MODELING PROTON-BOUND METHANOL, AMMONIA, AND AMINE COMPLEXES OF 12-CROWN-4-ETHER AND DIMETHOXYETHANE (GLYME) USING DENSITY-FUNCTIONAL THEORY

The association reactions undergone by 12-crown-4-ether, 12c4H(+), wit h NH3, CH3OH, CH3NH2, (CH3)(2)NH, and (CH3)(3)N have been studied usin g the B3LYP density functional method and a variety of basis sets. For comparison purposes the insertion reactions for the same bases into p rotonated dimethoxyethane (''glyme''), Gl.H+, and protonated glyme dim er, (Gl)(2)H+, have also been modeled. The B3LYP/aug-cc-pVDZ//B3LYP/4- 21G() level of theory was found to be a particularly favorable compro mise between accuracy and computational expense for the calculation of proton affinities of medium-sized species. The protonated glyme, Gl.H + the protonated glyme dimer, (Gl)(2)H+, and the protonated crown ethe r, 12c4H(+), form two internal hydrogen bonds with NH3, CH3OH, CH3NH2, and (CH3)(2)NH, except for (Gl)(2)H+. NH3 which has four O ... H bond s. In Gl.NH(CH3)(3)(+), there is a single O ... H bond and the protons of the methyl groups assist weakly in O ... HC bonding. The insertion energy of methanol, ammonia, and the series of amines into 12c4H(+) i ncreases with increasing proton affinity of the inserting base. A simi lar trend is observed for insertion into (Gl)(2)H+. Trimethylamine doe s not follow the expected trend because it forms proton-bound complexe s that have a single O ... HN bond instead of two. The association ene rgy of CH3OH2+, NH4+, etc., with 12c4 or Gl(2) decreases with increasi ng proton affinity (of methanol, ammonia, etc.).