Microwave Spectra and Structure of Ar–1,3-Difluorobenzene

The microwave spectrum of the dimer
Ar–1,3-difluorobenzene
from 2 to 18 GHz is reported. The spectrum has been observed using
a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer
that has recently been expanded to include the 2–6 GHz region
of the electromagnetic spectrum. Details of this upgraded spectrometer
are reported. Eighty-seven transitions were observed for the parent
dimer spectrum, which was adequately fit to a semirigid rotational
Hamiltonian consisting of A, B,
and C as well as four quartic centrifugal distortion
constants. Observations of 13C species in natural abundance
were aided by utilizing smaller chirp ranges of 7–9 and 9–11
GHz for 1.9 million and 3.73 million averages, respectively. Assignment
of 13C isotopologues allowed for determination of the Kraitchman
coordinates of the carbon atoms as well as inertial fits of the complex.
The quantum-chemical structure predicts an Ar to monomer center of
mass distance of 3.48 Å, compared with 3.564(1) Å determined
from experimental structural analysis. This new study indicates that
in fluorinated benzene–Ar dimers, when the fluorines are separated
by more carbon atoms, the Ar–ring center distance decreases.