UV spectrum of the high energy conformer of 1,3-butadiene in the gas phase

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 degreesC range. The shape of the resolved U-V 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 DeltaH degrees = 2.926 kcal/mol, in excellent agreement with previously reported values.