Vibrational energy levels for the electronic ground state of the diazocarbene (CNN) molecule

Abstract

The vibrational energy levels of diazocarbene (diazomethylene) in its electronic ground state, CNN, have been predicted using the variational method. The potential energy surfaces of CNN were determined by employing ab initio single reference coupled cluster with single and double excitations (CCSD), CCSD with perturbative triple excitations [CCSD(T)], multi-reference complete active space self-consistent-field (CASSCF), and internally contracted multi-reference configuration interaction (ICMRCI) methods. The correlation-consistent polarised valence quadruple zeta (cc-pVQZ) basis set was used. Four sets of vibrational energy levels determined from the four distinct analytical potential functions have been compared with the experimental values from the laser-induced fluorescence measurements of Wurfel et al. obtained in 1992. The CCSD, CCSD(T), and CASSCF potentials have not provided satisfactory agreement with the experimental observations. In this light, the importance of both non-dynamic (static) and dynamic correlation effects in describing the ground state of CNN is emphasised. Our best theoretical fundamental frequencies at the cc-pVQZ ICMRCI level of theory, ν₁ = 1230, ν₂ = 394, and ν₃ = 1420 cm^− 1, are in excellent agreement with the experimental values of ν₁ = 1235, ν₂ = 396, and ν₃ = 1419 cm^− 1, and the mean absolute deviation between the 23 calculated and experimental vibrational energy levels is only 7.4 cm^− 1. It is shown that the previously suggested observation of the ν₃ frequency at about 2847 cm^− 1 was in fact the first overtone 2ν₃.

Keywords:

• vibrational energy levels
• electronic ground state
• diazocarbene
• ICMRCI
• CASSCF

Acknowledgements

This research was supported by the US Department of Energy, Office of Basic Energy Sciences, Fundamental Interactions Team, Chemical Sciences Division, Grant No. DE-FG02-97ER14748. One of us (SC) would like to acknowledge support from the US Office of Naval Research under grant N00014-01-1-0235. Dedicated to Professor Raphael D. Levine on the occasion of his 70th birthday.