Hydrogen Bonding of Secondary Amides. the Cyclic Dimerization of Cis Isomers
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چکیده
(1) (2) merism of the amide group has been the subject of many spectroscopic studies.2 There has also been widespread interest in the self-association of N-methylacetamide, which is predominantly trans,l into linear hydrogen-bonded chains.3 The self-association of cis amides has only motivated one study,4 in which Bhaskar and Rao compared the hydrogen bonding properties of hr-methylacetamide with those of N-phenylurethane, which was presumed to be 95% cis.1 Their failure to observe any clear-cut difference between the hydrogen bonding behaviour of the two amides is readily explained by revised spectroscopic and theoretical data, which show that N-phenylurethane actually exists as the trans isomer.5~6 Indeed, no noncyclic secondary amide is known to occur exclusively in the cis form, but there are several for which the cis and trans isomers coexist in significant proportions. The self-associations of two of these, formanilide and o-methylformanilide, have been investigated in CC14 using infrared spectroscopy. The fundamental nonbonded V N H bands of secondary amides occur at 3400-3440 and 3430-3470 cm-1 for the cis and trans isomers respectively,l and bonded N-H stretching peaks are observed near 3080 and 3200 om-1 (cis) and 3300 cm-1 (trans) in related compounds.2 The infrared spectrum of o-methylformanilide, which has been interpreted by Siddall et a1.,7 has peaks in each of these regions, but as the concentration is lowered the bonded V N H peaks due to cis association persist after the corresponding trans peak vanishes.
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تاریخ انتشار 2005