Ion Cyclotron Resonance Spectroscopy
Authors: Baldeschwieler John D. 
Published: October 2006

Ion cyclotron resonance spectroscopy yields information on many aspects of ion-molecule chemistry. The method is ideally suited for experiments involving ion energies below several electron volts, and hence provides a valuable complement to other techniques. Cyclotron double resonance is uniquely suitable for establishing relationships between reactant ions and their product ions in complex ion-molecule reaction sequences. The double-resonance experiments with isotopic species yield information on reaction mechanisms and the nature of intermediate species.

Ion-molecule reactions which occur at low energies are quite sensitive to the nature of functional groups and the details of molecular structure. Reactions of ions or neutral molecules with specific reagents in the cyclotron spectrometer can thus be used to characterize unknown species. Once the systematic ion-molecule chemistry of useful reagents has been worked out, it should be possible to proceed in a manner directly analogous to classical chemical methods. Suppose, for example, that reagents A
+
, B
+
, C
+
, and D
+
each have characteristic reactions with different functional groups. Then these reagents can all be mixed with an unknown neutral species, X, and each of the reactions, X + A
+
→ ?, X + B
+
→ ?, . . . . can be examined. In contrast to solution chemistry, all the reagents can be added simultaneously to the unknown, since each of the specific reactions can be examined by cyclotron double resonance. The reactions which occur, the species synthesized, and the products of degradation then characterize X.

The same methodology can be applied to characterize an unknown ionic species X
+
, through use of neutral reagents A, B, C, and D. For example, proton transfer reactions to neutral species have been applied in studying ions of mass 45 produced from various sources. The order of the proton affinities of the neutral reagent molecules are as follows: NH
3
> isobutylene > propene. Ions of mass 45 can be produced by the protonation of ethylene oxide (see structure III), the protonation of acetaldehyde (see structure IV), and the fragmentation of dimethyl ether (see structure V). Those ions might be expected to have, respectively, the three structures:

Proton transfer from the mass-45 ions from sources III and IV to NH
3
and to isobutylene occurs readily, but not proton transfer to propene. For the ion from source V, proton transfer to NH
3
occurs, but not proton transfer to isobutylene or propene. Thus the proton transfer reactions to various neutral reagents demonstrate that the mass-45 ions from the various sources are different. This example is only a rudimentary version of an approach to the characterization of unusual ionic species; niore sophisticated applications can follow when the systematic chemistry of more reagents is available. This approach should be ideal for comparing nonclassical carbonium ions produced by different routes. Some very interesting ionic species are produced by rearrangements in the fragmentation of molecules, following electron impact. Such molecular rearrangements frequently result in the fragmentation of an ion radical to another ion radical with the elimination of a small neutral species. It should be possible to run these reactions in reverse to check the postulated mechanisms. An interesting result of the systematic study of proton transfer to various functional groups is the finding that the proton affinity of various amines and pyridine is extremely high. Species such as VI and VII: might be expected to be very stable; they are in fact so stable that they are unreactive with respect to subsequent chemistry at the charge center. Thus, if there are other functional groups on the ion, the important reactions should occur at these functional groups. It should be possible to design species for which the presence of the charge has little influence on the reactivity of a neutral functional group. In this case the charge functions simply as an inert label which makes the study of neutral-neutral reactions accessible by cyclotron resonance:

Various routes for development of the basic technique also appear to be very promising. Echo phenomena following sequences of pulsed excitation have been observed in electron cyclotron resonance. Analogous transient phenomena should also occur in ion cyclotron resonances. Pulsed-cyclotron-resonance techniques of course have intriguing analogies to nuclear-magnetic-resonance spin-echo experiments and may be the technique of choice for making accurate measurements of ion-molecule-reaction cross sections as a function of energy for low ion energies.

Finally, many ion-molecule reactions yield products in excited electronic states. For example, the reaction N
2
+
+ CO → N
2
+ CO-
+
has been studied by beam techniques. A straightforward procedure is to observe optical emission from the cyclotron spectrometer by placing a window at the end of the cyclotron cell. The emission can be analyzed with a crude set of optical filters, or with a high-speed spectrograph. Optical emission from the cyclotron cell can of course originate from many sources. The radiation from a specific excited product ion can be selected by a radio-frequency-optical double-resonance experiment. If, in the general reaction A
+
+ B → *C
+
+ D, ion A
+
is irradiated at its cyclotron resonance frequency, the number density of optical emitters *C
+
is changed. If the irradiating frequency is modulated, then the number of optical emitters will be modulated, so that the intensity of emission from *C
+
will also be modulated. When the optical emission from *C
+
is analyzed in a spectrograph with a photoelectric cell, the output of the photoelectric cell can be detected with a phase sensitive detector referenced to the modulation frequency. This highly specific modulation-detection scheme should discriminate against other sources of light in the cyclotron cell.