Raman and infrared, microwave spectra, conformational stability, adjusted r 0 structural parameters, and vibrational assignments of cyclopentylamine

Fourier transform microwave spectrum of cyclopentylamine, c–C 5 H 9 NH 2 has been recorded, and seven transitions have been assigned for the most abundant conformer, and the rotational constants have been determined: A  = 4909.46(5), B  = 3599.01(4), and C  = 2932.94(4). From the determined microwave rotational constants and ab initio MP2(full)/6‐311 + G(d,p) predicted structural values, adjusted r 0 parameters are reported with distances (Å): rC α –C β  = 1.529(3), rC β –C γ  = 1.544(3), rC γ –C γ  = 1.550(3), rC α –N = 1.470(3), and angles (°) ∠CCN = 108.7(5), ∠C β C α C β  = 101.4(5), and τC β C α C β ′ C γ ′  = 42.0(5). The infrared spectra (4000–220 cm −1 ) of the gas have been recorded. Additionally, the variable temperature (−60 to −100 °C) Raman spectra of the sample dissolved in liquefied xenon was recorded from (3800–50 cm −1 ). The four possible conformers have been identified, and their relative stabilities obtained with enthalpy difference relative to t‐Ax of 211 ± 21 cm −1 for t‐Eq ≥ 227 ± 22 cm −1 for g‐Eq ≥ 255 ± 25 cm −1 for g‐Ax. The percentage of the four conformers is estimated to be 53% for the t‐Ax, 11 ± 1% for t‐Eq, 20 ± 2% for g‐Ax and 16 ± 2% for g‐Eq at ambient temperature. The conformational stabilities have been predicted from ab initio calculations by utilizing several different basis sets up to aug‐cc‐pVTZ from both MP2(full) and density functional theory calculations by the B3LYP method. Vibrational assignments have been provided for the observed bands for all four conformers, which are predicted by MP2(full)/6‐31G(d) ab initio calculations to predict harmonic force constants, wavenumbers, infrared intensities, Raman activities, and depolarization ratios for all of the conformers. The results are discussed and compared to the corresponding properties of some related molecules. Copyright © 2014 John Wiley & Sons, Ltd.