The Remarkable Structure of Lithium Cyanide/Isocyanide

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Abstract
Electron correlation corrections have a considerable influence on the relative stabilities of lithium isocyanide (1), lithium cyanide (2), and the bridged form, 3. While Hartree‐Fock theory finds 1 to be most stable and 3 not to be a minimum, MP2/6‐31G* optimization indicates 3 to be the global minimum. At higher levels employing full fourth‐order Møller‐Plesset theory and a quadruply split valence and polarized basis set (MP4STDQ/6‐311+G*), 2 is only about 2 kcal/mol less stable than 1 and 3, which are indicated to have nearly the same energy. LiNC thus is similar to C(Na)N and C(K)N, both of which are known to prefer T‐shaped (bridged) structures in the gas phase. However, to an even greater extent than formerly realized, rotation of the lithium cation around the cyanide anion nucleus should be practically free. ΔH (LiCN) = 32.8 kcal/mol is estimated from the calculated lithium cation affinity of 151.2 kcal/mol. In addition, we find at the MP4SDTQ/6‐31+G*//MP2/6‐31G* level that the bridged form of NaCN is favored by 2–3 kcal/mol over the corresponding linear forms, which have nearly the same energy.