One- and two-mode behaviour of the (CO) Raman band in isotopic liquid mixtures of N,N-Dimethylformamide. Observation and computational prediction.

After its observation in the phonon spectra of mixed crystals (alkali halides) and molecular crystals (metal-hexacarbonyls), the phenomenon of either one- or two-mode behavior of specific spectral bands has been observed in binary isotopic liquid mixtures. First noticed in the Raman spectrum of the amide I [(C=O)] band of formamide/formamide-d2 liquid mixtures [1], the one-mode behavior consists in the spectral merging of the bands associated with a given normal mode with slightly different intrinsic frequencies in the two neat liquids, to only one band with a frequency situated in between, in contrast to the expected observation of two separate (two-mode) bands attributable to the two species of the mixture. Useless to say that overlap of the two bands and dynamical D/H exchange (occurring in a timescale not accessible to Raman spectroscopy) can be excluded as possible origins of merged bands. Whether one or the other of the two behaviors is observed depends upon the occurrence of more or less strict resonance conditions, i.e., how close are the oscillator frequencies in the two neat liquids respect to the intermolecular vibrational coupling between them and the bandwidth. To understand how the spectral features (resonance conditions and bandwidths) of the two oscillator in the two species of the binary mixtures control the degree of appearance of the one- and two-mode behavior, a computational procedure has been envisaged [2] based on the parametrization of the vibrational modes in a model liquid coupled by transition dipole mechanism (TDC). The application of this procedure to the isotropic and anisotropic spectra of the amide I band of N,N-dimethylformamide/ N,N-dimethylformamide-d1 and to N,N-dimethylformamide(12C=O)/ N,N-dimethylformamide (13C=O) mixtures (xmf=0, 0.25, 0.5, 0.75, 1.0) indicates the occurrence of the one-mode in the former and the two-mode behavior in the latter. The results of preliminary Raman experiments carried out for the amide I band of the N,N-dimethylformamide/ N,N-dimethylformamide-d1 mixture at the same concentrations support the prediction of the one-mode behavior for this mode in this mixture. Experiments on the amide I band of the N,N-dimethylformamide(12C=O)/ N,N-dimethylformamide (13C=O) mixtures at the same concentrations are in progress. REFERENCES [1] Mortensen, A., Faurkov, O.F. , Yarwood, J., Shelley, V., ‘’Vibrational spectra of mixtures of isotopomers of formamide. Anomalies in the carbonyl stretching region ‘’, Journal of Physical Chemistry 98(20), 5221-5226(1994). [2] Torii, H., Osada, Y., Iwami, M., ‘Merged and separate band profiles arising from resonantly coupled vibrational modes of liquid mixtures: theoretical study’, J. Raman Spectrosc. 39, 1592 (2008).