Structural, spectroscopic, and theoretical studies of a very short OHO hydrogen bond in bis(4‐( N ‐methylpiperidinium)‐butyrate) hydrobromide

The molecular structure of bis(4‐( N ‐methylpiperidinium)‐butyrate) hydrobromide, (MPBu) 2 HBr, has been characterized by single crystal X‐ray diffraction, infrared and NMR spectroscopies, and by DFT calculations. The crystals of the title compound at 140 K are monoclinic, space group C 2/ c , with a  = 11.7118(4), b  = 7.8737(2), c  = 23.9240(8) Å 3  , β  = 90.431(3)°, V  = 2206.1(1) Å 3 , and Z  = 4. Two 4‐( N ‐methylpiperidinium)‐butyrate moieties are joined by a very short and centrosymmetric O.H.O hydrogen bond of 2.436(2) Å 3  . The piperidine ring adopts a chair conformation with the methyl group in the equatorial and the bulky (CH 2 ) 3 COO substituent in the axial position. The broad absorption band below 1500 cm −1 in the FTIR spectrum confirms the existence of a very short O H O hydrogen bond. The 1 H and 13 C NMR spectra are interpreted on the basis of 2D experiments and the calculated GIAO/B3LYP/6‐31G(d,p) magnetic isotropic shielding tensors. In the optimized structure of the complex, a 4‐( N ‐methylpiperidinium)‐butyrate zwitterion interacts with a 4‐( N ‐methylpiperidinium)‐butyric acid cation forming an O .HO hydrogen bond of 2.580 Å 3  . Copyright © 2008 John Wiley & Sons, Ltd.