Identification of CH3OH2+ and H3O+-centered cluster isomers from fragment-dependent vibrational predissociation spectra of H+(CH3OH)4H2O

Abstract

Cluster isomers of H ϩ ͑CH 3 OH͒ 4 H 2 O have been identified by vibrational predissociation spectroscopy in combination with mass-selected detection of photofragments. Ab initio calculations indicate that the cluster ion can exist in either CH 3 OH 2 ϩ ͑CH 3 OH͒ 3 H 2 O or H 3 O ϩ ͑CH 3 OH͒ 4 isomeric forms. They can dissociate via a methanol loss or water loss channel, depending on the structure of the isomers. While water loss is the dominant channel of the dissociation, the methanol loss channel is readily accessible by the H 3 O ϩ -centered cluster isomer. We demonstrate in this study that mass-selected detection of photofragments produced by vibrational excitation is an effective way of identifying cluster isomers in the gas phase. © 2000 American Institute of Physics. ͓S0021-9606͑00͒01917-6͔A central issue in the studies of atomic and molecular clusters has been the identification of structural isomers. Studies on this subject have been extensive in the past three decades, 1 leading to the discovery and suggestion of many interesting macromolecular structures. One noticeable example is the identification of the structure of carbon clusters by ion mobility measurements using a drift tube. Monocyclic as well as polycyclic rings have been suggested to be stable isomers of the C 60 ϩ cluster ion, 2 in addition to the spheroidal fullerenes. Unfortunately, up to now, detailed spectroscopic examination of these intriguing cluster isomers remains lacking. Identification of cluster isomers by spectroscopic means has been deemed a difficult task. This is particularly the case for neutral clusters whose sizes are difficult to determine unambiguously. Although there have been several successful examples in this direction, 3-5 the range of exploration is expected to be severely limited by the lack of proper mass-selection means. For charged clusters, this limitation is apparently removed since they can be easily sizeselected by mass spectrometry. We have recently reported on the observations of various cluster isomers of NH 4 ϩ ͑H 2 O͒ 2-7 , 6 H ϩ ͑CH 3 OH͒ 4,5 , 7 H ϩ ͓͑CH 3 ͒ 2 O͔͒ 2 ͑H 2 O͒, 8 and H ϩ ͑CH 3 OH͒͑H 2 O͒ 1-6 ͑Ref. 9͒ using vibrational predissociation spectroscopy. Identification of the isomers was accomplished by varying cluster temperature over a range of Ϯ20 K and observing the corresponding peak intensity changes with the temperature. Here, we report a new and alternative means of identifying cluster isomers and demonstrate the feasibility of this approach, which involves mass-selected detection of photofragments, using the mixed methanolwater cluster ions H ϩ ͑CH 3 OH͒ 4 H 2 O as a benchmark system. A mixed methanol-water cluster ion ͓H ϩ ͑CH 3 OH͒ m (H 2 O) n ͔ can exist in a variety of isomeric forms. Interestingly, they can be either centered with CH 3 OH 2 ϩ or with H 3 O ϩ , depending on the structure and size of the cluster. Previous experiments 9-12 and calculations 9 have shown that these two isomeric forms ͑CH 3 OH 2 ϩ and H 3 O ϩ -centered͒ are comparable in stability when the cluster is...

Hydrogen Bond Rearrangements and Interconversions of H+(CH3OH)4H2O Cluster Isomers

“…The isomers that are most likely to be responsible for the experimental spectra are WM4I and WM4III (cf. Figure ), as assigned previously is not found in the experimental spectra.…”

“…Previously, we reported results of our attempts to observe these intriguing HB rearrangements using vibrational predissociation spectroscopy in combination with density functional theory calculations. Both H 3 O + -centered and CH 3 OH 2 + -centered treelike isomers have been experimentally identified for H + (CH 3 OH) 4 (H 2 O); however, the thorough dynamical mechanism by which these isomers are interconverted is still not known.…”

(HCl)2(CH3OH)2(X) (X= NH3 VEYA H2O) KÜMELERİNDE PROTON DİNAMİĞİNİN TEORİK OLARAK İNCELENMESİ

“…Sıvı ve katı fazda bu nedenle 2 boyutlu zig-zag zincir oluşturur ve zincirler van der Waals kuvvetleri ile bir arada tutunurlar [5]. Nötral ve protonlanmış su ve metanol kümeleri bir çok grup tarafından hem deneysel hem de teorik olarak çalışılmıştır [6][7][8][9][10][11][12][13][14][15][16][17].…”

Temperature and Size Dependence of Characteristic Hydrogen-Bonded Network Structures with Ion Core Switching in Protonated (Methanol)6–10–(Water)1 Mixed Clusters: A Revisit

“…Protonated (methanol) n –(water) m mixed clusters (H + M n W m ) are one of fundamental model systems of protonated binary mixed clusters of protic molecules. Many spectroscopic and mass spectrometric studies combined with quantum chemical calculations have been performed on H + M n W m to understand the size dependence of their proton solvation motifs. − For the small sizes ( n + m ≤ ∼5), the exhaustive infrared (IR) studies have elucidated the competition between the two ion cores (protonated sites), H 3 O + and MeOH 2 + , with the change of the ratio of the two components. ,,,, The H-bonded network structure changes with the switching of the ion core type. In addition, it has also been demonstrated that the two different dissociation channels (water-loss and methanol-loss) compete with each other and they correlate to the ion core type.…”

“…39−60 For the small sizes (n + m ≤ ∼5), the exhaustive infrared (IR) studies have elucidated the competition between the two ion cores (protonated sites), H 3 O + and MeOH 2 + , with the change of the ratio of the two components. 52,53,55,59,60 The H-bonded network structure changes with the switching of the ion core type. In addition, it has also been demonstrated that the two different dissociation channels (water-loss and methanol-loss) compete with each other and they correlate to the ion core type.…”