Vibrational predissociation spectroscopy of protonated methanol clusters (tetramers and pentamers) reveals linear and cyclic structural isomers in a supersonic expansion. The cyclic pentamer, containing a five-membered ring, is identified by its characteristic free-OH stretch at 3647 cm -1 and hydrogen-bonded OH stretches at 3448 and 3461 cm -1 . Ab initio calculations indicate that the excess proton in these clusters can be either localized on one methanol unit in cyclic CH 3 OH 2 + (CH 3 OH) 3 and linear CH 3 OH 2 + (CH 3 OH) 4 or delocalized between two methanol molecules in linear C 2 H 9 O 2 + (CH 3 OH) 2 and cyclic C 2 H 9 O 2 + (CH 3 OH) 3 . Dynamic intracluster proton transfer can occur upon repeated ring opening and closing. The association of this process with the anomalously high proton mobility in liquid methanol is discussed.

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We have observed the IR spectra of the melamine molecule and its deuteriated counterpart in the gas phase at ca. 150 ¡C and in a solid argon-matrix at 10 K. The assignment of the vibrations of melamine has been facilitated by the calculated thirty nine normal modes using several ab initio and density functional methods. By scaling the calculated vibrational frequencies, the theoretical computations have been demonstrated to be in good agreement with the experimental observations. The optimized equilibrium structure of melamine has been shown to be a planar but distorted-hexagonal triazine ring with three pyramidal amino groups, which result in di †erent conformers. This has been supported by the comparison between the observed and the calculated spectra for non-planar conformers 1 and 2 vs. the planar structure 3. In view of the small energy di †erences D 3h between the calculated conformers 1 and 2 and the " transition state Ï 3 (corresponding to a third-order saddle point on the potential-energy hypersurface), the melamine molecule has a Ñat potential-energy hypersurface near the equilibrium structures and the conformers can rapidly rearrange.

Articles you may be interested inCommunication: He-tagged vibrational spectra of the SarGlyH+ and H+(H2O)2,3 ions: Quantifying tag effects in cryogenic ion vibrational predissociation (CIVP) spectroscopy Exploring the correlation between network structure and electron binding energy in the ( H 2 O ) 7 − cluster through isomer-photoselected vibrational predissociation spectroscopy and ab initio calculations: Addressing complexity beyond types I-IIIClustering of water on protonated molecular ions has been investigated by vibrational predissociation spectroscopy. Systematic measurements at different cluster sizes reveal a close resemblance of the OH stretch spectra between NH 4 ϩ ͑H 2 O͒ n , CH 3 NH 3 ϩ ͑H 2 O͒ n , and H 3 O ϩ ͑H 2 O͒ n . Particularly at nу6, a sharp feature, identical to that found on ice and water surfaces, emerges at 3690 cm Ϫ1 for free-OH stretching. The feature is distinguished from the other free-OH absorption, commonly observed for small-and medium-sized (H 2 O) n clusters at 3715 cm Ϫ1 . The results, in conjunction with ab initio calculations, provide compelling evidence for 2-and 3-coordinated H 2 O in the protonated ion-water clusters.

Ab initio calculations were performed to investigate the structures, energetics, and vibrations of NH4 +(H2O) n cluster ions at n = 0−5. Equilibrium geometries of NH4 + and NH4 +−H2O are optimized at the MP2, MP4, CCD, QCISD, and B3LYP levels using the 6-31G*, 6-31G**, 6-31+G*, 6-31++G**, 6-311+G**, and 6-311++G** basis sets. The benchmark calculations indicate that using MP2 and B3LYP approaches with the 6-31+G* basis set is well suited for characterizing large NH4 +(H2O) n clusters. The two approaches correspondingly find the existence of a number of structural isomers at n = 2−5, of which the isomer with a filled first solvation shell is lowest in energy at n = 4. The calculations further predict that, at n = 5, the lowest energy isomer contains a four-membered ring with the second-shell H2O acting as a double-proton acceptor (AA). The prediction is in good agreement with the observation of jet-cooled NH4 +(H2O)5, where a characteristic hydrogen-bonded-OH stretching absorption at ∼3550 cm-1 is identified for the AA−H2O molecule in the vibrational predissociation spectra (Wang, Y.-S.; Chang, H.-C.; Jiang, J. C.; Lin, S. H.; Lee, Y. T.; Chang, H.-C. J. Am. Chem. Soc. 1998, 10 2, 8777). In this study, in addition to energetics, how hydrogen-bonding nonadditivity influences the geometries and vibrations of these clusters is analyzed.

Characterization of protonated formamide clusters by vibrational predissociation spectroscopy confirms theoretical predictions that O-protonation occurs in preference to N-protonation in formamide. The confirmation is made from a close comparison of the infrared spectra of H + [HC(O)NH 2 ] 3 and NH 4 + [HC(O)NH 2 ] 3 produced by a supersonic expansion with the spectra produced by ab initio calculations. For NH 4 + [HC(O)NH 2 ] 3 , prominent and well-resolved vibrational features are observed at 3436 and 3554 cm -1 . They derive, respectively, from the symmetric and asymmetric NH 2 stretching motions of the three formamide molecules linked separately to the NH 4 + ion core via three N-H + ‚‚‚O hydrogen bonds. Similarly distinct absorption features are also found for H + [HC(O)NH 2 ] 3 ; moreover, they differ in frequency from the corresponding vibrational modes of NH 4 + [HC(O)NH 2 ] 3 by less than 10 cm -1 . The result is consistent with a picture of proton attachment to the oxygen atom, rather than the nitrogen atom in H + [HC(O)NH 2 ] 3 . We provide in this work both spectroscopic and computational evidence for the O-protonation of formamide and its clusters in the gas phase.

A unique spatial arrangement of amide groups for CO2 adsorption is found in the open-ended channels of a zinc(II)-organic framework {[Zn4(BDC)4(BPDA)4]·5DMF·3H2O}n (1, BDC = 1,4-benzyl dicarboxylate, BPDA = N,N'-bis(4-pyridinyl)-1,4-benzenedicarboxamide). Compound 1 consists of 4(4)-sql [Zn4(BDC)4] sheets that are further pillared by a long linker of BPDA and forms a 3D porous framework with an α-Po 4(12)·6(3) topology. Remarkably, the unsheltered amide groups in 1 provide a positive cooperative effect on the adsorption of CO2 molecules, as shown by the significant increase in the CO2 adsorption enthalpy with increasing CO2 uptake. At ambient condition, a 1:1 ratio of active amide sites to CO2 molecules was observed. In addition, compound 1 favors capture of CO2 over N2. DFT calculations provided rationale for the intriguing 1:1 ratio of amide sorption sites to CO2 molecules and revealed that the nanochamber of compound 1 permits the slipped-parallel arrangement of CO2 molecules, an arrangement found in crystal and gas-phase CO2 dimer.

Behaviors of an excess proton in solute-containing water clusters were investigated using infrared spectroscopy and ab initio calculations. This investigation characterized the structures of protonated methanol-water clusters, H+(CH3OH)(H2O)n with n=2–6, according to their nonhydrogen-bonded and hydrogen-bonded OH stretches in the frequency range of 2700–3900 cm−1. Ab initio calculations indicated that the excess proton in these clusters can be either localized at a site closer to methanol, forming a methyloxonium ion core (CH3OH2+), or at a site closer to water, forming a hydronium ion core (H3O+). Infrared spectroscopic measurements verified the calculations and provided compelling evidence for the coexistence of two distinct structural isomers, CH3OH2+(H2O)3 and H3O+(CH3OH)(H2O)2, in a supersonic expansion. The spectral signatures of them (either CH3OH2+ or H3O+ centered) are the free-OH stretching absorption band at 3706 cm−1 of a single-acceptor-single-donor H2O, and the band at 3673 cm−1 of a single-acceptor CH3OH. At n=4–6, the clusters adopt structures similar to their pure water analogs with five-membered rings starting to form at n=5. The position of the excess proton in them varies sensitively with the number of solvent water molecules as well as the geometry of the clusters. To further elucidate the behaviors of the excess proton in these clusters, we analyze in detail the potential energy surface along the proton transfer coordinate for two specific isomers of n=2 and 4: MW2II and MW4I. It is found that the proton can be nearly equally shared by methanol and the water dimer subunit in the form of CH3OH–H+–(H2O)2, as substantiated by hydrogen bond cooperativity and zero-point vibrational effects.