Gas Electron Diffraction Study Research Articles

Cyclic Dimers of 4-n-Propyloxybenzoic Acid with Hydrogen Bonds in the Gaseous State.

A comprehensive study of saturated vapors of 4-n-propyloxybenzoic acid (POBA) by gas electron diffraction (GED) and mass spectrometric (MS) methods supplemented by quantum chemical (QC) calculations was carried out for the first time. An attempt was made to detect dimeric forms of the acid in the gaseous state. It has been established that at the temperature of GED experiment, vapor over a solid sample contains up to 20 mol.% of cyclic dimers with two O-H...O hydrogen bonds. The main geometrical parameters of gaseous monomers and dimers of POBA are obtained. The distance r(O…O) = 2.574(12) Å in the cyclic fragment of the gaseous dimer is close to that in the crystal structure (2.611 Å). In the mass spectrum of the POBA recorded the ions of low intensity with a mass exceeding the molecular mass of the monomer were detected. The presence of ions, whose elemental composition corresponds to the dissociative ionization of the dimer, confirms the results of the GED experiment on the presence of POBA dimers in the gas state. The results of GED studies of acetic acid, benzoic acid, and POBA were compared. It is shown that the COOH fragment saves its geometric structure in monomers, as well as the COOH...HOOC fragment with two hydrogen bonds in dimers of different acids. The intermolecular interaction energy in considered acid dimers was estimated using QC calculations (B97D/6-311++G **). The significant value of last (>84 kJ/mol) is the reason for the noticeable presence of dimers in the gas phase.

Nuclear quadrupole coupling in SiH2I2 due to the presence of two iodine nuclei

The rotational spectrum of diiodosilane was measured with a jet-pulsed, cavity Fourier transform microwave (FTMW) spectrometer over the frequency range 8.8–15GHz and assigned for the first time. The complete nuclear quadrupole coupling (NQC) tensors for both iodine nuclei were obtained for the 28Si, 29Si, and 30Si isotopologues of diiodosilane. In addition to the nuclear quadrupole coupling constants (NQCCs), rotational constants, centrifugal distortion constants, and nuclear spin-rotation constants were determined for each silicon isotopologue. Subtle, yet unmistakable, changes in the NQCCs of iodine upon isotopic substitution were observed and will be discussed. A r0 structure of diiodosilane was also fit via isotopic substitution, which led to the determination of bond lengths and angles: r0(SiI)=2.4236(19)Å, r0(SiH)=1.475(21)Å, ∠(ISiI)=111.27(13)°, and ∠(ISiH)=105.9(19)°. These molecular parameters will be compared to the results of a previous gas electron diffraction study.

Gas-Phase Structures of Ketene and Acetic Acid from Acetic Anhydride Using Very-High-Temperature Gas Electron Diffraction.

The gas-phase molecular structure of ketene has been determined using samples generated by the pyrolysis of acetic anhydride (giving acetic acid and ketene), using one permutation of the very-high-temperature (VHT) inlet nozzle system designed and constructed for the gas electron diffraction (GED) apparatus based at the University of Canterbury. The gas-phase structures of acetic anhydride, acetic acid, and ketene are presented and compared to previous electron diffraction and microwave spectroscopy data to show improvements in data extraction and manipulation with current methods. Acetic anhydride was modeled with two conformers, rather than a complex dynamic model as in the previous study, to allow for inclusion of multiple pyrolysis products. The redetermined gas-phase structure of acetic anhydride (obtained using the structure analysis restrained by ab initio calculations for electron diffraction method) was compared to that from the original study, providing an improvement on the description of the low vibrational torsions compared to the dynamic model. Parameters for ketene and acetic acid (both generated by the pyrolysis of acetic anhydride) were also refined with higher accuracy than previously reported in GED studies, with structural parameter comparisons being made to prior experimental and theoretical studies.

Molecular structure and internal rotation of CH2Cl group of chloropropanone oxime: gas electron diffraction, microwave spectroscopy, and quantum chemical calculation studies

The molecular structure of chloropropanone oxime [ClCH2C(CH3)=NOH] has been determined by gas electron diffraction (GED), microwave spectroscopy (MW) and quantum chemical calculations. Potential energy curves for the internal rotation of CH2Cl group in (E)- and (Z)-isomers as well as the optimized geometries and force constants for four conformations have been calculated by the quantum chemical calculations. Combined data analysis of the GED and MW data revealed the conformational mixture of 68(4) % (E)-anticlinal and 32 % (Z)-synclinal conformers. The principal values of geometrical parameters of the E-anticlinal conformer are: r g(ClH2C–C) = 1.499(2) A, r g(C–CH3) = 1.503(2) A, r g(C=N) = 1.276(4) A, r g(C–Cl) = 1.805(3) A, r g(N–O) = 1.398(4) A, ∠αC–C=N = 113.2(16)°, ∠αC–C–Cl = 111.1°(5), ∠αH3C–C=N = 127.0°(13), ϕ(NCCCl) = 121.1°(21). Numbers in parentheses are three times standard deviations of the data fit.

Benchmark study of molecules with large-amplitude ring-twisting motion: accurate equilibrium structure of succinic anhydride from gas electron diffraction data and coupled-cluster computations

The gas electron diffraction (GED) study of succinic anhydride (dihydro-2,5-furandione) is used as a benchmark example for the reliable determination of the structure of floppy molecules with saturated non-rigid ring. According to results of B3LYP calculations, the molecule has a planar skeleton (C2v total symmetry), whereas the MP2 optimized structure has a non-planar ring with the torsional angle φ(C–C–C–C) of up to 11° (C2 symmetry). It has been revealed that the increase in the quality of ab initio calculations up to CCSD(T) level and the enlargement of basis set up to quadruple-ζ size decrease the non-planarity of the molecular skeleton, which is approximated to be planar in the best-estimated ab initio structure. The semiexperimental equilibrium structure ( $$r_{\text{e}}^{\text{se}}$$ ) has been determined from the GED data with the application of the so-called dynamic (pseudo-conformer) model describing the large-amplitude ring-twisting motion. Vibrational corrections Δ(r a − r e) to the experimental internuclear distances have been calculated from the B3LYP/cc-pVTZ quadratic and cubic force constants. The best-estimated ab initio structure agrees well with the $$r_{\text{e}}^{\text{se}}$$ structure. This fact points to the reliability and a high accuracy of both experimental data and applied theory.

Molecular structure and internal rotation of CF₃ group of methyl trifluoroacetate: gas electron diffraction, microwave spectroscopy, and quantum chemical calculation studies.

The molecular structure of methyl trifluoroacetate (CF3COOCH3) has been determined by gas electron diffraction (GED), microwave spectroscopy (MW), and quantum chemical calculations (QC). QC study provides the optimized geometries and force constants of the molecule. They were used to estimate the structural model for GED study and to calculate the vibrational corrections for GED and MW data. In addition, potential energy curves for the internal rotations of CF3 and CH3 groups have been calculated for anti (dihedral angle of α(CCOC) is 180°) and syn (α(CCOC) = 0°) conformers of methyl trifluoroacetate. Both the GED and MW data revealed the existence of the anti conformer. Molecular constants determined by MW are A0 = 3613.4(3) MHz, B0 = 1521.146(8) MHz, C0 = 1332.264(9) MHz, ΔJ = 0.09(2) kHz, and ΔJK = 0.23(6) kHz. The GED data were well-reproduced by the analysis in which a large-amplitude motion of the CF3 group was taken into account. The barrier of the internal rotation of the CF3 group was determined to be V3 = 2.3(4) kJ mol(-1), where V3 is the potential coefficient of the assumed potential function, V(ϕ) = (V3/2)(1 - cos 3ϕ), and ϕ is a rotational angle for the CF3 group. The values of geometrical parameters (re structure) of the anti conformer of CF3COOCH3 are r((O═)C-O) = 1.326(6) Å, r(O-CH3) = 1.421(4) Å, r(C-H(in-plane)) = 1.083(14) Å, r(C-H(out-of-plane)) = 1.087(14) Å, r(C═O) = 1.190(7) Å, r(C-C) = 1.533(4) Å, r(C-F(in-plane)) = 1.319(4) Å, r(C-F(out-of-plane)) = 1.320(6) Å, ∠COC = 116.3(5)°, ∠OCH(in-plane) = 105.2° (fixed), ∠OCH(out-of-plane) = 110.0° (fixed), ∠O═CC = 123.7° (fixed), ∠O-CC = 111.2(5)°, ∠OCO = 125.2(5)°, ∠CCF = 110.1(3)°, and OCCF (out-of-plane dihedral angles) = ± 121.5(1)°. Numbers in parentheses are three times the standard deviations of the data fit.

A study of the structure and conformation of 1,1-dicyclopropylethene by gas electron diffraction and ab initio calculations

Abstract The molecular structure of 1,1-dicyclopropylethene, c-(C3H5)2C CH2, has been determined by gas electron diffraction (GED) and ab initio calculations. The potential energy surface as a function of the two torsional angles was calculated to examine the conformation of the molecule. The structural model for the GED data analysis was constructed using ab initio calculations at the MP2(full)/aug-cc-pVTZ level of theory. The conformational mixture at about 293 K was determined to be 36(14)% for the trans-gauche form with C1 symmetry and 64(14)% for the gauche-gauche form (mixture of the gauche-gauche form with C2 symmetry (38(27)%) and the gauche-gauche’ form with Cs symmetry (26%)). The uncertainty of the gauche-gauche form with C2 symmetry was estimated by the Hamilton R-factor ratio test. The present result is in agreement with that reported in a previous GED study (Traetteberg et al. J. Mol. Struct. 485–486 (1999) 73). The conformational abundances determined by GED are different from those in liquid xenon determined by a vibrational spectroscopic study (Durig et al., J. Phys. Chem. A. 109 (2005) 1650).

Structure and Conformational Properties of 1,3,3-Trimethyl-1,3-Azasilinane: Gas Electron Diffraction, Dynamic NMR, and Theoretical Study

Structure and the conformational properties of 1,3,3-trimethyl-1,3-azasilinane have been studied. According to gas electron diffraction (GED), the molecule exists in a slightly distorted chair conformation with the N-Me group in equatorial position. High-level quantum chemical calculations excellently reproduce the experimental geometry. Employing variable temperature (1)H and (13)C NMR spectroscopy down to 103 K, the conformational equilibrium could be frozen and the barrier to ring inversion determined.

Gas electron diffraction and quantum chemical study of the structure of a 2-nitrobenzenesulfonyl chloride molecule

A combined gas electron diffraction and quantum chemical (B3LYP/6-311+G**, B3LYP/cc-pVTZ, B3LYP/cc-pVTZ, midix (Cl), and MP2/cc-pVTZ) study of the structure of a 2-NO2-C6H4-SO2Cl molecule is performed. It is found experimentally that at a temperature of 345(5) K the gas phase contains two conformers of the C1 symmetry. Conformer I with a nearly perpendicular arrangement of the S-Cl bond with respect to the benzene ring plane (the C(NO2)-C-S-Cl torsion angle is 84(3)°) is contained predominantly (69(12)%). In conformer II, the S-Cl bond is located near the benzene ring plane (the C(NO2)-C-S-Cl angle is 172(3)°). The following experimental internuclear distances (A) are obtained for conformer I: rh1(C-H) = 1.064(15), rh1(C-C)av = 1.397(3), rh1(C-S) = 1.761(6), rh1(S-O)av = 1.426(4), rh1(S-Cl) = 2.043(5), rh1(N-O)av = 1.222(4), rh1(C-N) = 1.485(16). In both conformers, the NO2 group is turned by more than 30° relative to the benzene ring plane.

Gas electron diffraction and quantum chemical studies of the molecular structure of 2-nitrobenzenesulfonic acid

A combined gas electron diffraction and quantum chemical (B3LYP/6-311+G**, B3LYP/cc-pVTZ) study of the molecular structure of 2-nitrobenzenesulfonic acid (2-NBSA) is performed. Quantum chemical calculations show that the 2-NBSA molecule has five conformers, and the Gibbs energy of one of them is lower by more than 4.5 kcal/mol than the energy of the other conformers. It is found experimentally that the saturated vapor of 2-NBSA at T = 394(5) K contains only the low-energy conformer that has an intramolecular hydrogen bond between the H atom of the hydroxyl group and one of the O atoms of the NO2 group. The C-C-S-O(H) torsion angle determining the position of the S-O(H) bond is −72(7)°, while the NO2 group is substantially turned relative to the benzene ring plane (C1-C2-N-O = 40(5)°). The following experimental values of the internuclear distances are obtained for this conformer (A): r h1(C-H)av = 1.07(2), r h1(C-C)av = 1.401(4), r h1(C-S) = 1.767(6), r h1(S=O)av = 1.412(4), r h1(S-O) = 1.560(6), r h1(N-O)av = 1.217(5), r h1(C-N) = 1.461(8), r h1(O-H) = 0.99(3).

The molecular structure of PrI3 and GdI3 as determined by synchronous gas-phase electron diffraction and mass spectrometric experiment assisted by quantum chemical calculations

A gas electron diffraction study of PrI3 and GdI3 has been carried out in combination with mass spectrometric vapour monitoring at 1110(10) K and 1100(10) K, respectively. Up to 3 mol.% of dimeric species was observed in addition to the dominating monomeric molecules. The change of the thermal-averaged r g configuration parameters of the molecules in the series LaI3 → LuI3 reflects the lanthanide contraction. A low value of the shrinkage δ(I···I) even at such a high temperatures may be considered due to vibration effects in molecule whose equilibrium geometric nuclear structure is planar and which correspond to configurationally averaged 4f n electronic state. B3LYP calculations performed in this study with large core potential for lanthanide atoms also resulted in equilibrium geometry of D 3h symmetry. According to the quantum chemical calculations, the potential function the non-planar vibration is essentially anharmonic, which is therefore to be taken into account to correctly describe nuclear dynamics in molecules such as LnI3.

The molecular structure of TbI 3, DyI 3, HoI 3 and ErI 3 as determined by synchronous gas-phase electron diffraction and mass spectrometric experiment assisted by quantum chemical calculations

A first gas electron diffraction study of TbI 3, DyI 3, HoI 3 and ErI 3 has been carried out in combination with mass spectrometric vapour monitoring at 1117(10), 1117(10), 1132(10) and 1102(10) K, respectively. Up to 4 mol.% of dimeric species was observed in addition to the dominating monomeric molecules. The change of the thermal-averaged r g-configuration parameters of the molecules in the series TbI 3 → ErI 3 reflect the lanthanide contraction. A low value of the shrinkage δ(I…I) even at such a high temperatures may be considered due to vibration effects in molecule whose equilibrium geometric nuclear structure is planar and which correspond to configurationally-averaged 4f n electronic state. B3LYP and МР2 calculations performed in this work with large core potential for lanthanide atoms also resulted in equilibrium geometry of D 3h symmetry. According to the both quantum chemical methods applied the potential function the non-planar vibration is essentially anharmonic, which is therefore to be taken into account to correctly describe nuclear dynamics in molecules such as LnI 3. Different models applied in this study predict the equilibrium distance r e(Ln–I) to be shorter than the thermal-averaged r g(Ln–I) distance by 0.002–0.018(1) Å.

A gas electron diffraction study of the conformational composition of 1,3,5-trimethyl-1,3,5-triazacyclohexane

The conformational composition of 1,3,5-trimethyl-1,3,5-triazacyclohexane was studied by gas electron diffraction and quantum-chemical calculations at the density functional theory (B3LYP) and MP2 levels. Conformers with the general chair, boat, and twist ring forms were considered possible. These structures differed in the arrangement of CH3 groups in the axial (a) and equatorial (e) positions. A chair conformer with the axial orientation of one CH3 group was found to satisfy the electron diffraction data. Its main structural parameters (mean values) were r g(C-N) = 1.463(3) A, r g(C-H) = 1.117(5) A, ∠(C-N-C) = 110.91(1)°, and ∠(N-C-N) = 111.1(1)°.

Structure and conformation of (trifluoromethyl)thiobenzene, C 6H 5SCF 3: Gas electron diffraction and quantum chemical calculations

The geometric structure and conformation of (trifluoromethyl)thiobenzene, C 6H 5SCF 3, has been determined by two independent gas electron diffraction (GED) studies at the Universities of Moscow and Ivanovo and by quantum chemical calculations. Both experimental studies result in perpendicular orientation of the S CF 3 bond relative to the benzene plane. This result is confirmed by several quantum chemical calculations (HF/6-31G(d), B3LYP/cc-pVTZ, MP2/6-31G(d), MP2/6-311+G(d,p) and MP2/cc-pVTZ) which predict single-minimum potential functions for internal rotation around the C(sp 2) S bond. A double-minimum potential function predicted by the B3LYP/6-31G(d) calculation is not compatible with the experiment. B3LYP and MP2 calculations with large basis sets (6-311+G(d,p) and cc-pVTZ) predict barriers to internal rotation around the C(sp 2) S bond between 2.4 and 2.6 kcal/mol. The geometric parameters of both experimental studies agree with each other within the experimental uncertainties, except for the C F bond distance. The effect of fluorination of the methyl group in thioanisole, C 6H 5SCH 3, i.e., conformational change from planar orientation of the S CH 3 bond to perpendicular orientation of the S CF 3 bond, is rationalized on the basis of a natural bond orbital (NBO) analysis. The predominant effect is loss of conjugation between the p-shaped sulfur lone pair and the π system of the ring upon fluorination.

Gas phase structure of ((fluoroformyl)imido)sulfuryl difluoride, FC(O)N [dbnd6]S(O)F 2

The geometric structure and conformational properties of gaseous ((fluoroformyl)imido) sulfuryl difluoride, FC(O)N S(O)F 2, was investigated by gas electron diffraction (GED) and quantum chemical methods (MP2 and B3LYP with 6-31G(d) and 6-311+G(2df) basis sets). In combination with earlier reported infrared spectra, the GED study results in a mixture of at least three conformers, 71(10)% syn(N S)– syn(N–C), 14(8)% anti(N S)– syn(N–C) and 15(5)% syn(N S)– anti(N–C). Syn(N S) or anti(N S) implies synperiplanar or anticlinal orientation of the FC(O) group with respect to the SF 2 bisector and syn(N–C) or anti(N–C) implies synperiplanar or antiperiplanar orientation of the C O bond with respect to the N S bond. The anti(N S)– anti(N–C) conformer has not been observed, but a small amount (<5%) cannot be excluded. These conformational properties are well reproduced by quantum chemical calculations with small basis sets and the geometric parameters are reproduced satisfactorily with large basis sets.

Gas electron diffraction study of the vapour over dimethylamine–gallane leading to an improved structure for dimeric dimethylamidogallane, [Me2NGaH2]2: a cautionary tale

Dimethylamine-gallane is relatively slow to decompose in a closed system and vaporises at low temperature primarily as Me2(H)N.GaH3 molecules which can be trapped in a solid Ar matrix and characterised by their IR spectrum. Under the conditions needed to secure a useful gas electron diffraction (GED) pattern, however, the vapour was found to consist of dimeric dimethylamidogallane molecules, [Me2NGaH2]2, formed from the secondary amine adduct by elimination of H2, and the most reliable structure for which has been determined. Salient structural parameters (r(hl) structure) were found to be: r(Ga-N) 202.6(2), r(Ga-H) 155.6(8), r(N-C) 148.0(3), r(C-H) 111.2(6) pm; Ga-N-Ga 90.7(1), C-N-C 109.3(5), N-C-H 109.9(10) and H-Ga-H 119.4(42) degrees.

Gas Electron Diffraction and Quantum-Mechanical Study of Dimethyloxalate

The geometrical structure and conformation of dimethyloxalate, CH3OC(O)–C(O)OCH3, have been studied by gas electron diffraction (GED) and quantum-chemical calculations (MP2 and B3LYP methods with 6-31G* and cc-pVTZ basis sets). The GED analysis with a dynamic model (T = 323 K) results in a mixture of two planar conformers, anti (C2h symmetry) and syn (C2v symmetry) orientation of the two C=O bonds. The energy difference between these conformers is 0.02(0.18) kcal/mol and barrier to internal rotation around the C–C bond is 0.44(0.41) kcal/mol. The CH3 groups occupy synperiplanar positions with respect to the C=O bonds. The following main geometrical parameters for the anti conformer (A and degrees) have been derived: rg(C–C) = 1.532(3), rg(C=O) = 1.203(2), rg(Csp3–O) = 1.436(3), rg(Csp2–O) = 1.333(3), ∠α(Csp2–Csp2–O) = 111.9(1.9), ∠α(Csp2–O–Csp3) = 116.3(1.6), ∠α(O–C= O) = 127.0(1.8).

Early Days of Gas Electron Diffraction Studies in Japan

Several episodes, never printed except in Japanese, related to the young days of Yonezo Morino are documented. Stories highlight how his personal contacts with his teachers and colleagues and his studies and experiences in spectroscopy, quantum theory etc. had influenced his early works on gas-phase electron diffraction (GED).

Gas electron diffraction study of the geometric structure of triallylborane molecule

The main structural parameters of the triallylborane molecule having the C3 symmetry were determined by gas electron diffraction and quantum-chemical calculations at the MP2/6-31G(d,p) and B3LYP/6-31G(d,p) levels. The parameters calculated by the MP2/6-31G(d,p) method are in better agreement with the experimental data than those calculated by the B3LYP/6-31G(d,p) method.

Molecular structure and nicotinic activity of arecoline. A gas electron diffraction study combined with theoretical calculations

The molecular structure of arecoline (methyl 1,2,5,6-tetrahydro-1-methylnicotinate, ▪ has been determined by gas electron diffraction. Diffraction patterns were taken at about 370 K. Structural constraints for the data analysis were obtained from MP2/6-31G** calculations. Vibrational mean amplitudes and shrinkage corrections were calculated from the force constants obtained from the gas-phase vibrational frequencies and the B3LYP/6-31G** calculations. The electron diffraction data were well reproduced by assuming the mixture of four conformers. The determined structural parameters ( r g (Å) and ∠ α (°)) for the main conformer with 3 σ in parentheses are as follows: 〈 r g(N–C ring)〉=1.456(4); r g(N–C methyl)=1.451 (d.p.); r g(C C)=1.339(9); 〈 r g(C–C)〉=1.512(3); r g(O–C methyl)=1.434(5); r g(C(O)–O)=1.355 (d.p.); r g(C O)=1.209(4); the out-of-plane angle of the methyl group=50.3(23); ∠C ringN ringC ring=112.8(30); ∠N ringC ringC ring(H 2)=110.5(16); 〈∠C ringC ringC ring〉=118.4(5); ∠C CC(O)=116.8(7); ∠CC O=127.6(9); ∠CC–O=109.8(8), where the angle brackets denote averaged values and d.p. denotes dependent parameters. Fixing the abundances of the minor conformers, Ax-s- cis and Ax-s- trans, at the theoretical values (13% in total), those of the Eq-s- cis and Eq-s- trans conformers were determined to be 46(16) and 41(16)%, respectively. Here Ax and Eq denote the axial and equatorial directions of the N–CH 3 bond and s- cis and s- trans show the orientation of the methoxycarbonyl group expressed by the configuration of the C O and C C bonds. The N⋯O carbonyl distances of the Eq-s- cis and Ax-s- cis conformers are 4.832(13) and 4.874(16) Å, respectively. They are close to the N⋯N distance of the most abundant conformer of nicotine, 4.885(6) Å, suggesting that the Eq-s- cis and Ax-s- cis conformers have nicotinic activity.