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HF infrared emission from the reactions of atomic fluorine with methylcyanide, methylisocyanide, dimethylsulfide and dimethyldisulfide
29 November 1996
CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical Physics Letters 262 (1996) 683-688
HF infrared emission from the reactions of atomic fluorine with methylcyanide, methylisocyanide, dimethylsulfide and dimethyldisulfide Katharina Dehe, Horst Heydtmann Institut ffir Physikalische urM Theoretische Chemie, Johann Wolfgang Goethe Universiti~t, Marie-Curie-Strasse 11, D-60439 Frankfurt am Main, Germany Received 25 July 1996
Abstract In the reactive systems F + CH3CN, F + CH3NC, F + CH3SCH 3 and F + CH3SSCH 3 infrared emission spectra were recorded from HF in the fundamental region under relaxation-free conditions. The reported vibrational distributions are for F + C H 3 C N : NL=1:N~.=2 : N~.=3 =0.38:0.42:0.20; for F + C H 3 N C : N,.=u:N~._2:NL.=3: N~.=4=0.46:0.32:0.17:0.05; for F + C H 3 S C H 3 : Nw I : N~=2 : Nt.=3 = 0.36 : 0.39 : 0.25; for F + C H 3 S S C H 3 : N~.=l : N~.=2 : Nt'=3 : N~'=4 = 0.43:0.35:0.19:0.04. Rotational distributions are drawn up and discussed for F + CH3CN and F + CH3SCH3. There is strong evidence for the existence of two microscopic channels in all four systems investigated.
1. Introduction
[4]. Such distributions indicate a direct abstraction mechanism with interaction times o f the order of
Infrared emission from vibrationally-rotationally excited states of a molecule emerging from an exothermic reaction can be studied by infrared chemiluminescence. One o f the methods developed is the low-pressure arrested relaxation (AR) technique which was introduced by Polanyi and coworkers [1,2] and is used in this work. Vibrational energy distributions o f H F produced by abstracting a hydrogen atom from a primary CH bond in saturated hydrocarbons by a fluorine atom typically show an inverted distribution for the first and second excited levels (No= i / N v = 2 < 1) [3,4]. In our group F + CH 4 and F + C 2 H 6 have been studied by the abovementioned method which rendered the vibrational distributions N v = l : N o = 2 : No= 3 = 0 . 1 5 : 0 . 6 7 : 0 . 1 8 and No= i : No=2 : No=3 = 0.10 : 0.47 : 0.43 respectively
10 -14 S.
Molecules which contain heteroatoms often show vibrational distributions which are different. The reaction F + azomethane (CH3N2CH3), studied with the same apparatus as the reactions reported in this Letter, results in a noninverted H F vibrational distribution [5]. This behaviour was also reported for F+CH3CN by Bogan and Setser who found No= l : No = 2 : Nv = 3 = 0.39 : 0.33 : 0.28 [6]. The same chemical reaction yielded a somewhat different result using the flowing afterglow (FA) technique [7]. This technique, however, does not always give results consistent with the A R method [4]; we therefore shall limit ourselves to results obtained with the latter. Duewer and Setser [8] found a similar result for F + C H 3 S C H 3 : No= i : No=2 : No= 3 =
0009-2614/96/$12.00 Copyright © 1996 Published by Elsevier Science B.V. All rights reserved PH S 0 0 0 9 - 2 6 1 4 ( 9 6 ) 0 1 1 4 8 - 7
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K. Dehe, H. Heydtmann / Chemical Physics Letters 262 (1996) 683-688
0.43 : 0.30: 0.20. Noninverted HF distributions obtained under AR conditions are postulated by us to indicate intermediates which have a lifetime of at least 10 -13 s. Most likely an addition-elimination microscopic mechanism always competes with a microscopic mechanism of direct abstraction. Other examples which yielded within error limits HF vibrational distributions with no clear inversions (i.e. Nv= l : Nv=2 >/ 1) are the reactions F + 2,2'azobisisobutane, F + azodicarbonic acid dimethyl ester and F + t-butylisonitrile [9]. Most of these reagents contain methyl groups, heteroatoms with lone electron pairs and also double bonds. To separate the effects of lone electron pairs and double bonds we recently studied the energy disposal in the reaction F + 2,3-d2-butene-2 ~ HF + C4D2H 5 for which mo=l : No= 2 = 1 + 0 . 2 was found [10,11]. In the present publication we show the results for two reactions without double bonds but with lone electron pairs: F + dimethylsulfide and F + dimethyldisulfide, and also for the systems F + CH3CN and F + CH3NC which we compare with our earlier investigations and with former work [6,8].
2. Experimental The experiments were performed using the lowpressure AR technique. The apparatus has been described before [12]. It consists of a cylindrical reaction vessel with liquid-nitrogen-cooled walls connected to an effective pumping unit. Typical background pressures measured behind the cooled walls were about 1 x 1 0 - 2 Pa. The residence time for products in the observation zone was estimated to be about 5 x 10 -5 s. The infrared radiation was collected by a White cell installed inside the reaction vessel. Emission spectra were recorded with a F'FIR spectrometer (FTS-14, BIORAD) equipped with an InSb detector cooled by liquid nitrogen. The reagent inlet system used was the concentric nozzle arrangement described in a previous paper [13]. Fluorine atoms were preferably produced by passing F 2 (MAN Neue Technologie 98% purity) through a microwave discharge. With CH3NC also CF4 (Messer Griesheim, 99% purity) was used as a precursor for F atoms which led to the same results. With the dimethylsulfide and dimethyldisulfide reagents, however, the
vibrational populations were dependant on the precursor used, presumably because of interfering reactions with CF 2 radicals in these cases. Therefore, in these systems only the experiments with F / F 2 are considered here. Methylisocyanide was synthesised as described in Ref. [14]; the substance was stored at 77 K, kept in a dark bulb and used the day following the synthesis. All other substances were used as purchased after degassing in the usual manner. The purities given by the producers are as follows: 99.5% for CH3CN (Aldrich), > 99% for CH3SCH 3 (FLUKA) and 99% for CH3SSCH 3 (Janssen).
3. Results 3.1. F + acetonitrile Some examples of HF vibrational distributions are given in Table 1. A total number of 18 runs was performed. The mean of the observed distributions shows a slight inversion with a maximum at v = 2; Nv = I : Nv = 2 : N o = 3 = 0 . 3 8 " 0.42 : 0.20. These relative populations are, from our experience, certain within 10% deviations, including systematic and statistical errors. The highest observed level HF (v = 3, J = 18) corresponds to an available energy of 207 kJ mo1-1. The vibrational distribution varies more in this experiment than in others mainly because the signalto-noise ratio was poor but the results are independent of the reagent flows within the error limits. The rotational distribution shows a strong non-Boltzmann behaviour. The mean rotational distribution is pictured in Fig. 1. The upper limit for the fraction of vibration is ( f v ) m a x = 0.41; this figure was obtained neglecting any ground state population. 3.2. F + methylisonitrile HF vibrational distributions for 5 experiments of a total number of 15 runs are given in Table 1. Though the experiments were done with the concentric nozzle and separated nozzle arrangements only experiments with the concentric nozzle arrangement are shown. Fluorine as well as CF4 were used as F atom precursors and no differences were found between
K. Dehe, H. Heydtmann/ Chemical Physics Letters 262 (1996) 683-688
685
these sources. The mean vibrational distribution shows no inversion and the highest populated level is V = 1: N v = t : N v = 2 : N w 3 : N v = 4 = 0 . 4 6 : 0 . 3 2 : 0.18:0.04. The highest observed level H F (v = 4, J = 10) corresponds to an available energy of 201 kJ mol -~. The rotational distribution which is not shown is non-Boltzmann. The upper limit for the fraction of vibration neglecting the ground state is
is v = 2 ; (Nv= l : N v f f i 2 : N o = 3 = 0 . 3 6 : 0 . 3 9 : 0 . 2 5 ) . The total available energy calculated from the highest observed level (v = 3, J = 15) is 185.6 kJ m o l - i The rotational distribution shown in Fig. 2 shows a strong non-Boltzmann behaviour. The upper limit for the fraction of vibration neglecting the ground state is (f~)max 0.45.
(fv)max = 0.41.
3.4. F + dimethyldisulfide
3.3. F + dimethylsulfide
HF distributions of 5 experiments of this reaction with fluorine as F atom source are given in Table 1. A total number of 10 runs with this F atom source was performed. Six runs with CF4 as F atom source were also performed but the signal-to-noise ratio in these experiments was so poor that only the results with molecular fluorine are discussed. The mean vibrational distribution shows no inversion and the highest populated vibrational level is v = 1; (No=1:No=2 : No=3:Nv=4 = 0.43 : 0.35 : 0.19 : 0.04). The highest observed level is v = 4, J = 21 which
Five examples of the vibrational distribution of a total number of 11 runs with fluorine as F atom source are given in Table 1. Also 13 runs with CF4 as F atom source were done and the results were the same within the error limits. Because of the poorer signal-to-noise ratio of the spectra obtained with CF4 only the results with fluorine as F atom source are discussed. The vibrational distribution shows a slight inversion, and the highest populated vibrational level
=
Table 1 Selected experimental results for the reactions F + CH3CN, CH3NC, CH3SCH3, CH2SSCH 3 ~ I-IF + products. F 2 was used as a precursor in most experiments; the use of CF4 is specified Reagent (RH)
Flows (ixmol s - i )
Background
Relative populations N, (HF)
F2/CF 4
RH
pressure (10 -4 mbar)
~=1
~-2
~=3
CH3CN
15.2 30.4 19.0 19.0
110 96 80 13.9
2.0 2.0 0.35 0.7
0.38 0.41 0.40 0.46
0.44 0.43 0.45 0.41
0.18 0.15 0.15 0.13
CH 3NC
7.6 14.9 a 14.9 a 7.6 7.5 a
16.9 9.7 6.9 < 3 < 3
3.0 7.4 6.8 2.0 6.8
0.46 0.46 0.49 0.54 0.50
0.36 0.35 0.32 0.27 0.28
0.16 0.16 0.15 0.13 0.16
CH 3SCH 3
3.2 3.2 3.2 3.2 3.2
27.7 18.1 13.7 9.1 < 2
0.03 0.03 0.03 0.03 0.03
0.36 0.31 0.37 0.33 0.33
0.38 0.43 0.35 0.41 0.42
0.26 0.25 0.26 0.26 0.25
CH 3SSCH 3
4.9 4.9 1.6 6.5 1.6
34.6 15.7 6.9 1.4 I. 1
7.0 6.0 6.0 1.5 5.0
0.46 0.46 0.38 0.44 0.38
0.32 0.33 0.41 0.29 0.37
0.18 0.18 0.19 0.20 0.22
a Experiments with CF4.
~=4
0.02 0.03 0.04 0.06 0.06
0.03 0.03 0.03 0.07 0.03
K. Dehe, H. Heydtmann / Chemical Physics Letters 262 (1996)683-688
686
corresponds to a total available energy of 261 kJ m o l - ]. The upper limit for the fraction of vibration neglecting the ground state is (fv)ma~ =0.31. The rotational distribution which is not shown exhibits a non-Boitzmann behaviour.
A 20z
15lO-
v=3 5O I I I
I
O-
4. Discussion
. . . . .
0
n
. . . .
5
*
. . . .
i
i0
. . . .
15
J
20-
4.1. F + acetonitrile
Z
The relative population of u = 1 agrees well with that of Bogan and Setser's initial distribution, which was obtained using a correction for radiative cascading [6]. For some unexplained reason, however, we find a maximum in v = 2 and an N,= 3 that is lower than Bogan's value. Bogan and Setser also published rotational distributions [6]. From Fig. 4 of their paper and our Fig. 1 we can only conclude that less rotational relaxation was present in our experiments, therefore the vibrational relaxation must also be less.
15lO-
v=2 5•
O-
6 ....
~
....
ilOo00
•
ib ....
j
i's ....
20Z
15-
20-
leeOt
0 z
~
15-
6 ....
I
v=3 105-
O-
alam-: . . . .
i
0
. . . .
10
-.
:
,' ""....::
Z I0-
•
I0 ....
• 5"
•
i
d}
O'
....
J
• •
6
I'5
•
V= 1
S
....
J
J
.
5 ....
I 5 ....
:~
15
1°1e ~ • 6 ....
I b ....
'
i
5
S ....
o.O
Fig. 2. Mean rotational distributions for HF from the reaction F + CH 3SCH 3. The error bars indicate standard deviations of the mean values. The curve shows Boltzmann distributions at standard temperature.
•
i
•
~ b ....
• •
ee i5
• o A J
Fig. I. Mean rotational distributions for HF from the reaction F+CH3 CN. The error bars indicate standard deviations of the mean values. The curve shows Boltzmann distributions at standard temperature.
Because of the uncertainty in the population figures stated above, a peak in the vibrational distribution at v = 2 is not certain and our results are not considered to disagree with the former results. The lack of clear inversion must be due either to the presence of a triple bond or to the presence of a lone electron pair at nitrogen or to both. The calculated (fv)max for this and all the reactions treated here is distinctly lower than for F + CH 4 (0.58 from Ref. [3]). Correction for populations in No= o is necessary for the systems presented here and would yield ( f v ) lower than the given (fv)max" It is worth noting that Bogan and Setser have already speculated about two microscopic channels, one of direct abstraction and one with F addition followed by HF elimination. In the rotational distributions Bogan et al. observed only two maxima for
K. Dehe, H. Heydtmann / Chemical Physics Letters 262 (1996) 683-688
each vibrational state. Our distributions look rather different, v = 1 shows three maxima, the first peak J = 2 is a sign of some rotational relaxation, v = 2 and v = 3 give evidence of only two peaks, but the first maximum shows certainly at higher J than a relaxation peak. Very likely, after proper correction for rotational relaxation two maxima remain in all v levels.
4.2. F + methylisonitrile The vibrational distribution observed in this work clearly shows No=l/No=2 = 1 . 4 4 + 0 . 2 7 > 1. The absence of inversion can again be explained by the influence of a multiple bond and a lone electron pair. The usual formulation of CH3NC with a triple bond also indicates partial charge separation with electronegativity at the carbon atom of the isonitrile group. This could promote the importance of the addition - elimination channel which is linked to a distribution No= l > No= 2 > No= 3. The qualitative features for this reaction are different from our observations for the system F + t-C4H9NC [7] where the direct abstraction is more effective due to the presence of three methyl groups (see Table 2).
error limits. The results for the homologous reaction F + CH3OCH 3 show clear vibrational inversion between v = 1 and v = 2 [15], the direct abstraction is the ruling channel. Duewer et al. [8] also picture rotational distributions in Fig. 3. These are somewhat different from our results shown in Fig. 2 of this Letter. Higher levels are observed and less relaxation is apparent in our work. For each of the three vibrational states three maxima are observed. In v = 1 and v = 3 the low J maximum most likely shows the Boltzmann peak arising from some relaxation. We have problems with the interpretation of the rotational structure in v = 2 where the low J maximum appears at J = 6. Most likely two maxima are merging below J = 7. The maximum at J = 9 although reproducible appears doubtful. In summary we claim to have discovered for each vibrational level two rotational maxima occurring above the Boltzmann peak ( J = 2). This again, as in the case of F + acetonitrile, indicates the presence of two microscopic mechanisms. There is new evidence that indeed dimethylsulfide forms adducts with the OH radical [16]. It is therefore reasonable to postulate adduct formation of atomic fluorine with DMS: CH3SCH
4.3. F + dimethylsulfide Duewer and Setser [8] have studied the same system and the vibrational distribution which shows no inversion was already mentioned in Section 1. The large error limits given by these authors in their Table 1, however, are also in agreement with a weak inversion like the one we have determined. Hence our vibrational distributions agree within the given
687
3+
F
~
CH3S(F)CH
3,
with a subsequent elimination of HF. The other channel is direct abstraction.
4.4. F + dimethyldisulfide Dimethyldisulfide contains four lone electron pairs in the disulfur bridge. This must be the reason for the lack of inversion in the vibrational states of HF.
Table 2 H F / D F vibrational distributions for F + R H / R D reactions Reagent (RH)
N, = i
N, = 2
N, = 3
IV, = 4
N, = 5
(fv)ma~
Ref.
C H 3CN CH 3NC
0.38 0.46
0.42 0.32
0.20 0.18
0.04
-
0.41 0.41
this work this work
CH3SCH 3 CH 3SSCH 3
0.36 0.43
0.39 0.35
0.25 0.19
0.04
-
0.45 0.31
this work this work
CH 3NNCH 3 CD3NNCD 3 N2(t-C4Hg)2 N2(COOCH 3)2 t-C4HgNC
0.50 0.40 0.46 0.43 0.46
0.33 0.29 0.49 0.47 0.51
0.15 0.20 0.05 0. I 0 0.03
0.02 0.09 -
0.02 -
0.39 0.37 0.42 0.47 0.44
[5] [9] [7] [7] [7]
688
K. Dehe, H. Heydtmann / Chemical Physics Letters 262 (1996) 683-688
Radical addition, as in the F + dimethylsulfide case, can be expected. Elimination from such adducts may happen via two different transition states one forming a ring involving the nearest methyl group another one with a bond forming between F and the more remote methyl group. The averaged rotational distribution is not shown here and exhibits population of higher J and a structure indicating the presence of three maxima in v = 1 and v = 2. This observation is in accord with the assumption of two or even three microscopic channels.
vibrational distributions (no or little N o= 2, No = 1 inversion) and secondly from the rotational distributions (two J peaks from emission of nonrelaxed states).
Acknowledgement We are grateful for the support of this work by the Deutsche Forschungsgemeinschaft.
References 5. Summary Summarising, we can state that many examples have been discovered in which the HF vibrational distribution from a reaction of the type F + CH3R shows no strong inversion between NHF,o=I and NHF,o=2. Often weak inversion is noticed as for CHaCN and CH3SCH 3 which was described in this Letter. This is also the case with the reagents cisbutene-2, trans-butene-2 and butyne-2 which we investigated in earlier studies [10,17]. The latter examples show the effect of multiple bonds on the vibrational energy disposal. Absence of inversion or presence of equal population, i.e. N o= i = No= 2, was only found in reagents including heteroatoms with lone electron pairs. In Table 2 we find four examples of this behaviour with close to statistical distributions: F + CH3NC, CH3SSCH3, CD3N=NCH3, CD3N= NCD 3. The dimethylsulfide and the dimethyldisulfide reactions show clearly the influence of lone electron pairs. In all other cases shown in Table 2 double bonds and lone electron pairs lead to enforcing effects. This enforcement is particularly strong with azomethane and azomethane-d6. The systems studied in this Letter all show double evidence of a dual channel mechanism characterised by direct attack of the CH bond or adduct formation followed by elimination. The evidence comes first from the
[1] K.G. Anlauf, J.P. Kunz, D.H. Maylotte, P.D. Pacey and J.C. Polanyi, Discussions Faraday Soc. 44 (1967) 183. [2] J.C. Polanyi, Science 236 (1987) 680. [3] B.E. Holmes and D.W. Setser, in: Physical chemistry of fast reactions, Vol. 2, ed. I.W.M. Smith (Plenum Press, New York, 1980). [4] M.A. Wickramaaratschi, D.W. Setser, B. Hildebrandt, B. Ki~rbitzer and H. Heydtmann, Chem. Phys. 94 (1985) 109. [5] U. Schwanke, H. Heydtmann and J.J. Sloan, Chem. Phys. 132 (1989) 413. [6] D.J. Bogan and D.W. Setser, J. Chem. Phys. 64 (1976) 586. [7] D.J. Smith, D.W. Setser, K.C. Kim and D.J. Bogan, J. Phys. Chem. 81 (1977) 898. [8] W.H. Duewer and D.W. Setser, J. Chem. Phys. 58 (1973) 2310. [9] K. Dehe, H. Heydtmann and U. Schwanke, Chem. Phys. Lett. 169 (1990) 603. [10] K. Dehe and H. Heydtmann, Ber. Bunsenges. Phys. Chem. 100 (1996) 1226. [I I] K. Dehe, PhD thesis, J.W. Goethe Universit~it, Frankfurt/ Main (1994). [12] B. Dill and H. Heydtmann, Chem. Phys. 35 (1978) 161. [13] U. Schwanke and H. Heydtmann, Ber. Bunsenges. Phys. Chem. 91 (1987) 1043. [14] J. Casanova Jr., R.E. Schuster and N.D. Werner, J. Chem. Soc. (1963) 4280. [15] D.J. Bogan, D.W. Setser and J.P. Sung, J. Phys. Chem. 81 (1977) 888. [16] A.J. Hynes, R.B. Stoker, A.J. Pounds, T. McKay, J.D. Bradshaw, J.M. Nicovich and P.H. Wine, J. Phys. Chem. 99 (1995) 16967. [17] U. Schwanke and H. Heydtmann, Ber. Bunsenges. Phys. Chem. 94 (1990) 1395.
CHEMICAL PHYSICS LETTERS
ELSEVIER
Chemical Physics Letters 262 (1996) 683-688
HF infrared emission from the reactions of atomic fluorine with methylcyanide, methylisocyanide, dimethylsulfide and dimethyldisulfide Katharina Dehe, Horst Heydtmann Institut ffir Physikalische urM Theoretische Chemie, Johann Wolfgang Goethe Universiti~t, Marie-Curie-Strasse 11, D-60439 Frankfurt am Main, Germany Received 25 July 1996
Abstract In the reactive systems F + CH3CN, F + CH3NC, F + CH3SCH 3 and F + CH3SSCH 3 infrared emission spectra were recorded from HF in the fundamental region under relaxation-free conditions. The reported vibrational distributions are for F + C H 3 C N : NL=1:N~.=2 : N~.=3 =0.38:0.42:0.20; for F + C H 3 N C : N,.=u:N~._2:NL.=3: N~.=4=0.46:0.32:0.17:0.05; for F + C H 3 S C H 3 : Nw I : N~=2 : Nt.=3 = 0.36 : 0.39 : 0.25; for F + C H 3 S S C H 3 : N~.=l : N~.=2 : Nt'=3 : N~'=4 = 0.43:0.35:0.19:0.04. Rotational distributions are drawn up and discussed for F + CH3CN and F + CH3SCH3. There is strong evidence for the existence of two microscopic channels in all four systems investigated.
1. Introduction
[4]. Such distributions indicate a direct abstraction mechanism with interaction times o f the order of
Infrared emission from vibrationally-rotationally excited states of a molecule emerging from an exothermic reaction can be studied by infrared chemiluminescence. One o f the methods developed is the low-pressure arrested relaxation (AR) technique which was introduced by Polanyi and coworkers [1,2] and is used in this work. Vibrational energy distributions o f H F produced by abstracting a hydrogen atom from a primary CH bond in saturated hydrocarbons by a fluorine atom typically show an inverted distribution for the first and second excited levels (No= i / N v = 2 < 1) [3,4]. In our group F + CH 4 and F + C 2 H 6 have been studied by the abovementioned method which rendered the vibrational distributions N v = l : N o = 2 : No= 3 = 0 . 1 5 : 0 . 6 7 : 0 . 1 8 and No= i : No=2 : No=3 = 0.10 : 0.47 : 0.43 respectively
10 -14 S.
Molecules which contain heteroatoms often show vibrational distributions which are different. The reaction F + azomethane (CH3N2CH3), studied with the same apparatus as the reactions reported in this Letter, results in a noninverted H F vibrational distribution [5]. This behaviour was also reported for F+CH3CN by Bogan and Setser who found No= l : No = 2 : Nv = 3 = 0.39 : 0.33 : 0.28 [6]. The same chemical reaction yielded a somewhat different result using the flowing afterglow (FA) technique [7]. This technique, however, does not always give results consistent with the A R method [4]; we therefore shall limit ourselves to results obtained with the latter. Duewer and Setser [8] found a similar result for F + C H 3 S C H 3 : No= i : No=2 : No= 3 =
0009-2614/96/$12.00 Copyright © 1996 Published by Elsevier Science B.V. All rights reserved PH S 0 0 0 9 - 2 6 1 4 ( 9 6 ) 0 1 1 4 8 - 7
684
K. Dehe, H. Heydtmann / Chemical Physics Letters 262 (1996) 683-688
0.43 : 0.30: 0.20. Noninverted HF distributions obtained under AR conditions are postulated by us to indicate intermediates which have a lifetime of at least 10 -13 s. Most likely an addition-elimination microscopic mechanism always competes with a microscopic mechanism of direct abstraction. Other examples which yielded within error limits HF vibrational distributions with no clear inversions (i.e. Nv= l : Nv=2 >/ 1) are the reactions F + 2,2'azobisisobutane, F + azodicarbonic acid dimethyl ester and F + t-butylisonitrile [9]. Most of these reagents contain methyl groups, heteroatoms with lone electron pairs and also double bonds. To separate the effects of lone electron pairs and double bonds we recently studied the energy disposal in the reaction F + 2,3-d2-butene-2 ~ HF + C4D2H 5 for which mo=l : No= 2 = 1 + 0 . 2 was found [10,11]. In the present publication we show the results for two reactions without double bonds but with lone electron pairs: F + dimethylsulfide and F + dimethyldisulfide, and also for the systems F + CH3CN and F + CH3NC which we compare with our earlier investigations and with former work [6,8].
2. Experimental The experiments were performed using the lowpressure AR technique. The apparatus has been described before [12]. It consists of a cylindrical reaction vessel with liquid-nitrogen-cooled walls connected to an effective pumping unit. Typical background pressures measured behind the cooled walls were about 1 x 1 0 - 2 Pa. The residence time for products in the observation zone was estimated to be about 5 x 10 -5 s. The infrared radiation was collected by a White cell installed inside the reaction vessel. Emission spectra were recorded with a F'FIR spectrometer (FTS-14, BIORAD) equipped with an InSb detector cooled by liquid nitrogen. The reagent inlet system used was the concentric nozzle arrangement described in a previous paper [13]. Fluorine atoms were preferably produced by passing F 2 (MAN Neue Technologie 98% purity) through a microwave discharge. With CH3NC also CF4 (Messer Griesheim, 99% purity) was used as a precursor for F atoms which led to the same results. With the dimethylsulfide and dimethyldisulfide reagents, however, the
vibrational populations were dependant on the precursor used, presumably because of interfering reactions with CF 2 radicals in these cases. Therefore, in these systems only the experiments with F / F 2 are considered here. Methylisocyanide was synthesised as described in Ref. [14]; the substance was stored at 77 K, kept in a dark bulb and used the day following the synthesis. All other substances were used as purchased after degassing in the usual manner. The purities given by the producers are as follows: 99.5% for CH3CN (Aldrich), > 99% for CH3SCH 3 (FLUKA) and 99% for CH3SSCH 3 (Janssen).
3. Results 3.1. F + acetonitrile Some examples of HF vibrational distributions are given in Table 1. A total number of 18 runs was performed. The mean of the observed distributions shows a slight inversion with a maximum at v = 2; Nv = I : Nv = 2 : N o = 3 = 0 . 3 8 " 0.42 : 0.20. These relative populations are, from our experience, certain within 10% deviations, including systematic and statistical errors. The highest observed level HF (v = 3, J = 18) corresponds to an available energy of 207 kJ mo1-1. The vibrational distribution varies more in this experiment than in others mainly because the signalto-noise ratio was poor but the results are independent of the reagent flows within the error limits. The rotational distribution shows a strong non-Boltzmann behaviour. The mean rotational distribution is pictured in Fig. 1. The upper limit for the fraction of vibration is ( f v ) m a x = 0.41; this figure was obtained neglecting any ground state population. 3.2. F + methylisonitrile HF vibrational distributions for 5 experiments of a total number of 15 runs are given in Table 1. Though the experiments were done with the concentric nozzle and separated nozzle arrangements only experiments with the concentric nozzle arrangement are shown. Fluorine as well as CF4 were used as F atom precursors and no differences were found between
K. Dehe, H. Heydtmann/ Chemical Physics Letters 262 (1996) 683-688
685
these sources. The mean vibrational distribution shows no inversion and the highest populated level is V = 1: N v = t : N v = 2 : N w 3 : N v = 4 = 0 . 4 6 : 0 . 3 2 : 0.18:0.04. The highest observed level H F (v = 4, J = 10) corresponds to an available energy of 201 kJ mol -~. The rotational distribution which is not shown is non-Boltzmann. The upper limit for the fraction of vibration neglecting the ground state is
is v = 2 ; (Nv= l : N v f f i 2 : N o = 3 = 0 . 3 6 : 0 . 3 9 : 0 . 2 5 ) . The total available energy calculated from the highest observed level (v = 3, J = 15) is 185.6 kJ m o l - i The rotational distribution shown in Fig. 2 shows a strong non-Boltzmann behaviour. The upper limit for the fraction of vibration neglecting the ground state is (f~)max 0.45.
(fv)max = 0.41.
3.4. F + dimethyldisulfide
3.3. F + dimethylsulfide
HF distributions of 5 experiments of this reaction with fluorine as F atom source are given in Table 1. A total number of 10 runs with this F atom source was performed. Six runs with CF4 as F atom source were also performed but the signal-to-noise ratio in these experiments was so poor that only the results with molecular fluorine are discussed. The mean vibrational distribution shows no inversion and the highest populated vibrational level is v = 1; (No=1:No=2 : No=3:Nv=4 = 0.43 : 0.35 : 0.19 : 0.04). The highest observed level is v = 4, J = 21 which
Five examples of the vibrational distribution of a total number of 11 runs with fluorine as F atom source are given in Table 1. Also 13 runs with CF4 as F atom source were done and the results were the same within the error limits. Because of the poorer signal-to-noise ratio of the spectra obtained with CF4 only the results with fluorine as F atom source are discussed. The vibrational distribution shows a slight inversion, and the highest populated vibrational level
=
Table 1 Selected experimental results for the reactions F + CH3CN, CH3NC, CH3SCH3, CH2SSCH 3 ~ I-IF + products. F 2 was used as a precursor in most experiments; the use of CF4 is specified Reagent (RH)
Flows (ixmol s - i )
Background
Relative populations N, (HF)
F2/CF 4
RH
pressure (10 -4 mbar)
~=1
~-2
~=3
CH3CN
15.2 30.4 19.0 19.0
110 96 80 13.9
2.0 2.0 0.35 0.7
0.38 0.41 0.40 0.46
0.44 0.43 0.45 0.41
0.18 0.15 0.15 0.13
CH 3NC
7.6 14.9 a 14.9 a 7.6 7.5 a
16.9 9.7 6.9 < 3 < 3
3.0 7.4 6.8 2.0 6.8
0.46 0.46 0.49 0.54 0.50
0.36 0.35 0.32 0.27 0.28
0.16 0.16 0.15 0.13 0.16
CH 3SCH 3
3.2 3.2 3.2 3.2 3.2
27.7 18.1 13.7 9.1 < 2
0.03 0.03 0.03 0.03 0.03
0.36 0.31 0.37 0.33 0.33
0.38 0.43 0.35 0.41 0.42
0.26 0.25 0.26 0.26 0.25
CH 3SSCH 3
4.9 4.9 1.6 6.5 1.6
34.6 15.7 6.9 1.4 I. 1
7.0 6.0 6.0 1.5 5.0
0.46 0.46 0.38 0.44 0.38
0.32 0.33 0.41 0.29 0.37
0.18 0.18 0.19 0.20 0.22
a Experiments with CF4.
~=4
0.02 0.03 0.04 0.06 0.06
0.03 0.03 0.03 0.07 0.03
K. Dehe, H. Heydtmann / Chemical Physics Letters 262 (1996)683-688
686
corresponds to a total available energy of 261 kJ m o l - ]. The upper limit for the fraction of vibration neglecting the ground state is (fv)ma~ =0.31. The rotational distribution which is not shown exhibits a non-Boitzmann behaviour.
A 20z
15lO-
v=3 5O I I I
I
O-
4. Discussion
. . . . .
0
n
. . . .
5
*
. . . .
i
i0
. . . .
15
J
20-
4.1. F + acetonitrile
Z
The relative population of u = 1 agrees well with that of Bogan and Setser's initial distribution, which was obtained using a correction for radiative cascading [6]. For some unexplained reason, however, we find a maximum in v = 2 and an N,= 3 that is lower than Bogan's value. Bogan and Setser also published rotational distributions [6]. From Fig. 4 of their paper and our Fig. 1 we can only conclude that less rotational relaxation was present in our experiments, therefore the vibrational relaxation must also be less.
15lO-
v=2 5•
O-
6 ....
~
....
ilOo00
•
ib ....
j
i's ....
20Z
15-
20-
leeOt
0 z
~
15-
6 ....
I
v=3 105-
O-
alam-: . . . .
i
0
. . . .
10
-.
:
,' ""....::
Z I0-
•
I0 ....
• 5"
•
i
d}
O'
....
J
• •
6
I'5
•
V= 1
S
....
J
J
.
5 ....
I 5 ....
:~
15
1°1e ~ • 6 ....
I b ....
'
i
5
S ....
o.O
Fig. 2. Mean rotational distributions for HF from the reaction F + CH 3SCH 3. The error bars indicate standard deviations of the mean values. The curve shows Boltzmann distributions at standard temperature.
•
i
•
~ b ....
• •
ee i5
• o A J
Fig. I. Mean rotational distributions for HF from the reaction F+CH3 CN. The error bars indicate standard deviations of the mean values. The curve shows Boltzmann distributions at standard temperature.
Because of the uncertainty in the population figures stated above, a peak in the vibrational distribution at v = 2 is not certain and our results are not considered to disagree with the former results. The lack of clear inversion must be due either to the presence of a triple bond or to the presence of a lone electron pair at nitrogen or to both. The calculated (fv)max for this and all the reactions treated here is distinctly lower than for F + CH 4 (0.58 from Ref. [3]). Correction for populations in No= o is necessary for the systems presented here and would yield ( f v ) lower than the given (fv)max" It is worth noting that Bogan and Setser have already speculated about two microscopic channels, one of direct abstraction and one with F addition followed by HF elimination. In the rotational distributions Bogan et al. observed only two maxima for
K. Dehe, H. Heydtmann / Chemical Physics Letters 262 (1996) 683-688
each vibrational state. Our distributions look rather different, v = 1 shows three maxima, the first peak J = 2 is a sign of some rotational relaxation, v = 2 and v = 3 give evidence of only two peaks, but the first maximum shows certainly at higher J than a relaxation peak. Very likely, after proper correction for rotational relaxation two maxima remain in all v levels.
4.2. F + methylisonitrile The vibrational distribution observed in this work clearly shows No=l/No=2 = 1 . 4 4 + 0 . 2 7 > 1. The absence of inversion can again be explained by the influence of a multiple bond and a lone electron pair. The usual formulation of CH3NC with a triple bond also indicates partial charge separation with electronegativity at the carbon atom of the isonitrile group. This could promote the importance of the addition - elimination channel which is linked to a distribution No= l > No= 2 > No= 3. The qualitative features for this reaction are different from our observations for the system F + t-C4H9NC [7] where the direct abstraction is more effective due to the presence of three methyl groups (see Table 2).
error limits. The results for the homologous reaction F + CH3OCH 3 show clear vibrational inversion between v = 1 and v = 2 [15], the direct abstraction is the ruling channel. Duewer et al. [8] also picture rotational distributions in Fig. 3. These are somewhat different from our results shown in Fig. 2 of this Letter. Higher levels are observed and less relaxation is apparent in our work. For each of the three vibrational states three maxima are observed. In v = 1 and v = 3 the low J maximum most likely shows the Boltzmann peak arising from some relaxation. We have problems with the interpretation of the rotational structure in v = 2 where the low J maximum appears at J = 6. Most likely two maxima are merging below J = 7. The maximum at J = 9 although reproducible appears doubtful. In summary we claim to have discovered for each vibrational level two rotational maxima occurring above the Boltzmann peak ( J = 2). This again, as in the case of F + acetonitrile, indicates the presence of two microscopic mechanisms. There is new evidence that indeed dimethylsulfide forms adducts with the OH radical [16]. It is therefore reasonable to postulate adduct formation of atomic fluorine with DMS: CH3SCH
4.3. F + dimethylsulfide Duewer and Setser [8] have studied the same system and the vibrational distribution which shows no inversion was already mentioned in Section 1. The large error limits given by these authors in their Table 1, however, are also in agreement with a weak inversion like the one we have determined. Hence our vibrational distributions agree within the given
687
3+
F
~
CH3S(F)CH
3,
with a subsequent elimination of HF. The other channel is direct abstraction.
4.4. F + dimethyldisulfide Dimethyldisulfide contains four lone electron pairs in the disulfur bridge. This must be the reason for the lack of inversion in the vibrational states of HF.
Table 2 H F / D F vibrational distributions for F + R H / R D reactions Reagent (RH)
N, = i
N, = 2
N, = 3
IV, = 4
N, = 5
(fv)ma~
Ref.
C H 3CN CH 3NC
0.38 0.46
0.42 0.32
0.20 0.18
0.04
-
0.41 0.41
this work this work
CH3SCH 3 CH 3SSCH 3
0.36 0.43
0.39 0.35
0.25 0.19
0.04
-
0.45 0.31
this work this work
CH 3NNCH 3 CD3NNCD 3 N2(t-C4Hg)2 N2(COOCH 3)2 t-C4HgNC
0.50 0.40 0.46 0.43 0.46
0.33 0.29 0.49 0.47 0.51
0.15 0.20 0.05 0. I 0 0.03
0.02 0.09 -
0.02 -
0.39 0.37 0.42 0.47 0.44
[5] [9] [7] [7] [7]
688
K. Dehe, H. Heydtmann / Chemical Physics Letters 262 (1996) 683-688
Radical addition, as in the F + dimethylsulfide case, can be expected. Elimination from such adducts may happen via two different transition states one forming a ring involving the nearest methyl group another one with a bond forming between F and the more remote methyl group. The averaged rotational distribution is not shown here and exhibits population of higher J and a structure indicating the presence of three maxima in v = 1 and v = 2. This observation is in accord with the assumption of two or even three microscopic channels.
vibrational distributions (no or little N o= 2, No = 1 inversion) and secondly from the rotational distributions (two J peaks from emission of nonrelaxed states).
Acknowledgement We are grateful for the support of this work by the Deutsche Forschungsgemeinschaft.
References 5. Summary Summarising, we can state that many examples have been discovered in which the HF vibrational distribution from a reaction of the type F + CH3R shows no strong inversion between NHF,o=I and NHF,o=2. Often weak inversion is noticed as for CHaCN and CH3SCH 3 which was described in this Letter. This is also the case with the reagents cisbutene-2, trans-butene-2 and butyne-2 which we investigated in earlier studies [10,17]. The latter examples show the effect of multiple bonds on the vibrational energy disposal. Absence of inversion or presence of equal population, i.e. N o= i = No= 2, was only found in reagents including heteroatoms with lone electron pairs. In Table 2 we find four examples of this behaviour with close to statistical distributions: F + CH3NC, CH3SSCH3, CD3N=NCH3, CD3N= NCD 3. The dimethylsulfide and the dimethyldisulfide reactions show clearly the influence of lone electron pairs. In all other cases shown in Table 2 double bonds and lone electron pairs lead to enforcing effects. This enforcement is particularly strong with azomethane and azomethane-d6. The systems studied in this Letter all show double evidence of a dual channel mechanism characterised by direct attack of the CH bond or adduct formation followed by elimination. The evidence comes first from the
[1] K.G. Anlauf, J.P. Kunz, D.H. Maylotte, P.D. Pacey and J.C. Polanyi, Discussions Faraday Soc. 44 (1967) 183. [2] J.C. Polanyi, Science 236 (1987) 680. [3] B.E. Holmes and D.W. Setser, in: Physical chemistry of fast reactions, Vol. 2, ed. I.W.M. Smith (Plenum Press, New York, 1980). [4] M.A. Wickramaaratschi, D.W. Setser, B. Hildebrandt, B. Ki~rbitzer and H. Heydtmann, Chem. Phys. 94 (1985) 109. [5] U. Schwanke, H. Heydtmann and J.J. Sloan, Chem. Phys. 132 (1989) 413. [6] D.J. Bogan and D.W. Setser, J. Chem. Phys. 64 (1976) 586. [7] D.J. Smith, D.W. Setser, K.C. Kim and D.J. Bogan, J. Phys. Chem. 81 (1977) 898. [8] W.H. Duewer and D.W. Setser, J. Chem. Phys. 58 (1973) 2310. [9] K. Dehe, H. Heydtmann and U. Schwanke, Chem. Phys. Lett. 169 (1990) 603. [10] K. Dehe and H. Heydtmann, Ber. Bunsenges. Phys. Chem. 100 (1996) 1226. [I I] K. Dehe, PhD thesis, J.W. Goethe Universit~it, Frankfurt/ Main (1994). [12] B. Dill and H. Heydtmann, Chem. Phys. 35 (1978) 161. [13] U. Schwanke and H. Heydtmann, Ber. Bunsenges. Phys. Chem. 91 (1987) 1043. [14] J. Casanova Jr., R.E. Schuster and N.D. Werner, J. Chem. Soc. (1963) 4280. [15] D.J. Bogan, D.W. Setser and J.P. Sung, J. Phys. Chem. 81 (1977) 888. [16] A.J. Hynes, R.B. Stoker, A.J. Pounds, T. McKay, J.D. Bradshaw, J.M. Nicovich and P.H. Wine, J. Phys. Chem. 99 (1995) 16967. [17] U. Schwanke and H. Heydtmann, Ber. Bunsenges. Phys. Chem. 94 (1990) 1395.