Appendix I: Equilibrium Structures for the three conformers of allyl-NCO (conf. I, II and III) at the B3LYP-D3(BJ)/cc-pVQZ level of theory.Appendix II: Basis set benchmark using the B3LYP-D3(BJ) method.Appendix III: Measured line lists for conf. III and conf. I, including their parent and minor isotopologues ( 13 C, 15 N and 18 O).Appendix IV: Kraitchman coordinates for conf. I and conf. III.

The strong gauche preference along with the F-C-C-N(+) fragment in 3-fluoropiperidinium cation and analogues, in the gas phase, is dictated by electrostatic interactions, which can be both hydrogen bond F···H(N(+)) and F/N(+) attraction. In aqueous solution, where most biochemical processes take place, electrostatic effects are strongly attenuated and hyperconjugation is calculated to be at least competitive with Lewis-type interactions.

The pure rotational spectra of phenyl isocyanate (PhNCO) and phenyl isothiocyanate (PhNCS) were investigated using Fourier transform microwave (FTMW) spectroscopy in the range from 4 to 26 GHz. For each molecule, rotational transitions due to the parent species and nine minor isotopologues including seven 13 C, one 15 N and one 18 O/ 34 S have been observed in natural abundance. The rm (1) geometries were derived from the resulting sets of rotational constants and are consistent with the equilibrium structures (re) from ab initio calculations performed at the MP2/aug-cc-pVTZ level. NBO and Townes-Dailey analyses were conducted to better understand the electronic structure and geometry of each compound. In the case of PhNCS, the nitrogen atom displays more sp-like character resulting in shorter C-N bonds and a larger CNC angle relative to those of PhNCO.

The rotational fingerprint of the thiophene–water complex was investigated for the first time using Fourier transform microwave spectroscopy (7–20 GHz) aided by quantum mechanical calculations. Transitions for a single species were observed, and the rotational constants for the parent and 18O isotopomers are consistent with a geometry that is highly averaged over a barrierless large-amplitude motion of water that interconverts two equivalent forms corresponding to the global minimum (B2PLYP-D3­(BJ)/def2-TZVP). In this effective geometry, the water lies above the thiophene ring close to its σv plane of symmetry. The observed transitions are split by a second water-centered tunneling motion that exchanges its two protons by internal rotation about its C2 axis with a calculated barrier of ∼2.7 kJ mol–1 (B2PLYP-D3­(BJ)/def2-TZVP). Based on quantum theory of atoms in molecules, noncovalent interaction, and symmetry-adapted perturbation theory analyses, the observed geometry enables two intermolecular interactions (O–H···π and O–H···S) whose electrostatic and dispersive contributions favor formation of the thiophene–water complex.

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