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Relaxation of excited methyl radicals produced in the flash photolysis of dimethyl mercury
Volume 5, number 1
CHEI\;IICAL PHYSICSJ,ETTERS
RELAXATION IN THE
OF FLASH
EXCITED
METHYL
PHOTOLYSIS
OF
15 February L970
RADICALS
PRODUCED
DIMETHYL
MERCURY
A. B. CALLEAR and H. E. VAN DEN BERGH Pll_ysical Chenuktry Laboratory,
Lensfield
Road, Cambridge,
UK
Received 1?1January 1970
In the flash photolysis of Hg(CH3:2it is shown that approsimntely 85% of the CH3 radicals are produced in escited vibrational states. Hate coefficients were measured for relaxation of the free radicals by He, Xr. 02, CH4. C2H6 and SFg. and it is suggested that the rate is controlled by the out-of-plane bending mode. The combination of CH? radicals showed no pressure dependence do\\nto total pressures of 3 torr.
We have recently discovered that approximately 85% of the methyl radicals, produced in the flash photolysis of dimethyl mercury, are formed in excited states. From a study of the relaxation rate in various gases, it is evident that the free radicals are excited vibrationally. The measurements consisted solely of determinations of the time dependence of the concentration o,f radicals with zero-point energy, via the 2160A Sand [ 11; no electronic transitions of excited radicals (hot bands) were detected. Excess vibrational energy was shown to have negligibie effect on the combination rate and caused no measurable enhancement of reactivity with a variety of substrates. There are no previous reports of determinations of the rate of vibrational relaxation of the methyl radical. The experimental technique was identical to that which ye employed to determine the f-value of the 2160A band [2]. Practically all the experiments were conducted with a 20 kV flash unit at 855, [Hg(CH3)2] = 0.03 torr, an optical path length of 80 cm, and an initial gas temperature of 295(+2)OK. With the low flash energy and helium added to 3 torr, from the observed degree of photolysis [2] the temperature rise was computed to be -4OOK immediately after flashing and -8OoK following combination. Within an experimental error of &lo%, the rate coefficient for CH3 combination, with high inert gas pressures, was shown to be the same at 2950K as at 400oK [2]: therefore with the conditions given above, any effect of temperature rise on the CHQ can be neglected
with
[H&l % 3 torr.
1 shows the time aependence of the concentration of methyl radicals with zero-point enFig.
zgo
I
I
10
20
I 30
I
I
I
I
I
40
50
60
70
80
Time
delay
I 90
I
1
100
110
(psec)
Fig. 1. Time dependence of the pop&tion of methyl rndicals with zero-point vibrational energy in the flash photolysis of dimcthyl mercury. Photol_ytic flash czerDimcthyl mercury pressure 0.03 torr. -7 85 joules.
ergy ([CH:]) at various [He]. The form of the photolytic flash approximated closely to the analytical function t exp (-at) (t =time) where cx = = 7x 105 set-1 for the far ultraviolet. The spectroscopic flash was of slightly longer duration and the ‘set’ time-delays of fig. I strictly correspond to a mean [CH!$] eve; a 6 Ctsec interval. With [He] = 50 torr, the [CH3] is close to the maxipum at zero delay; with [He] = 3 torr, the 2160A absorption is about a of that with [HeI high at zero delay, and continues to rise after the termination of the initiating flash. Dimethyl mercury absorbs ligi$ in a banded system between 1850 and 2150A corresponding to 23
Volume 5, number
CIIEMICAL PHYSICS LETTERS
1
-85 kcal/Einstein
in excess of the heat of the
reaction Hg(CH3)2 - Hg + 2CH3. and the Hg(CH3)2 should dissociate within roughly IO-10 set of excitation. The rate of formation of atomic mercury was independent of the [He] and was consistent with instantaneous generation by the photoflash. In the flash photolysis of 0.03 Torr of dimethyl mercury with 2 Torr of a hydrocarbon, the time profile of the [CH:] was similar to that found with [He] = 50 torr. Evidently the methyl radicals are predominantly formed in excited states which populate the zero-point following collisional relaxation. The experimental results can be accounted for with a single relaxation time for deeactivation of the excited methyl radicals ([CH3]), and the rate coefficient for relaxation by helium is given by kHe = [He]-ld
Relative
by various
15
February 19’70
rate constants for relaxation of CH$ gases are listed in table 1. The [CH$
was measured at 5psec delay either with 15 > [He] 2 3 torr or with [He] = 3 torr + var-
ious pressures of added gases (M); relative coefficients were recorded fr2m relative d[CH!j]/d[M] as [M] - 0. Efficient
deactivation
by the polyatomic
rate
mole-
cules and comparatively slow deactivation by monatomic gases and 02 is characteristic of vibration relaxation. In general both CH4 and SF6 are extremely inefficient at inducing electronic transitions. The first three molecules listed in table 1 cause vibration-translation relaxation. whereas the polyatomic molecules can accept energy internally by vibrational exchange. The Lambert-Salter [4] empirical correlation predicts that in the relaxation by Ar the rate controlling frequency is about 450 cm-l which may suggest that the vibrational relaxation of the methyl radical is controlled by de-excitation of the out of plane bending mode. The frequency is not known for the free radical in the gas phase [5]. Since the Hg(CH3); is formed via a
after the termination of the photolytic flash. The results of fig. 1 for [He] = 3 torr yield he = = 8(*2) x lo-l3 cm3 set-l where the quoted error is twice the standard deviation. We have also
distributed
computed solutions of the differential rate equations with the Runge-Kutta method to obtain es-
quency modes of CH3 (- 1500 and 3000 cm-l) would be excited. To account for the results of
sentially the same result. T$e best fit to the experimental [CH$lt was found with 0.85 of the methyl radicals formed in excited states. In both methods it was assumed that excitation of the methyl radical does not affect the combination rate with [He] 3 3 torr. which is clearly inOdicated by the invariance on [He] of the 2160A nbsorption at the long delay times. This also shows that there is negligible total pressure dependence of the combination rate down to [He] = = 3 torr; the calculations of Rabinovitch and Setser [3] predict a decrease of -40% in the recombination rate at this pressure (298oK). Table 1 Relative rate coefficients and collision numbers for relaxation of excited methyl radicals Deactivator
Relative rate coefficient
He
Ar 02
24
1
Number of collisions for relaxation 500
1.55 (*O.lS)
200
3.4
(i’O.34)
100
CR4
13.9
(* 1.4)
35
CZHS
17.2
(I 1.7)
30
SFIZ
19,3
(i 149)
30
banded system. it should be sufficiently long lived for the excess energy to be statistically before
dissociation
and
the
high
fre-
table 1 we need to postulate that intramolecular transfer, converting energy in high frequency modes to multiple quanta in the out-of-plane bending mode, is easily induced in collision. Vibrationally excited methyl radicals do not exhibit any degree of enhanced chemical reactivity that is observable with this technique. With 5 torr OI added C2H4. H2, 02 or saturate6 hydrocarbons, at long delay times the 2160A absorption merged Co within 10% of that found in 50 torr of helium. Quantum yields for reaction of photochemically produced, hot radicals are generally small [6]. Rebbert and Ausloos [Y] concluded that hot ethyl radicals are deactivated in a single collision with CO2. Presumably ‘deactivation’ corresponds to removal of a small fraction of the energy of the radical and is expected to occur much more rapidly than full vibrational relaxation. Oxygen is curiously inefficient at including either vibrational relaxation or reaction of methyl radicals. Nitric oxide, however, efficiently relaxes and destroys the free radicals, even with low total pressures, as shown in fig. 1. Absolute reaction rates of CH3 with 02 and NO are not accurately known [8] and we plan 10 investigate the reactions in more detail with this method.
Volume
5, number
1
We are presently emission of infrared Hg(CH3)2.
CHEMICAL PHYSICS LETTERS attempting to stimulate radiation from flashed
the
We are grateful
to the European Space Research Organisation for a Research Fellowship awarded to H. E. Van den Bergh.
REFERENCES and .J.Shoosmith. Can. J. Phys. 34 (19%) 523: G. Herzberg. Proc. Roy. Sot. A2ti2 (1961) 291. [Z] - _ H. E. Van den Beroh. A. B. Callear and Ii. J. Kor[l]
G. iierzberg
Strom, Chem. Phys: Letters 4 (1969) 101. [3] B. S. Rnbinovitch and D. W. Setser. in: Advances in Vol. 3 onterscience. Ncxv York. photochemistry. 190’4) p. 1. [4] J.D. Lnmbert and S.Salter, Proc. 110~.Sot. A253
15 Februar?_ 1970
[5] G. Herzberg, Electronic spectra of polatomic molecules (van Nostrand--New York. I-966) p_ 5LA. WI D. P. DIXDIW nnd P. Ausloos. J. Chem. Phvs. 41
(19G4) 18k: IL D. Schultz and ii. A. Taylor, J. Chem. I?h>-s. lS (1930) 197: G.M. Harris and J. E. Willard, J. Am. Chem. Sot. 76 (1934) 46’78: R. D. Souffie, R. R. Williams Jr. and \V. Ii. Efzmill. J. Am. Chem. Sot. 78 (1936) 917. [71 R. E. Rebbert and P. Ausloos, J. Chem. Phys. 17 (1967) 2849. E. R. Allen and K. Ii’. Baglex, Bel;. Bunsenges. PhysikChem. 72 (1969) 22i: M. I. Christie, Proc. Roy. Sot. A249 (1959) 256: 0. E. Boarc and C.S. Pearson, -Advances in photochemistry. Vol. 3 (Interscience. New So&, 196i) p. 8: J. Heicklen and N. Cohen. in: Advances in photochemistry. Vol. 5 (lnterscience. New York. LSBS) p. 157.
(1957) 277.
25
CHEI\;IICAL PHYSICSJ,ETTERS
RELAXATION IN THE
OF FLASH
EXCITED
METHYL
PHOTOLYSIS
OF
15 February L970
RADICALS
PRODUCED
DIMETHYL
MERCURY
A. B. CALLEAR and H. E. VAN DEN BERGH Pll_ysical Chenuktry Laboratory,
Lensfield
Road, Cambridge,
UK
Received 1?1January 1970
In the flash photolysis of Hg(CH3:2it is shown that approsimntely 85% of the CH3 radicals are produced in escited vibrational states. Hate coefficients were measured for relaxation of the free radicals by He, Xr. 02, CH4. C2H6 and SFg. and it is suggested that the rate is controlled by the out-of-plane bending mode. The combination of CH? radicals showed no pressure dependence do\\nto total pressures of 3 torr.
We have recently discovered that approximately 85% of the methyl radicals, produced in the flash photolysis of dimethyl mercury, are formed in excited states. From a study of the relaxation rate in various gases, it is evident that the free radicals are excited vibrationally. The measurements consisted solely of determinations of the time dependence of the concentration o,f radicals with zero-point energy, via the 2160A Sand [ 11; no electronic transitions of excited radicals (hot bands) were detected. Excess vibrational energy was shown to have negligibie effect on the combination rate and caused no measurable enhancement of reactivity with a variety of substrates. There are no previous reports of determinations of the rate of vibrational relaxation of the methyl radical. The experimental technique was identical to that which ye employed to determine the f-value of the 2160A band [2]. Practically all the experiments were conducted with a 20 kV flash unit at 855, [Hg(CH3)2] = 0.03 torr, an optical path length of 80 cm, and an initial gas temperature of 295(+2)OK. With the low flash energy and helium added to 3 torr, from the observed degree of photolysis [2] the temperature rise was computed to be -4OOK immediately after flashing and -8OoK following combination. Within an experimental error of &lo%, the rate coefficient for CH3 combination, with high inert gas pressures, was shown to be the same at 2950K as at 400oK [2]: therefore with the conditions given above, any effect of temperature rise on the CHQ can be neglected
with
[H&l % 3 torr.
1 shows the time aependence of the concentration of methyl radicals with zero-point enFig.
zgo
I
I
10
20
I 30
I
I
I
I
I
40
50
60
70
80
Time
delay
I 90
I
1
100
110
(psec)
Fig. 1. Time dependence of the pop&tion of methyl rndicals with zero-point vibrational energy in the flash photolysis of dimcthyl mercury. Photol_ytic flash czerDimcthyl mercury pressure 0.03 torr. -7 85 joules.
ergy ([CH:]) at various [He]. The form of the photolytic flash approximated closely to the analytical function t exp (-at) (t =time) where cx = = 7x 105 set-1 for the far ultraviolet. The spectroscopic flash was of slightly longer duration and the ‘set’ time-delays of fig. I strictly correspond to a mean [CH!$] eve; a 6 Ctsec interval. With [He] = 50 torr, the [CH3] is close to the maxipum at zero delay; with [He] = 3 torr, the 2160A absorption is about a of that with [HeI high at zero delay, and continues to rise after the termination of the initiating flash. Dimethyl mercury absorbs ligi$ in a banded system between 1850 and 2150A corresponding to 23
Volume 5, number
CIIEMICAL PHYSICS LETTERS
1
-85 kcal/Einstein
in excess of the heat of the
reaction Hg(CH3)2 - Hg + 2CH3. and the Hg(CH3)2 should dissociate within roughly IO-10 set of excitation. The rate of formation of atomic mercury was independent of the [He] and was consistent with instantaneous generation by the photoflash. In the flash photolysis of 0.03 Torr of dimethyl mercury with 2 Torr of a hydrocarbon, the time profile of the [CH:] was similar to that found with [He] = 50 torr. Evidently the methyl radicals are predominantly formed in excited states which populate the zero-point following collisional relaxation. The experimental results can be accounted for with a single relaxation time for deeactivation of the excited methyl radicals ([CH3]), and the rate coefficient for relaxation by helium is given by kHe = [He]-ld
Relative
by various
15
February 19’70
rate constants for relaxation of CH$ gases are listed in table 1. The [CH$
was measured at 5psec delay either with 15 > [He] 2 3 torr or with [He] = 3 torr + var-
ious pressures of added gases (M); relative coefficients were recorded fr2m relative d[CH!j]/d[M] as [M] - 0. Efficient
deactivation
by the polyatomic
rate
mole-
cules and comparatively slow deactivation by monatomic gases and 02 is characteristic of vibration relaxation. In general both CH4 and SF6 are extremely inefficient at inducing electronic transitions. The first three molecules listed in table 1 cause vibration-translation relaxation. whereas the polyatomic molecules can accept energy internally by vibrational exchange. The Lambert-Salter [4] empirical correlation predicts that in the relaxation by Ar the rate controlling frequency is about 450 cm-l which may suggest that the vibrational relaxation of the methyl radical is controlled by de-excitation of the out of plane bending mode. The frequency is not known for the free radical in the gas phase [5]. Since the Hg(CH3); is formed via a
after the termination of the photolytic flash. The results of fig. 1 for [He] = 3 torr yield he = = 8(*2) x lo-l3 cm3 set-l where the quoted error is twice the standard deviation. We have also
distributed
computed solutions of the differential rate equations with the Runge-Kutta method to obtain es-
quency modes of CH3 (- 1500 and 3000 cm-l) would be excited. To account for the results of
sentially the same result. T$e best fit to the experimental [CH$lt was found with 0.85 of the methyl radicals formed in excited states. In both methods it was assumed that excitation of the methyl radical does not affect the combination rate with [He] 3 3 torr. which is clearly inOdicated by the invariance on [He] of the 2160A nbsorption at the long delay times. This also shows that there is negligible total pressure dependence of the combination rate down to [He] = = 3 torr; the calculations of Rabinovitch and Setser [3] predict a decrease of -40% in the recombination rate at this pressure (298oK). Table 1 Relative rate coefficients and collision numbers for relaxation of excited methyl radicals Deactivator
Relative rate coefficient
He
Ar 02
24
1
Number of collisions for relaxation 500
1.55 (*O.lS)
200
3.4
(i’O.34)
100
CR4
13.9
(* 1.4)
35
CZHS
17.2
(I 1.7)
30
SFIZ
19,3
(i 149)
30
banded system. it should be sufficiently long lived for the excess energy to be statistically before
dissociation
and
the
high
fre-
table 1 we need to postulate that intramolecular transfer, converting energy in high frequency modes to multiple quanta in the out-of-plane bending mode, is easily induced in collision. Vibrationally excited methyl radicals do not exhibit any degree of enhanced chemical reactivity that is observable with this technique. With 5 torr OI added C2H4. H2, 02 or saturate6 hydrocarbons, at long delay times the 2160A absorption merged Co within 10% of that found in 50 torr of helium. Quantum yields for reaction of photochemically produced, hot radicals are generally small [6]. Rebbert and Ausloos [Y] concluded that hot ethyl radicals are deactivated in a single collision with CO2. Presumably ‘deactivation’ corresponds to removal of a small fraction of the energy of the radical and is expected to occur much more rapidly than full vibrational relaxation. Oxygen is curiously inefficient at including either vibrational relaxation or reaction of methyl radicals. Nitric oxide, however, efficiently relaxes and destroys the free radicals, even with low total pressures, as shown in fig. 1. Absolute reaction rates of CH3 with 02 and NO are not accurately known [8] and we plan 10 investigate the reactions in more detail with this method.
Volume
5, number
1
We are presently emission of infrared Hg(CH3)2.
CHEMICAL PHYSICS LETTERS attempting to stimulate radiation from flashed
the
We are grateful
to the European Space Research Organisation for a Research Fellowship awarded to H. E. Van den Bergh.
REFERENCES and .J.Shoosmith. Can. J. Phys. 34 (19%) 523: G. Herzberg. Proc. Roy. Sot. A2ti2 (1961) 291. [Z] - _ H. E. Van den Beroh. A. B. Callear and Ii. J. Kor[l]
G. iierzberg
Strom, Chem. Phys: Letters 4 (1969) 101. [3] B. S. Rnbinovitch and D. W. Setser. in: Advances in Vol. 3 onterscience. Ncxv York. photochemistry. 190’4) p. 1. [4] J.D. Lnmbert and S.Salter, Proc. 110~.Sot. A253
15 Februar?_ 1970
[5] G. Herzberg, Electronic spectra of polatomic molecules (van Nostrand--New York. I-966) p_ 5LA. WI D. P. DIXDIW nnd P. Ausloos. J. Chem. Phvs. 41
(19G4) 18k: IL D. Schultz and ii. A. Taylor, J. Chem. I?h>-s. lS (1930) 197: G.M. Harris and J. E. Willard, J. Am. Chem. Sot. 76 (1934) 46’78: R. D. Souffie, R. R. Williams Jr. and \V. Ii. Efzmill. J. Am. Chem. Sot. 78 (1936) 917. [71 R. E. Rebbert and P. Ausloos, J. Chem. Phys. 17 (1967) 2849. E. R. Allen and K. Ii’. Baglex, Bel;. Bunsenges. PhysikChem. 72 (1969) 22i: M. I. Christie, Proc. Roy. Sot. A249 (1959) 256: 0. E. Boarc and C.S. Pearson, -Advances in photochemistry. Vol. 3 (Interscience. New So&, 196i) p. 8: J. Heicklen and N. Cohen. in: Advances in photochemistry. Vol. 5 (lnterscience. New York. LSBS) p. 157.
(1957) 277.
25