Bonds and Intermolecular Interactions – The Return of Cohesion to Chemistry

Sason Shaik

The Hebrew University of Jerusalem, Institute of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, Edmond J. Safra Campus, Givat Ram, Jerusalem, 91904 Israel

Email: sason.shaik@gmail.com

1.1 Introduction

One of the fundamental territories of chemistry is the chemical bond, the glue from which an entire chemical universe is constructed, resulting in a rich and enticing variety of species held together by stabilizing interactions. Let me first define the interactions and the manner in which they will be classified in this introductory chapter.

There are interactions that pair-up or delocalize electrons over a two or more atoms, and thereby give rise to entities we call molecules. This chapter refers to these interactions as chemical bonds. The chemical bond defines a chemical identity. It also accounts for the 'magic' of chemistry, whereby one molecule disappears and a new one appears. Take for example the soft gray solid, sodium, (Na)s, and mix it with the yellow-greenish gas, chlorine (Cl2). And within seconds, lo and behold, you get a white solid, (Na+Cl-)s. All three species have distinct identities, being defined by their chemical bonds, and as such the magic is a manifestation of breaking bonds and making new ones. Clearly, then, the term chemical bond defines the molecule, and hence also the chemical identity. Usage of this term is also a tribute to Gilbert Newton Lewis who invented it 100 years ago as a quantum unit of bonding in molecules. By so doing, Lewis has ushered "the electronic-structure revolution in chemistry".

There are however, stabilizing interactions also between and among molecules with saturated valences, such as hydrogen bonds, dipole–dipole and dispersion interactions, and so on. These interactions are responsible for another type of 'magic'- the magic of aggregation, crystallization, gelation, coagulation, micelle formation, self-assembly, and evolution of large mesoscopic bodies. Consider for example discrete water molecules in the gas phase (vapor). As you lower the temperature, gradually you see water appearing as a liquid, and as temperature is further lowered, lo and behold, the liquid becomes a beautiful translucent solid, ice. Thus, the interactions between the discrete molecules bring about the formation of mesoscopic and macroscopic matter, and as such, we refer to these cohesive forces as intermolecular interactions.

Despite the identity of the forces involved in these two kinds of interactions and the fact that their energy representations are entirely identical (Figure 1.1), the distinction of bonds and intermolecular forces is not artificial. Thus, chemical bonding obeys magic numbers (electron pairs, octets, Hückel's 4n+2 numbers, etc.), and defines chemical identity, while the intermolecular interaction, as a creator of aggregates, is an extensive property that does not obey magic numbers and has much lesser selectivity. In other words, bonds are formed to satisfy the atomic valences, while intermolecular interactions are residual interactions formed between species, which have satisfied their formal valences. Consequently, diatomic bonds are generally stronger than the respective diatomic intermolecular interactions, and as such, 'bonds' make molecules, while 'intermolecular interactions' create aggregates of molecules.

It is deemed necessary therefore to conserve this distinction in view of the current vogues of calling every stabilizing interaction, even as in He, a bond; a tendency that has already been criticized. This distinction is also didactic, playing an important role in the comprehension and teaching processes of the structure of chemistry. In fact, it is this distinction that has enabled the development of chemistry as a molecular science.

In many ways, bonding and intermolecular interaction is a field in a midst of a great renaissance. There are new interesting theoretical approaches to probe the origins of bonds, many novel bonding motifs, and experimental studies that describe imaging of bonds being broken and remade using atomic force microscopy. The bond is becoming again a central intellectual arena, and one can even find allusions to the bond as an elementary particle of chemistry, the so-called "bondon". Likewise, the interest in intermolecular interactions has moved to new frontiers with bonding motifs such as "halogen bonds" and CH ... HC interactions, and methods to calculate and analyze them. There exist also new chemical bonds that are supported by dispersion interaction, such as very long C–C bonds. There are even some claims...