Ab initio MO calculation of hydration energies of ammonium ions and their solvation effect

The hydration energies for the NH⁺₄ and CH₃NH⁺₃ ions were calculated by an ab initio MO method. The aqueous solvation energy difference between these two ions was found to be accounted for by the interactions of the ions with a few solvent molecules.     

Structure and dynamics of the CrIII ion in aqueous solution: Ab initio QM/MM molecular dynamics simulation

Structural and dynamical properties of the Cr(III) ion in aqueous solution have been investigated using a combined ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulation. The hydration structure of Cr(III) was determined in terms of radial distribution functions, coordination numbers, and angular distributions. The QM/MM simulation gives coordination numbers of 6 and 15.4 for the first and second hydration shell, respectively. The first hydration shell is kinetically very inert but by no means rigid and variations of the first hydration shell geometry lead to distinct splitting in the vibrational spectra of Cr(H(2)O)(6) (3+). A mean residence time of 22 ps was obtained for water ligands residing in the second hydration shell, which is remarkably shorter than the experimentally estimated value. The hydration energy of -1108 +/- 7 kcal/mol, obtained from the QM/MM simulation, corresponds well to the experimental hydration enthalpy value.     

Enthalpies of hydration of N-methylacetamide by one, two, and three waters and the effect upon the C=O stretching frequency. An Ab initio DFT study

DFT calculations at the B3LYP/D95++(d,p) level of clusters of N-methylacetamide (NMA) with one, two, and three waters that were geometrically optimized on the counterpoise-corrected potential energy surfaces show that the gas-phase enthalpy of the interactions of NMA with three waters is -14.11 kcal/mol. Since the interactions between the three waters is 0.99 kcal/mol, the interaction enthalpy would become -15.10 if these interactions were subtracted. The internal geometry of the NMA molecule is sensitive to the degree of hydration, as are the H-bond lengths. Changes in the internal bond lengths with degree of hydration are approximately additive. The calculated C=O stretching frequencies correlate extremely well with the calculated C=O bond lengths, which suggests that the solvent effect upon this stretch could not be a purely electrostatic interaction. The calculated C=O stretch for NMA solvated by three waters in the gas phase agrees very well with that experimentally observed in aqueous solution.     

Ab initio mo study on base stacking: Adenine-adenine interaction in single-stranded polyadenylic acid (Poly A)

The interaction energy for base stacking in single-stranded poly A is calculated by an ab initio MO method. The calculated value agrees well with the observed stacking enthalpy. Dispersion and charge transfer interactions are the main factors stabilizing the polynucleotide; the contribution of the former is dominant.     

Cooperative effect in hydrogen bonding of N-methylacetamide in carbon tetrachloride solution confirmed by NMR and IR spectroscopies

Recently we carried out ab initio molecular orbital calculations of the hydrogen bond energies in the dimers and trimers of N-methylacetamide (NMA), and found the existence of a cooperative effect in the hydrogen bonding, by which formation of one hydrogen bond in a hydrogen-bonded chain enhances the strength of another hydrogen bond. In order to confirm the existence of such a cooperative effect, we have determined experimentally the enthalpy changes occurring upon hydrogen bonding of NMA in CCl4 solution. First, following the method proposed by us, the population fractions of free (non-bonded) NH protons are obtained from the observed amide proton NMR chemical shifts and the IR intensities of the free NH stretching bands. Next, the enthalpy changes are evaluated by analyzing the equilibrium between the free and bonded states of an NH proton. In this analysis, the existence of the CCl4 solvent is taken into account. The stabilization energy of hydrogen bonds in a trimer, as compared with twice the hydrogen bond energy in a dimer, is 5.4 kJ mol(-1), in good agreement with the calculated value (5.9 kJ mol(-1)). This result provides experimental confirmation of the existence of a cooperative effect in hydrogen bonding.     

New statistical mechanical model for calculating Kirkwood factors in self-associating liquid systems and its application to alkanol+cyclohexane mixtures

A new statistical mechanical model for calculating Kirkwood factors in self-associating molecular liquids and their mixtures with nonassociating components has been developed in a consistent way which is based on an extended version of the Flory-Huggins model taking into account chemical association equilibria. The majority of molecular parameters involved into the theory has been fixed by independent quantum mechanical ab initio calculations of associated molecular clusters. The model is also able to predict other thermodynamic mixture properties such as the enthalpy of mixing and also the infrared absorbance of monomer alcohol species as function of concentration. Experimental results of nine alcohol+cyclohexane mixtures taken from the literature have been used to test the new model. The Kirkwood correlation factor gK, the molar enthalpy of mixing HmE, and the monomer IR absorbance can be described simultaneously in excellent agreement with experimental data covering the whole range of mole fraction including temperature dependence of gK, HmE, and the IR absorbance. Two parameters have been adjusted freely for each system. A third parameter for the nonspecific intermolecular dispersion interactions has been adjusted within a limited range of possible values given by physical arguments. The model opens a new way of a more reliable understanding of structures and equilibrium properties of hydrogen bonded systems in the condensed liquid state.     

A theoretical study of the hydration of Rb+ by Monte Carlo simulations with refined ab initio-based model potentials

An infinitely diluted aqueous solution of Rb⁺ was studied using ab initio-based model potentials in classical Monte Carlo simulations to describe its structural and thermodynamic features. An existing flexible and polarizable model [Saint-Martin et al. in J Chem Phys 113(24) 10899, 2000] was used for water–water interactions, and the parameters of the Rb⁺–water potential were fitted to reproduce the polarizability of the cation and a sample of ab initio pair interaction energies. It was necessary to calibrate the basis set to be employed as a reference, which resulted in a new determination of the complete basis set (CBS) limit energy of the optimal Rb⁺–OH₂ configuration. Good agreement was found for the values produced by the model with ab initio calculations of three- and four-body nonadditive contributions to the energy, as well as with ab initio and experimental data for the energies, the enthalpies and the geometric parameters of Rb⁺(H₂O) _n clusters, with n = 1,  2,…, 8. Thus validated, the potential was used for simulations of the aqueous solution with three versions of the MCDHO water model; this allowed to assess the relative importance of including flexibility and polarizability in the molecular model. In agreement with experimental data, the Rb⁺–O radial distribution function (RDF) showed three maxima, and hence three hydration shells. The average coordination number was found to be 6.9, with a broad distribution from 4 to 12. The dipole moment of the water molecules in the first hydration shell was tilted to 55° with respect to the ion’s electric field and had a lower value than the average in bulk water; this latter value was recovered at the second shell. The use of the nonpolarizable version of the MCDHO water model resulted in an enhanced alignment to the ion’s electric field, not only in the first, but also in the second hydration shell. The hydration enthalpy was determined from the numerical simulation, taking into account corrections to the interfacial potential and to the spurious effects due to the periodicity imposed by the Ewald sums; the resulting value lied within the range of the various different experimental data. An analysis of the interaction energies between the ion and the water molecules in the different hydration shells and the bulk showed the same partition of the hydration enthalpy as for K⁺. The reason for this similarity is that at distances longer than 3 Å, the ion–water interaction is dominated by the charge-(enhanced) dipole term. Thus, it was concluded that starting at K⁺, the hydration properties of the heavier alkali metal cations should be very similar.     

Theoretical calculation of electrode potentials: Electron-withdrawing compounds

The electrode potential of 2,3-dicyanobenzoquinone in aqueous solution has been calculated relative to parabenzoquinone using a thermodynamic cycle approach that includes accurate gasphase ab initio calculations and calculation of differences in free energies of hydration using the free-energy perturbation method. The discrepancy between the calculated and experimental electrode potential is disappointingly large (99 mV) compared to previous studies using this approach. This, along with the experimental evidence, suggests that the experimental value itself is too large and that theoretical approaches may indeed be as reliable as experimental ones for determining redox properties of molecules such as 2,3-dicyanobenzoquinone. In the light of this discrepancy we have examined the variation of the results with the basis set, inclusion of electron correlation and changes in the parameters used in the molecular dynamics free-energy simulations. The results are shown to be dependent upon the torsional parameters and especially dependent upon the basis set or semiempirical method used to obtain the electrostatic potential-derived charges. The best charge set was determined using the ab initio criteria of completeness—as far as it can be applied to large molecules—and also by studying the effect of hydration on these charges. This was done by allowing the solvent to perturb the wave function prior to the electrostatic potential determination. Thus, 3-21G and 6-31G * basis sets were found to give satisfactory results. Similar results were obtained using semiempirical and ab initio geometries.     

Quantitative orbital analysis of ab initio SCF=MO computations : Part II. Conformational preferences in H2N---OH and H2N---SH

The results obtained in a quantitative orbital analysis of the ab initio SCF=MO computations performed on hydroxylamine and thiohydroxylamine are described. The analysis is based on the use of fragment localized MOs and the energy effects associated with their interactions are estimated, in the framework of the ab initio SCF—MO computations, using PMO expressions. In particular the factors are analyzed which control the conformational preference in these molecules, and the effect of the sulphur 3d orbitals upon conformational stability is discussed.     

Thermodynamic study of poly(N-vinyl caprolactam) hydration at temperatures close to lower critical solution temperature

The lower critical solution temperature of aqueous solutions of poly(N-vinyl caprolactam) falls in the 305–307 K range and depends on the molecular weight of the polymer. The thermodynamic functions of mixing at 298 K have been calculated from measurements of vapor pressures and heats of dissolution and dilution. Partial Gibbs energy, partial enthalpy, and partial entropy of mixing were negative over the entire range of composition. Increasing temperature resulted in a decrease in the exothermal character of mixing. Excessive heat capacity values, calculated from the dependencies of enthalpy of mixing on temperature, were positive over the entire composition range. Heat capacity of dilute solutions was measured at 298 K and partial heat capacity of poly(N-vinyl caprolactam) at infinite dilution was shown to be positive. The data obtained point out the hydrophilic and hydrophobic hydration of poly(N-vinyl caprolactam) in aqueous solutions. Hydrophobic hydration dominates at temperatures close to binodal curve. As a result, the mutual mixing of the polymer with water is decreased and phase separation takes place.     

过渡金属离子液体的热化学性质研究BMIC/ZnCl2体系

在干燥的氩气氛中, 于363 K下缓慢混合等摩尔的氯化1-甲基-3-丁基咪唑(BMIC)和高纯无水ZnCl2, 得到了无色透明的离子液体BMIC/ZnCl2. 在298.15 K下, 用具有恒温环境的溶解反应热量计测定了不同浓度离子液体BMIC/ZnCl2在水中的溶解焓, 依据Pitzer方程拟合得到它们的标准摩尔溶解焓ΔsH0m和Pitzer溶解焓参数. 利用标准摩尔溶解焓估算了离子液体的水化焓.     

Electronic control of the stereochemistry of electrophilic and nucleophilic attack on double bonds in 6-membered rings

CNDO and ab initio MO calculations reveal a deformation of the π^* and π orbitals of cyclohexene in the axial directions, thus providing a reasonable explanation for the axial attack on cyclohexene either by electrophiles or by nucleophiles. It is shown in the case of 1-butene by an ab initio calculation that this orbital deformation is a result of the mixing of the π and σ orbitals of the double bond under the influence of the allylic C-C bond.     

Tl(I)‐the strongest structure‐breaking metal ion in water? A quantum mechanical/molecular mechanical simulation study

Structural and dynamical properties of the Tl(I) ion in dilute aqueous solution have been investigated by ab initio quantum mechanics in combination with molecular mechanics. The first shell plus a part of the second shell were treated by quantum mechanics at Hartree-Fock level, the rest of the system was described by an ab initio constructed potential. The radial distribution functions indicate two different bond lengths (2.79 and 3.16 A) in the first hydration shell, in good agreement with large-angle X-ray scattering and extended X-ray absorption fine structure spectroscopy results. The average first shell coordination number was found as 5.9, and several other structural parameters such as coordination number distributions, angular distribution functions, and tilt- and theta-angle distributions were evaluated. The ion-ligand vibration spectrum and reorientational times were obtained via velocity auto correlation functions. The Tl-O stretching force constant is very weak with 5.0 N m(-1). During the simulation, numerous water exchange processes took place between first and second hydration shell and between second shell and bulk. The mean ligand residence times for the first and second shell were determined as 1.3 and 1.5 ps, respectively, indicating Tl(I) to be a typical "structure-breaker". The calculated hydration energy of -84 +/- 16 kcal mol(-1) agrees well with the experimental value of -81 kcal mol(-1). All data obtained for structure and dynamics of hydrated Tl(I) characterize this ion as a very special case among all monovalent metal ions, being the most potent "structure-breaker", but at the same time forming a distinct second hydration shell and thus having a far-reaching influence on the solvent structure.     

Theoretical studies on the hydration of formic acid by ab initio and ABEEMσπ fluctuating charge model

The interaction between formic acid (FA) and water was systemically investigated by atom-bond electronegativity equalization method fused into molecular mechanics (ABEEMσπ/MM) and ab initio methods. The geometries of 20 formic acid–water complexes (FA–water) were obtained using B3LYP/aug-cc-pVTZ level optimizations, and the energies were determined at the MP2/aug-cc-pVTZ level with basis set superposition error (BSSE) and zero-point vibrational energy (ZPVE) corrections. The ABEEMσπ potential model gives reasonable properties of these clusters when compared with the present ab initio data. For interaction energies, the root mean square deviation is 0.74 kcal/mol, and the linear coefficient reaches 0.993. Next, FA in aqueous solution was also studied. The hydrogen-bonding pattern due to the interactions with water has been analyzed in detail. Furthermore, the ABEEMσπ charges changed when H₂O interacted with the FA molecule, especially at the sites where the hydrogen bonds form. These results show that the ABEEMσπ fluctuating charge model is fine giving the overall characteristic hydration properties of FA–water systems in good agreement with the high-level ab initio calculations.     

Spectra and structure of silicon containing compounds. XXXIX. Raman and infrared spectra,conformational stability,vibrational assignment and ab initio calculations of n-propyltrifluorosilane

The infrared (3100-40 cm(-1)) spectra of gaseous and solid and Raman (3200-20 cm(-1)) spectra of liquid with qualitative depolarization values and solid n-propyltrifluorosilane, CH(3)CH(2)CH(2)SiF(3), have been recorded. Additionally the infrared spectra of the sample in nitrogen and argon matrices have been recorded. Both the anti and gauche conformers have been identified in the fluid phases but only the anti conformer remains in the solid. Variable temperature (-105 to -150 degrees C) studies of the infrared spectra of the sample dissolved in liquid krypton have been recorded and the enthalpy difference has been determined to be 135+/-14 cm(-1) (1.62+/-0.17 kJ mol(-1)) with the anti conformer the more stable form. At ambient temperature it is estimated that there is 51+/-2% of the gauche conformer present. Also the enthalpy difference in the liquid was obtained from variable temperature studies of the Raman spectra and from three conformer pairs an average value of 179+/-18 cm(-1) (2.14+/-0.22 kJ mol(-1)) was obtained again with the anti form the more stable conformer. Relatively complete vibrational assignments are proposed for both conformers based on the relative infrared and Raman spectral intensities, infrared band contours, depolarization ratios which are supported by normal coordinate calculations. The geometrical parameters, harmonic force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios, and energy differences have been obtained for the anti and gauche conformers from ab initio MP2/6-31G(d) calculations. Structural parameters and energy differences have also been obtained utilizing the larger 6-311+G(d, p) and 6-311+G(2d, 2p) basis sets. By utilizing the previously reported microwave rotational constants for five isotopomers of CH(3)SiF(3) along with ab initio predicted structural values, r(0) parameters have been obtained for methyltrifluorosilane. Similarly, from the ab initio predicted parameters "adjusted r(0)" parameters have been estimated for both conformers of n-propyltrifluorosilane. The results are discussed and compared with those obtained for some similar molecules.     

碱基与氮甲基乙酰胺相互作用的从头算和ABEEMσπ研究

选取氮甲基乙酰胺(N-Methylacetamide,NMA)和DNA碱基为研究对象,应用ABEEMσπ/MM方法研究了NMA与碱基之间形成的二聚体的几何构型、电荷分布及结合能等性质,并进行了相应的从头算(abinitio)MP2水平研究,在B3LYP/6-311 ++G(d,p)水平下对结构进行了优化.将2种方法的研...     

An ab initio study of the hydrated positron

Ab initio calculations are presented for the hydration energy of the positron. Tetrahedral molecular-dipole-oriented clusters e⁺(H₂O)₄ are considered. In performing these calculations, the Hartree—Fock MO LCAO SCF approximation with the 4-31G split-valence basis set is used. The method was modified to treat the positron problem. It is shown that e⁺ in liquid water, like an electron, can be strongly solvated, with the hydration energy 0.2–0.3 eV greater than that of e⁺.     

Ab initio MO study of dipole—dipole interactions in acetonitrile dimers

MO STO-3G ab initio calculations have been carried out for the antiparallel dipole and the head-to-tail dipole model of acetonitrile dimers. The optimized interaction enthalpy is about half of the lowest experimental estimate. The calculated interaction distance for the antiparallel dipole model is very close to the sum of intermolecular radii of N and C; the distance for the head-to-tail model is about 20% higher than the sum of N and H intermolecular radii. The discussion of the interaction in terms of the supermolecule MO's suggests for both models a bonding of mainly electrostatic character. The shortcomings of the STO-3G basis set in dealing with this problem are compared with those reported in the literature. The influence of the basis set on the calculated electron distribution in acetonitrile monomer was examined as a preliminary part of the present study, and is also reported in the paper.