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

Abstract

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.

Photoelectron spectrum of the pyridyl radical

“…31,[35][36][37]64 The heat of reaction D r H1 for (4) at 298 K ranges from 0.8 to 1.1 eV depending on the isomer. Meyer et al 14 Taking the FWHM of bands into account, the IE of the X +1 A 0 ' X 2 A 0 transition is determined to be 7.70 AE 0.02 eV. The calculated IE is 7.72 eV for oB97X-D3 and 7.61 eV for CCSD(T) and is in agreement with the experimental IE.…”

“…The neutral geometry on the DFT level of p-pyridyl is in agreement with the CCSD(T) geometry. 14 The bond lengths differ by a maximum of 0.005 Å and the bond angles by a maximum of 11.…”

“…13 Rotational spectra have been predicted from high-level coupled cluster calculations. 14 N-containing heterocycles are also relevant in combustion of coal and biofuels. During coal burning, fuel-bound nitrogen, which consists mainly of pyrrole and pyridine rings, is oxidized and burned, 15 and produces CO 2 , NO x as well as HCN.…”

The reactivity of pyridine in cold interstellar environments: The reaction of pyridine with the CN radical

“…The most exothermic reaction for this set corresponds to the formation of 1-pyridyl. The rotational and vibrational spectra of pyridyl radicals was recently reported using vibrational perturbation theory (VPT2) (Meyer et al, 2022) using highly correlated coupled cluster methods (CCSD(T)/ANO0). In their study, the authors reported that the 1-pyridyl radical is more stable by 4.14-5.57 kcal mol −1 with respect to 2-3-pyridyl, in very good agreement with our values of ∼ 5.7 kcal mol −1 .…”

10 Years of the ACS PHYS Astrochemistry Subdivision

“…Examples of other novel molecules with potential astrochemical significance include 2-aza-1,3-butadiene, the C 7 H 5 radical, functionalized buckyballs in the form of C 60 O + and C 60 OH + , the 1-quinolinium (C 9 H 7 NH + ) cation along with its corresponding neutral and hydrogenated radicals, cyano-cyclopentadiene ions, the simplest aminooxycarbene in aminohydroxymethylene (H 2 N–C̈–OH), 1-cyanocyclobutene (C 5 H 5 N), monocyclic carbon ring cations of the form C 2 n + , the phenalenyl radical, and isomers of the PAH C 16 H 10 + . Purely theoretical studies are providing characterization for other unique molecules including glycolic acid, pyridyl radicals, and diazirine along with its cyclic isomers . Additionally, the molecular structure and spectra of noncovalent interactions are also characterized through a mixture of theory and experiment for glycerol–water clusters, HCN and CH 3 Cl clusters, and complexes of NH 3 ···CO …”