The pure rotational spectrum of allyl isocyanate (CH 2 = CHCH 2 NCO) was studied using chirped pulse and Balle-Flygare Fourier Transform microwave (FTMW) spectroscopy. Besides the previously reported gauche conformer, a the lowest energy conformer was identified for the first time with the assistance of the quantum-chemical calculations performed at the B3LYP-D3(BJ) and MP2 levels of theory with Dunning's cc-pVQZ basis set. The assignments were confirmed by the resolved hyperfine structure due to the 14 N quadrupole moment and the spectra of the corresponding 13 C, 15 N and 18 O singly substituted isotopologues in natural abundance. Rotational transitions of the most stable conformer revealed a tunneling splitting due to the interconversion motion between its two mirror images, and the tunneling path was established theoretically. In addition, benchmark calculations of various density functionals with and without dispersion corrections were carried out to investigate the effect of the short-range dispersion energy on the conformational structures.

The rotational spectrum of the thiophene-water (thiophene-w) complex was studied for the first-time using Fourier transform microwave (FTMW) spectroscopy from 7 to 20 GHz. Supported by density functional theory (DFT) calculations, transitions belonging to a single dominant conformer were observed in the spectrum. By comparing the experimentally derived spectroscopic parameters, which includes data for the singly substituted 18 O isotopic species, with the results from quantum chemical calculations at the B2PLYP-D3(BJ)/def2-TZVP level of theory, we show that the observed pattern of transitions is consistent with a structure that is highly averaged over a large amplitude rocking motion of the water. This effective geometry, stabilized via a primary O-H...π hydrogen bond, has the oxygen atom of water sitting above the plane of the thiophene molecule centered on the ring's σ v plane of symmetry. The spectrum also reveals a tunneling splitting with a characteristic 3:1 intensity ratio that arises from a water-centered internal rotation about its C 2 axis which exchanges its hydrogen atoms with an estimated barrier of approximately 2.7 kJ/mol (B2PLYP-D3(BJ)/def2-TZVP). Based on symmetry-adapted perturbation theory (SAPT) calculations, electrostatic and dispersive interactions are shown to be the most stabilizing contributors behind the formation of thiophene-w.

A spectroscopic analysis of allyl isothiocyanate (CH 2 = CHCH 2 NCS) was conducted via chirped-pulse and Balle-Flygare Fourier Transform microwave (FTMW) spectroscopy in the 7-25 GHz range. Rotational transitions associated with the different conformers were assigned based on information gathered from quantum-chemical calculations computed at B3LYP-D3(BJ) and MP2 levels of theory using the Dunning cc-pVTZ basis set. The results showed the existence of two conformers, the gauche conformer, which was reported in previous work a as well as the first reported existence of a second conformer which corresponds to the global minimum. Spectroscopic assignments included transitions due to both parent conformers, 34 S, 13 C and 15 N singly substituted isotopic species as well as hyperfine splitting due to the presence of 14 N.

The monohydrated complexes of trimethylene oxide and sulfide were investigated in the gas phase using Fourier transform microwave spectroscopy (8-18 GHz) aided by quantum mechanical calculations. In the assigned trimethylene oxide-water complex, the water subunit binds exclusively to the oxygen atom of the ring via a OH. . . O hydrogen bond (HB). In the sulfur containing complex, in addition to the primary OH. . . S HB, a secondary CH. . . O interaction is observed. The experimental results for the sulfur analog are consistent with the predictions from both ab initio and density functional theory (DFT) calculations with the aug-cc-pVTZ basis set whereas for the oxygen analog, both planar and puckered ring configurations are possible depending on the level of theory. Our experimental data aligns better with the results from DFT calculations which predict the trimethylene oxide ring to adopt a planar configuration in the complex. The nature and strength of the HBs are studied in detail using topological and energy decomposition analyses. Comparisons are also made computationally with the selenium analog to evaluate the characteristics of the HB along the chalcogen series. The results show that the HBs formed by these rings with water are mostly electrostatic and dispersive in nature and that the HB strength decreases from oxygen to selenium.