Absorption frequencies compounds

The role of IR spectroscopy in the early penicillin structure studies has been described (B-49MI51103) and the results of more recent work have been summarized (B-72MI51101). The most noteworthy aspect of a penicillin IR spectrum is the stretching frequency of the /3-lactam carbonyl, which comes at approximately 1780 cm" This is in contrast to a linear tertiary amide which absorbs at approximately 1650 cm and a /3-lactam which is not fused to another ring (e.g. benzyldethiopenicillin), which absorbs at approximately 1740 cm (the exact absorption frequency will, of course, depend upon the specific compound and technique of spectrum determination). The /3-lactam carbonyl absorptions of penicillin sulfoxides and sulfones occur at approximately 1805 and 1810 cm respectively. The high absorption frequency of the penicillin /3-lactam carbonyl is interpreted in terms of the increased double bond character of that bond as a consequence of decreased amide resonance, as discussed in the X-ray crystallographic section. Other aspects of the penicillin IR spectrum, e.g. the side chain amide absorptions at approximately 1680 and 1510 cm and the carboxylate absorption at approximately 1610 cm are as expected. 

Source Specific absorption frequencies vary from compound to compound. 

Table 1 records the frequencies of the carbonyl absorption of compounds III-VI in 0-02 molal carbon tetrachloride solutions, along with the zlp(C=0) values for V and VI. 

Redox potential data frequently correlate with parameters obtained by other spectroscopic measurements. The correlation of E° potentials with gas-phase ionization potentials has already been briefly discussed. Electronic transitions observed by UV-visible spectroscopy involve the promotion of an electron from one orbital to another and this can be viewed as an intramolecular redox reaction. If the promotion involves the displacement of an electron from the HOMO to the LUMO, then the redox potentials for the reduction of the compound, °REd, and for its oxidation, °ox, are of importance. For a closely related series of compounds, trends in oxidation and reduction potentials can be related to shifts in the absorption frequency, v. If the structural perturbation causes the HOMO and the LUMO to rise or fall in energy in tandem, then (E°RED — E°ox) will remain constant in such cases the HOMO—LUMO frequency (energy) will be essentially independent of the structural perturbation. Where there is a differential influence of the perturbation on the HOMO and the LUMO, then ( °red E°ox) will vary as will the energy of the electronic transition. In such cases a linear correlation of °red or E°0x may result. In the limit the energy of the HOMO, or more usually the LUMO, will be unaffected by structural perturbation where the acceptor orbital is pinned, direct linear correlation of E°Gx with v should be apparent. With E°ox and v in a common energy unit, the plot E°0x versus v should have a slope close to one.33-36... 

This technique measures the optical spectrum of light which is diffusely scattered off the catalyst sample. Absorption frequencies are characteristic of certain arrangements of molecules and their environment. Even pure compounds give rather broad diffuse spectra, and catalysts show even broader spectra. Hence, results are only semiquantitative at best. In regards to molybdena catalysts, the information derived with this technique is the coordination environments of Mo and Co in the catalyst. 

Thioformyl and thiocarbonyl cyanides. The long-sought for IR spectrum of this compound was obtained by Wentrup and coworkers162. S=CH—CN was prepared by Flash Vacuum Pyrolysis (FVP) of allyl cyanomethyl sulfide and condensed in an argon matrix. The absorption frequencies(in cm-1) and the intensities (in km mol-1) corresponding to the five bands observed were V2, CN stretch (2221, 17) vj, CH rock (1320, 11) U4, CS stretch (1103, 10) 05, CC stretch (889, 8) vg, CH wag (824, 26). These assignments were made on the basis of MP2/6-31G(d) calculations. The spectrum of formyl cyanide is reported and discussed elsewhere163. 

The characteristic absorption frequencies of this varied group of sulphur compounds may be inferred by reference to the data given in Appendices 2, 3 and 4. 

Figure 6.3 Effects of hydrocarbon chain modifications on melting points of similar-sized cuticular lipids. When lipids melt, the absorption frequency of C-H symmetric stretching vibrations increases from -2849 cm1 to -2854 cm4. From right to left, compounds are (chemical change relative to n-alkane, molecular mass in daltons) filled circles, n-dotnacontane (no change, 450) open circles, palmitic acid myristyl ester (wax ester, 452) filled triangles, 13-methylhentriacontane (methyl-branched alkane, 450) open triangles, (Z)-13-tritriacontene (double bond, 462) filled squares, 9,13-dimethylhentriacontane (2 methyl branches, 464) open squares, oleic acid oleyl ester (2 double bonds and an ester link, 532). Data from Gibbs and Pomonis (1995) and Patel el al. (2001).

Further, the ultraviolet absorption of compounds of this kind does not show the characteristic bathochromic shift. Thus, e.g., trimethyl-conkurchine (9) shows the same absorption as the corresponding saturated tertiary amine (213 mp in ether), whereas the frequency of typical enamines is shifted 28,30,37 to 225-238 m/x. 

As a result, specific kinds of vibration modes always appear at similar frequencies for different molecules then, it is feasible to construct a table of characteristic absorption frequencies. Typical absorption band tables for significant bonds included in polyatomic molecules are reported in ref. [58], Subsequently, using the typical absorption frequencies, it is relatively simple to recognize the occurrence of functional groups in unidentified compounds, and use this information to get structural information about these substances in order to identify them. 

L.C. Thomas and R.A. Chittenden, Characteristic infra-red absorption frequencies of organophos-phorus compounds-I. Phosphorus-oxygen bonds, Spectrochim. Acta, 20, 467-487 (1964). 

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