Coupled Cluster Characterization of 1‑, 2‑, and 3‑Pyrrolyl: Parameters for Vibrational and Rotational Spectroscopy

Pyrrolyl
(C4H4N) is a nitrogen-containing
aromatic radical that is a derivative of pyrrole (C4H5N) and is an important intermediate in the combustion of biomass.
It is also relevant for chemistry in Titan’s atmosphere and
may be present in the interstellar medium. The lowest-energy isomer,
1-pyrrolyl, has been involved in many experimental and theoretical
studies of the N–H photodissociation of pyrrole, yet it has
only been directly spectroscopically detected via electron paramagnetic
resonance and through the photoelectron spectrum of the pyrrolide
anion, yielding three vibrational frequencies. No direct measurements
of 2- or 3-pyrrolyl have been made, and little information is known
from theoretical calculations beyond their relative energies. Here,
we present an ab initio quantum chemical characterization of the three
pyrrolyl isomers at the CCSD­(T) level of theory in their ground electronic
states, with an emphasis on spectroscopic parameters relevant for
vibrational and rotational spectroscopy. Equilibrium geometries were
optimized at the CCSD­(T)/cc-pwCVTZ level of theory, and the quadratic,
cubic, and partial quartic force constants were evaluated at CCSD­(T)/ANO0
for analysis using second-order vibrational perturbation theory to
obtain harmonic and anharmonic vibrational frequencies. In addition,
zero-point-corrected rotational constants, electronic spin–rotation
tensors, and nuclear hyperfine tensors are calculated for rotational
spectroscopy. Our computed structures and energies agree well with
earlier density functional theory calculations, and spectroscopic
parameters for 1-pyrrolyl are compared with the limited existing experimental
data. Finally, we discuss strategies for detecting these radicals
using rotational and vibrational spectroscopy on the basis of the
calculated spectroscopic constants.