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Cesiated complexes of the aliphatic amino acids (Gly, Ala, hAla, Val, Leu, and Ile) were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light from a free-electron laser (FEL). To identify structures, the experimental spectra were compared to linear spectra calculated at the B3LYP-GD3BJ/def2-TZVP level of theory. Relative energies at 0 and 298 K for various possible conformers of all complexes were calculated at B3LYP, B3LYP-GD3BJ, and MP2(full) levels using the def2-TZVP basis set. Spectral comparison for all complexes indicates that the dominant conformation has the cesium cation binding to the carbonyl and hydroxyl oxygens, [CO,OH]. This conclusion contrasts with previous work for Cs+(Gly), which suggested that the [CO] binding motif was prevalent. This dichotomy is explored theoretically in detail using coupled-cluster calculations with single, double, and perturbative triple excitations, CCSD(T), as well as advanced density functional theory (DFT) approaches. The comparisons show that the [CO,OH] – [CO] double-well potential found for most DFT approaches disappears at the higher level of theory with only the [CO,OH] well remaining. An exploration of this effect indicates that electron correlation is critically important and that DFT approaches incorrectly handle the internal hydrogen bonding in these molecules, thereby over-delocalizing the charges on the amino acid ligands. 

Using light generated by an infrared free electron laser, action spectroscopy was performed on doubly charged complexes of the metalated dipeptide histidyl-histidine (HisHis). Metal cations used were zinc, cadmium, and copper. Molecular dynamics and quantum-chemical calculations were used to screen a large number of conformers, whose theoretical infrared spectra were compared to the recorded action spectra of these metalated complexes. The zinc and cadmium spectra display dominant features associated with an iminol binding motif of the HisHis ligand, where the metal ion coordinates with both pros () nitrogens of the imidazole sidechains, the terminal carbonyl oxygen, and the backbone nitrogen for which the hydrogen ordinarily bound here has migrated to a carbonyl. The copper complex was difficult to assign to a single species, because a few predicted bands are absent from the experimental spectrum. The theoretical single point energies were also calculated for all structures examined, and DFT methods were found to describe the ion conformer populations in the case of the zinc and cadmium chelates better than the MP2 prediction. 

The gas-phase structures of cationized glycine (Gly), including complexes with Li + , Na + , K + , Rb + , and Cs + , are examined using infrared multiple-photon dissociation (IRMPD) spectroscopy utilizing light generated by a free electron laser, in conjunction with ab initio calculations. To identify the structures present in the experimental studies, measured IRMPD spectra are compared to spectra calculated at B3LYP/6-311+G(d,p) for the Li + , Na + , and K + complexes and at B3LYP/def2TZVP for the Rb + and Cs + complexes. Single-point energy calculations were carried out at the B3LYP, B3P86, and MP2(full) levels using the 6-311+G(2d,2p) basis set for Li + , Na + , K + and the def2TZVPP basis set for Rb + and Cs + . The Li + and Na + complexes are identified as metal cation coordination to the amino nitrogen and carbonyl oxygen, [N,CO]-tt, although Na + (Gly) may have contributions from additional structures. The heavier metal cations coordinate to either the carbonyl oxygen, [CO]-cc, or the carbonyl oxygen and hydroxy oxygen, [CO,OH]-cc, with the former apparently preferred for Rb + and Cs + and the latter for K + . These two structures reside in a double-well potential and different levels of theory predict very different relative stabilities. Some experimental evidence is provided that MP2(full) theory provides the most accurate relative energies. 

The gas-phase structures of protonated unsymmetrical 1,1-dimethylhydrazine (UDMH) and the proton-bound dimers of UDMH and hydrazine are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy utilizing light generated by a free electron laser and an optical parametric oscillator laser system. To identify the structures present in the experimental studies, the measured IRMPD spectra are compared to spectra calculated at the B3LYP-GD3BJ/6-311+G(d,p) level of theory. These comparisons show that protonated UDMH binds the proton at the methylated nitrogen atom (α) with two low-lying α conformers probably being populated. For (UDMH) 2 H + , the proton is shared between the methylated nitrogen atoms with several low-lying α conformers likely to be populated. Higher-lying conformers of (UDMH) 2 H + in which the proton is shared between α and β (unmethylated) nitrogen atoms cannot be ruled out on the basis of the IRPMD spectrum. For (N 2 H 4 ) 2 H + , there are four low-lying conformers that all reproduce the IRMPD spectrum reasonably well. As hydrazine and UDMH see usage as fuels for rocket engines, such spectra are potentially useful as a means of remotely monitoring rocket launches, especially in cases of unsuccessful launches where environmental hazards need to be assessed.