NMR spectral properties of the tetramantanes - nanometer-sized diamondoids.

Tetramantanes, and all diamondoid hydrocarbons, possess carbon frameworks that are superimposable upon the cubic diamond lattice. This characteristic is invaluable in assigning their (1)H and (13)C NMR spectra because it translates into repeating structural features, such as diamond-cage isobutyl moieties with distinctively complex methine to methylene signatures in COSY and HMBC data, connected to variable, but systematic linkages of methine and quaternary carbons. In all tetramantane C22H28 isomers, diamond-lattice structures result in long-range (4)JHH, W-coupling in COSY data, except where negated by symmetry; there are two highly symmetrical and one chiral tetramantane (showing seven (4)JHH).

Partitioned-formula periodic tables for diamond hydrocarbons (diamondoids).

Isomeric diamond hydrocarbons (diamondoids or polymantanes) with the same number n of adamantane units share the same molecular formula C(Q)(CH)(T)(CH(2))(S) and can be divided into valence isomers (denoted as Q-T-S) by partitioning the number C = Q + T + S of their carbon atoms according to whether they are quaternary, tertiary, or secondary. Vertices of dualists are the centers of adamantane units, and dualist edges connect vertices of adjacent adamantane units (sharing a chair-shaped hexagon). Dualists of diamondoids are hydrogen-depleted skeletons of staggered alkane or cycloalkane rotamers.

Conjugated circuits currents in hexabenzocoronene and its derivatives formed by joining proximal carbons.

We report on calculated CC bond currents for a dozen derivatives of hexabenzocoroenene in which one or more proximal carbon atoms at the molecular periphery have been bridged. The approach that we use is graph-theoretical in nature, following our outline of this method in 2003, which is based on finding all conjugated circuits in all Kekulé valence structures of these molecules. To the π-electrons having 4n + 2 π-electrons are assigned anticlockwise π-electron currents and to conjugated circuits having 4n π-electrons are assigned π-electron currents.

Applying the conjugated circuits method to Clar structures of [n]phenylenes for determining resonance energies.

We consider the aromaticity of biphenylene and structurally related linear or angular [n]phenylenes for which the direct application of the model of conjugated circuits does not offer valid expressions for resonance energy and aromaticity. We located the cause of this problem as being due to Kekulé valence structures in which neighboring benzenoid rings are connected by two CC double bonds. By restricting the selection of Kekulé valence structures to those that contribute to Clar structures of such systems, we were able to show that linear and angular [n]phenylenes have approximately similar resonance energies, with angular [n]phenylenes being slightly more stable due to second order contributions arising from disjoint conjugated circuits.

Using Clar sextets for two- and three-dimensional aromatic systems.

After a brief history of the aromaticity concept, the use of Clar sextet circles is reviewed for explaining various aspects of planar aromatic systems (benzenoids, heterocycles) and of tridimensional carbon aggregates. When folding a graphene sheet for obtaining nanotubes, nanotori, or nanocones, the congruence of Clar sextet circles allows the classification of all such aggregates into two classes (congruent or incongruent) with marked differences in properties; this is in agreement with the well-known condition h - k ≡ 0 (mod 3), equivalent to congruence, in terms of the chiral vectors h and k for graphene sheets.

Defining rules of aromaticity: a unified approach to the Hückel, Clar and Randić concepts.

The molecular structure of any system may be unambiguously described by its adjacency matrix, A, in which bonds are assigned entry a(ij) = 1 and non-bonded pairs of atoms entry a(ij) = 0. For π-electron-containing conjugated hydrocarbons, this matrix may be modified in order to represent one of the possible Kekulé structures by assigning entry 1 to double bonds and entry 0 to single bonds, leading to the Kekulé matrix K which can be obtained from the A matrix by subtracting 1 from elements a(pq) that represent single bonds in the Kekulé structure. The A and K matrices are the boundary cases of a general matrix A(ε), named perturbation matrix, in which from elements a(pq) that represent single bonds is subtracted a value ε∈<0,1> representing the magnitude of the perturbation.

Use of mathematical structural invariants in analyzing combinatorial libraries: a case study with psoralen derivatives.

In this paper, calculated topological indices have been used to cluster a large virtual library of 12⁵ psoralen derivatives into 25 clusters in an effort to select a subset of mutually dissimilar structures from a large collection of molecules. Inspection of the 25 structures, one closest to the respective centroid of each cluster, shows that the molecules are structurally more diverse as compared to a subset of 25 selected randomly. It is expected that such methods based on easily calculated descriptors may find applications in new drug discovery from the analysis of libraries of interesting lead compounds.

Patterns of ring current in coronene isomers.

The ipsocentric pseudo-p model is used to predict maps of induced current density for isomeric variations of coronene in which the central hexagonal ring is surrounded by 5, 6, and 7-membered rings. All isomers in the set are predicted to support strong diatropic perimeter ring currents, thereby conforming to the magnetic criterion of aromaticity.

Correlations between local aromaticity indices of bipartite conjugated hydrocarbons.

Local aromaticity in rings of conjugated hydrocarbons can be measured in a variety of ways. In the present paper, we concentrate on two of these, namely, EC, i.e.

Discovery of low nanomolar and subnanomolar inhibitors of the mycobacterial beta-carbonic anhydrases Rv1284 and Rv3273.

A series of 2-(hydrazinocarbonyl)-3-aryl-1H-indole-5-sulfonamides has been derivatized by reaction with 2,4,6-trimethylpyrylium perchlorate, leading to pyridinium derivatives. The new sulfonamides were evaluated as inhibitors of two beta-carbonic anhydrases (CAs, EC 4.2.

Are thermodynamic and kinetic stabilities correlated? A topological index of reactivity toward electrophiles used as a criterion of aromaticity of polycyclic benzenoid hydrocarbons.

A topological index of reactivity (TIR) of benzenoid hydrocarbons is defined basing on an approximate value of the bicentric localization energies. TIR values correlate with all known (24) Hammett-Streitwieser position constants, based on kinetic data for electrophilic substitution in benzenoid hydrocarbons. The maximum value of the index, denoted by TIR(max), defines the stability of a molecule toward electrophiles.

Carbonic anhydrase inhibitors. Synthesis of 2,4,6-trimethylpyridinium derivatives of 2-(hydrazinocarbonyl)-3-aryl-1H-indole-5-sulfonamides acting as potent inhibitors of the tumor-associated isoform IX and XII.

A series of 2-(hydrazinocarbonyl)-3-aryl-1H-indole-5-sulfonamides possessing various 2-, 3- or 4- substituted phenyl groups with methyl-, halogeno- and methoxy-functionalities, or a perfluorophenyl moiety, has been derivatized by reaction with 2,4,6-trimethylpyrylium perchlorate. The new sulfonamides were evaluated as inhibitors of four mammalian carbonic anhydrase (CA, EC 4.2.

A new yardstick for benzenoid polycyclic aromatic hydrocarbons.

The newly introduced signature of benzenoids (a sequence of six real numbers Si with i = 6-1) shows the composition of the pi-electron partition by indicating the number of times all rings of the benzenoid are assigned 6, 5, 4, 3, 2, or 1 pi-electrons. It allows the introduction of a new ordering criterion for such polycyclic aromatic hydrocarbons by summing some of the terms in the signature. There is an almost perfect linear correlation between sums S6 + S5 and S4 + S3 for isomeric cata- or peri-fused benzenoids, so that one can sort such isomers according to ascending s 6 + S5 or to descending S4 + S3 sums (the resulting ordering does not differ much and agrees with that based on increasing numbers of Clar sextets and of Kekule structures).

Pi-electron partitions, signatures, and Clar structures of selected benzenoid hydrocarbons.

It is shown for a representative set of isomeric benzenoids that pi-electron partitions and signatures can serve for characterizing and ordering benzenoids. Benzenoid signatures (sequences s6 through s1 where the subscripts correspond to numbers of pi electrons in all rings) are obtained by examining the numbers of assigned pi electrons ranging from 6 to 1 for each ring in all resonance structures. The pi-electron partitions and signatures of all 33 non-isoarithmic peri-condensed benzenoid hydrocarbons with eight rings and four contiguous internal carbon atoms allow an ordering of these benzenoids that agrees fairly well with increasing numbers of Kekulé valence structures and Clar sextets.

Using variable and fixed topological indices for the prediction of reaction rate constants of volatile unsaturated hydrocarbons with OH radicals.

Volatile organic compounds (VOCs) play an important role in different photochemical processes in the troposphere. In order to predict their impact on ozone formation processes a detailed knowledge about their abundance in the atmosphere as well as their reaction rate constants is required. The QSPR models were developed for the prediction of reaction rate constants of volatile unsaturated hydrocarbons.

QSPR for physical properties of cata-condensed benzenoids using two simple dualist-based descriptors.

Dualists of benzenoids are a special type of graphs that have as vertices the centers of hexagons and as edges the pairs of condensed rings. Catafusenes (cata-condensed benzenoids) have acyclic dualists. Codes of catafusenes with h benzenoid rings start from one end of a dualist and consist of digits 0 (indicating linear annelation, angle of 180 degrees between two adjacent edges) and 1 or 2 for kinks (120 degrees or 240 degrees angles) with the convention of choosing the lowest number formed from these h-2 digits.

Four new topological indices based on the molecular path code.

The sequence of all paths pi of lengths i = 1 to the maximum possible length in a hydrogen-depleted molecular graph (which sequence is also called the molecular path code) contains significant information on the molecular topology, and as such it is a reasonable choice to be selected as the basis of topological indices (TIs). Four new (or five partly new) TIs with progressively improved performance (judged by correctly reflecting branching, centricity, and cyclicity of graphs, ordering of alkanes, and low degeneracy) have been explored. (i) By summing the squares of all numbers in the sequence one obtains Sigmaipi(2), and by dividing this sum by one plus the cyclomatic number, a Quadratic TI is obtained: Q = Sigmaipi(2)/(mu+1).

2-phenoxathiinyl-5-phenyloxazole and 5-phenoxathiinyl-2-phenyloxazole derivatives: experimental and theoretical study of emission properties.

The absorption and emission spectra in cyclohexane and methanol of the title phenoxathiinyl-phenyloxazole derivatives are presented and discussed. Comparing to the unsubstituted diphenyloxazole (PPO), the experimental results show a bathochromic shift of the emission band, a significant dependence of the maximum on the solvent polarity and a drastic decrease of the fluorescence quantum yield. Semiempirical MO calculations (AM1) in both the ground and excited states support the experimental findings.

Strain-free sextet-resonant benzenoids and their antisextet dualists.

By introducing antisextet dualists of sextet-resonant benzenoids that have only "full" and "empty" rings as defined by Clar, it was possible to devise a simple algorithm for an exhaustive enumeration of such sextet-resonant benzenoids (SRBs). Several relationships between invariants of such SRBs are described. All possible SRBs with up to 10 sextet rings are enumerated.

Lipophilic pyrylium salts in the synthesis of efficient pyridinium-based cationic lipids, gemini surfactants, and lipophilic oligomers for gene delivery.

Several new classes of pyridinium cationic lipids were synthesized and tested as gene delivery agents. They were obtained through a procedure that generates simultaneously the heterocyclic ring and the positively charged nitrogen atom, using lipophilic pyrylium salts as key intermediates that react with primary amines, yielding pyridinium salts. The choice of the appropriately substituted primary amine, diamine or polyamine, allows the design of the shape of the final lipids, gemini surfactants, or lipophilic polycations.

The eight classes of positive-curvature graphitic nanocones.

The eight different classes of single-wall positive-curvature graphitic nanocones are emphasized, characterized, and illustrated. Local transformations among different possible apex-region structures within a common class are investigated, it being noted that there is no short sequence of such transformations which convert from any one of these eight classes to another. Attention is directed to class identification and to nanocone codes, especially for "buckycones" comprised solely from pentagonal and hexagonal rings.

Partitioning of pi-electrons in rings for Clar structures of benzenoid hydrocarbons.

Resonance structures of polycyclic aromatic hydrocarbons can be associated with numerical formulas by assigning pi-electrons of C=C double bonds to individual benzenoid rings. Each C=C double bond in a resonance structure assigns two pi-electrons to a ring in a fused-benzenoid system if it is not shared by adjacent rings and one pi-electron when it is common to two rings, obtaining thus a "local" characterization of rings in polycyclic conjugated hydrocarbons. In the present contribution we extend this approach to the aromatic pi-sextet model of Clar, which offers an alternative description of benzenoid hydrocarbons.

Can topological indices transmit information on properties but not on structures?

A survey of several topological indices (TIs) is provided according to the nature (integers or real numbers) of the local vertex invariants (LOVIs) and of the resulting molecular descriptor (TI). The 1st generation TIs such as the Wiener index when both the LOVIs and the TI are integers have a very high degeneracy. This fact can become an asset for the problem under discussion: when confronted with the "inverse problem" (reverse engineering) such indices lead to a combinatorial explosion of possible solutions.

Experimental and theoretical study of 2,5-diaryloxazoles whose aryl are para-substituted phenyl groups.

Absorption and emission properties of some phenoxy derivatives of diphenyloxazole (PPO) are presented and discussed. The photophysical properties reflect a dependence on the substituted 2 or 5 position of the oxazole ring. The experimental data were correlated with molecular parameters such as molecular flexibility, electronic structure and the singlet-triplet gap.