Generalized Spin Orbital Density Functional Study of Multicenter Metal Systems

Electronic structure calculations at the correlated molecular orbital theory and density functional theory levels have been used to generate a reliable set of clustering energies for up to three water molecules in carbon dioxide clusters up to n = 12. The structures and energetics are dominated by Lewis acid-base interactions with hydrogen-bonding interactions playing a lesser energetic role. The actual binding energies are somewhat larger than might be expected. The correlated molecular orbital MP2 method and density functional theory with the ωB97X exchange-correlation functional provide good results for the energetics of the clusters, but the B3LYP and ωB97X-D functionals do not. Seven CO(2) molecules form the first solvent shell about a single H(2)O with four CO(2) molecules interacting with the H(2)O via Lewis acid-base interactions, two CO(2) interacting with the H(2)O by hydrogen bonds, and the seventh CO(2) completing the shell. The Lewis acid-base and weak hydrogen bond interactions between the water molecules and the CO(2) molecules are strong enough to disrupt the trimer ring configuration for as few as seven CO(2) molecules. Calculated (13)C NMR chemical shifts for mCO(2)·(H(2)O)(n) show little change with respect to the number of H(2)O or CO(2) molecules in the cluster. The O-H stretching frequencies do exhibit shifts that can provide information about the interactions between water and CO(2) molecules.

Geometries and energies of various metal aqua ions have been computed with Hartree-Fock (HF), single-reference second-order perturbation theory according to Møller and Plesset (MP2), complete active space self-consistent field (CAS-SCF) methods, multi-configurational quasi-degenerate second-order perturbation theory (MCQDPT2), and density functional theory (DFT), whereby for the latter the most widely used functionals BLYP, B3LYP, LDA ("local density approximation"), and another functional (SOP) exhibiting Slater exchange, have been investigated. The geometries as well as the energies are sensitive to the computational method, results of which can lead to different conclusions. The geometries of certain reactants, products, intermediates, or transition states cannot be obtained with all of the above-mentioned computational methods: in some cases, the structures are very poor, exhibiting large errors in the bond parameters, and in others, the targeted species cannot be calculated at all. The different computational techniques do not always predict the same coordination number of a given aqua ion or the same preferred water exchange mechanism. Inaccuracies arising from inadequate basis sets are also discussed.

The literature of quantum chemistry, covering both molecular orbital theory and density functional theory, is cluttered with abbreviations, acronyms, and jargon. Some of the more common terminology is explained in this glossary.

This review discusses methods for the incorporation of quantum mechanical effects into enzyme kinetics simulations in which the enzyme is an explicit part of the model. We emphasize three aspects: (a) use of quantum mechanical electronic structure methods such as molecular orbital theory and density functional theory, usually in conjunction with molecular mechanics; (b) treating vibrational motions quantum mechanically, either in an instantaneous harmonic approximation, or by path integrals, or by a three-dimensional wave function coupled to classical nuclear motion; (c) incorporation of multidimensional tunneling approximations into reaction rate calculations.

While CCSD(T) with spin-restricted Hartree-Fock (RHF) orbitals has long been lauded for its ability to accurately describe closed-shell interactions, the performance of CCSD(T) on open-shell species is much more erratic, especially when using a spin-unrestricted HF (UHF) reference. Previous studies have shown improved treatment of open-shell systems when a non-HF set of molecular orbitals, like Brueckner or Kohn-Sham density functional theory (DFT) orbitals, is used as a reference. Inspired by the success of regularized orbital-optimized second-order Møller-Plesset perturbation theory (κ-OOMP2) orbitals as reference orbitals for MP3, we investigate the use of κ-OOMP2 orbitals and various DFT orbitals as reference orbitals for CCSD(T) calculations of the corrected ground-state harmonic vibrational frequencies of a set of 36 closed-shell (29 neutrals, 6 cations, 1 anion) and 59 open-shell diatomic species (38 neutrals, 15 cations, 6 anions). The aug-cc-pwCVTZ basis set is used for all calculations. The use of κ-OOMP2 orbitals in this context alleviates difficult cases observed for both UHF orbitals and OOMP2 orbitals. Removing two multireference systems and 12 systems with ambiguous experimental data leaves a pruned data set. Overall performance on the pruned data set highlights CCSD(T) with a B97 orbital reference (CCSD(T):B97), CCSD(T) with a κ-OOMP2 orbital reference (CCSD(T):κ-OOMP2), and CCSD(T) with a B97M-rV orbital reference (CCSD(T):B97M-rV) with RMSDs of 8.48 cm-1, and 8.50 cm-1, and 8.75 cm-1 respectively, outperforming CCSD(T):UHF by nearly a factor of 5. Moreover, the performance on the closed- and open-shell subsets shows these methods are able to treat open-shell and closed-shell systems with comparable accuracy and robustness. CCSD(T) with RHF orbitals is seen to improve upon UHF for the closed-shell species, while spatial symmetry breaking in a number of restricted open-shell HF (ROHF) references leads CCSD(T) with ROHF reference orbitals to exhibit the poorest statistical performance of all methods surveyed for open-shell species. The use of κ-OOMP2 orbitals has also proven useful in diagnosing multireference character that can hinder the reliability of CCSD(T).

Electrochemical properties of Li[C nF 2 n+1 BF 3] as electrolyte salts for lithium-ion cells

RETRACTED: Molecular parameterization and determination of the electrode potentials of anticoagulant derivatives: Electrochemical and quantum chemical study

The formation mechanism, geometric structures, and electronic properties of a metal-substituted fullerene C58Fe2 have been studied using frontier orbital theory (FOT) and density functional theory (DFT). FOT predicts that two Fe atoms prefer to substitute the two carbons of a [6,6] double bond of C60 yielding a structure denoted as C58Fe2-3, which is different from the two equivalent substitution sites, i.e., the sites on the opposite of C60 cage or in the nearest neighboring sites of a pentagonal ring for C58X2 (X=N and B), and also different from the cross sites of a hexagonal ring for C58Si2. Five possible structures of C58Fe2 are optimized using DFT to see whether FOT works. The DFT calculations support the prediction of FOT. The Mulliken charge of Fe atom in C58Fe2-3 shows that the two Fe atoms of C58Fe2-3 lose 0.70 electron to the carbons of the cage, and the net spin populations of Fe atom indicate that each Fe atom has 1.11 μB magnetic moments, while each of the four nearest neighboring carbons has $-0.064~\mu_B$ magnetic moments. Thus, the two Fe atoms have ferromagnetic interaction with each other, and have weak antiferromagnetic interaction with their four nearest neighboring carbons, leaving 2.0 μB magnetic moments for the molecule.

We report calculated 2H nuclear quadrupole coupling constants and asymmetry parameter in deuterated crystalline oxalic acid dihydrated polymorphs within a full-potential linear combination of Gaussian-type orbitals employing both Density Functional Theory and Hartree-Fock approximation. The Becke's hybrid exchange B3 and the Lee, Yang and Parr correlation functionals were employed. Their dependence with basis set, interelectronic correlation, structural changes and crystal field environment are reported.

The photo-, thermo- and solvatochromic properties of 2,3-dihydro-1',3',3'-trimethyl-6-nitrospiro-[1-benzopyran-2,2'-1H-indole] (BSP-NO(2)) were studied in ILs containing the anion [NTf(2)](-) by UV-Vis absorption spectroscopy, ab initio molecular orbital theory and density functional theory (DFT) calculations. It was found that the kinetics and thermodynamics of the BSP-NO(2) <--> MC (merocyanine) equilibrium was sensitive to the nature of the cation. It was also observed that the imidazolium cation can form a through-space orbital interaction with the MC isomer, rather than a simple electrostatic interaction, thus preventing the MC conversion back to the BSP-NO(2) isomer. The BSP-NO(2) <--> MC equilibrium thus serves as a model system for studying modes of interaction of the cations in ionic liquids.

The potential energy surfaces for the insertion reactions of germylene into XHn molecules have been characterized in detail using ab initio molecular orbital theory and density functional theory. The model system Ge(CH3)2 + XHn (X = C, N, O, F, Si, P, S, and Cl; n = 1−4) has been chosen for the present study. All the interactions involve the initial formation of a donor−acceptor complex, followed by a high-energy transition state, and then an insertion product. The agreement between MP2 and B3LYP results indicates that the latter provides an adequate theoretical level for further investigations of molecular geometries, electronic structures, and kinetic features of the germylene reactions. The following conclusions emerge from this work: (i) the X−H insertion reactions of germylene occur in a concerted manner via a three-membered-ring transition state, and that the stereochemistry at the heteroatom X center is preserved; (ii) the stabilization energies of the germylene−XHn complexes increase in the order NH3 > H2O > PH3 > H2S ∼ HF > HCl ≫ SiH4 ∼ CH4; (iii) the order of reactivity for X−H bonds toward germylene insertion is Cl > F > S > O > P > N ≫ Si > C. In other words, the greater the atomic number of heteroatom (X) in a given row, the easier the insertion reaction of XHn hydrides and the larger the exothermicity. Moreover, the present study demonstrates that both electronic and steric effects play a major role in the course of insertion reactions of germylene into X−H bonds. This work also indicates that the chemical behavior of germylene should be more similar to that of silylene than to that of carbene species.

Hydrogenation of aluminium hexamer: Ab initio molecular orbital theory and density functional theory study

Ab initio molecular orbital theory and density functional theory have been applied to study the isoelectronic-liganded NiL2 (L = N42-, C4H42-, and C2O22-) sandwich complexes at the MP2/6-31G*, B3LYP/6-31G*, B3LYP/6-311+G*, and BHLYP/6-311+G(3df,3pd) levels of theory. The stable structure for Ni(N4)2 is a staggered conformer with D4d symmetry. The dissociation barriers for one N2 elimination and two N2 eliminations for Ni(N4)2 are 37.1 and 84.9 kcal/mol, respectively, at the B3LYP/6-31G* level of theory, which suggest that Ni(N4)2 is kinetically stable enough to resist dissociation. The calculated reaction energies for the dissociation of Ni(N4)2, Ni(C4H4)2, and Ni(C2O2)2 at the B3LYP/6-31G* level of theory suggest that both Ni(N4)2 and Ni(C2O2)2 are high-energy species; however, Ni(C4H4)2 is not.

Hydrogen tunneling plays a critical role in many biologically and chemically important processes. The nuclear-electronic orbital multistate density functional theory (NEO-MSDFT) method was developed to describe hydrogen transfer systems. In this approach, the transferring proton is treated quantum mechanically on the same level as the electrons within multicomponent DFT, and a nonorthogonal configuration interaction scheme is used to produce delocalized vibronic states from localized vibronic states. The NEO-MSDFT method has been shown to provide accurate hydrogen tunneling splittings for fixed molecular systems. Herein, the NEO-MSDFT analytical gradients for both ground and excited vibronic states are derived and implemented. The analytical gradients and semi-numerical Hessians are used to optimize and characterize equilibrium and transition state geometries and to generate minimum energy paths (MEPs), for proton transfer in the deprotonated acetylene dimer and malonaldehyde. The barriers along the resulting MEPs are lower when the transferring proton is quantized because the NEO-MSDFT method inherently includes the zero-point energy of the transferring proton. Analysis of the proton densities along the MEPs illustrates that the proton density can exhibit symmetric or asymmetric bilobal character associated with symmetric or slightly asymmetric double-well potential energy surfaces and hydrogen tunneling. Analysis of the contributions to the intrinsic reaction coordinate reveals that changes in the C-O bond lengths drive proton transfer in malonaldehyde. This work provides the foundation for future reaction path studies and direct nonadiabatic dynamics simulations of a wide range of hydrogen transfer reactions.

This paper looks at the possibility that the peptide bond may be more common than originally thought, leading to molecules of prebiotic interest containing heavier atoms of the second row of the periodic table. Ab initio Moller–Plesset (MP2) coupled-cluster molecular orbital methods and density functional theory have been used. A first investigation of the six-atom system [C,3H,O,N] showed that formamide, NH 2 [bond]CH[double bond]O, is the most stable system that can be formed. Systematic studies on this same system in which C, O and N were respectively replaced by Si, S and P were then carried out. It has been found that the peptide-like linkage is the most stable for [C,3H,S,N] and [Si,3H,O,N] where NH 2 [bond]CH[double bond]S and NH 2 [bond]SiH[double bond]O are about 10–14 kcal mol −1 more favourable than the corresponding enol tautomers and well below other isomers on the energy scale. For [C,3H,O,P], the most stable species is CH 3 [bond]P[double bond]O, which is found 18 kcal mol −1 below the PH 2 [bond]CH[double bond]O peptide analogue. By correcting the known inadequacies in the calculations with the average theoretical to experimental ratio from the benchmark molecules of the system, it is possible to obtain a best estimate of rotational constants and infrared frequencies that should be precise enough to initiate laboratory experiments and/or observations. The corrected values of B =6.0342 GHz and C =5.4921 GHz for NH 2 [bond]CH[double bond]S; B =9.2292 GHz and C =6.1164 GHz for NH 2 [bond]SiH[double bond]O; B =8.0275 GHz and C =6.4779 GHz for CH 3 [bond]P[double bond]O should be accurate to within a few tenths of a per cent. Theoretical infrared spectra are also provided to assist in identification of these exotic species.