Spectroscopic and theoretical studies on intramolecular OH-pi hydrogen bonding in 4-substituted 2-allylphenols.

2-Allylphenol (1) constitutes a mixture of conformers, in which an OH-pi hydrogen bonded closed (1a) and open form (1b) can be distinguished. 4-Substituted 2-allyphenols (2-9) have been synthesised and investigated by theoretical and spectroscopic methods. In 1-9, the energy and the structure of the hydrogen bonds show distinct variation with substituents. In the PE spectra of most compounds, two ionisations can be distinguished which are related to the allylic pi(C[double bond]C) orbitals of the two conformers a and b and differ in energy by DeltaIP(C[double bond]C). Alternatively, DeltaIP(C[double bond]C) can be determined indirectly from comparison of the PE spectra of the respective phenols and anisoles with the same substituents. DeltaIP(C[double bond, length as m-dash]C) values between 0.3 and 1.1 eV were found. Frequency shifts Deltanu(OH) of the O-H vibration in CHCl3 solution were measured by IR spectroscopy. By means of correlation analysis of the relationship between the strength of the intramolecular hydrogen bond, DeltaIP(C[double bond]C), Deltanu(OH) values and substituent constants it is established how substituents in 4-position affect the intramolecular OH-pi hydrogen bond. The investigations demonstrate that the DeltaIP(C[double bond]C) data can be used as descriptors for this intramolecular interaction.