Page 298 - 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

Microwave Molecular Spectroscopy 815

a. Rotational energy transfer. As indicated above, to which parameters are expected to differ significantly
rotational energy transfer can be studied by use of double- between conformers, or to unique structural effects, (iv)
resonance techniques. For example, by using infrared a basis to fix particular bond distances or angles or their
pump radiation with probe radiation in the millimeter- differences in the structural analysis, (v) initial structural
or submillimeter-wave region, both rotational and vibra- parameters for prediction of spectra of new and unusual
tional state-changing collisions can be studied. Here, a molecules, (vi) calculation of quadratic force constants
CO 2 laser coincidence with an infrared molecular tran- for the estimation of quartic distortion constants or the
sition provides a nonthermal population distribution in a evaluation of average structures, (vii) estimation of the
particular rotational state of the excited vibrational state. dipole components µ a , µ b , and µ c from which transition
The infrared radiation can be introduced into the measure- intensities may be computed, and (viii) calculation of the
ment cell along with the microwave probe radiation via a cubic force field to evaluate the rotation–vibration interac-
dichroic window. The probe radiation probes a single ro- tion constants α i and subsequently equilibrium rotational
tational transition within the excited vibrational state. The constants from the effective rotational constants.
change in intensity of the rotational line, or time response, The computation of semiexperimental equilibrium ro-
is recorded after the pumping laser is pulsed. The probe tational constants from calculated cubic force constants
hence monitors the population change as the system is allows the important equilibrium structure to be obtained.
perturbed and returns to equilibrium. Results of this approach have been quite satisfactory and
In the case of studies on the symmetric top CH 3 F, for ex- this can be expected to be a very useful approach where
ample, the CO 2 laser line 9P(20) pumps the CH 3 F energy limited vibrational data are available.
level population from J = 12, K = 2 of the ground vibra- In the case of weakly bound complexes, ab initio calcu-
tional state to the v 3 = 1, J = 12, K = 2 state. The excess lations can be very useful to limit the number of possible
population in the excited vibrational state produced by the configurations that need to be considered in the structural
pump is quickly redistributed by several CH 3 F CH 3 F col- determination. Furthermore, the ab initio results can help
lisional processes to reestablish rotational thermal equi- to remove other ambiguities which arise and to identify
librium within v 3 = 1. Also, several vibrational collision possible large-amplitude motions.
processes reestablish equilibrium among the vibrational In addition, various microwave-derived internal rota-
levels. The measured time-varying responses for the vari- tion barriers and fine structure and hyperfine structure
ous rotational transitions provide data to characterize the coupling constants all provide a wealth of data to test the
complicated internal energy transfer. Both J-changing ro- accuracy of quantum mechanical calculations.
tational collisions, where v 3 = 1, J = n, K = 0(n inte-
ger), and K-changing rotational collisions, where v 3 = 1,
E. Structural Information
J = m, K = 3n (m, n integers) have different behavior
but can be accurately modeled with four parameters. Structures of many hundreds of molecules—diatomic, lin-
Many more studies on rotational energy transfer can ear, symmetric top, and asymmetric top—have been eval-
be expected in the future. Pressure broadening effects ob- uated by microwave spectroscopy. Illustrative structures
served in rotational lines due to self-broadening or foreign are given in Tables III–V and XV (see also Section VIII).
gas broadening also provide information on energy trans- Structures of some small molecules are given in Table IX.
fer processes during molecular collisions; however, the Over the last 10 years, a considerable amount of work
information is much less than the method described here. has been devoted to the study of ions, radicals, molecu-
Various pressure broadening studies have been carried out lar complexes, and other transient molecular species due
over the years. Pressure broadening is discussed further in primarily to improved experimental techniques, which are
Section XI. discussed in Section XI. Results of some of these studies
will be addressed in this section. Structures of a few un-
stable molecules along with methods for their preparation
2. Ab Initio Quantum Calculations
are given in Table X. Methods of evaluating molecular
High-quality ab initio calculations have proven to be a structures and the complications arising from effects of
helpfulaidintheanalysisofrotationalspectra,particularly molecular vibration are discussed in Section VIII.
for predictive purposes and where there exists ambigui- By means of the very precise structural parameters that
ties. In general, ab initio calculations can provide useful can be obtained from microwave spectroscopy investiga-
information for structural determinations. These include tions, trends in bond distances or angles among a series
(i) estimates of the most stable conformers and the low- of structurally similar molecules can be readily studied.
est energy form, (ii) insight into the difference expected At present the most reliable parameters for many large
in structural parameters between conformers, (iii) a guide molecules are those derived by the substitution method