AZAAROMATIC CHARGE-TRANSFER COMPLEXES
931
Azaaromatic Charge-Transfer Complexes with s-Electron Acceptors: Evidence for Formation of (n, s) Complexes
by David R. Kearns,' Phillip Gardner, and John Carmody Department of Chemistry, University of California, Riverside, California
(Received September 19, 1966)
The spectroscopic properties of complexes formed between eight azaaromatic donors and the s-electron acceptors trinitrobenzene, p-benzoquinone, and tetrachloro-1,Pbenzoquinone have been investigated a t 77°K. From a consideration of donor ionization potentials, energy of the charge-transfer bands, and substituent effects on the energy of the charge-transfer absorption bands, it is concluded that quinoline, 6-bromo-, 8-bromo-, 1,2,3,4-tetrahydro- and 2-methylquinoline, and isoquinoline function as n-electron donors, whereas acridine and 8-hydroxyquinoline function as s donors. Azine complexes with dichlorodicyano-p-benzoquinone appear to be ionic.
Introduction Although there is a variety of different types of electron-donating molecules (s,n, and u ) , ~most of the work on charge-transfer complexes has involved the use of aromatic hydrocarbons which function only as s-electron donors. In spite of their importance in biology and elsewhere, there have been surprisingly 'few investigations of the properties of donor molecules containing nonbonding electrons (n electron^).^ The few studies which have been carried out with this latter group of donors have usually involved molecules such as amines, ethers, or alcohols which function only as n donor^.^^^ The azaaromatics are a particularly interesting class of donor molecules, because potentially they can function either as n donors or as A donors. Theory suggests that the azines should function as n donors with a-electron acceptors because overlap between n and a orbitals is larger than the overlap between s and u orbitals, and this seems to be borne out by experiment.6-g When A acceptors are used, however, it is not clear from overlap considerations alone whether azines should function as n or as 7~ donors. Unfortunately, the experimental situation is not much clearer. Although the electron-donating properties of azines have been the subject of several investigations, the results appear to be somewhat inconclusive.10-13 Miller and Wynne-Jones,11J2 for example, examined
the spectral properties of pyridine complexes with the s-electron acceptor trinitrobenzene (TNB) and found that these systems were chemically unstable at room temperature. Although they presented some evidence for the formation of a short-lived pyridine-TNB charge-transfer complex, even they did not consider the evidence conclusive. From an investigation of azine complexes with the a-electron acceptor tetrachlorophthalic anhydride (TCPA) , Chowdhury and Basu concluded that the azines were functioning as ?T donors rather than as n donors.' Mukherjee and (1) Alfred P. Sloan Fellow. (2) R. 9. Mulliken, J. Phys. Chem., 5 6 , 801 (1952). (3) R. S. Mulliken and W. B. Person, Ann. Rev. Phya. Chem., 13, 107 (1962). (4) M. Tamres and M. Brandon, J . A m . Chem. SOC.,82, 2134 (1960). (5) G. Briegleb, "Elektronen-Donator-Acceptor-Komplexe,"Springer Verlag, Berlin, 1961. (6) V. G. Krishna and M. Chowdhury, J. Phya. Chem., 67, 1067 (1963). (7) M. Chowdhury and S. Basu, Trans. Faraday Soc., 56, 335 (1959). (8) E. K. Plyler and R. S. Mulliken, J . A m . Chem. SOC.,81, 823 (1959). (9) C. Reid and R. S. R'lulliken, ibid., 76, 3869 (1954). (10) M. Chowdhury, J . Phge. Chem., 65, 1899 (1961). (11) R. E. Miller and W. E". K. Wynne-Jones, J. Chem. SOC.,2372 (1959). (12) R. E. Miller and W. F. K. Wynne-Jones, ibid., 4886 (1961). (13) D. C. Mukherjee and A. K. Chandra, SpeCtTOChim. Acta, 19,731 (1963).
Volume 71, Number 4
March 1967
D. R. K E A R N P. ~ , GARDNER, AND J. CARMODY
932
Chandra,13 on the other hand, assigned the weak absorption bands which they observed in azine haloquinone solutions to (n, ). charge-transfer complexes. Certainly it is possible for an azine to function as an n donor with one IT acceptor and as a T donor with a different IT acceptor, but there were other aspects of the above-mentioned experiments which were disturbing. I n the first place, most of the authors noted that the azine-acceptor solutions were unstable at room temperat~re;"-'~ secondly, in certain of the experiments, the spectra of the complexes were obtained by a difference method; thirdly, with the haloquinoneazine complexes, the absorption bands which were assigned as charge-transfer transitions of (n, T ) complexes were extremely weak (emax < 1).ls Because of the lack of definitive experimental information on the properties of azine-.-acceptor complexes, we decided to carry out a more thorough investigation of this type of charge-transfer complex. The multiple electron-donation properties of the azines and the possible importance of such complexes in biological systems provided &%her inducement for this study. Experimental Section All spectra were measured with a Cary 14 speetrometer using either a 1-cm cell or 0.2-cm "lollypop." Through the use of solvent mixtures which formed glasses at liquid nitrogen temperatures, we were able to carry out spectral measurements at 77OK. For these low-temperature measurements, the lollypop was first filled with a glassing solution of the donor and acceptor molecules. This cell was then placed in a partially silvered dewar which was filled with liquid nitrogen and mounted in $416 sample compartment of the Cary. Spectrograde d v e n t s were used without further purification, except io the case of ethyl ether where it w:ts necessary to carry out a d i s t i l W n to remove peroxides formed during storage. Quinoline and other liquid quinoline derivatives were purified by low-temperature vacuum distillation of the commercial material. Acridine, 8-hydroxyquinoline1 and the acceptors tetrachloro-114-benzoquinoneand TNB were purified by recrystallization. Dichlorodicyano-p-benzoquinone (DDQ) and p-benzoquinone were used as obtained commercially.
Results and Discussion Since previous studies by others and preliminary investigations of our own demonstrated that mixtures of azines with various acoepbm were unstable at room temperature, most of our spectral measurements were carried out a t 77°K to prevent chemical reactions The Journal of P h & d
Chemistry
from occurring. This was a particularly fortuitous decision, because complex formation was found to be highly temperature dependent. Most of the complexes were stable only at the reduced temperatures. Upon cooling to 77°K the azine-acceptor solutions usually became brightly colored, and the low-temperature absorption spectra of these solutions are presented in Figures 1-5. The donor and acceptor molecules were transparent in the spectral regions of interest for most of the complexes studied, although there was significant "tail" absorption in the case of the TNB complexes and the acridine complexes. The spectral properties of these donor-acceptor solutions are summarized in Table I along with similar data for the parent hydrocarbon complexes with the same acceptor. Table I: Position of Charge-Transfer Band of Azine-*-Acceptor Complexes (mp) Acceptor
Donor
Naphthalene (8.12 ev)" Quinoline (8.2 ev) Quinaldine Isoquinoline 6-Bromoquinoline 8-Chloroquinoline 1,2,3,4-Tetrahydroquinoline 8-Hydroxyquinoline (8.1 ev)" Anthracene (7.38 ev) Acridine (7.78 ev)
-TNB-Predb Obsd
p-Benzo-quinone Pred Obsd
Tetrachloro1,Cbenzo-quinonePred Obsd
370
370
480
355
505
350
500 510
510
...
565
...
370
,
..
460
457
...
...
605 635 650 570 590 650
...
...
400
450
530
... 370
520
480
500 625
525
500
'
' The n-ionization potential given in parentheses. Predicted location of the T-T charge-transfer band obtained using the Cz)/(I.P. - CI), where I.P. expression,' ~ V C T= I.P. - (C1 is the gas-phase r-ionization potential of the donor and where C, = 5.7 ev, Cz = 0.44ev for tetrachloro-1,Pbenaoquinone and Cl = 5.0 ev, C2 = 0.7 ev for trinitrobenzene. Estimated.
+
Evidence for Reuersible Formation of Charge-Transfer Complexes. Because of the known room-temperature instability of the azine-acceptor solutions, it was important first to establish that the colored species which we observed a t 77°K were actually reversibly formed donor-acceptor complexes and secondly, that the complexes were actually charge-transfer complexes. The reversibility of the complex formation was established by the following experimental observations.
AZAAROMATIC CHARGE-TRANSFER COMPLEXES
2,kO
4d00
sob0
5d00
6dOO
6dOO
7obO
933
7/00
WAVELENGTH t i )
WAVELENGTH
Figure 1. Absorption spectra of charge-transfer complexes with tetrachloro-1,Pbenzoquinone at 77°K (0.2-cm cell): a, 0.3 M isoquinoline and 5 X 10-4 tetrachloro-1,Pbenzoquinone in 3: 1 ether-isopropyl alcohol; b, 0.3 M quinoline and 4.3 x 10-4 M tetrachloro-l,4benzoquinonein 3: 1 ether-isopropyl alcohol; c, 0.3 M quinaldine M tetrachloro-1,4benzoquinone and 4 X in 3: 1 ether-isopropyl alcohol.
>-
.5
(A)
Figure 3. Absorption spectra of charge-transfer complexes with trinitrobenzene at 77'K: a, 0.3 M isoquinoline and 5 X 10-2 M TNB in 3 : 1 ether-isopropyl alcohol, 1-cm cell; b, 0.3 M quinoline and 5 x 10-2 M TNB in 3: 1 ether-isopropyl alcohol, 0.2-cm cell; c, 0.3 M quinaldine and 5 X 10-2 TNB in 3 : 1ether-isopropyl alcohol, 0.5-cm cell.
-
5000 WAVELENGTH 4000
6000
7000
6000
5000
WAVELENGTH
(A)
(8)
Figure 2. Absorption spectra of charge-transfer complexes with tetrachloro-1,Pbenzoquinone a t 77°K (0.2-cm cell): a, 1,2,3,4tetrahydroquinoline and tetrachloro-1,Pbenzoquinone in 1:1:0.4 ethanol-tetrahydrofuran-methanol; b, 0.4 M 8-chloroquinoline and 2 X lo-* M tetrachloro-1,4benzoquinone in 1:1:0.4 ethanol-tetrahydrofuran-methanol ; c, 6-bromoquinoline and 2 X 10-8 M tetrachloro-1,4benzoquinone in 1:1:0.4 ethanol-tetrahydrofuran-me thanol.
When the azine-acceptor solutions were warmed from 77 to 200°K, the intensity of the visible absorption bands always decreased (factor of -lo), but upon recooling to 7 7 O f ( , the solutions immediately returned to their original intensity; secondly, when the concentration of the acceptor was reduced, the intensity
Figure 4. Absorption spectra of charge-transfer complexes with p-benzoquinone a t 77'K: a, 1,2,3,4tetrahydroquinolineand p-benzoquinone in 1:1:0.4 ethanol-tetrahydrofuran-methanol; b, quinaldine and p-benzoquinone in 1: 1:0.4 ethanol-tetrahydrofuran-methanol; c, 6-bromoquinoline and p-benzoquinone in 1:1:0.4 ethanol-tetrahydrofuran-methanol ; d, quinoline and p-benzoquinone in 1:1:0.4 ethanol-tetrahydrofuran-methanol.
of the visible absorption band decreased. Since the solutions always contained an excess of the donor molecules, there was generally no significant change in the intensity of the visible absorption band when the concentration of the donor was varied over a small range. Finally, we noted that although the solutions appeared to be indefinitely (days) stable at Dry Ice temperature, they were not stable at room temperature. This room-temperature instability was evidenced by the Volume 71, Number 4 March 1967
934
O
D. R. KEARNS,P. GARDNER, AND J. CARMODY
5r--
i
04-
t k 03-
z 0 J
. l
:o z I-
O
O I -
L
'3000
3500
4000
4500
5000
L
5500
WAVELENGTH
6000
6500
7000
J
7500
lil
Figure 5. Absorption spectra of charge-transfer complexes (0.2-cm cell): a, S-hydroxyquinoline and 3 x 10-2 AT tetrachloro-l,4-benzoquinonein 1: 1 : 0.4 ethanol-tetrahydrofuran-methanol a t room temperature; b, 10-2 LIT acridine and 2 X 10-8 M tetrachloro-1,4-benzoquinone in 1 :1 ether-isopentane; c, 8-hydroxyquinoline and TNB in 1 : 1:0.4 ethanol-tetrahydrofuran-methanol a t room temperature.
slow appearance of new, temperature-independent absorption bands and by the disappearance of the lowtemperature absorption bands which we have attributed to formation of a complex. These various observations established that the new light-absorbing species which we observed a t low temperature were formed reversibly as the result of an interaction between the azines and the electron acceptors and that the chemical reactions which occurred at or near room temperature were detrimental to the formation of the complexes. Although the temperature and concentration dependence of lhe low-temperature absorption bands definitely indicated reversible formation of complexes, these data did not distinguish between the various types of complexes (ion-pair, charge-transfer) which might be formed. Evidence that the low-temperature azineacceptor complexes are charge-transfer complexes is obtained from a consideration of variation of the energy of the complex absorption band with the nature of the acceptor and the donor. If the absorption bands of the complex were due to charge-transfer transitions of ion pairs, the bands should blue shift, with increasing acceptor strength. From the data presented in Table I, it is evident that with a given donor, the stronger the electron acceptor, the lower the energy of the transition. This shift in the absorption is opposite that expected for a chargetransfer transition of an ion-pair complex, but as expected for the charge-transfer band of a complex between neutral molecules.6 The Journal of Physical Chemistry
The spectral data definitely rule out the possibility of ion formation in the case of the tetrachloro-1,4benzoquinone complexes. l4 The tetrachloro-1,4-benzoquinone radical anion has absorption bands a t 425 and 455 mp,14916whereas the tetrachloro-1,4-benzoquinone complexes which we studied exhibited absorption bands in the region 570-650 mp, with minima in the 450-mp region. Similar consideration applies to the pbenzoquinone complexes, since the benzoquinone radical anion should have a spectrum only slightly blue shifted from that of the tetrachloro-1,4-benzoquinone radical anion.I4 The spectrum of the T N B radical anion has not been reported so that it is not possible to apply the same test in the case of the TNB complexes. However, since the benzoquinone complexes are not ionic, it is unlikely that the corresponding complexes with the weaker electron acceptor TNB will be ionic.16 These various considerations all indicate that the azine-donor complexes are charge-transfer complexes in the usual sense. Nature of the Charge-Transfer Complex. The next problem was to determine whether the complexes were (n, a) or (a, T) type of charge-transfer complexes. In order to distinguish experimentally an (n, T) from a (a, T) complex, it is necessary to find some property or set of properties which are predictably different for the two types of complexes. Because of our inadequate knowledge of the properties of (n, T) complexes, most of the properties which one might like to use (heat of formation, molar extinction coefficient, solvent effects on A), to distinguish between (n, a) and (T,T) complexes are not useful for this purpose. With the exception of an X-ray structure determination, about the only reliable criterion which we have for distinguishing between these two types of complexes is based on the relation between the energy of the chargetransfer transitions and the a-ionization potential of the d o n ~ r . ~ , ~ From previous studies of (a, T) charge-transfer complexes it has been established that there is a fairly linear relation between the a-ionization potential of the donor and the frequency of the charge-transfer band. Consequently, if the a-ionization potential of a particular donor is known, it is possible to predict with a fair degree of accuracy the energy of the chargetransfer band for a (a, a) complex of this donor with a particular a a ~ c e p t o r . ~Accordingly, we have used (14) R. Forster and T. J. Thomas, Trans. Faraday SOC.,58, 860 (1962). (15) I. M.Kolthoff, D. Stocesoca, and T. S. Lee, J. Am. Chem. SOC., 75, 1834 (1953). (16) G.Briegleb, Angew. Chem., 76, 326 (1964).
935
AZAAROMATIC CHARGE-TRANSFER COMPLEXES
data obtained from studies of (n, n) complexes of various aromatic hydrocarbon donors along with known or estimated n-ionization potentials of the azines to predict the probable energy of the charge-transfer transition for (a, n) complexes of the azines with the electron acceptors TNB, p-benzoquinone, and tetrachloro-1,4-benzoquinone. The predicted chargetransfer bands are compared with the observed bands in Table I. We note from this comparison that the complexes of 8-hydroxyquinoline and acridine exhibit charge-transfer bands a t about the position expected for (n, n) complexes. In marked contrast to the behavior of these two donors, the rest of the quinoline derivatives exhibit charge-transfer bands which appear about 5000 em--' (-180 mp) lower in energy than predicted for n donation. The discrepancy between the predicted and observed values is clearly much larger than any experimental error. We conclude that with the exception of acridine and 8-hydroxyquinoline, the azines which we studied function as n donors with the T acceptors. Although we cannot predict with certainty which of the azines will form T , T complexes and which will form n, a complexes, our observations can easily be rationalized in the following way. The formation of (a, a) complexes with acridine can be understood in terms of the lower T ionization of acridine relative to most of the quinoline derivatives. Certainly 8-hydroxyquinoline is a better a-electron donor because the OH substituent lowers the n-ionization potential.20 Furthermore, it is expected to be a poorer n-electron donor than quinoline because of the possibility of intramolecular hydrogen bonding with the lone-pair electrons. It is therefore not surprising that 8-hydroxyquinoline does not function as a nelectron donor. We have shown that 8-hydroxyquinoline is not forming an n, T complex with tetrachloro-1,4-benzoquinone, but the true nature of the complex remains to be established. Bailey, et al., observed a band a t 500 mp with an impure, solid sample of the S-hydroxyquinoline-tetrachloro - 1,4 - benzoquinone which they attributed to a charge-transfer complex.21 Although we observed a band a t 500 mp in solution of these two components, we found that the intensity of the band increased (factor of 10) upon standing for several hours. Furthermore, we were unable to observe this band a t low temperatures when freshly prepared solutions were used. The clear implication is that the 500-mp band observed with 8-hydroxyquinolinetetrachloro-l,4-benzoquinone mixtures is not due to formation of a charge-transfer complex. The TNB-Shydroxyquinoline complex, on the other hand, appeared
to behave normally, exhibiting a charge-transfer band a t about 400 mp. In addition to those mentioned above, there are other experimental considerations which support our assignments. The different character of the ( A , n) and the (n, T ) complexes is also evidenced by the low stability of the (n, n) complexes as compared with the ( n , n) charge-iransfer complexes. Although acridine and possibly 8-hydroxyquinoline formed charge-transfer complexes with TNB and tetrachloro-l,4-benzoquinone a t room temperature, the (n, n) complexes of the other quinoline derivatives could be observed only a t reduced temperatures. In most of our experiments, the concentration of the azine was considerably larger than that of the acceptor. By assuming that all of the acceptor molecules were complexed a t 77"K, it was possible to obtain from the spectral data an approximate lower limit on the value of the extinction coefficient for the charge-transfer transition. For the tetrachloro-1,4-benzoquinonecomplexes which we have assigned as n, a complexes, the apparent extinction coefficients fall in the range 2500-4000. The apparent e values for the TNB complexes were generally quite small (e ili loo), but this probably can be attributed to incomplete complexation of all of the TNB. Interestingly enough, the extinction coefficient for the acridine tetrachloro-1,4-benzoquinone complex was 500, quite close to the value of 457 observed for the anthracene-tetrachloro-1,4-benzoquinone complex. This is, of course, consistent with the assignment of the acridine-tetrachloro-1,4-benzoquinone complex as a ( r ,a) type charge-transfer complex. Although there has been much discussion of the intensities of charge-transfer band^,^^,^^,^^ they still are not very well understood. Consequently, it is not possible a t this time to use the intensity of the chargetransfer band to distinguish between different types of complexes. On the basis of the various experimental results summarized above, we conclude that TNB, p-benzoquinone, and tetrachloro-1,4-benzoquinoneform (n, n) chargetransfer complexes with quinoline, quinaldine, isoquinoline, 6-bromoquinoline, 8-chloroquinoline, and (17) J. N. Murrell, Quart Rev. (London), 15, 191 (1961). (18) M. A. El-Sayed, M. Kasha, and Y. Tanaka, J . C h m . Phys., 34, 334 (1961). (19) J. E. Parkin and K. K. Innes, J . Mol. Spectry., 15, 407 (1965). (20) F. I. Vilesov, Soviet Phys. Usp., 6 , 888 (1964). (21) A. S.Bailey, R. P. J. Williams, and J. D. Wright, J . C h a . Soc.. 2579 (1956). (22) J. N. Murrell, J . Am. C h m . Sac., 81, 5037 (1959). (23) R. S. Mulliken, {bid., 79, 4839 (1957).
Volume 71,Number 4
March 1967
936
D. R. KEARNS,P. GARDNER, AND J. CARMODY
1,2,3,4-tetrahydroquinoline.Acridine, on the other hand, forms (n,n) complexes with these same acceptors and 8-hydroxyquinoline apparently reacts with tetrachloro-1,4-benzoquinone. We now want to discuss briefly some other azine-acceptor systems which did not appear to form charge-transfer complexes.
Other Azine--Electron Acceptor Systems Azine Complexes with Dichlorodicyano-p-benzoquinone (DDQ). We observed that low-temperature solutions of DDQ with various azines (quinoline, isoquinoline, dipyridyl) all exhibited absorption bands with maxima a t 700 mp suggesting perhaps that the DDQ complexes are ion pairs, that free DDQ ions are produced, or perhaps that DDQ undergoes some reaction to produce a species which absorbs a t 700 mp. DDQ is a much stronger electron acceptor than tetrachloro1,4-benzoquinone or TNBZ4so that ion formation is certainly a possibility. Unsuccessful Attempts to Observe Azine-a-Electron Acceptor Complexes. In contrast to the behavior of the quinoline derivatives, there were a number of azines (pyridine, pyrazine, phenanthridine, 5,6-benzoquinoline, tetraniethylpyrazine, 2,3-dimethylquinoxaline, dipyridyl, p-napht$hoquinoline) which did not appear to form colored charge-transfer complexes with TNB or tetrack~loro-l,4-benzoquinone.We believe that several factors (not necessarily the same in each case) were probably responsible for this. In general, the donors which did not form complexes (i) were somewhat less soluble in the solvents used than the donor molecules which did form charge-transfer complexes, (ii) have pK, values which are smaller than those which did 'arm complexes and, (iii) often appeared to be somewhat more reactive at room temperature toward the acceptors than donor molecules which formed complexes. Other investigators have also noted similar solubility and reactivity problem~.'0-1~
Relation to Previous Investigations of Azine-a-Electron Acceptor Complexes As some of our conclusions differ with those presented elsewhere hy others, it is perhaps worthwhile to mention these :md to point out why these differences might arise. Kinoshita measured the absorption spectra of acridine-tetrahromo-l,4-benzoquinonesolutions in carbon tetrachloride and observed an absorption band at 493 mp,24 but made no attempt to determine whether this was due to the formation of an (n, T) or a (a, T) complex. As noted in Table I, the acridine-tetrachloro1,4-benzoqiiinone complex has an absorption band a t The Journal
Ojr Phyaieal
Chemistry
500 mp. Because of the similarity in the electronaccepting properties of tetrabromo-l,4-benzoquinone and tetrachloro-1,4-benzoquinone and the similarity in the position of the absorption bands for the complexes with acridine, it appears likely that the tetrabromo-l,4-benzoquinone-acridine complex is a (a, n) charge transfer. Miller and Wynne-Jones studied the wridine-TNB _ " system in pyridine and observed slow formation (order of 10-20 min) of new absorption bands at 470 and 570 mp which they attributed to the formation of pyridineTNB ion pairs and the TNB ion.11J2 They also reported some evidence for a short-lived species which absorbed in the region of 3000 A and suggested this might be due to the pyridine-TNB charge-transfer complex. In contrast to the pyridine complex, the TNB complexes of the quinoline derivatives which we studied were formed only when the solutions were cooled to 77°K; they formed immediately upon cooling and they absorbed light in the vicinity of 490-510 mM, not a t 570 mp. Mukherjee and Chandra studied the absorption spectra of various azines with the acceptors tetrachloro1,4-benzoquinone, tetrabromo-l,4-benzoquinone, and p-iodanil and observed new absorption bands in the region of 470-490 mp which they attributed to the formation of (n, 7) charge-transfer complexes. l 3 There are several reasons for questioning this assignment. In the first place, the spectra were obtained by a difference method. I n view of the exceedingly small extinction coefficients obtained (emax N 0.015-1.0) the use of a difference method would appear questionable. Furthermore, it is difficult to believe that the (n, n) charge-transfer transition could be so weak, particularly in view of the fact. that even contact charge-transfer complexes of 0 2 with aniline, benzene, and ethanol have extinction coefficients which are greater than 100.25s26It is interesting to note that extinction coefficients for singlet-triplet transitions in halogenated aromatic molecules are on the order of 0.12' and that the lowest singlet-triplet transitions of quinoline and isoquinoline occur in the same region as the weak absorption bands which Mukherjee and Chandra attributed to (n, T) charge-transfer complexes.28 Finally, although they noted that the azines reacted with the
(24) M. Kinoshita, Bull. Chem. Sac. Japan, 35, 1609 (1962). (25) H. Tsubomura and R. S. Mulliken, J. Am. C h a . SOC.,82,
5966 (1960). (26) E.C.Lirn and V. L. Kowalski, J. Chem. Phys., 36, 1729 (1962). (27) D.S. McClure, ibid., 17, 665 (1949). (28) D.R. Kearns and A. Case, unpublished data.
ULTRAVIOLET-VISIBLE ABSORPTION SPECTRUM OF BROMINE
quinones, they made no attempt to prove that the faint absorption bands which they observed were not due to some reaction products. Acknowledgment. The support of the U. S. Public
937
Health Service (Grant No. GM-10499) is most gratefully acknowledged. The assistance of Mr. Jess Long in some of the temperature-dependence studies is also acknowledged with thanks,
The Ultraviolet-Visible Absorption Spectrum of Bromine between Room Temperature and 440"
by A. A. Passchier, J. D. Christian, and N. W. Gregory Department of Chembtry, Universay of Washington, Seattle, Washington
98106 (Received September 19, 1966)
The absorption spectrum of Brt(g) at wavelengths between 200 and 750 mp and at temperatures between 25 and 440' has been measured. Molar absorptivities suitable for quantitative use are reported. The temperature dependence is compared with that predicted by a Sulzer and Wieland equation. A marked temperature and concentration dependence in the ultraviolet region and evidence for Br4 are discussed.
We have found it convenient and sometimes necessary in the study of certain equilibrium systems to determine the concentration of bromine vapor from its absorption of light in the visible region of the spectrum. The bromine spectrum has been studied previously by a number of investigators.'" Although results are in general qualitative agreement, no two independent studies give values of the molar absorptivity at various wavelengths with sufficient consistency for quantitative use. Only Ribaud' and Acton, Aickin, and Bayliss2 report experimental results for the temperature dependence in the visible region. Sulzer and Wielandg have developed a semiempirical theory for the temperature dependence of a continuous absorption spectrum for diatomic molecules. They conclude that the shape of a given absorption peak is essentially determined by three characteristic parameters which may be evaluated from the observed spectrum a t any one temperature. They test their equation with results for chlorine, bromine, and iodine, using the data of Acton, Aickin, and Bayliss for bromine. Recently, Seery and Britton' have reported
a study of the bromine spectrum at 25" and their results differ substantially from those of Acton, Aickin, and Bayliss. Seery and Britton have used their data to find new parameters for the Sulzer-Wieland equation but did not experimentally verify the molar absorptivities which it predicts for higher temperatures. Hence for quantitative use we have felt it essential to make an additional experimental study of the (1) G . Ribaud, Ann. Phys., 12, 107 (1909). (2) A. P. Acton, R. G. Aickin, and N. S. Bayliss, J . Chem. Phys., 4, 474 (1936). (3) L. T. M. Gray and D. W. G. Style, Proc. Roy. SOC.(London), A126, 603 (1929). (4) R. G. Aickin and N. S. Bayliss, Trans. Faraday Soc., 34, 1371 (1938). (5) D. F. Evans, J . Chem. Phys., 23, 1426 (1955). (6) G . Burns and R. G . W. Norrish, Proc. Roy. SOC.(London), A271, 289 (1963). (7) D. J. Seery and D. Britton, J . Phys. Chem., 68, 2263 (1964). (8) E. A. Ogryzlo and B. C. Sanctuary, J . Phys. Chem., 69, 4423 (1965). (9) P. Sulzer and K. Wieland, Helv. Phys. Acta, 25, 653 (1952).
Volume 71, Number 4
March 1967
