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Pulse radiolysis observations of solvated electrons in liquid hydrocarbons
Votime 12,number2
CHEMICAL PHYSICSLETTERS
15December1971
PULSE RADIOLYSIS OBSERVATIONS OF SOLVATED ELECTRONS IN LIQUID HYDROCARBONS J.H. BAXENDALE, C. BELL and P. ~‘ARDMA~ Chemistry Department, The U~i~e~~t~~~ranc~~e~te~~i~39PL. UK Rcceivecl 5 October 1971
The absorption spectra of solmted electronsh ~e~~ylcyc~~e~ne ham been obtained at -113°C and 20°C and the decay kinetics observed. The ~omogeneons re~mb~atjons of ekctrons and cations in IQ&d methy~cyc~ohe~ne and n-hexme at 20°C observed by pulse radiolysis occur with k = 8 X lot3 and 2 X lOi M-t sx-’ respectiveiety.
Th_“,reactcm of electrons with pyTene and urban tetrachloride in methylcyclohexane at 20°C have k = 1 X IO’* M set . These values are consistent with electron mobilities measuredby conductivity.
Measurements of irradiated glasses or liquids at ca. -190°C show that solvated electrons produced by ionising radiation on hydrocarbons absorb strongly in the red up to at least 2000 nm [I-4]. The kinetics of decay of radiation-produced electrons have been followed by pulse radiofysis of nlethyicycio~~exane ]3] at -80°C and of liquid propane [43 at -183°C where their lifetimes are in the tenth and one microsecond ranges respectively. We find similar absorptions in me-
thylcyc!ohexane at -113°C (fig. 1) but we consider the apparent maximum at ca. 1.550 nm to be an artefact of the Ge pbotodjode detector which we used. We fmd no dose effect on the electron decay kinetics and the concentration at longer times follows a tw1j2 decay law [5], both of which indicate that the decay is due to electron-solvent cation recombination in “spurs”, i.e., it is a non-homogeneous process. Measuremenis of the effect of carbon tetrachloride on the electron decay give a rate constant of 1.2 X 109 M-1 set-l for the e- + CC14 reaction. We also wish to report our observationson the absorptions due to electrons in hydrocarbons at room temperature which pre~ous.~vestigafions were unable to detect f3f. These were made possible by the development of a photo diode-amplifier system having an overall rise time of ca. 3 &SC and capable of meaningful measurements down to 0.05% absorption. Methykyclohexane, n-hexane, cycIohexane and iso-
Fiq. 1. Absorption spectra of e- in mcthyIcycIohesane at -1 I3’C and 20°C (the ttvo spectra are on different CID. scabs). Inset: CR0 txace of absorption at IO00 nm produced by a 10 nsec, 2 krad pulse in met~yl~c~ohe~ane at 20°C.
pentane aUshow,absorptions which increaz with increasing wavelength as far as 1500 nm, and respond to the usual tests for solvated electrons. fn me~ylcycIoh~xane the absorption spectrum is generally similar to that at Iow temperature (fig. 1) but has relatively higher absorption at shorter wavelengths and the decay process is now in the nanosecond aange:After an initial “spike”(fig. I, inset) the ‘_
347
Volume 12, number 2
CHEhUCALPHYSICSLETTERS.
15 December
1971
50 I-IS --I+
f
TO*3% absn.
FC. 3. a) CR0 trace of absorption of e- at 1000 nm in pure methylcyclohexane after a 10 nscc, 2 krad pulse; b) methylcyclohcxane containing 10 PM CC4, same conditions as a).
150
time
2aJ
250
(ns)
Fig. 2. Second-order decay of e- in jr-hexane and methylcyclohexane at 20°C (the two decays are on difTcrent O.D.-’ scale!). Inset: CR0 trace of absorption at 1000 nm produced by 50 nsec, 4 krad pulse in rr-hexane at 20°C.
decay kinetics for Aexane and methylcyclohexane are second order (fig; 2). The “spike” is probably part of the very fast “spur” recombination process seen at low temperature and the slower decay that which occurs homogeneously between the “free” ions. In methylcyclohexane at 20°Cat 1000 rim we find GE is
the ratio of mobilities of electrons and negative ions in I!-hexane is found to be ca. 80. The estimated value for c-hexane is also consistent with its measured mobility. _4llowing for this enhanced mobility of the electron, the calculated diffusion controlled rate constant for its reaction with a neutral species (e.g., pyrene or CC14) in methylcyclohexane is ca. 5 X 101* M-l set-l. By measuring the effect of 5 PM and 10 ,uM concentrations of these &JO solutes on the decay of the electron absorption (fig. 3) we fmd k = 1 X lOL* M-l set-l for each. Contrary to previous observations [3] at low temperature we fmd that the rate of appearance of the pyrene anion is in line with the disappearance of the solvated electron.
I .l X IO3 (G is yield of free ions per 100 eV; E is mo-
lar decadic absorption coefficient) aEd assuming G = 0.12 as for c-hexane [5], E = 0.95X 104. Using this value of E, fig. 2 gives k = 8 X 1013 M-l set-l for the ion recombination process in met!rylcyclohexane and 2 X 1Ol4 M.-l set-1 in n-hexane. We also estimate ca. 1 X 1015M-l set-’ in c-hexane. In n-hexane at 2O’C having viscosity 0.3 cP, the calculated diffusion controlled rate constant between neutral species is 2 X 10’” M-l set-l and including the Debye factor for reaction between oppositely charged small ions, this increases to 3.0 X 1012 M-l set-I. Hence it would seem that here the electron has a diffusion constant some 60 X greater than that for normal ions. This is in reasonable agreement with thz conclusion from conductivity measurements [7,8] where
Reverences [ 11 W.H. Hamill, in: Radical ions, eds. E.T. KaiSer and L. Kevan (Wiley, New York, 1968) ch. 9. [Z] I-4. Taub and HA. Gillis, J. Am. Chem. Sot. 91 (1969)
650. [3] J.T. Richards and J.K. Thomas, Chem. Phys. Letters 10 10 (1971) 317. [4] H.A.Gillis, N.V. Kksan,G.G. Teather and K.H. Lokan, Chem. Phjs. Letters 10 (1971) 481. [S] SJ. Rzad, P.P. Infelta, J.M. Warman and R.H. Schuler, J. Chem. Phy?. 52 (1970) 3971. [6] SJ. Rwd and Jhl.Wasman, J. Chem. Phys. 49 (1969) 2861. [7] W.F. Schmid! and A.O. Allen, .J. Chem. Phys. 52 (1970) 4788. [8] RM. Mbday, LD. Schqidt arid H.T. Datis, J. Chem. Phys. 54 (1971) 3112.
.
348,
CHEMICAL PHYSICSLETTERS
15December1971
PULSE RADIOLYSIS OBSERVATIONS OF SOLVATED ELECTRONS IN LIQUID HYDROCARBONS J.H. BAXENDALE, C. BELL and P. ~‘ARDMA~ Chemistry Department, The U~i~e~~t~~~ranc~~e~te~~i~39PL. UK Rcceivecl 5 October 1971
The absorption spectra of solmted electronsh ~e~~ylcyc~~e~ne ham been obtained at -113°C and 20°C and the decay kinetics observed. The ~omogeneons re~mb~atjons of ekctrons and cations in IQ&d methy~cyc~ohe~ne and n-hexme at 20°C observed by pulse radiolysis occur with k = 8 X lot3 and 2 X lOi M-t sx-’ respectiveiety.
Th_“,reactcm of electrons with pyTene and urban tetrachloride in methylcyclohexane at 20°C have k = 1 X IO’* M set . These values are consistent with electron mobilities measuredby conductivity.
Measurements of irradiated glasses or liquids at ca. -190°C show that solvated electrons produced by ionising radiation on hydrocarbons absorb strongly in the red up to at least 2000 nm [I-4]. The kinetics of decay of radiation-produced electrons have been followed by pulse radiofysis of nlethyicycio~~exane ]3] at -80°C and of liquid propane [43 at -183°C where their lifetimes are in the tenth and one microsecond ranges respectively. We find similar absorptions in me-
thylcyc!ohexane at -113°C (fig. 1) but we consider the apparent maximum at ca. 1.550 nm to be an artefact of the Ge pbotodjode detector which we used. We fmd no dose effect on the electron decay kinetics and the concentration at longer times follows a tw1j2 decay law [5], both of which indicate that the decay is due to electron-solvent cation recombination in “spurs”, i.e., it is a non-homogeneous process. Measuremenis of the effect of carbon tetrachloride on the electron decay give a rate constant of 1.2 X 109 M-1 set-l for the e- + CC14 reaction. We also wish to report our observationson the absorptions due to electrons in hydrocarbons at room temperature which pre~ous.~vestigafions were unable to detect f3f. These were made possible by the development of a photo diode-amplifier system having an overall rise time of ca. 3 &SC and capable of meaningful measurements down to 0.05% absorption. Methykyclohexane, n-hexane, cycIohexane and iso-
Fiq. 1. Absorption spectra of e- in mcthyIcycIohesane at -1 I3’C and 20°C (the ttvo spectra are on different CID. scabs). Inset: CR0 txace of absorption at IO00 nm produced by a 10 nsec, 2 krad pulse in met~yl~c~ohe~ane at 20°C.
pentane aUshow,absorptions which increaz with increasing wavelength as far as 1500 nm, and respond to the usual tests for solvated electrons. fn me~ylcycIoh~xane the absorption spectrum is generally similar to that at Iow temperature (fig. 1) but has relatively higher absorption at shorter wavelengths and the decay process is now in the nanosecond aange:After an initial “spike”(fig. I, inset) the ‘_
347
Volume 12, number 2
CHEhUCALPHYSICSLETTERS.
15 December
1971
50 I-IS --I+
f
TO*3% absn.
FC. 3. a) CR0 trace of absorption of e- at 1000 nm in pure methylcyclohexane after a 10 nscc, 2 krad pulse; b) methylcyclohcxane containing 10 PM CC4, same conditions as a).
150
time
2aJ
250
(ns)
Fig. 2. Second-order decay of e- in jr-hexane and methylcyclohexane at 20°C (the two decays are on difTcrent O.D.-’ scale!). Inset: CR0 trace of absorption at 1000 nm produced by 50 nsec, 4 krad pulse in rr-hexane at 20°C.
decay kinetics for Aexane and methylcyclohexane are second order (fig; 2). The “spike” is probably part of the very fast “spur” recombination process seen at low temperature and the slower decay that which occurs homogeneously between the “free” ions. In methylcyclohexane at 20°Cat 1000 rim we find GE is
the ratio of mobilities of electrons and negative ions in I!-hexane is found to be ca. 80. The estimated value for c-hexane is also consistent with its measured mobility. _4llowing for this enhanced mobility of the electron, the calculated diffusion controlled rate constant for its reaction with a neutral species (e.g., pyrene or CC14) in methylcyclohexane is ca. 5 X 101* M-l set-l. By measuring the effect of 5 PM and 10 ,uM concentrations of these &JO solutes on the decay of the electron absorption (fig. 3) we fmd k = 1 X lOL* M-l set-l for each. Contrary to previous observations [3] at low temperature we fmd that the rate of appearance of the pyrene anion is in line with the disappearance of the solvated electron.
I .l X IO3 (G is yield of free ions per 100 eV; E is mo-
lar decadic absorption coefficient) aEd assuming G = 0.12 as for c-hexane [5], E = 0.95X 104. Using this value of E, fig. 2 gives k = 8 X 1013 M-l set-l for the ion recombination process in met!rylcyclohexane and 2 X 1Ol4 M.-l set-1 in n-hexane. We also estimate ca. 1 X 1015M-l set-’ in c-hexane. In n-hexane at 2O’C having viscosity 0.3 cP, the calculated diffusion controlled rate constant between neutral species is 2 X 10’” M-l set-l and including the Debye factor for reaction between oppositely charged small ions, this increases to 3.0 X 1012 M-l set-I. Hence it would seem that here the electron has a diffusion constant some 60 X greater than that for normal ions. This is in reasonable agreement with thz conclusion from conductivity measurements [7,8] where
Reverences [ 11 W.H. Hamill, in: Radical ions, eds. E.T. KaiSer and L. Kevan (Wiley, New York, 1968) ch. 9. [Z] I-4. Taub and HA. Gillis, J. Am. Chem. Sot. 91 (1969)
650. [3] J.T. Richards and J.K. Thomas, Chem. Phys. Letters 10 10 (1971) 317. [4] H.A.Gillis, N.V. Kksan,G.G. Teather and K.H. Lokan, Chem. Phjs. Letters 10 (1971) 481. [S] SJ. Rzad, P.P. Infelta, J.M. Warman and R.H. Schuler, J. Chem. Phy?. 52 (1970) 3971. [6] SJ. Rwd and Jhl.Wasman, J. Chem. Phys. 49 (1969) 2861. [7] W.F. Schmid! and A.O. Allen, .J. Chem. Phys. 52 (1970) 4788. [8] RM. Mbday, LD. Schqidt arid H.T. Datis, J. Chem. Phys. 54 (1971) 3112.
.
348,