On the HCN - HNC Energy Difference

Thanh L. Nguyen, Joshua H. Baraban, Branko Ruscic, John F. Stanton

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32 Scopus citations


The value for the HCN → HNC 0 K isomerization energy has been investigated by combining state-of-the-art electronic structure methods with the Active Thermochemical Tables (ATcT) approach. The directly computed energy difference between HCN and HNC at the HEAT-456QP level of theory is 5236 ± 50 cm-1. This is substantially lower (by ∼470 cm-1 or ∼1.3 kcal/mol) than the recently proposed high-level multireference configuration interaction value of 5705 ± 20 cm-1 of Barber et al. (Mon. Not. R. Astron. Soc. 2014, 437, 1828-1835). The discrepancy was analyzed by the ATcT approach, using several distinct steps, which (a) independently corroborated the current single-reference HEAT-456QP result, (b) independently found that the recent multireference-based value is highly unlikely to be correct within its originally stated uncertainty, and (c) produced a recommended value of 5212 ± 30 cm-1 for the HCN → HNC isomerization energy at 0 K, based on all currently available knowledge. The ATcT standard enthalpies of formation at 0 and 298 K for HCN, HNC, and their cations and anions are also presented.

Original languageEnglish
Pages (from-to)10929-10934
Number of pages6
JournalJournal of Physical Chemistry A
Issue number44
StatePublished - 8 Oct 2015
Externally publishedYes

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry


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