Valence and Rydberg states of CH3Cl: a MR-CISD study

Abstract

In this work ten singlet and nine triplet states are studied through multi-reference configuration interactions with singles and doubles (MR-CISD), including Davidson extensivity correction (MR-CISD+Q). For the first time the excited states whose energies are larger than ∼9.5 eV have been calculated using highly correlated methods. The energies, spatial extent (〈r²〉), configurations weights and oscillator strengths (f) have been computed. At the MR-CISD+Q level the excited states energies vary from ∼7.51 to 11.98 eV. The lowest (nσ*) excited singlet state is significantly mixed with the n3p_a1 and n3s Rydberg states, while the next (n3s) has a non-negligible mixture with the nσ* state. The next three singlet states obtained result from the (n_Cl)³(3p_e)¹ configuration and are almost degenerate. The next (n3p_a1) singlet state is significantly mixed with the nσ* state, while the last three have σ_C–Cl → Rydberg (3s or 3p) as the main configurations. According to the f values the most intense transition is to the 4¹A₁ state, a σ3p_a1 Rydberg state mixed with the σσ* and σ3s configurations. Our results indicate that the σσ* configuration is responsible for the high f value of the gs → 4¹A₁ transition. The Rydberg-valence mixing is greatly reduced in the triplet states whose singlet counterparts have significant multiconfigurational character. The 2³A₁ state (σσ*) does not have its singlet counterpart, while the 4¹A₁ state (σ3p_a1 + σσ* + σ3s) does not have its triplet counterpart. The obtained results are in good agreement with experimental results and with previous CASPT2 results.