Photochemistry of Matrix-Isolated and Thin Film Acid Chlorides: Quantum Yields and Product Structures

Abstract

Ultraviolet photoexcitation of matrix-isolated CH3COCl, CH3CH2COCl, and CH3CH2CH2CH2COCl produces HCl·CH2CO, HCl·CH3CHCCO, and HCl·CH3CH2CH2CHCCO complexes. We report precursor and matrix dependent reaction quantum yields. Quantum yield values decrease with increasing alkyl chain length due to a reduced number of α H-atoms available for the elimination reaction and steric considerations. We found quantum yields in neat matrixes to be roughly half that in argon or xenon matrixes and assign structures for HCl and ketene complexes in argon and xenon matrixes by comparing IR spectra to ab initio electronic structure calculations. In argon matrixes, the product complex HCl frequency is strongly shifted whereas the ketene remains unshifted with respect to matrix-isolated ketene. In xenon matrixes, HCl·ketene complexes display absorption bands indicative of two distinct structures. Differences between HCl·ketene structures in argon and xenon matrixes are attributed to size differences of the matrix lattice.