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The most important geometric parameters and associated uncertainties (2σ) determined for F2POPF2 are the distances (rg) P-O = 1.631 ± 0.010 Å, P-F = 1.568 ± 0.004 Å, and angles POP = 135.2 ± 1.8°, OPF = 97.6 ± 1.2°, and FPF = 99.2 ± 2.4°. Amplitudes of vibration were also found. The large POP angle and relatively short P-O bond length are consistent with a significant degree of pπ-dπ bonding. Our structure interpretation differs from an earlier one reported by Arnold and Rankin in the relative P-O and P-F bond lengths and in the conclusion that the molecule exists in a distribution of not very rigid, probably staggered, conformers instead of one fairly rigid structure.  相似文献   

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The molecular structure of magnesium dibromide was investigated by high-level computational techniques and gas-phase electron diffraction. The vapor consisted of about 88% monomeric and 12% dimeric species at the electron diffraction experimental conditions at 1065 K. The geometrical parameters and vibrational characteristics of monomeric, dimeric, and trimeric magnesium dibromide species were determined by computations. Very high level computations with extended basis sets and relativistic pseudopotentials on bromine were needed to reach an agreement between computed and estimated experimental equilibrium geometries for the monomer. For both the dimer and the trimer, different geometrical arrangements were tested. Their ground-state structures have halogen bridges with four-membered ring geometries and D2h and D2d symmetry, respectively. Thermodynamic parameters have also been calculated.  相似文献   

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The molecular structure of axial and equatorial conformer of the 1-bromo-1-silacyclohexane molecule, CH2(CH2CH2)2SiH-Br, as well as thermodynamic equilibrium between these species are investigated by means of gas-phase electron diffraction and quantum chemistry on the MP2(full)/SDB-AUG-cc-PVTZ level of theory. It is revealed that according to electron diffraction data, the compound exists in the gasphase as a mixture of conformers possessing the chair conformation of the six-membered ring and C s symmetry and differing in the axial and equatorial position of the Si-Br bond (ax. = 80(5) mol %, eq. = 20(7) mol %) at 352 K, that corresponds to the value of A = (G ax ?? ? G eq ?? ) = ?0.82(32) kcal/mol. It is found that observed data agree well with theoretical ones. Using Natural Bond Orbital (NBO) analysis it is revealed that axial conformer of 1-bromo-1-silacyclohexane molecule is an example of the stabilization of the form that is unfavorable from the point of view of steric effects and effects of conjugations. It is concluded that stabilization is achieved due to electrostatic interactions.  相似文献   

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The molecular structures of dimethylamino[(dimethylboryl)methylamino]methylborane, Me2NBMeNMeBMe2 (1) and 1,1-bis(dimethylboryl)-2,2-dimethylhydrazine, (Me2B)2NNMe2 (2) have been determined by gas electron diffraction (GED), density functional theory calculations at the B3PW91/6-311++G** level and ab initio calculations at the MP2/6-311++G** level. 1 adopts an open structure similar to that of the isoelectronic hydrocarbon molecule permethylbutadiene; the central B-N bond distance at 148.0/149.3(7) pm (MP2/GED) corresponds to a single covalent N--B bond distance, the two terminal distances, 140.9/140.5(4) pm and 141.8/141.3(4) pm, correspond to the distance between N and B atoms joined by a covalent sigma-bond and a dative pi-bond. A closed form where the establishment of a dative bond between the terminal N and B atoms has led to the formation of a four-membered ring also corresponds to a minimum on the potential energy surface, but the energy is calculated to be 14.3 kJ mol(-1) higher at the MP2 level. This structure is also incompatible with the GED data. 2 adopts a structure in which a dative sigma-bond between the dimethylamino N atom and one of the boron atoms has led to the formation of a three-membered N(2)B ring. The dative sigma-bond distance is 165.5/164.0(13) pm, the two other bond distances in the ring are N-B=150.6/148.9(9) pm corresponding to a covalent sigma-bond and N-N=145.1/145.4(3) pm. The terminal B--N distance 139.6/138.9(9) pm is consistent with a covalent sigma-bond augmented by a dative pi-bond. An open Y-shaped structure also corresponds to a minimum on the potential energy surface, but the energy is 18.7 kJ mol(-1) higher (MP2) and it is incompatible with the GED data.  相似文献   

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An electron diffraction study of the MeC(O)PMe2 molecule has been carried out which shows that the bond configuration at phosphorus in this compound is flattened from that observed in methylphosphines and dimethylcyanophosphine. This finding, as well as some lowering of barriers to inversion at phosphorus in acylphosphines, may be attributed to a contribution from conjugation of the amide type.Two models with different angles of rotation of the acetyl group about the PCAc bond, ψ, are consistent with the experimental data, those with ψ of 5.7 ± 6.6° (model I) and 78.8± 12.0° (model II) (ψ is measured relative to the Cs conformation with the CMePCMe bisector being cis with respect to the CO bond). Other parameters of the two models agree within uncertainties represented by three times standard deviation values 3σ (distances, ra, in nm; angles in degrees): model I model II PC (average) 0.1863(2) 0.1862(2) CC 0.1536(9) 0.1540(9) CO 0.1219(4) 0.1218(3) CH 0.1094(6) 0.1094(6) CMePCMe 105.7 (0.9) 105.1 (3.3) CMePCMe 99.3 (2.0) 102.2 (7.5) PCO 123.0 (0.6) 121.9 (0.6) PCC 115.3 (0.9) 112.7 (0.9) The greater errors for phosphorus valence angles and the angle of rotation, ψ, in the case of model II are due to high correlation between these para A possible difference between the PCMe and PCAc bond lengths has been estimated to be. ± 0.001nm(model I) or ± 0.003 nm(model II). Neither of the models can be preferred on the basis of the electron diffraction data; the results'obtained are also compatible with' the suggestion of large amplitude torsion motions.  相似文献   

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The electronic distribution in the AlH2Al bridge of the dimethyl aluminium hybride dimer was computed through ab initio SCF calculation. Comparison with diborane shows an increased role of the ionic Al+H 2 2– Al+ structures with respect to the usual covalent three-center bonds.  相似文献   

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The molecular structure of (F3C)2Se has been determined in the vapour phase by the sector microphotometer method of electron diffraction. Two structures, differing essentially in the angles of rotation of the CF3- groups about the C-Se bonds, are in good agreement with the data. The mean C-Se and C-F bond lengths are 1.978 and 1.333 Å, respectively.  相似文献   

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Gas electron diffraction analysis on S-methyl thioacetate, CH3C(O)SCH3   总被引:1,自引:0,他引:1  
The molecular structure of S-methyl thioacetate, CH3C(O)SCH3, was determined by gas electron diffraction (GED) with the assistance of quantum chemical calculations (B3LYP/6-31G and MP2/6-31G). Experimental and theoretical methods result in a structure with syn conformation (C=O double bond syn with respect to the S-C(H3) single bond). The following skeletal geometric parameters were derived from the GED analysis (ra values with 3sigma uncertainties): C=O = 1.214(3), C-C = 1.499(5), S-C(sp2) = 1.781(6), S-C(sp3) = 1.805(6) angstroms, O=C-C = 123.4(8) degrees, O=C-S = 122.8(5) degrees and C-S-C = 99.2(9) degrees.  相似文献   

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The molecular structure of propargylgermane, HCCCH2GeH3, has been determined by gas-phase electron diffraction. The electron-diffraction investigation has been supported by density functional theory and ab initio calculations. The ra value of the bond lengths (pm) are: r(C–Ge)=197.2(1); r(C–C)=143.9(2); r(CC)=123.1(1); r(H–Cacetylene)=108.5(3); r(C–H)=111.6(3) and r(Ge–Haverage)=153.7(2). The Ge–C–C angle is 111.7(1)° and the C–CC angle is 178.3(4)°. The uncertainties are one standard deviation from the least-squares refinement.  相似文献   

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An electron diffraction determination of the molecular geometry of hexamethyldisiloxane has removed much of the uncertainty concerning this structure. The length of the SiO bond and the SiOSi bond angle were determined to be 1.631 ± 0.003 Å and 148 ± 3°, respectively. The experimental data are consistent with a staggered conformation (C2v symmetry) while a model with twist angles around the SiO bonds of about 30° cannot be excluded. The molecule is probably performing large amplitude intramolecular motion.  相似文献   

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