Hydrogen bond network structures of protonated dimethylamine clusters H+(DMA)n (n = 3-7).

Infrared spectroscopy of protonated dimethylamine clusters, H+(DMA)n, (n = 3-7), and their Ar-tagged clusters was performed in the NH and CH stretching vibrational region to explore their hydrogen bond network structures. A stable isomer search and vibrational spectral simulations of the clusters were also carried out to support the interpretations of the observed spectra. Weakly hydrogen-bonded NH stretching vibrational bands, which are characteristic of cyclic structures of small-sized protonated clusters, are observed in the spectra of the Ar-tagged clusters of n ≥ 5, while only linear chain type structures are suggested for the Ar-tagged clusters of n = 3-4 and the bare clusters of all the sizes. These results demonstrate that the size and temperature dependence of the hydrogen bond network structures of the protonated dimethylamine clusters is analogous to that of protonated monohydric alcohol clusters.

Near-infrared spectroscopy of H3O+⋯Xn (X = Ar, N2, and CO, n = 1-3).

Near-infrared (NIR) spectra of H3O+⋯Xn (X = Ar, N2, and CO, n = 1-3) in the first overtone region of OH-stretching vibrations (4800-7000 cm-1) were measured. Not only OH-stretching overtones but also several combination bands are major features in this region, and assignments of these observed bands are not obvious at a glance. High-precision anharmonic vibrational simulations based on the discrete variable representation approach were performed. The simulated spectra show good agreement with the observed ones and provide firm assignments of the observed bands, except in the case of X = CO, in which higher order vibrational mode couplings seem significant. This agreement demonstrates that the present system can be a benchmark for high precision anharmonic vibrational computations of NIR spectra. Band broadening in the observed spectra becomes remarkable with an increase of the interaction with the solvent molecule (X). The origin of the band broadening is explored by rare gas tagging experiments and anharmonic vibrational simulations of hot bands.

Infrared spectroscopy of [H2O-Xn]+ (n = 1-3, X = N2, CO2, CO, and N2O) radical cation clusters: competition between hydrogen bond and hemibond formation of the water radical cation.

The water radical cation H2O+ is an important intermediate in radiation chemistry and radiobiology, and its role in radical reactions has recently attracted much attention. However, knowledge of intermolecular interactions of H2O+ remains very limited due to its high reactivity. We focus on structures of [H2O-X]+, formed by H2O+ with a counter molecule X, as a model for intermediates in reactions of H2O+. Such structural information provides the basis for understanding reaction processes of H2O+. Two structural motifs for [H2O-X]+ have been known: hydrogen bond and hemibond, which are expected to have very different reactivities from each other. Due to the high acidity of H2O+, the H-bonded form is mostly considered to be preferred. However, it has recently been reported that the hemibonded form is preferred in some cases. We perform infrared photodissociation spectroscopy and quantum chemical calculations on [H2O-Xn]+ (n = 1-3, X = N2, CO2, CO, and N2O) to determine their structural motifs. The competition between the hydrogen bond and hemibond formation is systematically examined based on the firm structure information. The competition is interpreted in terms of the proton affinity (PA) and the ionization potential (IP) of X. The rough ranges of PA and IP for the priority of the hemibond motif are determined. The impact of other factors on the competition is also discussed.

Experimental confirmation of the Badger-Bauer rule in the protonated methanol clusters: weak hydrogen bond formation as a measure of terminal OH acidity in hydrogen bond networks.

We report a linear correlation between the OH stretch frequency shift of the protonated methanol cluster, H+(MeOH)n, upon the π-hydrogen bond formation with benzene and the enthalpy change in clustering of H+(MeOH)n to H+(MeOH)n+1. This result suggests a new method to explore hydrogen bond strength in hydrogen bond networks.

Infrared Spectroscopy of (Benzene-H2S-Xn)+, X = H2O (n = 1 and 2) and CH3OH (n = 1), Radical Cation Clusters: Microsolvation Effects on the S-π Hemibond.

An unconventional covalent bond in which three electrons are shared by two centers is called hemibond. Hemibond formation frequently competes with proton transfer (or ionic hydrogen bond formation), but there have been a few experimental reports on such competition. In the present study, we focus on the (benzene-H2S)+ radical cation cluster, which is a model system of the S-π hemibond. The stability of the S-π hemibond to the microsolvation by water and methanol is explored with infrared spectroscopy of (benzene-H2S-Xn)+, X = H2O (n = 1 and 2) and CH3OH (n = 1), clusters. We also perform energy-optimization and vibrational simulations of (benzene-H2S-Xn)+. By comparison among the observed and simulated spectra, we determine the intermolecular binding motifs in (benzene-H2S-Xn)+. While the S-π hemibonded isomer is exclusively populated in (benzene-H2S-H2O)+, both the hemibonded and proton-transferred isomers coexist in [benzene-H2S-(H2O)2]+ and (benzene-H2S-CH3OH)+. Breaking of the S-π hemibond by the microsolvation is observed, and its solvent and cluster size dependence is interpreted by the proton affinity and the coordination property of the solvent moiety.

Infrared spectroscopy and theoretical structure analyses of protonated fluoroalcohol clusters: the impact of fluorination on the hydrogen bond networks.

To explore the impact of fluorination on the hydrogen bond networks of protonated alkylalcohols, infrared spectroscopy and theoretical computations of protonated 2,2,2-trifluoroethanol clusters, H+(TFE)n, (n = 4-7), were performed. It has been demonstrated that the development of the hydrogen bond networks from a linear type to cyclic types occurs in this size region for the protonated alkylalcohol clusters. In contrast, infrared spectroscopy of H+(TFE)n in the OH/CH stretch region clearly indicated that the linear type structures are held in the whole size range, irrespective of temperature of the clusters. The extensive stable isomer structure search of H+(TFE)n based on our latest sampling approach supported the strong preference of the linear type hydrogen bond networks. Detailed analyses of the free OH stretching vibrational bands evidenced the intra- and intermolecular OH⋯FC interactions in the clusters. In addition, infrared spectra of protonated clusters of 2,2-difluoroethanol, 2,2-difluoropropanol, and 3,3,3-trifluoropropanol were measured for n = 4 and 5, and their spectra also indicated the effective inhibition of the cyclic hydrogen bond network formation by the fluorination.

How many methanol molecules effectively solvate an excess proton in the gas phase? Infrared spectroscopy of H+(methanol)n-benzene clusters.

An excess proton in a hydrogen-bonded system enhances the strength of hydrogen bonds of the surrounding molecules. The extent of this influence can be a measure of the number of molecules effectively solvating the excess proton. Such extent in methanol has been discussed by the observation of the π-hydrogen-bonded OH stretch bands of the terminal sites of protonated methanol clusters, H+(methanol)n, in benzene solutions, and it has been concluded that ∼8 molecules effectively solvate the excess proton (Stoyanov et al., Chem. Eur. J. 2008, 14, 3596-3604). In the present study, we performed infrared spectroscopy of H+(methanol)n-benzene clusters in the gas phase. The cluster size and hydrogen-bonded network structure are identified by the tandem mass spectrometric technique and the comparison of the observed infrared spectra with density functional theory calculations. Though changes of the preferred hydrogen bond network type occur with the increase of cluster size in the gas phase clusters, the observed size dependence of the π-hydrogen bonded OH frequency agrees well with that in the benzene solutions. This means that the observations in both the gas and condensed phases catch the same physical essence of the excess proton solvation by methanol.

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. The discrete variable representation (DVR) based ab initio anharmonic algorithm with potential energy surface (PES) at CCSD/aug-cc-pVDZ quality is able to capture all the experimentally observed spectral features across all three amines, and the constructed vibrational Hamiltonian was used to analyze the couplings that give rise to the observed FR patterns. It was observed that the vibrational coupling among CH stretch modes on different methyl groups is weak (less than 2 cm-1) and stronger vibrational coupling is found to localize within a methyl group. In MMA and DMA, the complex feature between 2850 and 2950 cm-1 is a consequence of closely packed overtone states that gain intensities by mixing with the stretching modes. The simplification of the spectral pattern of TMA can be understood by the red-shift of the symmetric CH3 stretching modes by about 80 cm-1 relative to MMA, which causes the symmetric CH3 stretch to shift outside the FR window.

Cooperativity of the Activated CH/π Interaction Probed through CH Stretching Vibrations in Phenol-(Acetylene)n (∼16 ≤ n ≤ ∼30) and (Acetylene)n+ (10 ≤ n ≤ 70) Clusters.

The acidity of acetylene CH is stronger than that of alkane CH, and the attractive interaction between an acetylene CH with π-electrons, which shows a clear hydrogen bond property, is called activated CH/π interaction. In this study, cooperative enhancement of the activated CH/π interaction has been probed through the cluster size dependence of the red shift of the acetylene CH stretching vibrational band in neutral phenol-(acetylene)n (∼16 ≤ n ≤ ∼30) and (acetylene)n+ (10 ≤ n ≤ 70). In both the clusters, the characteristic asymmetric (red-shaded) shape of the CH stretch band has been observed. This band shape means that the magnitude of the activated CH/π interaction is enhanced by its cooperativity in the interior moiety of the cluster. The red-shifted component of the band extends with increasing cluster size, and the edge of this component seems to reach to the CH stretch band position of crystalline acetylene at the size of n = 20-30, indicating that dozens of molecules need to interact each other to maximize cooperativity in the activated CH/π interaction of acetylene. On the other hand, the peak position of the band does not converge to that of crystalline acetylene in the observed size range. The present result suggests that the spectral convergence of acetylene clusters to the bulk may occur in the cluster size range of hundreds or larger.

Anharmonic Coupling Revealed by the Vibrational Spectra of Solvated Protonated Methanol: Fermi Resonance, Combination Bands, and Isotope Effect.

Intriguing vibrational features of solvated protonated methanol between 2400-3800 cm-1 are recorded by infrared predissociation spectroscopy. Positions of absorption bands corresponding to OH stretching modes are sensitive to changes in solvation environments, thus leading to changes in these vibrational features. Two anharmonic coupling mechanisms, Fermi resonance (FR) contributed by bending overtones and combination band (CB) associated with intermolecular stretching modes, are known to lead to band splitting of OH stretching fundamentals in solvated hydronium and ammonium. Theoretical analyses based on the ab initio anharmonic algorithm not only well reproduce the experimentally observed features but also elucidate the magnitudes of such couplings and the resulting interplay between these two mechanisms, which provide convincing assignments of the spectral patterns. Moreover, while the hydroxyl group plays the leading role in all the above-mentioned features, the role of the methyl group is also analyzed. Through the H/D isotope substitution, we identify overtones of the methyl-hydroxyl rocking modes and their participation in FR.

Strong Fermi Resonance Associated with Proton Motions Revealed by Vibrational Spectra of Asymmetric Proton-Bound Dimers.

Infrared spectra for a series of asymmetric proton-bound dimers with protonated trimethylamine (TMA-H+ ) as the proton donor were recorded and analyzed. The frequency of the N-H+ stretching mode is expected to red shift as the proton affinity of proton acceptors increases. The observed band, however, shows a peculiar splitting of approximately 300 cm-1 with the intensity shifting pattern resembling a two-level system. Theoretical investigation reveals that the observed band splitting and its extraordinarily large gap of around 300 cm-1 is a result of strong coupling between the fundamental of the proton stretching mode and overtone states of the two proton bending modes, that is commonly known as Fermi resonance (FR). We also provide a general theoretical model to link the strong FR coupling to the quasi-two-level system. Since the model does not depend on the molecular specification of TMA-H+ , the strong coupling we observed is an intrinsic property associated with proton motions.

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, ß 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. 2-Pentanone is the simplest ketone containing CH bonds at the γ position. In this study, infrared predissociation spectroscopy for both neutral and ionized 2-pentanone in the gas phase through vacuum ultraviolet ionization detection is performed to investigate the ionization-induced isomerization and to observe the enolized product. An OH stretch band is observed in the infrared spectrum of ionized 2-pentanone, and this demonstrates its enolization accompanying the rearrangement of an alkyl hydrogen. The enolization of ionized 2-pentanone is theoretically supported by the reaction path search based on the anharmonic downward distortion following method.

Vibrational spectroscopy of protonated amine-water clusters: tuning Fermi resonance and lighting up dark states.

Strong coupling between stretching fundamentals and bending overtones of vibrational modes, known as Fermi resonance (FR), has been observed for proton motions in the protonated trimethylamine-water cluster. To investigate the role of FR, we examined the vibrational spectra of other three protonated ammonia/amine-water clusters, including the NH4+ ion and its mono- and di-methylated analogues, respectively, with and without argon tagging. In these systems, a simple frequency-scaled harmonic oscillator model will predict only one strong band between 2600 and 3200 cm-1 uniquely due to the hydrogen-bonded NH stretching fundamental for a given conformer. In the experimental vibrational spectra, however, multiple sharp bands were observed. Such a discrepancy often leads to the notions of the coexistence of multiple conformers and/or the appearance of an overtone state as a result of FR. In this work, we applied a discrete variable representation (DVR) implementation of ab initio anharmonic algorithms and demonstrated how one N-H+ stretching fundamental can lead to multiple bands as a result of intrinsic anharmonic couplings. A prominent effect of tuning these FR bands and lighting up dark overtone states in this wide frequency range was investigated by changing the number of methyl groups in the protonated amine moiety. The effect of Ar-tagging was also analyzed and decent agreement between the experimental and simulated spectra certified the above-mentioned simple pictures. We also found that the coupling constant for trimethylamine is the largest among these protonated amine-water clusters, and the overall coupling strength decreases as the hydrogen-bonded NH stretching frequency redshifts in the order of dimethylamine, methylamine, and ammonia.

Effects of mixing between short-chain and branched-chain alcohols in protonated clusters.

The previous analysis of the neat protonated branched-chain alcohol clusters revealed the impact of steric repulsion and dispersion of the bulky alkyl group on the hydrogen-bonded (H-bonded) structures and their temperature-dependence. To further understand the influence of the alkyl groups in H-bonded clusters, we studied the mixing of the two extremes of alcohols, methanol (MeOH) and tert-butyl alcohol (t-BuOH), with an excess proton. Infrared spectroscopy and a structural search of first principles calculations on the size-selected clusters H+(MeOH)m(t-BuOH)t (m + t = 4 and 5) were conducted. Temperature-dependence of the dominant H-bonded structures was explored by the Ar-tagging technique and quantum harmonic superposition approach. By introducing the dispersion-corrected density functional theory methods, it was shown that the effects of dispersion due to the bulky alkyl groups in the mixed clusters cannot be ignored for t≥ 2. The computational results qualitatively depicted the characteristics of the observed IR spectra, but overestimation of the temperature-dependence with dispersion correction was clearly seen due to the unbalanced correction between linear H-bonded structures and compact cyclic ones. These results demonstrate the importance of extensive investigation and benchmarks on different levels of theory, and that a properly sampled structure database is crucial to evaluate theoretical models.

Migrations and Catalytic Action of Water Molecules in the Ionized Formamide-(H2O)2 Cluster.

Isomerization dynamics involving the migrations, proton transfer reaction, and catalytic actions of water molecules upon vertical ionization of the formamide (FA)-(H2O)2 cluster is investigated by the infrared spectroscopy and theoretical reaction path search calculation. The infrared spectroscopic result indicates the [FA-(H2O)2]+ 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. The isomerization paths involving this enolization in ionized FA-(H2O)2 are explored by using the anharmonic downward distortion following method. We found multiple enolization paths which accompany proton exchanges among the formamide moiety and water molecules through the catalytic actions of the water molecules.

Influence of the microsolvation on hemibonded and protonated hydrogen sulfide: infrared spectroscopy of [(H2S)n(X)1]+ and H+(H2S)n(X)1 (n = 1 and 2, X = water, methanol, and ethanol).

Changes of the excess charge accommodation motif in hemibonded and protonated hydrogen sulfide by microsolvation are studied by infrared spectroscopy of [(H2S)n(X)1]+ and H+(H2S)n(X)1 (n = 1 and 2, X = water, methanol, and ethanol) clusters. While the hemibond in the (H2S)2+ ion core is stable to the microhydration by a single water molecule, the hemibond is broken by the proton transfer with the microsolvation by a single methanol or ethanol molecule. Hetero hemibond formation between hydrogen sulfide and these solvent molecules is not observed. On the other hand, the excess proton in H+(H2S)n can be easily transferred to the solvent molecule, even though the proton affinity of the solvent molecule is lower than that of hydrogen sulfide. Implications of these results to the charge accommodation by sulfur under the biological conditions are discussed.

Effects of solvent molecules on hemi-bonded (CH3SH)2+: infrared absorption of [(CH3SH)2-X]+ with X = H2O, (CH3)2CO, or NH3 and (CH3SH)n+ (n = 3-6).

Three-electron two-center (3e-2c) hemi-bonds play important roles in the oxidation and electron transport of proteins and are implicated to be involved in some neurodegenerative diseases. Our previous investigations on infrared (IR) spectra of (CH3SH)2+ using vacuum-ultraviolet photoionization, infrared dissociation, and time-of-flight detection have shown that (CH3SH)2+ is (3e-2c)-bonded. To investigate the influence of the solvent molecules on the (3e-2c)-bonded (CH3SH)2+ in a supersonic jet, we added H2O or (CH3)2CO or NH3 or (CH3SH)n (n = 1-4) to (CH3SH)2+ and investigated their IR action spectra. The (3e-2c)-bonded (CH3SH)2+ ion core was maintained when a molecule of H2O or (CH3)2CO or CH3SH binds, indicating that the ion core is more stable than the hydrogen bond, whereas the (3e-2c)-bond became broken by a NH3 molecule because the proton transfer led to a more stable hydrogen-bonded structure. The spectral features of the SH-stretching modes of (CH3SH)n+ (n = 3-6) indicate that the (3e-2c)-bonded (CH3SH)2+ ion core is maintained and the first two additional CH3SH are H-bonded to the free SH groups of the ion core. For larger clusters with n = 5 and 6, the additional solvent molecules likely bind to the first solvation shell. These results show also that the (3e-2c)-bonded S∴S structure is more stable than the S∴O and S∴N structures in [(CH3SH)2-X]+ with X = H2O or (CH3)2CO or CH3SH or NH3.

Electronic and Infrared Spectroscopy of Benzene-(H2S)n (n = 1 and 2): The Prototype of the SH-π Interaction.

Benzene-(H2S)n (n = 1 and 2) clusters are the simplest prototype exemplifying the SH-π interaction. Electronic and infrared spectroscopies were applied to the benzene-(H2S)n clusters under the molecular beam condition. The S1-S0 electronic spectrum was observed by one-color resonant two-photon ionization combined with mass spectrometry. Ionization depletion infrared spectra were also observed in the CH and SH stretch regions. The isomer-selective infrared spectra demonstrated that at least two isomers of n = 1 coexist under the present beam condition, and both of them have the SH-π bound structures. One isomer showed a red-shift in the S1-S0 electronic transition relative to that of bare benzene, while the electronic transition of another isomer was slightly blue-shifted. For n = 2, we confirmed a structure, in which hydrogen-bonded H2S dimer is located on top of the aromatic ring.

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. In the spectrum of the cation, the CH stretch band shows a long tail of up to 2600 cm-1. This tail component is attributed to the CH bond hyperconjugated with the nonbonding orbital at the nitrogen atom through geometry deformation (excitation of molecular vibrations) with the excess energy upon photoionization. This hyperconjugation causes the delocalization of the σ electron of the CH bond to the singly occupied nonbonding orbital so that the proton-donating ability of the CH is enhanced. It is shown that the excitation of the CN stretching vibration is especially effective in promoting the hyperconjugation.

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 (HOCD2CD2OH). The IR spectroscopic results indicate that ionized ethylene glycol isomerizes to the protonated methanol-HCO complex, and the isomerization path involving the double proton transfer is identified. This isomerization path is also supported by the theoretical isomerization path search, which demonstrates that several reaction pathways are mutually intercommunicated.