&e&ochimics Aota, Vol. 27A, pp. 1171 to 1173. Pergamon Pmaz 1971. Printed in Northern Ireland

Optical determination of the enthalpies of formation of some electron donor-acceptor complexes

R. FOSTER and I. B. C. M~EESON Chemistry Department, University of Dundee, Dundee DDl4HN, Scotland

(Received 18 Ap-2 1970)

Abstract-The enthalpies of formation of a number of complexesof methylbenzenewith fluoranil, with 1,3,5kinitrobeuzene, and with 1,4-dinitrobenzene have been estimated from the tem- perature-dependence of the product term KIcl, where KI is the association constant for the 1: 1 complex and cl is the molar absorptivity of the complex at the wavelength of measurement. The results are compared with values of AHo obtained from NMR chemical shift measurements.

A STANDARD method for determining the enthalpy of complex formation between an electron donor (D) and an electron acceptor (A) is by applicstion of the van% Hoff equation to the temperature-dependence of the association constant (K,). In many electron donor-acceptor systems it is only practicable to obtain values of K, from sets of solutions in which the total concentrstion (free and complexed) of the donor, ED],,, is very much greater than the corresponding total concentration of the acceptor [A],,. One restricting factor is the solubility of the acceptor, which is usudly very low. It is also necessary to have a significant fraction of one component in the complexed state [l, 21. Except in very favourable cases, this is only achieved by having a large excess of the second component.’ Furthermore, for optical determi- nations, the presence of large concentrations of free acceptor are more likely to have electronic sbsorptions which seriously overlap the intermolecular charge-transfer band on which the measurements are made. In general, the donor absorptions are to higher energy and cause less interference. For a series of solutions with the concentration condition [D], > [A],, a plot of [A],/A agsinst [D&-l (where A is the absorbance due to the complex) has been used to evaluate K, and l 1 (the molar absorptivity of the complex at the wavelength of measurement) by an application of the BENESI-HILDEBRAND equation [3]. The plot should be linear with gradient equal to (K,+ and intercept with the ordintlte equal to e-l. Recently DERANLEAU [4]

has pointed out some of the consequences of the presence of higher order complexes in systems where only 1: 1 complexing is assumed to occur. In the particular case of the presence of a complex D,A(K, = [D,A]/[DA][D], and molar absorptivity Ed), the BENESI-HILDEBRAND plots will still yield, at the lower s&u&ion limit, slopes which are correct, but intercepts which are incorrect, namely: er--l(l - K2~2/K1~1). For a considerable time it had been well appreciated that, whereas there seems to be little problem in evaluating the product term RI+ the separation of these factors gives rise to many anomalies [l, 5, 61. However, it has been shown that in at least one or two

[I] W. B. PERSON, J. Am. Chmm. Sot. 87, 167 (1965). [2] D. A. DE- AU, J. Am. Chem. Sot. 91,4044 (1969). [3] H. A. BENESI and J. H. HILDEBRAND, J. Am. Ohem. Sot. 71, 2703 (1949). [4] D. A. DEXLANLE AU, J. Am. C?wm. Sot. 91,405O (1969). [5] S. CUTER, J. N. Munnnn~, end E. J. Rosen, J. C&m. Sot. 2048 (1966). [6] P. H. EXSLIE, R. FOSTER, C. A. Fvxx and I. HOFXAN, Ttih&ron al, 2843 (1966).

1171

1172 R. FOSTER and 1. B. C. &%.&THESON

cases, a reasonable estimate of AH” may be obtained by plotter In K1~i against T-I instead of the normal van% Hoff plots of In & against 17-l [7, 85 There is a alight difference in the values of AH” obtained because there is a temperature dependence of e1 due to vibrational band-broadeniug. Over the experimental temperature ranges used this difference is small; it htts the effect of increasing the estimated AH” values by co. O-2 kcal mole-l.

The AH’ values for several electron donor-acceptor complexes have been cal- culated in this way and compared with values obtained from chemical shift measure- ments [9].

%!XWLTS AbND Drscussro~

The AH’ values obtained from plots of ln K,al against T-1 are compared in Table 1 with corresponding values of AH” obtained from NMR chemical shift measurements of nuclei in the acceptor, as the concentration of the donor is altered. It is seen that, although the values of AH” increase with increasing methylation of the donor in a series of complexes with a common acee&or speoies, the values are not identical with those obtained from NMR experiments. However, closer inspection shows that, apart from the fluorauil-hexamethylbenxene complex, there is reasonable a~ement between the values for the stronger complexes. The ~~Gant differences occur with the complexes having AH* values below c(;G. 34 kcal mole-l (as measured by NMR), The reason for the observed differences may lie in the problem of es- timating the optical absorption of the complex alone. In general with the weaker complexes (smaller -AH” values) the charge-transfer band is less well-resolved from the abso~tions of the components, so that corrections resulting from the overlap of the component abortions, which are difEcult to estimate with accuracy, become of increasing importance.

Table 1. Ent~a~pi~ of fo~ation for various electron donor-~e~ptor complexes in carbon tetr~~o~de

Donor Acoeptor -AH0 (kc&l. mole-l)*

present work from NMRt from Kr valueejE

Toluene fluoranil p-xyhxle ffuaranil Mesitylene fluorsnil DUrene fluoranil Pentamethyl~nzane fluoranil Hexamethyl~nzene fluoranil Durene 1,3,5trinitrobenzene Pentamethylbenzene 1,3,5-trinitrobenzene Hexamethylbenzene 1,3,5&nitrobenzene Hexamethylbenzene l&dinitrobenzene

3.6 & 0.5 25 rf 0.2 36 f 0.5 2.9 f 0.2

3-9, f 0.5 32 f 0.2 4.4 * 0.5 4.1 * 0.2 4.6, f O-B 46 f 0.2 4.95 f 0.5 56 f 0.2 2-6 f 0.7 3.3 j, 0.2 2-24 & 0.80 3.6 f 0.5 3.6 zt: 0.2 2.44 fi 0.61

3.8s * 0.6 3.9 jf 0.2 4.18 f 0.18 2-1 f 0.7 3-2 f 0.2

* Referred to a standard state of 1 M. 7 Ref. [S] converted from data on a standard state of 1 M. $ Ref. [lo].

[73 R. FOSTER and I. B. C. ~UTEESON, ~~ec~~~. Acta. 88& 2037 (1967). [S] R, FOSTER and I. B. C. lkfM!HESON, J. Ohma. Sot. B, 1713 (1970). [Q] M. 1. FOREMAN, 33. FOSTER and C. A. FYR’E, J. Cti. Sot. B, 528 (L970).

[lo] C. C. THOWPSON, SR. and P. A. D. DJC MAINE, J. Php. Chen. 69, 2766 (1966).

Determination of the enthalpies of formation of some electron donor-acceptor complexes 1173

The regular behaviour of AH” values, obtained from NMR chemical shift measurements, with ionisation potential of the donor for series of complexes of related donors with a common acceptor species; also of the regular variation of AH” with AS” and AC” obtained by such techniques, gives support to the evaluation of thermodynamic constants from NMR chemical shifts [9]. Against this standard the present values for the stronger complexes compare favourably as opposed to the published values [lo] for optically-determined values of AH” using the van’t Hoff plot of In KI against T-l (Table 1) in which the prior separation of the factors of the term K1el had been attempted.

EXPERIMENTAL

Optical absorbance measurements were made by means of a manually-operated Optica Cl?4 spectrophotometer. The absorbance scale was calibrated using standard

Table 2. Values of K,E obtained from the slopes of Benesi-Hildebrand plots for various complexes in carbon tetrachloride

Donor Acceptor A *

Klel max (1.*mole-2 cm-l)

(-) 33.50 25.0° l&O0 6.60° -2.80”

fluoranil 364 844 986 1220 1460 1890 koranil 388 1500 2060 2220 2740 3630 fluoraalil 407 3450 4220 5130 6730 8330 fluoranil 453 8670 10,700 14,100 16,600 21,300

fluoranil 465 22,500 28,000 36,700 46,600 66,200

p-Xylene Mesitylene Durene Pentamethyl-

benzene Hexamethyl

benzene Durene Pentamethyl-

benzene Hexamethyl-

benzene Hexamethyl-

benzene

fluoranil 486 53,700 65,800 86,000 111,000 156,000 1,3,5-trinitrobenzene 345 3000 3680 3920 4580 6130

1,3,&trinitrobenzene 5400 6900 8300 9900 12,500

1,3,6-trinitrobenzene 396

1,4dinitrobenzene 385

12,000 14,400 17,400 22,700

840 960 1160 1280

* Wavelength of measurement.

potassium dichromate-potassium hydroxide solutions. All the weighed-out solutions were measured using the same 1 cm cuvette against air as the reference. Absorption by the cuvette, free donor, and the solvent were obtained from separate measurements and the corrections applied arithmetically. This procedure was found to be more reliable than attempts to use so-called ‘matched cells’. The temperature of the solutions was controlled to &O-05” by circulating water or glycol through a cuvette jacket. The gradients of plots of [A],/A g a ainst [D&-l, which are equal to K1el, at five different temperatures, for each of the systems studied, are listed in Table 2.

Carbon tetrachloride B.D.H. “pure’ for spectroscopy” was used without further treatment. Other materials were purified by crystallisation or fractionation and their purities checked by GLC or by their melting points.

Achowledge~!l!he authors wish to thank the Science Research Council for financial support (to I.B.C.M.).