DFT Formalism Research Articles

L-leucinium perchlorate: new molecular complex with nonlinear optical properties. Vibrational, calorimetric and theoretical studies

Abstract On the basis of prior X-ray crystallographic results published by J. Janczak and G. Perpetuo, detailed vibrational studies were performed. The FT-IR and Raman spectra at ambient temperature were measured. The NLO properties were determined with the Kurtz-Perry experiment. Theoretical vibrational spectra were calculated. A detailed potential energy distribution (PED) analysis was performed. Assignments of observed bands were made. On the basis of these results, the behaviour of hydrogen bonds in the investigated compound was analysed and discussed. The equilibrium geometry of L-leucinium perchlorate was obtained. The results were compared with experimental X-ray data. The DFT formalism was used in theoretical studies. Detailed TDDFT study of hyperpolarizbility of first and second order for the investigated molecule was performed. Results were compared with experiments. Theoretical population analysis was used to determine the local electron density and local charges in investigated molecule. Differential scanning calorimetric study (DSC) was performed.

Real-space density functional theory and time dependent density functional theory using finite/infinite element methods

We present a numerical approach using the finite element method to discretize the equations that allow getting a first-principles description of multi-electronic systems within DFT and TD-DFT formalisms. A strictly local polynomial function basis set is used in order to represent the entire real-space domain. Infinite elements are introduced to model the infinite external boundaries in the case of Hartree’s equation. The diagonal mass matrix is obtained using a close integration rule, reducing the generalized eigenvalue problem to a standard one. This framework of electronic structure calculation is embedded in a high performance computing environment with a very good parallel behavior.

Can water be a catalyst on the HO2 + H2O + O3 reactive cluster?

We report a theoretical investigation on the role of water in the reaction between the hydroperoxyl radical and ozone. Due to the encouraging results obtained in our previous work on the HO2+O3 reaction, and also because of the low computational cost involved, all calculations have employed the Kohn–Sham DFT formalism. In particular, functionals with a high percentage of exact exchange are utilized in order to attain a high accuracy at the saddle points for reaction. It is found that ozone reacts with the HO2·H2O complex and that the classical barrier height of the oxygen-abstraction mechanism is now lower than the barrier to the hydrogen-abstraction mechanism. The implications of the results on the mechanism of the HO2+O3 reaction are discussed.

FP-LAPW study of anhydrous cadmium and silver oxalates: electronic structure and electron density topology

Results of theoretical studies of electronic structure (full potential linearized augmented plane Wave method within DFT formalism as implemented in Wien2k package), electron density topology and bonding (Bader’s quantum theory of atoms in molecules topological analysis of total electron density obtained from FP-LAPW calculations) in anhydrous cadmium and silver oxalates are presented. Detailed analysis of calculated band structures, densities of states (total and projected ones) and bond critical points shows that despite the differences of crystal structure both these compounds reveal important similarities and suggests that one can expect that they should exhibit very similar properties and behavior during e.g. thermal decomposition process (which is in agreement with experiments).

Coumarin derived chromophores in the donor–acceptor–donor format that gives fluorescence enhancement and large two-photon activity in presence of specific metal ions

Two coumarin derived dyes (compounds L a and L) have been synthesized in high yields by Schiff base condensation. On probing with a femtosecond laser, none of these compounds show any two-photon activity in the wavelength range, 760–860 nm. However, L a in presence of Zn(II) and L in presence of Mg(II), exhibit large two-photon absorption as well as emission in the same wavelength range. Theoretical calculations at the B3LYP functional with 6-31G∗ and LanL2DZ mixed basis set under DFT formalism support experimental results.

Theoretical study of CO adsorption on the illuminated TiO 2 (0 0 1) surface

A quantum modeling of the CO adsorption on illuminated anatase TiO 2 (0 0 1) is presented. The calculated adsorption energy and geometries of illuminated case are compared with the ground state case. The calculations were achieved by using DFT formalism and the BH and HLYP. Upon photoexcitation, an electron–hole pair is generated. Comparing of natural population in the ground state and the exited state, shows that an electron is trapped in a Ti 4+ ion and a hole is localized in an oxygen ion. The photoelectron helps generation of a CO 2 molecule on the TiO 2 surface. As shown by optimization of these systems, the CO molecule adsorbed vertically on the TiO 2 (0 0 1) surface in the ground state case while the CO molecule made an angle of 134.3° to this surface at the excited state case. Based on the here used model the obtained adsorption energy was 0.36 eV which is in excellent agreement with the reported experimental value. In the present work the C–O stretch IR frequencies are calculated which are 1366.53 and 1423.16 cm −1. These results are in good agreement with the earlier reported works for the surface carbonaceous compounds, and oxygenated carbon species.

Covalent vs Electrostatic Interactions in Rare Earth Systems: A Comparative Study of U(III), U(IV), and U(V) and Nd(III) Bonding Properties by DFT and CAS-PT2 Approaches

A description of the electronic structure of F(3)UCO, F(3)NdCO, F(4)UCO, and F(5)UCO has been obtained by Complete Active Space second-order perturbation theory CASPT2 calculations using a relativistic effective core potential. These multiconfigurational calculations have been compared to the DFT description combined with a quasi-relativistic ZORA scalar approach. Geometries have been optimized for both levels of calculations and frequencies computed in the DFT formalism. The bonding properties of U(III) have been compared to those of Nd(III) and of higher oxidation states of U(IV,V). Both methodologies are consistent and show a decrease of the covalent character of the U-CO bonding with a higher oxidation state, U(IV) or U(V), as well as its absence for for the isoelectronic Nd(III) species.

Hydrogen Bonding and Stacking π−π Interactions in Solid 6-Thioguanine and 6-Mercaptopurine (Antileukemia and Antineoplastic Drugs) Studied by NMR-NQR Double Resonance Spectroscopy and Density Functional Theory

A chemotherapeutic drug 6-thioguanine (2-amino-1,7-dihydro-6H-purine-6-thione, 6-TG) has been studied experimentally in the solid state by NMR-NQR double resonance and theoretically by the density functional theory. Fourteen resonance frequencies on (14)N have been detected and assigned to particular nitrogen sites in the 6-TG molecule. A valid assignment of NQR frequencies for 6-mercaptopurine (6-MP) has been proposed. The effects of molecular aggregations, related to intermolecular hydrogen bonding and stacking pi-pi interactions on the NQR parameters have been analyzed within the DFT and AIM (atoms in molecules) formalism for 6-TG and 6-mercaptopurine (6-MP). The so-called global reactivity descriptors have been calculated to compare the properties of molecules of 6-TG and 6-MP, to check the effect of -NH(2) group as well as to identify the differences in crystal packing.

Electron density distribution in cladribine (2-chloro-2′-deoxyadenosine) – A drug against leukemia and multiple sclerosis – Studied by multinuclear NQR spectroscopy and DFT calculations

2-Chloro-2′-deoxyadenosine (Cladribine) chemotherapeutic drug has been studied experimentally in solid state by 35Cl NQR and NMR–NQR double resonance and theoretically by the Density Functional Theory. Fifteen resonance frequencies on 14N have been detected and assigned to particular nitrogen sites in the 2-CdA molecule. The effects of tautomerism, regioisomerism, conformations and molecular aggregations, related to intermolecular hydrogen bond formation, on the NQR parameters have been analysed within the DFT and AIM ( Atoms in Molecules) formalism. The properties of the whole molecule, the so-called global reactivity descriptors, have been calculated for a comparison of both syn and anti conformations of 2-CdA molecule to check the effect of crystal packing on molecular conformation.

On the performance of the hybrid TPSS meta-GGA functional to study the singlet open-shell structures: A combined theoretical and experimental investigations of the Ni 2O 2 molecule

Two isomers have been experimentally characterized for the Ni 2 + O 2 → Ni 2O 2 reaction: the first isomer is the cyclic ground state with equivalent NiO bonds, while the second one with an open-cyclic structure and non-equivalent NiO bonds corresponds to a metastable intermediate, obtained by photo-excitation. The photo-conversion between these species has been evidenced in the near-infrared and visible regions. Compared with the experimental data, we showed that DFT (and TDDFT) calculations provide a reliable description of the photophysical process, provided the following requirements. We have to: (a) use a hybrid meta-GGA functional within the unrestricted DFT formalism (UDFT) for all spin multiplets, (b) determine a stable electronic solution for each multiplet (particularly for the singlet state), in order to give a qualitatively correct description of a multireference system, (c) determine the exact exchange contribution in the hybrid exchange-correlation functional, as the most stable state of each isomer is the singlet. For the Ni 2O 2 molecules, we found that the last requirement could be satisfied for the Hartree–Fock exchange contribution ranging from 2% to 8%, notably smaller than the value (10%) suggested for the TPPSh hybrid meta-GGA functional. Consequently, it is shown that the UDFT description covers static correlation effects needed for the correct treatment of the Ni 2O 2 isomers.

Density functional studies on the exchange interaction of a dinuclear Gd(iii)–Cu(ii) complex: method assessment, magnetic coupling mechanism and magneto-structural correlations

Density functional calculations have been performed on a [Gd(iii)Cu(ii)] complex [L(1)CuGd(O(2)CCF(3))(3)(C(2)H(5)OH)(2)] () (where L(1) is N,N'-bis(3-ethoxy-salicylidene)-1,2-diamino-2-methylpropanato) with an aim of assessing a suitable functional within the DFT formalism to understand the mechanism of magnetic coupling and also to develop magneto-structural correlations. Encouraging results have been obtained in our studies where the application of B3LYP on the crystal structure of yields a ferromagnetic J value of -5.8 cm(-1) which is in excellent agreement with the experimental value of -4.42 cm(-1) (H = JS(Gd).S(Cu)). After testing varieties of functional for the method assessment we recommend the use of B3LYP with a combination of an effective core potential basis set. For all electron basis sets the relativistic effects should be incorporated either via the Douglas-Kroll-Hess (DKH) or zeroth-order regular approximation (ZORA) methods. A breakdown approach has been adopted where the calculations on several model complexes of have been performed. Their wave functions have been analysed thereafter (MO and NBO analysis) in order to gain some insight into the coupling mechanism. The results suggest, unambiguously, that the empty Gd(iii) 5d orbitals have a prominent role on the magnetic coupling. These 5d orbitals gain partial occupancy via Cu(ii) charge transfer as well as from the Gd(iii) 4f orbitals. A competing 4f-3d interaction associated with the symmetry of the complex has also been observed. The general mechanism hence incorporates both contributions and sets forth rather a prevailing mechanism for the 3d-4f coupling. The magneto-structural correlations reveal that there is no unique parameter which the J values are strongly correlated with, but an exponential relation to the J value found for the O-Cu-O-Gd dihedral angle parameter is the most credible correlation.

Impact of tautomers on the absorption spectra of neutral and anionic alizarin and quinizarin dyes

We have used the PCM-(TD)DFT formalism to evaluate the tautomeric effects on the absorption spectra of the neutral and anionic forms of both alizarin and quinizarin dyes. Our procedure is able to predict an excitation energy with an accuracy of 0.08 eV. It is shown that for the monoanionic form of alizarin, two tautomers can coexist in solution. A resulting λ max was simulated, and was found to be in good agreement with experiment. We have subsequently quantified the extent to which excitation energy errors correlate with the spatial overlap between the HOMO and the LUMO, as measured by the quantity S HL.

Attachment of Different Donor Groups to a Cryptand for Modulation of Two‐Photon Absorption Cross‐Section

Two-photon absorption (TPA) properties of a laterally nonsymmetric aza cryptand with attached side arms have been investigated. This series of Schiff base derivatives supports the mechanistic approach for enhancing the TPA process, which is usually dictated by molecular geometry, pi-bridging, delocalization length, and corresponding charge-transfer possibilities. The results described here suggest that on increasing the branching units, the TPA cross-section, sigma((2)), can be tuned to a larger value. The TPA activity is "switched on" when a metal atom enters the cavity and serves as a conduit of electronic delocalization. The sigma((2)) value increases as the donor strength increases. The maximum value is obtained on moving from the single-branched system to the nearly threefold symmetry. This serves as a useful synthetic strategy for designing novel octupolar molecules with high sigma((2)) values. Theoretical calculations at the B3LYP functional with the 6-31G* basis set under DFT formalism provide supporting evidence that the communication between the side arms through the metal d orbital and more ordered geometry of chromophores leads to a smaller HOMO-LUMO gap, which has a great influence upon the electronic properties of the molecules.

Theoretical modeling of electrochemical interactions in bimetallic molybdenum nitrosyl complexes incorporating saturated bridges

Hybrid B3LYP and non-hybrid OLYP DFT formalism has been applied to neutral and reduced forms of bimetallic hydrotris(3-methylpyrazol-1-yl)borato (Tp 3-Me) molybdenum nitrosyl complexes incorporating ethane-1,2-diolate bridges. Direct evidence for localization of an extra electron in mixed-valence compounds {16 e:17 e} − is based on the analysis of electron density, energetic stabilization of asymmetric structures with an electron trapped on one Mo and the splitting of both calculated and experimental ν NO stretching frequencies. Differences in the first and second electron affinities calculated in PCM solvent model have been successfully related to cyclic voltammetry measurements. Electronic interactions through saturated ethanediolato bridges are evidenced by the extent of spin density delocalization towards the second Mo center.

Na N + (N ⩽ 12) clusters fragmentation channels: A conceptual DFT approach

In this paper we study the small Na$_{N}^{+}$ (N l 12) cationic clusters fragmentation channels, with all electronic calculations performed at the B3LYP/6-311G+(d,p) DFT level of theory. For these cluster sizes, we obtain for all neutral, cationic, bi-cationic and anionic species the optimized geometries, total energies and electronic properties such as the adiabatic ionization potentials, electron affinities and global hardnesses. Furthermore, we propose the novel concept of global hardness change, Δ η, within the conceptual DFT formalism, based on the maximum hardness principle of Chattaraj, Lee, and Parr. Thereafter we compute for all possible fragmentation channels the involved energetics, ΔE, and the global hardness change, Δη, both methods are shown to be in agreement with available experimental findings for all but one case. We show that hardness and the global hardness change are good DFT descriptors to assess the preferred fragmentation channels, where formation of the hardest fragments seems to be the driving force.

Diaza-18-crown-6 based chromophores for modulation of two-photon absorption cross-section by metal ions

Two chromophores with diaza-18-crown-6 as receptor have been synthesized in high yields. The electronic structure, one-photon absorption (OPA) spectra, and two-photon absorption (TPA) properties have been studied in detail. When no metal ion is added as input, both show negligible TPA cross-section ( σ 2). However, in the presence of Zn(II)/Cd(II)/Mg(II)/Ca(II) ion, each exhibits large TPA cross-section value. Binding of metal ion in the receptor increases the symmetric charge transfer leading to large σ 2 values. Theoretical calculations at the B3LYP functional with 6-31G ∗ and LanL2DZ mixed basis set under DFT formalism support experimental results.

DFT modelling of the [M–Pd–M] 6+ metal atom chains (M = Ni, Pd): Structural, electronic and magnetic issues

Following the recent findings on heterometallic string complexes, we extend the recently published work on NiPdNi(dpa) 4Cl 2 to discuss the Ni–Pd–Ni and Pd 3 chains with equatorial ligands (L) being dipyridylamide (dpa), 2,6-bis(phenylamido)pyridine (BPAP) and N, N′-bis-( p-toluenesulfonyl)-pyridyldiamide (Lpts), using the DFT formalism. The analysis of such a hypothetical series of linear trimetallics anticipates that, for NiPdNi(dpa) 4Cl 2, the extended valence shell of palladium strengthens the antiferromagnetic coupling between high-spin terminal nickel atoms. For L = BPAP the system, as expected, becomes diamagnetic, and antiferromagnetism reappears for L = Lpts. The theoretical modelling of the coupling following the Heisenberg Hamiltonian applied to two magnetic centres ( H ^ = - 2 J AB S ^ A · S ^ B ) gives −2 J = 320 and 497 cm −1 for L = dpa and Lpts, respectively. Pd 3 chains display an enhanced tendency to be diamagnetic with various ligands. More specifically with dpa, Pd 3(dpa) 4Cl 2—should it be synthesized—could be magnetically inactive since the strongly antiferromagnetic state generated by the coupling of two terminal, high-spin Pd atoms (−2 J = 1393 cm −1) is computed to be in close competition with the diamagnetic, closed-shell state. For L = Lpts as for BPAP, the hypothetic [Pd 3] 6+ chain is predicted to be diamagnetic, resulting from the high energy of the antibonding d(Pd)–p(N) orbital. The shift toward diamagnetism induced by the replacement of Ni by Pd in terminal position is therefore assigned to a stronger N → M donation interaction with M = Pd.

Prediction of the Vapor−Liquid Interfacial Tension of Nonassociating and Associating Fluids with the SAFT-VR Density Functional Theory

The SAFT-VR DFT Helmholtz free energy density functional [Gloor, G. J.; Jackson, G.; Blas, F. J.; Martín del Río, E.; de Miguel, E. J. Chem. Phys. 2004, 121, 12740] is used to describe the vapor−liquid interface of nonassociating and associating molecules ranging in size from small molecules to long chains. The functional, which is based on the statistical associating fluid theory for attractive potentials of variable range (SAFT-VR) description of the homogeneous fluid [Gil-Villegas, A.; Galindo, A.; Whitehead, P. J.; Mills, S. J.; Jackson, G.; Burgess, A. N. J. Chem. Phys. 1997, 106, 4168], is constructed by partitioning the free energy density into a reference term (which incorporates all of the short-range interactions and is treated locally) and an attractive perturbation (which incorporates the long-range dispersion interactions). This functional accounts explicitly for the correlations between the segments using a density-averaged correlation function incorporated into the perturbative term in a similar way to that proposed by Toxvaerd. The SAFT-VR DFT formalism is used to examine the vapor−liquid interfacial tension of a number of pure components, including: n-alkanes; small associating molecules, such as water; linear alkan-1-ols; and some selected replacement refrigerants. In the case of the hydrocarbons and other weakly polar substances, the surface tension can be predicted from intermolecular parameters derived in the usual way from the bulk fluid phase equilibria (vapor-pressure and saturated liquid density). By contrast, when one describes the interfacial properties of associating compounds, it is important to include the surface tension data as well as the bulk vapor−liquid phase equilibria in developing the intermolecular potential model. This provides a means of determining the balance between the dispersive and associative (hydrogen bonding) contributions to the intermolecular potential. The use of interfacial data in the refinement of the potential model allows one to obtain a reliable set of parameters, which can be used to predict the bulk and interfacial properties of mixtures for a broad range of thermodynamic conditions.

Metal induced enhancement of fluorescence and modulation of two-photon absorption cross-section with a donor–acceptor–acceptor–donor receptor

A metal ion sensing fluorophore L that exhibits a large two-photon absorption cross-section has been synthesized in good yields. The influences of different metal ion inputs, on the one- and two-photon spectroscopic properties of L, have been investigated. The ligand itself does not show any fluorescence although in presence of a metal ion like Zn(II), Cd(II), Mg(II) or Ca(II), a ∼25 time enhancement of fluorescence is observed. The ligand with symmetrical “donor–acceptor–acceptor–donor” characteristics exhibits a large two-photon absorption cross-section measured by femtosecond open-aperture Z-scan technique at 880 nm. However, presence of any of the above metal ions lowers its two-photon absorption cross-section ( δ) to different extents at 880 nm. Theoretical calculation carried out in DFT formalism on the ligand and its Zn(II) complex corroborate experimental results.

Theoretical modeling of photocatalytic active species on illuminated TiO 2

A theoretical study of the H 2O and O 2 adsorption on an illuminated TiO 2 anatase surface is presented. The electronic structure and the spin distribution were examined by employing the DFT formalism and the BHandHLYP functional. The adsorbates geometries were fully optimized, including the cluster relaxation. Our results show the dissociative adsorption of the H 2O molecule on the photoactivated TiO 2 (0 0 1) surface. This reaction produces one hydroxyl group with radical character (OH ) and an unpaired electron localized in the 5c-Ti atom. In case of the O 2 molecule, the non-dissociative chemisorption was obtained. This molecule shows one unpaired electron and a negative charge. In these sense, the adsorbed O 2 acts as a superoxide species (O 2 −).