Equilibrium structures of selenium compounds: The torsionally flexible molecule of selenophenol

Abstract

The equilibrium structure of selenophenol has been investigated using rotational spectroscopy and high-level quantum mechanical calculations, offering electronic and structural insight into the scarcely studied selenium compounds. The jet-cooled broadband microwave spectrum was measured in the 2–8 GHz cm-wave region using broadband (chirped-pulse) fast-passage techniques. Additional measurements up to 18 GHz used narrow-band impulse excitation. Spectral signatures were obtained for six isotopic species of selenium (80Se, 78Se, 76Se, 82Se, 77Se, and 74Se), together with different monosubstituted 13C species. The (unsplit) rotational transitions associated with the non-inverting μa-dipole selection rules could be partially reproduced with a semirigid rotor model. However, the internal rotation barrier of the selenol group splits the vibrational ground state into two subtorsional levels, doubling the dipole-inverting μb transitions. The simulation of the double-minimum internal rotation gives a very low barrier height (B3PW91: 42 cm−1), much smaller than for thiophenol (277 cm−1). A monodimensional Hamiltonian then predicts a huge vibrational separation of 72.2 GHz, justifying the non-observation of μb transitions in our frequency range. The experimental rotational parameters were compared with different MP2 and density functional theory calculations. The equilibrium structure was determined using several high-level ab initio calculations. A final Born–Oppenheimer (reBO) structure was obtained at the coupled-cluster CCSD(T)_ae/cc-wCVTZ level of theory, including small corrections for the wCVTZ → wCVQZ basis set enlargement calculated at the MP2 level. The mass-dependent method with predicates was used to produce an alternative rm(2) structure. The comparison between the two methods confirms the high accuracy of the reBO structure and offers information on other chalcogen-containing molecules.

Rotational Spectroscopy and Conformational Flexibility of 2‐Phenylethanethiol: The Dominant S−H⋅⋅⋅π Intramolecular Hydrogen Bond

“…Here we present a multi‐isotopic rotationally resolved analysis of PET, leading to an unambiguous discrimination of its conformational landscape and structure. This investigation complements previous rotational studies on other aromatic chalcogens like thiophenol, [18–20] benzyl mercaptan, [21,22] or selenophenol [23] and will permit future studies of intermolecular complexes involving PET.…”

“…More advanced calculations, in particular Watson's mass‐dependent r m method, [35] were not successful. This is attributed to the need of three to six additional isotopic‐dependent parameters, which would require additional isotopic data [23] . Alternatively, the substitution method approximates the equilibrium coordinates using the Kraitchman equations, [36] based on differences between isotopic moments of inertia.…”

“…The second conformer thus differs from the alcohol, where both the terminal group and the carbon skeleton adopt an antiperiplanar C s ‐symmetric structure. This difference is associated to the smaller torsional barriers on descending the chalcogen group, as illustrated in the cases of phenol ($${{B}_{2}^{exp}=1207{\rm \ }{{\rm c}{\rm m}}^{-1}}$$ ), [18] thiophenol ($${{B}_{2}^{exp}=289{\rm \ }{{\rm c}{\rm m}}^{-1}}$$ ) [19] and selenophenol ($${{B}_{2}^{theory}=42{\rm \ }{{\rm c}{\rm m}}^{-1}}$$ ) [23] . The three higher‐energy conformers were not observed experimentally, either because of conformational relaxation (in case of thiol torsion paths) or depopulation.…”