Quantum chemical exploration of dimeric forms of polycyclic aromatic hydrocarbons, naphthalene, perylene, and coronene

Highlights

• Polycyclic aromatic hydrocarbon dimers were obtained by geometry optimization.

• Two PAH molecules placed in parallel were directly bonded between them.

• Cage structures with four-membered rings on the side faces were constructed.

• PAH dimers were found to be located in sufficiently deep potential energy wells.

• PAH dimers were found to have massive energies larger than two isolated molecules.

Abstract

We have found dimeric forms of polycyclic aromatic hydrocarbon (di-PAH) molecules by quantum chemical calculations. Geometry optimization starting from short distances of ca. 0.15 nm between two PAH molecules placed in parallel gave cage structures of di-PAH with CC bond connections forming four-membered rings between the PAH molecules. The di-PAH molecules are located in sufficiently deep potential energy wells surrounded by high energy barriers, although their energies are much higher than those of the isolated two PAH molecules.

Introduction

A carbon atom can be bonded with several atoms to yield various sizes of chains, rings, and cages, which have interesting structures and valuable properties. Many of hydrocarbon and carbon structures, such as benzene, cyclohexane, adamantane, graphene, and diamond, are composed of stable six-membered rings. In some other cases, such as azulene, corannulene, fullerenes, and nano tubes, their carbon skeletons include five-membered rings or seven membered rings. Besides these intensively studied structures with five-, six-, or seven membered rings, another type of structures also with four- or three-membered rings has been explored. For example, hydrocarbons called prismanes C_2nH_2n, formed by two parallel regular n-gons connected by n rectangular faces, are to be noted due to their peculiar cage structures and high chemical energies [1], [2], [3], [4], [5], [6]. Although some prismanes, such as prismane C₆H₆ [1], cubane C₈H₈ [2], and pentaprismane C₁₀H₁₀ [3] were really synthesized, many other systems, such as hexaprismane C₁₂H₁₂ [5], [6] and the larger analogues have been studied only by theoretical calculations. In connection with hexaprismane C₁₂H₁₂, various benzene dimers (C₆H₆)₂ have been studied by quantum chemical calculations [7], [8], [9], [10]. Computational explorations revealed fifteen isomeric forms of benzene dimers [8], which include four types of synthesized dimers. Recently, a systematic exploration for the benzene isomers C₆H₆ on the ground-state potential energy surface (PES) by quantum chemical calculations found numerous numbers, more than two thousand, of isomeric forms [11], [12]. In the view of these studies, the search for further new types of hydrocarbon is still challenging.

Possible structures for carbon skeletons in hydrocarbon molecules can be related to those for carbons. Carbon structures were classified on the basis of known structures [13], for instance, various fullerene types, prisms, spiral chains, zigzag tubes, double layers, and diamond-like structures. More recently, quantum chemical calculations suggested the existence of a new class of carbon allotropes with four-membered rings [14], [15], [16], [17]: A prism-C_2n series (n = 8–10, 12, 14, 16, 18 and 20) like hamster wheels [14] can be related to the [n]-prismanes C_2nH_2n (n = 3–10) [6]. The prism-C_2n structures can connect to each other by facing at four-membered rings to give various sheet structures called the prism-C_2n sheets (n = 6, 8, and 12) [15]. The prism-C₁₂ sheet corresponds to a honeycomb shaped double carbon layers with covalent CC-bond connections between two graphene sheets [13], and it may be related to face-fused poly-hexaprismanes. The prism-C_n tubes are carbon allotropes with four-membered rings on the side faces and axially piled up n-membered carbon polygons (n = 3–8, 10, 12, 14, 16, 18, and 20) [16]. The prism-C_n tubes can be related to poly[n]prismanes (n = 3–6) [5]. The wavy carbons with condensed four-membered rings were also found by quantum chemical calculations [17].

In connection with the above studies, interactions between two benzene molecules [10] should be noted. Intermolecular π/π interactions between two benzene molecules cause weak attraction due to van der Waals forces at interplane distances of ca. 0.37 nm. In shorter distance around 0.25–0.30 nm, the exchange repulsion between the two aromatic molecules rises. However, once they come much closer at ca. 0.16 nm, strong covalent interactions take over, which yield cyclodimers of benzenes [10]. Considering the existence of the covalent bonds between polygons in [n]-prismanes and prism-C_2n, covalent interactions between polycyclic aromatic hydrocarbon (PAH) molecules are also expected to obtain at such a short distance.

Regarding computational methods, automatic explorations on PES for all the possible isomeric structures and transition state structures, as well as reaction pathways had been a significantly difficult issue. In recent two decades, a global exploration of equilibrium structures (EQ), transition state structures (TS), and dissociation channels (DC) on PES for a given chemical composition became possible by the global reaction route mapping (GRRM) techniques [18], [19], [20], [21], [22], [23]. Based on the GRRM techniques, many studies concerning explorations on PES [11], [12], [14], [15], [16], [17], [24], [25], [26], [27] were performed. The GRRM techniques are useful not only to list up EQ, TS and DC but also to determine the lowest energy barrier from an EQ, which gives essential information on the stability of the EQ. The capability accomplishing these tasks is a notable advantage of the GRRM techniques.

In this study, we have explored hydrocarbon structures with four-membered rings in a cage form by using the GRRM program [23]. They consist of a pair of PAH structures placed in parallel, which have many bonds between them. We have investigated dimers of naphthalene C₁₀H₈ as the smallest PAH in polyacene systems, perylene C₂₀H₁₂ as a typical PAH with concave perimeters, and coronene C₂₄H₁₂ as a representative PAH with fully convex perimeters. The study on acene dimerization [28] investigated various dimers, which have two bonds in between, for naphthalene, anthracene, pentacene, and heptacene, but no study on the dimers with more than two bonds has been yet reported. As will be shown, our quantum chemical calculations predicted that those structures have high energy but can exist in a deep potential energy well surrounded by high energy barriers.