Site-selective ionization of ethanol dimer under the tunable synchrotron VUV radiation and its subsequent fragmentation

A plethora of isomerization processes and hydrogen scrambling in the fragmentation of the methanol dimer cation: a PEPICO study

The valence photoionization of light and deuterated methanol dimers was studied by imaging photoelectron photoion coincidence spectroscopy in the 10.00-10.35 eV photon energy range. Methanol clusters were generated in a rich methanol beam in nitrogen after expansion into vacuum. They generally photoionize dissociatively to protonated methanol cluster cations, (CH₃OH)_nH⁺. However, the stable dimer parent ion (CH₃OH)₂⁺ is readily detected below the dissociation threshold to yield the dominant CH₃OH₂⁺ fragment ion. In addition to protonated methanol, we could also detect the water- and methyl-loss fragment ions of the methanol dimer cation for the first time. These newly revealed fragmentation channels are slow and cannot compete with protonated methanol cation formation at higher internal energies. In fact, the water- and methyl-loss fragment ions appear together and disappear at a ca. 150 meV higher energy in the breakdown diagram. Experiments with selectively deuterated methanol samples showed H scrambling involving two hydroxyl and one methyl hydrogens prior to protonated methanol formation. These insights guided the potential energy surface exploration to rationalize the dissociative photoionization mechanism. The potential energy surface was further validated by a statistical model including isotope effects to fit the experiment for the light and the perdeuterated methanol dimers simultaneously. The (CH₃OH)₂⁺ parent ion dissociates via five parallel channels at low internal energies. The loss of both CH₂OH and CH₃O neutral fragments leads to protonated methanol. However, the latter, direct dissociation channel is energetically forbidden at low energies. Instead, an isomerization transition state is followed by proton transfer from a methyl group, which leads to the CH₃(H)OH⁺⋯CH₂OH ion, the precursor to the CH₂OH-, H₂O-, and CH₃-loss fragments after further isomerization steps, in part by a roaming mechanism. Water loss yields the ethanol cation, and two paths are proposed to account for m/z 49 fragment ions after CH₃ loss. The roaming pathways are quickly outcompeted by hydrogen bond breaking to yield CH₃OH₂⁺, which explains the dominance of the protonated methanol fragment ion in the mass spectrum.

Wavelength dependent photoionisation of ethanol clusters: Generation of hydrogen like C5+ ions at terawatt laser intensity

Photoionisation of ethanol clusters has been investigated at different laser wavelengths over intensity range of ~1012- 1013 W/cm2. At 266 nm ionisation, singly charged fragments and protonated cluster ions (up...

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.

Barrierless Proton Transfer in the Weak C-H···O Hydrogen Bonded Methacrolein Dimer upon Nonresonant Multiphoton Ionization in the Gas Phase

Intermolecular proton transfer (IMPT) in a C-H···O hydrogen bonded dimer of an α,β-unsaturated aldehyde, methacrolein (MC), upon nonresonant multiphoton ionization by 532 nm laser pulses (10 ns), has been investigated using time-of-flight (TOF) mass spectrometry under supersonic cooling condition. The mass peaks corresponding to both the protonated molecular ion [(MC)H⁺] and intact dimer cation [(MC)₂]^•+ show up in the mass spectra, and the peak intensity of the former increases proportionately with the latter with betterment of the jet cooling conditions. The observations indicate that [(MC)₂]^•+ is the likely precursor of (MC)H⁺ and, on the basis of electronic structure calculations, IMPT in the dimer cation has been shown to be the key reaction for formation of the latter. Laser power dependences of ion yields indicate that at this wavelength the dimer is photoionized by means of 4-photon absorption process, and the total 4-photon energy is nearly the same as the predicted vertical ionization energy of the dimer. Electronic structure calculations reveal that the optimized structures of [(MC)₂]^•+ correspond to a proton transferred configuration wherein the aldehydic hydrogen is completely shifted to the carbonyl oxygen of the neighboring moiety. Potential energy scans along the C-H···O coordinate also show that the IMPT in [(MC)₂]^•+ is a barrierless process.

"Uphill" cation transport: A bioinspired photo-driven ion pump

Biological ion pumps with active ionic transport properties lay the foundation for many life processes. However, few analogs have been produced because extra energy is needed to couple to this "uphill" process. We demonstrate a bioinspired artificial photo-driven ion pump based on a single polyethylene terephthalate conical nanochannel. The pumping process behaving as an inversion of zero-volt current can be realized by applying ultraviolet irradiation from the large opening. The light energy can accelerate the dissociation of the benzoic acid derivative dimers existing on the inner surface of nanochannel, which consequently produces more mobile carboxyl groups. Enhanced electrostatic interaction between the ions traversing the nanochannel and the charged groups on the inner wall is the key reason for the uphill cation transport behavior. This system creates an ideal experimental and theoretical platform for further development and design of various stimuli-driven and specific ion-selective bioinspired ion pumps, which anticipates wide potential applications in biosensing, energy conversion, and desalination.

Communication: Protonation process of formic acid from the ionization and fragmentation of dimers induced by synchrotron radiation in the valence region

The ionization and fragmentation of monomers of organic molecules have been extensively studied in the gas phase using mass spectroscopy. In the spectra of these molecules it is possible to identify the presence of protonated cations, which have a mass-to-charge ratio one unit larger than the parent ion. In this work, we investigate this protonation process as a result of dimers photofragmentation. Experimental photoionization and photofragmentation results of doubly deuterated formic acid (DCOOD) in the gas phase by photons in the vacuum ultraviolet region are presented. The experiment was performed by using a time-of-flight mass spectrometer installed at the Brazilian Synchrotron Light Laboratory and spectra for different pressure values in the experimental chamber were obtained. The coupled cluster approach with single and double substitutions was employed to assist the experimental analysis. Results indicate that protonated formic acid ions are originated from dimer dissociation, and the threshold photoionization of (DCOOD)⋅D(+) is also determined.

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.

Site-selective dissociation processes of cationic ethanol conformers: the role of hyperconjugation

In present report, we explored hyperconjugation effects on the site- and bond-selective dissociation processes of cationic ethanol conformers by the use of theoretical methods (including configuration optimizations, natural bond orbital (NBO) analysis, and density of states (DOS) calculations, etc.) and the tunable synchrotron vacuum ultraviolet (SVUV) photoionization mass spectrometry. The dissociative mechanism of ethanol cations, in which hyperconjugative interactions and charge-transfer processes were involved, was proposed. The results reveal Cα-H and C-C bonds are selectively weakened, which arise as a result of the hyperconjugative interactions σCα-H → p in the trans-conformer and σC-C → p in gauche-conformer after being ionized. As a result, the selective bond cleavages would occur and different fragments were observed.