C-N bond stretching

IR spectroscopy was mainly used for the identification of specific absorptions caused by an N-H single bond and endo- or exocyclic double bonds. The endocyclic C=N bond stretching was observed in the IR spectra of compounds 39 and 48 at 1612 and 1590 cm-1, respectively <1997CC1553, 1995TL8187>, while the exocyclic C=N bond of 43 gave an absorption at 1617 cm-1 <19960M753>. The IR spectrum of the compound 60 contained characteristic absorption bands at 3271 (N-H) and 1372, 1154 cm-1 (S02) < 1999S1731 >. 

As a result of this strain, C N bond stretching in the decomposition of such a compound is believed to be more advanced in the transition state than is usual, so that the elimination acquires some of the character of an Ei process. Product stability, particularly in respect of jr-overlap, is therefore reflected in the transition state to an unusual degree, and the A -isomer is formed more readily than the less stable A -olefin cf. p. 105). 

The Sn-F bond stretching vibrations appear in the IR spectra for trifluoroboron-capped clathrochelates from 300 to 500 cm-i. The Sn-Br bond stretching vibrations are usually observed at much lower frequencies and, therefore, were not found in the range studied. Numerous intense bands from 2800 to 3200 cm- for these complexes are essentially intensified and shifted toward low frequencies. The C=N bond stretching vibrations appear at 1568-1580 cm- for all tin-capped clathrochelates except for glyoximates (their IR spectra contain vc=n bands at 1534-1544 cm- , Table 25) [71]. 

Similarly, a C=0 bond stretches at a higher frequency (-1700 cm" ) than does a C—O bond (-1100 cm" ), and a C=N bond stretches at a higher frequency (-2200 cm" ) than does a C=N bond (-1600 cm" ), which in turn stretches at a higher frequency than does a C—N bond (-1100 cm" ) (Table 14.4). 

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