Gas-phase electron diffraction study of cyclic dimer of dimethylphosphinic acid (Me 2P(O)OH) 2 using quantum chemical data and a priori force field

Abstract

The molecular structure of the hydrogen bonded cyclic dimer of dimethylphosphinic acid (Me 2P(O)OH) 2 was determined by gas-phase electron diffraction (GED) at 433 K. The presence of monomer cannot be determined at this temperature within the error limits for the GED method. Structural analysis was performed with consideration of non-linear kinematic effects at the first-order level of perturbation theory ( h1). The vibrational characteristics of internuclear distances were calculated from a priori scaled quantum chemical (RHF/6-311G**) force field. The analysis aided by a constraint based on the RHF/6-311G** calculations yielded the following r h1 -parameters of the C 2-symmetry dimer configuration: PO 1.497(3); P–O 1.573(4); P–C 1.806(1) and 1.811(1) Å; (C–H) av. 1.109(3) Å; ∠O–PO 120(1)°. Unlike PO and P–O bonds, whose lengths in the gas phase and in the solid state differ insignificantly, the –O⋯O distance in the gas phase ( r h1 2.81(4) Å) is considerably longer than in the solid state ( r α 2.48(2) Å). The latter is in accordance with the conclusion based on the IR spectra that transition from gas to a solid sample leads to strengthening of the H-bonds. Due to its small contribution to the diffraction pattern, the donor O–H bond length ( r h1 0.99(1) Å) was forcedly bound up with the parameters of C–H bonds. With this assumption, the other geometrical parameters characterizing the H-bond fragment have the following r h1 values: O⋯H 1.84(4) Å, ∠–OHO164(6)°, and ∠P–O–H 117(4)°. Conformational flexibility of the non-planar eight-atom ring of the dimer is experimentally verified by absence of any apparent peaks of the f( r) curve at the r-region of more than 4.2 Å.