Conformational and structural studies of n‐propylamine from temperature dependent Raman and far infrared spectra of xenon solutions and ab initio calculations

Abstract

The Raman and infrared spectra (4000 to 50 cm–1) of the gas, liquid or solution, and solid have been recorded of n‐propylamine, CH3CH2CH2NH2. Variable temperature (−60 to −100 °C) studies of the Raman (1175 to 625 cm–1) and far infrared (600 to 10 cm–1) spectra dissolved in liquid xenon were carried out. From these data, the five possible conformers were identified and their relative stabilities obtained with enthalpy difference relative to trans–trans (Tt) for trans–gauche (Tg) of 79 ± 9 cm–1 (0.9 ± 0.1 kJ/mol); for Gg of 91 ± 26 cm–1 (1.08 ± 0.3 kJ/mol); for Gg′ of 135 ± 21 cm–1 (1.61 ± 0.2 kJ/mol); for Gt of 143 ± 11 cm–1 (1.71 ± 0.1 kJ/mol). The percentage of the five conformers is estimated to be 18% for the Tt, 24 ± 1% for Tg, 23 ± 3% for Gg, 18 ± 1% for Gg′ and 18 ± 1% for Gt at ambient temperature. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug‐cc‐pVTZ from both second‐order Møller–Plesset (MP2, full) and density functional theory calculations by the Becke, three‐parameter, Lee–Yang–Parr method. Vibrational assignments were provided for the observed bands for all five conformers, which are supported by MP2(full)/6‐31G(d) ab initio calculations to predict harmonic force constants, wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. Estimated r0 structural parameters were obtained from adjusted MP2(full)/6‐311+G(d,p) calculations. The results are discussed and compared with the corresponding properties of some related molecules. Copyright © 2012 John Wiley & Sons, Ltd.

Conformational and structural stability of n and 2-propylthiols: a revisit

“…Experimental measurements reported T–G as the global minimum (although one earlier experiment based on IR absorption identified T–T as the global minimum) with one local minimum conformer G–T or T–T conformers, respectively. 2–8 The scan with initial geometry as the optimized G–G conformer resulted in T–G as the global minimum conformer and G–G as the local minimum which is similar to the prediction by Gorai et al 1 and Choi et al 9 Furthermore, the results reported herein match exceedingly well with the experimental results. Table ST2† summarizes all conformational scans for the nP molecule at the CCSD/cc-pVDZ level of theory with a step size of 5°.…”

“…In experiments, not all these conformers are found, though. Different researchers reported varying numbers and structures of conformers using different spectroscopic techniques like microwave, 2–5 infrared, 6,7 Raman, 8 and vacuum ultraviolet mass-analyzed threshold ionization spectroscopy. 9 On the other hand, literature pertaining to computational research using methods such as Urey–Bradley force field theory, 10 quantum molecular force field theory, 11 MP2/cc-pVTZ, and B3LYP/6-311++G(2df, 2pd) 9 differed significantly among each other as well as from the experiments.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part VII

“…Durig et al . described conformational and structural studies of n ‐propylamine from temperature‐dependent Raman and far IR spectra of xenon solutions and ab initio calculations . In a related paper, Guirgis, Durig, and co‐workers carried out conformational, vibrational, and structural studies of 2,2,3,3,3‐pentafluoropropylamine from Raman and IR spectra of gas, liquid, xenon solutions, and solid supported by ab initio calculations .…”

Raman and infrared, microwave spectra, conformational stability, adjusted r0 structural parameters, and vibrational assignments of cyclopentylamine

“…In the amine substituted alkanes previously studied, the amine groups are usually within 0.003 Å of the values predicted by the ab initio MP2 calculation with full electron correlation at the 6‐311 + G(d,p) basis set. The largest difference reported is for cyclobutylamine where the difference between the experimental and predicted C–N bonding distance are 0.005 Å.…”