Investigation of the C-Cl Bonds in Trichloroacrylic Acid, Cl2C = CClCOOH, by Single Crystal Zeeman Split NQR

The Cl NQR spectrum of trichloroacrylic acid, Cl C = CClCOOH, was studied by single crystal Zeeman NQR spectroscopy at 303 K (high temperature phase I) and at 275 K (low temperature phase II). At 303 K for the = CCl group chlorines it was found eQΦ Cl ) = 76.179 MHz, η ( Cl) = 0.1597; eQΦ h ( Cl )= 74.693 MHz, η( Cl) = 0.1868 and for the = CClC - group chlorine eQΦ h Cl ) = 75.145 MHz, η ( Cl) = 0.1568. The difference in the electric field gradients at the stereochemically inequivalent chlorines Cl and Cl is rather large. The asymmetry parameter η ( Cl ) shows an unusual temperature dependence which probably originates from the change of the hydrogen dynamics and position within the dimeric H-bonded unit (Cl C = CClCOOH) during the phase transition