Relating the Radical Stabilization Energy and Steric Bulk of a Hydrocarbyl Group to the Strength of its Bonds with Metals and Hydrogen. A Theoretical Study
Bibliographic record
Abstract
We have analyzed the strength of the XR bond for a number of hydrocarbyl groups (R) attached to X = hydrogen, X = (CNCH 3 )(H)(Tp)Rh [Tp = H-B(pyrazolyl) 3 ], and X = (OSi(CH 3 ) 3 )(OSi(CH 3 ) 3 )(NH-Si(CH 3 ) 3 )Ti with the help of a density functional theory (DFT) based energy decomposition scheme (EDA). The hydrocarbyl groups included had a radical sp 3 carbon with unsaturated aromatic or olefinic bonds (R 1 = Ph-CH 2, mesityl, Me-allyl, allyl) or saturated alkyl substituents (R 2 = Me, Et, Pe, i-Pr, t-Bu), a radical carbon as part of a saturated ring (R 3 = c-Pr, c-Bu, c-Pe, c-He), or a radical sp 2 carbon (R 4 = Ph, t-Bu-vinyl, Me-vinyl, vinyl). The EDA scheme was used to rationalize the relative order of the X−R i bond energies between groups ( i = 1,4), within groups (same i ), and between metals X = Rh, Ti, and hydrogen (X = H). It was found that the average bond energy within each group increases as R 4 > R 3 ∼ R 2 > R 1 for the two metals as well as H. This trend correlates with the radical stabilization energy of R i that decreases in absolute terms as R 4 < R 3 ∼ R 1 < R 2 . This trend enables one to make rough correlations between M−C bonds and M−H links on going from one group to another. Within each of the XR i systems where i = 1, 3, 4 there is also a correlation between the trend in the X−R i bond energies and the R i distortion energy. However, for the R 2 group trends in the X−R 2 bond energies are not determined by the radical stabilization but directly (X = H) or indirectly (X = Rh, Ti) by increasing steric bulk on R 2 . For X = H, steric bulk directly destabilizes the H−R 2 bond by increasing the steric interaction between H and R 2 without significantly changing the H−C bond distance. For X = Rh, Ti steric bulk indirectly destabilizes the M−R 2 bond by increasing the M−C bond distance in order to relieve the steric strain. This leads to a reduction in the M−C bonding overlap and the M−C bond strength. We note finally that the Rh−R i and Ti−R i bonds are some 60 kcal/mol weaker than the H−R i link. The major contributing factor here is the poorer overlap and larger energy gap between the orbitals involved in forming the M−C bond compared to the H−C link. Additional factors are larger steric interactions and the need for some distortion of the Rh and Ti fragments.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.005 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".