Torsional potential of 1,3-butadiene:ab initiocalculations

Abstract

Accurate torsional potentials and torsional barriers are derived for the 1,3-butadiene molecule using state-of-the-art coupled-cluster (CC) methods. The basis set effect, and the performance of different ab initio methodologies with respect to the CC calculations, have been carefully addressed. The CC results obtained here provide an excellent compromise between accuracy and computational cost, which can be extended to other systems.

Intrinsic Information Content of NMR Dipolar Couplings: A Conformational Investigation of 1,3‐Butadiene in a Nematic Phase

“…Here, we will discuss with some details what was obtained when using the U int ({f}) value of ref. [7], calculated at the MP2/cc-pVTZ level (anyway, very similar results were obtained when using U int ({f}) from more recent theoretical investigations [9,11,12] ). The two distributions differ slightly in the 908 < f < 1808 range in that the experimental curve is more peaked at a lower f with respect to the quantum mechanical calculations; the latter technique predicts also a higher population of the planar s-cis form.…”

“…More "intriguing" is the existence of a symmetry-related pair of nonplanar conformers, commonly referred to as s-gauche, whose higher stability with respect to the planar s-cis has been attributed to a decrease of the steric hindrance between protons H7 and H8 (see Figure 1) due to the loss of planarity. Recent theoretical studies [7][8][9][10][11][12] agree with this latter hypothesis whereas experimental evidences for the existence of the s-gauche pair are scanty in liquids (isotropic NMR [1,13,14] ) but more clear in the gas phase [microwave, [15] infrared spectroscopy, [16,17] and UV technique [17] ). The experimental investigation of such a kind of conformational equilibrium is, in general, quite complex and becomes very challenging when, as for the NMR technique, the observables are averaged quantities.…”

“…This class of compounds adopts a planar structure with respect to the double bonds to maximize orbital overlap and electron delocalization. Experimental evidence (see [1][2][3][4] and references therein) and theoretical investigations [5][6][7][8][9] on the internal rotational potential of 1,3-butadiene all agree that the planar s-trans (C 2h point group) is, by far, the most stable conformer ( % 99 %). More "intriguing" is the existence of a symmetry-related pair of nonplanar conformers, commonly referred to as s-gauche, whose higher stability with respect to the planar s-cis has been attributed to a decrease of the steric hindrance between protons H7 and H8 (see Figure 1) due to the loss of planarity.…”

“…A percentage of the s-cis conformer in the 1.1-2.6 % range is predicted, very close to the experimental value [2] of 1.9 % and to theoretical data at different levels of approximation. [7,8] When the torsional frequency is included, R 2,3 is longer than the MP2/cc-pVTZ value (1.470 vs 1.453 ) [but not so far from the MP4/6-31G* value (1.464 )]. A lower value of R 2,3 (1.444 ) is calculated when the torsional frequency is not included.…”

“…We must then conclude that the information content of the data is not enough to discriminate between the two hypothesis (s-cis vs s-gauche). Normalized CPDF curve obtained for AP(II) and AP(III) (excluding the torsional motion), compared with the theoretical curve (THEO) at the MP2/cc-pVTZ [7] level and with the experimental [2] curve (EXP).…”