3× Axial vs 3× Equatorial: The ΔGGA Value Is a Robust Computational Measure of Substituent Steric Effects

Research output: Contribution to journalArticle (Academic Journal)peer-review

Abstract

We define ΔGGA as the free energy change for the formal equilibrium: [13]G-H + 1-X-adamantane → [13]G-X + adamantane, where [13]G-H is the C13H22 fragment of all-trans graphane with 3-fold symmetry. This compares with a situation where the group X is equatorial to three cyclohexane rings with one where it is axial to three rings. ΔGGA values vary from 2.9 (CN) to 145.7 kJ mol–1 (CCl3), and this wide range means that ΔG can be calculated with confidence. ΔGGA values for Me, Et, i-Pr, and t-Bu form a regular series, 34.9, 63.3, 101.6, and 142.0, and clearly reflect the steric size of the groups. We propose a model where the six axial hydrogens surrounding X on [13]G-X provide a nearly circular constriction on the substituent close to its point of attachment but which does not extend far above this. We compare these results with A values and with calculations on 2- and 7-substituted [1(2,3)4]pentamantanes. We show that electronic effects on ΔGGA values are negligible but that they correlate well with computed cone and solid angles subtended by the substituent.
Original languageEnglish
Pages (from-to)13573-13578
Number of pages6
JournalJournal of the American Chemical Society
Volume143
Issue number34
Early online date17 Aug 2021
DOIs
Publication statusE-pub ahead of print - 17 Aug 2021

Bibliographical note

Funding Information:
We thank the Bristol Chemical Synthesis Centre for Doctoral Training, funded by EPSRC (EP/L015366/1) and the University of Bristol, for a Ph.D. studentship for A.W.M.

Publisher Copyright:
© 2021 American Chemical Society.

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