The diazocarbene (CNN) molecule:Characterization of the X ~3SIGMA~-and A ~3PI electronic states

摘要

The ground (X ~3SIGMA~-) and first excited triplet (A ~3PI) electronic states of diazocarbene (CNN) have been investigated systematically starting from the self-consistent-field theory and proceeding to the coupled cluster with single,double,and full triple excitations (CCSDT) method with a wide range of basis sets.While the linear X ~3SIGMA~- ground state of CNN has a real degenerate bending vibrational frequency,the A ~3PI state of CNN is subject to the Renner-Teller effect and presents two distinct real vibrational frequencies along the bending coordinate.The bending vibrational frequencies of the A ~3PI state were evaluated via the equation-of-motion coupled cluster (EOM-CC) techniques.The significant sensitivity to level of theory in predicting the ground-state geometry,harmonic vibrational frequencies,and associated infrared intensities has been attributed to the fact that the reference wave function is strongly perturbed by the excitations of 1pi->3pi followed by a spin flip.At the highest level of theory with the largest basis set,correlation-consistent polarized valence quadruple zeta (cc-pVQZ) CCSDT,the classical X-A splitting (T_e value) was predicted to be 68.5 kcal/mol (2.97 eV,24000 cm~(-1)) and the quantum mechanical splitting (T_o value) to be 69.7 kcal/mol (3.02 eV,24400 cm~(-1)),which are in excellent agreement with the experimental T_0 values,67.5-68.2 kcal/mol (2.93-2.96 eV,23600-23900 cm~(-1)).With the EOM-CCSD method the Renner parameter (epsilon) and averaged bending vibrational frequency (omega_2) for the A ~3PI state were evaluated to be epsilon=-0.118 and omega_2=615 cm~(-1),respectively.They are in fair agreement with the experimental values of epsilon=-0.07 and v_2=525 cm~(-1).