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

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Title	Gas phase enthalpies of formation, acidities, and strain energies of the [m,n]polyprismanes (m≥2; n=3-8; m×n≤16): A CBS-Q//B3, G4MP2, and G4 theoretical study
Authors/Editors*	S Rayne, K Forest
Where published*	Theoretical Chemistry Accounts
How published*	Journal
Year*	2010
Volume	127
Number
Pages	697-709
Publisher	Springer
Keywords	Polyprismanes; Enthalpy of formation; Gas phase acidity; Strain energy; Composite methods
Link	http://www.springerlink.com/content/h451270wm0743t68/
Abstract	Gas phase standard state (298.15 K, 1 atm) enthalpies of formation (ΔfH°(g)) and enthalpies (ΔacidH°(g)) and free energies (ΔacidG°(g)) of acid dissociation were calculated for the [m,n]polyprismanes (m≥2; n=3-8; m×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 (Estr) increase with larger [m,n]polyprismane size, and strain energies per one C-C bond (EstrCC) increase with increasing stack height within a given ring size series. Within a given stack height, EstrCC reaches a minimum with a ring size of five, increasing with either decreasing or increasing ring size away from this minima. Estr and EstrCC do not correlate with gas phase acidity, likely because the deprotonated anionic [2,n]polyprismane geometry retains approximately equal strain as the undissociated species despite substantial geometry changes due to carbon acid dissociation.

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