Conformational analysis of 5-piperidinevaleric acid, 5-(N-methylpiperidine)valerate and their hydrogen halides by MO calculations, X-ray diffraction and FTIR spectroscopy

Abstract

The most stable conformers of 5-(piperidine)valeric acid (1), 5-(N-methylpiperidine)valerate (2) and their hydrogen halides (3 and 4) were analyzed by the semiempirical PM3 method and selected compounds by the B3LYP/6-31G(d,p) method. As some of the investigated compounds are charged and the others can be neutral, some have acidic proton, others do not. They are capable of forming ionic bonds (via Coulombic attraction between the oppositely charged groups) or of forming the various types of hydrogen bonded conformers. As a result these compounds are ideally suited to study the importance of electrostatic interactions and hydrogen bonding on the relative stabilities of conformers. In the case of compounds containing N-methylpiperidine unit, for a particular conformer, the intramolecular attractive electrostatic interactions between the charged group play key roles in their relative stability in the gas phase. The electrostatic interaction of the X− ion with the positively charged nitrogen atom decreases their proton-acceptor properties and COOH⋯X− hydrogen bonds are present in all hydrogen halides (3). 5-Piperidine valeric acid with HF forms a molecular complex, while with HCl, HBr and HI an ion pair, according to the B3LYP calculations. The PM3 calculations predict a molecular complex also with HCl. The crystal structure of 5-(piperidine)valeric acid hydrogen bromide (4HBr), space group of crystals P21/n with a=6.204(1), b=32.777(7), c=6.416(1)Å, β=106.21(3)°, Z=4 and R=0.0685 was characterised by X-ray crystallography methods. Br− ion forms two types of hydrogen bonds: Br⋯N(1), 3.247(14)Å, and O(1) ⋯Br, 3.118(11)Å. Moreover, C–H⋯Br short contacts, which can be recognized as weak hydrogen bonds, exist in the crystal. The FTIR spectrum of 1 in the solid state shows an intense broad absorption in the 1600–400cm−1 region typical for a very short NHO hydrogen bonds. In solution the hydrogen bond seems to be longer. The bands of νCO at 1708cm−1 and νasCOO− at 1615cm−1 in CD3CN solution show that OH⋯N⇌O−⋯HN+ equilibrium exists. Ab initio calculations predict molecular structures of three most stable conformers of 1 in the gas phase.