VIBRATIONAL-SPECTRA OF MIXTURES OF ISOTOPOMERS OF FORMAMIDE - ANOMALIES IN THE CARBONYL STRETCHING REGION

The infrared and Raman spectra of liquid mixtures of HCONH2 and HCOND2 are presented. Contrary to what is expected, the isotropic Raman spec tra of the mixtures show only one carbonyl stretching band, whose posi tion depends upon the composition of the mixture. It is shown that the apparent collapsing of bands is not due to fast exchange between diff erent ''sites'' (i.e. the fast exchange limit). Similar phenomena have been observed previously in solids. Thus, the phenomenon might reflec t ''solid-like'' behavior of liquid formamide close to its melting poi nt. The depolarized component of the Raman Spectrum and the IR spectru m of the carbonyl stretching band are observed at approximately the sa me frequencies, whereas the isotropic components are observed at a low er frequency (noncoincidence). The same is observed for the NH2 bendin g mode. The noncoincidence splitting of this mode is shown to follow a model for binary mixtures of isotopomers developed by Logan. The asym metry of the IR and Raman bands can be explained in terms of two ''sit es'', one belonging to a hydrogen-bonded species and the other to a '' free'' species without hydrogen bonding to the carbonyl group, but may be forming hydrogen bonds to the carbonyl group of other formamide mol ecules.