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Lindqvist hexaniobate, [Nb

The substituent effect on the covalent character of C-H bonds in methane derivatives is evaluated by means of local descriptors based on the topology of the electron density. Halogens, -OH, -SH, =O, =S, -NO2, -NH2, and -OCH3 increase the covalent character of the remaining C-H bonds, while alkaline metals (-Li and -Na) result in the opposite trend. This study proposes that the inductive effect is due to polarization changes driven by substituent charges.

The performance of Hartree–Fock and twenty-three exchange–correlation functionals from Density Functional Theory was evaluated by means of experimental values of vertical excitation energies in metal complexes (tetraoxos, carbonyls, and halides) by using augmented basis sets of two different sizes, double- and triple- $$\zeta$$ . The analysis is carried out in terms of linear correlation parameters, that is, coefficients of determination and slope errors. In general, the results present only a small dependency with the basis set size. Range-separated hybrids (RSHs), mainly by LC-PBEPBE and LC-QTP, show the best performance for tetraoxo complexes. In addition, all methods provide poor results for carbonyls, where functionals from Local Density Approximation and Generalized Gradient Approximation (GGA) are better, followed by hybrids and RSHs. Moreover, hybrids, hybrid meta-GGAs (except for M06-HF), and mainly RSHs are more successful for halide complexes. Finally, considering all systems together, the functionals that provide the largest coefficients of determination (R2 $$\ge$$ 0.98) along with the smallest slope errors (Error( $$\alpha$$ )

The reactivity of gold has been investigated for a long time. Here, we performed an in-depth analysis of relativistic effects over the chemical kinetic properties of elementary reactions associated with methane activation by gold(I) cations, CH4 + Au+ ↔ AuCH2+ + H2. The global reaction is modeled as a two-step process, CH4 + Au+ ↔ HAuCH3+ ↔ AuCH2+ + H2. Moreover, the barrierless dissociation of the initial adduct between reactants, AuCH4+, is discussed as well. Higher-order relativistic treatments are used to provide corrections beyond the commonly considered scalar effects of relativistic effective core potentials (RECPs). Although the scalar relativistic contributions predominate, lowering the forward barrier heights by 48.4 and 36.1 kcal mol−1, the spin–orbit coupling effect can still provide additional reductions of these forward barrier heights by as much as 9% (1.0 and 2.2 kcal mol−1). The global reaction proceeds rapidly at low temperatures to the intermediate attained after the first hydrogen transfer, HAuCH3+. The relativistic corrections beyond the ones from RECPs are still able to double the rate constant of the CH4 + Au+ → HAuCH3+ process at 300 K, while the reverse reaction becomes five times slower. The formation of global products from this intermediate only becomes significant at much higher temperatures (∼1500 K upward). The scalar relativistic contributions decrease the dissociation energy of the initial adduct, AuCH4+, into the global products by 105.8 kcal mol−1, while the spin–orbit effect provides an extra lowering of 1.8 kcal mol−1.

Exchange-correlation (XC) functionals from Density Functional Theory (DFT) developed under the rigorous arguments of Correlated Orbital Theory (COT) address the Devil's Triangle of prominent errors in Kohn-Sham (KS) DFT. At the foundation of this triangle lie the incorrect one-particle spectrum, the lack of integer discontinuity, and the self-interaction error. At the top level, these failures manifest themselves in incorrect charge transfer and Rydberg excitation energies, along with poor activation barriers. Accordingly, the Quantum Theory Project (QTP) XC functionals have been created to address several of the long-term issues encountered in KS theory and its Time Dependent DFT (TDDFT) variant for electronic excitations. Recognizing that COT starts with a correct one-particle spectrum, a condition imposed on the QTP functionals by means of minimum parameterization, the question that arises is how does this affect the electronically excited states? Among up to 28 XC functionals considered, the QTP family provides one of the smallest mean absolute deviations for charge-transfer excitations while also showing excellent results for Rydberg states. However, there is some room for improvement in the case of excitation energies to valence states, which are systematically underestimated by all functionals investigated. An alternative path for better treatment of excitation energies, mainly for valence states, is offered by using orbital energies from QTP functionals, especially by CAM-QTP-02 and LC-QTP. In this case, the deviations from the reference data can be reduced approximately by half.

Polyoxoniobates are polyoxometalates with applications in catalysis, water splitting and organic reactions such as condensation of Knoevenagel and Cycloaddition of CO2. Actually, range-separated hybrids (RSHs) are being indicated for the description of electronic excitations within Time Dependent Density Functional Theory (TDDFT), mainly in systems presenting charge – transfer transitions. Here, this protocol is applied for studying ultraviolet – visible (UV/Vis) bands of polyoxoniobates in aqueous solution. Although the experimental data for comparison are scarce, the RSHs investigated were particularly successful, providing wavelength deviations usually smaller than 30 nm. Results indicate that wavelengths change slightly among all these compounds (22 – 35 nm), with the following sequence: [Nb10O28]6−

The rates of numerous activated reactions between neutral species increase at low temperatures through quantum mechanical tunneling of light hydrogen atoms. Although tunneling processes involving molecules or heavy atoms are well known in the condensed phase, analogous gas-phase processes have never been demonstrated experimentally. Here, we studied the activated CH + CO

Advanced relativistic quantum chemistry calculations were used for the first time to provide accurate determinations of fundamental molecular properties for two moscovium halides (McX, X = F and Cl). The recommended values presented here were obtained with the X2C-MMF-CCSD-T/RPF-4Z level of theory. Thus, we determined an equilibrium bond length (re) of 2.287 A, an equilibrium dipole moment (μe) of 6.59 D and a harmonic vibrational frequency (ωe) of 405 cm−1 for McF. On the other hand, the results for McCl show re, μe and ωe values equal to 2.728 A, 7.46 D and 244 cm−1, respectively. The same Mc+–X− polarity is predicted in molecules of both halides. The equilibrium dissociation energies attained in X2C-MMF-(FS)CCSD/RPF-4Z calculations are 4.04 and 3.45 eV for McF and McCl, respectively. Therefore, the Mc–F and Mc–Cl bonds are predicted to be slightly stronger than Bi–F and Bi–Cl ones, respectively. Finally, the huge values obtained for dipole moments in the molecules studied strongly suggest that moscovium halides are predominantly ionic compounds.

The forward and reverse H + CO ↔ HCO reactions are important for combustion chemistry and have been studied from a wide variety of theoretical and experimental techniques. However, most of the chemical kinetic investigations concerning these processes are focused on low pressures or fall-off regions. Hence, a high-level electronic structure treatment was employed here in order to provide accurate rate constant values for these reactions at the high-pressure limit along temperatures from 50 to 4000 K. In relative terms, the variational effects on rate constants are shown to be almost as important at high temperatures as quantum tunneling corrections at the lowest temperatures investigated. The activation energies fitted by using modified and traditional Arrhenius' equations for the forward rate constants from 298 to 2000 K are, respectively, equal to 2.64 and 3.89 kcal mol-1, while similar fittings provided, respectively, 1.96 and 3.22 kcal mol-1, considering now forward rate constants from a temperature range of 298-373 K. This last activation energy determination (3.22 kcal mol-1) is in better agreement with the commonly referenced experimental value of 2.0 ± 0.4 kcal mol-1, also obtained from traditional fittings in the range 298-373 K, than the value attained from a broader temperature range fitting (3.89 kcal mol-1). However, some additional care must be considered along these comparisons once the experimental reaction rate measurements have been done for the trimolecular H + CO + M → HCO + M reaction instead. Anyway, the usage of appropriate temperature ranges is fundamental for reliable activation energy comparisons, although the remaining deviation between theory and experiment is still large and is possibly caused by the different pressure regimes assessed in each case. Finally, we roughly estimated that the high-pressure limit for the HCO decomposition into H and CO can be achieved along temperatures ranging from ∼246 and ∼255 K downward, respectively, at pressures of 1.1 and 9.6 atm, although further experiments should be carried out in order to improve these estimates. On the other hand, pressures larger than 9.8 × 104 atm are required for the aforementioned dissociation reaction to attain the high-pressure limit at 700 K. Therefore, the rate constants determined here are probably applicable in combustion studies at lower temperatures.

The rates of numerous activated reactions between neutral species increase at low temperatures through quantum mechanical tunneling of light hydrogen atoms. Although tunneling processes involving molecules or heavy atoms are well known in the condensed phase, analogous gas-phase processes have never been demonstrated experimentally. Here, we studied the activated CH + CO2 -> HCO + CO reaction in a supersonic flow reactor, measuring rate constants that increase rapidly below 100 K. Mechanistically, tunneling is shown to occur by CH insertion into the C-O bond, with rate calculations accurately reproducing the experimental values. To exclude the possibility of H-atom tunneling, CD was used in additional experiments and calculations. Surprisingly, the equivalent CD + CO2 reaction accelerates at low temperature as zero point energy effects remove the barrier to product formation. In conclusion, heavy-particle tunneling effects might be responsible for the observed reactivity increase at lower temperatures for the CH + CO2 reaction, while the equivalent effect for the CD + CO2 reaction results instead from a submerged barrier with respect to reactants., Accepted for publication in the Journal of Physical Chemistry A. 30 pages including supplementary materials

Based on typical physical and chemical conditions expected in massive and dense hot cores during the protostar collapse, the formation of glycolaldehyde (CH2OHCHO) and methanol (CH3OH) was investigated from H2 and CO and formaldehyde (H2CO) as an intermediate. Thermochemical properties and rate constants were obtained for gas-phase reactions using high-level electronic structure methods and chemical kinetic calculations, and the concentrations of the molecules were evolved along time. The chemical equilibrium was reached in minutes at 1500 K, a time interval much shorter than that required time for a protostar formation process. The results indicate that the formaldehyde and methanol abundances are always larger than those for glycolaldehyde, for example, at 2000 K and [H2]0 equals to 1023 molecule cm−3, the abundances of H2CO, CH3OH, and CH2OHCHO relative to H2 are equal to 3 × 10−6, 5 × 10−6, and 1 × 10−12, while for [H2]0 equals to 1020 molecule cm−3 these abundances are 3 × 10−9, 5 × 10−12, and 2 × 10−21, respectively. Considering that our results can be applied to explain the proximity of methanol and formaldehyde maser emissions, from the whole set of results, the CH3OH abundance relative to H2CO ranges from 10−3 to 102.

The relativistic effects on energetic quantities of the HNgF → HF + Ng (Ng = Ar, Kr, Xe, and Rn) reactions were evaluated by means of advanced four-component calculations with high-level electron correlation treatment and adequate relativistic basis sets. The results indicate that the scalar relativistic corrections can provide classical barrier height increments of as much as 4.8% (1.9 kcal mol−1) for the reaction with the heaviest noble gas, radon, while the spin-orbit coupling is almost as important in this case, lowering the value by 3.8% (−1.5 kcal mol−1). Therefore, these two relevant relativistic corrections cancel almost completely. A similar picture is seen for the energy variation from the reactant (HRnF) to the products (HF + Rn), but the contribution signs are reversed now (−4.0 and 3.4 kcal mol−1). A partial cancelling is also noticed for the HXeF → HF + Xe reaction. Such pattern is surprising since scalar relativistic effects usually predominate by magnitude orders over the spin-orbit coupling. These conclusions are reflected in the changes observed in rate constant values. Finally, an alternative decomposition route catalyzed by HF is suggested for these HNgF compounds, which can shed some light on the controversies regarding their thermal stability.

Correction for ‘Successive protonation of Lindqvist hexaniobate, [Nb6O19]8−: electronic properties and structural distortions’ by Fernando Steffler et al., Phys. Chem. Chem. Phys., 2022, 24, 13083–13093, https://doi.org/10.1039/d2cp00607c.

The properties of carbon monoxide in its ground and two lowest lying excited states are investigated by conventional and time-dependent (TD) density functional theory (DFT). The dipole moment is decomposed into atomic polarisation (AP) and inter-atomic charge transfer (CT) contributions according to the quantum theory of atoms in molecules (QTAIM). Considerable AP and CT contributions cancel in the ground state (S0) resulting in its known negligible dipole moment. This balance is disturbed in the lowest triplet (T1) and singlet (S1) excited states, resulting in dipole moments of 1.57 D for T1 and 0.49 D for S1 . The AP decreases by 2.5 D on excitation to either state but the opposing CT is reduced by 0.8 D for T1 and by 1.8 D for S1, insufficient for cancellation. These excitations induce charge transfer from O to C which drives dipole changes accompanied by a weakening of the C–O bond, more pronounced in the S1 state, as trends in bond lengths, vibrational frequencies, and QTAIM properties sugges...

Two treatments of relativistic effects, namely effective core potentials (ECP) and all-electron scalar relativistic effects (DKH2), are used to obtain geometries and chemical reaction energies for a series of ruthenium complexes in B3LYP/def2-TZVP calculations. Specifically, the reaction energies of reduction (A-F), isomerization (G-I), and Cl- negative trans influence in relation to NH3 (J-L) are considered. The ECP and DKH2 approaches provided geometric parameters close to experimental data and the same ordering for energy changes of reactions A-L. From geometries optimized with ECP, the electronic energies are also determined by means of the same ECP and basis set combined with the computational methods: MP2, M06, BP86, and its derivatives, so as B2PLYP, LC-wPBE, and CCSD(T) (reference method). For reactions A-I, B2PLYP provides the best agreement with CCSD(T) results. Additionally, B3LYP gave the smallest error for the energies of reactions J-L. © 2017 Wiley Periodicals, Inc.

The nuclear electric quadrupole moment (NQM) of ruthenium was reevaluated by means of the molecular method. Therefore, four-component relativistic electronic structure calculations of the electric field gradient at the Ru nucleus in ruthenium monocarbide, which were done with the coupled cluster methodology and carefully augmented large basis sets, were combined with the respective nuclear quadrupole coupling constant from experiments. This provided a recommended NQM value of (425 ± 13) mbarn for 101Ru.

The Charge-Charge Flux-Dipole Flux (CCFDF) model in terms of multipoles from the quantum theory of atoms in molecules (QTAIM) was used to investigate the variations in infrared intensities of hydroxyl (OH) stretching modes during the dimerization of carboxylic acids. The hydrogen bond formation in these systems results into bathochromic shifts of vibrational frequencies for all the OH stretching modes along with huge infrared intensity increments for some of them. These bands become more intense on dimerization due mainly to changes in the cross-term contribution between charge and charge flux. In addition, interaction energies for the pair of atoms directly involved in individual hydrogen bonds (O…H) are linearly correlated to electron densities at their bond critical points (BCPs). Therefore, the hydrogen bonds between the carbonyl group (CO) of acetic acid and the hydroxyl group of halogenated monomers show the largest electron density values at their BCPs. The formation of these intermolecular interactions is also accompanied by ionic character enhancements of OH bonds and electron density decrements at their BCPs. We finally noticed that the hydrogen atom belonging to the hydroxyl group loses electronic charge, while the oxygen from the CO end becomes more negatively charged during dimerization. © 2019 Wiley Periodicals, Inc.

This investigation uses atomic properties derived from the quantum theory of atoms in molecules formalism to rationalize the infrared intensity of the stretching vibration that arises as a Lewis base (B) is protonated (B-H mode). Moreover, the interacting quantum atom (IQA) partition is employed to evaluate the energetics of protonation. All calculations are performed at the CCSD/cc-pVQZ level except by the IQA analysis, which is carried out by means of the B3LYP/cc-pVQZ//CCSD/cc-pVQZ treatment. First, an efficiency scale is established for Lewis bases in terms of the electronic charge transfer potential. Next, this study shows that the intensity of the B-H stretching depends mostly on the electronic charge amount transferred to the proton. Thus, intensity data provide empirical assessment of Lewis base charge transfer efficiency. Finally, the group separation observed during correlation of proton affinities and electronic charge transfer potential is explained by the interaction energy between fragments of the protonated system.

3,3′-Difluorodiazirine is a precursor of difluorocarbene radical (:CF 2 ) which is used in organic synthesis and photo affinity labelling. This molecule possesses no dipole moment in the ground electronic state ( S 0 ) but has a significant dipole moment (of magnitude ~0.97 D) in both its first (triplet, T 1 ) and second (singlet S 1 ) excited states. These equal dipole moments are shown to originate from widely differing atomic polarization and inter-atomic charge transfer terms (defined by the Quantum Theory of Atoms in Molecules (QTAIM)). The calculated vertical/adiabatic excitation energies for the T 1 and S 1 states are 2.81/2.63 and 3.99/3.78 eV, respectively. Geometries, vibrational frequencies, atomic charges and spin populations, and the localization–delocalization matrices (LDMs) (Matta, J. Comput. Chem. 35 (2014) 1165) of the excited states are compared with those of the ground state. All calculations have been conducted at the (U)QCISD/aug-cc-pVTZ level of theory.

This investigation combines theoretical results and experimental data to elucidate the main kinetic and mechanism for the ring opening metathesis polymerization of norbornene by means of the [RuCl2(PPh3)2(piperidine)] complex as catalyst initiator. Structural species were defined and a kinetic profile of the entire reaction is depicted. The determined mechanism showed an associative reaction between the [RuCl2(PPh3)2(piperidine)] complex initiator and ethyl diazoacetate as the first step, resulting in a metal-carbene species. This species loses one triphenylphosphine molecule (PPh3) and the global rate-determining step is associated with a removal of the second PPh3 molecule due to an electronic trans-synergism between piperidine and this PPh3 molecule. Entropic factors are crucial to the spontaneity observed for the dissociation of both PPh3 ligands. The step following the rate-determining one is associated with the binding of a norbornene molecule and a similar trans-synergism between this monomer and piperidine activates the catalysis. This electronic effect maintains the system in a very important conformation along the catalytic chain.

This work shows that vertical excitation energies (characterized as single-electron processes) can be expressed in terms of one-particle solutions from a self-consistent field problem built by means of correlated operators. There are two alternative ways of enforcing this proposal for i → a transitions in a system (M): (1) by using only eigenvalues obtained for the cationic species reached after the removal of an electron from orbital i (M+) or (2) by combining these quantities with the eigenvalue associated with orbital i from the neutral M system. We demonstrate that those eigenvalues derived from the equation-of-motion formalism in terms of the coupled cluster approach including single and double substitutions for ionization potentials and electron affinities show excellent performance in reproducing these electronic transition energies by either path, with mean absolute deviations (MADs) between 0.02 and 0.06 eV. Moreover, the Kohn-Sham Density Functional Theory (KS-DFT) methods from the Quantum Theory Project (QTP) family provide nice results in terms of the second approach (MADs from 0.21 to 0.47 eV). However, DFT is not as successful as long as one takes into account only the eigenvalues of M+, although the respective excitation energies from QTP functionals are still reasonable (MADs between 0.55 and 0.74 eV). Ultimately, these relations can be used as a new consistency condition to develop KS-DFT approximations to the correlated orbital theory.

We present a theoretical study, so that molecular data (geometrical parameters, vibrational frequencies, infrared intensities, electronic energies, enthalpies, and Gibbs energies) of four formamidic acid (FA) isomers (labeled here as FA1, FA2, FA3, and FA4) and formamide (HCONH2) are obtained from CCSD/cc-pVTZ, CCSD/aug-cc-pVTZ, CCSD/cc-pVQZ, and CCSD(T)/cc-pVTZ calculations. Furthermore, on the basis of insufficient or even lacking theoretical and experimental results in the literature, we employed the aforementioned theory levels to determine benchmark values of dipole moments and rotational constants for these four FA isomers in order to contribute for their detection in astrophysical environments. Besides, we provide for the first time data about forward and reverse rate constants (200–4000 K) and Arrhenius’ parameters for each interconversion reaction between pairs of FA isomers as well as for the tautomeric process involving FA4 and formamide, which were calculated from a Complete Basis Set (CBS) extrapolation equation obtained at CCSD/cc-pVTZ optimized geometries. Our kinetic analysis indicated a faster interconversion between the FA structures in comparison with the FA4 ↔ HCONH2 process, suggesting that these isomers could co-exist in astrophysical media. Finally, we estimated that these isomers may be detected with relative abundances, [FAx]/[HCONH2] (x = 1, 2, 3, and 4), between ∼0.01 and ∼0.1% in astrophysical sources at chemical equilibrium conditions and temperatures around 1000 K. However, these ratios can become as high as ∼1, ∼3, and ∼5%, respectively, in hotter regions with temperatures around 2000, 3000, and 4000 K (expected, for example, in massive star-forming regions).

In this study, intramolecular electronic charge fluxes in chlorofluoromethanes, which take place during atomic stretching displacements, were analyzed in terms of bond character descriptors. Topological parameters of the electron density at the critical points of C–X bonds (X = H, F or Cl) and charge fluxes were obtained through the Quantum Theory of Atoms in Molecules (QTAIM). All calculations were done at the CCSD/cc-pVQZ level of theory. Thus, we have noticed that the covalent character of C–H, C–Cl and C–F bonds seems to increase as hydrogen atoms are replaced by halogens in other bonding sites of those molecules. This is attributed to polarizations induced over the carbon atom by the negatively charged substituents. It was also observed that the charge fluxes at the atoms of enlarged C–H bonds are linearly related to these bond character parameters. A similar pattern of charge fluxes is likewise seen for C–Cl and C–F bonds. However, the displacement of halogen atoms results in a larger delocalization of the charge fluxes (long range electronic charge transfers). This probably leads to a further observed separation into subgroups of molecules with appropriate linear relationships between halogen charge fluxes and bond character descriptors.

The f-block elements are addressed in this third part of a series of prolapse-free basis sets of quadruple-ζ quality (RPF-4Z). Relativistic adapted Gaussian basis sets (RAGBSs) are used as primitive sets of functions while correlating/polarization (C/P) functions are chosen by analyzing energy lowerings upon basis set increments in Dirac-Coulomb multireference configuration interaction calculations with single and double excitations of the valence spinors. These function exponents are obtained by applying the RAGBS parameters in a polynomial expression. Moreover, through the choice of C/P characteristic exponents from functions of lower angular momentum spaces, a reduction in the computational demand is attained in relativistic calculations based on the kinetic balance condition. The present study thus complements the RPF-4Z sets for the whole periodic table (Z ≤ 118). The sets are available as Supporting Informationand can also be found at http://basis-sets.iqsc.usp.br.

Nonnuclear attractors (NNAs) are observed in the electron density of a variety of systems, but the factors governing their appearance and their contribution to the system's properties remain a mystery. The NNA occurring in homo- and heteronuclear diatomics of main group elements with atomic numbers up to Z = 38 is investigated computationally (at the UCCSD/cc-pVQZ level of theory) by varying internuclear separations. This was done to determine the NNA occurrence window along with the evolution of the respective pseudoatomic basin properties. Two distinct categories of NNAs were detected in the data analyzed by means of catastrophe theory. Type "a" implies electronic charge transfer between atoms mediated by a pseudoatom. Type "b" shows an initial relocation of some electronic charge to a pseudoatom, which posteriorly returns to the same atom that donated this charge in the first place. A small difference of polarizability between the atoms that compose these heteronuclear diatomics seems to favor NNA formation. We also show that the NNA arising tends to result in some perceptible effects on molecular dipole and/or quadrupole moment curves against internuclear distance. Finally, successive cationic ionization results in the fast disappearance of the NNA in Li2 indicating that its formation is mainly governed by the field generated by the quantum mechanical electronic density and only depends parametrically on the bare nuclear field/potential at a given molecular geometry.

The difluorocarbene radical (:CF2), used in organic synthesis and in photoaffinity labeling, can be generated by the pyrolytic or photolytic decomposition of 3,3-difluorodiazirine (CF2N2, DFD). DFD possesses no dipole moment in the ground electronic state S 0 but has an experimental dipole of 1.5 ± 0.2 debye (D) in its first singlet excited state S 1. These observations have been ascribed to the shift in electron population between orbitals (Frenking et al. in J Comp Chem 28:117–126, 2007). An alternative real-space explanation is presented, which shows that the vanishing dipole moment in S 0 results from a balance between a charge transfer contribution due to the flow of charge between atoms and an atomic polarization term due to the non-sphericity of atoms in molecules. This balance is altered in S 1. This orbital-free description is shown to be consistent with an incipient dissociation of DFD to :CF2 and N2 upon excitation. The Laplacian of the electron density and the molecular electrostatic potential exhibit significant reorganization on excitation, mirroring one another, with consequential changes in chemical reactivity. Conforming to Hund’s rule, the lowest excited state is a triplet state (T 1), and the next level, the one examined in this work, is the first singlet excited state (S 1) with vertical excitation energies of 2.81 and 3.99 eV, respectively. The calculated dipole moment magnitudes (in D) are 0.05 (S 0), 0.973 (T 1), and 0.969 (S 1) all pointing their negative end toward the nitrogens. The maximal average lifetime of S 1 (in absence of non-radiative de-excitation) is ca. 30 ps, sufficient for its slowest vibrational normal mode to complete 400 oscillations. From a comparison of Hartree–Fock, MP2, QCISD, CCSD, and TD-DFT/B3LYP calculations with experiment (all using an aug-cc-pVTZ basis set), for both the ground and excited states of DFD, the method of choice appears to be QCISD, the one used in this work.

This study provides a new determination of the nuclear electric quadrupole moment (NQM) for 131 Xe, which is achieved by the molecular method. Dirac-Coulomb Coupled Cluster calculations with a Gaunt correction (DC+G-CC) of electric field gradients (EFGs) and experimental nuclear quadrupole coupling constants of six molecular systems (XeH + , XeCuF, XeCuCl, XeAgF, XeAgCl and XeAuF) were considered. The best NQM obtained by our DC+G-CCSD-T EFGs was −114.6(1.1) mbarn, which is recommended as the new reference value for this nuclide given the high level electron structure calculations done here.

The molecular method is used to obtain nuclear electric quadrupole moment (NQM) values for hafnium through electric field gradients (EFGs) at this nucleus in HfO and HfS. Dirac–Coulomb calculations with the Coupled Cluster approach, DC-CCSD (T) and DC-CCSD-T, were carried out to achieve the most accurate estimates of these EFGs. Higher order corrections are also added. Hence, the most reliable values for 177 Hf and 179 Hf determined here are 3319(33) and 3750(37) mbarn, respectively, in nice accordance with the best currently accepted NQMs for this element.

Infrared fundamental vibrational intensities and quantum theory atoms in molecules (QTAIM) charge-charge flux-dipole flux (CCFDF) contributions to the polar tensors of the fluorochloromethanes have been calculated at the QCISD/cc-pVTZ level. A root-mean-square error of 20.0 km mol(-1) has been found compared to an experimental error estimate of 14.4 and 21.1 km mol(-1) for MP2/6-311++G(3d,3p) results. The errors in the QCISD polar tensor elements and mean dipole moment derivatives are 0.059 e when compared with the experimental values. Both theoretical levels provide results showing that the dynamical charge and dipole fluxes provide significant contributions to the mean dipole moment derivatives and tend to be of opposite signs canceling one another. Although the experimental mean dipole moment derivative values suggest that all the fluorochloromethane molecules have electronic structures consistent with a simple electronegativity model with transferable atomic charges for their terminal atoms, the QTAIM/CCFDF models confirm this only for the fluoromethanes. Whereas the fluorine atom does not suffer a saturation effect in its capacity to drain electronic charge from carbon atoms that are attached to other fluorine and chlorine atoms, the zero flux electronic charge of the chlorine atom depends on the number and kind of the other substituent atoms. Both the QTAIM carbon charges (r = 0.990) and mean dipole moment derivatives (r = 0.996) are found to obey Siegbahn's potential model for carbon 1s electron ionization energies at the QCISD/cc-pVTZ level. The latter is a consequence of the carbon mean derivatives obeying the electronegativity model and not necessarily to their similarities with atomic charges. Atomic dipole contributions to the neighboring atom electrostatic potentials of the fluorochloromethanes are found to be of comparable size to the atomic charge contributions and increase the accuracy of Siegbahn's model for the QTAIM charge model results. Substitution effects of the hydrogen, fluorine, and chlorine atoms on the charge and dipole flux QTAIM contributions are found to be additive for the mean dipole derivatives of the fluorochloromethanes.

The generator coordinate method was implemented in the unrestricted Hartree-Fock formalism. Weight functions were built from Gaussian generator functions for 1s, 2s, and 2p orbitals of carbon and oxygen atoms. These weight functions show a similar behavior to those found in the generator coordinate restricted Hartree-Fock method, i.e., they are smooth, continuous, and tend to zero in the limits of integration. Moreover, the weight functions obtained are different for spin-up and spin-down electrons what is a result from spin polarization. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012

We employed the Density Functional Theory along with small basis sets, B3LYP/LANL2DZ, for the study of FeTIM complexes with different pairs of axial ligands (CO, H 2 O, NH 3 , imidazole and CH 3 CN). These calculations did not result in relevant changes of molecular quantities as bond lengths, vibrational frequencies and electronic populations supporting any significant back-donation to the carbonyl or acetonitrile axial ligands. Moreover, a back-donation mechanism to the macrocycle cannot be used to explain the observed changes in molecular properties along these complexes with CO or CH 3 CN. This work also indicates that complexes with CO show smaller binding energies and are less stable than complexes with CH 3 CN. Further, the electronic band with the largest intensity in the visible region (or close to this region) is associated to the transition from an occupied 3 d orbital on iron to an empty π ∗ orbital located at the macrocycle. The energy of this Metal-to-Ligand Charge Transfer (MLCT) transition shows a linear relation to the total charge of the macrocycle in these complexes as given by Mulliken or Natural Population Analysis (NPA) formalisms. Finally, the macrocycle total charge seems to be influenced by the field induced by the axial ligands.

Some of the exact conditions provided by the correlated orbital theory are employed to propose new non-empirical parameterizations for exchange-correlation functionals from Density Functional Theory (DFT). This reparameterization process is based on range-separated functionals with 100% exact exchange for long-range interelectronic interactions. The functionals developed here, CAM-QTP-02 and LC-QTP, show mitigated self-interaction error, correctly predict vertical ionization potentials as the negative of eigenvalues for occupied orbitals, and provide nice excitation energies, even for challenging charge-transfer excited states. Moreover, some improvements are observed for reaction barrier heights with respect to the other functionals belonging to the quantum theory project (QTP) family. Finally, the most important achievement of these new functionals is an excellent description of vertical electron affinities (EAs) of atoms and molecules as the negative of appropriate virtual orbital eigenvalues. In this case, the mean absolute deviations for EAs in molecules are smaller than 0.10 eV, showing that physical interpretation can indeed be ascribed to some unoccupied orbitals from DFT.

We investigated noble gas-copper bonds in linear complexes represented by the NgCuX general formula in which Ng and X stand for a noble gas (neon, argon, krypton, or xenon) and a halogen (fluorine, chlorine or bromine), respectively, by coupled cluster methods and modified cc-pVQZ basis sets. The quantum theory of atoms in molecules (QTAIM) shows a linear relation between the dissociation energy of noble gas-copper bonds and the amount of electronic charge transferred mainly from the noble gas to copper during complexation. Large changes in the QTAIM quadrupole moments of copper and noble gases resulting from this bonding and a comparison between NgCuX and NgNaCl systems indicate that these noble gas-copper bonds should be better interpreted as predominantly covalent. Finally, QTAIM atomic dipoles of noble gases in NgNaCl systems agree satisfactorily with atomic dipoles given by a simple model for these NgNa van der Waals bonds.

Four atomic charge formalisms are compared using highly ionic diatomic molecules, such as LiF, NaF, KF, LiCl, NaCl, KCl, BF, AlF, GaF, BeO, and MgO. All calculations were done at the QCISD/6-311G(2df) level. The only formalism consistent with the characteristics of all these systems is Quantum theory of atoms in molecules (QTAIM). Absolute Mulliken charge values are small. ChelpG charges are not reliable for systems in which the atoms are largely anisotropic. Generalized atomic polar tensor values are contaminated with charge fluxes and atomic dipole fluxes and fail when these contributions are important and do not cancel each other. Finally, the charge–charge flux–dipole flux model was applied to dipole moment derivatives with QTAIM. This analysis shows that charge flux and atomic dipole flux contributions during bond stretching are almost null, except for oxides. There are also evidences that the lone electron pair at Group 13 elements in fluorides becomes less localized as the bond is stretched. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

A relativistic four-component study was performed for the XeF2 molecule by using the Dirac–Coulomb (DC) Hamiltonian and the relativistic adapted Gaussian basis sets (RAGBSs). The comparison of bond lengths obtained showed that relativistic effects on this property are small (increase of only 0.01 A) while the contribution of electron correlation, obtained at CCSD(T) or CCSD-T levels, is more important (increase of 0.05 A). Electron correlation is also dominant over relativistic effects for dissociation energies. Moreover, the correlation–relativity interaction is shown to be negligible for these properties. The electron affinity, the first ionization potential and the double ionization potential are obtained by means of the Fock-space coupled cluster (FSCC) method, resulting in DC-CCSD-T values of 0.3 eV, 12.5 eV and 32.3 eV, respectively. Vibrational frequencies and some anharmonicity constants were also calculated under the four-component formalism by means of standard perturbation equations. All these molecular properties are, in general, in satisfactory agreement with available experimental results. Finally, a partition in terms of charge–charge flux–dipole flux (CCFDF) contributions derived by means of the quantum theory of atoms in molecules (QTAIM) in non-relativistic QCISD(FC)/3-21G∗ calculations was carried out for XeF2 and KrF2. This analysis showed that the most remarkable difference between both molecules lies on the charge flux contribution to the asymmetric stretching mode, which is negligible in KrF2 but important in XeF2.

Atomic multipoles from the Quantum Theory of Atoms in Molecules (QTAIM), up to quadrupoles, and CHELPG charges are employed in the description of electrostatic properties of some small linear molecules: H2, HF, HCl, HBr, HCN, HNC, and CO. A proton is placed on the molecular axis in distances within the range 3–8 A from the terminal atoms. The polarization of molecular electronic densities induced by the proton is readily included by the calculation of atomic multipoles in each proton-molecule arrangement. Electrostatic potentials at the proton position as given by QTAIM multipoles are always in better agreement with the reference results, calculated directly at the B3LYP/6-311G(3d,3p) level, than those predicted with CHELPG charges, particularly for cases in which QTAIM multipoles showed that the contribution of atomic charges to these potentials is not clearly predominant over other contributions as in H2, HCl, HBr, HNC, and CO. The lone pair at the carbon atoms in CO and HNC, as described by QTAIM atomic dipoles, is indispensable in studies of electrostatic properties of these molecules. Moreover, QTAIM multipoles are also able to describe properly the large polarizations induced by the proton along the proton-molecule distances studied. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

The electric nuclear quadrupole moment (NQM) of lutetium was determined by means of the molecular method. This method combines accurate calculations of electric field gradients (EFGs) at the nucleus with experimental values of nuclear quadrupole coupling constants (NQCCs). Relativistic Dirac–Coulomb coupled cluster calculations were performed to obtain the EFGs at lutetium in LuF and LuCl. These values were supplemented by a Gaunt interaction correction and a correction for the effect of truncating the active space on the electron correlation energy. Our results indicate that the NQMs of 175 Lu and 176 Lu are given by 3415(34) and 4818(48) mbarn, respectively.

The potential energy curve for the ground-state of radium dimer (Ra2) is provided by means of atomic and molecular relativistic coupled cluster calculations. The short-range part of this curve is defined by an equilibrium bond length of 5.324 Å, a dissociation energy of 897 cm-1, and a harmonic vibrational frequency of 20.5 cm-1. The asymptotic behavior at large interatomic distances is characterized by the van der Waals coefficients C6 = 5.090 × 103, C8 = 6.978 × 105, and C10 = 8.786 × 107 atomic units. The two regions are matched in an analytical potential to provide a convenient representation for use in further calculations, for instance, to model cold collisions between radium atoms. This might become relevant in future experiments on ultracold, optically trapped, radioactive radium atoms that are used to search for a permanent electric dipole moment.