2602
J. Phys. Chem. 1980, 84, 2602-2605
Correlation between the Dipole Moments and Thermodynamical Data of the Iodine Complexes with Organic Oxides, Sulfides, and Selenides J. Glera, L. Sobczyk,"
F. LUX,+and R. Paetroldt
Institute of Chemistry, University of WrocYaw, WrocYaw,Poland, and Section of Chemistry, Friedrich-Schiiler University, Jena, German Democratic Republic (Received: March 28, 1979)
Thermodynamicaldata and dipole moments were determined for a series of electron donor-acceptor complexes of iodide with organic oxygen bases, phosphine sulfides, and phosphine selenides. An attempt was made to correlate the enthalpy of complex formation with the dipole moments.
Introduction One of the more important properties of the electron donor-acceptor (EDA) complexes is the enhancement of the dipole moment with respect to the vectorial sum of moments of its c~mponents.l-~This enhancement is a direct measure of charge displacement from the donor to the acceptor and varies over a wide range depending on the iype of complexes. The enthalpy of formation also varies over a wide range, from the values typical of van der Waals complexes to those characteristic of the chemical bond. Determination of the nature of relationships between these basic data would permit one to make more general considerations on the nature of the EDA interactions. The experimental results reported until now and the Mulliken charge transfer theory4 seem to indicate that an increase in complex stability is always accompanied by an increase in the dipole moment. The results of studies of the iodine complexes with sulfides: amines and ethers! and TCNE complexes with methyl benzene^^ may indicate that there is a rectilinear relationship between Ap and AH. The results of studies of the hydrogen bond systems of phenols with aniline, amidine, and pyridine derivatives reported by Foubert and HuyskensS also would confirm the linear relationship between Ap and AH in certain intervals. However, the above mentioned examples of correlation have no theoretical base. Guryanovag attempted to find an explanation on the basis of the Mulliken theory4 and obtained an expression which binds together the values of AH,hvCT, and ~ D A :
_-AH - -~ V C T e+A
I.LDA
(1)
where ~ D =A Aii, e-r = (the dipole moment resulting from complete electron transfer), hvCTis the electron transfer band energy, and A is a constant depending on the a, b, a*, and b* coefficients and the overlap integral Sol. The following relationship determined experimentally for various complexes of the n-6 and n--P type PDA
-AH = 35.3-
e.r is in agreement with eq 1if the value of humIA is assumed to be constant, which seems to be a too rough approximation. In particular, neglecting the term responsible for nocharge-transfer interactions, Wo = 0, and omitting the dipole moment value in the ground state restricts the apt Friedrich-Schiller University.
0022-3654/80/2084-2602$01 .OO/O
plicability of the above-mentioned relation to a small number of EDA complexes. Another attempted correlation was reported by Ratajczak and Orville-Thornas.l0 On the basis of the Mulliken theory4 these authors obtained the following relationship:
(3) where Po is the resonance integral and A p c ~is the contribution of the moment resulting from charge transfer in the overall increase of the dipole moment, which is fairly well satisfied by the complexes of amines with iodine. However, it seems unjustified to include the complex of iodine with benzene (a-a type) in this series. From this relation W o= 10.4 kcal/mol was determined. A similar value of 11.99 kcal/mol was obtained by Sambhi and Khool' from the linear relationship AH = f(RN),(RNis the resonance energy) for the complexes of aliphatic amines with iodine. The authorslOJ1explain this unexpectedly high value of W oas due to the prevailing contribution of repulsive forces resulting from small distances between the donor and the acceptor in these complexes. However, it should be emphasized that eq 3 is valid only for weak complexes, whereas in this case strong complexes were analyzed. Extrapolation of the rectilinear relationship to Ap 0 seems to be too risky. Under these circumstances it seemed justified to undertake further systematic studies of a selected series of complexes covering possibly the widest range of donoracceptor properties. It seems that such conditions are satisfied by the organic oxygen bases and also with sulfides and selenides as donors in the complexes with iodine. Numerous experiments indicate that these compounds form stable complexes of defined compositions and, as should be expected, of the same structure.
-
Experimental Section The dipole moments of the complexes were determined either in benzene (B) or in carbon tetrachloride (C) by the method described earlier.12 It consists in determining the permittivity and density gradients in donor and acceptor solutions extrapolated to infinite dilutions of all equilibrium components. This procedure requires thorough knowledge of equilibrium concentrations at various proportions of the donor and the acceptor. Equilibrium constants were measured by the BenesiHildebrand13 and Rose-Drago and Lang14 methods. The enthalpies of formation were determined either by the direct calorimetric method14or spectrophotometrically, i.e., from the relation of the equilibrium constant to temperature. The results obtained by both of these methods 0 1980 Amerlcan Chemical Society
The Journal of Physical Chemistty, Vol. 84, No. 20, 1980 2803
Thermodynamic Data of Iodine EDA Complexes
appeared to ble similar for several systems within the experimental errors. Synthesis of donors and their preparation were described earlier.14J6
Results and Discussion The results, of measurements of the equilibrium constants K,,formation enthalpies AH, dipole moments of donors pD, and their complexes with iodine /LAD are summarized in Tinble I. Let us attempt to find a general correlation between AH and the dipole moment in the ground state pN = pm from the Mulliken theory. The groundl-state energy of the complex WN is defined, according to :Mulliken, by eq 4,where Solis the overlap (4) integral, A = W1 - Wo (subscript 1 refers to the fullcharge-transfer state and 0 to the no-charge-transfer state) and /lo = Hol.- W$,, where Holis the resonance integral. The ratio of the a and b coefficientei in the ground-state wave-function equation $N = a$o + bq1 is defined by
w, - MlN
b
( 5)
P = a = - Wo1- W K l and, hence PO
P = -
A(1 -
(6)
sol2)+ SOlPO
and, finally P'
AH- w, Po - SOlW -
(7)
w,,
If one takes into account that the dipole moment in its ground state pN is defined by pN
:= po(a2
+ abSo1) +
+ abSo1)
(8)
+ +
and allows for the normalization condition a2 b2 2abSol = 1 an expression is derived which relates in a general way p N with bH: P N - PO
-po2
Pl - Po
Pl - PN
=(1 - SO12)arrz 1.11 - Po
2(11 - SO12) Wo- P1 - I*N - S
