Pyrrolyl (C 4 H 4 N) is a nitrogen-containing aromatic radical that is a derivative of pyrrole (C 4 H 5 N) 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-pointcorrected 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.

Isoprene (2-methyl-1,3-butadiene) is highly abundant in the atmosphere, second only to methane in hydrocarbon emissions. In contrast to the most stable trans rotamer, structural characterization of gauche-isoprene has proven challenging: it is weakly polar, present at the level of only a few percent at room temperature, and structurally complex due to both torsional and methyl tunneling motions. gauche-Isoprene has been observed by two distinct but complementary experimental approaches: chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy coupled with cryogenic buffer gas cooling, and cavity-enhanced FTMW spectroscopy with a pulsed discharge source. Thermal enhancement of the gauche population (from 1.7% to 10.3%) was observed in the cryogenic buffer gas cell when the sample was preheated from 300 to 450 K, demonstrating that high-energy rotamers can be efficiently isolated under our experimental conditions. Rotational parameters for the inversion states (0+/0–) have been determined for the first time, aided by calculations at increasing levels of theoretical sophistication. From this combined analysis, the inversion splitting ΔE and the F bc Coriolis coupling constant between the two inversion states have been derived.

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.

Metrics & MoreArticle Recommendations * sı Supporting Information R elevant Supporting Information files were missing in the final publication of the manuscript. The missing files are included here. ■ ASSOCIATED CONTENT* sı Supporting InformationThe Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpca.2c04094.ZMAT/Cartesian coordinates, normal coordinates of vibration, vibrational corrections, vibrational frequencies, SPCAT input files, GUINEA input files, variational coefficients (ZIP)

Methyl tert-butyl ether is a gasoline additive and a water pollutant. Its rotational spectrum was measured from 26.5-40 GHz using Chirped-Pulse Fourier Transform Microwave Spectroscopy. Measurements were conducted at low temperature via supersonic expansion and room temperature via static cell. The molecule was previous reported in Suenram et. al 1997 in a range of 9-18.6 GHz. This work expands that fit and converts it to the Rho Axis Method utilizing the program RAM36. The improved ground torsional state measurements as well as the room temperature data allowed for tentative assignments of torsionally excited transitions.