Isomerism in dihalogenometal porphyrins and phthalocyanines: a theoretical study of dihalogenotitanium complexes

Trans- and cis-isomers of dihalogenotitanium porphyrins and phthalocyanines, TiX₂P and TiX₂Pc (X = F, Cl, Br, I; P = porphyrin, Pc = phthalocyanine), which provide a good example of isomerism in dihalogenometal porphyrin and phthalocyanine complexes, have been studied by density functional theory calculations using the B3LYP hybrid method and triple-ζ valence basis sets. All eight complexes have two isomers, trans-isomer of D _4h symmetry and cis-isomer of C _2v symmetry with halogen atoms located in the same plane with two meso-atoms. In the case of difluorotitanium phthalocyanine the cis-isomer is preferable in the gas phase, while in all porphyrin complexes and remaining three phthalocyanine complexes the trans-isomers have lower energies than cis-isomers. Electrostatic repulsion between the halogen atoms and central nitrogen atoms of the macroheterocycle seems to play the major role in determining the preference of the trans- or cis-isomer.     

Theoretical investigation of ethylene/1-butene copolymerization process using constrained geometry catalyst (CpSiH2NH)-Ti-Cl2

The ethylene/1-butene copolymerization using constrained geometry catalyst CpSiH₂-NH-TiCl₂ (CGC) was investigated by the density functional theory and molecular dynamics. Structures and energetics of reactants, π-complexes, transition states, and products during insertion of ethylene and 1-butene monomers into the catalytic reactive site of the CGC were investigated by the density functional theory (DFT) using the software Dmol³, while dynamics of atoms during copolymerization process was investigated by classical molecular dynamics (MD) using the New-Ryudo-CR program. The calculated results were compared with the available experimental and theoretical ones. It was found that the ethylene insertion into Ti-Me active species is energetically more favorable than the butene one and the 2,1-butene insertion is more favorable than 1,2-butene one. Once the initial ethylene insertion has taken place, the further ethylene insertion occurring with a less energy barrier, in good agreement with experimental findings.     

Rhodium‐catalysed hydrogenation of enamides with monodentate phosphorous ligands. A density functional theory study

The hydrogenation of enamides catalysed by rhodium complexes of monodentate phosphorous ligands has been studied by density functional theory. The role of trans intermediates, made accessible by the non‐chelating nature of the ligand, has been taken into account. The findings here reported show that cis intermediates play the major role in the mechanism of the reaction, suggesting that data obtained with chiral monodentate phosphorous ligands or with a mixture of them should be interpreted excluding the intervention of structures with trans phosphine arrangement. Thus, results observed with monodentate phosphorous ligands must be interpreted in the light of the exclusive intervention of cis intermediates, without involvement of structures with trans phosphine arrangements. Copyright © 2010 John Wiley & Sons, Ltd.     

Electron affinities of cyano-substituted ethylenes

Electron affinities of ethylene and six cyano-substituted ethylenes (cyanoethylene, 1, 1-dicyanoethylene, cis-1, 2-dicyanoethylene, trans-1, 2-dicyanothylene, tricyanoethylene, and tetracyanoethylene) were determined using six different density functional or hybrid Hartree-Fock/density functional methods. Equilibrium geometries and harmonic vibrational frequencies for each species were determined with each density functional method. Experimental electron affinities exist for three of the six systems studied (cyanoethylene, trans-1, 2-dicyanoethylene, and tetracyanoethylene); for the three systems, the absolute average EA errors for the different methods are 0.10eV (BLYP), 0.19ev (BHLYP), 0.22eV (B3LYP), 0.20eV (BP86), 0.78eV (B3P86), and 0.81eV (LSDA). The electron affinities of gem-dicyanoethylene, cis-discyanoethylene, and tricyanoethylene are not known from experiment but are predicted here to be 1.23eV (gem-dicyanoethylene), 1.32eV (cis-dicyanoethylene), and 2.41eV (tricyanoethylene). Contrary to earlier suggestions, tetracyanoethylene is predicted to be planar, rather than twisted. Density functional theory predicts that the ²B_1u state of the ethylene anion lies lower than the ²B_2g state, which is reported by experimentalists as the (transient) ground state, and lower than the ²A_g state. Coupled-cluster results indicate that the ²A_g state is lower than either the ²B_2g or ²B_1u states. The energetic stabilization of cyano substitution on ethylene results from the π and π? conjugation of multiple cyano groups. The HOMO-LUMO gap in ethylene decreases with increasing cyano substitution, from 7.2eV in C₂H₄ to 3.8eV in C₂(CN)₄, explaining the extreme difference between the electron affinities of ethylene (negative) and tetracyanoethylene (～T3.0eV).     

A systematic angle-resolved photoelectron study of π orbitals in unsaturated organic molecules

The angular distribution parameter β has been determined for the π orbitals of 1,1-dichloroethylene, tetrachloroethylene, tetrafluoroethylene, propylene, 1-butene, isobutylene, cis-2-butene, trans-2-butene, 1,3-butadiene, cyclopentene, methylacetylene and furan in the photon energy range 10–30 eV using dispersed polarized synchrotron radiation. The energy dependence of β in the photoelectron energy range 2–10 eV was found to be similar for all the π orbitais investigated. The potential use of the energy dependence of β as an aid in making molecular-orbital assignments is discussed. Resonance photoionization features similar to those observed for the π orbitals of acetylene, ethylene and benzene were found in many of the π orbitals of the molecules investigated herein. Possible explanations for these resonance features are discussed.     

Application of activation hardness in perturbed pericyclic reactions: a case study involving electrocyclic ring opening reactions of heterocyclobutenes

Cyclobutene undergoes electrocyclic ring opening through conrotatory mode to give 1,3‐butadiene and similarly its skeletally substituted analogues viz. phosphetene, thietene, oxetene and azetene isomerize to give their respective heterodienes. Quantum mechanical investigations on these perturbed pericyclic isomerizations at B3LYP level with 6‐31G(d) and 6‐311 + G(d,p) basis sets reveal that hetero ‐ atoms significantly lower the barrier and alter the reaction free energies. Interestingly frontier orbital correlation diagram shows that σ and σ* levels are perturbed significantly while π and π* levels undergo little changes during the reaction; the lone pairs on hetero – atoms are slightly stabilized in the TS. This results in σ becoming HOMO in the TSs of phosphetene and thietene isomerizations and σ* becoming LUMO in all the cases. This necessitates defining hardness values based on closely interacting frontier orbitals for reactants and TSs and use them to compute activation hardness values to interpret relative reactivity. Copyright © 2010 John Wiley & Sons, Ltd.     

Irreversible cis-trans-Photoisomerization of Porphyrin Dimers with an Ethylene Bridge

On the basis of a comparison of the stationary absorption, fluorescence, and fluorescence excitation spectra of solutions of synthetic dimers of octaethylporphyrin (H₂OEP) and its zinc complex (ZnOEP) of composition ZnOEP–ZnOEP and ZnOEP–H₂OEP linked by an ethylene bridge in cis- and trans-geometry, we conclude that the cis-isomers of the above dimers, when illuminated, irreversibly change to their corresponding trans-isomers.     

Concerted SN2 mechanism for the hydrolysis of acid chlorides: comparisons of reactivities calculated by the density functional theory with experimental data

DFT computations have been performed in acetone and water solvents in order to investigate the mechanism of hydrolysis of acid chlorides. Acetyl chloride and chloroacetyl chloride hydrolyze via concerted, one‐step S_N2 mechanism, with the attack of water at the sp² hybridized carbon atom of the C?O group, and the transition state (TS) has distorted tetrahedral geometry. Solvent molecules act as general base and general acid catalysts. The TS of chloroacetyl chloride is tighter and less polar than the TS of acetyl chloride. The structure of the S_N2 TS for the hydrolysis of benzoyl chlorides changes with the substituents and the solvent. Tight and loose TSs are formed for substrates bearing electron withdrawing (e‐w) and electron donating (e‐d) groups, respectively. In acetone, only the e‐w effect of the substituents increase the reactivity of the substrates, and the change of the structure of the TSs with the substituents is small. In water, polar and very loose TSs are formed in the reactions of benzoyl chlorides bearing e‐d substituents, and the rate enhancing effect of both e‐d and e‐w groups can be computed at higher level of theory. Calculated reactivities and the changes of the structure of the TSs with substituents and solvent are in accordance with the results of kinetic studies. In S_N2 nucleophilic substitutions late/early TSs are formed if the attacking reagent is poorer/better nucleophile than the leaving group, and loose/tight TSs are formed for substrates bearing e‐d/e‐w substituents and in protic/aprotic solvents. Copyright © 2010 John Wiley & Sons, Ltd.     

On the photophysics of four heteroleptic iridium(III) phenylpyridyl complexes investigated by relativistic multi-configuration methods

ABSTRACT

In this work, we present a detailed analysis of the photophysical properties of four phosphorescent iridium(III) complexes, i.e. trans-N,N- and cis-N,N-(ppy)₂Ir^III(acac) as well as their fluorinated derivatives trans-N,N- and cis-N,N-(F₂ppy)₂Ir^III(acac). These properties include absorption and emission characteristics, intersystem crossing rates from the lowest singlet excited state, phosphorescence lifetimes of the individual triplet sublevels as well as the orientations of the transition dipole vectors. To this end, we have carried out combined density functional theory and multi-reference configuration interaction studies including spin–orbit coupling by perturbational as well as variational procedures. For the experimentally known complexes, we observe excellent agreement between our computed data and literature data. Also the blueshifts of the emission maxima occurring upon fluorination of the (ppy)₂Ir(acac) compounds are well reproduced. To our surprise, we find the experimentally not yet investigated cis-N,N-(F₂ppy)₂Ir(acac) isomer to be thermodynamically more stable than the well-known blue phosphorescent emitter trans-N,N-(F₂ppy)₂Ir(acac).     

Structures and Vibrational Spectra of the Hydrogen-Bonded Complexes Between Nitrous Acid and Acetone: Ab Initio and DFT Studies

ABSTRACT

The structures, stability, and vibrational spectra of the binary complexes formed between acetone and nitrous (trans and cis) acid have been investigated using ab initio calculations at the SCF and MP2 levels and B3LYP calculations with 6-311++G(d,p) basis set. Full geometry optimization was made for the complexes studied. It was established that the complex (CH₃)₂CO···HONO-trans is more stable than the complex (CH₃)₂CO···HONO-cis by 0.5–0.8 kcal·mol^?1. The accuracy of the calculations has been estimated by comparison between the predicted values of the vibrational characteristics (vibrational frequencies and infrared intensities) and the available experimental data. It was established, that the methods, used in this study are well adapted to the problem under examination. The predicted values with the B3LYP/6-311++G(d,p) calculations are very near to the results, obtained with MP2/6-311++G(d,p). The calculated frequency shift Δν(O[sbnd]H) for the complex (CH₃)₂CO···HONO-trans (1A) is larger than for the complex (CH₃)₂CO···HONO-cis (1B). In the same time the intensity of this vibration increases dramatically upon hydrogen bonding. The calculated increase for the complex 1A is up to 15 times and for the complex 1B is up to 30 times. The changes in the vibrational characteristics (vibrational frequencies and infrared intensities) of (CH₃)₂CO upon the complexation are more insignificant than the changes in the vibrational characteristics of HONO-trans and HONO-cis.     

Luminescence Study of cis- and trans-Stilbenes in the Sol-Gel System

The emission spectra of cis- and trans- stilbene molecules doped in sol-gels(tetraethoxysilane system) have been measured during the sol-gel transformation. Changes in the emission spectra of both molecules were observed as the system changed from sol stage to dried glass over a period of 14 days. Our results indicate that the motion of the cistrans photoisomerization decreases in accord with the rigidity changes of the surrounding system. Although the transcis photoisomerization was not observed in the sol-gel system, the twisting motion of the single bond between the phenyl group and the ethylene moiety during the photoexcitation was detected in this system. The existence of the transient intermediates will be discussed to interpret the subtle changes of the emission spectra of the stilbene molecules encapsulated in the sol-gel system.     

A thorough understanding of the Diels–Alder reaction of 1,3‐butadiene and ethylene

The Diels–Alder (DA) reaction is one of the most important reactions in organic chemistry. The controversy surrounding this reaction as to whether it follows a concerted or stepwise mechanism has existed for a long time. The reaction of 1,3‐butadiene and ethylene is the paradigmatic example of the DA reaction. We have reinvestigated the mechanism of this reaction using density functional theory. The theoretical study considered all types of possible pathways for the reaction of 1,3‐butadiene and ethylene using six functionals at different rungs of Jacob's ladder. Therefore, a complete picture is given for a thorough understanding of the iconic DA reaction, and a new stationary point during the reaction processes has been reported for the first time. The calculated results indicated that three functionals, ωB97X‐D, M06‐2X, and B2‐PLYP, of the fourth and fifth rungs of Jacob's ladder performed well in the investigation of the mechanism of this reaction and that the reliable basis set should be larger than 6‐311+G(2d,p). The cis‐1,3‐butadiene more easily reacted with ethylene compared with 1,3‐butadiene in the trans conformation. The concerted mechanism was found to be energetically favorable, whose energy barrier is around 10 kcal/mol lower than that of the stepwise mechanism. Two investigated solvents, toluene and CH₃CN, had little impact on this simple DA reaction. Copyright © 2014 John Wiley & Sons, Ltd.     

Study of satellite structure found in the photoelectron spectra of gaseous alkenes

The satellite structure in the photoelectron spectrum of the C(1s) shell has been studied for the following compounds: ethylene, propylene, 1-butene, trans-2-butene, cis-2-butene, butadiene and, for comparison, propane. The structure can be divided into two parts: a relatively sharp peak due to π → π* excitation and a broad complex band involving excitation of σ orbitals. Calculation of electron shakeup probabilities has been carried out using CNDO wave functions and compared with experiment. Reasonably good agreement was obtained, and the general problem of characterizing carbon—carbon double bond behavior by satellite structure due to electron shakeup is discussed.     

The Proton NMR Assignment of 1-Aminocyclo-propane-1-carboxylic Acid

The two sets of diastereomeric hydrogen atoms cis and trans to the carboxylate of 1-aminocyclopropane-1-carboxylic acid (ACC) were differentiated by a ¹H-nmr study. The measurement of the ¹H-nmr spectrum of ACC at pH 3.8 during the successive addition of non-chiral lanthanide reagents such as Eu(NO₃)₃, Pr(NO₃)₃ or Gd(NO₃)₃ demonstrated that the hydrogen atoms cis to the carboxylate function of ACC resonate at δ=1.42 and that the hydrogen atoms trans to the carboxylate function of ACC resonate at δ=1.20. The mono-substituted cyclopropanes, cyclo-propanecarboxylic acid (CPC) and cyclopropyl-amine (CPA) were used in complementary lanthanide-reagent shift titrations along with the back titration of an ACC-Eu⁺⁺⁺ complex with DCl to further substantiate the assignment. This assignment allows for the non-destructive, nonisotopic diluting analysis of various biosynthetically derived deuterated ACC's formed from the corresponding deuterated S-adenosyl-L-methionine.     

The cis-effect using the topology of the electronic charge density

We provide a physics-inspired coupling mechanism explaining the cis-effect in terms of electronic and nuclear degrees of freedom and explore the implications for three families of molecules. The cis- or trans-effect is related to the tendency of electronic charge density to move away from the bond critical point (BCP) and towards the associated nuclear attractors. A quantitative measure of this effect is given by the λ ₃ eigenvalue of the Hessian matrix of the electronic charge density. The physical origin of the cis-effect is tied to the observation that the central X=X, X=C or N bond-paths of the cis-isomers are more bent (they are up to 1.5% longer than the internuclear distance) than the bond-paths of the corresponding trans-isomers. Greater bond-path bending is associated with a stronger cis-effect; the direction of bond deformation can in all cases be predicted by the most facile (least compressible) mode of the electronic stress tensor. Further to this, the ellipticity ε of the X=X BCPs of a molecule displaying the cis-effect is lower in the cis-isomer than for the corresponding trans-isomer, suggesting that the cis-effect is less counterintuitive than previously thought. The molecules that exhibit the greatest cis-effect are those with fluorinated double bonds; this is because the most facile modes of the C–F bond couple with the highest-symmetry normal mode of vibration. Qualitative agreement is found with existing experimental data and predictions are made where experimental data is lacking.     

Mechanism of nucleophilic substitutions at phenacyl bromides with pyridines. A computational study of intermediate and transition state

DFT computations have been performed on nucleophilic substitutions of phenacyl bromides with pyridines to investigate the mechanism of the reaction. In contrast with earlier suppositions, tetrahedral intermediate is not formed by the addition of pyridine on the C?O group of phenacyl bromide, because the total energy of the reacting species increases continuously, when the distance between the N and C(?O) atoms of reactants is shorter than 2.7 Å. At a greater distance, however, a bridged complex of the reactants is observed, in which the N atom of pyridine is slightly closer to the C atom of the C?O, than to the C atom of the CH₂Br group of phenacyl bromide, the distances are 2.87 and 3.05 Å, respectively. The attractive forces between the oppositely polarized N and C(?O) atoms in the complex decrease the free energy of activation of the S_N2 attack of pyridine at the CH₂Br group. The calculated structural parameters of the S_N2 transition states (TS) indicate, that earlier TSs are formed when the pyridine nucleophile bears electron‐donating (e‐d) groups, while electron‐withdrawing (e‐w) groups on phenacyl bromide substrate increase the tightness of the TS. Free energies of activation computed for the S_N2 substitution agree well with the data calculated from the results of kinetic experiments and correlate with the σ_Py substituent constants, derived for pyridines, and with the Hammett σ constants, when the substituents (4‐MeO‐4‐NO₂) are varied on the pyridine or on the phenacyl bromide reactants. Copyright © 2008 John Wiley & Sons, Ltd.     

Long-Lived Luminescent Re(I) Complexes Containing cis-Carbonyl and Bidentate Phosphine Ligands

Highly emissive Re(I) complexes, hydrophobic cis-Re(CO)₂(c-dppene)(dpphen) and water-soluble cis-Re(CO)₂(c-dppene)(SO₃-dpphen) with a negative charge, were synthesized [where c-dppene is cis-(bis(diphenylphosphino)ethylene, dpphen is 4,7-diphenyl-1,10-phenanthroline, and SO₃-dpphen is its disulfonate derivative]. These Re(I) complexes have significantly higher molar absorption in the 350- to 490-nm region compared to their tricarbonyl Re(I) counterparts and their emission spectra are similar to those of Ru(II) polypyridyl complexes. The luminescence lifetimes of these Re(I) complexes approach 10 s in the absence of oxygen in both aqueous and less polar solvents. The complexes have limiting anisotropy values exceeding 0.3. As both ligands, the diimine and the bidentate phosphine, can be modified by adding different substituents, it should be possible to make cis-Re(CO)₂(diimine)(P-P) derivatives which are capable of labeling biomacromolecules for biochemical and biophysical studies.     

High diastereoselectivity induced by intermolecular hydrogen bonding in [3 + 2] cycloaddition reaction: experimental and computational mechanistic approaches

A diastereoselective [3 + 2] cycloaddition of N‐aryl substituted maleimides with N,α‐diphenyl nitrone possessing 11‐hydroxyundecyloxy as a flexible substituent was performed. Experimental and comprehensive mechanistic density functional theory studies reveals that intermolecular H‐bonding and steric repulsive interaction predominate exo‐Z and exo‐E cycloaddition transition states, respectively. The reaction proceeded smoothly depending on the reactants and gave a good yield of (syn) cis‐isoxazolidine or (anti) trans‐isoxazolidine as a single diastereomer.     

Diffusion of small penetrant molecules in polybutadienes

The diffusion coefficient D in the dilute limit for three different penetrants – oxygen, water, and methanol – in three different conformations of polybutadiene (all cis-1,4, all trans-1,4, and a random copolymer containing 50% trans-1,4, 40% cis-1,4, and 10% vinyl-1,2 repeat units) has been computed using molecular dynamics simulations for temperatures in the range T?=?300–400?K. Simulation runs of 25 and 50?ns were made for each of the 45 combinations of penetrant, conformation, and temperature studied. Over this temperature range the density of the all-cis-1,4 conformation is higher than that of the all-trans-1,4 and random copolymer conformations, which are approximately equal. For all three conformations, D for oxygen and water are comparable and larger than that of methanol. However for a given penetrant, strong differences were observed in the rate of increase of D for the three conformations. We find that the activation barriers for the three penetrants are generally between 20 and 30?kJ?mol^?1, in agreement with experimental results. The magnitude of the activation energy is directly proportional to the size, rather than the mass, of the penetrant molecule.     

Structural and electronic effects in metallocene catalysts studied by x‐ray techniques

A series of metallocenes, namely, [Cp₂ZrCl₂], [(MeCp)₂ZrCl₂], [(nBuCp)₂ZrCl₂], [(iBuCp)₂ZrCl₂], [(tBuCp)₂ZrCl₂], [Et(Ind)₂ZrCl₂], [Et(IndH₄)₂ZrCl₂], and [MeSi₂(Ind)₂ZrCl₂)] were analyzed by extended x‐ray absorption fine structure (EXAFS) and x‐ray photoelectron spectroscopy (XPS). Complementary techniques, UV–vis spectroscopy and cyclic and differential pulse voltammetry, were employed to characterize the organometallic complexes. The catalysts were evaluated in ethylene polymerization, having methylaluminoxane (MAO) as the cocatalyst, and the resulting polymers were characterized by gel permeation chromatography. The structural and electronic effects caused by the coordination sphere around the metal center and their effects on the catalytic activity and polymer characteristics are discussed. Copyright © 2008 John Wiley & Sons, Ltd.