P. Van Nuffel

The molecular structure of gaseous methyl vinyl ether at room temperature, studied by molecular orbital constrained electron diffraction and microwave spectroscopy

Abstract The gas phase molecular structure of methyl vinyl ether at room temperature has been studied by joint analysis of electron diffraction and microwave data. Constraints on geometrical and thermal parameters were derived from the geometry and force field of the s-cis form, obtained by ab-initio calculations (4–21 G basis set) after complete geometry relaxation. A range of models was investigated that fits all available data (infrared, microwave and electron diffraction). The following r g / r α -parameters were obtained: Cue5fbC: 1.337 A, C( sp 2 )ue5f8O: 1.359 A, C( sp 3 )ue5f8O: 1.427 A, : 1.102 A ∠Cue5fbCue5f8O : 127.3° and ∠COC: 116.8°. Experimental r g ue5f8 r e (ab initio) corrections are given for Cue5fbC, C( sp 2 )ue5f8O and C sp 3 )ue5f8O. This investigation demonstrates that molecular orbital constrained electron diffraction is sufficiently reliable and in such a manner that it can be applied to more complicated problems.

Structure determination of propanal by joint analysis of gas electron diffraction, microwave and infrared spectroscopy, including constraints and a valence force field from geometry relaxed ab-initio calculations

Abstract The structure and conformational equilibrium of CH 3 -SYN and CH 3 -SKEW conformers of propanal were studied by joint analysis of gas electron diffraction, microwave and infrared data, including constraints obtained after ab-initio relaxations (4-21G basis set) of the molecule. A valence force field was calculated for both conformers and scaled upon experimental IR frequencies; theoretical frequencies and band intensities are compared to experimental ones. Rotamer populations of CH 3 -SYN and CH 3 -SKEW were found to be 81 and 19%, respectively, at 300 K. The following best-fitting r g / r α parameters were obtained as: r (Oue5fbC) = 1.209(4) A, r (C2ue5f8C3) = 1.515(9) A, r (C3ue5f8C4) = 1.521(9) A for CH 3 -SYN and (with lower reliability) 1.569(45) A for CH 3 -SKEW, = 1.127(4) A, ∠O=Cue5f8C = 124.5(3)°(SYN) and 125.1(3)° (SKEW), ∠Cue5f8Cue5f8C = 113.8(4)° (SYN) and 110.2(4)° (SKEW), torsion angle around C2ue5f8C3 = 123.7(2.6)° for CH 3 -SKEW.

The effect of non-bonded attractive electronic (anomeric) interactions on conformation and geometry of organic phosphates and thiophosphates

Abstract Substituted 1,3,2-dioxaphosphorinane-2-oxides and -2-sulfides are classified according to the conformation of the ring and the orientation (axial or equatorial) of the P=O/P=S bond. Geometrical parameters (torsion angles, valence angles and bond lengths) of 37 compounds are analyzed and several systematic class differences are observed. The anomeric effect, with emphasis on the n π (O)—α * (P-axial substituent) interaction, provides the basis for rationalizing conformational preferences as well as geometrical details. Mean experimental geometries are compared to ab initio calculated geometries for conformers of (HO) 2 PH(O) and (HO) 2 PH(S) mimicking the dioxaphosphorinanes.

Gasphase conformational analysis of dimethylsulphite

Abstract The newly recorded gasphase infrared spectrum of the title compound is analysed conformationally based on the known existence of two conformers in the liquid phase. A quantitative profile simulation of the (S = 0)-stretching mode, using a self-written FORTRAN IV-program for use on a mini-computer, is applied to deduce the structure of the two conformers present. Out of a total of six theoretically possible structures, we propose the conformers present in the gasphase to be (pro-R +sc,pro-S +sc) and (pro-R +sc,ṕro-S±ap).