The manuscript studies the enantioselectivity and stereoselectivity of Diels-Alder (DA) cycloaddi... more The manuscript studies the enantioselectivity and stereoselectivity of Diels-Alder (DA) cycloadditions between cyclopentadiene (CPD) and a variety of dienophiles (ranging from halo-ethenes to cyano-ethenes), under oriented external electric fields (OEEFs). Applying OEEFs oriented in the X/Y-directions, perpendicular to the reaction axis (Z), will achieve complete isomeric and enantiomeric discrimination of the products. Unlike the Z-OEEF, which involve charge-transfer from the diene to the dienophile, and thereby brings about catalysis due to increased intramolecular bonding, an OEEF along X, aligned with the C=C bond of the dienophile, will lead to R/S enantiomeric discrimination by means of intramolecular-bond polarization. A Y-field will discriminate endo/exo stereoisomers in a similar mechanism. The XY field-combination will resolve both R/S and endo/exo. The resolution is complete and can be achieved at will by flipping the direction of the field along the X and Y axes. The preconditions for achieving the enantiomeric and isomeric discrimination are discussed, and require fixing of the CPD onto a surface. In so doing the chiral discrimination is achieved by dipole-moment selection rules, such that the field filters out one of the enantiomers, which is highly raised in energy by dipole selection. The dependence of the discrimination on the polarity of the dienophiles leads to a predictive trend.
We have carried out nonempirical (4-31G) and semiempirical (MNDO and AM1) calculations for pyrazo... more We have carried out nonempirical (4-31G) and semiempirical (MNDO and AM1) calculations for pyrazole. imidazole, and their 1-vinyl derivatives. The SCF calculations significantly overestimate the value of the orbital energy for n-type MO's. For the 1-vinylpyrazole molecule, all the methods incorrectly give the sequence of energy levels. The best agreement with experimental ionization potentials is obtained when using a Green's function approximation for the outer valence MO's together with the AM1 method.
The nature of the single bond in the three isoelectronic coinage metal dimers Cu2, Ag2 and Au2 is... more The nature of the single bond in the three isoelectronic coinage metal dimers Cu2, Ag2 and Au2 is investigated by means of the ab initio Breathing Orbital Valence Bond (BOVB) method, which allows one to calculate the respective contributions of the covalent and ionic structures to the total wave function, as well as the resonance energy arising from their mixing. It is shown that the BOVB method at its highest level provides bond dissociation energies in very good agreement with reference CCSD(T) values for the three dimers. It is also found that the covalent/ionic resonance energy is important in all three cases, contributing to 40-50% to the total bonding energy, thus qualifying the bonds in Cu2 and Au2 as quasi-charge-shift bonds, and that of Ag2 as a borderline case in-between classical covalent bond and charge-shift one. These results are further confirmed by analyses of the wave functions in terms of the Atoms-in-Molecule theory, which show that the Laplacian of the density at the bond critical point is large and positive in all three cases, which classifies the three bonds as charge-shift bonds within this theory.
Journal of the Chemical Society, Perkin Transactions 2, 1995
This paper addresses the question of an electron transfer mechanistic manifold by ab initio compu... more This paper addresses the question of an electron transfer mechanistic manifold by ab initio computations of the model systems Nu: + C2H6˙+→ Nu˙++ C2H6 where, NuH2S, PH3, H2O and NH3. The computations show that there exist two major classes of ET mechanisms. The first is a concerted ET mechanism which proceeds along a maximum bonding trajectory. The second class is a stepwise ET mechanism which involves shuttles of redox pairs, e.g., H+/H˙ and CH3+/CH3+˙, and thereby results in a single electron transfer from the nucleophile to the cation radical. Thus, an apparent ET may be a net result of consecutive steps which by themselves are non-ET steps. The genesis of the mechanistic manifold and variable transition state structure from valence bond (VB) configurations is discussed. It is found that all these mechanisms are typified by electronic structures that share the same set of VB configurations with variable proportions, giving rise thereby to variable transition state structure and an ET mechanistic family. The situation is reminiscent of the mechanistic manifold encountered in physical organic chemistry, e.g., the SN2 and SN1 mechanisms in the classical nucleophilic substitution process. For all the Nu:/C2H6˙+ combinations we also identified outersphere transition state analogues that avoid the bonding between the reactants. All the outer-sphere saddle points are found to be higher in energy than the bonded mechanisms of the concerted and stepwise varieties. It appears therefore, that outer-sphere mechanisms should be regarded as default options rather than natural mechanisms of ET-reactions of cation radicals. Indeed, all the computational trends exhibited by the ET mechanistic manifold are shown to be the consequences of maximum bonding.
The wide-ranging applications of the halogen bond (X-bond), notably in self-assembling materials ... more The wide-ranging applications of the halogen bond (X-bond), notably in self-assembling materials and medicinal chemistry, have placed this weak intermolecular interaction in a center of great deal of attention. There is a need to elucidate the physical nature of the halogen bond for better understanding of its similarity and differences vis-à-vis other weak intermolecular interactions, e.g. hydrogen bond, as well as for developing improved force-fields to simulate nano-and bio-materials involving X-bonds. This understanding is the focus of the present study that combines the insights of a bottom-up approach based on ab initio valence bond (VB) theory, and the block-localized wave-function (BLW) theory that uses monomers to reconstruct the wave function of a complex. To this end and with an aim of unification , we studied the nature of X-bonds in 55 complexes enumerated in Scheme 1 using the combination of VB and BLW theories. Our conclusion is clear-cut, most of the X-bonds are held by charge transfer interactions (i.e., intermolecular hyperconjugation) as envisioned more than 60 years ago by Mulliken. This is consistent with the experimental and computational findings that X-bonds are more directional than H-bonds. Furthermore, the good linear correlation between charge transfer energies and total interaction energies partially accounts for the success of simple force fields in the simulation of large systems involving X-bonds.
The ionization potentials of different molecules have been calculated with the outer valence Gree... more The ionization potentials of different molecules have been calculated with the outer valence Green's function (OVGF) technique, coupled with semiempirical MNDO, AM1 and PM3 methods. It is found that the OVGF method gives significantly better agreement with the experimental data than do results obtained with semiempirical calculations using Koopman's theorem including a new SAM1 and MNDO/d methods. Of the three semiempirical methods tested (MNDO, AMl, PM3) the OVGF (AMl) method gives the best agreement with experiment. 0 1997 Elsevier Science B.V.
ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance t... more ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
Accurate gas-phase and solution-phase valence bond calculations reveal that protonation of the hy... more Accurate gas-phase and solution-phase valence bond calculations reveal that protonation of the hydroxyl group of aliphatic alcohols transforms the C-O bond from a principally covalent bond to a complete charge-shift bond with principally "no-bond" character. All bonding in this charge-shift bond is due to resonance between covalent and ionic structures, which is a different bonding mechanism from that of traditional covalent bonds. Until now, charge-shift bonds have been previously identified in inorganic compounds or in exotic organic compounds. This work showcases that charge-shift bonds can occur in common organic species.
We model the interaction of side-chain and end-chain groups of poly(4-vinylpyridine) by a 5:1 mol... more We model the interaction of side-chain and end-chain groups of poly(4-vinylpyridine) by a 5:1 molar ratio mixture of 4-isopropylpyridine (side-chain model) and 4-propylpyridine (end-chain model). We find that the 4-isopropylpyridine in the mixture is oxidized in a slow air flow to produce 4-isopropylpyridine hydroperoxide which in turn precipitates as lamellar crystals with monoclinic structure. The fact that the peroxide group is exchanged for the hydrogen of the tertiary carbon demonstrates the high activity of the latter and gives strong support for its involvement in the self-protonation mechanism proposed earlier for the poly(4-vinylpyridine)/pyridine gel.
Bulletin of the Academy of Sciences of the USSR Division of Chemical Science, 1988
Conclusions A long-range effect of the nitrogen unshared pair on the direct 13 C-1 H coupling con... more Conclusions A long-range effect of the nitrogen unshared pair on the direct 13 C-1 H coupling constants is seen in the case of approximation of the corresponding proton and heteroatom to a distance on the order of their Van der Waals radii. The magnitude of this ...
Lanthanide complexes from La to Yb with IpW, ,039]7-ligands were studied by "0, 3'P and "'W NMR m... more Lanthanide complexes from La to Yb with IpW, ,039]7-ligands were studied by "0, 3'P and "'W NMR methods. The complexes are non-labile on the NMR time scale, and their 1: 2 composition in solution was confirmed by 3'P NMR. Contact and dipolar cont~b~tions to the lanthanide induced shift (LIS) of ligand nuclei have been estimated. The geometry of the ligand was shown to change in the lanthanide series and the dipolar contribution to LIS increases compared to the contact contribution, as the distance from the lanthanide ion increases.
94) that metallo−alkylidyne complexes decompose in aqueous solution and give rise to free carbyne... more 94) that metallo−alkylidyne complexes decompose in aqueous solution and give rise to free carbynes, which couple to yield acetylenes, is examined here theoretically. On the basis of the known marker reactions of carbynes in the doublet and quartet state, it is concluded that most of the reactivity patterns observed in the Bino experiment arose from quartet carbynes. Indeed, theory shows that quartet carbynes can be funneled to acetylene via a conical intersection. Moreover, many of the minor products are also identified as markers of the quartet carbynes. Carbynes formation in their doublet state is a minor channel that branches from the conical intersection and leads to the formation of dienes and olefins in the Bino experiment. Thus, we show that conical intersections are important also in thermally initiated reactions. Coupled to the experimental approach, the study opens a window to studies of carbynes under mild conditions.
The transition metal-dependent spin-orbit coupling (SOC) and outer-core (5s5p) correlation effect... more The transition metal-dependent spin-orbit coupling (SOC) and outer-core (5s5p) correlation effects in Ir- and Pt-catalyzed C-H activation processes are studied here using high level ab initio computations. The catalysts involve complexes with oxidation states: Ir(I), Ir(III), Pt(0), and Pt(II). It is demonstrated that for these heavy 5d transition metal-containing systems, the SOC effect and outer-core correlation effect on C-H activation are up to the order of ∼1 kcal/mol, and should be included if chemical accuracy is aimed. The interesting trends in our studied systems are: (1) the SOC effect consistently increases the C-H activation barriers and is apparently larger in higher oxidation states (Pt(II) and Ir(III)) than in low-oxidation states (Pt(0) and Ir(I)); and (2) the magnitude of outer-core (5s5p) correlation effects is larger in less coordinate-saturated system. The effect of basis set on the outer-core correlation correction is significant; larger basis sets tend to increase the C-H activation barriers.
To understand the dispersion stabilization of hydrocarbons in solids and of encumbered molecules,... more To understand the dispersion stabilization of hydrocarbons in solids and of encumbered molecules, wherein CH•••HC interactions act as sticky fingers, we developed here a valence bond (VB) model and applied it to analyze the H•••H interactions in dimers of H 2 and alkanes. The VB analysis revealed two distinct mechanisms of "dispersion." In the dimers of small molecules like H−H•••H−H and H 3 CH•••HCH 3 , the stabilization arises primarily due to the increased importance of the VB structures which possess charge alternation, e.
One of the landmark achievements of quantum chemistry, specifically of MO-based methods that incl... more One of the landmark achievements of quantum chemistry, specifically of MO-based methods that include electron correlation, was the precise calculation of the barrier for the hydrogenexchange reaction (B.
The manuscript studies the enantioselectivity and stereoselectivity of Diels-Alder (DA) cycloaddi... more The manuscript studies the enantioselectivity and stereoselectivity of Diels-Alder (DA) cycloadditions between cyclopentadiene (CPD) and a variety of dienophiles (ranging from halo-ethenes to cyano-ethenes), under oriented external electric fields (OEEFs). Applying OEEFs oriented in the X/Y-directions, perpendicular to the reaction axis (Z), will achieve complete isomeric and enantiomeric discrimination of the products. Unlike the Z-OEEF, which involve charge-transfer from the diene to the dienophile, and thereby brings about catalysis due to increased intramolecular bonding, an OEEF along X, aligned with the C=C bond of the dienophile, will lead to R/S enantiomeric discrimination by means of intramolecular-bond polarization. A Y-field will discriminate endo/exo stereoisomers in a similar mechanism. The XY field-combination will resolve both R/S and endo/exo. The resolution is complete and can be achieved at will by flipping the direction of the field along the X and Y axes. The preconditions for achieving the enantiomeric and isomeric discrimination are discussed, and require fixing of the CPD onto a surface. In so doing the chiral discrimination is achieved by dipole-moment selection rules, such that the field filters out one of the enantiomers, which is highly raised in energy by dipole selection. The dependence of the discrimination on the polarity of the dienophiles leads to a predictive trend.
We have carried out nonempirical (4-31G) and semiempirical (MNDO and AM1) calculations for pyrazo... more We have carried out nonempirical (4-31G) and semiempirical (MNDO and AM1) calculations for pyrazole. imidazole, and their 1-vinyl derivatives. The SCF calculations significantly overestimate the value of the orbital energy for n-type MO's. For the 1-vinylpyrazole molecule, all the methods incorrectly give the sequence of energy levels. The best agreement with experimental ionization potentials is obtained when using a Green's function approximation for the outer valence MO's together with the AM1 method.
The nature of the single bond in the three isoelectronic coinage metal dimers Cu2, Ag2 and Au2 is... more The nature of the single bond in the three isoelectronic coinage metal dimers Cu2, Ag2 and Au2 is investigated by means of the ab initio Breathing Orbital Valence Bond (BOVB) method, which allows one to calculate the respective contributions of the covalent and ionic structures to the total wave function, as well as the resonance energy arising from their mixing. It is shown that the BOVB method at its highest level provides bond dissociation energies in very good agreement with reference CCSD(T) values for the three dimers. It is also found that the covalent/ionic resonance energy is important in all three cases, contributing to 40-50% to the total bonding energy, thus qualifying the bonds in Cu2 and Au2 as quasi-charge-shift bonds, and that of Ag2 as a borderline case in-between classical covalent bond and charge-shift one. These results are further confirmed by analyses of the wave functions in terms of the Atoms-in-Molecule theory, which show that the Laplacian of the density at the bond critical point is large and positive in all three cases, which classifies the three bonds as charge-shift bonds within this theory.
Journal of the Chemical Society, Perkin Transactions 2, 1995
This paper addresses the question of an electron transfer mechanistic manifold by ab initio compu... more This paper addresses the question of an electron transfer mechanistic manifold by ab initio computations of the model systems Nu: + C2H6˙+→ Nu˙++ C2H6 where, NuH2S, PH3, H2O and NH3. The computations show that there exist two major classes of ET mechanisms. The first is a concerted ET mechanism which proceeds along a maximum bonding trajectory. The second class is a stepwise ET mechanism which involves shuttles of redox pairs, e.g., H+/H˙ and CH3+/CH3+˙, and thereby results in a single electron transfer from the nucleophile to the cation radical. Thus, an apparent ET may be a net result of consecutive steps which by themselves are non-ET steps. The genesis of the mechanistic manifold and variable transition state structure from valence bond (VB) configurations is discussed. It is found that all these mechanisms are typified by electronic structures that share the same set of VB configurations with variable proportions, giving rise thereby to variable transition state structure and an ET mechanistic family. The situation is reminiscent of the mechanistic manifold encountered in physical organic chemistry, e.g., the SN2 and SN1 mechanisms in the classical nucleophilic substitution process. For all the Nu:/C2H6˙+ combinations we also identified outersphere transition state analogues that avoid the bonding between the reactants. All the outer-sphere saddle points are found to be higher in energy than the bonded mechanisms of the concerted and stepwise varieties. It appears therefore, that outer-sphere mechanisms should be regarded as default options rather than natural mechanisms of ET-reactions of cation radicals. Indeed, all the computational trends exhibited by the ET mechanistic manifold are shown to be the consequences of maximum bonding.
The wide-ranging applications of the halogen bond (X-bond), notably in self-assembling materials ... more The wide-ranging applications of the halogen bond (X-bond), notably in self-assembling materials and medicinal chemistry, have placed this weak intermolecular interaction in a center of great deal of attention. There is a need to elucidate the physical nature of the halogen bond for better understanding of its similarity and differences vis-à-vis other weak intermolecular interactions, e.g. hydrogen bond, as well as for developing improved force-fields to simulate nano-and bio-materials involving X-bonds. This understanding is the focus of the present study that combines the insights of a bottom-up approach based on ab initio valence bond (VB) theory, and the block-localized wave-function (BLW) theory that uses monomers to reconstruct the wave function of a complex. To this end and with an aim of unification , we studied the nature of X-bonds in 55 complexes enumerated in Scheme 1 using the combination of VB and BLW theories. Our conclusion is clear-cut, most of the X-bonds are held by charge transfer interactions (i.e., intermolecular hyperconjugation) as envisioned more than 60 years ago by Mulliken. This is consistent with the experimental and computational findings that X-bonds are more directional than H-bonds. Furthermore, the good linear correlation between charge transfer energies and total interaction energies partially accounts for the success of simple force fields in the simulation of large systems involving X-bonds.
The ionization potentials of different molecules have been calculated with the outer valence Gree... more The ionization potentials of different molecules have been calculated with the outer valence Green's function (OVGF) technique, coupled with semiempirical MNDO, AM1 and PM3 methods. It is found that the OVGF method gives significantly better agreement with the experimental data than do results obtained with semiempirical calculations using Koopman's theorem including a new SAM1 and MNDO/d methods. Of the three semiempirical methods tested (MNDO, AMl, PM3) the OVGF (AMl) method gives the best agreement with experiment. 0 1997 Elsevier Science B.V.
ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance t... more ABSTRACT ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
Accurate gas-phase and solution-phase valence bond calculations reveal that protonation of the hy... more Accurate gas-phase and solution-phase valence bond calculations reveal that protonation of the hydroxyl group of aliphatic alcohols transforms the C-O bond from a principally covalent bond to a complete charge-shift bond with principally "no-bond" character. All bonding in this charge-shift bond is due to resonance between covalent and ionic structures, which is a different bonding mechanism from that of traditional covalent bonds. Until now, charge-shift bonds have been previously identified in inorganic compounds or in exotic organic compounds. This work showcases that charge-shift bonds can occur in common organic species.
We model the interaction of side-chain and end-chain groups of poly(4-vinylpyridine) by a 5:1 mol... more We model the interaction of side-chain and end-chain groups of poly(4-vinylpyridine) by a 5:1 molar ratio mixture of 4-isopropylpyridine (side-chain model) and 4-propylpyridine (end-chain model). We find that the 4-isopropylpyridine in the mixture is oxidized in a slow air flow to produce 4-isopropylpyridine hydroperoxide which in turn precipitates as lamellar crystals with monoclinic structure. The fact that the peroxide group is exchanged for the hydrogen of the tertiary carbon demonstrates the high activity of the latter and gives strong support for its involvement in the self-protonation mechanism proposed earlier for the poly(4-vinylpyridine)/pyridine gel.
Bulletin of the Academy of Sciences of the USSR Division of Chemical Science, 1988
Conclusions A long-range effect of the nitrogen unshared pair on the direct 13 C-1 H coupling con... more Conclusions A long-range effect of the nitrogen unshared pair on the direct 13 C-1 H coupling constants is seen in the case of approximation of the corresponding proton and heteroatom to a distance on the order of their Van der Waals radii. The magnitude of this ...
Lanthanide complexes from La to Yb with IpW, ,039]7-ligands were studied by "0, 3'P and "'W NMR m... more Lanthanide complexes from La to Yb with IpW, ,039]7-ligands were studied by "0, 3'P and "'W NMR methods. The complexes are non-labile on the NMR time scale, and their 1: 2 composition in solution was confirmed by 3'P NMR. Contact and dipolar cont~b~tions to the lanthanide induced shift (LIS) of ligand nuclei have been estimated. The geometry of the ligand was shown to change in the lanthanide series and the dipolar contribution to LIS increases compared to the contact contribution, as the distance from the lanthanide ion increases.
94) that metallo−alkylidyne complexes decompose in aqueous solution and give rise to free carbyne... more 94) that metallo−alkylidyne complexes decompose in aqueous solution and give rise to free carbynes, which couple to yield acetylenes, is examined here theoretically. On the basis of the known marker reactions of carbynes in the doublet and quartet state, it is concluded that most of the reactivity patterns observed in the Bino experiment arose from quartet carbynes. Indeed, theory shows that quartet carbynes can be funneled to acetylene via a conical intersection. Moreover, many of the minor products are also identified as markers of the quartet carbynes. Carbynes formation in their doublet state is a minor channel that branches from the conical intersection and leads to the formation of dienes and olefins in the Bino experiment. Thus, we show that conical intersections are important also in thermally initiated reactions. Coupled to the experimental approach, the study opens a window to studies of carbynes under mild conditions.
The transition metal-dependent spin-orbit coupling (SOC) and outer-core (5s5p) correlation effect... more The transition metal-dependent spin-orbit coupling (SOC) and outer-core (5s5p) correlation effects in Ir- and Pt-catalyzed C-H activation processes are studied here using high level ab initio computations. The catalysts involve complexes with oxidation states: Ir(I), Ir(III), Pt(0), and Pt(II). It is demonstrated that for these heavy 5d transition metal-containing systems, the SOC effect and outer-core correlation effect on C-H activation are up to the order of ∼1 kcal/mol, and should be included if chemical accuracy is aimed. The interesting trends in our studied systems are: (1) the SOC effect consistently increases the C-H activation barriers and is apparently larger in higher oxidation states (Pt(II) and Ir(III)) than in low-oxidation states (Pt(0) and Ir(I)); and (2) the magnitude of outer-core (5s5p) correlation effects is larger in less coordinate-saturated system. The effect of basis set on the outer-core correlation correction is significant; larger basis sets tend to increase the C-H activation barriers.
To understand the dispersion stabilization of hydrocarbons in solids and of encumbered molecules,... more To understand the dispersion stabilization of hydrocarbons in solids and of encumbered molecules, wherein CH•••HC interactions act as sticky fingers, we developed here a valence bond (VB) model and applied it to analyze the H•••H interactions in dimers of H 2 and alkanes. The VB analysis revealed two distinct mechanisms of "dispersion." In the dimers of small molecules like H−H•••H−H and H 3 CH•••HCH 3 , the stabilization arises primarily due to the increased importance of the VB structures which possess charge alternation, e.
One of the landmark achievements of quantum chemistry, specifically of MO-based methods that incl... more One of the landmark achievements of quantum chemistry, specifically of MO-based methods that include electron correlation, was the precise calculation of the barrier for the hydrogenexchange reaction (B.
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