Hydrogen bonding in methanol clusters probed by inner-shell photoabsorption spectroscopy in the carbon and oxygen K-edge regions

Carbon K-edge x-ray absorption spectra of liquid alcohols from quantum chemical calculations of liquid structures obtained by molecular dynamics simulations

For reproducing the carbon K-edge x-ray absorption spectra of liquid alcohols, inner-shell quantum chemical calculations based on the Hartree-Fock method were performed with the snapshots of the liquid structures obtained by molecular dynamics simulations. The C K-edge inner-shell spectrum of liquid ethanol (EtOH) was obtained by the summation of one thousand calculated spectra of EtOH molecules including neighbor EtOH molecules within the CH₂-CH₂ distance of 6 Å. For the C K-edge inner-shell spectrum of liquid methanol (MeOH), we have calculated one thousand spectra of MeOH molecules including neighbor MeOH molecules within the CH₃-CH₃ distance of 6 Å. The calculated C K-edge inner-shell spectra of liquid alcohols well reproduced the spectral shapes of the experimentally obtained x-ray absorption spectra and the spectral changes from gas to liquid phases.

Femtosecond Soft-X-ray Absorption Spectroscopy of Liquids with a Water-Window High-Harmonic Source

Femtosecond X-ray absorption spectroscopy (XAS) is a powerful method to investigate the dynamical behavior of a system after photoabsorption in real time. So far, the application of this technique has remained limited to large-scale facilities, such as femtosliced synchrotrons and free-electron lasers (FEL). In this work, we demonstrate femtosecond time-resolved soft-X-ray absorption spectroscopy of liquid samples by combining a sub-micrometer-thin flat liquid jet with a high-harmonic tabletop source covering the entire water-window range (284-538 eV). Our work represents the first extension of tabletop XAS to the oxygen edge of a chemical sample in the liquid phase. In the time domain, our measurements resolve the gradual appearance of absorption features below the carbon K-edge of ethanol and methanol during strong-field ionization and trace the valence-shell ionization dynamics of the liquid alcohols with a temporal resolution of ∼30 fs. This technique opens unique opportunities to study molecular dynamics of chemical systems in the liquid phase with elemental, orbital, and site sensitivity.

Microheterogeneity in Aqueous Acetonitrile Solution Probed by Soft X-ray Absorption Spectroscopy

Chemical processes in solution are influenced by microheterogeneity (MH), where two liquids seem to be mixed in a macroscopic scale but are microscopically inhomogeneous. We have investigated one of the simplest MH systems, aqueous acetonitrile solution, using soft X-ray absorption spectroscopy (XAS). Molecular interactions of acetonitrile were revealed by C and N K-edge XAS at different concentrations, and those of solvent water were separately revealed by O K-edge XAS. The energy shift of the C≡N π* peak at the C K-edge shows three characteristic concentration regions and a phase-transition-like behavior between them. By comparing the energy shifts in XAS spectra with ab initio quantum chemical inner-shell calculations, we have determined local structures of acetonitrile-water mixtures in three concentration regions and found that the dipole interaction between acetonitrile and water is the key structure to emerge the MH state in the middle concentration region.

Site-specific X-ray induced dynamics in liquid methanol

Complex chemical and biochemical systems are susceptible to damage from ionising radiation. However, questions remain over the extent to which such damage is influenced by the nature of the surrounding chemical environment, which can consist of both hydrophobic and hydrophilic domains. To gain fundamental insight into the first crucial mechanistic steps of radiation damage in such systems, we need to understand the initial radiation response, i.e. dynamics occurring on the same timescale as electronic relaxation, which occur in these different environments. Amphiphilic molecules contain both hydrophobic and hydrophilic domains, but the propensity for charge delocalisation and proton dynamics to occur in these different domains has been largely unexplored so far. Here, we present carbon and oxygen 1s Auger spectra for liquid methanol, one of the simplest amphiphilic molecules, as well as its fully deuterated equivalent d4-methanol, in order to explore X-ray induced charge delocalisation and proton dynamics occurring on the few femtosecond timescale. Unexpectedly, we find a similar propensity for proton dynamics to occur at both the carbon and oxygen site within the lifetime of the core hole. Our results could serve as a model for decay processes that are likely to occur in other more complex amphiphilic systems.

Intermolecular Interactions of Pyridine in Liquid Phase and Aqueous Solution Studied by Soft X-ray Absorption Spectroscopy

Intermolecular interactions of pyridine in liquid and in aqueous solution are studied by using soft X-ray absorption spectroscopy (XAS) at the C, N, and O K-edges. XAS of liquid pyridine shows that the N 1s→π* peak is blue shifted and the C 1s→π* peak of the meta and para sites is red shifted, respectively, as compared with XAS of pyridine gas. These shifts in liquid are smaller than those in clusters, indicating that the intermolecular interaction of liquid pyridine is weaker than that of pyridine cluster, as supported by the combination of quantum chemical calculations of the core excitation and molecular dynamics simulations of the liquid structure. On the other hand, XAS spectra of aqueous pyridine solutions (C5H5N)x(H2O)1−x measured at different molar fractions show that in the pyridine rich region, x>0.7, the C and N 1s→π* peak energies are not so different from pure liquid pyridine (x=1.0). In this region, antiparallel displaced structures of pyridine molecules are dominant as in pure pyridine liquid. In the O K-edge XAS, the pre-edge peaks sensitive to the hydrogen bond (HB) network of water molecules show the red shift of −0.15 eV from that of bulk water, indicating that small water clusters with no large-scale HB network are formed in the gap space of structured pyridine molecules. In the water rich region, 0.7>x, the N 1s→π* peaks and the O 1s pre-edge peaks are blue shifted, and the C 1s→π* peaks of the meta and para sites are red-shifted by increasing molar fraction of water. The HB network of bulk water is dominant, but quantum chemical calculations indicate that small pyridine clusters with the HB interaction between the H atom in water and the N atom in pyridine are still existent even in very dilute pyridine solutions.

Direct HRTEM observation of ultrathin freestanding ionic liquid film on carbon nanotube grid

Direct imaging of liquids by electron microscopy has been extremely difficult due to their high volatility. Ionic liquids are a unique liquid material with almost zero vapor pressure, which allows us to characterize them under high-vacuum conditions. Here we report the first direct observation of the microstructure and phase behavior of an imidazolium-based room-temperature ionic liquid by high-resolution transmission electron microscopy with the aid of a special carbon nanotube network, which is able to support a freestanding ultrathin ionic liquid film on its nanosized holes. It was found that the existence of cluster structures is one of the intrinsic properties of the ionic liquid in its whole liquid phase range. Furthermore, the size and mobility of the clusters play an important role during phase transition of the ionic liquid. We show that the direct HRTEM imaging on freestanding liquid film is a powerful technique to obtain insight into the structure of ionic liquids and their phase behavior. The present study can provide an important starting point for more sophisticated structural studies on the microstructure of liquidus materials.

Pathways for hydrogen bond switching in a tetrameric methanol cluster

Computational techniques (second order Møller-Plesset MP2 perturbation theory in conjunction with medium and large size basis sets) are applied to explore structural aspects of a hydrogen-bonded tetrameric cluster of methanol molecules, based geometrically on a tetrahedral arrangement of the four oxygen atoms of the cluster. The hydrogen-bonded structures that represent minima on the potential energy surface are established, and the complete set of pathways that allow interconversion between these structures through "switching" of the hydrogen bonding arrangement are elucidated. The implications of these results in terms of dynamic properties of the cluster are discussed.