Time-resolved spectroscopic and density functional investigation of selected quinone methide and carbene intermediate

Abstract

In this thesis, spectroscopic techniques and computational methods were employed to provide adequate characterization of electronically excited states and transient intermediates. The most common way to detect transient intermediates is through their absorption. Femtosecond and nanosecond transient absorption measurements were carried out for 6-(hydroxydiphenylmethyl) naphthalen-2-ol (NQMP) in aqueous conditions (with varied pH) and 3-(1a,9b-dihydro-1H-cyclopropa[l]phenanthren-1-ylidene) tetrahydrofuran with ambient acetonitrile solvent. The B3LYP method at the 6-311+G (d, p) level of theory were used for the density functional theory (DFT) calculations done for the NQMP project. Similar DFT calculations using the 6-311+G (2d, p) basis set were used for the Carbene project part of this thesis. Theoretical analyses of the transition states enable the determination of the reaction mechanism's quantitative energy barriers.

A new type of precursor 6-(hydroxydiphenylmethyl) naphthalen-2-ol (NQMP) for generating the Naphthoquinone Methides (NQM) is described with the design of inhibited "proximity" of the naphthol OH ESIPT with the neighboring carbon atom. I studied the photo acidity of NQMP in aqueous solutions provided with different pHs. NQMP and its naphtholate anion can both be photolyzed to produce the NQM intermediate, with a persistent existence up to 200 µs. The NQM persisting lifetime in aqueous conditions can be explained by the large energy barrier (27.9 kcal mol-1) for the nucleophilic addition by H2O according to the results of the DFT calculation. Secondly, I investigated in this thesis the ylide formation of alkylcarbenes with the solvent acetonitrile with time-resolved femtosecond spectroscopy techniques. To further support the assignments of the ylide formation, theoretical DFT studies were carried out at the level of 6-311+G(2d,p) to examine the optimized structures, and to simulate the UV-Vis absorbance of possible excited states, intermediates and byproducts. Additionally, when these UV-Vis spectra are compared, a mismatch with the λmax 450 nm peak is revealed. except that the ylide adduct with acetonitrile matches the peak well. I report the spectroscopic analysis of ylides generated by the nucleophilic addition of acetonitrile into the empty p orbital of the 3-oxacyclopentylidenecarbene 12.