Infrared multiple-photon dissociation of small organic sulfoxides




Banjavčiʹc, Marko Peter

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The infrared multiple-photon dissociation of several small organic sulfoxides was studied in order to provide some insight in the dissociative product channels for this class of molecules. Vibrational excitation was achieved with a TEA CO₂ laser and photofragments were identified with a laser ionized time of light mass spectrometer. A beam of 10.5 eV photons generated in a static xenon gas tripling cell from a tightly focused Nd:YAG third harmonic beam of light was used to ionize the molecules and fragment species. The photoionization mass spectra were found to contain fewer fragmentation species than the corresponding electron impact ionization mass spectra. Numerous product species were observed from skeletal rearrangement reactions despite the lower level of energy excitation from the photoionization process. The infrared photolysis wavelength dependence of dimethyl sulfoxide indicated that the maximum abundance for the major fragmentation species occurred at (or near) 1085.8 cm⁻¹. The major products at 1085.8 cm⁻¹ were [H₃CSO]⁺ and [CH₃]⁺. Minor products were also observed at a higher output power threshold like the skeletal rearrangement species [H₃CS]⁺ and [OCH₃]⁺. With the infrared photolysis wavelength dependence of methyl phenyl sulfoxide, the major product species ([CH₃]⁺ and [OSCH₃]⁺) had a maximum abundance near 1086.9 cm⁻¹ . Small mass fragment species (like [HCS]⁺, [C₄H₅]⁺ and [C₄H₃]⁺) had a maximum abundance near 1080 cm⁻¹ while large mass fragment species ([C₆H₈S]⁺ and [C₆H₅SO]⁺) had a maximum abundance that was further red-shifted (1058.9 and 1050.4 cm⁻¹ , respectively). The photolysis at 1085.8 cm⁻¹ generated the high mass fragment species ([C₆H₅SO]⁺, [C₆H₈S]⁺ and [C₅H₅S]⁺) at all CO₂ laser output powers but a plateau in the abundance was observed at higher output powers. The smaller mass fragmentation species only appeared after a threshold power was surpassed. The maximum abundance for the various fragmentation species of sec-butyl methyl sulfoxide occurred near 1072 cm⁻¹. The CO₂ laser output power dependence for the formation of the butane, butyl and butene ions indicated all three were generated at all output powers with an abundance plateau or decline occurring at higher output powers. Fragmentation species were observed from the butyl group primary products with the power threshold increasing for the sequentially smaller secondary species.



Sulfoxides, Photoionization