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

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).Key words: diazoacetophenones, phenylketenes, He(I) photoelectron spectroscopy, thermolysis, quantum chemical calculations.

2-Diazo-1-(4-hydroxyphenyl)ethanone: a versatile photochemical and synthetic reagent

“…1 H NMR of 1a displays one singlet at δ 6.30 (D 2 O-CD 3 CN, 1H, s) for the α-methine proton consistent with a single s-Z configuration deduced from the crystallographic data (SI1 ‡) and consistent with several other diazoacetophenones [25][26][27][28][29][30][31] and semi empirical (AM1) and ab initio (HF/6-31G(d)) determinations. 32,33…”

UV photoelectron spectroscopic and computational study of 7-substituted cycloheptatrienes

“…As it is well established that the correct geometry optimizations of molecules containing chalcogen atoms require the addition of d-polarization functions (19)(20)(21)(22)(23), we have used the above triple zeta split valence basis set augmented by two dpolarization functions, one p-polarization function on hydrogen atoms, as well as one diffuse function on hydrogen atoms. The interpretation of the PE spectra was based on our successfully applied method (11,24,25,26) that is basically an extension of Koopman's theorem. The simulated PE spectra of all compounds were obtained by anchoring the KohnSham MO orbital energies to the calculated first IP using our Fortran program PESPEC (12(a)).…”

Ionization potentials of nitriles — Photoelectron spectra of succinonitrile and glutaronitrile

“…To remove water from succinonitrile and glutaronitrile (both purchased from Sigma-Aldrich), samples were pumped in the PE spectrometer at elevated probe temperatures (50-60°C) and the process was followed by monitoring the PE spectra until the prominent ionization of water at 12.62 eV (7) had disappeared and no further change between subsequent spectra was discernible. The final spectra (averages of 20 scans) were also run with a probe temperature of 50-60°C with nitrogen as an internal calibrant as well as make-up gas (8) to increase the pressure and improve the signal-to-noise ratio.…”