Raman and DFT Study on N-H+…Cl- Hydrogen Bonding in 1,1,3,3-Tetra-Methylguanidinium Chloride forming an Ion-pair Molecule in the Vapor Phase

Rolf W. Berg, Anders Riisager, Rasmus Fehrmann

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Abstract

The chemistry of 1,1,3,3-tetramethylguanidinium ([TMGH]+) chloride, a low temperature (molten) ionic liquid, is discussed, based on its Raman spectra associated with ab initio molecular orbital DFT-type quantum mechanical calculations (with 6-311+G(d,p) basis sets) on “molecules” in isolated gaseous free states without any assumed symmetry. The calculations on the monomeric [TMGH]+ ion and the dimeric ion pair converged to give geometries near the established crystal structure of the [TMGH]Cl salt. This salt is known to contain dimeric ion pairs of the kind [TMGH]ClCl[TMGH](Fig.1).Experimentally obtained Raman scattering spectra of the compound (as the solid, as solutes in ethanolic and aqueous solutions and as a vapor at 225 ºC) are presented and assigned, by comparing to the ab initio vibrational analyses (calculated IR and Raman band positions and intensities). It is concluded that dimeric molecular ion pairs with four N-H+…Cl- hydrogen bonds seem to exist also in the solutions, and probably are responsible for the relatively high solubility of the “salt” in ethanol. The “salt” can be easily sublimed at about 200-230 oC. The Raman spectrum of the vapor at 225 ºC has a characteristic strong band at 2229 cm-1 that shows that the gas phase consists of monomeric ion pair “molecules” (Fig.2) held together by one N-H+…Cl- hydrogen bond, the stretching band of which is causing the band (Fig.3).
Original languageEnglish
Publication date2009
Publication statusPublished - 2009
EventConference on Molten Salts and Ionic Liquids - Copenhagen, Denmark
Duration: 24 Aug 200829 Aug 2008
http://www.euchem2008.dk/

Conference

ConferenceConference on Molten Salts and Ionic Liquids
CountryDenmark
CityCopenhagen
Period24/08/200829/08/2008
Internet address

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