Structure of the High-Energy Conformer of 1,3-Butadiene

Abstract

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.

Torsional potential of 1,3-butadiene:ab initiocalculations

“…The s-cis energy is underestimated by these methods but, interestingly and in contrast to the transition state, the barrier does not feel the effect of the triple excitations. The experimental values of [38] have also been considered the most reliable ones by other studies [7, 9,111. We are aware of the limitations of the basis set treatment beyond the MP2 level, but the CCSD and CCSD(T) methods usually provide an excellent description of the correlation effects, being the most sophisticated methods available for this system in combination with a moderate size basis set.…”

High Level ab Initio Energies and Structures for the Rotamers of 1,3-Butadiene

“…Earlier predictions for the gauche rotamer were made by De Maré et al 22 by transferring to the gauche rotamer scale factors for internal coordinate force constants fitted to the experimental data for the trans rotamer. 50,51 Their predictions, which are listed in Table 6, include the effects of anharmonicity and are thus not directly comparable to the harmonic frequencies in Table 4.…”

“…22 However, a recent re-examination of this band with much higher resolution showed the band comes from a difference band of the trans rotamer. 23 In addition, no subbands with rotational structure of the gauche rotamer, which is only 5% or less abundant at room temperature, 24 could be identified in the high-resolution infrared spectrum.…”

Ab InitioDetermination of the Roto-Torsional Energy Levels oftrans-1,3-Butadiene

“…Butadiene has been studied several times from ab initio calculations (4,(13)(14)(15)(16)(17)(18)(19)(20). In a previous paper (4), it was demonstrated that the torsional frequencies (21-23) could be reproduced with accuracy using ab initio calculations and the one-dimensional model.…”