Conformational analysis of 1,3-butadiene

Abstract

The conformational analysis of 1,3-butadiene has been accomplished using the temperature dependences of two nuclear magnetic resonance spectral parameters, SAB and Jcc, obtained from 2,3-dideuterio-l,3-butadiene (2) and 1,1,4,4-tetradeuterio-l,3-butadiene (3), respectively. These parameters have been correlated with the trans cis conformational equilibrium (eq 1). The magnitude of the derived vicinal 2,3 proton-proton coupling of the minor conformer indicates that it is nonplanar (skew). The enthalpy difference for the trans ^skew equilibrium (la ^lc, Id) was found to be +2105 ± 44 cal/mol at AS = +1.376 eu. The compound 1,3-butadiene is generally represented as a mixture of two planar conformers, la (s-trans) and lb (s-cis), in mobile equilibrium.3 Hiickel, using theoretical considerations, suggested the possibility of two such forms as early as 1932.4

UV Spectrum of the High Energy Conformer of 1,3-Butadiene in the Gas Phase

“…[47][48][49][50][51][52] Analysis of recently remeasured Raman 51 and IR 52 spectra of 1,3-butadiene by Engeln et al and by De Maré et al, respectively, supports a gauche geometry for s-cis-1,3-butadiene in the gas phase. Early evidence for a gauche geometry of the minor conformer was provided by temperaturedependent NMR experiments of deuterated 1,3-butadienes in CS 2 solution, 53 but large correlated errors made this conclusion unconvincing. High level ab initio and density functional calculations have generally supported the gauche geometry with a CCCC dihedral angle between 30°and 41°for the minor conformer in the gas phase.…”

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

“…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.…”

“…Single-quantum (SQ), 5-quantum (5Q) and refocused-5-quantum (5QR) 1 H spectra were recorded for the anisotropic sample (Figures 2, 3, and 4, respectively). 5QR and 5Q spectra were obtained using the pulse sequences shown in Figure 5.…”

“…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.…”

Effect of Confinement on Chemical Reactions

“…The first example is the rotational isomerization of 1,3-butadiene. This reaction is a good candidate to study the effect of the geometrical constraints imposed by the pore walls for several reasons: (1) it has an easily identifiable reaction coordinate (the dihedral angle between the four carbon atoms); (2) the mechanism involves a three-dimensional motion of the molecule, which is likely to be affected by the reduced dimensionality of the porous environment, and (3) the reaction in the bulk has been extensively studied both experimentally (Arnold et al, 1990;Fisher and Michl, 1987;Squillacote et al, 1979;Engeln et al, 1992;Carreira, 1975;Lipnick and Garbisch, 1973) and through simulations (Murcko et al, 1996;De Maré et al, 1997;Sancho-García et al, 2001a;Sancho-García et al, 2001b), and hence there is data available for comparison. The reaction is also made interesting by the fact that the structure of the high-energy rotamer is determined by an interplay between the steric impediment of the methylene end groups and the stabilization due to sp 2 -sp 2 conjugation.…”