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.

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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