<b>Dimer and Trimer in Pyridine–Ethanol Mixture Reinvestigated Applying the Scanning Multi‐Channel Raman Difference Technique and AM1 Molecular Orbital Calculations</b>

High-precision measurements on the ν1 (ring-breathing) mode in pyridine–ethanol mixtures with varying mole fraction of the reference molecule pyridine were made using a device for multi-channel and scanning multi-channel Raman difference spectroscopy (SMT-RDS) with CCD detection developed recently in this laboratory. The high quality of the data permits one to make precise estimates of wavenumber shifts, linewidth (FWHM) changes and integrated intensities. The spectral features observed in the range 970–1010 cm−1 in mixtures having different molar ratios of C5H5N and C2H5OH were assigned to ‘free’ C5H5N, dimer [C5H5N···(HOC2H5)] and trimer [C5H5N···(HOC2H5)2]. These three species co-exist in the mixture along with the solvent in concentration-dependent ratios. Geometry optimization and vibrational frequency calculations performed using the AM1 molecular orbital method were used to interpret the experimental observations.

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