Page 328 - 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 845

hand, for linear and symmetric rotors, the field gradient is splittings are possible in singlet electronic ground states
symmetric (χ xx = χ yy ) about the bond axis z, which cor- and have been observed. This arises from the interaction
responds to a principal axis, and from Eq. (130) it follows ( =−µ · H) of the nuclear spin magnetic moment µ I
I
1
1
that χ x = χ y =− x z =− χ. and a weak magnetic field H generated by rotation of the
2 2
molecule. Hence, H is proportional to the angular momen-
C. Quadrupole Coupling Constants tum. High-resolution techniques are required to observe
and the Chemical Bond the small magnetic splittings required for evaluation of the
nuclear magnetic coupling constants. The theory for com-
Considerable information on chemical bonding has been
bined nuclear quadrupole and magnetic hyperfine interac-
obtained from the quadrupole coupling constants. The ba-
tions has been developed. For CH 2 O, the observed hyper-
sic approach is to relate the nuclear coupling in molecules
fine structure arises solely from the magnetic moments of
to the known coupling in free atoms. The field gradient
the two protons, since neither hydrogen nor oxygen has a
in atoms is primarily determined by the p electrons, and
quadrupole moment. On the other hand, for CHDO there
in a molecule any factor that affects the p electron dis-
is a quadrupole interaction from D which has a quadrupole
tribution gives rise to a change in the atomic coupling
moment as well as a magnetic interaction from the nuclear
constant. Relations between the charge distribution near
spins of H and D.
the coupling nucleus and the coupling constants are avail-
able. These analyseshave provided additional insightsinto
the concepts of ionic character, hybridization, π-bonding, XI. ADVANCED EXPERIMENTAL METHODS
and electronegativities. Table XXVII illustrates the depen-
dence of the field gradientq on the electronic environment, Continued developments in experimental and theoretical
showing eQq for the Cl nucleus for a series of molecules. techniques have ensured that microwave spectroscopy re-
The value for FCl is larger than for atomic Cl, which indi- mains a very active field. Since its inception shortly after
cates a positive charge on the Cl atom, F Cl . At the other World War II, numerous experimental improvements and
−
+
+
extreme, for the highly ionic bond in K Cl , where the Cl technological advances have been made. These and vari-
−
nucleus is surrounded by a symmetric charge distribution, ous theoretical improvements are discussed and well doc-
we find q ≡ 0. On the other hand, the coupling constant umented in other articles and books (see Bibliography).
for BrCl is near that for atomic chlorine, which indicates Here we highlight a few of the more recent developments.
essentially a covalent bond with little ionic character. As
one proceeds from ICl to AlCl, the bond becomes less A. Spectroscopy of Isotope-Substituted
covalent and more ionic in character. Nonpolar Molecules
It was often observed that microwave spectroscopy could
D. Magnetic Interactions not be applied to nonpolar molecules. This observation
must be somewhat clarified. Improvements in detection
Although we shall not treat magnetic interactions, the
sensitivity have opened the way to measurements on
reader should be aware that small magnetic hyperfine
very slightly polar molecules with electric dipole mo-
ments approaching 10 −5 D. Moreover, mechanisms for
TABLE XXVII Comparison of eQq (in MHz) for a the generation of such small moments have now been rec-
35
Chlorine ( Cl) Nucleus in Various Molecules
ognized and exploited.
Molecule eQq Small dipole moments can be induced in nonpolar
molecules by three mechanisms. First, small dipole mo-
ClF −145.9
ments can be vibrationally induced when, for example,
Cl (atomic) −109.74
symmetric or spherical tops are in excited, degenerate vi-
BrCl −102.4
brational states. Second, for nonlinear molecules without
ICl −85.8
a center of symmetry, centrifugal distortion can distort the
ClCN −83.4
molecule, hence generating a small dipole moment. This
HC CCl −79.7
mechanism,asdiscussedinSectionIV.B,hasnowallowed
CH 3 Cl −74.8
the observation of rotational transitions in the ground vi-
SiH 3 Cl −40.0
brational state of spherical tops. Third, nonsymmetric iso-
AgCl −37.3
topic substitution of a nonpolar molecule can give rise to a
TICl −15.8
small molecular dipole moment by vibrational averaging
AlCl −8.8
effects. This case is the most general and is applicable to
KCl 0
a large number of nonpolar molecules.