Gas-phase Electron Diffraction Study Research Articles

Picosecond infrared laser driven sample delivery for simultaneous liquid-phase and gas-phase electron diffraction studies.

Here, we report on a new approach based on laser driven molecular beams that provides simultaneously nanoscale liquid droplets and gas-phase sample delivery for femtosecond electron diffraction studies. The method relies on Picosecond InfraRed Laser (PIRL) excitation of vibrational modes to strongly drive phase transitions under energy confinement by a mechanism referred to as Desorption by Impulsive Vibrational Excitation (DIVE). This approach is demonstrated using glycerol as the medium with selective excitation of the OH stretch region for energy deposition. The resulting plume was imaged with both an ultrafast electron gun and a pulsed bright-field optical microscope to characterize the sample source simultaneously under the same conditions with time synchronization equivalent to sub-micrometer spatial resolution in imaging the plume dynamics. The ablation front gives the expected isolated gas phase, whereas the trailing edge of the plume is found to consist of nanoscale liquid droplets to thin films depending on the excitation conditions. Thus, it is possible by adjusting the timing to go continuously from probing gas phase to solution phase dynamics in a single experiment with 100% hit rates and very low sample consumption (<100 nl per diffraction image). This approach will be particularly interesting for biomolecules that are susceptible to denaturation in turbulent flow, whereas PIRL–DIVE has been shown to inject molecules as large as proteins into the gas phase fully intact. This method opens the door as a general approach to atomically resolving solution phase chemistry as well as conformational dynamics of large molecular systems and allow separation of the solvent coordinate on the dynamics of interest.

The molecular species in saturated and overheated vapors of Sc(acac)3: A first structural study of bis-acetylacetonate scandium Sc(acac)2 radical

Mass spectrometric and gas-phase electron diffraction (GED/MS) studies of overheated Sc(acac)3 vapor at temperature 812(5) K were performed. Experimental data indicate that overheated vapor contains, along with tris-form Sc(acac)3, bis-complex Sc(acac)2, which is the result of partial thermal dissociation of tris form. Testing of three models D2h(2Ag), D2h(2B3g) and D2d(2B1) of bis-complex geometric and electronic structure showed that the planar model D2h(2B3g) best fits GED/MS data. Reinterpretation of GED/MS data for saturated vapor of Sc(acac)3 at T = 428 K was carried out. Temperature dependence of rg(Sc-O) parameters of effective temperature-averaged geometric configuration Sc(acac)3 is noted.

Joint gas-phase electron diffraction and quantum chemical study of conformational landscape and molecular structure of sulfonamide drug sulfanilamide

Conformational composition and molecular structure of sulfanilamide (para-aminobenzenesulfonamide, SA) has been investigated by means of gas electron diffraction (GED) and quantum chemical (QC) calculations. Conformations with eclipsed orientation of the S=O and the N–H bonds in the sulfonamide moiety have been found to be predominant in vapor at the average temperature of the GED experiment of 184(5) °C. The structural parameters of the most stable conformer are the following (rh1 in A and ∠h1 in ° with 3σ in parenthesis): r(C=C) av = 1.410(4), r(S=O)av = 1.433(4), r(C–S) = 1.763(6), r(S–N) = 1.649(6), ∠CSN = 104.7(15), (∠CSO)av = 109.0(8). The orientation of the S–N bond of the sulfonamide group about the anilinic ring plane has been found to be different from orthogonal by about 13°. It has been shown that QC calculations tend to overestimate the values the S=O bond lengths as well as are not always accurate in the prediction of mutual orientation of the sulfonamide group and the anilinic ring plane. While in the gas phase, low energy conformations are found to be the most abundant; the molecular structure of SA in the crystal phase resembles a high energy conformation with staggered orientation of the N–H and the S=O bonds for all polymorph modifications. The mechanisms of mutual transformations of different SA conformers into each other have also been considered and discussed.

The unusual symmetry of hexafluoro-o-xylene-A microwave spectroscopy and computational study.

The rotational constants and quartic centrifugal distortion coefficients of hexafluoro-o-xylene were precisely derived from the 8 GHz to 18 GHz gas phase microwave spectrum. In addition, the rotational constants of all singly substituted 13C isotopologues were determined. Instead of the intuitively expected symmetry of C2v, as in o-xylene, calculations with a variety of methods (B3LYP, CAM-B3LYP, ωB97XD, MP2, and coupled-cluster singles, doubles, and perturbative triples) predict a C2 symmetry structure in which the two CF3 groups rotate in opposite directions by about 16°. While the experimental results in this study are not capable of proving this unusual symmetry, they can support future microwave, gas phase electron diffraction or nuclear magnetic resonance studies. From the presented data, a preliminary r0 structure was determined, reproducing the experimental rotational constants with deviations of no more than 15 kHz. Analysis of the interactions between the two CF3 groups using an effective fragment potential approach identifies two major contributions to their interaction, due to exchange-repulsion and electrostatic repulsion, with electrostatic repulsion responsible for the barrier at the C2v geometry.

Small Neutral Geminal Silicon/Phosphorus Frustrated Lewis Pairs

Two new silicon/phosphorus‐based frustrated Lewis pairs (FLP), F3SiCH2PtBu2 (1) and Cl3SiCH2PtBu2 (2) were prepared from lithiated di‐tert‐butylmethylphosphane, LiCH2PtBu2, and the corresponding silicon tetrahalides. They were characterised by NMR spectroscopy and by single‐crystal X‐ray diffraction. A gas phase electron diffraction study of 1 identified two conformers of similar energy in the vapor. The reactivity of both, 1 and 2, towards a series of small molecules was investigated. Phenyl isocyanate was found to undergo addition reactions of its C‐N unit to afford five‐membered SiCPC(O)N‐ring structures. Cl3SiCH2PtBu2 (2) was converted with perfluorophenyllithium and 3,5‐bis(trifluoromethyl)phenyllithium. The reaction with LiC6F5 resulted in the ortho‐fluoride substitution product (C6F5)2SiCH2(C6F4)PtBu2 (5), while the aryl reagent without ortho‐fluoride groups, leads to the intramolecular Lewis pair (C8H3F6)3SiCH2PtBu2 (6). The FLP reactivity of 6 was tested.

The equilibrium molecular structure of 2-cyanopyridine from combined analysis of gas-phase electron diffraction and microwave data and results of ab initio calculations

The gas-phase electron diffraction study of 2-cyanopyridine was carried out for the first time. Results of ab initio structure calculations performed at the CCSD(T) level of theory agree well with the equilibrium structure determined by the electron diffraction method in combination with vibrational spectroscopy data and microwave rotational constants. The deviations between them are only a few thousandths of A units and a few tenths of degree in the bond lengths and bond angles, respectively. The structure in the solid state is more different from that in the gas phase. The observed discrepancies between these structures are up to 0.02 A and 2° in the bond lengths and bond angles, respectively. The influence of the ortho-, meta-, and para-cyano substituents on the geometry of pyridine ring is discussed. The pyridine ring is noticeably distorted due to cyano substituents. The Cipso–N and/or Cipso–C bond lengths are elongated both in 2-CNP and 4-CNP by 0.004 A in comparison to those in pyridine, whereas the Cipso–C bond lengths are increased by 0.005 and 0.009 A in 3-CNP.

Internal rotation and intramolecular hydrogen bonding in thiosalicylamide: gas phase electron diffraction study supported by quantum chemical calculations

The molecular structure of thiosalicylamide (2-hydroxythiobenzamide) was investigated in the gas phase at 401 K by means of gas electron diffraction (GED) combined with quantum chemical (QC) calculations. Special attention was paid to the internal rotation of the thioamide group. Structural refinement was performed taking into account rovibrational corrections to the thermal-average internuclear distances calculated with harmonic and anharmonic (cubic) MP2/cc-pVTZ force constants in terms of static and dynamic models. It was shown that both models fitted the GED data equally well. The results of the GED refinement revealed that in the equilibrium structure, the thioamide group is twisted by about 30° with respect to the phenol ring plane. This is the result of an interatomic repulsion of hydrogen atom in the amide group from the closest hydrogen atom of the benzene ring, which overcomes the energy gain from the π−π conjugation of the thioamide group and the aromatic system of thiosalicylamide. Natural bond orbital (NBO) analysis and comparison of the thiosalicylamide molecular structure with those of related compounds revealed hydrogen-bonded fragment between the hydroxyl and thiocarbonyl groups. The structure of thiosalicylamide in the gas phase was found to be markedly different from that in the solid phase due to the effect of intermolecular hydrogen bonding in the crystal.

The perfluorinated alcohols c-C6F11OH, c-C6F10-1,1-(OH)2 and c-C6F10-1-(CF3)OH.

The thermally unstable α-fluoroalcohol undecafluorocyclohexanol (c-C6F11OH) was prepared by addition of hydrogen fluoride to the corresponding ketone. c-C6F10(CF3)OH was obtained by protonation of its alkoxide [NMe4]+[C7F13O]-. Decafluorocyclohexane-1,1-diol (c-C6F10(OH)2) was prepared by acidic workup of the corresponding alkoxide [NMe4]+[C6F11O]- with sulfuric acid, which yielded (c-C6F10(OH)2) and fluorosulfonic acid. The structures of c-C6F10(CF3)OH·2H2O and of (c-C6F10(OH)2) were elucidated by single-crystal X-ray and gas-phase electron-diffraction studies.

Combined Electron-Diffraction and Spectroscopic Determination of the Structure of Spiropentane, C5H8.

Gas-phase electron-diffraction (GED) data have been combined with recent spectroscopic rotational constants to determine the rα0 structural parameters for spiropentane, C5H8. The structure has D2d symmetry, and the results yield values of 1.105(2) Å for the CH bond length, 1.557(3) Å for the distal CC bond length, and a smaller value of 1.482(1) Å for the four lateral CC bonds that connect to the central carbon atom. The HCH angle is 113.7(13)°, and the HCH flap angle, defined as the angle of the HCH bisector and the distal CC bond, is 150.2(16)°. Corresponding rg values are 1.122(2) Å, 1.560(3) Å, 1.485(1) Å, 115.1(13)°, and 148.9 (16)°. The results are in good accord with values from density functional calculations (B3LYP/cc-pVTZ) and resolve some questions about the structure reported in an earlier GED study, in particular about the HCH angle and anomalous rotational constants calculated for the structure.

Molecular structure of (cyanomethoxy)(dimethylamino)methane as studied by gas-phase electron diffraction and quantum-chemical calculations

The molecular structure of (cyanomethoxy)(dimethylamino)methane in the gas phase is investigated by means of gas-phase electron diffraction and quantum-chemical studies for the first time. It is shown that in the gaseous state, (cyanomethoxy)(dimethylamino)methane is a mixture of gauche-anti (65%) and anti-gauche (35%) conformers. The slight influence of the anomeric effect on the compound’s structural parameters was noted.

Substituent effect on geometric and electronic structure of benzenesulfonic acid: gas-phase electron diffraction and quantum chemical studies of 4-CH3C6H4SO3H and 3-NO2C6H4SO3H molecules

A combined gas-phase electron diffraction/mass-spectrometric and quantum chemical (B3LYP/cc-pVTZ, MP2/cc-pVTZ) study of the molecular structures of para-methylbenzenesulfonic acid (4-MBSA) and meta-nitrobenzenesulfonic acid (3-NBSA) was carried out. On the basis of mass spectrometric analysis, it was found that the substituted benzenesulfonic acids are thermostable at least up to 431(3) K. The fragmentations of 4-MBSA and 3-NBSA molecules under electron impact were analyzed. Quantum chemical calculations show that the 4-MBSA molecule exists as an enantiomeric pair, which is formed as a result of rotation of OH group about the S–O(H) bond. The 3-NBSA molecule has two conformers with different orientations of the O–H bond with respect to the nitro group and two corresponding enantiomers. The equilibrium configurations of 4-MBSA and both conformers of 3-NBSA have similar structures of the SO3H group, with the O–H bond eclipsing one of the S=O bonds. Selected experimental bond distances for 4-MBSA/3-NBSA are (A) rh1(C–C)av = 1.403(3)/1.395(4); rh1(C–S) = 1.765(5)/1.784(5); rh1(S=O)av = 1.433(4)/1.438(4); and rh1(S–O) = 1.618(4)/1.620(4). The potential functions for the internal rotation of SO3H, OH, and CH3 or NO2 groups were calculated, and the transition states between enantiomers (conformers) were determined. The influence of substituent's nature on molecular geometry as well as on the energies of frontier orbitals and red-ox properties of the compounds is discussed. The inductive and mesomeric substituent effects were estimated from the donor–acceptor interaction energies of the natural bond orbitals of substituent and benzene frame. The correlation between group electronegativities and cooperative energetic characteristics of inductive and mesomeric effects of substituents is shown.

Molecular structure and relative stability of trans and cis isomers of formanilide: gas-phase electron diffraction and quantum chemical studies

Molecular structure of formanilide is determined by gas-phase electron diffraction (GED) augmented by quantum chemical calculations (B3LYP/cc-pVTZ and MP2/cc-pVTZ) and literature microwave (MW) data. The combined GED and MW data are well reproduced for the mixture of trans and cis isomers with the relative abundance of 59 ± 5 and 41 ± 5 %, respectively, at T = 410 K. The trans isomer (C s symmetry) is planar, while the cis isomer (C 1 symmetry) has the twisted structure with the amide group rotated by 36.7 ± 2.7° with respect to the phenyl ring. In accord with theoretical calculations, the amide bond –NH–C(O)– is planar in trans formanilide and a somewhat nonplanar in cis isomer. Accurate structural parameters were obtained by a simultaneous fit of the rotational constants reported in the literature and GED intensities obtained in this study. The N–C(O) and N–CPh bond dissociation energies in formanilide are calculated using Gaussian-4 method. It is revealed that the strength of N–C(O) bond in formanilide is 50 kJ/mol less than that in benzamide. On the contrary, the strength of adjacent bond (N–CPh) increases by 35 kJ/mol compared to aniline. It is rather unexpectedly that the bond strength weakening does not result in the bond elongating, and vice versa.

The influence of steric hindrance on conformation properties and molecular structure of 2,4,6-trinitrobenzenesulfonic acid: gas electron diffraction and quantum chemical calculations

A combined gas-phase electron diffraction and quantum chemical (B3LYP/cc-pVTZ, MP2/cc-pVDZ) study of molecular structure of 2,4,6-trinitrobenzenesulfonic acid (2,4,6-tri-NBSA) was carried out. Quantum chemical calculations showed that 2,4,6-tri-NBSA possesses six conformers, which form three pairs of enantiomers with the relative energy of 0, 4.4/3.9, and 2.5/2.5 kcal/mol. It was experimentally established that at T = 444(5) K a saturated vapor over 2,4,6-tri-NBSA is, predominantly (up to 93 mol.%), represented by a low-energy enantiomers II and II′ characterized by intramolecular hydrogen bond between an H atom of the hydroxyl group and one of the O atoms of the NO2 group. Experimental internuclear distances for the low-energy enantiomers are (A): r h1(C–C)av. = 1.387(4), r h1(C–S) = 1.811(6), r h1(S=O)av. = 1.424(4), r h1(S–O) = 1.579(4), r h1(N–O)av. = 1.214(3), r h1(C–N)av. = 1.491(5). Geometry of the conformer II points on existance of strong steric interactions between SO2OH group and two ortho-nitro groups. Analysis of the orbital interactions between the substituents and benzene ring was carried out. Geometric parameters and energies of transition states between conformers were calculated (B3LYP).

Determination of molecular structure of succinic acid in a very complex conformational landscape: Gas-phase electron diffraction (GED) and ab initio studies

The molecular structure of succinic acid has been investigated by the gas-phase electron diffraction (GED) method for the first time. According to predictions of MP2/cc-pVTZ calculations, the molecule has 18 stable conformers with the C C C C chain in the gauche ( G) or anti ( A) configuration, and four of them, I ( G), II ( A), III ( G) and IV ( A) belonging to the C 2, C 2h, C 1 and C 1 point groups, respectively, with relative energies ΔE ZPE within 2.2 kcal/mol can be present at the experimental temperature of 445 K in noticeable amounts. The ratio of the conformers I:II:III:IV = 45(15):20(15):10(assumed):25(15) (in %) has been determined in the GED analysis guided by theoretical predictions. To take into account vibrational effects, the corrections Δ( r e − r a ) to the experimental r a bond lengths were calculated from the MP2/cc-pVTZ quadratic and cubic force constants. The obtained equilibrium structural parameters of the dominant conformer I are the following (bond lengths in Å, angles in degrees): r e (C sp3 C sp3) = 1.508(3), r e (C sp3 C sp2) = 1.499(2), r e (C O) = 1.343(2), r e (C O) = 1.202(1), e (C C C) = 111.8(4), e (C C O) = 112.0(4), e (O C O) = 123.0(1), τ(C C C C) = 69.9(11). Yielding the best agreement with the GED structure, the MP2/cc-pVQZ approximation overestimates the C O and C O bond lengths by ca. 0.005(2) Å.

Conformational properties of ortho-nitrobenzenesulfonamide in gas and crystalline phases. Intra- and intermolecular hydrogen bond

A combined gas-phase electron diffraction and quantum chemical (B3LYP/6-311+G**, B3LYP/cc-pvtz, MP2/cc-pvtz) study of molecular structure of 2-nitrobenzenesulfonamide (2-NBSA) was carried out. Quantum chemical calculations showed that 2-NBSA has four conformers, two of which are stabilized by intramolecular hydrogen bond. The latter (with the S–N bond in a close to orthogonal position around the phenyl ring and differing from each other by staggered or eclipsed positions of the N–H and S=O bonds in the SO2NH2 group) presented in a saturated vapor over 2-NBSA at T = 433 (3) K in commensurable amounts. Experimental internuclear distances (Ǻ) for the staggered conformer are (A): r h1(C–H)av. = 1.071(9), r h1(C–C)av. = 1.390(4), r h1(C–S) = 1.789(8), r h1(S=O)av. = 1.427(6), r h1(S–N) = 1.644(6), r h1(N–O)av. = 1.221(4), r h1(C′–N) = 1.487(8), r h1(N–H)av. = 1.014. Calculations at B3LYP/cc-pvtz level were performed to determine the structure and the energies of the transition states between conformers. It was shown that the conformer structures of free molecule differ from those of a molecule stabilized by intermolecular hydrogen bonds in a crystal. Influence of a substituent X (X = –CH3, –NO2) on conformational features of the ortho-substituted benzenesulfonamide was established.

Anomeric effect in N-azidomethylpyrrolidine: gas-phase electron diffraction and theoretical study

Gas-phase electron-diffraction data and high-level quantum chemical calculations have been used to study the conformational behaviour of N-azidomethylpyrrolidine. The two most stable conformers with a relative abundance of about 80% at 298 K possess gauche orientation of the azidomethyl group around the C-N(pyr) bond (C-N(azido)gauche with respect to the endocyclic N(pyr)-C bond). This orientation is a strong manifestation of an anomeric effect. The influence of the anomeric effect is also reflected in shortening of the C-N(pyr) bond and lengthening of the C-N(azido) bond as compared to such bonds in other compounds.

Use of imaging plates (IPs) in the gas-phase electron diffraction (GED) experiments on the EG-100 M apparatus. The tetrachloromethane molecule as a test object

The gas-phase electron diffraction (GED) experiment at the Moscow State University was carried out for the first time using imaging plates (IPs). The response of the Fuji BAS-MP IPs calibrated by means of 14C standard sources was found to be linear in a broad dynamic interval (larger than 104). The GED study of tetrachloromethane was carried out in order to test the applied technique (IP reader FLA7000) and the developed software. The determined structural parameters of the CCl4 molecule are in excellent agreement with those obtained in experiments with photographic registration.

Molecular structure and stabilities of fumaric acid conformers: Gas phase electron diffraction (GED) and quantum-chemical studies

The molecular structure of fumaric acid was reinvestigated by the gas phase electron diffraction (GED) method because it was determined several years ago assuming the existence of one conformer only. According to MP2/cc-pVTZ predictions, the molecule has six stable conformations, and three of them, sp,sp,sp,sp (I), ap,sp,sp,sp (II) and ap,ap,sp,sp (III), with relative energies within ca. 3 kJ mol −1 should be present in detectable amounts at the experimental temperature of ca. 480 K. The best fit to the GED intensities was achieved at the approximately equal amounts of the conformers I–III. To take into account vibrational effects, the corrections to the experimental r a bond lengths ( r e − r a ) were calculated using quadratic and cubic force constants from high-level ab initio calculations (MP2/cc-pVTZ). The deduced equilibrium bond lengths of the conformer II with the estimated total errors (in Å) are the following: r e (C C) = 1.331(3), r e (C C) s- trans = 1.473(3), r e (C C) s- cis = 1.479(3), r e (C O) s- trans = 1.345(3), r e (C O) s- cis = 1.344(3), r e (C O) s- trans = 1.205(3), r e (C O) s- cis = 1.204(3). The structural effects arising due to presence of the C C double bond were analysed. In comparison to the experiment, the MP2/cc-pVQZ approximation overestimates some bond lengths in fumaric acid and similar molecules by up to 0.005 Å. The rotational constants of the conformers I–III were derived from the GED equilibrium geometries.

Tautomeric and conformational properties of dibenzoylmethane, C6H5–C(O)–CH2–C(O)–C6H5: gas-phase electron diffraction and quantum chemical study

Tautomeric and structural properties of dibenzoylmethane, C6H5–C(O)–CH2–C(O)–C6H5, have been investigated by gas-phase electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximation with different basis sets up to cc-pVTZ). Analysis of GED intensities resulted in the presence of 100(5)% enol tautomer at 380(5) K. The enol ring possesses C S symmetry with a strongly asymmetric hydrogen bond. The two phenyl rings are rotated with respect to the enol ring by 15.1(5.0) and 12.1(5.8)°. The experimental geometric parameters are reproduced very closely by the B3LYP/cc-pVTZ method.

On the Molecular and Electronic Structures of AsP3and P4

The molecular and electronic structures of AsP(3) and P(4) have been investigated. Gas-phase electron diffraction studies of AsP(3) have provided r(g) bond lengths of 2.3041(12) and 2.1949(28) A for the As-P interatomic distances and the P-P interatomic distances, respectively. The gas-phase electron diffraction structure of P(4) has been redetermined and provides an updated value of 2.1994(3) A for the P-P interatomic distances, reconciling conflicting literature values. Gas-phase photoelectron spectroscopy provides experimental values for the energies of ionizations from the valence molecular orbitals of AsP(3) and P(4) and shows that electronically AsP(3) and P(4) are quite similar. Solid-state (75)As and (31)P NMR spectroscopy demonstrate the plastic nature of AsP(3) and P(4) as solids, and an extreme upfield (75)As chemical shift has been confirmed for the As atom in AsP(3). Finally, quantum chemical gauge-including magnetically induced current calculations show that AsP(3) and P(4) can accurately be described as strongly aromatic. Together these data provide a cohesive description of the molecular and electronic properties of these two tetraatomic molecules.