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

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.