Ab initio NQR study of piperidine umbrella inversions M.Z. Kassaee * , B.N. Haerizade, Z.S. Hossaini Department of Chemistry, Tarbiat Modarres University, P.O. Box 14155-4838, Tehran, Iran Received 15 August 2004; revised 6 October 2004; accepted 19 October 2004 Abstract Two conformers of piperidine interconvert through nitrogen inversion. In this equilibrium, conformers with equatorial N–H appear dominant over those with axial N–H. In this study, substituent (X) effects are investigated on nitrogen inversions, for axially substituted 3-X- piperidines, using ab initio calculated NQR parameters, where XZH, F, Cl, Br, CH 3 , CF 3 , OH, CHO, COOH, CN, NH 2 and NO 2 . These 12 species are in three forms. They may have an axial sp 3 N–H (1 X ), a planar sp 2 N–H (2 X ), and/or an equatorial sp 3 N–H (3 X ). The axial minima, 1 X , appear to go through transition states (TS), 2 X , and convert to equatorial minima, 3 X . Ab initio and DFT methods are employed to study these equilibria (1 X (min)#2 X (TS)#3 X (min)). Energies, structural parameters as well as 2 H( 2 H–N) and 14 N quadrupole coupling constants (c 2H and c 14N , respectively) are calculated for 1 X , 2 X and 3 X . The order of c 2H magnitude appears to be: 2 X O3 X O1 X . The same order is obtained for c 14N when XZF, Cl, Br, CH 3 , CF 3 , COOH, CN, NH 2 and NO 2 . For XZH, OH and CHO, c 14N trend changes to: 2 X O 1 X O3 X . According to our B3LYP/6-31 g (d,p) and MP2/6-31g(d,p) data, the equilibrium shifts in favor of 1 X over 3 X form is demonstrated, when X is F, Cl, Br, COOH, NO 2 , CN or CF 3 . On the other hand, equatorial 3 X dominate over axial 1 X when XZH, CH 3 , OH and NH 2 . q 2004 Published by Elsevier B.V. Keywords: Piperidines; Quadrupole coupling constant; NQR; Nitrogen inversion; Ab initio; DFT 1. Introduction Piperidines are among the most important six-mem- bered saturated heterocyclics that occur in many alkaloids and pharmaceutics [1]. Non-intra-hydrogen bonded piperidines are expected to have ‘chair confor- mational reversal barrier’ and ‘nitrogen inversion barrier’ ranges similar to those of piperidine itself (10.4 and 6.1 kcal/mol, respectively) [2,3]. Such is the case for N- methylpiperidine in which the N-methyl group prefers the equatorial position, but the extent of such preference was long in doubt [4–11]. A low-temperature NMR study gives a KDG 0 value of 0.36 kcal/mol for the axial- equatorial N–H equilibrium of unsubstituted piperidine with the equatorial position being preferred [12]. An infrared study suggests an equatorial N–H for 8-tert- butyl-trans-decahydroquinolines [13]. Even though 99.63% of naturally occurring nitrogen atoms is 14 N, with the spin of unity, its quadrupole moment is only 1.67!10 K30 m 2 [14,15]. Consequently, the corresponding coupling constants are small and seldom more than 6 Mc/s. A low 14 N coupling constant is the main reason for the difficulty often encountered for acquiring data on nitrogen-14 nuclei. Hence, the intensities of nitrogen pure quadrupole resonance lines are very low and extremely sensitive spectrometers and much patience are required to get a result [16]. For instance, 14 N coupling constants (Mc/s) obtained by pure quadrupole resonance in the solid phase for NH 3 , CH 3 NH 2 (CH 3 ) 2 NH and (CH 3 ) 3 N are 3.161 [17], 3.986 [18], 4.650 [18] and 5.194 [19], respectively. On the other hand, the 2 H nucleus is present to the extent of 0.015 percent in naturally occurring hydrogen atoms. Its spin is unity and its quadrupole moment is even smaller than 14 N, about 2.87!10 K31 m 2 [14,15]. As a conse- quence its pure quadrupole resonance lines are much smaller and experimentally more out-of-the-way. Perhaps due to these difficulties, up to this point not many reports have appeared on the utility of NQR parameters for the analyses of configuration inversions in cyclic amines. 0166-1280/$ - see front matter q 2004 Published by Elsevier B.V. doi:10.1016/j.theochem.2004.10.034 Journal of Molecular Structure: THEOCHEM 713 (2005) 245–254 www.elsevier.com/locate/theochem * Corresponding author. Tel.: C98 912 1000392; fax: C98 21 8006544. E-mail address: kassaeem@modares.ac.ir (M.Z. Kassaee).