The molecular structure and conformation of dichlorotetramethyldisiloxane as determined by gas phase electron diffraction

Dichlorotetramethyldisiloxane is studied by gas-phase electron diffraction at room temperature. The least-squares values of the bond distances (r_g) and bond angles () are: r(C---H)=1.084(5) Å, r(Si---O) = 1.624(2) Å, r(Si---C) = 1.852(2) Å, r(Si---Cl) = 2.067(2) Å, SiOSi = 154.0° (1.5), ClSiO = 110.2° (0.8), ClSiC = 109.6°(0.7), HCSi = 111.7°(1.5), OSiC = 110.0°(0.8), τ₁ (zero corresponds to the Si---Cl bond trans to the Si---O---Si linkage) = 78°(6) and τ₂ = 141°(19). A two-conformer model cannot be ruled out.     

The gas phase molecular structure of silyl formate,determined by electron diffraction

The structure of silyi formate, HCOOSiH₃, in the gas phase is determined by electron diffraction. The principal bond lengths and angles (r_a) are r(Si-O) = 169.5 ± 0.3 pm, r(C-O) = 135.1 ± 0.6 pm, r(C  O) = 120.9 ± 0.7 pm, ∠(C-O-Si) = 116.8 ± 0.5°, ∠(OC-O) = 123.5 ± 0.5°. The silyi group is twisted by 21° away from the planar cis conformation but there is nevertheless a very short (286.5 ±1.0 pm) non-bonded Si ·O contact.     

The molecular structure and conformational composition of epichlorohydrin as determined by gas phase electron diffraction

The molecular structure of gaseous epichlorohydrin has been investigated using electron diffraction data obtained at 67°C. The conformational composition at this temperature is such that the molecules exist predominantly in a gauche-2 conformer (where the C---Cl bond is 160° away from the C---O) bond). Refinements showed that 33% (σ = 4) of the molecule exist in the gauche-1 form. The important distances (r_g) and angle () with the associated uncertainties are r(C---H) = 1.095(5) Å, r(C---O) = 1.442(3) Å, r(C---C) = 1.475(8) Å, r(C---C_M) = 1.523(7) Å, r(C---Cl) = 1.788(2) Å, CCO = 114° (1), CCC_M = 119°(1), ClCC = 108.9° (7), and Tau(ClCCO) = −150°(10) (gauche-2) and Tau(ClCCO) = 78° (10) (gauche-1).     

The molecular structure and conformations of oxepane as determined by gas electron diffraction

The electron-diffraction data for gaseous oxepane, collected at 310 K, can be explained in terms of a 53:47% mixture of two twist-chair conformations. Using the nomenclature of Crerner and Pople [1], the conformations are characterised by q₂ = 0.579 å, q₃ = 0.685 Å, φ₂ = 13.3°, φ₃, = 63.0° and q₂ = 0.511 Å. q₃ = 0.588 Å, ø₂ = 116.1°, ø₃ = 217.6°. The other structural parameters (r_a-structure) are r_CO = 1.419 Å, r_cc = 1.531 Å, r_CH = 1.105 Å, ∠H-C-H = 106.0°, with a mean ring valency angle of 112-0° for the former conformation, and of 116.2° for the latter. There is a good agreement between the experimental geometries and the results from molecular mechanics calculations.     

3-Bromo-2-methyl-1-propene: molecular structure and conformation in the gas phase as determined by electron diffraction and molecular mechanics calculation

A gas phase electron diffraction study of 3-bromo-2-methyl-1-propene shows that there is predominantly a gauche conformer present. Data recorded at 20 and 180°C show 4(8) and 5(4)% respectively of a second confomer with a planar heavy atom skeleton. The gauche structural results in terms of r_a distances and angles at 20°C were found to be: r(C---C) = 1.331(9) Å, r(C---CH₂Br) = 1.484(6) Å, r(C---CH₃) — r(C---CH₂Br) = 0.017 Å, (assumed), r(C---Br) = 1.965(6) Å, C=C---CH₂Br = 121.5(0.7)°, C=C---CH₂Br — C=C---CH₃ = 0.7° (constraint from molecular mechanics calculation), C---C---Br = 112.2(0.5)°, torsional ANGLE = 112.5(2.2)°. Uncertainties are given as 2σ, where σ includes uncertainties due to correlation among observations, electron wavelength and other parameters used in the data reduction. The results obtained from the 180°C data agree very well with those given above. The molecular mechanics calculations yield information consistent with the experimental results.     

The molecular structure of fluoromalononitrile as determined by gas-phase electron diffraction and ab initio calculations

The molecular structure of fluoromalononitrile was studied by means of gas-phase electron diffraction and quantum mechanical methods using HF/6-31G(d), MP2/6-311++G(2df,2pd) and DFT/B3LYP/6-31G(d), B3PW91/6-31G(d), B3LYP/6-311++G(2df,2pd) and B3PW91/6-311++G(2df,2pd). The r(g) and angle(alpha) structural parameters we obtained from the present analysis are: CC=1.487(5) A, CN=1.157(3) A, CF=1.386(5) A, CH=1.096 A (ass.), angleCCC=106.7(1.0) degrees , angleCCF=108.0(0.7) degrees , angleCCN=177.6(2.0) degrees . Uncertainties in parenthesis are 3sigma.     

Molecular structure of caffeine as determined by gas electron diffraction aided by theoretical calculations

The molecular structure of caffeine (3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione) was determined by means of gas electron diffraction. The nozzle temperature was 185 °C. The results of MP2 and B3LYP calculations with the 6-31G⁷ basis set were used as supporting information. These calculations predicted that caffeine has only one conformer and some of the methyl groups perform low frequency internal rotation. The electron diffraction data were analyzed on this basis. The determined structural parameters (r_g and ∠_α) of caffeine are as follows: <r(NC)_ring> = 1.382(3) Å; r(CC) = 1.382(←) Å; r(CC) = 1.446(18) Å; r(CN) = 1.297(11) Å; <r(NC_methyl)> = 1.459(13) Å; <r(CO)> = 1.206(5) Å; <r(CH)> = 1.085(11) Å; ∠N₁C₂N₃ = 116.5(11)°; ∠N₃C₄C₅ = 121. 5(13)°; ∠C₄C₅C₆ = 122.9(10)°; ∠C₄C₅N₇ = 104.7(14)°; ∠N₉–C₄=C₅ = 111.6(10)°; <∠NCH_methyl> = 108.5(28)°. Angle brackets denote average values; parenthesized values are the estimated limits of error (3σ) referring to the last significant digit; left arrow in parentheses means that this parameter is bound to the preceding one.     

The molecular structure of diethyl(silyl)amine in the gas phase,determined by electron diffraction

The molecular structure of N(C₂H₅)₂(SiH₃) in the gas phase has been determined by electron diffraction. The SiNC₂ skeleton is a shallow pyramid, with angles CNC 114.5(12)° and SiNC 120.9(5)°, and the methyl groups lie so that one CC bond lies close to the CNC plane, but the other is almost perpendicular to it. Other important parameters (r_a) are: r(SiN) 171.5(3), r(CN) 145.6(4), r(CC) 154.3(8) pm, and ∠NCC 113.6(6)°.     

The structure of cyclopentene oxide determined by gas phase electron diffraction

The r_g structure of cyclopentene oxide has been determined by the simultaneous least squares analysis of electron diffraction and microwave spectroscopic data. The investigation has reaffirmed previous studies indicating that the molecule prefers a boat conformation. The methylene and epoxide flap angles obtained are 152.3±2.1° and 104.7±1.0° respectively. Other structural parameters determined are: r_g (C-H avg.) = 1.120±0.004 Å; r_g (C-C avg.) = 1.538±0.002 Å; r_g (C-O) = 1.443±0.003 Å, and r_g (C-C) = 1.482±0.004 Å for the carbon-carbon bond in the three membered epoxide ring. These results compare favorably with the reported structures of ethylene oxide and cyclohexene oxide. A tentative rationalization of the unusual boat conformation is also offered.     

The molecular structure and conformation of trichloronitromethane as determined by gas-phase electron diffraction and theoretical calculations

The molecular structure of trichloronitromethane has been studied in the gas phase using electron diffraction data. The molecules are found to undergo low barrier rotation about the CN bond with a planar CNO₂ moiety in agreement with HF/MP2/B3LYP/6-311G(d,p) calculations. The experimental data are consistent with a dynamic model using a potential function for the torsion of V = (V₆/2)(1 − cos 6τ). The major geometrical parameters (r_g and ) for the eclipsed form, obtained from least squares analysis of the data are as follows: r(NO₃) = r(NO₄) = 1.213(2) Å, r(CN) = 1.592(6) Å, r(CCl)_av = 1.749(1) Å, Cl₅CN/Cl₆CN = 109. 6°/106.3°(2), O₃NC/O₄NC = 117. 6°/114.1°(4), τCl₅C₁N₂O₃ = 0.0°, and V₆ = 0.20(25) kcal/mol.     

The structure of 1-methyl-1-silaadamantane as determined by gas phase electron diffraction

The structure of 1-methyl-1-silaadamantane (MSA) has been determined by gas phase electron diffraction. There appears to be somewhat less ring strain at the silicon bridgehead of MSA than in the previously studied 1-methyl-1-silabicyclo[2.2.1]heptane (MSBH). The average SiC bond length [1.879(3) Å is comparable to those found in acyclic organosilicon systems. Also, the average CC bond length (1.547(2) Å) is only slightly longer than that observed for adamantane (1.540(2) Å). Valence angles at the silicon bridgehead experience only a moderate perturbation away from their unstrained tetrahedral values. On this basis it is expected that MSA should be somewhat less reactive than MSBH under S_N2 conditions according to the reaction mechanism suggested by L.H. Sommer.     

3-chloro-1-butene: gas-phase molecular structure and conformations as determined by electron diffraction and by molecular mechanics and ab initio calculations

Gaseous 3-chloro-1-butene has been studied experimentally by electron diffraction (ED) at 20 and 180°C, and at these temperatures, 76(10)% and 62(10)%, respectively, of the most stable conformer i.e. the one having a hydrogen atom eclipsing the double bond, were found. The conformer with the chlorine atom eclipsing the C=C bond was also present. However, from the experimental data it was not possible to establish conclusive evidence for the conformer with an eclipsed CH₃ group. Molecular mechanics (MM) calculations and ab initio calculations using a 4-21 basis set were carried out with complete geometry optimization, and calculated parameters from each of the methods were used in combination with the ED data. Such calculations indicated the existence of all three conformers mentioned above. Least-squares analysis including constraints from the ab initio calculation gave as a result the following molecular structure (r_a distances and ??? angles) for the predominant conformer: r(C=C) = 1.337(6) Å, r(=C---C) = 1.503(4) Å, r(C---CH₃) = 1.522 Å, R(C---Cl) = 1.813(4) Å, <r(C---H)> = 1.089(18) Å, ???C=C---C = 122.9(2.1)°, ???C---C---C = 112.6(2.2)°, ???=C---C---Cl = 109.9(0.2)°, ???Cl---C---CH₃ = 109.3°. = 121.9° and = 110.0(1.3)°. The torsional angles were then τ(C=C---C---Cl> = −119.4° and τ(C=C---C---CH₃) = 120.3(2.1)°. Error limits are 2σ (σ includes estimates of systematic errors and correlations), parameters without quoted uncertainties are dependent or were constrained relative to another parameter. Combining the ED data with MM results yielded parameters consistent with those given above.     

The structure of trimethoxymethane in the gas phase,an electron diffraction,ab initio and molecular mechanics study

The structure of trimethoxymethane in the gas phase was studied by electron diffraction, ab initio molecular orbital calculations and molecular mechanics. The molecule was found to exist almost exclusively as an asymmetric all-staggered TGG conformer. The electron diffraction structural parameters (r_g distances, r_α angles) as obtained from geometrically consistent r_α-refinements are: r(C-O) central 1.382(6) Å, r(C-O) terminal 1.418(6) Å, r(C-H) 1.112(1) Å, ∠(O-C-O) in the gauche—gauche chain 115.0(1.0)°, in the gauche-anti chains 109.2(0.6)° ∠(C-O-C) 114.3(0.6)°, ∠(O-C-H)_Me 109.9(0.3)°, methyl torsion 68(6)°. The structure is adequately reproduced by molecular mechanics calculations applying Allinger's force field. The structures of methoxymethanes can be explained in terms of the anomeric effect. This is confirmed by ab initio calculations.     

The molecular structure of S-triazine in the gas phase determined from electron diffraction, infrared/raman data and ab initio force field calculations

The gas phase molecular structure of s-triazine has been determined from electron diffraction data. Experimental vibrational parameters proved consistent with those from the 4-21G force field after scaling onto infrared/Raman frequencies, as well as after direct scaling on electron diffraction data. The analysis resulted in the following r_g/r^°-parameters CN = 1.338(1) Å, CH = 1.106(8) Å, CNC = 113.9(1), NCN = 126.1, HCN = 116.9. The (new) r_g – r_e (4-21G) correction for aromatic CN is 0.006(1) Å.     

The molecular structure and the puckering potential function of silacyclobutane as determined by gas electron diffraction and relaxation constraints from ab initio calculations

Gas electron diffraction is applied to determine the geometric parameters of the silacyclobutane molecule using a dynamic model where the ring puckering was treated as a large amplitude motion. The structural parameters and the parameters of the potential function were refined taking into account the relaxation of the molecular geometry estimated from ab initio calculations at the MP2/6-311+G(d, p) level of theory. The potential function has been described as V() = V₀[(/_e)² − 1]² with the following parameters V₀ = 0.82 ± 0.60 kcal/mol and _e = 33.5 ± 2.7°, where is a puckering angle of the ring.

The geometric parameters at the minimum V() (r_a in Å, in degrees and uncertainties given as three times the standard deviations including a scale error) are: r(Si–H_ax) = 1.467(96), r(Si–H_eq) = 1.468(96), r(Si–C) = 1.885(2), r(C–C) = 1.571(3), r(C–H) = 1.100(3), CSiC = 77.2(9), HSiH = 108.3, SiCH_eq = 123.5(16), SiCH_ax = 111.9(16), CC₅H_eq = 118.4(24), CC₅H_ax = 112.3(24), HC₃H = 107.7, δ(HSiH) = 6.6, δ(HC₃H) = 7.0, where the tilts δ, HSiH, and HC₃H are estimated from ab initio constraints. The structural parameters are compared with those obtained for related compounds.     

The gas-phase molecular structure of 1,1-diethynylsilacyclobutane as determined by means of electron diffraction and ab initio calculations

As a continuation of our systematic investigation of the effect of substituents on the ring geometry and dynamics in silacyclobutanes and in order to explore the role of the silicon atom as a mediator for electronic interactions between the attached fragments, we studied the molecular structure of 1,1-diethynylsilacyclobutane (DESCB) by means of gas-phase electron diffraction and ab initio calculations. The structural refinement of the electron diffraction data yielded the following bond lengths (r_a) and bond angles (uncertainties are 3σ): r(Si–C)=1.874(2) Å, r(Si–C)=1.817(1) Å, (C–Si–C)=79.2(6)°, (C–Si–C)=106.5(6)°. The geminal Si–CC moieties were found to be bent outwards by 3.1(15)° and the puckering angle was determined to be 30.0(15)°. The evidently short Si–C bond length, which was also reproduced by the ab initio calculations, could be rationalized as being the consequence of the electronic interaction between the outer π charges of the triple bond and the 3pπ orbitals at the silicon atom. It is also likely that the conjugation of the geminal ethynyl groups leads to an enhancement of this bond contraction. Electrostatic interactions and the subsequent reduction of the covalent radius of the silicon atom may also contribute to this bond shortening. It has been found that the endocyclic Si–C bond length fits nicely within a scheme describing a monotonous decrease of the Si–C bond length with the increase of the electronegativity of the substituent in various geminally substituted silacyclobutanes.A series of related silacyclobutanes and acyclic diethynylsilanes have been studied by applying various ab initio methods and their optimized structures were compared to the structure of DESCB. Among these compounds are 1,1-dicyanosilacyclobutane (DCYSCB), which is isoelectronic to DESCB, 1,1-diethynylcyclobutane (DECB) which is isovalent to DESCB, monoethynylsilacyclobutane (MESCB) and monocyanosilacyclobutane (MCYSCB). Searching for reasonable support for the explanation of the structural results of DESCB we performed detailed natural population analysis as well as Mulliken population analysis (MPA) on DESCB and other related molecules. In contrast to the Mulliken charges, the natural atomic charges provided helpful information concerning the bonding properties in DESCB and the corresponding compounds. By varying the size of some basis sets, we could demonstrate the validity of the repeatedly discussed dependency of the Mulliken MPA on the basis set.For the performance of the quantum mechanical calculations we employed the following methods and basis sets: HF/6-31G(d,p), DFT/B3PW91/6-31G(d), DFT/B3PW91/6-311++G(d,p), MP2/6-31G(d,p) and MP2/6-311++G(d,p).     

Molecular structure of 1,1-difluoroethylene as determined by gas phase electron diffraction and microwave data

The structure of 1,1-difluoroethylene was determined, from gas phase electron diffraction data obtained independently in Leiden and Tokyo and the rotational constants of F₂CCH₂, F₂CCHD and F₂CCD₂ derived from the microwave study by Chauffoureaux. The two electron diffraction data agreed without significant discrepancy. From a joint least squares analysis of the diffraction and microwave data, the following r_g bond distances and r_z bond angles were derived: CC = 1.340 ± 0.006 Å, C-F = 1.315 ± 0.003 Å, C-H = 1.091 ± 0.010 Å, ∠C-C-F = 124.7 ± 0.3°, ∠C-C-H = 119.0 ± 0.4°, where the uncertainties represent estimated limits of error.     

The molecular structure of norbornene as determined by electron diffraction and microwave spectroscopy

The molecular structure of norbornene has been investigated in the gas phase by combining electron diffraction data with microwave spectroscopic rotational constants. The interatomic distances (r_g) and bond angles were obtained by applying a least squares program to the refined experimental molecular diffraction intensities. The CC bond length was found to be 1.336 ± 0.002 Å while the

) bond length was 1. 529 ± 0.007 Å. Other bond lengths and angles included (IUPAC numbering system was used for norbornene): C₁-C₆ = 1.550 ± 0.020 Å, C₁-C₇ = 1.566± 0.005 Å, C₅-C₆ = 1.556 ± 0.005 Å, C-H_ave. = 1.103 ± 0.003 Å, ∠C₁C₂C₄ = 95.3°. The dihedral angle between planes C₁C₂C₃C₄ and C₁C₆C₅C₄ is 110.8 ± 1.5° while that between C₁C₂C₃C₄ and C₁C₇C₄ is 122.3°. The moments of inertia calculated from ED structure are in good agreement with microwave spectroscopic values.     

The molecular structure of tetramethoxysilane in the gas phase,an electron diffraction study

The molecular structure of tetramethoxysilane was determined in the gas phase by electron diffraction. The molecule has S₄ symmetry, slightly flattened along the axis. The SiO bonds are shorter than in methylsilylether, demonstrating the effect of electronegative substituents on the Si atom. The geometrical parameters (r_a structure) are: Si-O bond 1.613 Å; C-O bond 1.414 Å; C-H bond 1.12 Å; O-Si-O angle bisected by S₄ axis 115.5°; Si-O-C angle 122.3°; O-C-H angle 111°; methoxyl torsional angle 64°; methyl torsional angle 60°.