Dehydrophenylnitrenes:  Quartet versus Doublet States

2003-07-19T00:00:00Z (GMT) by Holger F. Bettinger Wolfram Sander
The geometries and energies of 4-, 3-, and 2-dehydrophenylnitrenes (<b>3</b>, <b>4</b>, and <b>5</b>) are investigated using complete active space self-consistent field (CASSCF), multiconfiguration quasi-degenerate second-order perturbation (MCQDPT), and internally contracted multiconfiguration−reference configuration interaction (MRCI) theories in conjunction with a correlation consistent triple-ζ basis set. 4-Dehydrophenylnitrene <b>3</b> has a quartet ground state (<sup>4</sup>A<sub>2</sub>). The adiabatic excitation energies to the <sup>2</sup>A<sub>2</sub>, <sup>2</sup>B<sub>2</sub>, <sup>2</sup>A<sub>1</sub>, and <sup>2</sup>B<sub>1</sub> states are 5, 21, 34, and 62 kcal mol<sup>-1</sup>, respectively. The <sup>2</sup>B<sub>2</sub> state has pronounced closed-shell carbene/iminyl radical character, while the lowest-energy <sup>2</sup>B<sub>1</sub> state is a combination of a planar allene and a 2-iminylpropa-1,3-diyl. The MCQDPT treatment overestimates the excitation energy to <sup>2</sup>B<sub>2</sub> significantly as compared to CASSCF and MRCI+Q. Among quartet states, <sup>4</sup>A<sub>2</sub>-<b>3</b> is the most stable one, while those of <b>4</b> and <b>5</b> (both <sup>4</sup>A‘ ‘) are 3 and 1 kcal mol<sup>-1</sup> higher in energy. <b>5</b> also has a quartet ground state and a <sup>2</sup>A‘ ‘ state 7 kcal mol<sup>-1</sup> higher in energy. On the other hand, the doublet-quartet energy splitting is −6 kcal mol<sup>-1</sup> for <b>4</b> in favor of the doublet state (<sup>2</sup>A‘ ‘). Hence, <sup>2</sup>A‘ ‘-<b>4</b> is the most stable dehydrophenylnitrene, 3.5 kcal mol<sup>-1</sup> below <sup>4</sup>A<sub>2</sub> of <b>3</b>. The geometry of <sup>2</sup>A‘ ‘-<b>4</b> shows the characteristic features of through-bond interaction between the in-plane molecular orbitals at N and at C3. The <sup>2</sup>A‘ state of <b>4</b> resembles the <sup>2</sup>A<sub>1</sub> state of <b>3</b> and lies 32 kcal mol<sup>-1</sup> above <sup>4</sup>A‘ ‘-<b>4</b>. The lowest-energy <sup>2</sup>A‘ state of <b>5</b>, on the other hand, resembles the <sup>2</sup>B<sub>2</sub> state of <b>3</b> and lies 22 kcal mol<sup>-1</sup> above <sup>4</sup>A‘ ‘-<b>5</b>.