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

Abstract Raman spectra of the Cl 3 CCHO/CCl 4 and Cl 3 CCHO/C 6 D 12 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(CO) and ν(CH) 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(CH) stretching vibration band in dimers was blue‐shifted by nearly 18 cm −1 , whereas the ν(CO) 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(CH) vibration band and − 5.5/− 10.1 cm −1 shifts for the ν(CO) stretching vibrations of the two conformers present. To facilitate the interpretation of experimental findings, the optimized geometries and vibrational wavenumbers of the Cl 3 CCHO/HCl 2 CCClO molecules and (Cl 3 CCHO) 2 /(HCl 2 CCClO) 2 dimers were calculated at the B3LYP/6‐311 + + G(3df,3pd) level. Copyright © 2010 John Wiley & Sons, Ltd.