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Gas-Phase Lithium Cation Basicity of Histamine and its Agonist 2-(β-aminoethyl)-pyridine. Chelation Effect

Małgorzata Hallmann 1Ewa D. Raczyńska 1Pierre-Charles Maria 2Jean-François Gal 2

1. Agricultural University, Dep. of Chemistry, Nowoursynowska 159c, Warszawa 02-776, Poland
2. Université de Nice-Sophia Antipolis, LRSAE, Parc Valrose, Nice 06108, France

Abstract

The gas-phase lithium cation basicities (LCBs) were obtained for histamine (HA) and its agonist 2-(β-aminoethyl)-pyridine (AEP) from collision-induced dissociation of lithium adducts (kinetic method) using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). For measurements, (MeOCH2)2, Et3PO and HMPA, the strongest bases in the LCB scale, were used as the reference compounds. The experimental LCB of AEP is between those of Et3PO and HMPA. The experimental LCB of HA is higher than those of AEP and HMPA by 1.5-2 kcal mol-1 indicating that it is outside of the current LCB scale.

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In parallel, quantum-chemical calculations [DFT(B3LYP/6-31G*] were performed for HA, AEP and their lithium adducts using the Gaussian'98 program package. Among twenty two conformations of HA-Li+ adduct, only one structure is significantly more stable than others. This is also the case for one structure among seven conformations of AEP-Li+ adduct. These two stable structures have the 'scorpio' conformation, where Li+ cation is almost equally chelated by two basic nitrogens, the ring N-aza and the chain N-amino. Other HA-Li+ and AEP-Li+ conformations have considerably higher energies than the 'scorpio' structures.

The DFT calculated difference between LCB of HA and AEP (4 kcal mol-1) is in agreement with that experimentally obtained (> 2 kcal mol-1). High experimental and theoretical values of LCB for HA and AEP confirm the strong chelation of Li+ by both ligands in the gas phase. This chelation effect was also observed previously for the proton gas-phase basicity.


Acknowledgments. DFT calculations were carried out at the Interdisciplinary Center for Molecular Modeling (ICM, Warsaw).

 

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Presentation: poster at 18th Conference on Physical Organic Chemistry, Posters, by Ewa D. Raczyńska
See On-line Journal of 18th Conference on Physical Organic Chemistry

Submitted: 2006-05-31 18:35
Revised:   2009-06-07 00:44