CF2XCF2X and CF2XCF2• Radicals (X = Cl, Br, I): Ab Initio and DFT Studies and Comparison with Experiments

Rotational Isomerization of Carbon-Carbon Single Bonds in Ethyl Radical Derivatives in a Room-Temperature Solution

The rotational isomerization of 1,2-disubstituted ethyl radical derivatives, reaction intermediates often found in the reaction of 1,2-disubstituted ethane derivatives, has never been measured because of their short lifetime and ultrafast rotation. However, the rotational time constant is critical for understanding the detailed reaction mechanism involving these radicals, which determine the stereoisomers of compounds produced via the intermediates. Using time-resolved infrared spectroscopy, we found that the CF₂BrCF₂ radical in a CCl₄ solution rotationally isomerizes with a time constant of 47 ± 5 ps at 280 ± 2 K. From this value and the rotational barrier heights of related compounds, CH₃CH₂ and CH₃CH₂CHCH₃ radicals in CCl₄ were estimated to rotationally isomerize within 1 ps at 298 K, considerably faster than ethane and n-butane, which rotationally isomerize with time constants of 1.8 and 81 ps, respectively. The time constant for the rotational isomerization was similar to that calculated using transition state theory with a transmission coefficient of 0.75.

Structural Dynamics of C2F4I2 in Cyclohexane Studied via Time-Resolved X-ray Liquidography

The halogen elimination of 1,2-diiodoethane (C2H4I2) and 1,2-diiodotetrafluoroethane (C2F4I2) serves as a model reaction for investigating the influence of fluorination on reaction dynamics and solute-solvent interactions in solution-phase reactions. While the kinetics and reaction pathways of the halogen elimination reaction of C2H4I2 were reported to vary substantially depending on the solvent, the solvent effects on the photodissociation of C2F4I2 remain to be explored, as its reaction dynamics have only been studied in methanol. Here, to investigate the solvent dependence, we conducted a time-resolved X-ray liquidography (TRXL) experiment on C2F4I2 in cyclohexane. The data revealed that (ⅰ) the solvent dependence of the photoreaction of C2F4I2 is not as strong as that observed for C2H4I2, and (ⅱ) the nongeminate recombination leading to the formation of I2 is slower in cyclohexane than in methanol. We also show that the molecular structures of the relevant species determined from the structural analysis of TRXL data provide an excellent benchmark for DFT calculations, especially for investigating the relevance of exchange-correlation functionals used for the structural optimization of haloalkanes. This study demonstrates that TRXL is a powerful technique to study solvent dependence in the solution phase.

Formation and Infrared Spectrum of the Open-Form 2-Bromoethyl Radical (2-C2H4Br•) from Ultraviolet Irradiation of a C2H4/Br2/p-H2 Matrix

The addition reaction of halogens to alkenes is important in organic synthesis, but the reaction intermediate has rarely been detected. Whether the structure of the intermediate bromoethyl (C₂H₄Br^•) radical is a bridged form or an open form is unclear. We took advantage of the diminished cage effect of solid p-H₂ and employed infrared (IR) absorption to record the IR spectrum of C₂H₄Br^• after photolysis of a C₂H₄/Br₂/p-H₂ matrix at 254 nm, followed by annealing. New spectral features at 676.9, 776.7, 1068.5, 1148.0, 3041.8, and 3126.8 cm^-1 are assigned to the open-form 2-bromoethyl radical, according to their photolytic behavior and comparison with scaled harmonic vibrational wavenumbers and IR intensities calculated with the B2PLYPD3/6-311++G(2df,2p) method.

Photodissociation of CF2ICF2I in solid para-hydrogen: infrared spectra of anti- and gauche-˙C2F4I radicals

Ultraviolet photolysis of 1,2-diiodotetrafluoroethane (CF₂ICF₂I) produced anti- and gauche-˙C₂F₄I radicals.

Perspective: 4D ultrafast electron microscopy--Evolutions and revolutions

In this Perspective, the evolutionary and revolutionary developments of ultrafast electron imaging are overviewed with focus on the "single-electron concept" for probing methodology. From the first electron microscope of Knoll and Ruska [Z. Phys. 78, 318 (1932)], constructed in the 1930s, to aberration-corrected instruments and on, to four-dimensional ultrafast electron microscopy (4D UEM), the developments over eight decades have transformed humans' scope of visualization. The changes in the length and time scales involved are unimaginable, beginning with the micrometer and second domains, and now reaching the space and time dimensions of atoms in matter. With these advances, it has become possible to follow the elementary structural dynamics as it unfolds in real time and to provide the means for visualizing materials behavior and biological functions. The aim is to understand emergent phenomena in complex systems, and 4D UEM is now central for the visualization of elementary processes involved, as illustrated here with examples from past achievements and future outlook.

Synthesis, crystal structure, DFT/TDDFT calculation, photophysical properties and DNA binding studies of morpholino moiety ligand based two Cu(ii) complexes in combination with carboxylates

Two Cu(II) compounds have been characterized by structure analyses and DFT/TD-DFT calculations. Both the complexes potentially bind with CT-DNA and corresponding binding constants are in the order of 10⁵ M⁻¹.

Synthesis, characterization, TDDFT calculation and biological activity of tetradentate ligand based square pyramidal Cu(ii) complexes

Fluorescent Cu(ii) compounds have been synthesized, and characterized by structure analyses and DFT/TD-DFT calculations. Compounds show catecholase and anticancer activities.

Density functional and spin-orbit ab initio study of CF3Br: molecular properties and electronic curve crossing

Quantum chemical calculations of CF(3)Br and the CF(3) radical are performed using density functional theory (DFT) and time-dependent DFT (TDDFT). Molecular structures, vibrational frequencies, dipole moment, bond dissociation energy, and vertical excitation energies of CF(3)Br are calculated and compared with available experimental results. The performance of six hybrid and five hybrid meta functionals in DFT and TDDFT calculations are evaluated. The ωB97X, B3PW91, and M05-2X functionals give very good results for molecular structures, vibrational frequencies, and vertical excitation energies, respectively. The ωB97X functional calculates well the dipole moment of CF(3)Br. B3LYP, one of the most widely used functionals, does not perform well for calculations of the C-Br bond length, bond dissociation energy, and vertical excitation energies. Potential energy curves of the low-lying excited states of CF(3)Br are obtained using the multiconfigurational spin-orbit ab initio method. The crossing point between 2A(1) and 3E states is located near the C-Br bond length of 2.45 Å. Comparison with CH(3)Br shows that fluorination does not alter the location of the crossing point. The relation between the calculated potential energy curves and recent experimental result is briefly discussed.

Unimolecular reactions of chemically activated CF2BrCF2CH3 and CF2BrCF2CD3: evidence for 1,2-FBr interchange

Vibrationally excited CF2BrCF2CH3 and CF2BrCF2CD3 molecules were prepared with 96 kcal mol-1 energy at room temperature by the recombination of CF2BrCF2 and CH3 (CD3) radicals. The observed unimolecular reactions are 1,2-BrF interchange to give CF3CFBrCH3 (CD3) molecules and 2,3-FH (FD) elimination; the rate constants are 2.2 x 10(5) (1.5 x 10(5)) s(-1) and 2.0 x 105 (0.75 x 10(5)) s(-1), respectively. The CF3CFBrCH3 (CD3) molecules rapidly, relative to the reverse reaction, eliminate HBr or DBr to give the observed product CF3CF=CH2 (CD2). Density functional theory at the B3PW91/6-311+G(2d,p) level was used to obtain vibrational frequencies and moments of inertia of the molecule and transition states for subsequent calculations of statistical rate constants for CF2BrCF2CH3 and CF2BrCF2CD3. Matching experimental and calculated rate constants gave threshold energies of 62 and 66 kcal mol-1 for 1,2-BrF interchange and 2,3-FH elimination, respectively. The BrF interchange reaction is compared to ClF interchange from CF2ClCF2CH3 and CF2ClCHFCH3.

4D ultrafast electron diffraction, crystallography, and microscopy

In this review, we highlight the progress made in the development of 4D ultrafast electron diffraction (UED), crystallography (UEC), and microscopy (UEM) with a focus on concepts, methodologies, and prototypical applications. The joint atomic-scale resolutions in space and time, and sensitivity reached, make it possible to determine complex transient structures and assemblies in different phases. These applications include studies of isolated chemical reactions (molecular beams), interfaces, surfaces and nanocrystals, self-assembly, and 2D crystalline fatty-acid bilayers. In 4D UEM, we are now able, using timed, single-electron packets, to image nano-to-micro scale structures of materials and biological cells. Future applications of these methods are foreseen across areas of physics, chemistry, and biology.

Unimolecular reactions of vibrationally excited CF2ClCHFCH3 and CF2ClCHFCD3: evidence for the 1,2-FCl interchange pathway

Chemically activated CF2ClCHFCH3 and CF2ClCHFCD3 molecules were prepared with 94 kcal mol-1 of vibrational energy by the recombination of CF2ClCHF and CH3(CD3) radicals at room temperature. The unimolecular reaction pathways were 2,3-FH(FD) elimination, 1,2-ClF interchange and 1,2-ClH elimination; the interchange produces CF3CHClCH3(CF3CHClCD3) with 105 kcal mol-1 of vibrational energy. Rate constants for CF2ClCHFCH3 [CF2ClCHFCD3] were (3.1+/-0.4)x10(6) s-1 [(1.0+/-0.1)x10(6) s-1] for 2,3-FH [FD] loss, (1.5+/-0.2)x10(6) s-1 [(8.3+/-0.9)x10(5) s-1] for 1,2-ClF interchange, and (8.2+/-1.0)x10(5) s-1 [(5.3+/-0.6)x10(5) s-1] for 1,2-ClH [DCl] loss. These correspond to branching fractions of 0.55+/-0.06 [0.43+/-0.04] for 2,3-FH [FD] loss, 0.29+/-0.03 [0.35+/-0.04] for 1,2-ClF interchange, and 0.16+/-0.02 [0.22+/-0.02] for 1,2-ClH [ClD] loss. Kinetic-isotope effects were 3.0+/-0.6 for 2,3-FH [FD] loss, 1.6+/-0.3 for 1,2-ClH loss, and 1.8+/-0.4 for 1,2-ClF interchange. The CF3CHClCH3 (CF3CHClCD3) molecules formed by 1,2-FCl interchange react by loss of HCl [DCl] with rate constants of (5.6+/-0.9)x10(7) s-1 [(2.1+/-0.4)x10(7)] s-1 for an isotope effect of 2.7+/-0.4. Density functional theory was employed to calculate vibrational frequencies and moments of inertia for the molecules and for the transition-state structures. These results were used with RRKM theory to assign threshold energies from comparison of computed and experimental unimolecular rate constants. The threshold energy for ClF interchange is 57.5 kcal mol-1, and those for HF and HCl channels are 2-5 kcal mol-1 higher. Experiments with vibrationally excited CF2ClCF2CF3, CF2ClCF2CF2Cl, and CF2ClCF2Cl, which did not show evidence for ClF interchange, also are reported.

Ultrafast Electron Diffraction: Dynamical Structures on Complex Energy Landscapes

In this contribution, we report studies in ultrafast electron diffraction (UED), with the aim of exploring new directions. The main focus is on the determination of complex structures and their dynamics with spatial and temporal resolutions sufficient to give an atomic-scale picture for the evolution in chemical or biological change. We also provide the theoretical framework for UED, and compare the experimental findings of UED to those predicted by density functional and charge density calculations. Selected applications are given in order to highlight phenomena related to concepts such as bifurcation of trajectories in dynamics, far-from-equilibrium coherent structures, and conformational robustness in biological structures. For the former two cases, we consider chemical systems, and, for the latter, we examine proteins of 200 atoms (angiotensin I) or more.

Photodissociation Reaction of 1,2-Diiodoethane in Solution: A Theoretical and X-ray Diffraction Study

Various molecular species are known to form during the photoreaction of C2H4I2 in the gas phase and in solution. We have studied all species involved in this reaction by ab initio and density functional theory (DFT) calculations: Geometries, energies, and vibrational frequencies of C2H4I2, bridged C2H4I*, anti C2H4I*, C4H4, I2, I3-, and the isomer C2H4I-I were calculated. The absorption peaks and oscillator strengths of selected species along the potential energy surface (PES) were calculated using time-dependent DFT and were compared with available experimental results. The calculated PES satisfactorily describes the observed reactions of the photoexcited C2H4I2 molecule. In the gas phase, there is only one reaction pathway: the first C-I bond ruptures followed by a secondary C-I breakage in the haloethyl radical C2H4I*. In solution, by contrast, another reaction channel, which is energetically more favored over the secondary dissociation, is switched on due to a solvation effect: the bridged C2H4I* can bind to the free iodine atom to form a C2H4I-I isomer without any energy barrier. The isomer can then break into C2H4 and I2. The rotational barriers in the gas phase and in solution were also calculated and compared. To provide experimental data on the structure of C2H4I2 in solution, the ground state structure of C2H4I2 in methanol was determined from static X-ray diffraction data using 88 keV (lambda = 0.14 A) X-rays. The structural parameters are compared with those from the theoretical results.

Density Functional Theory Predicts the Barriers for Radical Fragmentation in Solution

[reaction: see text] N-Methoxypyridyl radicals formed by one-electron reduction of the corresponding cationic heterocycles undergo N-O bond cleavage. Experimental activation free energies for a series of these bond fragmentations are compared to corresponding barriers determined from electronic structure calculations. The DFT barriers agree well with those from experiment, being smaller than the latter values by an average value of ca. 1 kcal/mol, for rate constants varying over almost 3 orders of magnitude, or within ca. 3 kcal/mol over 8 orders of magnitude of rate constant. For a model compound, the B3PW91/6-31+G hybrid density functional method is also found to be in good agreement with the MCSCF-MRMP2 method. One of the reactions is found by DFT to have no minimum for the reactant radical, consistent with a truly barrierless reaction.