Effective Fragment Potential Model Research Articles

Modeling Absolute Redox Potentials of Ferrocene in the Condensed Phase.

Absolute thermodynamic quantities for critical chemical reactions are needed to determine the role of solvents and reactive environments in catalysis and electrocatalysis. Theoretical methods can provide such quantification but are often hindered by the innate complexity of electron correlation and dynamic relaxation of solvent environments. We present and validate a protocol for calculating the redox potentials of the ferrocene/ferrocenium redox pair in acetonitrile. Equation-of-motion and effective fragment potential (EFP) methods are used to characterize the adiabatic and vertical ionization potentials as well as the electron affinity processes. We benchmark molecular mechanics against the EFP model to show the differences in the ferrocene electronic polarizability in two redox states. Our best estimate of the redox potential (4.94 eV) agrees well with the experimental value (4.93 eV). This demonstrates the ability of modern computational methods to predict absolute redox potentials quantitatively and to quantify the correlation of dynamic effects, which underlie their origin.

Excited state hydrogen atom transfer in micro-solvated dicoumarol: A TDDFT/EFP1 study.

Ground (S0) and excited (S1) state properties of dicoumarol (DC) are investigated by applying density functional theory (DFT) and time dependent DFT (TDDFT) interfacing with the effective fragment potential (EFP) method of solvation. Benzene and pyrone rings of the each 4-hydroxy coumarin (4HC) moiety are in a plane and these planes are twisted by 180° with respect to each other. Two intra-molecular hydrogen bonds (HB) CO⋯HO exist between the carbonyl (CO) and hydroxyl (OH) groups of different 4HC moieties (4HC-1 and 4HC-2). DC(H2O)3 complex is formed using the original EFP model (EFP1). Four inter-molecular HBs are established by the carbonyl and hydroxyl oxygen atoms of 4HC-1 and 4HC-2 moieties; two HBs with two solvent molecules on one side of the complex and other two HBs with one solvent molecule at the other side. In S1 state, the hydrogen atomtransfer takes place only from the hydroxyl group of 4HC-1 to the carbonyl group of 4HC-2. The natural charge analysis and the modification of HBs manifest the intra-molecular charge transfer (ICT) from one 4HC moiety to another. Theoretical and experimental studies of the absorption spectra, and the theoretical study of potential energy curves of OH bonds at both S0 and S1 states affirm the hydrogen atom transfer from the hydroxyl group of 4HC-1 to the carbonyl group of 4HC-2 moiety.

Electronic Polarization Effect of the Water Environment in Charge-Separated Donor-Acceptor Systems: An Effective Fragment Potential Model Study.

The electronic polarization (POL) of the surrounding environment plays a crucial role in the energetics of charge-separated systems. Here, the mechanism of POL in charge-separated systems is studied using a combined quantum mechanical and effective fragment potential (QM/EFP) method. In particular, the POL effect caused by charge separation (CS) is investigated at the atomic level by decomposition into the POL at each polarizability point. The relevance of the electric field generated by the CS is analyzed in detail. The model systems investigated are Na+-Cl- and guanine-thymine solvated in water. The dominant part of the POL arises from solvent molecules close to the donor (D) and acceptor (A) units. At short D-A distances, the electric field shows both positive and negative interferences. The former case enhances the POL energy. At longer distances, the interference is weakened, and the local electric field determines the POL energy.

Accurate First Principles Model Potentials for Intermolecular Interactions

The general effective fragment potential (EFP) method provides model potentials for any molecule that is derived from first principles, with no empirically fitted parameters. The EFP method has been interfaced with most currently used ab initio single-reference and multireference quantum mechanics (QM) methods, ranging from Hartree-Fock and coupled cluster theory to multireference perturbation theory. The most recent innovations in the EFP model have been to make the computationally expensive charge transfer term much more efficient and to interface the general EFP dispersion and exchange repulsion interactions with QM methods. Following a summary of the method and its implementation in generally available computer programs, these most recent new developments are discussed.

Effect of Solvation on the Vertical Ionization Energy of Thymine: From Microhydration to Bulk

The effect of hydration on the vertical ionization energy (VIE) of thymine was characterized using equation-of-motion ionization potential coupled-cluster (EOM-IP-CCSD) and effective fragment potential (EFP) methods. We considered several microsolvated clusters as well as thymine solvated in bulk water. The VIE in bulk water was computed by averaging over solvent-solute configurations obtained from equilibrium molecular dynamics trajectories at 300 K. The effect of microsolvation was analyzed and contrasted against the combined effect of the first solvation shell in bulk water. Microsolvation reduces the ionization energy (IE) by about 0.1 eV per water molecule, while the first solvation shell increases the IE by 0.1 eV. The subsequent solvation lowers the IE, and the bulk value of the solvent-induced shift of thymine's VIE is approximately -0.9 eV. The combined effect of the first solvation shell was explained in terms of specific solute-solvent interactions, which were investigated using model structures. The convergence of IE to the bulk value requires the hydration sphere of approximately 13.5 Å radius. The performance of the EOM-IP-CCSD/EFP scheme was benchmarked against full EOM-IP-CCSD using microhydrated structures. The errors were found to be less than 0.01-0.02 eV. The relative importance of the polarization and higher multipole moments in EFP model was also investigated.

Solvent Effects on the Electronic Transitions ofp-Nitroaniline: A QM/EFP Study

Solvatochromic shifts of the electronic states of a chromophore can be used as a measure of solute-solvent interactions. The shifts of the electronic states of a model organic chromophore, p-nitroaniline (pNA), embedded in solvents with different polarities (water, 1,4-dioxane, and cyclohexane) are studied using a hybrid quantum mechanics/molecular-mechanics-type technique in which the chromophore is described by the configuration interaction singles with perturbative doubles (CIS(D)) method while the solvent is treated by the effective fragment potential (EFP) method. This newly developed CIS(D)/EFP scheme includes the quantum-mechanical coupling of the Coulomb and polarization terms; however, short-range dispersion and exchange-repulsion terms of EFP are not included in the quantum Hamiltonian. The CIS(D)/EFP model is benchmarked against the more accurate equation of motion coupled cluster with singles and doubles (EOM-CCSD)/EFP method on a set of small pNA-water clusters. CIS(D)/EFP accurately predicts the red solvatochromic shift of the charge-transfer π → π* state of pNA in polar water. The shift is underestimated in less polar dioxane and cyclohexane probably because of the omission of the explicit quantum-mechanical treatment of the short-range terms. Different solvation of singlet and triplet states of pNA results in different probabilities of intersystem crossing (ISC) and internal conversion (IC) pathways of energy relaxation in solvents of different polarity. Computed singlet-triplet splittings in water and dioxane qualitatively explain the active ISC channel in dioxane and predict almost no conversion to the triplet manifold in water, in agreement with experimental findings.

Alanine: Then There Was Water

An ab initio study of the addition of successive water molecules to the amino acid l-alanine in both the nonionized (N) and zwitterionic (Z) forms are presented. The main focus is the number of waters needed to stabilize the Z form and how the solvent affects conformational preference. The solvent is modeled by ab initio electronic structure theory, the EFP (effective fragment potential) model, and the isotropic dielectric PCM (polarizable continuum method) bulk solvation techniques. The EFP discrete solvation model is used with a Monte Carlo algorithm to sample the configuration space to find the global minimum. Bridging structures are predicted to be the lowest energy Z minima after 3-5 discrete waters are included in the calculations, depending on the level of theory. Second-order perturbation theory and PCM stabilize the Z structures by approximately 3-6 and 7 kcal/mol, respectively, relative to the N global minimum through the addition of up to 8 waters. Subsequently, the contributions of each are approximately 1 kcal/mol relative to the N global minimum. The presence of 32 waters appears to be close to converging the N-Z enthalpy difference, DeltaH(N-Z).

The onset of dissociation in the aqueous LiOH clusters: a solvation study with the effective fragment potential model and quantum mechanics methods

The first systematic study on the solvation of the LiOH(H 2O) n ( n=1–6,8) clusters is herein presented by using both the molecular-mechanics effective fragment potential (EFP) model and full quantum mechanics methods. Aqueous clusters are sequentially calculated first with the LiOH solute molecule described by a restricted Hartree–Fock (RHF) wavefunction and the H 2O solvent molecules by the EFP model (EFP/RHF), and then with the full quantum mechanics RHF and Møller–Pleset second-order perturbation (MP2) theories. Calculated properties include equilibrium geometries, Mulliken charges, bond lengths and orders, and relative energies inter alia. Present results indicate that at the least six H 2O molecules are necessary to be added to one LiOH molecule to cause its spontaneous dissociation into a Li +/OH − separated ion pair. Another instance of dissociation is also observed in one of the LiOH(H 2O) 8 isomers. EFP/RHF reasonably reproduces RHF structural properties and dissociation patterns but discrepancies arise in the solute charge description. Comparisons of the present theoretical results with previous EFP studies and with a few experimental data are also discussed.

Solvent Effect on Electronegativity, Hardness, Condensed Fukui Functions, and Softness, in a Large Series of Diatomic and Small Polyatomic Molecules: Use of the EFP Model

The effect of solvent on the electronegativity, hardness, and condensed Fukui function, and atomic softness for a set of diatomic and small polyatomic molecules and ions has been studied using the effective fragment potential (EFP) model. The binding function was used for monitoring the solvation of the molecule. We do not observe a decrease in the HOMO−LUMO gap in the solvent. All anions show a significant change in the chemical potential. Both HOMO and LUMO energy levels decrease in the solvent phase as compared to the gas phase. For the major part of the acids, the increase in the LUMO orbital energy is larger than in the HOMO orbital energy. For the group of salts, we observe an increase in the LUMO energy level and a similar decrease in the HOMO energy level, resulting in a small change in the chemical potential. The importance of the change in the wave function upon solvation was shown through an analysis of the relaxation part in the hardness and condensed Fukui function. Very close values found fo...

Study of Small Water Clusters Using the Effective Fragment Potential Model

The recently developed effective fragment potential (EFP) model is applied to the description of a series of small water clusters, (H2O)n, n = 3−5. These results are compared with those found in the literature. The model accurately reproduces results obtained at ab initio levels of theory, while the computational cost is comparable to that of models employing empirical potentials. The EFP model thus offers significant promise as an inexpensive alternative to the Hartree−Fock methodology in the treatment of small water clusters.