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

822 Microwave Molecular Spectroscopy

TABLE XII Selected Molecular Ion Studies a
Ion Information/comments Reference
SH − Spectroscopic constants; method to distinguish between positively Civis, S., Walters, A., Yu Tretyakov, Yu., Bailleux, S., and Bogey,
and negatively charged ions and neutrals in plasma M. (1998). J. Chem. Phys. 108, 8369
HN + Spectroscopic constants, quadrupole coupling, infrared-microwave Ho, W. C., Pursell, C. J., Weliky, D. P., Takayi, K., and Oka, T.
2
double resonance (1990). J. Chem. Phys. 93, 87
PO + Spectroscopic constants, r e = 1.424993 ˚ A, ω e = 1411.5cm −1 , Petrmichl, R. H., Peterson, K. A., and Woods, R. C. (1991).
ab initio calculations, comparison to SiF + J. Chem. Phys. 94, 3504
XeH + Spectroscopic constants, r e = 1.6028 ˚ A, ω e = 2270.2cm −1 , Peterson, K. A., Petrmichl, R. H., McClain, R. L., and Woods,
quadruple coupling, Zeeman analysis, spin–rotation coupling R. C. (1991). J. Chem. Phys. 95, 2352
He···Ar + Zeeman analysis, microwave–microwave double-resonance study Carrington, A., et al. (1995). J. Chem. Phys. 102, 2379
FN + Spectroscopic constants, ab initio calculations, r(F N) = 1.2461 ˚ A, Botschwina, P., Sebald, P., Bogey, M., Demuynck, C., and
2
r(N N) = 1.1034 ˚ A Destombes, J.-L. (1992). J. Mol. Spectrosc. 153, 255
FCO + Spectroscopic constants, ab initio calculations, r(F C) = 1.2014 ˚ A, Botschwna, P., et al. (1992)
r(C O) = 1.1151 ˚ A
Ar···H + 3 Spectroscopic constants, large-amplitude internal rotation motion, Baillex, S., et al. (1998). J. Mol. Spectrosc. 190, 130
+
120 planar rotation of the H group relative to Ar atom;
◦
3
r = 2.3852 ˚ A is the distance from the center of mass of the H +
3
triangle to Ar
HNCCN + Spectroscopic constants, linear configuration Amano, T., and Scappini, F. (1991). J. Chem. Phys. 95, 2280
a See also references cited. Spectroscopic contants: Rotational constants, distortion constants, various other interaction constants.
for example, the small induced dipole is 0.0022 D. These 4. Rotational Isomerism and Ring Conformations
and other weakly bonded complexes are usually pro-
duced by the rapid expansion of a pressurized mixture Microwave spectroscopy has also provided considerable
of the constituents through a supersonic nozzle into an information on rotational isomerism and conformations
evacuated cell of the spectrometer (see Section XI). In- of ring compounds. The existence of one or more distinct
formation like that outlined above for H-bonded com- rotational isomers for a large number of molecules has
plexes can also be attained for these complexes. These been demonstrated. Similarly, the stable conformations
studies of weakly bonded complexes provide important for numerous ring compounds have been derived. Detailed
information about intermolecular forces. Recently, the structural information has also been obtained for many
range of complexes studied has been extended to include rotamers and ring systems.
clusters such as (Ar) n HF (n = 2,3,4) and (CO 2 )n HF The sensitivity of the rotational constants to the ring
(n = 2,3). conformation is illustrated in Fig. 16. The boat confor-
In the analysis of complexes, the accurate structures, mation is clearly the correct conformation. Similarly, the
barriers to internal rotation, etc., of the molecular species rotational isomers evident in Fig. 10 may be identified
involved in complex formation are important in order to by comparison of the observed and calculated separation
understand the subtle changes that may occur upon com- (B + C) between the bands of a given series.
plexation. Also, it may be noted that for a complex formed Distortion of small rings due to substitution has also
by an asymmetric-top molecule and a rare gas atom, there been observed. In going from cyclopropene,CH CHCH 2 ,
are, in general, eight structures which are compatible with to 3,3-difluorocyclopropene, CH CHCF 2 , for example,
˚
the moments of inertia. This ambiguity can often be clar- the C C double bond is increased in length by 0.025 A,
ified or reduced by use of symmetry arguments or other while the other ring bonds are shortened. On the
parameters such as the dipole moment, nuclear quadrupole other hand, axiridine,CH 2 CH 2 NH, and chloroaziridine,
structure, isotopic information, or ab initio calculations. CH 2 CH 2 NCl, are remarkable in that detailed structural
The latter has been particularly useful in clarifying struc- studies reveal that the ring structures are virtually iden-
turalambiguities and providinginsightinto the most stable tical. Isoxazole,OCH CHCH N, is a good example of
form. A large number of weakly bound complexes have the tendency of C H bonds adjacent to ring heteroatoms
been studied by microwave spectroscopy. to tilt slightly toward these atoms.
Table III gives a few molecular structures. In addi- A detailed study of various isotopomers of 1,2,3-
tion, Table XIII provides the geometries of some selected trifluorobenzene showed that the bond angles at C 1 and
complexes. C 3 are enlarged, while the C 1 C 2 and C 2 C 3 bonds are