CASSCF and multireference CI with singles and doubles study of low‐lying valence and Rydberg states of 2H‐tetrazole

Abstract

Complete active space self-consistent field (CASSCF) and multireference CI with singles and doubles (MR-CISD) calculations [including extensivity corrections, at MR-CISD+Q and multireference averaged quadratic coupled cluster (MR-AQCC) levels] have been performed to characterize the low-lying valence and the Rydberg states of 2H-tetrazole. The highest level results (MR-AQCC/d'-aug'-cc-pVDZ) indicate the following ordering of the valence singlet excited states: S(1) (n-pi*), 6.06 eV; S(2) (n-pi*), 6.55 eV; S(3) (pi-pi*), 6.55 eV. The MR-CISD+Q/d'-aug'-cc-pVDZ results indicate the same ordering, but at slight higher energies: 6.16, 6.68, and 6.69 eV, respectively. According to our MR-CISD+Q/d'-aug'-cc-pVDZ results, the next two states are Rydberg states, at 7.69 eV (pi-3s) and 7.89 eV (n-3s). The calculated energies of these two states, as well as their proximity, are consistent with the conclusion reached by Palmer and Beveridge (Chem Phys 1987, 111, 249) that the first band of the photoelectron spectrum of 2H-tetrazole is likely to be associated to the first two ionizations processes (of pi and N lone pair electrons), at energies close to 11.3 eV.

Accurate calculation of the ionization energies of the chlorine lone pairs in 1,1,1-trifluoro-2-chloroethane (HCFC-133a)

“…Calculating Rydberg states of molecules is a very challenging task using quantum chemistry methods, since it requires the use of very diffuse basis sets and highly correlated methods, especially when they are close to valence states [19,20]. In many cases, however, the energies of Rydberg states can be estimated through the Rydberg formula [21]:…”

Ultrafast Dynamics of Photochemical Nitrile Imine Formation

“…Note that the lowest singlet excited state of the simplest congener of 1, 2H-tetrazole (R = R' = H in Figure 1b), is a nπ*-state and not a π-π*-state as evidenced by complete active space self-consistent field and multireference configuration interaction methods. [16] The DFT-level-of-theory chosen here is able to reproduce this result fully (see Figure S6 for a detailed discussion of the electronic structure of 1 and its relation to that of 2H-tetrazole). However, according to the very same DFT-method, the lowest n-π*state of 1 is actually S 5 and as such it is much higher in energy that the lowest π-π* state.…”

“…Figures S3 to S5 for optimized geometries and their DFT‐predicted vibrational spectra). Note that the lowest singlet excited state of the simplest congener of 1 , 2 H ‐tetrazole (R=R′=H in Figure 1b), is a n–π*‐state and not a π–π*‐state as evidenced by complete active space self‐consistent field and multireference configuration interaction methods [16] . The DFT‐level‐of‐theory chosen here is able to reproduce this result fully (see Figure S6 for a detailed discussion of the electronic structure of 1 and its relation to that of 2 H ‐tetrazole).…”