Ground state and vertical excitation energies of the diazene isomers with the coupled cluster method

Abstract

State-of-the-art coupled cluster methods were used to study ground and excited states of the diazene isomers. Three coupled cluster variants were applied in the ground state calculations: CCSD, CCSDT, and CCSDT(Q f ) for basis sets up to aug-cc-pV5Z. The ground state equilibrium geometry was found using combination of the analytical and numerical gradients. The computed values compare very well with experimental data if the latter are available. The vertical excitation energies were obtained with the EOM-CCSD and EOM-CCSDT approaches for the same set of basis sets. The importance of the connected triple excitations in the accurate evaluation of the excitation energy is discussed. Out of the four lowest lying excited states studied for each isomer at least one belongs to the category for which an inclusion of triple excitations is crucial for its adequate description.

Coordination Properties of Bridging Diazene Ligands in Unusual Diiron Complexes

“…Whereas hydrazine is a well-studied and stable compound in its free state, free diazene is quite unstable. Only the trans -isomer of this molecule has been synthesized, under low temperatures. , Theoretical study of the diazene isomers also predicts the trans -isomer to be the most stable …”

“…34,35 Theoretical study of the diazene isomers also predicts the trans-isomer to be the most stable. 36 The coordination chemistry of diazene is rather interesting. As a ligand, diazene displays at least four binding modes, illustrated in Chart 1.…”

Combined Electron-Diffraction, Spectroscopic, and Theoretical Determination of the Structure of N-Deuterio-trans-Methyldiazene, CH3N═ND. Conformational Effects of the N═N Double Bond

“…trans- HNNH and its various deuterium and nitrogen-15 isotopomeric forms have been studied extensively by infrared and ultraviolet spectroscopy and, from rotational constants determined by high-resolution methods, the structure is well-established. − Its equilibrium r e parameters are 1.247(1) Å and 1.029(1) Å for the NN and NH bond lengths and 106.3(1)° for the NNH angle . The structural and spectroscopic results are generally in good agreement with predictions from theoretical calculations. − A review of much of the work on the diazenes by Craig et al contains a description of their efforts to detect the cis form of diazene. These attempts were not successful, but the cis arrangement is thought to play a transitory role in the stereospecific hydrogen reduction of carbon and nitrogen multiple bonds. , It is also believed to occur as an intermediate complex with the nitrogenase enzyme in the reduction of atmospheric nitrogen to ammonia. , …”

“…Only two of the orbitals are of A″ symmetry; these are bonding combinations of the carbon and nitrogen out-of-plane p z orbitals with the two out-of-plane H atoms (1s A −1s B combination). One of these (8) favors the S arrangement; the other (11), the E geometry. Overall, the A′ in-plane orbital combinations are the main determining factor in lowering the energy of the eclipsed orientation of the methyl group.…”

“…As nitrogen analogs to ethylene compounds, the diazenes are of considerable interest to chemists, both experimentally and theoretically. − The simplest of these are shown in Figure , with a trans arrangement displayed since in all cases this structure is theoretically lower in energy than the cis form. We note also that, for the trans forms, rotamers with a methyl CH bond eclipsing ( E ) the NN bond have a lower energy than the ones with staggered ( S ) CH bonds with respect to the NN bond.…”