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The binding energies at 0 K of sodium and silver ions to ammonia, methylamine, ethylamine, acetonitrile, and benzonitrile were determined using threshold collision-induced dissociation (CID) and molecular orbital calculations at the ab initio and density functional theory levels. There is good agreement between experimental and calculated binding energies. For the five ligands, threshold CID/CCSD(t)(fu)/6-311++G(2df,p)//MP2(fu)/6-311++G(d,p) Na+ binding energies are the following: ammonia, 25.6 ± 2.8/24.8; methylamine, 27.0 ± 1.4/25.9; ethylamine, 27.7 ± 2.3/27.1; acetonitrile, 30.0 ± 2.3/30.3; and benzonitrile, 32.7 ± 1.4/35.0 (B3LYP/6-311++G(d,p)//B3LYP/6-311++G(d,p)) kcal/mol. Threshold CID and B3LYP/DZVP Ag+ binding energies are the following: ammonia, 40.6 ± 3.0/38.9; methylamine, 41.5 ± 2.3/41.1; ethylamine, 42.9 ± 1.4/43.2; acetonitrile, 40.8 ± 2.0/39.3; and benzonitrile, 41.5 ± 2.8/43.1 kcal/mol. Wherever comparisons with literature data are possible, the Na+ binding energies determined in this ...

The complexes of silver ion, Ag+, with the twenty naturally occurring amino acids have been calculated using hybrid density functional theory at the B3LYP/DZVP level. For all of these silver complexes, several possible structures were examined, but as there are remarkable similarities between all the structures at the global minima, only summarized data are reported. All of the complexes, except that with proline, are solvated ions. Amino acids containing only hydrocarbon side chains are bidentate, coordinating through the amino and carbonyl groups and the remaining amino acids (with the exception of proline) are tricoordinate with the same two interactions as in the simpler amino acids and an additional interaction through the side chain. The proline complex contains zwitterionic proline with the Ag+ ion attached to the carboxylate anion. Enthalpies (at 298 K) for dissociation of Ag+ from the complexes range from 49.3 kcal mol-1 for glycine to 80.4 kcal mol-1 for arginine. Free energies for these reactio...

The binding enthalpies at 0 K of the silver ion to water, methanol, ethanol, diethyl ether, and acetone were calculated using density functional theory (DFT) using the hybrid B3LYP level of theory with the DZVP basis set; they were also measured using the threshold collision-induced dissociation (CID) method. There is good agreement between the two sets of data. For the five ligands, the DFT/threshold CID values are: water, 28.1/31.6 ± 2.5; methanol, 30.1/33.0 ± 3.7; ethanol, 32.0/33.9 ± 3.5; diethyl ether, 33.3/33.2 ± 1.5; and acetone, 36.2/38.0 ± 1.4 kcal/mol. The average of the absolute differences between the DFT and threshold CID results is 2.0 kcal/mol, a value smaller than the average experimental uncertainty of 2.5 kcal/mol. For identical ligands, the silver ion binding energies are lower than the lithium ion binding energies, but higher than the sodium ion binding energies.

Molecular orbital calculations at B3LYP/6-31++G(d,p) are reported for bases 2-(aminomethyl)-5-oxazolone, 2-(aminomethyl)-4-methyl-5-oxazolone, 2-phenyl-5-oxazolone, and 2-phenyl-4 methyl-5-oxazolone and for the cations formed by protonation of these bases on their imino nitrogens. Structures and relative energies for isomers generated by protonation at each of the four heteroatoms of 2-(aminomethyl)-5-oxazolone are reported. Lithium and silver cations both add to 2-(aminomethyl)-5-oxazolone, but unlike the proton, they bind with two heteroatoms simultaneously. For both the lithiated and argentinated 2-(aminomethyl)-5-oxazolone cations the lowest energy isomers have the metal coordinated with the two nitrogen atoms. Proton affinities of these bases are in the range 217.0-221 kcal mol -1 , with the methyl group at C4 increasing the proton affinity by 3 kcal mol -1 . Single-point calculations were performed at MP4(fc)/6-311++G(2df,p)//B3LYP/6-31++G(d,p) for 2-(aminomethyl)-5-oxazolone, diketopiperazine, glycine, and alanine and their conjugate acids. The proton affinities from this level of theory are lower by as much as 2.7 kcal mol -1 than those calculated at B3LYP/6-31++G(d,p). Enthalpies of formation calculated at B3LYP/6-31++G(d,p) from isodesmic reactions for glycine, alanine, and their conjugate acids are all within 1 kcal mol -1 of the experimental values, but those calculated at MP4 deviate by as much as 4.8 kcal mol -1 . Enthalpies of formation from atomization reactions at the MP4 level are in larger disagreement with experimental values.

The presence of an interacting water or methanol molecule has been shown to catalyze the 1,3-proton shift in a peptide linkage between the tautomers of protonated formamide and glycylglycylglycine. Density functional theory calculations at the B3LYP/6-31++G(d,p) level of theory show that, for glycylglycylglycine, the forward barrier of this shift decreases from a free energy at 298 K of 39.6 kcal/mol in the absence of solvent to 26.7 kcal/mol in the presence of water and to 22.0 kcal/mol in the presence of methanol. Protonation at the amide nitrogen of the second residue results in a large increase in the C -N bond distance from 1.336 to 1.519 A, whereas protonation at the carbonyl oxygen leads to a decrease in the C-N bond distance from 1.336 to 1.321 A. Solvent-catalyzed tautomerism may play an important role in the fragmentation of electrosprayed, protonated peptides in the gas phase.