Molecular structure of carbonyl fluoride as studied by gas electron diffraction and microwave data

The molecular structure of carbonyl fluoride has been determined by electron diffraction. The results have been used in conjunction with the rotational constants reported by Carpenter in a combined structure analysis. The values so obtained are r_z (C=O) = 1.1717 ± 0.0013 Å, r_z (C-F) = 1.3157 ± 0.0005 Å, and ∠_zF-C-F = 107.71 ± 0.08°. These agree with the corresponding parameters estimated by Carpenter from the rotational constants alone. The effective constants, α₃, representing the cubic anharmonicity of bond stretching vibrations have been estimated.     

The structure of 1,3-dichloropropyne as determined by gas electron diffraction and comparison with microwave data

The structure of 1,3-dichloropropyne has been studied by gas electron diffraction. The resulting parameters r_a have been converted into r_α^o distances. A geometrical structure has been fitted to these internuclear distances. Thus the following parameters (r_α^o) have been determined: r(C₁-Cl₁) = 1.629 (10) A, r(C₁C₂) = 1.201 (13) Å, r(C₃-Cl₂) = 1.791 (6) A, ∠(C₂-C₃-Cl₂) = 111.1° (1.0°), ∠(H-C₃-H) = 98.8° (3.1°), ∠(C₂-C₃-H) = 108.7° (3.2°). ∠(Cl₁-C₁C₂) = 176.6° (1.1°), ∠(C₁C₂-C₃) = 182.7° (1.4°). Inconsistencies have been detected between our results and the rotational constants reported by Günther and Zeil. Discussion of the problem including rotational constants of the first excited vibrational state leads to the conclusion that the observed discrepancies are due to temperature effects.     

Molecular structure of bis(acetylacetonato)beryllium in the gas phase as determined from electron diffraction data

The molecular structure of bis(acetylacetonato)beryllium has been determined by gas electron diffraction. The experimental data were found to be consistent with the D_2d model in which the oxygen atoms are arranged tetrahedrally around the central beryllium atom (∠OBeO = 106.0 ± 1.0°). The structural parameters are as follows: r_g(Be-O) = 1.615 ± 0.006 Å, r_g (C-O) = 1.270 ± 0.004 Å, r_g (C-C_ring) = 1.397 ± 0.004 Å, r_g (C-C_meth) = 1.499 ± 0.005 Å. The mean amplitudes of vibration were calculated from the normal-vibration treatment using the modified Urey—Bradley force field     

Molecular structure of 1,1-dichloroethylene as studied by gas electron diffraction and microwave data. Estimation of equilibrium structure by an extended anharmonic model analysis

The r_z structure of 1,1-dichloroethylene has been determined by a joint analysis of the electron diffraction intensity and the rotational constants as follows: $\text{r}{}_{\mathrm{<mtext>z</mtext>}}\text{(}\text{CH}\text{) = 1.088 ± 0.011, r}{}_{\mathrm{<mtext>z</mtext>}}\text{(}\text{CC}\text{) = 1.329 ± 0.003, r}{}_{\mathrm{<mtext>z</mtext>}}\text{(}\text{CCl}\text{) = 1.725 ± 0.002}\text{A}\text{?}\text{, ∠}{}_{\mathrm{<mtext>z</mtext>}}\text{HCH}\text{= 121.4 ± 0.7}\text{and}\text{∠}{}_{\mathrm{<mtext>z</mtext>}}\text{ClCCl}\text{= 114.1 ± 0.2°}$. The uncertainties represent estimated limits of error. The observed structural parameters are compared with those for related compounds and the systematic trends in the bond lengths and bond angles are discussed. The effective constants representing anharmonicity have been obtained from an analysis of the isotopic differences in the r_z structure. By using the r_z parameters and the effective constants, the equilibrium structure has been estimated as follows: $\text{r}{}_{\mathrm{<mtext>e</mtext>}}\text{(}\text{CH}\text{) = 1.079 ± 0.012, r}{}_{\mathrm{<mtext>e</mtext>}}\text{(}\text{CC}\text{) = 1.324 ± 0.005, r}{}_{\mathrm{<mtext>e</mtext>}}\text{(}\text{CCl}\text{) = 1.721 ± 0.003}\text{A}\text{?}\text{, ∠}{}_{\mathrm{<mtext>e</mtext>}}\text{HCH}\text{= 120.5 ± 0.8}\text{and}\text{∠}{}_{\mathrm{<mtext>e</mtext>}}\text{ClCCl}\text{= 114.0 ± 0.3°}$.     

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 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 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 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.     

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).     

Molecular structure of 3-methylthiophene studied by gas electron diffraction combined with microwave spectroscopic data

The molecular structure of 3-methylthiophene

has been determined by gas electron diffraction (GED) combined with microwave (MW) spectroscopic data. Ab initio calculations at the HF/3–21G* level were carried out and used as structural constraints in the data analysis. The torsional vibration of the methyl group was treated as a large-amplitude motion. The structural parameters were determined to be: r_g(S---C₂) = 1.719(2) Å, r_g(C₂=C₃) = 1.370(3) Å, r_g(C₃---C₆) = 1.497(6) Å, r_g(C₂---H) = 1.101(5) Å, CSC = 91.6(2)°, SC₂C₃ = 113.3(5)°, SC₅C₄ = 111.3(3)°, C₂C₃C₆ = 123.2(11)° and C₃C₆H = 112(2)°. The values of r(S---C₂) − r(S---C₅) and r(C₂=C₃) − r(C₄=C₅) were fixed at the 3–21G* value of 0.002Å. Parenthesized values are the estimated limits of error (3σ) referring to the last significant digit.     

Molecular structure of 3-methylthiophene studied by gas electron diffraction combined with microwave spectroscopic data

The molecular structure of 3-methylthiophene has been determined by gas electron diffraction (GED) combined with microwave (MW) spectroscopic data. Ab initio calculations at the HF/3–21G* level were carried out and used as structural constraints in the data analysis. The torsional vibration of the methyl group was treated as a large-amplitude motion. The structural parameters were determined to be: r_{g(S---C2) = 1.719(2) Å}, r_{g(C2=C3) = 1.370(3) Å}, r_{g(C3---C6) = 1.497(6) Å}, r_{g(C2---H) = 1.101(5) Å}, CSC = 91.6(2)°, SC₂C₃ = 113.3(5)°, SC₅C₄ = 111.3(3)°, C₂C₃C₆ = 123.2(11)° and C₃C₆H = 112(2)°. The values of r(S---C₂) - r(S=C₅) and r(C₂=C₃)-r(C₄ =C₅) were fixed at the 3–21G* value of 0.002 Å. Parenthesized values are the estimated limits of error (3σ) referring to the last significant digit.     

Molecular structure of bis(acetylacetonato)nickel(II) in the gas phase as determined from electron diffraction data

The molecular structure of bis(acetylacetonato)nickel(II) has been determined by a sector-microphotometer gaseous electron-diffraction method. The experimental data were found to be consistent with a monomeric square-planar structure. The structural parameters of the chelate were determined as follows: ∠ ONiO = 93.6 ± 1.1°, r_g(Ni-O) = 1.876±0.005A Å, r_g(C-0) = 1.273± 0.007 Å, r_g(C-C_ring) = 1.401 ± 0.010 Å, r_g(C-C_methyl) = 1.504 ± 0.013 Å. The mean amplitudes of vibration and the shrinkage effects were calculated from normal-vibration treatment using the Urey-Bradley force field.     

Molecular structure and conformational composition of gaseous 1,1-dichloro-2-bromomethyl-cyclopropane and 1,1-dichloro-2-cyanomethyl-cyclopropane as determined by electron diffraction

The molecular structure of the title compounds have been investigated by gas-phase electron diffraction. Both molecules exist as about equal amounts of the two gauche conformers. There is no evidence for the presence of a syn conformer, but small amounts of this form cannot be excluded. Some of the important distance (r_a) and angle (∠_α) parameters for 1,1-dichloro-2-bromomethyl-cyclopropane are: r(CH) = 1.095(19) Å, r(C₁C₂) = 1.476(11) Å, r(C₂C₃) = 1.517(31) Å, r(CCH₂Br) = 1.543(32) Å, r(CCl) = 1.752(6) Å, r(CBr) = 1.950(13) Å, ∠CCBr = 110.5(1.9)°, ∠ClCCl = 111.9(6)°, ∠CCC = 117.5(1.3)°, σ₁ (CC torsion angle between CBr and the three-membered ring for gauche-1) = 116.2(5.6)°, σ₂ = −132.7(7.6). For 1,1-dichloro-2-cyanomethyl-cyclopropane the parameter values are: r(CH) = 1.101(16) Å, r(C₁C₂) = 1.498(9) Å, r(C₂C₃) = 1.544(21) Å, r(C₂C₄) = 1.497(33) Å, r(CCN) = 1.466(26) Å, r(CN) = 1.165(8) Å, r(CCl) = 1.754(5) Å, ∠CCCN = 113.7(2.0)°, ∠CCC = 122.8(1.6)°, ClCCl = 112.5(4)°, σ₁ = 113(13)°, σ₂ = −124(10)°.     

Molecular structure and conformation of cis-1,3- dichloro-1-propene as determined by gas phase electron diffraction

The molecular structure and conformation of cis-1,3-dichloro-1-propene have been determined by gas phase electron diffraction at a nozzle temperature of 90°C. The molecule exists in a form in which the chlorine atom of the methyl group and the carbon-carbon double bond are gauche to one another. The results for the distance (r_g) and angle (∠_α) parameters are: r(C-H) = 1.078(10)Å, r(CC) = 1.340(5)Å, r(C-C) = 1.508(7)Å, r( =C-Cl) = 1.762(3)Å, r(C-Cl) = 1.806(3)Å, ∠Cl-C-C = 111.7°(1.8), ∠(CC-C) = 125.5°(1.5), ∠Cl-CC = 124.6°(1.6) and ∠H-C-Cl = 111°(5). The torsion-sensitive distances close to the gauche form can be approximated using a dynamic model with a quartic double minimum potential function of the form V(Φ) = V₀[1 + ($\text{Φ}\text{Φ}{}_0$⁴ - 2($\text{Φ}\text{Φ}{}_0$)²], where V_o = 1.1(8) kcal mol^?1 and Φ₀ = 56°(5) (Φ = 0 corresponds to the anti form).     

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.     

Molecular structure and conformation of gaseous vinyldimethylchlorosilane as determined by electron diffraction

The molecular structure of vinyldimethylchlorosilane has been determined by gas phase electron diffraction at room temperature. The least squares values of the bond lengths (r_g) and bond angles (∠_α) are : r(CH) = 1.086(6) Å, r(CC) = 1.347(5) Å, r(SiC=) = 1.838(6) Å, r(SiC) = 1.876(3) Å, r(SiCl) = 2.078(2) Å, ∠CCSi = 127.8° (1.2) and ∠=CSiCl = 107° (1). Models with pure syn form and a mixture of syn and gauche gave equally good agreement with the diffraction data.     

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 of cyclohexanone determined by gas-phase electron diffraction,including microwave data

The electron diffraction data for gaseous cyclohexanone, collected at 371 K, combined with microwave rotational constants, can be explained by a single chair conformation. Least-squares analysis of the observed data led to an r_g, r_α-structure with the following geometrical parameters: r_CO = 1.229 Å, r_C1C2 = 1.503 Å, r_C1C2 = 1.542 Å, r_C3C4 = 1.545 Å, r_CH = 1.088 Å, ∠ C-CO-C = 115.3°, ∠ CO-C-C = 111.5°, ∠ C-C-C = 110.8°, ∠ H-C-H = 106°. The sp₂ -hybridized part of the ring is less puckered, whereas the sp³ part is more puckered than in cyclohexane.     

The molecular structure of trifluoroethene in the gas-phase as determined from electron diffraction and microwave data

The molecular structure of trifluoroethene was determined from electron diffraction data and the microwave rotational constants of the parent and deuterated molecule, corrected for zero-point vibrational motion. A GVFF adjusted to fit the vibrational frequencies was used for the correction. The molecule was found to be planar. Assuming equal geminal C1—F bond lengths, the following r_g distances and r_av angles are found: C1—F = 1.316 ± 0.011 Å, C2—F = 1.342 ± 0.024 Å, CC = 1.341 ± 0.012 Å, C—H = 1.100 ± 0.02 Å, ∠C—C—F1 = 123.1 ± 1.5°. ∠C—C—F2 = 124.0 ± 0.6°, ∠C—C—F3 = 120 ± 0.7° (Fl trans to F3) and ∠C—C—H = 124.0 ± 1.7°.The error limits include 3σ (σ = estimated standard deviation) and estimates of the systematic errors. The analysis suggests that all the C1—F distances are not equivalent, neither are the C2—C1—F angles, though the differences are not significant (10% level).