Modeling proton-bound methanol, ammonia, and amine complexes of 12-crown-4-ether and dimethoxyethane ("glyme") using density functional theory

Abstract

The association reactions undergone by 12-crown-4-ether, 12c4H + , with NH 3 , CH 3 OH, CH 3 NH 2 , (CH 3) 2 NH, and (CH 3) 3 N have been studied using the B3LYP density functional method and a variety of basis sets. For comparison purposes the insertion reactions for the same bases into protonated dimethoxyethane ("glyme"), Gl‚H + , and protonated glyme dimer, (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 compromise 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 ether, 12c4H + , form two internal hydrogen bonds with NH 3 , CH 3 OH, CH 3 NH 2 , and (CH 3) 2 NH, except for (Gl) 2 H + ‚NH 3 which has four O‚‚‚H bonds. In Gl‚NH(CH 3) 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 + increases with increasing proton affinity of the inserting base. A similar trend is observed for insertion into (Gl) 2 H +. Trimethylamine does not follow the expected trend because it forms proton-bound complexes that have a single O‚‚‚HN bond instead of two. The association energy of CH 3 OH 2 + , NH 4 + , etc., with 12c4 or Gl 2 decreases with increasing proton affinity (of methanol, ammonia, etc.).