| dc.description.abstract | (1) Infrared spectra of the high?temperature phases of
dimethylammonium chloride and of the methylammonium tetrachlorometallates
show evidence for the presence of an essentially “undistorted” C?H?NH??
ion of C?v symmetry. The ordered, low?temperature phases show the
expected site?group splittings. Hydrogen bonding is stronger in the
low?temperature phases, and these phases also show the torsional mode
frequency, 2??, as well as the combination bands involving ?.
(2) Infrared spectra of ethylammonium chloride and those
of ethylammonium tetrachlorometallates show the expected splittings
of vibrational modes in the low?temperature phases along with the
C–H torsional modes and other modes. Bands are broad in the
high?temperature phases, suggesting appreciable motion
of the C?H?NH?? ion.
(3) The close correspondence between the changes in the
infrared spectra across the phase transitions of alkylammonium
chlorides and those of the M²?(alkylammonium) tetrachlorometallates
clearly indicates that the vibrational states of the ammonium ions
are different in the different phases. It is
possible that the metal–halogen clusters have little role to play
in the phase transitions of the tetrachlorometallates.
(4) Alkylammonium bromides and bis?(alkylammonium) tetra?
bromometallates (II) exhibit phase transitions similar to those
observed in the chloro compounds. Infrared spectra of the bromo
derivatives across their phase transitions clearly establish the
key role played by the alkylammonium cations in the phase transitions.
III.3. Experimental
Alkylammonium chlorides and bromides were prepared by
exactly neutralizing aqueous alkylamine solutions with hydrochloric
and hydrobromic acids respectively. The salts obtained by evaporation
were twice recrystallized, dried and stored under vacuum over
silica gel. Bis?(alkylammonium) tetrahalogenometallates (II) were
made by mixing stoichiometric amounts of pure hydrated metal(II)
halides and alkylammonium halides (a slight excess of the latter) in
a minimum of deionized water at 90?°C to form clear solutions, which
upon very slow cooling formed well?defined crystals. The crystals were
washed with a little deionized water, dried and stored under
silica gel.
The tetrabromometallates, prepared by us
for the first time, were characterized by standard procedures of
chemical analysis. Bromine was estimated by Volhard’s method,
cadmium and manganese were weighed as their anthranilates and pyro?
analysed. The results were accurate within 2%.
Differential scanning calorimetry (D.S.C.) was carried out
on a Perkin?Elmer DSC?2 instrument. Low?temperature studies were
conducted in hydrogen atmosphere using liquid nitrogen as coolant.
Temperature calibration was done using transition temperature of
cyclohexane (186.1?K) and the melting point of indium (429.7?K) as
standards. The heat of transition of indium (?H = 28.5?kJ?mol?¹)
was taken as standard for calorimetric calibration.
Infrared spectra of mulls (in Nujol or Fluorolube) and thin
films were recorded at different temperatures with a Perkin?Elmer 580
spectrophotometer fitted with Specac variable?temperature cell and
temperature controller. Qualitative features of the bands are indicated
by the symbols s (strong), m (medium), w (weak) and sh (shoulder).
The band half?widths were measured at the geometric mean of the baseline
transmittance and the transmittance at the centre. | |
