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

Abstract

Principal component analysis and singular value decomposition (PCA and SVD) with self-modeling (SM) based on the van't Hoff equation constraint, were applied to 1,3-butadiene UV spectra measured in the 5.0-93.0°C range. The shape of the resolved UV spectrum of the high-energy conformer of 1,3-butadiene is sensitive to broadening of the spectra of the individual conformers with increasing T. A simulated spectral set, in which temperature broadening was purposely included, was used to develop a procedure that allows precise recovery of the pure component spectra. A set of Gaussians is derived that, on convolution with the spectrum for each corresponding T, creates a uniformly broadened spectral set. Exact recovery of the "s-cis spectrum" from the simulated spectral set is accomplished. Treatments of the experimental spectra using this method yields a minor conformer 1,3-butadiene spectrum that is only slightly red-shifted relative to the s-trans-1,3-butadiene spectrum and is consistent with the s-gauche structure. Application of PCA-SM to the onsets of the simulated spectral set showed that temperature broadening does not significantly alter the conformer enthalpy difference derived from the best-fit van't Hoff plot (compare 3.00000 kcal/mol assumed with 2.99992 kcal/mol recovered). The experimental onset spectra give ∆H°) 2.926 kcal/mol, in excellent agreement with previously reported values.

Coexistence of 1,3-butadiene conformers in ionization energies and Dyson orbitals

“…The energy difference between the two structures is in fact very small: 3.97 and 2.75 kcal/ mol, using the B3LYP/TZVP and MP2/TZVP models, respectively, which agree well with the believed range of 2.3-3.5 kcal/mol, 42,48,59 and the experimental energy of 2.93 kcal/ mol. 3 A small and competitive energy barrier from the local minimum well ͓5.98 kcal/ mol obtained using the MP2 model, which agrees well with the experimental value of 5.93 kcal/ mol ͑Ref. 13͔͒ makes it possible to interconverse between the conformers at the experimental conditions.…”

“…This property from theory can be used as a guideline to design a new experiment to differentiate the conformers. 3,14,49 Finally, the differences between the OVGF calculations of the present work and Ref. 5 need to be addressed.…”

“…3,[49][50][51] The barrier to rotation about the central bond is small and rotation to the less favorable, but reactive s-cis conformation is rapid. 3 Although the molar percentage for the s-gauche conformer is very small, the existence of the s-gauche conformer in the 1,3-butadiene sample makes it possible to induce the s-trans/s-gauche equilibrium ͓Eq. ͑2͔͒ to move toward the product under certain conditions.…”

“…7-12 and references therein the view of interconversion or coexistence of the planar s-trans conformer and nonplanar s-gauche-1,3-butadiene conformer of the less stable local minimum structure has received strong support from experimental evidence. 3,5,6 Both such structures of the conformer pair are almost identical in their geometries except for the dihedral angle formed by the four-carbon frame: C=C−C=C, where the s-trans conformer exhibits a dihedral angle of 180°, whereas the planar s-cis conformer possesses an angle of 0°. It was predicted theoretically that the dihedral angle of the gauche conformer of 1,3-butadiene lies in the range of 30°-41°, 7 depending on the model employed.…”

“…As a prototypical example of molecules with conjugated bonds, 1,3-butadiene exhibits, by virtue of facile rotation about the CC-CC single bond at ambient temperatures, an equilibrium mixture of s-trans and higher energy s-cis/s-gauche conformers. 3 Therefore, it is increasingly important to comprehend the electronic structures of 1,3-butadiene conformers in order to provide more confidence in detailed theoretical understanding of the experimental observations. Experimental evidence of ionization energies of L-phenylalanine 4 revealed that the conformer-dependent ionization energy ͑lowest ionization state͒ could range from 8.80 to 9.15 eV.…”