Xay, 1963
IONIZATION POTEXTIALS OF
The structures with two adjacent charges would be much less probable due to electrostatic repulsion. Confirmation of the form (11) is supplied by the lower -AHl.-z value for the trien (22.30 kcal./mole) as compared to that of diethylenetriamine (23.16 kcal./mole). I n fact if only form (I) did exist one would expect a greater heat of formation for trienHzz+than for clenH2+, due t o the lower electrostatic repulsion. The low - A l l s value (9.50 kcal./mole) may be attributed to the preferential protonation of the secondary nitrogen and a greater electrostatic repulsion of the positive charges. These two factors may also account easily for the sequence --AHs (trien) > .AHB(den) > --AH, (trien) . The entropy changes for the first neutralization stage are positive whereas those for the successive steps decrease till they become negative. Such a trend has already been found for the neutralization of ethylenediamine and diethylenetriamine2 arid for other stepwise reactions as the formation of metal complexes with neutral’*l1or anionic ligands.12 (11) hl. Ciampolini, P. Paoletti, and L. Saci.oni, J . Chem. Soc., 4553 (1960). (12) J. K. Kury, -4.D Paul, L. C. Iiepler, and R. E. Comirk, J . Am. Chem. Soc., 81,4186 ( l e s s ) , P.K. Gallagher and E. L. King, tbzd., 82, 3510 (1960).
1067
AZINES
en
den
trien
Pi P
I 12
-AH,
(kcaljmole). 11
1
0
Ratlo Nsec./Ntotai,
Fig. 1.-Plot of -AH1 ( 0 )and A& ( A ) against the ratio of the number of secondary nitrogen atoms to the total number of nitrogen atoms.
Acknowledgmen ts.-The authors are indebted to Prof, L. Sacconi for helpful suggestions and criticism. Acknowledgment is made to the Italian “Consiglio Nazionale delle Flicerche” for the support of this research.
CHARGE TRASSFER IKTERACTION BETWEEN IODIXE AXD AZIXES : IONIZATION POTENTIALS OF AZINES1 BY V. G. KRISHNA~ AND MIHIRCHOWDHURY Whitmore Chemical Laboratory, T h e Pennsylvania State University, University Park, Pennsylvania Received November 1, 1962 Charge transfer data on four azine-iodine complexes are reported. Azines are shown to be n-donors toward iodine. The trend of the charge transfer maxima indicates that the experimentally observed ionization potentials of the azines correspond to r-electron ionization.
Introduction The following note will be concerned with the complexes formed between iodine and monocyclic azines : pyridine, pyrimidine, pyrazine and s-triazine. The aspects considered are (1) the perturbation of the visible band of iodine and the ultraviolet and infrared spectra of donor, (2) the equilibrium constants of the molecular complexes formed, and (3) the energy and extinction coefficient of the charge transfer (CT) bands. Previous studies3y5have indicated that the N-heterocyclics are n-donors toward iodine. I n the first ~ W O a thorough investigation of the pyridine-iodine complex and in the third5 the electron donating properties of substituted monoazines have been reported. The n-donor properties of monoazines are indirectly perturbed by the n-electronic structure.6 I n the present case of polyazines, in addition to the variation of the ~ ~ interaction of the nn-electronic s t r u c t ~ r e , ’the (1) This investigation was supported by a grant from the Air Force Office of Scientific Research t o the University. (2) Institute of Molecular Biophysics, The Florida State University, Tallahassee, Florida; from whom reprints can be obtained. (3) C. Reid and R. S. Mulliken, J . A m . Chem. Soc., 7 6 , 3869 (1954). (4) E. K. Plyler and R. S. Rlulliken, ibid., 81, 823 (1959). ( 5 ) J. Nag-Chaudhuri and 5. Basu, Trans. Faraday Soc., 55, 898 (1959). ( 6 ) H. C. Longuet-Hipgins, J . Chem. Phys., 18, 275 (1950). (7) N. Mataga and K. Nishimoto, Z. physik. Chem. (Frankfurt), 18, 140 (19573.
orbitals also changes markedlyg and the n-donor properties of these molecules should reflect both these changes. An important aspect of this investigation is its relation to the ionization potentials of azines. The experimentally observedlO-ll ionization potentials of azines have received a varied interpretation by different authors as due to n-lo-l3and n-electron7-l4ionizations. A more unambiguous interpretation is needed before the ionization potential data could be used as a n ex~ perimental criterion for various theoretical models proposed for K-heterocyclics. I n the following investigation an attempt will be made to resolve this ambiguity through the use of charge transfer data. A linear relation between the charge transfer maxima and the ionization potentials of the donors in a series of molecular complexes with a common acceptor has been e~tablished.~5.’6Since the azines are n-donors toward (London), 1170, (8) R. McWeeny and T. E. Peacock, Proc. Phys. SOC. 41 (1957). (9) L. Goodman, J . Mol. Spectry., 6, 109 (1961). (10) I. Oinura, H. Baha, K. Higasi, and Y. Kanaoka, Bull. Chcm. SOC. Japan, 30, 633 (1957). (11) K. Higasi, I. Omura, and H. Baba, J . Chem. Phys., 24, 623 (1956). (12) T. S a k a j i m a and A. Pullman, J . chzm. phys., 66,793(1958). (13) IC Watanabe. T. Nskayama, and J. afottl, quoted in ref. 14. (14) &I. A. El-Sayed, M. Kasha, and Y. Tsnaka, J . Chem. Phys., 34, 334 (1961).
V. G. KRISHNA AND MIHIRCHOWDHURY
IO68
4
1
1
5200 4800 4400 4000 WAVELENGTH. Fig. 1.-Perturbation of the 520 mp iodine band by pyrazine. Concentration of iodine, 5.06 X 10-4 M; concentration of pyrazine: (A) 0; (B) 4.11 X 10-* '34; ( C ) 9.50 X 10- Z M . Cyclohexane is used as solvent.
6000
5600
iodine, the trend in CT maxima of the azine-iodine complexes should indicate the trend of n-ionisation potentiah of azines. Experimental A . Materials.-Absorption spectra were recorded on Warren Electronics, Inc., Spectrocord with fused silica cells of optical path 2.0 and 0.1 cm. The latter cells were used to investigate the CT bands and the perturbation of the donor bands with excess iodine. The rest of the absorption measurements were carried out with 2.0-cm. cells. All measurements were performed a t room temperature. Infrared spectra were measured with PerkinElmer Model 21 spectrophotometer. Matheson, Coleman and Bell Spectroquality cyclohexane was used as solvent without further purification. The azines used were pyridine (Eastman Kodak, Spectrograde), pyrazine (Aldrich Chemicals), pyrimidine (Nutritional Biochemicals), and striazine. The sample of s-triazine was previously obtained as a gift from Dr. R. C. Nirt of American Cyanamid Company. Pyridazine was found unsuitable for investigation: pyridazineiodine solutions developed turbidity rapidly. B. Procedures. 1. Shifted Iodine Band and K Values.Visible spectra of cyclohexane solutions with fixed concentration of iodine but different amounts of donor were recorded with cyclohexane as blank. The equilibrium constants listed in Table I were calculated-assuming a 1: 1 complex-from the decrease in optical density at 520 mp. Any absorption due to the shifted iodine band at 520 mp could be corrected because of its Ganssian shape.17 Each K in Table 1 is an average of values on fifteen different solutionB, except in the case of s-triazine, where it is an average of only two values. The emex of shifted iodine bands were obtained from the optical density at the band maximum and the concentration of the complex. 2. CT Bands.-The CT bands of a,zine-iodine complexes were found in the spectral region where both donor and acceptor have considerable absorption (Fig. 2). For obtaining the wave lengths and extinction coefficients of CT bands a procedure similar to that of Reid and Mulliken3 was used. Spectra of cyclohexane solutions with excess of donor and small amounts of iodine were recorded; cyclohexane solutions with an equal amount of donor were used as blanks. The absorption due t o uncomplexed iodine was subtracted to get the absorption of azine-iodine complex. Since the light intensities reaching the phototube were low due to the donor absorption, high gain settings were used. CT band maxima could be located with the same accuracy as in the ordinary ultraviolet spectral measurements. However (15) H. McConnell, J. S. Ham, and 3. R. Pllrstt, J. Chem. Phys., 11, 66 (1953). (16) S. H. Hastings, J. L. Franklin, J. C . Schiller, and I'. 4.Matsen, J . Am. Chem. Soc., 76, 2900 (1953). See ref. 23 for further examples. (17) Thib procedure cannot give accurate K for s-trlazine where the shift is small and the complexation is poor. I n spite of this inaccuracy, i t Can be compared with other K values in Table I.
1701. 67
the extinction coefficients measured by this procedure could not be accurate. An error of 25y0wae estimated. CT spectra of three different solutions were recorded for each azine. The C T bands of s-triazine could not be detected. 3 . Perturbation of Donor Bands.-The perturbation of the n -+ T * bands and the first T -+ T * bands of the donor was studied by measuring the absorption of the donors in a concentrated iodine solution against an iodine blank of an equal concentration. Donor to iodine ratio was 1 :60 for n + T * absorption at 330 m p and 1:10 for T -+ T* absorption at 260 mp. Bbsorption of iodine a t shorter wave lengths permitted only ten times excess of iodine in the region of 7r + T* spectra.
Discussion The Shifted Iodine Band.-Iodine in non-polar solvents shows a visible band with the maximum at 520 mp. In presence of the donors this band undergoes a blue shift. A few spectra of the pyraziiie-iodine system are presented in Fig. 1. Siiice the rest of the spectra are similar they will not be reproduced. Table I shows that the blue shift of the visible hand of iodine on complexation follows the order of the K values of the complexes and the pK, values of the donors. However neither the n -+ T* nor the T -+ T* absorption of the donors showed aiiy perturbation with excess of iodine concentratioa.la The infrared spectra of pyraaiiie (* 0.2 M)in carbon tetrachloride between 3 and 15 4 showed very little perturbation with iodine concentrations up to 0.2 11.1. These facts indicate that the blue shift of the iodine band is mainly due to a repulsive interaction in the excited state of iodine. This is consistent with Nulliken'slg explanation of the blue shift of the 520 mp band of iodine. TABLE I PERTURBATIOX OF THE VISIBLEBANDOF IODINE Donor
P&"
K
Shifted iodine band Ah
fIn.%X
Pyridine 5.2 9Bb 100 2150 1.3 17 90 1170 Pyrimidine 0.6 12 80 1250 Pyrazine 0.0 2 50 .. +Triazine a Taken from: S. F. Mason, J . Chern. Soc., 1240 (1969). Previously reported values for the K of pyridine-iodine complex a t room temperature are: 160 (ref. l), 45 (ref. 3), 107 (A. G. Maki and E. K. Plyler, J. Phys. Chenz., 66, 766 (1962), and 101 (A. I. Popov and R. H. Rygg, J . A m . Chem. Soc., 79, 4622 (1957)). The K increases with pyridine concentration beyond a pyridine to iodine ratio of 100.
From Table I it can be seen that the emox of the shifted iodine band increases with the stability of the complex. The explanation for this phenomenon is lacking at present. Borrowing of intensity from CT transition has been suggested as a possibility.20 Ionization Potentials.-Table I shows that K values of the azine-iodine complexes follow the trend of pKa values of the donors. Thus the electrons that enter into hydrogen bond and CT interaction should be of the same type. The azines, then, should be n-donors and not a-donors. A similar conclusion has been drawn in two previous investigation^.^,^ The CT maxima of azine-iodine complexes are: pyridine-235 mp, pyrazine-242 mw and pyrimidine237 and 246 mp. (18) R. P. Lang. J . Am. Chem. S o c . , 84, 1186 (1962). I n this paper a blue shift of the thioacetamide band a t 269 mp by 3250 c m . 7 on complexation with iodine has been reported. The thioacetamide-iodine complex is much stronger than azine-iodine complexes studied here. .4 greater interaction between donor and acceptor in the pround states is PO& sible in the former aase. (le? R. S.Mulliken, Ree. trau. chzm , 76, 846 (1956). (20) H. Tsubomura and R . P. Lang, J. Am. Chem. Soc., 83, 2085 (1961).
May, 1963
IONIZATION POTENTIALS OF AZINES
The two maxima for pyrimidine-iodine complex indicate two CT transitions. The width of pyrimidineiodine CT bands, which is about twice that of pyridineiodine band support the view of two CT transitions. We interpret them, tentatively, as the two CT transitions corresponding to two non-bonding MO’s. The energy difference of 1600 cm.-l between the two bands is higher than what is expected of n orbital interaction alone. The fact that the center of gravity of the two CT bands of pyrimidine-iodine system does not coincide with the CT maximum of pyridine-iodine complex shows that this split of 1600 cm.-l involves more than n orbital interaction. I n pyrazine-iodine system only a single absorption band is observed. This does not necessarily indica,te the lack of two CT transitions. The energy difference of two C T bands in pyrazine-iodine systems corresponding to the two non-bonding MO’s should be lower than in pyrimidine-iodine system. The resolution of two broad CT bands with small energy difference would not be possible a t the high slits used. The broadness of the pyrazine-iodine CT band, which is comparable to that of pyrimidine-iodine CT bands, strongly indicate two CT transitions. The observed CT band maximum, then, is the average of the two CT maxima. Two different (n, r*) states in pyrazine with an energy difference of about 500 ern.-' have been observed experimentally.21-22 Using this value for energy difference of the two non-bonding MO’s, we infer that the maxima of two CT transitions in pyrazine-iodine complex should be at 244 and 241 m r . However, the conclusions to be drawn in the following paragraph are independent of the value chosen for the energy difference of the two non-bonding MO’s of pyrazine as long as it is less than the energy difference of the two non-bonding MO’s in pyrimidine. Now, it can be seen that for the CT bands involving the higher of the two non-bonding MO’s the CT maxima of azine-iodine complexes follow the donor order : pyridine > pyrazine > pyrimidine; with the CT bands involving the lower of the non-bonding MO’s the order is the pyridine = pyrimidine > pyrazine. The same order is expected for the n-ionization potentials of these compounds from the theoretical relation between CT maxima and ionization potentials.15,18 The ionization potentials observed by electron (21) M. Ito, R. Sbimada, T. Kuraishi, and M. Mizushima, J. Chem. Phys.1 26, 1508 (1957).
(22) M. A. El-Sayed and G. W. Robinson, MOL Phys., 4, 273 (1961).
I
1069
I
I
I
5.0 -
‘
I
4.0 -
ug3.0-
-0 0
2.0-
I /f I.Or
2700
2500 2300 WAVELENGTH.
2100
Fig. 2.-Charge transfer bands of (A) pyridine-iodine, (B) pyrimidine-iodine, and (C) pyrazine-iodine systems. See text for the details of the experimental procedure.
impact method follow the order: pyridine < pyrimidine < pyrazine. The trend of experimentally observed ionization potentials differs from that of the n-ionization potentials inferred from the CT spectra. These considerations indicate that the observed ionization potentials in azineslO* l1 correspond to r-electron ionization but not to n-electron ionization. More experimental work on ionization potentials and CT maxima of different azines and their derivatives will serve to make this conclusion more trustworthy. Figure 2 shows that the tmax of the CT band decreases with the stability of the complex. This anomalous relation has been noted before in several cases. 23 A detailed explanation is lacking at present even though two mechanisms were p r o p o ~ e d . ~ * ~ ~ ~ Acknowledgments.--We wish to express our indebtedness to Professor Lionel Goodman for constant encouragement and for his helpful criticism of the manuscript. Our thanks are also due t o Miss Beverly Jamattona for translating the article by Nakajima and Pullman. (23) S. P. McGlynn, Chem. Rev., 58, 1113 (1958). (24) 1,. E. Oreel and R. 8. Illulliken. J. Am. Chem. Soc., 79,4839 (1957 ). (25) J . N.Murrell, %bid.,81,5037 (1959).
