The molecular structures and conformational compositions of 1,3 cyclohexanedione and 1,4 cyclohexanedione as determined by gas-phase electron diffraction and theoretical calculation

Abstract

► The molecular structures of 1,3 and 1,4 cyclohexanedione have been studied in the gas phase. ► For 1,4 cyclohexanedione a mixtures of twist (70(9)%) and chair (30(9)%) forms was observed. ► For 1,3 cyclohexanedione a mixture of chair (60(13)%) and boat (40(13)%) forms was observed. ► The results are consistent with theoretical MP2 and B3LYP calculations using 6-311G(d,p) basis set. The molecular structures of 1,3 and 1,4 cyclohexanedione have been investigated in the gas phase at nozzle temperatures of 379 K and 383 K respectively. These two molecules exist in conformation mixtures of: twist (70(9)%) and chair (30(9)%) forms for 1,4 cyclohexanedione; and chair (60(13)%) and boat (40(13)%) forms for 1,3 cyclohexanedione. The results are consistent with theoretical MP2 and B3LYP calculations using 6-311G(d,p) basis set where the stable forms found were: twist and chair for 1,4 cyclohexanedione and chair, boat and twist for 1,3 cyclohexanedione. The geometrical parameter values ( r g and ∠ α ) and their error limits (3 σ ) obtained from least squares refinements for the twist form of 1,4 cyclohexanedione are : r (C H) av = 1.116(5) Å, r (C O) = 1.220(2) Å, r (C 1 C 2 ) = 1.528(8) Å, r (C 2 C 3 ) = 1.535(17) Å, ∠ C 1 C 2 C 3 = 113.3(5) Å, ∠ C 6 C 1 C 2 = 117.9(10)°, τ C 6 C 1 C 2 C 3 = 24.6(6)° and Flap (angle between planes formed by atoms C 3 , C 4 , C 5 and C 3 , C 5 and the mid-point of atoms C 6 and C 2 ) = 180°. and for the chair form of 1,3 cyclohexanedione: r (C H) av = 1.105(5) Å, r (C O) = 1.220(2) Å, r (C 1 C 2 ) 1.528(2) Å, r (C 3 C 4 ) = r (C1 C2) − 0.008 Å, r (C 4 C 5 ) = r (C1 C2) + 0.010 Å, ∠ C 1 C 2 C 3 = 114.0(8)°, ∠ C 6 C 1 C 2 = 116.2(5)°, τ C 6 C 1 C 2 C 3 = 40.1(2)°, and Flap(angle between planes C 4 C 5 C 6 and C 4 C 6 C 2 ) = 142.8°(assumed).