The r0 structural parameters, conformational stability, and vibrational assignment of equatorial and axial cyanocyclobutane

Abstract

Variable temperature (−55 to −100 °C) studies of the infrared spectra (3500–400 cm −1) of cyanocyclobutane, c-C 4H 7CN, dissolved in liquid xenon have been carried out. The infrared spectra of the gas and solid, as well as, the Raman spectrum of the liquid have also been recorded. By utilizing eight pairs of conformers at 10 different temperatures, the enthalpy difference between the more stable equatorial conformer and the axial form has been determined to be 254 ± 12 cm −1 (3.03 ± 0.15 kJ/mol). The percentage of the axial conformer present at ambient temperature is estimated to be 23 ± 1%. The ab initio MP2(full) average predicted energy difference for a variety of basis sets is 238 ± 23 cm −1 in excellent agreement with the experimental Δ H value, but the average value of 331 ± 7 cm −1 from density functional theory predictions by the B3LYP method is significantly larger than the experimentally determined enthalpy value. By utilizing previously reported microwave rotational constants for both the equatorial and axial conformers combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r 0 parameters have been obtained. The determined heavy atom structural parameters for the equatorial[axial] conformer are: the distances (Å) C N = 1.160(3) [1.160(3)], C α (CN) = 1.461(3) [1.468(3)], C α C β = 1.557(3) [1.562(3)], C β C γ = 1.549(3) [1.550(3)] and angles (°) ∠ C α C β C γ = 87.8 ( 5 ) [ 89.1 ( 5 ) ] , ∠ C β C α C β = 88.3 ( 5 ) [ 87.9 ( 5 ) ] , ∠ NC ( C β C α C β ) = 130.9 ( 5 ) [ 120.0 ( 5 ) ] and a puckering angle of 28.5(5) [24.1(5)]. The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for both conformers from MP2(full)/6-31G(d) ab initio calculations and compared to experimental values where available. The results are discussed and compared to the corresponding properties of some similar molecules.