Dec., 19G2
COMPLEXES IN PHOTOHALOGENATIOS PROCESSES
2423
THE PRESENCE OF TRAXSIEXT CHARGE-TRAXSFER COMPLEXES IS HALOGEN ATOM RECOMBINATIOK AND PHOTOHALOGENATION PROCESSES BYROBERT L. STRONG Department of Chemistry, Rensselaer Polytechnic Institute, Troy, N . Y. Received A4au 85, 1962
Transient halogen atnm-aromatic complexes are formed after the flash photolysis of iodine in benzene, toluene, o- and p-xylene, mesitylime, and hexamethylbenzene, and of bromine in benzene. These complexes are detected by their very strong charge-transfer spectra in the visible region. The processes are completely reversible for iodine in benzene, toluene, and xylene, while very slight photoiodination occurs in the iodine-mesitylene and iodine-hexamethylbenzene systems. Disappearance of the complex following decay of the flash is second-order and diffusion-controlled. Rate constants baaed on dissociation quantum yields from a simple continuum solvent model are lower than those predicted by simple diffusion theory, and indicate that molecular complex formation can lead to lower primary quantum yields of dissociation through deactivation of the halogen molecule. Photobromination of the benzene occurs in the bromine-benzene system; the extent of reaction per flash is only slightly increased by oxygen, while the rate of disappearance of the bromine atom-benzene complex (which follows concurrent first- and second-order kinetics) is greatly decreased by oxygen. Presumably photobromination occurs in the primary process through an activated bromine molecule.
Introduction 0.01-0.02% of the molecular iodine (initial conThe flash photolysis technique has been used centration about 2 X 10-6 M ) disappears per flash extensively for the study of halogen atom recom- by photoiodination of the mesitylene. The concentration of the complex is related to bination with various third bodies in the gas phase and in inert solvents. A relatively large fraction the change in absorbance a t ware length X by of the halogen molecules is dissociated by the flash 4Ax = E ~ , ~ ~ [ D * I ] ( 2 ) resulting in a transitory decrease in the absorbance of the cell in the visible region; kinetics of the atom where is the molar extinction coefficient of recombination process are determined by following the complex and d is the absorbing light path. spectrophotometrically as a function of time the Integration of the second-order rate expression subsequent increase in absorbance to its initial in terms of AAxleads to value. However, there is a transient increase in absorbance following the flash photolysis of iodine in various aromatic solvents in the visible region’J that has been attributed to the charge-transfer spectrum3of a 1: 1 complex between an iodine atom From slopes of the (1/4-4) us. t plots, values of the (acceptor) and the donor aromatic molecule, D. “relative extinction coefficients,” e C J k , were Similar transient spectra have been observed by calculated and plotted as a function of wave length Bridge4 and by Gover and Porters in a rariety of for the pur2 solvents benzene, toluene, o-xylene, solvents. p-xylene, and mesitylenez; wave lengths of The formation of the complex is comp!etely re- maximum absorption were 495, 51.5, 570, 520, and versible for iodine in benzene, toluene, or xylene; ,590 rnp, respectively. These shifts in A,, exdisappearance of the complex after the flash has pressed in terms of differences in energy, A(hv,ax), decayed to negligible intensity is second-order, relative to that of the benzene complex, were shown as shown bv linear time plots of the reciprocal of the to be almost identical with those of comparable change in absorbance, A24 ( E - A1/2.31 for small molecular halogen and interhalogen complexes, changes in transmittancy, where I is the transmitted providing further confirmation of the charge-translight intensity) over a t least a fivefold change in fer nature of these spectra observed in the iodine A A . This is consistent with the second-order re- atom systems. rombination mpchanism (1) if it is assumed that the The overlap of the De1 charge-transfer and IZ spectra for each of these five aromatic donors k prevents independent measurements of the rate of appearance of Ip and the simultaneous rate of 2D.I +1:(+2D) (1) disappearance of D.1. With hexamethylbenzene observed change of the absorbance is due entirely as the donor species, however, the shift of A,, to change in the concentration of D.I--i.e., if one toward the infrared has been showno to he so large neglects the accompanyihg change in absorbance that there is no absorption by D . 1 a t 489 mw resulting from the concurrent formation of I?, over a hexamethylbenzene concentration range in either “free” or complexed with D. Disappearance carbon tetrachloride of from 0.062 to 0.434 Jf. of the mesitylene-iodine atom complex also follows By combining change in absorbance measurements second-order kinetics, although approximately at a given time after initiation of the flash at this wave length, 4A0, with those a t a wave length ( 1 ) 9. J. Rand and R. L. Strong, J . A m Chem. Soc.. 82, 5 (1960). where absorption changes, Ail A, are due largely ( 2 ) R. L. Strong, S J. Rand. and J S Britt, thzd., 82, 5053 (1960) ( 3 ) R S. Nullikc-n, 2Czd.. ‘74, 811 (1952). to the complex (for (wLmpl(\, f N T , mp), it \\as (4) N K. Bridge. I. Chrm. Phua., Sa, 945 (1960). fi1rthc.r shown that ( 5 ) T. 4. Go’ier and G. Porter. I’roc. Roll S o r . (1.urriluii). -62, 476 (1961).
(6)
R. I..
S t i u u g ~ r r c J. l 1’PIuriu. .I. d i n . Clreni. Sou.. 83. 2843 (1961).
ROBERT L. STRONG
2424
where and ec,X are the molar extinction coefficients of I? and D.1 at 605 mp, €1: is the extinction coefficient of I? at 489 mp, and K , is the equilibrium constant for the complex
I
+ DJ-1.D
(5) Yalues for K , of 2.7 l./mole and for €,,A of lG00 l./ mole em. at GO5 mp and room temperature ivere obtained from the best straight line through the points plotted according t o equation 4 as a function of hexamethylbenzene concentration ([D I) in carbon tetrachloride. Linear plots were also obtained for this system when the reciprocals of AA' or AAA were plotted against time, again indicative of the second-order kinetic behavior of absorbance changes for the absorbing spccies. At 489 mp
where [IItotal( = -2A[I2]) is the total iodine atom concentration-either as free iodine atoms or complexed with hexamethylbenzene ; this assumes that the rate of formation of 1 2 is the same regardless of the form of the iodine atoms. In terms of Ail0
At 60.5 mp
(8) Values for 1; were calculated using equations 7 and 8 from slopes of l / A A cis. t plots a t 489 and 605 mp, respectively, at room temperature, and are given in Table I. TABLE I SVMVARY O F
RCCOMBlNATION RATECoSSTANTs, MEASURED AT 489 mp AND 605 mp
[D 1 (moles/l )
k
k
(from eq. 7) (I./rnole see.)
(from eq. 8 ) (I./rnole see.)
0 431 ,434
7.6 7.0
.431 .372
6.6 6 2 7 1 9 0 8 6 10.1 7 8 10 4 6 9
.310 310 "18
.I86 .l86 .I24
.Ofi2 Avrrage
x
7 9 X
109
lo9
7.6 6.4 6.0 7.1 6 0 8 0 9.6 7.3 8. 7 T.9 4.3
x
109
7 2 X lo9
The fact that there ic; no significant trend in li a t either W ~ V Plcngf h o w r t h e concentration range
Yol. 66
studied (where the ratio [ I ] / [ D . I ] varies from approximately unity to five) indicates that the rate of recombination is approximately the same for complexed and "free" iodine atoms, and is not a significant funct'ion of the encount'er diameters. (This conclusion was also expressed by Gover and Porter.s) 1:urther verification is found in t'he fact that the average value of k determined a t either wave length is in fair agreement xyith the values obtained for the recombination of iodine in pure CC14 in earlier flash work"* and in work involving the combination of mean iodine atom lifetime and quantum yield meas~rements.~ Experimental The apparatus was of conventional design for flash production and rapid spectrophotometric measurement of absorbance changes as a function of time a t a single wave length,1° and has been described in detail in reference 1. The flash lamp consisted of four parallel quartz discharge tubes (each 18 mm. 0x1. with aluminum electrodes 12 cm. apart) arranged in cylindrical form and surrounded by an aluminum reflector. The continuous analyzing beam from a 6.3-volt (battery-operated) automobile headlamp passed through the cylindrical reaction cell (10-cm. light path, 28 mm. 0.d.) positioned along the cylindrical axis of the flash lamp, and rendered approximately monochromatic by the appropriate Bausch and Lomb interference filter. (Characteristics of the nine interference and necessary auxiliary filters are included in reference 2 . ) The output from a 9 3 1 4 photomultiplier was fed through a capacitance-coupled cathode follower amplifier to a Tektronix Type 531 oscilloscope equipped with either a type 53B or 1) plug-in preamplifier, and the resulting oscillogram corresponding to a rapid change in transmittancy of the cell was photographically recorded. Four General Electric Pyranol capacitors (total capacitance 30 microfarads) were discharged through t,he flash lamp by a GL-5830 mercury thyratron tube controlled by the gate-out from the oscilloscope sweep circuit. Although rated a t 10,000 volts, thePe capacitors were generally charged t o 8,000 volts (960 joules); under these conditions the flash light intensity reached a maximum in less than 40 psec., and decayed with a half time of approximately 20 psec. Scattered flash light decayed to a negligible value by 150-200 psec. after initiation of the flash in most cases, although reliable measurements could be made after 70 psec. by applying a suitable scattered light correction. hfethods of solvent purification and drying are given in references 1 and 2 . Highest purity hexamethylbenzene, bromine, and iodine were used without further purification, except that the iodine used in cells filled in vacuo was resublimed from a XI-12 mixture. Indine-benzene and bromine-benzene cells were filled both in air and in ~ C U (following rigorous outgassing consisting of trap-to-trap distillation and a t least eight freezing, pumping, and thawing cycles). 811 other cells were filled in air but were tightly stoppered with ground-glass caps. The cells were wrapped with Eastman Kodak Co. gelatin Rratten filters to limit loa-wave length absorption by the corresponding molecular halogen complexes; K2 and 2B filters were used for the iodine and bromine cells, respectiveIy.
O
Discussion Estimation of k and Ec.--_llt8hough t'he extinction coefficient of the complex can be expressed in terms of AAAand d by equation 2 , the concentration of tmhecomplex is not known and (as pointed out above) the overlap of the 1-isible spectra prevents determination of k for the donors benzene, toluene, xylene, and mesitylene. If it is assumed, however, that recombination in each system is (7) R. Marshall and N. Davidson, J . Chem. Phys., 21, 2086 (1953). ( 8 ) R. L. Strong and J. E. Willard, J . A m . Chem. Soc.. 79, 2098 (19.57). (9) 13. Rosrnan and R. hl. Noyes, ihid., 80, 2410 (1958). (10) ill. I. Christie, R. 0. IT. Norrish, and 0. Porter, I'ror.. Soc. (London), 8 2 1 6 , 152 ( 1
KO?/.
COMPLEXES I?; PHOTOHALOGEY ITIOY PROCESSES
nec.. 1962
diffusion-controlled and independent of the encounter diameters of the recombining species and that the diffusion process can be represented by a sphere moving through a continuous visvous medium, the rate (ionstants at constant temperature should be inversely proportional to the coefficients of ~ i s r o s i t y . ~T-aluw for kl mere calculated using an average value of 7.5 x IOg l./mole sec. from Table I for the recombination process in carbon tetrachloride (7 = 0.07 centipoise at Z O O ) , and extinction coefficients at A,, n-ere calculated from 1, ec values given in reference 2 for air-filled cells. Thme are tahula ted in Talile 11.
843.5
TABLE I11 RATESOF RE(OMBINATION FROM
+
A V D EXTIW~YO\ C'OEFFICIEYTS EQUATION 10
L
re
Solvent
(from eq. 11)
(l./mole sec.)
(1 /mole cm.)
Benzene Toluene o-Xylene p-Xvlene Xesitvlme
0.29 .31 .24 .29 .2i
1 0 . 5 x 109 6 6 3.7 4 4 4.7
3300 1830 1480 1160 "600
Because of the extreme sensitivity of the computer integration to errors in I , and i L 4 x at a given time after the flash,l the absolute \values for 1; TABLE I1 and cc are qualitative only. However, the l o w r RATES O r IbECOXF%IYATIOV .4ND EXTINCTION COEFFICIENTS values for k relative to that of benzene, in contrast ~ ~ S h T Z I I UI)IFFUSlO~-CONTROLI,EU G RECOMBIN.4TION to those given in Table 11,for the stronger molecular 'i iodine charge-transfer complexes (lower donor k/a, a t XmTx (centik et poir,e) (cm./sec ) (I./niole (I./mole ionization potentials) indicate that the primary Solvent a t 20' (from ref. 2) sec.) cni.) quantum vields calculated from equation I 1 Benzene 0 632 3 2 X IO6 11 2 X lo9 3500 are too high. Presumably charge-transfer interacToluene 590 3 6 12 3 3400 tion increases the probability of deactivation of the 810 2 5 9 0 3600 o-Xylene excited iodine molecule through a radiationless 2930 p-Xylene 648 3 8 11.2 transition. Mr~sitvlenc~ 702 18 10 4 5800 The Flash Photolysis of Bromine in Benzene.As in systems involving iodine, the flash photolysis The rate constants can be estimated in another of bromine in benzene is accompanied by a transimanner. The complete rate expression for the tory increase in the visible-region absorbance, photochemical formation of the complex (assumed again indicative of the formation of the atomto be instantaneous and therefore proportional to benzene (D-Br) charge-transfer complex. There the light ahsorlwd, Ia)*and its disappearance is are, however, several important differences between this and the iodine-benzene system: (a) d [ D . I ] 'dt = 2+Ia - 2k[D.IIz (!I) appreciable photobromination of the benzene occurs (en. 0.04% of the bromine disappears per flash Or, in terms of AL4h at an initial bromine concentration of 2 X J I ) : (b) the rate of disappearance of the complexdetermined by following A/Ix as a function of time at wave lengths above the region of absorption by where 6 is the primary quantum yield of dissocia- Br2-is complex, apparently being neither firsttion of the iodine molecule. A theory has been nor second-order : and (c) the rate of disappearance developed by X o y ~ s , 'based ~ also on the model of the complex in thoroughly outgassed cells is of the liquid b4ng a continuous medium of vis- murh faster, hut the extent of reaction per flash only slightly less, than in air-filled cells with well cosity 7 . that predicts dried benzene. $ = l A possible mechanism for the diqappearnnce of DTRrinvolving ronciirrrn 1 11 1
where m is the mass of the atom, a is its radius, and ( h v - E ) is the excess kinetic energy of the atoms heading in opposite directions. Although this theory predicts a higher quantum yield (0.21) for iodine in carbon tetrachloride than is observed 10.13),8.9it giws quite good agreement for lowviscosity hydrocarbons.'? Equation 11 has been used to calculate for the five aromatic qolvents assuming an average flash light wave length of 500 m p , and equation 10 has been solved for the F: 'ec values given in Table I1 using an IRhI-650 digital computer as previously described.' Resulting values of k and cr are listed in Table 111.
+
R. 31. NOJ~PS. Z . EZektrochen.. 64, 1.53 11460). (12) I ) . Booth nnd I?. A I . Noyes. .I. A m . Chrm. ,Soc., 82, 1868 (11)
(l!)60).
D . R r --+X
(12)
1iI 2D.Rr --+ Rr? (+ 2D)
(13)
The expression for the rate of disappearance of I).Rris -djD.Br] dt
=
l;l[D.Br]
+ 2X.L.[D.Rr\2(14)
At wavf' lengths above the absorption region of Rr2 Add, = e,,d[n.nrl
and
( 1 .?)
2426
G. H. BROWN, S. R.ADISESH,A N D J, E. TAYLOR
The integrated rate law then is
where C is the constant of integration. The exponential term eklt is plotted as a function of l / A A x for varying assumed values of kl until a straight line results, the constant e c , ~ / kthen 2 being determinable from the slope/intercept ratio. At wave lengths greater than ,500 mp, kl is approximately constant for air-filled cells, independent of wave length, and equals ca. 1.8 X lo3sec.-’. In thoroughly outgassed cells it is approximately ten times larger, and almost completely masks the second-order contribution. The constant E ~ , X / ~ : ! is a function of wave length with a maximum value of 6 X cm.’sec. a t approximately 560 mp in air-filled cells; the spectrum is quite broad, however, extending over the complete visible region. Initially it was assumed that the product of reaction 12 (X) eventually leads to bromination products. However. the fact that the extent of bromination per flash is (approximately) the same in outgassed and in air-filled cells, but that k , is an order of magnitude larger in the former, suggests that recombination to form molecular bromine must occur by reactions involving X in addition to reaction 13. Partial photobromination of the benzene presumably occurs through a primary process involving an artivated bromine molecule, as suggested in earlier studies of benzene’3 and bromo(13) E. Rabinoaitsch, Z. phgsik. Chem., B19, 190 (1932).
Vol. 66
benzene14 photobrominations, rather than a complexed bromine atom. Reaction 12 might bc a transition from the a-complex (D.Br) to a morc localized a-complex that can also react with another a-complex, a D.Br, or a free Br atom to produce molecular bromine, although there is no experimental evidence for this second intermediate. Products of the aromatic bromination have not as yet been completely identified, although th? proton magnetic resonance spectrum of a residue from vacuum evaporation of the benzene indicates that tetrahromocyclohexene may be a major produc t. Neither bromobenzene nor hexabromocyclohexane was found to be a product, in contrast to earlier low-intensity benzene photobromination s t ~ d i e s . ‘ ~ , ‘It~ is possible that moisture and higher bromine concentrations can lead to acidcatalyzed ring substitution and complete addition reactions.’C Indeed, early run3 in which the benzene mas not rigorously dried gave greater amounts of bromine reacting per flash than in the runs reported here. Acknowledgments.-Experimental details and further equilibrium results on these investigations of the bromine-benzene system will be reported in a later publication by Mrs. Jeanne S. Bartlett. This work was supported in part by National Science Foundation Grants NSF G-4181 and G9988. We especially want to thank Professor hI. J. S. Dewar for many helpful suggestions made during the Symposium. (14) D L. Hamrnirk, J. n l Hutson, and G. I Jenkins
J.
Chem.
1959 (1918). (15) W Meidinner, 2. phgazk. Chem , B6,29 (1929). (16) R Cornuhert and A Rio. Bull soe c h z m France, 60 (19S5).
SOC,
KINETIC STUDIES OF PHOTOTROPIC REACTIONS. I. EVIDESCE FOR SN IOX-PAIR IXTERMEDIATE IN THE REACTION OF METHYL VIOLET WITH CYANIDE I O Y BY GLESNH. BROWS, SETTYR.ADISESH,AND JAY E . TAYLOR Department of Chemistry, Kent State Cniz~ersity,Kent, Ohio Received M a g 86, 1962
Methyl violet and cyanide ion react to form the leuconitrile, w-hich reaction is reversed by the action of ultraviolet lighl . For the forward reactiofi in ethanol-water solutions, a decrease in the second order rate constant is observed as cyanide ion is inrreased over a wide range of concentration. The phenomenon is esplained bv ion-pair formation betwren the reactant ions. Assuming that a specific ion-pair is a reaction intermediate, a simple equgtion, kobsd = k * K / ( K [ C N - ] I ) , whew /;ah+ and k*.are.the observed and the corrected rate ronstants and K is the association constant for ion-pair formation, explains the kinetics of the reaction. As the dielectric constant of the solvent mixture is decreased, both the rate of reaction and ion-pair formation increase. The various rate constants at 25.0, 35.0, and 43.5’ are given along with values for AF,,,, AHaot,and AS,,,. The values of the association constants are given a t these same temperatures as well as the values for AI**$AH, and A S for ion-pair formation.
+
Introduction ;Ilarckwald2 gave the name “phototropy” to the phenomenon in which a solid changes color when exposed to certain wave lengths of electromagnetic radiation but reverts to its original color on removal of the exciting radiation. This phenomenon now (1) This work was supported by t h e Aerospace nledlcai Research Laboratorley Wright 4ir D e ~ r l o p m e n tCenter, M’riglit-PdttFi sun Air I’orce Basr, Ohio ( 2 ) W.Marchuald. Z. p h ~ s z l Chem., SO, 110 (1899).
goes under the name of phototropism or photochromism. The field has been reviewed recently by Brown and Shaw.3 Holmes4 has proposed that the triarylmethvl leuconitriles on excitation with ultraviolet radiation show phototropism by the formation of either triarylmethyl radical or the corresponding dye, (3) G. I I . B r o u n a n d K,G. Sliau. Rei, P u r e 1 p p 1 Chri,. , 11, 1 (1961). (4) E. 0. IInimes, J . P/iz/s. Ciiern, 61, 4J4 (19571
