Vibrational analysis, conformational stability, force constants, barriers to internal rotations, RHF, MP2 and DFT calculations of trans,trans‐2,4‐hexadiene

Abstract

AbstractSix conformers are proposed for the trans,trans‐2,4‐hexadiene (TTHD) molecule. Utilizing the 6–31G(d) and 6–311 + G(d) basis sets at the levels of RHF, MP2 with full electron correlation and DFT‐B3LYP, five conformers are found to be transition states, due to the calculated imaginary wavenumber(s). Accordingly, the totally symmetric TTT conformer where the two methyl groups are staggered to all trans vinylic hydrogen's (C2h symmetry) is the stable conformer in the gaseous, liquid and crystalline phases. Aided by MP2(full)/6–311 + G(d) geometric parameters, a chosen value of the kinetic parameter F = 6.11284856 was determined. Utilizing the observed band at 148 cm−1 in the far‐infrared spectrum of the gas, a value of 1.41 ± 0.06 kcal mol−1 was obtained for the C3v internal rotor (V3), compared with 1.83 ± 0.07 kcal mol−1 predicted from a potential energy surface scan (1 kcal = 4.184 kJ). Aided by normal coordinate analysis and quantum chemistry (QC) calculations, the scaled quantum mechanical (SQM) force field was obtained employing five scaling factors for different types of vibrational motions. The scaled vibrational wavenumbers from both MP2(full) and B3LYP levels using the 6–31G(d) basis set provides an excellent approach to the observed fundamentals in either infrared or Raman spectra of the TTT conformer. Thus, confident vibrational assignments and potential energy distributions (PEDs) were provided for all fundamental modes of the TTT conformer. Moreover, the conformational stabilities, structural parameters, rotational constants, infrared intensities and Raman scattering activities are also reported. These results are discussed and compared with the experimentally determined values for some related conjugate systems when appropriate. Copyright © 2004 John Wiley & Sons, Ltd.