Theoretical Model Chemistry Research Articles

Delocalization-to-Localization Charge Transition in Diferrocenyl-Oligothienylene-Vinylene Molecular Wires as a Function of the Size by Raman Spectroscopy

In going from short to large size thienylene-vinylene diferrocenyl cations, the transition from a charge delocalized to a localized state is addressed by resonance Raman spectroscopy and supported by theoretical model chemistry. The shorter members, dimer and tetramer, display conjugated structures near the cyanine limit of bond length equalization as a result of the strong interferrocene charge resonance, producing a full charge delocalized mixed valence system. In the longest octamer, charge resonance vanishes and the cation is localized at the bridge center (the mixed valence property disappears). The hexamer is at the delocalized-to-localized turning point. Solvent and variable-temperature Raman measurements highlight this borderline property. A detailed structure-property correlation of bond length alternation data and Raman frequencies is proposed to account for the whole set of spectroscopic properties, with emphasis on the changes observed with the size of the molecular wire.

Solvent effect on keto–enol tautomerism in a new β-diketone: a comparison between experimental data and different theoretical approaches

The novel β-diketo compound (3-acetyl-4-oxopentanoic acid) OPAA is here synthesized and completely characterized in the solid state by means of X-ray crystallography and in solution by potentiometry and 1H and 13C NMR spectroscopy. In the solid state, OPAA exhibits the di-keto (DK) structure, however, in solution, we can observe a strong solvent dependent tautomeric equilibrium. Theoretical ab initio calculations employing DFT at the B3LYP/6-311G** level, and different methods of theoretical model chemistry (CBS-4M, G3MP2, CBS-QB3) are used to extensively investigate the tautomeric equilibrium and compare it with experimental data. Solvent effects are evaluated using a CPCM continuum solvation method; among all applied methods, CBS-4M is the one that better predicts experimental data and is able to qualitatively describe tautomeric equilibrium in solution, allowing thermodynamic calculations of pKa. Furthermore a supermolecular solvent approach is used to better analyze solvent–solute interactions in order to predict chemical properties.

High-Accuracy Theoretical Study on the Thermochemistry of Several Formaldehyde Derivatives

In the case of several formaldehyde derivatives, with importance in atmospheric and combustion chemistry, the currently available thermochemical values suffer from considerably large uncertainties. In this study a high-accuracy theoretical model chemistry has been used to provide accurate thermochemical data including heats of formation at 0 and 298 K and standard molar entropies at 298 K for CF(2)O, FCO, HFCO, HClCO, FClCO, HOCO, and NH(2)CO. For most of the thermochemical quantities studied here, this investigation delivers the best available estimate.

Multireference Model Chemistries for Thermochemical Kinetics.

By combining the generalized valence bond ansatz of correlated participating orbitals (CPO) with the complete-active-space prescription for selecting configurations and with the use of multireference second order perturbation theory (MRMP2) for including dynamical correlation, we define three levels of multireference (MR) theoretical model chemistries for electronic structure calculations of chemical reaction energies and barrier heights. The three levels differ in their choice of which orbitals are considered to be participating; the choices are called nominal (nom-CPO), moderate (mod-CPO), and extended (ext-CPO). Combining any of these three choices with a method for treatment of dynamical correlation energy and a one-electron basis set yields a theoretical model chemistry. Unlike the full-valence choice of active orbitals, the CPO choices lead to active spaces that contain the orbitals needed to include important static correlation effects on chemical reactions but do not increase with the size of the nonparticipating portion of the system, and hence they remain viable computational options even for many large and complex reacting systems. The accuracies of the new levels, combined with the MG3S basis set (a partially augmented, multiply polarized valence triple-ζ basis with appropriately tight d functions for 3p-block elements) and with the fully augmented correlation-consistent aug-cc-pVTZ basis set, are assessed against a previously presented database of barrier heights for diverse reaction types. We find that nom-CPO level captures the bulk of the static correlation energy, and MRMP2/nom-CPO calculations have an average error of only 1.4 kcal/mol in barrier heights, which may be compared to 5.0 kcal/mol for single-reference MP2 theory, 2.5 kcal/mol for CCSD, and 4.1 and 1.0 kcal/mol for the B3LYP and M06-2X density functionals, respectively. The accuracy of MRMP2/CPO for transition structure bond lengths and donor-acceptor distances is excellent, with a mean unsigned error of only 0.007 Å as compared to 0.018 Å for CCSD, 0.019 Å for M06-2X, and 0.039 Å for MP2 and B3LYP. We also introduce a new multireference diagnostic, called the M diagnostic, that allows one to measure the importance of static correlation in a given reagent or transition state.

High-accuracy extrapolated ab initio thermochemistry. III. Additional improvements and overview

Effects of increased basis-set size as well as a correlated treatment of the diagonal Born-Oppenheimer approximation are studied within the context of the high-accuracy extrapolated ab initio thermochemistry (HEAT) theoretical model chemistry. It is found that the addition of these ostensible improvements does little to increase the overall accuracy of HEAT for the determination of molecular atomization energies. Fortuitous cancellation of high-level effects is shown to give the overall HEAT strategy an accuracy that is, in fact, higher than most of its individual components. In addition, the issue of core-valence electron correlation separation is explored; it is found that approximate additive treatments of the two effects have limitations that are significant in the realm of <1 kJ mol(-1) theoretical thermochemistry.

Ab initio thermochemistry of some geochemically relevant molecules in the system Cr-O-H-Cl

A complete theoretical model chemistry algorithm (TMCA) for the prediction of thermodynamic properties of geochemically relevant gaseous and aqueous complexes, based on molecular quantum mechanics, is presented and discussed. Cr species are selected as a case study due to the high nuclear mass and the complex electronic structure of this transition metal. The various derived magnitudes are internally consistent and sufficiently accurate to warrant comparison with the existing (and often conflictual) experimental data and literature estimates. The TMCA is based on density functional theory (DFT) B3LYP/6-31G(d,p) gas phase computations followed by computation of solvation effects by the integral polarized continuum model approach at HF/STO-3G level. Energy corrections due to relativistic effects and electron-electron correlation are accounted for by a newly developed periodic function based on computed ionization potentials and electron affinity of the central metal. Electrostatic entropy contributions to the bulk solvation entropy are accounted for by a Born-model equation based on the electrostatic component of the Integral Equation Formalism—Polarized Continuum Model (IEFPCM) coupling work. As an ancillary result, the TMCA model confirms the validity of the absolute solvation energy terms of the aqueous proton. The TMCA model is of general validity and could be eventually adopted as a standard procedure in the ab initio assessment of gas-phase and aqueous-phase energetics of geochemically relevant species.

HEAT: High accuracy extrapolated ab initio thermochemistry

A theoretical model chemistry designed to achieve high accuracy for enthalpies of formation of atoms and small molecules is described. This approach is entirely independent of experimental data and contains no empirical scaling factors, and includes a treatment of electron correlation up to the full coupled-cluster singles, doubles, triples and quadruples approach. Energies are further augmented by anharmonic zero-point vibrational energies, a scalar relativistic correction, first-order spin-orbit coupling, and the diagonal Born-Oppenheimer correction. The accuracy of the approach is assessed by several means. Enthalpies of formation (at 0 K) calculated for a test suite of 31 atoms and molecules via direct calculation of the corresponding elemental formation reactions are within 1 kJ mol(-1) to experiment in all cases. Given the quite different bonding environments in the product and reactant sides of these reactions, the results strongly indicate that even greater accuracy may be expected in reactions that preserve (either exactly or approximately) the number and types of chemical bonds.

Ab initio study of the oxidation of CH 3 SH to CH 3 SSCH 3

The thermodynamic and kinetic aspects of the oxidation of methanethiol to dimethyl disulfide by oxygen were studied using ab-initio molecular techniques. The reaction was considered as a multi-step process. Geometrical parameters and molecular energies for critical points were determined and characterized at the MP2(Full)/6–31G* level. Afterwards, a study was carried out employing different methods of theoretical model chemistry (G2, G2(MP2), G3 and CBS-4 MB), for which the equilibrium geometry at MP2(Full)/6–31G* was assumed as starting point. The steps governing the rate of the whole reaction were determined and prove to be the same for any level of theory employed. Although the MP2(Full)/6–31G* level is able to afford a good qualitative description of the reaction pathway, the quantitative aspect is less satisfactory. Results from the methods of theoretical model chemistry, on the other hand, accord well with experimental data, if available, and are very useful for studying the thermodynamic and kinetic aspects of chemical reactions.

Chemistry without Coulomb tails

The theoretical model chemistry that results from a molecular Schrödinger equation in which the Coulomb terms are strongly attenuated is investigated for a range of molecules. Little deterioration is found and we propose that this represents a very natural approach to achieving linear cost scaling in quantum chemical calculations.

Transformation of the Hamiltonian in excitation energy calculations: Comparison between Fock-space multireference coupled-cluster and equation-of-motion coupled-cluster methods

A use of the transformed form of the Hamiltonian in excitation energy calculations is discussed and a comparison between methods using this form is performed. It is shown that use of a similarity-transformed Hamiltonian can lead to separation of the ground state and excited state eigenvalue problems. Then standard approaches for determination of excited state energies can be used. Since these energies are usually quasidegenerate, it is convenient to use the effective Hamiltonian scheme. For different approximate methods, various advantages and disadvantages referring to desirable features of a ‘‘theoretical model chemistry’’ are discussed.