Computational investigation of the conformer selective complexes of 1,2,3,4-tetrahydroisoquinoline: Ammonia (THIQ: NH3) in S0

Abstract

Three low-energy conformations are identified in 1,2,3,4-tetrahydroisoquinoline (THIQ): NH3 complexes. The lone pairs on N atoms strongly affects the conformational landscape. Both density functional theory (DFT) and MP2 levels of computations yield consistent results. Multiple PESs corresponding to different types of hydrogen bonds are drawn to account for all possible complexes. Ten complexes are identified. The global minimum contains a strong N-H⋅⋅⋅N and a weak N-H⋅⋅⋅π hydrogen bonds where NH3 is an H bond acceptor. In the remaining two complexes ammonia acts as a donor. All the low-energy complexes which can show intermolecular proton transfer were studied in detail although none shows PT with a single NH3 molecule. The intermolecular vibrational modes are critically analyzed and the relative shifts in the characteristic modes of ammonia are explained. Types and sites for H-bonding can be ascertained from the vibrational analyses. Changes in molecular electrostatic potentials (MEP) on complex formation are explained and confirm our computational results. We suggest further experiments to confirm our key observations.