title: Resolution of conformer-specific all-trans-1,6-diphenyl-1,3,5-hexatriene UV absorption spectra.
abstract: All-trans-1,6-diphenyl-1,3,5-hexatriene (ttt-DPH) exists in solution as a mixture of s-trans,s-trans and s-cis,s-trans conformers. The latter is higher in energy, and its contribution increases with increasing temperature. ttt-DPH UV absorption spectra broaden with increasing temperature and undergo blue shifts with decreasing polarizability. We describe here the resolution of two spectrothermal matrices of ttt-DPH UV absorption spectra into two conformer-specific components. The first matrix consists of DPH spectra measured in n-dodecane in the 283 to 374 K T range and the second of ttt-DPH absorption spectra measured in the even numbered n-alkanes (n-C(8)-n-C(16)) at temperatures selected to achieve isopolarizability (284-372 K). Principal component analysis (PCA) treatments showed that reasonable two-component systems are attained by compensation for T-induced broadening and shifting in the pure conformer spectra. The self-modeling (SM) method used to resolve the n-C(12) matrix is successfully tested on a simulated matrix closely mimicking ttt-DPH experimental spectra in n-C(12). Compensation for nonlinear effects yields robust two-component matrices from the experimental spectra. Their resolution into pure component spectra is based on the application of the Lawton and Sylvestre (LS) non-negativity criterion at the spectral onset to define the spectrum of the low energy s-trans-conformer and the optimum linearity van't Hoff (vH) plot criterion to find the spectrum of the higher-energy s-cis-conformer. Resolved spectra are somewhat sensitive to the choice of the spectral region in which the LS criterion is applied. The surprising result is that both resolutions lead to the conclusion that the molar fraction of the s-cis-conformer equals, or even exceeds, the molar fraction of the s-trans-conformer as the highest T's employed in our study are approached.
authors.affiliation: Faculty of Chemistry, Jagiellonian University, 30 060 Cracow, Poland.
authors.forename: Andrzej M
authors.initials: AM
authors.lastname: Turek
authors.forename: Jack
authors.initials: J
authors.lastname: Saltiel
authors.forename: Tallapragada R S
authors.initials: TR
authors.lastname: Krishna
authors.forename: Govindarajan
authors.initials: G
authors.lastname: Krishnamoorthy
mesh_terms: D000042:Absorption; D003198:Computer Simulation; D004161:Diphenylhexatriene / Q000737:chemistry*; D008956:Models, Chemical; D008968:Molecular Conformation; D025341:Principal Component Analysis; D012997:Solvents / Q000737:chemistry; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet / Q000379:methods*; D013696:Temperature; D013816:Thermodynamics
chemical_list: D012997:Solvents; D004161:Diphenylhexatriene