Ab Initio Level Research Articles

Synthesis, spectroscopic, thermal and quantum chemical studies on trivalent erbium NO chelating sulfamonomethoxine–cyclophosph(V)azane complex

Novel hexachlorocyclophosph(V)azane of sulfamonomethoxine (L), was prepared and their coordinating behavior toward the lanthanide ion erbium (III) was studied. The structures of the isolated products were proposed based on elemental analyses, IR, UV–VIS, 1H NMR, 31P NMR, SEM, TEM, XRD, mass spectra, effective magnetic susceptibility measurements and thermogravimetric analysis (TGA). The coordination polyhedra of the eight donor atoms around the two erbium (III) ions are best described as distorted dodecahedral. Computational studies were carried out at the DFT-B3LYP/6-31G(d) level of theory on the structural and spectroscopic properties of L and its binuclear erbium (III) complex. Different tautomers of the ligand were optimized at the ab initio DFT level. Keto-form structure is about 30.5kcal/mol more stable than the enol form (taking zpe correction into account). Simulated IR frequencies were scaled and compared with that experimentally measured. TD-DFT method was used to compute the UV–VIS spectra in gas phase and DMF.

Adsorption of Propane and Propylene on CuBTC Metal–Organic Framework: Combined Theoretical and Experimental Investigation

A combined experimental and theoretical investigation of propane and propylene adsorption in the metal–organic framework CuBTC is presented. The dependence of adsorption enthalpies on the adsorbed amount was determined by microcalorimetry up to 8 mmol g–1 coverage (roughly C3Hn/Cu2+ ratio of 1.5). Trends observed experimentally were interpreted on the basis of accurate calculations carried out at the hybrid DFT–ab initio level. Three types of adsorption sites were identified; however, qualitatively different results were obtained for propane and propylene. Propane preferentially adsorbs at the cage center sites (−ΔH° = 43 kJ mol–1), followed by adsorption at the cage window sites (31 kJ mol–1), while the interaction with the coordinatively unsaturated sites (CUS) is relatively weak (24 kJ mol–1). On the contrary, propylene preferentially interacts with the CUS (56 kJ mol–1), while the adsorption at the cage center and cage window sites was found to be only 45 and 34 kJ mol–1, respectively. Due to the topology of CuBTC, lateral interactions are significantly more important among the adsorbates located at the cage center and cage window sites (populated in the case of propane) than among adsorbates at the CUS and cage center sites (populated in the case of propylene). Therefore, adsorption energies obtained for coverages above 6 mmol g–1 of adsorbed amount were larger for propane than for propylene. Consequently, the presence of small cages makes the CuBTC MOF less suitable for propane/propylene separation than MOFs having the Cu2+ CUS but without small cages (e.g., CPO-27).

Interpretation and Application of Reaction Class Transition State Theory for Accurate Calculation of Thermokinetic Parameters Using Isodesmic Reaction Method

We present a further interpretation of reaction class transition state theory (RC-TST) proposed by Truong et al. for the accurate calculation of rate coefficients for reactions in a class. It is found that the RC-TST can be interpreted through the isodesmic reaction method, which is usually used to calculate reaction enthalpy or enthalpy of formation for a species, and the theory can also be used for the calculation of the reaction barriers and reaction enthalpies for reactions in a class. A correction scheme based on this theory is proposed for the calculation of the reaction barriers and reaction enthalpies for reactions in a class. To validate the scheme, 16 combinations of various ab initio levels with various basis sets are used as the approximate methods and CCSD(T)/CBS method is used as the benchmarking method in this study to calculate the reaction energies and energy barriers for a representative set of five reactions from the reaction class: R(c)CH(R(b))CR(a)CH2 + OH(•) → R(c)C(•)(R(b))CR(a)CH2 + H2O (R(a), R(b), and R(c) in the reaction formula represent the alkyl or hydrogen). Then the results of the approximate methods are corrected by the theory. The maximum values of the average deviations of the energy barrier and the reaction enthalpy are 99.97 kJ/mol and 70.35 kJ/mol, respectively, before correction and are reduced to 4.02 kJ/mol and 8.19 kJ/mol, respectively, after correction, indicating that after correction the results are not sensitive to the level of the ab initio method and the size of the basis set, as they are in the case before correction. Therefore, reaction energies and energy barriers for reactions in a class can be calculated accurately at a relatively low level of ab initio method using our scheme. It is also shown that the rate coefficients for the five representative reactions calculated at the BHandHLYP/6-31G(d,p) level of theory via our scheme are very close to the values calculated at CCSD(T)/CBS level. Finally, reaction barriers and reaction enthalpies and rate coefficients of all the target reactions calculated at the BHandHLYP/6-31G(d,p) level of theory via the same scheme are provided.

Adsorption of lactic acid on chiral Pt surfaces—A density functional theory study

The adsorption of the chiral molecule lactic acid on chiral Pt surfaces is studied by density functional theory calculations. First, we study the adsorption of L-lactic acid on the flat Pt(111) surface. Using the optimed PBE - van der Waals (oPBE-vdW) functional, which includes van der Waals forces on an ab initio level, it is shown that the molecule has two binding sites, a carboxyl and the hydroxyl oxygen atoms. Since real chiral surfaces are (i) known to undergo thermal roughening that alters the distribution of kinks and step edges but not the overall chirality and (ii) kink sites and edge sites are usually the energetically most favored adsorption sites, we focus on two surfaces that allow qualitative sampling of the most probable adsorption sites. We hereby consider chiral surfaces exhibiting (111) facets, in particular, Pt(321) and Pt(643). The binding sites are either both on kink sites-which is the case for Pt(321) or on one kink site-as on Pt(643). The binding energy of the molecule on the chiral surfaces is much higher than on the Pt(111) surface. We show that the carboxyl group interacts more strongly than the hydroxyl group with the kink sites. The results indicate the possible existence of very small chiral selectivities of the order of 20 meV for the Pt(321) and Pt(643) surfaces. L-lactic acid is more stable on Pt(321)(S) than D-lactic acid, while the chiral selectivity is inverted on Pt(643)(S). The most stable adsorption configurations of L- and D-lactic acid are similar for Pt(321) but differ for Pt(643). We explore the impact of the different adsorption geometries on the work function, which is important for field ion microscopy.

Zero-field splitting in nickel(II) complexes: A comparison of DFT and multi-configurational wavefunction calculations

The zero-field splitting (ZFS) is an important quantity in the electron spin Hamiltonian for S = 1 or higher. We report calculations of the ZFS in some six- and five-coordinated nickel(II) complexes (S = 1), using different levels of theory within the framework of the ORCA program package [F. Neese, Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2, 73 (2012)]. We compare the high-end ab initio calculations (complete active space self-consistent field and n-electron valence state perturbation theory), making use of both the second-order perturbation theory and the quasi-degenerate perturbation approach, with density functional theory (DFT) methods using different functionals. The pattern of results obtained at the ab initio levels is quite consistent and in reasonable agreement with experimental data. The DFT methods used to calculate the ZFS give very strongly functional-dependent results and do not seem to function well for our systems.

Sampling the energy landscape of Pt13 with metadynamics

The potential energy surface of a metallic nanoparticle formed by 13 atoms of platinum is efficiently explored using metadynamics in combination with empirical potential molecular dynamics. The scenario obtained is wider and more complex of what was previously reported: more than thirty independent energy basins are found, corresponding to different local minima of Pt. It is demonstrated that in almost all the cases these motifs are local minima even at ab-initio level, hence proving the effectiveness of the method to sample the energy landscape. A classification of the minima in structural families is proposed, and a discussion regarding the shape and the connections between energy basins is reported.

Structure Prediction of Binary Pernitride MN2 Compounds (M=Ca, Sr, Ba, La, and Ti)

Metal-pernitride compounds belong to a class of chemical systems in which both the complex ions and the non-bonding electrons may play roles in the formation of their modified crystalline structures. To investigate this issue, the energy landscapes of pernitrides of metals with different maximum valence (M=Ca, Sr, Ba, La, and Ti) were globally explored on the ab initio level at standard and high pressures, thereby yielding possible (meta)stable modifications in these systems together with information on how the landscape changed as function of the valence of the metal cation. For all of the systems in which no compounds had been synthesized so far, we predicted the existence of kinetically stable modifications that should, in principle, be experimentally accessible. In particular, TiN2 should crystallize in a new structure type, TiN2-I.

Solvent Effects on One- and Two-Photon Optical Properties of a Novel 2, 2':6', 2''-Terpyridine-Based Chromophore

At ab initio level, time-dependent hybrid density functional theory and re- sponse theory have been applied to study the solvent effect on the geometrical and elctronic structure as well as one- and two-photon absorption (TPA) properties of 2,2':6',2-terpyridine-based chromophore L. It is found that solvatochromic shift of charge-transfer (CT) appears nonmonotonic behavior with regard to the polarity of solvent. Besides, the one-photon absorption (OPA) strengths are enlarged slightly and are more dependent on the optical dielectric constant. The TPA cross sections are en- hanced when considering solvent effect compared to that in gas phase and the TPA cross sections show an increasing trend with the increase of solvent polarity. All the theoretical results are in reasonable agreement with experimental data.

Identification of promising chemical systems for the synthesis of new materials structure types: An ab initio minimization data mining approach

In this research we performed data exploring for binary compounds with elements from groups V, IV-VI, and III-VII, with the goal to identify chemical systems where the recently proposed ?5-5? structure type might be experimentally accessible. Among others, TlF, SnO, SnS, SnSe, GeS, GeSe, PbO, PbS, ZnO and ZnS, were chosen for the study. For each of these systems, a local optimization on ab initio level with the LDA functional was performed for the 5-5 structure type, plus other experimentally observed and theoretically proposed structure types, for comparison. Afterwards, the results were combined with earlier theoretical work involving the 5-5 structure in the earth alkaline metal oxides and the alkali metal halides. As a result, we suggest the GeSe and the ZnO systems as the most suitable ones for synthesizing the 5-5 structure type.

Photophysics of fulvene under the non-resonant stark effect. Shaping the conical intersection seam

We introduce a mechanistic strategy to control the excited state lifetime of fulvene based on shaping the topography of an extended seam of intersection with the non-resonant dynamic Stark effect. Fulvene has a very short excited state lifetime due to an energetically accessible seam of intersection which lies along the methylene torsion coordinate, and the initial decay occurs at the seam segment around the planar conical intersection structure. We have followed a three-step approach to simulate the control. First, we have calculated the effect of a non-resonant electric field on the potential energy surface at the ab initio level, including the field in a self-consistent way. The relative energy of the planar segment of the seam is increased by the non-resonant field. In the second step we simulate the control carrying out MCTDH quantum dynamics propagations under a static non-resonant field to derive the main control mechanisms. At moderately intense fields (epsilon < or = 0.03 a.u.) the decay is faster as compared to the field free case because the vibrational overlap between the excited and ground state vibrational functions is increased. However, at more intense fields (epsilon = 0.04 a.u.) the planar conical intersection is energetically inaccessible and the decay occurs at a slower time scale, at the segment of the seam with more twisted geometries. In the third step, the control over the dynamics is exerted with a non-resonant dynamic field. The acceleration of the decay due to the improved vibrational overlap does not occur, but the decay can be made slower with a dynamic field of 0.08 a.u. The results show the viability of our approach to control the photophysics shaping the topology of the conical intersection seam, and they prove that the extended nature of the seam is crucial for simulating and understanding the control.

The fundamental chemical equation of aromaticity

In the search for the chemical measure of molecular aromaticity, namely the energetic gain of a cyclic delocalized structure for just being cyclic, the numerous definitions of generic aromatic stabilisation energies proposed hitherto stand as approximates of the conceptual limit devised within the framework of spectral graph theory, i.e. the topological resonance energy (TRE). After a 36 year challenge, the TRE acyclic reference of any π-cyclic molecule, originally merely defined by an abstract matching polynomial, is now given a real chemical structure: the Möbius-twisted head-to-tail metathesis cyclo-dimer of the parent ring. The original treatment at the Hückel molecular orbital level of theory can now be extended to DFT or ab initio levels. The corresponding ring opening-closing-twisting chemical transformation provides the observable basis for the measure of aromaticity under either vertical or adiabatic conditions.

Binding of Noble Metal Clusters with Rare Gas Atoms: Theoretical Investigation

Binding of noble metal clusters (M(n), M = Cu, Ag, and Au; n = 2-4) with rare gas atoms (Rg = Kr, Xe, and Rn) has been investigated at the density functional (CAM-B3LYP) and ab initio (MP2) levels of theory. The calculation shows significant affinity of neutral metal clusters for interaction with rare gas atoms. The binding energies indicate that gold clusters have the highest and silver clusters have the lowest affinity for interaction with rare gas atoms, and for the same metal clusters, there is a continuous increase in E(b) from Kr to Rn. The M-Rg bonding mechanism have been interpreted by means of the quantum theory of atoms in molecules (QTAIM), natural bond orbital (NBO), and energy decomposition analysis (EDA). According to these theories, the M-Rg bonds are found to be partially electrostatic and partially covalent. EDA results identify that these bonds have less than 40% covalent character and more than 60% electrostatic, and also NBO calculations predict the amount of charge transfer from the lone pair of rare gas to σ* and n*orbitals of metal clusters.

The High Selectivity of the Cp2ZrHCl Reducing Agent for Imides: A Combined Experimental and Theoretical Study on γ‐Lactam and Isoxazolidinone Derivatives

AbstractSelective reductions of semicyclic imides either to hemiaminals (endo carbonyl reduction) or to aldehydes and amines (exo carbonyl reduction) in high yields by Schwartz's reagent (Cp2ZrHCl) are reported. Mechanistic aspects of these reactions have been investigated at the DFT ab initio level with consideration of implicit solvent effects and thermal and pressure corrections to 298 K/1 atm. The reactions proceed from the reagents to very stable final Zr‐oxo intermediates through the formation of σ complexes and subsequent four‐center transition state structures (1,2‐migratory insertion). The energetic barriers for the insertion steps depend strongly on the natures of the ancillary groups at the reducing carbonyl groups. N‐Carbamoyl groups are reduced to hemiaminals whereas N‐acyl groups react at the exo carbonyl groups. The absence of the second carbonyl group in the simple N‐methyl‐2‐pyrrolidone (less oxidized system) strongly reduces the thermodynamic drive for the formation of the metallocene oxo intermediate and allows an explanation of the remarkable chemoselectivity of the hydrozirconation. The zirconocene‐oxo intermediates hydrolyze quickly throughout an ionic mechanism in which water molecules strongly assist the process by stabilizing ion pair separation. The computational results are consistent with a large set of experimental data on reduction of γ‐lactam and isoxazolidinone derivatives under mild conditions. They provide a way to explain and predict the relative importance of the two competitive endo/exo reduction channels of semicyclic imides by considering the electronic and steric features of substituents.

Rate coefficients of the CF3CHFCF3 + H → CF3CFCF3 + H2 reaction at different temperatures calculated by transition state theory with ab initio and DFT reaction paths

The minimum energy path (MEP) of the reaction, CF(3)CHFCF(3) + H → transition state (TS) → CF(3)CFCF(3) + H(2), has been computed at different ab initio levels and with density functional theory (DFT) using different functionals. The computed B3LYP/6-31++G**, BH&HLYP/cc-pVDZ, BMK/6-31++G**, M05/6-31+G**, M05-2X/6-31+G**, UMP2/6-31++G**, PUMP2/6-31++G**//UMP2/6-31++G**, RCCSD(T)/aug-cc-pVDZ//UMP2/6-31++G**, RCCSD(T)/aug-cc-pVTZ(spd,sp)//UMP2//6-31++G**, RCCSD(T)/CBS//M05/6-31+G**, and RCCSD(T)/CBS//UMP2/6-31++G** MEPs, and associated gradients and Hessians, were used in reaction rate coefficient calculations based on the transition state theory (TST). Reaction rate coefficients were computed between 300 and 1500 K at various levels of TST, which include conventional TST, canonical variational TST (CVT) and improved CVT (ICVT), and with different tunneling corrections, namely, Wigner, zero-curvature, and small-curvature (SCT). The computed rate coefficients obtained at different ab initio, DFT and TST levels are compared with experimental values available in the 1000-1200 K temperature range. Based on the rate coefficients computed at the ICVT/SCT level, the highest TST level used in this study, the BH&HLYP functional performs best among all the functionals used, while the RCCSD(T)/CBS//MP2/6-31++G** level is the best among all the ab initio levels used. Comparing computed reaction rate coefficients obtained at different levels of theory shows that, the computed barrier height has the strongest effect on the computed reaction rate coefficients as expected. Variational effects on the computed rate coefficients are found to be negligibly small. Although tunneling effects are relatively small at high temperatures (~1500 K), SCT corrections are significant at low temperatures (~300 K), and both barrier heights and the magnitudes of the imaginary frequencies affect SCT corrections.

Vibrationally Resolved Absorption and Emission Spectra of Dithiophene in the Gas Phase and in Solution by First-Principle Quantum Mechanical Calculations.

The absorption and emission spectra of dithiophene have been computed in different environments (gas phase, apolar, and polar solvents) and at different temperatures, including Duschinsky, temperature and solvent effects at full ab initio level, and considering the anharmonicity of the double well potential associated with the inter-ring torsional mode. The computed spectra are in very good agreement with the experimental ones, allowing for a complete assignment of the main vibrational features. Five different density functionals (BLYP, B3LYP, CAM-B3LYP, BHLYP, and PBE0) have been tested, and CAM-B3LYP and PBE0 are the most accurate.

Effects of α-mono heteroatoms (N vs. P), and β-conjugation on cyclic silylenes

Following the quest for stable silylenes, we have found high thermodynamic and kinetic viabilities for a series of novel cyclic alkylaminosilylenes [CAASs: including 1N–4N)] over their corresponding cyclic alkylphosphinosilylenes (CAPSs: 1P–4P, respectively), at ab initio and DFT levels, coupled with appropriate isodesmic reactions. Among silylenes scrutinized, 4N immerged the most promising for its higher singlet–triplet energy difference (ΔES–T), wider band gap (ΔEHOMO–LUMO), higher heat of hydrogenation (ΔEH), significant resistance to dimerization and resistance to rearrangement to its full valence octet isomer, etc. Unsaturation destabilized 2N, 3N, and 3P through cross conjugation.

Vibrational spectral analysis, computation of thermodynamic functions for various temperatures and NBO analysis of 2,3,4,5-tetrachlorophenol using ab initio HF and DFT calculations

FT-IR and FT-Raman spectra of 2,3,4,5-tetrachlorophenol (TCP) have been recorded in the regions 4000–400cm−1 and 3500–100cm−1 respectively. The total energy calculations of TCP were tried for the possible conformers. The molecular structure, geometry optimization, vibrational frequencies were obtained by the ab initio and DFT levels of theory (B3LYP and B3PW91) with the standard basis sets, 6-311++G(d, p) and 6-311+G(d, p) for C1 and C2 conformers. The harmonic frequencies were calculated and the scaled values were compared with experimental FT-IR and FT-Raman spectra. The observed and the calculated frequencies are found to be in good agreement. Stability of the molecule arising from hyper conjugative interactions, charge delocalization has been analyzed using natural bond orbital (NBO) analysis. The thermodynamic functions (heat capacity, entropy, vibrational partition function and Gibbs energy) from spectroscopic data by statistical methods were obtained for the range of temperature 100–1000K. The polarizability, first hyperpolarizability, anisotropy polarizability invariant has been computed using quantum chemical calculations. The chemical parameters were calculated from the HOMO and LUMO values.

Prediction of Absolute Hydroxyl pKa Values for 3-Hydroxypyridin-4-ones.

pKa values have been calculated for a series of 3-hydroxypyridin-4-one (HPO) chelators in aqueous solution using coordination constrained ab initio molecular dynamics (AIMD) in combination with thermodynamic integration. This dynamics-based methodology in which the solvent is treated explicitly at the ab initio level has been compared with more commonly used simple, static, approaches. Comparison with experimental numbers has confirmed that the AIMD-based approach predicts the correct trend in the pKa values and produces the lowest average error (∼0.3 pKa units). The corresponding pKa predictions made via static quantum mechanical calculations overestimate the pKa values by 0.3-7 pKa units, with the extent of error dependent on the choice of thermodynamic cycle employed. The use of simple quantitative structure property relationship methods gives prediction errors of 0.3-1 pKa units, with some values overestimated and some underestimated. Beyond merely calculating pKa values, the AIMD simulations provide valuable additional insight into the atomistic details of the proton transfer mechanism and the solvation structure and dynamics at all stages of the reaction. For all HPOs studied, it is seen that proton transfer takes place along a chain of three H2O molecules, although direct hydrogen bonds are seen to form transiently. Analysis of the solvation structure before and after the proton transfer event using radial pair distribution functions and integrated number densities suggests that the trends in the pKa values correlate with the strength of the hydrogen bond and the average number of solvent molecules in the vicinity of the donor oxygen.

Theoretical investigation on structures, stability and properties of [P, X, Y] (X=C, Si; Y = O, S) isomers

The structures, energetics and stability of the [P, X, Y] (X = C, Si; Y = O, S) radicals are explored by means of the density functional theory and ab initio levels. Seventeen [P, X, Y] isomers and 14 interconversion transition states are obtained at the B3LYP/6-311G(d) level. At the CCSD(T)/6-311 + G(2df)//QCISD/6-311G(d) + ZPVE level, the lowest-lying isomers are the linear PCO 1a (0.0 kcal/mol), PCS 1b (0.0) and the three-membered ring cPSiO 1c (0.0), cPSiS 1d (0.0) on their respective potential energy surfaces. These four isomers exhibit considerably not only thermodynamic but also kinetic stabilities. Additionally, the cyclic cPCS 2b (32.8) and linear PSiS 2d (18.6) possess also high kinetic stability. All of six isomers 1a, 1b, 2b, 1c, 1d and 2d are considerably stabilized by a barrier of at least 20 kcal/mol, and may be detected in the laboratory or interstellar space. Their valence bond structures and possible formation strategies in the laboratory and space are discussed in detail. Finally, the similarities and discrepancies on structures and stabilities between [P, X, Y] (X = C, Si; Y = O, S) isomers are compared. These predicted results are highly expected to be informative for the future identification of [P, X, Y] (X = C, Si; Y = O, S) in the laboratory and space.

Natural Bond–Bond Polarizability: A Hückel-Like Electronic Delocalization Index

We show how the bond-bond polarizability index, as originally introduced by Coulson and Longuet-Higgins in the Hückel-theoretic context, can be generalized in the natural bond orbital (NBO) framework to ab initio molecular orbital and density functional theory levels. We demonstrate that such a "natural bond-bond polarizability" (NBBP) index provides a flexible and quantitative descriptor for a broad spectrum of delocalization effects ranging from strong π aromaticity to weak intra- and intermolecular hyperconjugative phenomena. Illustrative applications are presented for representative delocalization effects in saturated and unsaturated species, chemical reactions, and hydrogen-bonding interactions.