Before delving into the infrared spectrum (IR) of benzoic acid, it is important to demystify the concept. Infrared spectrum (IR spectroscopy or vibrational spectroscopy) entails the communication of infrared radiation with matter. It includes a range of mechanisms, usually relying on absorption spectroscopy. As with all of these techniques, it can be utilized to point out and analyze chemical substances. Such samples may be solid, liquid, or gas.
An instrument called an infrared spectrometer (or spectrophotometer) is used to provide an infrared spectrum and lead the process of infrared spectroscopy. An IR spectrum can be seen in a graph of infrared light absorbance (or transmittance) on the vertical axis vs. frequency or wavelength on the horizontal axis.
Typical units of frequency used in IR spectra are reciprocal centimeters (sometimes called wave numbers), with the symbol cm−1. Units of IR wavelength are commonly given in micrometers (formerly called “microns”), symbol μm, which are distinguished into wave numbers in a reciprocal way. A common laboratory instrument that uses this technique is a Fourier transform infrared (FTIR) spectrometer.
Benzoic acid, on the other hand, is a clear (or colorless) solid with the formula C6H 5CO 2H. It is the simplest aromatic carboxylic acid. The name is gotten from gum benzoin that was for an extended time its only source. Benzoic acid comes about naturally, in many plants and becomes an intermediate in the biosynthesis of many secondary metabolites. Salts of benzoic acid are utilized as food preservatives. Benzoic acid is a vital forerunner for the industrial synthesis of many other organic substances. The salts and esters of benzoic acid are known as benzoates.
Infrared spectra of benzoic and deuterobenzoic acids were quantified at low temperatures. The feature bands of the COOH group near 1700, 1300, and 950 cm.-1 were observed as pairs. This connotes that two kinds of configurations having diverse energies and diverse spectra cohabit in the crystal. The energy difference was evaluated to be about 0.1 Kcal. /mole from measurements of intensity ratios of pairs at various temperatures. The nearly equal distances of the two C—O bonds in benzoic acid, i.e. 1.29 and 1.24 A as determined by X-ray measurements, were interpreted as the average for the mixture of the two configurations.
The crystal and molecular outlooks of benzoic acid were meticulously determined by “Sim, Robertson and Goodwin”, who documented that the molecules form nearly planar, centrosymmetrical dimers with hydrogen bonds (2.64A) in between adjacent carboxyl groups. They allocated the hydrogen atom of the carboxyl group to O2, though the resolution of this hydrogen atom was not good enough to ascertain the assignment. This lack of resolution may indeed be attributed to the nearly similar distances of the two C-0 bonds, i.e. 1.29 and 1.24 A, and indicate a ready transfer of hydrogen to the other oxygen across the hydrogen bond.
The infrared spectra of benzoic and deuterobenzoic acids measured at low temperatures produce different results as the case may be. These have been identified below:
Frequencies Characteristic of the COOH and COOD Groups
There are two bands at 1706 and 1684 cm.-1 in the region of the stretching vibration of C=0 bonds of the dimeric units. The band at 1432 cm.-1 or 1421 cm.-1 at room temperature is because of one of the coupled vibrations of C—O stretching and OH bending, the two bands at 1334 and 1298 cm.-1, or 1324 and 1288 cm.-1 at room temperature, respectively, are in the region of another coupled vibration of C—O stretching and OH bending, and those at 959 and 948 cm.-1, which are equal to the broadband at about 935 cm.-1 at room temperature are in the region of O—H out-of-plane bending vibration2'. These bands disappear on deuteration.
A polarized infrared spectrum is measured at —150 °C and is shown that bands at 1184 and 1174 cm.-1 have high dichroic ratios. This implies that the configuration of samples is good. Therefore, the splitting of the bands of low dichroic ratios at 1706 and 1684 cm.-1 must not be because of the “factor group splitting”, occasioned by the interaction with neighboring molecules of the crystal.
A pair of bands at 1334 and 1298 cm.-1 alongside as the doublet at 959 and 948 cm.-1 must not be because of the “factor group splitting” either. In fact, it is difficult to determine that a lot of pairs of bands come about by the “Fermi resonance”.
Another type of explanation of the splitting of bands of the carboxylic group is the tunneling motion of hydrogen atoms in the O—H groups of the acid dimers from one equivalent position to another. Although such a phenomenon would give rise to the splitting of the Raman-active VO—H energy levels into pairs’, infrared-active bands of VC=O, VC-0+ (5OH and oOH would be difficult to give rise to the splitting by tunneling effect.
If the two configurations cohabit and the ratio of absorption coefficients of a pair is constant, the energy difference (4E0) between both A and B can be gotten by quantifying intensity ratios of the pair at various temperatures.
Interpretation for Nearly Equal C—O Distances
The C—O distances of gaseous dimers of formic and acetic acids have been achieved by using the electron diffraction G. The longer and shorter values for these acids are 1.36 and 1.25A, and the same goes for nicotinic acid are 1.18 and 1.34A, respectively. The differences between these distances are wider than those for the benzoic acid. The nearly equal C—O distances for the benzoic acid, i.e. 1.29 and 1.24A, may be interpreted as averages for the mixture of the two configurations with a larger difference in C—O distances than that actually observed
Finally, in a recall, the infrared spectrum (IR) of benzoic acid is a combination of two properties — infrared spectrum and benzoic acid. While IR spectroscopy or vibrational spectroscopy includes the communication of matter viz: solid, liquid or gas including a range of mechanisms, benzoic acid is the simplest aromatic carboxylic acid, with the name gotten from gum benzoin, that comes about in a natural way, in many plants and becomes an intermediate in the biosynthesis of a lot of secondary metabolites, that is active in salts and used as food preservatives.
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