Conformational stability from variable-temperature infrared spectra of xenon solutions, r0 structural parameters, and vibrational assignment of pyrrolidine

The infrared spectra of gaseous and variable-temperature liquid xenon solutions of pyrrolidine have been recorded. The enthalpy difference has been determined to be 109 ± 11 cm(-1) (1.30 ± 0.13 kJ mol(-1)) with the envelope-equatorial conformer more stable than the twist form with 37 ± 3% present at ambient temperature. Ab initio calculations utilizing various basis sets up to MP2(full)/aug-cc-pVTZ have been used to predict the conformational stabilities, energy at the equatorial-axial saddle point, and barriers to planarity. From previously reported microwave rotational constants along with MP2(full)/6-311+G(d,p) predicted structural values, adjusted r(0) parameters have been obtained for both conformers. Heavy atom distances (?) of equatorialtwist] conformer are as follows: N(1)-C(2) = 1.469(3)1.476(3)], N(1)-C(3) = 1.469(3)1.479(3)], C(2)-C(4) = 1.541(3)1.556(3)], C(3)-C(5) = 1.541(3)1.544(3)], C(4)-C(5) = 1.556(3)1.543(3)]; and angles (deg)∠N(1)C(2)C(4) = 102.5(5)107.6(5)], ∠N(1)C(3)C(5) = 102.5(5)105.4(5)], ∠C(2)C(4)C(5) = 104.3(5)104.6(5)], ∠C(3)C(5)C(4) = 104.3(5)103.7(5)], ∠C(2)N(1)C(3) = 104.1(5)103.9(5)], τC(2)C(4)C(5)C(3) = 0.0(5)13.5(5)]. A complete vibrational assignment is proposed for both conformers.