A study of the vacuum pyrolysis of para-substituted ...

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Title A study of the vacuum pyrolysis of para-substituted diazoacetophenones with He(I) ultraviolet photoelectron spectroscopy
Author(s) N. H. Werstiuk, H. M. Muchall, J. G. Ma, Michael T. H. Liu
Journal Canadian Journal of Chemistry / Revue Canadienne De Chimie
Date 1998
Volume 76
Issue 8
Start page 1162
End page 1173
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 (la) and its p-methyl, p-methoxy, p-chloro, and p-nitro analogues Ib, Ic, Id, and le. Analysis of the pyrolysate with He(I) ultraviolet PE spectroscopy shows that at a laser power Level of 26 W (500 +/- 50 degrees C) la, Ib, Ic, and Id, 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 (le) thwarted our attempts to generate 2e and obtain its spectrum. Calculations at semiempirical (AMI) and ab inito (WF/G-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/G-31G(d)) of la-ld and the synthesized PE spectra of la, Ib, and Ic correlate well with the PE spectroscopic data supports this finding. Shifts observed in the three low-energy ionizations of ketenes 26, 2c, and 2d induced by the para-substitution can be related to the character of the corresponding occupied molecular orbitals of phenylketene (2a).

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