Conformational stability from variable temperature FT-IR spectra of xenon solutions and ab initio calculations of trans 3-pentenenitrile and 3-methyl-3-butene nitrile

Abstract

The infrared (3500–40 cm −1) spectra of gaseous, liquid and solid 3-methyl-3-butene nitrile, CH 2C(CH 3)CH 2CN, and trans 3-pentenenitrile, CH 3CHCHCH 2CN, have been recorded. Both the cis and gauche conformers have been identified for both conformers in the fluid phases but only the cis form remains in the solid. Variable temperature (−55 to −100°C) studies of the infrared spectra of the samples dissolved in liquid xenon have been carried out. From these data, the enthalpy difference has been determined to be 174±20 cm −1 (2.08±0.24 kJ/mol) for 3-methyl-3-butene nitrile with the cis conformer the more stable rotamer and 187±20 cm −1 (2.23±0.24 kJ/mol) for trans 3-pentenenitrile, again with the cis rotamer the more stable form. Complete equilibrium geometries have been obtained for both rotamers for both molecules by ab initio calculations employing the 6-31G(d), 6-311G(d,p), 6-311+G(d,p) and 6-311+G(2d,2p) basis sets at the levels of restricted Hartree–Fock and/or Moller–Plesset to second order. Only the MP2/6-311+G(2d,2p) calculation gives the correct conformer stability for 3-methyl-3-butene nitrile.