Supermolecule Approach Research Articles

DFT study of polarizabilities and dipole moments of water clusters

AbstractDensity functional theory (DFT) calculations with different exchange‐correlation functionals, Becke's three‐parameter exchange functional and the gradient‐corrected functional of Lee, Yang, and Paar (B3LYP) and Becke's three‐parameter functional with Perdew–Wang correlational functional (B3PW91), are performed to study the dielectric properties of small and medium‐sized water clusters. For these H‐bonded systems, we optimize the geometries and compute the dipole moments and polarizabilities using a supermolecule approach. The corresponding properties of the individual water molecules in the clusters are extracted from the molecular properties using the Hirshfeld expansion of the electronic density. Calculations at Hartree–Fock and second‐order Møller–Plesset (MP2) levels are also presented for a comparison with the DFT results. The dependence of the dielectric properties on the cluster structure and size is given for both clusters and individual molecules and is compared with the available experimental data. The intermolecular interactions and their influences on the dielectric properties are discussed. The current results could serve to improve existing polarizable force fields for water. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Computational study of the Grignard reaction with alkynes

The complexation of MgX 2, MeMgX, and Me 2Mg (X=Cl, Br) with propyne have been studied both in the gas phase and in the presence of dimethyl ether (Me 2O) by means of the density functional theory B3LYP/6-31+G* method. Solvation was modeled using the supermolecule approach. The complex formation between disolvated magnesium compound and alkyne is connected with replacement of a coordinated solvent molecule by the alkyne, being necessary for the reaction to proceed to the end. The reaction towards end products, an acetylenic Grignard reagent and an alkane, is exothermic. The transition state has a cyclic structure, which is almost triangular with practically linear proton transfer. The transition state resembles more the reagents than the products.

Theoretical Study of Magnesium Compounds: The Schlenk Equilibrium in the Gas Phase and in the Presence of Et2O and THF Molecules

The Schlenk equilibrium involving RMgX, R2Mg, and MgX2 (R = Me, Et, Ph and X = Cl, Br) has been studied both in the gas phase and in diethyl ether (Et2O) and tetrahydrofuran (THF) solutions by means of the density functional theory (DFT) B3LYP/6-31+G* method. Solvation was modeled using the supermolecule approach. The stabilization due to interaction with solvent molecules decreases in the order MgX2 > RMgX > R2Mg and among the groups (R and X) Ph > Me > Et and Cl > Br. Studied magnesium compounds are more strongly solvated by THF compared to Et2O. The magnesium halide is solvated with up to four solvent molecules in THF solution, assuming that trans-dihalotetrakis(tetrahydrofurano)magnesium(II) complex forms. The formation of cis-dihalotetrakis(tetrahydrofurano)magnesium(II) is energetically less favorable than the formation of corresponding disolvated complexes. The predominant species in the Schlenk equilibrium are RMgX in Et2O and R2Mg + MgX2 in THF, which is consistent with experimental data.

Test study on the excitation spectra of the N2?He van der Waals molecule

We have performed ab initio fourth-order Moller–Plesset perturbation theory calculations in the framework of the supermolecule approach on the vertical excitation spectra of the weakly bound van der Waals N2–He dimer. They indicate a ``T-shaped'' stablest ground N2(X1Σ g +)−He(1S) electronic state with a well depth, D e, of 21.63 cm−1 at a minimum distance, R e, of 3.44 A and zero-point vibration correction, D o, of 7.07 cm−1. They also indicate a ``T-shaped'' stablest excited conformer with R e = 3.25 A, D e=36.85 cm−1 and D o=17.06 cm−1 for the N2(B3Π g )–He(1S) triplet electronic level. In order to investigate the use of less-demanding correlation methods, test density functional theory calculations using the mPW1PW exchange–correlation functional are also presented for comparison.

Test Study on the Excitation Spectrum of the CO ⃛Ar van der Waals Molecule

In the present contribution, we performed ab initio Møller-Plesset perturbation theory second-order (MP2) calculations in the framework of the supermolecule approach on the vertical excitation spectra of the weakly bound van der Waals CO-Ar molecule in its "near T-shaped" most stable ground state structure, as a guideline for future theoretical and experimental work. These test calculations indicate a ground CO ( X 1 D )- Ar ( 1 S ) interaction with R e =3.72 Å, D e =109 cm -1 and D o =92 cm -1 . They also indicate an excited CO ( A 1 @ )- Ar ( 1 S ) interaction with R e =3.55 Å, D e =141 cm -1 and D o =121 cm -1 . A red shift of 29 cm -1 for the CO ( X 1 D )- Ar ( 1 S ) M CO ( A 1 @ )- Ar ( 1 S ) vertical excitation energy, with respect to the corresponding CO ( X 1 D ) M CO ( A 1 @ ) excitation in the absence of Ar, can be obtained by comparing the corresponding D o values.

Thermochemistry of arylselanyl radicals and the pertinent ions in acetonitrile.

Reduction and oxidation potentials of a series of parasubstituted phenylselanyl radicals, XC(6)H(4)Se(*), have been measured using photomodulated voltammetry in acetonitrile. The thermodynamic significance of these data was substantiated through a study of the oxidation process of the pertinent selenolates in linear sweep voltammetry. Both the reduction and the oxidation potentials correlate linearly with the Hammett substituent coefficients sigma and sigma(+) leading in the latter case to slopes, rho(+), of 2.5 and 3.8, respectively. Through comparison of these slopes with those published previously for the O- and S-centered analogues, it is revealed that the pi-interaction becomes progressively smaller as the size of the radical center increases in the order O, S, and Se. Solvation energies of the pertinent selenolates and selanylium ions have been extracted from thermochemical cycles incorporating the measured electrode potentials for XC(6)H(4)Se(*) as well as electron affinities and ionization potentials obtained from theoretical calculations at the B3LYP/6-31+G(d) level. The extracted data show the expected overall substituent dependency for both kinds of ions; that is, the absolute value of the solvation energy decreases as the charge becomes more delocalized. The data have also been compared with solvation energies computed using the polarizable continuum model (PCM). Interestingly, we find that, while the model seems to work well for selenolates, it underestimates the solvation of selanylium ions in acetonitrile by as much as 25 kcal mol(-)(1). These large deviations are ascribed to the fact that the PCM method does not take specific solvent effects into account as it treats the solvent as a continuum described solely by its dielectric constant. Gas-phase calculations show that the arylselanylium ions can coordinate covalently to one or two molecules of acetonitrile in strong Ritter-type adducts. When this strong interaction is included in the solvation energy calculations by means of a combined supermolecule and PCM approach, the experimental data are reproduced within a few kcal mol(-)(1). Although the energy difference of the singlet and triplet spin states of the arylselanylium ions is small for the gas-phase structures, the singlet cation is undoubtedly the dominating species in solution because the triplet cation lacks the ability to form covalent bonds.

Density functional theory test study on the CO ···· He dimer

AbstractIn the current paper we have investigated the bonding properties of the weakly bound van der Waals CO ···· He dimer by means of the Perdew–Wang91 exchange and Perdew–Wang correlation (PWPW) and Adamo and Barone's Becke‐style one‐parameter functional using the modified Perdew–Wang91 exchange and Perdew–Wang correlation (mPW1PW) exchange‐correlation functionals in the framework of the supermolecule approach, using density functional theory local‐spin‐optimized atom‐centered basis sets, complemented with bond functions optimized at the mPW1PW level of theory. These calculations show that the use of the mPW1PW functional using bond functions gives a realistic representation of the interaction‐energy potential for this van der Waals molecular complex, as compared with reference Møller–Plesset perturbation theory calculations. In contrast, the PWPW functional is unable to describe the bonding properties of the system, and all values obtained with this functional approximation are considered out‐of‐scale compared with the rest of the calculations presented in this contribution. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 95: 177–183, 2003

An ab initio study of solvent polarity and hydrogen bonding effects on the nitrogen NMR shieldings of N,N‐dimethylacetamidine

AbstractDensity functional theory combined with the polarizable continuum model (PCM) and continuous set of gauge transformations method is applied to investigate the effects of solvent polarity on the nitrogen NMR shieldings of N, N‐dimethylacetamidine. Hydrogen bonding effects on shielding are likewise calculated using a supermolecule approach, where the imino group of the solute is hydrogen bonded with solvent. Theoretical results are compared with published experimental data. The PCM shielding calculations utilizing PCM‐optimized solute geometries yield results comparable to those obtained with the supermolecule approach. Geometry optimization of the solute appears to be more important in PCM shielding calculations than in the supermolecule approach. The large solvent shifts observed in water can only be reproduced when the N·H distance used in the calculation indicates full proton transfer from water to the imino nitrogen of the solute. Copyright © 2002 John Wiley & Sons, Ltd.

Solvation of Sulfur-Centered Cations and Anions in Acetonitrile

The solvation of substituted phenylsulfenium ions and thiophenoxides in acetonitrile has been analyzed on the basis of experimental and theoretical data. Experimental solvation energies are obtained from previously reported oxidation and reduction potentials of the corresponding arylthiyl radicals in combination with theoretically calculated ionization potentials and electron affinities at the B3LYP/6-31+G(d) level. These calculations provide a consistent set of values in contrast to the data sets obtained in our previous paper (Larsen et al., J. Am. Chem. Soc. 2001, 123, 1723). The extracted solvation data show the expected substituent dependency for both kinds of ions, i.e., the absolute value of the solvation energy decreases as the charge becomes more delocalized. For the thiophenoxides there is good agreement between the experimental solvation energies and solvation energies computed using the polarizable continuum model (PCM). The solvation of the arylsulfenium ions is much stronger than predicted by the PCM method. This can be attributed to the formation of a strong covalent bond of the Ritter type between the arylsulfenium ion and one molecule of acetonitrile. When this interaction is included in the solvation energy calculations by means of a combined supermolecule and PCM approach the experimental data are reproduced within a few kcal mol-1. While the energy difference of the singlet and triplet spin states of the arylsulfenium ions is almost negligible in gas phase, the singlet cation is undoubtedly the dominating species in solution, since the triplet cation lacks the ability to form a covalent bond with acetonitrile.

The influence of the solute/solvent interactions on the first-order hyperpolarizability in urea molecule. A quantum chemical study

The results of quantum chemical computations of first-order hyperpolarizability ( β) of urea molecule in the gas phase and in polar solvents are presented. The environmental effects were included via the quantum-mechanical Langevin dipoles/Monte Carlo (QM/LD/MC) method as well as supermolecule approach. The results of calculations confirm the existense of the substantial solvent effect on β value for urea molecule. The calculated β values, including solvent effect, are compared with experimental data determined in solution phase by EFISH measurements.

Effects of hydration on the molecular structure of atrazine dimers: A MOPAC (PM3) study

A semi-empirical (PM3) study of atrazine dimer hydration in which the supermolecule approach is used to simulate a solvent sheath of water molecules varying from 7 to 24 H 2O's around the atrazine dimer. Enthalpies of complex formation and reaction are reported along with entropies of complex formation and free energies of reaction and formation. The hydration sphere around the atrazine dimer is reduced stepwise by removing the outermost water molecule followed by calculation of the new structure. In the case of the hydrated dimer with 21 water molecules, the removal of one water molecule results in rearrangement of the majority of the solvent sheath. This is accompanied by a change in the number of hydrogen bonds and the average hydrogen bond distance between water molecules, ring chlorines, a ring nitrogen and other water molecules. There are no additional major changes in the solvent sheath as one continues the stepwise removal of water In addition, reaction enthalpies and entropies show a systematic change at a point where the solvent sheath contains 17 water molecules. The atrazine rings and their attached nitrogens maintain their planarity during the solvation process, unlike the case where metal ion binding to the atrazine dimer occurs.

Ab Initio Study of the CH3F···H2O Complex

The CH3F···H2O complex has been studied using both the supermolecule approach through fourth-order Møller−Plesset perturbation theory (MP4) and perturbation theory of intermolecular forces. Nine configurations have been examined, seven of which were found to be attractive. The global minimum occurs when a bent C−F···H−O hydrogen bond is formed with the C···O distance of 6.15 a0 and the water molecule in the same plane as the hydrogen bond. The binding energy for this geometry is equal to 5291 μEh (3.32 kcal/mol) at the MP4 level of theory. When bond functions are included in the basis set, this configuration is further stabilized to 5739 μEh (3.60 kcal/mol). The two configurations where a hydrogen atom of water is closest to the carbon atom of fluoromethane are repulsive at all distances examined due to electrostatic interactions. The increase of the magnitude of the binding energy when the basis set includes bond functions is primarily due to increased attractiveness of dispersion energy. The electrostatic interaction is the most significant energy component for all seven attractive configurations at their radial minima, particularly for configurations where the C−F bond points toward the H2O molecule. The exchange and dispersion energies are, respectively, the second and third most important contributions to the interaction energy for the seven attractive configurations at their radial minima. The MP2 interaction energy is found to approximate the MP4 interaction energy qualitatively, but underestimates the attraction of the seven attractive configurations at their optimal intermolecular separations by 8−82 μEh. A model potential for the CH3F···H2O system has been developed.

1,10-Phenanthroline and Its Complexes with Magnesium Compounds. Disproportionation Equilibria

The solvation, complexation, and disproportionation equilibria, which might be important during titration of a Grignard reagent RMgX with an alcohol in the presence of 1,10-phenanthroline (phen), have been studied both in the gas phase and solution using the density functional theory (DFT) B3LYP/6-31+G* method. Solvation was modeled using the supermolecule approach. NBO atomic charge analyses were performed at the B3LYP/6-31G* level. The absorption spectra of the complexes were calculated by the DFT TD/MPW1PW91/6-311+G** method. According to our calculations the complexation of magnesium halide MgX2 with 1,10-phenanthroline is the reason for the disappearance of the red color of the complex RMgX(phen) near the titration end point.

Self-Consistent Reaction Field Calculations of Aqueous Al3+, Fe3+, and Si4+: Calculated Aqueous-Phase Deprotonation Energies Correlated with Experimental ln(Ka) and pKa

Reaction energies for the deprotonation of Al3+·6(H2O), Fe3+·6(H2O), and Si(OH)4 were calculated using Hartree−Fock and density functional methods with 6-311+G(d,p) (for Al3+ and Si4+) and 6-311G(d) (for Fe3+) basis sets. Theoretical energies were calculated using a supermolecule approach (i.e., explicit hydration of the solute) combined with the Integral Equation Formalism Polarized Continuum Model (IEFPCM; Cancès et al., 1997, J. Chem. Phys. 107, 3032) and the Self-Consistent Isodensity Polarized Continuum Model (SCIPCM; Keith and Frisch, 1994, ACS Symp. Ser. 56, 22) in Gaussian 98. Tests on the effects of increasing the number of water molecules explicitly included in the supermolecule were also carried out. Additional water molecules in the energy minimizations of Al(OH)3, Fe(OH)3, and [Si(OH)3(OH2)]+ resulted in 5-coordinate complexes for all three species. Correlations of deprotonation energies with observed ln(Ka) values are good for individual cations. These correlations suggest that the combined supermolecule/continuum approach can give reliable pKa estimates, provided that the structural optimization reflects the aqueous-phase solute and that the basis set includes polarization and diffuse functions. An estimate is made of the pKa of the reaction[Si(OH)3(OH2)]+(aq) ↔ Si(OH)4(aq) + H+(aq)which has not yet been measured. A value of pKa ≈ −2 is predicted in this study.

A density functional theory test study on the N 2 ��� He dimer

In the present contribution we report a study of the weakly bound van der Waals N2–He molecule in the framework of the supermolecule approach by means of the PWPW and mPW1PW exchange–correlation functionals, using density functional theory local-spin-optimized atom-centered basis sets complemented with bond functions optimized at the mPW1PW level of theory. Calculations show that the mPW1PW functional using bond functions gives a realistic representation of the interaction-energy potentials for this van der Waals dimer, comparable to reference Moller–Plesset perturbation theory calculations. In contrast, the PWPW functional is unable to describe the bonding properties of this system and all values of the bonding properties obtained at different geometries with this functional are considered out-of-scale compared with the rest of the calculations presented in this study.

Test study on the excitation spectra of the CO–He van der Waals molecule

In the present contribution we performed ab initio Møller–Plesset perturbation theory to fourth order (MP4) and coupled-cluster CCSD(T) calculations in the framework of the supermolecule approach on the vertical excitation spectra of the weakly bound van der Waals CO–He molecule in its “near T-shaped” most stable ground state structure. They indicate that the excited Co(a3Π)–He(1S) triplet interaction has Re=3.47Å,De=22.87cm−1 and Do=6.07cm−1 for CO(3A′)–He(1S), and Re=3.41Å,De=24.90cm−1 and Do=7.46cm−1 for the CO(3A″)–He(1S) electronic state. They also indicate an excited CO(A1Π)–He(1S) singlet interaction with Re=3.49Å,De=20.92cm−1 and Do=5.05cm−1 for CO(1A′)–He(1S), and Re=3.37Å,De=27.21cm−1 and Do=8.24cm−1 for the CO(1A″)–He(1S) state.

Semiempirical INDO/S study on the solvatochromism of merocyanine dyes

AbstractWe report results of semiempirical intermediate neglect of differential overlap (INDO/S) calculations on the spectroscopy of 4′‐hydroxy‐1‐methylstilbazolium betaine (HMSBB). Solvent effects were included using both the continuum and supermolecule approaches. The calculated spectra are compared with experimental ultraviolet‐visible spectra for several solvents. The odd bathochromic shift of the low energy band (for nonpolar solvents) is tentatively discussed in terms of the contributions of different solvation mechanisms, and an assignment based on INDO/S calculated spectra is presented. We start from the assumption that in solution, conformations other than the completely planar structure (A) are possible. For protic solvents, in which hydrogen bonding constitutes the main contribution to solvation, calculations using the planar, fully optimized AM1 geometry reproduce fairly well the observed spectra. The intense, π→π* band is calculated close to 20,000 cm−1, in good agreement with the experimental values in water and methanol. For the very polar formamide, the INDO/S+SCRF result is also very accurate, suggesting that formamide's strong polarity induces HMSBB to adopt the planar structure A. For polar solvents with nonspecific interactions such as acetonitrile or dimethylformamide, the situation is not as clear. We speculate that the experimental numbers can be reproduced only by a very large supermolecule. Finally, for nonpolar solvents such as cyclohexane, benzene and CCl4, the experimental number of 16,141 cm−1 (previously obtained from extrapolation of solvent mixtures by Jacques and coworkers) is reproduced by the INDO/S method only considering structures B and C. These results can be considered a confirmation of Jacques' suggestion that the solvatochromism of HMSBB can be explained by the solvents inducing a geometry change in HMSBB. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001

Calculation of the UV Absorption Spectra of As(III) Oxo- and Thio- Acids and Anions in Aqueous Solution and of PF3 in the Gas-phase

Changes in the UV spectra of As(OH)3 solutions with variations in pH and temperature have recently been used to determine the temperature dependence of the pKa of the acid. In previous studies I used quantum mechanical techniques to study changes in structure and vibrational spectra as a function of pH for arsenites and thioarsenites. I previously calculated UV spectra for ``molecular'' minerals, like realgar As4S4. Here I use a number of different quantum mechanical methods, both Hartree-Fock and density functional theory based, to calculate the UV spectra for both a related simple well-characterized gas-phase molecule PF3 and for As(OH)3 and As(SH)3 and their conjugate anions and some neutral and anionic oligomers in aqueous solution. For the monomeric species small numbers of water molecules have been explicitly included, in a supermolecule or microsolvation approach. I find that UV absorption energies accurate to a few tenths of an eV can be obtained both for gas- phase PF3 and for neutral arsenious acid in aqueous solution, for which the UV absorption maximum is calculated to occur around 6.5 eV, consistent with experiment. Accurate calculation of the UV energies for arsenite anions in aqueous solution is much more difficult, since basis set size and solvation effects are considerably larger than for the neutral molecules, but fairly reliable results can still be obtained. Deprotonation is found to reduce the lowest calculated UV transition energy by about half an eV. Oligomerization also reduces the lowest calculated UV energy by at least half an eV. Replacement of one or all the –OH groups by –SH groups reduces the lowest calculated UV energies by about 2 eV. UV excitation energies have been calculated for oligomeric species as large as As3E3(EH)3 and As4E6, where E = O, S, and may be useful for identifying such species in solution.

Coupled cluster ab initio potential energy surfaces for CO . . . He and CO . . .H 2

Ab initio potential energy surfaces including the vibrational coordinate dependence are presented for CO… He and CO… H2 using the coupled cluster method with Brueckner orbitals. The interaction energy is calculated using the supermolecule approach. The calculation of rate constants for the vibrational relaxation of CO(v = 1) by He and their comparison with the experimentally measured values over the temperature range 40–300 K is used to test the accuracy of the CO… He surface. Comparison with results from an earlier surface calculated by symmetry adapted perturbation theory shows that the two surfaces have similar scattering characteristics and reproduce the experimental measurements to a similar level of accuracy. The potential surface for the CO… H2 system is presented as raw data in anticipation of future calculations.

Coupled cluster ab initio potential energy surfaces for CO… He and CO… H2

Ab initio potential energy surfaces including the vibrational coordinate dependence are presented for CO… He and CO… H2 using the coupled cluster method with Brueckner orbitals. The interaction energy is calculated using the supermolecule approach. The calculation of rate constants for the vibrational relaxation of CO(v = 1) by He and their comparison with the experimentally measured values over the temperature range 40–300 K is used to test the accuracy of the CO… He surface. Comparison with results from an earlier surface calculated by symmetry adapted perturbation theory shows that the two surfaces have similar scattering characteristics and reproduce the experimental measurements to a similar level of accuracy. The potential surface for the CO… H2 system is presented as raw data in anticipation of future calculations.