Phase Electron Diffraction Data Research Articles

Molecular structure of 6-cyclopropyl-1,5-diazabicyclo[3.1.0]hexane: gas phase electron diffraction and theoretical study

The molecular structure and conformational composition of 6-cyclopropyl-1,5-diazabicyclo[3.1.0]hexane were determined by gas phase electron diffraction and quantum chemical calculations. The gas phase electron diffraction data were well reproduced for the mixture of two conformers with anti-boat and gauche-boat mutual ring orientation having 15 and 85% relative abundance, respectively. The standard enthalpy of formation of substance under study was calculated using atomization reactions, yielding value of 307.9 ± 3.3 kJ mol-1 in gas phase.

Equilibrium molecular structure and spectra of 6-methyl-1,5-diazabicyclo[3.1.0]hexane: joint analysis of gas phase electron diffraction, quantum chemistry, and spectroscopic data.

The equilibrium geometry of the boat conformation (Cs point group symmetry) of the 6-methyl-1,5-diazabicyclo[3.1.0]hexane (MDABH) molecule, absolutely dominating under normal conditions, was studied by the gas-phase electron diffraction (GED) method at 20 °C with the involvement of NMR, IR, and Raman spectroscopic data and quantum chemical calculations. The potential function of ring-puckering deformation for the MDABH bicyclic system was calculated at the MP2/aug-cc-pVTZ and B3LYP/cc-pVTZ levels. It was found by MP2 calculation that the total energy of the boat conformation is 3.52 kcal mol-1 lower than that of the chair conformation. For the first time, we recorded the IR and Raman spectra for liquid samples of MDABH and assigned their peculiarities only to boat conformation vibrations using the Pulay technique of scaling quantum chemical force fields. In the case of the chair form, transferability of the refined scale factors was used for reliable prediction of the location of its fundamental frequencies. According to the joint structural analysis of the above data, the most important equilibrium geometric re-parameters for the boat conformation of the MDABH molecule were determined to be (bond lengths in Å; angles in degrees, Cs symmetry): C2N1 = 1.466(2), C2C3 = 1.523(2), N1N5 = 1.512(2), C6N1 = 1.440(2), C6C7 = 1.487(2), ∠C2N1N5 = 106.1(2), ∠N1C2C3) = 110.2(4), ∠C2C3C4 = 99.9(4), ∠N1N5C6 = 58.3(1), ∠N1C6N5 = 63.3(1), ∠N1C6C7 = 114.9(6), ∠C6N1C2 = 111.8(1), ∠N5N1C2C3 = 17.3(1), ∠N1C2C3C4 = -26.8(2), θ = C2C3C4/C2N1N5C4 = -26.2(3), φ = N1C6N5/C2N1N5C4 = 74.0(1). Comparison of these and earlier results showed that the NN bond length in the diaziridine ring is very weakly dependent on the cis- or trans-arrangement of substituents at the nitrogen atoms.

Equilibrium structures of the tetramezine diastereomers and their ratio: joint analysis of gas phase electron diffraction, quantum chemistry, and spectroscopic data.

For the first time, we applied a gas-phase electron diffraction (GED) method together with vibrational spectroscopy and quantum chemical calculations to investigate the equilibrium geometries of achiral meso and enantiomeric diastereomers of tetramezine [1,2-bis-(3,3-dimethyldiaziridin-1-yl)ethane] and their ratio in the mixture. In the joint structural analysis of these data, a new approach based on PES parameters is used in the framework of a static molecular model (small amplitude motion approximation). The agreement between the theoretical and experimental molecular intensities is characterized by a divergence factor Rf of 5.9%. The experimental re-parameters of tetramezine diastereomers agreed with our B3LYP/cc-pVTZ and MP2/cc-pVTZ calculations, which predicted the total energy of the meso form (Ci point group symmetry) to be lower than that of the enantiomeric form (C2 point group symmetry), by 6.4 and 4.7 kJ mol-1, respectively. The experimentally measured percentages of the meso and both enantiomeric diastereoisomers at 360 K were 70% and 30%, respectively. We characterized the meso form using 2D NMR spectra. Our GED data are in good agreement with the X-ray diffraction analysis of the meso form. This result reflects the weak effect of intermolecular interactions in the crystal. We assigned the IR spectrum bands of the crystalline meso form using the Pulay technique of scaling quantum chemical force fields. In the case of the enantiomeric form calculated at the same level, transferability of the refined scale factors was used for more reliable prediction of the mutual location and interpretation of its fundamental frequencies.

Combined Electron-Diffraction, Spectroscopic, and Theoretical Determination of the Structure of N-Deuterio- trans-Methyldiazene, CH3N═ND. Conformational Effects of the N═N Double Bond.

Gas phase electron-diffraction (GED) data obtained at a nozzle-tip temperature of 273 K have been combined with spectroscopic vibrational-rotational constants to determine the structure of trans-methyldiazene, an important prototype for the N═N double bond. The N-deuterio form CH3N═ND was used in the study since it is appreciably more stable than CH3N═NH. Both the theoretical and experimental results are consistent with a planar C s trans-CNND framework. The experimental results ( rα0/ rg273) are 1.465(2)/1.467(2) Å for the CN bond, 1.248(1)/1.251(1) Å for the N═N double bond, and 1.037(17)/1.048(17) Å for the ND bond. The NND angle is 105.9(20)/105.6(20)° and the CNN angle is 112.4(5)/112.2(5)°, where the uncertainties in parentheses are twice the standard deviation from a simultaneous least-squares fit of the GED and microwave data. For the methyl group, both theory and experiment indicate that two CH bonds are symmetrically arranged out of the molecular plane while the third CH' lies in the plane in an eclipsed (not staggered) cis-H'CNN arrangement. Theoretical calculations (B3LYP/cc-PVnZ and CCSD(T)/cc-PVnZ) suggest a slight distortion of the methyl group, with a tilt of the methyl top axis about 5° away from the N═N bond. The experimental data are consistent with this picture but are equally consistent with an undistorted methyl group. Inclusion of distortions predicted by theory in a complete basis set limit (CBS) lead to a preferred analysis with average values of 1.086(5)/1.106(5) Å for the CH bond length and an average HCH angle of 108.3(8)/107.8(8)°. Features of the structure of methyldiazene and related compounds are discussed. It is found that the short N═N bond length in the diazenes produces much greater steric repulsion than in analogous ethylene compounds and this effect leads to some interesting conformational and distortion differences for attached CH3 groups.

Internal rotation and equilibrium structure of the 2-methyl-2-nitropropane molecule from joint processing of gas phase electron diffraction data, vibrational and microwave spectroscopy data, and quantum chemical calculation results

The structure and internal rotation of the 2-methyl-2-nitropropane molecule is studied by electron diffraction and quantum chemical calculations with the use of microwave and vibrational spectroscopy data. The electron diffraction data are analyzed within the general intramolecular anharmonic force field model and the quantum chemical pseudoconformer model, considering the adiabatic separation of the degree of freedom of large amplitude motion, i.e., the internal rotation of the NO2 group. The equilibrium eclipsed configuration of the C s symmetry molecule has the following experimental bond lengths and valence angles: r e(N=O) = 1.226//1.226(8) A, r e(C–N)//r e(C–C) = 1.520//1.515/1,521(4) A, ∠еC–C–N = = 109.1/106,1(8)°, ∠еO=N=O = 124.2(6)°, ∠eC–C–Havg = 110(3)°. The equilibrium geometry parameters are well consistent with MP2/cc-pVTZ quantum chemical calculations and microwave spectroscopy data. The thermally average parameters previously obtained within the small vibration model show a satisfactory agreement with the new results. The electron diffraction data used in this work do not allow a reliable determination of the barrier to internal rotation. However, at a barrier of 203(2) cal/mol, which is derived from the microwave study, it follows from the electron diffraction data that the equilibrium configuration must correspond to an eclipsed arrangement of C–C and N=O bonds, which is also consistent with the results of quantum chemical calculations of various levels.

Structure and Energetics of β–Diketonates. XI. Molecular Structure of Sc(thd) 3 from Gas–Phase Electron Diffraction Data

Electron–diffraction and mass–spectrometric studies of saturated vapor of scandium tris–dipivaloyl–methanate showed that at 135(5)°, the vapor contains only monomeric Sc(thd)3, whose structural parameters r a , r g , and rα were determined. The internuclear distances in the chelate ring were found to be rαSc=O) = 2.066(5)A, rα(O=C) = 1.272(3)A, and rα(C=Cr) = 1.385(3)A. The ScO6 coordination polyhedron has a D3 symmetry configuration close to a regular antiprism. The angle of rotation of the O=O=O trigonal faces relative to their position in a regular prism is 25.7(1.5)°.

Equilibrium structure and large amplitude motion investigation of 1,4-disilacyclohexa-2,5-diene by means of electron diffraction, vibrational spectroscopic data, and ab initio calculations

Equilibrium and thermal average structure of 1,4-disilacyclohexa-2,5-diene (DSCHD) was determined by applying procedures that have been recently developed for joint treatment of gas phase electron diffraction and molecular spectroscopy data guided by ab initio calculations. Both large amplitude motion and anharmonic vibrational effects were taken into account. Similar structural results were obtained by considering DSCHD molecule as a unique equilibrium conformer and as a set of quasi-conformers describing large amplitude motion. The determined equilibrium structural parameters for DSCHD (planar configuration) are: r e ( Si– C)=1.861(2) A ̊ , r e ( CC)=1.346(3) A ̊ , r e ( Si– H)=1.498(10) A ̊ , r e ( C– H)=1.074(10) A ̊ , ∠(C–Si–C)=109.9(3)°, ∠(H–CC)=116.7(10)°, and ∠(H–Si–H)=107.9(10)°. Uncertainties given in parentheses include three times standard deviation and a systematic error.

1,2-Diiododisilane and 1,1,2,2-tetraiododisilane: a reinvestigation of the molecular structures and vibrational properties by gas-phase electron diffraction, temperature dependent Raman spectroscopy, and ab initio molecular orbital- and density functional calculations

The molecular structure, internal rotation and vibrational properties of 1,2-diiododisilane (DIDS), IH2Si–SiH2I, and 1,1,2,2-tetraiododisilane (TIDS), I2HSi–SiHI2, have been reinvestigated using gas phase electron diffraction (GED) data at temperatures of 55 (DIDS) and 155 (TIDS) °C, together with Raman spectroscopy, and ab initio molecular orbital (MO)- and density functional (DFT) calculations. The title compounds exist in the gas and liquid phases as a mixture of two conformers, anti (C2h), with a torsion angle φ (XSiSiX)=180°, and gauche (C2), with a torsion angle φ (XSiSiX) ≈60° (X=I (DIDS), X=H (TIDS)). Some structural parameter values obtained from the GED dynamic model refinements, using results from the theoretical calculations as constraints, were as follows (anti values with estimated 2σ uncertainties): For DIDS: Bond lengths (rg): r(Si–Si)=2.315(26) Å, r(Si–I)=2.447(6)Å. Bond angles (∠α): ∠SiSiI=107.5(10)°, ∠ISiH=109.8(5)°. For TIDS: Bond lengths (rg): r(Si–Si)=2.364(30)Å, r(Si–I)=2.450(6)Å, Bond angles (∠α): ∠SiSiI=107.2(4)°, ∠ISiH=109.9(2)°, ∠ISiI=111.7(3)°. Raman vibrational spectra for various temperatures are presented and analysed aided by normal coordinate calculations and the ab initio MO- and DFT results. From intensity variations with temperature of four band pairs for DIDS and two pairs for TIDS, conformational energies in the liquids were determined from van't Hoff plots. ΔH0(≈ΔE0) values of 0.31±0.14kcalmol−1 and 0.14±0.10kcalmol−1 were obtained for DIDS and TIDS respectively, gauche being the low energy conformer for both disilanes.

Improved quantum mechanical study of the potential energy surface for the bithiophene molecule

The potential energy surface (PES) for the 2,2′-bithiophene molecule was investigated using Hartree–Fock, correlated MP2, MP4(SDQ), CCSD, and density functional theory levels. Distinct basis sets ranging from double-zeta to triple-zeta quality, with polarization functions added on all atoms, were employed as well as the Dunning correlated consistent polarized valence double-zeta (cc-pVDZ) basis set. Single point configuration interaction CISD calculations were also performed using the cc-pVDZ basis set. Harmonic frequency calculations were performed for the unambiguous characterization of the stationary points located on the PES and also to calculate thermal Gibbs free energy corrections. Regarding the structural predictions we found that the B3LYP/6-311G** and MP2/cc-pVDZ fully optimized geometries exhibit the best agreement with the gas phase electron diffraction data. The calculated B3LYP/6-311G**, MP2/cc-pVDZ and experimental torsional angle for the syn-gauche structure are, respectively, 37.4° (B3LYP), 39.9° (MP2), and 36°±5° (expt.) with the corresponding values for the anti-gauche form being, respectively, 150.3° (B3LYP), 146.0° (MP2), and 148°±3° (expt.). The relative energy between the two minima and torsional barriers are sensitive both to the size of the basis set and the level of the quantum mechanical method used. Therefore, larger basis sets are needed to assess the ability of the DFT approach for describing torsional barriers. The MP4(SDQ) and CCSD relative energy results, reported in this work, can be considered as the most reliable torsional potential data available for the 2,2′-bithiophene molecule. Our results indicate that the experimentally estimated relative energy value for the two equilibrium structures present on the PES for the bithiophene molecule, and consequently the relative abundance of the anti-gauche species, is somewhat underestimated. By comparison with MP4(SDQ) and CCSD results we have shown that single point DFT/6-311G** calculations using HF/6-31G* geometries is the most computationally efficient procedure to study bithiophene like systems, with energy barriers agreeing within 2 kJ/mol.

Molecular structure of dimethylthiocarbamoyl chloride according to gas phase electron diffraction data andAB INITIO calculations

Molecular structure of dimethylthiocarbamoyl chloride according to gas phase electron diffraction data andAB INITIO calculations

Structure and energetics of β-diketonates. IX. Molecular structure of Sr(DPM)2 according to gas phase electron diffraction data

Simultaneous electron diffraction and mass spectrometric investigation of overheated vapor of strontium dipivaloyl methanate was carned out. It was found that at 437(5)°C one molecular form, Sr(DPM)2, prevails in the vapor. The ra, rg, and rα structural parameters of the Sr(DPM)2 monomeric molecule were determined; the molecule was found to have D2d symmetry. The structural characteristics are compared in the series Ca(DPM)2—Sr(DPM)2—Ba(DPM)2.

1,1,2-Triiododisilane (I2HSi–SiH2I): molecular structure, internal rotation and vibrational properties determined by gas-phase electron diffraction, infrared and Raman spectroscopy, and ab initio molecular orbital- and density functional calculations

The molecular structure, internal rotation and vibrational properties of 1,1,2-triiododisilane (TIDS), I2HSi–SiH2I, have been studied using gas phase electron diffraction (GED) data at an average temperature of 52°C, together with infrared and Raman spectroscopy, and ab initio molecular orbital- and density functional calculations. The title compound exists in the gas and liquid phases as a mixture of two minimum conformers, anti, with a torsional angle φ(HSiSiI)=180°, and gauche, with a torsional angle φ(HSiSiI)≈80°. Discrepancies were found between the experimental results and the theoretical calculations regarding conformational stability. The GED analysis, using a cosine potential function in describing the torsional motion, gives about 51% contribution of the anti conformer, while the corresponding spectroscopic result contributes about 64%. The theoretical calculations give the gauche conformer a lower energy of average value 1.1kcalmol−1, corresponding to 90% of this conformer in the gaseous mixture. Some structural parameter values obtained from the GED refinements, using results from the theoretical calculations as constraints, are as follows (torsional vibrational average values with estimated 2σ uncertainties): Bond lengths (rg): r(Si–Si)=2.329(12) Å, r (Si–I) =2.449(3) Å (average value of the three Si–I bonds), r(Si–H)=1.527Å (estimated value). Bond angles (∠α): ∠(SiSiI)=109.9(4)° (average value), ∠ISiI=110.5(4)°, ∠ISiH=109.0(2)°. The torsional cosine potential-function parameters V1, V2 and V3 were obtained from the GED data analysis. Vibrational spectra were presented and analysed, aided by normal coordinate calculations and ab initio results.

1,1,2,2-tetrachlorodisilane (Cl2HSi–SiHCl2): molecular structure, conformation and torsional potential as determined by gas-phase electron diffraction, vibrational spectroscopic data and ab initio molecular orbital calculations

The molecular structure, conformational composition and torsional potential of 1,1,2,2-tetrachlorodisilane (TCDS), Cl2HSi–SiHCl2, were studied using gas phase electron diffraction (GED) data at 23°C, together with earlier recorded spectroscopic data and normal coordinate- and ab initio molecular orbital calculations. The title compound exists in the gas phase at room temperature as a mixture of two conformers, anti, with a torsion angle φ(HSiSiH)=180°, and gauche, with a torsion angle φ(HSiSiH)≈60°. The gauche conformer predominates, occupying approximately 80% of the gas composition at 23°C. Some structural parameter values obtained from the GED refinements, using results from the earlier spectroscopic work and ab initio molecular orbital calculations as constraints, are as follows (gauche conformer with estimated 2σ uncertainties): bond lengths (rg): r(Si–Si)=2.310(8)Å, r〈(Si–Cl)〉=2.039(2)Å (average value), r(Si–H)=1.511Å (assumed value). Bond angles (∠α): ∠〈SiSiCl〉=108.9(4)° (average value), ∠ClSiCl=109.7(3)°, ∠SiSiH=111.5° (assumed value).

Investigation of the structure and energetics of Β-diketonates. VIII. Molecular structure of the Ba(dpm)2 monomer according to gas phase electron diffraction data

A combined electron diffraction and mass spectrometric study of the overheated vapor over barium dipivaloylmethanate was performed. It was found that at 488°C the monomeric molecular form predominates in the vapor. The Ba(dpm)2 molecule has D2d symmetry. Its ra, rg, and rα parameters were determined.

Investigation of the structure and energetics ofΒ-diketonates. VI. Molecular structure of calcium dipivaloylmethanate Ca(DPM)2 according to gas phase electron diffraction data

Combined mass spectrometric and electron diffraction study of the overheated vapor over calcium dipivaloylmethanate was performed. It was established that one molecular form, Ca(DPM)2, predominates in the vapor at 420–460‡C. The ra, rg and ra parameters of the Ca(DPM)2 molecule were determined, and the symmetry of the molecule was found to be D2d.

Investigation of the structure and energetics ofΒ-diketonates. VII. Molecular structure of Ga(DPM)3 according to gas phase electron diffraction data

A combined electron diffraction and mass spectrometric study of the saturated vapor over gallium tris-dipivaloylmethanate was performed. It was found that the vapor contains only the monomeric molecules Ga(DPM)3of C3 symmetry, for which the ra, rg,and ra parameters were determined. The force constant f(Ga-O) was estimated from the experimental value of the vibration amplitude 1(Ga-O). 1999.

C(sp2)−C(sp3) Rotational Barriers in Simple Amides: H2N−C(O)R (R = Methyl, Ethyl, i-Propyl, tert-Butyl)

Potential energy surfaces for rotation about the C(sp2)−C(sp3) bond are reported for acetamide, propanamide, 2-methylpropanamide, and 2,2-dimethylpropanamide at different levels of ab initio electronic structure theory with correlation effects included. In all cases, fully optimized geometries of rotational minima are consistent with gas phase electron diffraction data and crystal structure data. The experimental barrier height for methyl rotation in acetamide is reproduced to within 0.1 kcal/mol. This study yields a set of improved criteria for the construction of rotational potentials for the Ca−C bond which are used to obtain improved MM3 torsional parameters. In addition, we find that the use of higher levels of theory leads to significantly different results than those obtained in prior Hartree−Fock studies on acetamide and 2-methylpropanamide.

An MP2 and density functional study of the oxides of nitrogen

Equilibrium geometries, harmonic vibrational frequencies, atomization energies and heats of formation have been calculated for seven neutral oxides of nitrogen and compared with experimental values. We used standard Hartree-Fock and MP2 wavefunctions, together with local (SVWN), non-local (BP, BLYP) and hybrid HF-DFT (ACM or B3-PW91) density functionals. A good quality triple-zeta double polarization (TZ2P) gaussian basis was used throughout. Results using the ACM functional are by far the best, further emphasizing the quality and practical utility of hybrid density functionals for the prediction of molecular geometries and energetics. Local DFT predicts strongly exothermic heats of formation for all the nitrogen oxides except NO, in direct contradiction to experiment. Our ACM calculations predict that N 2O 5 has a C 2 equilibrium structure with the NO 2 groups twisted with respect to the central ONO plane by 31.6 °, in excellent agreement with gas phase electron diffraction data.

CC and CO bond conformations in 1,2-dimethoxyethane, bis-(2-methoxyethyl)ether and poly(ethyleneoxide): dependence on solvent and temperature

Data from 3 J HCCH and 3 J HCOC couplings in 1,2-dimethoxyethane and bis-(2-methoxyethyl)ether, plus 3 J HCCH in poly(ethylene oxide), have been obtained in five solvents by iterative fitting. The resulting proportions of gauche rotamers are higher than previous estimates, and higher still after allowance is made for the pentane effect. They fit well with gas phase electron diffraction data, with current gas phase theoretical calculations and with standard RIS parameters for the polymer, but less well with calculations for the liquid state. The influence of solvent arises more from variations in its H-bond donor properties than in its dielectric.

The molecular structure of tetrakis(trimethylstannyl)methane by gas electron diffraction

The gas phase electron diffraction data of C * (SnMe 3) 4 recorded with a nozzle temperature of about 120°C are consistent with a molecular model of T symmetry; the bond distances ( r a) are C *-Sn= 217.7(3),Sn C= 216.2(4), and C H= 112.2(6)pm; the valence angles<C *SnC= 112.3(4)° and<SnCH= 111.5(6)°.