Temperature-Dependent Crystal Structure Study of a Small Heterobicycle, C₉H₁₂O₄

Abstract

1,7,7-Trimethyl-2,6-dioxabicyclo[2.2.2]octan-3,5-dione is a dipseudoacyl ketal formed by a double intramolecular dehydration/cyclization of 2-carboxy-3,3-dimethyl-5-oxohexanoic acid. The compound is a volatile, colorless crystalline solid (mp 137 °C) which forms prisms and plates from ethanol by slow solvent evaporation. Crystals are orthorhombic, space group P2₁2₁2₁ (#19), a = 8.0557(2) Å, b = 10.3545(3) Å, c = 10.7583(3) Å, V = 897.37(11)ų (101 K). The crystal and molecular structure has been determined on a single prismatic specimen at 280, 250, 200, 150 and 101 K (±1 K). The ostensibly rigid molecule possesses mirror symmetry, which is not retained in the solid state. Each six-membered ring has a boat conformation, with torsions-0.2°, 1.3°, −2.7° (101 K). Carbonyl groups are eclipsed with a common methine C–H bond, and the OCC(H) bond angles are enlarged to 126.9°, 127.0°, while the intraring CCO angles are contracted to 112.0, 112.1° (uncorrected for libration). The coefficients of thermal expansion are 3.34 × 10⁻⁴, 6.97 × 10⁻⁴ and 6.54 × 10⁻⁴Å/K for the three orthorhombic axes, respectively, over the range studied. A rigid-body librational model fit to the anisotropic librational parameters grows worse at lower temperatures, suggesting at least two lower frequency internal modes make contributions to the librational terms.

Graphical Abstract

A bicyclic rigid dipseudoacyl ketal shows libration which on analysis of the thermal motion from 280 to 101 K suggest two active low frequency vibrational modes.