Frontier Orbital Considerations Research Articles

Neutrophil Leukocyte: Combustive Microbicidal Action and Chemiluminescence

Neutrophil leukocytes protect against a varied and complex array of microbes by providing microbicidal action that is simple, potent, and focused. Neutrophils provide such action via redox reactions that change the frontier orbitals of oxygen (O2) facilitating combustion. The spin conservation rules define the symmetry barrier that prevents direct reaction of diradical O2 with nonradical molecules, explaining why combustion is not spontaneous. In burning, the spin barrier is overcome when energy causes homolytic bond cleavage producing radicals capable of reacting with diradical O2 to yield oxygenated radical products that further participate in reactive propagation. Neutrophil mediated combustion is by a different pathway. Changing the spin quantum state of O2 removes the symmetry restriction to reaction. Electronically excited singlet molecular oxygen (1O2 *) is a potent electrophilic reactant with a finite lifetime that restricts its radius of reactivity and focuses combustive action on the target microbe. The resulting exergonic dioxygenation reactions produce electronically excited carbonyls that relax by light emission, that is, chemiluminescence. This overview of neutrophil combustive microbicidal action takes the perspectives of spin conservation and bosonic-fermionic frontier orbital considerations. The necessary principles of particle physics and quantum mechanics are developed and integrated into a fundamental explanation of neutrophil microbicidal metabolism.

Rational Design, Synthesis, and Characterization of Deep Blue Phosphorescent Ir(III) Complexes Containing (4′-Substituted-2′-pyridyl)-1,2,4-triazole Ancillary Ligands

On the basis of the results of frontier orbital considerations, 4-substituted-2'-pyridyltriazoles were designed to serve as ancillary ligands in 2-phenylpyridine main ligand containing heteroleptic iridium(III) complexes that display deep blue phosphorescence emission. The iridium(III) complexes, Ir1-Ir7, prepared using the new ancillary ligands, were found to display structured, highly quantum efficient (Φp = 0.20-0.42) phosphorescence with emission maxima in the blue to deep blue 448-456 nm at room temperature. In accord with predictions based on frontier orbital considerations, the complexes were observed to have emission properties that are dependent on the electronic nature of substituents at the C-4 position of the pyridine moiety of the ancillary ligand. Importantly, placement of an electron-donating methyl group at C-4' of the pyridine ring of the 5-(pyridine-2'-yl)-3-trifluoromethyl-1,2,4-triazole ancillary ligand leads to an iridium(III) complex that displays a deep blue phosphorescence emission maximum at 448 nm in both the liquid and film states at room temperature. Finally, an OLED device, constructed using an Ir-complex containing the optimized ancillary ligand as the dopant, was found to emit deep blue color with a CIE of 0.15, 0.18, which is close to the perfect goal of 0.15, 0.15.

ChemInform Abstract: Bond Length and Reactivity: The gauche Effect. A Combined Crystallographic and Theoretical Investigation of the Effects of β -Substituents on C-OX Bond Length.

The ‘variable oxygen probe’ is applied to systems with the general structure Y–C–C–OX (Y = F, H, SiR3), using both crystal structure correlations (including 25 new structures) and ab initio calculations (SCF[DZP]), for 20 structures, Y–CH2–CH2–OX: Y = H, F and SiH3(gauche and trans); X = CH3, CHO (E and Z) and NO2. The calculations reproduce conformational preferences well (all our 2-fluoroethyl derivatives crystallise with F gauche to OX). Both crystal and calculated structures give linear bond-length/pKHOX correlations. From the crystal structures: definitive correlations are derived for primary and secondary alcohol derivatives; no significant difference is observed between axial and equatorial cyclohexyl systems; β-fluorine has a small bond-shortening effect on the C–OX bond, which is greatest for the best leaving groups OX; and data for two systems with β-silicon are consistent with a bond-lengthening effect. The inductive effect of β-fluorine, observed as its effect on the C–OX bond length, shows no significant dependence on geometry.Calculated bond lengths give similar results, but show the different sensitivities to varying OX expected from frontier orbital considerations (σY–C–σ*C–OX for Y = Si > H > F). The length of the C–OX bond in Y–CH2–CH2–OX is a linear function of the electronegativity of Y. 1,3-Interaction energies calculated (at the MP2[DZP] level) for the isodesmic reaction Y–CH2–CH2–OX + CH3–CH3→ CH3–CH2–OX + Y–CH2–CH3 give a measure of the destablising effect of β-fluorine and the stabilising effect of β-silicon in these systems. In three different systems which react with participation of adjacent σ-bonds, the lengthening of the C–OX bond, observed in the ground state using the variable oxygen probe, is not accompanied by significant involvement of the adjacent orbitals. It is suggested that C–OX bond breaking, and the further structural reorganisation, are not closely coupled in such reactions.

The coordination chemistry of dipyridylbenzene: N-deficient terpyridine or panacea for brightly luminescent metal complexes?

1,3-Di(2-pyridyl)benzene (dpybH) structurally resembles the widely-used ligand terpyridine (tpy), with which it is isoelectronic. In this critical review, following a brief overview of synthetic strategies for dpybH and derivatives, we survey the different types of complex that are possible with these ligands. Whilst metals such as ruthenium(ii), osmium(ii) and platinum(ii) give a terdentate N--C--N binding mode in which cyclometallation occurs at C(2), the ions iridium(iii), rhodium(iii) and palladium(ii) favour C(4) metallation. The latter process can be blocked by appropriate ligand modification, to allow the N--C--N mode to be accessed with these metal ions too. The luminescence properties of the complexes are discussed. A huge range of emission efficiencies are encountered amongst Ir(iii) complexes containing dpyb derivatives, according to the other ligands present. Trends can be rationalised with the aid of simple frontier-orbital considerations. The Pt(ii) complexes of dipyridylbenzenes are also intensely luminescent. Their application to contemporary organic light-emitting device (OLED) technology is discussed, including white light emitters exploiting excimer emission. Their potential as cell imaging agents amenable to time-resolved detection procedures on the microsecond timescale has also been demonstrated (118 references).

Negative-Working Lithographic Diazo Resins: The Structure of 4-Diazodiphenylamines (Diazo S and Diazo MS) and Their Polymeric Formaldehyde Condensates

The structure of negative-working diazo resins has been investigated by means of quantum mechanical modeling of models for the polymerization processes leading to their formation. Comparison of the predicted heats of formation, the ultraviolet-visible (UV−Vis) spectra, and the infrared spectra with experimental results leads to the conclusion that the diazonium structure of 4-diazodiphenylamines is most stable in acidic media. The regioselectivity for the condensation polymerization reaction of such species with O-protonated formaldehyde cation was examined using atomic charge (electrostatic potential (ESP)), frontier molecular orbital, and product development approaches. Although methylol substitution at both rings seems possible, a reaction that leads to ortho- and para-substitution of the non-diazo-containing ring (β-ring) seems to be favored, according to product development and frontier orbital considerations. During subsequent polymerization, similar conclusions are reached regarding the electrophil...

Principal Component Analysis on the effect of nucleophiles on the reactivity of α-acylenaminoketones

This report shows the importance of Principal Component Analysis for grouping types of products observed when α-acylenaminoketones K1–K3 react with four substituted hydrazine nucleophiles in five organic solvents. The reactions were carried out with the goal of obtaining substituted pyrazoles and determining which of the carbonyls would preferentially be attacked by the nucleophile. The reaction products were submitted to GC-MS analysis and the results were subjected to Principal Component Analysis (PCA). The data set was separated in four groups (scores). The deacetylated pyrazoles P5 were separated from the other pyrazoles by the first principal component PC1. The second principal component PC2 separated the pyrazoles P4, derived from nucleophilic attack on the acetyl carbonyl group, from the pyrazoles P6, derived from nucleophilic attack on the carbonyl bonded to the more bulky group R (loadings analysis). The simultaneous analysis of the scores-loadings shows the relationship between the mechanisms (types of reaction products-loadings) and the reaction conditions (solvent, nucleophile). Frontier orbital considerations were also included to complete the analysis.

Anodic oxidation of 5-amino-1,4-naphthoquinone (ANQ) and synthesis of a conducting polymer (PANQ)

The electrooxidation of 5-amino-1,4-naphthoquinone (ANQ) at glassy carbon and Pt electrodes in acetonitrile leads to functionalized polymer films. Results from different spectroscopic techniques (FT-IR, UV-Vis, X-ray photoelectron spectroscopy (XPS)) and frontier orbital considerations converge to a definition of poly(5-amino-1,4-naphthoquinone) (PANQ) structure which is of the polyaniline-type bearing one quinone group per ANQ moiety.

Addition of electrophiles to unsymmetric alkenes. Effects of β oxygen substituents on the stereo- and regio-chemistry of positive halogen addition

The stereo- and regio-chemistry of two step additions of positive halogen electrophiles to 5-methylene-2-phenyl-1,3-dioxane, 1-tert-butyl-4-methylenecyclohexane and 3-methoxy-2-(methoxymethyl)prop-1-ene have been determined. The direction of initial electrophile attack is in line with the frontier orbital and electrostatic considerations described by us previously. The regiochemistry of addition is strongly affected by hyperconjugative effects, acting between the intermediate epihalonium ion and the β C–X bonds. Where the β C–X bonds bear a fixed periplanar relation to the epihalonium ion and X is more electronegative than hydrogen, anti-Markownikoff addition is strongly promoted and becomes exclusive when two such β C–X bonds are present. If the β C–X bond is free to rotate away from periplanarity, then β C–H bonds will adopt the geometry required for hyperconjugation and Markownikoff regiochemistry will be favoured. The results are consistent with ab initio theoretical calculations and can be rationalised using a simple electrostatic model.

A pentuply-bridging carbonyl group: crystal and molecular structure of a salt of the 1-oxo-2-phenyl-1,2-dicarbadodecaborate(12) anion, [LH]+[O(Ph)C2B10H10]–(L = 1,8-N,N,N′,N′-tetramethylnaphthalenediamine)

Deprotonation of the C-hydroxy ortho-carborane 1,2-HO(Ph)C2B10H10 by 1,8-N,N,N′,N′-tetramethylnaphthalene-diamine gives a salt whose anion, [O(Ph)C2B10H10]–, effectively consists of a nido-shaped [PhCB10H10]– residue capped by a pentuply-bridging carbonyl group whose C–O distance of 1.245(3)A and position over the open carborane face, 2.001(3)A from the cage carbon atom, are readily rationalized by frontier orbital considerations.

Enzyme inhibition by electrophilic cyclopropane derivatives

Electrophilic cyclopropane-containing compounds have been found in our previous studies to be latent, irreversible inhibitors of horse liver alcohol dehydrogenase, lactate dehydrogenase, and carboxypeptidase A. In addition, cyclopropyl methanols have provided information concerning the mechanism of hydrogen transfer by nicotinamide coenzymes and dehydrogenases. These results are evaluated with the aid of molecular graphics and frontier orbital considerations in the light of the established stereochemical courses of reactions involving cyclopropane ring opening. It is concluded that mechanisms for nucleophilic inhibition consistent with chemical theory and precedent are available at each enzyme's active site. Potential enzymic nucleophiles are identifed.

Frontier orbital control in hydrazino bridged [14]annulenes

[10b.10c]Diazapyraceheptylenes 1b and 1c were obtained from olefinic precursors 6. Treatment of 7 under the same conditions does not allow a synthesis of the parent compound 1a which may be explained by frontier orbital considerations.

ChemInform Abstract: 1,3-DIPOLAR CYCLOADDITION OF NITRONES WITH α,β-UNSATURATED SULFONES

C,N-Diphenyl nitrones react with substituted α,β-unsaturated phenylsulfones yielding isoxazolidinic cycloadducts whose structure and stereochemistry were assigned on the basis of 1H and 13C nmr data. The ycloaddition regioselectivity is discussed in accordance with frontier orbital considerations.

Comments on the electronic structure and reactivity of chromones

AB initio calculations of ground-state wavefunctions of chromones and thiochromones have been carried out and correlated with the observed reactivity of these molecules. Quantum mechanical electrostatic potentials correctly predict nucleophilic attack to occur preferentially at C-2 and C-4 in chromones and thiochromones respectively. Frontier-orbital considerations correctly predict the site selectivity of the photochemical cycloaddition to the pyrone double bond of chromone.

1,3‐Dipolar cycloaddition of nitrones with α,β‐unsaturated sulfones

AbstractC,N‐Diphenyl nitrones react with substituted α,β‐unsaturated phenylsulfones yielding isoxazolidinic cycloadducts whose structure and stereochemistry were assigned on the basis of 1H and 13C nmr data. The ycloaddition regioselectivity is discussed in accordance with frontier orbital considerations.

Selectivity in cycloadditions-IX: Cycloadditions of nitrile oxides to indoles. Reactivity and regiochemistry

Cycloadditions of benzonitrile oxide and mesitonitrile oxide to N-methylindole and indole yield the acid sensitive cycloadducts 1 a-d with high regioselectivity. With N-carbethoxyindole the stable cycloadducts 1 e,f and minor amounts of the regioisomeric 2 e,f are isolated. The electron withdrawing substituent reduces both the regioselectivity and the reactivity of the cycloadditions. Frontier orbital considerations, based on MINDO/3 calculations, allow elucidation of the observed changes in reactivity and regiochemistry.

Selectivity in cycloadditions—VII : Cycloadditions of nitrile oxides to thiophene and benzo[b]thlophene, regiochemistry

Thiophene adds benzo- and mesitonitrile oxide yielding mainly cycloadducts 1 or products derived from them. With benzothiophene the regioselectivity of the cycloadditions of the two nitrile oxides is lower and both the cycloadducts 10 and 11 could be isolated in a 78:22 and 26:74 ratio, respectively. Frontier orbital considerations, using EH and MINDO/3 calculations, and a comparison with the regioselectivities of related systems allowed elucidation of the observed regiochemistry. Some differences in orientation between cycloadditions and electrophilic substitution reactions of benzothiophene are pointed out and discussed.

Kinetic control of stereochemistry in the addition of unsymmetrical ketens to cyclopentadiene

Assumption of an initial crosswise approach of cyclopentadiene and a keten, combined with simple frontier orbital considerations, leads to a prediction of stereochemistry in the electrocyclic addition which has been confirmed by experiment.