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JUAN C. LÓPEZ, SUSANA BLANCO, RAQUEL SÁNCHEZ, ALBERTO LESARRI AND JOSÉ L. ALONSO, Grupo de Espectroscopía Molecular (GEM), Departamento de Química Física y Química Inorgánica, Facultad de Ciencias, Universidad de Valladolid, E-47005 Valladolid, Spain.
Microsolvated clusters constitute models that allow the investigation of molecular properties in the transition from isolation to solvation and the understanding of solute-solvent interactions and solvent-enhanced chemical reactivity. In this work we have analyzed the jet-cooled rotational spectra of the formamide-(H2O)n and 2-azetidinone-(H2O)n (n=1, 2) clusters in order to investigate the diverse hydrogen bond interactions of water with the peptide functional group. Formamide (NH2--CHO) and 2-azetidinone (C3H5NO), a lactam four-membered ring that forms part of penicillin- and cephallosporin-type antibiotics, are two of the most simple molecules carrying a peptide group. A molecular beam Fourier Transform microwave spectrometer\footnote ~J. L. Alonso, F. Lorenzo, J. C. López, A. Lesarri, S. Mata and H. Dreizler; Chem. Phys. , \textbf218, 267 (1997) equipped with a heating nozzle\footnote ~S. Blanco, J. C. López, J. L. Alonso, P. Ottaviani and W. Caminati; J. Chem. Phys. , \textbf119, 880 (2003) has been used for this research. We have observed thre 1:1 forms of the formamide-H2O complex, two 1:1 forms of the 2-azetidinone-H2O complex and the trimers formamide-(H2O)2 and 2-azetidinone-(H2O)2. The most stable conformer of the formamide-H2O complex, previously observed by Lovas et al. , and the most stable form of the 2-azetidinone-H2O complex are stabilized by two hydrogen bonds O-H
O=C and N-H
O with water closing a cycle with the peptide functional group. In the second most stable 1:1 conformers water is placed on the other side of the carbonyl group stabilized by O-H
O=C hydrogen bonds and by C-H
O weak hydrogen bonds. In the third formamide-H2O form, water is bonded to the amino group by a N-H
O hydrogen bond. For the trimers, the two molecules of water close a cycle with the peptide group with three different hydrogen bonds (O-H
O=C, O-H
O and N-H
O). The spectra of the parent and several D/H, 15N/14N, 13C/12C and 18O/16O isotopomers have been measured in order to investigate the structures of these clusters.