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A study of the vacuum pyrolysis of para-substituted diazoacetophenones with He(I) ultraviolet photoelectron spectroscopy

Title:

A study of the vacuum pyrolysis of para-substituted diazoacetophenones with He(I) ultraviolet photoelectron spectroscopy

Werstiuk, Nick H and Muchall, Heidi M. and Ma, Jiagong and Liu, Michael TH (1998) A study of the vacuum pyrolysis of para-substituted diazoacetophenones with He(I) ultraviolet photoelectron spectroscopy. Canadian Journal of Chemistry, 76 (8). pp. 1162-1173. ISSN 0008-4042

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Official URL: http://dx.doi.org/10.1139/cjc-76-8-1162

Abstract

An ultraviolet photoelectron (PE) spectrometer apparatus that utilizes a tuneable 50 W CW CO2 laser as a directed heat source was used to study the vacuum pyrolysis of diazoacetophenone (1a) and its p-methyl, p-methoxy, p-chloro, and p-nitro analogues 1b, 1c, 1d, and 1e. Analysis of the pyrolysate with He(I) ultraviolet PE spectroscopy shows that at a laser power level of 26 W (500 ± 50°C) 1a, 1b, 1c, and 1d, cleanly yield the corresponding phenylketenes 2a, 2b, 2c, and 2d, respectively, the products of the Wolff rearrangement of the incipient ketocarbenes. Of this group of highly reactive ketenes, which cannot be isolated in the condensed phase at ambient temperature, only 2a has been the subject of a previous PE spectroscopic study. But our work indicates that the sample of 2a prepared in the earlier study was impure. The low volatility of p-nitrodiazoacetophenone (1e) thwarted our attempts to generate 2e and obtain its spectrum. Calculations at semiempirical (AM1) and ab inito (HF/6-31G(d)) levels of theory established that the diazoacetophenones prefer to adopt twisted syn conformations. That the calculated ionization potentials (HAM/3 and Becke3LYP/6-31+G(d)//HF/6-31G(d)) of 1a-1d and the synthesized PE spectra of 1a, 1b, and 1c correlate well with the PE spectroscopic data supports this finding. Shifts observed in the three low-energy ionizations of ketenes 2b, 2c, and 2d induced by the para-substitution can be related to the character of the corresponding occupied molecular orbitals of phenylketene (2a).

Divisions:Concordia University > Faculty of Arts and Science > Chemistry and Biochemistry
Item Type:Article
Refereed:Yes
Authors:Werstiuk, Nick H and Muchall, Heidi M. and Ma, Jiagong and Liu, Michael TH
Journal or Publication:Canadian Journal of Chemistry
Date:August 1998
Funders:
  • Natural Sciences and Engineering Research Council of Canada
Keywords:diazoacetophenones, phenylketenes, He(I) photoelectron spectroscopy, thermolysis, quantum chemical calculations
ID Code:6768
Deposited By:DANIELLE DENNIE
Deposited On:12 Jul 2010 17:02
Last Modified:08 Dec 2010 18:09
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