Gas-phase molecular structures of substituted 1,3-bisketenes: A challenge for theory and experiment

Molecular structures of dimethylbis(trimethylsilylketyl)silane (Me₂Si[C(SiMe₃)CO]₂), dimethylbis(trimethylgermylketyl)silane (Me₂Si[C(GeMe₃)CO]₂), and dimethylbis(trimethylstannylketyl)germane (Me₂Ge[C(SnMe₃)CO]₂) have been studied in the gas phase by electron diffraction accompanied by high level ab initio and DFT calculations. Extensive theoretical conformational analyses of the molecules in the vapour predicted a possibility of existence of two types of conformers with small energy differences. The first type had gauche-gauche arrangements of the ketenyl groups in the central C(CO)XC(CO) fragments directed away from each other. The second type had nearly syn-gauche arrangements of the ketenyl groups. In addition, the energy differences were found to depend on the level of computations used. The experimental analysis, in turn, was unable to distinguish between different conformers due to the large number of similar overlapping distances. The experimental data were fitted by an averaged single-conformer model, which nevertheless allowed reliable determination of bonds and bonded angles in the molecules. Main experimental (r_h1) structural parameters for Me₂Si[C(SiMe₃)CO]₂, Me₂Si[C(GeMe₃)CO]₂, and Me₂Ge[C(SnMe₃)CO]₂, i.e. Me₂X[C(YMe₃)CO]₂ (X,Y = Si, Ge, Sn), are (X-C)_mean 187.7(1) pm, 194.6(2) pm, 216.1(3) pm; (Y-C)_mean, 187.7(1) pm, 188.8(8) pm, 194.6(4) pm; (CC)_mean, 135.3(5) pm, 131.6(5) pm, 131.5(13) pm; (CO)_mean, 117.0(7) pm, 117.4(7) pm, 119.0(11) pm; (C-H)_mean, 110.6(7) pm, 110.0(4) pm, 109.1(13) pm; (X(Y)-CC)_mean, 114.4(2)°, 115.6(1)°, 115.6(2)°; (C-X(Y)-C_Me)_mean, 108.3(3)°, 108.4(3)°, 108.9(13)°; C(2)-C(1)-Y(4)-C(10), −19(6)°, 5(4)°, −9(10)°; C(7)-C(6)-Y(9)-C(38),−22(7)°, −32(3)°, −9(10)°; C(2)-C(1)-X(5)-C(6), 128(4)°, 142(1)°, 108(9)°; C(7)-C(6)-X(5)-C(1), 92(6)°, 115(2)°, 108(9)°, respectively.