IR+vacuum ultraviolet (118 nm) nonresonant ionization spectroscopy of methanol monomers and clusters: Neutral cluster distribution and size-specific detection of the OH stretch vibrations

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.

Laser Spectroscopy of Helium Solvated Clusters of Methanol and Methanol-Water in the Symmetric Methyl Stretching Band

Mid-infrared spectra of methanol and methanol-water clusters have been investigated in the symmetric CD₃ stretching band of CD₃OH and CD₃OD. We find that the position of this band provides a useful signature of the general type of hydrogen-bonded cluster it is associated with. Our results are consistent with those previously reported in the OH stretching region (Sulaiman, M. I.; Yang, S.; Ellis, A. M. J. Phys. Chem. A 2017, 121, 771-776) in that methanol clusters from the trimer to the pentamer are cyclic and that mixed clusters with one water molecule (and at least two methanol molecules) are also cyclic. We additionally provide evidence that the methanol trimer adopts a chair-like structure (as opposed to bowl-like), that mixed clusters with a larger number of water molecules are also cyclic, and that branched methanol clusters contribute to the depletion signal in larger methanol clusters. We performed double-hybrid DFT calculations which support these interpretations.

The evolution towards cyclic structures in the aggregation of aromatic alcohols: the dimer, trimer and tetramer of 2-phenylethanol

Gas-phase spectroscopic studies of alcohol clusters offer accurate information on the influence of non-covalent interactions on molecular recognition, and are of paramount importance to model supramolecular and biological chemical processes. Here, we examine the role of the aliphatic side chain in the self-aggregation of aromatic alcohols, using a multi-methodological gas-phase approach which combines microwave spectroscopy and mass-resolved electronic and vibrational laser spectroscopy. Spectroscopic and electronic structure computations were carried out for the dimer, trimer and tetramer of 2-phenylethanol, extending previous investigations on smaller aromatic alcohols. While the conformational flexibility of the ethyl group anticipates a variety of torsional isomers, the intra- and inter-molecular interactions restrict molecular conformations and favour particularly stable isomers. The conformational landscape of the clusters is very shallow and multiple competing isomers were rotationally and/or vibrationally detected, including three dimer species, two trimers and two tetramers. Cluster growth is associated with a tendency to form cyclic hydrogen bond structures.

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.

Proton Transfer in Nitromethane-Ammonia Clusters under VUV Single-Photon Ionization Explored by Infrared Spectroscopy and Theoretical Calculations

It is known that the acidity and reactivity of the CH bond can be enhanced after ionization. Also, this property plays a pivotal role in proton transfer reaction and in the formation of new molecules. Herein, infrared spectroscopy and high-precision quantum chemical calculations are used to study the neutral and cationic clusters of nitromethane-ammonia (CH₃NO₂-NH₃). It is found that in the neutral cluster, CH₃NO₂ and NH₃ are mainly bonded by three intermolecular hydrogen bonds, in which electrostatic contribution plays a major role. After vacuum ultraviolet (VUV) single-photon ionization of CH₃NO₂-NH₃, the positive charge redistributes from the ionized nitrogen atom of NH₃ to the CH₃NO₂ molecule immediately. Then, the proton of CH₃NO₂ transfers to NH₃ to form a proton-transferred type structure CH₂NO₂-NH₄⁺, without any effective energy barrier, due to the positive hyperconjugation of cationic nitromethane. A closed loop of positive charge transfer takes place in the CH₃NO₂-NH₃ cluster after VUV ionization. The present work demonstrates that both the proton transfer reaction and charge transfer process have occurred in the ionized CH₃NO₂-NH₃ cluster. Moreover, it is found that the proton transfer reaction is a result of the highly acidic CH bond caused by hyperconjugation between the σ (CH) bond and π orbital.

Predicting OH stretching fundamental wavenumbers of alcohols for conformational assignment: different correction patterns for density functional and wave-function-based methods

A model is presented for the prediction of OH stretching fundamental wavenumbers of alcohol conformers in the gas phase by application of a small set of empirical anharmonicity corrections to calculations in the harmonic approximation. In contrast to the popular application of a uniform scaling factor, the local chemical structure of the alcohol is taken into account to greatly improve accuracy. Interestingly, different correction patterns emerge for results of hybrid density functional (B3LYP-D3 and PBE0-D3) and wave-function-based methods (SCS-LMP2, LCCSD(T*)-F12a and CCSD(T)-F12a 1D). This raises questions about electronic structure deficiencies in these methods and differences in anharmonicity between alcohols. After its initial construction on the basis of literature assignments the model is tested with Raman jet spectroscopy of propargyl alcohol, cyclohexanol, borneol, isopinocampheol and 2-methylbutan-2-ol. For propargyl alcohol a spectral splitting attributed to tunneling is resolved. PBE0-D3 is identified as a well performing and broadly affordable electronic structure method for this model. A mean absolute error of 1.3 cm-1 and a maximum absolute error of 3 cm-1 result for 46 conformers of 24 alcohols in a 60 cm-1 range, when a single parameter is adjusted separately for each alcohol substitution class (methanol, primary, secondary, tertiary).

Revisiting the Spectroscopy of Water Dimer in Jets

Laser spectroscopy in jets is one of the main sources of structural data from molecular aggregates. Consequently, numerous and sophisticated experimental systems have been developed to extract precise information, which is usually interpreted in the light of quantum mechanical calculations. However, even with the most sophisticated experiments, it is sometimes difficult to interpret the experimental results. We present here the example of water dimer and how after almost 70 years, the assignment of its mass-resolved IR spectrum still generates controversy that extends toward the mechanism of ionization of water aggregates.

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.

Conformational Landscapes and Infrared Spectra of Gas-phase Interstellar Molecular Clusters [(C3H3N)(CH3OH)n, n = 1-4]

Acrylonitrile (A) is one of the important interstellar molecules, which is considered closely related to the origin of life. And methanol (M) is one of the commonly used solvents, which is also found in outer space. Herein, we obtained the infrared (IR) spectra of size-selected AM_n (n = 1-4) clusters in supersonic jet by monitoring their fragments of H⁺AM_n-1 (n = 1-4) with vacuum ultraviolet single-photon soft ionization/IR-depletion technique. IR spectra of AM_n (n = 1-4) clusters were recorded in the CH and OH vibration bands in the range of 2700-3800 cm^-1. Spectra of AM_n (n = 1-4) clusters are similar in the CH stretching regions, while those show significant variations in the OH stretching regions with the increase of methanol molecules. Calculated IR spectra, which were predicted with the B3LYP-D3(BJ)/aug-cc-pVDZ method, were employed to compare with the experimental results. For AM, AM₂, and AM₃, the structures with the methanol cyclic hydrogen bonded with [N1-C4(H6)] of acrylonitrile are more stable than the other H-bonded structures. For the most stable structures of AM₄, however, the results show that the acrylonitrile is binding to a H-bonded ring formed by OH groups of four methanol molecules. The AM, AM₂, and AM₃ conformers with the single ring on the C₁ side of acrylonitrile, such as C₁-AM-a, C₁-AM₂-a, and C₁-AM₃-a, are dominant in the gas phase, while the C₂-AM₄-a conformer with the H-bonded ring formed by the OH groups on the C₂ side of acrylonitrile is more stable than that of CM₄-A-a in our experimental conditions (>130 K). These findings may provide valuable insight into the microsolvation process of the interstellar molecules and other biomolecules in gas phase.

Infrared + vacuum ultraviolet two-color ionization spectroscopy of neutral metal complexes based on a tunable vacuum ultraviolet free-electron laser

This paper describes an experimental technique for studying neutral metal complexes using infrared + vacuum ultraviolet (IR+VUV) two-color ionization spectroscopy based on a tunable VUV free-electron laser (VUV-FEL). The preliminary IR spectroscopy results of mass-selected nickel tetracarbonyl are reported in this work. The results demonstrate that the tunable VUV-FEL light allows the selective ionization of a given neutral cluster free of confinement along with the recording of well-resolved IR spectra. As the ionization energies of many neutral clusters are accessible by a broadly tunable VUV-FEL (50-150 nm) and near-threshold ionization can be readily achieved, the proposed experimental method offers unique possibilities for the size-specific study of a wide variety of confinement-free neutral clusters.

Infrared Spectroscopy of Neutral Water Dimer Based on a Tunable Vacuum Ultraviolet Free Electron Laser

Infrared (IR) spectroscopy provides detailed structural and dynamical information on clusters at the fingerprint level. Herein, we demonstrate the capability of a tunable vacuum ultraviolet free electron laser (VUV-FEL) for selective detection of a wide variety of neutral water clusters and for recording the size-dependent IR spectra. The present technique does not require the presence of an ultraviolet chromophore or a dipole moment and is generally applicable for IR spectroscopy of neutral clusters free from confinement. To show the features of our technique, we report here the IR spectra of neutral water dimer in the OH stretch region, providing benchmarks for theoretical study of the accurate description of hydrogen bonding structures involved in liquid water and ice. Quantum mechanical calculations on a 12-dimensional ab initio potential energy surface are utilized to simulate the anharmonic vibrational spectra of water dimer. These results help to resolve the controversy of the exact vibrational assignment of each band feature of the water dimer.

An integrated instrument of DUV-IR photoionization mass spectrometry and spectroscopy for neutral clusters

We have developed an integrated instrument combining deep ultraviolet laser ionization mass spectrometry (DUV-LIMS) and infrared multiphoton dissociation (IR-MPD) spectroscopy, abbreviated as DUV-IR. The 177.3 nm DUV laser (7 eV single-photon energy) has short pulse duration (15 ps) and appropriate pulse energy (∼20 µJ), which is found to be highly efficient for low-fragment photoionization of neutral metal clusters and molecules. A home-made cluster source is designed with an adjustable formation channel suitable for the generation of different cluster series. The well-aligned components of the reflection time-of-flight mass spectrometer, as well as the coaxial design of DUV laser and molecular beam, bring forth high sensitivity and high resolution of the DUV-LIMS. Taking these advantages, well-resolved neutral V_n (n = 1-43) and (Benzene)_n (n = 1-25) clusters have been generated free of fragmentation. In addition to the generation and detection of neutral clusters, a fast-flow reaction tube is also designed downstream of the cluster source allowing to study their reactivity. In particular, a broad-range tunable IR laser (1.3-16 µm) is coupled with the DUV laser to attain IR-MPD spectroscopic analysis. This integrated system offers a general protocol to prepare various clusters to study their gas-phase reactivity and to determine their structures.

Competition between hydrogen bonds and van der Waals forces in intermolecular structure formation of protonated branched-chain alcohol clusters

To investigate the influence of bulky alkyl groups on hydrogen-bonded (H-bonded) network structures of alcohols, infrared (IR) spectra of protonated clusters of 2-propanol (2-PrOH) and tert-butyl alcohol (t-BuOH) were observed in the OH and CH stretch regions. In addition, by varying the tag species, the temperature dependence profile of the isomer population of H+(t-BuOH)n was revealed. An extensive search for stable isomers was performed using dispersion-corrected density functional theory methods, and temperature-dependent IR spectral simulations were done on the basis of the harmonic superposition approximation. The computational results qualitatively agreed with the observed size and temperature dependence of the H-bonded network structures of these protonated bulky alcohol clusters. However, the difficulty in the quantitative evaluation of dispersion was also demonstrated. It was shown that H+(2-PrOH)n (n = 4-7) have essentially the same network structures as the protonated normal alcohol clusters studied so far. On the other hand, H+(t-BuOH)n (n = 4-8) showed a clear preference for the smaller-membered ring structures, that is very different from the preference of the protonated normal alcohol clusters. The origin of the different structure preferences was discussed in terms of the steric effect and dispersion.

Hydrogen bond network structures of protonated short-chain alcohol clusters

Protonated alcohol clusters enable extraction of the physical essence of the nature of hydrogen bond networks.

Solvation energies of the proton in methanol revisited and temperature effects

Various functionals assessing solvation free energies and enthalpies of the proton in methanol.

Infrared absorption of methanol-water clusters (CH3OH)n(H2O), n = 1-4, recorded with the VUV-ionization/IR-depletion technique

We recorded infrared (IR) spectra in the CH- and OH-stretching regions of size-selected clusters of methanol (M) with one water molecule (W), represented as M_nW, n = 1-4, in a pulsed supersonic jet using the photoionization/IR-depletion technique. Vacuum ultraviolet emission at 118 nm served as the source of ionization in a time-of-flight mass spectrometer to detect clusters M_nW as protonated forms M_n-1WH⁺. The variations in intensities of M_n-1WH⁺ were monitored as the wavelength of the IR laser light was tuned across the range 2700-3800 cm^-1. IR spectra of size-selected clusters were obtained on processing of the observed action spectra of the related cluster-ions according to a mechanism that takes into account the production and loss of each cluster due to IR photodissociation. Spectra of methanol-water clusters in the OH region show significant variations as the number of methanol molecules increases, whereas those in the CH region are similar for all clusters. Scaled harmonic vibrational wavenumbers and relative IR intensities predicted with the M06-2X/aug-cc-pVTZ method for the methanol-water clusters are consistent with our experimental results. For dimers, absorption bands of a structure WM with H₂O as a hydrogen-bond donor were observed at 3570, 3682, and 3722 cm^-1, whereas weak bands of MW with methanol as a hydrogen-bond donor were observed at 3611 and 3753 cm^-1. For M₂W, the free OH band of H₂O was observed at 3721 cm^-1, whereas a broad feature was deconvoluted to three bands near 3425, 3472, and 3536 cm^-1, corresponding to the three hydrogen-bonded OH-stretching modes in a cyclic structure. For M₃W, the free OH shifted to 3715 cm^-1, and the hydrogen-bonded OH-stretching bands became much broader, with a weak feature near 3179 cm^-1 corresponding to the symmetric OH-stretching mode of a cyclic structure. For M₄W, the observed spectrum agrees unsatisfactorily with predictions for the most stable cyclic structure, indicating significant contributions from branched isomers, which is distinctly different from M₅ of which the cyclic form dominates.

Molecular structures of gas-phase neutral morpholine and its monohydrated complexes: experimental and theoretical approaches

Morpholine (NH(CH₂CH₂)₂O) is a typical six-membered aliphatic heterocyclic compound.

Exploration of hydrogen bond networks and potential energy surfaces of methanol clusters using a two-stage clustering algorithm

A two-stage algorithm based both on the similarity in shape and hydrogen bond network is developed to explore the potential energy surface of methanol clusters.

Gas-phase conformational preference of the smallest saccharide (glycolaldehyde) and its hydrated complexes with bridged hydrogen bonding

Glycoaldehyde (GA, HOCH₂CHO) is the simplest sugar unit of the carbohydrates and the only sugar to have been detected in interstellar space to date.

Photoinduced intermolecular dynamics and subsequent fragmentation in VUV-ionized acetamide clusters

Photofragmentation of small gas-phase acetamide clusters (CH₃CONH₂)_n (n ≤ 10) produced by a supersonic expansion source has been studied using time-of-flight ion mass spectroscopy combined with tunable vacuum-ultraviolet (VUV) synchrotron radiation. Fragmentation channels of acetamide clusters under VUV photoionization resulting in protonated and ammoniated clusters formation were identified with the discussion about the preceding intramolecular rearrangements. Acetamide-2,2,2-d3 clusters were also studied in an experiment with a gas discharge lamp as a VUV light source; comparison with the main experiment gave insights into the mechanism of formation of protonated acetamide clusters, indicating that proton transfer from amino group plays a dominant role in that process. Geometry of the acetamide dimer was discussed and the most stable arrangement was concluded to be achieved when subunits of the dimer are connected via two N-H⋯O -C hydrogen bonds. Also, the influence of the photon energy on the stability of the clusters and their fragmentation channels has been examined.

All-solid-state deep ultraviolet laser for single-photon ionization mass spectrometry

We report here the development of a reflectron time-of-flight mass spectrometer utilizing single-photon ionization based on an all-solid-state deep ultraviolet (DUV) laser system. The DUV laser was achieved from the second harmonic generation using a novel nonlinear optical crystal KBe2BO3F2 under the condition of high-purity N2 purging. The unique property of this laser system (177.3-nm wavelength, 15.5-ps pulse duration, and small pulse energy at ∼15 μJ) bears a transient low power density but a high single-photon energy up to 7 eV, allowing for ionization of chemicals, especially organic compounds free of fragmentation. Taking this advantage, we have designed both pulsed nanospray and thermal evaporation sources to form supersonic expansion molecular beams for DUV single-photon ionization mass spectrometry (DUV-SPI-MS). Several aromatic amine compounds have been tested revealing the fragmentation-free performance of the DUV-SPI-MS instrument, enabling applications to identify chemicals from an unknown mixture.

Dissociative ionization of the 1-propanol dimer in a supersonic expansion under tunable synchrotron VUV radiation

Different C–C bond cleavage of the 1-propanol dimer induced by site-selective photoionization under tunable synchrotron VUV radiation.

Proton transfer in acetaldehyde–water clusters mediated by a single water molecule

Bridging molecules: a single water molecule enhances the stability of symmetric acetaldehyde water clusters, and acts as a bridge for the transport of a proton between two acetaldehyde molecules.

Alcohol dimers – how much diagonal OH anharmonicity?

The massive infrared attenuation and increased anharmonicity of OH stretching overtones in alcohol dimers is experimentally quantified.

Mass-selected IR-VUV (118 nm) spectroscopic studies of radicals, aliphatic molecules, and their clusters

Mass-selected IR plus UV/VUV spectroscopy and mass spectrometry have been coupled into a powerful technique to investigate chemical, physical, structural, and electronic properties of radicals, molecules, and clusters. Advantages of the use of vacuum ultraviolet (VUV) radiation to create ions for mass spectrometry are its application to nearly all compounds with ionization potentials below the energy of a single VUV photon, its circumventing the requirement of UV chromophore group, its inability to ionize background gases, and its greatly reduced fragmenting capabilities. In this review, mass-selected IR plus VUV (118 nm) spectroscopy is introduced first in a general manner. Selected application examples of this spectroscopy are presented, which include the detections and structural analysis of radicals, molecules, and molecular clusters in a supersonic jet.

Generation of intense single-order harmonic pulse in the vacuum ultraviolet region using a deep ultraviolet driving laser

A 90 nm single-order harmonic pulse with a 200 nJ on-target pulse energy at 1 kHz was realized through a harmonic generation process with a 35 fs Ti:Sa third harmonic in a Kr gas cell.