No abstract

Various isomeric structures of the hydrated clusters of sulfuric acid, H2SO4(H2O) n (n = 1−5), are examined using a density functional molecular orbital method. Due to the small energy difference between trans and cis conformations about two OH groups of sulfuric acid, there are three types of isomeric forms of the hydrated clusters of sulfuric acid which involve the proton nontransferred trans conformer, the proton transferred trans conformer, and the proton nontransferred cis conformer of sulfuric acid. In the case of transoid H2SO4, the proton transferred ion-pair structures become more stable than the proton nontransferred structures as the number of water molecules increases. The hydrated clusters of the cis conformation remain neutral hydrogen-bonded structures even if the number of water molecules increases. All stable clusters tend to form multi-cyclic structures. While both protons of sulfuric acid participate in cyclic hydrogen bonding in the neutral structures, the OH group of HSO4 - in the ion-pair structures remains dangling because the counterion H3O+ prefers to make strong hydrogen bonds with water molecules and/or the HSO4 - moiety. The energy difference between the most stable structures of proton transferred and proton nontransferred isomers is found to be less than 1 kcal/mol in the case of n = 3 and 4 clusters. The ion-pair structure of HSO4 -H3O+(H2O)4 becomes 2 kcal/mol more stable than the hydrogen-bonded neutral cluster H2SO4(H2O)5 in the case of n = 5. Analyzing the interaction energies, many-body interaction is shown to be essential to describe the stability between neutral and ionic clusters owing to the difference of charge flow on the neutral and ion-pair structures in multi-cyclic hydrogen bonding. The calculated IR spectra of stable isomers of H2SO4(H2O) n clusters clearly demonstrate the significant red-shift of OH stretching of sulfuric acid and hydrogen-bonded OH stretching of water molecules as the number of cluster size increases. The IR spectra of the OH stretching of hydrated sulfuric acid are predicted to appear in three regions, hydrogen-bonded OH stretching of H3O+ (2500∼2800 cm-1), hydrogen-bonded OH stretching of water molecules (3100∼3500 cm-1), and nonhydrogen-bonded OH stretching of water molecules (3800∼3900 cm-1).

The molecular structures of the hydrated clusters of the HCl molecule, HCl(H2O)n, n=1–5, are examined by employing density functional molecular orbital methods. The most stable structures of the n=1–3 clusters are found to be of the proton nontransferred type. In the case of the n=4 cluster, the proton nontransferred and proton transferred structures have nearly similar energies. There are several stable isomers for the n=5 case and the structures of these isomers are found to be all proton transferred. The relative stabilities of the direct ion-pair H+Cl−(H2O)n and the indirect ion-pair H3O+(H2O)n−1Cl− are discussed in conjunction with their structures. The prediction of the IR spectra of the stable HCl(H2O)n clusters clearly indicate the large red-shifts of the H–Cl stretching and hydrogen-bonded O–H stretching frequencies.

Pure methane ices (CH 4 ) were irradiated at 10 K with energetic electrons to mimic the energy transfer processes that occur in the track of the trajectories of MeV cosmic-ray particles. The experiments were monitored via an FTIR spectrometer (solid state) and a quadrupole mass spectrometer (gas phase). Combined with electronic structure calculations, this paper focuses on the identification of CH x (x ¼ 1Y4) and C 2 H x (x ¼ 2Y6) species and also investigates their formation pathways quantitatively. The primary reaction step is determined to be the cleavage of a carbonhydrogen bond of the methane molecule to form a methyl radical (CH 3 ) plus a hydrogen atom. Hydrogen atoms recombined to form molecular hydrogen, the sole species detected in the gas phase during the irradiation exposure. In the matrix two neighboring methyl radicals can recombine to form an internally excited ethane molecule (C 2 H 6 ), which either can be stabilized by the surrounding matrix or was found to decompose unimolecularly to the ethyl radical (C 2 H 5 ) plus atomic hydrogen and then to the ethylene molecule (C 2 H 4 ) plus molecular hydrogen. The initially synthesized ethane, ethyl, and ethylene molecules can be radiolyzed subsequently by the impinging electrons to yield the vinyl radical (C 2 H 3 ) and acetylene (C 2 H 2 ) as degradation products. Upon warming the ice sample after the irradiation, the new species are released into the gas phase, simulating the sublimation processes interstellar ices undergo during the hot core phase or comets approaching perihelion. Our investigations also aid the understanding of the synthesis of hydrocarbons likely to be formed in the aerosol particles and organic haze layers of hydrocarbon-rich atmospheres of planets and their moons such as Titan.