Probing the interaction between solid benzene and water using vacuum ultraviolet and infrared spectroscopy

We present results of a combined vacuum ultravioloet (VUV) and infrared (IR) photoabsorption study of amorphous benzene:water mixtures and layers to investigate the benzene-water interaction in the solid phase. UV spectra of 1:1, 1:10 and 1:100 benzene:water mixtures at 24 K reveal a concentration dependent shift in the energies of the 1B2u, 1B1u and 1E1u electronic states of benzene. All the electronic bands blueshift from pure amorphous benzene towards gas phase energies with increasing water concentration. IR results reveal a strong dOH-π benzene-water interaction via the dangling OH stretch of water with the delocalised π system of the benzene molecule. Although this interaction influences the electronic states of benzene with the benzenewater interaction causing a redshift in the electronic states from that of the free benzene molecule, the benzene-benzene interaction has a more significant effect on the electronic states of benzene. VUV spectra of benzene and water layers show evidence of non-wetting between benzene and water, characterised by Rayleigh scattering tails at wavelengths greater than 220 nm. Our results also show evidence of benzene-water interaction at the benzene-water interface affecting both the benzene and the water electronic states. Annealing the mixtures and layers of benzene and water show that benzene remains trapped within in/under water ice until water desorption near 160 K. These first systematic studies of binary amorphous mixtures in the VUV, supported with complementary IR studies, provide a deeper insight into the influence of intermolecular interactions on intramolecular electronic states with significant implications for our understanding of photochemical processes in more realistic astrochemical environments

Ionic strength and calcium regulate membrane interactions of myelin basic protein and the cytoplasmic domain of myelin protein zero

The formation of a mature myelin sheath in the vertebrate nervous system requires specific protein-membrane interactions. Several myelin-specific proteins are involved in stacking lipid membranes into multilayered structures around axons, and misregulation of these processes may contribute to chronic demyelinating diseases. Two key proteins in myelin membrane binding and stacking are the myelin basic protein (MBP) and protein zero (P0). Other factors, including Ca2+, are important for the regulation of myelination. We studied the effects of ionic strength and Ca2+ on the membrane interactions of MBP and the cytoplasmic domain of P0 (P0ct). MBP and P0ct bound and aggregated negatively charged lipid vesicles, while simultaneously folding, and both ionic strength and calcium had systematic effects on these interactions. When decreasing membrane net negative charge, the level and kinetics of vesicle aggregation were affected by both salt and Ca2+. The effects on lipid membrane surfaces by ions can directly affect myelin protein-membrane interactions, in addition to signalling effects in myelinating glia.publishedVersio

Graphene oxide layer-by-layer films for sensors and devices

Funding Information: Funding: The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007–2013) CALIPSO under grant agreement number 312284 and from the Portuguese funding agency FCT—Fundação para a Ciência e a Tecnologia—within projects UID/EEA/50014/2019, UID/FIS/00068/2019, PTDC/FIS-NAN/0909/2014, UID/FIS/00068/2019, the Bilateral Project entitled “Deteção de Estrogénio-um Contaminante Emergente em Corpos Hídricos” within the scope of “Cooperação Transnacional_FCT (Portugal)-CAPES (Brazil) 2018”.Layer-by-layer films of poly (allylamine hydrochloride) (PAH) and graphene oxide (GO) were characterized, looking at growth with the number of bilayers, morphology, and electrical properties. The PAH/GO films revealed a linear increase in absorbance with the increase in the number of deposited bilayers, allowing the determination that 10.7 ± 0.1 mg m−2 of GO is adsorbed per unit of area of each bilayer. GO absorption bands at 146, 210, 247 and 299 nm, assigned to π-π* and n-π* transitions in the aromatic ring (phenol) and of the carboxylic group, respectively, were characterized by vacuum ultraviolet spectroscopy. The morphological characterization of these films demonstrated that they are not completely uniform, with a bilayer thickness of 10.5 ± 0.7 nm. This study also revealed that the films are composed of GO and/or PAH/GO fibers and that GO is completely adsorbed on top of PAH. The electrical properties of the films reveal that PAH/GO films present a semiconductor behavior. In addition, a slight decrease in conduction was observed when films were prepared in the presence of visible light, likely due to the presence of oxygen and moisture that contributes to the damage of GO molecules.publishersversionpublishe

Systematic investigation of CO₂ : NH₃ ice mixtures using mid-IR and VUV spectroscopy – part 1: thermal processing

The adjustment of experimental parameters in interstellar ice analogues can have profound effects on molecular synthesis within an ice system. We demonstrated this by systematically investigating the stoichiometric mixing ratios of CO2 : NH3 ices as a function of thermal processing using mid-IR and VUV spectroscopy. We observed that the type of CO2 bonding environment was dependent on the different stoichiometric mixing ratios and that this pre-determined the NH3 crystallite structure after phase change. The thermal reactivity of the ices was linked to the different chemical and physical properties of the stoichiometric ratios. Our results provide new details into the chemical and physical properties of the different stoichiometric CO2 : NH3 ices enhancing our understanding of the thermally induced molecular synthesis within this ice system

The ground and ionized states of azulene : a combined study of the vibrational energy levels by photoionization, configuration interaction, and density functional calculations

A synchrotron-based photoionization spectrum of azulene shows significant additional vibrational fine structure when compared to previous studies. This spectrum was successfully analysed by Franck-Condon (FC) methods. Previously reported zero-kinetic-energy electron spectra (ZEKE) for azulene, have been reinterpreted in FC terms, leading to some alternative assignments to the earlier work. The sequence of ionic states has been determined by ab initio configuration interaction (CI) methods, leading to reliable theoretical values for both the calculated adiabatic and vertical ionization energies (AIE and VIE respectively). VIE were calculated by both symmetry-adapted cluster (SAC-CI), together with Green’s function (GF) and Tamm Dancoff approximation (TDA), single excitation CI methods; AIE for highest states of each symmetry, were determined by open-shell self-consistent field (SCF) methods at the restricted Hartree-Fock level. Complete active space SCF(CASSCF) was used for the pairs of states 12A2 + 22A2 and 12B1 + 22B1 each of which occurs as antisymmetric and symmetric (higher energy) combinations. The combined ionic state sequences (AIE and VIE) from these methods are 12A2 < 12B1 < 22A2 < 22B1. The PES shows a series of broad bands above 11 eV, each of which are attributed to more than one ionisation. The calculated PES sequence of states up to 19 eV shows the SAC-CI and GF results are in almost exact agreement. The internal spacing of the bands is best reproduced by the simpler GF and TDA methods. States involving simultaneous ionization and electronic excitation are considered by both SAC-CI and TDA methods.PostprintPeer reviewe

The vacuum ultraviolet absorption spectra of norbornadiene : vibrational analysis of the singlet and triplet valence states of norbornadiene by configuration interaction and density functional calculations

A synchrotron-based vacuum ultraviolet absorption spectrum (VUV) of norbornadiene (NBD) is reported, and the extensive vibrational structure obtained has been analysed. The previously known 5b13s-Rydberg state has been reinterpreted by comparison with our recent high resolution photoelectron spectral (PES) analysis of the X2B1 ionic state. Additional vibrational detail in the region of this Rydberg state is observed in its VUV spectrum, when compared with the PES 2B1 ionic state; this is attributed to underlying valence state structure in the VUV. Valence and Rydberg state energies have been obtained by configuration interaction (CI) and time dependent density functional theoretical methods (TDDFT). Several low-lying singlet valence states, especially those which arise from ππ* excitations, conventionally termed NV1 to NV4, have been examined in detail. Their Franck-Condon (FC) and Herzberg-Teller (HT) profiles have been investigated and fitted to the VUV spectrum. Estimates of the experimental 00 band positions have been made from these fits. The anomaly of observed UV absorption by the 1A2 state of NBD is attributed to HT effects. Generally the HT components are less than 10% of the FC terms. The calculated 5b13s lowest Rydberg state also shows a low level of HT components. The observed electron impact spectra of NBD have been analysed in detail in terms of triplet states.PostprintPeer reviewe

The ground and ionic states of cyclohepta-1,3,5-triene and their relationship to norcaradiene states. New 1H and 13C NMR spectra, and analysis of a new experimental photoelectron spectrum by ab initio methods

The strong inter-relationship between cyclohepta-1,3,5-triene (CHT) and norcaradiene (NCD) observed in some reactions, has been extended to include the energy surfaces for some low-lying ionic states. Whilst equilibrium structures of 2A/ symmetry containing the CHT skeleton were routinely found, the structures emerging with 2A// symmetry were found to be NCD ionic states. Surface studies, by variation of the C1 to C6 distance, showed minima for both state symmetries. Curve crossing which occurs in CS symmetry, is avoided by distortion to C1 symmetry. The CHT → NCD structural change is attributed to initial conrotatory closure of the singly occupied molecular orbital. A new synchrotron-based study of the photoelectron spectrum (PES) for CHT up to 25 eV shows little vibrational structure. We have assigned the overall PES up to 17 eV in considerable detail, using a variety of theoretical methods. The vertical ionization energy (VIE) sequence in the PES for CHT ions, is predicted to be: 12A/ < 12A// < 22A/ < 22A//. The calculated lowest ionic state, with lowest vibrational frequency 87 cm-1, leads to a high density of vibrational states. The Franck-Condon (FC) envelopes of the two lowest PES bands have been analysed. The identity of the PES spectrum, as derived from CHT rather than NCD, was demonstrated by 1H and 13C nuclear magnetic resonance (NMR) spectra of the sample; this agrees with the predicted PES spectra of CHT and NCD. The NCD predicted spectrum shows significant differences from CHT.PostprintPostprintPeer reviewe

On the electronic structure of methyl butyrate and methyl valerate

No. 2019-A0010806820 UIDB/00068/2020 PTDC/FIS-AQM/31281/2017Abstract: We present novel results of the analysis of the electronic structure of two aliphatic esters: methyl butyrate and methyl valerate. High-resolution photoabsorption spectra were collected and analyzed over the energy range 4.0–10.8 eV and showed for both the molecules not only a clear band of the HOMO to LUMO transition, but also vibronic structure associated with the first Rydberg-valence transition. Photoelectron spectra recorded from 9 to over 28 eV revealed many ionization states with the first adiabatic ionization energies found to be 9.977 eV and 9.959 eV for methyl butyrate and methyl valerate, respectively. Ab initio calculations have been performed in order to help assign the photoabsorption and photoelectron features. Photolysis life times in the atmosphere were calculated revealing that photolysis is not competitive over hydroxyl radical scavenging in the process of removal of these esters from the atmosphere. Graphical abstract: [Figure not available: see fulltext.]publishersversionpublishe