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

An ultraviolet photoelectron (PE) spectrometer apparatus that utilizes a tu neable 50 W CW CO2 laser as a directed heat source was used to study the va cuum pyrolysis of diazoacetophenone (la) and its p-methyl, p-methoxy, p-chl oro, and p-nitro analogues Ib, Ic, Id, and le. Analysis of the pyrolysate w ith 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 correspon ding phenylketenes 2a, 2b, 2c, and 2d, respectively, the products of the Wo lff rearrangement of the incipient ketocarbenes. Of this group of highly re active ketenes, which cannot be isolated in the condensed phase at ambient temperature, only 2a has been the subject of a previous PE spectroscopic st udy. But our work indicates that the sample of 2a prepared in the earlier s tudy was impure. The low volatility of p-nitrodiazoacetophenone (le) thwart ed our attempts to generate 2e and obtain its spectrum. Calculations at sem iempirical (AMI) and ab inito (WF/G-31G(d)) levels of theory established th at the diazoacetophenones prefer to adopt twisted syn conformations. That t he calculated ionization potentials (HAM/3 and Becke3LYP/6-31+G(d)//HF/G-31 G(d)) of la-ld and the synthesized PE spectra of la, Ib, and Ic correlate w ell with the PE spectroscopic data supports this finding. Shifts observed i n 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 occ upied molecular orbitals of phenylketene (2a).