TY - JOUR
T1 - X-ray Crystal Structure, Raman Spectroscopy and Ab Initio Density Functional Theory Calculations on 1,1,3,3-Tetramethylguanidinium Bromide
AU - Berg, Rolf W.
AU - Riisager, Anders
AU - Nguyen van Buu, Olivier
AU - Fehrmann, Rasmus
AU - Harris, Pernille
AU - Kristensen, Steffen Buus
AU - Brunetti, Anna Chiara
PY - 2010
Y1 - 2010
N2 - The salt 1,1,3,3-tetramethylguanidinium bromide, [((CH3)2N)2C=NH2]+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 P21/n, with a = 7.2072(14), b = 13.335(3), c = 9.378(2) Å, beta = 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.
AB - The salt 1,1,3,3-tetramethylguanidinium bromide, [((CH3)2N)2C=NH2]+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 P21/n, with a = 7.2072(14), b = 13.335(3), c = 9.378(2) Å, beta = 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.
U2 - 10.1021/jp107152x
DO - 10.1021/jp107152x
M3 - Journal article
SN - 1089-5639
VL - 114
SP - 13175
EP - 13181
JO - Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory
JF - Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory
ER -