Local Minimum Geometry Research Articles

New benzene dimers: an MRMP2 study

The reaction energies of one cis-fused and four new trans-fused benzene dimers have been studied with state-specific multi-reference Moller-Plesset perturbation theory of the second order (MRMP2) based on reference CASSCF(12,12) wavefunctions. The cis-fused dimer 1, the product of symmetry-allowed [4 + 2] cycloaddition process, has been previously characterized experimentally and theoretically and, in this study, was used as reference. Dimer 1 lies 45 kcal/mol above two isolated benzene molecules, and its retro-dimerization activation energy is ~ 8 kcal/mol. In contrast, the [4 + 2] formation of trans-fused dimer 2 is symmetry-forbidden with an energetic driving force (Erel) of 57.6 kcal/mol. An activation barrier for corresponding retro-dimerization calculated to be 31.0 kcal/mol. The thermal [2 + 2] formation of dimers 3 and 4 are also symmetry-forbidden. They have been calculated to be 79.0 kcal/mol and 105.1 kcal/mol higher in energy than two isolated benzene molecules, respectively. However, the activation energies of 23.2 and 20.5 kcal/mol could allow for kinetic persistence of these compounds. Dimer 5 is the only symmetry-allowed [2 + 2] cycloaddition product. With Erel of 67.5 kcal/mol and Ea for retro-dimerization reaction of 19.2 kcal/mol, it shows high similarity with the symmetry-forbidden dimers. The wave function for each of the dimers at their local minimum geometry contains greater than 80% of the configuration $$\sigma^{4} \pi^{8} \sigma^{*0} \pi^{*0}$$. The transition state for the retro-dimerization of 1 (1TS) also contains more than 80% of the closed-shell configuration $$\sigma^{4} \pi^{8} \sigma^{*0} \pi^{*0}$$. For 2TS–5TS systems, the wavefunctions exhibit significant biradical character (20%, 20%, 14% and 26%, respectively).

Modern Valence-Bond Description of Homoaromaticity.

Spin-coupled (SC) theory is used to obtain modern valence-bond descriptions of the electronic structures of local minimum and transition-state geometries of three species that have been considered to exhibit homoconjugation and homoaromaticity: the homotropenylium ion, C8H9+, the cycloheptatriene neutral ring, C7H8, and the 1,3-bishomotropenylium ion, C9H11+. The resulting compact SC wave functions are of comparable quality to complete-active-space self-consistent field constructions that are based on the same "N electrons in M orbitals" active spaces, but they are much easier to interpret directly. Analysis of the forms of the SC orbitals and of the overlaps between them, as well as an examination of the compositions of the associated resonance patterns, strongly suggest that both of the homotropenylium and 1,3-bishomotropenylium ions are homoaromatic at their local minimum geometries, with all of the other cases that were considered being nonaromatic. The SC results also show that the differences between "no-bond" and "bond" homoconjugated systems are very likely to be much smaller than previously thought.

Atomic scale insights into ethanol oxidation on Pt, Pd and Au metallic nanofilms: A DFT with van der Waals interactions

In this work, we use dispersion corrected DFT calculations (DFT+D3) to explore the ethanol decomposition on Pt, Pd and Au metallic nanofilms. The structural and energetic properties of ethanol and the most common intermediate products in ethanol oxidation were investigated, namely acetaldehyde, acetic acid, acetyl and CO. Our results suggests that even when starting from an initial condition very close to the equilibrium geometry it is not possible to obtain the correct adsorption properties from standard DFT calculations; the system is trapped in a local minimum geometry and the correct adsorption geometry cannot be accessed. Therefore, the inclusion of vdW interactions is fundamental to assure correct adsorption properties and agreement with experimental data. By analyzing the adsorption of ethanol and its oxidation products on metallic nanofilms we found that Pt (111), Pt/Pd (111), Pt/Au (111) and Pd/Pt (111) nanofilms present enhanced adsorption properties and seem to be good candidates for ethanol catalysis.

Shifting Paradigms: Electrostatic Interactions and Covalent Bonding

The role of electrostatic interactions in covalent bonding of heavier main group elements has been evaluated for the exemplary set of molecules X(2)H(2) (X=C, Si, Ge, Sn, Pb). Density functional calculations at PBE/QZ4P combined with energy decomposition procedures and kinetic energy density analyses have been carried out for a variety of different structures, and two factors are responsible for the fact that the heavier homologues of acetylene exhibit doubly hydrogen-bridged local minimum geometries. For one, the extended electronic core with at least one set of p orbitals of the Group 14 elements beyond the first long period is responsible for favorable electrostatic E-H interactions. This electrostatic interaction is the strongest for the isomer with two bridging hydrogen atoms. Secondly, the H substituent does not posses an electronic core or any bonding-inactive electrons, which would give rise to a significant amount of Pauli repulsion, disfavoring the doubly bridged isomer. When one of two criteria is not met the unusual doubly bridged structure no longer constitutes the energetically preferred geometry. The bonding model is validated in calculations of different structures of Si(2)(CH(3))(2).

Revisiting the origin of the preferential π-π stacking conformation of the (+)-8-phenylmenthyl acrylate

In this paper we report a theoretical study aiming to revisit the origin of the preferential π-π stacking conformation of the (+)-8-phenylmentyl acrylate (2). For this, we have applied several DFT and ab initio methods to calculate local minimum geometries, vibrational frequencies, thermochemical properties and ¹H NMR chemical shifts for 2 and the model compound 3-phenylpropyl acrylate (3). We have observed that the MPW1B95 functional was the most suitable method to explain ¹H NMR experimental data which show the stacking conformation of 2 (2S) is more stable that trans conformation (2T) and the stacking conformation of 3 (3S) is less stable that trans conformation (3T). After that, geometrical and energetic features of the intermolecular complex benzene...methylacrylate (4) have also been studied using MPW1B95 method. From our results, we have noticed that both steric and dispersion effects play a key role in the conformational equilibrium of 2.

Theoretical Study of Pyrazolate-Bridged Dinuclear Platinum(II) Complexes: Interesting Potential Energy Curve of the Lowest Energy Triplet Excited State and Phosphorescence Spectra

Four kinds of 3,5-dialkylpyrazolate(R2pz)-bridged dinuclear platinum(II) complexes [Pt2(mu-R2pz)2(dfppy)2] (dfppy=2-(2,4-difluorophenyl)pyridine; R2pz=pyrazolate in 1, 3,5-dimethylpyrazolate in 2, 3-methyl-5- tert-butylpyrazolate in 3, and 3,5-bis(tert-butyl)pyrazolate in 4) were theoretically investigated by the DFT(B3PW91) method. The Stokes shift of their phosphorescence spectra was discussed on the basis of the potential energy curve (PEC) of the lowest energy triplet excited state (T1). This PEC significantly depends on the bulkiness of substituents on pz. In 1 and 2, bearing small substituents on pz, one local minimum is present in the T1 state besides a global minimum. The local minimum geometry is similar to the S0-equilibrium one. The T1 state at this local minimum is characterized as the pi-pi* excited state in dfppy, where the dpi orbital of Pt participates in this excited state through an antibonding interaction with the pi orbital of dfppy; in other words, this triplet excited state is assigned as the mixture of the ligand-centered pi-pi* excited and metal-to-ligand charge transfer excited state ((3)LC/MLCT). The geometry of the T1-global minimum is considerably different from the S0-equilibrium one. The T1 state at the global minimum is characterized as the triplet metal-metal-to-ligand charge transfer ((3)MMLCT) excited state, which is formed by the one-electron excitation from the dsigma-dsigma antibonding orbital to the pi* orbital of dfppy. Because of the presence of the local minimum, the geometry change in the T1 state is suppressed in polystyrene at room temperature (RT) and frozen 2-methyltetrahydrofuran (2-MeTHF) at 77 K. As a result, the energy of phosphorescence is almost the same in these solvents. In fluid 2-MeTHF at RT, on the other hand, the geometry of the T1 state easily reaches the T1-global minimum. Because the T1-global minimum geometry is considerably different from the S0-equilibrium one, the phosphorescence occurs at considerably low energy. These are the reasons why the Stokes shift is very large in fluid 2-MeTHF but small in polystyrene and frozen 2-MeTHF. In 3 and 4, bearing bulky tert-butyl substituents on pz, only the T1-global minimum is present but the local minimum is not. The electronic structure of this T1-global minimum is assigned as the (3)MMLCT excited state like 1 and 2. Though frozen 2-MeTHF suppresses the geometry change of 3 and 4 in the T1 state, their geometries moderately change in polystyrene because of the absence of the T1-local minimum. As a result, the energy of phosphorescence is moderately lower in polystyrene than in frozen 2-MeTHF. The T1-global minimum geometry is much different from the S0-equilibrium one in 3 but moderately different in 4, which is interpreted in terms of the symmetries of these complexes and the steric repulsion between the tert-butyl group on pz and dfppy. Thus, the energy of phosphorescence of 3 is much lower in fluid 2-MeTHF than in frozen 2-MeTHF like 1 and 2 but that of 4 is moderately lower; in other words, the Stokes shift in fluid 2-MeTHF is small only in 4.

Theoretical Study of Electronic Structures of [Peroxoporphinato]manganate [Mn(P)(O2)]− Anion

Abstract Geometries and electronic structures of the quintet and septet states of the [Mn(P)(O2)]− (P: porphinato) anion were theoretically investigated. Four local minimum geometries with two side-on additions of O2 to MnP, 1 (5B2) and 3 (7A2), and two end-on additions of O2, 2 (5A″) and 4 (5A′), were found. One of the side-on forms, 1, corresponds to that observed by X-ray crystallographic study. 3 is a septet state with higher spin than the quintet states of 1, 2, and 4. Each of the two end-on forms has the corresponding transition state with the side-on geometry. Energy differences among 1, 2, and 3 are less than 1 kcal mol−1. The electronic structures of four local minimum states were essentially different. 1 has an electronic structure presented as [MnIII(O22−)]−. 2 has antiferromagnetically coupled diradical character presented by the formal chemical formula of [MnII•(O2•−)]−. 3 has ferromagnetically coupled diradical character. 4 is presented by [MnII(O2−)]−.

Shape resonance of H2− anion stabilized in a molecular trap

The possibility of stabilizing an electronically metastable anion in a molecular trap is studied using ab initio electronic structure methods. The low-energy H2− p-wave Σ2u+ shape resonance state is used as the metastable anion solute, and a pair of inwardly oriented LiCN molecules (NCLi⋯LiCN) is used as a molecular trap. It is found that for the (NCLi⋯LiCN) trap at its local minimum geometry, the B22u state (that corresponds to the Σ2u+ state for the isolated H2−) becomes electronically stable and is the ground state of the system. It is also found that, in this model trap, two other (excited) anionic states are bound (2Ag and B23u). Detailed numerical results are presented for the (NCLi⋯H2⋯LiCN)− species (i.e., the trapped H2−) whose vertical electron detachment energy is 3.429 eV at the CCSD(T) level with the aug-cc-pVDZ basis sets.

Shape resonance of the ethylene anion stabilized in a molecular trap

The possibility of stabilizing an electronically metastable anion in a molecular trap is studied using ab initio electronic structure methods. The 2 B 2 g C 2 H 4 − d-wave shape resonance state is used as the metastable anion solute, and a pair of inwardly oriented LiCN molecules (NCLi⋯LiCN) is used as a molecular trap. Although the global minimum of the C 2H 4+2LiCN+e system is the (LiCN) 2 − anion with a weakly (van der Waals) bound C 2H 4 molecule, it was found that for the (NCLi⋯LiCN) trap at its local minimum geometry, the 2 B 2 g state becomes electronically stable and is the ground state of the system. It is also found that, in this model trap, three other (excited) anionic states are bound ( 2 A g , 2 B 1 u , and 2 B 2 u ). Detailed numerical results are presented for the (NCLi⋯H 2CCH 2⋯LiCN) − species (i.e., the trapped ethylene anion) whose vertical electron detachment energy is 2.97 eV at the CCSD(T) level with the augcc-pVDZ basis sets.

Symmetry breaking effects in NO3−: Raman spectra of nitrate salts and ab initio resonance Raman spectra of nitrate–water complexes

Ground-state structures and vibrational frequencies are calculated for complexes of the nitrate anion with one and two water molecules at the ab initio Hartree–Fock level with a basis set including diffuse and polarization functions. Two local minimum geometries are found for each complex. Calculations of the electronically excited states at the CIS level are then used to find the forces on each of the atoms upon vertical excitation to the two lowest-lying (near-degenerate) strongly allowed electronic transitions. These forces are converted to gradients of the excited-state potential surfaces along the ground-state normal modes and compared with the parameters obtained previously from empirical simulations of the experimental resonance Raman intensities of NO3− in dilute aqueous solution. The calculations on two-water clusters agree well with the experimental excited-state geometry changes along the totally symmetric N–O stretch. The calculations underestimate the frequency splitting of the antisymmetric stretching vibration (degenerate in the isolated D3h ion) and the resonance Raman intensity in this mode, suggesting that bulk solvent polarization enhances the asymmetry of the local environment for NO3− in water. Comparison of the ground-state vibrational frequency splitting of the antisymmetric stretch with the corresponding values for the nitrate ion in salts having known crystal structures suggests that the rms difference among the three N–O bond lengths for nitrate anion in water probably exceeds 0.01 Å.

Molecular-dynamics simulations of water clusters

The local minimum geometries and corresponding energy values of water clusters, [(H 2O) n , n=2–8], have been investigated by using the molecular dynamics simulation method. In the simulations two different potential energy functions of central-force model, CF and CF2, have been used. Particular attention was paid to investigate the effectiveness of these two empirical potential energy functions. CF has been used for n=2 only, whereas CF2 has been used for n=2–8. The cage structure of the water clusters appear for n⩾6.

An Unstable Anion Stabilized in a Molecular Trap

The possibility of stabilizing an electronically metastable anion in a molecular trap is studied using ab initio electronic structure methods. The 2Πg N2- d-wave shape resonance state is used as a prototype for the metastable anion solute, and a pair of inwardly oriented LiCN molecules (NCLi···LiCN) is used as a prototypical trap. It is found that for the (NCLi···LiCN) trap at its local minimum geometry, the 2Πg state becomes electronically stable and is the ground state of the system. It is also found that, in this model trap, two other (excited) anionic states are bound (2Σg and 2Πu) one of which is a trap-bound state and the second of which is another stabilized resonance state. Moreover, calculations performed on traps of varying strength (which we alter by varying the trap size) show that the number of bound anionic states strongly depends on the trap's strength. Detailed numerical results are presented for the (NCLi···N⋮N···LiCN)- trap plus solute anion whose vertical electron detachment energy is 3...

On the structure of the monohydrated superoxide molecular anion, O2-·(H2O). An ab initio molecular orbital study

The ground-state potential energy surface of the monohydrated superoxide ion–dipole complex O2-·(H2O) was investigated viaabinitio molecular orbital theory in order to establish firmly the relative energies of stationary points on this surface. Using the configuration interaction (CI) theoretical approach to accommodate the effects of electron correlation, the global minimum on the potential energy surface corresponds to a structure adopting Cs molecular symmetry with one end of the water molecule forming a hydrogen bond with one end of the superoxide. A more symmetric structure adopting C2v molecular symmetry is shown to be a transition state linking two equivalent forms of the Cs geometry via a water rocking motion. Using a highly flexible triple-zeta basis set with quadratic configuration interaction theory incorporating all single and double electron substitutions [QCISD/6–311++G**], the scaled zero point energies for the C2v and Cs geometries are 64.6 and 64.1 kJ mol-1, respectively. The energy barrier to the water rocking motion along the reaction coordinate is 3.9 kJ mol-1. The frequencies of the symmetric and asymmetric water O–H stretches in the C2v structure are 3731 and 3765 cm-1, respectively. The water O–H stretching frequencies in the Cs structure are 3178 cm-1 for the “hydrogen bonded’' OH and 3937 cm-1 for the “free’' OH. The geometry of the global minimum of O2-·(H2O) on the equivalent of the first electronically excited potential energy surface of the bare superoxide was also determined using the complete active space self-consistent field (CASSCF) theoretical approach. A vibrational frequency analysis confirms that the excited-state stationary point is a local minimum geometry. The excited-state geometry differs significantly from that in the ground electronic state. The overall molecular symmetry in the excited state remains as Cs, but the water molecule adopts an orientation approximately midway between the ground-state Cs and C2v configurations.

A Theoretical Approach to Analytical Properties of 2,4-Diamino-5-phenylthiazole in Water Solution. Tautomerism and Dependence on pH

Theoretical models are used to study pH-dependent equilibria of 2,4-diamino-5-phenylthiazole tautomer molecules in water. A complete screening of semiempirical SCF multiple minimums of hypersurfaces, corresponding to several solute−water supermolecules, has been made. Multiple minimum hypersurface searching confirms experimental NMR results indicating that the native diamine tautomer predominates in aqueous neutral and basic media. This tautomeric structure, protonated either in N3 and N4, also predominates in aqueous acid media with a minor presence of a protonated monoimine tautomer, in agreement with 1H NMR results in D2O. High-level ab initio SCF MO of the main structures, where solvent reaction field effects are taken into account with a dielectric constant equivalent to that of water, predict a nonconjugated protonated monoimine tautomer in nonprotic solvents, according to 1H NMR data in polar aprotic solvents. Calculated electron excitation patterns of hydrated species in water agreed with the experimental UV spectra at different pH values. The quantum chemical procedures for calculating total energies and frontier orbital eigenvalues in local minimum geometries of the relevant supermolecules provide an appropriate model for comparisons of theoretical results with experimental facts in the case of analytical voltametry. The frontier orbital eigenvalues of the most populated minimums discard the appearance of electroanalytic signals in the case of acidic samples because of the similarity of the predicted values for all protonated isomers and water. Experimental measurements confirm the oxidative character of electroanalytic signals.