J. 1. Franklin Humble Oil and Refining Company Boytown, Texas
The Chemical Behavior of Ions in Gases
This paper will discuss ions in gases. ~t is, I fear, a rather esoteric topic to chemists who are used to thinking of ions as existing in solution where the solvent supplies the rather large amount of energy required to bring about the formation of ions from neutral molecules. The solvent, of course, accomplishes this by vigorous interaction with the electric charge of the ion, with the result that the i311 is never isolated but is alwiys accompanied by its retinue of solvent molecules. Indeed, it was because we wished to study the behavior of pure ions, unaccompanied by solvent, that we engaged in these researches into the chemistry of ions in gases. I n order to generate a po;itive ion in the gas phase we must supply the necessary energy by some means other than solvation. In our studies we have employed low energy electrons to supply the necessary energy; and in order to introduce the enerev into the molecules somewhat selectively and to oKain interpretable measurements for individual ionic species, we have worked at rather low pressures. In our studies we consider 10-b10-6 mm of mercury as low pressure and 0 . 0 1 4 . 5 mm of mercury as high pressure. Except at the very highest end of this pressure range we can look upon each ion or molecule as an isolated system that will have very small probability of undergoing collision during the brief time of retention in our apparatus. Consequently, whatever energy is imparted to the molecule must be imparted a t a single collision with an electron; any change that occurs must result from this one impact and must occur in quite a short time (about lo-' sec); energy transferred to the molecule by the electron will go to excite internal modes and will not alter the momentum of the molecule. The most convenient and satisfactory device for accomplishing our desired objectives was a massspectrometer and in all of our investigations one of several such instruments has been employed. Most of our work has been conducted with sector field instruments but some employed a cycloidal instrument as well. A sector This address was given by Dr. Franklin on the occasion of t h e presentation of the Southwest Award to him a t the Southnest Regional Meeting of the ACS in Dallas on December 7, 1962. The author had this to say by way of introduction. "The only duty required of the winner of the Southwest Award is t o present a n address t o the Southwest Regional Meeting of the American Chemical Society upon some aspect of his research. Our labomtory has heen engaged principally in studies of the chemistry of ions in gases a t low pressures and i t therefore seems in keeping ujth the spirit of this occasion that I present a.hrief survey of our studies of these phenomena. Before doing so, however, I must neknoukdge my indebtedness and my profound gratitude t o my colleagues in researrh and especially to Drs. Field, Lampe, and Munson, and Mr. Geiger, whose collaboration hits gone far toward making this occasion possible."
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field instrument is shown diagrammatically in Figure 1 together with typical operating conditions. When a narrow beam of electrons passes through the gas in the ion source some will collide with molecules of the gas. If the energy of the electrons is sufficient, electrons in the molecule will be excited to higher orbitals or removed completely, forming an ion. In some instances more than one electron may be affected. Thus a track of positive ions is formed along the course
F g r r e 1. Scnemots d ogrom of lector field m m r >pertromeler. Electron energy 8-70 v. Electron u r r e n l - 3 ~ Repeller elcorooe vo loge 5-20 v. Ion occslerotina- rolloae 2000 V. on wrrent 10 -
-
-
0.1
amp.
-10-'2amos.
Source temperature 150°C. Ga, pressure-ionimtion chambers -1 0 - 6 -1.0 mm Hg. andyrer tube 10'-10-%m Hg. Magnetic field 3003000 gauss. Ionization chamber dimensionr: length, 7 m m source A; 20 mm source B; depth, 4 mm; distance, center of electron beom to ion exit slit, 2 mm.
The Cover
The photograph shows Mr. Wilhurn C. Gieger operating the mass soertrometer l r I i I k l l i m e d . The heavy installatit~uo n tllc tnwks is the mngnrt,. Tire upper part of the analyzer bul,e and ion cdlectnr are in the top renter of the picture. The lower end of the analyzer tohe emerges from between the magnets in the eent,er. The ion source and gas inlet tuhe ran he seen extending diagon;tlly downward in the lower center hehind Mr. Geiger.
of the electron beam and these are impelled toward the exit slit by a small electric potential applied a t the repeller electrode. The ions pass out into the ion gun where they are accelerated by an electric field to a controlled energy and passed into a magnetic field where their paths are caused to curve, the greater the e/m (charge to mass ratio) the greater being the curvature. By properly adjusting the accelerating potential and the magnetic field, ions of a given elm can be focused upon the collector where their abundance may be measured as an electric current. If the analyzer is focused upon a certain ion and the energy of the electron beam is gradually reduced, the intensity of the ion beam will diminish correspondingly along a curve similar to those shown in Figure 2. If the reduction of electron energy is carried far enough, the ion current will diminish to zero. The electron energy a t which this occurs is the appearance potential. The curvature a t the foot of the curve is attributable in part to the thermal spread in energy of the electrons and in part to any low-lying excited states of the ion. If the curve in Figure 2 is extended upward sufficiently, it will eventually reach a broad maximum and then gradually decline as the electron energy increases. We will not be concerned, however, with the upper portion of the ionization efficiency curve. An electron traveling with sufficient energy to ionize a molecule will he "in contact" with the molecule for a period in the order of to lo-'* sec. Since even rather rapid vibrations require time greater than 10-I3 sec, it is evident that the time for ionization is much less than the time of a vibration. As a consequence, the configuration of the ion immediately after its formation will be essentially the same as that of the molecule
at the time of impact. Any decomposition or adjustment of the ion into a more favorable configuration will necessarily occur later. This is the well-known FranckCondon principle' and, expressed simply, requires that ionization by electron impact involve a vertical transition from molecule to ion. Figure 3 a-c give an example of the behavior of several simple systems according to the Franck-condon principle. For the system in Figure 3a, the ground state of AB+ falls within the Franck-Condon region and one would expect the appearance potential of AB+ to correspond to the ionization potential of AB. On the other hand, the repulsive portion of the attractive curve does not intersect the Franck-Condon region opposite to the decomposition asymptote and so the A+ion will not he formed in the ground state. It will, however, be formed from the repulsive curve with excess energy. Figure 3b shows a system in which the FranckCondon region does include the decomposition asymptote. Careful measurement of the appearance potential of A+ should yield the energy required to form A+ and B in their ground states. Figure 3c shows a system in which the Franck-Condon region intersects the potential energy curve of AB+ above the decomposition asymptote. Here AB+ would not be observable and the decomposition products would be formed with excess energy. We can, of course, treat the appearance potential as a heat of reaction so that for the reaction AB-A++B
(1)
'For a more complete discussion, see FIELD,F. H., Press, New York, 1957, p. 157.
,
A(Nei) = 21.56 Exp = 21.46
Figure 2.
Ionization efficiency curve..
AND
FRANKLIN, J. L., "Electron Impact Phenomena," Academic
distance 30
,
distance 3b
Figure 3.
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285
we may write AP
=
AHJ(A+)
+ aH,(B) - aH,(AB) + E.,
(2)
where E,, is excess energy. An equivalent statement is AP
=
D(A
- B ) + I(A) + E,,
(3)
where D and I are bond strength and ionization potential, respectively. From these considerations one mould suppose that the appearance potential of most parent ions would be greater than the adiabatic ionization potential, and indeed this is usually the case although with most molecules the differenceis small (.
were calculated directly from the appearance potential; two mere corrected for excess translational energy. The precision, although far poorer than is routinely obtained by calorimetry, is probably about as good as could be expected for measurements by several experimenters employing a variety of instruments. We conclude that AH,(CH,+) is close to 262 kcal/mole and that this represents the ion in its ground state. It is noteworthy that AH,(CH,+) is the same when determined from fragmentation processes as from direct ionization of methyl radical. Molecular quantum theory predicts a planar configuration for CH3+and one expects CH3 radical to be planar also. This expectation was borne out by spectroscopic studies of methyl radical ( I ) and the agreement of AH,(CHz+) from the two sources implies that the radical and ion have the same configuration, presumably a planar one. To further elucidate this problem we undertook in our laboratories to compare the energy required to form a planar carbonium ion with that required to form a carbonium ion that is sterically constrained from achieving planarity (8). Tahle 2 shows the appearance potential of three tertiary ions formed from the tertiary bromide. The t-butyl ion is free to achieve its preferred configuration, which should he planar. The ions are and H
H
Unless otherwbe stated, energy values were taken from Field and Franklin, see footnote 1.
286 / lournol o f Chemicol Education
However, there is no reason to believe that the ethyl radical has such a structure hut rather that it has the conventional structure CHa-CHz, where the bonds to the carbon having the unpaired electron are planar, as in methyl. Now, appearance potentials of C2H5+by ionization of the radical and by various fragmentation processes give the same value for AH,(C2H5+) within the limits of accuracy of the measurements (see Table 3). Since the radical probably has a planar configuration the ion obtained from the radical must have one:
For a discussion of complex theory, see DEWAR, M. J. S., "Electronic Theory of Organic Chemistry," Oxford University Press, London, 1949.
Toble 3.
'q
Heat of Formotion of CzHsf
--- - +++ +++ + - + +
CIHr CIHst e e C2H6 C1HSt H GH,+ NO1 e CaH,ONO e C8H8 C.H5+ CH. n-C4Hia CIHsf CzH5 e Averaee value for framnentation~rocesses
AHI (kcal/mole) 226 223 229 224 224 225
Further since all AH, were the same, either all of the ions had the planar structure or else the 11 complex has the same heat of formation as the planar ion. If the latter is true then both may be present but there is certainly no reason to consider that the n complex structure would predominate. Thus, it seems highly improbable that the n complex can be the principal structure of the ethyl carbonium ion and, indeed, the existence of the complex as a structural entity seems quite improbable. Although their separate existence in solution is difficult to establish, for many years alkyl carbonium ions have been postulated to play an important part in organic solution chemistry where the evidence points to increasing stability and ease of formation in the sequence methyl, ethyl, see-propyl, tert-butyl. Ionization potentials and heats of formation amply support this view as will be seen from Table 4. Toble 4.
Alkyl Carbonium Ions
heat of formation of the ion obtained by a fragmentation process is greater than that by direct ionization, then the excess heat of formation may be the activation energy for the decomposition reaction. Certainly the activation energy can be no greater than this excess energy. In Table 6 we compare the heat of formation of several ions by fragmentation and direct ionization processes and show the possible maximum energy of activation. These are all four-center or more complex processes and it is evident that the activation energy is quite small, in many cases probably zero. I n no instance is it comparable in magnitude to the activation energy one might expect for a similar reaction of neutral molecules. It will be obvious that the activation energy here is taken as the barrier to reaction in the exothermic direction. Any endothermicity, of course, will show up as an activation energy, also, but here we distinguish between them. Table 6.
Activation Energies for 4-Center Reoctions (kcol/mole) -AHn-
Obs
It has long been known also that strong acids catalyze the isomerization of paraffins and it is generally postulated that the reaction proceeds through the medium of carbonium ions. Similar rearrangements of primary to secondary propyl ions are observed in Table 5 . Here AH,(C3H7+) from several sources including n-propyl and see-propyl radicals are given. I t is evident that the ion prefers the secondary configuration and that the parent ion in almost all of the fragmentation reactions must undergo rearrangement to yield secondary ions in the process of decomposition. Further, little or no activation could be involved in the decomposition since this would show up as excess energy and result in a high value for AH,(C3HI+). Similar examples can be shown for the butyl ion. Table 5.
Heat of Formation of Propyl Ion
Reactant
AH!
Min
mnx
This absence, or near absence, of activation energy for reactions of gaseous ions naturally leads one to suspect that solution reactions involving ions may also he free of activation energy. With this in mind r e undertook to calculate the rate constant for several SN1solvolysis reactions on the assumption that the reaction rate is limited by the ionization step and that the only activation energy will he the endothermicity of this step. The heats of formation of the ions in the gas phase are all known, but to he useful with solution reactions it is necessary to obtain a value for the heat of solvation. Only rather indirect methods based upon modifications of the Born equation are available, and we employed the method developed by Latimer, Pitzer, and Slansky (3). This rests ultimately upon the establishment of a set of consistent ionic radii. The details may be obtained elsewhere (4). In Table 7, the results are given for typical calculations cf the rate of solvolysis of several alkyl halides. To indicate the validity of the method the heat of the reaction is also given. The calculated values in all cases agree quite well m-ith the experimental ones; indeed, the agreement is probably better than the method justifies. However, it certainly suggests that the method is Toble 7.
The small energy of activation noted for the decomposition-rearrangement reactions leading to the formation of see-propyl is not unique; activation energies appear to be small or nonexistent for a large number of decomposition reactions, even for some that involve quite complex rearrangements. Direct ionization will usually form an ion in its ground state. If the
Activotion Energy of Solvolysis (kcol/mole)
+ + -- + +
tert-C*HpC1 lerl-CaHs+ C1sec-CsHJ see-CsHi 1CHICI CHJ+ CINHCI, NH4,,+ CL-
23 25 73 2.0
23.1 24.3
...
3 .6*
" Experimental heat of reartion. Volume 40, Number 6, June 1963
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287
reasonably valid and that our observation of small activation energy barriers (other than endothermicity) in the gas phase applies as well t o ionic reactions insolution.
Where reactions are obviously occurring a chemist mould naturally like to know the nature of the reactions. If we have a process
Ion-Molecule Reactions
it is obvious that C+ should have the same appearance potential as A+. Consequently, we have depended largely upon comparing appearance potentials of secondary ions with those of primary ions in the mass spectrum in order to adduce the chemical reactions leading to the formation of the secondary ion. Thus, in Table 9 we listed the appearance potential of several primary and secondary ions from CD4. I t will he evident from these that CD,+ is the precursor of CD6+ and CD3+ is the precursor of CzD5+. From this information then we write the following equations to represent the reactions in question:
Thus far this discussion has dealt with information obtained at quite low pressures in the ion source. At these conditions the probability that an ion will undergo collision in the source is negligibly small. As pressure is increased, the probability of collision increases and, a t pressures in the order of 500 microns, will approximate unity. Table 8 gives the approximate mean free path of a typical small ion in our source a t various pressures. Collision, of course, does not necessarily result in reaction, hut in many instances reaction of the ion and Table 8.
Approximate Mean Free Path of lons at Ionization Chamber Conditions
Pressure (mm He)
Mean free path, A (in mmi
At+B-C++D
+ CD4
CD4+
+ Clh
CDa+
--
CDlf
(5)
+ CD,
Cznst
+ D1
(6) (7)
dlA" Table 9. Appearance Potentials of Primary and Secondary lons from CDI
' 11, 111rdi~lnuwfn.n. the rrnrrr vf
rlrvtnm hr:Ixr! 11, 1t.c i r m enit slit i, 2 ulln 11 h i* i~ppr~~iwnlc:I\ lhr nlln>hrrcii~
