P—O—C Stretching Vibrations

P—O—C (Aromatic) vibrations. Just as there are marked differences between the absorption frequencies of C—O—C (alkyl) and C—O—C (aryl) linkages, so are there marked differences between the two types of P—O—C linkage. 

The differentiation between the two classes of P—O—C linkage has been clearly demonstrated by studies on related series of compounds such as triphenyl, diphenyl-ethyl, phenyl-diethyl and triethyl phosphates. Examination of these materials at the same concentration in the same cell reveals a gradual weakening and ultimate disappearance of a strong band near 1200 cm , whilst, at the same time, a band at 1030 cm that is absent in triphenylphosphate shows a corresponding increase in strength throughout the series [6]. 

On the basis of this and other related series [6, 7], and especially of the work of Nyquist [42] who has correlated the data on over 100 compounds, the P—O—C (aromatic) absorption has been assigned to the 1200 cm region, and it has been shown that a band 

This band is always accompanied by a second absorption which has been attributed to either the symmetric stretch of the P—O—C system (the antisymmetric mode being that at 1200 cm ), or to a separate P—0 stretch which is not so coupled. Thomas [43] strongly favours the latter explanation which is supported by the persistence of this band in both P—O—P and P—OH compounds, and by the fact that in the latter the frequency is a linear function of the tt values of the substituents, Nyquist [42] places this band between 994 and 914 cm and notes that the band is at a lower frequency in trivalent phosphorus compounds. Thomas [43] widens this range to 905—996 cm for pentavalent compounds and suggests 860—850 cm for trivalent. He points out that this band is often complex and the values quoted therefore refer to the strongest band. 

In unsaturated esters of the type P—O—C=C or P—O—C the same correlation applies. 

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A very strong band occurs at 1050-970 cm" in all compounds having the P—0—C (aliphatic) link. This band is probably due to an asymmetric P—O—C stretching vibration. In most ethoxy phosphorus compounds this band is a doublet. In methoxy and ethoxy phosphorus compounds a second strong band appears at 830-740 cm" probably due to symmetric P—0—C stretching. This band is usually absent in other alkoxy phosphorus compounds. 

Functionalized monosaccharides, peracetylated aryl glycosides for example, exhibit bands in the 800-1000 cm region and are characteristic for differentiating a- and p anomers. The relative intensities of C-O-C stretching vibrations in the 1000-1100 cm region and a band near 300 cm for -anomers are criteria for differentiating anomeric peracetylated alkyl and aryl glycosides [11]. 

Huisken, F., Kulcke, A., Laush, C., and Lisy,. 1. M., Dissociation of small methanol clusters after excitation of the O—H stretch vibration at 2.7 p, J. Chem. Phys. 95, 3924-3929 (1991). Bleiber, A., and Sauer, J., The vibrational frequency of the donor OH group in the H-bonded dimers of water, methanol and silanol. Ab initio calculations including anharmonicities, Chem. Phys. Lett. 238, 243-252 (1995). 

Far-Infrared Bands and Shifts of the vC=C Stretching and SC-H o.p. Bending Vibrations of Benzene upon Complexation (171)... 

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