Conformational and structural studies of ethynylcyclopentane from temperature dependent Raman spectra of xenon solutions, infrared spectra, and ab initio calculations

Variable temperature (−50 to −100 °C) Raman spectra (3500–136 cm−1) were recorded of ethynylcyclopentane, c-C5H9CCH in liquid xenon. The envelope-equatorial (Eq) conformer was determined more stable than envelope-axial (Ax) form with enthalpy difference of 94 ± 9 cm−1 (1.12 ± 0.11 kJ/mol) and 39 ± 2% Ax conformer present at ambient temperature. The conformational stabilities have been predicted from ab initio calculations with basis sets up to aug-cc-pVTZ. From previously reported microwave rotational constants and ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters are reported Eq [Ax] distances (Å) rCC = 1.211(3) [1.211(3)], rCαCC = 1.461(3) [1.467(3)], rCαCβ = 1.542(3) [1.542(3)], rCβCγ = 1.541(3) [1.542(3)], rCγCγ′ = 1.556(3) [1.555(3)] and angles (°) ∠CαCC = 179.4(5) [179.9(5)], ∠CβCαCC = 113.7(5) [111.5(5)], ∠CβCαCβ′ = 102.6(5) [102.1(5)], ∠CαCβCγ = 103.7(5) [103.7(5)], ∠CβCγCγ′ = 106.0(5) [105.9(5)] and τCβCαCβ′Cγ′ = 40.8(5) [41.6(5)]. Vibrational assignments are reported, supported by ab initio predictions and results are discussed.

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• The heavy atom structural parameters have been determined for both forms.
• Vibrational assignments have been obtained for both conformers.
• Energy differences have been predicted for both forms and the planar transition state.
• Experimental enthalpy difference has been determined between the two conformers.