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Why the ΔνCH blue shift is larger in chloral than in dichloroacetyl chloride dimers?


  • Roman Szostak

    Corresponding author
    1. Department of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland
    • Department of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland.
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Raman spectra of the Cl3CCHO/CCl4 and Cl3CCHO/C6D12 binary systems were recorded as a function of the mole fraction. Features originating from self-aggregates of chloral (trichloroethanal, trichloroacetaldehyde—TCAA) molecules were detected in different spectral regions. The most pronounced changes were observed in the vicinity of the ν(C[DOUBLE BOND]O) and ν(C[BOND]H) stretching vibration bands. Using two-dimensional correlation spectroscopy (2D-COS), evolving-factor analysis (EFA) and multivariate curve resolution (MCR), dimer bands were identified, and their positions were determined. The ν(C[BOND]H) stretching vibration band in dimers was blue-shifted by nearly 18 cm−1, whereas the ν(C[DOUBLE BOND]O) dimer band was red-shifted by more than 5 cm−1. For these bands, the observed shifts were accompanied by an almost twofold change in the bandwidth, from approximately 19 and 6 cm−1 for dilute solutions (x = 0.05) to 36.6 and 11.5 cm−1, respectively, in pure TCAA. The formation of dimers was confirmed by multivariate analysis of the Raman spectra of chloral recorded as a function of temperature. Analogous analysis of dichloroacetyl chloride (DCAC) spectra gave an 8.9 cm−1 blue shift for the ν(C[BOND]H) vibration band and − 5.5/− 10.1 cm−1 shifts for the ν(C[DOUBLE BOND]O) stretching vibrations of the two conformers present. To facilitate the interpretation of experimental findings, the optimized geometries and vibrational wavenumbers of the Cl3CCHO/HCl2CCClO molecules and (Cl3CCHO)2/(HCl2CCClO)2 dimers were calculated at the B3LYP/6-311 + + G(3df,3pd) level. Copyright © 2010 John Wiley & Sons, Ltd.