The quantum chemical calculation of NMR two-bond spin–spin coupling constants in the N?···H–OH?→?N–H···OH? switching

A quantum chemical study has been carried out to investigate the effects of the size of H₂O cluster and substituents (X = H, Me, OMe, CHO, NO, and NO₂) in the para position of the anilide ion on the two-bond spin–spin coupling constants (SSCCs) ^2h J _N···O across in the N–H–O switching at B3LYP/6-311 ++G(2d,2p) level of theory. The changes in ^2h J _N···O SSCCs due to variation of substituent and H₂O cluster size were well monitored by changes in binding energy, structural parameter, electron density topography, natural charge, charge transfer, and percentage of p-character of N atom in the C–N bond. Linear correlations were found between ^2h J _N···O and above-mentioned properties.     

A quantum chemical study of the infrared absorption intensities of the isoelectronic C2v systems H2F+, H2O and NH−2

The results of a comparative theoretical study of the dipole moment derivatives and infrared absorption intensities within the double harmonic approximation are presented for the isoelectronic, isostructural C_2v molecules: H₂F⁺, H₂O and NH⁻₂. The calculations, performed at the ab initio SCF and CI levels of theory, utilize basis sets of triple zeta+two polarization functions quality. For the ions H₂F⁺ and NH⁻₂, in the absence of adequate experimental data the equilibrium geometries and force constants were also calculated. The trends observed in the dipole moment derivatives for the three systems are indicative of the amount of electronic charge associated with the hydrogen atoms and are very similar to the trends noted for a set of C_3v hydrides.     

Simultaneous interactions of pyridine with substituted benzene ring and H–F in X-ben⊥pyr···H–F complexes: a cooperative study

The cooperative effects between T-shape stacking and hydrogen bond interactions in X-ben⊥pyr···H–F complexes were investigated in this work. The results indicate that the electron-withdrawing/donating substituents decrease/increase the magnitude of the binding energies compared to the unsubstituted X-ben⊥pyr···H–F (X = H) complex. The cooperative effects have been studied while using the atoms in molecules (AIM) and natural bond orbital (NBO) methods, allowing us to evaluate the interplay between T-shape stacking and hydrogen bond interactions. There are good relationships among binding energies, Hammett constants, geometrical parameters, and the results of AIM and NBO analysis in X-ben⊥pyr···H–F complexes.     

A quantum chemical study of aluminum–carbon bonds in three-coordinate aluminum compounds

The spatial and electronic structures of three-coordinate aluminum molecules with Al–C bonds are calculated using the GAMESS-Firefly program package at DFT and HF levels of theory. By NBO and AIM methods the main characteristics of Al–C bonds in these molecules are determined. It is shown that by their topological characteristics the Al–C bonds can be characterized as weakened intermediate bonds close to the bonds between closed shell atoms.     

A quantum chemical study of aluminum–nitrogen bonds in three-coordinate aluminum compounds

DFT and HF methods implemented in the PC GAMESS-Firefly program package are employed to calculate the spatial and electronic structures of molecules of three-coordinate aluminum compounds with Al–N bonds. NBO and AIM methods are applied to determine the main characteristics of Al–N and N–C bonds in these molecules. It is shown that Al and N atoms interact with each other as closed shell atoms. The N–C bonds are close to covalent ones.     

Physical–chemical study of water in contact with a hydrophilic polymer: Nafion

The study detailed in this paper is about the determination of the physical–chemical parameters of water, after keeping it in prolonged contact with the Nafion polymer. The parameters under study are: electrical conductivity, χ (μS cm^?1); heat of mixing with acid (HCl), ΔQ _mix ^HCl (J kg^?1) or basic (NaOH) solutions, ΔQ _mix ^NaOH  (J kg^?1), and pH. χ increases of up to two orders of magnitude, ΔQ _mix ^NaOH  (J kg^?1) is exothermic and increases as the electrical conductivity increases, with a roughly linear trend, up to one order of magnitude. The analogous ΔQ _mix ^HCl  (J kg^?1), on the contrary, is found to be null. The pH is quite acid and shows a very good linear correlation with log χ. The linear correlations hint at a single cause for the variation of the three very different physical–chemical parameters. This complex and hard to rationalize phenomenology, finds a good theoretical support in the work hypothesis of the formation of dissipative structures within the liquid. These are far-from-equilibrium systems outside the paradigm of classical thermodynamics. The work hypothesis of the formation of molecular aggregates of water molecules (dissipative structures, aqueous nanostructures, clusters, coherence domains, etc.) is shared with two other aqueous systems obtained with different preparation protocols, so we briefly recall them here: (1) EDS (extremely diluted solutions): obtained through an iterative process of successive dilutions and agitations. (2) IFW (iteratively filtered water): obtained through an iterative process of successive filtrations through sintered glass filters. (3) INW (iteratively nafionized water): obtained through an iterative process of successive drying and wetting of the Nafion polymer. Each protocol produces water exhibiting its own peculiarities, to the point that they can be considered different, albeit with the common element of a variation of the super-molecular structure of the water solvent. The physical–chemical properties of these perturbed waters cannot be framed by the paradigm of classical thermodynamics, but rather require the use of the thermodynamics of systems far from the equilibrium and of irreversible processes.     

Structural variation, π-charge transfer, and transmission of electronic substituent effects in (E)-β-substituted styrenes: a quantum chemical study

The nature and transmission mechanism of substituent effects in (E)-β-substituted styrenes, C₆H₅–CH=CH–X, have been investigated from the structural changes induced by a variable substituent on the phenyl group. The molecular structures of 46 (E)-β-substituted styrenes were determined from MO calculations at the B3LYP/6-311++G** level of theory. The structural variation of the phenyl probe is best represented by two orthogonal linear combinations of the internal ring angles, S _F ^STY and S _R ^STY . Regression analysis of S _F ^STY using appropriate explanatory variables reveals a composite field effect, the main component of which originates from the long-range effect of the substituent enhanced by field-induced π-polarization of the vinylene spacer and resonance-induced field effects. The electronegativity of the substituent also plays a role in determining the value of S _F ^STY . Comparison with coplanar 4-substituted biphenyls reveals that the components of the field effect in the two molecular systems are of the same nature (apart from the electronegativity contribution, which is not present in biphenyl derivatives). However, the structural variation of the phenyl probe is more pronounced in (E)-β-substituted styrenes due to the shorter distance between substituent and probe. Analysis of π-charge distribution shows that the aptitude of the substituents to exchange π-electrons with the styrene and coplanar biphenyl frames is nearly the same. Nevertheless, the π-charge variation on the phenyl probe of (E)-β-substituted styrenes is 57 % greater than the corresponding quantity in coplanar 4-substituted biphenyls. Thus, the vinylene spacer is more effective than the phenylene spacer in transmitting π-charges. The S _R ^STY parameter is related to the amount of π-charge transferred from the –CH=CH–X moiety into the π-system of the benzene ring, or vice versa, due to the resonance effect of the variable substituent and, to a lesser extent, to field-induced π-polarization.     

Ab initio study of the cooperativity between NH···N and NH···C hydrogen bonds in H3N–HNC–HNC complex

Ab initio calculations at the MP2/aug-cc-pVTZ level have been performed to study the cooperativity of hydrogen bonds in homoclusters (HNC–HNC–HNC and HNC–HNC–HNC–HNC) and heteroclusters (H₃N–HNC–HNC and H₃N–HNC–HNC–HNC). The cooperative energies in the HNC–HNC–HNC and H₃N–HNC–HNC trimers are –2.05 and –2.56 kcal/mol, respectively. The result shows that the cooperativity in the heterotrimer is larger than that in the homotrimer. A similar result also happens in the tetramers. The energy decomposition scheme indicates that orbital interaction is a major contribution to the cooperative energy of N···HN hydrogen bond, whereas the electrostatic and orbital interactions to that of C···HN hydrogen bond. The effect of HNC chain length on the strength of N···HN hydrogen bond has also been considered at the MP2/aug-cc-pVDZ level. It is indicated that the interaction energy of N···HN hydrogen bond trends to be a fixed value when the HNC number tends to be infinite, and the strength of N···HN hydrogen bond is regulated mainly through the electrostatic and polarization interactions although the charge transfer interaction also has an effect on it.     

Host–guest complexes of mixed glycol-phenanthroline cryptands: prediction of ion selectivity by quantum chemical calculations IV

Abstract Structures and complex-formation energies, calculated with DFT (B3LYP/LANL2DZp) for the cryptands [2.2.phen] and [2.phen.phen] with endohedrally complexed alkali and alkaline earth metal ions, were utilized to predict their ion selectivity. Both cryptands [2.2.phen] and [2.phen.phen] have a cavity size smaller than [2.2.2], [phen.phen.phen] and [bpy.bpy.bpy], and prefer to bind K⁺ and Sr²⁺, whereas [2.2.phen] that is larger than [2.phen.phen], has a preference for Ba²⁺, and [2.phen.phen] favours Na⁺ and Ca²⁺. The cryptand flexibility is mainly attributed to the presence of CH₂–N_SP3···N_SP3–CH₂ groups. Graphical abstract Host–Guest Complexes of mixed Glycol-Phenanthroline Cryptands—Prediction of Ion Selectivity by Quantum Chemical Calculations III Ralph Puchta* and Rudi van Eldik Keywords Cation selectivity Host–guest DFT DFT-studies allow a sensitive analysis of selectivity and cage size. Calculations predict a favourable binding of K⁺, Sr²⁺ and Ba²⁺ by [2.2.phen], and binding of K⁺, Na⁺, Ca²⁺ and Sr²⁺ by [2.phen.phen]. The cryptands fold around the ions by twisting their torsion angles in order to reach the best coordination mode for each cation. For “Prediction of ion selectivity by quantum chemical calculations III” see, R. Puchta, R. van Eldik. Aust. J. Chem. 60, 889–897 (2007).     

Mechanism of a chemical classic: quantum chemical investigation of the autocatalyzed reaction of the serendipitous wöhler synthesis of urea

The detailed reaction pathways for the ammonium cyanate transformation into urea (W?hler's reaction) in the gas phase, in solution, and in the solid state have exhaustively been explored by means of first-principles quantum chemical calculations at the B3LYP level of theory using the 6-31G(d,p) basis set. This serendipitous synthesis of urea is predicted to proceed in two steps; the first step involves the decomposition of the ammonium cyanate to ammonia and isocyanic or cyanic acid, and the second one, which is the main reaction step (and probably the rate-determining step), involves the interaction of NH(3) with either isocyanic or cyanic acid. Several alternative pathways were envisaged for the main reaction step of W?hler's reaction in a vacuum involving the formation of "four-center" transition states. Modeling W?hler's reaction in aqueous solution and in the solid state, it was found that the addition of NH(3) to both acids is assisted (autocatalyzed) by the active participation of extra H(2)O and/or NH(3) molecules, through a preassociative, cooperative, and hydrogen-transfer relay mechanism involving the formation of "six-center" or even "eight-center" transition states. The most energetically economic path of the rate-determining step of W?hler's reaction is that of the addition of NH(3) to the C=N double bond of isocyanic acid, directly affording urea. An alternative pathway corresponding to the anti-addition of ammonia to the Ctbd1;N triple bond of cyanic acid, yielding urea's tautomer HN=C(OH)NH(2), seems to be another possibility. In the last case, urea is formed through a prototropic tautomerization of its enolic form. The energies of the reactants, products, and all intermediates along with the barrier heights for each reaction path have been calculated at the B3LYP/6-31G(d,p) level of theory. The geometry optimization and characterization of all of the stationary points found on the potential energy hypersurfaces was performed at the same level of theory.     

Progress in the non-equilibrium vibrational kinetics of hydrogen in magnetic multicusp H− ion sources

The non-equilibrium vibrational kinetics of H₂ in multicusp magnetic discharges has been studied by improving a previous model developed by our groups. In particular, a complete set of V-T (vibrational translation) rates involving H-H₂(v) collisions, calculated by using a three-dimensional dynamics approach, has been inserted into our self-consistent model for better representing the corresponding relaxation. Different experimental situations are simulated with special emphasis on the temporal scales necessary for the different distributions (electron energy and vibrational distributions) to reach stationary values. Finally, a comparison between theoretical and experimental quantities such as vibrational temperature, electron temperature, electron number density and concentration of negative ions (H⁻) shows a satisfactory agreement, thus indicating the basic correctness of our model.     

Intramolecular O-H··N hydrogen bonds in 2-hydroxybenzaldimine compounds: Spectroscopic and quantum chemical investigations

Summary Spectroscopic and quantum chemical data that characterize the intramolecular hydrogen bonds in a series of 2-hydroxybenzaldimine compounds (Ph(OH)(CH=N-R);R=-NH(Ph), -OH,-OCH₃, -NH₂, -Ph, -CHO, -H, -CH₃, -(CO)(CH₃) are reported. Optimized geometries and vibrational spectra were calculated at HF/6-31G(d,p) and B3LYP/6-31G(d,p) levels, NMR spectra at GIAO-BLYP/6-311++G(d,p)//B3LYP/6-31G(d,p) level of theory. The sequences of calculated frequencies (v(OH) andv(CN)) and chemical shifts ((OH)) agree reasonably well with the corresponding experimental data. The consistency of the calculated data is demonstrated by exploiting several correlations between bond distances, vibrational frequencies, chemical shifts, and hydrogen bond energies. In particular, there exists an almost perfect relation between the hydrogen bond strengths, as measured byr(O-H) distances, and the hydrogen bond distancesr(H··N) andr(O··N) It is shown that electrostic potentials and several kinds of partial charges (Mulliken, CHELPG, MK, and NPA) of the nitrogen atoms, to a first approximation, may serve as a means for characterizing the proton acceptor capabilities of the different imino groups.

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Intramolekulare O-H··N-Wasserstoffbrückenbindungen in 2-Hydroxybenzaldiminverbindungen: Spektroskopische und quantenchemische Untersuchungen

Zusammenfassung Intramolekulare Wasserstoffbrückenbindungen in einer Reihe von 2-Hydroxybenzaldiminverbindungen (Ph(OH)(CH=N-R);R=-NH(Ph), -OH, -OCH₃, -NH₂, -Ph, -CHO,-H, -CH₃, -(CO)CH₃) werden mit Hilfe von spektroskopischen und quantenchemischen Daten charakterisiert und untersucht. Optimierte Geometrien und Schwingungsspektren wurden mittelsab initio (HF/6-31G(d,p)) und DFT Methoden (B3LYP/6-31G(d,p)) berechnet, NMR-Spektren mittels GIAO-BLYP/6-311++G(d,p)//B3LYP/6-31G(d,p). Die berechneten Frequenzen (v(OH) undv(CN)) sowie die chemischen Verschiebungen ((OH)) stimmen mit den entsprechenden experimentellen Daten gut überein. Die innere Konsistenz der Rechnungen wird durch verschiedene Korrelationen zwischen Bindungsabständen, Schwingungsfrequenzen, chemischen Verschiebungen und Wasserstoffbrückenbindungsenergien demonstriert. Insbesondere findet man eine nahezu perfekte Relation zwischen den Bindungsabständenr(O-H), die als Maß für die Stärken der Wasserstoffbrückenbindungen dienen, und den Wasserstoffbrückenbindungsabständenr(H··N) undr(O··N). Es wird gezeigt, daß die Protonenakzeptorfähigkeiten der unterschiedlichen Iminogruppen in erster Näherung durch elektrostatische Potentiale und verschiedene Partialladungen (Mulliken, CHELPG, MK und NPA) der Stickstoffatome charakterisiert werden können.

     

A quantum chemical study on the reactivity of four licorice flavonoids scavenging ·OOCl3C

Structural Chemistry - Isoliquiritigenin, liquiritigen, uralenol, and neouralenol are four natural licorice flavonoid compounds extracted from licorice, which have the ability of scavenging free...     

Directionality of intermolecular C–F···F–C interactions in crystals: experimental and theoretical charge density study

Bonding intermolecular F···F interactions were analyzed in crystal of a fluorinated benzene derivative using topological analysis of experimental charge density distribution. It was found that the values of electron density and potential energy density in bond critical points of the F···F interactions in crystal depended on the distance between the fluorine atoms, rather than on the corresponding C–F···F angles. The directionality of these F···F interactions was demonstrated by DFT-D and MP2 calculations for model hexafluorobenzene dimers. Possible discrepancies between dimerization energies computed by ab initio methods and via empirical correlations are discussed.     

FTIR study of the sol–gel synthesis of cementitious gels: C–S–H and N–A–S–H

The study explored the compatibility between the main product of Portland cement hydration and the main product of the alkali activation of fly ash: C–S–H and N–A–S–H gels, respectively. Both gels were synthesized with laboratory reagents at different pH values. Blends of the two were synthesized as well, using the sol–gel procedure. All the gels were characterized with Fourier transform IR spectroscopy (FTIR). The gels synthesized with this procedure were shown to precipitate together with a silica-rich gel. In addition, the pH level was found to play a determinant role in both C–S–H and N–A–S–H gel synthesis. The C–S–H gel is the major phase formed at pH > 11 and N–A–S–H gel for pH > 12. The results relating to the joint synthesis of the two (C–S–H and N–A–S–H) gels were not conclusive. Technique used for the characterization failed to differentiate between them in the blended material.

A quantum chemical study of the infrared absorption intensities of the isolectronic C3v systems NH3, H3O+ and CH−3

The dipole moment derivatives and the infrared absorption intensities for the isoelectronic, isostructural species NH₃, H₃O⁺ and CH⁻₃, calculated by ab initio quantum methods within the double harmonic approximation, are reported. The calculations were performed at the SCF, CI and CPA″ levels of theory using basis sets of triple zeta+two polarization functions quality. For the ions H₃O⁺ and CH⁻₃, in the absence of adequate experimental information, the calculations are fully ab initio, since the equilibrium geometries as well as the force constants had to be computed. The applicability of the harmonic treatment to systems with inversion potentials is discussed, especially with regard to H₃O⁺. The dipole moment derivatives of the three systems show interesting, regular trends in accordance with the amount of electronic charge associated with the hydrogen atoms.     

Vibrational study of the contact ion pair formation in the NaClO3–DMF system

Infrared and Raman studies of sodium chlorate in N,N-dimethylformamide (DMF) at different concentrations allowed to observe changes that are interpreted in terms of the contact ion pair formation. As the DMF is an ionising solvent it was possible to observe the equilibrium between associated and non-associated chlorate. In addition, depolarisation measurements were used to distinguish between the asymmetric and symmetric band representations in almost all regions of this anion. The vibrational assignment is entirely satisfactory indicating that in the Na⁺ClO₃⁻ contact ion pair the local symmetry around ClO₃⁻ changes from C_3v to C_S. In addition, our results do not show any evidence for the formation of solvent separated ion pairs and aggregates in this system.     

Theoretical study of the N—H···O red-shifted andblue-shifted hydrogen bonds

Theoretical calculations are performed to study the nature of the hydrogen bonds in complexes HCHO···HNO, HCOOH···HNO, HCHO···NH3, HCOOH···NH3, HCHO···NH2F and HCOOH···NH2F. The geomet- ric structures and vibrational frequencies of these six complexes at the MP2/6-31 G(d,p), MP2/6-311 G(d,p), B3LYP/6-31 G(d,p) and B3LYP/6-311 G(d,p) levels are calculated by standard and counterpoise-corrected methods, respectively. The results indicate that in complexes HCHO···HNO and HCOOH···HNO the N—H bond is strongly contracted and N—H···O blue-shifted hydrogen bonds are observed. While in complexes HCHO···NH3, HCOOH···NH3, HCHO···NH2F and HCOOH···NH2F, the N—H bond is elongated and N—H···O red-shifted hydrogen bonds are found. From the natural bond orbital analysis it can be seen that the X—H bond length in the X—H···Y hydrogen bond is controlled by a balance of four main factors in the opposite directions: hyperconjugation, electron density redistribu- tion, rehybridization and structural reorganization. Among them hyperconjugation has the effect of elongating the X—H bond, and the other three factors belong to the bond shortening effects. In complexes HCHO···HNO and HCOOH···HNO, the shortening effects dominate which lead to the blue shift of the N—H stretching frequencies. In complexes HCHO···NH3, HCOOH···NH3, HCHO···NH2F and HCOOH···NH2F where elongating effects are dominant, the N—H···O hydrogen bonds are red-shifted.