Gas-phase Electron Diffraction Experiment Research Articles

Molecular structure of 1,2-diethyldiaziridine studied by gas electron diffraction supported by quantum chemistry calculations

The molecular structure of 1,2-diethyldiaziridine was studied by gas-phase electron diffraction (GED) and quantum chemical calculations. Best fitting to GED experiment scattering intensities was obtained for three conformer model. The experimental structural parameters of most abundant conformer of 1,2-diethyldiaziridine are following (rh1 bond lengths in Å, bond angles in degrees with 3σ in parentheses): r(NCcycl), 1.446(4); r(NN), 1.492(5); r(CC), 1.528(3); r(NCacycl), 1.459(6); ∠NCC, 112.3(11); ∠CNC, 114.7(6). The gas phase standard enthalpy of formation of 1,2-diethyldiaziridine was calculated using Gaussian-4 theory, giving the value of 148.3 ± 3.3 kJ/mol. 1H and 13C NMR spectra obtained in CDCl3 solution were compared with DFT predictions.

Preparation and Properties of Chlorosulfuryl Chloroformate, ClC(O)OSO2Cl.

The novel chlorosulfuryl chloroformate, ClC(O)OSO2Cl, was prepared by the reaction of CCl4 and SO3. Alternatively, the compound was obtained from the direct insertion reaction of SO3 to Cl2C═O. The latter reaction constitutes also a confirmation of the proposed mechanism for the former one. Density functional theory and MP2 theoretical approximations predict the existence of two conformers, syn-gauche and syn-anti, depending on the value adopted by the dihedral angles ϕ(Cl-C-O-S) and ϕ(C-O-S-Cl). The structure of the syn-gauche conformer was determined by gas-phase electron diffraction (GED). The existence of the syn-anti conformer can be neither confirmed nor excluded by the GED experiment. Vibrational spectra (vapor-phase and argon-matrix Fourier transform infrared and liquid Fourier transform Raman) were interpreted by an equilibrium mixture between both conformers.

Improved methods and calibration technique in gas-phase electron-diffraction experiments: Use of nitrogen as a wavelength standard

Improved methods are described for extracting electron-scattering data from films used in gas-phase electron-diffraction (GED) experiments. These involve use of a precision transmission scanner in conjunction with powerful, public-domain image-processing software (ImageJ). Procedures for finding an accurate center of the diffraction ring pattern are given and the software permits flexible determination of the intensity average of each radial circle or any arc thereof. In addition, nitrogen gas is proposed as the ideal calibration molecule because virtually all molecules are in the ground vibrational state at all temperatures normally encountered in GED experiments. For this state the rotational constant is very accurately known, as are several small corrections needed to obtain an appropriate calibration value for the GED experiment. The result from theory is ra(N≡N) = 1.10087 Å with a standard deviation (σ) of 0.00035 Å from fitting experiments.

Diffractive imaging of a rotational wavepacket in nitrogen molecules with femtosecond megaelectronvolt electron pulses.

Imaging changes in molecular geometries on their natural femtosecond timescale with sub-Angström spatial precision is one of the critical challenges in the chemical sciences, as the nuclear geometry changes determine the molecular reactivity. For photoexcited molecules, the nuclear dynamics determine the photoenergy conversion path and efficiency. Here we report a gas-phase electron diffraction experiment using megaelectronvolt (MeV) electrons, where we captured the rotational wavepacket dynamics of nonadiabatically laser-aligned nitrogen molecules. We achieved a combination of 100 fs root-mean-squared temporal resolution and sub-Angstrom (0.76 Å) spatial resolution that makes it possible to resolve the position of the nuclei within the molecule. In addition, the diffraction patterns reveal the angular distribution of the molecules, which changes from prolate (aligned) to oblate (anti-aligned) in 300 fs. Our results demonstrate a significant and promising step towards making atomically resolved movies of molecular reactions.

Tridentate Lewis Acids Based on 1,3,5-Trisilacyclohexane Backbones and an Example of Their Host-Guest Chemistry.

Directed tridentate Lewis acids based on the 1,3,5-trisilacyclohexane skeleton with three ethynyl groups [CH2Si(Me)(C2H)]3 were synthesised and functionalised by hydroboration with HB(C6F5)2, yielding the ethenylborane {CH2Si(Me)[C2H2B(C6F5)2]}3, and by metalation with gallium and indium organyls affording {CH2Si(Me)[C2M(R)2]}3 (M = Ga, In, R = Me, Et). In the synthesis of the backbone the influence of substituents (MeO, EtO and iPrO groups at Si) on the orientation of the methyl group was studied with the aim to increase the abundance of the all-cis isomer. New compounds were identified by elemental analyses, multi-nuclear NMR spectroscopy and in some cases by IR spectroscopy. Crystal structures were obtained for cis-trans-[CH2Si(Me)(Cl)]3, all-cis-[CH2Si(Me)(H)]3, all-cis-[CH2Si(Me)(C2H)]3, cis-trans-[CH2Si(Me)(C2H)]3 and all-cis-[CH2Si(Me)(C2SiMe3)]3. A gas-phase electron diffraction experiment for all-cis-[CH2Si(Me)(C2H)]3 provides information on the relative stabilities of the all-equatorial and all-axial form; the first is preferred in both solid and gas phase. The gallium-based Lewis acid {CH2Si(Me)[C2Ga(Et)2]}3 was reacted with a tridentate Lewis base (1,3,5-trimethyl-1,3,5-triazacyclohexane) in an NMR titration experiment. The generated host-guest complexes involved in the equilibria during this reaction were identified by DOSY NMR spectroscopy by comparing measured diffusion coefficients with those of the suitable reference compounds of same size and shape.

Conformational properties of 1-tert-butyl-1-silacyclohexane, C5H10SiH(t-Bu): gas-phase electron diffraction, temperature-dependent Raman spectroscopy, and quantum chemical calculations

The conformational preference of the t-butyl group in 1-t-Bu-1-silacyclohexane was studied experimentally by means of gas-phase electron diffraction (GED), and temperature-dependent Raman spectroscopy as well as by quantum chemical calculations applying density functional theory and ab initio methods. According to the GED experiment at 283 K, the vapor of the title compound contains only the equatorial conformer. At 99.5 % level of confidence, up to 4 % of axial conformer cannot be completely excluded, however. The Raman spectroscopy experiment in the temperature range of 295–375 K of the neat liquid indicated that the equatorial conformer is favored over the axial one by 0.56 (15) kcal mol−1 (ΔH values). The experimental values are fairly well reproduced by the calculations. CCSD(T) calculations predict the equatorial conformer to have a 1.19 kcal mol−1 lower Gibbs free energy (corresponding to about 89 % equatorial preference) and a 0.99 kcal mol−1 lower enthalpy than the axial conformer at 300 K. According to natural bond orbital analysis, the equatorial conformer of the title compound is an example of a molecular stabilization, which is not favored by steric and conjugation effects but favored by electrostatic interactions. Results from dynamic NMR experiments were inconclusive.

Molecular structure and barriers to internal rotation of α-naphthalenesulfonyl chloride: a study by gas-phase electron diffraction and quantum chemical calculations

α-Naphthalenesulfonyl chloride, α-NaphSC, was studied by gas-phase electron diffraction (GED) and quantum chemical calculations (HF/6-311 + G**, HF/aug-cc-pVDZ, B3LYP/cc-pVDZ, B3LYP/cc-pVTZ, B3LYP/aug-cc-pVDZ, B3LYP/aug-cc-pVTZ, MP2/cc-pVDZ, and MP2/cc-pVTZ). The calculations predict the existence of two conformers with C1 (I) and Cs (II) symmetries. The most stable conformer I has an enantiomer. The experimental data of α-NaphSC obtained at 370(5) K could be best fitted by a C1 symmetry model indicating that only this form exists in the gas-phase. In this model the Cα–S–Cl plane deviates from the perpendicular orientation relative to the plane of the naphthalene skeleton. Under the applied experimental conditions, the mole fraction of a second less stable conformer II of α-NaphSC predicted by calculations is no more than 1 %. The following geometrical parameters of conformer I were obtained from the experiment (A and °; uncertainties are in parentheses): rh1(C–H) = 1.082(6), rh1(C–C)cp = 1.407(3), rh1(C–S) = 1.764(5), rh1(S–O)av = 1.425(3), rh1(S–Cl) = 2.051(5), ∠C–Cα–C = 122.5(1), ∠Cα–S–Cl = 101.5(10); C9–C1–S–Cl = 71.4(21). The calculated barriers to internal rotation of the sulfonyl chloride group exceed considerably the thermal energy values corresponding to the temperatures of the GED experiments. Natural bond orbitals analysis of the electron density distribution was carried out to explain the peculiarities of the molecular structure of the studied compound and the deviation from the structures of β-NaphSHal molecules and their benzene analogs.

Interrelation of nonequivalent C[sbnd]C bonds of naphthalene frame and spatial orientation of substituents: Beta-naphthalene sulfonyl fluoride and beta-naphthalene sulfonyl chloride

β-naphthalene sulfonyl fluoride, β-NaphSF, and β-naphthalene sulfonyl chloride, β-NaphSCl, were studied by gas-phase electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 in combination with cc-pVDZ, aug-cc-pVDZ and cc-pVTZ basis sets). For each compound the calculations predicted the existence of two conformers which are enantiomers. On the basis of the experimental data it was found that the gas phase over β-NaphSF and NaphSCl at 357(5)K and 395(5)K, respectively, consists of molecular species of C1 symmetry in which the CβSHal plane deviates from the perpendicular orientation relative to the naphthalene skeleton plane. The following geometrical parameters (Å and degrees) were obtained from the experiment (uncertainties are in parentheses): rh1(CH)aver.=1.097(7), rh1(CC)aver.=1.410(3), rh1(CS)=1.753(6), rh1(SO)aver.=1.414(4), rh1(SF)=1.559(5), ∠CCβC=122.8(3), ∠CβSF=103.3(30); Φ(CαCβSF)=104(6) for β-NaphSF, and rh1(CH)aver.=1.089(4), rh1(CC)aver.=1.411(3), rh1(CS)=1.757(5), rh1(SO)aver.=1.419(3), rh1(SСl)=2.053(4), ∠CCβC=122.8(1), ∠CβSCl=102.2(7), Φ(CαCβSCl)=108(3) for β-NaphSCl. The calculated barriers to internal rotation of the sulfonyl halide groups exceed considerably the thermal energy values corresponding to the temperatures of the GED experiments. Natural bond orbital (NBO) analyses of the electron density distribution were applied to explain the peculiarities of the molecular structure of the studied compounds and the deviation from the structures of their benzene analogs.

Prediction of conformational population of large cycloalkanes using ab initio correlated methods: Cycloundecane, cyclododecane, and cyclotridecane

AbstractWe report MP4(SDTQ) and CCSD(T) ab initio temperature‐dependent conformational populations for a series of cycloalkanes with increasing ring size, cycloundecane (11‐C), cyclododecane (12‐C), and cyclotridecane (13‐C), using extended basis sets. Our aim is to analyze the commonly used quantum chemical procedure for evaluating the thermal energy contribution to Gibbs free energy particularly the adequacy of standard harmonic oscillator vibrational partition function for large molecular systems. Experimental population data for cycloundecane, obtained from low temperature (90.1 K) 13C NMR, and cyclododecane, from gas phase electron diffraction experiment at 393.15 K, are available, and we found a good agreement with MP4(SDTQ) and CCSD(T) ab initio calculated conformational population. Our results indicate that the procedure of considering all 3N‐6 vibrational modes as harmonic oscillators seems adequate for the prediction of the preferred conformer of cycloundecane, cyclododecane, and cyclotridecane, in the absence of a more appropriate approach to treat the low frequency modes that is applicable for large molecules. Therefore, it can be expected to work satisfactorily in room temperature conformational analysis studies on large molecules containing a great number of vibrational frequencies below 625 cm−1, such as in supramolecular chemistry, biological, and material science applications. © 2012 Wiley Periodicals, Inc.

Comprehensive study of the structure of aluminum trihalides from electron diffraction and computation

The molecular geometries of the complete series of aluminum trihalides, AlX3 (X = F, Cl, Br, and I), are discussed based on new gas-phase electron diffraction (GED) experiments (AlCl3 and Al2Cl6) and on the reanalysis of our previous GED data (for the rest of the series) with an improved and analogous treatment of the temperature effect on the average bond lengths. This is the first time that all structure analyses are based on GED experiments of homogeneous vapors; either as pure monomers or dimers. The structures of other small oligomers as well as those of the crystals of aluminum trihalides are also discussed.

Molecular structure of N,N′-o-phenylene-bis(salicylideneaminato)copper(II) studied by gas-phase electron diffraction and quantum-chemical calculations

The molecular structure of N,N′-o-phenylene-bis(salicylideneaminato)copper(II) (Cu(saloph)) was determined using the combination of gas-phase electron diffraction (GED), mass spectrometry, and quantum-chemical calculations. According to both experimental and theoretical approaches the molecule of Cu(saloph) is planar and possesses C2v symmetry. Main structural parameters determined by GED experiment are the following (total error is given in a brackets): rh1(Cu–N) = 1.960(20) A, rh1(Cu–O) = 1.913(17) A, ∠NCuN = 82.7(18)°, ∠OCuO = 91.6(21)°, ∠NCuO = 92.9(9)°. The experimental structural parameters of Cu(saloph) molecules determined by X-ray single crystal analysis and GED experiments were discussed and compared to the theoretical ones.

Gas electron diffraction: the direction of development of the method

The problems of gas phase electron diffraction experiments are considered. The operating conditions for a reconstructed electron diffraction apparatus are determined in a scattering regime with an effective cross section of σ = 1.06 × 102 b on a molecular beam with Kn > 1. On the example of the I2 molecule it is shown that at a vapor pressure over a substance equal to 10−3 mmHg, one can sort out a desired signal at scattering angles up to s = 30 A−1. A nomographic chart for the choice of the optimal operating conditions of electrons scattering is proposed.

A gas-phase electron diffraction and quantum chemical investigation of the molecular structure of 1-bromosilacyclobutane

The gas-phase electron diffraction experiment has shown that 1-monobromosilacyclobutane (MBSCB) exists in two conformational forms, the axial and equatorial with a significantly higher prevalence of the latter form (73(6)%). Various quantum mechanical procedures have been applied to investigate the thermodynamic equilibrium of the two conformers as well as the geometrical parameters of MBSCB. Among these methods was MP2/6-311++G(2df,2pd). This level of theory provided an axial to equatorial ratio of 29:71% and values for the geometrical parameters that are in good agreement with the experimental values except for the C–C bond length, which is by 0.02 Å shorter than in the experiment. The main geometrical parameters obtained from the experiment are: ( r a Å, ∠ a°): Si–C 1.872(3); C–C 1.583(6); Si–Br 2.225(3); ∠ CSiC 79.3; dihedral angle φ 28.8(52) ( axial) and 39.9(2) ( equatorial). Natural bond orbital (NBO) and atoms in molecule (AIM) analyses have been performed. Both, the donor–acceptor (Lewis and non-Lewis) orbital interactions and the topological properties of the charge density at the critical points ρ( r) have consistently confirmed the experimental results and facilitated their interpretation. For the purpose of comparison and systematic investigation, we optimized the geometries and analyzed the NBOs and the topological properties of silacyclobutane, 1-monofluorosilacyclobutane (MFSCB), and 1-monochlorosilacyclobutane (MCSCB). Simple relationship has been found between the puckering angle θ and the puckering amplitude q, which allows for the prediction of either θ or q for mono- and dihaloginated silacyclobutanes. Additionally, NBO deletion analysis comprising NOSTAR, NOVIC, and NOGEM deletion algorithms have been conducted. Interesting conclusions regarding structure and conformational stability of the studied monohalogenated silacyclobutanes could be drawn from this analysis.

Geometries of Second-Row Transition-Metal Complexes from Density-Functional Theory.

A data set of 19 second-row transition-metal complexes has been collated from sufficiently precise gas-phase electron-diffraction experiments and used for evaluating errors in DFT optimized geometries. Equilibrium geometries have been computed using 15 different combinations of exchange-correlation functionals in conjunction with up to three different effective core potentials. Most DFT levels beyond the local density approximation can reproduce the 29 metal-ligand bond distances selected in this set with reasonable accuracy and precision, as assessed by the mean and standard deviations of optimized vs experimentally observed bond lengths. The pure GGAs tested in this study all have larger standard deviations than their corresponding hybrid variants. In contrast to previous findings for first-row transition-metal complexes, the TPSSh hybrid meta-GGA is slightly inferior to the best hybrid GGAs. The ranking of some popular density functionals, for second-row transition-metal complexes, ordered according to decreasing standard deviation, is VSXC ≈ LSDA > BLYP > BP86 > B3LYP ≈ TPSSh > PBE hybrid ≈ B3PW91 ≈ B3P86. When zero-point vibrational corrections, computed at the BP86/SDD level, are added to equilibrium bond distances obtained from a number of density-functional/basis-set combinations, the overall performance in terms of mean and standard deviations from experiment is not improved. For a combined data set comprised of the first- and second-row transition-metal complexes the hybrid functionals B3P86, B3PW91, and the meta-GGA hybrid TPSSh afford the lowest standard deviations.

Accurate Equilibrium Structures Obtained from Gas-Phase Electron Diffraction Data: Sodium Chloride

A novel method has been developed to allow the accurate determination of equilibrium gas-phase structures from experimental data, thus allowing direct comparison with theory. This new method is illustrated through the example of sodium chloride vapor at 943 K. Using this approach the equilibrium structures of the monomer (NaCl) and the dimer (Na(2)Cl(2)), together with the fraction of vapor existing as dimer, have been determined by gas-phase electron diffraction supplemented with data from microwave spectroscopy and ab initio calculations. Root-mean-square amplitudes of vibration (u) and distance corrections (r(a) - r(e)) have been calculated explicitly from the ab initio potential-energy surfaces corresponding to the vibrational modes of the monomer and dimer. These u and (r(a) - r(e)) values essentially include all of the effects associated with large-amplitude modes of vibration and anharmonicity; using them we have been able to relate the ra distances from a gas-phase electron diffraction experiment directly to the re distances from ab initio calculations. Vibrational amplitudes and distance corrections are compared with those obtained by previous methods using both purely harmonic force fields and those including cubic anharmonic contributions, and the differences are discussed. The gas-phase equilibrium structural parameters are r(e)(Na-Cl)(monomer) = 236.0794(4) pm; r(e)(Na-Cl)(dimer) = 253.4(9) pm; and <(e)ClNaCl = 102.7(11) degrees. These results are found to be in good agreement with high-level ab initio calculations and are substantially more precise than those obtained in previous structural studies.

Conformations, Structures, and Vibrational Spectra of Triethylchloro- and Triethylbromosilane Using Theoretical Methods, Gas Phase Electron Diffraction, and IR and Raman Spectroscopy

Conformational studies of triethylchlorosilane (TECS) and triethylbromosilane (TEBS) and the elucidation of their gas phase molecular structures have been accomplished by the combined use of theoretical (ab initio and density functional theory) calculations and experimental data from gas phase electron diffraction experiments. Additionally, analysis of the experimental features observed in the IR and Raman spectra recorded for both compounds has allowed us to propose a complete description of the vibrational spectra of both compounds, including an explanation of certain bands, which can only be correctly assigned when more than one conformer is considered to be present.

Structure and conformation of thioanisole: Gas electron diffraction and contradicting quantum chemical calculations

The geometric structure and conformational properties of thioanisole, C 6H 5SCH 3, have been studied by gas-phase electron diffraction (GED) and quantum chemical calculations. The GED intensities are fitted very well with a single conformer with planar orientation of the S–CH 3 bond ( φ(C–S) = 0°). The experimental potential function for internal rotation around the C(sp 2)–S bond derived with a dynamic model possesses a single minimum for planar conformation, but a second shallow minimum for perpendicular orientation cannot be excluded. Calculated potential functions depend strongly on the computational method. The MP2 approximation with small basis sets (6-31G(d)) predict a single minimum for perpendicular orientation, ( φ(C–S) = 90°), with intermediate basis sets (6-311G(d,p)) an additional shallow minimum for planar orientation and with large basis sets (cc-pVTZ) the global minimum for planar orientation and an additional shallow minimum at φ(C–S) = 90°. The B3LYP/cc-pVTZ calculations result in a single minimum for planar orientation, in agreement with the GED experiment. The experimental and calculated barrier to internal rotation is about 1 kcal/mol. The following geometric parameters ( r a and ∠ h1 with 3 σ uncertainties) were derived: r(C–C) av = 1.395(3) Å, r(C(sp 2)–S) = 1.775(4) Å, r(S–CH 3) = 1.813(4) Å, ∠CSC = 104.5(5), ∠C2C1C6 = 119.3(3), ∠C1C2C3 = 120.2(4)°.

Investigation of structure of liquid 2,2,2 trifluoroethanol: Neutron diffraction, molecular dynamics, and ab initio quantum chemical study

The molecular conformation and intermolecular H bonding in liquid 2,2,2 trifluoroethanol (TFE) have been studied by neutron diffraction with hydrogen/deuterium isotopic substitution at room temperature. For comparison, conformations of molecules and their dimers in the gas phase have also been calculated, based on the density functional theory. Energies, geometry, and vibrational frequencies of dimers were analyzed. Diffraction data analyzed by the "Monte Carlo determination of g(r)" (MCGR) method resulted in a molecular structure in agreement with the findings from gas phase electron diffraction experiments and density functional calculations. The intermolecular structure functions were compared to the same functions obtained from a molecular dynamics simulation. All of the composite radial distribution functions are in good agreement with the simulation results. According to our calculation the hydrogen-bonded aggregation size is smaller in pure liquid TFE than in pure liquid ethanol.

The molecular structure of Mg(acac) 2 determined by gas-phase electron diffraction and quantum mechanical calculations

The molecular structure of magnesium bis-acetylacetonate has been determined by synchronous gas-phase electron diffraction (GED) and mass spectrometric experiment and quantum mechanical calculations (HF/6-311+G(d,p), B3LYP/6-311+G(d,p), and MP2/6-311+G(d,p)). Both experimental and theoretical approaches yielded the structure of D 2 d symmetry with chelate rings located in perpendicular planes. Major structural parameters determined by GED experiment are the following: bond distances r α(Mg–O)=1.966(4), r α(O–C)=1.279(2), r α(C–C′)=1.408(3), r α(C–C m )=1.534(4) Å; valence angles ∠ α(O–Mg–O)=93.3(2), ∠ α(O–C′–C)=125.3(2), ∠ α(O–C–C m )=116.2(4)°.

Ethylene glycol dimers in the liquid phase: A study by x-ray and neutron diffraction

Molecular conformations and intermolecular H bonding in liquid ethylene glycol have been studied by neutron diffraction with hydrogen/deuterium isotopic substitution, and by x-ray diffraction measurements at room temperature. For comparison, conformations of molecules and their dimers in the gas phase have also been calculated, based on the density-functional theory. Energies, geometries, and vibrational frequencies of dimers were analyzed. Diffraction data analyzed by the Monte Carlo method resulted in a molecular structure in agreement with the findings from gas-phase electron diffraction experiments and density-functional calculations. Conformers of ethylene glycol molecules in the liquid phase were identified as a gauche OCCO arrangement, stabilized by intramolecular hydrogen bonding, which is in agreement with the results of gas-phase density-functional calculations. Data analysis resulted in an average of three hydrogen-bonded nearest neighbors of molecules in liquid ethylene glycol. In addition, experimental composite pair correlation functions RHO and RR have been extracted from the diffraction data for future comparison with computer simulation results.