Ultrafast spectroscopy and dynamics of nitrenes and carbenes
Abstract (Summary)Time resolved spectroscopic techniques including Ultrafast Transient Absorption, Laser flash photolysis with UV-visible and infrared detection and modern quantum chemical calculations were used to directly observe and identify reactive intermediates involved in the photolysis of azido and diazo compounds. In this dissertation the photolysis of 8-azidoadenosine and its derivatives were studied in water and in buffer solutions. Using ultrafast spectroscopic techniques it was shown that the corresponding singlet nitrene undergoes a hydrogen shift to form a closed azaquinodimethane faster then it can relax to a triplet nitrene or react with biological target molecules in a photoaffinity labeling experiment. The spectroscopic characteristics of azaquinodimethane were determined using nanosecond laser flash photolysis in the UV-visible and IR regions. The lifetime of this species was estimated to be several minutes in the absence of good nucleophiles, and it is the species which forms adducts in cross-linking and photoaffinity labeling experiments via selective reactions with nucleophiles. Quantum chemical calculations were utilized to identify reactive intermediates produced upon photolysis. Implications of the results of this work on the interpretation of results obtained in photoaffinity labeling and cross-linking experiments are discussed. Ultrafast spectroscopy and nanosecond time resolved Laser Flash Photolysis methods were used to study the photochemistry of 3,5-dichloro 2-biphenyl azide in different solvents. The corresponding singlet nitrene was detected and corresponding spectroscopic characteristics were determined. The photochemistry of diphenyldiazomethane in series of alcohols solvent reveals the fastest known proton transfer reaction to a carbon atom. Studies performed using ultrafast transient absorption spectroscopy revealed the formation and decay of the benzhydryl carbocation. It was shown that benzhydryl carbocation is a short lived intermediate in neat alcohol solvents. Rates of protonation were measured directly and shown to correlate with solvation times in alcohols from methanol to 2-propanol. The rate-limiting step in the overall O-H insertion reaction was determined from a study of the kinetic isotope effect. Transient absorption actinometry measurements were performed on the femtosecond time scale and the contribution of the protonation channel to the O-H insertion reaction was estimated for several alcohols.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Date of Publication:01/01/2005