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 r0 parameters have been obtained for both conformers. Heavy atom distances (A) of equatorial[twist] conformer are as follows: N1–C2 = 1.469(3)[1.476(3)], N1–C3 = 1.469(3)[1.479(3)], C2–C4 = 1.541(3)[1.556(3)], C3–C5 = 1.541(3)[1.544(3)], C4–C5 = 1.556(3)[1.543(3)]; and angles (deg)∠N1C2C4 = 102.5(5)[107.6(5)], ∠N1C3C5 = 102.5(5)[105.4(5)], ∠C2C4C5 = 104.3(5)[104...