Raman formic acid

Beltramo G, Shubina TE, Koper MTM. 2005. Oxidation of formic acid and carbon monoxide on gold electrodes studied by surface-enhanced Raman spectroscopy and DFT. ChemPhysChem 6 2597-2606. 

P. Zielke and M. A. Suhm, Raman jet spectroscopy of formic acid dimers Low frequency vibrational dynamics and beyond. Phys. Chem. Chem. Phys. 9, 4528 4534 (2007). 

Abstract—Low frequency lines in the Raman spectrum of single crystals of formic acid and ice are interpreted as arising from the vibrations of the hydrogen bond H.O. 

Some years ago when studying the low frequency region in the Raman spectra of crystals and liquids we paid attention to some weak bands situated in the region of 150-300 cm-1 which appeared with various substances containing the hydroxyl group [1], With different substances these bands have different width and intensities. In Table 1 some examples are given and the approximate positions of the bands are listed. Such a band of low frequency is particularly strong in the spectrum of formic acid (Fig. 1). 

Fig. 4. Microphotometrie record of low frequency Raman spectrum of formic acid crystal...

Most fundamental work on the vibrational spectra of azoles appeared in the period 1960-1980. Examples of more recent work include (i) a complete assignment of the gas-phase IR spectrum of indazole (93JCS(F1)4005) (ii) IR spectral data were used to determine the enthalpies of 0—H. . . N and N—H. . . O bonds in complexes of formic acid and 3,5-dimethylpyrazole (87MI301-01) (iii) the vibrational assignment of the Raman spectrum of polycrystalline pyrazole (92MI301-01) based on 3-21G calculations. 

Formic Acid, In 1938 Davies and Sutherland predicted that the H bond stretching mode p would be very low and proposed that the band width of p is due to slow vibrations of one-half of the dimer with respect to the other. (500). J. O. Halford (856) gives reference to three Raman studies of liquid formic acid which reveal a shift near 200 cm" , and speculates that this might be caused by the symmetric Pa mode of a cyclic dimer. He shows by entropy arguments that this frequency is reasonable. Further examination by Simova (1884, 1883), and particularly by Gross and Val kov (830), revealed Raman shifts... 

Other Carboxylic Acids, The low frequency modes of acetic acid and higher carboxylic acids are not as well understood as those of formic acid, and will not be discussed in detail. Raman shifts are reported for solid benzoic acid at 190 and 400 cm (1693, 1694) and for solid tartaric acid at 52, 80, 101, 115, 144, and 164 cm (1891, 1695). Gross and Val kov conclude that the frequency of the O—H 0 vibration is near 200 cm and is unchanged either by deuterium substitution or by increase in the mass of the attached alkyl groups (830). Batuev s studies seem to be in disagreement with this conclusion (162, 163). 

The carboxylic acid carbonyl frequency is shifted to lower frequency by 1 to 6 percent in carboxylic acid dimers, which is much less than the relative shifts of The mechanical splitting of the symmetric and antisymmetric carbonyl stretching modes is evident in the difference between the Raman and IR spectra of formic acid. Of course the average of these two is of interest, but often only one is measured. 

Raman low frequency shifts, formic acid, methanol. 

Figure 11. Raman spectra of the formic-biological system (A) shown with the vibration peaks of formic and diamond anvils used in this study. The outlined boxed region is shown at higher resolution (B) to quantify the successive decrease in the peak intensity of the C-H stretch of formic acid at pressures of 68,142, and 324 MPa. The equivalent formate concentrations (C), corresponding to each peak height change, are based on comparisons with a known calibration curve. All experiments were performed at 25°C, with diamond anvil cells with gold-lined sample chambers. Pressures were estimated using Raman shifts in quartz used as an internal calibrant.

Figure 11. (a) Raman spectra (ambient conditions) of the C-H bend region of two different recovered products from formic acid. The inset is a photomicrograph of the 4.0 GPa products, intensely orange in color, (b) FTIR spectra of the samples described in (a). 

The in situ study in electrochemical cells of the catalyst surface is challenging due to low stuface sensitivity through the electrolyte. Several surface-sensitive techniques have been employed to probe the abundance and/or state of adsorbed surface species formed during formic acid electrooxidation broadband sum frequency generation [89, 90], surface-enhanced Raman spectroscopy [21], scanning tunneling microscopy [91], and Fourier transform infrared spectroscopy [19,26,27,31,32, 41,92-99],... 

Figure 4.1 Comparison of the IR (upper) and Raman (lower) spectra of nylon-6,6. The IR spectrum was obtained by transmission through a film cast from formic acid, while the Raman spectrum was obtained from a chip of polymer. Note the domination of bands due to polar (amide) groups in the IR data, while the Raman spectrum is dominated by backbone and C-H modes. In fact, below 1650 cm the Raman spectrum is reminiscent of that of polyethylene.

There are, however, indications for a phase transition in formic acid around 208 K jl10,111. Which kind of structural change actu ally occurs, a transition to structure IV, an order-disorder transition involving structures I and IV, or a significant change in interchain orientation or -distances is still an open question lll-113. An ab initio calculation of vibrational spectra of formic acid chains is currently out of reach. We compiled therefore available Raman results on 0-H frequencies in isolated formic acid, in the cyclic dimer and in the crystal in table 11 in order to enable a comparison with the previously discussed cases of hydrogen fluoride and hydrogen cyanide. 

Raman half-width of i, formic, acetic, valeric acids. 

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