1662
Huyskens, D'Hondt, Govaerts, and Zeegers-Huyskens
Energy Parameters and Charge-Transfer Spectra of the Complexes of Bromine with Substituted Pyridines P. Huyskens," J. D'Hondt, F. Govaerts, and Th. Zeegers-Huyskens Laboratory for Physical Chemistry, University of Louvain, 3030 Heverlee, Belgium (Received July 27, 7972) Publication costs assisted by KatholiekiUniversiteit te Leuven and F. K. F. 0. Belgium
The frequencies uCT at maximal absorption, the extinction coefficients emax, and the bandwidths l u l l 2 of the charge-transfer bands were determined for complexes of bromine with several substituted pyridines. These values were compared with the force constants kBr-Br and &...Br2 previously determined by farinfrared spectrometry, with the complexation enthalpy W , and with the pK, of the donor. The pecularities of the relations that exist between these experimental values can be explained in terms of the simplified resonance theory of Mulliken. From the above experimental data the weight of dative structure F1 and the energy parameters Wol, WO,and W1 of this theory were computed. While for all the complexes studied here WOand W1 can be described by quadratic functions of F1, systematic positive deviations occur for Wol in the case of the ortho-substituted compounds. These deviations are caused by the increase in the N--.Br interdistance and are responsible for the deviations these compounds show relatively to the linear relationship of huCT us. F1 observed for the other complexes. When WOand Wl are plotted us. pK,, the ortho-substituted compounds also show significant deviations. The observed positive deviations of the complexation enthalpy WN us. pKa for these compounds are the consequence of the fact that the positive deviations of WI, caused by the increasing of N-S-Br interdistance and by an additional repulsion between the ortho-methyl group and the Br- ion, are not completely compensated by the decreasing of WO.The transition moment pcT is roughly proportional to the weight of dative structure: pcT N 17F1.
The complexes of bromine with several substituted pyridines were previously studied in our laboratory by means of various experimental techniques: far-infrared1 and visible spectrometry,2 determination of dipole moments,l and adiabatic c a l ~ r i m e t r y From .~ these studies it was possible to show the existence of a linear relationship between the logarithm of the complexation constants and the weight of dative structure in the complex. A similar relationship holds for the complexation enthalpy. In this work, the charge-transfer bands of these complexes that appear in the ultraviolet region are studied. The characteristics of these bands are then compared with the previous experimental data and the peculiarities are explained in terms of the simplified resonance theory of Mulliken.4 Characteristics of the Charge-Transfer Bands The ultraviolet spectrum was studied in the region between 210 and 300 mp where the charge-transfer bands appear. The solvent was 1,2-dichloroethane and the temperature was 25". At a given wavelength, the absorbance A of the ternary solutions can be related to the formal concentrations CDO and CAO of the donor and of the acceptor, and to the real concentration of the complex CK through the expression where d is the cell thickness (0.025 cm) and t D , € A , and C K are the molar extinction coefficients of the free donor, the free acceptor, and the complex. The reference cell was filled with a solution of the same concentration CDO of the donor. The value of t K is then found from the difference in absorbance A - A o between the two cells, using the expression The Journaiof Physicai Chemistry, Voi. 77, No. 13, 1973
The concentration of the complex is computed from the values of the formal concentrations, using the complexation constants determined in the previous work. In our determinations C,O and CDO are of the order of 10-3 mol L - l . Equation 1 assumes that only 1:l complexes are present in the solution. In a previous work,2 it was shown by the method of Briegleb-Liptay, by this method of continuous variations and by the method of the isosbectic points, that under the circumstances of our experiments, the appearance of higher complexes is negligible. It is then possible to determine the wavelength, where cK is maximal (e,,), and also the bandwidth Aullz. The transition moment pcT was then computed using the empirical equation of Tsubomura and Lang.5 pCT = 0.0958[--]
tmaxA~1/2 Vmax
(3)
In Table I, the experimental values determined in this work are shown against the other characteristics of the complexes which were determined in the previous works: KBr-Br, the Br-Br stretch force constant (directly related to the weight of dative structure), hN...Br, the force constant of the vibration associated with the charge-transfer (1) J. D'Hondt and Th. Zeegers-Huyskens, J. Mol. Struct.. 10, 135 (1971). (2) J. D'Hondt. C. Dorval. and Th. Zeegers-Huyskens, J. Chim. Phys.,
68,516 (1972). (3) 8.Tilquin and L. Lamberts, Z. Phys. Chem., in press. (4) R. S. Mulliken and W. 8 . Person, "Molecular Complexes," Wiley-lnterscience, London, 1969. (5) H. Tsubomura and R. Lang, J. Amer. Chem. SOC.,83,2085 (1961)
1663
Charge-Transfer Spectra of Bromine Complexes bond, and WN, the enthalpy of the bond. In this table we also give the pKa value of the donor taken from the literature.6 The complexes are classified by decreasing kBr-Br force constant, and thus increasing weight of dative structure.
Comparison between the Direct Experimental Data The comparison of the experimental data in Table I brings some interesting peculiarities to light. For all the complexes studied here which do not bear substituents in the ortho position, one observes the following relationships between k N . . . B ~ ,WN, pKa, huCT, and k B r - B r
WN = 1.35 hv,,
W N=
0 . 6 1 k ~ ~(maX - ~ ~ dev 0.01 e v ) (4)
- 0.049pK, (max dev 0.025 eV) (5) -0.22 - 0.045pKa (max dev 0.015 eV) (6)
= 5.50
hvCT = 4.34 kN
-
+
0.67kBr-Br (max dev 0.025 eV) (7)
lated F1 from kBr-Br using the value of 2.29 mdyn A-1 for kBr-Bro and 0.25 for the ratio kBr-Br-O/kBr-Bro. From F1 it is then possible to calculate the coefficients a and b, using eq 10 and taking normalizing requirements into account. According to the simplified resonance theory of Mulliken, the bond energy WN and hvCT are related by the expression
-WN =
1 +CT
+
1 F
2
SO,
'
(Wo,So1 - W 0 2
(13)
where Wol is the integral.
WOl =
p 0 H A d7
(14)
Wol, WO,and W1 can be calculated from the coefficients a and b and from the experimental values of WN and h U C T , using the following equations derived from the basic expressions of the theory of Mulliken (ref 4, p 12). c
: Br -
2.43 - l . g 2 k ~ ~(max - ~ ~dev 0.02 mdyn
A-')
(8)
It must be noted here that these analytical expressions hold only for the studied range of the variables (see Table I) and that these quantities do not necessarily exhibit linear relationships in the whole range of values. It appears that, within the same limits, eq 4 and 5 also hold for the ortho-substituted complexes. This is not the case, however, for expressions 6 and 7: the ortho-disubstituted compounds show WN values that are 0.06-0.09 eV too high and hvCTvalues that are 0.05-0.09 too low. For the orthosubstituted compounds the deviations are of the order of 0.03 eV. Significant negative deviations also occur for the orthodisubstituted complexes for eq 8. These pecularities can be explained in terms of the simplified resonance theory of M ~ l l i k e n On . ~ the other hand, the data in Table I show the existence of a rough linear relationship between the transition moment pCT and WN. This relation can be ugitten PCT
-
-lGWN
(9)
Determinationof the Parameters of the Simplified Mulliken Theory The wave function $N of the complex in the ground state is considered as a combination of only two resonance structures: the no-bond wave function $0 with the energy WO,and the dative function $1 with the energy W1 *N = a*o + b*l The fraction in dative structure F1 is given by
(10)
c
7
In Table I1 we give the values of these energy parameters with those of F1, a, and b for the various complexes studied in this work.
Discussion of the Results It is interesting to.consider the evolution of the various parameters with increasing weight of dative structure. The complexes which do not bear ortho substituents obey within 0.01 eV the relation Wol = -1.59 - 0.90FI (18) Again, this linear relation only holds for the studied range of Fi. The ortho-substituted complexes and the ortho-disubstituted complexes show values that are systematically higher by a few hundredths of an electron volt. These deviations are responsible for the deviations in the hvCT k B r - B r relations. As a matter of fact, huCT is very sensitive to variations in Wol, according the Mulliken equation
+
P, = b 2 abSo, (11) where S o l is the overlap integral. It can be expected tnat this integral does not differ appreciably for the various complexes studied here. We assumed therefore in all cases that S o l was equal to 0.3, a value generally admitted for this kind of complexes.4 Following a method presented by Person,' F1 can be computed from the force constants of the Br-Br stretching vibration respectively in the complex (kBr..Br) and in the free molecule ( k B r - B r o ) by means of the equation
kBr-Br-o is the force constant in the Brz- ion. We calcu-
If we recalculate huCT from this expression replacing only the experimental value of Wol by that of expression 18, the deviations of the ortho-substituted complexes in the huCT-kBr-Br relations disappear. The reason for the effect of ortho substitution on Wol must be sought in an increase in the interdistance N-Br caused by the steric hindrance of the ortho substituent. This increase in the inter(6) D. D. Perrin, "Dissociation Constants of Organic Bases in Aqueous Solutions," Butterworths,London, 1965. (7) H. B. Friedrich and W. B. Person,J. Chem. Phys., 44,2161 (1966). The Journal of Physical Chemistry, Vol. 77, No. 13, 1973
1664
Huyskens, D'Hondt, Govaerts, and Zeegers-Huyskens
TABLE I: Experimental Characteristics of Bromine-Substituted Pyridine Complexes
3-CI 3-Br 3-1 2,6 Me2 2-Me 2-Et 3-Et 3-Me 4-Me 4-Et 2,5-Me2 4-f-C4H9 3,5-Me2 3,4-Me2 2,4-Me2 2,4,6-Me3
1.70 1.67 1.62 1.54 1.52 1.51 1.50 1.49 1.48 1.45 1.45 1.44 1.43 1.42
0.38 0.40 0.46 0.42 0.49 0.16 0.48 0.50 0.49 0.48 0.51 0.52 0.51 0.51
-0.51
1.42 1.41
0.51 0.44
-0.51 -0.50
- 0.35 - 0.43 -0.46 0.45 -0.48 -0.47 -0.49 -0.49
-
2.84 2.84 3.25 6.60 5.96 5.89 5.22 5.56 5.63 5.98 5.87 6.40 5.99 6.15 6.16 6.63 7.43
2270
5.46
2340 2330 2340 2310 2331 2327 2339 2341 2355
5.28 5.32 5.30 5.37 5.32 5.33 5.30 5.30 5.27
2341 2362 2359 2372
5.30 5.26 5.26 5.23
39480 37600 39420 37600 361 50 42900 43920 38240 44200
6740 7010 6580 6500 6320 7220 641 0
7.5 7.5 7.1 7.7 7.7 7.7 7.8
46090 6550 741 0
45680 35800
8.1 7.6
Reference 1. Reference 3. Reference 6 TABLE I I : Parameters of the Simplified Mulliken Theory for Pyridine-Bromine Complexes (Energy Terms in eV)
Substituent of pyridine 3-81: 2,6-Me2 2-Et None 3-Et 3-Me 4-Me 4-Et 3,5-Me2 2,4-Me2 2,4,6-Me3
Fi
0.338 0.410 0.425 0.430 0.435 0.441 0.458 0.459 0.476 0.476 0.481
a
b
wo 1
wo
w1
0.745 0.691 0.680 0.675 0.671 0.666 0.654 0.653 0.639 0.639 0.635
0.481 0.544 0.558 0.562 0.567 0.572 0.586 0.587 0.601 0.601 0.604
- 1.89 - 1.94 - 1.96 - 1.99 - 1.98 - 1.99
0.80 1 .oo 1.05 1.10 1.07 1.12 1.16 1.17 1.24 1.22 1.25
2.40 1.87 1.77 1.78 1.70 1.68 1.57 1.56 1.46 1.44 1.43
distance for a given F1 value for ortho-disubstituted complexes also explains the lowering of the h N . . . B r force constant in this case. It can be pointed out here that the extrapolated value of -2.04 eV at F1 = 0.5 seems to be little dependent on the approximations made in calculating the weight of dative structure. For instance, if in eq 12 the term k g r - ~ r - O / h ~ r - ~ r O is disregarded the extrapolated value becomes -1.95 eV. If the overlap integral is 0.2 instead of 0.3 a value of -2.10 is found. It must be noted here that, under these circumstances, all the values of Wol of Table I1 will be shifted in a similar manner and that the calculated effect of ortho substituents on Wol will remain the same. When the values of WN in Table I are compared with those of F1 in Table 11, the data are observed to obey the following relation W , = -1.07F1 (max dev 0.015 eV) (20) This linea; relationship between the complexation enthalpy and the weight of dative structure was observed by Tilquin3 and holds for all the complexes studied here. Although this result involves a long linear extrapolation it seems unlikely that it is the consequence of good luck. This result can be expected when the classical, noncharge-transfer forces between donor and acceptor are equal to the separate solvent-solute forces. However, as pointed out by the reviewer of this work, this result does The Journal of Physical Chemistry, Vol. 77, No. 73, 7973
-1.99 - 1.99 -2.01 -1.99 -1.98
not exclude the possibility that experimental WN values, and consequently, the calculated WO,W1, and Wol could involve some solvent contribution. Owing to the fact that in the absence of charge transfer WOequals WN, Wo must also vanish when FI equals zero. However, it is not possible to fit the experimental data in Table I1 with a linear relation without a constant term. It is necessary to introduce a quadratic term. As a matter of fact, all the experimental points obey the relationship
Wo
= 1.80F1
+
1.68FI2 (max dev 0.02 eV) (21)
Let us first consider the case of the complexes without ortho substituents. Owing to the very similar neighborhood of the N-a-Br-Br group it may be expected that the stabilizing energy WO,of the no-bond structure varies in the same manner with the N-Br distance for all these complexes, This means that for all the complexes without ortho substituents Wo depends only on r. We can then write
When F1 increases the N-Br interdistance decreases, passing from the sum of van der Waals radii of the N and of the Br atom (1.5 1.9 = 3.4 A) in the absence of charge transfer to an order of magnitude corresponding to
+
Charge-Transfer Spectra of Bromine Complexes
1665
+
of pyridine and of substituted pyridines show differences of several hundredths of a n electron volt. Moreover, there is some doubt in the attribution of the values to the loss of an n or a e l e ~ t r o n . ~ On the other hand, there must exist some correlation between the ability of a base to take up a proton from water and the energy needed to lose an electron. We may therefore expect that the deviations predicted in the relations of F1 us. IDshould also appear in the relations of F1 us. the pKa of the donor. This behavior is in fact observed where F1 is plotted against the values of PKa. While for nonortho-substituted complexes the relation
the sum of the N+ radius and of the Br- radius (0.25 1.95 E 2.20 A) when the charge transfer is complete. From this point of view it is noticeable that the force constant k~ ...B ~ ,which is related to the equilibrium distance rN ...Br, is found in the previous work to be proportional to F1. The variation of the slope dWo/dF1 when F1 increases can then be related to the marked increase in the repulsion force (sWo/sr) when the interdistance N-Br decreases. Another noticeable point is that the ortho-substituted compounds do not show deviations from eq 21, even though owing to the deductions concerning Wol the N-Br interdistance must be greater for the same values of F1. This can be explained if the lowering of the repulsion energy between the N and the Br atom arising from the increase of the distance is somewhat compensated by a new repulsion term arising from the interaction between the ortho methyl group and the bromine atom. Owing to the fact that, for F1 = 0.5, W1 must be equal to WO,all the experimental values fit with the equation
F,
+
+ IDv- E
A
where I D V is the vertical ionization potential of the donor and E A V is the electron affinity of the acceptor. EA" depends on Fl, passing in the gas phase from a value of -1.2 eV8 when F1 = 0 and the Br-Br interdistance is that of the neutral molecule, to a value of -2.6 eV when FI = 1 and the Br-Br interdistance is that of the Brz- ion. These values must be corrected when passing from the gas to the solution. However, at a given value of F1, the electron affinity E A V must show a given value, independent of the other terms of eq 24. For ortho-substituted and -disubstituted compounds GI must show positive deviations: the repulsion energy of the interaction of the methyl group and Brz- must be taken into account (and may be more important than in the case of WOowing to the increase in the van der Waals radius of Br- comparative to Br) but in addition the increase in the N-..Br interdistance must here have a n opposite effect relative to WO, reducing the attraction energy between the charges and the exothermic polarization effects to the ions upon the molecule of the partner. If there is no marked difference in W1 for a given value of FI, it must then be concluded that the reduction in GI is compensated by the lowering of ID'. I n order to obtain the same W1, the same weight of dative structure F1 and the same WN value for ortho-substituted and ortho-disubstituted complexes, the ionization potential of the donor has to be lower. This should therefore appear in the curves of FI us. ID.Unfortunately, the data of the literature concerning the ionization potentials
+
0.040pKa (max dev 0.008) (25)
holds, the disubstituted compounds show deviations as high as 0.08 and the deviations for the monosubstituted ortho derivatives are significant, These deviations of F1 us. PKa for ortho-substituted compounds produce deviations in WO,WI, and Wol. However, the deviations of Wol compensate these of WO and WI in expression 19 in such a manner that eq 5 still retains its validity in the same case of ortho-substituted compounds. In expression 13 the positive deviations of ortho-substituted complexes of W1 for a given PKa value are not compensated by the negative deviations of WO (and to a smaller extent by the deviations of Wol) so that the complexation energy for these complexes remains lower. It can consequently be concluded that the deviations of WN us. pKa for the ortho-substituted complexes are due to the increase in the energy W1 of the dative structure brought about by the additive repulsion between the acceptor and the methyl group and by the increase in the N+-Br- interdistance that is not entirely compensated by the decrease in the energy WOof the no-bond structure. Owing to eq 20, expression 9 can be written
W1 = 5.66 -. 11.65F1 5.94F12 ( m a x dev 0.03 eV) (23) It must be noted here that this relation cannot be extrapolated to low values of F1. W1 is related to the stabilizing energy GI of the dative structure'relative to the free ions by the expression W1 = G,
= 0.220
,
The transition moment is proportional to the weight of dative structure.
Experimental Section The spectroscopic measurements were made with the Unicam SP 700 spectrophotometer, with a monochromator held at constant temperature. Pyridine and its methyl and ethyl derivatives were Fluka purissium products. 3-Bromopyridine was a Reilly purum product. These compounds were distilled before using. Bromine was a Fluka purissimum product. The solvent CH&'l-CH&l was a Merck purissimum product. Acknowledgment. The authors are indebted to the Instituut voor Aanmoediging van het Wetenschappelijk onderzoek in de Nijverheid en in de Landbouw and to the Fonds voor Kollektief Fundamenteel Onderzoek for their financial support for this work. (8) W. B. Person,!;
Chem. Phys., 38, 109 (1963). (9) D. W. Turner, Molecular Photoelectron Spectroscopy," Wiley-interscience, London, 1970, p 324.
The Journal of Physical Chemistry, Vol. 77, No. 13, 1973
