Conformational stabilities from variable temperature Raman spectra, r0 structural parameters and vibrational assignments of 1,2-diphosphinoethane

Infrared spectra (3600–500 cm−1) of gas, amorphous, crystalline solid and Raman spectra (3000–200 cm−1) of liquid and solid were recorded. Variable temperature Raman spectra (30 to −38 °C) of liquid were recorded which indicates the presence of significant amounts of six conformers at ambient temperature. Rotation about CC bond gives Trans (T) and Gauche (G) conformers and rotation around both CP bonds gives trans (t) and gauche (g) forms. Enthalpy differences for five of the lower energy conformers w.r.t. the lowest energy tTt form were determined to be 272 ± 39 cm−1, 434 ± 40 cm−1, 443 ± 30 cm−1, 663 ± 60 cm−1 and 671 ± 34 cm−1 for g′Tt, g′Tg′, g′Tg, tG′g′ and tG′t conformers, respectively, which are in the increasing order of energy. Ab initio calculations with various basis sets up to aug-cc-pVTZ were performed and to support the spectroscopic studies MP2(full)/6-31G(d) was used to predict Raman activities, infrared intensities, vibrational frequencies and band contours. By utilizing the previously reported microwave rotational constants along with MP2(full)/6-311+G(d,p) predictions, adjusted r0 parameters were obtained for two Trans and two Gauche (CC bond) conformers. The determined heavy atom structural parameters for g′Tt [g′Tg′] {tG′g′} |tG′t| conformers are: distances (Å) P1C2 = 1.870(3) [1.866(3)] {1.858(3)} |1.859(3)|, C2C3 = 1.535(3) [1.536(3)] {1.534(3)} |1.536(3)|, C3P4 = 1.863(3) [1.866(3)] {1.866(3)} |1.859(3)|, and angles (°) ∠P1C2C3 = 110.5(5) [110.4(5)] {117.0(5)} |118.5(5)|, ∠C2C3P4 = 115.5(5) [110.4(5)] {113.7(5)} |118.5(5)|. The results are discussed and compared to the corresponding properties of some similar molecules.

► Six lower energy conformers have been identified from their vibrational spectra.
► Enthalpy differences have been determined among the lowest energy forms.
► Adjusted r0 structures were obtained for four lower energy conformers.
► Ab initio and DFT calculations were performed for all the conformers.