Kinetics of reactions between methyl and dimethyl-aminyl radicals formed in the flash photolysis of tetramethylhydrazine, tetramethylurea and tetramethyltetrazene in the gas phase

Journal of Photochemistry

and Photobiology,

A:

Chemistry,

43 (1988)

31-

31

41

KINETICS OF REACTIONS BETWEEN METHYL AND DIMETHYLAMINYL RADICALS FORMED IN THE FLASH PHOTOLYSIS OF TETRAMETHYLHYDRAZINE, TETRAMETHYLUREA AND TETRAMETHYLTETRAZENE IN THE GAS PHASE JORMA

SEETULA,

KAARLO

Physical Helsinki

Chemistry (Finland)

Laboratory,

(Received

August

18, 1987;

KALLIORINNE Helsinki

and JOUKO

University,

in revised form October

KOSKIKALLIO

Meritullinkatu

1 C, SF-001

70

28, 1987)

Summary Methyl and dimethylaminyl radicals are formed in the flash photolysis of tetramethylhydrazine, tetramethylurea and tetramethyltetrazene in the gas phase. The rate constants of radical combination and hydrogen exchange reactions between methyl and dimethylaminyl radicals were calculated with a computer integration program, using a least-squares iteration method, from the amounts of final products determined by gas chromatography. The ratio of the rate constants of the combination and hydrogen abstraction reactions between two dimethylaminyl radicals (1.55 f 0.2) is close to the value (1.59 + 0.2) for the ratio of the rate constants of the respective reactions between methyl and dimethylaminyl radicals.

1. Introduction Dimethylaminyl radicals are formed in the photolysis of the nitrogencontaining compounds tetramethylurea (TMU) [ 11, tetramethyl-2tetrazene (TMT) [ 21, dimethylacetamide [ 1, 3, 41, trimethylamine [ 51, dimethylamine [6 - 81 and dimethylnitrosamine [ 91, and in the pyrolysis of tetramethylhydrazine (TMH) [lo] and TMT [2,11 - 143. Methylaminyl radical is formed in the photolysis of methylamine [ 151. Dimethylaminyl and methyl radicals react by forming both addition and hydrogen exchange products. 2MezN’ -

MezNNMez

(1)

2Me2N* -

Me,NH + MeN=CH2

(2)

Me*N’ + Me. __+

Me3N

(3)

Me2N’ + Me- -

CH4 + MeN=CH,

(4)

2Me. +

Me 2

lOlO-6030/88/$3.50

(5) 0 Elsevier Sequoia/Printed

in The Netherlands

32

[l, 151 N-Methylenemethylamine, CH3N=CH2, polymerizes trimerizes [2, 9, 15 - 171 to 1,3,5-trimethylhexahydro-1,3,5-triazine.

and

3CH3N=CH2

(6)

-

(CHSNCHZJS

IV-Methylenemethylamine tetramethylmethylenediamine Me’

+ MeN=CH,

MeNCH;

-

+ Me2NS -

also react may by the reactions

MeZN-CH; MeNCH2NMe2

with

radicals

producing

(7) (8)

Tetramethylmethylenediamine is one of the main products of the reactions of methyl and dimethylaminyl radicals in the liquid phase [ 6 - 8, 121 whereas in the gas phase it is a minor product [ 21. The rate constants of reactions (1) - (4) have been calculated previously from an analysis of the products of the flash photolysis of dimethylacetamide [3). The flash photolysis produces high radical concentrations and the radical-radical reactions are fast compared with the radical-molecule reactions, which therefore can be neglected when calculating the rate constants of the radical-radical reactions. To test the validity of these earlier results [3] we carried out new experiments with TMH, TMU and TMT,

2. Experimental

details

2.1.Ma teria Is TMH and TMU (Fluka AG) were used as received. TMT was synthesized by the method described by Watson [Z]. Helium (AGA Co., 99.995%), NO (AGA Co., 99.0%) and NO2 (AGA Co., 98%) were used as received. 2.2. Methods A gas mixture of TMH, TMT or TMU together with 100 kPa He was introduced into a quartz tube 200 cm long and 9 mm in diameter. A quartz flash lamp situated parallel to the tube was connected to a 14.7 PF capacitor and charged at about 16 kV. The half-width of the flash was about 20 J_IS. The gas mixture was electrically heated and the temperature was kept constant during the experiments within about 2 K [ 181. The reaction products were analysed with an Aerograph 1520-1B gas chromatograph equipped with a 2 cm3 gas inlet system and a glass column (350 cm X l/16 in) filled with 80/100 mesh Chromosorb 103. Peak areas were calculated with a Luxor ABC 800 microcomputer. The products were identified by comparing the retention times with those of known compounds. Ethane was used as reference for the calculations since only relative amounts of products were needed for the calculation of rate constants. The amounts of products formed in one flash were extrapolated from the total amounts of products obtained after one to eight flashes. The

33 TABLE

1

Relative amounts of products tures of tetramethylhydrazine T

pTMH

(K)

&Pa)

295 295 295 346 402

0.67 1.33 4.00 1.33 1.33

Mean

(CzH6 = 100 units) formed (TMH) and 100 kPa He

by flash photolysis

of gas mix-

cH4

CzH6

(MeNCH2)3

MezNH

Me3N

MeNNMe

MezNNMeNMe2

37 42 43 35 38

100 100 100 100 100

40 29 26 31 29

155 141 155 143 131

51 50 60 68 67

22 27 19 20 17

4 5 9 8 5

39

100

31

145

59

21

6

(73)

The amount of dimethylamine formed by radical reactions is shown in parentheses. calculated relative amount of tetramethylhydrazine formed by reaction (1) is 109.

The

results are collected in Table 1. The small amounts of pentamethyltriazane observed in the gas chromatogram were identified by mass spectroscopy (M+ 117). The response factor of azomethane was used in calculating the amount of pentamethyltriazane. Small amounts of nitrogen were formed in the photolysis of TMH, after several flashes, owing to a secondary photolysis reaction of the product azomethane. About 5% of the TMH, 3% of the TMU and 40% of the TMT were decomposed in one flash.

3. Results The reaction products obtained in the flash photolysis of TMH are formed by the following primary photochemical reactions: Me2NNMe2 + hv +

MezNH + MeN=CH2

Me2NNMe2 + hv +

2Me2N’

(10)

Me2NNMe2 + hv +

Me’ + MezNNMe’

(11)

Trimethylhydrazyl radical may decompose and methyl radicals.

(9)

or react with dimethylaminyl

Me2NNMe’ +

Me* + MeN=NMe

(12)

MezNNMe’ -

Me2N’ + CH2=NH

(13)

Me2N’ + Me,NNMeMe’ + MezNNMe’ +

-

MelNNMeNMez Me2NNMe2

(14) (15)

Equal amounts of methyl and trimethylhydrazyl radicals are formed by photolysis of TMH (reaction (II)). The relative amount of methyl

34

radicals can be calculated from the relative amounts (ethane = 100 units) of final products measured by gas chromatography (Table 1) Me’ = 2Me2 + CH4 + MesN = 299 During the radical-radical reactions of the flash photolysis experiment trimethylhydrazyl radical partly dissociates forming 21 units of azomethane by reaction (12) and partly reacts with dimethylaminyl radical forming 6 units of pentamethyltriazane by reaction (14). The extent of the recombination of radicals to give TMH by reaction (15) cannot be measured. The extent of the reaction is expected to be small and similar to that of reaction (14). Because only small amounts of trimethylhydrazyl radicals are consumed by reactions (12), (14) and (15), trimethylhydrazyl radicals react to a large extent by other reactions, for example by reaction (13). The product of reaction (13), CH,=NH, polymerizes at room temperature and cannot be detected by gas chromatography [9] _ Reaction (13) does not interfere with the calculations of rate constants of reactions (1) - (5). According to the material balance, equal amounts of products should be obtained from hydrogen-donating and hydrogen-accepting reactions between radicals. The experimental result Me*NH + CH4 = 119 differs somewhat from the calculated value 3(CHsNCH& = 93, which may be due to the polymerization [ 1,151 of CHsN=CH2. The relative contributions of the three simultaneous reactions (9), (10) and (11) of electronically excited TMH can be estimated from the amounts of products obtained in flash photolysis experiments. The amount of dimethylamine formed by flash photolysis of TMH was reduced by about 50% when NO was added (Table 2), indicating that dimethylamine is formed partly by the non-radical reaction (9). 73 units of dimethylamine were formed by non-radical photodissociation of TMH (Table 1). The relative extents of the photodissociation reactions (10) and (11) of TMH can be estimated from the amounts of methyl and dimethylaminyl radicals. The initial amounts of these two radicals can be calculated from the amounts of products formed by reactions (1) - (5) together with reactions (12) (15). Me2NS = MesN + 2MeNH + CH4+ 2Me2NNMe2 + MezNNMeNMe2 = 443 TABLE

2

Relative amounts of products (TMH = 10 000 units) formed by flash photolysis mixtures of 0.67 kPa tetramethylhydrazine, 100 kPa He and NO at 295 K pNO

cd36

fMeNC&)

121 0.91 1.23

31 43 53

3

Me2NH

Me3N

MeNNMe

Me2NN0

Me2NNMe2

176 83 94

71 1.5 0.23

21 17 19

0 406 498

10000 10000 10000

W’a) 0 1.73 2.66

of gas

35

The amount of TMH formed by reaction (15) is unknown. It is expected to be small and comparable with the 6 units of Me,NNMeNMe* (Table 1). The amount of TMH photolysed by one flash via reaction (11) is equal to 288, which is the difference between the 299 units of methyl radicals calculated from the amounts of products of reactions (3) - (5) and the 21 units of azomethane formed by reaction (12). The relative amount of TMH photolysed by reaction (10) is equal to half the difference between the 443 units of dimethylaminyl radicals calculated from the amounts of products of reactions (1) - (4) and (14) and the relative amount of dimethylarninyl radical formed by reaction (13), which is equal to the difference between the 278 units of methyl radicals formed by reaction (11) and the 21 units of azomethane and 6 units of MezNNMeNMez (Table 1). The amount of product of reaction (10) is accordingly equal to {443 - (278 216)}/2 = 69. (In the following, the relative extent of reaction is equivalent to the amount of starting material consumed in it.) The relative extents of primary photodissociation of TMH by reactions (9), (10) and (11) obtained by this method are 17% 17% and 66% respectively. The excess energy of the methyl and dimethylaminyl radicals formed by photodissociation reactions (10) and (11) can be estimated from the N-N bond dissociation energy (176 kJ mol-l) of TMH (reaction (10)) and the C-N bond dissociation energy of TMH, which is assumed to be approximately equal to the respective bond dissociation energy (288 kJ mol-‘) of trimethylamine [lo]. The energy of the first excited electronic state S1 of TMH is estimated from the onset of the broad UV absorption at about 250 nm to be about 480 kJ mol-‘. If the excess energy is equally distributed between the products, the excess energy of the methyl radicals must be about 96 kJ malll and the excess energy of the dimethylaminyl radicals about 152 kJ mol-‘. The enthalpy, 49.1 kJ mol-‘, of reaction (16) was calculated from the values mfo((CH,),N’ ) = 161.1 kJ mol-’ [lo], M,“(CH;) = 142 kJ mol-l [22] and Mf”(CH,=NH) = 68.2 kJ mol-’ [21]. The dissociation reaction (16) of hot dimethylaminyl radicals conexcess energy could be fast and compete with taining about 152 kJ mol-’ the radical-radical reactions (1) - (4) 193. Me2N

l*

-

Me’

+ CH2=NH

(16)

The thermally equilibrated dimethylaminyl radical does not decompose during fast radical-radical reactions [9]. We used a large excess of 100 kPa He in the reaction mixtures of the flash experiments in order to obtain thermal equilibration of the radicals involved in reactions (1) - (5). Approximate thermal equilibration is obtained because each radical is expected to collide more than lo3 times with the helium atoms and about 20 times with TMH molecules before colliding and reacting with other radicals. One advantage of the flash photolysis method owing to the high radical concentrations and correspondingly fast radical-radical reactions is that the radical-molecule reactions and the wall reactions can be neglected in calculating the rate constants of the radical-radical reactions (1) - (5) from

36

the relative amounts of products. The typical radical-molecule reaction (17) is about three orders of magnitude slower than the radical-radical reactions in our experiments [ 193. CH; + CH,NDNDCH3 -

CH, + CH,NDNDCH;

(17)

The rate constants of reactions (1) - (4) were calculated from values of initial radical concentrations by integrating the rate equations, using a computer program to fit the calculated amounts of reaction products with the experimental values by least-squares iteration. The values of rate constants are calculated using the value of the rate constant k5 = 2.70 X 10” dm3 mol-r s-l obtained previously [IS]. This value is in good agreement with the value k5 = 2.79 X lOlo dm3 mol-’ s-l, which is the mean of nine different values reported previously [ 23 1. Since the amount of TMH formed in reaction (1) cannot be measured experimentally it is not possible to calculate the value of the rate constant kl. The value kl = 1.70 X 1O1* dm3 mol-l s-l was used in calculating the rate constants of reactions (2) - (4). If for example the value k 1 = 1.20 X lOfo dm3 mol-’ s-’ is used instead, the calculated values of the rate constants decrease: k2 from 1.14 to 1.09, kQ from 1.10 to 1.07 and ka from 0.69 to 0.67, all in units of 10” dm3 mol-’ s-l. The ratio of the rate constants k3/k4 remains constant, however, independent of the value of kl. The calculated values of rate constants are shown in Table 3. Photodissociation of TMU produces dimethylaminyl and methyl radicals. Me*NCONMe,

+

Me,N- + MezNCO*

Cl@

Me2NCONMe2 + hv __f

Me* + MezNCONMe’

(19)

Me*NCONMe, + hv +

Me2NH + CO + CH,N=CH,

(20)

Me2NCO- -

TABLE

hv +

Me2N’ + CO

(21)

3

Values of rate constants a gas mixture containing

of reactions (Z), (3) and (4) TMH and 100 kPa He

obtained

by the flash photolysis

Reactiorz

lo-'Ok (dm3

mol-1

s-l)

lo--‘Ok (dm3 mol-l

s-l)

of

Reference

(1)

(2)

(3)

(4)

(5)

(1.70)

1.14

1.10

0.69

(2.70)

This work

1.67

0.27

1.75

0.79

(2.70)

3

The values are based [18] kl = 1.70 x lOlo dm3 mol-’

on s-l.

k5 = 2.70

x lOlo

dm3 mol-1

se1 and the estimated

value

37

Me2N-CO-NCH;

+

Me*N’ + CO + CH2=NH

(221

When TMU is photolysed together with NO the products of radical reactions (1) - (5) except dimethylamine are almost completely quenched (Table 4). We found that 17% of dimethylamine was not quenched by NO and it is assumed to be formed by non-radical photodissociation reaction (20) of TMU. The relative amounts of radicals (CO = 100 units) formed by the photodissociation of TMU (reactions (18) - (21)) can be calculated from the relative amounts of products (Table 4). Me* = 2Me, + MesN f MeCONMe* + CH4 = 22 The amount of methane could not be measured owing to the large amount of CO formed in the photolysis of TMU. The relative amount of methane is estimated to be about 2. MezN* = 2Me2NNMe2 + 2MezNH + x = 186 + x The relative amount x of TMU formed by reaction (23) cannot be measured. Me2NCO’ + Me’ -

MeCONMe*

(23)

The fact that 259 units of MezNNO are formed by reaction between Me*N’ and NO indicates that x is about 70. Because no products of the radical Me2NCONMe’ formed by reaction (19) were observed we assume that it is dissociated completely by reaction (22). The radical Me2NCO’ is, however, more stable because small amounts of N,N-dimethylacetamide were formed by reaction (23). According to the material balance the relative amount of CO = 100 should be equal to the amount 104 calculated from the products. Me’ + (Me*N’ - Me* - MeCONMe*)/2 + MezNH(non-radical) = 104 The relative extents of the three primary photodissociation reactions (18), (19) and (20) of TMU can only be estimated because the amount x of Me2NCONMe2 formed in the reverse of reaction (18) is not known. The relative extent of reaction (18) is TABLE

4

Relative amounts of products (CO = 100 units) formed by flash photolysis ture of 0.133 kPa tetramethylurea and 100 kPa He and of a mixture with at 295 K

of a gas mix1.33 kPa NO

NO

CO

CzH6

(MeNCH,)s

MepNH

MesN

Me2NNMe2

MeCONMe2

MeNNO

0 1.33

100

7.2 0

5.0 0

42 (35) 7.1

1.6 0

58 3.1

4.2 0

0 259

The relative theses.

amount

of

dimethylamine

formed

by radical

reactions

is shown

in paren-

38

(Me*N’ +x - Me-)/Z + MeCONMes = 131 if it is assumed that x = 70. The relative extent of reaction (19) is equal to the amount of Me* = 22. The relative extent of reaction (20) is 7.1. The relative extents of photodissociation reactions (18), (19) and (20) are 81.9%, 13.7% and 4.4% respectively. The flash photolysis of TMT produces dimethylaminyl and methyl radicals by reactions (24) - (27) : Me2NN=NNMe2

+ hv __f

Me2N’ + Me,NN=N’

(24)

Me,NN=NNMe*

+ hv __f

Me’ + Me2NN=NNMe’

(25)

Me2NN=N’

MesN’ + Nz

(26)

-

(27)

-

MezNN=NNMe’

MesN- + N2 + MeN=CH,

The methyl and dimethylaminyl radicals then react by reactions (1) - (5), forming the products shown in Table 5. The material balance of the amounts of products of hydrogen transfer reactions is in agreement within the experimental error: CH.,, + Me,NH = 22.8 and 3(MeN=CH), = 21.3. When a mixture of 0.133 kPa TMT, 100 kPa He and 1.21 or 2.76 kPa nitrogen oxide was photolysed the products of radical reactions (1) - (5) were almost completely scavenged (Table 6). Only about 0.23% of TMH is formed in the presence of NO compared with the amount formed in the absence of NO. A relatively large amount of about 6.7% of dimethylamine was obtained in the flash experiment with NO compared with the experiment without NO. It is expected that TMT is partly photolysed by a nonradical intramolecular rearrangement reaction: Me2NN=NNMe2

+ hv -

Me,NH + Nz + MeN=CH*

(28)

The 121 units of MezNNO formed in the flash photolysis of TMT with NO is approximately equal to the 112 units of dimethylaminyl radicals formed in the flash photolysis of TMT without NO calculated from the relative amounts of products. Me2N’ = 2(23.3 -

1.5) + (2 X 32.6) + 2.6 + 0.8 = 112.2

The relative extents of the three different photolysis reactions (24), (25) and (28) of TMT can be estimated from the amounts of methyl and TABLE

5

Relative amounts of products (IV2 = 100 units) formed by flash photolysis ture of 0.133 kPa tetramethyltetrazene and 100 kPa He at 295 K

of a gas mix-

~-N2

CH4

C2H6

PfeNCH2)

100

0.82

1.05

7.1

The relative theses.

amount

3

of dimethylamine

Me2NH

Me3N

MeNNMe

MeqNNMe2

23.3

2.6

0.55

32.6

(21.8)

formed

by radical

reactions

is shown

in paren-

39 TABLE

6

Relative amounts (CzH6 = 100 units) of mixture of 0.133 kPa tetramethyltetrazene,

products formed by flash photolysis 100 kPa He and NO at 295 K

of

a gas

NO

MezNH

MeJN

MeN=NMe

Me2NNMe2

Me*NNO

0 1.21 2.76

23.3 1.9 1.2

2.6 0 0

5.5 5.4 6.5

32.6 0.10 0.05

0 126 114

dimethylaminyl radicals and dimethylamine formed in the three reactions respectively. Me- = 0.82 + (2 X 1.05) + 2.6 = 5.5 The relative extent of reaction (24) is 112.2 - 5.5 = 106.7, the extent of reaction (25) is 5.5 and that of reaction (28) is 1.5, or 93.9%, 4.8% and 1.3% respectively. Rate constants of the reactions (1) - (4) between methyl and dimethylaminyl radicals were not calculated from the values of the amounts of products obtained by flash photolysis of TMT because part of the small amounts of methyl radicals and their reaction products methane, ethane and trimethylamine could have been formed by reactions other than (1) (4), such as radical-molecule reactions involving hot radicals. These reactions would strongly affect the calculated values of rate constants of reactions (1) - (4). When TMT in an argon matrix at 18 K was photolysed using a mercury lamp, only nitrogen, dimethylamine and ALmethylmethaneimine were formed, and no TMH. The ratio of rate constants kl/k2 is zero at 18 K, owing to some kind of interaction with the argon matrix. No temperature effect would be expected because the activation energy should be approximately zero for the fast radicalradical reactions. 4. Discussion The values of rate constants of reactions (1) - (4) obtained in the present work can be compared with the values obtained in our earlier work [3] involving the flash photolysis of dimethylacetamide (Table 3). The values of the rate constants of reactions (3) and (4) between methyl and dimethylaminyl radicals are in agreement within the experimental error. However, the values of the rate constants for reactions (1) and (2) between two dimethylaminyl radicals differ widely in the two studies. In this work the rate constant k, = 1.70 X lOfo dm3 mole1 s-l was estimated in order to calculate the amount of TMH formed by reaction (1). When a larger value of k 1 was used in the calculations, the values of k3 and k4 were unreasonably small.

40

The ratio of rate constants k,/kz should be equal to the ratio of the amounts of products TMH to dimethylamine of reactions (1) and (2) if these products are formed by reactions (1) and (2) only. Using the values given in Table 1 we calculated kI/kz = 1.49. From the amounts of products obtained by Watson [2] in the photolysis of TMT in the gas phase the ratio k,/kz = 0.71 can be calculated, and from the amounts of products obtained in the pyrolysis of TMT, k1/k2 = 0.95. Radical concentrations were low in Watson’s experiments and dimethylamine may have been formed by a hydrogen abstraction reaction between dimethylaminyl radical and TMT as well as by reaction (2). Values of the ratio kI/k, of the rate constants calculated from Watson’s data should therefore be taken as a lower limit. Similarly, using the amounts of products obtained by Hancock [6] by photolysing dimethylamine in carbon tetrachloride, a lower limit value of kl/ka = 0.69 can be calculated. From the amounts of products obtained by Jones and Gesser [l] by photolysing TMU and dimethylacetamide, lower limit values of k Jk2 = 0.13 and 1.47 respectively can be calculated. These values show that our value k,/k, = 1.49 is acceptable and could not be much smaller. It is nevertheless uncertain because kl could not be calculated. The rate constants of reactions (1) - (4) could not be calculated from the experimental results of the flash photolysis of TMU because the amount of methane could not be measured. However, the ratio of rate constants k Jkz = 1.66 was obtained from the amounts of products of reactions (1) and (2). This value is comparable with the value kl/kz = 1.49 estimated from the amounts of products obtained by flash photolysis of TMH. The value of the ratio of rate constants kI/kz = 32.6/21.8 = 1.49 was calculated from the amounts of products of reactions (1) and (2) formed by flash photolysis of TMT as shown in Table 5. The amount of dimethylamine was corrected for the small amount by the non-radical reaction (28). This value is comparable with the values of the same ratio, 1.66 and 1.49, obtained by flash photolyses of TMU and TMH respectively. The mean value of the ratio of the rate constants of the addition and disproportionation reactions between two dimethylaminyl radicals, k Jkz = 1.55 * 0.2, is close to the ratio of the rate constants of the reactions between methyl and dimethylaminyl radicals, k3/k4 = 1.59. The difference in the enthalpies of reactions (1) and (2) can be calculated from the values of the bond dissociation enthalpies as follows [lo, 20] : AIfN_--N(

Me2NNMe2) -

= 140 kJ mol-l

-

AHc_n(Me2N’)

+ MN_n(MeZNH)

x

And the difference in the enthalpies of reactions (3) and (4) can be calculated as A.EI= -

AEIc_,( Me sN) -

A%-u(MeZN-

= 147 kJ mol-1 -x where x = AZYc-,(Me,N’

).

) + A&-&H,)

41

From the similarity of these two values (140 and 147 kJ mallI) it is not unexpected that the values of the ratio of the rate constants k,/kz and k3/k4 are similar: 1.55 and 1.59 respectively. Hancock and Dickenson [7,8] found only minor amounts of about 1% of the dimethylaminyl radical combination product TMH in non-polar solvents at room temperature. We obtained a similar result by photolysing TMH in a solid argon matrix at about 10 K. Rice and Gnelecki [24] found that dimethylaminyl radicals were stable when trapped at 77 K. The stability is due to slow diffusion of the radicals in the solid matrix. The value of the ratio of the extents of combination and hydrogen abstraction reactions of dimethylaminyl radicals is 1.55 in the gas phase. The different values of the ratio of the two reactions of dimethylaminyl radicals in the gas phase and in liquid or solid phase could be due to different kinds of collision leading to different products. For the combination reaction a close contact between the two radicals during the collision is necessary whereas the hydrogen abstraction reaction could occur at somewhat larger distances, for example when the radicals are separated by one solvent molecule in solution. In the gas phase both close contact collisions and distant collisions occur and may favour either a combination or a hydrogen abstraction reaction respectively. References 1 P. W. Jones

and H. D. Gesser, J. Chem. Sot. B, (1971) 1877. 2 J. S. Watson, J. Chem. Sot., (1956) 3677. 3 J. Seetula, K. Blomqvist, K. Kalliorinne and J. Koskikailio, Acta Chem. &and., Ser. A, 40 (1986) 658. 4 C. H. Nichols and P. A. Leermakers, J. Org. Chem., 35 (1970) 2754. 5 H. Gesser, J. T. Muilhaupt and J. E. Griffiths, J. Am. Chem. Sot., 79 (1954) 4834. 6 K. G. Hancock and D. A. Dickenson, J. Chem. Sot., Chem. Commun., (1973) 783. 7 K. G. Hancock and D. A. Dickenson, J. Org. Chem., 39 (1974) 331. 8 K. G. Hancock and D. A. Dickenson, J. Org. Chem., 40 (1975) 969. 9 G. Geiger and J. R. Huber, Helu. Chim. Acta, 64 (1981) 989. 10 J. A. Kerr, R. C. Sekhar and A. F. Trotman-Dickenson, J. Chem. Sot., (1963) 3217. 11 P. Heymanns and P. Rademacher, Tetrahedron, 42 (1986) 2511. 12 A. Good and J. C. J. Thynne, J. Chem. Sot. B, (1967) 684. 13 B. G. Gowenlock, P. Jones and D. R. Snelling, Can J. Chem., 41 (1963) 1911. 14 C. J. Michejek and W. P. Hoss, J. Am. Chem. Sot., 92 (1970) 6298. 15 E. P. Gardner and J. R. McNesby, J. Phys. Chem., 86 (1982) 2640. 16 C. R. C. Lindley, J. G. Calvert and J. H. Shaw, Chem. Phys. Lett., 67 (1979) 57. 17 B. G. Gowenlock and K. E. Thomas, J. Chem. Sot. B, (1966) 409. H. Lemmetyinen and J. Koskikallio, Finn. Chem. 18 M.-L. Pohjonen, L. Leinonen, Lett., (1974) 139. 19 P. Gary, A. A. Herod, A. Jones and J. C. J. Thynne, Trans. Faraday SOC., 62 (1966) 2774. 20 D. M. Golden, R. K. Solly, N. A. Gac and S. W. Benson, Int. J. Chem. Kinef., 4 (1972) 433. 21 S. W. Benson, Thermochemical Kinetics, WiIey, New York, 1976, 2nd edn., pp. 278, 299. 22 M.-L. Pohjonen and J. Koskikallio, Acta Chem. Stand., Ser. A, 33 (1979) 449. 23 J. A. Kerr and S. J. Moss, Handbook of Bimolecular and Termolecular Gas Reactions, Vol. II, CRC Press, Cleveland, OH, 1981, pp. 33, 77, 78. 24 F. 0. Rice and C. J. Gnelecki, J. Am. Chem. Sot., 79 (1957) 2679.