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

Abstract

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 P2 1/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 (CH 2) 3COOH substituent in the axial and the CH 3 group in the equatorial positions. In the crystal, the Br − anion is engaged in a medium-strong hydrogen bond with the COOH group (O H⋯Br − = 3.141(1) Å), N +⋯Br − electrostatic interactions and in several C H⋯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 (CH 2) 3COOH substituent in the equatorial position and the CH 3 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 νC O band at 1712 cm −1.