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

摘要

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 (3σ) obtained from least squares refinements for the twist form of 1,4 cyclohexanedione are: r(CH)_(av) = 1.116(5), r(CO) = 1.220(2), r(C~1C~2) = 1.528(8), r(C~2C~3) = 1.535(17), ∠C~1C~2C~3 = 113.3(5), ∠C~6C~1C~2 = 117.9(10)°, τC ~6C~1C~2C~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(CH)_(av) = 1.105(5), r(CO) = 1.220(2), r(C ~1C~2) 1.528(2), r(C~3C~4) = r(C~1C~2) - 0.008, r(C~4C~5) = r(C~1C~2) + 0.010, ∠C ~1C~2C~3 = 114.0(8)°, ∠C~6C ~1C~2 = 116.2(5)°, τC~6C~1C ~2C~3 = 40.1(2)°, and Flap(angle between planes C ~4C~5C~6 and C~4C~6C ~2) = 142.8°(assumed).