Computational study of ${\rm X-H}\cdots {\rm C}$ and ${\rm C-H}\cdots{\rm X}$ hydrogen bonds in n-alkane-${\rm HX}$ complexes (${\rm X} = {\rm F,OH}$, alkane=propane, butane, pentane) has been carried out in this work. Ab initio and density functional theories were used for this study. For $n$-alkane-${\rm H}$₂${\rm O}$ complexes both ${\rm O} \cdots {\rm H-C}$ and ${\rm O-H}\cdots {\rm C}$ hydrogen bonded complex have been found, while for 𝑛-alkane-${\rm HF}$ complexes, our attempt to optimize ${\rm F}\cdots {\rm H-C}$ ${\rm H}$-bond was not successful. Like most of the hydrogen bonded systems, strong correlation between binding energy and stretching frequency of ${\rm H-F}$ and ${\rm O-H}$ stretching mode was observed. The values of electron density and Laplacian of electron density are within the accepted range for hydrogen bonds. In all these cases, ${\rm X-H}\cdots {\rm C}$ hydrogen bonds are found to be stronger than ${\rm C-H}\cdots {\rm X}$ hydrogen bonds.

Theoretical and experimental studies of bonding in the main group homonuclear diatomic molecules have been pursued for many years, and they possess serious challenges for scientists. Most of the early experimental work have been carried out by Herzberg.^1,2 We take a relook at the bonding motifs of Period 2 homonuclear diatomic molecules (from Li₂ to Ne₂) using varieties of quantum chemical tools, commonly used for intermolecular bonding/interactions now. The methods employed include Atoms in Molecules (AIM), Non-covalent Index plot (NCI), Electrostatic potential (ESP), and Potential Acting on oneElectron in a Molecule (PAEM). The spectroscopic constants i.e., equilibrium bond distances (re), harmonic frequencies (x), bond dissociation energies (D_e) have all been evaluated using high-level ab initio methods and critically compared with the experimental results. Multi-reference calculations (CASSCF) on B2 and C2 have been carried out as they have a large number of low lying electronic states. Bonding within these homonuclear diatomic molecules show all the diversities that are encountered in inter/intra-molecular bonding in chemistry. Based on the AIM analysis, these 8 homonuclear diatomic molecules could be divided into three different groups, based on the correlation between binding energy and the electron density at the bond critical point. However, PAEM/ESP analysis allows us to analyse all eight of them as one group havinga good correlation between binding energy and the PAEM/ESP at the critical point between the two atoms. Our results highlight the arbitrariness in relying on some computational tools to characterize a bond as covalent (shared) or ionic/electrostatic (closed). In contrast, they also show the usefulness of the various methods in exploring similarities and differences in bonding. We propose that from Li₂ to Ne₂, all homonuclear diatomic molecules are bound by ‘chemical bonds’.