The molecules 1,4-cyclohexadiene (unconjugated 1,4-CHD) and 1,3-cyclohexadiene (conjugated 1,3-CHD) both have two double bonds, but these bonds interact in different ways. These molecules have long served as examples of through-bond and through-space interactions, respectively, and their electronic structures have been studied in detail both experimentally and theoretically, with the experimental assignments being especially complete. The existence of Rydberg states interspersed with the valence states makes the quantum mechanical calculation of their spectra a challenging task. In this work, we explore the electronic excitation energies of 1,4-CHD and 1,3-CHD for both valence and Rydberg states by means of complete active space second-order perturbation theory (CASPT2), extended multi-state CASPT2 (XMS-CASPT2), and multiconfiguration pair-density functional theory (MC-PDFT); it is shown by comparison to experiment that MC-PDFT yields the most accurate results. We found that the inclusion of Rydberg orbitals in the active space not only enables the calculation of Rydberg excitation energies but also improves the accuracy of the valence ones. A special characteristic of the present analysis is the calculation of the second moments of the excited-state orbitals. Because we find that the CASPT2 densities agree well with the CASSCF ones and since the MC-PDFT methods gets accurate excitation energies based on the CASSCF densities, we believe that we can trust these moments as far as giving a more accurate picture of the diffuseness of the excited-state orbitals in these prototype molecules than has previously been available.
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Highly-excited state properties of cumulenone chlorides in the vacuum-ultraviolet
Recent observations of chloromethane in interstellar environments suggest that other organohalogens, which are known to be critically important in Earth's atmosphere, may also be of significance beyond our own terrestrial veil. This raises the question of how such molecules behave under extreme conditions such as when exposed to vacuum ultraviolet (VUV) radiation. VUV photons promote molecules to highly excited states that fragment in non-statistical patterns controlled by the initial femtosecond dynamics. A detailed understanding of VUV-driven photochemistry in complex organic molecules that consist of more than one functional group is a particularly challenging task. This quantum chemical analysis reports the electronic states and ionization potentials up to the VUV range (6–11 eV) of the chlorine-substituted cumulenone series molecules. The valence and Rydberg properties of lone-pair terminated, π-conjugated systems are explored for their potential resonance with lone pairs from elsewhere in the system. The carbon chain elongation within the family ClHC n O, where n = 1–4, influences the electronic excitations, associated wavefunctions, and ionization potentials of the molecules. The predicted geometries and ionization potentials are in good agreement with the available experimental photoelectron spectra for formyl chloride and chloroketene, n = 1–2. Furthermore, comparison between the regular cumulenone species and the corresponding chlorinated derivatives exhibit similar behaviors especially for n = 3, where the allene backbone in propadienone chloride is severely bent. Most notably for the excited states is that the Rydberg character becomes more dominant as the energy increases, with some retaining valence characters.
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- Award ID(s):
- 1757220
- NSF-PAR ID:
- 10251874
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 22
- Issue:
- 21
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 11838 to 11849
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0CP01835J
