Far-Infrared Synchrotron Spectroscopy and Quantum Chemical Calculations of the Potentially Important Interstellar Molecule, 2‑Chloroethanol

The high brightness
of the Australian synchrotron allowed for detailed
spectra to be collected at high resolution (0.00096 cm–1) in the vicinity of the a/b/c-type ν19 band of 2-chloroethanol, which
involves O–H torsional motion about the C–O bond. A
rovibrational analysis was performed for both chlorine isotopologues
in the ν19 fundamental (centered at ∼344 cm–1) which involved the assignment of 7153 lines (J ≤ 90, Ka ≤ 41). A global fit to these lines in addition to 119
microwave lines (J ≤ 29, Ka ≤ 11) led to the determination
of spectroscopic constants up to the sextic level in both the ground
and excited states using Watson’s A-reduction
Hamiltonian. The constants agree well with those calculated at the
anharmonic MP2/cc-pVTZ level and allow for spectroscopically accurate
predictions of rotational transitions in the ground vibrational state
to be made over a broad range of rotational energies (TR < 1000 K). We explored the role that 2-chloroethanol
might play in interstellar molecular clouds by performing calculations
on the substitution reaction between HCl and ethylene glycol, and
the addition reaction between HCl and oxirane, all of which have been
observed in Sagittarius B2­(N) and are expected to play important roles
in the chemistry that occurs on the icy mantles of interstellar dust
grains. While both reactions have relatively high activation barriers,
the HCl + oxirane reaction was found be much more exothermic; further
calculations on it indicate that a water-like environment significantly
reduces the barrier while slightly increasing its exothermicity. These
results suggest that 2-chloroethanol could be efficiently produced
from the cosmic ray bombardment of common interstellar ices.