Are they linear, bent, or cyclic? Quantum chemical investigation of the heavier group 14 and group 15 homologues of HCN and HNC.

ABSTRACT

The singlet potential-energy surface (PES) of the system involving the atoms H, X, and E (the (H, X, E) system) in which X=N-Bi and E=C-Pb has been explored at the CCSD(T)/TZVPP and BP86/TZ2P+ levels of theory. The nature of the X-E bonding has been analyzed with charge- and energy-partitioning methods. The calculations show that the linear isomers of the nitrogen systems lin-HEN and lin-HNE are minima on the singlet PES. The carbon compound lin-HCN (HCN=hydrogen cyanide) is 14.9 kcal mol(-1) lower in energy than lin-HNC but the heavier group 14 homologues lin-HEN (E=Si-Pb) are between 64.8 and 71.5 kcal mol(-1) less stable than the lin-HNE isomers. The phosphorous system (H, P, E) exhibits significant differences concerning the geometry and stability of the equilibrium structures compared with the nitrogen system. The linear form lin-HEP of the former system is much more stable than lin-HPE. The molecule lin-HCP is the only minimum on the singlet PES. It is 78.5 kcal mol(-1) lower in energy than lin-HPC, which is a second-order saddle point. The heavier homologues lin-HPE, in which E=Si-Pb, are also second-order saddle points, whereas the bent-HPE structures are the global minima on the PES. They are between 10.3 (E=Si) and 36.5 kcal mol(-1) (E=Pb) lower in energy than lin-HEP. The bent-HPE structures possess rather acute bending angles H-P-E between 60.1 (E=Si) and 79.7° (E=Pb). The energy differences between the heavier group 15 isomers lin-HEX (X=P-Bi) and the bent structures bent-HXE become continuously smaller. The silicon species lin-HSiBi is even 3.1 kcal mol(-1) lower in energy than bent-HBiSi. The bending angle H-X-E becomes more acute when X becomes heavier. The drastic energy differences between the isomers of the system (H, X, E) are explained with three factors that determine the relative stabilities of the energy minima 1) The different bond strength between the hydrogen bonds H-X and H-E. 2) The electronic excitation energy of the fragment HE from the X (2)Π ground state to the (4)Σ(-) excited state, which is required to establish a E≡X triple bond in the molecules lin-HEX. 3) The strength of the intrinsic X-E interactions in the molecules. The trends of the geometries and relative energies of the linear, bent, and cyclic isomers are explained with an energy-decomposition analysis that provides deep insight into the nature of the bonding situation.