Infrared Evidence of Polymorphism in Formic Acid Crystals

Low temperature Raman spectra of KHCO3 single crystal

“…The bands shift towards higher frequencies and the halfwidths decrease considerably as expected. This is helpful in assigning the 300K bands near 137 and 186 cm-' which appear to be superposed A, which are expected to have strong c1LW, cchw and cc:u coefficients respectively [12]. They should show following In the observed low temperature spectrum the strongest X Z component is shown for 196 cm-' band which may thus correspond to a R: motion.…”

“…Strong YZ components on the other hand are exhibited by the bands at 80 and 142 cm-'. These are The hydrogen bond vibrations identified at 222, 196 and 142 cm-' can be described as stretching v (OH)... 0 and bending y (OH)... 0 (R'v) and 6 (OH)... 0 (R'w) vibrations respectively using the same symbols as for formic acid dimer [12,131. The 142 cm-' band with its 300K equivalent at 137 cm-' is close to the value (139 crn-') calculated by Nakamoto [3] for a H.B.…”

“…The 222 cm-' band satisfies this condition and its frequency appears too high for a potassium ions motion but can be favorably compared to the antitranslational H.B. stretching mode of the formic acid dimer at 224 cm-'[12, 131. The mass of the HCO; ion is somewhat higher than that of HCOOH but this appears to be compensated by a stronger hydrogen bond (FH...o force constant) in the former: the OH stretching frequencies are 31 10 [I41 and 2630 cm-' and the O... 0 distances 2.70 [ 151 and 2.61 A for HCOOH and KHCO, respectively.…”

Pressure-dependent studies on hydration of the C–H group in formic acid

“…Formic acid and acetic acid are known to form low-temperature crystal characterized by infinite hydrogen-bonded chains tightly packed in layers, while all the other molecules among the monocarboxylic acid series form isolated dimer pairs linked by weak intermolecular bonding. [15][16][17][18]29 Although the structural properties of formic acid in low-temperature crystalline form are understood, controversial conclusions about the configuration of formic acid in high-pressure crystalline form ( PϾ0.8 GPa) still remain. Recently Allan et al 29 found the high-pressure crystal structure of formic acid is quite different from that formed at low temperature.…”

“…Other workers 8 also suggested that the role of the cyclic dimers, characteristic of the gas phase with two O-H---O hydrogen bonds, is minor ͑7% of the molecules͒ in the liquid formic acid. The formic acid dimer in the gas phase 7-11 and the long chains in the crystals [15][16][17][18][19][20] show the variety of hydrogen bonds which formic acid can form. Knowledge of the microscopic details of neat formic acid has been greatly enhanced in the literature, [9][10][11][12][13] while experimental evidence of C-H---O interaction among formic acid cluster has been difficult to obtain.…”