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Abstract The symmetry breaking in octahedral silsesquioxane and its germanium analogues (Si8O12H8and Ge8O12H8) has been investigated using the M06-2X/6-31++G(3df, 3pd) method and group theory. Both structures undergo$${O}_{h}\downarrow {T}_{h}$$symmetry breaking, characterized by pseudo-Jahn−Teller stabilization energies of 0.22 kcal/mol for Si-POSS and 9.82 kcal/mol for Ge-POSS. Under the influence of the pseudo-Jahn–Teller effect, the distortion vector involves the vibrational a2gmode with imaginary frequency. The distortion forces in Oh-POSS are predominantly localized on the oxygen atoms and driven by the coupling between the lowest unoccupied molecular orbital (a1g) and the highest occupied molecular orbital (a2g). The symmetry breaking is attributed to a pseudo-Jahn–Teller mechanism of type (a2gx a1g) = a2g. The symmetrical substitution of oxygen atoms by X (where X = C, N, P) results in viable Th-Si8X12H8and Th-Ge8X12H8structures. The observed pseudo-Jahn–Teller distortion and substitutional symmetry breaking caused by X indicates a consistent electronic relaxation mechanism, characterized by the formation of C=C, N=N and P=P bonds on the POSS cubic faces, which serves as hallmarks of stability. Additionally, we find that the volume of substituted Th-symmetrical POSS is sufficiently large to accommodate small ions.
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Excited‐State Distortions Promote the Photochemical 4π‐Electrocyclizations of Fluorobenzenes via Machine Learning Accelerated Photodynamics Simulations
Abstract Benzene fluorination increases chemoselectivities for Dewar‐benzenes via 4π‐disrotatory electrocyclization. However, the origin of the chemo‐ and regioselectivities of fluorobenzenes remains unexplained because of the experimental limitations in resolving the excited‐state structures on ultrafast timescales. The computational cost of multiconfigurational nonadiabatic molecular dynamics simulations is also currently cost‐prohibitive. We now provide high‐fidelity structural information and reaction outcome predictions with machine‐learning‐accelerated photodynamics simulations of a series of fluorobenzenes, C6F6‐nHn, n=0–3, to study their S1→S0decay in 4 ns. We trained neural networks with XMS‐CASPT2(6,7)/aug‐cc‐pVDZ calculations, which reproduced the S1absorption features with mean absolute errors of 0.04 eV (<2 nm). The predicted nonradiative decay constants for C6F4H2, C6F6, C6F3H3, and C6F5H are 116, 60, 28, and 12 ps, respectively, in broad qualitative agreement with the experiments. Our calculations show that a pseudo Jahn–Teller distortion of fluorinated benzenes leads to an S1local‐minimum region that extends the excited‐state lifetimes of fluorobenzenes. The pseudo Jahn–Teller distortions reduce when fluorination decreases. Our analysis of the S1dynamics shows that the pseudo‐Jahn–Teller distortions promote an excited‐statecis‐transisomerization of a πC‐Cbond. We characterized the surface hopping points from our NAMD simulations and identified instantaneous nuclear momentum as a factor that promotes the electrocyclizations.
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- Award ID(s):
- 2118201
- PAR ID:
- 10368799
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Chemistry – A European Journal
- Volume:
- 28
- Issue:
- 38
- ISSN:
- 0947-6539
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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