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

We define Δ 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 CH fragment of all- 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. Δ values vary from 2.9 (CN) to 145.7 kJ mol (CCl), and this wide range means that Δ can be calculated with confidence. Δ 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 values and with calculations on 2- and 7-substituted [1(2,3)4]pentamantanes. We show that electronic effects on Δ values are negligible but that they correlate well with computed cone and solid angles subtended by the substituent.