title: Crystalline and solution phases of N,N-dimethylethylenediamine complexed with lithium triflate and sodium triflate: intramolecular and intermolecular hydrogen bonding.
abstract: Infrared and Raman spectroscopy were used to study hydrogen-bonding interactions and the cation coordination effect in solutions of N,N-dimethylethylenediamine (DMEDA) with lithium triflate (LiTf) and sodium triflate (NaTf). A comparison of pure DMEDA with DMEDA dissolved in carbon tetrachloride enabled the separation of the relative contributions of intermolecular and intramolecular hydrogen-bonding interactions to the N-H stretching frequencies. The addition of LiTf and NaTf to DMEDA shifts the N-H stretching frequencies through two competing effects: the cation coordination effect lowers the frequencies, while the disruption of the hydrogen-bonding interactions increases the frequencies. These two effects were distinguished in a study of the concentration dependence of both salts dissolved in DMEDA; the differentiation was based on the difference in the spectral sensitivities of the symmetric and the antisymmetric stretch in both the Raman and infrared spectra. During this study, DMEDA-LiTf and DMEDA-NaTf crystals were discovered, and their structures were solved by X-ray diffraction techniques. The analysis of the vibrational spectra of these crystals was greatly enhanced by unambiguous knowledge of the structural details of cation-molecule and anion-cation interactions. These structure-spectra correlations were used to complement analogous spectroscopic studies in the solution phases. Analysis of spectral regions in both crystalline and solution phases particularly sensitive to the nature and strength of cation-molecule interactions clearly established that the interaction of the lithium ion with the nitrogen atoms of DMEDA was stronger than the sodium ion-DMEDA interaction, as expected from charge density arguments.
authors.affiliation: Department of Chemistry and Biochemistry, University of Oklahoma, Norman, Oklahoma 73019, USA.
authors.forename: Nathalie M
authors.initials: NM
authors.lastname: Rocher
authors.forename: Roger
authors.initials: R
authors.lastname: Frech
authors.forename: Douglas R
authors.initials: DR
authors.lastname: Powell