Verification of the Experimental Equilibrium Structure of 3,4-Dicyanofuroxan and 3-Cyano-4-Aminofuroxan Molecules by MP2 and CCSD(T) Methods

Abstract

The equilibrium (r_e) structure of 3-cyano-4-aminofuroxan (3,4-CAFO) and 3,4-dicyanofuroxan (3,4-DCFO) molecules is determined for the first time by combined QC calculations using post-Hartree–Fock methods (MP2, CCSD(T)) with cc-pVXZ (X = T, Q) basis sets up to the CCSD(T)/cc-pVQZ level of theory. The CCSD(T) single-reference approximation is applied using the Lee–Taylor criterion (<0.02). Theoretical and experimental values of geometric r_e-parameters of these molecules in the gas phase are compared. QC calculations (CCSD(T)) generally agree with the experimental r_e-structure of these molecules determined by gas phase electron diffraction (GED). Discrepancies between the values of theoretical and experimental parameters in 3,4-DCFO (4 distances) and 3,4-CAFO (2 distances and 3 angles) molecules are several times higher than the available statistics (CCSD(T)) and the reported experimental errors. The revealed systematic inconsistencies between theoretical and experimental values of some bond lengths and bond angles make these molecules interesting objects for theoretical and experimental structural studies in order to obtain refined geometric r_e-parameters of furoxan compounds and to accumulate the statistics of r_e-structures reproduction using high-level quantum chemical methods. When solving the structural inverse problem (IP), it is desirable that balanced molecular models and optimization strategies are selected and applied on a competitive basis.