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REVIEW 3 major objections 4 minor 36 references

Chemical bonding in three-membered ring systems

T0 review · 3 major / 4 minor · reviewed 2026-08-12 · deepseek-v4-flash

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Pith's one-line read In high symmetry, three-membered rings open and close along two different energy paths with no saddle-point barrier; only when symmetry is lowered do the paths merge.

desk verdict Solid OVB/CASSCF study of three-membered ring additions and eliminations; the C2v 'no saddle point' claim outruns the constrained-scan evidence. read the letter →

arxiv 2411.09399 v2 pith:AXYHERXX submitted 2024-11-14 physics.chem-ph

Nina Strasser , Alexander F. Sax This is my paper · ORCID
classification physics.chem-ph
keywords three-memberedringcompoundscyclotrisilanessilacyclopropanesCASSCForthogonalvalencebondanalysisdiabaticreactionsminimumenergypathsPauliexclusionprinciple
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper tries to establish why three-membered rings made of carbon and silicon—cyclopropane, silirane, disilirane, and cyclotrisilane—are sometimes surprisingly stable. Using CAS(4,4) wave functions and an orthogonal valence bond (OVB) analysis, it argues that in high C2v symmetry, the addition of a carbene analog (methylene or silylene) to a double bond and the reverse elimination do not pass over a saddle point at all. Instead, the forward and reverse reactions follow two different minimum energy paths; the energy rises along one electronic state until the system jumps to the other, like two crossing diabatic states. When the system is allowed to bend into the lower Cs symmetry, the two states mix into one smooth adiabatic path, and the reaction proceeds without a barrier. If this picture is right, it explains why bulky substituents that lock a cyclotrisilane into high symmetry make it kinetically stable, and why that stability is a symmetry effect rather than an intrinsic thermodynamic one.

What carries the argument

The central object is the orthogonal valence bond (OVB) analysis of CAS(4,4) wave functions: delocalized CASSCF molecular orbitals are localized onto fragment molecular orbitals (FMOs) by a Procrustes transformation, and the wave function is expanded in configuration state functions (CSFs) with definite local charge and spin distributions, such as the no-bond CSF, the local-triplet CSF, and ionic charge-transfer CSFs. Tracking CSF weights and energies along the approximate reaction coordinate R reveals which fragment states dominate in the bonded and dissociated regions and shows that the two reaction valleys keep distinct electronic characteristics, the signature of diabatic behavior. The CSF labels provide the diagnostic: in C2v only twelve of the twenty CSFs are totally symmetric, so the single-excitation and charge-shift CSFs that could mix the valleys are absent; in Cs all twenty CSFs enter and the states combine adiabatically.

What would settle it

Run an unconstrained transition-state search for one of the C2v reactions, such as methylene elimination from cyclopropane at the CAS(4,4) level, starting from the crossing-point geometry; if an intrinsic reaction coordinate connects the ring to the separated fragments through a first-order saddle point, the central claim fails.

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Extended reading notes

Core claim

Using CAS(4,4) wave functions and an orthogonal valence bond (OVB) analysis of the four ring systems cyclopropane, silirane, disilirane, and cyclotrisilane, the paper finds that in C2v symmetry the forward addition and reverse elimination reactions are diabatic reactions: they follow different minimum energy paths whose potential energy curves cross at a point where the system jumps from one electronic state to the other. There is no saddle point and no conventional reaction barrier on the adiabatic ground-state surface; the energy simply rises along one diabatic curve, then falls after the jump. In Cs symmetry the two diabatic states combine into a single adiabatic ground state and the reaction follows one smooth minimum energy path without a barrier. The authors conclude that the kinetic stability of substituted cyclotrisilanes and related three-membered rings is therefore a symmetry effect: substituents that prevent deformation away from C2v force the elimination reaction to climb the diabatic curve, while unhindered systems relax through Cs and react easily.

Load-bearing premise

The central claim rests on the assumption that constraining the fragment-fragment distance R and optimizing all other coordinates is enough to trace the true minimum energy path; if the real path bends through asymmetric distortions that the C2v scan freezes out, a saddle point could still exist.

Editorial extensions

If this is right

  • With large substituents that freeze the ring into C2v geometry, methylene or silylene elimination has to climb the diabatic curve; the paper's crossing-point estimates (up to 492 kJ/mol for cyclopropane and 311 kJ/mol for cyclotrisilane) explain why such three-membered rings can be isolated.
  • When the system can relax to Cs, the diabatic states mix into a single adiabatic ground state, so unhindered rings open and close along one smooth path without a saddle-point barrier.
  • The well-known stability of substituted cyclotrisilanes is kinetic rather than thermodynamic: cyclotrisilane is not intrinsically unstable toward elimination, and the long-standing difficulty in making it came from kinetic protection, not from an unfavorable reaction energy.
  • In C2v symmetry, addition of a carbene analog to a double bond requires both fragments to change from low-spin to high-spin character, often with an umbrella inversion of the pyramidal fragment; these electronic rearrangements, not a conventional barrier, govern the reaction cost.
  • All C2v reactions studied are orbital-symmetry forbidden in the Woodward-Hoffmann sense, while the Cs versions are allowed; the paper's diabatic picture gives a local, spin-resolved account of what the symmetry rules summarize globally.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the paper's picture transfers to other cheletropic additions, constrained symmetric scans that show cusps should be reinterpreted as diabatic crossings, and transition-state searches should be run in the lower-symmetry group before concluding that a barrier exists.
  • The mechanism predicts a substituent test: rigid bridges that enforce C2v should raise elimination barriers toward the computed crossing-point energies, while floppy substituents that permit Cs folding should erase them; this could be checked by comparing tethered, bulky, and flexible substituents on known cyclotrisilanes.
  • The paper's closing suggestion that jumps between troughs need an electron-phonon description points to a concrete dynamical follow-up: compute nonadiabatic couplings or surface-hopping rates between the two diabatic states in the crossing region to see whether the C2v reaction actually crosses or tunnels.
  • The OVB weight analysis implies a falsifiable electronic-structure marker: spin-sensitive measurements should see the fragments acquire triplet-like geometry at the same fragment separation where the C2v potential energy curve jumps, rather than gradually.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 4 minor

Summary. The manuscript reports CAS(4,4) calculations with orthogonal valence bond (OVB) analysis for the formation and cleavage of four three-membered rings, c-(CH2)3-k(SiH2)k, by addition/elimination of methylene or silylene to ethene, disilene, or silaethene. The central claim is that in C2v symmetry the addition and elimination reactions follow different minimum energy paths and are diabatic: the energy rises monotonically along one branch until the system switches to the other branch, so that there are no saddle-point barriers on the potential energy surface. In Cs symmetry, by contrast, the diabatic states combine into a single adiabatic path. The authors use this to explain the kinetic stability of substituted cyclotrisilanes and related rings, and they provide PECs, geometry curves, CSF weights and energies, corrected reaction energies, and comparisons across the four ring systems.

Significance. If the no-saddle-point claim is correct, the paper offers a physically concrete explanation for the kinetic stability of substituted cyclotrisilanes, with a local charge/spin interpretation that goes beyond a Woodward-Hoffmann symmetry label. The calculations are carried out at a consistent CAS(4,4) level, with CAS(6,6) checks for two of the reactions, and the OVB transformation is well defined and yields interpretable CSF weights and energies. The paper is also candid about the limitations of its reaction-coordinate picture, explicitly noting that the true reaction coordinate is replaced by an approximate coordinate R and that continuous curve representations hide discontinuous MEP switching. These strengths are real; the main weakness is that the headline topological conclusion is inferred from constrained one-dimensional scans rather than from a full or sampled potential energy surface.

major comments (3)
  1. [VII; IV A] The central claim that C2v reactions have "no energy barriers corresponding to saddle points" is based entirely on constrained scans along the approximate reaction coordinate R (Section VII: "all other geometry parameters were optimized"; Section IV A: "the true reaction coordinate lambda is mostly replaced by an approximate reaction coordinate R"). The scan protocol deliberately conserves the electronic structure along R, and the C2v constraint freezes out asymmetric distortions. Such a one-dimensional scan cannot rule out first-order saddle points whose transition vector involves a combination of coordinates other than R, nor saddle points that connect the two apparent MEPs through symmetry-broken geometries. Section VI states that "the energy at the crossing point is not an adiabatic reaction barrier," but that statement is interpretive and does not replace a transition-state search. I therefore do not regard the no-saddle-point assertion as established; the authors should either perform unconstrained transition-state searches and/or IRC calculations in the relevant regions, or explicitly restrict the claim to "no barrier along the constrained R coordinate."
  2. [IV B; VI] The paper mixes adiabatic and diabatic levels of description in a way that matters for the conclusion. Section IV B says that the PECs in the two-trough case "are indeed adiabatic PECs because the energies are the lowest eigenvalues of the Hamiltonian," yet the reactions are then called diabatic. The physically important question, whether the two C2v valleys are disjoint on the adiabatic ground-state PES, requires knowledge of the ridge between them. No ridge height, second-order saddle point, or minimum-energy crossing seam is reported. Please provide such a characterization, or frame the conclusion as "no barrier along the constrained R scan" rather than "no saddle point on the PES." This is not a semantic quibble, because Section VI and Table S7 use the crossing-point energies as estimates of kinetic barriers for substituted rings.
  3. [V; Supporting Information B] CAS(4,4) is validated against CAS(6,6) only for R1v and R2v, i.e., cyclopropane and cyclotrisilane. The heteronuclear systems R3-R6 involve polarized pi bonds and charge-asymmetric fragments, and the unusual result for c-CSi2H6, that methylene elimination remains diabatic and yields triplet fragments without a jump, is not covered by those checks. A CAS(6,6) test for at least one heteronuclear reaction would materially strengthen the claim that the reported PES topology is not an artifact of the smaller active space.
minor comments (4)
  1. [Throughout] There are several typographical errors, e.g., "0f" in Section F, "ist" in Section VI, and "Reuter et al.some" in Section VI; these should be corrected.
  2. [Figure 2] The caption says that CSFs labelled in red contribute only in Cs symmetry, but the figure appears in grayscale; please ensure the color coding is visible or use another marker such as boldface or an asterisk.
  3. [IV B; Figures] The continuous representation of discontinuous PECs is acknowledged in Section IV B and in the Supporting Information, but the main-text figures are still drawn as continuous curves; a consistent notation such as dashed segments or vertical jump markers would help readers distinguish the two MEP branches.
  4. [Supporting Information] The SI lists fragment energies and selected energy differences, but not the full set of optimized geometries and total energies along the R scans; providing these data would make the PECs reproducible and would also allow readers to check the claimed cusps and jumps.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: the central energies and geometries are ab initio outputs, not fitted inputs; the main weakness is an under-supported no-saddle-point inference, not a reduction of results to assumptions.

full rationale

The derivation chain starts from CAS(4,4) and CAS(6,6) electronic structure calculations. Total energies, optimized geometries, and CSF weights are computed outputs, not parameters fitted to the OVB concepts. The OVB analysis is a post-processing transformation of the same CASSCF wave functions, and the paper states that diagonalizing the CI matrix built from OVB CSFs reproduces the CASSCF energy exactly ('the lowest eigenvalue of the CI matrix must be identical with the CAS(4,4) energy obtained with delocalized MOs'), so the interpretive layer does not generate the energetics. The diabatic/different-MEP classification is an interpretation of computed PECs and CSF weights, and the self-citations (refs. 7, 13, 19, 20) describe the OVB methodology and prior applications rather than supplying an unverified uniqueness theorem. The weakest claim — that C2v reactions have no saddle-point barriers — is not circular but under-supported: it rests on constrained scans along one approximate coordinate R with serial MO seeding ('With this strategy, the electron structure along the approximate reaction coordinate was conserved'), and the paper itself concedes the ridge between valleys is unknown ('As long as one does not study the potential energy surface in detail, one can only assume that the two skew troughs are separated by a ridge of unknown height'). That is a completeness/validation concern about missing transition-state searches or full PES sampling, not a case where a prediction reduces to a fitted parameter or to a self-citation by construction. The mild self-referential method basis justifies a low nonzero score, but no circular step meeting the quoted-evidence standard was found.

Assumptions & free parameters 0 free parameters · 5 assumptions · 0 invented entities

The central claim does not depend on fitted constants or newly invented physical entities. The key assumptions are the sufficiency of the CAS(4,4) active space, the validity of the constrained R scan for locating MEPs, and the transferability of the OVB and Procrustes localization scheme. These are domain assumptions rather than ad hoc additions.

assumptions (5)
  • standard math Born-Oppenheimer approximation is valid for the studied reactions.
    Invoked in Section IV A to define the potential energy surface and the concept of minimum energy paths.
  • domain assumption CAS(4,4) active space is sufficient to describe the reactions, including the spin rearrangements.
    Applied in Sections V and VII; only tested against CAS(6,6) for two systems (R1v and R2v), and for heteronuclear systems equivalent bonds are not equivalently correlated (admitted in Section V).
  • domain assumption Constrained scans along the approximate reaction coordinate R, with all other coordinates optimized, capture the true minimum energy paths and any existing saddle points.
    The method in Section VII uses 0.1 Angstrom steps along R and this is the basis for the claim that no saddle-point barriers exist in C2v.
  • domain assumption The orthogonal Procrustes localization yields fragment MOs that preserve the essential electronic structure for interpretation.
    The OVB analysis in Section VII relies on this transformation, citing prior work by the same author; the validity of the localized picture is assumed.
  • domain assumption The configurational uniformity criterion identifies diabatic states and supports the interpretation of separate MEPs as diabatic reactions.
    Used in Section IV B to justify why the reactions with different MEPs are called diabatic; this criterion is taken from Ruedenberg and Atchity.

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Pith. "Pith review of Chemical bonding in three-membered ring systems." pith.science (2026). https://pith.science/paper/AXYHERXX

@misc{pith2026241109399,
  author       = {Pith},
  title        = {Pith review of: Chemical bonding in three-membered ring systems},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/AXYHERXX}},
  note         = {Machine review of arXiv:2411.09399}
}
abstract

The formation of the four three-ring systems \ce{c-(CH2)_{3-k}(SiH2)_{k}}, ($k=0$: cyclopropane, $k=1$: silirane, $k=2$: disilirane, $k=3$: cyclotrisilane) by addition of methylene and silylene to the double bond in ethene, disilene, and silaethene, as well as the elimination of the carbene analogs from the three-rings, was studied with CAS(4,4) wave functions in both $C_{2v}$ and $C_s$ symmetry. To reveal charge and spin redistribution during these reactions the CAS(4,4) wave functions were analyzed using the orthogonal valence bond method (OVB). The potential energy curves, different internal coordinates, and the results of the OVB analysis show, that frequently the addition and elimination reactions follow different minimum energy paths, because they are indeed diabatic reactions. In these cases, there are no energy barriers corresponding to saddle points on the potential energy surfaces but the energy increases during one diabatic reaction until, at a certain point, the system jumps to the other diabatic state and, in the following, the energy decreases. This happens for reactions in $C_{2v}$ symmetry; as soon as the system can change to the lower symmetry, the diabatic states combine to an adiabatic one and the reaction follows a single minimum energy path.

Figures

Figures reproduced from arXiv: 2411.09399 by the authors.

Figure 1
Figure 1. FIG. 1. PECs obtained with the adiabatic wave function Ψ [PITH_FULL_IMAGE:figures/full_fig_p014_1.png] view at source ↗
Figure 2
Figure 2. FIG. 2. The types of product FCSFs used to interpret a CAS(4,4) singlet wave function. CSF [PITH_FULL_IMAGE:figures/full_fig_p017_2.png] view at source ↗
Figure 3
Figure 3. FIG. 3. A sketch of the molecular geometry of the investigated 3-rings and of important geometry [PITH_FULL_IMAGE:figures/full_fig_p021_3.png] view at source ↗
Figures from the paper (43 more)
Figure 4
Figure 4. Figure 4: FIG. 4. Total energies for the reactions c-C [PITH_FULL_IMAGE:figures/full_fig_p022_4.png]
Figure 5
Figure 5. Figure 5: FIG. 5. Geometry parameters of the methylene and the ethene fragment in reactions [PITH_FULL_IMAGE:figures/full_fig_p023_5.png]
Figure 6
Figure 6. Figure 6: FIG. 6. CSF energies and weights, and sum of weights of the largest contributions to the wave [PITH_FULL_IMAGE:figures/full_fig_p024_6.png]
Figure 7
Figure 7. Figure 7: FIG. 7. Geometry parameters of the methylene and the ethene fragment in reactions [PITH_FULL_IMAGE:figures/full_fig_p030_7.png]
Figure 8
Figure 8. Figure 8: FIG. 8. CSF energies, weights, and sum of weights of the largest contributions to the wave functions [PITH_FULL_IMAGE:figures/full_fig_p031_8.png]
Figure 9
Figure 9. Figure 9: FIG. 9. Total energies for reactions [PITH_FULL_IMAGE:figures/full_fig_p031_9.png]
Figure 10
Figure 10. Figure 10: FIG. 10. Left: Structure S1. Right: Structure S2. [PITH_FULL_IMAGE:figures/full_fig_p032_10.png]
Figure 11
Figure 11. Figure 11: FIG. 11. Geometry parameters of the silylene and disilene fragments in reactions [PITH_FULL_IMAGE:figures/full_fig_p034_11.png]
Figure 12
Figure 12. Figure 12: FIG. 12. CSF energies, weights, and sum of weights of the largest contributions to the wave [PITH_FULL_IMAGE:figures/full_fig_p035_12.png]
Figure 12
Figure 12. Figure 12: The change of the wave function from high-spin to low-spin characteristics at [PITH_FULL_IMAGE:figures/full_fig_p036_12.png]
Figure 13
Figure 13. Figure 13: FIG. 13. The active FMOs [PITH_FULL_IMAGE:figures/full_fig_p037_13.png]
Figure 14
Figure 14. Figure 14: FIG. 14. The active FMOs [PITH_FULL_IMAGE:figures/full_fig_p038_14.png]
Figure 15
Figure 15. Figure 15: FIG. 15. Geometry parameters of the silylene and disilene fragments in reactions [PITH_FULL_IMAGE:figures/full_fig_p039_15.png]
Figure 16
Figure 16. Figure 16: FIG. 16. Geometry parameters of the disilene fragment in reactions [PITH_FULL_IMAGE:figures/full_fig_p039_16.png]
Figure 17
Figure 17. Figure 17: FIG. 17. Geometry parameters of the disilene fragment in reactions [PITH_FULL_IMAGE:figures/full_fig_p040_17.png]
Figure 18
Figure 18. Figure 18: FIG. 18. The active FMOs [PITH_FULL_IMAGE:figures/full_fig_p042_18.png]
Figure 19
Figure 19. Figure 19: FIG. 19. CSF energies, weights, and sum of weights of the large contributions to the wave function [PITH_FULL_IMAGE:figures/full_fig_p043_19.png]
Figure 20
Figure 20. Figure 20: FIG. 20. Total energies for the reactions [PITH_FULL_IMAGE:figures/full_fig_p044_20.png]
Figure 21
Figure 21. Figure 21: FIG. 21. Geometry parameters of c-CSi [PITH_FULL_IMAGE:figures/full_fig_p046_21.png]
Figure 22
Figure 22. Figure 22: FIG. 22. CSF energies, weights, and sum of weights of the large contributions to the wave functions [PITH_FULL_IMAGE:figures/full_fig_p047_22.png]
Figure 23
Figure 23. Figure 23: FIG. 23. Geometry parameters of the disilene fragment in reactions [PITH_FULL_IMAGE:figures/full_fig_p049_23.png]
Figure 24
Figure 24. Figure 24: FIG. 24. Geometry parameters of the disilene fragment in reactions [PITH_FULL_IMAGE:figures/full_fig_p050_24.png]
Figure 25
Figure 25. Figure 25: FIG. 25. Geometry parameters of the methylene fragment in reactions [PITH_FULL_IMAGE:figures/full_fig_p051_25.png]
Figure 26
Figure 26. Figure 26: FIG. 26. CSF energies, weights, and sum of weights of the largest contributions to the wave [PITH_FULL_IMAGE:figures/full_fig_p052_26.png]
Figure 27
Figure 27. Figure 27: FIG. 27. Total energies for the reactions [PITH_FULL_IMAGE:figures/full_fig_p053_27.png]
Figure 28
Figure 28. Figure 28: FIG. 28. Geometry parameters of both fragments. [PITH_FULL_IMAGE:figures/full_fig_p053_28.png]
Figure 29
Figure 29. Figure 29: FIG. 29. CSF energies, weights, and sum of weights of the large contributions to the wave functions. [PITH_FULL_IMAGE:figures/full_fig_p054_29.png]
Figure 30
Figure 30. Figure 30: FIG. 30. Geometry parameters of both fragments. [PITH_FULL_IMAGE:figures/full_fig_p055_30.png]
Figure 31
Figure 31. Figure 31: FIG. 31. Geometry parameters of both fragments. [PITH_FULL_IMAGE:figures/full_fig_p056_31.png]
Figure 32
Figure 32. Figure 32: FIG. 32. Geometry parameters of both fragments. [PITH_FULL_IMAGE:figures/full_fig_p056_32.png]
Figure 33
Figure 33. Figure 33: FIG. 33. CSF energies and weights and the sum of the weights of the largest contributions to the [PITH_FULL_IMAGE:figures/full_fig_p057_33.png]
Figure 34
Figure 34. Figure 34: FIG. 34. From left to right: The four possible structures S1, S2, S3, and S4. In CSi [PITH_FULL_IMAGE:figures/full_fig_p058_34.png]
Figure 35
Figure 35. Figure 35: FIG. 35. Total energies for the elimination reaction and the four recombination reactions. [PITH_FULL_IMAGE:figures/full_fig_p058_35.png]
Figure 36
Figure 36. Figure 36: FIG. 36. Geometry parameters of the silaethene fragment. [PITH_FULL_IMAGE:figures/full_fig_p061_36.png]
Figure 37
Figure 37. Figure 37: FIG. 37. Geometry parameters of the silylene fragment. [PITH_FULL_IMAGE:figures/full_fig_p062_37.png]
Figure 38
Figure 38. Figure 38: FIG. 38. CSF energies and weights for all five reactions. [PITH_FULL_IMAGE:figures/full_fig_p063_38.png]
Figure 39
Figure 39. Figure 39: FIG. 39. CSF energies and weights for all five reactions. [PITH_FULL_IMAGE:figures/full_fig_p064_39.png]
Figure 40
Figure 40. Figure 40: FIG. 40. Total energies for the elimination reaction and the four recombination reactions. [PITH_FULL_IMAGE:figures/full_fig_p066_40.png]
Figure 41
Figure 41. Figure 41: FIG. 41. Geometry parameters of the molecular system. [PITH_FULL_IMAGE:figures/full_fig_p067_41.png]
Figure 42
Figure 42. Figure 42: FIG. 42. Geometry parameters of the molecular system. [PITH_FULL_IMAGE:figures/full_fig_p068_42.png]
Figure 43
Figure 43. Figure 43: FIG. 43. CSF energies and weights for all four recombination reactions. [PITH_FULL_IMAGE:figures/full_fig_p070_43.png]
Figure 44
Figure 44. Figure 44: FIG. 44. CSF energies and weights for all four recombination reactions. [PITH_FULL_IMAGE:figures/full_fig_p071_44.png]
Figure 45
Figure 45. Figure 45: FIG. 45. Comparison of the PECs of all investigated reactions in [PITH_FULL_IMAGE:figures/full_fig_p072_45.png]

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