INFRARED SPECTROPHOTOMETRY AND ORGANIC CHEMISTRY
Infrared Spectrophotometry and Organic Chemistry
Infrared (IF) Spectrophotometry is designed to determine the sample by measuring infrared radiation absorption of wavenumbers within the region of 4,000-400 cm-1 at different wavenumbers, as it passes through the sample. This technique employs the property that the IF absorption spectrum of the material is typical of its chemical structure. IR spectra are demonstrated in charts drawn via plotting on the abscissa and the absorbances or transmittances on the ordinate. Except when indicated, when the spectrum of the sample is the same in the intensity of absorption at a similar wavenumber to the spectrum of the reference standard, the sample is recognized as a concordant with the anticipated material (Adams, 2014).
Consistent with Gailey (2011) findings, IR absorption bands are presented as wavelengths (λ) or wavenumbers (V). Wavenumber represents the number of waves per unit length. Therefore, the wavenumbers are directly relative to frequency, in addition to the IR absorption energy. The wavenumber unit (cm-1) is more regularly used in contemporary IR devices that are linear in the cm-1 scale. Contrarily, wavelengths are contrariwise proportional to frequencies and their related energy. Presently, the recommended unit of wavelength is micrometers (μm) (Bradshaw & Monk, 2012).
Figure 2: Infrared spectroscopy (Ewing, 2013, p. 15).
At temperature beyond absolute zero, all atoms within the molecule are in constant vibration relating to one another. When the frequency of a specified vibration is equivalent to the frequency of infrared radiation focused on the molecule, the molecule will absorb the radiation. Every atom comprises three degrees of freedom in proportional to the motions along any of the 3 Cartesian coordinate axes (including x, y and z) (Griffiths, 2015). A polyatomic molecule of atoms contains 3n total degrees of freedom. Nevertheless, 3 degrees of freedom are needed to define translation, the mobility of the whole molecule via space. As well, 3 degrees of freedom corresponds to rotation of the whole molecule. Thus, the other 3n-6 degrees of freedom become true, fundamental vibrations for the nonlinear molecule. Linear molecule has 3n-5 fundamental vibrational methods as only degree of freedom is enough to describe a rotation. Among the 3n-5 or 3n-6 fundamental vibrations (similarly called normal vibration modes), those that generate a net transformation in the dipole moment can cause an IR action, and those that provide polarization changes can result in Roman activity. As expected, some vibrations may be both Roman and IR active (Ewing, 2013).
The number of resultant absorption bands is different from the overall number of fundamental vibrations. It is diminished since some are not IR active and one frequency may contribute multiple modes of motion to occur (Debruin, 2012). On the contrary, more bands are produced by the appearance of overtones (essential multiples of the fundamental absorption frequency), variations of fundamental frequencies, combinations of fundamental frequencies, coupling interplays between 2-fundamental absorbance frequencies, and coupling interplays between overtones and fundamental vibrations or combination bands (such as Fermi resonance). The intensity of combinations, overtones, and multiple bands are below those of fundamental bands. The blending and combination of each factor hence form a unique IR spectrum for all the compounds (Adams, 2014).
Advantages and Disadvantages of Nacl Plates and Kbr Pallets in Sample Preparation of IR
For numerous organic compounds, NaCl functions well although it is prone to attack from moisture. For metal synchronization complexes KBr characteristically works appropriately to their huge windows. An amount of NaCL and KBr are utilized in IR sample preparation because they both soft salts. Also, there is no overlapping signal from the matrixes (Bradshaw et al., 2012).
The only disadvantage of KBr and NaCl is that they are hydroscopic. Due to this, it is often a good concept to get a spectrum run as Nujol mull on the samples too (as cited in Mcdowell, 2014, p. 69).
Overtone Bands
Overtones bands are the absorption of energy contributed by a shift of instead of 1 within the vibrational quantum number. Whereas overtones are often forbidden changes and thus are weakly absorbing, they provide additional bands than projected. Overtones are easily determined by the availability of powerfully absorbing fundamental change at considerably more than half the frequency of the overtone. Because the energy is proportionate to the frequency absorbed, and this is proportionate to the wavenumber, the initial overtone will be observed in the spectrum at two times the wavenumber of the fundamentals (Jayne, 2014).
Roman Spectroscopy
It is a method centered on inelastic scattering of monochromatic light, often from laser sources. In elastic scattering refers to the frequency of the photons within monochromatic transitions after interaction with a sample (Gailey, 2011). Photons of the laser light get absorbed and later reemitted. The reemitted photon's frequency is moved up and down compared with an initial monochromatic frequency that is known as the Roman Effect. This movement provides detail concerning rotational, vibrational and other low-frequency transitions within molecules. The Roman spectroscopy may be used to analyze liquid, solid and gas samples (Griffiths, 2015).
According to Philips (2011) studies, a Roman spectrum encompasses three parts, the strong Rayleigh line and less strong Roman bands within the strokes (low energy, red shifted) and Anti-Strokes (high-energy, blue shifted) parts of the spectrum, in which that latter two parts have equal energy. Only the stroke bands are measured since they are easily detectable because of the higher intensities. The Rayleigh line adds to 0 Roman shifts so that Anti-Strokes lines possess negative wavenumbers and stroke likes possess positive wavenumbers. The wavenumber shifts are typical for material.
Figure 2: Roman transition schemes (Jayne, 2014, p. 180).
Why Aspirin Follows a Pseudo- First-Order Kinetic
Hydrolysis of the drug substance could be a principal factor in the solutions instability. Aspirin, for instance, goes through hydrolysis with the resulting degradation products becoming acetic acid and salicylic acid.
Aspirin → Salicylic Acid + Acetic Acid
When salicylic acid is mixed with ferric ion, it shows a characteristic purple color. The kinetic of hydrolysis was identified to follow pseudo-first-order reaction, because among the polar solvents examined, aspirin was the most stable in phosphate buffer with the half –life of about 537.21 ± 8.42 hrs., and lowest stability in water/glycerol system (of half-life, 155.31 ± 2.33 hrs.). Boric acid buffer and ten-percent dextrose solution hydrolyzed aspirin to a similar extent.
Reason for pH dependence in fluorescence dependence on quinine structure
The pH influences the molecule structure which indirectly influences the fluorescence intensity. At low pH, quinine will be available as a deprotonated molecule. When the pH rises from 2.48 to 3.75, a rise in fluorescent is showed. This is due to as pH rises, there are more quinine molecules within the solution has a change to monoprotonated. This suggests that the monopronated state of quinine comprise a higher fluorescence intensity value greater than that of the disprotonated state. Nevertheless, as the pH surpasses 3.75, a rise in pH value causes a drop in fluorescence intensity. This is due to as the pH rises further, increasingly monoprotonated quinine be totally deprotonated. This means that the deprotonated form contains lower fluorescence intensity in comparison with the monoprotonated state (Fiorenza & Bonomi, 2013).
Why radiation from fluorescence is measured at 90 degrees from the excitation radiation
Fluorescence refers to a phenomenon where photons absorbed by particles or molecules excite that molecule. Then, as the molecule emits that extra energy, photons are radiated at a slightly longer wavelength. The reason fluorescence is measured at 90 degrees is to be accurate and sure you are looking at it, instead of any energy passing through. Thus, to get accurate measurements, go for the maximum all the time (Jayne, 2014).
References
Adams, M. L. (2014). Quantitative mixed solvent analysis by infrared spectrophotometry. Ft. Belvoir: Defense Technical Information Center.
Philips, L. K. (2011). British pharmacopoeia: Infra-red reference spectra. (2011). London: Her Majesty's Stationery Office.
Bradshaw, T., & Monk, P. M. (2012). Chemistry for the biosciences: The essential concepts. Oxford: Oxford University Press.
Debruin, H. J. (2012). Determination of Traces of Oxygen in Sodium Metal by Infrared Spectrophotometry. Analytical Chemistry Anal. Chem., 32(3), 360-362. Retrieved September 5, 2015.
Ewing, G. W. (2013). Topics in chemical instrumentation; a volume of reprints from the journal of chemical education. Easton, PA: Chemical Education Pub.
Fiorenza, A., & Bonomi, G. (2013). Identification of Elastomers by Infrared Spectrophotometry. Rubber Chemistry and Technology, 36(4), 1129-1147. Retrieved February 29, 2016.
Gailey, J. A. (2011). Determination of molecular orientation in polymer films by infrared spectrophotometry. Analytical Chemistry Anal. Chem., 33(13), 1831-1834. Retrieved February 29, 2016.
Griffiths, P. R. (2015). Automated measurements of infrared spectra of chromatographically separated fractions. Athens, GA: Environmental Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency.
Jayne, J. (2014). Hydration numbers by near-infrared spectrophotometry. Hidden and erroneous assumptions. The Journal of Physical Chemistry J. Phys. Chem., 87(3), 527-528. Retrieved February 29, 2016.
Mcdowell, R. S. (2014). Determination of carbon-13 by infrared spectrophotometry of carbon monoxide. Analytical Chemistry Anal. Chem., 42(11), 1192-1193. Retrieved February 18, 2015.
