Elizete Ventura

Accurate reference data for electronically excited states are essential for assessing multireference electronic-structure methods. We benchmark state-averaged completeactive-space self-consistent field (SA-CASSCF), exchange-scaled CASSCF (XS-CASSCF), fully uncontracted multireference configuration interaction with single and double excitations (MR-CISD), and four a posteriori size-consistency corrections against the theoretical best estimates of QUESTDB. The dataset comprises 262 valence excitations of 41 small and medium-sized molecules, including singlet and triplet n*, *, and doubly excited states. All calculations used a consistent COLUMBUS protocol with molecule-specific active spaces and the aug-cc-pVDZ basis set. For the 232 safe transitions, SA-CASSCF and MR-CISD yield mean absolute errors of 0.57 and 0.31 eV, respectively, with predominantly positive biases. The Davidson–Silver/Siegbahn (+DV3) and Pople (+P) corrections reduce the mean absolute errors to 0.10 and 0.11 eV and the root-mean-square errors to 0.13 eV. They also provide a more balanced description across spin multiplicities and excitation characters. Both XSCASSCF improve singlet * states, particularly those with pronounced ionic character, but degrade n* states and do not improve the overall statistics relative to SA-CASSCF. For 36 selected unsafe or doubly excited transitions, MR-CISD+DV3 and MR-CISD+P retain mean absolute errors of 0.16 eV. Their agreement with exFCI-based reference values and discrepancies from several CASPT3- and PC-NEVPT2-based estimates identify states whose current best estimates merit independent re-examination. Overall, size-consistencycorrected MR-CISD provides highly accurate and broadly balanced valence excitation energies.

For many years researchers believed that hydrocarbons only contain covalent bonds. However, since 1985 Okamoto et al. demonstrated the formation of hydrocarbon salts in several systems, demolishing the structural principle that hydrocarbons only contain covalent bonds. Despite the great importance of this outcome to the study of chemical bonds, quantum chemical calculations on these systems are essentially nonexistent. The stability of the hydrocarbon ions along with the steric hindrance associated with the formation of the covalent bond contribute to their occurrence either in solution (dissociated) or in the solid state. These facts along with the common formation of ion pairs in solvents of low polarity motivated us to search for hydrocarbon ion pairs in the gas phase. Its energetics has also been studied in four nonprotic solvents, through a continuum solvation model (CPCM). DFT and CASSCF calculations indicate a metastable and highly polar ion pair between the tricyclopropylcyclopropenylium cation and a simplified Kuhn's anion. The barrier to the covalent structure varies from ∼4.8 to 14.4 kcal/mol, while the energy difference between the ion pair and the covalent form varies from ∼4.3 to 25.4 kcal/mol. The obtained theoretical results along with previous experimental results suggest the following strategy to obtain kinetically and thermodynamically stable hydrocarbon ion pairs: choose very stable hydrocarbon ions and systematically increase the steric hindrance between them.

Although CH₂FCl (HCFC‐31) recently became of great atmospheric importance, studies concerning its excited states are almost nonexistent. Several excited singlet states were studied (valence nσ* and Rydberg n3s, n3p, σ3s, and σ3p) through highly correlated multireference configuration interaction with singles and doubles, including extensivity correction. Comparison with the states of CH₃Cl indicates a strong influence of the F atom. Potential energy curves suggest formation of an electrostatically bound complex that relaxes to a hydrogen‐bonded contact ion‐pair (HBCIP) which can decay yielding CH₂F + Cl or to the ground state minimum of CH₂FCl. The HBCIP has a dipole moment of 9.57 D, a CI wavefunction described as 0.65ionic + 0.20biradical and it is strongly bonded by 4.72 eV. Its H bond has characteristics of moderate and strong H bonds. The simulated absorption spectrum confirms the nσ* assignment for the first and suggests the n3s + n3p_σ assignment for the second band.

An active environment changes the photophysics of an excited chromophore. Surface hopping simulations are crucial to understanding how.

A complex environment is any atomic or molecular system changing a chromophore's nonadiabatic dynamics compared to the isolated molecule. The action of the environment on the chromophore occurs by changing the potential energy landscape and triggering new energy and charge flows unavailable in the vacuum. Surface hopping is a mixed quantum-classical approach whose extreme flexibility has made it the primary platform for implementing novel methodologies to investigate the nonadiabatic dynamics of a chromophore in complex environments. This Perspective paper surveys the latest developments in the field, focusing on charge and energy transfer processes.

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.

Organic molecules are a potential source of prebiotic chemistry in the interstellar medium (ISM). Methanol (MetOH) is a very important source of more complex molecules. H₃O⁺(aq) and Cl⁻(aq) are fundamental to living organisms and can be generated in the ISM from the dissociation of HCl with just four water molecules, yielding the (H₃O)⁺(H₂O)₃Cl⁻ ion‐pair. Here, a detailed mechanism, based on density functional theory (DFT) and ab‐initio (2^nd order Mϕller‐Plesset perturbation theory, MP2) calculations, is suggested for the substitution reactions of these water molecules by MetOH. The time required for formation of an appreciable amount of the product ((H₃O)⁺(MetOH)₃Cl⁻) can be only few years. Such reaction can take place in Sagittarius B2, where HCl, H₂O and MetOH have already been identified and it can be an important source for the formation of more complex prebiotic structures.

Correction for ‘Modeling the heating and cooling of a chromophore after photoexcitation’ by Elizete Ventura et al., Phys. Chem. Chem. Phys., 2022, 24, 9403–9410, https://doi.org/10.1039/D2CP00686C.

The heating of a chromophore due to internal conversion and its cooling down due to energy dissipation to the solvent are crucial phenomena to characterize molecular photoprocesses. In this work, we simulated the ab initio nonadiabatic dynamics of cytosine, a prototypical chromophore undergoing ultrafast internal conversion, in three solvents—argon matrix, benzene, and water—spanning an extensive range of interactions. We implemented an analytical energy-transfer model to analyze these data and extract heating and cooling times. The model accounts for nonadiabatic effects, and excited- and ground-state energy transfer, and can analyze data from any dataset containing kinetic energy as a function of time. Cytosine heats up in the subpicosecond scale and cools down within 25, 4, and 1.3 ps in argon, benzene, and water, respectively. The time constants reveal that a significant fraction of the benzene and water heating occurs while cytosine is still electronically excited.

The occurrence of a hydrogen‐bonded contact ion pair (HBCIP, in the gas phase) between the dibenzotropylium cation and chloride has been suggested previously. However, the low barrier (ΔE^‡, of ~1 kcal/mol) to its covalent counterpart can preclude its detection. In this work it is shown (through density functional theory calculations) that ΔE^‡ can be increased significantly, based on larger but similar carbocations containing more benzene rings. Using the electrostatic potential as a measure of carbocation stability, along with its linear correlation with ΔE^‡, one obtains a ΔE^‡ value as high as ~3.7 kcal/mol for the HBCIP between the heptabenzotropylium cation and chloride. The trend for ΔE^‡ can be explained by the deformation energy of the carbocation, from its structure in the HBCIP to that in the transition state.