Structural and vibrational properties of trigonellinium perchlorate dimer, (TRGH·ClO 4 ) 2 M. Szafran * , Z. Dega-Szafran, A. Katrusiak, K. Stobiecka, M. Pankowski Faculty of Chemistry, Adam Mickiewicz University, ul Grunwaldzka 6, 60780 Poznan ´, Poland Received 1 July 2002; revised 21 October 2002; accepted 11 November 2002 Abstract The crystal structure of the complex of trigonelline with perchloric acid (TRGH·ClO 4 ) 2 , was determined by X-ray diffraction as monoclinic, space group P2 1 /n with a ¼ 5.285(1), b ¼ 14.662(3), c ¼ 12.494(2) A ˚ , b ¼ 95.96(3)8 and Z ¼ 4. The complexes occur in the crystal as a centrosymmetric hydrogen bonded dimers with the O – H· · ·O distance of 2.635(9) A ˚ . The COOH group is inclined at 15.5(1)8 to the pyridine ring plane. The ClO 4 anion interacts electrostatically with four positively charged nitrogen atoms. The optimized bond lengths, bond and torsion angles of trigonelline moiety calculated by MP2/6- 31G(d,p), B3LYP/6-31G(d,p), PM3 and SAM1 approaches are in good agreement with the X-ray data. Relatively large divergences are observed between the calculated and experimental bond lengths and angles of the ClO 4 anion and its distances to the N þ atoms. The AM1 method reproduces quite a different structure with a bifurcated donor hydrogen bond between the COOH group and the ClO 4 anion. The FTIR spectra of TRG·HClO 4 and TRG·DClO 4 confirmed the dimeric structure. q 2003 Elsevier Science B.V. All rights reserved. Keywords: Trigonellinium perchlorate; X-ray diffraction; MP2; B3LYP; PM3; SAM1 and AM1 calculations; Geometry; FTIR and Raman spectra; H-bond; Electrostatic interactions 1. Introduction Trigonelline (TRG), N-methyl-3-carboxy-pyri- dine, is a natural betaine isolated as monohydrate from various plants, seeds and the western rock lobster [1–5]. Its structure has been determined by X- ray diffraction [5] and ab initio calculations [6]. We have recently synthesised the 1:1 and 2:1 complexes of TRG with HBr, HI, HNO 3 , HClO 4 and HBF 4 [7]. The 2:1 complexes crystallize with one molecule of water and their FTIR spectra are characterised by a broad and strong absorption in the 1500 – 400 cm 21 region, typical of hydrogen bonds shorter than 2.5 A ˚ . The 1:1 complexes were divided into two groups. The first group includes complexes with HBr, HI and HNO 3 , and the second group with HClO 4 and HBF 4 . FTIR spectra of the first group are very similar to the analogous complexes of betaine [8–10] and pyridine betaine [11–12]. In the spectra of the first group the frequency and shape of the nOH absorption correlate well with the proton acceptor properties of the anion and indicate that a proton from the acid is transferred to TRG and the anions are bonded to the carboxyl group, COOH· · ·X. 0022-2860/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved. PII: S0022-2860(02)00611-7 Journal of Molecular Structure 649 (2003) 43–53 www.elsevier.com/locate/molstruc * Corresponding author. Tel: þ48-61-829-1320; fax: 48-61-865- 8008. E-mail address: szafran@main.amu.edu.pl (M. Szafran).