Conformational Dynamics of Tetraisopropylmethane and of Tetracyclopropylmethane¹

Abstract

Tetraisopropylmethane (1) exists in solution as a mixture of two types of conformers (D2d and S4 time-averaged symmetry) in the ratio 93:7 at -110 degrees C, interconverting with a barrier of 9.7 kcal mol-1. Molecular mechanics calculations and the multiplicity of NMR signals at low temperature allow the assignment of these conformations. The only conformation populated in tetracyclopropylmethane (2) is the same type as the minor conformation (S4 time-averaged symmetry) populated in 1. 13C NMR spectra at about -180 degrees C show that degenerate versions of this conformation interconvert with a barrier of 4.5 kcal mol-1. Molecular mechanics calculations that characterize the six possible conformational types for these molecules, and the most important interconversion pathways, are reported. Calculated and experimental barriers match satisfactorily well.

Recent Advances in Stereodynamics and Conformational Analysis by Dynamic NMR and Theoretical Calculations

“…According to MM3 calculations, the most stable, called T1, is threefold degenerate and the second-most stable (T3) is sixfold degenerate. [84] The low-temperature (-115°C) 1 H NMR spectrum of 12 shows two distinguishable CH multiplets with a 93:7 intensity ratio. The ∆G ϶ value for the interconversion is 9.7 kcal mol -1 , as derived from the rate constants (k) used for the line shape simulation (Figure 17).…”

Conformational Studies on Oligosubstituted Adamantane Derivatives – Structural Features of Tetravinyl‐, Tetracyclopropyl‐, and Tetraisopropyladamantane

“…[1,2] A recent report shows that the parent tetracyclopropylmethane (1) exists as an S 4 -symmetric conformer in the solid state as well as in solution, while tetraisopropylmethane (2) adopts a D 2d -symmetric conforma-not give any crystals suitable for X-ray crystal structure analysis, even when using the Optical Heating and Crystallization Device (OHCD). This method was rather successful when applied to the tetravinyl derivative 17, X-ray crystal structure analysis of which revealed an approximately C 2 -symmetric conformation in the solid state at 150 K. However, no well-defined orientation was detected for the cyclopropyl groups in 8, and the molecules were severely disordered even at 30(1) K. In contrast to this, X-ray crystal structure analysis of the tetraisopropyl derivative 9 revealed an S 4 -symmetric conformation for this hydrocarbon at 203 K.…”

“…[1,2] Far less complete and systematic studies exist for the conformational behavior of small molecules in which four identical sterically demanding groups are symmetrically attached to a tetrahedral organic skeleton such as an adamantane core. In a sense such molecules can be regarded as steric analogues of 1 and 2, with the central carbon atom being replaced by the wider hydrocarbon skeleton.…”

New Structurally Interesting Cyclopropane Derivatives: A World of Wonders and Surprises

“…The dynamics of this molecule was revealed by studying the temperature dependence of its 1 H NMR spectrum down to -115°C. A minor conformer was clearly detected at this low temperature with a proportion of 6.5 %, and a complete line-shape analysis revealed a free-energy barrier to conformational changes in this molecule of 9.7 kcal/mol [37].…”

“…The interesting dynamic behavior of this molecule was eventually disclosed by 13 C NMR spectra taken at -165 and -180°C [37].…”