M. S.B. MUNSON
2034
Ionic Reactions in Gaseous Amines
by M. S. B. Munson Esso Research and Engineering Company, Baytown Research and Development Dieision, Baytown, Texas (Received January 21, 1966)
Ionic reactions are reported for ammonia and the three methylamines. The major product ions are the protonated molecule ions and the higher solvates of these ions. Protontransfer, charge-transfer, and hydride-transfer reactions were observed in the trimethylamine system. The rate constants for ionic reactions show a slight decrease in the order NH3, CH3NH2, (CH&NH, (CH8)N. This decrease parallels the dipole moment which decreases in the same order but is in the opposite direction to the polarizabilities which increase in this order.
Introduction Previous work has been reported from this laboratory on gaseous ionic reactions for a few simple oxygenated compounds’ and on proton-transfer reactions in mixtures of simple polar compounds.2 The purpose of this paper is to report on the ionic reactions in gaseous ammonia and the methylamines. Studies have been made of ionic reactions in a m m ~ n i a ~which - ~ show that the dominant product ions are NH4+ and higher solvated protons, H+(NH&. The present studies (up to 2 torr) extend the pressure range of Derwish and others4 and partially bridge the gap toward the studies of Hogg and KebarleS5 Work has been done on methylamine which indicates that the protonated molecule-ion is the major productionP
Experimental Section The apparatus and experimental procedure have been adequately described previously. For the experiments on the methylamines at pressures as high as 0.3-0.4 torr, the source design was that used for earlier work from this laboratory,’ The repeller voltage was 2.5 v (FS = 6.25 v/cm) and the ion path was 2 mm. The pressure calibration curves for the amines were taken from those for structurally related compounds.* For the experiments with ammonia a t pressures as high as 2 torr, the tight source which has been described in more recent work9was used. The gases were obtained from the Matheson Co. and were further purified by a single distillation within the gas handling system. The mono- and dimethylThe Journal of Physical Chemistry
amines contained small amounts of the higher amine as impurities. Appearance potentials were measured for (14.W 1)+ions for all four compounds at pressures for which (MW l)+/(MW)+ 1, and in each case A(MW 1)+ = I(MW)-the minimum energy of formation of the protonated molecule-ions is equal to the ionization potential of the molecule.
+
+ +
Results NHI. The predominant second-order product ion in ammonia is NH4+ whose relative concentration passes through a maximum at 0.1-0.2 torr as the pressure is increased at a substantially constant reaction time. Since the maximum concentration of NH4+ is about 90% of the total ionization, a large fraction of NHz+ ions must also give NH4+. The previously suggested sequence4 (1) M .S.B. Munson, J . Am. Chem. Soc., 87, 5313 (1965). (2) M. S.B. Munson, ibid., 87, 2332 (1965). (3) L. M.Dorfman and P. C. Noble, J . Phys. Chem., 63, 980 (1959). (4) G. A. W. Derwish, A. Galli, A. Giardini-Guidoni, and G. G. Volpi, J . Chem. Phys., 39, 1599 (1963). (5) A. M.Hogg and P. Kebarle, ibid., 43, 449 (1965). (6) 11.Henchman and C. J. Ogle, private communication. (7) F. H. Field, J. L. Franklin, and M.S.B. Munson, J . Am. Chem. SOC.,85, 3575 (1963). (8) F. H. Field and hf. S. B. hlunson, paper presented a t the 11th ASTiM Conference on Mass Spectrometry, San Francisco, Calif., May 1963. (9) F. H. Field and M. S. B. Munson, J . Am. Chem. Soc., 87, 3289 (1965).
IONIC REACTIONS IN GASEOUS AMINES
+ NH3 +NH3+ + NH2 NH3+ + KH3 +NH4+ + NH2
NH2+
2035
(1)
I
I
I
I
1
1
(2)
is adequate to explain the present results. There is an appreciable scatter in the data so that the rate constants are not precise, but the rate constant for the reaction of NH3+ with NH3 is about 1 X cc/molecule sec and the rate constant for reaction of NH2+ with NH3 is 2 X cc/molecule sec. Previous determinations gave 1.3 and 1.8 X cc/molecule sec for these rate c o n ~ t a n t s . ~The reaction for the solvation of NH4+ is also very rapid, although no value can be given. The other product ions whose composition and origin can be established are iYHl+.SH3,m/e 35, whose relative concentration passes through a broad maximum of approximately 60y0 of the total ionization near 1 Figure 1. Relative concentrations of ions torr and NH4+.2NH3, m/e 52, whose relative concenin methylamine; -1.5-v electrons. tration increases continuously to 20% of the total ionization a t 2 torr. Very small concentrations of Traces of ions of m/e as high as 127 were observed, ions were observed at m/e 69, presumably NH4+.3NH3, but their relative concentrations varied markedly in approximately 0.1% of the total ionization. different batches of ammonia; therefore, the ions must The present distribution of solvated NH4+ ions conbe formed from impurities. tains much larger fractions of the lower solvated ions CHINH2. Representative data for the pressure dethan that of Hogg and Keba~-le;~however, this difpendence at constant reaction time of the relative conference is not disconcerting and is likely the result of a centrations of the major ions in methylamine are shown higher temperature in the present case (220” compared in Figure 1. This experiment was done with electrons with 20’) and a much shorter reaction time for the of approximately 15 v energy and the two major pripresent experiments (lo+ sec compared with mary ions in the electron impact spectrum are CH3sec). The two sets of data are mutnally corroborative NH1+ and CH2NH2+. It is assumed that the ions a t and show the increasing extent of solvation with inm/e 30 are CH2NH2+and not CH3NH+ since loss of D creasing density. to give CH3CDNH2+ is 12 times more probable than Other ions were observed in these experiments a t loss of H on ionization of CH8CDzNH2.12Since CH3m/e 32 and 49 and na/e 46 and 63, of the order of a few NH3+ passes through a maximum value of 90% of the per cent of the total ionization. Since these experitotal ionization, it is apparent, that both of the ions must ments were performed, however, it has been shown form CH3NH3+ that at pressures of 1-2 torr and higher, the distribuCH3NH2+ CH3NH2 + tion of ions in the mass spectrum is strikingly sensitive CH3NH3+ CH2NHz (3) to the purity of the gas.5s10r11Since extreme precautions concerning gas purity were not taken in these CH2NH2+ CHsNHz +CH3NH3+ CH3N (4) experiments and methylamine and dimethylamine I n experiments with 50-v electrons, m/e 28 was also had both been used in the instrument shortly before one of the abundant ions, approximately 20y0 of the these experiments on NH3 were done, it is not possible total ionization by electron impact, and the maximum to be certain whether the ions at m/e 32 were NZH4+ relative concentration of CH3NH3+was 90% of the or CH31VH3+and those a t m/e 46, N3H4+ or (CH& total ionization again; therefore NH2+. However, it seems likely that the major part of the ion currents at these masses are caused by the CHZN+ CHgNH2 --j CH3NH3+ HCN ( 5 ) protonated amines. The ratios 133/132and I ~ T / I ~ ~ also occurs. were always larger than one would expect for either (10) M. S. B. Munson and F. H. Field, J . Am. Chem. SOC.,87, 3294 13C or 15K isotopes. If the ions at m/e 32 and 46 are (1965). the protonated amines, then it is likely that the ions (11) M. S. B. Munson and F. H. Field, ibid.,87,4242 (1965). a t m/e 49 and 63 are the partially solvated protonated (12) J. Collin, “Advances in Mass Spectrometry,” J. Waldron, amines. Ed., Pergamon Press, New York, N. Y.,1959, pp 384-391.
+
+
+
+
+
+
Volume 70,Number 6 June 1966
M. 8. €3. >!IUNSON
2036
Since CH3+, HCN+, and CH3K+ are formed in the electron impact spectrum (50 v) t o the extent of 3-5% of the total ionization and there are no secondary product ions except CH3NH3+of relative concentration large enough to account for the decrease in relative concentration of these ions, these ions must either react to give CH3NH3+directly by proton transfer or they must react to give an ion which will give CH3NH3+for example, charge transfer or hydride transfer. It is not possible t o distinguish between the two processes in the present experiments, because the small amounts of CH3NHa+and CH2NH2+produced by ionic reactions could not be detected in the presence of the large amounts of CH3SH2+and CHzNHz+in the primary spectrum. From plots of log (Ii/ZIi), the logarithms of relative ionic concentrations, us. P, it is possible to determine rate constants for reactions of the primary ions. Reasonable straight lines were obtained in all the cases reported in this paper. These values are given in Table I. Table I : Reactions in Methylamine k, 10-9 cc/molecule sec
Ion
Product
CH3N?dZf CH,NHz+ CH3X + CH2N + CHN+ CH3+
CHINHI' CHINHI
+
CH3?JH3+
0.9
* 0.1
0.7
0.1 0 . 9 =t0 . 1 0.9 0 . 1 1.0 1.2
*
CH3NH2, is very rapid. Traces of ions (0.1%) a t m/e 94, presumably CH3NH3+.2(CH3NHz),were observed a t the highest pressures of these experiments, but conversion to the higher solvated protons was not sufficient to discuss the relative stabilities of these species. (CH3)dVH. Figure 2 shows representative data for gaseous ionic reactions in dimethylamine. Since (CH&NH+ and CH3NHCH2+(loss of methyl H rather than amine H is assumed by analogy with CH&D2NHa) are the predominant primary ions a t this electron voltage (nominally 13 v) and (CH3)&H2+is the overwhelmingly predominant second-order product ion, the reactions must be (CH3)zNH+
+ (CH3)2NH + (CH3)2NH2+
CH3NHCH2+
+ (CHa)*NH+
+ CH3NHCH2
(6)
+ C2H&
(7)
(CH3)2NHz+
The relative abundance of m/e 28 is sufficiently large with 50-v electrons that the observation of no additional product ions means that it reacts by proton transfer CHzN+
+ (CH3)zNH
(CH3)2SHz+
1
+ CHSSHz +CHZNHz+ + CHI
could have been detected in the curve for the disappearance of CHzSHzf because of the small concentration of CH3+relative to CH2NH2+. Small amounts (2%) of ions at m/e 45, 46, 58, 59, and 60 were observed in the mass spectrum and the ions at m/e 46 and 60 were the major ones a t the highest pressures. Since there were impurities of a few tenths of a per cent of (CH3)2NHand (CH3)3N in the monomethylamine, it is likely that these impurities are the source of these ions, particularly since it has already been shown that proton transfer reaction from CHsNH3+to (CH3)zNH is very rapid.2 It is apparent from Figure 1 that further solvation t o give the next higher solvated proton, CHaNH3+. The Journal of Physical Chemistry
(8)
There are several other ions each present as 2 4 % of the total ionization with 50-v electrons and (CH3)2NH2+ is the overwhelmingly predominant product ion a t this electron voltage as well, so each of these ions must react to give (CH3)2NH2+ directly or each must react by hydride or charge transfer followed by (6) or (7). I t seems likely that proton transfer is dominant.
The variation of rate constants in this series is probably real, but it is certainly not an appreciable variation, I t is unlikely that a hydride transfer reaction CH3+
+ HCN
i
Figure 2. Relative concentration of ions in dimethylamine; -13-v electrons.
IONIC REACTIONS IN GASEOUS AMINES
2037
Table I1 shows rate constants for several of the primary ions with dimethylamine. NH4+ is formed as a rearrangement ion in the electron impact spectrum of dimethylamine so that it is possible to estimate the rate constant for this proton transfer reaction of what is normally only a secondary product ion.
-4
Table 11: Reaction of Ions in Dimethylamine k, 10-9 cc/molecule Ion
Product
(CHa)zNH+ CHaNHCH2+ CzHJ'I CzHaN + CHaN + CHzN + NHa + CH, +
(CHa)2NH2+ (CHa)zNH2
+
880
+
0 . 6 i0 . 1 0 . 4 & 0.1 0.5 0.4 0.6 f 0 . 2 0.5
Figure 3. Relative concentration of ions in trimethylamine; -50-v electrons.
(MW)+ but not the (MW - 1)+ions react to give (CH&NH+. The relative concentration of (CH3)aNCHa+ is constant (f575, which is a reasonable exThe disolvated proton (CH&NHz+. (CH&NH is perimental precision) at pressures as high as 0.3 torr. readily formed under the present experimental condiFigure 3 shows representative data for the pressure tions as one would expect by analogy with the previous dependence at substantially constant reaction time of results, but the formation of the trisolvated proton several ions in trimethylamine with 50-v electrons. under these conditions is doubtful (less than 0.1% of If one recalls that the relative concentration of (CH& the total ionization). The extent of conversion to the NCHz+, m/e 58, is independent of pressure with lower disolvated proton is not large, so that the failure to energy electrons, then it is apparent that hydride transobserve the trisolvated proton does not indicate any fer reactions are occurring. From the ratio of the inspecial instability of the higher solvated protons. crease in relative Concentration of (CH$2NCHz+ to There are small concentrations of three other ions, the increase in relative concentration of (CH3),NH+, m/e 58, 60, and 103, which are probably (CH&NCHz+, one may estimate that hydride transfer is about one(CH&NH+, and (CH&NCH2+. (CH&NH. There fifth of proton transfer. is a small amount of trimethylamine present as an imAlso in Figure 3, there is a slight maximum in the purity (several tenths of a per cent) which will probably pressure plot for (CH3)3N+and the pressure plot for account for the presence of m/e 60, (CH3)3NH+, a t this ion is different in shape from the expected behavior 1-2% of the total ionization by proton transfer reacof ions which react by pseudo-first-order processes. tions to this impurity. However, 158/IBo is much larger A typical curve for ions disappearing by pseudo-first in these experiments than it is in the experiments with order processes is shown for CH4N+ in Figure 3. trimethylamine and since (CH3)2NCHa+is present as At the lower voltages of the experiments previously de%lo% of the total ionization at the highest pressures, scribed, the pressure plot for the disappearance of it appears that it must be largely the product of ionic (CH3)3N+ is similar to that shown for CH4N+ in reactions with dimethylamine. The pressure dependFigure 3; consequently, it is apparent that there are ence of the relative concentration of this ion indicates charge-transfer reactions occurring in this system from that it is formed by processes of different order and it ions produced at high electron energy which produce was not possible to determine the origin of this ion. (CH3)3N+. However, since (CH3)3N+reacts rapidly A likely explanation for the ion at m/e 103 (1-2% of the itself, it is not possible to determine the extent of chargetotal ionization) is that it is a solvated (CH3)2NCHz+ transfer processes occurring. ion. It was suggested earlier in this paper that chargeand hydride-transfer reactions might be occurring for (CH3)dV. I n experiments with low-energy electrons (12 v) the only primary ions formed by electron impact methylamine and dimethylamine, but if so, the reacare (CH&N+ and (CH,)zNCHa+. With trimethyltions were occurring only to a very small extent. In amine, unlike the other methylamines, only the trimethylamine all three types of reactions are obviously Volume 70,Number 6 June 1066
n1. S. B. MUNSON
2038
occurring, but the precursors could not be convincingly established. Table I11 summarizes the data for the rate constants for reactions in trimethylamine.
Table 111: Ionic Reactions in Trimethylamine k, 10-9 cc/molecule Ion
sec
Comments
(CH3)3N ( CH3)2NCH2 +
0.33 0.000 & 0.005
(CHs)aNH+ No evidence for any reaction
CSHTN + CzHsN CH4N CHIN
0.10 0.35 0.40 0.50
CH3 +
0.63
+
+
+ +
(CH3)$NH+, by analogy to other methylamines
Higher molecular weight ions were also observed as products. (CH&NH+. (CH3)3N,m/e 119, was observed as 1-2y0 of the total ionization. I l 1 9 / I ~varied approximately as P2in these experiments. The relative abundance of this solvated ion is notably less than the relative abundances of the analogous ions for the other amines. Since the relative abundances of these higher solvated species are very sensitive functions of pressure, this observation may not be meaningful. However, since the differences are appreciable, they may be real and indicate a lower stability of the disolvated proton in trimethylamine compared with the other methylamines. Small amounts of ions a t m/e 117 were observed also (-1%) which may be attributed to (CHJ2NCHz+. (CH3)3N, t’he solvated species corresponding to the other major ion. 111,/158 was approximately the same with 12 and 50-v electrons and increased with increasing pressure; t,hese observations support the assignment. Small concentrations (0.5%) of ions were observed a t m/e 74 with both 12 and 50-v electrons. These ions were not secondary product ions since In/Iw increased with increasing pressure, and since they were found with both high- and low-voltage electrons they are probably formed from (CH3)3NH+or its solvate. A very likely assignment is (CH3)*N+,the quaternary ammonium ion.
Discussion The rate constants for reactions of CH3+ and CH2N+ with the methylamines decrease slightly with increasing methyl substitution. This decrease in rate constants parallels the decrease in dipole moments with increasing The Journal of Physical Chemistry
methyl ~ubstitution‘~ rather than the increase in polarizability.14 The decrease, however, is small and might be an instrumental artifact. Other work on ionic reactions with polar molecules has shown that the cross sections for reaction vary in a fairly complicated way with ion energy and in particular that for low values of the ion energy, ion-dipole forces become important.15 Similarly, the work on methylamine has shown that iondipole forces become important in determining the nature of the reaction products a t low ion energies.6 The effect of ion energy on rate constants was not studied systematically in the present experiments, but the maximum ion energy is only 1.2 v and a t high pressures the large majority of the ions react before they attain this energy. Most of the ions, then, have very low energies when they react and ion-dipole forces may be dominant in determining the rate constants. The structures and heats of formation of the ions and radicals produced in these reactions are not sufficiently well known to warrant much discussion. The reactions give predominantly the protonated molecule-ions and their solvates so that for radiolysis experiments on these compounds one would consider the radicals and products from proton transfer of the primary ions produced by electron impact and those radicals produced by the neutralization of the solvated protonated molecule-ions. It is of interest that the disolvated, and in some cases the trisolvated, protons were observed for these compounds as one would expect on the basis of previous experiments on oxygenated compounds’ and a m m ~ n i a . ~ Unfortunately, the pressures of the present experiments were not high enough to give indications of the existence and stability of the higher solvated species. In particular, it would be worthwhile to check at higher pressures to see if there is an appreciable decrease in stability of the solvated species on filling the “inner solvation shell”: NH4+04NH3, CH3NH3+-3CHaNH9, (CH3)2NH2+. (CH3)2NH,and (CHB)3NH+. (CH3)N, as one would predict for this series of compounds.
Acknowledgments. This work was supported in part by Project SQUID under Contract Nonr-3623 (S-18). The author is very grateful to Mr. W. C. Gieger for performing these experiments with his accustomed competence and to Drs. F. H. Field and J. L. Franklin for their helpful discussions. (13) A. L. McLellan, “Tables of Experimental Dipole Moments,” W. H. Freeman and Co., San Francisco, Calif., 1963. (14) H. H. Landolt and R. Bornstein, “Zahlenwerte und Funtionen,” Part 3, 6th ed, Springer-Verlag, Berlin, 1950. (15) T. F. Moran and W. H. Hamill, J . Chem. Phys., 39, 1413 (1963).
