Hammett parameters (σp) provide a means to quantify a substituent’s capacity to perturb the electronic environment in a molecule. They represent a measurement of the electron-donating or withdrawing properties of a moiety. These parameters are based on the equilibrium of benzoic acid and its para-substituted derivatives. In an analogous manner, this study sought to use para-substituted benzonitrile derivatives to explore the relationship between the electronic effects of different substituents and the C≡N absorption frequency in the infrared region. Specifically, the nitrile was utilized as an electronic-environment probe through the measurement of vibrational Stark effects. The effects of sixteen different substituents on the C≡N absorption were analyzed using a Bruker Vertex 80 FT-IR spectrometer. All spectra were taken using a CaF2 liquid flow cell, with each p-substituted benzonitrile derivative tested in three different solvents chosen to cover a wide range of dielectric constants (1,4-dioxane ε = 2.25; N,N-dimethylformamide ε = 36.7; propylene carbonate ε = 65.5). By obtaining spectra in different media, any dependency of the nitrile absorption shift on solvent selection could be identified. Findings suggest that there seems to be a roughly linear correlation between a substituent’s σp value and the C≡N absorption frequency. As σp increases (electron-withdrawing quality increases) the C≡N absorption blue-shifts. Furthermore, data suggest that substituent effects on nitrile absorption appear to be independent of the solvent choice.

Correlation between Hammett parameters and vibrational Stark effects in benzonitrile derivatives

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.