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 NH3, CH3OH, CH3NH2, (CH3)2NH, and (CH3)3N 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)2H+, 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)2H+, and the protonated crown ether, 12c4H+, form two internal hydrogen bonds with NH3, CH3OH, CH3NH2, and (CH3)2NH, except for (Gl)2H+·NH3 which has four O···H bonds. 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+ increases with increasing proton affinity of the inserting base. A similar trend is observed for insertion into (Gl)2H+. 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 CH3OH2+, NH4+, etc., with 12c4 or Gl2 decreases with increasing proton affinity (of methanol, ammonia, etc.).