Ion-Molecule Reactions in CH3CI- and C,H,CI-NH,
kinetics for first-order initiation. More than likely chain termination, if homogeneous, is some complex mixture of the above possibilities. Clearly, more detailed experimental studies are needed for the elucidation of the complete mechanism of the uninhibited decomposition. In this regard, a conventional static system should prove more fruitful than the present apparatus. However, the main aims of this work. have been achieved. T h a t is, we have shown that the thermal decomposition of EtCN is essentially a radical-chain reaction and the unimolecular elimination of HCN and H2 do not appear to occur a t rates indicated by the results of other workers; we have shown also that the first-order rate constant for reaction 1is very close to predictions based on our previous VLPP studies of alkyl cyanides which, in essence, confirms the value of e 2 1 kJ mol-l for the stabilization energy of the cyanomethyl radical. Achnowledgment. R.D.G. acknowledges the receipt of a Commonwealth Post-Graduate Research Award.
References and Notes (1) K. D. King and R. D. Goddard, J. Am. Chem. Soc., 97, 4504 (1975). (2) K. D. King and R. D. Goddard, Int. J. Chem. Kinet., 7, 837 (1975); E for C-C fission in EtCN is 334.7 kJ mol-' at 300 K and 338.9 kJ mol-' at 1100 K.
The Journal of Physical Chemistry, Vol. 82, No. 15, 1978
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K. D. King and R. D. Goddard, J. Phys. Chem., 80, 546 (1976). M. Hunt, J. A. Kerr, and A. F. Trotman-Dickenson, J . Chem. Soc., 5874 (1965). P. N. Dastoor and E. U. Emovon, Can. J . Chem., 51, 388 (1973). H. E. O'Neal and S. W. Benson, J. fhys. Chem., 71, 2903 (1967). D. A. Luckrafl a d P. J. Robinson, Int. J. Chem. Kinet., 5, 137 (1973). K. D. King and R. D. Goddard, Int. J . Chem. Kinet., 7, 109 (1975). K. D. King and R. D. Goddard, Int. J . Chem. Klnet., in press. Y. Gonen, L. A. Rajbenbach, and A. Horowitz, Int. J. Chem. Kinet., 9, 361 (1977). 2.B. Alfassi, D. M. Golden, and S. W. Benson, Int. J. Chem. Klnet., 5, 991 (1973). S. W. Benson and H. E. O'Neal, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., No. 21 (1970). K. D. King and R. D. Goddard, unpublished work. K. D. King, unpublished work. W. C. Herndon and L. L. Lowry, J. Am. Chem. Soc., 88, 1922 (1964); W. C. Herndon, M. B. Henley, and J. M. Sullivan, J . fhys. Chem., 67, 2842 (1963). M. F. R. Mulcahy and D. J. Williams, Aust. J. Chem., 14, 534 (1961). T. W. Asmus and T. J. Houser, J . fhys. Chem., 73, 2555 (1969). Y. Gonen, A. Horowitz, and L. A. Rajbenbach, J. Chem. Soc., Farachy Trans. 1 , 72, 901 (1976). J. A. Kerr and M. J. Parsonage, "Evaluated Kinetic Data on Gas Phase Addition Reactions", Butterworths, London, 1972; G. E. Bullock and R. Cooper, Trans. Faraday Soc., 67, 3285 (1971). W. Forst and C. A. Winkler, Can. J . Chem., 33, 1814 (1955); D. E. McElcheran, M. H. J. Wijnen, and E. W. R. Steacie, ibid., 38, 321 (1958); J. W. S. Jamieson, G. R. Brown, and J. S. Tanner, ibid., 48, 3619 (1970). K. J. Laidler, "Reaction Kinetics", Vol. I, Pergamon Press, London, 1963.
Positive Ion-Molecule Reactions in the Methyl Chlorideand Ethyl Chloride-Ammonia Systems Zygmunt Luczynskl' and Jan A. Herman* Centre de Recherches sur les Atomes et les Mol6cules et D6parlement de Chimie, Universit6 Laval, Qusbec, GIK 7 f 4 , Canada (Received February 6, 1978) Publication costs assisted by the Universit6 La Val
The ion-molecule reactions of ions formed by photoionization in the 11.6-11.8-eV region in CH3C1-NH3and C2H5C1-NH3binary systems were investigated by high-pressure mass spectrometry (0.01-1 Torr). The dialkylchloronium ions formed at higher pressures are very effectively transformed in the presence of small quantities of ammonia into solvated alkylammonium ions [RNH3+(NH3),],where R = CH3 or C2H5 and m I3. In the presence of high concentrations of ammonia, practically the only observed positive ions in both binary systems are solvated ammonium ions, [NH4+(NH3),],where n I 4.
Introduction Ionic processes in gaseous methyl and ethyl chlorides have been intensively investigated in the past decade by various mass spectrometric techniques,2-10in conventional radi~lysis,~ and in vacuum ultraviolet photolysis.ll Results obtained by mass spectrometry allowed kinetic data and a kinetic scheme to be determined rather precisely for ion-molecule reactions in both pure compounds and their mixtures with hydrogen and nitrogen. Most of the positive parent ions formed in irradiated or photoionized CH&l or C2H6C1react with the parent molecules to give CH3C1H+ and C2H5C1H+species, which in turn condense very efficiently with a second parent molecule with HC1 elimination, thus forming dialkylchloronium ions, [ (R),Cl]+. These ions and their solvates, [(R),Cl+(RCl),], are unreactive toward CH3C1and C2H5C1neutrals, and, therefore, they seem to be the end-product positive ions, their yields being almost 100% a t a pressure of 1 Torr and moderate temperatures. It is interesting for the radiation chemistry 0022-365417812082-1679$0 1.OO/O
of gases to investigate the modifications of the described ionic processes in pure chloroalkanes in the presence of scavengers. The interpretation of results obtained by the scavenging-of-positive-ions technique generally used in radiation chemistry may be difficult in view of the unknown processes occurring between ions and the scavenger molecules. However, the direct observation of the ionic process sequence is feasible in a high-pressure mass spectrometer, and some assumptions made in stationary radiolysis experiments can now be confirmed or disproved. The object of the present study is to obtain information on positive ion-molecule reactions in CH&1 and C2HSC1 in the presence of NH3, which is a typical positive ion scavenger.
Experimental Section The measurements were done in the photoionization high-pressure mass spectrometer already described, where some minor modifications were incorporated.12 In order 0 1978 American
Chemical Society
1680
The Journal of Physical Chemistry, Vol. 82, No. 15, 1978
to ionize the chloroalkanes [IP (CH3C1,C2H5C1,and NH,) = 11.28, 10.98, and 10.16 eV, re~pectivelyl~] an argon resonance-line (11.6-11.8-eV) lamp was used as a photoionizing agent.14 The principal modification in the present mass spectrometer consists of the separation of the ion region from the quadrupole mass filter (Extranuclear Laboratories, Model 15) by a partition with a small hole (diameter 2 mm), allowing an efficient pumping of the ion-detection region Torr) from the reaction zone, where pressures up to 10 Torr were used. In order t.0 minimize the mass discrimination effect of the quadrupole mass filter (QMF) and of the detector, the running parameters of the QMF were fixed after the mass pattern of acetone obtained for the same pressure and the photoionization conditions in a magnetic scanning highpressure mass ~pectr0meter.l~With some reduction in mass resolution, intensities of ion peaks in the QMF within 3% of those of the mass pattern of the magnetic sector mass spectrometer were obtained. The measurements in field free conditions in the reaction chamber were done a t room temperature for a length of 2.0 mm between the entrance slit, of the photons and the exit orifice of ions. In order to minimize the collision induced dissociation of the ions outside the reaction chamber, the experiments were done at the lowest possible value of the electrical field (-4 V/cm) of the ion optics. The chemicals from Matheson Co. were degassed by the usual freezing and thawing technique. The gasses were mixed in calculated proportions in a vacuum line connected to the mass spectrometer just before admission to the reaction chamber. The analyses by gas chromatography of the pure gaseous compounds or their mixtures with ammonia (maintained at room temperature for an interval of time corresponding to the time of t h experiment) showed a concentration of less than W4%of impurities originating either from the manufacturing processes, or from reactions of chloroalkanes with ammonia.
2.Luczynski and J. A. Herman
[(CH,
l2C t!] + CH3NH:
n U
nn? V.V I
n A-
0.03
(",I2
0.04
a05
Mole fraction of NH3 Figure 1. Relative intensities of the major ions resulting from the photoionization of CH,CI-NH, gas mixtures as a function of NH, concentration at 0.2 Torr total pressure.
Results The "high pressure" mass spectra of photoionized pure CH3C1 and C2H5C1of the present experiments are in agreement with those obtained with an instrument using 1-keV electrons as the ionizing agent.s,g For both chloroalkanes almost 90% of the total ion current results from the dialkylchloronium ions, the balance being composed of a small percentage of parent ions, protonated parent ions, and in the case of C2H5C1the C4H9+species. The most probable source of the last species is CzH4+ions, which are the only ionic fragments having a sufficiently low appearance potential [AP(C2H4+)= 11.33 eV"] and which are able to react with the neutral parent molecule, as shown by ion cyclotron resonance (ICR) ~ t u d i e s .On ~ the other hand the absence of CH2CI+and its ion-molecule reaction products observed in other laboratories is explained by the fact that its appearance potential [AP(CH2C1+)= 13.98 eV3] is higher than the energy delivered by the resonance doublet lines of a r g ~ n . ~ , ~ , ~ However, the described behavior changes dramatically when even small quantitites of ammonia are admixed to the systems. In Figures 1and 2 are shown typical behavior as a function of ammonia concentration of the relative intensities of the most prominent ions observed in CH3C1-NH3 and C2H5C1-NH3systems under a pressure of 0.2 Torr. It can be seen that the presence of a molar fraction of 0.01 NH, causes a strong decrease in the intensities of dialkylchloronium ions concomitant with the formation of important quantities of NH4+(NH3), and RNH3+(NH3), species, where R = CH3 or CzH5and m 5 3 and n 5 4. The disapearance of [(R),Cl]+ ions is also
Mole fraction of NH, Figure 2. Relative intensities of the major ions resulting from the photoionization of C2H,CI-NH3 gas mixtures as a function of the NH3 concentration at 0.2 Torr total pressure.
observed in experiments where the concentration ratio of the components RC1/NH3 is constant, but the total pressure is steadily increased. Such behavior is illustrated in Figure 3 for changes of relative intensities of ions as a function of the pressure in the range 0.01-1 Torr for 98% CH3C1-2% NH3. In the same figure the insert shows the behavior of the 95% CH3C1-5% NH3 mixture in the 0.01-0.5-Torr pressure region. A similar behavior is noted for the C2H6C1-NH3system. The influence of either the total pressure, or the composition of the mixtures on the relative yields of formation of ammonium and alkylammonium ions and their solvates is depicted in Figure 4. One can see that
The Journal of Physical Chemistty, Vol. 82, No. 15, 1978
Ion-Molecule Reactions in CH3CI- and C2H5CI-NH3
1681
Scheme I ii
Cf+
N H;
E
-e 0
0 .-+
e
0.4
LL
0.2
0
0.2
0.4
Pressure
0.8
0.6
1.o
, Torr
Flgure 3. Dependences of the relative intensities of the major ions in the 2 % NH3-98% CH3Cl gas mixture on the total pressure. The insert shows analogous dependences obtained for the 5 % NH3-95% CH3CI mixture in the low pressure region. Mole fraction of NH3 0
1.01
0.1
0.2
0.3
0.4
I
I
0.5
I
constitute 95% of the total ion current. On the other hand, after an initial increase in the intensities of both kinds of ionic species, the increase in the total pressure at constant composition (Figure 4B) leads to a stationary state. In this stationary state the final intensity of methylammonium ions is practically equal to the initial intensity of dialkylchloronium ions. It is worthwhile to note that the C4Hgt species formed in photoionized C2H5C1react with NH3 to give C4Hy NH3t(NH3),-1 ions, where n I 3. However, on account of the low intensity of the C4H9+precursor ion, the observed intensities of the ammoniated species were within 1-2% of the total ion current. The ammoniated C4H9+species has been written here in a form analogous to the C4H9OH2+(H20),ions identified in the t-C&g+-H20 systems.16
Discussion The observed decrease of dialkylchloronium ions in the CH3C1-NH3 and CpH,C1-NH3 systems can result either from their direct reaction with NH, molecules, or from the decreasing availability of their ionic precursor species, RC1+ or RCIHt. In the region of low pressure ( p I0.02 Torr), where the condition are not favorable for the formation of the third generation of ions, Le. [(R)2C1+],the only species originating from ammonia is NH4+. The alkylammonium ions appear at a higher pressure of the mixture and are always preceded by the presence of dialkylchloronium ions, suggesting a direct conversion: [(R),Cl]+ NH, RNH3+ + RC1 (1)
+
.-c
I
0
0.2
I
0.4
I
0.6
~-",-1
I
0.8
1.0
Pressure , Torr Flgure 4. Relative intensities of the dialkylchloronium ions and the total fraction of solvated ammonium ions, NH4'(NH3),, where n = 0, 1, 2, 3, 4, and methylammonium ions, CH3NH3'(NH3),, where m = 0, 1, 2, 3, resulting from photoionization of CH3CI-NH3 gas mixtures as a function of ammonia concentration at 0.2 Torr total pressure (A), and as function of total gas pressure at a constant 0.02 mole fraction of ammonia (B).
the behavior of these species is different in both types of experiment. With increasing concentration of NH3 at constant total pressure (Figure 4A), the intensity of methylammonium ions initially increases, then steadily diminishes, while a monotonic increase of ammonium ions is noted. For a 1:l mixture of CH3C1-NH3 the latter
-
Such substitution reactions in solution are well known.17 The photoionization yields for all three compounds of interest are comparable.18 Therefore, the dominating contribution of ammonium ions and its solvated ion species in mixtures of high ammonia concentration cannot result only from an increased participation of NH3+ ions formed in primary ionization of ammonia and their subsequent reaction with NH3 or RC1 molecules to give NH4+species. However the high intensities of NH4+and their solvates correlate well with the two concurrent reactions paths for RClH+ disappearance: (a) reaction with parent molecules to form [(R),Cl]+ (first channel), and (b) proton transfer to ammonia (second channel). With increasing NH, concentration the probability of the disappearance of RClH+ ions by the second channel increases, the most probable reason being that the rate constant for proton transfer to ammonia are one order of magnitude higher than those for the first channel condensation reaction involving RClH+, whose values are 1.4 X and 5.5 X cm3 molecule-' s-l for CH3Cl and C2H,C1, respectively.' A corresponding decrease in the yield of dialkylchloronium ions leads to the observation of low intensities of products from reaction 1, and consequently, one observes an almost complete conversion of all ions present in the system into NH4+(NH3)species. According to this reasoning, the overall scheme of the reaction of the
1682
Z.Luczynski and J. A. Herman
The Journal of Physical Chemistry, Vol. 82, No. 15, 1978
Scheme I1
L
,+
r
-1+
J
L
J
+
species. For example, the y radiolysis of the C2H6C1 NH3 O2 system shows an increase in the yield of formation of CzH4 compared to that of the radiolysis of the C2HBCl Oz system.21 It is possible that this increase results from the reaction C2H5NH3,sols+ Cls0< CzH4 (NH3,HCl)
+ +
4
r
,+
L
J
positive ions in the RC1-NH, system can be illustrated as shown in Scheme I. The proposed mechanism of the influence of NH, on the ionic processes in the methyl and ethyl chlorides, which accounts for the proton transfer by RClH+ species and for the nucleophylic substitution in [(R)zC1]sions, fulfills the empirical law formulated by Beauchamp, which states that a nucleophylic substitution takes place only when an absence of a concurrent channel of proton transfer exists.lg In the case of the RClH+ ions the energetics and structural reasons favor the proton transfer in the direction of a stronger Brransted base, such as NH, (the proton affinities of CH,Cl, CzH6C1,and NH, are, respectively, 160,167, and 202 k c a l / m 0 1 ) . ~ >In~ the ~ case of dialkylchloronium ions the structure and a uniform distribution of the positive charge on each hydrogen atomg seems to-favor the nucleophylic substitution (Scheme 11). In conventional scavenging experiments in radiation chemistry the relative ammonia concentration is usually low: a few percent of the investigated compound. Therefore, from the point of view of the mechanism of ion scavenging by NH3 in chloroalkanes, an important result of this work may be the observed conversion of almost all ions present in the system into alkylammonium ions selectively solvated by ammonia. In the radiolysis and vacuum-UV photolysis of chloroalkane-NH, systems the products of the ionic neutralization processes are not known. However, it can be assumed that some of the products are formed through recombination of the RNH3'
+
Acknowledgment. This work was made possible through the financial assistance of the National Research Council of Canada and of the Ministere de 1'Education du QuBbec.
References and Notes (1) Permanent address: Department of Radiation Chemistry, Institute of Nuclear Research, Warsaw, Poland. (2) S. K. Gupta, E. G.Jones, A. G. Harrison, and J. J. Myher, Can. J. Chem., 45, 3107 (1967). (3) A. G. Harrison and J. 6. J. Thyme, Trans. Faraday SOC.,64, 1287 (1968). (4) T. 0. Tiernan and B. M. Hughes, Adv. Chem. Ser., No. 82,412 (1968). (5) N. A. McAskill, Aust. J . Chem., 22, 2275 (1969). (6) A. A. Herod, A. G. Harrison, and N. A. McAskill, Can. J . Chem., 49, 2217 (1971). (7) J. L. Beauchamp, D. Holtz, S. D. Woodgate, and S. L. Patt, J. Am. Chem. Soc., 94, 2798 (1972). (8) Z.Luczynski and H. Wincel, Inf. J . Mass Specfrom. Ion Phys., 14, 29 (1974). (9) Z.Luczynski, W. Malicki, and H. Wincel, Inf. J . Mass Specfrom. Ion Phys., 15, 321 (1974). (10) H. S. Tan, M. J. K. Pabst, and J. L. Franklin, Int. J. Mass. Specfrom. Ion Phys., 21, 297 (1976). (11) L. Cremieux and J. A. Herman, Can. J . Chem., 52, 3098 (1974). (12) H. Wincel and J. A. Herman, J . Chem. SOC.,Faraday Trans. 1 , 6 9 , 1797 (1973). (13) H, M. Rosenstock, K. Draxl, B. W. Steiner, and J. T. Herron, J. Phys. Chem. Ref. Data, 6, Suppi. 1, (1977). (14) R. Gordon, Jr., R. E. Rebbert, and P. Ausloos, Natl. Bur. Stand., Tech. Note, No. 496 (1969). (15) Z.Luczynsk1and H. Wincel, Inf. J. Mass Specfrom. Ion Phys., 23, 37 (1977). (16) K. Hiroaka and P. Kebarle, J . Am. Chem. Soc., 99, 360 (1977). (17) G. Olah in "Halonium Ions", Wiley-Interscience, New York, N.Y., 1975, p 22. (18) In our laboratory the measured photoionizationyiekls for CH,CI, C&lHsCI, and NH, are respectively 0.38, 0.39, and 0.29 k 0.03 for argon resonance lines. The method used is described in ref 14. (19) J. L. Beauchamp in "Interaction between Ions and Molecules", P. Ausloos, Ed., Plenum Press, New York, N.Y., 1975, p 913. (20) P. Kebarle, Annu. Rev. Phys. Chem., 28, 445 (1977). (21) J. Herman, results from this laboratory.
