Experimental and theoretical studies of noncovalent metal -ligand complexes: Applications to solvation and nucleobase tautomerization and modification.

摘要

Threshold collision-induced dissociation of M+(ligand) and Cu+(solvent)x complexes with xenon are studied as a function of kinetic energy using guided ion beam mass spectrometry. M+ include alkali metal ions (Li+, Na+ and K+), and transition metal ions (Fe +, Co+, Ni+, Cu+, and Zn+). The ligands studied belong to two classes: solvent molecules (acetone, methanol, and ethanol), and the nucleic acid bases (cytosine, guanine, uracil, and thymine), including modified uracils (methyluracils, halouracils, and thiouracils), and model systems for the nucleic acid bases (2-, 3-, and 4-hydroxylpyridine and 2-hydroxylpyrimidine). For the Cu+(solvent) x and alkali metal ion-nucleobase systems, endothermic loss of an intact neutral ligand is observed as the dominant and lowest energy dissociation channel. Thresholds analyses provide a direct measure of the bond dissociation energies (BDEs) for M+-nucleobase systems. Systematic examination of the Cu+(solvent)x systems as a function of the extent of salvation, i.e., variations in x, provide sequential BDEs. For the transition metal ion-nucleobase complexes, endothermic loss of an intact neutral nucleobase is observed at high energies, whereas a variety of low-energy activated dissociation processes are also observed. Accurate determination of the transition metal ion-nucleobase BDEs can only be accomplished via characterization of the transition states (TSs) for all low-energy pathways and competitive modeling. As these have not yet been characterized, analysis of the total and metal ion cross section provides lower and upper bounds to the BDEs. These analyses are performed using the phase space limit (PSL) for simple metal ion-ligand bond cleavage reactions, while tight transition state (TTS) models are employed for activated dissociation pathways. All analyses take into account the effects of multiple ion-neutral collisions, kinetic and internal energy distributions of the reactants, and dissociation lifetimes. Ab initio calculations are performed at various levels of theory to obtain the structures, molecular constants, and energetics of the associated species (i.e. reactants, transition states, products) involved in the reactions studied, the properties (i.e. proton affinities, acidities, bases paring energies) of the species of interest, characteristics of Cu+(ligand) x interactions (by Natural Bound Orbital Analyses), potential energy landscapes of reactions of interest, and theoretical estimated BDEs of these complexes. In order to assist the metal ion-nucleobase studies, ab initio calculations are applied to investigate the tautomerization of neutral nucleobases and alkali metal ion-nucleobase complexes at various levels of theory. The formation of dimers or cationic clusters of neutral nucleobase, and the gas phase association of metal ions with nucleobases may facilitate tautomerization, such that these energy barriers can be overcome under our experimental conditions.