Understanding Fermi resonances in the complex vibrational spectra of the methyl groups in methylamines.

Vibrational spectra of the methyl groups in mono-methylamine (MMA), dimethylamine (DMA), and trimethylamine (TMA) monomers and their clusters were measured in three experimental set-ups to capture their complex spectral features as a result of bend/umbrella-stretch Fermi resonance (FR). Multiple bands were observed between 2800 and 3000 cm-1 corresponding to the methyl groups for MMA and DMA. On the other hand, the corresponding spectrum of TMA is relatively simple, exhibiting only four prominent bands in the same frequency window, even though TMA has a larger number of methyl groups.

Infrared spectroscopic observation of the McLafferty rearrangement in ionized 2-pentanone.

The McLafferty rearrangement is a well-known process in mass spectrometry. In ionization of organic molecules containing a carbonyl group, β cleavage occurs following transfer of a hydrogen atom of aliphatic CH at the γ position to the carbonyl group. Although the McLafferty rearrangement has undergone numerous mass spectrometric investigations, no spectroscopic investigation of the enolized radical cation generated in the hydrogen atom transfer has been carried out.

Migrations and Catalytic Action of Water Molecules in the Ionized Formamide-(HO) Cluster.

Isomerization dynamics involving the migrations, proton transfer reaction, and catalytic actions of water molecules upon vertical ionization of the formamide (FA)-(HO) cluster is investigated by the infrared spectroscopy and theoretical reaction path search calculation. The infrared spectroscopic result indicates the [FA-(HO)] cation has the hydrogen-bonded structure of the enol isomer cation of formamide and the water dimer. This structure is formed by proton transfer from the CH bond to the carbonyl group through the catalytic action of the water molecules.

Infrared Spectroscopic Study on Trimethyl Amine Radical Cation: Correlation between Proton-Donating Ability and Structural Deformation.

Barrierless intermolecular proton transfer from a CH bond has recently been reported in the vertical ionization of the trimethyl amine (TMA) dimer. This result indicates the remarkable enhancement of the proton-donating ability of the CH bond in its cationic state. In the present study, we have carried out an infrared spectroscopy of the neutral and cationic TMA in the CH stretch region and their theoretical calculations to investigate the mechanism of enhancement of the proton-donating ability in the cationic state.

A liquid crucible model for aggregation of phenylacetylene in the gas phase.

The aggregation of phenylacetylene in the gas phase was investigated by selectively recording the IR spectra of clusters consisting of up to six monomer units. Analysis of the IR spectra with the aid of B97-D3/aug-cc-pVDZ level calculations reveals the formation of an anti-parallel π-stacked structure of the dimer and a hitherto unknown assembly of clusters incorporating exclusively aromatic C-Hπ interactions between various units of the trimer and higher clusters. The aggregation behaviour of phenylacetylene in the gas phase is fundamentally different from benzene, phenol and aniline vis-à-vis their crystal structures.

IR Spectroscopic Investigation on Isomerization of Ionized Ethylene Glycol.

It has been known that photoionization of ethylene glycol generates protonated methanol when the ionization energy is in the vicinity of the vertical ionization energy. Although two different isomerization paths have been proposed for the protonated methanol production, the isomerization path has not yet been identified. To investigate the isomerization of ionized ethylene glycol, infrared (IR) predissociation spectroscopy based on vacuum ultraviolet photoionization is carried out for neutral and cationic ethylene glycol and partially deuterated isotopomer (HOCDCDOH).

An integrated experimental and theoretical reaction path search: analyses of the multistage reaction of an ionized diethylether dimer involving isomerization, proton transfer, and dissociation.

An ionization-induced multistage reaction of an ionized diethylether (DEE) dimer involving isomerization, proton transfer, and dissociation is investigated by combining infrared (IR) spectroscopy, tandem mass spectrometry, and a theoretical reaction path search. The vertically-ionized DEE dimer isomerizes to a hydrogen-bonded cluster of protonated DEE and the [DEE-H] radical through barrierless intermolecular proton transfer from the CH bond of the ionized moiety. This isomerization process is confirmed by IR spectroscopy and the theoretical reaction path search.

Infrared Spectroscopic Study of the Acidic CH Bonds in Hydrated Clusters of Cationic Pentane.

Infrared spectroscopy of the hydrated clusters of cationic pentane, which are generated through the vacuum ultraviolet photoionization in the gas phase, is carried out to probe the acidic properties of their CH bonds. The monohydrated pentane cation forms the proton-shared structure, in which the proton of CH in cationic pentane is shared between the pentyl radical and water molecule. In the di- and trihydrated clusters, the proton of CH is completely transferred to the water moiety so that the clusters are composed of the pentyl radical and protonated water cluster.

Antifreeze Activity of Xylomannan from the Mycelium and Fruit Body of Flammulina velutipes.

An identified class of antifreeze, a xylomannan-based thermal hysteresis (TH)-producing glycolipid, has been discovered from diverse taxa, including plants, insects, and amphibians. We isolated xylomannan from the mycelium and fruit body of the basidiomycete Flammulina velutipes using successive hot extraction with water, 2% and 25% aqueous KOH, and gel filtration chromatography. The xylomannan from the fruit body had a recrystallization inhibiting (RI) activity (RI=0.

Infrared Spectroscopic Investigation of the Acidic CH Bonds in Cationic n-Alkanes: Pentane, Hexane, and Heptane.

Radical cations of n-alkanes (pentane, hexane, and heptane) in the gas phase are investigated by infrared predissociation spectroscopy with the argon or nitrogen tagging. All-trans and gauche-involving conformers are identified for these cations by comparisons of observed infrared spectra and vibrational simulations. Intense CH stretch bands are observed in the frequency region lower than the normal alkyl CH stretch frequency.

Hyperconjugation in diethyl ether cation versus diethyl sulfide cation.

Ionization of a molecule can greatly alter its electronic structure as well as its geometric structure. In this collaborative experimental and theoretical study, we examined variance in hyperconjugation upon ionization of diethyl ether (DEE) and diethyl sulfide (DES). We obtained the experimental gas phase vibrational spectra of DEE, DES, DEE(+), DES(+), DEE(+)-Ar, and DES(+)-Ar in the wavenumber region of 2500 to 3600 cm(-1).

Infrared spectroscopic investigation of photoionization-induced acidic C-H bonds in cyclic ethers.

Infrared (IR) predissociation spectroscopy based on vacuum-ultraviolet photoionization detection is performed for the neutral and cationic tetrahydrofuran (THF) and tetrahydropyran (THP). The CH bonds in neutral THF and THP are regarded as aprotic, even though the CH bonds are weakened by the negative hyperconjugation. After 118 nm photoionization, however, the negative hyperconjugation changes to the positive hyperconjugation and their CH bond acidities remarkably increase.

The large variation in acidity of diethyl ether cation induced by internal rotation about a single covalent bond.

In the IR spectrum of the diethyl ether cation, an extraordinarily intense band, with an extremely broad bandwidth, was observed at 2700 cm(-1), much lower frequency than normal CH stretch frequencies. This band is assigned to the stretch band of the CH bond, which is hyperconjugated with the singly occupied molecular orbital of the oxygen atom. The hyperconjugation causes the delocalization of the σ electron of the CH bond so that it enhances the acidity of the CH bond as well as the CH stretch band intensity.

Acid-base formalism in dispersion-stabilized S-H···Y (Y═O, S) hydrogen-bonding interactions.

The role of sulfhydryl (S-H) group as hydrogen bond donor is not as well studied as that of hydroxyl (O-H). In this work we report on the hydrogen-bonding properties of S-H donor in 1:1 complexes of H2S with diethyl ether (Et2O), dibutyl ether (Bu2O), and 1,4-dioxane (DO). The complexes were prepared in supersonic jet and investigated using infrared predissociation spectroscopy based on VUV photoionization detection.

Isomer-selective infrared spectroscopy of the cationic trimethylamine dimer to reveal its charge sharing and enhanced acidity of the methyl groups.

Infrared predissociation spectroscopy of the trimethylamine dimer cation generated by the vacuum-ultraviolet photoionization is isomer-selectively carried out by monitoring two main fragment channels, protonated trimethylamine and the trimethylamine monomer cation. The spectral carriers monitored by these two channels are assigned to different isomers of the trimethylamine dimer cation. One is the charge-shared (hemibond) structure, in which the positive charge is intermolecularly delocalized over the dimer through the interaction between the nonbonding orbitals of the nitrogen atoms.

The Intermolecular S−H⋅⋅⋅Y (Y=S,O) hydrogen bond in the H2S dimer and the H2 S-MeOH complex.

The nature of the S−H⋅⋅⋅S hydrogen-bonding interaction in the H2 S dimer and its structure has been the focus of several theoretical studies. This is partly due to its structural similarity and close relationship with the well-studied water dimer and partly because it represents the simplest prototypical example of hydrogen bonding involving a sulfur atom. Although there is some IR data on the H2 S dimer and higher homomers from cold matrix experiments, there are no IR spectroscopic reports on S−H⋅⋅⋅S hydrogen bonding in the gas phase to-date.

Experimental and theoretical investigations of isomerization reactions of ionized acetone and its dimer.

Ionization dynamics of acetone and its dimer in supersonic jets is investigated by a combination of experimental and theoretical techniques, both of which have recently been developed. In experiments, the neutral and the cationic structures are explored by infrared predissociation spectroscopy with the vacuum-ultraviolet photoionization detection schemes. Reaction paths following the one-photon ionization of the acetone monomer and its dimer have been studied by the joint use of several theoretical methods including the ab initio molecular dynamics, the global reaction route mapping, the intrinsic reaction coordinate, and the artificial force induced reaction calculations.

Long-range migration of a water molecule to catalyze a tautomerization in photoionization of the hydrated formamide cluster.

The dynamics on the vacuum-ultraviolet one-photon ionization of a formamide-water cluster is investigated by a combination of theoretical reaction-path search and infrared spectroscopic methods. A keto-enol tautomerization of the formamide moiety occurs after photoionization by a catalytic action of the water molecule accompanied with its long-distance migration; the water molecule in the cluster migrates almost one turn around the formamide moiety. During the migration, the water molecule abstracts the proton of CH in the formamide moiety and carries it to the O atom side in the carbonyl group through a "catch and release"-type catalytic action.

Intermolecular proton-transfer in acetic acid clusters induced by vacuum-ultraviolet photoionization.

Infrared (IR) spectroscopy based on vacuum-ultraviolet one-photon ionization detection was carried out to investigate geometric structures of neutral and cationic clusters of acetic acid: (CH(3)COOH)(2), CH(3)COOH-CH(3)OH, and CH(3)COOH-H(2)O. All the neutral clusters have cyclic-type intermolecular structures, in which acetic acid and solvent molecules act as both hydrogen donors and acceptors, and two hydrogen-bonds are formed. On the other hand, (CH(3)COOH)(2) (+) and (CH(3)COOH-CH(3)OH)(+) form proton-transferred structures, where the acetic acid moiety donates the proton to the counter molecule.

Vibrational spectroscopy of size-selected neutral and cationic clusters combined with vacuum-ultraviolet one-photon ionization detection.

Various vibrational spectroscopic techniques combined with vacuum-ultraviolet one-photon ionization mass spectrometry have recently been developed for jet-cooled molecules and clusters. These techniques open general applications of size-selected infrared and also Raman spectroscopies to neutral clusters by overcoming difficulties in the conventional vibrational spectroscopic methods for jet-cooled clusters. Their spectroscopic principles have been demonstrated by investigations for neutral clusters of simple protic molecules without an ultraviolet chromophore, which is necessary for the conventional vibrational spectroscopic methods of gas phase clusters based on the fluorescence or multiphoton ionization detection.

Infrared predissociation spectroscopy of cluster cations of protic molecules, (NH3)n+, n=2-4 and (CH3OH)n+, n=2,3.

Infrared predissociation spectroscopy is carried out for the structure investigation of unprotonated cluster cations of protic molecules such as ammonia and methanol, which are generated through vacuum-ultraviolet one-photon ionization of their jet-cooled neutral clusters. The observed spectral features show that the cluster cations have the proton-transferred type structures, where a pair of a protonated cation and a neutral radical, NH(4) (+)..

Size-selected infrared predissociation spectroscopy of neutral and cationic formamide-water clusters: stepwise growth of hydrated structures and intracluster hydrogen transfer induced by vacuum-ultraviolet photoionization.

Vibrational spectroscopy of size-selected formamide-water clusters, FA-(H2O)n , n = 1-4, prepared in a supersonic jet is performed with vacuum-ultraviolet-ionization detected-infrared predissociation spectroscopy (VUV-ID-IRPDS). The cluster structures are determined through comparisons of the observed IR spectra with theoretical calculations at the MP2/6-31++G** level. The FA-(H2O)n , n = 1-3, clusters have ring-type structures, where water molecules act as both single donor and single acceptor in the hydrogen-bond network between the amino and carbonyl groups of FA.

Infrared predissociation spectroscopy of ammonia cluster cations (NH3)n+ (n=2-4) produced by vacuum-ultraviolet photoionization.

Infrared predissociation spectroscopy of vacuum ultraviolet-pumped ion (IRPDS-VUV-PI) is performed on ammonia cluster cations (NH3)n+ (n=2-4) that are produced by VUV photoionization in supersonic jets. The structures of (NH3)2+ and (NH3)4+ are determined through the observation of infrared spectra and vibrational calculations based on ab initio calculations at the MP2/6-31G** and 6-31++G** levels. (NH3)2+ is found to be of the "hydrogen-transferred" form having the (H3N+-.