980
J . B.
The attractive coefficients A determined by us show that the parameters of the hydrogen bond potential functions to be used in the empirical energy calculations vary greatly according to whether neutral molecules, zwitterions; or ions participate in the hydrogen. bonding. Further work is needed to test the applicability of UHBand to extend it to shorter distances; for this purpose lattice energy calculations of 'crystals of the above molecules may be useful.
Acknowledgment. One of us (R. D. S.) wishes to express his thanks to the National Research Council of Italy for having assisted him financially throughout the course of this investigation. References and Notes (1) R . F. McGuire, F. A. Momany, and H . A. Scheraga, J. Phys. Chem., 76, 375 (1972)
Nagy, 0 . B. Nagy, and A . Bruyiants
(2) H. A. Scheraga, Advan. Phys. Org. Chenr.. (3) In this work we are not concerned with ?he torsional energies. It suffices to say that usually they are represented by a cosine function of the dihedral angle, such that when added to nonbonded and electrostatic contributions reproduce the experimental barriers to rotation in small model systems. (4) (a) P. A. Kollman and L. C:Allen, J. Amer. Chem. Soc., 92, 6101 (1970); (b) P. J . Hay, W. J. Hunt, and W. J. Goddard, ibid., 94, 8301 (1972) ( 5 ) R . E. Dickerson and I. Geis, "The Structure and Action of Proteins." Harper and Row, New York, N. Y . , 1969, pp 73,83, and 93. (6) M . Yudkin and R . Offord, "A Guidebook to Biochemistry," University Press, Cambridge, 1971, pp 24 and 46-49. (7) A. S. V . Burgen, 0. Jardetzsky, J. C. Metcalfe, and N. W. WadeJardetzsky, Proc. Nat. Acad. Sci. U. s., 58, 447 (1967). (8) R . J . Weinkam and E. C. Jorgensen, J. Amer. Chem. SOC., 93, 7033 (1971);ibid., 93,7038 (1CiTlI. (9) J. A. Pople and G. A. Segal, J. Cfiem. Phys., 44, 5289 (1966). (10) G. A. Segai, Quan?um Chemica! Exchange Program. No, 91, QCPE, Indiana University. (11) P. A . Kollman and L. C. Allen. Chem. Rev., 72, 283 (1972). (12) J . F. Yan. F. A. Momany, R. Hoffmann, and H. A. Scheraga, J. Phys. Chem., 74, 420 (1970). (13) W. R . Oegerle and J . R. Sabin, J. Mol. Slrucr., 15, 131 (1973)
r Complexes in Organic Chemistry. XI.' Effect of Acceptors on the
harge-Transfer Complexes Formed by Cyclic Anhydrides agy,2 6.B. Nagy," and A. Bruylants i a b o r a f o i r e de Chimie Generale e t Organique. Universite Catholique d e Louvain. lnstitut Lavoisier, Place L. Pasteur. B-1348 louvain-la-Neuve, Belgium (Received July 27. 1973: Revised Manuscript Received January 74. 1974)
r-T and ri-r type charge-transfer complexes with fixed donor moiety and variable acceptor moiety were examined. The thermodynamic and spectroscopic properties of these complexes were analyzed as a function of the properties of the acceptors. Several new electron affinity values for the acceptors were determined and their magnitude was interpreted in the light of the molecular electronic structure.
In a previous paper3 we examined the variation of the properties of charge-transfer complexes (CT complexes) formed by tetrachlorophthalic anhydride (TCPA) with aromatic T donors when the latter were changed. In the present paper we wish to report a similar study on the n-a and r--s type CT complexes of several cyclic anhydrides (Table I). The main purpose i s to examine the behavior of various CT complexes when their acceptor moiety is varied. Although the different presently known acceptors were already compared and their acceptor strength carefully a n a l y ~ e d only , ~ a few studies were devoted to closely related acceptors. The homologous series of polynitrobenzenes4-6-8 and of substituted p-benzoquinones4 7-9 were studied in detail. The importance of this type of study should be emphasized since it permits one to establish how the properties of the acceptor molecule may influence those of the whole CT complex. According to the theoiy o f weak complexes a direct relationship exists between the CT band position ymax and the electron affinity of the acceptor, EA4
The Journal of Physical Chemistry, Vol. 78, No. 10, 1974
C1 and Cz are constants characteristic mainly of the donor moiety. Since the last term on the right-hand side of eq 1 turns out to be negligible, one should obtain a straight line with unit slope when plotting hvcr against E A both expressed in eV units. This prediction has already been verified experimentally by BriegleblO and by Foster8 who used either the actual electron affinity values or the half-wave reduction potentials, E I : ~ ,which are closely related to themlo
E,
= -E,,>
4- 1.41
(2)
The intercept of eq 1is given approximately by
c, = I ,
4- E , --
w,,
(3)
where I , is the ionization energy of the donor; Ec represents the Coulomb interaction energy of the two oppositely charged ions resulting from the complete transfer of one electron from the donor to the acceptor; Wo represents the interaction energy between the donor and the acceptor due to other factors than charge-transfer. Since in practice 1 I D + &I >> 1 Wol, eq 3 reduces to
c, =: I" IEc (41 It is noteworthy that only very few E l values are known at present and their reliability is often questionable.10
Charge-Transfer Complexes in Organic Chemistry TABLE I: Cyclic AxnkcydridesUsed the ~ ~ e ~ e
iin
~~~~~
_I
-__. Name
~
~ Symbol
Tetrachlorophthdic anhydride TCPA Tetrabromopbthalir anhydride TEPA 3 , 6 - D i c h l o r o ~ ~ t ~ ianhydride a~ic DCPA 4-N itrophthlie anhydride 4NPA 3,5-DinitropbthRlic anhydride 3,5NPA Konzophthalic anhydride HPA Phthalic anbydrde PA 1.8-Naplithalenedicarboxyllc anhydride NA 2,3-Pyridinedicaii,boxylic anhydride PCA Maleic anhydride MA The study of CT complexes presents an extremely useful approach to o v ~ r c ~ ) mthe e difficulties. If a series of CT complexes with fixed donor and variable acceptor moieties is exammeld, new PiecI ronaffinity values can be extracted from the p113t hvc r CIS.Ki(A
All the CT complexes studied in 1,2-dichloroethane (DCE) presented a new electronic absorption band either well separated from the components' absorption or showing up as a shoulder (Figure I). Table Il gives the results obtained by using the Scatchard equation Cor the 77-7r type CT complexes of acenaphthene (ACE) with the various anhydrides. The 1:l stoichiometric composition of such complexes was previously eslabljshed.1,3 The corkcentrationof the anhydrides varied kiptween 3 x IO-" and 2 x M The range of concentration of the donor is 0.i-1.1 M The saturation factors are 0.2-0.7, except for DCPA (0.15) and this value rnust be regarded witb caution. The extinction coefficient of the complexes does not vary with temperature showing that contact interactions are unimportant.133.11 The theirmodynamic and spectroscopic data are also collected ira Table lil. The enthalpies of complex formation do not show any change with temperature. This precludes the presence of isomeric CT complexes of different AHo The low values for A N " indicate that ACE fl3Ym5 only relatively weak complexes with the anhydrides under study. This 1 5 corroborated by the low ionic character (i%) of the C'H' bond The entropy values, ranging from -2 to -12 eu, are negative in all cases. U seems that the increase in entropy due to partial desolivation of the donor and acceptor molecules cannot countcv balance the considerable entropy loss acmrnpanying the complex formation. The desolvation efi s the greatest for MA and the smallest for TCPA, A, and 3,.5NYA4..This is in accord with the different solvation of thc acceptors due to their different size. More important, however, rci the solvation of the CT complex which is governed mainly by its polarity due to the charge-transfer, that YS the more polar the complex the better it is solvated This effect works in an opposite manner to the effect of the partial desolvation resulting in the observed entropy :sequence. Good correlation exists between AN" and TAS" (Figure 2 ) ~The slope is close to unity (1.20 f 0 08) showing that the entropy varies almost as i - ~ ~ as c hthe enthalpy. Therefore it cannot be supposed tlhat the entropy factor remains constant when a series of C'T complexes with variable acceptor moiety is considered. IVevertheless, the observed AN"/TAS" relationship is rather surprising as no such relationship could be obtained for complexes of variable donor moiety.3 The molar extinction coefficient t does not vary regular-
Figure 1. Electronic spectra of the CT complexes of cyclic anhydrides with ACE in 1,2-dichloroethane at 7' = 20" (concentrations in parentheses in M): 1, TCPA (3 X -k ACE (0.436); 2, 4 N P A (6.0 X ) f ACE (0.416); 3, DCPA (1.2 X l o - * ) 4- ACE (0.140); 4, PA (2.4 X l o - * ) -1- ACE (0.555); 5, M A (2 X lo-') 4- ACE (0.313);6, 3,5NPA (1,666 X 15-3) ACE (0.555); 7, TBPA (3 x 10-3) ACE (0.60).
+
+
DCPA
ob
1
-
I 2
I
-
3 -4 T A S"(kKal Mor-')
Figure 2. Relationship between AN" and TAS" for the complexes of ACE with various cyclic anhydrides in DCE.
133,12
ly with , , ,c while the oscillator strength f gives an excellent correlation (Figure 3) (f = -0.105 (lt0.013)V,,, 6.12 (lt0.31)). Thus f represents much better than c the charge-transfer transition intensity.l Since the correlation obtained satisfies the general t h e ~ r y ,i ~e., f decreases with increasing U,, no correction was carried out for solvent complexation effects.l This latter is supposed to vary only slightly from acceptor to acceptor. The transition moment pT of the c arge-transfer band is about 2 W5for all complexes studied. The complexes of 1,4-diazabicyclo[2.2.2]oetane(DBO) with the anhydrides are of the n-a type. They are stronger than the corresponding a-T type complexes of ACE as indicated by the lower bmax values. The thermodynamic data show clearly that this is indeed the case: K for the complex DBO-PA16 is equal to 0.9 M-I at 40", AN" = -4.2 kcal mol-I, TAS" = -4.4 kcal mol-l on the other hand for the complex ACE-PA we have K c- 0.09 M-1 at 44", AHo = -1.3 kcal mol-', and TAS" -2.8 kcal mol-1. TCPA also forms stronger complexes with n donors: the value Kkln = 5.6 M - l a t 20" for the complex DBOTCPAI7 is to be compared with K,, = 2 M - l at 20" for the complex ACE-TCPA.l8 The half-band width for the complexes of ACE increases slightly with ijmax. The relationship is far from
+
The Journal of Physical Chemistry, Vol. 78,No. 70. 7974
J. €3. Nagy, 0. B. Nagy, and A. Bruylants
982
TABLE 11: Variation of Thermodynamic and Spectroscopic Data with Acceptor Properties for the Complexes ACE-Anhydrides at 20" in DCE' Fmax
-TAP,
Anhydride
E A ,SV
TCPA 0.58" TBPAa 0.7' 0,se DCPA 3,5NPA* l . l d 4NPA 0.5a MA 0.57' PA 0.1gc
K, M - '
2.1 i O . l 1.8 h0.05 0.8 f 0 . 0 4 2.5 1 0 . 1
-0.43 -0.33 +0.13 -0.53
10.03 10.03 10.05 h0.04
-0.6 0.7 k 0 . 0 3
+ O . l &0.03
-0.1
-
emax,
F1/2,
M-'
f X
kcal mol-1
kK
cm-1
cm-1
-3.6 1 0 . 5 -3.4 f 0 . 4 -2.4 1 0 . 4 -4.0 h O . 4
3.2 k O . 5 3.1 h 0 . 4 2.5 h 0 . 4 3.6 k 0 . 4
24.3 24.25 26.15 20.3 23.75
2300 2300 2600 2400 2700
940 960 800 1030
-0.5 l t O . 1 -1.0f
0.6 zk0.l
AH', kcalmol-1
AGO, kcal mol-'
%ax,
BT~
D
i%h
3.5
1.8
3.6
1.8 1.7 2.0
5 5 3 6
102
3.4 4.0
220
a Reference 13. Reference 14. Reference 10. Calculated from the relationship Bmax/E2y of the complexes formed with pyrene.'!''l e Calculated from eq 6 and verified by eq '7. f Calculated from A = eKC
