C-0 stretching frequency

The C=0 stretching frequency in 2 hexa none appears at 1720 cm To view this VI bration on Learning By Mod elmg select the calculated value of 1940 cm ... 

The general trends of structural variation on the position of C=0 stretching frequencies may be... 

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... 

C = 0 stretching frequencies are typically lowered by about 20-30 cm 1 from the values given when the carbonyl group is conjugated with an aromatic ring or an alkene group. 

Carbon monoxide on metals forms the best-studied adsorption system in vibrational spectroscopy. The strong dipole associated with the C-O bond makes this molecule a particularly easy one to study. Moreover, the C-0 stretch frequency is very informative about the direct environment of the molecule. The metal-carbon bond, however, falling at frequencies between 300 and 500 cm1, is more difficult to measure with infrared spectroscopy. First, its detection requires special optical parts made of Csl, but even with suitable equipment the peak may be invisible because of absorption by the catalyst support. In reflection experiments on single crystal surfaces the metal-carbon peak is difficult to obtain because of the low sensitivity of RAIRS at low frequencies [12,13], EELS, on the other hand, has no difficulty in detecting the metal-carbon bond, as we shall see later on. 

We illustrate the sensitivity of the C-0 stretching frequency for the bonding configuration with a perhaps somewhat dated but still very instructive study of the adsorption sites of alloy surfaces. Soma-Noto and Sachtler [18] reported an infrared investigation of CO adsorbed on silica-supported Pd-Ag alloys some of their spectra are shown in Fig. 8.5. On pure palladium, CO adsorbs mainly in a twofold position, evidenced by the intense peak around 1980 cm 1, although some CO appears to be present in threefold and linear geometries as well. This is a common feature in adsorption studies on supported catalysts, where particles exhibit a variety of surface... 

Adsorption of 0.05 monolayers (ML) of CO on this surface gives rise to a peak at 2015 cm-1 corresponding to the internal C-0 stretch frequency of the molecule in the on-top adsorption site and one at 470 cm-1 due to the metal-molecule bond. The latter is not easily observable in infrared spectroscopy. Increasing the CO coverage to 0.33 ML enhances the intensity of the HREELS peaks. In addition, the C-O stretch frequency shifts upward because of dipole-dipole coupling [16, 17]. The LEED pattern corresponds to an ordered (V3xV3)R30° overlayer in Wood s notation (see the Appendix) in accordance with the coverage of 0.33 ML. 

The top curve shows the spectrum of adsorbed CO that is observed when no nitrile compound is added to the electrolyte. The C-0 stretching frequency occurs at 2085 cm, which is characteristic of a saturated CO adlayer at this potential. The next three spectra were recorded in solutions which contain 1.0 M CH.CN, 0.2 M C-H.CN, and 0.1 M HOOCCH.CN, respectively. The intensity of the vfcO) band is reduced about 50% in each case. This indicates that the amount of CO adsorbed on the electrode is reduced by the... 

The negative VCD observed in dilute solution at the urethane C==0 stretching frequency, and the negative bias observed at higher concentrations is due to an intrinsic contribution from the urethane carbonyl stretch. In either the cis or trans conformation, the urethane C=0 stretch can generate current around an... 

Rh-Rh vector may donate electron density to the empty Rh-Rh (T -orbital and/or overlap through back-bonding with either of the fiUed n MOs [103]. Infrared measurements of Rh2f4-CO adducts demonstrate clearly a red shift in the C=0 stretching frequency relative to imhgated carbon monoxide [104]. These results are consistent with electron delocalization from the Rh-Rh n into the low-lying Similarly, back-... 

Table 7-6 Mean Absolute Errors in C=0 Stretching Frequencies ...

The different C=0 stretching frequencies of the two systems is a reflection of the stronger basicity of the pyran-4-one. The effects of substitution on this absorption band are smaller in pyran-4-ones, and this feature has been attributed to a more significant contribution of the aromatic structure (100) to the hybrid structure than the contribution of (101) to the structure of pyran-2-one (B-77MI22201). 

The C=0 stretching frequency of a number of 4-hydroxy- and 4-alkoxy-coumarins, including several anticoagulant drugs, has been identified by isotopic replacement of the carbonyl carbon atom by 3C as the highest frequency band in the 1750-1550 cm-1 region. Introduction of the isotopic atom causes a reduction in the C=0 frequency of ca. 30 cm-1 (82JHC475). 

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