Stretching frequencies, infrared, 372 Table

Use the infrared stretching frequencies in Table 12.4 to distinguish between different classes of organic compounds. 

Infrared stretching frequencies for a range of 2-aryl-6-phenyl-47/-thiopyran-4 -ones, -thiones, and oximes and some related 1-oxides and 1,1-dioxides have been reported (Table 14) <1993PS(81)101>. 

Table 9 Infrared stretching frequencies for exocyclic double bonds on small rings...

In Table VII are recorded mean values for in a number of ethylene-metal carbonyl complexes and parent metal carbonyls as well as values for the double-bond infrared stretching frequency rc c the magnetic shielding parameter t for ethylene in those transition metal complexes for which data are available. Although with the metal carbonyl complexes, differences of geometry, oxidation state, etc., do not permit a correlation to be drawn between the absolute values of rco and for the various complexes, it is quite apparent from the tabulated data for the Mo, Mn, and Fe complexes that for a given metal. 

Infrared studies on DjO solutions of carbamyl phosphate led to the same conclusioa Table 19.7 shows infrared bands of acetyl phosphate and carbamyl phosphate in the presence and absence of Mg ions. The carbonyl group in acetyl phosphate dianion and carbamyl phosphate dianion absorbs at 1708 and 1666 cm , respectively. Table 19.7 shows that the positions of these bands are increased only slightly in the presence of Mg ions. These very small increases contrast sharply with large decreases in the infrared stretching frequencies of the carbonyl group when it coordinates with metallic ions. 

Table 12.4 Infrared Stretching Frequencies of Some Typical Bonds...

Infrared spectroscopy can also be used to probe resonance in carboxylic acid derivatives. The dipolar resonance structure weakens the C=0 bond and causes a corresponding decrease in the carbonyl stretching frequency (Table 20-2). The IR data for carboxylic acids reported in Section 19-3 refer to the common dimeric form, in which hydrogen bonding reduces the stretching frequencies of both the 0-H and C=0 bonds to about 3000 and 1700 cm respectively. A special technique—vapor deposition at very low temperature—allows the IR spectra of carboxylic acid monomers to be measured, for direct comparison with the spectra of carboxylic acid derivatives. Monomeric acetic acid displays vc=o at 1780 cm similar to the value for carboxylic anhydrides, higher than that for esters, and lower than that of halides, consistent with the degree of resonance delocalization in carboxylic acids. 

A criterion for the position of the extent of the mesomerism of type 9 is given by the bond order of the CO bond, a first approximation to W hich can be obtained from the infrared spectrum (v C=0). Unfortunately, relatively little is known of the infrared spectra of amide anions. How-ever, it can be assumed that the mesomeric relationships in the anions 9 can also be deduced from the infrared spectra of the free amides (4), although, of course, the absolute participation of the canonical forms a and b in structures 4 and 9 is different. If Table I is considered from this point of view, the intimate relationship betw-een the position of the amide band 1 (v C=0) and the orientation (0 or N) of methylation of lactams by diazomethane is unmistakeable. Thus the behavior of a lactam tow ard diazomethane can be deduced from the acidity (velocity of reaction) and the C=0 stretching frequency (orientation of methylation). Three major regions can be differentiated (1) 1620-1680 cm h 0-methylation (2) 1680-1720 cm i, O- and A -methylation, w ith kinetic dependence and (3) 1730-1800 em , A -methylation, The factual material in Table I is... 

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