The Complex Formation of Boron Fluoride with Lower Olefin at Low Temperatures. I. The Isotope Effects and Absorption Spectra of Boron Fluoride-Propylene and -Ethylene Systems

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The equilibrium constant of the isotopic exchange between boron fluoride gas and boron fluoride in a propylene solution at －112°C has been determined by means of a shaking-type gas-liquid contactor. When the mole ratio of propylene to boron fluoride in the liquid phase was larger than 1, the value of the constant obtained was 1.018. In the boron fluoride-ethylene system, the value of the constant is equal to the vapor pressure ratio of the isotopic boron fluorides, i.e., 1.011. In the measurement of the absorption spectrum of liquid propylene at low temperatures, two weak absorptions were observed at 43650 and 45000 cm－1. They were assigned to the TR←N transition. When boron fluoride was added to liquid propylene at low temperatures, these absorption bands disappeared, although the V←N bands were still observed. The above findings lead to the following conclusion: boron fluoride can form stable liquid σ-complexes with polar mono-olefins such as propylene at low temperatures; these complexes are not the intermediate products in polymerization by a boron fluoride catalyst. On the other hand, ethylene, which is a non-polar mono-olefln and which does not polymerized at atmospheric pressure, cannot form a stable σ-complex with boron fluoride, even at low temperatures. The authors wish to express their deep thanks to Dr. Shizu Tonomura for her kind advice on measuring the absorption spectra.
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