Chemistry Reference
In-Depth Information
until they undergo collision with the walls of the vacuum chamber. The com-
plexes can absorb microwave radiation while in the collisionless expansion
phase and their rotational spectra can be detected. The results presented here
have been established mainly by using a pulsed-nozzle, F-T microwave spec-
trometer, but modified to incorporate a so-called fast-mixing nozzle [15]. The
latter device allows complexes of B
XY to be formed from two reactive com-
ponents B and XY (e.g. ethyne and ClF) and achieve collisionless expansion
in the vacuum chamber before the reaction (often violent) that would at-
tend mixing under normal conditions. A detailed description of this nozzle
is available elsewhere [25].
The form of the observed spectrum of B
···
XY can often give a clue to the
symmetry of the species responsible for it. Thus asymmetric-top molecules,
symmetric-top molecules and linear molecules give rise to different spectral
patterns. Once the rotational spectrum of a complex B
···
XY has been as-
signed, the observed transition frequencies may be fitted to give a range of
precise spectroscopic constants, usually for the zero-point state, which can
then be interpreted to give various molecular properties of B
···
XY. Of princi-
pal interest here are the rotational constants, centrifugal distortion constants
and nuclear quadrupole coupling constants.
Rotational constants
G
=
A
,
B
or
C
are inversely proportional to principal
moments of inertia
I
α
···
2
I
α
,where
refers
to one of the three principal inertia axis directions
a
,
b
or
c
.The
I
α
are re-
lated to the coordinates of the atoms
i
in the principal axis system via the
relations
I
α
=
i
through the expressions
G
=
h
/
8
π
α
i
i
), where
m
i
(
β
+
γ
α
,
β
and
γ
aretobecyclicallypermuted
over
a
,
b
and
c
. Hence, the principal moments of inertia are simple functions
of the distribution of the masses of the atoms of the complex in space. Ac-
cordingly, these quantities can be used to determine the separation of the two
subunits B and XY and their relative orientation in space, i.e. the radial and
angular geometries of the complex, respectively. All molecular geometries of
B
XY considered here are of the
r
0
-type, that is, are obtained by fitting the
zero-point principal moments of inertia of a limited number of isotopomers
as though they are equilibrium quantities. Moreover, the geometry of each
component is assumed to survive complex formation.
Although there are several centrifugal distortion constants that can be de-
termined from the rotational spectrum of a complex B
···
···
XY, one is of special
importance, namely,
D
J
( for linear or symmetric top molecules) or, equiva-
lently,
∆
J
are inversely
proportional to the intermolecular stretching force constant
k
σ
,accordingto
simple and convenient expressions presented by Millen [26] in the approxi-
mation of rigid, unperturbed subunits B and X and with the neglect of terms
higher than quadratic in the intermolecular potential energy function. Thus,
k
σ
offers a measure of the strength of the interaction, given that it is the
restoring force per unit infinitesimal extension of the weak bond.
∆
J
(for an asymmetric-rotor molecule). Both
D
J
and
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