Structure elucidation of fluorescent H1 antihistamines by ultraviolet resonance Raman spectroscopy: solvent structures of tripelennamine and mepyramine

Ultraviolet Resonance Raman (UVRR) spectroscopy—a Raman technique that combines high sensitivity with high selectivity and does not suffer from background fluorescence—is applied to the fluorescent H₁ antihistamines tripelennamine (TRP) and mepyramine (MEP) in aqueous solution to elucidate their molecular structure as a function of pH. In a previous investigation of these compounds (C. Tardioli, G. Gooijer G. van der Zwan, J. Phys. Chem. B, 113 , (2009), 6949), the presence of gauche conformers caused by intramolecular interaction of the protonated alkylamine tail with the pyridine nitrogen was assumed to explain the pH dependence of the fluorescence properties. In order to validate this assumption, use is made of the resonant excitation of the aminopyridine chromophore in TRP and MEP. In that way, structural information associated with the vibrations of that moiety can be obtained, and the changes it undergoes upon protonation can be monitored. Assignment of the vibrations was achieved with the help of a number of other compounds, and quantum chemical calculations. N,N‐Dimethylaminopyridine (2DMP) and its mono‐protonated form (2DMPH⁺) were investigated, since this molecule was shown to have optical properties closely resembling those of the aminopyridine moiety in TRP and MEP. Assignment of the vibrations of 2DMP was accomplished by comparison with the resonance Raman spectra of two other reference structures, 2‐aminopyridine and dimethylaniline—for which ordinary Raman data are available—and by Gaussian calculations. UVRR spectra of TRP and MEP could be readily interpreted on the basis of vibrational assignments of the parent chromophores, i.e. 2DMP and 2DMPH⁺. Vibrations of the aminopyridine chromophore in TRP and MEP at neutral pH, where the aminoalkyl chain is protonated, are modified when compared to the vibrational pattern recorded for a fully neutral molecule in alkaline solution. This implies an electronic redistribution in the ring originating from internal hydrogen bonding between the aminoalkyl tail and the aminopyridine chromophore. Copyright © 2010 John Wiley & Sons, Ltd.