Correlation charts, reference

The value of infrared spectrometry as a means of identification of unknown compounds and to investigate structural features is immense. Spectra are used in an empirical manner by comparison of samples with known materials and by reference to charts of group frequencies. A simplified correlation chart is shown in Table 9.8. The interpretation of infrared spectra is best considered by discussing the prominent features of a representative series of compounds. 

Attempt to identify the unknown labeled with a letter by matching the spectrum with a spectrum from the IR spectral library. First, take a preliminary look at the spectrum to check for obvious signs of the various functional groups (refer to Table 8.1 or a correlation chart). This will help reduce the number of possibilities. Report your decision regarding its identity to your instructor and justify your decision. 

Process the basic ID H data and find signals representative of a particular type of functional group. Search for characteristic chemical shifts, multiplet structures, signal shapes and check the spectrum for dynamically broadened signals. To confirm your first (tentative) assignments use suitable reference data if available and/or check with standard H correlation charts (see recommended reading). 

For an unknown compound without a reference standard, important structural information can be obtained from the IR spectrum. Fig. 9 is a simplified illustration of the correlation between the absorption frequency incm and the functional groups (a more comprehensive description of this type of correlation chart is given in Ref. ). By observing the presence or absence of certain group frequencies, related to common functional groups such as -OH, -NH2, -CH3, -C=0, -CN, -C-O-C, -COOH, etc., the gross structural features of an unknown compound can be quickly determined. 

To extract structural information from infrared spectra, you must be familiar with the frequencies at which various functional groups absorb. You may consult infrared correlation tables, which provide as much infonnation as is known about where the various functional groups absorb. The references listed at the end of this chapter contain extensive series of correlation tables. Sometimes the absorption information is presented in the form of a chart called a correlation chart. Table 2.3 is a simplified correlation table a more detailed chart appears in Appendix 1. 

Chemical shifts are sensitive to hydrogen bonding and are solvent dependent, as seen in case of pyridine. Consequently, the reference as well as the solvent should always accompany chemical-shift data. No data are given on peptides and other biochemical compounds. All shifts given in these correlation charts are relative to ammonia unless otherwise specified. A section of miscellaneous data gives the chenfical shift of special compounds relative to unusual standards. Reference 17 contains a compilation of publications that involve various nuclei. 

Figure 22 shows a chemical-shift correlation chart for C, illustrating the 5c ranges for functional groups frequently found in polymers [2,16] all shifts are cited relative to the usual reference standard, tetramethylsilane (TMS), at 0 ppm. Aliphatic carbons bonded to silicon (C - Si) also resonate near this point. C-C aliphatic carbons appear at somewhat higher 8c s, between 5 and 50 ppm. As a rule of thumb, 5c increases as the number of protons decreases ... 

While phosphorus is not a common element in polymeric materials, other than in the polyphosphazenes, it does occur in a number of additives, such as secondary antioxidants and some plasticizers. The ehemieal-shift correlation chart of Fig. 26 illustrates that 8p depends largely on the oxidation state of the phosphorus, with P(III) species (such as phosphites) resonating between —450 and +250 and P(V) species (such as phosphates), between —50 and +100 ppm [17,18,60]. These shifts are relative to the reference... 

More recently, the fundamental frequencies of all types of phosphate ions, such as metaphosphates, pyrophosphates, hypo-phosphites, etc., have been studied by several groups of workers, notably by Corbridge [31,32], by Lecomte [33,34] and by Tsuboi [42]. Their findings have been condensed in the form of a correlation chart for phosphorus oxyacids by Corbridge [32], and sufficient data are now available to enable most of the individual types to be differentiated. These absorptions Jiave already been discussed in Chapter 18, to which further reference should be made. The spectra of some phosphate high polymers in molten, glaceous and crystalline states have been reported by Dues and Gehrke [35]. 

The following charts provide characteristic middle-range infrared absorptions obtained from particular functional groups on molecules (Refs. 1 and 2). These include a general mid-range correlation chart, a chart for aromatic absorptions, and a chart for carbonyl moieties. Charts for near infrared absorptions and for inorganic moieties can be found in the cited references. 

An important parameter derived from carbon-13 spectra is the chanical shift. The correlation chart in Figure 6.1 shows typical chanical shifts, listed in parts per million (ppm) from tetramethylsilane (TMS) the carbons of the methyl groups of TMS (not the hydrogens) are used for reference. Approximate chemical shift ranges for selected types of carbon are also given in Table 6.1. Notice... 

In the introductory course, you learned to use four basic kinds of information from NMR spectra number of signals, chemical-shift values, integrated signal area, and splitting patterns. We will now delve furtha- into some of these, particularly the splitting patterns. A brief correlation chart is provided in Table 10.7. More extensive listings can be found in the references. 

The various characteristics of carbon NMR spectra have been discussed in the preceding section. An abbreviated correlation chart is provided in Table 10.10, and more detailed ones are available in the general references and online [29]. A good collection of spectra is available online as well [30]. 

D. Good reference on oxides Nyquist and Kagel, Infrared Spectra of Inorganic Compounds. Pages 206-233 show the spectra of 55 oxides and p. 9 gives a correlation chart for 48 of them. 

Objective Identify the compounds with the help of their characteristic frequencies and the correlation charts. Once the spectrum has been established by comparison to catalog data such as the Coblentz Society Desk Book by C. D. Graver (Part 1 in the Bibliography) or the Aldrich Gas Phase Handbook, refer to the Answers to the Exercises for a detailed discussion. 

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