Monitoring the Formation and Decay of Transient Photosensitized Intermediates Using Pump-Probe UV Resonance Raman Spectroscopy. II: Kinetic Modeling and Multidimensional Least-Squares Analysis

Abstract

Analysis of transient excited-state Raman spectra is a challenging spectroscopic measurement since transient spectral features are often overlapped with dominant ground-state and solvent bands. In the previous manuscript, resolution of component Raman spectra from the time-resolved amine quenching of excited-triplet benzophenone was accomplished using self-modeling curve resolution, a model-free factor analysis technique that relies on correlation in the data along a changing composition dimension. The results are consistent with the production of diphenylketyl radicals by H-atom abstraction from the amine and subsequent free-radical decay by recombination reactions. A kinetic model for this chemistry is developed in the present work, based on the observed Raman scattering data and the structures of product species confirmed by mass spectral analysis. The model is applied to the analysis of the time-dependent Raman scattering data using multidimensional least-squares methods, and it yielded well-resolved spectra of benzophenone excited-triplet states, diphenyl ketyl radical, and the solvent and ground-state precursors. The best-fit kinetic parameters agree well with the time-dependent triplet-state and ketyl-radical concentration profiles.

Monitoring the Formation and Decay of Transient Photosensitized Intermediates Using Pump-Probe UV Resonance Raman Spectroscopy. I: Self-Modeling Curve Resolution

“…The evolution of both excited-triplet state scattering and its relationship to the ketyl radical intensity suggest a mechanism for this system that can be further analyzed with a kinetic model. 40 This is an important use of self-modeling curve resolution: to provide a modelfree analysis of a set of data that can suggest a m echanism and allow m ore detailed evaluation of the reaction kinetics. In m ore complex kinetic systems, where a simple homogeneous reaction model is not feasible (such as polymerization with accompanying viscosity changes during reaction), self-modeling of pump-probe Raman scattering data could still resolve spectra of intermediates and report their time evolution during the course of reaction.…”

Calorimetry-Derived Composition Vectors to Resolve Component Raman Spectra in Phospholipid Phase Transitions

“…2); the quality of fit is optimized by minimizing the sum of the squared residuals, given by the sum of the square of the residuals = tr(R T R) , where R=D-A∧C∧. Examples of physical models that have been successfully applied to the analysis of component vibrational spectra from unknown data sets include the resolution of ground and excited state Raman spectra from their excitation intensity dependence, 5 the determination of donor and acceptor triplet-state Raman spectra through modeling their time-resolved energy transfer kinetics, 6 the separation of adsorbate infrared (IR) spectra on heterogeneous surfaces by modeling their concentration-dependent isotherms, 7,8 and finally, by modeling differences in the time-dependent accumulation of adsorbed versus surface-bound silane reagents in sol-gel silica films, the IR spectra of these components could be resolved. 9…”