Frontier orbital positions

It is well known that ligands strongly influence the electrochemical properties of phthalocyanines [143]. Electron-withdrawing groups lead to a positive shift of the 

In the second reduction, protons compensated the charge over the whole pH range, but the increased slope pointed towards a number larger than stoichiometric indicating the additional interaction with anions [172]. 

In summary, the position of the frontier electron energy levels (HOMO and LUMO) of porphyrins and phthalocyanines can be finely tuned by the appropriate combination of central metal and substituted ligand as detected in photoelectron spectroscopy or in the redox potentials. A rich redox chemistry in interplay with a number of reactants and counterions has been estabhshed. A systematic consideration of the electrochemical and photoelectrochemical characteristics of porphyrins and phthalocyanines as individual molecules in solution, as molecules adsorbed at surfaces, and as molecular thin films serving to mimic the characteristics of molecular aggregates is of much relevance to the choosing or designing of optimized porphyrin or phthalocyanine sensitizers for DSSCs. 

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Correlation of frontier orbital positions and conduction type of molecular semiconductors as derived from ups in combination with electrical and photoelectrochemical experiments. 

Schlettwein, D., and N.R. Armstrong (1994). Correlation of frontier orbital positions and conduction type of molecular semiconductors as derived from UPS in combination with electrical and photoelectrochemical experiments. J. Phys. Chem. 98, 11771-11779. 

When you request an orbital, yon can use the cardinal number of the orbital (ordered by energy and starting with number=l) or an offset from either the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LL MO). Offset from the HOMO are negative and from the LUMO are positive. Often these frontier orbitals are the ones of most chemical interest. 

Frontier orbital theory predicts that electrophilic substitution of pyrroles with soft electrophiles will be frontier controlled and occur at the 2-position, whereas electrophilic substitution with hard electrophiles will be charge controlled and occur at the 3-position. These predictions may be illustrated by the substitution behaviour of 1-benzenesulfonylpyr-role. Nitration and Friedel-Crafts acylation of this substrate occurs at the 3-position, whereas the softer electrophiles generated in the Mannich reaction (R2N=CH2), in formylation under Vilsmeier conditions (R2N=CHC1) or in formylation with dichloromethyl methyl ether and aluminum chloride (MeO=CHCl) effect substitution mainly in the 2-position (81TL4899, 81TL4901). Formylation of 2-methoxycarbonyl-l-methylpyrrole with... 

The positively charged allyl cation would be expected to be the electron acceptor in any initial interaction with ethylene. Therefore, to consider this reaction in terms of frontier orbital theory, the question we need to answer is, do the ethylene HOMO and allyl cation LUMO interact favorably as the reactants approach one another The orbitals that are involved are shown in Fig. 1.27. If we analyze a symmetrical approach, which would be necessary for the simultaneous formation of the two new bonds, we see that the symmetries of the two orbitals do not match. Any bonding interaction developing at one end would be canceled by an antibonding interaction at the other end. The conclusion that is drawn from this analysis is that this particular reaction process is not favorable. We would need to consider other modes of approach to analyze the problem more thoroughly, but this analysis indicates that simultaneous (concerted) bond formation between ethylene and an allyl cation to form a cyclopentyl cation is not possible. 

In all three frontier orbital combinations shown above, the upper orbital components are the same sign, and their overlap is positive. In the two cases on the left, the lower orbital components also lead to positive overlap. Thus, the upper and lower interactions reinforce, and the total frontier orbital interaction is non-zero. Electron movement (chemical reaction) can occur. The right-most case is different. Here the lower orbital components lead to negative overlap (the orbitals have opposite signs at the interacting sites), and the total overlap is zero. No electron movement and no chemical reaction can occur in this case. 

The addition of dimethyl acetylenedicarboxylate to 5-/er/-butyl-Ar,Ar,Ar, Ar -tetramethyl-2-aza-pentalene-1,3-diamine (20) is frontier orbital rather than charge controlled, and results initially in attack at the 3a-position to give, via a dipolar intermediate, tricycle 21, which undergoes valence isomerization to the cyclopent[c]azepine 22.107... 

Naphthalene undergoes electrophihc substitutions at the a rather than p position. The Hueckel molecular orbital calculations show that all the carbons have the same jt electron density 1.0. This is not in agreement with the theory of organic reactions based on the Coulombic interaction that electrophilic attack occurs on the most negatively charged atom. Fukui [7] proposed the frontier orbital theory for the discrepancy between the theory and the experimental observation. The importance of... 

The amplitude of the frontier orbitals determines the selectivity. The most reactive atom in a molecule has the largest amplitude of the frontier orbitals. The frontier orbitals overlap each other to the greatest extent at the sites with the largest amphtudes. Reactions occur on the atoms in the electron donors and acceptors, where the HOMO and LUMO amplitudes are largest, respectively. Electrophiles prefer the a position of naphthalene, an electron donor, with the larger HOMO amplitude (Scheme 21). Nucleophiles attack the carbons of the carbonyl groups, an electron acceptor, with the larger LUMO amplitude (Scheme 7). 

Reduced nicotinamide-adenine dinucleotide (NADH) plays a vital role in the reduction of oxygen in the respiratory chain [139]. The biological activity of NADH and oxidized nicotinamideadenine dinucleotide (NAD ) is based on the ability of the nicotinamide group to undergo reversible oxidation-reduction reactions, where a hydride equivalent transfers between a pyridine nucleus in the coenzymes and a substrate (Scheme 29a). The prototype of the reaction is formulated by a simple process where a hydride equivalent transfers from an allylic position to an unsaturated bond (Scheme 29b). No bonds form between the n bonds where electrons delocalize or where the frontier orbitals localize. The simplified formula can be compared with the ene reaction of propene (Scheme 29c), where a bond forms between the n bonds. 

The electrostatic mixing by the positive charge polarizes rin the same direction (Scheme 12b, cf. Scheme 8a), possibly more significantly than the overlap mixing. The n orbital is the frontier orbital. The proton attacks on C. The regioselectivity is reversed. 

The frontier orbital theory was developed for electrophilic aromatic substitution (Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume). Application is successful to the ortho-para orientation (Scheme 23a) for the benzenes substituted with electron donating groups. The ortho and para positions have larger HOMO amplitudes. The meta orientation (Scheme 23b) for the electron accepting groups is under control of both HOMO and the next HOMO [25]. 

According to these consideration the diamino-substituted phosphenium (an alternative suggestion for its nomenclature is phosphanylium) cation, 5, and the phosphanetriylammonium (iminophosphenium) cation, 6, possess the largest intrinsic (gas phase) stabihties. Since in the X-ray structures the molecules are to a first-order isolated, this theoretical stability scale determined for the gas phase should also mimic the various trends of the stabilities of the cations and their chelation behaviour. The methylenephosphenium, 7, and the PjH cations, 8, suffer from poor stabihties. On the other hand the phosphirenium cation, 11, is considered to be fairly well stabilized. It is due to n-electron delocalisation of the positive charge in the phosphirenium cation. Intermediate cases in stabihty are the PO+ (9) and PS+ cations (10). Of further interest are the frontier orbital considerations, as shown in Fig. 2. 

Eq. (3.25) stands for Mulliken s overlap and orientation principle. The charge-transfer interaction takes place according to the way in which the overlap of HO of the donor and LU of the acceptor becomes maximum. Particularly, the single-site interaction will occur at the position of the greatest HO density of the donor and at the position of the greatest LU density of the acceptor, as is seen from Eq. (3.26). In such cases the particular role of the frontier orbitals is evident. 

It is thus evident that the reaction path is controlled by the frontier-orbital interaction. The position of reaction will be determined by the rule of maximum overlapping of frontier orbitals, that is, HO and LU MO s of the two reacting molecules. Sometimes SO takes the place of HO or LU in radicals or excited molecules. Hence, the general orientation principle would be as follows ... 

The model predicts that frontier orbital-controlled addition at position (1) should be favorable for octahedral or pseudooctahedral complexes of... 

It is tempting to conclude from their epr spectra that the bond angles for ANM and BNM are the same in both syn and anti isomers and that the different behavior of these carbenes is due solely to electronic effects. The frontier-orbital electron population at the a-position of naphthalene is nearly twice that at the P-position (Fleming, 1976). We will detail below how this difference can influence AGST and thereby the properties of these carbenes. 

Thus we find that the reaction is a syn (suprafacial) addition with respect to both the diene and dienophile. The frontier orbitals involved shows that the reaction occurs by interaction of HOMO and LUMO. So there is no possibility of substituents to change their position. Substituents which are on the same side of the diene or dienophile will be cis on the newly formed ring as is seen between the reaction of dimethyl maleate (a cis dienophile) with 1,3 butadiene. The product formed is cis 4,5 dicarbomethoxy cyclohexane. 

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