3098
M.S. B. MUNSON,J. L. FRANKLIN, AND F. H. FIELD
High Pressure Mass Spectrometric Study of Alkanes’
by M. S. B. Munson, J. L. Franklin, and F. H. Field Humble Oil and Refining Company, Research and Developmentq Baytown, Texas
(Received July 10; 1964)
-1lass spectra and ion-molecule reactions have been studied in ethane, propane, and the butanes at source pressures up to 150 p. I n all four compounds the most important reactions involve hydride ion transfer leading to the formation of the CnHZnfl+ ion. With propane and the butanes this ion increases to 40-60% of the total ionization above 100 p pressure. With propane and butane the next most important reaction involves the transfer of Hz- to forin CnHzn+. CZI-I,+ in small concentrations was observed in ethane and its precursor was deduced to be an excited ethane ion. Rate constants for a number of reactions have been determined. Primary ions formed with excess kinetic energy exhibit anomalous rate behavior. Several collision-induced metastable transitions occur with all four compounds.
Introduction We have recently completed studies on ionic reactions of gaseous niethaneza using the wide pressure range accessible to the mass spectrometer at Hunible.2b As a continuation of these and earlier studies on the reactions of gaseous hydrocarbon ions, we are reporting in this paper reactions in ethane, propane, n-butane, and isobutane a t source pressures as high as 150 p. The major ionic reactions in methane have been eswith good agreement of the tablished for some values of the rate constants or cross sections among different workers and general agreement on the less prominent reactions and the processes occurring a t high pressures. 2a,6--8 The reactions in ethylene are more coniplex and the agreement among different w ~ r k e r s is ~ not b ~ so ~ good ~ ~ ~as~in~ the case of methane. Ionic reactions in acetylene have also been studied a t low to moderate pressures in mass with fair agreement as to the general pattern of reactivity For the homologous series of paraffins, few dataon total reactivity are available7** although hydride transfer reactions were reported for several paraffin^.'^ Since ionic reactions niay be of importance in the radiation chemistry of hydrocarbons, we are undertaking a study of these reactions at as high pressures as possible in our mass Spectrometer. Although we cannot interpret all of our data nor are the data as precise as we would like, The Journal of Physical Chemistry
we feel that the observations which we are able to make will be of interest. Experimental The mass spectrometer was that described by Fieldzb with an electron multiplier for ion detection. Ifass spectra were obtained for different source pressures for different repeller field strengths and different electron (1) Supported in part by Project SQUID under Contract No. Nonr-3623(S-18). (2) (a) F. H. Field, J. L. Franklin, and hf. S. B. Munson, J . A m . Chem. Soc., 85, 3575 (1963); (b) F. H. Field, ibid., 83, 1523 (1961). (3) V. L. Tal’roze and A. K. Lyumbimova, Dokl. Akad. Nauk SSSR, 86,909 (1952). (4) (a) D. P . Stevenson and D. 0. Schissler, J . Chem. Phys., 23, 1353 (1955); (b) D. 0. Schissler and D. P. Stevenson, ibid., 24, 926 (1956). (5) F. H. Field, J. L. Franklin, and F. W. Lampe, J . Am. Chem. Soc., ’79,2419 (1957). (6) S. Wexler and N. Jesse, ibid., 84, 3425 (1962). (7) R. Fuchs, 2. Naturforsch., 16a, 1026 (1961). (8) G. A. W. Derwish, A. Galli, A. Giardini-Guidoni, and G. G. Volpi, J . Chem. Phgs., 40,5 (1964). (9) C. E. Melton and P. 9. Rudolph, ibid.. 32, 1128 (1960). (10) P. Kebarle and E. W.Godbole, ibid., 39, 1131 (1963). (11) F. H. Field, J. L. Franklin, and F. W. Lampe, J . Am. Chem. Soc., 79, 2665 (1957). (12) R . Barker, W H. Hamill, and R. R. Williams, Jr., J . Phys. Chem., 63, 825 (1959). (13) A. Bloch, “Advances in Mass Spectrometry,” Vol. 11, R. M . Elliott, Ed., Pergamon Press, Oxford, 1963, p. 48. (14) F. H. Field and F. W. Lampe, J . Am. Chem. Soc., 80, 5587 (1958).
HIGHh E S S L R E
I I A S H SPECTltOhlETRIC STV1)Y O F
ALKANES
energies. Appearance. pot (mt ials wcre nieasured by a rriodification of thc retarding potential difference techniclucI5 which has bcen described clsewhcrc. lfi The source pressures wcrc dcterriiiried from t hc rcscrvoir pressures by calibration curves for each gas obtairietl by t he method previously dcscribed.l7 Thc source t crripclraturt: was 180-200°. Electron currents of 0.02 Ma. arid elrct ron criergies of 70 e.v. were normally uscd, but higher currents wcrc uscd in expcriments with low elect roil cricrgy to give grcatcr ion concentrations. The hydrocarbons uscd in these cxperiinents wer(' I'hillips research gradc (stated purity 9 9 . 9 s mole yo) which wcre twicc distilled fro111a Liride lIolecular Sieve into carefully cvacuatcd storage bulbs, after discarding light arid heavy fractions.
3099
t h c k
Results h'thane. Figures I , 2 , arid 3 show relative conceniratioris of several of thc ions of ethane as functions of pressure. C2H5+ may be considered formally as involving transfer of H - from cthane to the reactant ions but CJH6+must involve a complex of suficicnt stability that additional C-C bonds are formed. Figure 3 shows sccoridary ions, C3H3+and C4H7+,which perhaps react further, and C2H7+ which is also formed by a sccondorder process. We did not undertake high resolution studies to verify that thc mass 31 was C2&+ and not CH30+,but the relative concentration of mass 31 was some 50-100 timc.s that of 32(02+) or lS(H30+)so we feel that the observed mass 31 was not caused by reactions of trace aiiiounts of oxygen or watcr. I'rotonated ethane, C2H7+, the sccorid of the homologous series beginning with CH6+, has been observed previously in high prcssure mass spectrometric studies of ~ n c t h a r i cand ~ ~ ethane* ~~ and some "Cermak-type" experiments on ethancI* as well as in a-radiolysis of
0.4
EV 70 ev
0 Figure 1.
20
40
60
FS 12.5 v/cm
80 100 p (C2H6) I p
120
140
Relative concentrations of ions in ethane.
160
P(C2H6I1 /l
Figure 2.
Relative concentrations of ions in ethane.
Figure 3.
Relative concentrations of ions in ethane.
p (C2H6) I
ethylene.I0 The relative conccntration of protonated ethane, C2H7+ (about O.3y0of the total ionization for 70-e.v. clectroris a t about 1.50 p of ethane), is much lower t,han that of thc homologous protonated methane, CH6+, under similar conditions (about TiO%). The vcry much lower rclativc concentration was also noted by the other workers who observed it and is compatible with previous failures to observe this ion in ethane7 arid cthanc-hydrogen mixturcs.lg The other two major product ions are C3H7+and C4H9+ (Fig. 2 ) . C3H6+(Fig. 2 ) exhibits thc characteristic behavior of a second-order ion over this pressure range. The fact that the relative concentrations of C3H7+ and C4Hg+ continuously increase over this same pressure range means that they have a higher order dependence or1 pressure than C3H6+. From thc shapes of and 1 6 7 / 1 4 , < thcsc curves and the observation that 143/141 (15) 1%. E. F o x , R. M .Hickam, D. ,J. Grove, and T. Kjeldaas, J r . , Rev. Sci. Instr., 2 6 , 1101 (1955). (16) M. S. B. Munson, J . L. Franklin, and I?. I I . Field, J . I'hys. C'hrm., 67, 1542 (1963). (17) F. H . Field and M. S. R. Munson, paper presented :it 11th ASTiM Conference on Mass Spectrometry, San Francisco, Calif,, May. 1963. (18) A. Henglein and G. A . Muccini, %. A'aturjnrsch., 17a, 452 (1962). (19) V. I,. Tal'roze and E. L. Frankevich, .I. A m . Chem. Soc., 80, 2344 (1958).
M. S. B. MUNSON, J. L. FRANKLIN, AND F. H. FIELD
3100
both increase approximately linearly with ethane pressure, we conclude that C3H7+ increases as the third power of the pressure and C4Hg+ as the fourth power, even though the stoichiometry requires only two ethane molecules. The observation that these ratios of concentrations do not pass through the origin, however, suggests that there is a small amount of both formed by second-order reactions. Small relative concentrations of C3H4+, C4H5+, C.&+, and CdH*+were observed as second-order ions. Small amounts of C3H6+and perhaps trace amounts of CZHsf were observed as third-order ions. No mass 58 in excess of the 13C isotope of C4H9+ or any of the higher protonated alkanes were observed. Ions of mass as high as 78 were observed, but the relative, concentrations (roughly times that of C4H9+)were too low for us to be certain that they were products of ionic reactions. CzH4+ is constant within f 3 % for all of the experiments with 70-e.v. electrons at field strengths of 12.5 and 2.5 v./cm. (corresponding to a change in residence time of a factor d 5 ) . The other primary ions except CzH6+ decrease in an approximately exponential manner as shown for C2H3+, CzH2+,and CH3+ (Fig. 1 and 2). A reaction sequence
PI+
(P1M+) (PIM+)
--
+ M +(PIM+)
+M
--
(P~[M]z+)
SI,
+M
SI,+
+ SI,
(P1[Mlz+) Tlg+
+
(S1,A4+)
(SijM+) +Turn +
N1g
kll
(1)
k21,
(2)
k31
(3)
k41g
(4)
k61,
(5)
k6ijrn
(6)
which involves the reaction of the intermediate complex (P,M+) in (3) and (4),as well as the reaction of the secondary ions in (5) and (6), to produce the tertiary ions has been analyzed by Lampe, Franklin, and Field.20 For primary ions, one obtains the simple exponential decay curve (PI+)= (p,+)oe-kll(M)tl (7) (Pl+)o is the concentration of P I +at zero time; k l , is the rate constant for reaction of the ion P I + ; (AI) is the concentration of neutral species and t , is the residence time of the ion, (2dM,/eF.S.)1"; d is the ion path in the source; M I the mass of PI+; e the electronic charge; and F.S. the field strength in the source. With the additional assumptions that ion current is proportional to ion concentration with the same factor for all ions and that the fragmentation pattern of the initially excited ion is independent of pressure, then The Journal of Physical Chemistry
In (Ii/EIi)
=
AiO - ki(M)ti
(8)
since (Pi+)o = A,OBaIi and (Pi+)
= aIi
and A ? is the fraction of ionization of Pi+ at very low pressure for which no reaction occurs (which is equivalent to zero time). ~~~~~
~~~
Table I : Rate Constants for Reactions of Ions in Ethane
Ion
k, cc./ molecule-aec. x 108
2 6 f 0 5 2 6 f 0 7 1 7 f O 3
2 2 0 0 0
9 f O 6 1 f 0 4 4 8 f O 09 01-0 03 07-0 18
Q(12.5 v./cm.), cm.2 X 1016
Products
86
88 59 131 96 25 0 5-1 7 3 3-8 9
CzHs
+
CZH4+, C&+ CzH6+, CaH5+ CzH7+, C2H6+from CzHe +*
The rate constants reported in Table I are calculated from the slopes of the initial linear segments of the plots of logarithms of relative intensity against source pressure. The values of the rate constants are in sufficient agreement at field strengths of 2.5 and 12.5 v./cm. that they are listed as average values in Table I, together with the cross sections (calculated from Q = Ict/d). From the essentially zero slope of the curve for CzH4+ (with 70-e.v. electrons) we estimate that kzg cannot be greater than about 1 X 10-l1 cc./molecule-see. The agreement of our appearance potentials for primary ions with the literature data is good, with the exception of A(C2H+) for which we obtained 23.4 e.v. in contrast to the literature value of 27.1 e.v.21 A(Cz&+) was determined as 12.7 e.v. CzD6 was used rather than Cz& to eliminate the complications due to the 13Cisotope which interferes seriously with Cz&+ a t mass 31. This value corresponds perhaps to A(CZH5') (12.8 e.v.) or possibly A(C2H4+)(12.3 e.v.) from ethane. Henglein and Muccinil* observed CzH7+ in ethane but did + the not identify its precursor. If Cz&+ or C Z H ~ is precursor, then AHf(C2H7+)is less than 193 or 170 kcal./mole, respectively. For either value the formation of CzH7+ from Cz&+ is exothermic and would be (20) F. W. Lampe, J. L. Franklin, and F. H. Field, "Progress in Reaction Kinetics," Vol. I, Pergamon Press, New York, N. Y., 1961, pp. 73-79. (21) Appearance potentials and heats of formation of ions are generally taken from F. H. Field and J. L. Franklin, "Electron Impact Phenomena," Academic Press, New York, N . Y . , 1957.
HIGHPRESSURE MASSSPECTROMETRIC STUDY OF ALKANES
expected to occur. From appearance potentials C2H6 (ground state) does not react to form CZH7+. An ion with a second-order pressure dependence which saturates at high pressure cannot be formed from either CzH4+ (which remains essentially constant with pressure) or C & , + (which is a product ion itself). We are, therefore, forced to postulate that C&+ is formed by reaction of excited CZH6+, the excited state probably being that leading to the formation of the primary CzH6+ion. This mechanism yields AHf(CzH7+) I 229 kcal./mole. The reactions involving the complex (CZH7+) and the ions CzH6+ and CZH7+ in methane are probably f
CH3+
+ CH4
(C2H7+)
(CzH7+) +CzH5+
+ 'CH,
--+
CzH7+
+ Hz
+ CH4
(9) (10)
We observed CzH7+ as a third-order ion in methane, but of intensity too low to prove by appearance potentials that it was formed from CH3+. The observation of a small amount of CzH7+ implies that the decomposition of C&7+ to CzH5+ has an activation energy, but the very small amounts of C2H7+ formed in CH4 and CzHa suggest that the activation energy for decomposition is very small. On the basis of all these considerations we are inclined to the opinion that AHf(CzH7+) is slightly less than 225 kcal./mole and that the proton affinity of ethane, P(CZH,J, is about 120 kcal./mole. Since the appearance potential of C3H7+is 12.8 v. (corresponding to Cz&+ as a primary ion) and C3H7+ is a third-order ion and CzH5+ is a second-order ion, we assume that C3H7+is formed from CzH6+. There is, however, a small amount of C3H7+which is formed by a second-order reaction. From the relative concentrations involved, CzH3-+and CzHz+must react to give predominantly CzH6'+ and the other ions of lower concentration may well do so. The orders of the reactions involved suggest that CzH5 + forms C3H7+ which then forms C4H9+. Experiments were also done with lower energy electrons (e.v. Ei 12.5 v.) with a low field strength (F.S. = 2.5 v./cm.). CzH4+ decreased slightly under these conditions although with 70-e.v. electrons a t the same field strength the rela,tive concentration of CzH4+ was independent of pressure. From this small decrease in CzH4+ for which log; ( I ~ ~ / Z Idecreased 1) reasonably linearly in P, a value of 3.5 f 1.2 X 10-I' cc./moleculesec. was obtained for the rate constant for reaction of CzH4+. CZHz+ may react to form CzH4+ exothermally and we feel that the constancy of CzH4+ with 70-e.v. electrons is probably the result of its being formed as well as its reacting. Even this rate constant for reaction of CzH4+ is unexpectedly low since CzH4+ may produce several ions in exothermic reactions.
3101
The value for the rate constant for reaction of C2H6+ was 6.7 f 1.0 X lo-" cc./molecule-sec., lower than the value obtained with 70-e.v. electrons, 1.8 X 10-Io cc./molecule-sec. The increase in CZH6 + concentration was about equal to the decrease in C&+ concentration, but the formation of CzH6+ from ground-state C2H6+ is endothermic by about 20 kcal./mole. The formation of CzH6+ from C2H4+ is also endothermic by about 15 kcal./mole. No bther primary ions are present in sufficient concentration to produce this amount of C2H6+ so we suggest that C2H5+ is formed from second-order reactions of excited ethane ions, probably the excited state which leads to the formation of the primary C2H5+ ion. If excited C2Ha+ ions are the major reactive group of C&+ ions, then, since the concentration of excited species will decrease relative to the ground-state species with decreasing voltage of the ionizing electrons, the apparent rate constant calculated on the assumption that all of the C2&+ ions react should be lower with 12.5 than with 70-e.v. electrons. Fuchs' has made a study of the ionic reactions in ethane at fairly low pressures such as that the secondary ions of molecular weight greater than the neutral species were only of the order of 1-5 X of that of the primary ions. He observed in the secondary spectrum of ethane that C3H3+,C3H7+, and C4H7+ were about 40, 20, and lo%, respectively, of C3H6+. From our data we estimate the ratios for second-order processes for these product ions to be about 35,12, and lo%, respectively. He also reports a small cross section (about 1 X 10-l6 cm.2, each) for reaction of CzH4+to give C3H6+ and C3H7+. He attributes the other products to CzH2+ or CzH3+but is unable to decide between the two reactants. The sum of the partial cross sections for the reactions which he reports is 30-60 X 10-l6 cm.2,depending upon the choice of reactants. The general pattern of reactivity of ethane in this work is the same as that given by Volpi and coworkerss but the agreement of cross sections for reactions of ions is poor; our values are about twice as large as theirs although they are in about the same relative order. This disagreement is surprising since there is good agreement for the values of the rate constants for ionic reactions in methane among the different workers. Fuchs' values' for reaction cross sections are also higher than those of Volpi and co-workers.8 Schissler and Stevenson22report cross sections of 78 and 16 X 10-l6 for reaction of C2H3+with CzH6to give C3H6+at average ion energies of 0.10 and 1.0 e.v. These values are of the order of our value of 25 X 10-l6 cm.2for reac(22) D. 0. Schissler and D. P. Stevenson, J . Chem. Phys., 24, 926 (1956).
Volume 08,Number 1 1
November, 1964
3102
tion of CzH3+ with ethane at an average ion energy of 1.3 e.v. and very different from Volpi’s value of 4 X 10-la cm.2. Assignment of product-reactant sequences by Volpi and co-workers which are different from ours are also determined by relative concentration or “mass balance” considerations which are not conclusive in either case. Propane. Figure 4 shows plots of the relative concentrations (as fractions of total ionization) for some of the ions in propane as a function of pressure. Obviously, the dominant reaction is that which can be considered as “hydride ion transfer” (H-) from neutral
M. S.B. ~IUNSOS, J. L. FRAKKLIN, AND F. H. FIELD
dominantly the odd mass ions. These ions involve three propane molecules in their formation since the ratio I 7 1 / 1 6 7 increases approximately linearly with an increase in pressure. The highest mass ion noted was 85, CBHU + ( G H + seems highly improbable) probably formed by a third-order process. For the (26’s and CB’S,CnHzn+l+is the most abundant ion, which is perhaps a reflection of the lower heats of formation of these ions relative to other ions of the same carbon number, an observation which has been made for Cl-c4.” The low ratio, 167/A143,despite the presumed permissibility of reactions to give C4H9+,indicates that reactions of these primary ions with C3H8 proceed through complexes which form very weak C-C bonds and transfer H- (similar to the CZ&+ complex from CH4+and CH4 which forms essentially no C-C bonds1 but rather transfers a proton18).
Table I1 : Reactions of Ions in Propane Q(12.6 hi.
Ion
CzH4’
cc./moleculesec. X log
0.63 f 0.10
v./cm.), cm.2 X
lo’@
Products
30
CaHe+ CaH7 C4H7 C46 C4Hs C& +
+
+
+
+
Fraction of secondorder products
-0.3
-0.7
