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PRODUCTION AND UNIMOLECULAR DYNAMICS OF ALKYLCARBENES

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Abstract

Despite considerable efforts over the past many decades, reliable production and experimental characterizations of alkylcarbenes have proven exceedingly challenging. Here, we report that alkylketenes can be utilized as precursors for generating a wide range of alkylcarbenes. Using a few prototypical members, we have attempted to develop a better understanding of the intrinsic stability and unimolecular reaction dynamics of this elusive class of molecules. In all our experiments, we employed vacuum ultra-violet (VUV) photoionization in conjunction with photofragment translational energy mass spectrometry. At 355 nm excitation, photolysis of gaseous methylketene leads to CO loss producing ethylidene (CHCH3). The measured product translational energy distributions are consistent with the theoretically calculated singlet-triplet energy gap of ~ 12.5 kJ/mol. Our most significant finding is that triplet ethylidene is a long-lived species, if its inherent vibrational excitation is below the energetic barrier for intersystem crossing. Next, we studied the C-C bond fission channel at 248 nm excitation for 1-propyl and 2-propyl radicals, respectively. The primary photoproducts for this channel are methylene (CH2) and ethyl radicals (C2H5) for 1-propyl, and ethylidene and methyl radicals (CH3) for 2-propyl. Our results indicate that a ubiqutous feature of alkyl radical photodynamics is the formation of the isomer-selective carbene and radical products that involve site-specific C-C bond cleavage in the excited state at the radical carbon. Thereafter, we undertook a study of the photodissociation dynamics of gaseous dimethylketene at four near-UV excitation wavelengths: 320.0, 340.0, 355.1 and 371.0 nm. In each case, we monitored the competition between the two CO elimination channels, producing dimethylcarbene (CH3CCH3) and propene (C2H4), respectively. Our results indicate that propene photolytic products are formed prior to CO elimination, with a barrier height that is ~ 24 kJ/mol above the energetic asymptote for dimethylcarbene formation. Finally, we performed an analogous study of the competing pathways in photolysis of gaseous ethylketene at excitation wavelengths of 320.0, 340.0 and 355.1 nm. Here too, we observed propene photoproducts that are formed prior to CO elimination, in addition to the expected ethylcarbene (C2H5CH) products. Our results suggest that energy barrier for this direct propene channel lies below the energetic threshold for ethylcarbene formation.

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174 pages

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Date Issued

2022-12

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Keywords

alkylcarbenes; dimethylcarbene; ethylcarbene; ethylidene; ketenes; triplet carbenes

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Committee Chair

Davis, Harry

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Committee Member

Ezra, Gregory
DiStasio, Robert

Degree Discipline

Chemistry and Chemical Biology

Degree Name

Ph. D., Chemistry and Chemical Biology

Degree Level

Doctor of Philosophy

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Government Document

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Attribution 4.0 International

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dissertation or thesis

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