Dehydrophenylnitrenes:  Quartet versus Doublet States

Abstract

The geometries and energies of 4-, 3-, and 2-dehydrophenylnitrenes (3, 4, and 5) 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-zeta basis set. 4-Dehydrophenylnitrene 3 has a quartet ground state ((4)A(2)). The adiabatic excitation energies to the (2)A(2), (2)B(2), (2)A(1), and (2)B(1) states are 5, 21, 34, and 62 kcal mol(-1), respectively. The (2)B(2) state has pronounced closed-shell carbene/iminyl radical character, while the lowest-energy (2)B(1) state is a combination of a planar allene and a 2-iminylpropa-1,3-diyl. The MCQDPT treatment overestimates the excitation energy to (2)B(2) significantly as compared to CASSCF and MRCI+Q. Among quartet states, (4)A(2)-3 is the most stable one, while those of 4 and 5 (both (4)A'') are 3 and 1 kcal mol(-1) higher in energy. 5 also has a quartet ground state and a (2)A' ' state 7 kcal mol(-1) higher in energy. On the other hand, the doublet-quartet energy splitting is -6 kcal mol(-1) for 4 in favor of the doublet state ((2)A''). Hence, (2)A''-4 is the most stable dehydrophenylnitrene, 3.5 kcal mol(-1) below (4)A(2) of 3. The geometry of (2)A''-4 shows the characteristic features of through-bond interaction between the in-plane molecular orbitals at N and at C3. The (2)A' state of 4 resembles the (2)A(1) state of 3 and lies 32 kcal mol(-1) above (4)A''-4. The lowest-energy (2)A' state of 5, on the other hand, resembles the (2)B(2) state of 3 and lies 22 kcal mol(-1) above (4)A''-5.

EPR spectroscopic and computational characterization of the 2‐dehydro‐m‐xylylene and 4‐dehydro‐m‐xylylene triradicals

“…Compared to CISD + Q and AQCC energies which generally agree nicely, RS2 energies are even worse than CASSCF data for these states. Similar findings have been reported previously for some closed‐shell doublet states of dehydrophenylnitrenes 10–12 as well as for closed‐shell singlet states of phenylnitrene . It is also evident that the differences between computations based on (7,7) and (9,9) reference wavefunctions are generally rather small (for RS2c smaller than for SCF), so that truncation of the active π space in the reference wavefunction by leaving the lowest and highest orbitals uncorrelated offers a viable way to reduce computational demands without significant loss of numerical accuracy in computations on this type of triradicals (see also, the corresponding data for the singlet–triplet energy splitting of 1 , given in Note ).…”

“…The 8b 2 orbital is largely localized on the nitrogen atom, whereas the 3b 1 orbital closely resembles the singly occupied molecular orbital of the benzyl radical. Thus, analogous to the dehydrotoluenes, 10 and 12 feature 4 A″ and 4 A 2 quartet ground states, respectively, with similar doublet–quartet splittings ( 10 : 5.3 kcal/mol; 12 : 7.9 kcal/mol; MRMP2/cc‐pVTZ//CASSCF/cc‐pVTZ), whereas the 2 A″ ground state of 11 lies 3.5 kcal/mol below the 4 A″ state . By a careful analysis of the zero‐field splitting (ZFS) parameters of derivatives of 10 and 12 , direct evidence for the contribution of carbenoid resonance structures 10b and 12b (i.e., polarization of the π system with an increase of spin density at the dehydrocarbon) could be obtained, as will be discussed in more detail below …”

“…Triradicals of the σσπ type have been realized as (fluorinated) dehydrophenylnitrenes 10–12 (Chart ) . Phenylnitrene 13 is well known to have a 3 A 2 ground state dominated by the electronic configuration (8b 2 ) 1 (3b 1 ) 1 , whereas the 1 A 2 open‐shell singlet state with the same orbital occupation pattern lies 15–16 kcal/mol higher in energy .…”

3,4,5,6‐Tetrafluorophenylnitren‐2‐yl: A Ground‐State Quartet Triradical

“…Density functional calculations predict for 2 a and 4 a high-spin quartet (Q) ground states, while for 3 a a low-spin doublet (D) ground state is calculated. [1] For 2 a and 4 a quinoid resonance structures 2 a' and 4 a' can be formulated, which suggest that these triradicals also have some carbene (cyclohexadienylidene) character. In contrast, for 3 a no such resonance structure is possible.…”

5‐Dehydro‐1,3‐quinodimethane: A Hydrocarbon with an Open‐Shell Doublet Ground State

“…[23] On the other hand, recently reported computational studies by Clark and Davidson [24] suggest a strong interaction between the s and p systems in 2,6-dehydro-m-quinone (DMQ), somewhat surprising for electrons in formally orthogonal regions of space. These results indicate a myriad of mechanisms for the interactions between electrons in openshell systems, a factor that will become more important as their size continues to grow.…”