R. G. Cooks ond J. H. Beynon Purdue University West Lofayette, Indiana 47907
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I
Metastable lorn and Ion Kinetic Energy Spectrometry The development of a new research area
This article has two objectives. The first is to sketch the eenesis of a new research area. Too often this process is seen as the culmination of measured progress a d rational planning; which it can be but seldom is. The role of serendipity, ;andom choice, and even error in advancing research has been noted before, but, even among scientists, it tends to be underestimated. In part this results from the instinct of the authors to appear in the best possible light. The introductory section of the average paper, written after completion of the experiments and consideration of their significance, illustrates this well. The apparent prescience of the authors in taking just the approach which would extend their own earlier results and interlock with those of other investigators, is often as impressive as the work itself. These comments are not intended as a reproach, nor do we claim perfect innocence ourselves. However, a more realistic viewpoint would better serve those who do not contribute to the literature, as well as the students who mav soon beein to do so. The second objective, the-achievement of which runs parallel with the first, is more traditional. It is to present the current status of t h e field of ion kinetic energy spectrometrv, a widely applicable technique recently developed a s a k outgrowth ofwork on mass sp&rometry~ Measurements of ion masses and abundances form the essence of mass spectrometry, a technique which grew from those fruitful years before the turn of the century when studies on electrical discharges in gases led J. J. Thomson in 1897 (I) to the discovery of the electron and to glory and, in the same year (2) led Willy Wien to the discovery of gaseous ions and oblivion. Mass spectrometry (3) is frequently used to acquire qualitative or quantitative information on the sample introduced into the instrument. That is, it is often employed as an analytical technique. Applications extend throughout chemistry, physics, geology, and biology and typical uses include 1) The determination of the exact mass of nuclides, as well as
isotonic ahundancesand hence of chemical atomic weiehts. ~~~
~
~
2 , Molrcular u,rlght, molecular formula, and molecular structure drterminatiuns on organrr and hiolug~ralcompwnds. 3, The determumtion of trace elements at levels extendrng below
the parts per billion range. 4) The determination of bath the amount and the position of in-
corporation of isotopically labeled atoms in complex compounds. 5) The dating of geological samples by accurate measurements of isotope ratios (e.g., 36Arand 40Ar). 6) The identification of the constituents of complex mixtures of organic compounds (gas chromatography/mass spectrometry). 7) The qualitative and quantitative analysis of surfaces (ion microprabe). 8) Thermoehernieal determinations including heats of formation of ions, ionization potentials, electron affinities, proton affinities and, most recently, the measurement of acidity and basicity of isolated species in the gas phase. 9) Kinetic analysis of fast reactions (time-of-flight mass spectrometry). The fact that the masses, kinetic energies, and abundances of ions can be readily measured, together with the
Figure 1. High resolution mass spectrometer showing regions in which reactions of metastable ions are examined. The time scale appropriate to metastable ion reaction is of the order of 10 pr
Figure gies.
2.
Elecirostatic energy analyzer
for
analysis of ion kinetic ener-
ease with which gaseous ions can be directed in space and time, has made possible the study of fundamental processes in chemistry at the molecular level. It is from this branch of mass spectrometry, especially as regards the acquisition of data on thermochemistry, reaction mechanisms, and kinetics, that we find the budding of ion kinetic energy spectrometry. Since ion kinetic energy spectrometry (IKES) has developed from mass spectrometry, it may be helpful to review some of the characteristics1 of a double focusing mass spectrometer (Fig. 1). Molecular ions are generated in the ion source with a range of internal energies (typically the average energy is several electron volts). After a time of approximately 1 ps, these ions, plus any ionic products generated by way of unimolecular fragmentation or bimolecular ion-molecule reactions, leave the ion source and acquire a kinetic energy of several kilovolts. The ions have a range of kinetic energies of the order of an electron volt and they "drift" at velocities of the order of 10' cm/s through the f i n t field-free region and into the electrostatic analyzer. This analyzer (Fig. 2) consists of a radial electric field which separates ions according to their kinetic energy to charge ratios. Ions of different kinetic energies are therefore focused a t different points in the plane of the energy-resolving 8-slit and, if the 0-slit is wide enough, all the ions drift through the second field-free region and enter the magnetic analyzer. The radial magnetic field separates ions according to their momentum to charge To supplement the present discussion on the instrumentation and applications of mass spectrometry the reader is referred to earlier papers in this Journal (4). Volume 51, Number 7. July 1974 / 437
ratio; so if all ions had the same kinetic energy, it would analyze for mass to charge ratio. Even though all the incoming ions do not have the same kinetic energy, i t is possible to arrange the geometry of the system so that the imperfect focus produced by the electric sector is counterbalanced by that of the magnetic sector. Hence, all ions of the same mass to charge ratio are brought to a single point of focus, independently of their kinetic energy. Since ions which are spreading in direction are also focused by such a geometry, instruments based upon this principle are termed double focusing mass spectrometers. They focus for energy and direction and analyze for mass to charge ratio. The discovery of ion kinetic energy spectrometry (IKES) occurred in the course of the study of what are in one sense instrumental artefacts of mass spectra, metastable peaks. In the next section we consider the properties of metastahle ions which give rise to these peaks. It is interesting to note that in one widely used type of mass spectrometer, the quadrupole, metastable peaks are not observed. Fortunately, in the view of those who deem IKES to be a technique of some value, magnet technology was ahead of r.f. electronics 20 years ago and metastable peaks were encountered daily in the course of using mass spectrometers.
a fraction of an electron volt and depends mainly upon the voltage used to accelerate these ions. The kinetic energy of an ion formed by a spontaneous fragmentation in the analyzer, however, is determined by the masses of the reactant and the daughter ions, the total kinetic energy of the reactant being shared between the fragments on decomposition. Hence, when metastable ions are studied it is possible to study a reaction not merely a reaction product. The best known application of this principle is in the elucidation of fragmentation patterns and hence molecular structures. For example, the observation of a peak at mass 58 in the mass spectrum of an unknown compound of molecular weight 128 tells little about the structure of the compound. However, the presence of metastahle peaks which demonstrate the occurrence of the reactions 128+ 86+ + 42 and 86+ 58+ + 28 is much more useful. These correspond to loss of the neutral alkenes, propene (mass 42) and ethene (mass 28), and suggest that the unknown is a ketone which can undergo two successive McLafferty rearrangements (six-membered cyclic hydrogen transfer reactions). Only 4-octanone fulfills this re-
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H
Metastable Ions
The mass spectrometer was developed during the 1920's and 30's primarily as a physicist's tool. The first strictly chemical use i t found was as a means of analyzing hydrocarbon mixtures in the petroleum industry. Efficient separation procedures such as gas chromatography had not then been developed and yet the enormous task of qualitative and quantitative analysis of complex petroleum mixtures was tackled. By careful control of instrumental conditions and extensive use of standards, considerable success was achieved. Into this realm of accurate peak intensity measurements there intruded broad low abundance peaks which were not restricted to integral mass positions but could occur at any position on the mass scale. Hipple and his colleagues (5) were able to show that these peaks were due to the fragmentation of metastahle ions, that is, ions which are more stable than those which fragment in the ion chamber hut less stable than those which survive, intact, the entire journey through the instrument. Metastable ions fragment after acceleration but hefore mass analysis. Since the ejected neutral species (m3) carries its share of the kinetic energy of the parent ion (ml+), the product of the metastahle reaction (mz+) is distinguished from daughter ions generated in the ion chamber by its lower kinetic energy. Because a magnetic sector is a momentum analyzer, such ions will appear to have lower mass to charge ratios than they really do possess. A simple, if somewhat lengthy calculation (Appendix), shows that they occur at a position on the mass scale given by mz2/ml. Although Hipple, Fox, and Condon were able to estimate the lifetimes of metastable ions (6) and so to initiate the study of the kinetics of ion decomposition, metastable peaks had nuisance value for much of the mass spectrometry community and they were therefore suppressed. This was done by applying a retarding field prior to the final collector which results in a potential barrier large enough to prevent transmission of the (low kinetic energy) ions formed in metastahle ion reactions. Later, as interest in the chemistry underlying the formation of a mass spectrum grew, the unique value of metastable peaks began to be appreciated. Unlike daughter ions formed by reactions occurring in the ion source, those formed in the analyzer bear indelible evidence, in their kinetic energies, of their parentage. Thus, the kinetic energy of all ions formed in the source is the same to within 438
/ Journal of Chemical Education
quirement. Where intensity will permit, any measurement on a metastahle peak is preferred to one on a normal daughter ion since the amhiguity regarding the reaction in question is removed. This applies, for example, to appearance potential measurements in which thermochemical data are derived from measurements of ion abundance as a function of the energy of the ionizing agent. Although the mass spectra of numerous organic compounds were measured during the 1950's and metastahle peaks were used in assigning molecular structures from fragmentation patterns, little use was made of the fact that metastahle peaks come in a variety of shapes. Indeed, for many years the rich variety of shapes passed unnoticed. In 1964 one of us observed a flat-topped metastable peak for the first time and showed that the width was due to the conversion of internal energy to kinetic energy during fragmentation of the parent ion (7). Although the kinetic energy with which the fragments separate is very small relative to the total kinetic energy of the system, the energy release occurs in the center of mass system of reference, while the product ion is detected in the lahoratory system. This conversion from one reference system to another causes the range of kinetic energies possessed by the daughter ion to he far greater than the kinetic energy release. Thus, if the change in the velocity of the product ion due to kinetic energy release is 6u in the center of mass system then the energy release must he proportional to (U 6113~- uz where u is the velocity in the absence of any kinetic energy release. The kinetic energy release as measured in the laboratory system is therefore amplified by a factor which is proportional to
+
2u6u
+ (6")'
=
g
(6u)' 6" the constant of proportionality involving the masses of the reactant and products. This amplification factor can he extremely large when u is large and 6u is small; it always results in a marked broadening of the metastable peak. (Compare the metastahle peak shapes given in Figure 3.) The relationship between peak width and kinetic energy release can he derived quite simply from the laws of conservation of energy and of momentum. The kinetic energy (T) released when an ion ml+ fragments spontaneously to give products mz+ and ma is related to the width ( d in
Figure 4, ion kinetic energy spectrum of benzyi chloride. The reactions giving rise to several of the peaks are shown. Note the reactions of doubly charged ions which appear at higher energy to charge ratios than the main beam of stable ions (1.0 El.
- ,
zza
tio
zdo
ib
i b ~ ~
Figure 3. Peak shapes due to fragmentation of various metastable ions. A great variety of information is embodied in these peaks which were obtained by scanning the accelerating voltage and observing reactions Occurring in the first field-free region of a double focusing mass spectrometer.
units of atomic mass to charge ratio) of the metastable peak in the mass spectrum by the equation
where Ve is the ion accelerating voltage multiplied by the unit of charge. in other words the kinetic energy of the parent ion ml+. This equation is derived in the Appendix. Kinetic Energy Spectrometry
Although the preceding observation on kinetic energy release had immense notential value for thermochemical measurements, it was of little practical use so long as metastable peaks continued to be recorded as part of the mass spectrum. The problem is not so much the low abundances of the metastable ion ~ r o d u c t sas that of interference. The solution was to d o t o normal mass peaks what had previously been done to metastable peaks-suppress them. This is most readily achieved by observing metastahle ion decompositions which occur in the fieldfree region prior to the electric sector. At any given ratio of electric sector voltage E and ion accelerating voltage V, only ions of a precisely defined energy will be transmitted through the electric sector and the energy resolving 8-slit. As soon as this ratio is altered from the normal value, stable ions which have experienced the full ion accelerating voltage will no longer be transmitted; ions with higher or lower kinetic energy to charge ratios can be transmitted hy selecting the appropriate ratio of V and E. In particumz+ +ma occurs in the fieldlar, if the reaction ml+ free region preceding the electric sector, the product mz+ will have kinetic energy mzVe/ml where V is the ion accelerating voltage. This product ion will he transmitted if the accelerating voltage V is increased by a factor of ml/mz or, conversely, if the electric sector voltage is lowered by a factor mz/ml. If an electron multiplier is placed immediately behind the energy resolving 6-slit (Fig. 1) and mass analysis is not attempted, it is convenient to scan the electric sector voltage and so to obtain an IKE (ion kinetic energy) spectrum (8). For the highest energy resolution and the most accurate measurement of ion kinetic energies it is essential that the enerrr -. resolvina slit be verv narrow. IKE spectra provide a convenient summary of the very considerable amount of data related to metastahle ion decom~ositions for a given compound. These complex spectra (Fig.
-
4) frequently show differences even when the correspond-
ing mass spectra are identical and thev can therefore he used to distinguish isomers. More significantly, these spectra define all the fragmentation reactions occurring for the compound in question and they therefore have value in molecular structure analysis. The sequencing of bio-oligomers is just one area of application of IKE spectra. The discovery of these spectra catalyzed a good deal of the work described later and the background to their discovery is therefore of some interest. The instrument used in the development work a t Purdue was the prototype Hitachi/Perkin-Elmer RMH-2. This had been designed both as a high resolution mass spectrometer, with special consideration to computer control, and as a sensitive instrument for studying metastable peaks. During early tests the electric sector voltage was scanned over a small range to determine the energy homogeneity of the ion beam. Peaks due to ions with energy to charge ratios greater and smaller than that of the stable ion beam issuing from the ion source were seen, but the detector a t the &slit (a Faraday cup) was inadequate to observe them with good sensitivity. So, i t was replaced by an electron multiplier. Spectra observed with organic compounds were then so complicated, even for simple compounds like methane (where 17 peaks were observed when the electric sector voltage was scanned), that argon was studied. Instead of just one peak being observed, even in this case there were several peaks corresponding to interaction of the ion beam with background gas even under the high vacuum conditions employed. With the increased sensitivity afforded by the electron multiplier it was also possible to make very accurate measurements of peak positions. When this was done it was found that some peaks did not occur a t exactly the calculated positions. In any field of activity, it is necessary to make careful measurements in order to check the performance of instruments. When unexpected, often second-order, deviations from expected results are seen, one can either ignore them or accord them special attention. The small deviations in peak positions do indeed contain a great deal of information since they are due to conversion of kinetic energy into internal energy (discussed later in the section on ion-molecule reactions). The preceding paragraphs have dealt with studies made without mass analysis. In most work, however, mass analysis is done in conjunction with kinetic energy analysis since it requires two independent measurements to uniquely define both the reactant and the reaction product. If a mass spectrometer of conventional geometry is used, it is then more convenient to leave the electric sector set a t a fixed value (so as to fix the mass scale) and to vary the ion accelerating voltage in order to transmit the daughter ion mz+ formed as the result of a decomposition of a metastahle ion in the first field-free regiom2 This technique, which was used to produce the metastahle Volume 51, Number 7, July 1974
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439
peaks shown in Figure 3, was first reported by Barber and Elliott 19) and develoned hv Jennines " .110). . It was also independently discovered h y " ~ u t r e l l(11) who was using a double focusine mass suectrometer as the detector staee of a tandem mass spec&meter for study of ion-mol&de reactions. Exueriencine difficulties in the tandem oueration and seeking otherkses for the sophisticated detector, he discovered that hv scannine the accelerating voltaee he could study metastable ion reactions. The development of the accelerating voltage and electric sector voltage scan techniques opened the way to the study of the properties of metastahle ions and particularly to the accurate measurement of their ahundances and widths. In turn, these parameters reveal much regarding ion chemistry as we shall now try to show. Applications of Ion Kinetic Energy Spectrometry
Among applications of IKES, both fundamental and analytical, we shall delineate a few, chosen to illustrate the detailed knowledge which is being accumulated about species as inaccessible and complex as organic ions with lifetimes in the microsecond range. Kinetic Energy Release
A most striking (and useful) feature of the kinetic energy release, T,is the range of values observed for unimolecular fragmentations. The molecular ion of vinyl fluoride, for example, fragments with the elimination of a molecule of HF, and the reaction is accompanied by the release of 1.0 eV of kinetic energy, i.e., by 23 kcal mole-'. Large energy releases are commonly associated with elimination reactions which proceed via cyclic activated complexes of small ring size.
Figure 5 . Structured metastable peak for loss of NO. from various substituted nitrobenzenes.
I t is concluded that the metastahle ions generated by these two routes are structurally dissimilar and exist as the keto and en01 forms, (I)and (II), respectively.
;;
1'
o c x c ~ , By way of contrast, when Ted Ast, then a graduate student here, determined T for the elimination of HCN from the molecular ion of symmetrical triazine and found a value of less than 0.2 meV (yes, milk electron volts, 0.2 meV = 5 cal mole-') his competence was seriously, although mistakenly, questioned. Only because of the amplification effect discussed above can a quantity this small he measured with a mass spectrometer, where ion kinetic energies are typically thousands of electron volts. Anyone familiar with calorimetry will appreciate the envy felt in thermodynamic circles at measurements which can he reported3 a s 4 2 + 0.2 cal mole-'. A more typical value of the kinetic energy release is 22 meV, the value for HCN loss from the molecular ion of henzonitrile. In this case the kinetic energy is amplified by a factor of 919 so that the total range of laboratory energies at a reactant ion energy of 6 keV is 20.2 eV. The wide range of values of T encountered in practice and the accuracy with which this quantity can he measured make it a very useful parameter for characterizing ions in the gas phase. Ion Structure The ion CsHsO+. generated in the ionsource from acetophenone (the molecular ion) or from n-hutyrophenone (as a daughter ion formed by alkene elimination from the molecular ion) undergoes loss of a methyl radical in the field-free region to give C ~ H J O +However, . the kinetic energy release accompanying the reaction in acetophenone is 7 meV, whereas that associated with the corresponding reaction in the product from n-hutyrophenone is 46 meV. ZForinstruments with reversed magnet and electric sector positions, sector scans are much more useful. See concluding section. Note that here, as elsewhere, the kinetic energy release referred to is the most probable value. 440
/ Journal of Chemical Education
V
%%Co
(11 tn) Information can also he obtained on ionic reaction mechanisms and this is illustrated by the reaction in which NO. is lost from para-substituted nitrohenzenes. The metastahle peak for this process consists of two superimposed peaks (Fig. 51, one broad and the other narrow (12). Both are due to unimolecular reactions. As the para-suhstituent is made more electron donating the relative contribution of the broad peak increases, as does its width. This and ancillary data show that the reaction involves formation of the aryloxy cation (111) which can he effectively stabilized by an electron donating para-suhstituent. The loss of NO. with release of much less kinetic energy occurs by a competitive reaction and gives a much less stable product. Substituent effect studies (12) suggest that this process involves oxygen migration to the ortho site to give the unstable product (IV). Thus NO. loss occurs via competitive 3- and 4-centered rearrangement mechanisms and the nature of the suhstituent determines the relative importance of each process.
Y
O
6
-
$
0
0
im)
/'
iIV)
lntercharge Distance in Doubly-Charged Ions Ions hearing two positive charges can be generated by electron impact. For example, p-toluidine yields, among others, an ion of mass 106 and charge 2. Some of these C7HsNZ+ ions have lifetimes, to unimolecular fragmentation, of the order of s, and they decompose spontaneously in the field-free region which precedes the electric
sector in the double focusing mass spectrometer. Among the reactions which doubly charged ions undergo are charge separation reactions in which the charges are shared between the fragment ions; for example, in the present case C,H,N2+ C1H8N2+
-
+
C,&'
C,H,N+
+ CH,N+ + C,H;
The reverse reaction, bringing the two positively charged ions together, has an activation energy which is largely due to the electrostatic repulsion of like charges. Hence the kinetic energy released during the unimolecular fragmentation represents the difference in coulombic energies at infinity and that in the ionic species (strictly in the activated complex). Measurement of this kinetic energy by the methods already described is equivalent to measurement of the intercharge distance. For the two reactions of p-toluidine shown above the energy releases are 3.3 eV and 2.3 eV, respectively. Expressing the coulomhic relation in the appropriate units we get eqn. (2).
Hence, the intercharge distances are 4.4 A and 6.2 A, respectively. I t therefore seems probable that these reactions occur, respectively, from the intact cyclic form of the molecular ion and from a ring-opened form witb the charges located a t the ends of the structure. Energy Partitioning An area in which measurements on metastable ions can make a unique contribution is concerned with the partitioning of the reverse activation energy for unimolecular decomposition between kinetic and internal energy of the products (Fig. 6). The advantages of employing ions in studies such as these have already been noted. Metastable ions have the additional characteristic that they bave very little energy in excess of that required for the reaction in question (higher energy ions will have fragmented before reaching the analyzer). Their properties therefore relate directly to the potential surface for the reaction, essentially without complications due to excess internal energy (13). For reactions in which the reverse activation energy (Fig. 6) can he determined by standard mass spectrometric methods, energy partitioning can be elucidated by measuring the kinetic energy release T. Not only does such data have fundamental significance, but it also indicates the magnitude of the correction for the reverse acti-
Reoetlon Caardinofe Figwe 6. Potential energy diagram showing the origin of Me kinetic energy release (TJand its relationship to the reverse activation energy.
vation energy which must be applied to thermochemical values determined by appearance potential methods. Frequently, such corrections greatly improve the accuracy of the tbermochemical data. lon-Molecule Reactions In this section we consider ion-molecule reactions which, like the unimolecular reactions of metastable ions, occur in the field-free region of a mass spectrometer and which are also amenable to analysis by the techniques of ion kinetic energy spectrometry. The analyzer tube of a mass spectrometer is usually maintained a t the lowest pressure attainable since the presence of residual gas molecules has the effect of defocusing the beam. The actual analyzer pressure in modern instruments is typically 10-7 torr hut Aston, who build one of the first mass spectrometers, did so without the benefits of modem vacuum technique. One consequence of his high analyzer pressures was the occurrence of various reactions between the high energy ions and residual gas molecules which led to anomalous peaks, since referred to as Aston bands. These peaks are related to metastable peaks in that they involve reaction of a fully accelerated ion occurring in the analyzer. Processes such as which occur without the transmission of a significant amount of kinetic energy to the product N+ ions, can very conveniently be studied in a double focusing mass spectrometer. The product ions, m + , have twice the kinetic enerw of normal sindv charged ions because thev were accec&ated as douh1;charged~ions. Ions arising frorn this reaction (and only this reaction) will be transmitted if the electric sector voltage is raised from its normal value E to 2 E. The mass spectrum of all doubly charged ions formed in the ion source can be obtained by using this reaction as a means of separating these ions from all others (14). The sector is set at 2 E, collision gas is added to the first fieldfree region, and the doubly charged ion (2 E) spectrum is recorded (Fig. 7). These new types of mass spectra bave potential value in chemical analysis and the individual charge exchange reactions provide a valuable source of information on ion structures and thermochemistry (15). To take just one example: Ions of the general formula C.HeZ+ where n > 6, occur witb high abundance in the 2 E mass spectra of hydrocarbons. A particularly stable structure must be involved and speculation as to what form the bonding takes in such an ion is currently going on. An interesting event occurred in our laboratories about two years ago in connection with the study of 2 E mass spectra. Setting out one morning to look at the processes described by eqn. (3) in benzene, one of the authors mis-
Figure 7. The mass spectrum solely of doubly-charged ions from m-xylene (the 2 E spectrum) is compared with the ordinary mass spectrum.
Volume 51, Number 7.July 1974 / 441
takenly halved instead of douhling the electric sector voltage in attempting to transmit the product ion. A strong signal was ohserved at the P-multiplier. We realized that the opposite process must be occurring and the ions must be acquiring extra charges. Considerable confusion resulted when we tried to mass analyze this peak and indeed only after two hours of discussion involving T. Ast and R. M. Caprioli, as well as the authors, was the relationship between ion accelerating voltage, electric sector voltage, and mass scale clearly understood. This led to the study of the charge stripping reaction (4) both for simple and for more complex ions. The particular merit of the modified mass spectrometric methods in the study of reactions such as this is that they allow the accurate determination of ion kinetic energies. Moreover, the process can he studied a t high energy resolution using an instrument such as the RMH-2. m++N-mW+N+e(4) Under these conditions rare gas ions were studied and found to undergo charge stripping with formation of three distinct but close lying kinetic energy peaks transmitted with the electric sector voltage set near to the value El2 (16). These correspond to formation of the doubly charged product ion with the accompanying loss of different amounts of kinetic energy. The difference (Q') hetween the kinetic energy of the reactant ion, m+, and that of the product ion, m2+, is a valuable source of thermochemical information not only for this, but also for other high energy ion-molecule reactions. To illustrate,
4
0
3
0
2
0
1
0
I
a o'kvr
Figure 8. Translational energy distribution of ions Ar2+ formed tram A r t in a high energy collision.
Ar+ shows three peaks for the charge stripping reaction (Fig. 8). I t is concluded that there are three states (or groups of states) in the incident argon ion beam 10-5 s after formation: (1) ground state ions which lose a large amount of kinetic energy in being excited to the doubly charged ion level, (2) ions in long-lived excited states (4D, 4F, and 2F) which need to lose less energy, and (3) ions in high Rydberg states which have the same internal energy as does ArZ+ and which can be ionized without conversion of kinetic energy into internal energy. For organic ions, the kinetic energy loss is found to he equal to the difference between the double and the single ionization potentials of the species in question (17). Therefore, a simple measurement using the IKE technique suffices to determine double ionization potentials, provided single ionization potentials are known (they usually are while double ionization potentials seldom are known). This method has considerable advantages over those previously available and provides another illustration of the range of applications of ion kinetic energy measurements. It is also possible to studyother kinds of charge permutation reactions by changing the voltage across the electric sector. For example, if this voltage is changed from its 442
/ Journal of Chemical Education
normal value + E to a value -E, product ions ml- having the same mass hut opposite charge from the incident ions ml+ can he observed. In this case, two electrons are being given to the ion ml+ and taken from the collision gas (18). Such quantities as electron affinities may become accessible from measurements made on these reactions. High energy ion-molecule reactions may also occur with excitation of the reactant ion rather than charge transfer. The collisionally excited ion may then fragment in the field-free region and all the advantages inherent in studying metastable ions devolve on these processes. The degree of excitation of the ion can he measured from the loss in translational energy. Although the observation of such reactions goes back to J. J. Thomson's day (19) they are now providing valuable information on ionatruc; tures and hence on molecular structures. The characteristic enthusiasm with which mass spectrometry is pursued was displayed a t the beginning of this new phase of interest in collision-induced dissociations. Rather than wait until a separate gas introduction system could be built, eas was introduced into the analvzer in McLaffertv's lahoiatory by loosening a bolt and deliberately allowing air to leak into the vacuum svstem: while in Jennines' laboratory the heaters used to bake-out the vacuum system were turned on to secure the necessary increase in pressure. Other methods are now used.
-
Summary
Ion kinetic enerrr is a new form of sDec-. s~ectrometw . trometry which is emerging frommass spectrometry. The technique can be used to study both unimolecular reactions a i d also ion-molecule reactions which occur at high kinetic energy, all the processes of interest occurring in the analyzer of a mass spectrometer. From the average kinetic energy of the product ions it is possible to characterize the reaction involved and from the kinetic energy distribution the conversion of internal to kinetic energy during fragmentation can be monitored. The reverse process, conversion of kinetic to internal energy in an ionmolecule reaction can also he studied. These simple measurements have an astonishingly wide range of applications (20) in molecular structure determination, in thermochemical determinations including immoved values for heats of formation of ions. double ionization potentials and electron affinities, in the determination of the structures of c o m ~ l e xions and their reaction mechanisms, in the assignment of electronic and even vibrational enerm levels in simnle s~ecies.and in ~rovidine data on eneripartitioning. considerable development is foreseen in the immediate future as instruments soecifically designed as ion kinetic energy spectrometers are used (21). The principles of this design are that ions should be mass analyzed first and that energy resolution should be maximized. This leads to a geometry in which the magnetic sector precedes the electric sector and to the name MIKE (mass-analyzed ion kinetic energy) spectrometry. Part of a MIKE spectrum taken on the Purdue instrument is shown in Figure 9. All unimolecular fragmentations of the selected ion are recorded in a MIKE spectrum. Any of these processes can also be studied a t high energy resolution and the collision-induced fragmentation and charge exchange reactions of any selected ion can be examined. MIKE spectrometers provide an interesting illustration of the Tact that scientific instruments are frequently designed for one purpose and used for another. Commercial mass spectrometers in which the magnetic sector precedes the electric sector were in use before the MIKES principle was discovered. If there is any lesson at all in the foregoing pages it is that commercial scientific instruments are anything hut sacrosanct. They should be modified, rearranged, run backwards, prodded, and coaxed to perform
Hence the kinetic energyof the product ions ml+ is given by (9)
m ~ u , 2 / 2 = (m,u,1/2)
The radius of curvature ir) of these ions in a magnetic field, H, is given by the standard equation for magnetic sectors as
Eliminating us between eqns. (9)and ( 1 0 )
.. . Figure 9. MIKE spectrum of the benzene molecular ion. Only the major peaks in the spectrum are shown.
tasks other than those envisaged by the designer. One can hardly be doing something new if one is using a n instrument in a manner foreseen when the instrument was designed. Finally, it should be noted that mainly work done a t Purdue has been emphasized in this article. However, the groups of Durup, Newton, McLafferty, Kerwin, Jennings, Los, Futrell, and many others have contributed greatly to the field of study discussed in this article. Acknowledgment
We thank the National Science Foundation for its support of the Purdue work described in this article and Professor D. A. Davenport for valuable discussion. Appendix. Derivation of Relationship between Metastable Peak Width and Kinetic Energy Release (Equation ( 1 ) )
Consider an ion ml+ which after acceleration through a potential V fragments spontaneously to m ~ and + ms. Let the velocities of ml+, mz+, and ma in the laboratory frame of reference be UI, "2, and UQ. By the law of conservation of energy
T
+ m,u,1/2
= mzu?/2
+ m,u:/2
By the law of conservation of momentum
Eliminating US gives
Suhstituting'h m,u12 = Ve gives
(5)
The left-hand side of this equation describes the position of e peak in a mass spectrum, in this case the metastable peak, and the right-hand side shows that the center of the peak is given almost exactly by m22/ml where the much smaller term maT/ m~Veis neglected relative to ( m 3 T / m 2 V e ) ' l Z ; while the width, d, is given by
Rearrangement of eqn. (12) gives eqn. ( 1 ) . Literature Cited (1) Thomson, J. J., Phil. Mag., 44.293, 311 (16971. (2) Wien, W., V h n d l . Dem. Physik. C n . au Berlin 16. 165 l16971: Wirn, W.. Ann. Phya.. 6% 167 118981. (3) FOX general treatment8 of ma^ weefmmetry at an intmductory level see: Kiser. R. W.. "Introduefian to Mass spectmmetry and IU Applieatiom." pnntice-Hall. In=., N.J.. 1965; Hill. H. C.. "lnfmduction to Mass Spochometry." Heyden, London. 1566; Shrader, S. R.. "lntmduetory Maas Spectrometry." Allyn and Bamn, Ine., Bolton, 1971: McLafferty, F. W.. '4nbrpretafion of Mass Spectre." W. A. Benjamin. he.. New York. 1966; Wiiliams, D. H., and Howe. I.. "Ptineip l ~ o f o r g a n i eM a s Spectrametry,"McGraw-HillBmkCo., London. 1972. (4) E e n g . G. W., J. CHEM. EDUC., 46, A69. A149. A213 11969): Ahramson, F. P., J. CHEM. EDUC.. 49.A283(19721. 151 Hioole. J.A.. and Condon. E. U..Phva. Rsu.. 68.54 119451. H ~ ~ P IJ.. ; A,; FOX.R. E.. A d cokoi.E u..'P~~s. RW, 69.347 119461. (71 Beynan, J . H., Saundera. R. A,. and Williams. A. E.. Nnture. 204. 67 (19641: Beyno", J. H., Saundera. R. A,. and Williams. A. E.. Z a r l Noiurfmsch., 20% 180 (19651. (81 Beynon, J. H., Amy, J. W., and Baitinger, W. E.. Chem. Commun., 723 (19691; Beynon. J. H..Amy, J. W.. Baiting% W. E.. and Kornsteu, T.. Inr. J. M a * Spaefrom. IanPhya.. 3.55 (19691. (9) Barber, M., and Elllatt, R. M.. Twelfth Annual Conference on Mass Spectrometry and Allied Tonics. ASTM CornmittecE-14. Mantrod. 1964. (I01 Jcnnings.K.~.:~.c h a m Phys., 43.4176(19651. , (111 Futrell. J. H.,Ryan, K.R.,sndSieck,L. W..J. Chrm. P h ~ s .43.1832(19651. (12) Beynon, J. H., Bertrsnd, M., and Cmks, R. G., J. Amer Cham. Soc.. 95, 1739
i6i
,."?,%
\'>,",.
(131 Jones, E. G., Beynon, J. H., and C m b , R. G.,J . Chrm. Phys.. 57. 2652 119721; Jones. E. G.,Bauman, L. E., Cmks, R. G.,and Beynon, J . H.. 0%-Moss Spect m m . 7. 185l19731. I141 Bcynon. J. H.. Mathias. A.. and Williams. A. E., OF#. Masa Spectrom.. 5. 303 (19711. (151 ht,T,.Beynoo. J.H.,andCmks, R.G.. Or8 Ma8Spectmm.. 6.749l19721. I161 Cooks, R. G..Baynon, J. H., and Ast, T.. J. Amer. Chem. Sor., 94, IN4 (19721: Aaf,T.,Beynon. J.H., s n d C m b , R.G., J.Amer. Chem Soc.. 94,6611 119721. I171 Aaf, T., Bcynon. J. H., and Cmka, R. G., Inf. J. Moss Sp~clmm.Ion Phys.. 11. Wl19731. (181 Appell. J., Fournier, P. G.. Fehsonfeld, F. C.. and Durup. J., presented at the 20th Annual Conference on Mass Speefmmefry and Allied Topics. Dsllss. 1972; Keough. T.. Beynon, J. H.. and Cmks. R. G.. J . A m e i Cham. S o r . 95. 1695 119731. I191 CompanAsfon, F.W.,Pmc. CambridgePhil. Sor., 19, 317 (19191. (201 For a full treatment soe Cmks, R. G.,Beynon. J. H.. Caprioli. R. M.. and Lester. R. G.."Mefasfahle Ions." Elsevier. Amsterdam. 1973. (211 Beyno". J. H., Cooks. R. G.. Amy. J. W.,waitinger, W. E.. and Ridlcy. T. Y.. Anal. Chem.. 45.10231\(19731.
Volume 57, Number 7. July 1974
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