Structural studies of pyrido[1,2-c]pyrimidine derivatives by 13C CPMAS NMR, X-ray diffraction and GIAO/DFT calculations

New 1-aryl- or 1-heteroaryl-piperazinylbutyl derivatives with pyrido[1,2-c]pyrimidine imide moiety, with the expected higher selectivity to 5-HT1A receptors, have been synthesized. Five hydrochlorides and one base were studied by solid-state 13C CPMAS NMR spectroscopy. 13C chemical shifts indicated that the piperazine ring nitrogen N-1 was protonated. The crystal structure of the base (with 4′F, 3″-CF3 substituents) was determined by X-ray crystallography. The pyridopyrimidine fragment is essentially planar, and aromatic substituent at C4 is twisted by 59.0°. The crystals are stabilized mainly by the CO…HC interactions. The analysis was completed by theoretical calculations of the shielding constants at the GIAO/DFT (B3LYP/6-311+G∗∗) level.