1H and 13C NMR chemical shift assignments of agmatine analogues, (3-aminopropyl)guanidine and (trans-4-aminocyclohexyl)guanidine

An extensive theoretical study of two agmatine analogues, (3-aminopropyl)guanidine and (trans-4-aminocyclohexyl)guanidine, has been performed at the ab initio HF/6-31G**, HF/6-311G**, B3LYP/6-311G** and B3LYP/6-311G**/IEFPCM levels. Bond lengths, bond angles and dihedral angles of the optimized structures of (3-aminopropyl)guanidine and (trans-4-aminocyclohexyl)guanidine are very close to each other. The 1H and 13C NMR chemical shifts of (3-aminopropyl)guanidine and (trans-4-aminocyclohexyl)guanidine were calculated by using GIAO (with and without solvent effects), CSGT and IGAIM methods. The calculated chemical shifts were compared with the two-dimensional NMR experimental data. Overall, the calculated chemical shifts show very good agreement with the experimental results. The harmonic vibrational frequencies for (3-aminopropyl)guanidine and (trans-4-aminocyclohexyl)guanidine were calculated at the B3LYP/6-311G** level.