State-of-the-art electronic structure calculations (MR-CISD) are used to map five different dissociation channels of CH3Cl along the C-Cl coordinate: (i) CH3(X̃(2)A2″) + Cl((2)P), (ii) CH3(3s(2)A1') + Cl((2)P), (iii) CH3(+)((1)A1') + Cl(-)((1)S), (iv) CH3(3p(2)E') + Cl((2)P), and (v) CH3(3p(2)A2″) + Cl((2)P). By the first time these latter four dissociation channels, accessible upon VUV absorption, are described. The corresponding dissociation limits, obtained at the MR-CISD+Q level, are 3.70, 9.50, 10.08, 10.76, and 11.01 eV. The first channel can be accessed through nσ* and n3s states, while the second channel can be accessed through n(e)3s, n(e)3p(σ), and σ3s states. The third channel, corresponding to the CH3(+) + Cl(-) ion-pair, is accessed through n(e)3p(e) states. The fourth is accessed through n(e)3p(e), n(e)3p(σ), and σ3p(σ), while the fifth through σ3p(e) and σ(CH)σ* states. The population of the diverse channels is controlled by two geometrical spots, where intersections between multiple states allow a cascade of nonadiabatic events. The ion-pair dissociation occurs through formation of CH3(+)···Cl(-)and H2CH(+)···Cl(-) intermediate complexes bound by 3.69 and 4.65 eV. The enhanced stability of the H2CH(+)···Cl(-) complex is due to a CH···Cl hydrogen bond. A time-resolved spectroscopic setup is proposed to detect those complexes.
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 (hr 2 i), 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 (ns*) 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 ns* 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 ns* state, while the last three have s C-Cl / Rydberg (3s or 3p) as the main configurations. According to the f values the most intense transition is to the 4 1 A 1 state, a s3p a1 Rydberg state mixed with the ss* and s3s configurations. Our results indicate that the ss* configuration is responsible for the high f value of the gs / 4 1 A 1 transition. The Rydberg-valence mixing is greatly reduced in the triplet states whose singlet counterparts have significant multiconfigurational character.The 2 3 A 1 state (ss*) does not have its singlet counterpart, while the 4 1 A 1 state (s3p a1 + ss* + s3s) does not have its triplet counterpart. The obtained results are in good agreement with experimental results and with previous CASPT2 results.
The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calculations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of -conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy (ACME) multiconfiguration self-consistent field (MCSCF) method and the Graphically Contracted Function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, the development of
Pyrene fluorescence after a high-energy electronic excitation exhibits a prominent band shoulder not present after excitation at low energies. The standard assignment of this shoulder as a non-Kasha emission from the second-excited state (S2) has been recently questioned. To elucidate this issue, we simulated the fluorescence of pyrene using two different theoretical approaches based on vertical convolution and nonadiabatic dynamics with nuclear ensembles. To conduct the necessary nonadiabatic dynamics simulations with high-lying electronic states and deal with fluorescence timescales of about 100 ns of this large molecule, we developed new computational protocols. The results from both approaches confirm that the band shoulder is, in fact, due to S2 emission. We show that the non-Kasha behavior is a dynamic-equilibrium effect not caused by a metastable S2 minimum. However, it requires considerable vibrational energy, which can only be achieved in collisionless regimes after transitions into highly excited states. This strict condition explains why the S2 emission was not observed in some experiments.
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