Describing and predicting molecular properties via vibrational spectroscopy in combination with electron density analysis
The major aim of the present work is the correlation of electron density investigations with vibrational spectroscopic studies. In particular, Raman spectroscopy was applied to qualitatively approve DFT-calculated changes in the electron density distribution induced by structural modi¯cations. Moreover, a method was derived to predict properties of the electron density distribution quantitatively via combinations of vibrational spectroscopic and NMR spectroscopic data. Furthermore, the initial changes in the electron density distribution upon photoexcitation and related changes in the molecular structure were investigated via resonance Raman spectroscopy. The results of all these studies are shortly summarized in the following. After the impact and the limitations of electron density studies are outlined in the "Introduction" (chapter 1), the basic tools to calculate and analyze the electron density distribution ½(r) are summarized in chapter 2 "Theoretical details". In section 4.1 an il- lustrative example of ½(r)-studies in the life sciences was discussed in detail. This example is related to an investigation of Schirmeister and Luger, who studied the selectivity of an inhibition reaction of an aziridine derivative functioning as a protease inhibitor. For that purpose they determined the electron densities at the carbon atoms within the aziridine ring via high resolution x-ray measurements of an aziridine single crystal and via DFT-calculations of an isolated aziridine molecule. Continuing the work of Schirmeister and Luger the in°uences of neighboring molecules on ½(r) and therewith on the electrophilicity of the aziridine carbons were studied to shed light on the impact of intermolecular interactions on the electron density distribution ½(r). It turned out that NHN-hydrogen bridges and intermolecular interactions between dimethylmalonate moieties exhibit opposite in°uences on the aziridine ring. In particular, an electrophilic attack to a protease enzyme would occur at C2 if hydrogen bridges at the aziridine-N are ruling. In contrast, C1 would be more electrophilic than C2 if intermolecular interactions of the dimethylmalonate group are dominating. The results and the study of an aziridine in a simulated aqueous environment support the assumption of Schirmeister and Luger, who supposed the electrophilic attack occurs via C1.