J. Phys. Chem. 1995, 99, 13126-13131
13126
Rate Constants for Reactions of Substituted Methyl Radicals (CH20CH3, CH2NH2, CH21, and CH2CN) with 0 2 Akira Masaki and Shigeru Tsunashima Department of Applied Physics, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152, Japan
Nobuaki Washida" Division of Atmospheric Environment, The National Institute for Environmental Studies, 16-2, Onogawa, Tsukuba, Ibaraki, 305, Japan Received: October 28, 1994; In Final Form: May 8, 1995@
Reactions of the substituted methyl radicals CH20CH3, CH2NH2, CH21, and CH2CN with 0 2 have been studied by a combination of pulsed laser photolysis and photoionization mass spectrometry. The reactivity is greatly enhanced by substituting one of the hydrogen atoms of a methyl radical with the electron-donating substituents, and (3.5 f 0.4) x -0CH3 and -NH2. The rate constants obtained at 298 f 5 K were (6.5 & 0.7) x lo-' I in units of cm3*molecule-'-s-' for CH20CH3 and CH2NH2, respectively, and no pressure dependence was observed between 0.6 and 6 Torr (80-800 Pa) of N2 as a bath gas. The reactivity is reduced for CH2I and CHzCN, which have the electron-withdrawing substituents -I and -CN. The rate for the reaction of CH2I with 0 2 was independent on the total pressure between 2 and 15 Torr (270-2000 Pa) of N2, and the rate constant was determined to be (1.6 0.2) x 10-l2 ~m~*molecule-~*s-'. The rate constants of CH2CN with 0 2 depended on the total pressure, and were (6.8-11.0) x ~m~*molecule-~*s-l for total pressures 2-6 Torr (270-800 Pa) of N2 0 2 . The reactivity of substituted methyl radicals is discussed by the correlation with ionization potentials and electronegativities of radicals.
*
+
1. Introduction There have been numerous kinetic studies on termolecular reactions of alkyl type free radicals with molecular oxygen. Rate constants for these reactions range from 1 x to 30 x lo-'* cm3molecule-'.s-' at room temperature. The limiting high-pressure rate constants for alkyl radicals 0 2 reactions are known to correlate with the ionization potentials of the radicals;, a radical whose ionization potential is low tends to react with a large rate constant. Recently, we proposed a similar correlation for rates of reactions of hydroxyalkyl radicals with
+
02.11
Since hydroxyl is an electron-donating substituent, ionization potentials of hydroxyalkyl radicals are relatively lower than those of alkyl radicals, and the reaction rate constants with 0 2 increase. In the present study, the rate constants for the reactions of methoxymethyl, aminomethyl, iodomethyl, and cyanomethyl radicals with 0 2 have been measured:
+ 0, + M - 02CH20CH, + M CH2NH2+ 0, + M - 02CH2NH2+ M CH21+ 0, + M - 02CH21+ M CH2CN + 0, + M - 02CH2CN+ M
CH,OCH,
(1) (2)
2. Experimental Section The details of the experiment had already been shown in previous articles." The radical precursors and 0 2 were introduced into a tubular Pyrex reactor (13 mm i.d.) with N2 as carrier gas. Pulsed 193 nm radiation from an ArF excimer laser (Lambda Physik, LPX 120 icc) was directed along the axis of the reactor. CH20CH3 was generated by 193 nm pulse photolysis of CC14 followed by the H atom abstraction reaction of C1 atom with CH3OCH3:
+ hv,,, - CCl, + C1 CC1, + hv,,, - CCl, + 2C1 CH,OCH, + C1- CH,OCH, + HCl CCl,
H,NCH,CH,NH,
(4)
* To whom correspondence
should be addressed. Abstract published in Advance ACS Abstracts, August 15, 1995.
0022-3654/95/2099-13126$09.00/0
(5) (6)
(7)
CHzNH2 was generated by pulsed photodissociation of ethylenediamine, NH2CH2CH2NH2, using the ArF laser:
(3)
-0CH3 and -NH2 are typical electron-donating substituents, while -I and -CN are electron-withdrawing substituents. The present study focused on (i) measuring the kinetics of these four new radicals and (ii) examining how fast the rate constants of
@
the above four reactions are, since the ionization potentials of the former two radicals are expected to be lower and the latter two higher than the unsubstituted methyl radical.
+ hv,,, - 2CH,NH2
(8)
CH2I and CHzCN were produced by the photodecomposition of chlorinated precursor molecules by ArF laser pulse.
+ hv,,, - CH21+ C1 C1CH2CN + hv,,, - CH2CN + C1 CH2ClI
(9) (10)
CH3OCH3 (99%, Tokyo Kasei), CCl4 (98%, Wako Pure Chemical), ethylenediamine (EDA) (99%, Wako Pure Chemical), CH2ClI (99%, Wako Pure Chemical), ClCH2CN (99%, 0 1995 American Chemical Society
Reactions of Substituted Methyl Radicals 1
'
40
'
"
42
'
'
44
'
'
46
J. Phys. Chem., Vol. 99,No. 35, 1995 13127 " I
48
MASS NUMBER(m/e)
81
[021 = 0 P a
bl
[021,=
I
0 . 2 2 1 Pa
1
Figure 1. Part of the mass spectra of the products of 193 nm photolysis: [total] = 560 Pa, [CH30CHJ = 0.27 Pa, [CC14] = 0.075 Pa, and [ C H Z O C H ~= ] ~1.2 x 10" mole~ules.cm-~.
Wako Pure Chemical), 0 2 (99.9%, Nippon Sanso), and N2 (99.9995%, Nippon Sanso) were used as delivered. Part of the gas in the reactor was sampled through a Pyrex orifice (diameter = 0.3 mm) located in the wall of the reactor, and the time dependences of the radical concentrations were recorded directly by photoionization mass spectrometry. For a few experiments with the CH2I 0 2 reaction, a Pyrex orifice of smaller pin hole size (diameter = 0.2 mm) was used in order to carry out the higher total pressure experiments. A Xe resonance lamp having a sapphire window (8.44 eV) was used to ionize CH20CH3 and CH2NH2. A Kr resonance lamp with a MgF2 window (10.3 and 10.64 eV) and an Ar resonance lamp having a LiF window (1 1.62 and 11.83 eV) were used to ionize CH2I and CH2CN, respectively. The rate constants for reactions 1-4 were determined by measuring the decay rates of the time dependences of radical concentration under various 0 2 concentrations. All the measurements were carried out at 298 f 5 K.
+
8
0
16
TIME / m s e c Figure 2. Typical temporal profile of CHlOCH3+: (a) in the absence of 02; (b) [02] = 0.221 Pa. The experimental condition is shown in the fifth line in Table 1. Solid lines through the data are the exponential function (eq 11) fitted to these points by a least-squares method. The first-order decay constants (k') of CH20CH3+ are 34 1 and 418 k 9 s-' for (a) and (b), respectively.
*
3. Results and Discussion 1. Reaction of CH~OCHJwith 0 2 . The mixture of CCld CH3OCH3iN2 flowed into the reactor and was photolyzed by the 193 nm laser pulse. An ion signal at mle = 45 was observed when the spray of gas emanating from the orifice was photoionized by the Xe resonance lamp (8.44 eV). The mass spectra of the ion signal is shown in Figure 1. Only the signal at mle = 45 was observed; this can be attributed to CH20CH3 radicals generated by processes 5-7, since the signal intensity was proportional to the CCld concentration in the presence of excess amount of CH30CH3. The ionization potential of the CH20CH3 radical is reported to be 6.94 eV.I2 From the observed signal to noise ratio, the detection limit of CH20CH3 was estimated to be about lo9 molecule~cm-~. A small signal at mle = 46 (about 2% of the mle = 45 signal) is originates in I3C isotope of the CH3OCH3 radical. A signal at mle = 46 due to the parent CH3OCH3+ ion was not observed (no signals at mle = 45 and 46 were observed in the absence of C1 atoms). This is reasonable because the ionization potential of CH30CH3 (9.96 eV)I3 is higher than the energy of ionizing light (8.44 eV). Figure 2, a and b, shows the time profiles of the CH20CH3+ signal in the absence and in the presence of 0 2 . The rate constant for reaction 7 has been reported to be 1.76 x ~m~.molecule-~*s-' . I 4 Therefore, the rapid rise of the signal of CH20CH3+, as shown in Figure 2, is expected. The decay shown in Figure 2b reflects the decrease of CH20CH3 by reaction 1. Other side reactions, such as radical-radical reactions could be disregarded, since the initial radical concentration was always kept low ( < 2 x 10" molecule~cm-~).
0
0.2
0.1
02 PRESSURE / Pa
Figure 3. Typical plot of the decay rate of CH20CH3' against
0 2
pressure. The experimental condition is shown in the seventh line in Table 1.
Hence, the decay should be governed by a single-exponential function: [CH20CH3]= [CH20CH3], exp(-k't)
(1 1)
Here, [CH~OCH~IO stands for the initial radical concentration and 'k stands for the decay rate of the radical written as
+
k' = k [ 0 2 ] k,
(12)
where k is the overall rate constant for reaction 1, and k, represents other loss processes (pumpout from the cell, loss on the walls and reactions with parent molecules or with impurities), which are responsible for the decay shown in Figure 2a (e40 s-l). The values of k' were determined for various 0 2 concentrations by fitting the decay profiles of CH3OCH3+ to eq 11. The value of k was obtained from the slope of the plot of k' against the 0 2 pressure according to eq 12. One of the typical plots of k' vs [ 0 2 ] is shown in Figure 3. In this case the slope gives the value of kl to be (6.3 f 0.5) x ~ m ~ m o l e c u l e - ~ . and s - ~ k, to be 43 f 9 sec-I. The measurements were carried out under various total pressures and initial radical concentrations. The results are summarized in Table 1. Indicated error limits of the rate
13128 J. Phys. Chem., Vol. 99, No. 35, 1995 TABLE 1: Rate Constants for CH20CH3
+
Masaki et al.
0 2
and CH2NH2
+
0 2
10-'6[total], molecules cm-3
10-'2[Cc14], molecules cm-3
10-"[CH3OCH~], molecules cm-3
laser, mJ1 pulse cm-?
10-"[CH2OCH3]o," molecules cm-3
1.9 3.2 9.6 16 16 16 19
26.5 16.6 13.8 6.4 13.8 17.3 12.2
134 83 67 115 64 118 112
5.8 5.0 5.0
19.6 10.6 8.8 4.1 8.8 12.8 7.8
5.0 5 .O 5.8 5.0
10-16[total], molecules cm-3
10-'2[EDA], molecules cm-3
laser, mJ1 pulse cm-?
6.4 9.6 13 16 19 19
0.86 1.06 0.90 0.77 0.64 0.80
4.2 5.8 5.8 5.8 4.6 5.8
10I2k,cm3 molecule-' s-l 6.1 & 0.7 6.4 f 1.0 6.1 f 1.0 6.7 & 0.7 6.7 & 0.7
7.0 i 0.5 6.3 f 0.5 av 6.5 i 0.7
~O-'O[CH~NH~]O," molecules cm-3
101'k,cm3 molecule-' S K I 3.4 iz 0.6 3.6 & 0.5 3.5 f 0.6 3.2 f 0.8 3.6 i 0.3 3.5 iz 0.4 av 3.5 It 0.4
3.5 6.0 5.1 4.4 2.9 4.5
" Initial radical concentration estimated by absorption cross section of CCll and EDA at 193 nm and laser power. 1
I
35
30
40
~ 8 1 1 0 2 1= 0 Pa
45
M A 5 5 NUMBER / m / e
-.-
Figure 4. Mass spectra of the products of photolysis of ethylenediamine by 193 nm.
constants represent the 95% confidence limits of the leastsquares fitting of the plots. No total pressure dependence of the rate constants was observed between 0.6 and 6 Torr (80-800 Pa). Therefore, the measured rate constants should be close to the high-pressure limit. Consequently, the limiting high-pressure rate constant for reaction 1 was determined by averaging all the data listed in Table 1 as k(CH,OCH,
+ 0,) = (6.5 f 0.7) x
cm3molecule-'*s-'
The indicated error limit represents the 95% confidence limit of the averaged value. The rate for the CH20CH3 02 reaction" is about a half of that for CH20H 0 2 , 11.7 x cm3.molecule-'.s-', although the ionization potential of CH2OCH3 is lower than that of CHzOH, 7.5 eV.I5.I6 2. Reaction of CH2NH2 with 0 2 . When ethylenediamine (EDA) was photolyzed by a 193 nm laser pulse, an ion signal at mle = 30 was observed by using the Xe resonance lamp (8.44 eV), which confirms the formation of CH2NH2 radicals by reaction 8. A mass spectrum of the signal is shown in Figure 4. No signal of C H ~ C H ~ N Hwas Z + observed at mle = 44. Therefore, there appears to be no significant branching to form CH2CH2NH2 by reaction 13.
+
NH2CH,CH,NH,
+ hv,,, - CH2CH2NH2+ NH,
+
(13)
The ionization potential of the CH2NH2 radical is not known.
0
TIME
8 / msec
16
Figure 5. Typical temporal profile of CH2NH2+: (a) [total] = 530 Pa, [EDA] = 0.010 Pa, and the laser power was 4.7 mJ/(pulsecm-?); the experimental condition of (b) and (c) is shown in the last line in Table 1; (b) in the absence of 0 2 ; (c) [O?] = 0.0521 Pa. Solid lines through the data are the exponential function fitted by using a leastsquares procedure. The first-order decay constants (k') for CH*NH2+ in the displayed ion signal profiles are 182 f 8, 87 i 2, and 477 f 14 s-I for (a), (b), and (c), respectively.
From values of the heat of formation of CH2NH2+ and CH2N H 2 , 7.7213and 1.45 eV,I7 respectively, the ionization potential of CH2NH2 was calculated to be 6.3 eV. The energy of the Xe lamp (8.84 eV) should be high enough to ionize CH2NH2. The detection limit of CH2NH2 was estimated to be about lo9 molecule~cm-~.The radical concentrations were estimated by absorption cross section of EDA at 193 nm and laser power. Rate constants for reaction 2 were determined by the same procedure as described in the case of CH20CH3. Time profiles of the signal at mle = 30 are shown in Figure 5. When the concentration of the precursor molecule (EDA) is high, Figure 5a, the CH2NH2+ signal does not decay as a single exponential. This decay was too rapid to be explained by radical-radical reaction of CH2NH2. Probably heterogeneous reactions among the parent EDA molecule adsorbed on the surface of Pyrex tube
Reactions of Substituted Methyl Radicals
J. Phys. Chem., Vol. 99, No. 35, 1995 13129 TABLE 2: Rate Constants for CH2I CHiCN 0 2
+
l0-'6[total] molecules cm-3
Figure 6. Part of the mass spectra of the products of photolysis of C H X l by 193 nm.
and orifice and laser light are involved. Such strange phenomena were observed sometimes when high boiling point molecules, such as EDA or aromatics, were used as precursor molecules. When the concentration of EDA was kept less than 1 x 10l2molecules~m-3,the decay profile of the signal of CH2NH2+ was satisfactory, as shown in Figure 5b. In the presence of 0 2 (Figure 5c), the time profile of the CH2NH2+ signal shows a faster exponential decay due to reaction 2. Results were analyzed using eqs 11 and 12. All the results of rate measurements are summarized in Table 1. No total pressure dependence was observed, and the limiting high-pressure rate constant for reaction 2 was determined by averaging all the data:
+ 0,) =
1.1 0.92 0.88 0.67 0.81 0.77 0.91 46 42
(3.5 f 0.4) x lo-" ~m~*molecule-l*s-~ The rate of reaction 2 was very rapid as expected. The rate was faster than that for the reaction of the isoelectronic CH2OH 0 2 , and as fast as that for reactions of C3 hydroxyalkyl radicals, such as CH3C(OH)CH3 and CH3CH(OH)CH2, with
+
02.11
3. Reaction of CH2I with 0 2 . Ion signals for CH2I at mle = 141 were detected in the photolysis of CH2ClI by ArF excimer laser pulses. CH2I radicals were photoionized by the Kr lamp with a MgF2 window (10.03 and 10.64 eV). Formation and detection of CH2I radicals have been reported by using the photolysis of CH2ClI and photoionization mass spectrometry.l 8 Mass spectra are shown in Figure 6. In addition to the signal of CH21, signals of I+ (mle = 127) and HIf (mle = 128) can be seen. Since the signals of CH235C1and CH237C1were seen at mle = 49 and 51, respectively (not shown in Figure 6 ) , iodine atoms are also produced by photolysis of CH2ClI. Processes 9 and 14 are competing. CH,ClI
+ hv,,, - CH,Cl+
I
(14)
The origin of HI molecules is not clear (but not important in this study). The ionization potential of the CH2I radical is not known. The appearance potential to form CH2I+ from CH212 has been reported to be 10.55 eV.I9 Using values of heat of formation of CH&, CHzI, and I, which are 1.27,172.27,20and 1.11 eV,*' respectively, the ionization potential of CH2I radical was calculated to be less than 8.4 eV. This value is consistent with the ionization potential of CHzCl radicals, 8.80 eV.22 Decay of the signal of the CH# by the addition of 0 2 was observed and rates of decay were measured by the same method as in the cases of CH20CH3 and CH2NH2. Rate constants obtained under nine different conditions are listed in Table 2. No pressure dependence was observed between 2 and 15 Torr (270-2000 Pa) total pressures (mostly N2). The rate constant
and
3.9 3.2 3.2 3.7 3.2 3.9 3.2 4.2 4.2
5.5 3.8 3.6 3.2 3.3 3.9 3.7 25 23
lo'%, cm3 molecule-1s-I 1.6 f 0.3 1.8 f 0.3
1.5 f 0.2 1.5 f 0.3 1.6 f 0.2 1.6 i 0.3 1.5 f 0.3 1.6 f 0.2b 1.5 f O . l b av 1.6 f 0.2
10-'6[total] lO-'*[CICH2CN], laser, 10-'"[CH2CN]o," molecules molecules mJ/pulse molecules 1015k,cm3 cm-3 cm-3 cm-* cw3 molecule-I s-I ~
6.4 13 19
190 83 320
4.6 4.6 4.6
~
~
~~
6.8 f 2.0 7.5 f 1.1 11.0 f 1.0
22 9.7 37
Initial radical concentration estimated by absorption cross section
of CHzClI and ClCHzCN at 193 nm and laser power. Used the q5 = 0.2 mm Pyrex orifice, sensitivity of the instrument was lo-' of other measurements.
for reaction 3 was determined by averaging all the data: k(CH21
k(CH2NH2
0 2
10-"[CH2I]o,O 10-'2[CH2C11], laser, molecules mUpulse molecules cm-3 cm-2
6.4 6.4 9.6 13 16 19 21 32 48
130 135 I40 MASS NUMBER/ m / e
125
+
+ 0,) = (1.6 f 0.2) x lo-'* ~ m ~ m o l e c u l e - l * s - ~ +
It is interesting to compare the rate of the CH2I 0 2 reaction with the limiting high-pressure rate constant, k,, for the CH3 0 2 reaction. Values of k, for the CH3 0 2 reaction were reported to be 2.2 x lov1*~ m ~ m o l e c u l e - ~ *measured s-~ over the pressure range 0.25-150 bar,9 and 1.3 x ~m~molecule-~-s-' measured from 3 to IO4 torr (4 x 13 bar).23 The value of (1.8 f 0.2) x lo-'* cm3~molecule-'.s-' was recommended by the NASA Panel for Data E v a l ~ a t i o n . ~ ~ The ionization potential of CH3 radical has been reported to be 9.84 eV.25 Therefore, the previous correlation between ionization potentials and rate constants does not apply to CH2I and CH3. In this case, however, if two reactions of CH2I 0 2 and CH3 0 2 are compared by reaction cross sections, 0, the ratio of a(CH2I 0 2 ) to a(CH3 0 2 ) is about 1.4.26 This value satisfies the correlation between ionization potentials and cross sections. It is interesting that the rate for the CH2I 0 2 reaction does not depend on the pressure even in this low-pressure region. In the CHzCl 0 2 reaction, the rate constant is not reached the limiting high-pressure rate constant (km)at 1 atm of N2.27 And for CH3 0 2 , the limiting high-pressure rate constant is not approached until 100 bar of total pressure. One possible reason is the occurrence of two-body processes in the reaction of CH2I
+
+
+
+
+
+
+
+
+
+02.
CH,I
+ 0, - C H 2 0 + IO -HCOOH + I -CHI0 + OH
(154 (133) (15c)
Reactions 15a and 15b are exothermic by 152 and 491 kJlmo1, respectively. However, the UV absorption of the 02CH2I radical formed by the CH2I 0 2 reaction has been measured using pulse radiolysis at 750 Torr total pressure of SFs.28 4. Reaction of CH2CN with 0 2 . When ClCH2CN was photolyzed at 193 nm, an ion signal at d e = 40 due to CH2CN+ was observed. The Ar resonance lamp (1 1.62 and 11.83
+
Masaki et al.
13130 J. Phys. Chem., Vol. 99, No. 35, 1995 [ai
1021=
o
!
Pa
m \
*
c
1
0
,
16
8
2 Figure 7. Typical temporal profile of CH?CN+. The experimental conditions are [total] = 530 Pa, [CICH2CN] = 0.448 Pa, and [ C H r CN]o = 2.2 x IO” m o l e c u l e ~ c m - ~(a) ; in the absence of 0 2 ; (b) [O?] = 187 Pa. Solid lines through the data are the exponential function (eq 18) fitted by a least-squares method. The first-order decay constants (k’) for CH2CN+ are 5 5 iz 3 and 393 & 60 s-’ for (a) and (b), respectively.
eV) was used as the photoionization light source. When the Kr lamp (10.03 and 10.64 eV) was used, no signal of CH2CN was observed. Detection of CHzCN radicals by the photoionization mass spectrometer using the Ar lamp has been investigated by Park and G ~ t m a n .The ~ ~ ionization potential of CH2CN has been reported to be 10.87 eV.30 The appearance potential of CH2CN from CH3CN has been reported to be 14.01 eV.31 From heats of formation of CH2CN, CH3CN, and H, 2.53, 0.67, and 2.26 eV,2’respectively, the ionization potential of CH2CN was calculated to be 9.89 eV. Although there is a discrepancy of 1 eV, CH2CN radicals should be ionized by the Ar lamp. The detection limits of CHzCN was estimated to be about 6 x lo9 molec~les*cm-~.No other signal was observed except CH2CN. Figure 7, a and b, shows typical time dependences of the CH2CN+ signal. In Figure 7b, the CH,CN+ signal decayed exponentially in the presence of 0 2 , but a residual signal was present at reaction time longer than 10 ms. Since the initial radical concentration was always kept below 4 x 10” mole~ules-cm-~, the decay of the radical would not be affected by radical-radical reactions. The observed rate for reaction 4 was very slow, and about 1.5 Torr (200 Pa) of 0 2 was needed to observed the decay of CH2CN. An equilibrium between CH2CN, 02, and 02CH2CN was considered as was observed in the reaction of allyl radicals with 0 2 . 3 2 In this case, the residual signal reflects the regeneration of CHlCN by the reverse reaction: 0,CH2CN
+ M - 0, + CH,CN + M
(16)
However, reaction 16 was ruled out because the height of the residual signal did not increase when the temperature of the flow reactor was elevated up to 368 K. Another possible origin of residual signal at mle = 40 is a fragmentation of the 02CH2CNf ion. 02CH,CN
+ hv - CH2CNf + 0, + e-
(17)
Similar unidentified residual signals have been observed in the cases of the CH302 NO33 and HO2 NO34 reactions. At
+
+
Z40 = A exp(-k‘t)
+B
(18)
where 140 stands for the signal intensity at mle = 40, k’ stands for the decay rate shown by eq 12, and A and B are fitted parameters. The values of k’ were determined by fitting the time dependence of the CH,CN+ signal to eq 18. Then, the values of k’ were plotted against the 0 2 concentration, and the rate constants for reaction 4 were determined from the slope. The results are summarized in Table 2 together with the experimental conditions. As can be seen in Table 2, the rates for reaction 4 were very slow and they depended on the total pressure. 5. Trend of the Reactivity. The present results agree qualitatively with the correlation noted previously between the rate constants at the high-pressure limit and the ionization potentials of radicals. For CH20CH3 and CH2NH2 radicals having electron-donating substituents ( -OCH3 and -NH2), the ionization potentials are low (6.94 and 6.3 eV, respectively) and the reaction rates with 0 2 are enhanced compared to CH3 02. On the other hand, for CHZIand CHZCNradicals having electron-withdrawing substituents (-I and -CN), the ionization potentials are high (8.4 and 10-11 eV, respectively) and the rates of reaction with 0 2 are slower. In Figure 8, the limiting high-pressure rate constants obtained in the present work are plotted against the ionization potentials of radicals together with previous results for a l k ~ l ~and .~.~ hydroxyalkyl” radicals. As can be seen in Figure 8, the CH2NH2 radical falls close to the correlation line while CH20CH3 and CHJ radicals do not. Recently, Seetula et a1.’8.35.36 used a so-called TSED parameter as an indicator of the reactivity of substituted methyl radicals with halogen molecules. TSED parameters are calculated by the simple algebraic sum of the Pauling electronegativities of the atoms or groups attached to the radical carbon, and they indicate the strengths of the dipole-induced dipole interaction in the R 0 2 reaction. In Figure 9, the limiting high-pressure rate constants are plotted against the TSED parameters. The TSED parameter for C X I X ~ X was ~ calculated as
+
+
AEN(CX,X,X3) = x ( E N x , - EN,)
(19)
J. Phys. Chem., Vol. 99, No. 35, 1995 13131
A A
vi
-“
10.
6’
D(H-X) = [D(X-X)
e
. A
+ D(H-H)]/2 + 23(ENx, - EN,)’ (20)
Here, values of 104.2 kcal mol-’ and 2.1 were used for D(HH) and ENH, respectively. The signs (+, -) of the TSED parameter were determined by whether Hammett’s empirical parameter is positive (electron donating) or negative (electron withdrawing). Seetula has pointed out3’ that estimate of values of electronegativity (AEN) for CH20H and CH20CH3 radicals is difficult because of the inductive effect of the radical. In the present study, values of AEN of each radical were just calculated by eq 20. As can be seen in Figure 9, the correlation is not satisfactory only for the CH2NH2 radical and that for CH2OH and CHrOCH3 seems not so bad. The above results show that the reactivities of substituted methyl radicals are not just a function of the ionization potentials and electronegativities of radicals. Further studies of substituted methyl radicals, such as CH2N02, CHzF, CH2C1, CH2Br, and CH>(CO)CH3,are desirable. A determination of the limiting high-pressure rate constant of very slow reactions, such as CH2CN 02, would be especially important.
+
Acknowledgment. We thank Prof. Kyle D. Bayes (UCLA) for his kind and helpful suggestions and comments regarding this study. References and Notes ( I ) Lenhardt, T. M.; McDade, C. E.; Bayes, K. D. J . Chem. Phys. 1980, 72, 304. (2) Washida, N.; Bayes, K. D. J . Phys. Chem. 1980, 84, 1309. (3) Ruiz, R. P.; Bayes, K. D. J . Phys. Chem. 1984, 88, 2592. (4) Wu, D.; Bayes, K. D. Int. J . Chem. Kinet. 1986, 18, 547. ( 5 ) Xi. Z.; Han, W.-J.; Bayes, K. D. J . Phys. Chem. 1988, 92, 3450. (6) Wagner, A. F.; Slagle, I. R.; Sarzynski, D.; Gutman, D. J . Phys. Chem. 1990, 94. 1853.
(7) Smith, M. J. C.; Pilling, M. J. J . Phys. Chem. 1985, 89, 4713. (8) Keiffer, M.; Pilling, M. J.; Smith, M. J. C. J . Phys. Chem. 1987, 91, 6028. (9) Cobos, C. J.; Hippler, H.; Luther, K.; Ravishankara, A. R.; Troe, J. J. Phys. Chem. 1985, 89, 4332. (10) Plumb, I. C.: Ryan, K. R. In?. J . Chem. Kinet. 1981, 13, 1011. (11) (a) Miyoshi, A.; Matsui, H.; Washida, N. Chem. Phys. Lett. 1989, 160, 291. (b) Miyoshi, A.; Matsui, H.; Washida, N. J . Phys. Chem. 1990, 94, 3016. (12) Lossing, F. P. J. Am. Chem. Soc. 1977, 99, 7526. (13) (a) Rosenstock, H. M.; Drax, K.; Steiner, B. W.; Herron, J. T. J . Phys. Chem. Re$ Data 1977,6, Suppl. 1. (b) Lide, D. R. CRC Handbook of Chemist/ nnd Physics, 73rd ed.; CRC Press: Boca Raton, 1992- 1993. (14) Michael. J. V.: Nava. D. F.: Pavne. W. A.: Stief. L. J. J . Chem. Phis. 1979, 70, 3652. (15) Ruscic. B.: Berkowitz. J. J . Chem. Phvs. 1991. 95. 4033 (16) Kus, S. C.; Zhang, Z.: Klemm, R. B.; Liebman, J. F.; Stief, L. J.; Nesbitt, F. L. J . Phys. Chem. 1994, 98, 4026. (17) Benson, S. W. Thermochemical Kinetics, 2nd ed.; John Wiley and Sons: New York, 1976; pp 299. (18) Seetula, J. A.; Gutman. D.; Lightfoot, P. D.; Rayes, M. T.; Senkan, S. M. J . Phys. Chem. 1991, 95, 10688. (19) Tsai. B. P.: Baer. T.: Werner. A. S.: Lin. S . F. A. J . Phv.y. Chem, 1975, 7 9 , 570. (20) DeComo. J. J.: Bafus. D. A,: Franklin. J. L. J . Chem. Thermodvn. 1971, 3, 125. (21) Atkinson, R.: Baulch. D. L.; Cox, R. A,: Hamson, R. F., Jr.; Kerr, J. A.; Troe, J. J. Phys. Chem. Ref. Data 1992, 21, 1125. (22) Lossing, F. P. Bull Soc. Belg. 1972, 81, 125. (23) Kaiser, E. W. J . Phys. Chem. 1993, 97, 11681. (24) DeMore, W. B.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kulylo, M. J.; Howard, C. J.; Ravishankara, A. R.; Kolb, C. E.; Molina, M. J. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling, JPL Publication 92-20; Jet Propulsion Laboratory, California Institute of Technology: Pasadena, 1992. (25) Herzberg, G.; Shoosmith, J. Can. J . Phys. 1956, 34, 523. (26) A rate constant is expressed as a product of relative velocity, u, and a cross section, u. k = uu (A) Since u is proportional to I/p”?,wherep is reduced mass, the various values of u can be easily calculated. and various u should correlate with IP. Normally u does not change very much, and so it can just be ignored. But comparing CH3 0 2 with CH?I 02, the reduced masses are a lot different, 10.2 and 26.1, respectively. If the value of 1.8 x IO-’? cm3.molecule-’.s-’ was used fork, of the CH3 0 2 reaction:?l
+
+
+
So the u correlation with IP may not be so bad. (27) Fenter, F. F.; Lightfoot, P. D.; Caralp, F.; Leschaux, R.; Niiranen, J. T.; Gutman, D. J . Phys. Chem. 1993, 97, 4695. (28) Sehested, J.; Ellermann, T.; Nielsen, 0. J. Int. J . Chem. Kine?. 1994, 26, 259. (29) Park, J. Y.; Gutman, D. J . Phys. Chem. 1983, 87, 1844. (30) Pottie, R. F.; Lossing, F. P. J . Am. Chem. Soc. 1961, 83, 4737. (31) Dibeler, V. H.; Liston, S. K. J. Chem. Phys. 1968, 48, 4765. (32) (a) Ruiz, R. P.; Bayes. K. D.; Macpherson, M. T.; Pilling. M. J. J . Phys. Chem. 1981, 85, 1323. (b) Morgan. C. A,; Pilling, M. J.; Tulloch, J. M.; Ruiz, R. P.; Bayes, K. D. J . Chem. Soc., Faraday Trans. 2 1982, 78, 1323. (33) Masaki, A.; Tsunashima, S.; Washida, N. Chem. Phys. Lett. 1994, 218. 523. (34) Imamura, T.: Washida, N. Laser Chem. 1995, 16, 43. (35) Seetula, J. A,; Gutman, D. J . Phys. Chem. 1991, 95, 3626. (36) Timonen, R. S.; Seetula, J. A.; Niiranen, J.; Gutman, D. J . Phys. Chem. 1991, 95, 4009. (37) Seetula, J. A. Ann. Acad. Sci. Fenn., Ser. A , II. Chem. 1991, 234, 35-37.
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