deMon Density-functional Theory Research Articles

DFT Simulation of Electron Spectra for Auger Electron and Photoelectron Spectra of Lithium Compounds

(Li, O, F)-Auger electron, and X-ray photoelectron spectra (AES, VXPS) of solid lithium compounds (Li metal, LiCl, LiF, Li2O) are simulated by deMon density functional theory (DFT) calculations using the model molecules of the unit cell. Calculated valence XPS, core-electron binding energies (CEBE)s, and Li-, O-, and F-KVV AES for the substances correspond considerably well to experimental results. For the calculation of VXPS, the observed spectra of Li2O pellet with chemisorbed CO2 almost show agreement with simulation curve of the valence XPS according to the model for the 1/1 ratio of Li2O/Li2CO3. In the case of AES calculation, we analyze the experimental AES with our modified Auger electron kinetic energy calculation method which corresponds to the two final-state holes at the ground state and at the transition-state in DFT calculation by removing 1 and 2 electrons, respectively. Experimental KVV AES of the Li atom, and (O, F) KVV AES of (Li2O and LiF) in the substances almost agree well to the AES calculated with maximum kinetic energies at the ground state, and at the transition-state, respectively.

Kinetic energies to analyze the experimental auger electron spectra by density functional theory calculations

In the Auger electron spectra (AES) simulations, we define theoretical modified kinetic energies of AES in the density functional theory (DFT) calculations. The modified kinetic energies correspond to two final-state holes at the ground state and at the transition-state in DFT calculations, respectively. This method is applied to simulate Auger electron spectra (AES) of 2nd periodic atom (Li, Be, B, C, N, O, F)-involving substances (LiF, beryllium, boron, graphite, GaN, SiO2, PTFE) by deMon DFT calculations using the model molecules of the unit cell. Experimental KVV (valence band electrons can fill K-shell core holes or be emitted during KVV-type transitions) AES of the (Li, O) atoms in the substances agree considerably well with simulation of AES obtained with the maximum kinetic energies of the atoms, while, for AES of LiF, and PTFE substance, the experimental F KVV AES is almost in accordance with the spectra from the transitionstate kinetic energy calculations.

Analysis of valence XPS and AES of C, N, O, and F-containing substances by DFT calculations using the model molecules

Experimental valence X-ray photoelectron spectra (VXPS) and Auger electron spectra (AES) of (Li, C, N, O, F) elements of four solid substances [graphite, GaN, SiO2, LiF] are analyzed by density-functional theory calculations using the model molecules of the unit cell. For the calculations, we use deMon density functional theory (DFT) program to estimate VXPS, core-electron binding energies, and (Li, C, N, O, F)-KVV AES of the solid substances. In the AES simulations, we evaluate theoretical kinetic energies of the AES with our modified calculation method. The modified kinetic energies correspond to two final-state holes at the ground state and at the transition-state in DFT calculations, respectively. Experimental KVV AES of the (Li, C, N, O) atoms in the substances agree considerably well to simulation of AES obtained with the maximum kinetic energies of the atoms, while, the experimental F KVV AES of LiF is almost in accordance with the spectra from the transition-state kinetic energy calculations.

Evaluation of antioxidant activity and electronic structure of aspirin and paracetamol

We present a study of electronic structure, chemical bonding, and antioxidant activity of phenolic antioxidants (aspirin and paracetamol). X-ray photoelectron and emission spectra of the antioxidants have been simulated by deMon density functional theory (DFT) calculations of the molecules. The chemical bonding of aspirin is characterized by the formation of oxygen ‘lone-pair’ π-orbitals which can neutralize free radicals and thus be related to antioxidant properties of the drug. In the case of paracetamol the additional nitrogen ‘lone pair’ is formed which can explain toxicity of the drug. We propose an evaluation method of antioxidant activity based on the relationship between experimental half-wave oxidation potential ( E p/2 ) and calculated ionization potentials ( I P ) by the DFT calculations, and can conclude that paracetamol has the higher antioxidant activity than aspirin.

Analysis of Electron Spectra of Carbon Allotropes (Diamond, Graphite, Fullerene) by Density Functional Theory Calculations Using the Model Molecules

X-ray photoelectron, emission, and Auger electron spectra of diamond, graphite, and fullerene have been analyzed by deMon density-functional theory (DFT) calculations using the model molecules adamantane derivative (C 1 0 H 1 2 (CH 3 ) 4 ), pyrene (C 1 6 H 1 0 ), and C 6 0 , respectively. The theoretical valence photoelectron, C Kα X-ray emission, and Auger electron spectra for the allotropes are in good accordance with the experimental ones. The combination analysis of the valence X-ray photoelectron and C Ka X emission spectra enables us to divide the valence electronic distribution in the individual contributions for pa- and pπ-bonding MOs of the carbon allotropes, respectively. The experimental Auger electron spectra of the allotropes can be classified in each range of ls-2p2p, ls-2s2p, and 1s-2s2s transitions for C KVV spectra, and in individual contributions of the chemically different carbon atoms from the theoretical analysis.

Surface Damage of PVC Polymers during XPS Analysis

We examined surface damage for polyvinyl chloride (PVC) sample after 260 minutes at an unmonochromated Al Kα X-ray source power at 600 W, because PVC decomposes very quickly due to the X-ray irradiation. The pendant Cl groups of the PVC were estimated to cleave more rapidly than C-C bonds in the main chains from two-center bond energies calculated by AM1 method using the model pentamer. The valence XPS of the polymer at the initial time, and after 120 and 260 minutes of X-ray irradiation were analyzed by deMon density-functional theory (DFT) calculations using the model pentamer. The simulated results afforded us that the pendant Cl groups of PVC cleave mainly due to the X-ray radiation.

Analysis of XPS and AES of Carbon Allotrope (Diamond, Graphite, C<SUB>60</SUB>) by DFT Calculations using the Model Molecules

X-ray photoelectron, and Auger electron spectra of diamond, graphite, and C60 fullerene have been analyzed by deMon density-functional theory (DFT) calculations using the model adamantane derivative (C10H12(CH3)4), pyrene (C16H10), and C60 molecules, respectively. The theoretical valence photoelectron, and Auger electron spectra for the allotrope show good accordance with the experimental ones, although we couldn’t also observe the Auger electron spectrum for solid C60. The experimental AES of the allotrope can be classified in each range of 1s-2p2p, 1s-2s2p, and 1s-2s2s transitions for C KVV spectra.

Analysis of X-ray photoelectron spectra of electrochemically prepared polyaniline by DFT calculations using model molecules

In order to clarify the electronic state of polyaniline synthesized electrochemically, we analyzed valence X-ray photoelectron spectra (XPS) and core-electron binding energies of N, N′-Diphenyl-1,4-phenylenediamine and polyaniline by deMon density-functional theory calculations (DFT) using the molecule and model molecule, respectively. Polyaniline has been synthesized electrochemically from two different thin films on platinum plates in oxidation and reduction states, respectively. Valence XPS of the conductive polymers were analyzed by deMon DFT calculations using model tetramers of the four chemical structural formulae (1S, 2S, 1A, and 2A) which MacDiarmid and co-workers proposed. We can conclude that the oxidized polyanililine corresponds to the 2A type, while the reduction form approximated the 1S type.

Analysis of Valence XPS of Hydrocarbon Polymers Modified by He-Ion Bombardment

Surface analysis of hydrocarbon polymers (polyethylene (PE), polypropylene (PP), polystyrene (PS)) by He-ion bombardment was performed from the theoretical and experimental valence X-Ray photoelectron spectra (XPS). The polymers were He-ion bombarded at an energy of 150 keV with fluences between 1 × 1012 and 1 × 1015 ions cm-2. The bombardment destroys the chemical bonds of the polymer surface and activates carbon atoms near the surfaces. They may produce radicals and recombine with oxygen in the residual gas and/or atmospheric oxygen. We consider that experimental results for valence XPS of the polymers due to the bombardment are simulated by deMon density-functional theory (DFT) calculations using the model trimers, or dimer of neutral, radical, cationized, and oxidized compounds.

Electronic structure of thiophenes and phtalocyanines

The results of x-ray fluorescence measurements of thiophenes [regioregular poly 3-hexylthiophene (P3HT) and {alpha},{omega}-dihexylquaterthiophene (DH{alpha}4T)] and phtalocyanines [copper phtalocyanine (CuPc) and copper hexadecafluorophtalocyanine (F{sub 16}CuPc)] are presented. Experimental carbon, nitrogen, fluorine K{alpha} and sulphur, copper L{sub 2,3} x-ray emission spectra are compared with ultraviolet photoemission spectroscopy spectra and deMon density-functional theory calculations of model molecules. We find that the carbon K{alpha} emission is almost identical for P3HT and DH{alpha}4T. This indicates that the electronic structure of the {pi} system is not affected by the presence of the side group. The ratio of emission intensities of the Cu L{sub 2} to Cu L{sub 3} intensities is found to be smaller by a factor of 2 for CuPc and F{sub 16}CuPc than it is for pure metal. This demonstrates the strong covalency in phalocyanines.

Theoretical X-ray photoelectron and emission spectra of Si- and S-containing polymers by density-functional theory calculations using model molecules

The X-ray photoelectron and emission spectra (XPS and XES) of Si- and S-containing polymers [(–Si{–CH 2–} 3) n (PCS), (–Si(CH 3)(OH)–O–) n (PMHSO), (–Si(C 6H 5)(OH)–O–) n (PTES), (–Si(C 6H 5)(OH)–O–Si(CH 3)(OH)–O–) n (PPMHSO), (–C 6H 4–O–C 6H 4–SO 2–) n (PES)] were simulated by the deMon density-functional theory (DFT) calculations using the model molecules. The theoretical valence photoelectron and C Kα X-ray emission spectra showed good accordance with some experimental ones. The combined analysis of the valence XPS and C and O Kα XES enables us to divide the observed valence electronic distribution into the individual contributions for pσ-, pπ- and non-bonding MOs of the polymers.

Soft-x-ray fluorescence study of the quasi-one-dimensional Heisenberg antiferromagnet tetraphenylverdazyl

Soft-x-ray fluorescence measurements have been performed on a single crystal of the organic antiferromagnet 2,4,6-triphenylverdazyl. Resonant and nonresonant C $K\ensuremath{\alpha}$ and N $K\ensuremath{\alpha}$ $(2\stackrel{\ensuremath{\rightarrow}}{p}1s$ transition) x-ray emission spectra (XES) were measured and compared with x-ray photoelectron valence band spectra and deMon density-functional theory calculations. It is shown that intramolecular interactions are much stronger than intermolecular ones and give the main contribution to the formation of C $2p$ density of states. We present evidence of a delocalization of unpaired N $2p$ electrons over the verdazyl ring. The excitation energy dependence of C $K\ensuremath{\alpha}$ and N $K\ensuremath{\alpha}$ XES observed below the C $1s$ and N $1s$ thresholds, respectively, is discussed in terms of symmetry selective resonant inelastic x-ray scattering.

Theoretical X-Ray Photoelectron and Emission Spectra of C-, N-, and O-Containing Polymers by Density-Functional Theory Calculations using Model Molecules

The X-Ray photoelectron and emission spectra (XPS, XES) of C-, N-, and O-containing polymers [polyvinyl alcohol (PVA), polyethylene oxide (PEO), and dianhydride-4,4′-oxydianiline (PMDA-ODA)] were simulated by de-Mon density-functional theory (DFT) calculations using the model molecules. The theoretical valence photoelectron and C, N, and O Kα X-Ray emission spectra showed good agreement with some experimental ones. The combined analysis of the valence XPS and C, N, and O Kα XES enables us to divide the observed valence electronic distribution into the individual contributions for pσ-, pπ- and non-bonding MOs of the polymers.

Accurate Density-Functional Calculation of Core-Electron Binding Energies of Some Substituted Benzenes

Abstract The core electron binding energies (CEBE's) of benzene, seven monosubstituted benzenes (Ph-X) and one disubstituted benzene (p-NH2-C6H4-NO2) were calculated using density-functional theory (DFT). The unrestricted generalized transition-state (uGTS) model was employed. The DeMon DFT program with a combined functional of Becke's exchange (B88) with Perdew's correlation (P86) was used. The average absolute deviation of the calculated CEBE's of the title compounds was 0.3 eV when the cc-pVDZ basis set was used. The ``CEBE shift'' of the ring carbon in Ph-X was calculated while taking the CEBE on the ring carbon in Ph-H as a reference. The thus-calculated CEBE shifts agree with experiment within the value of the average absolute deviation, 0.1 eV. The signs and quantitative numerical values of the CEBE shifts are very close to the corresponding Hammett σ constants.

Theoretical X-ray photoelectron spectra of polymers by deMon DFT calculations using the model dimers

Core-electron spectra of 15 polymers [(CH 2CH 2) n (PE), (CH 2CH(CH 3)) n (PP), (CH 2CH 2NH) n (PEI), (CH 2CH 2O) n (PEG), (CH 2CH(OCH 3)) n (PVME), (CH 2CHF) n (PVF), (CH 2CF 2) n (PVDF), (CF 2CF 2) n (PTFE), (CH 2CH(CN)) n (PAN), (CH 2C(CH 3)(CN)) n (PMAN), (CH 2CH 2S) n (PETHS), (CH 2CHCl) n (PVC), (CH 2CCl 2) n (PVDC), (Si(CH 3) 2) n (PDMS), (Si(CH 3) 2O) n (PDMSO)], and valence photoelectron spectra (XPS) of the six polymers (PP, PEI, PAN, PMAN, PVME, PTFE) were obtained by deMon density-functional theory (DFT) calculations using the model dimers. The core-electron spectra were simulated with the Gaussian lineshape functions with fixed linewidths of 0.5, and 1.0 eV for each C1s, and (N1s, O1s, F1s) MO value, respectively, and calculated Al-Kα valence photoelectron spectra were obtained using Gaussian lineshape functions of an approximate linewidth (0.08 I k ): I k ( I FL)= I′ k −WD, as indicated in previous works. The vertical ionization potential I′ k and each core-electron binding energy (CEBE) were calculated by restricted generalized diffuse ionization (rGDI) and unrestricted generalized transition-state (uGTS) models, respectively. The theoretical core-electron spectra showed better agreement with the experimental ones of the polymers than those due to Koopmans' theorem. The difference between the calculated and the experimental CEBEs reflected the reasonable WDs of the polymers.

Analysis of XPS and XES of diamond and graphite by DFT calculations using model molecules

X-ray photoelectron and emission spectra (XPS and XES) of diamond and graphite have been analyzed by deMon density-functional theory (DFT) calculations using the model adamantane derivative (C10H12(CH3)4) and pyrene (C16H10) molecules, respectively. The theoretical valence photoelectron and C Kα X-ray emission spectra for the allotrope are in good accordance with the experimental ones. The combined analysis of the valence XPS and C Kα XES enables us to divide the valence electronic distribution into the individual contributions for pσ-, and pπ-bonding MOs of the diamond and graphite, respectively. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 102–108, 2001

Radiation-induced degradation of polyethersulphone films studied by fluorescent X-ray emission spectroscopy

The electronic structure and chemical bonding of unirradiated and irradiated Poly-1,4-phenylene-ether-sulphone (PES) films have been studied with fluorescent X-ray emission spectroscopy. PES films have been irradiated with 150 keV Ar + ions at concentrations of 5 × 10 13, 1 × 10 14, 1 × 10 15 and 7.5 × 10 15 cm −2. The electronic structure of PES has also been calculated with MO LCAO calculations using a deMon density functional theory (DFT) program and shows good agreement with the experimentally obtained X-ray fluorescence spectra (S L 2,3, C Kα and O Kα). It is found that the C Kα X-ray emission spectra (XES) of irradiated PES films display marked changes at very high concentrations (1 × 10 15 − 7.5 × 10 15 cm −2), similar to the spectroscopic characteristics of amorphous carbon. According to our measurements, the low-energy shift of the S Kα 1,2 XES is observed for concentrations greater than 5 × 10 13 cm −2 which is related to the reduction of –SO 2– to –S– following the transfer of an oxygen atom from the –SO 2– functional group to the radical site.