Perfluoro effect on the electronic excited states of para-benzoquinone revealed by experiment and theory.

We report a comprehensive study on the electronic excited states of tetrafluoro-1,4-benzoquinone, through high-resolution vacuum ultraviolet photoabsorption spectroscopy and time-dependent density functional theory calculations performed within the nuclear ensemble approach. Absolute cross section values were experimentally determined in the 3.8-10.8 eV energy range. The present experimental results represent the highest resolution data yet reported for this molecule and reveal previously unresolved spectral structures. The interpretation of the results was made in close comparison with the available data for para-benzoquinone [Jones et al., J. Chem. Phys., 2017, 146, 184303]. While the dominant absorption features for both molecules arise from analogous π* ← π transitions, some remarkable differences have been identified. The perfluoro effect manifests in different ways: shifts in band positions and cross sections, appearance of features associated with excitations to σCF* orbitals, and spectrum broadening by quenching of either vibrational or Rydberg progressions. The level of agreement between experiment and theory is very satisfactory, yet that required the inclusion of nuclear quantum effects in the calculations. We have also discussed the role of temperature on the absorption spectrum, as well as the involvement of core-excited resonances in promoting dissociative electron attachment reactions in the 3-5 eV range.

Electronic structure and VUV photoabsorption measurements of thiophene.

The absolute photoabsorption cross sections for thiophene in the 5.0-10.7 eV range were measured using synchrotron radiation. New theoretical calculations performed at the time-dependent density functional theory level were used to qualitatively interpret the recorded photoabsorption spectrum. The calculations facilitated a re-analysis of the observed vibronic and Rydberg structures in the photoabsorption spectrum. Here a number of features have been re-assigned, while a number of other features have been assigned for the first time. This represents the most comprehensive and self-consistent assignment of the thiophene high-resolution photoabsorption spectrum to date.

Revisiting the photoabsorption spectrum of NH3 in the 5.4-10.8 eV energy region.

We present a comprehensive revisited experimental high-resolution vacuum ultraviolet (VUV) photoabsorption spectrum of ammonia, NH3, covering for the first time the full 5.4-10.8 eV energy-range, with absolute cross sections determined. The calculations on the vertical excitation energies and oscillator strengths were performed using the equation-of-motion coupled cluster method restricted to single and double excitation levels and used to help reanalyze the observed Rydberg structures in the photoabsorption spectrum. The VUV spectrum reveals several new features that are not previously reported in the literature, with particular reference to the vibrational progressions of the (D̃1E'←X̃1A1 '), the (F̃1E'←X̃1A1 '), and the (G̃1A2 ″←X̃1A1 ') absorption bands. In addition, new Rydberg members have been identified in nda1 '←1a2 ″D̃''1A2 ″←X̃1A1 ', where n > 3 has not been reported before as well as in nde″←1a2 ″F̃1E'←X̃1A1 ' and in nsa1 '←1a2 ″G̃1A2 ″←X̃1A1 '. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of ammonia in the Earth's atmosphere (0-50 km).

Probing the Lowest-Lying Electronic States of Acrylic Acid by Experimental and Theoretical Methods.

We report a combined experimental and theoretical study of the electronic state spectroscopy of acrylic acid (C3H4O2) in the gas phase, by high-resolution vacuum ultraviolet (VUV) photoabsorption measurements in the 4.0-10.8 eV energy range, together with ab initio calculations (vertical energies and oscillator strengths), which were used in the assignment of the valence transitions. We also discuss the Rydberg transitions for this molecular target, obtained using the experimental ionization energies available in the literature. The experimental spectrum presented in this paper represents the highest resolution data yet reported for acrylic acid and reveals new features not previously reported in the literature. The dominant transitions have been assigned to (π*(4a″) ← π(3a″)) and (π*(4a″) ← π(2a″)), the latter exhibiting excitation of the ν5'( a') C = O stretching mode with mean energy of 0.155 eV. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of acrylic acid in the upper stratosphere (20-50 km).

Protonation of aqueous alanine by photoionization of water.

Hydronium ions produced by photolysis of water are used to study the protonation dynamics of alanine zwitterions in water. The measurements are done by UV-VIS and UV-IR femtosecond transient absorption spectroscopy on alanine in H2O and D2O. It is estimated that the reaction rate constant for the deuteration of alanine zwitterions is 4 × 1010 M-1 s-1, while the reverse process has a rate constant of 2 × 108 s-1. In addition to hydronium ions the photolysis of water yields hydrogen atoms and hydrated electrons together with hydroxyl radicals and hydroxyl ions. However, no other products resulting from reactions between aqueous alanine and the photolysis products of water are positively identified during the first 530 ps after the photolysis. Potential secondary reactions that are not observed experimentally are discussed and an upper limit is set for their yield where possible.

The influence of the localised charge of C- and N-termini on peptide self-assembly.

The charge of a peptide influences final assembled structures. It is important to consider not only global charge, but also local, such as that found on the terminal residues. This work investigates the change of peptide self-assembly through the selection of different amino acid sequences and by varying the local charge of the residues on the C- and N-termini.

Dimensionality of carbon nanomaterial impacting on the modulation of amyloid peptide assembly.

A wide variety of inorganic nanomaterials have been exploited so far for their great potential for biological applications. Some of these materials could be valid candidates to modulate the assembly of amyloid peptides, which is relevant to amyloid-related diseases. In this work, we reveal that a carbon nanomaterial can indeed modulate the assembly of amyloid peptides and, additionally, we show that this modulating effect is closely related to the dimensionality of the nanomaterials.

Valence and Rydberg Excitations of 2,4- and 2,6-Difluorotoluene as Studied by Vacuum Ultraviolet Synchrotron Radiation and ab Initio Calculations.

Here we report novel comprehensive investigations on the electronic state spectroscopies of isolated 2,4- and 2,6-difluorotoluene in the gas phase by high-resolution vacuum ultraviolet (VUV) photoabsorption measurements in the 4.4-10.8 eV energy range, with absolute cross-section values derived. We also present the first set of ab initio calculations (vertical energies and oscillator strengths), which we have used in the assignment of valence transitions of the difluorotoluene molecules, together with calculated ionization energies to obtain the Rydberg transitions for both molecules. The measured absolute photoabsorption cross sections have been used to estimate the photolysis lifetimes of 2,4- and 2,6-difluorotoluene in the Earth's atmosphere.

Valence and lowest Rydberg electronic states of phenol investigated by synchrotron radiation and theoretical methods.

We present the experimental high-resolution vacuum ultraviolet (VUV) photoabsorption spectra of phenol covering for the first time the full 4.3-10.8 eV energy-range, with absolute cross sections determined. Theoretical calculations on the vertical excitation energies and oscillator strengths were performed using time-dependent density functional theory and the equation-of-motion coupled cluster method restricted to single and double excitations level. These have been used in the assignment of valence and Rydberg transitions of the phenol molecule. The VUV spectrum reveals several new features not previously reported in the literature, with particular reference to the 6.401 eV transition, which is here assigned to the 3sσ/σ(∗)(OH)←3π(3a″) transition. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of phenol in the earth's atmosphere (0-50 km).

Toluene Valence and Rydberg Excitations as Studied by ab initio Calculations and Vacuum Ultraviolet (VUV) Synchrotron Radiation.

The electronic spectroscopy of isolated toluene in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 4.0-10.8 eV energy range, with absolute cross-section measurements derived. We present the first set of ab initio calculations (vertical energies and oscillator strengths), which we use in the assignment of valence and Rydberg transitions of the toluene molecule. The spectrum reveals several new features not previously reported in the literature, with particular relevance to 7.989 and 8.958 eV, which are here tentatively assigned to the π*(17a') ← σ(15a') and 1π*(10a″) ← 1π(14a') transitions, respectively. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of toluene in the upper stratosphere (20-50 km).

Valence and Ionic Lowest-Lying Electronic States of Isobutyl Formate Studied by High-Resolution Vacuum Ultraviolet Photoabsorption, Photoelectron Spectroscopy, and Ab Initio Calculations.

The highest resolution vacuum ultraviolet photoabsorption spectrum of isobutyl formate, C5H10O2, yet reported is presented over the energy range 4.5-10.7 eV (275.5-118.0 nm) revealing several new spectral features. Valence and Rydberg transitions and their associated vibronic series observed in the photoabsorption spectrum have been assigned in accordance with new ab initio calculations of the vertical excitation energies and oscillator strengths. Calculations have also been carried out to determine the ionization energies and fine structure of the lowest ionic state of isobutyl formate and are compared with a newly recorded photoelectron spectrum (from 9.0 to 27.0 eV). The value of the first ionization energy was determined to be 10.508 eV (adiabatic) and 10.837 eV (vertical). New vibrational structure is observed in the first photoelectron band, predominantly resulting from C-O and C═O stretches of the molecule. The photoabsorption cross sections have been used to calculate the photolysis lifetime of isobutyl formate in the upper stratosphere (20-50 km), indicating that the hydroxyl radical processes will be the main loss process for isobutyl formate.

Electronic State Spectroscopy of Halothane As Studied by ab Initio Calculations, Vacuum Ultraviolet Synchrotron Radiation, and Electron Scattering Methods.

We present the first set of ab initio calculations (vertical energies and oscillator strengths) of the valence and Rydberg transitions of the anaesthetic compound halothane (CF3CHBrCl). These results are complemented by high-resolution vacuum ultraviolet photoabsorption measurements over the wavelength range 115-310 nm (10.8-4.0 eV). The spectrum reveals several new features that were not previously reported in the literature. Spin-orbit effects have been considered in the calculations for the lowest-lying states, allowing us to explain the broad nature of the 6.1 and 7.5 eV absorption bands assigned to σ*(C-Br) ← nBr and σ*(C-Cl) ← n(Cl) transitions. Novel absolute photoabsorption cross sections from electron scattering data were derived in the 4.0-40.0 eV range. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of halothane in the upper stratosphere (20-50 km).

Electronic excitation of carbonyl sulphide (COS) by high-resolution vacuum ultraviolet photoabsorption and electron-impact spectroscopy in the energy region from 4 to 11 eV.

The electronic state spectroscopy of carbonyl sulphide, COS, has been investigated using high resolution vacuum ultraviolet photoabsorption spectroscopy and electron energy loss spectroscopy in the energy range of 4.0-10.8 eV. The spectrum reveals several new features not previously reported in the literature. Vibronic structure has been observed, notably in the low energy absorption dipole forbidden band assigned to the (4π←3π) ((1)Δ←(1)Σ(+)) transition, with a new weak transition assigned to ((1)Σ(-)←(1)Σ(+)) reported here for the first time. The absolute optical oscillator strengths are determined for ground state to (1)Σ(+) and (1)Π transitions. Based on our recent measurements of differential cross sections for the optically allowed ((1)Σ(+) and (1)Π) transitions of COS by electron impact, the optical oscillator strength f0 value and integral cross sections (ICSs) are derived by applying a generalized oscillator strength analysis. Subsequently, ICSs predicted by the scaling are confirmed down to 60 eV in the intermediate energy region. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of carbonyl sulphide in the upper stratosphere (20-50 km).

Electronic excitation of furfural as probed by high-resolution vacuum ultraviolet spectroscopy, electron energy loss spectroscopy, and ab initio calculations.

The electronic spectroscopy of isolated furfural (2-furaldehyde) in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 3.5-10.8 eV energy-range, with absolute cross section measurements derived. Electron energy loss spectra are also measured over a range of kinematical conditions. Those energy loss spectra are used to derive differential cross sections and in turn generalised oscillator strengths. These experiments are supported by ab initio calculations in order to assign the excited states of the neutral molecule. The good agreement between the theoretical results and the measurements allows us to provide the first quantitative assignment of the electronic state spectroscopy of furfural over an extended energy range.

Electronic states of tetrahydrofurfuryl alcohol (THFA) as studied by VUV spectroscopy and ab initio calculations.

The electronic spectroscopy of isolated tetrahydrofurfuryl alcohol (THFA) in the gas phase has been investigated using high-resolution photoabsorption spectroscopy in the 5.0-10.8 eV energy-range, with absolute cross-section measurements derived. The He(I) photoelectron spectrum was also collected to quantify ionization energies in the 9-16 eV spectral region. These experiments are supported by the first high-level ab initio calculations performed on the excited states of the neutral molecule and on the ground and excited state of the positive ion. The good agreement between the theoretical results and the measurements allows us to quantify for the first time the electronic-state spectroscopy of THFA. The present work also considers the question of the lowest energy conformers of the molecule and its population distribution at room temperature.

Valence and ionic lowest-lying electronic states of ethyl formate as studied by high-resolution vacuum ultraviolet photoabsorption, He(I) photoelectron spectroscopy, and ab initio calculations.

The highest resolution vacuum ultraviolet photoabsorption spectrum of ethyl formate, C2H5OCHO, yet reported is presented over the wavelength range 115.0-275.5 nm (10.75-4.5 eV) revealing several new spectral features. Valence and Rydberg transitions and their associated vibronic series, observed in the photoabsorption spectrum, have been assigned in accordance with new ab initio calculations of the vertical excitation energies and oscillator strengths. Calculations have also been carried out to determine the ionization energies and fine structure of the lowest ionic state of ethyl formate and are compared with a newly recorded He(I) photoelectron spectrum (from 10.1 to 16.1 eV). New vibrational structure is observed in the first photoelectron band. The photoabsorption cross sections have been used to calculate the photolysis lifetime of ethyl formate in the upper stratosphere (20-50 km).

Anisotropy spectra for enantiomeric differentiation of biomolecular building blocks.

All biopolymers are composed of homochiral building blocks, and both D-sugars and L-amino acids uniquely constitute life on Earth. These monomers were originally enantiomerically differentiated under prebiotic conditions. Particular progress has recently been made in support of the photochemical model for this differentiation: the interaction of circularly polarized light with racemic molecules is currently thought to have been the original source for life's biological homochirality. The differential asymmetric photoreactivity of particular small molecules can be characterized by both circular dichroism and anisotropy spectroscopy. Anisotropy spectroscopy, a novel derivative of circular dichroism spectroscopy, records the anisotropy factor g = Δε/ε as a function of the wavelength. Anisotropy spectroscopy promisingly affords the wavelength-dependent determination of the enantiomeric excess (ee) inducible into chiral organic molecules by photochemical irradiation with circularly polarized light. Anisotropy spectra of small molecules therefore provide unique means for characterizing the different photochemical behaviors between enantiomers upon exposure to various wavelengths of circularly polarized light. This chapter will: (1) present the theory and configuration of anisotropy spectroscopy; (2) explain experimentally recorded anisotropy spectra of selected chiral biomolecules such as amino acids; and (3) discuss the relevance of these spectra for the investigation of the origin of the molecular homochirality observed in living organisms. This review describes a new chiroptical technique that is of significance for advances in asymmetric photochemistry and that is also highly relevant for the European Space Agency Rosetta Mission, which will determine enantiomeric excesses (ees) in chiral organic molecules in cometary ices when it lands on Comet 67P/Churyumov-Gerasimenko in November 2014.

Determination of degree of ionization of poly(allylamine hydrochloride) (PAH) and poly[1-[4-(3-carboxy-4 hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) in layer-by-layer films using vacuum photoabsorption spectroscopy.

Electrostatic and hydrophobic interactions govern most of the properties of supramolecular systems, which is the reason determining the degree of ionization of macromolecules has become crucial for many applications. In this paper, we show that high-resolution ultraviolet spectroscopy (VUV) can be used to determine the degree of ionization and its effect on the electronic excitation energies of layer-by-layer (LbL) films of poly(allylamine hydrochloride) (PAH) and poly[1-[4-(3-carboxy-4 hydroxyphenylazo)benzene sulfonamido]-1,2-ethanediyl, sodium salt] (PAZO). A full assignment of the VUV peaks of these polyelectrolytes in solution and in cast or LbL films could be made, with their pH dependence allowing us to determine the pK(a) using the Henderson-Hasselbach equation. The pK(a) for PAZO increased from ca. 6 in solution to ca. 7.3 in LbL films owing to the charge transfer from PAH. Significantly, even using solutions at a fixed pH for PAH, the amount adsorbed on the LbL films still varied with the pH of the PAZO solutions due to these molecular-level interactions. Therefore, the procedure based on a comparison of VUV spectra from solutions and films obtained under distinct conditions is useful to determine the degree of dissociation of macromolecules, in addition to permitting interrogation of interface effects in multilayer films.

Electronically excited states of formic acid investigated by theoretical and experimental methods.

Absolute cross-section values are reported from high-resolution vacuum ultraviolet (VUV) photoabsorption measurements of gas-phase formic acid (HCOOH) in the photon energy range 4.7-10.8 eV (265-115 nm), together with quantum chemical calculations to provide vertical energies and oscillator strengths. The combination of experimental and theoretical methods has allowed a comprehensive assignment of the electronic transitions. The VUV spectrum reveals various vibronic features not previously reported in the literature, notably associated with (3pa'←10a'), (3p'a'←10a'), (3sa'←2a″) and (3pa'←2a″) Rydberg transitions. The assignment of vibrational features in the absorption bands reveal that the C=O stretching, v3'a', the H'-O-C' deformation, v5'a', the C-O stretching, v6'a', and the O=C-O' deformation, v7'a' modes are mainly active. The measured absolute photoabsorption cross sections have also been used to estimate the photolysis lifetime of HCOOH in the upper stratosphere (30-50 km), showing that solar photolysis is an important sink at altitudes above 30 km but not in the troposphere. Potential energy curves for the lowest-lying electronic excited states, as a function of the C=O coordinate, are obtained employing time dependent density functional theory (TD-DFT). These calculations have shown the relevance of internal conversion from Rydberg to valence character governing the nuclear dynamics, yielding clear evidence of the rather complex multidimensional nature of the potential energy surfaces involved.

Electronic state spectroscopy of 1,4-pentadiene as studied by VUV photoabsorption spectroscopy and ab initio calculations.

We present high resolution VUV photoabsorption spectra of 1,4-pentadiene, C(5)H(8), over the wavelength range 115-247 nm (10.8-5.0 eV). These spectra reveal several new features not previously reported in the literature. These measurements are complemented by the first ab initio calculations for the three most abundant conformational isomers of 1,4-pentadiene, C(5)H(8), which we then use in the assignment of valence and Rydberg transitions. Calculations of the two lowest energy ionic states of 1,4-pentadiene are also presented and compared with the experimental data available in the literature. The measured absolute photoabsorption cross sections have been used to calculate the photolysis lifetime of 1,4-pentadiene in the upper stratosphere (20-50 km).