Accurate structures of aminoacids in the gas phase have been obtained by joint microwave and quantum-chemical investigations. However, the structure and conformational behavior of α-aminoacids once incorporated into peptide chains are completely different and have not yet been characterized with the same accuracy. To fill this gap, we present here an accurate characterization of the simplest dipeptide analogue (N-acetyl-glycinamide) involving peptidic bonds. State-of-the-art quantum-chemical computations are complemented by a comprehensive study of the rotational spectrum using a combination of Fourier transform microwave spectroscopy with laser ablation. The coexistence of the C7 and C5 conformers has been proved and energetically as well as spectroscopically characterized. This joint theoretical-experimental investigation demonstrated the feasibility of obtaining accurate structures for flexible small biomolecules, thus paving the route to the elucidation of the inherent behavior of peptides.

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Context. The formation of glycine is strongly relevant to our understanding of the interstellar medium and is most accuretely studied computationally. Aims. We carry out a theoretical study of the reactions between the radical cation of ammonia and CH 3 COOH/CH 2 COOH as possible processes leading to glycine derivatives. Methods. The gas-phase reactions were theoretically studied using ab initio methods. We employed the second-order Moller-Plesset level in conjunction with the cc-pVTZ basis set. In addition, the electronic energies were refined by means of single-point calculations at the CCSD(T) level on the MP2/cc-pVTZ geometries with the aug-cc-pVTZ basis set. Results. We report accurate potential energy surfaces for the reactions considered in this work. The different intermediate species as well as the most relevant transition states for these reactions are characterized. Conclusions. Formation of protonated glycine from the reaction of NH + 3 with acetic acid is an exothermic (−9.15 kcal/mol at CCSD(T) level) barrier free process. However, the results obtained indicate that the hydrogen-transfer process forming NH + 4 and CH 2 COOH is clearly the dominating path, in agreement with the experimental evidence. The formation of ionized glycine from the reaction of product CH 2 COOH with NH + 3 is a quasi-isoenergetic (2.03 kcal/mol at CCSD(T) level) barrier free process that leads to a highly stable intermediate: protonated glycine.

A systematic study of the three lowest-lying structures, namely, linear, fan, and cyclic, of neutral FeC(n) (n = 1-8) clusters on the singlet, triplet, quintet, and septet potential energy surfaces has been carried out. Predictions for their electronic energies, rotational constants, dipole moments, and vibrational frequencies have been made using the B3LYP method in conjunction with the 6-311+G(d) basis set. Triplet or quintet states are found as the lowest-lying ones for FeC(n) clusters, and the septet states are found more stable than the singlet ones. The incremental binding energies show an even-odd parity effect, with n-even clusters being more stable than the n-odd ones in the linear and fan clusters, whereas a decrease with n is found for cyclic ones. The most stable isomers for FeC(n) clusters correspond to a fan-type structure for n < or = 4, whereas for n > or = 6 cyclic structures are the most stable. Only in the case of n = 5 the most stable isomer is the linear structure. Comparing the results of FeC(n) clusters with the previously studied TMC(n) (TM = Sc, Ti, V, Co, and Zn) systems, we can see that, as it should be expected, iron doped carbon clusters present an intermediate behavior between early and late first-row transition metal doped clusters.

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