Valence one-electron and shake-up ionisation bands of polycyclic aromatic hydrocarbons. IV. The dibenzanthracene species

A study of the valence photoelectron spectrum of uracil and mixed water-uracil clusters

The valence ionization of uracil and mixed water-uracil clusters has been studied experimentally and by ab initio calculations. In both measurements, the spectrum onset shows a red shift with respect to the uracil molecule, with the mixed cluster characterized by peculiar features unexplained by the sum of independent contributions of the water or uracil aggregation. To interpret and assign all the contributions, we performed a series of multi-level calculations, starting from an exploration of several cluster structures using automated conformer-search algorithms based on a tight-binding approach. Ionization energies have been assessed on smaller clusters via a comparison between accurate wavefunction-based approaches and cost-effective DFT-based simulations, the latter of which were applied to clusters up to 12 uracil and 36 water molecules. The results confirm that (i) the bottom-up approach based on a multilevel method [Mattioli et al. Phys. Chem. Chem. Phys. 23, 1859 (2021)] to the structure of neutral clusters of unknown experimental composition converges to precise structure-property relationships and (ii) the coexistence of pure and mixed clusters in the water-uracil samples. A natural bond orbital (NBO) analysis performed on a subset of clusters highlighted the special role of H-bonds in the formation of the aggregates. The NBO analysis yields second-order perturbative energy between the H-bond donor and acceptor orbitals correlated with the calculated ionization energies. This sheds light on the role of the oxygen lone-pairs of the uracil CO group in the formation of strong H-bonds, with a stronger directionality in mixed clusters, giving a quantitative explanation for the formation of core-shell structures.

Spin-Component-Scaled ΔMP2 Parametrization: Toward a Simple and Reliable Method for Ionization Energies

A practical, accurate, and cost- and implementation-free method (ΔMP2-SOS(IP)) for the calculation of vertical ionization potentials is proposed. The simple method is based on a single-step, a diagonal, frequency-independent approximation to the second-order self-energy expression combined with the spin-component-scaled technique. The search for an optimal scaling factor is performed for a set of 50 moderately sized molecules, and the quality of the method is additionally assessed for a benchmark set of 24 organic acceptor molecules. The proposed ΔMP2-SOS(IP) method provides the best results of valence ionization energies as compared to the several standard self-consistent variants of the electron propagator methods at the second and higher orders (EP2, SCS-EP2, EP3, OVGF) with almost CCSD(T) or IP-EOM-CCSD accuracy and the cost of only a single opposite-spin ΔMP2-type calculation ( O( N³)). For core ionization energies, our new methods outperform the standard ΔMP2 results due to a better balanced treatment of the correlation and relaxation term in the second-order self-energy.

Benchmarking semiempirical, Hartree–Fock, DFT, and MP2 methods against the ionization energies and electron affinities of short- through long-chain [n]acenes and [n]phenacenes

Vertical and adiabatic ionization energies (IEs) and electron affinities (EAs) were calculated for the n = 1–10 [n]acenes using a wide range of semiempirical, Hartree–Fock, density functional, and second-order Moller–Plesset perturbation theory model chemistries. None of the model chemistries examined was able to accurately predict the IEs or EAs for both short- through long-chain [n]acenes, as well as for extrapolations to the polymeric limit, when compared to available experimental and benchmark theoretical data. Except for 6-31G(d), the choice of the basis set does not affect B3LYP results significantly. Analogous calculations using a suite of eight modern and (or) popular density functionals for the n = 1–10 [n]phenacenes revealed similar problems in estimating the IEs and EAs of these compounds, with the sole exception of the M062X functional for adiabatic IEs and potentially the APFD, B3LYP, and MN12SX functionals for adiabatic EAs. The poor IE/EA prediction performance for the parent [n]acenes and [n]phenacenes may extend to their substituted derivatives and heteroatom-substituted analogs. Consequently, caution should be exercised in the application of non-high-level calculations for estimating the IE/EA of these important classes of materials.

Electronic properties of FC(O)SCH2CH3. a combined helium(I) photoelectron spectroscopy and synchrotron radiation study

The valence electronic properties of S-ethyl flouromethanethioate (S-ethyl fluoromethsanethioate), FC(O)SCH2CH3, were investigated by means of He(I) photoelectron spectroscopy in conjunction with the analysis of the photofragmentation products determined by PEPICO (phtoelectron-photoion-coincidence) by using synchrotron radiation in the 11.1-21.6 eV photon energy range. The first band observed at 10.28 eV in the HeI photoelectron spectrum can be assigned with confidence to the ionization process from the HOMO [nπ(S) orbital], which is described as a lone pair formally localized on the sulfur atom, in agreement with quantum chemical calculations using the outer valence Green function method [OVGF/6-311++G (d,p)]. One of the most important fragmentation channels also observed in the valence region corresponds to the decarbonylation process yielding the [M-CO](·+) ion, which is clearly observed at m/z = 80. Moreover, S 2p and S 2s absorption edges have been examined by measuring the total ion yield spectra in the 160-240 eV region using variable synchrotron radiation. The dynamic of ionic fragmentation following the Auger electronic decay has been evaluated with the help of the PEPIPICO (photoion-photoion-photoelectron-coincidence spectra) technique.

Electron momentum spectroscopy of 1-butene: a theoretical analysis using molecular dynamics and molecular quantum similarity

The results of experimental studies of the valence electronic structure of 1-butene by means of electron momentum spectroscopy (EMS) have been reinterpreted on the basis of molecular dynamical simulations in conjunction with the classical MM3 force field. The computed atomic trajectories demonstrate the importance of thermally induced nuclear dynamics in the electronic neutral ground state, in the form of significant deviations from stationary points on the potential energy surface and considerable variations of the C-C-C-C dihedral angle. These motions are found to have a considerable influence on the computed spectral bands and outer-valence electron momentum distributions. Euclidean distances between spherically averaged electron momentum densities confirm that thermally induced nuclear motions need to be fully taken into account for a consistent interpretation of the results of EMS experiments on conformationally flexible molecules.

Photoelectron and electron momentum spectroscopy of tetrahydrofuran from a molecular dynamical perspective

The results of experimental studies of the valence electronic structure of tetrahydrofuran employing He I photoelectron spectroscopy as well as Electron Momentum Spectroscopy (EMS) have been reinterpreted on the basis of Molecular Dynamical simulations employing the classical MM3 force field and large-scale quantum mechanical simulations employing Born-Oppenheimer Molecular Dynamics in conjunction with the dispersion corrected ωB97XD exchange-correlation functional. Analysis of the produced atomic trajectories demonstrates the importance of thermal deviations from the lowest energy path for pseudorotation, in the form of considerable variations of the ring-puckering amplitude. These deviations are found to have a significant influence on several outer-valence electron momentum distributions, as well as on the He I photoelectron spectrum.

The band 12 issue of norbornane: a study of higher shake-up states

In line with a recent study of the electronic structure of the cage compound norbornane (J. Chem. Phys. 121 (2004), 10525; J. Phys. Chem. A 109 (2005), 4267), symmetry adapted cluster expansion configuration interaction (SAC-CI) general R calculations have been performed and compared with results obtained by the third order algebraic diagrammatic construction scheme [ADC(3)]. Comparison has been made with previously performed electron momentum spectroscopy (EMS) and ultraviolet photo-electron measurements. The region around ~25 eV (band 12), characterized by an elaborated band in the EMS spectrum which is missing in previous Green's function and ADC calculations, is investigated. This study is completed with outer-valence Green's function (OVGF) and SAC-CI/SD-R calculations, and results are obtained by employing (single and double) ionization extended second order ADC [ADC(2)-x]. Since ADC(3) only includes 2h-1p shake-up states, while SAC-CI general-R also includes higher order states, the agreement between both methods assures that the higher order shake-up states do not play an important role in the ionization spectrum of norbornane. While the band-12 issue of norbornane is therefore still open for further discussion, a tentative description in terms of ultrafast nuclear dynamical effects and autoionization processes has become more plausible.

Electronic properties and dissociative photoionization of thiocyanates. Part II. Valence and shallow-core (sulfur and chlorine 2p) regions of chloromethyl thiocyanate, CH2ClSCN

A combination of photoelectron spectroscopy and synchrotron based photoelectron photoion coincidence (PEPICO) spectra has been applied to investigate the electronic structure and the dissociative ionization of the CH(2)ClSCN molecule in the valence region. The PES is assigned with the electronic structure calculations at the outer-valence Green's function and symmetry adapted cluster/configuration interaction (SAC-CI) levels offer an explanation of our experimental results. Upon vacuum ultraviolet irradiation the low-lying radical cation, located at 10.39 eV is formed. The molecular ion is observed in the time-of-flight mass spectra, together with the CH(2)SCN(+) and CH(2)Cl(+) daughter ions. The total ion yield spectra have been measured in the S 2p and Cl 2p regions and several channels have been determined in dissociative photoionization events for the core-excited species. Thus, by using time-of-flight mass spectrometry and synchrotron radiation the relative abundances of the ionic fragments and their kinetic energy release values were obtained from both PEPICO and photoelectron photoion photoion coincidence spectra. Possible fragmentation processes are discussed and compared with that found for the related CH(3)SCN species.

Outermost and inner-shell electronic properties of ClC(O)SCH2CH3 studied using HeI photoelectron spectroscopy and synchrotron radiation

A study of valence electronic properties of S-ethyl chlorothioformate (S-ethyl chloromethanethioate), ClC(O)SCH(2)CH(3), using HeI photoelectron spectra (PES) and synchrotron radiation is presented. Moreover, the photon impact excitation and dissociation dynamics of ClC(O)SCH(2)CH(3) excited at the S 2p and Cl 2p levels are elucidated by analyzing the total ion yield (TIY) spectra and time-of-flight mass spectra acquired in multicoincidence mode [photoelectron-photoion coincidence (PEPICO) and photoelectron-photoion-photoion coincidence (PEPIPICO)]. The HeI photoelectron spectrum is dominated by features associated with lone-pair electrons from the ClC(O)S- group, the HOMO at 9.84 eV being assigned to the n(π)(S) sulfur lone-pair orbital. Whereas the formation of C(2)H(5)(+) ion dominates the fragmentation in the valence energy region, the most abundant ion formed in both the S and Cl 2p energy ranges is C(2)H(3)(+). Comparison with related XC(O)SR (X = H, F, Cl and R = -CH(3), -C(2)H(5)) species reveals the impact of the alkyl chain on the photodissociation behavior of S-alkyl (halo)thioformates.

Theoretical Study of Optical and Electronic Properties of Unsymmetrical Dendritic Molecules with Different Bridge Moieties

Molecules bearing polyphenylphenyl dendrons show great potential for applications in organic light-emitting diodes. In this paper, quantum-chemical calculations have been applied to investigate the structural and electronic properties of typical molecules bearing polyphenylphenyl dendrons, 2-(2′,3′,4′,5′-tetraphenyl)phenyl-5-(p-N,N-dimethyl)phenyl pyridine (A), 1-(2′,3′,4′,5′-tetraphenyl)phenyl-4-(p-N,N-dimethyl)phenyl benzene (B), 2-(2′,3′,4′,5′-tetraphenyl)phenyl-5-(p-N,N-dimethyl)phenyl thiophene (C), 1-(2′,3′,4′,5′-tetraphenyl)phenyl-2,5-dimethoxy-4-(p-N,N-dimethyl)phenyl benzene (D), and 2-(2′,3′,4′,5′-tetraphenyl)phenyl-5-(p-N,N-dimethyl)phenyl furan (E). The geometrical and electronic structures in the ground state and lowest singlet excited states have been optimized by B3LYP/6-31G(d) and TD-B3LYP/6-31G(d) methods, respectively. The important parameters, including ionization potential (IP), electron affinity, the reorganization energies (λ), hole extraction potential, and electron extraction potential, first lowest singlet excitation energies, maximum absorption and emission wavelengths are also systematically investigated. The result implied that the HOMO, energy gaps, and IP are affected by the central aromatic ring in the order of phenylene>pyrryl> 2,5-dimethoxyphenylene>thienyl>furyl. The solvent effects on absorption and emission spectra were further examined using the polarizable continuum model.

Probing the molecular and electronic structure of capsaicin: a spectroscopic and quantum mechanical study

The conformational preferences of capsaicin were investigated by using the hybrid meta density functional theory (DFT) method MPWB1K. Its flexible, pendant side chain allows for a multitude of conformations only slightly different in energy. The distinctive vibrational features of the most stable conformers were characterized. To elucidate the most favorable reaction sites of capsaicin for radical scavenging, various homolytic bond-dissociation energies were also calculated. Of the possible radical intermediates, the allyl and benzyl radicals are energetically preferred. The filled and empty electronic structures of capsaicin were investigated by exploiting the photoelectron and electron-transmission spectra also of reference molecules and suitable quantum-mechanical calculations. On this basis, a reliable pattern of the vertical ionization energies and electron-attachment energies of capsaicin was proposed. The frontier pi molecular orbitals are concentrated over the vanillyl moiety, with a modest influence of the amidic-aliphatic chain. The (negative) first vertical electron affinity is predicted to be similar to that of benzene. The absorption spectrum of capsaicin and its change by conversion into a phenolic deprotonated anion (modest bathochromic displacement) or a phenoxyl neutral radical (from colorless to red) were interpreted with time-dependent DFT calculations. ESR measurements following chemical or electrochemical reduction of capsaicin did not lead to detection of the corresponding radical anion. The spectra show fragmentation of the original molecule and formation of a variety of radical species which are believed to have a semiquinonic structure.

Imaging Momentum Orbital Densities of Conformationally Versatile Molecules: A Benchmark Theoretical Study of the Molecular and Electronic Structures of Dimethoxymethane

The main purpose of the present work is to predict from benchmark many-body quantum mechanical calculations the results of experimental studies of the valence electronic structure of dimethoxymethane employing electron momentum spectroscopy, and to establish once and for all the guidelines that should systematically be followed in order to reliably interpret the results of such experiments on conformationally versatile molecules. In a first step, accurate calculations of the energy differences between stationary points on the potential energy surface of this molecule are performed using Hartree-Fock (HF) theory and post-HF treatments of improving quality (MP2, MP3, CCSD, CCSD(T), along with basis sets of increasing size. This study focuses on the four conformers of this molecule, namely the trans-trans (TT), trans-gauche (TG), gauche-gauche (G+G+), and gauche-gauche (G+G-) structures, belonging to the C2v, C1, C2, and Cs symmetry point groups, respectively. A focal point analysis supplemented by suited extrapolations to the limit of asymptotically complete basis sets is carried out to determine how the conformational energy differences at 0 K approach the full CI limit. In a second step, statistical thermodynamics accounting for hindered rotations is used to calculate Gibbs free energy corrections to the above energy differences, and to evaluate the abundance of each conformer in the gas phase. It is found that, at room temperature, the G+G+ species accounts for 96% of the conformational mixture characterizing dimethoxymethane. In a third step, the valence one-electron and shake-up ionization spectrum of dimethoxymethane is analyzed according to calculations on the G+G+ conformer alone by means of one-particle Green's function [1p-GF] theory along with the benchmark third-order algebraic diagrammatic construction [ADC(3)] scheme. A complete breakdown of the orbital picture of ionization is noted at electron binding energies above 22 eV. A comparison with available (e,2e) ionization spectra enables us to identify specific fingerprints of through-space orbital interactions associated with the anomeric effect. At last, based on our 1p-GF/ADC(3) assignment of spectral bands, accurate and spherically averaged (e,2e) electron momentum distributions at an electron impact energy of 1200 eV are computed from the related Dyson orbitals. Very significant discrepancies are observed with momentum distributions obtained for several outer-valence levels using standard Kohn-Sham orbitals.