Resonance Stabilization Energy Research Articles

Carbene-Assisted Arene Ring-Opening.

Despite the significant achievements in dearomatization and C-H functionalization of arenes, the arene ring-opening remains a largely unmet challenge and is underdeveloped due to the high bond dissociation energy and strong resonance stabilization energy inherent in aromatic compounds. Herein, we demonstrate a novel carbene assisted strategy for arene ring-opening. The understanding of the mechanism by our DFT calculations will stimulate wide application of bulk arene chemicals for the synthesis of value-added polyconjugated chain molecules. Various aryl azide derivatives now can be directly converted into valuable polyconjugated enynes, avoiding traditional synthesis including multistep unsaturated precursors, poor selectivity control, and subsequent transition-metal catalyzed cross-coupling reactions. The simple conditions required were demonstrated in the late-stage modification of complex molecules and fused ring compounds. This chemistry expands the horizons of carbene chemistry and provides a novel pathway for arene ring-opening.

Atmospheric degradation mechanism of anthracene initiated by OH•: A DFT prediction

Density functional theory (DFT) calculations at the M06-2X/def2-TZVP level have been employed to investigate the atmospheric oxidation mechanism of anthracene (ANT) initiated by HO•. Direct hydrogen atom abstraction from the ANT using HO• takes place hardly at ambient conditions while addition of HO• to the C1, C2, and C4 sites are thermodynamically and kinetically more advantageous. The addition reactions are controlled by the aromaticity and the kinetic trends were justified by resonance stabilization energies. The rate constants were calculated by using the Rice-Ramsperger-Kassel-Marcus (RRKM) and canonical transition state theory (CTST) methods in conjugation with zero curvature tunneling (ZCT). The overall RRKM-bimolecular rate constant at ambient conditions is 6.72 × 10−12 cm3 molecule−1 s−1, is negatively dependent on the temperature and can be expressed as k250−3501bar=3.92×10−14exp(1534.9T). Contribution of the AD-C4 path in the overall reaction is about 70–80%, implying that the dependence of overall rate constant on pressure can be ignored. The kinetic data exhibit that the ANT is degraded during its long-range transport in the atmosphere and cannot be classified as persistent organic pollutants.

Fast and Bioorthogonal Release of Isocyanates in Living Cells from Iminosydnones and Cycloalkynes.

Bioorthogonal click-and-release reactions are powerful tools for chemical biology, allowing, for example, the selective release of drugs in biological media, including inside animals. Here, we developed two new families of iminosydnone mesoionic reactants that allow a bioorthogonal release of electrophilic species under physiological conditions. Their synthesis and reactivities as dipoles in cycloaddition reactions with strained alkynes have been studied in detail. Whereas the impact of the pH on the reaction kinetics was demonstrated experimentally, theoretical calculations suggest that the newly designed dipoles display reduced resonance stabilization energies compared to previously described iminosydnones, explaining their higher reactivity. These mesoionic compounds react smoothly with cycloalkynes under physiological, copper-free reaction conditions to form a click pyrazole product together with a released alkyl- or aryl-isocyanate. With rate constants up to 1000 M-1 s-1, this click-and-release reaction is among the fastest described to date and represents the first bioorthogonal process allowing the release of isocyanate electrophiles inside living cells, offering interesting perspectives in chemical biology.

Synthesis, spectra, electronic structure, molecular docking and cytotoxicity investigation on 2-(piperidin-1-ylmethyl)-isoindoline-1,3-dione – A Mannich base system

A Mannich base molecule of 2-(piperidin-1-ylmethyl)isoindoline-1,3-dione (PPF) has been synthesized using Mannich base condensation reaction. The molecular geometry of PPF molecule was optimized by B3LYP/6-311G(d,p) basis set. The vibrational frequencies have been assigned with the aid of normal coordinate analysis. Total energy distributions (TED) were used to explain each vibrational wavenumber assignments of a molecule. The 1H and 13C NMR spectroscopic analyses were predicted by using gauge independent atomic orbital (GIAO) method. The thermal stability was determined by applying the thermo gravimetric/differential scanning calorimetry (TG/DSC) studies. The HOMO-LUMO energies, population density of states (PDOS), overlap population density of states (OPDOS), reduced density gradient (RDG), Mulliken atomic charges and molecular electrostatic potential (MEP) surface were plotted to investigate the van der Waals, attraction, repulsion, non-covalent and covalent interactions of the compound. The natural bond orbital (NBO) charge analysis was used in explaining the intermolecular charge transfer and resonance stabilization energy of molecule. Besides, the molecular docking studies have been performed. The cytotoxic activity of the PPF molecule has been resolved.

238 - Training GPs to deliver the symptoms clinic

Semiconducting composite films have been formed by plasma enhanced chemical vapor deposition (PECVD) from orthocarborane and aromatic precursors. Aromatic/orthocarborane composite films exhibit exciting potential in areas such as neutron detection and photocatalysis. An important factor in determining the electronic properties of these materials is the existence of an intact π system in the deposited film, indicating deposition of a (more or less) intact aromatic precursor. X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) data for films formed by co-deposition of orthocarborane and pyrimidine, pyridine, aniline, indicate that the integrity or decomposition of the aromatic π electron network correlates closely with aromatic resonance stabilization energy. The resonance stabilization energy of pyrimidine is low (8 kcal/mol), and the resulting films exhibit no evidence of a π network. Pyridine and aniline have resonance stabilization energies of 24.2 and 39.5 kcal/mol, respectively, and both exhibit FTIR and XPS core level shakeup features indicating intact π networks. This trend permits the prediction of film properties for a broad variety of aromatic precursors in such films.

Migratory Insertion Strategies for Dearomatization

Development of strategies for molecule functionalization by dearomatization has surged in the last two decades. The benefits of overcoming the resonance stabilization energy outweigh the cost; diverse compounds could be accessed in a short number of steps. One approach that has been of interest in recent years is the dearomatization of indoles and other (hetero)aromatic compounds by migratory insertion. The chiral σ-bond palladium intermediate could be reduced or trapped by a second functionalization. In this short review we will summarize the recently discovered reactions from our group and others in this field of metal-catalyzed dearomatizations by migratory insertion.1 Introduction2 Monofunctionalizations: Heck and Reductive Heck Reactions2.1 N-Tethered Heterocycles2.2 Non-N-tethered Heterocycles2.3 Non-heterocycles3 Dearomative Difunctionalizations: Interrupted Heck Reaction3.1 N-Tethered Heterocycles3.2 Non-N-tethered Heterocycles4 Conclusion

INTERACTION OF DERIVATIVES OF 7-AMINO-1,5-BENZO-DIAZEPIN-2-ONES WITH α,β-UNSATURATED KETONES

Derivatives of new condensed heterosystems, the tetracyclic [1,4]diazepino[3,2,1- hi ]pyrido[4,3,2- cd ]indoles and tricyclic [1,4]diazepino[2,3- g ]- and -[2,3- f ]quinolines have been synthesized by the interaction of 7-amino-4-phenyl(or methyl)-1,3,4,5-tetrahydro(or 1,3-dihydro)-2 H -1,5-benzodiazepin-2-ones with dimethyl 2-oxoglutaconate and methyl 4-oxo-2-pentenoate. The corresponding carboxylic acids of the new derivatives were synthesized by alkaline hydrolysis of the methyl esters. The direction of the cyclocondensation reaction was determined by both the structure of the diazepine ring of the initial amines and also by the structure of the α,β-unsaturated ketones. Quantum-chemical calculations have been carried out of the minimal values of the local energy of ionization and of the resonance stabilization energy on specific atoms of the initial amines. How to Cite Janciene, R.; Stumbreviciute, Z.; Vektariene, A.; Sirutkaitis, R.; Podeniene, D.; Palaima, A.; Puodziunaite, B. Chem. Heterocycl. Compd. 2010 , 46 , 998. [ Khim. Geterotsikl. Soedin. 2010 , 1235.] For this article in the English edition see DOI 10.1007/s10593-010-0614-y

Coordination behavior of 3-amino-5,5′-dimethylhydantoin towards Ni(II) and Zn(II) ions: Synthesis, spectral characterization and DFT calculations

The interaction of 3-amino-5,5′-dimethylhydantoin with nickel and zinc ions was investigated at different conditions by means of UV–Vis, FTIR spectroscopy and electrochemical methods as well as DFT quantum chemical calculations. Through calculated DFT frequencies and experimental FTIR spectra of the complexes the correlation analysis has been made. NBO analysis of the ligand and complexes provides the information about intramolecular resonance stabilization energy and charge transfer between ligand and metal ions. First derivative UV–Vis spectra were done in order to resolve the π→π* and n→π* carbonyl electron transitions of the ligand and complexes. We have shown that complexes with 1:2 stoichiometry were formed at pH closed to physiological. The stability constant was determined for 3-amino-5,5′-dimethylhydantoin (L) and Ni(II) ions using differential pulse polarography data. The existence of one complex (n¯ = 2) is proved in the presence of high ligand concentration (CL = 2 × 10−3 ÷ 6 × 10−3 mol L−1) and the values of the total constant is: lgβNI(II)-L=9.5±0.4 (I = 0.1). The UV–Vis spectroscopy and DFT calculations confirm the stoichiometry of the complexes. The results of the study are useful for incorporation with chemical equilibrium models for evaluation of the speciation and the reactions of metals with hydantoin derivatives in order to develop methods for determination of these metals in different milieu.

N-C Axially Chiral Anilines: Electronic Effect on Barrier to Rotation and A Remote Proton Brake.

N-Aryl-N-methyl-2-tert-butyl-6-methylaniline derivatives exhibit a rotationally stable N-C axially chiral structure and the rotational barriers around an N-C chiral axis increased with the increase in electron-withdrawing character of para-substituent on the aryl group. X-ray crystal structural analysis and the DFT calculation suggested that the considerable change of the rotational barriers by the electron effect of para-substituents is due to the disappearance of resonance stabilization energy caused by the twisting of para-substituted phenyl group in the transition state. This structural property of the N-C axially chiral anilines was employed to reveal a new acid-decelerated molecular rotor caused by the protonation at the remote position (remote proton brake).

Terthieno[3,2‐b]Thiophene (6T) Based Low Bandgap Fused‐Ring Electron Acceptor for Highly Efficient Solar Cells with a High Short‐Circuit Current Density and Low Open‐Circuit Voltage Loss

AbstractA terthieno[3,2‐b]thiophene (6T) based fused‐ring low bandgap electron acceptor, 6TIC, is designed and synthesized for highly efficient nonfullerene solar cells. The chemical, optical, and physical properties, device characteristics, and film morphology of 6TIC are intensively studied. 6TIC shows a narrow bandgap with band edge reaching 905 nm due to the electron‐rich π‐conjugated 6T core and reduced resonance stabilization energy. The rigid, π‐conjugated 6T also offers lower reorganization energy to facilitate very low VOC loss in the 6TIC system. The analysis of film morphology shows that PTB7‐Th and 6TIC can form crystalline domains and a bicontinuous network. These domains are enlarged when thermal annealing is applied. Consequently, the device based on PTB7‐Th:6TIC exhibits a high power conversion efficiency (PCE) of 11.07% with a high JSC > 20 mA cm−2 and a high VOC of 0.83 V with a relatively low VOC loss (≈0.55 V). Moreover, a semitransparent solar cell based on PTB7‐Th:6TIC exhibits a relatively high PCE (7.62%). The device can have combined high PCE and high JSC is quite rare for organic solar cells.

Understanding Differential Interaction of Protic and Aprotic Ionic Liquids inside Molecular Confinement.

Considering the contemporary interests of water-free reverse micelles (RMs) in the field of organic reaction medium and potential drug delivery carrier, we synthesized two different classes of ionic liquids (ILs), protic N-methyl-2-pyrrolidonium hexanoate, [NMP][Hex], and aprotic choline hexanoate, [Chl][Hex], and subsequently incorporated them in a mixture of polyoxyethylene (20) sorbitan monooleate (Tween-80) and cyclohexane. In order to understand the differential nature of interinterionic interaction of two ILs, we performed DFT calculations on pure ILs to correlate with experimental results. The formation of IL-in-oil RMs was confirmed from phase behavior and DLS studies. Interestingly, [NMP][Hex]-based systems showed a larger monophasic region and droplet size along with higher shear viscosity compared to [Chl][Hex]-based systems. Stronger interaction between [NMP]+ and Tween-80 due to their protic nature might be the driving force for such observations which supported the resonance stabilization energy [E(2)] and charge population analysis by NBO calculation. Smaller E(2) values along with lesser NBO charges on atoms involved in H-bonding in pure [NMP][Hex] than [Chl][Hex] corroborated with the experimental observations. This primary hypothesis was further confirmed from FTIR and time-resolved fluorescence studies. These systems showed efficient thermal stability. Taking all of the results together, we anticipate that these RMs could be used as efficient delivery systems and for nanomaterial synthesis.

Dioxygen: What Makes This Triplet Diradical Kinetically Persistent?

Experimental heats of formation and enthalpies obtained from G4 calculations both find that the resonance stabilization of the two unpaired electrons in triplet O2, relative to the unpaired electrons in two hydroxyl radicals, amounts to 100 kcal/mol. The origin of this huge stabilization energy is described within the contexts of both molecular orbital (MO) and valence-bond (VB) theory. Although O2 is a triplet diradical, the thermodynamic unfavorability of both its hydrogen atom abstraction and oligomerization reactions can be attributed to its very large resonance stabilization energy. The unreactivity of O2 toward both these modes of self-destruction maintains its abundance in the ecosphere and thus its availability to support aerobic life. However, despite the resonance stabilization of the π system of triplet O2, the weakness of the O-O σ bond makes reactions of O2, which eventually lead to cleavage of this bond, very favorable thermodynamically.

PECVD of boron carbide/aromatic composite films: Precursor stability and resonance stabilization energy

Semiconducting composite films have been formed by plasma enhanced chemical vapor deposition (PECVD) from orthocarborane and aromatic precursors. Aromatic/orthocarborane composite films exhibit exciting potential in areas such as neutron detection and photocatalysis. An important factor in determining the electronic properties of these materials is the existence of an intact π system in the deposited film, indicating deposition of a (more or less) intact aromatic precursor. X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) data for films formed by co-deposition of orthocarborane and pyrimidine, pyridine, aniline, indicate that the integrity or decomposition of the aromatic π electron network correlates closely with aromatic resonance stabilization energy. The resonance stabilization energy of pyrimidine is low (8 kcal/mol), and the resulting films exhibit no evidence of a π network. Pyridine and aniline have resonance stabilization energies of 24.2 and 39.5 kcal/mol, respectively, and both exhibit FTIR and XPS core level shakeup features indicating intact π networks. This trend permits the prediction of film properties for a broad variety of aromatic precursors in such films.

Initiation stage of alkene metathesis: Insights from natural bond orbital and charge decomposition analyses

The RuPR3 bonds of 1–2(a-b)-PC, RuCHPh bonds of 1a-b, 2-Inact/Act and 1a-b, 2-RCB were analyzed by charge decomposition (CDA) and natural bond orbital (NBO). We have found that the dissociation step of the RuPR3 bond is driven by charge transfer, while the RCB by polarization effects. Furthermore, the π(Cipso)–π*(RuC) interaction was associated with delocalization effects in the benzylidene ring. Likewise, the nature of the rotameric changes in the carbene was studied through the resonance stabilization energy (ENLW). 2 presented a lower ΔENLW (Inactive→Active) than 1a-b, which confirms that the delocalization effects are related to a low carbene rotameric energy.

Resonance-assisted hydrogen bonds revisited. Resonance stabilization vs. charge delocalization

The origins of stabilization in the short strong hydrogen bonds commonly referred to as "resonance-assisted" (RAHB) have been revisited using the modern valence-bond theory, the hybrid variational-perturbational interaction energy decomposition scheme and atoms in molecules (AIM) analysis. Dimers of carboxylic acids and amides have been chosen as the model structures for intermolecular RAHBs, while for the intramolecular case malondialdehyde and its substituted derivatives have been selected. The estimated (negligible) resonance stabilization energies and relative magnitudes of interaction energy components indicate that the origin of stabilization in the studied complexes is charge-delocalization. Although in the case of intramolecular RAHBs the resonance effects are much more pronounced, still they are a relatively minor contribution to the total stabilization energy. In fact, the estimated resonance stabilization energies diminish with an increasing strength of the hydrogen bond (as indicated by AIM and structural descriptors).

Conjugation, Resonance, and Stability in N-Heterocyclic Silylenes and in Phosphorus Ylide Substituted Silylenes

The role of π conjugation in the thermodynamic stabilization of N-heterocyclic silylenes and phosphorus ylide substituted silylenes is analyzed using the block localized wave function (BLW) method. This method enables the direct calculation of the resonance stabilization energies caused by π-electron delocalization and/or by cyclic 6-π-electron delocalization (aromaticity). The major advantage of the BLW method is that there is no need for reference compounds, as the fully conjugated molecule itself serves as the reference compound. The observed high stability of the C═C unsaturated N-heterocyclic silylenes 1(E = Si; silaimidazol-2-ylidene) and the C–C saturated 2 (E = Si; silaimidazolin-2-ylidene) is rationalized by their high resonance energies (REs) of 79.4 and 53.4 kcal/mol, respectively. The nuclear independent chemical shift (NICSzz(1.0)) value of −22.0 for 1 (E = Si) indicates significant diatropic ring current, implying the existence of aromaticity. The additional stabilization of 1 due to aromaticity (6-π-electron delocalization) is 14 kcal/mol, which is only 18% of the total RE of 1 but is 55% of the aromatic stabilization of benzene. Thus, aromaticity contributes to the stability of 1, but it is not a prerequisite for the isolation of N-heterocyclic silylenes. Cyclic unsaturated silylenes substituted by exocyclic phosphorus ylide substituents, i.e., 12, have calculated cyclic resonance energies and thermodynamic stabilization energies similar to those of 1 (E = Si), and they are significantly larger than those of saturated 2 (E = Si); i.e., the REs of 12 (R″ = CH3, SiH3) are 73.4 and 75.6 kcal/mol, respectively. These data suggest that 12 is sufficiently stable to be isolated, as indeed was recently reported. The energy of the lone pair orbital on Si in 12 (R″ = Ph) is higher by 1.9 eV than that of 1 (E = Si, R = t-Bu), suggesting that 12 are better σ donors and thus may exhibit higher activity in transition-metal catalysis.

Structures of N,N-Dialkoxyamides: Pyramidal Anomeric Amides with Low Amidicity

The first X-ray structures of two anomeric N,N-dialkoxyamides (2 and 3) have been obtained, which confirm that they are highly pyramidalized at nitrogen and have long N-CO bonds, a characteristic of other anomeric amides and a consequence of drastically reduced amidicity. The crystals also demonstrate chirality at the amide nitrogen in the solid state. The structures are well-predicted by density functional calculations using N,N-dimethoxyacetamide as a model. The amidicity of N,N-dimethoxyacetamide has been estimated by two independent methods, COSNAR and a new transamidation method, which give almost identical resonance stabilization energies of -8.6 kcal mol(-1) and only 47% that of N,N-dimethylacetamide computed at the same level. The total destabilization is composed of a resonance and an inductive contribution, which we have evaluated separately. The electronegative oxygens at nitrogen are responsible for localization of the nitrogen lone pair on the amide nitrogen, a factor that contributes to a loss of resonance over and above the impact of pyramidalization at nitrogen, as well as the fact that N,N-dimethoxyacetamide is predicted to protonate on the carbonyl oxygen in preference to nitrogen.

Naphthiporphyrins

Benziporphyrins, cross-conjugated porphyrin analogues with a benzene ring in place of one of the usual pyrrole units, have varying degrees of macrocyclic aromaticity because the 6π electron arene needs to give up its aromatic characteristics to facilitate conjugation over the entire system. As naphthalene would lose less resonance stabilization energy in giving up one of its benzene units, it was proposed that naphthiporphyrins would exhibit enhanced diatropicity compared to the related benziporphyrins. A naphthiporphyrin was prepared using the "3 + 1" variant of the MacDonald condensation by reacting 1,3-naphthalenedicarbaldehyde with a tripyrrane in the presence of TFA, followed by oxidation with DDQ. Although the free base form of naphthiporphyrin showed no overall diatropicity, the corresponding dication in TFA-CDCl(3) demonstrated a significant diatropic ring current where the internal CH shifted upfield to between 4.0 and 4.6 ppm. Naphthiporphyrin was converted to the corresponding palladium(II) complexes by reaction with Pd(OAc)(2) in acetonitrile, and the complex was further characterized by X-ray crystallography. Oxynaphthiporphyrins were similarly prepared by the "3 + 1" methodology from 4-methoxy-1,3-naphthalene-dicarbaldehyde, and these showed slightly enhanced diatropic character compared to oxybenziporphyrins. Reaction of oxybenziporphyrins or oxynaphthiporphyrins with silver(I) acetate afforded the corresponding silver(III) organometallic derivatives. A meso-tetraphenyl naphthiporphyrin was also synthesized in 4% yield by reacting a 1,4-naphthalene dicarbinol with 2 equiv of benzaldehyde and 3 equiv of pyrrole in the presence of BF(3).Et(2)O, followed by oxidation with DDQ. However, this 1,4-naphthiporphyrin showed reduced diatropic character compared to the corresponding p-benziporphyrin system. The NMR spectra for the 1,4-naphthiporphyrin show that the naphthalene unit pivots over the macrocycle and this presumably leads to further steric interactions that reduce the planarity of the macrocycle. These results demonstrate that while naphthiporphyrins can show enhanced aromatic properties as predicted, other factors may overwhelm this effect.

Resonance Stabilization Energy of 1,2-Azaborines: A Quantitative Experimental Study by Reaction Calorimetry

Aromatic and single-olefin six-membered BN heterocycles were synthesized, and the heats of hydrogenation were measured calorimetrically. A comparison of the hydrogenation enthalpies of these compounds revealed that 1,2-azaborines have a resonance stabilization energy of 16.6 ± 1.3 kcal/mol, in good agreement with calculated values.

Interaction of derivatives of 7-amino-1,5-benzo-diazepin-2-ones with α,β-unsaturated ketones

Derivatives of new condensed heterosystems, the tetracyclic [1,4]diazepino[3,2,1-hi]pyrido-[4,3,2-cd]indoles and tricyclic [1,4]diazepino[2,3-g]- and -[2,3-f]quinolines have been synthesized by the interaction of 7-amino-4-phenyl(or methyl)-1,3,4,5-tetrahydro(or 1,3-dihydro)-2H-1,5-benzodiazepin- 2-ones with dimethyl 2-oxoglutaconate and methyl 4-oxo-2-pentenoate. The corresponding carboxylic acids of the new derivatives were synthesized by alkaline hydrolysis of the methyl esters. The direction of the cyclocondensation reaction was determined by both the structure of the diazepine ring of the initial amines and also by the structure of the α,β-unsaturated ketones. Quantum-chemical calculations have been carried out of the minimal values of the local energy of ionization and of the resonance stabilization energy on specific atoms of the initial amines.