Carbon-carbon triple bonds as nucleophiles: Adducts of ethyne and propyne with boron trifluoride

The mid-infrared (4000-400 cm(-1)) spectra of ethyne/BF3 and propyne/BF3 mi xtures, dissolved in liquefied argon are discussed. In all spectra, the for mation of a 1:1 van der Waals complex was observed, while at higher concent rations of BF3 absorption bands of a 1:2 species between propyne and BF3 we re also observed. Using spectra recorded at several temperatures between 96 and 125 K, the complexation enthalpy Delta H degrees for ethyne . BF3 was determined to be -9.3(4) kJ mol(-1). For propyne . BF3 and propyne .(BF3)(2 ) the Delta H degrees values are -13.2(4) and -19.8(12); kJ mol(-1), respec tively. For the 1:1 complexes the structures and harmonic vibrational frequ encies were calculated at the MP2/6-31+G(d) level, while for the 1:2 adduct similar calculations were carried out at the corresponding RHF level. Usin g a SCRF/SCLPCM scheme to correct for the solvent influences, and statistic al thermodynamics to account for the zero-point vibrational and thermal con tributions, approximate values for the gas-phase complexation energy were o btained from the experimental enthalpies. The resulting values, -14.3(10) k J mol(-1) for ethyne . BF3, -18.9(10) kJ mol(-1) for propyne . BF3, -30.5(1 2) kJ mol(-1) for propyne .(BF3)(2), are compared with the ab initio values .