Page 329 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN009N-447 July 19, 2001 23:3

846 Microwave Molecular Spectroscopy

TABLE XXVIII Microwave Studies of Isotopically Induced Polar Molecules
Molecule Reference
17 O C O, a Gripp, J., M¨ader, H., Dreizler, H., and Teffo, J. L. (1995). J. Mol. Spectrosc. 172, 430
µ = 0.0006 D
18 O C O ,
a
µ = 0.0011 D
D Bottcher, O., Heineking, N., and Sutter, D. H. (1990). J. Mol. Spectrosc. 139, 236
, a µ 0.0115 D
D
D 2 C C CH 2 , b Meyer, V., and Sutter, D. H. (1993). Z. Naturforsch. 48a, 725
µ = 0.0053 D
a Studied with a waveguide MWFT spectrometer.
b Studied with a waveguide bridge-type MWFT spectrometer equipped with flat, oversize Stark cells.

Deuterium substitution often provides dipole moments for such cells minimizes radical decomposition. A typical
of sufficient magnitude for ready detection, typically cell is illustrated in Fig. 26. This type of cell, or a variant of
0.01 D. Such isotopic substitution allows the direct mea- this cell, can be used to study molecular ions, semistable
surement by microwave spectroscopy of many funda- molecules, and molecules at high temperature.
mental molecules, such as CH 4 ,CH 3 CH 3 ,CH 2 CH 2 , Although molecular ions have been detected and as-
and CH CH. In addition to new structural information, signed (see Section IV.E.2), their number is still small,
other advantages are possible. Internal rotation informa- primarily because of the difficultly in producing signi-
tion is difficult to extract from symmetric-top spectra. By ficant concentrations. A new technique which increases
converting a symmetric top to an asymmetric top (say their density by about two orders of magnitude employs
(CH 3 CH 3 → CH 3 CHD 2 ), effects such as internal rotation a longitudinal magnetic field along the axis of a glow dis-
can be more readily studied. Heavy-atom substitution also charge tube. The addition of the magnetic field increases
produces a small dipole moment; however, the result of the length of the ion-rich negative glow and the concen-
this type of substitution will be much smaller than for tration of the molecular ions. The signal enhancement by
17
12
H-atom substitution. For example, in 16 O C O, the ob- application of a magnetic field for HCO is illustrated in
+
served dipole was estimated to be only 6 × 10 −4 D. Other Fig. 27. The inset depicts the glass pipe absorption cell
examples of small induced dipole moments from isotopic (5 ft long, 1.5 in. inside diameter). The glass pipe is con-
substitution are given in Table XXVIII. nected to short transition sections that flare to 4 in. The
transitionsectionshousecylindricalelectrodesofthesame
B. Unstable and Semistable Species inside diameter as the glass pipe to maximize microwave
transmission. The solenoid provides a field of up to 300 G.
Many molecular species studied by microwave spec-
By employment of high-temperature cells, numerous
troscopy are unstable to various degrees, and special
molecules have been studied that at room temperature
preparation techniques, absorption cells, and instrumenta-
would not have sufficient vapor pressure to give an ob-
tion methods have been developed for their investigation.
servable microwave spectrum. Various alkali halides, for
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 inter-
molecular 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 produc-
tion methods, a continuous flow of radicals is supplied to FIGURE 26 Details of a typical free space cell for the study of
radicals or other unstable species. The reactive species pass into
the microwave absorption cell. The use of glass absorp-
the cell and are subjected to microwave radiation, and the resulting
tion cells with Teflon windows is particularly useful for absorption is detected. The molecular species are continuously
such studies. The large volume-to-surface ratio possible replaced by the pumping system.