Page 324 - 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 841

1 0 0 This is found in a number of cases, but in HCP, for ex-
E 1 = E + E + µ 12 , (116)
2 1 2
ample, deuterium substitution increases the moment by
1 0 0
E 2 = E + E − µ 12 , (117) 0.007 D. This reflects the fact that the electronic structure
2 1 2
can also be affected by isotopic substitution. The use of
which is a first-order effect. Once the matrix element is
deuterium substitution to convert a nonpolar molecule to
specified µ 12 = µKM/J(J + 1), the above relations pro-
a slightly polar molecule can be illustrated by CH 3 CD 3 .
vide the Stark energies. Such effects are possible not only
The isotope-induced moment is about 0.01 D, and mi-
for asymmetric tops, they also can be observed for linear
crowave spectra for this molecule have been observed. The
molecules in excited bending modes, where µ 12 involves
effects of centrifugal distortion have also been observed.
the quantum number l rather than K.
The near-prolate asymmetric rotor HNCO has an excep-
tionally large energy of rotation about the a axis, which
C. Dipole Moment Information
is essentially along the NCO chain. The NCO chain is
◦
By analysis of the Stark effect, very accurate dipole mo- essentially linear and the HNC angle is 128 . The mo-
ments can be evaluated, and many hundreds of molecules ment µ a is found to decrease with increasing K. The
have now been studied. Illustrative dipole moments de- values are µ a (K = 0) = 1.577 D, µ a (K = 1) = 1.572 D,
termined by microwave spectroscopy are collected in µ a (K = 2) = 1.540 D, and µ a (K = 3) = 1.500 D for the
Table XXVI. Some of the interesting effects observed and J = 3 → 4 transition. This may be explained as follows:
the implications will be briefly discussed. Since the dipole as K increases, the rotation about the a axis increases and
moment obtained is associated with a particular isotopic the HNC angle decreases due to the centrifugal force; thus
species, vibrational state, and rotational transition, it is the projection of the N H bond moment on the a axis de-
possible to observe small changes in µ with these quanti- creases and hence µ a .Asa final example, we mention the
ties. In LiCl, the dipole moment is found to increase with use of microwave Stark effect measurements to determine
vibrational state µ(v = 0) = 7.120 D, µ(v = 1) = 7.20 D, the dipole moment direction in the molecule. The Stark
and µ(v = 2) = 7.296 D. This is consistent with a largely effect does not provide the signs of the dipole components,
ionic bond, where the average separation between the and therefore the direction must be obtained from other
atoms increases with v. Because deuterium substitution information, such as electronegativities. However, the ef-
tends to decrease the average separation between the fect of isotopic substitution, where the primary effect is to
atoms, a decrease in the dipole moment may be expected. rotate the principal axis system, has been used to specify
the directions of the dipole components and hence µ. This
method is illustrated in Fig. 23.
TABLE XXVI Illustrative Dipole Moments
Molecule µ (D) Molecule µ (D)
D. Zeeman Effect
Linear molecules Applied magnetic fields affect the microwave spectrum
LiCl 7.119 CO 0.112 in a manner analogous to the Stark effect. This is called
HCN 2.985 HCP 0.390
OCS 0.7152 HCCF 0.73
Symmetric-top molecules
CH 3 F 1.854 CH 3 Cl 1.869
CH 3 Br 1.797 CH 3 l 1.618
CH 3 CN 3.913 CH 3 CCH 0.784
(CH 3 ) 3 N 0.612 (CH 3 ) 3 P 1.192
Asymmetric-top molecules
Molecule µ a µ b µ c µ(D)
H 2 CO — — — 2.339
HFCO 0.58 1.91 — 1.99
HCOOH 1.391 0.26 — 1.415
CH 3 CFO 2.83 0.88 — 2.96
FIGURE 23 Principal inertial axes of HCOF. The direction of µ b
trans-CH 3 CH 2 CH 2 F 1.970 0.566 — 2.050 may be selected from chemical arguments, but not that of µ a .
gauche-CH 3 CH 2 CH 2 F 1.137 1.450 0.472 1.902 However, for µ a to increase on deuteration, which results in a
CH CHF 1.280 0.629 — 1.427 rotation of the b axis toward the D atom, the direction must be as
shown. The arrowhead indicates the negative end of the dipole
(CH 3 ) 2 PH — 1.15 0.44 1.23
moment components.