A comparative study of the energetics, structures, and mechanisms of the HCN ↔ HNC and LiCN ↔ LiNC isomerizations

Abstract

The potential energy surfaces of the HCN H HNC and LiCN w LiNC isomerization processes were determined by ab initio theory using fully optimized triple-zeta double polarization types of basis sets. Both the MP2 corrections and the QCISD level of calculations were performed to correct for the electron correlation. Results show that electron correlation has a considerable influence on the energetics and structures. Analysis of the intramolecular bond rearrangement processes reveals that, in both cases, H (or Li' ) migrates in an almost elliptic path in the plane of the molecule. In HCN H HNC, the migrating hydrogen interacts with the in-plane a , a * orbitals of CN, leading to a decrease in the C-N bond order. In LiCN H LiNC, Li+ does not interact with the corresponding a , a * orbitals of CN.Key words: potential energy surfaces, intra-molecular bond rearrangement, bond orders, elliptic path, migration of Li' .RCsumC : On a dCterminC les surfaces d'Cnergie potentielle des processus d'isomCrisation HCN H HNC et LiCN H LiNC B l'aide de la thCorie ab initio, en faisant appel B des ensembles de base de type double polarisation, triple zeta, totalement optimisCs. Dans le but de corriger la corrClation Clectronique, on a effectuC des corrections MP2 ainsi que des calculs au niveau QCISD. Les rCsultats montrent que la corrtlation Clectronique a une grande influence sur les energies et les structures. L'analyse des processus de rearrangement intramoltculaire des liaisons rCvkle que, dans chacun des cas, H (ou Li+) migre par une voie pratiquement elliptique dans le plan de la moltcule. Dans le rkarrangement HCN H HNC, l'hydrogkne qui migre interagit avec les orbitales a,ap* dans le plan CN conduisant B une diminution de l'ordre de la liaison C-N. Dans LiCN w LiNC, Li+ n'interagit pas avec les orbitales a , a * correspondantes du CN.

Molecular properties and potential energy surfaces of the cyanides of the groups 1 and 11 metal atoms

“…The potential energy surfaces for the isomerizations of HCN ↔ HCN and LiCN ↔ LiNC are available, which were obtained at the second-order Møller-Plesset perturbation ͑MP2͒ and quadratically convergent configuration interaction singles-and-doubles levels of theory. 13 Farantos and Tennyson 14 have reported a trajectory calculation for the LiCN and KCN systems with two degrees of freedom neglecting the C-N vibrations, where the metal atom appeared to move around the CN group in a chaotic way.…”

Magnesium Cyanide or Isocyanide?

“…[14] Experimental and calculation data indicate that the XÀCN/XÀNC ratio increases with increasing electronegativity of X, that is,t he cyanide isomer becomes more favorable for covalently bound CN groups (Scheme 1a). [13][14][15][16][17][18] Forionically bound CN À ,for example,LiCN,the cyanide/isocyanide energy differences become negligible while,atthe same time,the transition states for isomerization are lowered as well.…”

“…), [14] LiCN (calc.). [16] b) Relative energies (kcal mol À1 )for Ae(CN) 2 complexes(MP4SDTQ// MP2 including ZPE, true minimaw ith no imaginary frequencies); values taken from ref. [19].…”

Magnesium Cyanide or Isocyanide?

“…[10] More covalently bound main-group CN compounds generally prefer cyanide connectivity.F or example,o rganic nitriles (RCN) are thermodynamically more stable than the corresponding isonitriles (RNC). [13][14][15][16][17][18] Forionically bound CN À ,for example,LiCN,the cyanide/isocyanide energy differences become negligible while,atthe same time,the transition states for isomerization are lowered as well. [12] Likewise,t he anion [(CF 3 ) 3 B À CN] À is 8.4 kcal mol À1 lower in energy than [(CF 3 ) 3 B À NC] À .…”

“…), [13] Me 3 SiCN (calc. [16] b) Relative energies (kcal mol À1 )for Ae(CN) 2 complexes(MP4SDTQ// MP2 including ZPE, true minimaw ith no imaginary frequencies); values taken from ref. [16] b) Relative energies (kcal mol À1 )for Ae(CN) 2 complexes(MP4SDTQ// MP2 including ZPE, true minimaw ith no imaginary frequencies); values taken from ref.…”