Interpretation of an IR-spectrum

There exist no hard and fast rules with regard to the interpretation of an IR-spectrum, but based on the vast wealth of experience and wisdom of the analyst amalgamated with a storehouse of general observations go a long way towards the exact interpretation of the same. However, following different aspects must be taken into consideration while interpreting the spectrum ... 

Give a comprehensive account on the interpretation of an IR-Spectrum. Explain. 

A precise treatment of the vibrations of a complex molecule is not feasible thus, the IR spectrum must be interpreted from empirical comparison of spectra and extrapolation of studies of simpler molecules. Many questions arising in the interpretation of an IR spectrum can be answered by data obtained from the mass and NMR spectra. 

The complete interpretation of an IR spectrum is difficult because most organic molecules have dozens of different bond stretching and bending motions, and thus have dozens of absorptions. On the one hand, this complexity is a problem because it generally limits the laboratory use of IR spectroscopy to pure samples of fairly small molecules— little can be learned from IR spectroscopy... 

The first approach to interpretation of an IR spectrum is to consider the spectrum as the superposition of a number of group frequencies [35]. The second is to confirm the existence of specific chemical groups by looking for other modes of this group, that is, bending, twisting, and wagging modes. 

In an attempt to aid interpretation of the IR spectrum of MbCO we decided to model the full protein by use of a hybrid quantum mechanics/molecular mechanics approach (QM/MM), to evaluate changes in the CO stretching frequency for different protein conformations. The QM/MM method used [44] combines a first-principles description of the active center with a force-field treatment (using the CHARMM force field) of the rest of the protein. The QM-MM boundary is modeled by use of link atoms (four in the heme vinyl and propionate substituents and one on the His64 residue). Our QM region will include the CO ligand, the porphyrin, and the axial imidazole (Fig. 3.13). The vinyl and propionate porphyrin substituents were not included, because we had previously found they did not affect the properties of the Fe-ligand bonds (Section 3.3.1). It was, on the other hand, crucial to include the imidazole of the proximal His (directly bonded to the... 

Interpretation of the IR spectrum of an unknown compound is an art that requires experience and practice. The more spectra you examine, the easier it will become to recognize the absorption due to an O—H group and to differentiate between that band and one that results from an N—H group. 

The characteristic pattern of an IR spectrum, particularly of the fingerprint region between 1500 and 1000 cm ", can be used for reliable identification of a molecule. This appears to promise an easy automation of the interpretation of IR spectra. An important factor that prevents an easy automation of the interpretation process is the fuzziness of peak positions. For example, the position of a carbonyl peak can vary from 1650 to... 

Thus the prediction of the position of an amide carbonyl is one of the more difficult calls in the interpretation of an unknown spectrum. As these systems are seldom observed in solution (usually very insoluble in most nonpolar solvents used in IR sampling), the following comments may be of value. Studies of amide carbonyl frequencies in dilute nonpolar solution indicate that H-bonding effects are largely responsible for the low frequencies observed with primary and secondary amides but obviously play no role in tertiary amides. 

Spectrometers are designed to measure the absorption of electromagnetic radiation by a sample. Basically, a spectrometer consists of a source of radiation, a compartment containing the sfflnple through which the radiation passes, and a detector. The frequency of radiation is continuously varied, and its intensity at the detector is compar ed with that at the source. When the frequency is reached at which the sample absorbs radiation, the detector senses a decrease in intensity. The relation between frequency and absorption is plotted as a spectrum, which consists of a series of peaks at characteristic frequencies. Its interpretation can furnish structural information. Each type of spectroscopy developed independently of the others, and so the data format is different for each one. An NMR spectrum looks different from an IR spectrum, and both look different from a UV-VIS spectrum. 

As each functional group is discussed in future chapters, the spectroscopic properties of that group will be described. For the present, we ll point out some distinguishing features of the hydrocarbon functional groups already studied and briefly preview some other common functional groups. We should also point out, however, that in addition to interpreting absorptions that ore present in an IR spectrum, it s also possible to get structural information by noticing which absorptions are not present. If the spectrum of a compound has no absorptions at 3300 and 2150 cm-1, the compound is not a terminal alkyne if the spectrum has no absorption near 3400 cm -, the compound Is not an alcohol and so on. 

Compared to gc-mfe, gc-ir is much less sensitive. Whereas a mass spectrum can be recorded from as little as 10"10 g of sample, at least I0 4 g is required for an ir spectrum. Care is required in the interpretation of vapour phase ir spectra as they differ in certain respects from the corresponding liquid or solid phase spectra. Rotational fine structure may appear/band positions may be... 

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