitated 8A ai b%”, and it increases a t lower temperatures. and washed by CC1, and dried under high vacuum. This valw ia $ ~ ~ ~ ~ t ~lower n t i than a ~ ~that y reported Propionitrile was dried by refluxing it over Pz06, fnp. ~ ~ l ~ ~ ~ ~ (‘ahout i o r ~ 28). ~ t ~ Nevertheless, i~c6 it seems onto Na2C03, and finally Clistibled onto 4hak the ~ g ~ is ~rriuch~ less~ pronounced ~ t in ~ oredistilled ~ ~ ~ ~acid kbnn ~ in‘ propionitrile. ~ o ~ Probably ~ ~the ~ MgClOd. , ~ I t ~was then deaerated and distilled into a sealed container on a high-vacuum line. Before being former sdvant strongly interacts with the ions and this used for preparation of a solution, the solvent was dried &her favors the dissociation of ion pairs into free again with molecular sieves, 4X ,for 2 days. ions or ~o~~~~~ the ~ ~ h a r a cof t ~ ion r pairs from tight The stock solution (4.8 X 10+ M ) of thiaiithrinium c, ones (the respective anion might be ~ u and ~~ perchlorate was prepared on a h ~ g h ~ v a cline the other solutions were then prepared by dilution ~~~~~~~e~~~~~ ~ , r ~ ~ i ~ ~ ~ ~anhydride r o a c e t ~ is c a poorer technique, all the operations being performed again on a a o ~ agger4 ~ and y ~its dielectric ~ ~ ~constantG ~ ~ is only 2.7. high-vacuum line. ‘These~ ~ o ~ eaccount r ~ ~ for e sthe high degree of aggregation in the ~ ~ It is ~ also probable y that~ the ~ ~ ~ e ~ A c ~ n o ~ l ~ We ~ gwish ~ ~ to t .thank the Kational in this solvent is given by the equation Science Foundation and the Petroleum Research Fund, + @lO,-) (T. +, ClO,-), administered by the American Chemical Society, for the ~~~~~~~~~~~~~~~
Le., the aggregation is described as a dimerization and not t,etr,zmedzaLioa, because T +, Clod---ion pairs are not expected lo dissociatx in this medium. Firially, it an Pmown that some radical cations combine with the parent oompound into dimers,’ e.g., (naph+. W e ~~vestigated, therefore, the effect of thalene)~* addition of neertral thianthrene to the solution of T +,
