Centrosymmetric dimer of quinuclidine betaine and squaric acid complex

The complex of squaric acid (3,4-dihydroxy-3-cyclobuten-1,2-dion, H(2)SQ) with quinuclidine betaine (1-carboxymethyl-1-azabicyclo[2.2.2]octane inner salt. QNB), 1, has been characterized by single-crystal X-ray analysis, FTIR and NMR spectroscopies and by DFT calculations. In the crystal of 1, monoclinic space group P2(1)/n, one proton from H(2)SQ is transferred to QNB. QNBH(+) and HSQ(-) are linked together by a O-H center dot center dot center dot O hydrogen bond of 2.553(2) angstrom. Two such QNBW(+)center dot HSQ(-) complexes form a centrosymmetric dimer bridged by two O-H center dot center dot center dot O bonds of 2.536(2)angstrom. The FTIR spectrum is consistent with the X-ray results. The structures of monomer QNBH(+)center dot HSQ(-) (1a) and dimer [QNB-H(2)SQ](2) (2) have been optimized at the B3LYP/6-311++G(d,p) level of theory. Isolated dimer 2 optimized back to a molecular aggregate of H(2)SQ and QNB. The calculated frequencies for the optimized structure of dimer 2 have been used to explain the frequencies of the experimental FTIR spectrum. The interpretation of H-1 and C-13 NMR spectra has been based on the calculated GIAO/B3LYP/6-311++G(d,p) magnetic isotropic shielding constants for monomer 1a. (C) 2012 Elsevier B.V. All rights reserved.