X-ray Crystal Structure, Raman Spectroscopy, and Ab Initio Density Functional Theory Calculations on 1,1,3,3-Tetramethylguanidinium Bromide

Abstract

The salt 1,1,3,3-tetramethylguanidinium bromide, [((CH(3))(2)N)(2)C═NH(2)](+)Br(-) or [tmgH]Br, was found to melt at 135(5) °C, forming what may be referred to as a moderate temperature ionic liquid. The chemistry was studied and compared with the corresponding chloride compound. We present X-ray diffraction and Raman evidence to show that also the bromide salt contains dimeric ion pair "molecules" in the crystalline state and probably also in the liquid state. The structure of [tmgH]Br determined at 120(2) K was found to be monoclinic, space group P2(1)/n, with a = 7.2072(14), b = 13.335(3), c = 9.378(2) Å, β =104.31(3)°, Z = 2, based on 11769 reflections, measured from θ = 2.71-28.00° on a small colorless needle crystal. Raman and IR spectra are presented and assigned. When heated, both the chloride and the bromide salts form vapor phases. The Raman spectra of the vapors are surprisingly alike, showing, for example, a characteristic strong band at 2229 cm(-1). This band was interpreted by some of us to show that the [tmgH]Cl gas phase should consist of monomeric ion pair "molecules" held together by a single N-H(+)···Cl(-) hydrogen bond, the stretching vibration of which should be causing the band, based on ab initio molecular orbital density functional theory type calculations. It is not likely that both the bromide and chloride should have identical spectra. As explanation, the formation of 1,1-dimethylcyanamide gas is proposed, by decomposition of [tmgH]X leaving dimethylammonium halogenide (X = Cl, Br). The Raman spectra of all gas phases were quite identical and fitted the calculated spectrum of dimethylcyanamide. It is concluded that monomeric ion pair "molecules" held together by single N-H(+)···X(-) hydrogen bonds probably do not exist in the vapor phase over the solids at about 200-230 °C.

Vibrational Spectroscopy of Ionic Liquids

“…Thus, it was proposed instead that this band belongs to CN stretching mode of dimethylcyanamide, (CH 3 ) 2 N−CN, resulting from thermal decomposition of [TMGH]Cl and [TMGH]Br. 241 In the temperature-jump experiments of Chambreau et al, 257 the IR spectrum is recorded for the vapor phase generated after a discharge on a filament within a small sample of the ionic liquid. For instance, characteristic IR bands of CH 3 NCS indicated formation of this species upon heating of [C 4 C 1 im][SCN], and Chambreau et al 257 proposed reaction mechanisms of thermal decomposition for several ionic liquids based on 1-alkyl-3-methylimidazolium cations and cyanate-anions.…”

“…In Berg’s review on Raman spectroscopy of ionic liquids, he provided preliminary results for a system based on the tetramethylguanidinium cation, [TMGH] + . More detailed analysis of vibrations of [TMGH] + have been performed by Berg et al and Horikawa et al for [TMGH]­[NTf 2 ], and by Berg et al for [TMGH]­Cl and [TMGH]­Br . These works provide lists of vibrational frequencies experimentally observed and calculated by quantum chemistry methods.…”

Advancement of technology towards high-performance non-aqueous aluminum-ion batteries

“…In summary, the chloroaluminate AlCl 4 À anions could intercalate/de-intercalate from carbonaceous materials during cycling. This process has been observed by the measurement of XPS, XRD and electrochemical in-situ technology et al [50,81]. Hence, carbon-based materials are electrochemically active in AIBs.…”

N,N,N′,N′-Tetramethylguanidinium tetraphenylborate

“…For related structures, see: Fischer & Jones (2002); Berg et al (2010); Tiritiris et al (2011); Criado et al (2000); Kanters et al (1992); Bujak et al (1999); Wong et al (2004); Pajzderska et al (2002).…”