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

The most stable conformers of 5-(piperidine)valeric acid (1), 5-(N-methylpi peridine)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 o f forming ionic bonds (via Coulombic attraction between the oppositely char ged groups) or of forming the various types of hydrogen bonded conformers. As a result these compounds are ideally suited to study the importance of e lectrostatic 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 in teractions between the charged group play key roles in their relative stabi lity in the gas phase. The electrostatic interaction of the X- ion with the positively charged nitrogen atom decreases their proton-acceptor propertie s and COOH X- hydrogen bonds are present in all hydrogen halides (3). 5-Pip eridine valeric acid with HF forms a molecular complex, while with HCl, HBr and HI an ion pair, according to the B3LYP calculations. The PM3 calculati ons predict a molecular complex also with HCl. The crystal structure of 5-( piperidine)valeric acid hydrogen bromide (4HBr), space group of crystals P2 (1)/n with a = 6.204(1), b = 32.777(7), c = 6.416(1) Angstrom, beta = 106.2 1(3)degrees, Z = 4 and R = 0.0685 was characterised by X-ray crystallograph y methods. Br- ion forms two types of hydrogen bonds: Br ... N(1), 3.247(14 ) Angstrom, and O(1)... Br, 3.118(11) Angstrom. Moreover, C-H ... Br short contacts, which can be recognized as weak hydrogen bonds, exist in the crys tal. The FTIR spectrum of I in the solid state shows an intense broad absor ption in the 1600-400 cm(-1) region typical for a very short NHO hydrogen b onds. In solution the hydrogen bond seems to be longer. The bands of nu C=O at 1708 cm(-1) and nu(as)COO(-) at 1615 cm(-1) in CD3CN solution show that OH ... N reversible arrow O-... HN+ equilibrium exists. Ab initio calculat ions predict molecular structures of three most stable conformers of 1 in t he gas phase. (C) 1999 Elsevier Science B.V. All rights reserved.