Unstable and Semistable Species

Many molecular species studied by microwave spec-troscopy are unstable to various degrees, and special preparation techniques, absorption cells, and instrumentation methods have been developed for their investigation. These techniques and methods have been applied to the study of radicals, ions, and semistable molecules. The low operating pressure of typical microwave studies helps in minimizing decomposition from wall collisions and intermolecular collisions.

Radicals (see also Section IV.E.2) are very short-lived, reactive species. They are often produced as products of an RF electric discharge. Alternatively, the products of an electric discharge are allowed to react with another substance to produce the desired radical. In these production methods, a continuous flow of radicals is supplied to the microwave absorption cell. The use of glass absorption cells with Teflon windows is particularly useful for such studies. The large volume-to-surface ratio possible for such cells minimizes radical decomposition. A typical cell is illustrated in Fig. 26. This type of cell, or a variant of this cell, can be used to study molecular ions, semistable molecules, and molecules at high temperature.

Although molecular ions have been detected and assigned (see Section IV.E.2), their number is still small, primarily because of the difficultly in producing significant concentrations. A new technique which increases their density by about two orders of magnitude employs a longitudinal magnetic field along the axis of a glow discharge tube. The addition of the magnetic field increases the length of the ion-rich negative glow and the concentration of the molecular ions. The signal enhancement by application of a magnetic field for HCO+ is illustrated in Fig. 27. The inset depicts the glass pipe absorption cell (5 ft long, 1.5 in. inside diameter). The glass pipe is connected to short transition sections that flare to 4 in. The transition sections house cylindrical electrodes of the same inside diameter as the glass pipe to maximize microwave transmission. The solenoid provides a field of up to 300 G.

By employment of high-temperature cells, numerous molecules have been studied that at room temperature would not have sufficient vapor pressure to give an observable microwave spectrum. Various alkali halides, for

FIGURE 26 Details of a typical free space cell for the study of radicals or other unstable species. The reactive species pass into the cell and are subjected to microwave radiation, and the resulting absorption is detected. The molecular species are continuously replaced by the pumping system.

FIGURE 27 Apparatus for enhanced production of positive molecular ions. A 13-mA discharge in a 1:1 mixture of CO and H2 is used to produce HCO+. Enhancement of the line at 267.5 GHz is apparent when the field is turned on. [From De Lucia, F. C., Herbst, E., Plummer, G. M., and Blake, G. A. (1983). J. Chem. Phys. 78, 2312.]

FIGURE 27 Apparatus for enhanced production of positive molecular ions. A 13-mA discharge in a 1:1 mixture of CO and H2 is used to produce HCO+. Enhancement of the line at 267.5 GHz is apparent when the field is turned on. [From De Lucia, F. C., Herbst, E., Plummer, G. M., and Blake, G. A. (1983). J. Chem. Phys. 78, 2312.]

example, have been studied this way. In situ production in hot absorption cells has been useful for many cases where simple vaporization is not applicable because of dissociation or decomposition. Here, reactions are often made to occur within the hot cell by flowing appropriate constituents into the cell. Likewise, pyrolysis and thermolysis have been developed as useful techniques for producing numerous semistable molecular species.

The lifetimes of semistable molecules are usually on the order of 1 sec; hence, the molecular species are not isolable, but they live long enough to flow through conventional absorption cells at moderate flow rates. Such molecular species are not generally accessible by standard chemical methods and their properties and chemistry are often relatively unknown.

By coupling a reaction flow system with a microwave spectrometer, semistable molecules can be detected and readily identified. A simple production method would involve merely heating an appropriate precursor and passing the resulting decomposition products into an absorption cell. Alternatively, a compound may be heated (or not) and transported with (or without) an inert carrier gas into a reaction zone where reaction with another (heated) substance takes place. The reaction zone may be just before or inside the absorption cell. As general examples, we cite the production of thioketene, CH2=C=S, by pyrolysis of the trimer [(CH3)2CS]3 at 1000°C, preparation of bromoketene, BrHCCO, by pyrolysis ofBr2HCCOCl with zinc metal at 300°C, the synthesis of ClBSe by passing Cl2Se2 over boron at 1100°C, and of XNCO (X = Br, I) by passing the halide vapor, X2, over warm, dry AgNCO. Figure 28 shows an example of a high-temperature reaction flow system. The molecule or molecules to be reacted are passed through a small quartz tube heated to an appropriate temperature. The thermolysis products flow directly into and through the absorption cell of a microwave spectrometer. The flow rate is adjusted to maintain a suitable pressure (ca. 0.1-100 ¡m Hg). The low operating pressures characteristic of microwave spectroscopy help to reduce unwanted polymerization or decomposition with the metal walls, etc., and the flow technique continually replaces the sample with fresh reaction products.

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