The photoelectron spectrum of tetramethyldiazetine: The elucidation of ring size effects on azo group ionization potentials

The photoelectron spectrum of 3,3,4,4-tetramethyldiazetine (4-Me₄) has been measured. By comparison with the previously reported photoelectron spectra of 3,3-dimethyldiazirine and 3,3,5,5-tetramethylpyrazoline, 4-Me₄ has three resolved low energy ionization (8.87, 10.36, and 11.65 eV) with an anomalously low second ionization potential. MINDO/3-and ab initio STO-3G calculations on 3-, 4-, and 5-membered ring azo compounds (diazenes) have been carried out, and these allow definite conclusions about the order of n_?, n₊, and π ionization potentials to be made for the diazetine (n_? < N₊ < π) and for 3,3-dimethyldiazirine and 3,3,5,5-tetramethylpyrazoline(n_? < π n₊), in agreement with the predictions of Heilbronner. Ab initio STO-3G calculations on the nitrogen and alkyl fragments of the cyclic diazenes reveal the origin of ionization energy shifts with changes in ring size.