Structure, conformation and hydrogen bonding of 4-(N-methylpiperidinium)-butyric acid bromide

A 1:1 complex between 4-(N-methylpiperidinium)-butyrate inner salt (betaine) and hydrobromide, MPBUH·Br, has been characterized by single crystal X-ray analysis. The crystals are monoclinic, space group P21/c, with a = 8.284(1), b = 10.369(1), c = 13.809(2) Å, β = 92.26(1)°. The piperidine ring adopts a chair conformation with the (CH2)3COOH substituent in the axial and the CH3 group in the equatorial positions. In the crystal, the Br− anion is engaged in a medium-strong hydrogen bond with the COOH group (OH⋯Br− = 3.141(1) Å), N+⋯Br− electrostatic interactions and in several CH⋯Br− contacts with the aliphatic groups forming a cavity in which it is enclosed. Three conformers (2-4) were optimized by the B3LYP/6-31G (d, p) level of theory. The most stable is conformer 4 with the (CH2)3COOH substituent in the equatorial position and the CH3 group in the axial position. The stability of the investigated conformers is controlled by electrostatic interactions between the oppositely charged groups. The FTIR spectrum of MPBUH·Br shows a strong band at 2941 cm−1 due to νOH and the νCO band at 1712 cm−1.