Quadratic quantum-chemical force fields have been determined for s-trans-1,3-butadiene using B3LYP and MP2 methods. Basis sets included 6-311++G, cc-pVTZ, and aug-cc-pVTZ. Scaling of the force fields was based on frequency data for up to 11 isotopomers, some of these data being original. A total of 18 scale factors were employed, with, in addition, an alteration to one off-diagonal force constant in the A(u) species. MP2 calculations without f functions in the basis perform badly in respect of out-of-plane bending mode frequencies. Centrifugal distortion constants and harmonic contributions to vibration-rotation constants (alphas) have been calculated. Existing experimental frequency data for all isotopomers are scrutinized, and a number of reassignments and diagnoses of Fermi resonance made, particularly in the nu(CH) region. The three types of CH bond in butadiene were characterized in terms of bond length and isolated CH stretching frequency, the latter reflecting data in the nu(CD) region. Broad agreement was achieved with earlier results from local mode studies. Differences in CH bond properties resemble similar differences in propene. A simplified sample setup for recording FT-Raman spectra of gases was applied to four isotopomers of butadiene.

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The experimental vibrational frequencies of s-trans-1,3-butadiene, for which the assignments are well-established, are used to determine the scale factors for its quantum mechanical force field obtained at the MP2/6-31G*//MP2/6-31G* level of theory. The scale factors are then transferred to the MP2/6-31G*//MP2/6-31G* force fields of the s-cis and s-gauche rotamers and their theoretical frequencies calculated. Comparison of the vibrational frequencies of these three species indicates a special region of the IR spectrum of 1,3-butadiene in the gas phase (720−790 cm-1) in which only a band attributable to the s-gauche rotamer should be present; i.e., it should be free both of the observed IR bands of the s-trans and of the calculated vibrational frequencies of the s-cis conformer. Investigation of the medium- and high-resolution IR spectra of 1,3-butadiene in the gas phase reveals the presence of a band at 749.22(20) cm-1 possessing the typical B contour (consistent with A symmetry, C 2 group). Rotational analysis of the medium-resolution spectrum of this band yields the rotational constants A‘‘ − B̄‘‘ = 0.4478(27) cm-1 and A‘ − B̄‘ = 0.4455(25) cm-1, only about one-third of the experimental values for s-trans-1,3-butadiene. This identifies the band as belonging to the high-energy conformer of 1,3-butadiene. The agreement between the experimental and theoretical values of the band center (749 vs 735 cm-1), the clear B type contour, and the extremely complicated character of the high-resolution spectrum of the band at 749.22 cm-1 strongly suggest that the geometry of the high-energy conformer of 1,3-butadiene in the gas phase is nonplanar s-gauche and not planar s-cis.