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

Microwave Molecular Spectroscopy 819

constants for two isocyanopolyynes, H (C C) 2 NC and
H (C C) 3 NC.
Another illustration is provided by the common sub-
stance sulfuric acid, (HO) 2 SO 2 . The absorption spectrum
of discrete sulfuric acid molecules has been recently stud-
ied by microwave spectroscopy. A detailed molecular
structure determination indicates the free acid does not
have the expected C 2v symmetry, but rather the symmetry
is only C 2 . The stable conformation is depicted in Fig. 13b.
The interesting features found, which show the power of
◦
the method, are that the OH groups are rotated ca. 90 from
the compact C 2v form and the heavy-atom framework is
slightly twisted and its symmetry is also not precisely C 2v .
Structural studies of large molecules may be illus-
trated by cyclopentadienyl chromium dicarbonyl nitrosyl
(η 5 C 5 H 5 )Cr(CO) 2 NO, a simple piano-stool-type config-
uration. Interest in these metal–nitrosyl compounds arises
since NO is a neurotransmitter. A study of various iso-
tomersgivestheoverallmolecularconfigurationandstruc-
ture of Fig. 15.

FIGURE 13 (a) Structure, orientation of principal axis, and
dipole moment components in H 2 PC CH. [From Cohen, E. A., 2. Free Radicals and Ions
McRae, G. A., Goldwhite, H., Stefano, S. D., and Beaudet, R. A.
(1987). Inorg. Chem. 26, 4000.] (b) Conformation of sulfuric acid Microwave techniques have been applied to the study of
showing the C 2 axis. [From Kuczkowski, R. L., Suenram, R. D., free radicals such as CH, CN, OH, NO, SO (see Table III),
and Lovas, F. J. (1981). J. Am. Chem. Soc. 103, 2561.] ClO, SiF 2 , SiN, CCH, HO 2 , CuO, and other short-lived
species. In situ synthesis in the absorption cell is often
ab initio quantum calculations. The structural formu- used to produce these unstable species (see Section XI).
las through n = 5 are given in Fig. 14. The n = 0, 1 The free radicals OH, CH, and CCH have also been iden-
molecules have been detected in a cold molecular cloud. tified in interstellar space. Molecules with electronic an-
The molecules were studied by pulsed microwave Fourier gular momentum (spin or orbital) have rather complicated
transform (MWFT) spectroscopy. A low-current dc dis- rotational spectra that arise because of the interaction of
charge of methylcyanoacetylene in Ne was used to pro- the electronic angular momentum with the rotational an-
duce methylcyanopolyynes to n = 4. The rotational con- gular momentum. The details of the spectrum depend on
stants of Fig. 14 provide the basis for predicting radio
transitions of interest for astronomical searches. Sim-
ilarly, a recent study has provided accurate rotational

5
FIGURE 14 Methylcyanopolyyne structural formulas and micro- FIGURE 15 A view of η -C 5 H 5 Cr(CO 2 ) 2 NO. Some structural pa-
5
wave-determined rotational constants. For details of the analy- rameters are also given. Cp denotes η -C 5 H 5 ; asterisk indicates
sis see Chen, W., Grabow, J.-U., Travers, M. J., Munrow, M. R., assumed value. For details see Kukolich, S. G., McKay, R. T.,
Novick, S. E., McCarthy, M. C., and Thaddeus, P. (1998). J. Mol. Breckenridge, S. M., Flores, L. D., Morris, G. E., S`eckafoose, S. M.,
Spectrosc. 192, 1. and Morrison, D. L. (1995). Inorg. Chem. 34, 4182.