Pi-electronic Structure Research Articles

Synthesis, Reactions, and Electronic Properties of 16 π-Electron Octaisobutyltetraphenylporphyrin

The reaction of the doubly oxidized beta-octaisobutyl-meso-tetraphenylporphyrin (OiBTPP, 4), which has a 16 pi-electronic structure at the porphyrin core, with a variety of metal reagents was investigated. The reaction of 4 with SnCl(2) followed by ethanolysis afforded an 18 pi-electron tin complex, (OiBTPP)Sn(OEt)(2) (5), in a redox manner. No reactions were observed using zerovalent metals (Zn, Cu, and Pd). However, the reaction of 16pi [(OiBTPP)Li](+)[BF(4)](-) (6), which was easily derived from 4, with Zn, Cu, and Pd(2)(dba)(3) gave the corresponding 18pi metalloporphyrins (OiBTPP)M (7, M = Zn(EtOH); 8, M = Cu; and 9, M = Pd). One-electron oxidation of the copper complex 8 by AgSbF(6) afforded a 17 pi-electron cation radical complex, [(OiBTPP)Cu](*+)[SbF(6)](-) (10). The UV-visible and electron spin resonance spectra of 10 were quite similar to those of previously reported beta-octaethyl-meso-tetraphenylporphyrin (OETPP) derivatives, [(OETPP)Cu](*+)X(-) (X = ClO(4), I). In contrast to the reaction of 6 with Zn to give the 18pi complex 7, the reaction of 4 with divalent ZnCl(2) enabled us to isolate a new 16pi porphyrin-zinc(II) complex, [(OiBTPP)Zn(Cl)](+)[ZnCl(3)](-) (11), in 92% yield. The solid-state structures of 5 and 7-11 were unambiguously determined by single-crystal X-ray crystallography. The porphyrin cores of 10 (17pi) and 11 (16pi) are much more distorted than those of the 18pi derivatives 5 and 7-9. Furthermore, the bond distances of 10 and 11 clearly showed the presence of bond alternation in contrast to aromatic 18pi species 8 and 7, respectively. Nucleus-independent chemical shift calculations of 4 and some metalated porphyrins indicated that the highly distorted 16pi porphyrins are essentially nonaromatic, with only weak antiaromaticity. Magnetic circular dichroism studies in conjunction with ZINDO/S calculations assisted in identifying the electronic transitions of the UV-vis spectra of key porphyrins. Electrochemical and thin-layer UV-vis spectroelectrochemical experiments on 4 (16pi) and 11 (16pi) indicated that both compounds can be electroreduced to give the 18pi species, with the 16pi/18pi transition being reversible in the case of [(OiBTPP)Zn(Cl)](+)[ZnCl(3)](-) (11).

Self-Assembled Carbon Nanotubes on Gold: Polarization-Modulated Infrared Reflection−Absorption Spectroscopy, High-Resolution X-ray Photoemission Spectroscopy, and Near-Edge X-ray Absorption Fine Structure Spectroscopy Study

Recently we reported noncovalent functionalization of nanotubes in an aqueous medium with ionic liquid-based surfactants, 1-dodecyl-3-methylimidazolium bromide (1) and 1-(12-mercaptododecyl)-3-methylimidazolium bromide (2), resulting in positively charged single-wall carbon nanotube (SWNT)-1,2 composites. Thiolation of SWNTs with 2 provides their self-assembly on gold as well as templating gold nanoparticles on SWNT sidewalls via a covalent -S-Au bond. In this investigation, we studied the electronic structure, intermolecular interactions, and packing within noncovalently thiolated SWNTs and also nanotube alignment in the bulk of SWNT-2 dried droplets and self-assembled submonolayers (SAMs) on gold by high-resolution X-ray photoemission spectroscopy (HRXPS), C K-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS). HRXPS data confirmed the noncovalent nature of interactions within the nanocomposite of thiolated nanotubes. In PM-IRRAS spectra of SWNT SAMs on gold, the IR-active vibrational SWNT modes have been observed and identified. According to PM-IRRAS data, the hydrocarbon chains of 2 are oriented with less tilt angle to the bare gold normal in a SAM deposited from an SWNT-2 dispersion than those of 1 deposited from an SWNT-1 dispersion on the mercaptoethanesulfonic acid-primed gold. For both the dried SWNT-2 bulk and the SWNT-2 SAM on gold, the C K-edge NEXAFS spectra revealed the presence of CH-pi interactions between hydrocarbon chains of 2 and the pi electronic nanotube structure due to the highly resolved vibronic fine structure of carbon 1s --> R*/sigma*C-H series of states in the alkyl chain of 2. For the SWNT-2 bulk, the observed splitting and upshift of the SWNT pi* orbitals in the NEXAFS spectrum indicated the presence of pi-pi interactions. In the NEXAFS spectrum of the SWNT-2 SAM on gold, the upshifted values of the photon energy for R*/sigma*C-H transitions indicated close contact of 2 with nanotubes and with a gold surface. The angle-dependent NEXAFS for the SWNT-2 bulk showed that most of the molecules of 2 are aligned along the nanotubes, which are self-organized with orientation parallel to the substrate plane, whereas the NEXAFS for the SWNT-2 SAM revealed a more normal orientation of functionality 2 on gold compared with that in the SWNT-2 bulk.

Excited states of thiophene: ring opening as deactivation mechanism

(Time-dependent) Kohn-Sham density functional theory and a combined density functional/multi-reference configuration interaction method (DFT/MRCI) were employed to explore the ground and low-lying electronically excited states of thiophene. Spin-orbit coupling was taken into account using an efficient, nonempirical mean-field Hamiltonian. Phosphorescence lifetimes were calculated by means of spock.ci, a selecting direct multi-reference spin-orbit configuration interaction program. Throughout this paper, we use the following nomenclature: S1, S2,..., T1, T2,..., denominate electronic structures in their energetic order at the ground state minimum geometry, whereas S1, S2,..., T1, T2,..., refers to the actual order of electronic states at a given nuclear geometry. Multiple minima were found on the first excited singlet (S1) potential energy hypersurface with electronic structures S1 (piHOMO-1-->pi+piHOMO-->pi), S2 (piHOMO-->pi), and S3 (piHOMO-->sigma*) corresponding to the 2 1A1 (S1), 1 1B2 (S2), and 1 1B1 (S3) states in the vertical absorption spectrum, respectively. The S1 and S2 minimum geometries show out-of-plane deformations of the ring. The S3 electronic structure yields the global minimum on the S1 surface with an adiabatic excitation energy of merely 3.81 eV. It exhibits an asymmetric planar nuclear arrangement with one significantly elongated C-S bond. A constrained minimum energy path calculation connecting the S1 and S3 minima suggests that even low-lying vibrational levels of the S1 potential well can access the global minimum of the S1 surface. Nonradiative decay of the electronically excited singlet population to the electronic ground state via a close-by conical intersection will be fast. According to our work, this ring opening mechanism is most likely responsible for the lack of fluorescence in thiophene and the ultrafast decay of the S1 vibrational levels, as observed in time-resolved pump-probe femtosecond multiphoton ionization experiments. An alternative relaxation pathway leads from the S1 minimum via vibronic coupling to the S2 potential well followed by fast inter-system crossing to the T2 state. For an estimate of individual rate constants a quantum dynamical treatment will be required. The global minimum of the T1 surface has a chair-like nuclear conformation and corresponds to the T1 (1 3B2, piHOMO-->pi) electronic structure. Phosphorescence is weak here with a calculated radiative lifetime of 0.59 s. For the second potential well on the T1 surface with T3 (1 3B1, piHOMO-->sigma*) electronic structure, nonradiative processes are predicted to dominate the triplet decay.

Phthalocyanines: old dyes, new materials. Putting color in nanotechnology

Phthalocyanines are versatile building blocks for fabricating materials at the nanometer scale. These colored macrocycles exhibit fascinating physical properties which arise from their delocalized pi-electronic structure. This article describes why these molecules are targets for different scientific purposes and technological applications.

Retro-Leapfrog and Related Retro Map Operations

Operations on maps are well-known theoretical tools for transforming a given polyhedral tessellation. Several theoretical investigations of fullerenes, such as their pi-electronic structure and stability, need information on the original map which was transformed into a larger molecular structure. In this respect, retro-operations, particularly those of the most used leapfrog, chamfering, and capra operations, appear particularly useful in searching the associate graphs of fullerenes. A series of analyzed cages proved to be leapfrog transforms of smaller cages. This information was useful in understanding their closed pi-electronic structure and related properties including the local aromaticity. An index based on the optimized geometries enabled the evaluation of aromaticity of their various substructures. Pictorial images of the pi-electron distribution as the main Kekulé valence structures have been performed by the aid of the JSCHEM software package.

Relation between the substituent effect and aromaticity.

Molecular geometries of benzene and its 18 monosubstituted derivatives were optimized at B3LYP/6-311+G** level of theory. The changes of pi-electron delocalization of the benzene fragment were estimated by use of aromatic stabilization energies (ASE) based on different homodesmotic reaction schemes, geometry-based HOMA model, magnetism-based NICS, NICS(1), NICS(1)zz, and an electronic delocalization index, PDI, derived from the AIM theory. Apart from aromatic stabilization energies the other descriptors of aromaticity vary to a very small extent, indicating high resistance of the pi-electron structure to the substituent effect. This is somewhat analogous to a tendency of benzene systems to retain their initial pi-electron structure during the reaction course that leads to aromatic substitution.

Extent of cyclic pi-electron delocalization modification in exocyclically substituted fulvenes.

Fulvene derivatives, 20 mono- and 14 di-exocyclically substituted, were optimized at the B3LYP/6-311+G level of theory. Aromaticity indices include aromatic stabilization energy (ASE), Schleyer's Nucleus Independent Chemical Shift (NICS), (3)He chemical shifts, anisotropy and exaltation of magnetic susceptibility, and the Harmonic Oscillator Model of Aromaticity (HOMA), which is a geometry-based descriptor. These indices were used to estimate the extent of a cyclic pi-electron delocalization due to the substituent effect. A dramatic variation of these indices was found, indicating great sensitivity of the pi-electron structure of the ring. Except for anisotropy of magnetic susceptibility, all other indices exhibited perfect equivalence.

Metal chelation aptitudes of bis(o-azaheteroaryl)methanes as tuned by heterocycle charge demands.

We describe the synthesis of a number of 1,3-azol-2-yl-, 1,3-benzazol-2-yl-, and azinyl-based bis(o-azaheteroaryl)methanes (LH, L(-) = Het(2)CH(-)) and their coordinating properties toward divalent transition metals (Zn, Cu, Co, Ni, Hg, Pd). This extended investigation includes both symmetrical and unsymmetrical ligands based on several substituted and/or unsubstituted thiazole, benzothiazole, benzoxazole, benzimidazole, pyridine, and quinoline derivatives. Depending on the structure and electron properties of the ligand, a vast set of neutral chelates ML(2) were obtained, where the ligand is present in its carbanionic form L(-). Additionally, we have prepared salt complexes [M(LH)(n)]X(m), where the ligand is present as a neutral system. Neutral chelates were typically obtained by the reaction of the ligand with metal acetates in alcoholic solution; salt complexes were formed by reaction with other metal salts such as chlorides. By exploring the coordinating properties of several bisheteroarylmethane ligands based on heteroaromatics of variable pi-electron structure and substitution pattern, we demonstrate that the formation of neutral chelates is strictly dependent on the electron-withdrawing capacity (charge demand) of the heteroaromatic moiety. The latter primarily dictates the efficiency by which the negative charge of the anionic ligand L(-) is stabilized by delocalization in ML(2) and, therefore, the stability of the chelate itself. On the basis of the large number and the variable nature of the nitrogen ligands used, we confirm the general validity of the charge-demand-dependent formation of chelates. This key factor can therefore be used for the efficient design of new pi-deficient heteroaromatic nitrogen ligands in chelates of great potential in many synthetic, catalytic, and technological fields.

Pi electronic structure and spectra of benzothienobenzothiophenes

Pi electronic structure and spectra of benzothienobenzothiophenes

The preference of tryptophan for membrane interfaces.

One of the ubiquitous features of membrane proteins is the preference of tryptophan and tyrosine residues for membrane surfaces that presumably arises from enhanced stability due to distinct interfacial interactions. The physical basis for this preference is widely believed to arise from amphipathic interactions related to imino group hydrogen bonding and/or dipole interactions. We have examined these and other possibilities for tryptophan's interfacial preference by using 1H magic angle spinning (MAS) chemical shift measurements, two-dimensional (2D) nuclear Overhauser effect spectroscopy (2D-NOESY) 1H MAS NMR, and solid state 2H NMR to study the interactions of four tryptophan analogues with phosphatidylcholine membranes. We find that the analogues reside in the vicinity of the glycerol group where they all cause similar modest changes in acyl chain organization and that hydrocarbon penetration was not increased by reduction of hydrogen bonding or electric dipole interaction ability. These observations rule out simple amphipathic or dipolar interactions as the physical basis for the interfacial preference. More likely, the preference is dominated by tryptophan's flat rigid shape that limits access to the hydrocarbon core and its pi electronic structure and associated quadrupolar moment (aromaticity) that favor residing in the electrostatically complex interface environment.

Exact eigenstates of the Pariser-Parr-Pople model for anthracene

The pi-electronic structure of anthracene is discussed by combining exact solutions of the Pariser-Parr-Pople (PPP) model and semiempirical PM3 calculations. Symmetry adaptation of the 2.8 million singlet valence-bond (VB) diagrams is explicitly demonstrated for D2h and electron-hole symmetry. Standard PPP parameters provide a comprehensive fit to one- and two-photon anthracene spectra and intensities up to the strong 1 B-1(3u)-absorption at 5.24 eV, the 10th excited state in the dense correlated spectrum, and indicate a reassignment of two-photon absorptions. The singlet-triplet gap and fine-structure constants also agree with experiment. Fully-relaxed PM3 geometries are obtained for the anthracene ground state and for singlet, triplet, and charged bipolarons. The PM3 bond lengths correlate well with PPP bond orders for the idealized structure. Single-determinantal PM3 excitation and relaxation energies for bipolarons are consistent with exact PPP results and contrast all-valence electron with pi-electron calculations. Several correlation effects are noted in the rich pi-spectra of anthracene in connection with improved PPP modeling of conjugated molecules and polymers.

Resonance Raman studies of acetylheme-reconstituted myoglobins. Characterization of 2- versus 4-substituent/protein interactions.

Resonance Raman (RR) spectra are reported for the deoxy, oxy, and metcyano species of sperm whale myoglobin (Mb) reconstituted with deuteroheme, 2-acetyldeuteroheme, 4-acetyldeuteroheme, and 2,4-diacetyldeuteroheme. The functional inequivalence of the 2- and 4-positions of the heme moiety is manifested in the vibrational frequencies of the carbonyl and certain heme skeletal modes. The RR data indicate that the protein influences the porphyrin pi-system primarily through the 2-substituent. In the deoxy Mbs, the 2-acetyl group is more conjugated into the pi-macrocycle than the 4-acetyl group. In the ligated Mbs, the extent of conjugation of the 2-group is less than in the deoxy forms. This result indicates that ligand binding differentially alters the pi-electronic structure for 2- vs 4-substituted systems. The proximal histidine-iron stretching vibration of all four deoxy Mbs occurs at 222 +/- 1 cm-1, which indicates that the substituent-induced changes in the pi-electronic structure do not result in large changes of the electron density in the axial ligand bonds. RR spectra recorded immediately after heme reconstitution indicate the presence of two inequivalent forms of the protein. 1H NMR spectroscopy indicates that these two forms are the normal and reversed heme orientational isomers previously described by La Mar and co-workers [La Mar, G.N., Budd, D.L., Viscio, D.B., Smith, K.M., & Langry, K.C. (1978) Proc. Natl. Acad. Sci. U.S.A. 75, 5755-5759]. The RR spectra demonstrate that in the deoxy species the 2-acetyl group of the normal form is conjugated into the macrocycle to approximately the same extent as the 4-acetyl group of the reversed form and vice versa.(ABSTRACT TRUNCATED AT 250 WORDS)

Applications of molecular orbital-valence bond theory in chemistry

Abstract

Vibrational modes of flavin bound to riboflavin binding protein from egg white. Resonance Raman spectra of lumiflavin and 8-substituted riboflavin.

The resonance Raman (RR) spectra of 8-halogenated-riboflavin, 8-demethyl-riboflavin(8-H-RF), 8-amino-riboflavin(8-NH2-RF), 8-methoxy-riboflavin(8-OCH3-RF), lumiflavin, and 3-methyl-lumiflavin were observed. The Raman lines with the highest frequency are at 1624, 1620, and 1615 cm-1 for 8-chloro-riboflavin, 8-bromo-riboflavin, and 8-iodo-riboflavin, respectively. This systematic shift confirms that the 1631 cm-1 line of riboflavin is derived from the benzene part of isoalloxazine. Substitution at the 8-position by an amino or methoxy group, which has a large influence on the electronic structure of isoalloxazine, changes the RR spectrum markedly in comparison with that of 8-halogenated riboflavin. The 1583 cm-1 line of riboflavin, which involves the vibrational displacement of N(5) and C(4a) atoms of isoalloxazine, is shifted to the low frequency side by substitution at the 8-position with an amino or methoxy group. The corresponding line of 8-H-RF, on the contrary, shifts to the high frequency side. The RR spectrum of lumiflavin is very different from that of riboflavin in the range from 1200 to 1300 cm-1. Although the pi-electronic structure is little affected by the substitution at the 10-position, the Raman spectrum of lumiflavin in this region is very sensitive.

ChemInform Abstract: CALCULATION OF THE PI‐ELECTRONIC STRUCTURE AND SPECTRA OF HOMO‐TRISUBSTITUTED HALOBENZENES BY THE MIM AND PPP METHODS

Chemischer InformationsdienstVolume 5, Issue 22 Physical Organic Chemistry ChemInform Abstract: CALCULATION OF THE PI-ELECTRONIC STRUCTURE AND SPECTRA OF HOMO-TRISUBSTITUTED HALOBENZENES BY THE MIM AND PPP METHODS A. I. KISS, A. I. KISSSearch for more papers by this authorA. MARTIN, A. MARTINSearch for more papers by this author A. I. KISS, A. I. KISSSearch for more papers by this authorA. MARTIN, A. MARTINSearch for more papers by this author First published: June 4, 1974 https://doi.org/10.1002/chin.197422093Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume5, Issue22June 4, 1974 RelatedInformation

ChemInform Abstract: QUANTUM CHEMICAL STUDY OF THE EFFECTS OF SUBSTITUENTS ON THE PI‐ELECTRON STRUCTURE AND REACTIVITY OF CHALCONE

Chemischer InformationsdienstVolume 5, Issue 8 Physical Organic Chemistry ChemInform Abstract: QUANTUM CHEMICAL STUDY OF THE EFFECTS OF SUBSTITUENTS ON THE PI-ELECTRON STRUCTURE AND REACTIVITY OF CHALCONE J. LESKA, J. LESKASearch for more papers by this authorP. ZAHRADNIK, P. ZAHRADNIKSearch for more papers by this author J. LESKA, J. LESKASearch for more papers by this authorP. ZAHRADNIK, P. ZAHRADNIKSearch for more papers by this author First published: February 26, 1974 https://doi.org/10.1002/chin.197408079Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume5, Issue8February 26, 1974 RelatedInformation

The Pi electron structure and absorption spectra of chlorophylls in solution

The Pi electron structure and absorption spectra of chlorophylls in solution

The electronic spectra of the isomeric thienothiophenes

The chemical and physical behavior of the isomeric thienothiophenes is interpreted and correlated on the basis of semiempirical Pariser-Parr-Pople SCF-MO calculations followed by CI between all singly excited states. The results are in reasonable agreement with experiment and indicate a similarity in pielectronic structure to naphthalene.

SCF—MO—LCAO—CI Treatment of the Pi-Electronic Structure of Formaldehyde Using Ohno's Formula for βCO and Constrained to Satisfy Koopmans' Theorem Exactly

The Pople—Pariser—Parr simplification of Roothaan's matrix formulation of the Hartree—Fock self-consistent-field (SCF) equations is applied to the pi-electronic portion of the formaldehyde molecule as (1) a test of the use of Ohno's resonance integral formula as a basis for obtaining self-consistent resonance integrals for heterobonds and (2) as a test of the effect of constraining the energy of the highest occupied orbital to satisfy Koopman's theorem exactly. The calculated spectroscopic intervals, dipole moments, and charge distributions are not too far different from those obtained by the self-consistent-electronegativity method of Brown and Heffernan, but there are indications that the present method may form a more satisfactory basis for extension to other heteroatomic conjugated systems.

The PI electronic structure of thieno[3,4c]thiophene

The PI electronic structure of thieno[3,4c]thiophene