取代基对1-甲基尿嘧啶与N-甲基乙酰胺氢键复合物中氢键强度的影响

使用密度泛函理论B3LYP方法和二阶微扰理论MP2方法对由1-甲基尿嘧啶与N-甲基乙酰胺所形成的氢键复合物中的氢键强度进行了理论研究, 探讨了不同取代基取代氢键受体分子1-甲基尿嘧啶中的氢原子对氢键强度的影响和氢键的协同性. 研究表明: 供电子取代基使N－H…O＝C氢键键长r(H…O)缩短, 氢键强度增强; 吸电子取代基使N－H…O＝C氢键键长r(H…O)伸长, 氢键强度减弱. 自然键轨道(NBO)分析表明: 供电子基团使参与形成氢键的氢原子的正电荷增加, 使氧原子的负电荷增加, 使质子供体和受体分子间的电荷转移量增多; 吸电子基团则相反. 供电子基团使N－H…O＝C氢键中氧原子的孤对电子轨道n(O)对N－H的反键轨道σ^*(N－H)的二阶相互作用稳定化能增强, 吸电子基团使这种二阶相互作用稳定化能减弱. 取代基对与其相近的N－H…O＝C氢键影响更大.     

Trigger bond analysis of nitroaromatic energetic materials using wiberg bond indices

The identification of trigger bonds, bonds that break to initiate explosive decomposition, using computational methods could help direct the development of novel, “green” and efficient high energy density materials (HEDMs). Comparing bond densities in energetic materials to reference molecules using Wiberg bond indices (WBIs) provides a relative scale for bond activation (%ΔWBIs) to assign trigger bonds in a set of 63 nitroaromatic conventional energetic molecules. Intramolecular hydrogen bonding interactions enhance contributions of resonance structures that strengthen, or deactivate, the C NO₂ trigger bonds and reduce the sensitivity of nitroaniline‐based HEDMs. In contrast, unidirectional hydrogen bonding in nitrophenols strengthens the bond to the hydrogen bond acceptor, but the phenol lone pairs repel and activate an adjacent nitro group. Steric effects, electron withdrawing groups and greater nitro dihedral angles also activate the C NO₂ trigger bonds. %ΔWBIs indicate that nitro groups within an energetic molecule are not all necessarily equally activated to contribute to initiation. %ΔWBIs generally correlate well with impact sensitivity, especially for HEDMs with intramolecular hydrogen bonding, and are a better measure of trigger bond strength than bond dissociation energies (BDEs). However, the method is less effective for HEDMs with significant secondary effects in the solid state. Assignment of trigger bonds using %ΔWBIs could contribute to understanding the effect of intramolecular interactions on energetic properties. © 2018 Wiley Periodicals, Inc.     

取代基对N—H…O＝C氢键三聚体中氢键强度的影响

使用MP2方法研究了氢键三聚体中N-H…O=C氢键强度,探讨了氢键受体分子中不同取代基对N-H…O=C氢键强度的影响.研究表明,不同取代基对氢键三聚体中N-H…O=C氢键强度的影响是不同的:取代基为供电子基团,氢键键长r(H…O)缩短,氢键强度增强;取代基为吸电子基团,氢键键长r(H…O)伸长,氢键强度减弱.自然键轨道(NBO)分析表明,N-H…O=C氢键强度越强,氢键中氢原子的正电荷越多,氧原子的负电荷越多,质子供体和受体分子间的电荷转移越多.供电子基团使N-H…O=C氢键中氧原子的孤对电子n(O)对N-H的反键轨道σ~*(N-H)的二阶相互作用稳定化能增加,吸电子基团使这种二阶相互作用稳定化能减小.取代基对与其相近的N-H…O=C氢键影响更大.     

Push–pull effect on the geometries,electronic and optical properties of thiophene based dye-sensitized solar cell materials

Geometries, electronic structure and electronic absorption spectra of thiophene based dye-sensitized solar cells were performed using Density Functional Theory (DFT) and time dependent density functional theory (TD-DFT). Different electron donating and electron withdrawing groups have been substituted. Geometries and electronic properties have been computed at B3LYP/6-31G⁷ and absorption spectra at TD-B3LYP/6-31G⁷ level of theory. Major change in bond lengths and bond angles occurs in the system where there is electron withdrawing or electron donating groups have been substituted. In SYSTEM-2 and SYSTEM-3 intra charge transfer has been observed. HOMO of SYSTEM-2 and SYSTEM-3 is delocalized on left side while LUMO on right side of the molecule. In SYSTEM-1, HOMO is on left side while LUMO is in the center. The designed systems show two absorption peaks for each of the system. In short, choice of appropriate electron withdrawing and donating groups is very important for improving the performance of dye-sensitized solar cells.     

取代基对N-H…O=C氢键三聚体中氢键强度的影响

使用MP2方法研究了氢键三聚体中N—H…O=C氢键强度, 探讨了氢键受体分子中不同取代基对N—H…O=C氢键强度的影响. 研究表明, 不同取代基对氢键三聚体中N—H…O=C氢键强度的影响是不同的: 取代基为供电子基团, 氢键键长r(H…O)缩短, 氢键强度增强; 取代基为吸电子基团, 氢键键长r(H…O)伸长, 氢键强度减弱. 自然键轨道(NBO)分析表明, N—H…O=C氢键强度越强, 氢键中氢原子的正电荷越多, 氧原子的负电荷越多, 质子供体和受体分子间的电荷转移越多. 供电子基团使N—H…O=C氢键中氧原子的孤对电子n(O)对N—H的反键轨道滓*(N—H)的二阶相互作用稳定化能增加, 吸电子基团使这种二阶相互作用稳定化能减小. 取代基对与其相近的N—H…O=C氢键影响更大.     

Rigid or floppy water-containing dipole-bound dimer anions

We here report a comparative experimental and theoretical study of dipole-bound electron attachment on four polar hydrogen-bonded dimers containing one water molecule (water-water, ammonia-water, phenol-water and pyridine-water). When the water molecule is the proton acceptor in the neutral complex, with a trans-linear hydrogen bond (water-water and phenol-water), it is shown that the equilibrium geometry of the dipole-bound anion tends to a cis-linear hydrogen bond for which the total dipole moment is larger. On the contrary, when the water molecule is the proton donor to a nitrogen atom (ammonia-water and pyridine-water), dipole-bound electron attachment does not lead to subsequent modifications of the complex geometry.     

Exploring unusual antioxidant activity in a benzoic acid derivative: a proposed mechanism for citrinin

A mechanism is proposed for the unusual antioxidant activity in citrinin based on computed O-H bond dissociation enthalpies (BDE). These data suggest that citrinin itself is not the active species, but rather a pair of hydrated Michael addition products consisting of substituted 2,6-dihydroxy benzoic acids. These diastereomers act as radical scavengers via O-H bond dissociation with computed BDE's ranging from 78.9-80.9 kcal/mol for the active groups present. These data represent an unusually facile O-H bond dissociation for a phenol containing a strongly electron withdrawing group. This atypical reactivity arises from an intramolecular network of hydrogen bonds that both stabilize the incipient radical and facilitate extended delocalization through atoms external to the aromatic ring. The additional influence of stereochemistry on BDE is computed to be 2.0 kcal/mol. Data presented are for gas phase molecules, but solvents are unlikely to strongly modify these results since most polar groups are involved in intramolecular hydrogen bonds and thus less available for association with solvent. Citrinin and the Michael addition products are likely too toxic for use as antioxidants in organisms but this study clearly identifies specific reaction sites in the active form, thus guiding rational design of synthetic derivatives with more favorable biocompatibility.     

Oxidation of substituted benzaldehydes by quinolinium chlorochromate: A structure and solvent dependent kinetic study

The kinetics of oxidation of several para‐substituted benzaldehydes by quinolinium chlorochromate was studied under pseudo‐first‐order conditions in different (hydrogen bond donor and hydrogen bond acceptor) neat organic solvents. The operation of nonspecific and specific solvent–solvent–solute interactions was explored by correlating the rate data with solvent parameters through correlation analysis. Both electron‐releasing and electron‐withdrawing substituents enhanced the rate and the Hammett plot showed a break in the reactivity order indicating applicability of dual mechanism. An explanation in conformity with structure and solvent effects on reactivity has been proposed. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 154–158, 2003     

四唑衍生物异构化反应的理论研究

运用ＭＮＤＯＭＯ法对五种四唑衍生物的互变异构反应进行了理论计算,结果表明,过渡态的几何构型主要由迁移基团所决定,迁移基团的电性对反应速度影响很大,反应活化能与反应物中断裂键的键级和迁移基轩处净民荷之间均存在良好的线性关系,二取代基的性质均对反应平衡常数产生影响。     

Electron density topological analysis of hydrogen bonding in glucopyranose and hydrated glucopyranose

Topological analysis of the electron density profiles and the atomic basin integration data for the most energetically favorable (4)C(1) and (1)C(4) conformers of beta-D-glucopyranose, calculated at the B3LYP/6-31+G(d), MPWlPW91/6-311+G(2d,p), and MP2/6-31+G(d) levels, demonstrates that intramolecular hydrogen bonding between adjacent ring OH groups does not occur in glucopyranose, given the need to demonstrate a bond critical point (BCP) of correct (3,-1) topology for such an interaction to be termed a hydrogen bond. On the other hand, pyranose ring OH groups separated by three, rather than just two, carbon atoms are able to form an intramolecular hydrogen bond similar in topological properties and geometry to that found for propane-1,3-diol. Vicinal, equatorial OH groups in the (4)C(1) conformer of glucopyranose are, however, able to form strong bidentate hydrogen bonds with water molecules in a cooperative manner, each water molecule acting simultaneously as both hydrogen bond donor and acceptor, and characterized by (3,-1) bond critical points with increased values for the electron density and the Laplacian of rho(r) compared to an isolated ethane-1,2-diol/water complex.     

AIM analysis of intramolecular hydrogen bonding in O-hydroxy aryl Schiff bases

AIM analysis was applied to study the changes in such topological parameters as the electron density at critical points of all the bonds of the molecule during the so-called nonadiabatic proton transfer in intramolecular hydrogen bonding in o-hydroxy aryl Schiff bases. Proton transfer is presented by a stepwise elongation and fixing of the hydroxyl bond with complete optimization of the rest of the parameters of the molecule by the B3LYP/6-311++G(d,p) method. A more detailed study of electron density changes at the critical points of the chelate and phenol rings in the stepwise proton-transfer process is presented. It was shown that the dependency of the electron density at the critical point of the chelate ring on tautomeric equilibrium is of a complicated character, whereas it is linear for the phenol ring. A complex study of the changes in the total electron density at the hydrogen bond, the quasi-aromatic ring, and in the whole molecule has been accomlished. The calculations of the intramolecular hydrogen bond by means of conformational and topological methods are discussed.     

Addition polymerization of nitrile compounds by organotin catalysts

A study was made of the polymerization of NC(CH₂)_nCN (n = 1?4), fumaronitrile and tetracyanoethylene (TCNE) by organotin compounds such as, oxides and alkoxides, as catalysts. It was found that polymerization occurred with malononitrile, succinonitrile, fumaronitrile and TCNE, but not with glutaronitrile and adiponitrile. Initiation of the polymerization is by addition of the tin compounds to the CN bond leading to the formation of a SnN bond, and propagation is probably by insertion of this bond to CN bonds. Addition of the tin compounds occurred only to nitrile groups that were sufficiently activated by proximity to electron withdrawing groups. The mutual electron withdrawing effect of one nitrile on the other is sufficient to permit this addition if the nitriles are geminal or vicinal, otherwise no reasonable polymerization occurred. The polymerization of the nitrile bond leads to the formation of highly conjugated systems, which give a black colour to the polymers.     

The Grob/Eschenmoser fragmentation of cycloalkanones bearing β-electron withdrawing groups: a general strategy to acyclic synthetic intermediates

Introduction of a β-electron withdrawing group to cycloalkanones allows facile C-C bond fragmentation. The reaction has been demonstrated with a large range of ring sizes, bearing various leaving and electron withdrawing groups, and using a variety of nitrogen and oxygen containing nucleophiles (>30 examples). The application of fragmentation products to the preparation of substituted γ-lactones has been demonstrated. Mechanistic studies are reported which are suggestive of a Grob/Eschenmoser type reaction.     

A model for the Heyrovsky reaction as the second step in hydrogen evolution

On a number of electrodes the second step in hydrogen evolution is the reaction of a proton with an adsorbed hydrogen intermediate to form a molecule, which is also known as the Heyrovsky reaction. We have developed a model Hamiltonian for this reaction, which for concrete applications requires extensive calculations on the basis of density-functional theory. Explicit results are presented for a Ag(111) electrode. The rate-determining step is electron transfer to the proton that approaches the electrode from the solution. At the saddle point for this reaction the adsorbed hydrogen atom has moved a little away from the surface in order to reduce the repulsion of the product molecule. Electron transfer to the proton occurs when the distance between the two particles is close to the bond distance of the hydrogen molecule.     

Evidence for site‐specific intra‐ionic hydrogen/deuterium exchange in the low‐energy collision‐induced dissociation product ion spectra of protonated small molecules generated by electrospray ionisation

The experimental investigation of site‐specific intra‐ionic hydrogen/deuterium (H/D) exchange in the low‐energy collision‐induced dissociation (CID) product ion spectra of protonated small molecules generated by electrospray ionisation (ESI) is presented. The observation of intra‐ionic H/D exchange in such ions under low‐energy CID conditions has hitherto been rarely reported. The data suggest that the intra‐ionic H/D exchange takes place in a site‐specific manner between the ionising deuteron, localised at either a tertiary amine or a tertiary amine‐N‐oxide, and a γ‐hydrogen relative to the nitrogen atom. Nuclear magnetic resonance (NMR) spectroscopy measurements showed that no H/D exchange takes place in solution, indicating that the reaction occurs in the gas phase. The compounds analysed in this study suggested that electron‐withdrawing groups bonded to the carbon atom bearing the γ‐hydrogen can preclude exchange. The effect of the electron‐withdrawing group appears dependent upon its electronegativity, with lower χ value groups still allowing exchange to take place. However, the limited dataset available in this study prevented robust conclusions being drawn regarding the effect of the electron‐withdrawing group. The observation of site‐specific intra‐ionic H/D exchange has application in the area of structural elucidation, where it could be used to introduce an isotopic label into the carbon skeleton of a molecule containing specific structural features. This could increase the throughput, and minimise the cost, of such studies due to the obviation of the need to produce a deuterium‐labelled analogue by synthetic means. Copyright © 2010 John Wiley & Sons, Ltd.     

Strong effect of methyl group on the strength of ionic hydrogen bond between C2H2 and H3O+

The effect of methyl group on the strength of the ionic hydrogen bond between C₂H₂ and H₃O⁺ has been studied with quantum chemical calculations at the UMP2/6‐311++G(d,p) level. The presence of a methyl group in the proton acceptor results in an energetic increase of 6.02 kcal/mol, increased by about 39%, whereas that in the proton donor leads to an energetic decrease of 2.18 kcal/mol, decreased by 14%. The charge analyses indicate that the methyl group in the proton acceptor is electron‐donating and that in the proton donor is electron‐withdrawing. The former plays a positive contribution to the formation of ionic hydrogen bond and the latter plays a negative contribution to the formation of ionic hydrogen bond. The weakening effect of solvent on the role of methyl group in the ionic hydrogen bond has also been studied at the UB3LYP/6‐311++G(d,p) level. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

A comparative ab initio study of the primary hydration and proton dissociation of various imide and sulfonic acid ionomers

We compare the role of neighboring group substitutions on proton dissociation of hydrated acidic moieties suitable for proton exchange membranes through electronic structure calculations. Three pairs of ionomers containing similar electron withdrawing groups within the pair were chosen for the study: two fully fluorinated sulfonyl imides (CF(3)SO(2)NHSO(2)CF(3) and CF(3)CF(2)SO(2)NHSO(2)CF(3)), two partially fluorinated sulfonyl imides (CH(3)SO(2)NHSO(2)CF(3) and C(6)H(5)SO(2)NHSO(2)CF(2)CF(3)), and two aromatic sulfonic acid based materials (CH(3)C(6)H(4)SO(3)H and CH(3)OC(6)H(3)OCH(3)C(6)H(4)SO(3)H). Fully optimized counterpoise (CP) corrected geometries were obtained for each ionomer fragment with the inclusion of water molecules at the B3LYP/6-311G** level of density functional theory. Spontaneous proton dissociation was observed upon addition of three water molecules in each system, and the transition to a solvent-separated ion pair occurred when four water molecules were introduced. No considerable quantitative or qualitative differences in proton dissociation, hydrogen bond networks formed, or water binding energies were found between systems containing similar electron withdrawing groups. Each of the sulfonyl imide ionomers exhibited qualitatively similar results regarding proton dissociation and separation. The fully fluorinated sulfonyl imides, however, showed a greater propensity to exist in dissociated and ion-pair separated states at low degrees of hydration than the partially fluorinated sulfonyl imides. This effect is due to the additional electron withdrawing groups providing charge stabilization as the dissociated proton migrates away from the imide anion.     

A comparative computational study of the homolytic and heterolytic bond dissociation energies involved in the activation step of ATRP and SET‐LRP of vinyl monomers

A quantum‐chemical calculation of the homolytic and heterolytic bond dissociation energies of the model compounds of the monomer and dimer is reported. These model compounds include the dormant chloride, bromide, and iodide species for representative activated and nonactivated monomers containing electron‐withdrawing groups as well as for a nonactivated monomer containing an electron‐donor group. Two examples of sulfonyl and N‐halide initiators are also reported. The homolytic inner‐sphere electron‐transfer bond dissociation is known as atom transfer and is responsible for the activation step in ATRP. The heterolytic outer sphere single electron transfer bond dissociation is responsible for the activation step in single electron transfer mediated living radical polymerization (SET‐LRP). The results of this study demonstrated much lower bond dissociation energies for the outer sphere single electron transfer processes. These results explain the higher rate constant of activation, the higher apparent rate constant of propagation, and the lower polymerization temperature for both activated and nonactivated monomers containing electron‐withdrawing groups in SET‐LRP. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1607–1618, 2007     

Kinetico-mechanistic studies of the acidolysis of Rh-C bonds in monocyclometallated dirhodium(II) acetato complexes; influence of electronic and steric effects

The protonation/demetallation reaction, in CD(3)COOD, of phosphine monometallated triacetato-bridged dirhodium(ii) complexes of general formulae [Rh(2)(μ-OOCCH(3))(3){(RC(5)CH(3))P(RC(6)H(4))(2)}(2)] has been studied from a kinetico-mechanistic perspective. The process has been monitored via the disappearance of the nuclear magnetic resonance signals of the protons present in the non-metallated ortho positions of the phosphine ligand and taking advantage of the relatively fast back metallation process that follows the acidolysis reaction, the sequence behaving as an overall equilibrium reaction. The process has a high associative character with important entropy demands, increasing both for the larger and the more electron withdrawing phosphine ligands. For the complexes with electron withdrawing CF(3) substituents on the cyclometallated phenyl ring, this demand is compensated by very low activation enthalpies. The data agree with an intimate mechanism that corresponds to the reverse of the electrophilic C-H bond activation, which has already been studied for this type of system, and requires the presence of a protonated acetato ligand in the axial position, close to the metallated Rh-C(aromatic) bond. The involvement of external solvent molecules, interacting via hydrogen bonds with the system, is also evident for systems which include CF(3) groups with the correct geometric arrangement.     

The strength of hydrogen bonding to metal-bound ligands can contribute to changes in the redox behaviour of metal centres

A series of nine tripodal tetradentate ligands based on tris(pyridyl-2-methyl)amine TPA with hydrogen bond donors R in one, two and three of the pyridine 6-positions (R = NH2 amino, L(Am-1,2,3); NHCH2(t)Bu neopentylamino, L(Np-1,2,3); NHCO(t)Bu pivaloylamido, L(Piv-1,2,3)) and TPA are used to investigate the effect of different hydrogen bonding microenvironments on electrochemical properties of their LCuCl complexes. The hydrogen bond donors are rigidly preorganised and suitably oriented for intramolecular N-H...Cl-Cu hydrogen bonds. Cyclic voltammetry studies show that the reduction potential of the Cu(II)/Cu(I) couple as a function of the ligand follows the order TPA < L(Am-n) < or approximately L(Np-n) < L(Piv-n), and that the magnitude of the effect increases with the number of hydrogen bonding groups. These trends could be explained in terms of the steric and electronic effects exerted by these groups stabilising the Cu(I) oxidation state. In fact, the X-ray structure of the air-stable [(L(Piv-3))Cu(I)Cl] complex is reported and shows elongated Cu-N and Cu-Cl bonds, presumably due to the combination of steric and electron withdrawing effects exerted by the three pivaloylamido groups. We reasoned that the strength of hydrogen bonding in the Cu(I) and Cu(II) oxidation states could differ and therefore contribute also to the aforementioned redox changes; this hypothesis is tested using IR and NMR spectroscopy. IR studies of the [(L(Piv-1,2,3))Cu(I)Cl] and [(L(Piv-1,2,3))Cu(II)Cl]+ complexes in acetonitrile show that the intramolecular N-H...Cl-Cu hydrogen bonding weakens in the order L(Piv-1) > L(Piv-2) > L(Piv-3), and that it is stronger in the Cu(I) complexes. The 1H NMR spectra of the [(L(Piv1,2,3))Cu(I)Cl] complexes are in complete agreement with the IR data, and reveal that the stability of the Cu(I) complexes to oxidation in air increases in the order L(Piv-1) < L(Piv-2) < L(Piv-3). The hydrogen bonds in the Cu(I) complexes are stronger because of the higher electron density on the Cl ligand, when compared to the Cu(II) complexes. This is consistent with ab initio MP2 calculations performed on the complexes [(L(Piv-3))Cu(I)Cl] and [(L(Piv-3))Cu(II)Cl]+. Thus, the electron density of a metal-bound ligand acting as hydrogen bond acceptor is revealed as the major factor in determining the strength of the hydrogen bonds formed. From the IR data the energies of the N-H...Cl-Cu hydrogen bonds is estimated, as is the contribution of changes in hydrogen bond strength with the oxidation state of the copper centre and number of interactions to stabilising the Cu(I) state. Some of the implications of this result in dioxygen activation chemistry are discussed.