Heteroatom influence in substituted benzenes and pyridine-like heterocycles in terms of direct and indirect intramolecular interactions

The general expression for the one-electron density matrix obtained previously for saturated organic molecules (V. Gineityte, J. Mol. Struct. (Theochem), 343 (1995) 183) is shown to be applicable also to substituted benzenes and pyridine-like heterocycles. On this basis, a new interpretation of the influence of a heteroatom (substituent) upon the remaining fragment of an aromatic molecule is suggested. To this end, the occupation number of a 2p_z AO of the aromatic ring has been expressed as a sum of five terms, two of them describing the intramolecular charge transfer and the remaining ones representing the secondary (induced) dipole moments arising within the ring under the influence of heteroatom, viz. the so-called ipso–ortho (para–meta), para–ipso and ortho–meta dipole moments. Just the latter two moments proved to play the principal role in the formation of the observed picture of the electron density distribution, viz. of an increase (reduction) of occupation numbers of 2p_z AOs in the ortho and para positions after introducing an electron-donating (accepting) substituent. For pyridine-like heterocycles and substituted benzenes, these dipole moments are determined mostly by the direct and the indirect interactions, respectively, between the highest occupied and the lowest vacant MO of benzene. Orbitals of the heteroatom (substituent) play the role of mediators in the above-mentioned indirect interaction.     

Substituent effects in the benzene dimer are due to direct interactions of the substituents with the unsubstituted benzene

The prevailing views of substituent effects in the sandwich configuration of the benzene dimer are flawed. For example, in the polar/pi model of Cozzi and co-workers (J. Am. Chem. Soc. 1992, 114, 5729), electron-withdrawing substituents enhance binding in the benzene dimer by withdrawing electron density from the pi-cloud of the substituted ring, reducing the repulsive electrostatic interaction with the nonsubstituted benzene. Conversely, electron-donating substituents donate excess electrons into the pi-system and diminish the pi-stacking interaction. We present computed interaction energies for the sandwich configuration of the benzene dimer and 24 substituted dimers, as well as sandwich complexes of substituted benzenes with perfluorobenzene. While the computed interaction energies correlate well with sigmam values for the substituents, interaction energies for related model systems demonstrate that this trend is independent of the substituted ring. Instead, the observed trends are consistent with direct electrostatic and dispersive interactions of the substituents with the unsubstituted ring.     

Substituent effects on the edge-to-face aromatic interactions

The edge-to-face interactions for either axially or facially substituted benzenes are investigated by using ab initio calculations. The predicted maximum energy difference between substituted and unsubstituted systems is approximately 0.7 kcal/mol (approximately 1.2 kcal/mol if substituents are on both axially and facially substituted positions). In the case of axially substituted aromatic systems, the electron density at the para position is an important stabilizing factor, and thus the stabilization/destabilization by substitution is highly correlated to the electrostatic energy. This results in its subsequent correlation with the polarization and charge transfer. Thus, the stabilization/destabilization by substitution is represented by the sum of electrostatic energy and induction energy. On the other hand, the facially substituted aromatic system depends on not only the electron-donating ability responsible for the electrostatic energy but also the dispersion interaction and exchange repulsion. Although the dispersion energy is the most dominating interaction in both axial and facial substitutions, it is almost canceled by the exchange repulsion in the axial substitution, whereas in the facial substitution, together with the exchange repulsion it augments the electrostatic energy. The systems with electron-accepting substituents (NO2, CN, Br, Cl, F) favor the axial substituent conformation, while those with electron-donating substituents (NH2, CH3, OH) favor the facial substituent conformation. The interactions for the T-shape complex systems of an aromatic ring with other counterpart such as H2, H2O, HCl, and HF are also studied.     

Application of the semilocalized approach to chemical reactivity of butadiene

The semilocalized approach to chemical reactivity (J. Mol. Struct. (Theochem) 588 (2002) 99; Int. J. Quant. Chem. 94 (2003) 302) is applied to study the addition reaction of an electrophile or nucleophile to the butadiene molecule. In accordance with the classical concept of the reaction center and its neighborhood (substituent), only one of the two H₂C=CH-fragments of butadiene is supposed to be under a direct attack of the reagent, whereas the remaining H₂C=CH-group is assumed to play the role of the substituent and thereby to participate in the process indirectly by exerting certain electron-donating or accepting effect upon the former group and/or the reagent. The main aim of the study consists in revealing the role of the H₂C=CH-substituent in the formation of the known higher reactivity of the terminal carbon atom of the attacked C=C-bond (as compared to the internal atom) irrespective of the nature of the reagent. To this end, we seek to obtain an explicit algebraic representation of the interdependence between the direction and the extent of the total influence of the H₂C=CH-substituent, on the one hand, and the nature of the reagent, on the other hand. The expressions for electron density and bond order redistributions among separate fragments of contacting molecules derived previously in the form of power series are shown to yield the above-anticipated representation. On this basis, it is demonstrated that the electron-donating effect of the initially occupied (bonding) orbital of the substituent and the electron-accepting effect of its initially vacant (antibonding) orbital upon the remaining fragments of the whole reacting system may be considered independently whatever the nature of the reagent. However, a strong interdependence is established between the actual relative extents of these two components of the total effect of the H₂C=CH-group and the electron-donating (accepting) properties of the reagent. Moreover, this group of atoms is shown to manifest itself as an electron-donating (accepting) substituent under influence of an electrophilic (nucleophilic) attack. Using this principal result of the paper, the actual reactivity of butadiene with respect to electrophile (nucleophile) is interpreted by invoking a model system of a substituted ethene containing a simple (one-orbital) electron-donating (accepting) substituent, and a terminal addition easily follows for both types of the reagent.     

Topological analysis of the molecular charge density and impact sensitivy models of energetic molecules

Important explosives of practical use are composed of nitroaromatic molecules. In this work, we optimized geometries and calculated the electron density of 17 nitroaromatic molecules using the Density Functional Theory (DFT) method. From the DFT one-electron density matrix, we computed the molecular charge densities, thus the electron densities, which were then decomposed into electric multipoles located at the atomic sites of the molecules using the distributed multipole analysis (DMA). The multipoles, which have a direct chemical interpretation, were then used to analyze in details the ground state charge structure of the molecules and to seek for correlations between charge properties and sensitivity of the corresponding energetic material. The DMA multipole moments do not present large variations when the size of the Gaussian basis set is changed; the largest variations occurred in the range 10-15% for the dipole and quadrupole moments of oxygen atoms. The charges on the carbon atoms of the aromatic ring of each molecule become more positive when the number of nitro groups increases and saturate when there are five and six nitro groups. The magnitude and the direction of the dipole moments of the carbon atoms, indicators of site polarization, also depend on the nature of adjacent groups, with the largest dipole value being for C-H bonds. The total magnitude of the quadrupole moment of the aromatic ring carbon atoms indicates a decrease in the delocalized electron density due to an electron-withdrawing effect. Three models for sensitivity of the materials based on the DMA multipoles were proposed. Explosives with large delocalized electron densities in the aromatic ring of the component molecule, expressed by large quadrupole values on the ring carbon atoms, correspond to more insensitive materials. Furthermore, the charges on the nitro groups also influence the impact sensitivity.     

Lithium and sodium cation binding of cyclopentadienyl anions: Electronic effects of cyclopentadienyl substitution

The Li⁺ and Na⁺ binding of substituted cyclopentadienyl (Cp) anions were investigated using computational techniques. The ring centroid-metal distances and the binding energies of the Cp-metal complexes correlate very well with the ∑σ_m of the substituted Cp ring. These properties also correlate well with the Cp Θ_zz values. The trend in the correlations is the more electron-rich the Cp (negative ∑σ_m and Θ_zz values values), the shorter the Cp-metal bond and the stronger the binding energy. The NBO metal charges correlate, though not very well in either case, with the Cp Θ_zz and ∑σ_m values. However, there is a substantial increase in correlation when the sum of the absolute value of the Hammett σ_m (∑|σ_m|) is employed. The significantly improved correlation when the ∑|σ_m| values are employed leads us to propose a model for substituted Cp charge transfer upon Li⁺ or Na⁺ complexation, and it also informs us that the Hammett substituent constant σ_m contains information about substituent polarizabilities, at least in the case of Li⁺- and Na⁺-substituted Cp anions.     

Transmission of substituent effects through unsaturated systems. I—relative magnitudes of proton and carbon substituent chemical shifts in ethylenes,butadienes, α-enones and benzenes

The carbon-13 shifts of C-1, C-2 and C-3 are determined in a series of 1-cyclohexen-3-ones substituted in position 1. Linear relationships are demonstrated between the substituent chemical shifts of corresponding carbons in substituted ethylenes, butadienes, α-enones and benzenes. The substituent chemical shifts of proton H-2 are also reported and correlated with those of corresponding protons in ethylenes and benzenes. The slopes of the lines for the carbons directly linked to the substituent are close to unity, showing a relative independence of the substituent effect for this nucleus from the variation of the unsaturated framework. In contrast to this, the transmission of the substituent effect through one double bond (nuclei β to the substituents) decreases as the number of conjugated π bonds in the whole structure increases. This relationship is interpreted as being due to the ability of an unsaturated system to spread the variation of π electron density induced by the substituent.     

Substituent effects on the acidity of weak acids. 2. Calculated gas-phase acidities of substituted benzoic acids

To investigate the origin of substituent effects on the acidity of benzoic acids, the structures of a series of substituted benzoic acids and benzoates have been calculated at the B3LYP/6-311+G* and MP2/6-311+G* theoretical levels. The vibrational frequencies were calculated using B3LYP/6-311+G* and allowed corrections for the change in zero-point energies on ionization, and the change in energy on going from 0 K (corresponding to the calculations) to 298 K. A more satisfactory agreement with the experimental values was obtained by energy calculations at the MP2/ 6-311++G* level using the above structures. The resulting Delta H(acid) values agree very well with the experimental gas-phase acidities. The energies of compounds with pi-electron-accepting or -releasing substituents, rotated to give the transition state geometries, provided rotational barriers that could be compared with those found for the corresponding substituted benzenes. Isodesmic reactions allowed the separate examination of the substituent effects on the energies of the acids and on the anions. Electron-withdrawing groups stabilize the benzoate anions more than they destabilize the benzoic acids. Electron-donating groups stabilize the acids and destabilize the anions by approximately equal amounts. The gas-phase acidities of meta- and para-substituted benzoic acids are linearly related. This is also found for the acidities of substituted phenylacetic acids and benzoic acids. Since direct pi-electron interactions are not possible with the phenylacetic acids, this indicates that the acidities are mainly controlled by a field effect interaction between the charge distribution in the substituted benzene ring and the negative charge of the carboxylate group. The Hammett sigma(M) and sigma(P) values are also linearly related for many small substituents from NO(2) through the halogens and to OH and NH(2). Most of the other substituents fall on a line with a different slope     

Conformations of highly hindered aryl ethers: XV. Mesomeric moments in m-dinitrobenzenes

The dipole moments of fourteen m-dinitrobenzenes with different substituents (OMe, CN, CH₃, CHO, Cl, CF₃, COOMe, F, Br, NH₂) in various positions were measured in order to study mesomeric interactions. Inclusion of conformational considerations permitted estimates to be made for the mesomeric moments of these groups at given positions. The values obtained could be applied with consistent results to a total of forty m-dinitrobenzenes, including others previously reported. The mesomeric moment increments deduced, compared with substituted benzenes possessing no nitro groups, were found to be only slightly larger than in non-nitro substituted benzenes (compared with substituted alkanes), which would indicate a reluctance of the extended π system of m-dinitrobenzene to interact with other substituents present, in any way other than by direct extension of the π network.One apparent case of intermolecular electron donor-acceptor complex formation was detected. The measured dipole moment of o-iodoanisole indicates a planar anti conformation for the methoxyl group.     

Halogen bonding: a theoretical study based on atomic multipoles derived from quantum theory of atoms in molecules

Atomic multipole moments derived from quantum theory of atoms in molecules are used to study halogen bonds in dihalogens (with general formula YX, in which X refers to the halogen directly interacted with the Lewis base) and some molecules containing C–X group. Multipole expansion is used to calculate the electrostatic potential in a vicinity of halogen atom (which is involved in halogen bonding) in terms of atomic monopole, dipole, and quadrupole moments. In all the cases, the zz component of atomic traceless quadrupole moments (where z axis taken along Y–X or C–X bonds) of the halogens plays a stabilizing role in halogen bond formation. The effects of atomic monopole and dipole moments on the formation of a halogen bond in YX molecules depend on Y and X atoms. In Br₂ and Cl₂, the monopole moment of halogens is zero and has no contribution in electrostatic potential and hence in halogen bonding, while in ClBr, FBr, and FCl it is positive and therefore stabilize the halogen bonds. On the other hand, the negative sign of dipole moment of X in all the YX molecules weakens the corresponding halogen bonds. In the C–X-containing molecules, monopole and dipole moments of X atom are negative and consequently destabilize the halogen bonds. So, in these molecules the quadrupole moment of X atom is the only electrostatic term which strengthens the halogen bonds. In addition, we found good linear correlations between halogen bonds strength and electrostatic potentials calculated from multipole expansion.     

取代邻苯二腈的合成

在室温条件下合成了一系列芳氧基邻苯二腈化合物,应用无素分析、1HNMR、IR确定了它们的结构,讨论了不同取代基对反应的影响.结果表明,酚的芳环上有推电子基时反应容易进行;酚的芳环上有拉电子基时反应慢或不反应.     

Full configuration interaction calculation of singlet excited states of Be3

The full configuration interaction (FCI) study of the singlets vertical spectrum of the neutral beryllium trimer has been performed using atomic natural orbitals [3s2p1d] basis set. The FCI triangular equilibrium structure of the ground state has been used to calculate the FCI vertical excitation energies up to 4.8 eV. The FCI vertical ionization potential for the same geometry and basis set amounts to 7.6292 eV. The FCI dipole and quadrupole transition moments from the ground state are reported as well. The FCI electric quadrupole moment of the X (3)A(1) (') ground state has been also calculated with the same basis set (Theta(zz)=-2.6461 a.u., Theta(xx)=Theta(yy)=-1/2Theta(zz)). Twelve of the 19 calculated excited singlets are doubly excited states. Most of the states have large multiconfigurational character. These results provide benchmark values for electronic correlation multireference methods. (4ex6MO)CAS-SDCI values for the same energies and properties are also reported.     

Arene-cation interactions of positive quadrupole moment aromatics and arene-anion interactions of negative quadrupole moment aromatics

Intermolecular interactions involving aromatic pi-electron density are widely believed to be governed by the aromatic molecular quadrupole moment, Theta(zz). Arene-cation binding is believed to occur primarily with negative Theta(zz) aromatics, and arene-anion binding is believed to occur largely with positive Theta(zz) aromatics. We have performed quantum mechanical computations that show the cation binding of positive Theta(zz) aromatics and the anion binding of negative Theta(zz) aromatics is quite common in the gas phase. The pi-electron density of hexafluorobenzene, the prototypical positive Theta(zz ) aromatic (experimental Theta(zz) = 9.5 +/- 0.5 DA), has a Li+ binding enthalpy of -4.37 kcal/mol at the MP2(full)/6-311G**level of theory. The RHF/6-311G** calculated Theta(zz) value of 1,4-dicyanobenzene is +11.81 DA, yet it has an MP2(full)/6-311G** Li+ binding enthalpy of -12.65 kcal/mol and a Na+ binding enthalpy of -3.72 kcal/mol. The pi-electron density of benzene, the prototypical negative Theta(zz) aromatic (experimental Theta(zz) = -8.7 +/- 0.5 DA), has a F- binding enthalpy of -5.51 kcal/mol. The RHF/6-311G** calculated Theta(zz) of C6H2I4 is -10.45 DA, yet it has an MP2(full)/6-311++G** calculated F- binding enthalpy of -20.13 kcal/mol. Our results show that as the aromatic Theta(zz) value increases the cation binding enthalpy decreases; a plot of cation binding enthalpies versus aromatic Theta(zz) gives a line of best of fit with R2 = 0.778. No such correlation exists between the aromatic Theta(zz) value and the anion binding enthalpy; the line of best fit has R2 = 0.297. These results are discussed in terms of electrostatic and polarizability contributions to the overall binding enthalpies.     

Substituent Effect on the C–NO2 and N–NO2 Bond Dissociation Energies of Nitroaromatic Molecules

Six density function theory methods (B3LYP, B3P86, MPWB1K1, MPWPW91, PBEPBE, TPSS1KCIS3) were used to calculate bond dissociation enthalpies of nitro compounds, where the B3P86 method was found to give the most accurate predictions. Using the B3P86 method meta‐ and para‐substituted nitroaromatics were systematically studied for the first time. The remote substituent effects, Hammett relationships, and the origin of the substituent effects were discussed on the basis of the calculated results. Both meta‐ and para‐substituted nitromethyl‐benzenes showed significant substituent effects and a fair correlation against substituent constants σ_p⁺ The ground state effects were found to play the major role in determining the overall substituent effects. Meanwhile, nitroamino‐ benzenes showed irregular substituent effects and a poorer Hammett correlation, where both ground and radical state effects contributed to the overall substituent effects.     

Molecular quadrupole moments and magnetic anisotropies of non-dipolar 1,3,5-trisubstituted benzenes

The molecular quadrupole moments and magnetic anisotropies of a series of non-dipolar 1,3,5-trisubstituted benzenes (C₆H₃X₃; X=H, Me, t-Bu, F, Cl, Br) are analysed to obtain information concerning the charge distributions. Whereas alkyl groups have relatively little effect on benzene. halogen substituents are strongly interactive and the moments become progressively less negative from 1.3.5-tribromo-to 1.3.5-trifluoro-benzene.     

Electrostatic properties of aqueous salt solution interfaces: a comparison of polarizable and nonpolarizable ion models

The effects of ion force field polarizability on the interfacial electrostatic properties of approximately 1 M aqueous solutions of NaCl, CsCl, and NaI are investigated using molecular dynamics simulations employing both nonpolarizable and Drude-polarizable ion sets. Differences in computed depth-dependent orientational distributions, "permanent" and induced dipole and quadrupole moment profiles, and interfacial potentials are obtained for both ion sets to further elucidate how ion polarizability affects interfacial electrostatic properties among the various salts relative to pure water. We observe that the orientations and induced dipoles of water molecules are more strongly perturbed in the presence of polarizable ions via a stronger ionic double layer effect arising from greater charge separation. Both anions and cations exhibit enhanced induced dipole moments and strong z alignment in the vicinity of the Gibbs dividing surface (GDS) with the magnitude of the anion induced dipoles being nearly an order of magnitude larger than those of the cations and directed into the vapor phase. Depth-dependent profiles for the trace and z z components of the water molecular quadrupole moment tensors reveal 40% larger quadrupole moments in the bulk phase relative to the vapor which mimics a similar observed 40% increase in the average water dipole moment. Across the GDS, the water molecular quadrupole moments increase nonmonotonically (in contrast to the water dipoles) and exhibit a locally reduced contribution just below the surface due to both orientational and polarization effects. Computed interfacial potentials for the nonpolarizable salts yield values 20-60 mV more positive than pure water and increase by an additional 30-100 mV when ion polarizability is included. A rigorous decomposition of the total interfacial potential into ion monopole, water and ion dipole, and water quadrupole components reveals that a very strong, positive ion monopole contribution is offset by negative contributions from all other potential sources. Water quadrupole components modulated by the water density contribute significantly to the observed interfacial potential increments and almost entirely explain observed differences in the interfacial potentials for the two chloride salts. By lumping all remaining nonquadrupole interfacial potential contributions into a single "effective" dipole potential, we observe that the ratio of quadrupole to "effective" dipole contributions range from 2:1 in CsCl to 1:1.5 in NaI, suggesting that both contributions are comparably important in determining the interfacial potential increments. We also find that oscillations in the quadrupole potential in the double layer region are opposite in sign and partially cancel those of the "effective" dipole potential.     

Summation of nucleus independent chemical shifts as a measure of aromaticity

When the nucleus independent chemical shifts, NICS(1)zz, for a set of aromatic and antiaromatic hydrocarbons are summed, they show an excellent linear relationship with the magnetic susceptibility exaltation, Lambda, for neutral, cationic, and monoanionic species. Aromatic and antiaromatic dianions show a similar relationship but with a different slope. However, when both Lambda and the summation of NICS(1)zz are divided by the area of the ring squared, the vast majority of the aromatic and antiaromatic species fall on the same line, indicating that both NICS and Lambda are affected by the size of the ring. The species that deviate slightly from the line are a few of the anionic compounds, which may be a result of the difficulties of calculating magnetic properties of anions. This is the first report of the relationship of NICS to ring area. In addition, the excellent correlation between Lambda and the summation of NICS(1)zz demonstrates that summation of NICS(1)zz values for individual ring systems of polycyclic ring systems to give a measure of the aromaticity of the entire system is justified. By extension, the excellent correlation also serves to demonstrate that the NICS(1)zz values for individual ring systems are reliable measures of local aromaticity/antiaromaticity. Finally, the excellent correlation between experimental shifts and the 13C NMR shifts calculated with density functional theory, B3LYP/6-311+g(d,p), serves as an indirect validation of the accuracy of the NICS(1)zz calculated by the same method and basis set.     

C-C bond unsaturation degree in monosubstituted ferrocenes for molecular electronics investigated by a combined near-edge x-ray absorption fine structure, x-ray photoemission spectroscopy, and density functional theory approach

We present the results of an experimental and theoretical investigation of monosubstituted ethyl-, vinyl-, and ethynyl-ferrocene (EtFC, VFC, and EFC) free molecules, obtained by means of synchrotron-radiation based C 1s photoabsorption (NEXAFS) and photoemission (C 1s XPS) spectroscopies, and density functional theory (DFT) calculations. Such a combined study is aimed at elucidating the role played by the C-C bond unsaturation degree of the substituent on the electronic structure of the ferrocene derivatives. Such substituents are required for molecular chemical anchoring onto relevant surfaces when ferrocenes are used for molecular electronics hybrid devices. The high resolution C 1s NEXAFS spectra exhibit distinctive features that depend on the degree of unsaturation of the hydrocarbon substituent. The theoretical approach to consider the NEXAFS spectrum made of three parts allowed to disentangle the specific contribution of the substituent group to the experimental spectrum as a function of its unsaturation degree. C 1s IEs were derived from the experimental data analysis based on the DFT calculated IE values for the different carbon atoms of the substituent and cyclopentadienyl (Cp) rings. Distinctive trends of chemical shifts were observed for the substituent carbon atoms and the substituted atom of the Cp ring along the series of ferrocenes. The calculated IE pattern was rationalized in terms of initial and final state effects influencing the IE value, with special regard to the different mechanism of electron conjugation between the Cp ring and the substituent, namely the σ/π hyperconjugation in EtFC and the π-conjugation in VFC and EFC.     

可溶性氧钒酞菁的非共振三阶非线性光学性质

合成了不同取代基和不同取代基位置的可溶性氧钒酞菁化合物, 并采用简并四波混频方法测定了这些化合物在非共振条件下的<γ>值和X^(^3^)值, 它们分别在10^-^3^1esu及10^-^1^0~10^-^9esu数量级。酞菁环上取代基的性质将影响化合物的<γ>值, 取代基的给电子能力越强, <γ>值就越大, 而取代基位置对<γ>值的影响较小。     

Diels-alder reactions of hexafluoro-2-butyne with 2-heterosubstituted furans: A facile and general synthesis of 1, 4-disubstituted 2,3-Di(trifluoromethyl)benzenes

An electron-donating heteroatom substituent at position-2 of a furan promotes regiospecific opening of the 7-oxa bridge of the Diels-Alder cycloadduct with hexafluoro-2-butyne, producing a 4-heterosubstituted 2,3-di(trifluoromethyl)phenol building block in a single step. The phenol and heteroatom substituent are easily transformed to the corresponding iodide or triflate that readily undergoes Heck, Suzuki, and Stille reactions to install a variety of substituents in high yields. This methodology provides a facile and general synthesis of 1,4-disubsituted 2, 3-di(trifluoromethyl)benzenes.