Rotational and Vibrational Spectra of the Pyridyl Radicals: A Coupled-Cluster Study

Pyridyl is a prototypical nitrogen-containing aromatic radical
that may be a key intermediate in the formation of nitrogen-containing
aromatic molecules under astrophysical conditions. On meteorites,
a variety of complex molecules with nitrogen-containing rings have
been detected with nonterrestrial isotopic abundances, and larger
nitrogen-containing polycyclic aromatic hydrocarbons (PANHs) have
been proposed to be responsible for certain unidentified infrared
emission bands in the interstellar medium. In this work, the three
isomers of pyridyl (2-, 3-, and 4-pyridyl) have been investigated
with coupled cluster methods. For each species, structures were optimized
at the CCSD­(T)/cc-pwCVTZ level of theory and force fields were calculated
at the CCSD­(T)/ANO0 level of theory. Second-order vibrational perturbation
theory (VPT2) was used to derive anharmonic vibrational frequencies
and vibrationally corrected rotational constants, and resonances among
vibrational states below 3500 cm–1 were treated
variationally with the VPT2+K method. The results yield a complete
set of spectroscopic parameters needed to simulate the pure rotational
spectrum of each isomer, including electron–spin, spin–spin,
and nuclear hyperfine interactions, and the calculated hyperfine parameters
agree well with the limited available data from electron paramagnetic
resonance spectroscopy. For the handful of experimentally measured
vibrational frequencies determined from photoelectron spectroscopy
and matrix isolation spectroscopy, the typical agreement is comparable
to experimental uncertainty. The predicted parameters for rotational
spectroscopy reported here can guide new experimental investigations
into the yet-unobserved rotational spectra of these radicals.