On Cyclic Edge-Connectivity of Fullerenes

A graph is said to be cyclic $k$-edge-connected, if at least $k$ edges must be removed to disconnect it into two components, each containing a cycle. Such a set of $k$ edges is called a cyclic-$k$-edge cutset and it is called a trivial cyclic-$k$-edge cutset if at least one of the resulting two components induces a single $k$-cycle. It is known that fullerenes, that is, 3-connected cubic planar graphs all of whose faces are pentagons and hexagons, are cyclic 5-edge-connected. In this article it is shown that a fullerene $F$ containing a nontrivial cyclic-5-edge cutset admits two antipodal pentacaps, that is, two antipodal pentagonal faces whose neighboring faces are also pentagonal. Moreover, it is shown that $F$ has a Hamilton cycle, and as a consequence at least $15\cdot 2^{\lfloor \frac{n}{20}\rfloor}$ perfect matchings, where $n$ is the order of $F$.Comment: 11 pages, 9 figure

Automated conjecturing VI : domination number of benzenoids

We demonstrate the use of a conjecturing program that can be a tool for researchers investigating bounds of invariants of chemical graphs by investigating upper bounds for the domination number of a benzenoid. The program is open-source, of general use, and can be used to generate conjectured bounds for any invariant of any class of chemical graphs

European Journal of Combinatorics Index, Volume 27

BACKGROUND: Diabetes is an inflammatory condition associated with iron abnormalities and increased oxidative damage. We aimed to investigate how diabetes affects the interrelationships between these pathogenic mechanisms. METHODS: Glycaemic control, serum iron, proteins involved in iron homeostasis, global antioxidant capacity and levels of antioxidants and peroxidation products were measured in 39 type 1 and 67 type 2 diabetic patients and 100 control subjects. RESULTS: Although serum iron was lower in diabetes, serum ferritin was elevated in type 2 diabetes (p = 0.02). This increase was not related to inflammation (C-reactive protein) but inversely correlated with soluble transferrin receptors (r = - 0.38, p = 0.002). Haptoglobin was higher in both type 1 and type 2 diabetes (p &lt; 0.001) and haemopexin was higher in type 2 diabetes (p &lt; 0.001). The relation between C-reactive protein and haemopexin was lost in type 2 diabetes (r = 0.15, p = 0.27 vs r = 0.63, p &lt; 0.001 in type 1 diabetes and r = 0.36, p = 0.001 in controls). Haemopexin levels were independently determined by triacylglycerol (R(2) = 0.43) and the diabetic state (R(2) = 0.13). Regarding oxidative stress status, lower antioxidant concentrations were found for retinol and uric acid in type 1 diabetes, alpha-tocopherol and ascorbate in type 2 diabetes and protein thiols in both types. These decreases were partially explained by metabolic-, inflammatory- and iron alterations. An additional independent effect of the diabetic state on the oxidative stress status could be identified (R(2) = 0.5-0.14). CONCLUSIONS: Circulating proteins, body iron stores, inflammation, oxidative stress and their interrelationships are abnormal in patients with diabetes and differ between type 1 and type 2 diabetes</p

Forcing Independence

An independent set in a graph is a set of vertices which are pairwise non-adjacent. An independ-ent set of vertices F is a forcing independent set if there is a unique maximum independent set I such that F ⊆ I. The forcing independence number or forcing number of a maximum independent set I is the cardi-nality of a minimum forcing set for I. The forcing number f of a graph is the minimum cardinality of the forcing numbers for the maximum independent sets of the graph. The possible values of f are determined and characterized. We investigate connections between these concepts, other structural concepts, and chemical applications. (doi: 10.5562/cca2295

Forcing, Freedom, & Uniqueness in Graph Theory & Chemistry

Harary’s & Randić’s ideas of “forcing” & “freedom” involve subsets of double bonds of Kekule structure such as to be unique to that Kekule structure. Such forcing sets are argued to be greatly generalizable to deal with various other coverings, and thence forcing seems to be fundamental, and of notable potential utility. Various forcing invariants associated to (molecular) graphs ensue, with illustrative (chemical) ex-amples and some mathematical consequences being provided. A complementary “uniqueness” idea is not-ed, and the general characteristic of “derivativity” of “forcing” is established (as is relevant for QSPR fit-tings). Different ways in which different sorts of forcings arise in chemistry are briefly indicated.(doi: 10.5562/cca2000

Maximum cardinality resonant sets and maximal alternating sets of hexagonal systems

AbstractIt is shown that the Clar number can be arbitrarily larger than the cardinality of a maximal alternating set. In particular, a maximal alternating set of a hexagonal system need not contain a maximum cardinality resonant set, thus disproving a previously stated conjecture. It is known that maximum cardinality resonant sets and maximal alternating sets are canonical, but the proofs of these two theorems are analogous and lengthy. A new conjecture is proposed and it is shown that the validity of the conjecture allows short proofs of the aforementioned two results. The conjecture holds for catacondensed hexagonal systems and for all normal hexagonal systems up to ten hexagons. Also, it is shown that the Fries number can be arbitrarily larger than the Clar number

The Clar Structure of Fullerenes

A fullerene is a 3-regular plane graph consisting only of pentagonal and hexagonal faces. Fullerenes are designed to model carbon molecules. The Clar number and Fries number are two parameters that are related to the stability of carbon molecules. We introduce chain decompositions, a new method to find lower bounds for the Clar and Fries numbers. In Chapter 3, we define the Clar structure for a fullerene, a less general decomposition designed to compute the Clar number for classes of fullerenes. We use these new decompositions to understand the structure of fullerenes and achieve several results. In Chapter 4, we classify and give a construction for all fullerenes on |V| vertices that attain the maximum Clar number |V|/6 - 2. In Chapter 5, we settle an open question with a counterexample: we construct an infinite family of fullerenes for which a set of faces attaining the Clar number cannot be a subset of a set of faces that attains the Fries number. We develop a method to calculate the Clar number directly for many infinite families of fullerene

Currents in Carbon and Heterocyclic Networks

Current density maps are calculated within the ipsocentric approach for a variety of systems, to determine the nature of their aromatic magnetic response, and to probe the underlying principles governing ring current aromaticity. The first chapter briefly discusses the history of the term aromaticity, from the discovery of benzene, to ring current theory, the modern quantum mechanical ipsocentric approach, and the simple but powerful selection rules that are derived from it. Examples of systems displaying aromatic, antiaromatic and localised, non aromatic responses are provided in Chapter 2 to demonstrate the utility of the method, and the in depth analysis of aromaticity that it permits. Chapter 3 explores the possibility of designing tailored ring current responses on finite nanographene flakes via functionalisation by examining a variety of nanographene/nanographane hybrids derived from coronene and ovalene. This idea is extended in Chapter 4, by consideration of substitution of C6 cycles for borazine like B3N3 cycles, creating benzenoid/borazinoid hybrids. Chapter 5 investigates how BN heteroannulenes can be successfully aromatised by alteration of electronic charge. The approaches for altering current response introduced in Chapters 3 to 5 are unified in Chapter 6 in a case study of pyrene and structures derived from it by variation of charge, substitution, and functionalisation. Chapter 7 further examines how changing the electronic environment by substitution of carbon centres for heteroatoms alters ring current patterns, using linear polyacenes as the example systems. Chapter 8 moves away from the methods of controlling ring current by chemical manipulation and considers the effects of geometric change on aromaticity of the homotropenylium cation and the extended family of N homoannulenes. A brief discussion of the possibilities of designing aromatic systems using extended non standard ring architectures concludes the thesis