Page 308 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: GNH Final Pages
Encyclopedia of Physical Science and Technology EN009N-447 July 19, 2001 23:3

Microwave Molecular Spectroscopy 825

bonding in determining the conformations (often the more millimeter-, and submillimeter-wave regions. Active in-
stable one) of certain rotamers. For methoxyacetic acid, terplay between laboratory observations and astronomical
CH 3 OCH 2 COOH, for example, an intramolecular hydro- observations has helped to drive both areas.
gen bond is formed between the hydroxyl hydrogen and Laboratory measurements have been driven by con-
the ether oxygen, giving a planar five-membered ring. tinued improvements in experimental techniques such as
rapid cooling Fourier transform microwave spectroscopy
(see Section XI). This technique, coupled with improved
5. Methyl Group: Tilts and Asymmetry
species production methods, has provided capabilities to
Numerous examples of methyl group tilts have been found identify an increasing number of exotic molecular species.
where the symmetry axis of the methyl group does not Moreover, recent advances in methods to generate high-
correspond with the bond direction. The effect has been frequency microwave radiation, such as high-frequency
demonstrated from analysis of internal rotation splittings backward wave oscillators, have enabled laboratory mea-
and directly from structure determinations. In methyl ni- surements in the terahertz region (1 THz = 1000 GHz,
trate, CH 3 ONO 2 , for example, the CH 3 groups is tilted to- equivalent to λ = 0.3 mm). This technological develop-
◦
ward the unshared electron pair by 4.8 . The NO 2 group is ment will enable, for example, the direct detection of light
◦
also found to be tilted by 3.9 away from the methyl group. hybrids for which even the lowest rotational transitions
Asymmetric methyl groups for which the hydrogen are at high frequencies. In the case of the radical NH,
3
atom in-plane (plane of the heavy-atom skeleton) has a for example, with a electronic ground state, the
slightly smaller C H bond length than the out-of-plane ground-state transition is split into three fine-structure
hydrogens have been reported. Examples are methyl ni- transitions at frequencies 946, 974, and 1000 GHz.
14
trate and thioacetaldehyde. Each of these transitions is split by quadruple ( N) and
magnetic hyperfine (H) interactions.
The detection of molecules via their rotational spec-
6. Quasi-Linear Molecules
tra allows astrophysicists to probe interstellar clouds to
Finally, we mention HCNO, where the ground-state spec- provide information on their environment, star formation,
trum is consistent with a linear structure, but the equilib- interstellar chemistry, mechanisms for synthesis and de-
rium structure is slightly bent. It has been found that the struction of interstellar molecules, isotopic distributions,
bending potential function has a small hump well below etc.
the ground vibrational level at the linear configuration. Continued and increased advances in our knowledge of
HCNO is one of a handful of molecules, called quasi- interstellarspacecanbeexpected.ThesatelliteFIRST(Far
linear molecules, with this characteristic behavior. Other Infrared and Submillimeter Space Telescope) is planned
examples are HNCO, HNCS, and HNCSe. In a similar to be launched by 2003 and will cover the frequency range
way, CH 3 NCO, CH 3 NCS, and CH 3 NCSe are examples of 300–3000 GHz (λ = 1–0.1 mm). This telescope will also
quasi-symmetric-top molecules. provide information on planets and comets. These satel-
lite observations, coupled with high-frequency microwave
studies, will enable the identification of a host of other
7. Interstellar Space Applications
molecules in space.
The region between the stars contains interstellar clouds
composed primarily of dust and gas. Over 100 molecules
(neutrals, ions, and radicals), mostly carbon-containing V. CENTRIFUGAL DISTORTION EFFECTS
compounds, have been identified in interstellar molecular
clouds.Thelargestmoleculecontains13atoms,thecarbon The rigid-rotor treatment discussed in the previous section
chain HC 11 N. accounts for the general features of the rotational spec-
Interstellar spectroscopy is usually carried out with trum. These gross features are modified somewhat when
a single-dish radio telescope composed of a precision the effects of nonrigidity, nuclear coupling, and so forth
parabolic reflector with a highly sensitive microwave de- are taken into account. In this section, the effects of cen-
tector at the focal point of the reflector. Molecular identi- trifugal distortion are considered.
fications, usually via rotational emission frequencies, are The centrifugal force produced by rotation distorts the
made by comparison with precise laboratory frequency molecule from its equilibrium configuration and the bond
measurements or frequency predictions. Microwave spec- distance and angles change slightly. Hence, the rotational
troscopy plays a very important role in providing the data spectrum is no longer characterized by a set of equilibrium
for identification of new molecular species. Identifica- moments of inertia. Additional terms in the Hamilitonian
tions have been made mostly with data in the microwave, are required to account adequately for the observed