Spectroscopic and electrochemical properties of intramolecularly hydrogen-bonded compounds. Ortho-hydroxyazobenzenes

The gas-phase uv photoelectron spectra and the half-wave polarographic reduction potentials in acetonitrile solutions of a series of 2-hydroxy-5-methylazobenzene have been determined. The first three IP's and the E1/2 values were linearly correlated with σ+ and σ constants, respectively. E1/2 can also be linearly related to the energy of the lowest unoccupied molecular orbital. Effects of the intramolecular hydrogen bonding on the pe spectra and on the polarographic reduction of the title compounds were evaluated by comparing the experimental findings in 2-hydroxy-5-methylazobenzene, 2-methoxy-5-methylazobenzene, and p-hydroxyazobenzene. On H-bond formation, the antibonding combination of the two nitrogen lone-pairs, n−, and the oxygen lone-pair is ionized at a slightly higher energy and at a lower energy, respectively, than in the non H-bonded compound. The first π-ionization band is slightly lowered in energy in the non H-bonded structure, probably owing to a normal substituent effect. These features are not maintained in the 4'-nitro derivative, where the band encompassing the n− ionization remains at the same potential in both structures, while the first π band in the non H-bonded compound moves toward higher IP. Spectral differences between the bonded and nonbonded structures are the result of a balance of substituent effects, and of both hydrogen bond and orbital interactions. The solution electron affinity of the studied compounds increases on H-bond formation by an amount comparable with the experimental strength of the H-bonding. INDO/S-CI calculations are in qualitative agreement with the experimental results.