Non-alternant Hydrocarbons Research Articles

Vitamin B12 as an allosteric cofactor; dual fluorescence, hysteresis, oscillations and the selection of corrin over porphyrin.

Studies of the emission spectra of four Co(III) cobinamides (diaquo-, aquohydroxo-, dihydroxo- and dicyano-) show (1) that the excited states corresponding to the alphabeta and epsilon absorption bands behave like the S(1) and S(2) levels in the non-alternant hydrocarbon azulene (with emission from S(2)>> S(1) in violation of Kasha's rule) and (2) that the excited states include a TICT (twisted intramolecular charge transfer) mechanism, as in the simpler cyanines, but where the TICT state gives rise to dual fluorescence instead of cis-trans isomerisation. Combined with the previously reported dual fluorescence from the S(1) level in synthetic metal corrinoids and in the naturally-occurring metal-free corrin, this provides evidence that the existence of an additional (metastable) ground state with a significantly different vibronic splitting and nuclear configuration is an intrinsic property of the basic corrin ligand (irrespective of the nature of the side-chains and the metal ion or even the absence of a metal) which distinguishes it from porphyrin. The occurrence of hysteresis (and its associated oscillations) in redox reactions of the cobinamides involving both the Co(III/II) and Co(II/I) couples indicates that the corrin ligand also has an intrinsic ability to exist in different conformations or "allosteric" forms with differing redox potential, which further distinguishes it from the porphyrin ligand. Possible links between the existence of an additional metastable ground state and of allosteric changes and the likely reasons for the selection of corrin over a porphyrin for the vitamin B(12)-dependent enzymes are discussed.

Disjoint molecular orbitals in nonalternant conjugated diradical hydrocarbons.

The concept of disjoint NBMOs in diradical alternant hydrocarbons (AHs) is reviewed and extended to diradical nonalternant hydrocarbons (nonAHs). A valence-bond (VB) method for recognizing disjoint versus nondisjoint NBMOs (nonbonding molecular orbitals) in diradicals is presented. When circumscribed with hexagonal rings, disjoint diradicals produce nonradical polycyclic successors and nondisjoint diradicals produce diradical polycyclic successors. The interconnection between the topological VB terms of cross-conjugated, disjoint NBMOs, and essentially disconnected polycyclic hydrocarbons is delineated.

Valence-Bond and Hückel Molecular Orbital Diradicals—Alternant versus Nonalternant Effects

Two distinct classes of diradicals are examined and compared. Valence-bond diradicals are topologically enforced, whereas Hückel molecular orbital diradicals can undergo skeletal distortions and transform to a nonradical form and tend to gain or lose electrons to form stable polyions. Alternant hydrocarbon diradicals are nonpolar species and nonalternant hydrocarbon diradicals tend to be polar species and less prevalent.

Resonance Topology and Related Aspects of Fluoranthenoid / Fluorenoid and Indacenoid Hydrocarbon Radicals

Data on the number of resonance structures of free radical polycyclic conjugated hydrocarbons are tabulated and studied. The first examples of fluoranthenoid / fluorenoid and indacenoid polyradical systems are presented. Some comparative generalizations between benzenoid free radicals and fluoranthenoid / fluorenoid and indacenoid polyradical systems are formulated. For example, there is a tendency of these latter even-carbon nonalternant hydrocarbons to have fewer radical isomers compared to the former alternant hydracarbons. Examples where Fowler's leapfrog algorithm identifies the more stable fluorenoid anion and indacenoid dianion isomer are presented.

$$\pi $$ -Electron Long-Range Spin–Spin Coupling in the Full Configuration Interaction Method

A technique for calculating spin perturbation effects in the framework of the previously developed matrix version of the full configuration interaction method is presented. The method is adapted to calculations of \(\pi \)-electron atom–atomic spin polarizability defined as the \(\pi \)-contribution to the indirect spin–spin nuclear coupling constant. Particular calculations show that pronounced long-range spin–spin coupling is actually possible in some characteristic conjugated systems such as polymethine dye carbocations or some nonalternant hydrocarbons. At the same time, spin–spin correlators appearing in McKonnel's theory are inapplicable in describing such long-range effects.

Non-Alternant Hydrocarbon Polycycles: Signs of Unusual Reactivity

A connection is developed between typical characteristic polynomial coefficient sign patterns and Δ, the difference between the numbers of π-bonding and π-antibonding molecular orbitals for hydrocarbons. A link is revealed between Δ and dominant Sachs' subgraphs for the non-zero characteristic polynomial coefficient that is closest to the end of the polynomial. Through the intermediacy of the appropriate Sachs' subgraphs, the connection between a given Lewis structure and Δ is clarified. Exploiting the Sachs' subgraphs, one can redesign non-alternant hydrocarbon structures to alter π-system stability and, therefore, the kinetic stability of interesting molecules. A PM3 test of this methodology is provided in the accompanying paper.

A PM3 Test of Kinetic Stability Predictions for Selected Sets of Non-Alternant Hydrocarbons

An hypothesis, developed in the preceeding paper, permits recognition of non-alternant hydrocarbons for which the number of π-bonding molecular orbitals (MOs) does not equal the number of π-antibonding MOs. PM3 results fully support expectations for the kinetic instability of those systems examined herein. Two structures are classified as anti-aromatic as a result of their very small PM3-calculated ΔEST values.

One step Pd(0)-catalyzed synthesis, X-ray analysis, and photophysical properties of cyclopent[hi]aceanthrylene: fullerene-like properties in a nonalternant cyclopentafused aromatic hydrocarbon.

A simple procedure for the synthesis of cyclopentafused polycyclic aromatic hydrocarbons (CP-PAH) with Pd(PPh(3))(2)Cl(2) catalyst has been applied to the one-pot palladium(0)-catalyzed coupling of 9,10-dibromoanthracene (1) with 2-methyl-3-butyn-2-ol. Reactions carried out in refluxing benzene in the presence of CuSO(4)/Al(2)O(3) yielded 9,10-dialkynylanthracene 2a, alkynyl aceanthrylene 2b, and 2,7-disubstituted cyclopent[hi]aceanthrylene 2c in 13%, 23%, and 19% purified yields, respectively, with total conversions of 80-90%. Sealed tube reactions without copper at 110 degrees C improved the yield of 2c up to >75%. Single-crystal X-ray analyses of 2a and 2c reveal a three-dimensional hydrogen bonding network, producing a unique crystal packing. The packing structure of 2b is dominated by pi-pi stacking interactions between two aceanthrylene molecules. CP-PAHs 2b and 2c have potentially interesting fullerene-like photophysics. While the UV-vis and fluorescence spectra of 2a (Phi(F) = 0.87) show the characteristic vibronic structure of anthracene, the UV-vis spectra of ruby-red aceanthrylene 2b and greenish-black cyclopent[hi]aceanthrylene 2c extend well into the visible range. Isomers 2b and 2c showed no detectable fluorescence emission. Unlike fullerenes, compounds 2b and 2c are poor singlet oxygen sensitizers with measured (1)O(2) quantum yields of 0.02 and 0.06, respectively. As expected from a simple Hückel analysis, 2c has relatively low two-electron reduction potentials as determined by cyclic voltammetry.

Selective Fluorescence Quenching of Alternant vs. Non-Alternant Polycyclic Aromatic Hydrocarbons by Carbon Tetrachloride

Selective Fluorescence Quenching of Alternant vs. Non-Alternant Polycyclic Aromatic Hydrocarbons by Carbon Tetrachloride

Forensic Analysis of Commercial Petroleum Products Using Selective Fluorescence Quenching

A novel method for the forensic analysis of commercial petroleum products is presented. In this approach, the petroleum sample is extracted with nitromethane and then separated by capillary liquid chromatography with laser-induced fluorescence detection. The addition of selective fluorescence quenching agents allows the sample to be profiled by the distribution of alternant and nonalternant polycyclic aromatic hydrocarbons (PAHs). In preliminary studies, the quenching behavior of nitromethane and diisopropylamine was established by using a standard mixture of sixteen PAHs ranging in size from two to six aromatic rings. Subsequent examination of new and used motor oil demonstrated that characteristic differences arise in the PAH content, which may allow for the unique identification of oil from a particular engine or vehicle. In addition, three brands of petrolatum jelly were successfully distinguished. Although a number of alternant alkylated and heterocyclic PAHs were found in all petrolatum samples, there were significant differences in the relative concentrations of alternant as well as nonalternant PAHs. This allowed for clear differentiation of the samples through qualitative inspection of their chromatograms as well as quantitative statistical correlation techniques.

Dicyclopenta[a,d]cyclooctene: A [14]Annulene Containing Two Zero-Atom Cross-Links.

14π-Electrocyclization across the two pentafulvenoid moieties of pentafulvene 1 occurs upon heating to provide dicyclopenta[a,d]cyclooctene 2 [Eq. (1)]. The nonalternant hydrocarbon 2 shows diatropic character and a distinctive absorption spectrum with the longest wavelength maximum at 767 nm.

A structural bridge between alternant and non-alternant hydrocarbons

A simple set of trimethylene-substituted even, fully-p-bonded, non-alternant monocycles is shown to have several key features in common with acyclic, even alternant polyenes at the Huckel level. These non-alternant molecules provide a bridge between alternant and non-alternant hydrocarbons. This topic might serve as a useful addition to Huckel theory courses targeted at senior undergraduate students.

Mapping the modification of ring currents induced by cyclopenta-fusion on a naphthalene core

An analysis of the σ-, π- and total current density maps, calculated using a distributed-origin coupled Hartree–Fock method, of the non-alternant cyclopenta-fused polycyclic aromatic hydrocarbons (CP-PAH) acenaphthylene (1), pyracylene (2), fluoranthene (3) and cyclopenta[cd]fluoranthene (4) shows that, whereas the presence of a single pentagon in 1 and 3 has only minor effects on the core naphthalene unit, the strong cooperative effect of the two pentagons connected by a formal double bond in 2 and 4 results in intense paratropic currents in the pentagons, and markedly reduced aromaticity according to conventional criteria. The accuracy of the all-electron maps is supported by calculation of 1H and 13C NMR chemical shifts in agreement with experiment and their interpretation is consistent with nucleus-independent chemical shifts (NICS values). It is shown that the pattern of paratropic contributions can be explained qualitatively within Hückel–London π-electron theory.

Non-alternant Hydrocarbon Substituents as Powerful Carbenium-ion Stabilizers: A Graph-theoretical Study with PM3 Support

Graph-theoretical arguments establish that some non-fused, even, non-alternant bicycles have unequal numbers of bonding and antibonding orbitals. Both HUckel and PM3 semiempirical calculations support the contention that such molecules have non-additive π-polarizations relative to the simple π-polarizations of even, non-alternant monocycles. The dicyclopropenyl substituent derived from the corresponding symmetric, contiguous, non-fused, non-alternant bicycle is shown to be a powerful carbocation stabilizer at the PM3 level of theory. Methine cyclopropenyl is shown to be another potent non-alternant carbenium-ion stabilizer.

Spectrochemical evaluation of pyridinium chloride as a possible selective fluorescence quenching agent of polycyclic aromatic hydrocarbons in water and neat acetonitrile

Pyridinium chloride (PC) is examined as a selective, fluorescence quenching agent for alternant as opposed to nonalternant polycyclic aromatic hydrocarbons (PAHs) in two polar solvents — water and acetonitrile. Nine alternant and 13 nonalternant PAHs were dissolved in water and acetonitrile and a total of 0.2 M of pyridinium chloride was added. The resulting change in fluorescence intensity was observed and reported as the Stern–Volmer quenching constant. Results show that PC is a selective quencher in both polar solvents. It selectively quenches the fluorescence emission intensity of alternant PAHs while leaving the nonalternant PAH fluorescence emission virtually unchanged. These results agree with the selective quenching behavior seen for PC surfactant analogs, cetylpyridinium chloride (CPC) and dodecylpyridinium chloride (DDPC). Furthermore, these results illustrate that the presence of a surfactant or micelle is not a requirement for selective quenching. The selective quencher PC is applicable to situations where a surfactant is not desirable or soluble.

Phenyl migrations in dehydroaromatic compounds. A new mechanistic link between alternant and nonalternant hydrocarbons at high temperatures

Flash vacuum pyrolysis of benzo[b]biphenylene, an alternant polycyclic aromatic hydrocarbon (PAH), gives fluoranthene, a nonalternant PAH, as the major product at 1100 degrees C in the gas phase. The most reasonable mechanism to explain this isomerization involves equilibrating diradicals of 2-phenylnaphthalene that rearrange by the net migration of a phenyl group to give equilibrating diradicals of 1-phenylnaphthalene, one isomer of which then cyclizes to fluoranthene.

High temperature behavior of alternant and nonalternant polycyclic aromatic hydrocarbons

Three different flash vacuum pyrolysis (FVP) set-ups were employed for the study of the thermal behavior of alternant and nonalternant polyclic aromatic hydrocarbons (PAH). The 1,2-switch of carbon atoms in the thermal automerizations of PAH, previously observed in benzene, pyrene and benz[ a]anthracene, has been additionally observed for [1- 13C]anthracene, [6- 13C]benzo[ c]phenanthrene and [1- 13C]- and [4- 13C]phenanthrenes at temperatures between 900 and 1100°C. Bay region PAH (chrysene, benz[ a]anthracene, phenanthrene, picene, perylene, benzo[ ghi]perylene, benzo[ e]pyrene, and benz[ a,c]-anthracene) were discovered to undergo an intriguing transformation when heated at 1100°C; the transformation is believed to be triggered by cyclodehydrogenation across the bay region. Thermal formation of five-membered rings was used to synthesize a series of new nonalternant hydrocarbons. When the thermal closure of a five-membered ring is energetically unfavorable, migration of an ethynyl group from one benzene ring to another along the edge of a PAH (e.g. anthracene) can take place.

Correlation of substituent effects and energy levels of the two lowest excited states of the azulenic chromophore

[structure: see text] Experimental and calculated data show that the relative energy levels of the two lowest excited states of azulene are sensitive to the nature and position of substituents on the nonalternant hydrocarbon. Extending such investigations led to a rational explanation for some of the baffling data on azulenic bacteriorhodopsin analogues in the literature.

Proton chemical shifts in NMR. Part 14. Proton chemical shifts, ring currents and π electron effects in condensed aromatic hydrocarbons and substituted benzenes

The proton resonance spectra of a variety of condensed aromatic compounds including benzene, naphthalene, anthracene, phenanthrene, pyrene, acenaphthylene and triphenylene were obtained in dilute CDCl3 solution. Comparison of the proton chemical shifts obtained with previous literature data for CCl4 solution shows small but significant differences. A previous model (CHARGE6) for calculating the proton chemical shifts of aliphatic compounds was extended to aromatic compounds. This was achieved by including an automatic identification of both five- and six-membered aromatic rings based on atomic connectivities plus a dipole calculation of the aromatic ring current. The ring current intensity in the molecules was calculated by two alternative methods. a) The ring current intensity in the individual benzenoid rings was a function of the number of adjoining rings and b) the molecular ring current was proportional to the molecular area divided by the molecular perimeter. This, plus the inclusion of deshielding steric effects for the crowded protons in these molecules, gave a good account of the observed chemical shifts. The model was also applied successfully to the non-alternant hydrocarbons of fulvene and acenaphthylene and to the aliphatic protons near to and above the benzene ring in tricyclophane and [10]cyclophane.The Huckel calculation of the π electron densities in CHARGE6 was used to calculate the π electron densities in substituted benzenes. The π-inductive effect was used to simulate the effect of CX3 groups (X = H, Me, F) on the benzene ring. These together with the long range effects of the substituent groups identified previously allowed a precise calculation of the SCS of a variety of substituents on all the benzene ring protons.The model gives the first accurate calculation of the proton chemical shifts of condensed aromatic compounds and of the proton SCS in the benzene ring. For the data set of 55 proton chemical shifts spanning 3 ppm the rms error of the observed vs. calculated shifts was ca. 0.1 ppm. The model also allows the interpretation of the shifts in terms of the separate interactions calculated in the programme, i.e. π electron densities and steric, anisotropic and electric field effects. Previous correlations of the proton SCS with π electron densities and substituent parameters are shown to be over simplified. The relative proportions of these different interactions are very different for each substituent and for each ring proton. p

Spectrochemical investigations in dendritic media: evaluation of nitromethane as a selective fluorescence quenching agent in aqueous carboxylate-terminated polyamido amine (PAMAM) dendrimers

Alternant and nonalternant polycyclic aromatic hydrocarbons (PAHs) are employed to compare carboxylate-terminated polyamido amine (PAMAM) dendrimers to typical anionic micelles. Nitromethane is a known, selective, quenching agent of alternant PAHs. However, recent studies by Acree and co-workers have found that nitromethane will also quench the fluorescence emission intensity of nonalternant PAHs in the presence of anionic surfactants above the critical micelle concentration. The quenching of alternant and nonalternant PAHs by nitromethane is used to compare dendritic ‘unimolecular micelles’ to traditional micelles. Experimental results indicate that the PAHs’ association with these dendrimers does not appear to occur in the ‘palisade’ region, as seen in traditional micelles, but rather deeper within the dendrimer structure. Solvent polarity probe studies and quenching in the absence of nitromethane also support this conclusion. Due to the fact that the PAHs do not reside near the negatively charged surface groups, the nitromethane selective quenching rule is obeyed in carboxylate-terminated PAMAM dendrimers (10 mM in surface groups).