Volume 23, number 3 CHEMICAL PHYSICS LETTERS 1 December 1973

ANOMALOUSLY HIGH RATE CONSTANTS FOR THE REACTION OF SOLVENT POSITIVE IONS WITH SOLUTES

IN IRRADIATED CYCLOHEXANE AND METHYLCYCLOHEXANE

Erika ZADOR*, Jofin M. WARMAN and Andries HUMMEL Infenrniversiry Reacror Insrimre, Del/r, The Nerherlands

Received I Septemtzr 1973

The formation kinetics of the ions ThlPD+ and pyrene+ have been studied in pulse irradiated dilute solutions of TMPD and pyrene in cyclopentane, n-hexane, cyclohexane, methylcyclohexnnc. iso-octane and cycle-octane. In cyclo- hexane and methylcyclohexane the rates of formation indicate rate constants, (7 2 4) X 10” hi-‘.szc-’ in the case of cyclohexane, for the reaction of solvent positive ions with the solute which are more than tenfold greater than cx- petted for the reaction of molecular positive ions. In the other solvenb, howcvcr, rate constanls on the order of 10” &I-‘s-&l are found.

1. Introduction

Some years ago a study [I] was made of the effect of electron scavengers, in particular Ccl,, on the yield of positive ions scavenged by cyclopropane in irradiated liquid cyclohexane. From these experiments it was de- duced that the ratio, rD, of the sum of the diffusion ce efficients of the charged species before, D(+) + D(-), and after,D(+) + D(Cl-), electron scavenging had a val- ue of approximately 17. Since that time a value of 9 X 1V3cm2sec-l has been measured [2] for the diffu- sion coefficient of the electron, D(-), in cyclohexane by conductivity methods. Combining this value with the above VahE of rD leads t0 the COnClUSiOn that the diffusion coefficient of the positive charge carrier, D(+), initially formed in cyclohexane is much greater than that expected for the diffusion of a molecular en- tity.’

Recently, Beck and Thomas [3] have found direct evidence for the formation of a highly mobile positive

ion in cyclohexane by measuring the growth of the pyrene radic&cation on a picosecond timescale. The

l On leave from Thk Cenhal Research Institute for Physics of the Hungarian Academy’of Sciences, Budepest, Hungary.

rate constant required to explain the growth observed was estimated [3] to be 4 X 101lM-lsec-l which is al- most two orders of magnitude greater than that expect- ed for a diffusion controlled reaction of a molecular ion. Hummel and Luthjens [4] arrived at similar conclusions on the basis of a study of the pulse radiolysis of dilute solutions of biphenyl in cyclohexane. Infelta and Rzad [5] have recently discussed the consequences of a high- ly mobile positive ion for steady state and pulse radio- lysis experiments.

The displacement of a positive centre in other ways than by movement of an electron deficient molecule has beeri known to occur in crystalline solids for some time and “hole” migration has been invoked by Hamill [6] to explain effects observed in low temperature hy- drocarbon glasses. Resonance charge transfer has also been suggested as an explanation for the rapid growth of the nuns-stilbene cation in pulse irradiated 1 ,I -di- chloroethane solutions [7] _

In an attempt to clari@ the general question of the mobilities of positive ions &i liquid saturated hydrocar- bons we have studied the formation kinetics of positive ions derived from solutes dissolved in everal saturated hydrocarbdn solvents. We ptisent here,a preliminary report of the results.obtained.

Vdume 23, number 3 CHEMICAL PHYSICS LETTERS 1 December 1973

2. Experimental

The liquids were ionised by irradiation with 10 and 20 nsec, 1 ampere pulses of 3MeV electrons from a Van de Graaff accelerator. The total dose for a 10 nsec pulse was approximately 2 krad. The overall risetime of the optical detection system, which consisted of a

lP28 photomultiplier with the output fed directly in- to a Hewlett Packard I83A oscilloscope, was approxi- mately 2 nsec. The hydrocarbon solvents were passed through a column of activated silica gel immediately prior to use. The N, N, N’, N’ -tetrametbyl p-phenyl- ene cliamine (TMPD) used was obtained from the dihy- drochloride and was vacuum sublimed before use. The pyrene was used without further purification.

3. Results and discussion

When studying the formation kinetics of positive ions in hydrocarbon liquids, consideration must be given to the fate of the electrons for the following reasons. If the electrcns are not scavenged then a very rapid decay of the homogeneously distributed (“free”) ions will occur due to the very large values for the elec- tron-positive ion recombination rate constants in these liquids [8-IO]. If a high concentration cf electron scav- enger is present then the lifetimes of a large fraction of the geminately recombining ion pairs are extended in- to the nanosecond region [4, 1 I], resulting in a rapid- ly decaying “geminate spike”. Thus, under the limiting conditions of no electron scavenger and a large amount of electron scavenger, the rapid decay of ion pairs would considerably complicate the interpretation of concurrent formation kinetics. A concentration of ap- proximately 1 0b4 M of a good electron scavenger would appear to be a reasonable compromise between these two extremes since it has been found in the biphenyl- cyclohexane system [4] that at this concentration an end of pulse (1Onsec) yield of negative ions only slight- ly in excess of the free ion yield is observed and the re- sulting decay is gradual with a first half life greater than 500 nsec.

An effective “good” electron scavenger concentration of approximately l@M may be quite readily obtained in hydrocarbon liquids by saturation with 02 (concen- tration ca. IO-*M). By measuring the effect of 0, on the yield of triphenylmethyl radicals formed on electron

364

capture by triphenylmethyl chloride it has been deter- mined by the present authors that an 0, saturated sol-

ution is equivalent to a concentration of 3.5 X l@M, 1.9X l@M and 1.3 X l@M of tt-iphenylmethylcttlo- ride (an efficient electron scavenger [ 10, 12, 131) in w hexane, cyclohexane and iso-octane, respectively. In the caSe of the so111 te TMPD the use of a saturated O2 solution hzs the additional advantage that any triplet TMPD formed will be rapidly removed by reaction with

O2 &TMPD3+Oz) = 1 3 X 101oM-lsec-l in cyclohexane

[14]), thus preventing complications due to the very

similar absorption spectra of TMPD+ and TMPD3. Oscilloscope traces of the formation of TMPD+,

monitored at 560 run, in 0, saturated, l@M solutions

‘200 nsec’

cyclohexane

‘200 nsec

iso-octane

‘200 nsec ’

Fig.1. Oscilloscope traces of tie absorption produced at 560 m (TMpD+) by puke irradiation (20115.e~) of 10-M, 02 Sat- urated sdutions of TMPD in the liquids shown.

Volume 23, numbcr 3 CHEhlICAL PHYSICS LETTERS I December :973

normal hexane

cyclohexane

I 200 nsec’

iso-octane

methylcyciohexane

‘200 nsec ’

cycle-octane

I 1000 nsec

I

fig. 2. Oscilloscope traces of the absorption produced at 450 nm (pyrene? by pulse irradiation (10 nsec) of lo! M, O2 aturatcd solutions of pyrene in the liquids shown.

of TMPD in n-hexane, cyclohexane and iso-octane are shown in fig. 1. A considerable differenu: between the formation kinetics in cyclohexene and iso-octane is readily seen. In the latter solvent a growth occurs over several hundred nanoseconds whereas cyclohexane is characterized by a complete lack of growth. If it is as- sumed that at this low salute concentration reaction occurs only with the homogeneously distributed free ions, then a best fit to the TMPD data in isooctane is obtained using an ion recombination rate constant of 2 X 1012M-l,c-1 and a rate constant for scavenging of the solvent positive ions of (2.0 t 0.5) X 101oM-L

~6. The latter value is of the magnitude expected For a.difFusion controlled reaction of molecular ions in this liquid.

Although the absorption signal in n-hexane, fig.lA,

is much smaller than in iso-octane, a growth of the pos- itive ion on the same order as that found for iscmctane

is discernible, suggesting a close to diffusion controlled reaction rate for positive ion scavenging in h.hexane also. In the case of cyclohexane. if it is estimated that a growth with a half rise-time of 2Onsec could not be observed in the present experiments, a lower limit of 3.5 X 1011M-1sec-~ may be placed on the rate constant for scavenging of solven t positive ions.

Almost identical kinetics are found for the forma- tion of the pyrene radical cation, monitored at 450 nm, in lpM, O2 saturated solutions of pyrene in the above liquids, see figs.2a, b and c. It should be pointed out that the results in fig.1 are for 20 nsec pulses while those presented in fig.2 were obtained with 10 nSec pulses. The almost identical absorptions observed for

365

Volume 23, number 3 CHEMICAL PHYSICS LETTERS 1 December 1973

the two ions is therefore fortuitous and indicates an

extinction coefficient for pyrene+ approximately twice that for TMPD+ (eW = 12000 [ 151). On reduction of the pyrene concentration to 3 X 1W5M in cyclohexane a growth of the solute positive ion, over the first hun-

dred nanoseconds or so, does become apparent. Making

the assumption that pyrene” is formed by reaction of

the solute only with the free ions (C = 0.15 in cyclo- hexane [16]),a rate constant of(7 24)X 101lM-lsec-l is estimated for positive ion scavenging by pyrene from

the initial, rate of pyrene+ formation in the 3 X lWSM solution. Within the error limits this value is in agree-

ment with the value of 4 X I O1lM-lsec-L estimated by Beck and Thomas [3] _ Similar treatments of the l@

M data for lz-hexane and iso-octane yield rate constants of 4 X lOlo and 2 X 101OM-lsec-l, respectively.

In order to test the possib!e significance of a cyclic

structure of the hydrocarbon, experiments were carried out using as solvents cyclopen tane, cycle-octane and methylcyclohexane. The growth of the pyrene positive

ion over several hundred nanoseconds observed for cy- clopentane and cycle-octane, figs.2d and f leads to val- ues of 3 X lOLo and 8 X 109M-1sec-1, respectively, for the rate constants for positive ion scavenging in these liquids. For methylcyclohexane, however, rapid forma-

tion kinetics (fig.2e), similar to that for cyclohexane,

were found with a rate constant greater than 2 X 10” M-lsec-1 being indicated.

The present results clearly indicate the formation of positive charge carriers with mobilities far in excess

of those expected for molecular ions on irradiation of liquid cyclohexane and its methyl derivative. It may also be concluded, however, that this is not a general property of liquid saturated hydrocarbons.

The mechanism of rapid transport of positive charge through the liquid is unclear at present. While hole movement due to a band structure cannot be complete- ly excluded, it would seem that the explanation for this phenomenon is most likely to be found in the exchange of charge between positive molecular ions and neigh- bowing molecules. The latter process requires that the geometrical configurations cf the positive ion and ground

state molecule be closely similar. In this connection, it

has been suggested by Lipsky et al. [ 17j that a guide to:

the degree of similarity of the geometry of higher ex- cited states to that of the ground state may be reflect- ed in the magnitude of the fluorescence quantum yield- following photo-excitation of saturated hydrocarbons. In view of this, it is interesting to note that of the com- pounds studied here those which display anomalously high rate constants for reaction of positive ions also have been found to have relatively large fluorescence yields [17], viz., cyclohexane, 3.5 X 10e3 and methyl-

cyclohexane, 5.5 X 1 W3 versus rz-hexane, 2 X I @ , iso-, octane, cyclopentane and cycleoctane, < 1W5.

References

[1] S.J. Rzad, R.H. Schuler and A. Hummel, J. Chcm. Phys. 51 (1969) 1369.

(21 W.F. Schmidt and A.O. Allen, J. Chem. Phys. 52 (1970) 4780.

[3] G. Beck and J.R. T%omJs, J. Phys. Chcm. 76 (1972) 3856. [4] A. Hummcl andL.H. Luthjens, 1. Chem. Phys. 59 (19?3)

654. [S] P.P. Infelta and S.J. Rzad, J. Chem. Phys. 58 (1973)

3775. [6] W.H. Hamill, in: Radical ions, eds. E.T. Kaiser and L. Xevw

(Interscience, New York, 1968) ch. 9. [7] S. Arai, H. Ueda, RI. Firestone and L.M. Dorfman, J.

Chem. Phys. 50 (1969) 1072. [S] J.H. Bsxendak, C. Bell end P. Wardman, J. Chem. Sot.

Faraday Trans. I69 (1973) 776. [9] J.H. Baxendalc, C. Bell and P. Wardman, Chem. Phys.

Letters 12 (1971) 347. [IO] J.hl. Warman, M.P. de Haas and A. Hummel, to be pub

lished. [ 1 l] JX. Thomas, K. Johnson. T’. Klippert and R. Lowers, J.

Chem. Phys. 48 (1968) 1608. [ 121 E. Davids, A. Hummel and J.h¶. Warman, to be published. [ 131 C. Capellos and A.O. Allen, I. Phys. Chem. 73 (1969)

3264. [14] E. Zador, J.M. Warman and A. Hummel, to be published. [ 1.51 H.D. Burrows. D. Greatorex and T.J. Kemp, J. Phys.

Chem. 76 (1972) 20. [I61 W-F. Schmidt and A-0. Allen, J. Chem. Phys. 52 (1970)

2345. [ 171 W. Rothman, F. Hirayama and S. Lipsky, J. Chern. Phys.

58 (1973) 1300.

366