Interplay between conformation and crystal packing in aryl propargyl ethers: Structural and spectroscopic properties of 2-(prop-2-yn-1-yloxy)acene derivatives

Abstract

The 2-(prop-2-yn-1-yloxy)naphthalene (compound I) and 1-nitro-2-(prop-2-yn-1-yloxy)benzene (compound II) were prepared in excellent yields by the reaction of propargyl bromide with 2-naphthol and 2-nitrophenol, respectively. Structural and conformational properties of both compounds have been analyzed using a combined approach including single-crystal X-ray diffraction and quantum chemical calculations. Two forms are found to be stable and nearly isoenergetic conformers, depending on the mutual orientation of the C≡C triple bond and the O–C single bond of the prop-2-yn-1-yloxy group. The gauche conformation is the most stable form for the vacuum isolated species, with the anti conformation higher in energy by 1.00 and 1.89 kcal/mol computed at the M06-2X/6-311++G(d,p) level of approximation, for I and II, respectively. The gauche form is observed in the crystal of I but the nearly planar anti conformation is adopted in the crystalline phase of compound II. The occurrence of C–H⋅⋅⋅π and C–H⋅⋅⋅O intermolecular interactions involving the acidic -C≡C–H hydrogen donor and -C≡C- and -OCH2– acceptors is discussed. Furthermore, vibrational features of the prop-2-yn-1-yloxy moiety are analyzed by infrared and Raman spectroscopy. The UV-Vis spectrum shows the presence of well-defined absorption at ca. 280 and 320 nm assigned to the π→π* transitions in the naphtyl/benzyl and prop-2-yn-1-yloxy groups, respectively.