X-Ray, FTIR and quantum chemical studies of short and asymmetric hydrogen bonds in bis(2,6-dimethylpyridine-N-oxide) sulphate [2,6-(CH3)2C5H3N---OH]2[SO4]

Crystals of bis(2,6-dimethylpyridine-N-oxide) sulphate are monoclinic, space group P2₁/c, a = 14.098(2) Å, b = 7.855(1) Å, c = 15.203(3) Å, β = 104.84(1)°. The crystal structure has been refined to R = 0.0373 (2052 reflections). The disordered SO²⁻₄ anion accepts hydrogen bonds from two protonated 2,6-dimethylpyridine-N-oxides and two alternative conformations of the SO²⁻₄ group are distinguished. The occupancy factor of the predominant orientation is 0.63 and the O...O distances are 2.445(2) and 2.453(4) Å; in the second form (fraction, 0.37), these distances are 2.445(2) and 2.544(9) Å.

The PM3 and AM1 methods predict three minima for the title complex, whereas the SAM1 and BLYP/6-31G methods predict only one. All methods predict that molecular complex 3 is the most stable. The SAM1 geometry is very close to that of BLYP/6-31G.

The Fourier transform IR (FTIR) spectrum shows a very intense and broad (continuum) absorption within the 1600-400 cm⁻¹ region, typical of short hydrogen bonds. There is no absorption in the 3000-2000 cm⁻¹ region expected for the longer hydrogen bond (2.544(9) Å) in the less populated orientation. Isotope and solvent effects are discussed.