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The cyclization of a series of nonheterocyclic allenyl aryl ketones was examined using boron trifluoride etherate and indium triflate to mediate the reaction. Yields with BF3 were low in most instances due mainly to competitive destruction of the substrates. With In(OTf)3, there was less decomposition, and the yields of the cyclized product were much higher, but only for substrates with electron-donating substituents. Cyclization did not occur without those substituents. A computational study using the ωB97X-D/6-311+G(2d,p)//ωB97X-D/6-31+G(d,p) method confirmed better stability of the σ-complexed substrate by indium(III) and that meta-substituents on the phenyl ring of the substrate significantly influenced the activation barrier of the cyclization, whereas the effect of para-substituents was almost negligible. The computational results supported the idea that the cyclization is a 4π-electrocyclization and not a 5-endo-dig ring closure as had been proposed in the literature.

Since the 1960s, dithiolene complexes have been intensely studied; however, the same cannot be said about diselenolene complexes. Thus, the chemistry associated with the reduction of several Mo- and W-tris(diselenolene) complexes was investigated. In particular, relative reduction potentials, changes in key geometrical properties, the contribution of the metal center to the redox active MO, and HOMO–LUMO energy gaps are investigated. It is noted that the results obtained for the tris(diselenolene) complexes are compared to analogous Mo- and W-tris(dithiolene) complexes to understand the effect of substituting the sulfur atoms with selenium atoms. The reduction potentials of the complexes are more dependent upon the choice of the ligand than the metal. Overall, it is found that the substitution of chalcogen atom for the tris complexes investigated herein has only a subtle effect on the calculated reduction potentials between analogous redox couples. Upon reduction of the neutral and mono-anionic complexes, it is found that changes in key bond lengths, fold angles (θ), and trigonal twist angles (ΦAvg) are very similar for the tris(diselenolene) complexes investigated. Such changes have been previously observed for several Mo- and W-tris(dithiolene) complexes. Comparing the HOMO–LUMO energy gaps of the tris(diselenolene) complexes to the tris(dithiolene) complexes, the former complexes have on average a 0.07-eV smaller energy gap and are thus expected to be slightly more reactive to reduction. Lastly, in the neutral complexes, the Mo and W atoms contribute at most 26% to the redox active MO; thus, in the case of the tris(diselenolene) complexes, it can be concluded that the redox active MO is predominantly ligand based. The contribution of the metal-based AO becomes less as the complexes are reduced. In summary, given the high interest of dithiolene complexes in the areas of alternative energy and material science, the results presented herein provide motivation to further investigate the chemistry of diselenolene complexes.

The mechanism of the iridium-catalyzed functionalization of a primary C–H bond at the γ position of an alcohol 5 is investigated by density functional theory (DFT) calculations. A new IrIII–IrV mechanism is found to be more feasible than the previously reported IrI–IrIII mechanism. 10 In the IrIII–IrV mechanism, the reaction begins with the initial formation of (Me4phen)IrIII(H)[Si(OR)Et2]2 from the catalyst precursor, [Ir(cod)OMe]2 (cod = 1,5-cyclooctadiene). The catalytic cycle includes five steps: (1) the insertion of norbornene into the Ir–H bond to produce (Me4phen)IrIII(norbornyl)[Si(OR)Et2]2 (R = –CH(C2H5)C3H7); (2) the Si–H oxidative addition of HSi(OR)Et2 to form (Me4phen)IrVH(norbornyl)[Si(OR)Et2]3; (3) the reductive elimination of norbornane to furnish (Me4phen)IrIII[Si(OR)Et2]3; (4) the intramolecular C–H activation of the primary C–H bond at the γ position; and (5) the Si–C reductive elimination to produce the final product and regenerate the catalyst. The highest barrier in the IrIII–IrV mechanism is 7.3 kcal/mol lower than that of the IrI–IrIII mechanism. In addition, the regioselectivity of the C–H activation predicted by this new IrIII–IrV mechanism is consistent with experimental observation.

We recently described the binding of 1-β-phosphonomethylene-1-deoxy-d-glucopyranose, (S)-1-β-phosphonofluoromethylene-1-deoxy-D-glucopyranose (βG1CFSP), and (R)-1-β-phosphonofluoromethylene-1-deoxy-d-glucopyranose (βG1CFRP) to the enzyme β-phosphoglucomutase as transition state analogues of phosphoryl transfer through formation of stable MgF3− and AlF4− complexes (Proc. Natl. Acad. Sci. U.S.A. 2014, 111, 12384). Herein, we describe in detail the five-fold difference in acidity (pKa2) for the (S)- and (R)-configured diastereomeric fluorophosphonates through a series of NMR spectroscopy experiments. The differences in acidity were corroborated using computational quantum mechanical calculations to determine structures of lowest energy conformers and provide insight into why the (S) isomer is substantially more acidic.

Due to ligand non-innocence and reversible one-electron-transfer processes dithiolene complexes have been intensively studied both experimentally and computationally. While the substitution of the ligating sulfur atoms by selenium provides a means to delicately tune the behavior of dithiolene compounds, diselenolene complexes have not been as thoroughly examined. Yet, the search for such ligands has been ongoing since the 1970s. Thus, we have looked at several metal-bisdiselenolene complexes and have compared key properties of these complexes with their bisdithiolene analogues to determine the effect of substituting the chalcogen atom. The results herein show that substitution of the sulfur atoms by selenium within these complexes only subtly changes the thermodynamics and kinetic reactivity of bisdithiolene complexes while not significantly affecting the geometries of the complexes. The significance being that the relatively minor structural changes that occur upon redox is a key feature of dithiolene complexes. Due to ligand non-innocence and reversible one-electron-transfer processes dithiolene complexes have been intensively studied, however, diselenolene complexes have not. First-principles calculations show that substitution of the sulfur atoms by selenium within the investigated complexes does offer the ability to subtly tune the thermodynamics and kinetic reactivity of bisdithiolene complexes, while not significantly affecting the geometries of the complexes. © 2015 Wiley Periodicals, Inc.

Organotin compounds are very important in material design as well as in biomedical and biochemical applications. However, little is known about their BDEs experimentally or computationally. Thus, a variety of common quantum chemical methods in combination with several approaches to treating relativistic effects of the tin core electrons were used to calculate the BDEs of organotin compounds. Our results show that the BDEs are very sensitive to the choice of the computational method and to the treatment of relativistic effects.

The mutual interaction between beryllium bonds and halogen bonds within H2Be···FCl···Base complexes, where Base includes a wide set of N- and O-containing Lewis bases, has been studied at the M06-2X/6-31+G(d,p) level of theory. The reliability of this theoretical model was assessed by comparison with ab initio CCSD/aug-cc-pVTZ reference calculations. Cooperative effects were investigated within the framework of the atoms in molecules theory (AIM) by analyzing the topology of the electron density and the changes in the atomic energy components. The decomposition of the total stabilization energy into atomic components is found to be a very reliable tool to describe halogen bond interactions. Both the topological analysis of the electron density and the changes in the atomic energy components of the binding energy show the existence of strong cooperative effects between beryllium and halogen bonds, which are in some cases very intense. In general, there is a correlation between the intrinsic basicity of the Lewis base participating in the halogen bond and the resulting cooperativity in the sense that the stronger the base, the larger the cooperative effects.

Atomic energies are used to describe local stability in eight low-lying water hexamers: prism, cage, boat 1, boat 2, bag, chair, book 1, and book 2. The energies are evaluated using the quantum theory of atoms in molecules (QTAIM) at MP2/aug-cc-pVTZ geometries. It is found that the simple, stabilizing cooperativity observed in linear hydrogen-bonded water systems is diminished as clusters move from nearly planar to three-dimensional structures. The prism, cage, and bag clusters can have local water stabilities differing up to 5 kcal mol(-1) as a result of mixed cooperative and anticooperative interactions. At the atomic level, in many cases a water may have a largely stabilized oxygen atom but the net water stability will be diminished due to strong destabilization of the water's hydrogen atoms. Analysis of bond critical point (BCP) electron densities shows that the reduced cooperativity results in a decrease in hydrogen bond strength and an increase in covalent bond strength, most evident in the prism. The chair, with the greatest cooperativity, has the largest average electron density at the BCP per hydrogen bond, whereas the cage has the largest total value for BCP density at all hydrogen bonds. The cage also has the second largest value (after the prism) for covalent bond critical point densities and an oxygen-oxygen BCP which may factor into the experimentally observed stability of the structure.

Atomic energies are used to visualize the local stabilizing and destabilizing energy changes in water clusters. Small clusters, (H(2)O)(n), from n = 2-5, at MP2/aug-cc-pVTZ geometries are evaluated using energies defined by the quantum theory of atoms in molecules (QTAIM). The atomic energies reproduce MP2 total energies to within 0.005 kcal mol(-1). Oxygen atoms are stabilized for all systems and hydrogen atoms are destabilized. The increased stability of the water clusters due to hydrogen bond cooperativity is demonstrated at an atomic level. Variations in atomic energies within the clusters are correlated to the geometry of the waters and reveal variations in the hydrogen bond strengths. The method of visualization of the energy changes applied here is especially suited for application to large biomolecules.

Analogues of the antioxidant ethoxyquin are studied to determine the effects of changing the group 16 element in the reacting center of a glutathione peroxidase mimic. The energy barrier for hydrogen peroxide reduction decreases along the group, with sulfur showing the highest, selenium intermediate, and tellurium the lowest barrier. The ethyl side chain on the chalcogen is substituted with methyl and tertiary-butyl groups to determine the degree of steric effects on the barrier. A phenyl substituent results in a higher barrier than the alkyl side chains.

Density functional theory and second-order Møller–Plesset perturbation theory with effective core potentials have been used to calculate homolytic bond-dissociation enthalpies, D(Sn–X), of organotin compounds, and their performance has been assessed by comparison with available experimental bond enthalpies. The SDB-aug-cc-pVTZ basis set with its effective core potential was used to calculate the D(Sn–X) of a series of trimethyltin(IV) species, Me3Sn–X, where X = H, CH3, CH2CH3, NH2, OH, Cl, and F. This is the most comprehensive report to date of homolytic Sn–X bond-dissociation enthalpies (BDEs). Effective core potentials are then used to calculate thermodynamic parameters including donor–acceptor bond enthalpies, [Formula: see text], for a series of tin-ligand complexes, L2SnX4 (X = Br or Cl, L = py, dmf, or dmtf), which are compared with previous experimental and nonrelativistic computational results. Based on computational efficiency and accuracy, it is concluded that effective core potentials are appropriate computational methods to examine bonding in organotin systems.

Triply hydrogen-bonded complexes of the form AAA-DDD are shown to have the strongest interaction when the complex is substituted with electron withdrawing groups on the donor molecule (DDD) and electron donating groups on the acceptor molecule (AAA). In particular, the largest effects are observed when the withdrawing groups act through resonance. This serves to flatten the entire system resulting in more linear, and consequently stronger, hydrogen bonds. Furthermore, the present calculations show that the binding energy correlates with the electron density at the bond critical points and inversely with the hydrogen bond lengths.

A systematic comparison of the optimized geometries of five organotin compounds, Cl(n)Sn(CH(3))(4-n), n = 0-4, with the available gas-phase electron diffraction results is reported. All optimizations were carried out with the B3LYP density functional method. Comparison of 10 basis sets and three effective core potentials leads to the conclusion that the combination of the SDB-aug-cc-pVTZ basis set and the LANL2 effective core potential for tin, together with the 6-31G(d,p) basis set for the other atoms, is recommended for the prediction of the geometries of organotin compounds.

A type of cage critical point (CCP) that has remained so far only a ‘mathematical possibility’ is reported for the first time. A CCP enclosed by two ring surfaces is found in a single ring in each of three derivatives of 1,12-difluorobenzo[c]phenanthrene. The two fluorine nuclei are linked by a bond path which results in the closure of a seven-membered ring in the ‘bay’ region. The crowding of the fluorine atoms forces an α-helical (staircase) twisting of the molecule causing the ring surface to split into two giving rise to a CCP. The Poincare–Hopf relationship is satisfied in each case.

A bond path linking two saturated fluorine atoms is found to be ubiquitous in crowded difluorinated aromatic compounds. The bond path is shown to persist for a range of internuclear distances (2.3-2.8 A) and a range of relative orientations of the two C-F internuclear axes. The F. . .F bonding is shown to exhibit all the hallmarks of a closed-shell weak interaction. The presence of such a bond path can impart as much as 14 kcal/mol of local stabilization to the molecule in which it exists, a stabilization that can be offset or even overwhelmed by destabilization of other regions in the molecule. Several other weak closed-shell interactions were also found and characterized including F. . .C, F. . .O, and C. . .C interactions, hydrogen bonding, dihydrogen bonding, and hydrogen-hydrogen bonding. This study represents another example of the usefulness and richness of the bond path concept and of the theory of atoms in molecules in general.

Electron-withdrawing groups (EWGs) on the nitrogen atom of pyrroles have significant effects on the properties of the pyrrole. It has been suggested that the experimental 13C chemical shifts show a general increase in deshielding effect with the increase of N-EWGs strength [A. Thompson, S. Gao, G. Modzelewska, D.S. Hughes, B. Patrick, D. Dolphin, Org. Lett., 2, 3587 (2000)]. However, recently observed 15N chemical shifts of pyrroles do not correlate with the N-EWG strength. To elucidate the relationship between the electronic structures of pyrroles and their nitrogen and carbon chemical shifts, density functional theory calculations were performed on pyrroles with various substituents. A correlation between the paramagnetic shift and the 15N chemical shift was observed for the pyrroles, indicating that the nitrogen chemical shift trend for the pyrroles arises entirely from variations of the paramagnetic shift contribution. However, a general correlation between the 15N chemical shifts and the EWG strength...

We report a detailed characterization of the valence shell charge concentration (VSCC) of fluorine in methyl fluoride through the atoms in molecules theory. A comparison between the VSCCs of fluorine and neighboring elements (N, O, P, S, Cl) in the periodic table is also presented. The fluorine VSCC is shown to be tetrahedral and the most compact with the highest values of ρ and −∇ 2 ρ . Our results calculated at several post Hartree–Fock levels of theory using a large basis set show a remarkable insensitivity to the level of theory.

The performance of the DFT functionals B3LYP, B3PW91 and PBE1PBE in conjunction with selected basis sets for the prediction of 15N and 13C nuclear shieldings for small nitrogen-containing molecules is assessed. For molecules containing only singly bonded nitrogen and carbon atoms, the PBE1PBE/6-311+G(2df,p) level of theory is shown to provide reliable 15N and 13C nuclear shieldings. For the triply bonded nitrogen-containing molecules N2, CH3CN and HCN, all three levels of theory are much less satisfactory.

Density functional theory is used to study a series of model iron porphyrins in the gas phase. In the first part of this study, three range-separated hybrid density functionals developed by Chai and Head-Gordon were assessed; ωB97, ωB97X, and ωB97XD. The effects of including full Hartree-Fock exchange at long-range and dispersion corrections are reported with respect to the geometries and binding energies of oxygen to the iron porphyrin systems. The functionals all correctly predict the quintet ground state for the deoxy-iron porphyrins, where typically hybrid functionals fail and predict a triplet ground state. Including dispersion in ωB97XD is shown to give the best results for the O2 binding energy and geometrical parameters. The second part of the study employs ωB97XD to study iron porphine systems with different amino acids in the axial position. Geometrical parameters are reported and compared to experimental data, where available. Binding energies of the systems with oxygen are also reported and discussed.

The structure sensitivity and cluster size convergence for formate adsorption on the Cu(100), Cu(110) and Cu(111) surfaces have been investigated systematically using density functional theory and the cluster model containing up to 40 Cu atoms. The copper core–valence correlation effect on the adsorbate–surface interaction is examined by using three different basis sets and effective core potentials. The calculated geometries and vibrational frequencies are in good agreement with experimental data even on the small clusters and are not surface sensitive. However, the adsorption energies show strong dependence on the surface structure and the cluster size. The adsorption energies are shown to converge very well for the large clusters, and the activity of the Cu planes for formate adsorption is in the order of Cu(110)>Cu(100)>Cu(111), the same as that observed experimentally for methanol synthesis. Regardless of the basis set, cluster size and surface structure, all results show an anionic formate adsorption species. The chemisorption mechanism and the local structure of formate on the three copper surfaces are essentially very similar. Some discussion about cluster modeling is presented.

The hyperfine coupling constants (HFCCs) in the hydroxyl radical are investigated through comparison of results obtained from a variety of quantum chemical methods. The couplings obtained from the multi-reference configuration interaction (MRCI) wave function, built upon the restricted open-shell Hartree–Fock (ROHF) reference determinant, are investigated in terms of the basis set, the configuration selection energy threshold, and the size of the reference space. Overall results which converge to the experimental couplings are obtained for hydrogen, but not for oxygen. In particular, the MRCI method shows no improvement over density functional theory (the B3LYP functional), for the calculation of Aiso(17O). On the other hand, results in excellent agreement with experiment are obtained through the use of the quadratic configuration interaction (QCISD) method based on the unrestricted HF (UHF) reference determinant with the identical basis sets. Examination of UHF and ROHF based coupled-cluster methods, CCS...

Density functional theory is used to study the energetics, geometries, and hyperfine couplings in sugar radicals which are generated through irradiation of DNA. The C4‘−S and the C3‘−S radicals are determined to be the lowest lying species of the radicals formed through abstraction of a hydrogen or a hydroxyl group from a model of the sugar present in DNA, respectively. The C2‘ radical has the highest energy and the smallest degree of ring puckering of all possible carbon-centered radicals formed via hydrogen abstraction. In addition to the possible dehydrogenated and dehydroxylated products, various radicals which lead to substantial ring alterations, such as ring breaks or flattening of the ring, are also studied. In most cases, the calculated hyperfine coupling constants directly support the assignment of the experimentally observed couplings to the specific radicals. The effects of rotation about the C5‘C4‘ bond on the HFCCs in the C5‘ and O5‘ radicals are examined in order to compare the experimental...

Quantum-chemical studies (UMP2/6-311G*//UHF/6-311G*) on a series of phosphenium and arsenium derivatives confirm singlet ground states and reveal an unexpected model for the bonding in their lowest triplet state. Models of the novel solid-state dimer structures that are experimentally observed for some arsolanium cations reveal large positive dimerization energies (P, 3, 277 kJ/mol; As, 7, 207 kJ/mol), implying that the observed dimers are imposed by crystal-packing phenomena. The surprising contrast observed in the butadiene cycloaddition reactivities for phosphenium and arsenium cations are understood in terms of the calculated absolute energies of the observed structural arrangements for the cycloadducts in each case; the cheletropic adduct 4 is 52 kJ/mol more stable than the Diels−Alder adduct for the phospholanium cation, while in the case of the arsolanium cation the Diels−Alder adduct 9 is favored by 59 kJ/mol.

The geometries, relative energies, and hyperfine coupling constants of dehydrogenated, hydrogenated, and hydroxylated radiation products, as well as the anion and the cation, of thymine (T), 1-methylthymine (1-MeT), and uracil (U) are calculated through the use of density functional theory. The results for T and 1-MeT are subsequently compared to accurate ENDOR experiments performed by Sagstuen and co-workers (Sagstuen, E.; Hole, E. O.; Nelson, W. H.; Close, D. M. J. Phys. Chem. 1989, 93, 5974 and references therein). The theoretical coupling constants support the experimental assignment of the observed radicals. It is suggested that the hydrogen added to form the O4-hydrogenated product is located at an angle of 50° out of the molecular plane in T. Unfavorable interactions of this hydrogen with the N3 and the methyl hydrogens are shown to lead to this out-of-plane position for O4−H. For the similar radical in 1-MeT, hydrogen bonding in the crystal leads to an in-plane hydrogen position. Notable geometry ...

The thermal motion of CD+4 in the gas phase at 4 and 40 K is studied by means of a molecular dynamics method using the MP2/6‐31G(d,p) force field. The thermal motion of CD+4 in the temperature range from 4 to 40 K does not involve a deuterium permutation on the picosecond time scale. The electronic energy fluctuations of CD+4 due to thermal motion are less than 0.7 kcal/mol at 4 K and 1.6 kcal/mol at 40 K, which is less than the energy barrier (3.4 kcal/mol at MP2 level) required for the permutation between two energy minimum C2v structures. On the basis of these calculations it is suggested that the observation of four equivalent hydrogen atoms in the ESR spectrum of CH+4 can be attributed to fast quantum tunneling, with an estimated rate of 4.3×106 s−1 for CH+4 and 1.6×103 s−1 for CD+4.

A new class of calcium antagonists (dibenzotricyclic compounds) is studied by means of reaction field ab initio calculations and molecular dynamics simulations. The central ring of these tricyclic molecules is found to be more important to the calcium antagonistic potency than the two phenyl rings. The central ring with antagonistic potency shows hydrophobic character, thus the interaction between the drug and the binding sites is assumed to be dominated by hydrophobic interactions. Variation of the flexure angle, the angle between the two phenyl rings, does not change the hydrophobic property of the central ring significantly, therefore it is not expected to affect the interaction between the drug and binding site directly. The effect of the flexure angle on calcium antagonistic potency, the relation between drug affinity of these tricyclic molecules and their ionization energies, and the interaction of calcium ions with the central ring are discussed. © 1994 John Wiley & Sons, Inc.

The small and subtle effects of electron correlation in the Rydberg-like 3{sup 1}D and 3{sup 3}D states of the helium-like ions are studied using very accurate explicitly correlated wave functions. In particular, the effects of electron correlation on one-electron densities, probability distributions for the interelectronic distance, and their moments are examined with the help of density and Coulomb holes. Statistical electron correlation is examined with radial and angular correlation coefficients. The different methods of analysis provide complementary insight.

The influence of up to six Ne atoms on the isotropic hyperfine (hf) coupling constants of the hydrogen atoms in CH+4 is investigated using density functional theory. It is found that at very short NeH distances (R = 1.8 A), the hf structure is strongly affected, whereas at the more realistic van der Waals distance (R ≈ 2.5 A), the methane radical experiences a very weak field from the surrounding Ne atoms. At larger distances, the effects of the Ne matrix are negligible. It is furthermore found that at small R, the changes in aiso (H) are strongly dependent on the relative positions of the Ne atoms.

Statistical electron correlation coefficients provide overall measures of the difference between the electron pair density and the product of one-electron densities. Explicitly correlated wave functions are used to compute radial and angular correlation coefficients for 29 low-lying states (n1S, n3S, n1P, n3P, n1D, and n3D with n < 7) of the two-electron ions from helium through Ne8+. This data base of 1566 correlation coefficients permits a systematic study of trends with respect to variation of nuclear charge, degree of excitation, angular momentum, and spin multiplicity. © 1993 John Wiley & Sons, Inc.

Allylperoxyl radicals undergo a 1,3-transfer of oxygen, and both a concerted mechanism and a mechanism involving fragmentation of the peroxyl and readdition of oxygen have been previously proposed. In our theoretical study of this rearrangement, calculations were performed at the unrestricted Hartree-Fock, MP2, and MP3 levels: standard 6-31G(d) basis sets were used for geometry optimizations and pathway studies; single point energies for all optimized structures were obtained at the MP2/6-31G(d,p)//H-F/6-31G(d) level and for some structures at the MP3/6-31G(d,p)//HF/6-31g(d) level

The direct oxidation of ebselen and several derivatives by hydrogen peroxide is investigated using the B3LYP/6-31G(d,p) method to elucidate the effects of substituents on GPx-like activity. While previous studies have attributed the differences in GPx activity of substituted ebselen compounds to the electronic nature of the substituents, the influence of functional groups is poorly understood. The effects of various solvents are incorporated by employing the CPCM method. It is shown that a substituent in the ortho position to the selenium atom sterically hinders attack of a nucleophile at selenium and thus increases the barrier to reaction. The observed increase in GPx-like activity of an ebselen derivative with an ortho substituent is explained by the fact that the steric hindrance prevents thiol exchange reactions.

Configuration interaction calculations have been carried out for low-lying singlet and triplet states of LiH({sup 1,3}{Sigma}{sup +}), BH({sup 1,3}II), NH({sup 1}{Delta}, {sup 3}{Sigma}{sup {minus}}), and FH({sup 1,3}II), with the aim of evaluating singlet-triplet energy component differences. These calculations represent the first systematic study of the quantum mechanical interpretation of Hund's rule in a series of molecules. While early molecular work on Hund's rule suggested that the electron-nuclear attraction may always be deeper in the triplet than in the singlet, exceptions are found in the present work. Also, in certain cases, where configuration mixing and molecular charge separation play a key role, the singlet electron-electron repulsion exceeds that of the triplet.

Weakly bonded complexes of water with oxygen and nitrogen are studied using ab initio methods; their thermodynamic functions and infrared spectra are calculated. Infrared spectra for these complexe...

A mechanism involving a six-membered cyclic transition state where the hydrogen of the hydroxyl group interacts with the oxygen of the carbonyl group has been proposed previously to describe the thermolysis of many beta-hydroxyl compounds. In this paper, the proposed mechanism is studied for a series of beta-hydroxyl aldehydes. Rate constants and activation energies are reported as well as a study of the influence of tunneling on the reaction rates. The electron density at the ring critical points, population analyses by the theory of atoms in molecules (AIM) and the natural bond orbital (NBO) method, as well as atomic energy analyses are used to gain insight into this interesting mechanism and into the effects of substituents.

This paper describes a new empirical approach for the evaluation of fluorine-fluorine spin-spin coupling constants (J(FF)) in aromatic compounds. The correlations between J(FF) and the delocalization index calculated within the framework of the theory of atoms in molecules (AIM) and with the fluorine-fluorine internuclear separation are investigated. Both the internuclear separation and the delocalization index are found to be highly correlated with J(FF). A regression model in which the experimental J(FF) coupling constant is fitted exponentially to the internuclear separation and linearly to the delocalization index yields a squared correlation coefficient as high as 0.96 for a data set consisting of 33 coupling constants spread over a range of 85 Hz.

The interaction of BeX(2) (X = H, F) with water molecules has been analyzed at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. The formation of strong beryllium bonds between water molecules and the BeX(2) derivative triggers significant electron density redistribution within the whole system, resulting in significant changes in the proton donor and proton acceptor capacity of the water molecules involved. Hence, significant cooperative and anti-cooperative effects are present, explaining why there is no case in which the global minimum corresponds to a tetracoordinated beryllium atom. In fact, the most stable clusters can be viewed as the result of the attachment of BeX(2) to the water trimer and the water dimer, respectively, and not as the result of the solvation of the BeX(2) molecule. We have also shown that the decomposition of the interaction energy into atomic components is a reliable quantitative tool to describe all the closed-shell interactions present in the clusters investigated herein, namely hydrogen bonds, beryllium bonds and dihydrogen bonds. Indeed, we have shown that the changes in the atomic energy components are correlated with the changes in the strength of these interactions, and they provide a quantitative measure of cooperative effects directly in terms of energies.

The bonding interaction between non-hydrogen-bonded (NHB) hydrogen atoms and the non-nuclear attractor (NNA) observed in cavity-bound anionic water clusters has been investigated using ab initio methods. The clusters range in size from four to ten water molecules and generally consist of several water molecules hydrogen-bonded in a ring, with two of these rings stacked such that the NHB hydrogen atoms are directed towards a central cavity. Analysis of the electron density using the theory of atoms in molecules yields information about the bonding interaction that cannot be obtained with conventional computational techniques. The results indicate that this novel interaction, Ha...NNA, is a weak, closed-shell interaction. Further analysis of the atomic properties of the NHB hydrogen atoms indicate that this novel bond cannot be classified as a hydrogen bond. On the basis of the results from the current investigation, this interaction is best described as a weak dipole-ion interaction between the positively charged NHB hydrogen atoms and the negatively charged NNA.

Extensive abinitio molecular orbital calculations are reported for the hydrogen bonded complexes of the formula [Formula: see text] and, [Formula: see text] where m = 0, 1, 2, and 3. The one-proton ("linear") hydrogen bonded complexes of the ammonium ions and hydrogen fluoride are predicted to be more stable by about 1 or 2 kcal/mol than the two- and three-proton ("bent") complexes. Several systematic trends and a number of differences between the neutral and protonated series of complexes are observed. The basis set dependence of the results, basis set superposition errors and electron correlation effects are discussed.

Analytical expansions for the radial density D(r), as calculated within the Hartree–Fock approximation, are reported for the neutral atoms from helium to xenon. As part of a brief discussion of one-electron properties which may be calculated from D(r) or the related density distribution ρ(r), expectation values for rn, n = −2, −1, 1, 2, 3, 4, and 5 are presented.

A recent proposal for the evaluation of the shape of the Fermi hole in atoms is investigated. It is pointed out that the arbitrariness inherent in the Hartree-Fock orbitals introduces an ambiguity in the definition, which can be overcome by using the unorthogonalized Hartree orbitals. This is still not quite satisfactory, the use of a proper Hartree-Fock as well as a proper Hartree wavefunction, each independently optimized, being essential for obtaining the shape of the Fermi hole. Calculations demonstrating these points are presented for the 23S state of the helium isoelectronic sequence.

The density difference ΔD(r1), defined as the difference between the exact and the Hartree-Fock radial density functions is reported for the 2 3S and the 2 1S excited states of the helium atom. The density differences in both cases resemble the inverse of the helium ground-state density difference, but are more long-ranged due to the diffuseness of the excited states. The effect of electron correlation on one-electron distributions is also demonstrated by comparison of (r1n) expectation values and several indices of the relative sizes of atoms. Electron correlation leads to a contraction of the Hartree-Fock electron clouds.

The topological properties of the charge density of the hydrogen-bonded complexes between nitrites and hydrogen chloride correlate linearly with theoretical estimates of the hydrogen-bond energy. At the 6-31G** level, the hydrogenbond energies range from a low of 10 kJ mol m NCCN—HC1 to a high of 38 kJ mol in LiCN—HCl. A linear relationship between the charge density at the hydrogen-bond critical point and the NH internuclear distance of the RCN—HC1 complexes indicates that the generalization of the bond-length-bond-order relationship of CC bonds due to Bader, Tang, Tal and Biegler-Konig can be extended to intermolecular hydrogen bonding.

The pair distribution functions evaluated for the 23S state of the helium isoelectronic sequence from the Hart and Herzberg correlated wave functions and those corresponding to the Hartree-Fock approximation are used to determine the shape of the corresponding Coulomb holes. As a consequence of a discontinuity in the Hartree-Fock solution between He and Li+, the Coulomb hole has a different shape for He than for Li+ and the other isoelectronic ions.

The relative sizes of the neutral from hydrogen to xenon are discussed in terms of several properties based on the density distribution. Of the various indices of atomic size considered, a density contour approach is shown to be most consistent with the periodic behaviour of chemical and physical properties and to allow satisfactorily for the arbitrariness inherent in any theoretical approach to atomic dimensions. By means of an empirical scaling procedure, the theoretical radii are scaled to the range of values common to empirical radii.