A quantum chemistry study of ruthenabenzene complexes

The electronic structure and properties of the ruthenabenzenes and substituted ruthenabenzenes have been explored using the hybrid density functional B3LYP theory. Systematic studies on the substituent effect in para-substituted ruthenabenzenes complexes have been studied. The following substituents were taken into consideration: H, NO₂, CN, CHO, COOH, F, CH₃, OH, and NH₂. Basic measures of aromatic character were derived from the structure and nucleus-independent chemical shift (NICS). The NICS calculations indicate a correlation between NICS(1.5) and the hardness in all species. The atoms in molecule analysis indicates a correlation between r(Ru-C) bonds and the electron density of bond critical point in all species.     

Is Free Cyclooctatetraene Dianion an Aromatic System? A Quantum Chemistry Study

To investigate whether free cyclooctatetraene dianion (COT^2?) is aromatic, quantum chemistry methods were used to optimize its structure. Based on the optimized structures, the natural population analysis (NPA) charge, bond order, delocalization energy, nucleus‐independent chemical shift (NICS), and harmonic oscillator model of aromaticity (HOMA) values were computed by DFT‐B3LYP method with basis set 6‐311++G**, which shows that COT^2? is not aromatic as it is not planar and has different bond lengths and bond orders, smallest delocalization energy and positive NICS values. To further confirm the finding, the changes of NICS and energy against ring distortion angle were scanned. The COT^2? has positive NICS values all along the angle from 180° to 120° while other aromatic systems always have negative values. The energy scanning suggests that COT^2? should have the weakest capability to maintain its planar structure. All the calculations strongly indicate that COT^2? is not aromatic. This study also suggests that NICS scan might be a good approach to judge aromaticity.     

Theoretical study on platinabenzene and mono- and difluorinated platinabenzenes: Structure, properties, and aromaticity

The electronic structures and properties of the platinabenzene and mono- and difluorinated platinabenzenes isomers have been investigated using hybrid density functional B3LYP theory. Basic measures of aromatic character were derived from structure, molecular orbital, and nuclear independent chemical shift (NICS). An energetic criterion suggests that ortho isomer of monofluorinated and F15 isomer of difluorinated platinabenzenes enjoy conspicuous stabilization. The polarizability and molecular orbital analysis are compatible with this result. NICS values calculated at several points above the ring center fail to give the result consistent with that based on relative energy, polarizability, and molecular orbital analysis. The atoms in molecules analysis indicates a correlation between NICS (1.0) and the electron density of the ring critical point (ρ_rcp) in monofluorinated platinabenzenes. There is a similar correlation in difluorinated platinabenzenes (except for F12 and F24 isomers) between NICS (0.5) and ρ_rcp.     

DFT studies and AIM analysis of AlN-polycycles

The structure and properties of AlN-polycycles were studied by DFT (density functional theory) method. The results of calculations were obtained at B3LYP/6-311G(d, p) level on model species. Topological parameters such as electron density, its Laplacian, kinetic electron energy density, potential electron energy density, and total electron energy density at the ring critical points (RCP) from Bader’s ‘Atoms in molecules’ (AIM) theory were analyzed in detail. These results indicate a good correlation between ρ(3, +1), G(r), H(r), and V(r) averaged values and hardness of AlN-polycycles. The aromaticity of all molecules has been studied by nucleus-independent chemical shift. There is a linear correlation between ΣNICS(0.0)_molecule values and polarizability.     

Chemistry of amino acid thionoester derivatives. An unexpected change in the reaction course between heterocyclic-2-carboxylic acid hydrazides and N-substituted thionoglycinates. Preferential formation of 4-amino-1,2,4-triazole adducts over 1,3,4-oxadiazole products

The reaction between benzoic acid hydrazides and ethyl N-carbobenzyloxythionoglycinate produces the expected 2-aminomethyl-1,3,4-oxadiazoles in good yield. Heterocyclic carboxylic acid hydrazides give similar products when the hydrazide moiety is located at either the three or four position (relative to the heteroatom) in the ring. However, when heterocyclic-2-carboxylic acid hydrazides are utilized, oxadiazole formation is dramatically reduced. Instead, the intermediate imidates are usually isolated as the major products of the reaction from one equivalent of these hydrazides. These imidate products are accompanied by significant amounts of 4-amino-1,2,4-triazole derivatives which arise from incorporation of two equivalents of the hydrazide. The structure of these unexpected 4-aminotriazole products was confirmed by nmr and mass spectral data as well as an X-ray analysis. In the presence of a stoichiometric amount of these hydrazides, the 4-aminotriazoles become the major products of the reaction. This phenomenon was found to be general for 2-thienyl, 2-furoic, picolinic, and pyrazinoic acid hydrazides. The intermediate imidates for each of these systems were isolated, characterized and found to have a remarkable thermal stability. Conversion of these imidates to the corresponding 1,3,4-oxadiazoles could only be accomplished in hot acetic anhydride. A mechanistic rationale is presented which suggests that some stabilization of the intermediate imidate must occur in these examples which allows an intermolecular process to compete so effectively with an intramolecular cyclization. Since the cyclization to oxadiazole is presumed to be acid catalyzed, this stabilization is proposed to occur specifically by the formation of a hydrogen bond between the ring heteroatom and the proton-ated imino nitrogen present in the imidate prior to cyclization. The formation of such a hydrogen bond removes the carboxylate oxygen from its opportune position for cyclization, while the protonated imino nitrogen can still activate the imidate for subsequent reaction with a second equivalent of hydrazide. In all cases where this heteroatom is capable of hydrogen bond formation, 4-aminotriazoles predominate. The relative amount of 4-aminotriazole product is directly correlatable with the donor capability of the ring hetero-atom. This proposed model was tested by examining a system where steric congestion would be expected to prevent hydrogen bond formation. Indeed, when N-methyl-2-pyrrole carboxylic acid hydrazide was utilized in the reaction, the corresponding 1,3,4-oxadiazole was formed as expected in high yield. Conversely, an acyclic aliphatic hydrazide specifically bearing a beta heteroatom (N-carbobenzyloxyglycine hydrazide) produced the expected 4-aminotriazole adduct in high yield. This therefore appears to be a general phenomenon which provides a useful synthetic entry to several new unsymmetrically substituted 4-amino-1,2,4-triazole derivatives.     

A Theoretical Study of NBO,NICS, and 14N NQR Parameters of Adenine Tautomers in the Gas Phase via DFT

The natural bond orbital (NBO) analysis, nucleus independent chemical shift (NICS), and ¹⁴N NQR parameters of the most stable tautomers of adenine in the gas phase were predicted using density functional theory method. The NBO analysis revealed that the resonance interaction between lone pair of the nitrogen atom and empty non‐Lewis NBO increases with increasing the p character of the nitrogen lone pair. The present investigation indicated the π clouds in both the considered heterocyclic rings containing six electrons, and these tautomers has the aromatic character. The NICS study utilizing the gauge‐invariant atomic orbital method showed that there are diatropic currents in the heterocyclic rings of the tautomers, so we determined the order of overall aromaticity of these tautomers. The results of NQR parameter calculations showed three parameters are effective on nuclear quadrupole coupling constant; the p character value of lone pair electrons of nitrogens, and the related occupancies and whenever, the lone pair electrons of nitrogens participate in the formation of chemical bond and/or π system of the ring, the q_zz and consequently its χ decreases.     

Density functional theory investigation on the electronic structure of furo[3,4-b]pyridine-type heterocyclics: from monomer to polymer

The geometries and electronic properties of six polymers based on furo[3,4-b]pyridine-type heterocyclics were studied using density functional theory (DFT) at the B3LYP/6-31G(d) level. Bond lengths, bond critical point (BCP) properties, nucleus-independent chemical shift (NICS), and Wiberg bond indexes (WBIs) are analyzed and correlated with conduction properties. The changes of bond length, BCP properties, NICS, and WBIs all show that the conjugational degree is increased with main chain extension. The changes of NICS also show that the conjugation is stronger in the central section than in the outer section. And the HOMO–LUMO energy gap (E _g) is decreased steadily upon chain elongation. The results suggest that the six polymers all have lower energy gaps (in the range of 0.39–0.58 eV), which indicate that these proposed polymers are good candidates for the conductive materials.     

The NICS‐XY‐Scan: Identification of Local and Global Ring Currents in Multi‐Ring Systems

Nucleus‐independent chemical shift (NICS)‐based methods are very popular for the determination of the induced magnetic field under an external magnetic field. These methods are used mostly (but not only) for the determination of the aromaticity and antiaromaticity of molecules and ions, both qualitatively and quantitatively. The ghost atom that serves as the NICS probe senses the induced magnetic field and reports it in the form of an NMR chemical shift. However, the source of the field cannot be determined by NICS. Thus, in a multi‐ring system that may contain more than one induced current circuit (and therefore more than one source of the induced magnetic field) the NICS value may represent the sum of many induced magnetic fields. This may lead to wrong assignments of the aromaticity (and antiaromaticity) of the systems under study. In this paper, we present a NICS‐based method for the determination of local and global ring currents in conjugated multi‐ring systems. The method involves placing the NICS probes along the X axis, and if needed, along the Y axis, at a constant height above the system under study. Following the change in the induced field along these axes allows the identification of global and local induced currents. The best NICS type to use for these scans is NICS_πZZ, but it is shown that at a height of 1.7 Å above the molecular plane, NICS_ZZ provides the same qualitative picture. This method, namely the NICS‐XY‐scan, gives information equivalent to that obtained through current density analysis methods, and in some cases, provides even more details.     

Beiträge zur Chemie der Pyrrolpigmente, 22. Mitt.: Die Selektivität photochemischer Isomerisierungen bifunktioneller und bichromophorer Verbindungen mit geometrischer Isomerie — Studien an Gallenpigmentstrukturmodellsystemen

Bifunctional molecules of structural type5 and6 (resembling bilatrienes-abc) containing double bonds show a strong selectivity toward photochemical isomerization: Starting from the (Z,Z)-isomer the (Z,E)-isomer, is obtained but none of the (E,E)-isomer. The reason for this selectivity could be established by conformational analysis,PPP-calculations and the photochemical behaviour of N-methylated derivatives (8 and9). The configuration, (Z) at a particular double bond induces a certain amount of twisting at the attached, single bond on steric reasons. This twisting is smaller than the twisting observed for the double-single-bond system of configuration (E). It is this differrential twist which causes the observed selectivity.On the other hand bichromophoric systems like2 or4 give no selectivity on photochemical isomerization.Based on the experimental data an energy scheme for the singlet isomerization paths for bichromophoric and bifunctional systems was deduced.

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21. Mitt.:H. Falk, K. Grubmayr, K. Thirring undN. Gurker, Mh. Chem.109, 1183 (1978).     

Intermolecular interactions in uracil–nitrous acid complexes: structures,binding energy,topological properties,and nuclear magnetic resonance study

Molecular interactions between uracil and nitrous acid (U–NA) [C₄N₂O₂H₄? NO₂H] have been studied using B3LYP, B3PW91, and MP2 methods with different basis sets. The optimized geometries, harmonic vibrational frequencies, charge transfer, topological properties of electron density, nucleus‐independent chemical shift (NICS), and nuclear magnetic resonance one‐ and two‐bonds spin–spin coupling constants were calculated for U–NA complexes. In interaction between U and NA, eight cyclic complexes were obtained with two intermolecular hydrogen bonds N(C)HU…N(O) and OH_NA…O_U. In these complexes, uracil (U) simultaneously acts as proton acceptor and proton donor. The most stable complexes labeled, UNA1 and UNA2, are formed via NH bond of U with highest acidity and CO group of U with lowest proton affinity. There is a relationship between hydrogen bond distances and the corresponding frequency shifts. The solvent effect on complexes stability was examined using B3LYP method with the aug‐cc‐pVDZ basis set by applying the polarizable continuum model (PCM). The binding energies in the gas phase have also been compared with solvation energies computed using the PCM. Natural bond orbital analysis shows that in all complexes, the charge transfer takes place from U to NA. The results predict that the Lone Pair (LP)(O)_U → σ*(O? H) and LP(N(O)_NA → σ*(N(C)? H)_U donor–acceptor interactions are most important interactions in these complexes. Atom in molecule analysis confirms that hydrogen bond contacts are electrostatic in nature and covalent nature of proton donor groups decreases upon complexation. The relationship between spin–spin coupling constant (^1hJ_H…_Y and ^2hJ_H…_Y) with interaction energy and electronic density at corresponding hydrogen bond critical points and H‐bonds distances are investigated. NICS used for indicating of aromaticity of U ring upon complexation. © 2013 Wiley Periodicals, Inc.     

Density functional theoretical investigation of the aromatic nature of BN substituted benzene and four ring polyaromatic hydrocarbons

We have studied the topological and local aromaticity of BN-substituted benzene, pyrene, chrysene, triphenylene and tetracene molecules. The nucleus-independent chemical shielding (NICS), harmonic oscillator model of aromaticity (HOMA), para-delocalization index (PDI) and aromatic fluctuation index (FLU) have been calculated to quantify aromaticity in terms of magnetic and structural criteria. We find that charge separations due to the introduction of heteroatoms largely affect both the local and topological aromaticity of these molecules. Our studies show that the presence of any kind of heteroatom in the ring not only reduces the local delocalization in the six membered ring, but also affects strongly the topological aromaticity. In fact, the relative orders of the topological and local aromaticity depend strongly on the position of the heteroatoms in the structure. In general, more ring shared BN containing molecules are less aromatic than the less ring shared BN molecules. In addition our results provide evidence that the structural stability of the molecule is dominated by the σ bond rather than the π bond.     

Complexation of Indole-3-acetic Acid with Iron(III): Influence of Coordination on the π-Electronic System of the Ligand

 The iron(III) complex of indole-3-acetic acid (1) was prepared, and its physicochemical properties, mode of iron(III) coordination, and electronic structure were studied using UV/Vis, diffuse reflectance infrared Fourier transform (DRIFT), and transmission ⁵⁷Fe M?ssbauer spectroscopic techniques. The data obtained provide evidence that iron(III) is not only coordinated by the carboxylic O-donor atom, but also via the conjugated π-electronic system of the pyrrole moiety involving both the non-shared electronic pair of the heteroatom and the C(2)*C(3) double bond. Considering the well-known increased sensitivity of the pyrrole residue in indole derivatives to oxidation as compared to the benzene ring, as well as the formation of a triple complex (peroxidase-1-O₂) proposed for the enzymatic 1 oxidative degradation mechanism involving as a key step the Fe³⁺ → Fe²⁺ transition in the enzyme form as discussed in literature, it is concluded that iron(III) coordination with 1 can influence the redox properties of the pyrrole ring by affecting its π-electronic system.     

Complexation of Indole-3-acetic Acid with Iron(III): Influence of Coordination on the π-Electronic System of the Ligand

Summary.  The iron(III) complex of indole-3-acetic acid (1) was prepared, and its physicochemical properties, mode of iron(III) coordination, and electronic structure were studied using UV/Vis, diffuse reflectance infrared Fourier transform (DRIFT), and transmission ⁵⁷Fe M?ssbauer spectroscopic techniques. The data obtained provide evidence that iron(III) is not only coordinated by the carboxylic O-donor atom, but also via the conjugated π-electronic system of the pyrrole moiety involving both the non-shared electronic pair of the heteroatom and the C(2)*C(3) double bond. Considering the well-known increased sensitivity of the pyrrole residue in indole derivatives to oxidation as compared to the benzene ring, as well as the formation of a triple complex (peroxidase-1-O₂) proposed for the enzymatic 1 oxidative degradation mechanism involving as a key step the Fe³⁺ → Fe²⁺ transition in the enzyme form as discussed in literature, it is concluded that iron(III) coordination with 1 can influence the redox properties of the pyrrole ring by affecting its π-electronic system. Received September 17, 2000. Accepted (revised) October 31, 2000     

Conformational behaviour of non-fused biheterocycles. Part XV. Isomeric phenylisoxazoles

The conformational behaviour of isomeric phenylisoxazoles has been investigated at the STO-3G level optimizing the most significant geometrical parameters. The energy minima of the two isomers having an heteroatom in an ortho position correspond to planar structures, whereas the 4-isomer has a twisted equilibrium conformation. This confirms that H? H non-bonded repulsions are stronger than (lone-pair)-H ones. The similar conformational behaviour of corresponding phenylfurans and phenylisoxazoles shows that the second heteroatom has a negligible effect and that the conformational behaviour of non-fused biheterocycles is dictated by the nature of the ortho substituents.     

Asymmetric Synthesis of Octahydrobenzofuran Core Structure with Three Contiguous Stereogenic Centers and Development of the Absolute Configurations

Cyclization of enantiopure (S/R)-1 with halogen-reagent-constructed enantiopure octahydrobenzofuran core structure with contiguous three stereogenic centers of both enantiomers (SRR and RSS). The absolute configurations of all compounds have been established from x-ray analysis of the single crystal of (3aS,7aR,7R)-3. The chiral initiation in diastereoselective mode of 5-exo ring closure across C?C bond of pendant cyclohexene moiety has been proposed.

[Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for the following free supplemental resource(s): Full experimental and spectral details.]     

Substituent effect on intramolecular hydrogen bonding in 2‐hydroxybenzaldehyde

The effect of some substituents on intramolecular hydrogen bonding of 5‐X‐2‐hydroxybenzaldehyde (5‐X‐2‐HBA) has been studied by B3LYP and MP2 methods using 6‐311++G** and AUG‐cc‐PVTZ basis sets. The relationship between hydrogen bond energy E_HB and electron donation (or withdrawal) of substituents has been investigated. An approximately good linear relationship has been detected between Hammett coefficients and hydrogen bond formation energy (R² = 0.98). Herein, population analysis has been performed by atoms in molecules (AIM) and natural bond orbital (NBO) methods. The results of AIM and NBO analyses are in a good agreement with calculated energy values. Furthermore, correlation between ring aromaticity and hydrogen bonding has been investigated by nucleus‐independent chemical shift (NICS) at GIAO/B3LYP/6‐311++G** level of theory. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Deprotonation of C‐Alkyl Groups of Cationic Triruthenium Clusters Containing Cyclometalated C‐Alkylpyrazinium Ligands: Experimental and Computational Studies

The C‐alkyl groups of cationic triruthenium cluster complexes of the type [Ru₃(μ‐H)(μ‐κ²N¹,C² ‐L)(CO)₁₀]⁺ (HL represents a generic C‐alkyl‐N‐methylpyrazium species) have been deprotonated to give kinetic products that contain unprecedented C‐alkylidene derivatives and maintain the original edge‐bridged decacarbonyl structure. When the starting complexes contain various C‐alkyl groups, the selectivity of these deprotonation reactions is related to the atomic charges of the alkyl H atoms, as suggested by DFT/natural‐bond orbital (NBO) calculations. Three additional electronic properties of the C‐alkyl C? H bonds have also been found to correlate with the experimental regioselectivity because, in all cases, the deprotonated C? H bond has the smallest electron density at the bond critical point, the greatest Laplacian of the electron density at the bond critical point, and the greatest total energy density ratio at the bond critical point (computed by using the quantum theory of atoms in molecules, QTAIM). The kinetic decacarbonyl products evolve, under appropriate reaction conditions that depend upon the position of the C‐alkylidene group in the heterocyclic ring, toward face‐capped nonacarbonyl derivatives (thermodynamic products). The position of the C‐alkylidene group in the heterocyclic ring determines the distribution of single and double bonds within the ligand ring, which strongly affects the stability of the neutral decacarbonyl complexes and the way these ligands coordinate to the metal atoms in the nonacarbonyl products. The mechanisms of these decacarbonylation processes have been investigated by DFT methods, which have rationalized the structures observed for the final products and have shed light on the different kinetic and thermodynamic stabilities of the reaction intermediates, thus explaining the reaction conditions experimentally required by each transformation.     

Theoretical study on structures and aromaticities of P5(-) anion, [Ti (eta(5)-P5)]- and sandwich complex [Ti(eta(5)-P5)2]2-

The equilibrium geometries, energies, harmonic vibrational frequencies, and nucleus independent chemical shifts (NICSs) of the ground state of P5(-) (D(5h)) anion, the [Ti (eta(5)-P5)]- fragment (C(5v)), and the sandwich complex [Ti(eta(5)-P5)2]2- (D(5h) and D(5d)) are calculated by the three-parameter fit of the exchange-correlation potential suggested by Becke in conjunction with the LYP exchange potential (B3LYP) with basis sets 6-311+G(2d) (for P) and 6-311+G(2df) (for Ti). In each of the three molecules, the P-P and Ti-P bond distances are perfectly equal: five P atoms in block P5(-) lie in the same plane; the P-P bond distance increases and the Ti-P bond distance decreases with the order P5(-), [Ti(eta(5)-P5)2]2-, and [Ti (eta(5)-P5)]-. The binding energy analysis, which is carried out according to the energy change of hypothetic reactions of the three species, predicts that the three species are all very stable, and [Ti (eta(5)-P5)]- (C(5v)), more stable than P5(-) and [Ti(eta(5)-P5)2]2- synthesized in the experiment, could be synthesized. NICS values, computed for the anion and moiety of the three species with GIAO-B3LYP, reveal that the three species all have a larger aromaticity, and NICS (0) of moiety, NICS (1) of moiety, and minimum NICS of the inner side of ring P5 plane in magnitude increase with the order P5(-), [Ti(eta(5)-P5)2]2-, and [Ti (eta(5)-P5)]-. By analysis of the binding energetic and the molecular orbital (MO) and qualitative MO correlation diagram, and the dissection of total NICS, dissected as NICS contributions of various bonds, it is the main reason for P5(-) (D(5h)) having the larger aromaticity that the P-P sigma bonds, and pi bonds have the larger diatropic ring currents in which NICS contribution are negative, especially the P-P sigma bond. However, in [Ti (eta(5)-P5)]- (C(5v)) and [Ti(eta(5)-P5)2]2- (D(5h), and D(5d)), the reason is the larger and more negative diatropic ring currents in which the NICS contributions of P-P pi bonds and P5-Ti bonds including pi, delta, and sigma bonds, especially P5-Ti bonds, are much more negative and canceled the NICS contributions of P and Ti core and lone pair electrons.     

Studies of the Reactions of 2-Substituted Dimethyl Benzoylphosphonates with Trimethyl Phosphite

Abstract

Dimethyl benzoylphosphonates (1) react with trimethyl phosphite to give anionic intermediates (2) which decompose to give carbenes (3) and trimethyl phosphate [1]. When suitable ortho-substituents are present on the benzoylphosphonate, intramolecular carbene insertion reactions can occur to give cyclic systems. With alkyl substitutents, where the length of the chain provides a choice of cyclisation pathways, insertion into an appropriate C[sbnd]H bond to give a 5-membered ring has been found to be the preferred option. We have therefore been investigating the behaviour of systems, such as (4), where insertion into a C[sbnd]H bond to give a 5-membered ring is prevented.