Gas-phase enthalpies of formation, acidities, and strain energies of the [m, n]polyprismanes (m ≥2; n = 3-8; m x n ≤ 16): a CBS-Q//B3, G4MP2, and G4 theoretical study

摘要

Gas-phase standard state (298.15 K, 1 atm) enthalpies of formation (Δ_fH°_((g))) and enthalpies (Δ_(acid)H°_((g))) and free energies (Δ_(acid)G°_((g)))) of acid dissociation were calculated for the [m, n]polyprismanes (m ≥ 2; n = 3-8; m x n ≤ 16) using the composite method CBS-Q//B3, G4MP2, and G4 levels of theory. Excellent agreement was obtained between the theoretical estimates and experimental values for [2, 4]polyprismane (cubane), the only member with reliable experimental data. Gas-phase acidities of the [2, 3], [2, 4], and [2, 5]polyprismanes correlate well with percent s-character of the C-H bonds as determined by nuclear magnetic resonance spectroscopy. Based on this correlation, and calculations on the [2, n] (n = 3-8) series, no substantial change in the gas-phase acidity or C-H bond s-character is expected with increasing [2,n] polyprismane ring size at n = 6-8. Where three or more stacked [m, n]polyprismane rings are present (m > 2), geometry optimizations for the deprotonated anions converged on cage-opened non-prismatic geometries, suggesting these compounds may be structurally unstable to gas-phase deprotonation/protonation cycles and potentially preventing reliable experimental measurement or calculation of their acidities. Total strain energies (E_(str)) increase with larger [m, n]polyprismane size, and strain energies per one C-C bond (E_(str)~(CC)) increase with increasing stack height within a given ring size series. Within a given stack height, E_(str)~(CC) reaches a minimum with a ring size of five, increasing with either decreasing or increasing ring size away from this minima. E_(str) and E_(str)~(CC) do not correlate with gas-phase acidity, likely because the deprotonated anionic [2, n]polyprismane geometry retains approximately equal strain as the undis-sociated species despite substantial geometry changes due to carbon acid dissociation.