Non-alternant Hydrocarbons Research Articles

A theoretical determination of the electronic spectrum of Methylenecyclopropene

The vertical electronic spectrum of methylenecyclopropene, the prototype of the nonalternant hydrocarbons known as fulvenes, has been studied using multiconfigurational second-order perturbation theory. The calculations comprise three valence states and the 3s, 3p, and 3d members of the Rydberg series converging to the first π ionization limit. Vertical excitation energies to three valence states are found at 4.13, 6.12, and 6.82 eV. The second of them corresponds to an excitation from the highest occupied σ orbital to a π* orbital, while the other two are π → π* excitations. The third transition gives rise to the most intense feature in the electronic spectrum. The results are rationalized within the scheme of two interacting double bonds. Comparisons are made between this and the previous theoretical calculations of the electronic spectra of related systems and also between the available experimental data of methylenecyclopropene in solution.

Spectrochemical investigations in molecularly organized solvent media: Evaluation of nitromethane as a selective fluorescence quenching agent for alternant PAHs dissolved in micellar solvent media

Applicability of the nitromethane selective quenching rule for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons (PAHs) is examined for 29 representative PAH solutes dissolved in micellar cetyltrimethylammonium bromide (CTAB) and micellar sodium dodecylsulfate (SDS) solvent media. Experimental results show that nitromethane quenched fluorescence emission of only the 17 alternant PAHs in the cationic CTAB surfactant solvent media as expected. Emission intensities of nonalternant PAHs, except for the few exceptions noted previously, were unaffected by nitromethane addition. Unexpected quenching behavior was observed, however, in the case of nonalternant PAHs dissolved in micellar SDS solvent media. Nitromethane quenched fluorescence emission of nonalternant PAHs studied in the SDS solvent media, which is contrary to the selective quenching rule.

Cyclic Conjugation Effects in Cyclacenes

Abstract Cyclic conjugation is studied in Hückel and Möbius cyclacenes as well as in a few other related open-chain and cyclic belt-shaped hydrocarbons (e.g., zig-zag cyclarenes). the main conclusions are that (a) the energy effect of the conjugation along the perimeter of cyclacenes is relatively large, (b) conjugation along the 6 -, 10 - and 14-membered cycles has a much greater effect, in spite of the fact that these cycles embrace bonds that formally are not involved in cyclic conjugation. Möbius cyclacenes are non-alternant hydrocarbons and their odd-cycles have a significant contribution to conjugation. the contribution of conjugation in individual cycles to the total resonance energy is briefly discussed.

Unexpected electrochemical reduction of fluoranthene in the solvents DME and HMPA: new light onto the mechanism of hydrogenation to produce tetrahydrofluoranthene

The non-alternant aromatic hydrocarbon fluoranthene (Ar0) has been reduced, either chemically with Na or Li or by electrolysis, to the radical anion Ar˙– in the three solvents THF (tetrahydrofuran), DME (dimethoxyethane) and HMPA (hexamethylphosphoric triamide). The UV–VIS absorption spectrum of the orange–brown Ar˙– is quite similar in the three solvents and in all instances addition of H+–H2O has resulted in quantitative electron-back-donation along with H2 evolution and recovery of unchanged fluoranthene Ar0. Thus the usual Birch-type reduction to a dihydro derivative is totally inefficient in the cases under investigation. The two-electron reduction has also been achieved in these three solvents. The greenish-yellow dianion Ar2– produced in THF exhibits characteristic UV–VIS absorption patterns, disproportionates with Ar0 and reacts with H+–H2O only to evolve H2.With DME or HMPA a blood-red species is produced whose absorption spectrum is virtually the same and quite different from that observed in THF. In both solvents addition of H+–H2O leads to tetrahydrofluoranthene as a main reaction product but disproportionation is not observed at all in HMPA and this is not compatible with a regular dianion Ar2–.Reaction with D+–D2O instead of H+–H2O has shown that hydrogenation involves radical abstraction of H atoms from the solvent in both cases; this sheds new light onto the reaction mechanism. Furthermore, several other experiments indicate that the dianionic blood-red species is most likely a complex written as [Ar˙–⋯˙– Solvent], in which the Ar˙– moiety is bound to a solvated electron localized on a solvent molecule.

Synthesis and Characterization of the Three Dicyclopentapyrenes

Polycyclic aromatic hydrocarbons containing fully unsaturated five-membered rings as integral components of their trigonal carbon networks (CP-PAHs) have attracted considerable attention recently in a variety of scientific circles. Some of these novel nonalternant hydrocarbons exhibit unusual photophysical behavior, e.g., anomalous fluorescence, and/or pronounced biological activity, acute cyctotoxicity. Many have been identified or are believed to be formed as products of incomplete combustion, either in flames or in cigarette smoke, and a few have been produced by flash pyrolyses of other hydrocarbons. Five-membered rings fully encircled by benzene rings impart a curvature to the trigonal carbon network that gives rise to the bowl-shaped geometry of corannulene and to the closed polyhedral surfaces of the fullerenes. Herein, the authors report the syntheses and a preliminary spectroscopic examination of the three isomeric dicyclopentapyrenes.

Energy Component Analysis of the Pseudo-Jahn−Teller Effect in the Bicyclic Nonalternant Hydrocarbons: The Pentalenoid and Heptalenoid Systems

To gain insight into the nature of the pseudo-Jahn−Teller (JT) effect, an energy component analysis has been carried out for the bicyclic nonalternant hydrocarbons termed pentalenoid and heptalenoid systems by using the ab initio RHF method with 6-31G(d) basis set. Inspection of the energy component comprised in the total energy reveals that the stability of a less symmetrical nuclear configuration is largely responsible for the decrease in the internuclear repulsion energy and the interelectronic repulsion energy due to σ electrons. These observations are consistent with an expansion of the carbon skeleton brought about by the pseudo-JT distortion. Another energy component also plays an essential role in the pseudo-JT stabilization: For the examples, the preference for the C2h structure rather than the D2h one for the pentalene arises from a decrease in the interelectronic repulsion energy due to π electrons, while that for heptalene results from a lowering of the nuclear−electron attraction energy due to π electrons. This sharp distinction between the pentalenoid and heptalenoid systems is accounted for in terms of an electrostatic interaction combined with a charge relaxation attributed to π electrons.

Selective fluorescence quenching to discriminate between alternant and non-alternant polycyclic aromatic hydrocarbons: acephenanthrylene derivatives as exceptions to the nitromethane quenching rule

Nitromethane selectively quenches the fluorescence emission of alternant polycyclic aromatic hydrocarbons (PAHs)via an electron transfer mechanism. Emission intensities of non-alternant PAHs, for the most part, are unaffected. Results of previous measurements show that nitromethane does quench the fluorescence emission of the nine acenaphthylene solutes studied, which is completly contrary to what would be expected based on the fact that all nine solutes are non-alternant PAHs. To investigate this phenomenon further, the fluorescence behaviour of 1-, 3-, 4-, 5-, 6-, 7-, 8-, 9- and 10- methylacephenanthrylene dissolved in neat acetonitrile, acetonitrile–ethyl acetate and acetonitrile–toluene solvents was measured at different nitromethane concentrations. Experimental results support the notion that the double bond in the five-membered ring is fixed; therefore, the solutes act as alternant rather than non-alternant PAHs.

Study of geometry effects on heavy atom perturbation of the electronic properties of derivatives of the nonalternant polycyclic aromatic hydrocarbons fluoranthene and acenaphtho[1,2-k]fluoranthene

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStudy of geometry effects on heavy atom perturbation of the electronic properties of derivatives of the nonalternant polycyclic aromatic hydrocarbons fluoranthene and acenaphtho[1,2-k]fluorantheneBenjamin F. Plummer, L. Kraig Steffen, Tala L. Braley, W. Greg Reese, Kathryn Zych, Gregory Van Dyke, and Brock TulleyCite this: J. Am. Chem. Soc. 1993, 115, 24, 11542–11551Publication Date (Print):December 1, 1993Publication History Published online1 May 2002Published inissue 1 December 1993https://doi.org/10.1021/ja00077a061RIGHTS & PERMISSIONSArticle Views471Altmetric-Citations25LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts Get e-Alerts

Spectroscopic Investigation of Fluorescence Quenching Agents. Part IV: Selectivity of Nitromethane for Discriminating between Alternant versus Nonalternant Polycyclic Aromatic Hydrocarbons in Solvents of Differing Polarities

To further assess the applicability of nitromethane as a selective quenching agent for alternant vs. nonalternant polycyclic aromatic hydrocarbons in HPLC analysis, we measured the effect that it has on the fluorescence emission behavior of 96 different polycyclic aromatic hydrocarbons dissolved in a binary ethyl acetate/acetonitrile solvent mixture. Nitromethane quenching results are compared with previously reported acetonitrile, aqueous/acetonitrile, and toluene/acetonitrile solvent mixtures. Results of these measurements revealed that the “selective quenching” rule is obeyed for the vast majority of PAHs in all solvents considered thus far, with the coronene derivatives being the only major exceptions.

Eine Anmerkung zum langwelligen UV-Übergang in alternierenden und dazu isomeren nichtalternierenden polycyclischen aromatischen Kohlenwasserstoffen

Abstract Based on the HMO formalism and for an infinite number of carbon centers an analytical solution for the ratio x = ∆ε(HOMO-LUMO)[NAH]/∆c(HOMO-LUMO)[AH] of nonalternant polycyclic aromatic hydrocarbons (NAH) and its respective counterparts (AH) is given. It is shown that generally x ≥ 1, which is in agreement with experimental observations.

Polycyclic aromatic hydrocarbons with five-membered rings: Modeling of experimental X-ray and neutron-diffraction structures

Several of the readily available theoretical programs are evaluated as tools for modeling the structures of polycyclic aromatic hydrocarbons with five-membered rings (CPAHs). The experimentally determined bond lengths and angles are compared to calculated values. Experimental bond lengths are also compared to Pauling and Huckel molecular orbital (HMO) bond orders. Previously published experimental X-ray and neutron-diffraction structures of acenaphthene, acenaphthylene, fluoranthene, cyclopent[o,p,q,r]benz[c]phenanthrene, and corannulene are modeled by the programs MMX, AM1, MNDO, and PM3, and previously reported STO-3G and 6-31G * data are also evaluated. In general, the error differences between the experimental and calculated results for all of the semiempirical programs were small. However, PM3 performed slightly better than AM1 and MMX, while MNDO generated structures which exhibited the largest deviation from experiment. Although the standard deviations for all programs are shown to be of comparable magnitude, a particular bond length or bond angle in any given theoretical calculation can exhibit significant error from the experimental data. The scatter in the bond order data computed from Huckel molecular orbital theory and valence bond theory is contrary to results obtained with alternant systems. It appears that these approaches are less successful at modeling accurately the nonalternant hydrocarbon systems described in this paper.

Spectroscopic properties of polycyclic aromatic compounds: Part II. Fluorescence emission and quenching behavior of select acenaphthylene derivatives in organic nonelectrolyte solvents

Fluorescence emission spectra have been measured for acenaphthylene, aceanthrylene, acephenanthrylene, benz[ e]aceanthrylene, 3-methylbenz[ j]aceanthrylene, 6-methylbenz[ j]aceanthrylene, benzo[ def]cyclopenta[ hi]chrysene, cyclopenta[ cd]pyrene (also called acepyrylene) and acenaphth[1,2 a]acenaphthylene in organic nonelectrolyte solvents of varying polarity. Benz[ e]aceanthrylene was found to exhibit probe character for many of the solvents considered; however, emission intensity ratios in benzene, toluene, p-xylene and o-xylene were too small compared against nonelectrolyte solvents of similar polarity. The effect of nitromethane as selective quenching agent was also examined. Results of these measurements show that nitromethane does quench the fluorescence emission of the nine solutes studied, which is contrary to what would be expected based upon the fact that all nine solutes are nonalternant polycyclic aromatic hydrocarbons.

Spectroscopic Investigation of Fluorescence Quenching Agents. Part III: Effect of Solvent Polarity on the Selectivity of Nitromethane for Discriminating between Alternant versus Nonalternant Polycyclic Aromatic Hydrocarbons

To better assess the applicability of nitromethane as a selective quenching agent for alternant versus nonalternant polycyclic aromatic hydrocarbons in HPLC, TLC, and HPTLC analysis, we measured the effect that it has on the fluorescence emission behavior of 96 different polycyclic aromatic hydrocarbons dissolved in binary toluene/acetonitrile solvent mixtures. Results of these measurements revealed that the “selective quenching” rule is obeyed for the vast majority of PAHs, with the coronene derivatives being the only major exceptions. Fluorescence emission spectra are also reported for benzo[g]chrysene, naphtho[2,3g]chrysene, 4 H-benzo[c]cyclopenta[mno]chrysene, dibenzo[ghi,mno]fluoranthene (commonly called corannulene), rubicene, diacenaphtho[l,2j:l‘,2'l]fluoranthene, 10-methyl-benzo[b]fluoranthene, 3-methoxybenzo[k]fluoranthene, and 3-hydroxybenzo[k]fluoranthene in organic nonelectrolyte solvents of varying polarity. Calculated emission intensity ratios failed to vary systematically with solvent polarity, and all nine of the aforementioned solutes were thus classified as nonprobe molecules.

Graph generation: quasicrystal-like benzenoid systems

An equivalence relation is defined on the terminal rings of alternant and non-alternant hydrocarbons including catacondensed benzenoid systems with zero, one and two branched hexagons, benzocyclobutadienes, benzocy-cloctatetraenes and certain pentalene derivatives. This relation led to the generation of vertex-transitive regular graphs which allow the study of many properties of interest, viz. quasicrystal-like codes, ordering diagrams, build-up (aufbau) processes, combinatorial aspects of the number of Kekule structures and a novel postulate on the stability of these hydrocarbons. The graphs generated represent real chemical species such as cubane and certain transition-metal clusters or they may be thought of as depicting chemical transformations. The work is comparable with the use of permutation matrices as graph generators recently adopted by Randicet al.(M. Randic, W. L. Woodworth, A. F. Kleiner and H. Hosoya, J. Comput. Chem., 1987, 2, 522).

The conjugated-circuit model: application to nonalternant hydrocarbons and a comparison with some other theoretical models of aromaticity

The conjugated-circuit model with an improved set of Q n parameters is applied to non-alternant hydrocarbons. The resonance energy per π electron (REPE) model (with a corrected set of parameters) and the hardness model are also applied to the same group of non-alternants. It is found that the RE/e (the resonance energy per π electron calculated using the conjugated-circuit model) and the REPE are intercorrelated quantities, whereas RE/e or REPE do not correlate with absolute hardness at all for non-alternants. Therefore RE/e was selected as the criterion for aromatic stability, and hardness as the reactivity criterion because it is defined in terms of the frontier orbitals. The stability (high aromaticity and low reactivity) of a non-alternant molecule is then judged by both the RE/e and the hardness. In most cases the stability predictions were in agreement with the available experimental evidence. Buckminsterfullerene, which is a spherical non-alternant structure, is found to be a hard molecule, but not as hard as benzene.

A simple treatment to design NBMO degenerate systems in alternant and non-alternant hydrocarbons

A simple treatment for systems with many degenerate non-bonding molecular orbitals (NBMO's) in alternant and non-alternant hydrocarbons without performing computer calculations is presented at the Huckel level. Using Huckel theory, we have proven that the linkage between carbon atoms with coefficients of zero in an NBMO does not present any substantial action to the original NBMO. By using this simple rule, many polycyclic conjugated hydrocarbons possessing high spin multiplicity are hypothesized. We term these NBMO degenerate systems “NBMO network systems”. This treatment can be expected to provide information for designing novel organic high-spin molecules including possible ferromagnetic substances in particular those consisting of non-alternant polycyclic conjugated hydrocarbons.

Preparation and Spectroscopic Characterization of Cyclopenta[1,2-a : 3,4,5-b′c′]dicoronene

The non-alternant hydrocarbon cyclopenta[1,2-a : 3,4,5-b′c′]dicoronene (3) has been prepared by aluminium chloride catalyzed dimerization of coronene (1). The UV/VIS absorption spectrum, fluorescence spectrum and fluorescence quantum yield of 3 is reported and compared with that of the alternant isomer benzo[1,2,3-bc:4,5,6-b′c′]dicoronene (2). Relationships observed in the UV/VIS absorption spectra of alternant and non-alternant polycyclic aromatic hydrocarbons and preliminary theoretical investigations (PMO calculations) on two model systems (C, E) are also reported.

π-Electron “ring currents” in dipleiadiene

Calculations are effected of the “London” π-electron magnetic properties of dipleiadiene, the synthesis of which was reported some years ago by Vogel et al. who, at the same time, made deductions about the π-electron “ring currents” in this molecule on the basis of its NMR spectra. Six different methods of calculation are invoked; these range in sophistication from the method of McWeeny, based on a simple Hückel molecular orbital, to the method of Coulson et al. (which makes minimal appeal to the London integral-approximation), based on a Pariser-Parr-Pople self-consistent-field wavefunction in which resonance integrals are made iteratively self-consistent with respect to the corresponding calculated bond orders. Discussion is included on trends in the predicted magnetic properties observed when the sophistication of the method of calculation is increased. It is concluded that at least the seven-membered ring of dipleiadiene appears to bear a paramagnetic π-electron “ring current”, though the overall “London” contribution to its magnetic susceptibility perpendicular to the molecular plane is probably weakly diamagnetic. It is thought that the very high field shifts of the peripheral attached protons in dipleiadiene (observed by Vogel et al. in the 1H NMR spectrum of the molecule) could as easily be due to non-uniform π-electron charge density effects in this non-alternant hydrocarbon as to the presence of strongly paramagnetic π-electron “ring currents”. The dianion of dipleiadiene is calculated to be strongly diamagnetic in its “London” π-electron magnetic properties (of the same order, in fact, as the neutral condensed benzenoid hydrocarbons of similar size) and the extent of this diamagnetism is found to be remarkably insensitive to the sophistication of the method used to calculate it.

Correlation effects and electron delocalization in nonalternant hydrocarbon .pi. compounds

Many-particle contributions to the π-electronic structure of nonalternant hydrocarbons are investigated by a local correlation approach. Numerical results are derived by adopting a simple tight-binding model as well as a model operator in the zero-differential overlap (ZDO) approximation definded in terms of the Hartree-Fock hopping element. The different integrals are calculated ab initio in a Slater-type basis. The atomic π-electron density localization is quantified by the charge fluctuations corr in the correlated ground state |ψ 0 >

On the localization of π-electrons in linear and cyclic hydrocarbons

The localization properties of π-electrons in alternant and nonalternant hydrocarbons are investigated by calculating the mean-square deviation corr of the electronic charge at the ith π-centre in the correlated ground state |Ψcorr> The interatomic π-correlations are determined by the method of the local approach (LA) in the framework of a zero differential overlap (ZDO) model-Hamiltonian supplemented by the tight-binding approximation. The necessary integrals are calculated parameter-free, i.e. ab initio, in a Slater-type basis. Comparison of the corr numbers with the charge fluctuations predicted in the independent-particle picture |ΨSCF> allows for the definition of correlation-strength parameters Δ i , Σ i which measure the many-particle character of π-bonding. It is demonstrated that electronic correlations are sizeable in the π-network of hydrocarbons. This is also valid for conjugated polyenes and cyclic (4n + 2) systems. The π-electron localization exceeds throughout th...