Gas-phase Electron Diffraction Study Research Articles

Tetrakis(dimethylamido)vanadium(IV).

The title compound, [V(C(2)H(6)N)(4)], (I), has non-crystallographic D(2d) molecular symmetry and contains an approximately tetrahedrally coordinated V atom with dimethylamido ligands. Each N atom features a nearly trigonal planar geometry. There are two independent molecules of (I) in the asymmetric unit. The results are compared with those previously reported for gas-phase electron-diffraction studies [Haaland, Rypdal, Volden & Andersen (1992). J. Chem. Soc. Dalton Trans. pp. 891-895].

Low-temperature crystallization and X-ray structure determination of hexafluoropropene at 95 K

Crystals of hexafluoropropene, C3F6, melting point 115 K, were obtained by low temperature crystallization on the diffractometer in a cold N2 gas stream. The crystal structure was solved based on a dataset of 1347 reflections ((sinθ/λmax = 0.66 Å-1 ) measured at 95 K. C3F6 crystallizes in the triclinic space group P-1 (No. 2), a = 6.072(6) Å, b = 6.233(5) Å, c = 6.569(6) Å, α = 92.33(5)°, β = 108.75(7)°, γ = 94.31(5)°, V = 234.2(6) Å3, Z = 2, R1 = 0.050 for 729 reflections ( Fo 2 > 2σ(Fo 2)). The molecular structure has almost Cs symmetry with all atoms in a common plane except two fluorine atoms of the CF3 group. The fluorine atom of the CF3 group in the molecular plane is in trans position to the fluorine atom bonded to the central carbon atom. The geometrical parameters obtained from X-ray data are compared to those from an earlier gas phase electron diffraction study and, moreover, to those, obtained by theory from different high level ab initio calculations which were carried out as well for hexafluoropropene as for propene itself.

Density functional studies of molecular structures of N-methyl formamide, N,N-dimethyl formamide, and N,N-dimethyl acetamide

Density functional theory was applied to the calculation of molecular structures of N-methyl formamide (NMF), N,N-dimethyl formamide (DMF), and N,N-dimethyl acetamide (DMA). DFT calculations on NMF, DMF, and DMA were performed using a combination of the local functional of Vosko, Wilk, and Nusair (VWN) with the nonlocal exchange functional of Becke and the nonlocal correlational functional of Lee, Yang, and Parr (BLYP). The adiabatic connection method (ACM) of Becke has also been used, for the first time, for the calculation of molecular structures of NMF, DMF, and DMA. The calculated molecular structures are in excellent agreement with the experimental geometries of NMF and DMA derived from gas-phase electron-diffraction studies. Sparse experimental data on the gas-phase geometry of DMF reported in the literature compares well with the DFT results on DMF. DFT emerges as a powerful method to calculate molecular structures.

Molecular structure of MnI2: A gas-phase electron diffraction study

The molecular structure of MnI2 has been determined by gas-phase electron diffraction. The analysis confirmed the linearity of the equilibrium configuration of the monomer with a bond length (rg) of 2.538 ± 0.008 A. The presence of about 7% dimer was also indicated. The experimental data were consistent with the usual dimer model with two halogen bridges but they were insufficient to determine the dimer structure unambiguously.

Vibrational Overtone Spectroscopy of Cycloheptatriene Chromium Tricarbonyl, Benzene Chromium Tricarbonyl, and Corresponding Hydrocarbon Ligands

The vibrational overtone spectra of gaseous cycloheptatriene chromium tricarbonyl (CHTCT) and benzene chromium tricarbonyl (BCT) were recorded at the third overtone region using photoacoustic spectroscopy and compared to the spectra of the nonbonded ligands, cycloheptatriene (CHT) and benzene. Analysis of the spectra leads to the conclusion that the metal−ligand bonding lengthens the CH bond by 0.007 Å in the cycloheptatriene complex and by 0.003 Å in the benzene complex. The spectrum of BCT exhibits an additional transition which is absent in the spectrum of free benzene. The experimental peaks are assigned to the pure CH overtones and combination bands. The structural information obtained from the spectra is discussed and compared to the results of gas-phase electron diffraction and microwave spectroscopic studies. Analysis of experimental data indicates that μ6-bonding is more favorable for cycloheptatriene chromium tricarbonyl.

Ab initio conformational analysis of cyclooctane molecule

The potential energy surface (PES) for the cyclooctane molecule was comprehensively investigated at the Hartree–Fock (HF) level of theory employing the 3–21G, 6–31G, and 6–31G* basis sets. Six distinct true minimum energy structures (named B, BB, BC, CROWN, TBC, and TCC1), characterized through harmonic frequency analysis, were located on the multidimensional PES. Two transition state structures were also located on the PES for the cyclooctane molecule. Electron correlation effects were accounted for using the Moller–Plesset second-order perturbation theory (MP2) approach. The predicted global minimum energy structure on the ab initio PES for the cyclooctane molecule is the BC conformer. A gas phase electron diffraction study at 300 K suggested a conformational mixture while an NMR study in solution at 161.5 K predicted the BC conformer as the predominant form. The equilibrium constants reported in the present study, which were evaluated from the ab initio calculated total Gibbs free energy change values, were in good agreement with both experimental investigations. The ab initio results showed that the low temperature condition significantly favored the BC conformer while above room temperature both BC and CROWN structures can coexist. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 524–534, 1998

Structure and conformation of cyclopentene, cycloheptene and trans-cyclooctene

The molecular geometries of different conformations of cycloalkenes, C n H 2 n-2 , with n = 5, 7 and 8 were optimized by restricted Hartree-Fock calculations using the 6–31G∗ basis set followed by second-order Møller-Plesset perturbation theory (MP2) treatment of electron correlation. For cyclopentene, C 5H 8, the potential function for the ring-puckering motion was constructed, followed by solving for the vibrational eigenvalues in terms of distributed Gaussian bases. Good agreement was obtained with the observed frequencies in the far-infrared spectrum of the molecule. For cycloheptene, C 7H 12, geometries were optimized for the chair, boat, twist-boat, and three possible transition states. The chair-chair interconversion mechanism was investigated and compared with available experimental evidence and with the results of a previous molecular mechanics calculation. The computed potential barrier compares well with NMR evidence, but the conformation of the relevant transition state is found to be different from the one assumed in the experimental study. The structure of the smallest isolable trans-cycloalkene, trans-cyclooctene, C 8H 14, was optimized, yielding a structure in reasonable agreement with a previous gas phase electron diffraction study. The agreement includes the pyramidality of the olefinic carbon atoms which was also compared with available X-ray data on related compounds. The bond angles and torsion angles were in better agreement with the experiment than were those obtained in earlier molecular mechanics studies, although it is remarkable how well that method works for these highly strained cyclic systems.

Molecular geometry of 2-nitrotoluene from gas phase electron diffraction and quantum chemical study

The molecular geometry of 2-nitrotoluene has been determined by gas phase electron diffraction and quantum chemical computations at the MP2/6–31G∗ and Becke3-Lee-Yang-Parr (B3-LYP)/6–31G∗ levels of theory. Computed differences in CC bond lengths were utilized as constraints in the electron diffraction structure analysis. The scaled B3-LYP/6–31G∗ force field was used to generate the initial set of vibrational amplitudes. The electron diffraction study yielded the following bond lengths ( r g) and bond angles: C 1C 2, 1.405(8) Å; NO, 1.231(3) Å; C 1C 7, 1.508(8) Å; CN, 1.490(9) Å; C 7C 1C 2, 127.3(7)°; NC 2C 3, 113.8(6)°; C 1C 2C 3, 124.2(9)°; C 6C 1C 2, 114.8(6)°; C 5C 6C 1, 123.1(10)°; O-N-O, 124.9(3)°; ϕ(CN), 38(1)°. The structural features of the molecule point to steric interactions prevailing between the methyl and nitro groups.

Molecular structure of trimethylamine–gallane, Me3N·GaH3: ab initio calculations, gas-phase electron diffraction and single-crystal X-ray diffraction studies †

The structure of the gallane adduct Me3N·GaH3 has been investigated by ab initio quantum chemical calculations. The results of gas-phase electron-diffraction (GED) measurements, together with earlier microwave measurements, have been reanalysed using the SARACEN method to determine the most reliable structure of the gaseous molecule. Salient structural parameters (rαo structure) were found to be: r(Ga–H) 151.1(13), r(Ga–N) 213.4(4), r(N–C) 147.6(3), r(C–H) 108.4(4) pm; H–Ga–N 99.3(8) and Ga–N–C 108.8(2)°. Unlike the corresponding alane derivative, the adduct is monomeric in the crystalline phase with dimensions very close to those of the gaseous molecule, as confirmed by a redetermination of the structure of a single crystal at 150 K.

Intramolecular Motion and Molecular Structure of N-Nitropyrrolidine: A Gas-Phase Electron Diffraction and Ab Initio Molecular Orbital Study.

Intramolecular Motion and Molecular Structure of N-Nitropyrrolidine: A Gas-Phase Electron Diffraction and Ab Initio Molecular Orbital Study.

Molecular structures of tetraborane(10) derivatives: ab initio calculations for (CH3)2MB3H8 (M = B, Al, Ga or In) and gas-phase electron diffraction studies of (CH3)2AlB3H8 and (CH3)2GaB3H8

Structural trends in the family of compounds (CH3)2MB3H8 (M = B, Al, Ga or In) have been investigated by ab initio molecular orbital calculations. In addition, the gas-phase molecular structures of (CH3)2AlB3H8 and (CH3)2GaB3H8 have been re-determined by gas-phase electron diffraction using the SARACEN method of structural analysis. Salient structural parameters (rα0) for the aluminium and gallium compounds were found respectively to be: r[B(1)· · ·M(2)] 231.6(7), 234.2(8); r[B(1)–B(3)] 178.2(12), 178.9(23); r[B(1)–B(4)] 184.4(10), 184.3(23); r[B(1)–H(1,2)] 124.6(11), 121.6(18); r[M(2)–H(1,2)] 182.5(13), 186(6); r[B(1)–H(1,4)] 126.2(11), 122.9(18); r[B(4)–H(1,4)] 142.6(11), 140(3) pm; butterfly angle 123.8(20), 119.8(13)°.

Molecular structures of tetraborane(10) derivatives: ab initio calculations for H2MB3H8 (M = B, Al, Ga or In) and gas-phase electron diffraction study of H2GaB3H8

Structural trends in the family of compounds H2MB3H8 (M = B, Al, Ga or In) have been investigated by ab initio molecular orbital calculations. In addition, the molecular structure of the arachno borane H2GaB3H8 has been re-determined by gas-phase electron diffraction using the SARACEN method of structural analysis. Salient structural parameters (rα0) were found to be: r[B(1)· · ·Ga(2)] 231.0(2), r[B(1)–B(3)] 177.9(13), r[B(1)–B(4)] 184.0(13), r[B(1)–H(1,2)] 123.0(11), r[Ga(2)–H(1,2)] 181(4), r[B(1)–H(1,4)] 123.7(11), r[B(4)–H(1,4)] 142.2(18) pm; butterfly angle 117.1(7)°.

Microwave spectrum, molecular structure, and ab initio calculation of ( E)-chloroacetaldehyde oxime

The microwave spectra of ( E)- 35ClCH 2CHNOH, ( E)- 37ClCH 2CHNOH, ( E)- 35ClCH 2CHNOD, and ( E)- 37ClCH 2CHNOD have been observed in the frequency range from 26.4 to 40.5 GHz. The rotational and centrifugal distortion constants of the four isotopic species in the ground and excited vibrational states were determined. The values of the ΔI( = I c − I a − I b ) obtained for the normal ( 35Cl and 37Cl) and deuterated ( 35Cl and 37Cl) species in the ground vibrational state were found to be − 16.350(4), − 16.36(4), − 17.06(4), and − 17.07(6) uÅ 2, respectively. The molecular structure of this molecule was determined to be the anticlinal form ( ф 1:∠ ClCCN = 121.4° , ф 2:∠ CNOH = 180.0° ), as shown in Fig. 1(a). It was found that ab initio calculation of MP2 6-31 G∗∗ level can be used to predict the most stable rotational conformer of ( E)-ClCH 2CHNOH. The seven structural parameters of the heavy atom skeleton of this molecule were fitted to the eight rotational constants ( B and C) of the four isotopic species. The obtained structural parameters ( r 0) are almost the same as those ( r α ) obtained previously by a gas-phase electron diffraction study (K. Iijima, T. Miwa, T. Sakaizumi, O. Ohashi, J. Mol. Struct. 352/353 (1995) 161), within the errors. The r(CN and ∠CCN obtained for ( E)-ClCH 2CHNOH are slightly shorter and drastically narrower than those of ( Z)-ClCH 2CHNOH. The trend is consistent with those of CH 3CHNOH and CH 3CH 2CHNOH.

Thermal Conversion ofcloso−1,2-(SiMe3)2−1,2-C2B4H4 tocloso-1,6-(SiMe3)2−1,6-C2B4H4: Structure Determination by Ab Initio Calculations, Gas-Phase Electron Diffraction, and Low-Temperature X-Ray Diffraction

closo−1,2-(SiMe3)2−1,2-C2B4H4 undergoes thermal conversion to 1,6-(SiMe3)2−1,6-C2B4H4. The reaction pathway was monitored by 11B NMR spectroscopy. The structures of the 1,2- and 1,6-isomers were optimized at the HF/6-31 G* ab initio level. Gas-phase electron diffraction studies for both isomers are reported, as well as low-temperature X-ray crystal structure determinations. Comparison of calculated structural data with the data obtained experimentally shows good agreement between theory and experiment.

Microwave Spectrum and Molecular Structure of (Z)-Chloroacetaldehyde Oxime

The microwave spectra of (Z)-35ClCH2CH=NOH, (Z)-37ClCH2CH=NOH, (Z)-35ClCH2CH=NOD, and (Z)-37ClCH2CH=NOD have been observed in the frequency range from 16.5 to 40.0 GHz. The rotational and centrifugal distortion constants of four isotopic species and the nuclear quadrupole coupling constants of the35Cl species were determined. The values of planar moment (Pcc= (Ia+ Ib− Ic)/2) obtained for four isotopic species were found to be 1.761(14), 1.800(18), 1.777(19), and 1.770(18) u Å2in the ground vibrational state. The unchanged values ofPccshow that the chlorine and hydroxyl hydrogen atoms lie in or are very close to theabinertial plane of the molecule. The conformation of theZ-form was determined to be Calcd I (φ1= 180°, φ2= 180°), as shown in Fig. 1, from the comparison of the observed and calculated rotational constants and coordinates of the chlorine and hydroxyl hydrogen atoms. The determined conformation is consistent with that reported by an electron diffraction study (K. Iijima, T. Hanamori, T. Sakaizumi, and O. Ohashi,J. Mol. Struct.299,149–153 (1993).). One excited vibrational state is assigned to the C–C torsional mode, and it is anharmonic. This fact is also consistent with the result of a large amplitude motion about the C–C single bond reported by a gas-phase electron diffraction study. Assuming the skeleton of the heavy atoms is a planar molecule, the seven structural parameters of theZ-form (anti-form) were fitted to the eight rotational constants (BandC) of four isotopic species. The obtained structural parameters (r0) are almost the same as those obtained by electron diffraction study (rα) within the errors.

Gas phase electron diffraction study of the hafnium perchlorate molecule

The structure of the Hf(ClO4)4 molecule was investigated by electron diffractometry in the gas phase. The model with D4 symmetry is in best agreement with experimental data; it contains ClO4 fragments in the form of distorted tetrahedra bridged by two oxygen atoms to the central Hf atom.

Molecular Structures of Sulfur Cyanide Trifluoride, SF(3)CN, and Sulfinyl Cyanide Fluoride, FS(O)CN.

General valence force fields for SF(3)CN and FS(O)CN are derived from vibrational data taken from the literature and from theoretical calculations. Gas phase electron diffraction studies on both molecules yield the following geometric parameters (r(a) distances and angles with 3sigma uncertainties). SF(3)CN: r(S-F(e)) = 155.2(4) r(S-F(a)) = 165.7(3), r(S-C) = 173.6(8), r(C&tbd1;N) = 115.9(4) pm; angle(F(a)SF(e)) = 86.9(3), angle(F(a)SC) = 86.0(4) angle(F(e)SC) = 98.7(8), angle(F(a)SF(a)) = 169.0(6), angle(SCN) = 171(4) degrees. FS(O)CN: r(S-F) = 159.8(3), r(S=O) = 143.2(2), r(S-C) = 178.3(3), r(C&tbd1;N) = 115.0(3) pm; angle(FSO) = 104.9(4), angle(FSC) = 93.9(4), angle(CSO) = 105.3(5), angle(SCN) = 176(4) degrees. These experimental results are compared to ab initio values (HF/3-21G, HF/6-31G, and MP2/6-31G), and the bonding properties in these sulfur (IV) cyanides are discussed.

Intramolecular Hydrogen Bonding and Molecular Structure of 2,5-Dihydroxyterephthalaldehyde and 4,6-Dihydroxyisophthalaldehyde: A Gas-Phase Electron Diffraction and ab Initio Molecular Orbital Study

The molecular structure of 2,5-dihydroxyterephthalaldehyde has been determined from a joint electron diffraction/ab initio investigation, and the molecular structure of 4,6-dihydroxyisophthalaldehyde has been obtained from ab initio calculations at the MP2/6-31G* level. There is considerable intramolecular hydrogen bonding in these structures manifested by the O···H and O···O distances as well as by the structural changes in the rest of the molecule. These changes are consistent with the notion of resonance-assisted hydrogen bonding. The hydrogen bonding is somewhat stronger in 4,6-dihydroxyisophthalaldehyde than in 2,5-dihydroxyterephthalaldehyde, and this difference may be linked to the difference in the mutual positioning of the interacting formyl and hydroxy groups in these molecules.

The Structure of Hexamethyltungsten, W(CH3)6: Distorted Trigonal Prismatic with C3 Symmetry

Ab initio and density functional calculations show that the equilibrium structure of hexamethyltungsten is a distorted trigonal prism of C3 symmetry (with local C3v symmetry for the WC6 skeleton). A regular prismatic D3 structure (with D3h skeleton) is found to be ca. 20 kJ mol-1 higher in energy at correlated levels of theory. It is a transition state connecting two C3 minima. These results extend a recent gas-phase electron diffraction study which favored a regular prismatic structure but could not rule out a distortion to C3v. The failure of a previous theoretical study to locate the distorted minimum is due to the neglect of electron correlation and to some other restrictions during the structure optimizations. Correlation is important, e.g. for the description of hyperconjugative “agostic” C−H → W interactions which are found to be pronounced in W(CH3)6. Structures optimized with gradient-corrected or hybrid density functionals, or at the MP2 level, describe these interactions well. The observed single-line 13C and 1H NMR spectra are explained by dynamic motions due to the low D3 inversion and methyl rotation barriers. 13C chemical shifts calculated using density functional theory differ by ca. 18 ppm between the two nonequivalent sets of methyl groups in the distorted trigonal prismatic structure. Low-temperature NMR experiments could be useful to confirm this value and thus the distortion. Harmonic vibrational frequency analyses are consistent with experimental results and have been used to characterize stationary points on the potential energy surface. Differences between the structural preferences of W(CH3)6 and WH6 are investigated via detailed bonding analyses.

Conformational analyses of molecules and molecular clusters

Parallels are drawn between gas-phase electron diffraction studies of conformations of free molecules and of molecular clusters. A primary goal of cluster research is the measurement of nucleation rates in structural transformations. Advantages of cluster systems are outlined, and the significance of the results obtained is explained. Illustrative examples are given.