CF_2XCF_2X and CF_2XCF_2• Radicals (X = Cl, Br, I): Ab Initio and DFT Studies and Comparison with Experiments

1,2-dihalotetrafluoroethanes (CF_2XCF_2X, X = I, Br and Cl) and halotetrafluoroethyl radicals (CF_2XCF_2•, X = I, Br, and Cl) have been studied by ab initio molecular-orbital techniques using restricted Hartree−Fock and Density functional theory (DFT-B3PW91). For the optimized HF geometries, we carried out local MP2 calculations to account for electron correlation effects. Each CF_2XCF_2X molecule and CF_2XCF_2• radical has two conformational minima (anti and gauche) and two rotational transition structures in the rotational energy surface along the C−C bond. The rotational barriers of the  radicals are smaller than those of the parent molecules due to the absence of the nonbonded interaction between two halogen atoms. In contrast, the conformational energy difference between two stable rotamers (anti and gauche) of each radical is larger than that in the corresponding parent molecules. This stabilizing effect on the anti conformers of the radicals is rationalized in terms of hyperconjugation between the radical center and the σ^*(C−X) molecular orbital. The dissociation energies for breaking the first and second C−X bonds of CF_2XCF_2X were also calculated and compared with available experimental data. The CF_2XCF_2• radicals show dramatically different behavior compared with haloethyl radicals (CH_2XCH_2•). The CF_2XCF_2• radical has two minima and two saddle points, whereas CH_2XCH_2• radical has only one minimum and one saddle point in the rotational energy surface. In addition, the bridged structures are not stable for CF_2XCF_2• radicals in contrast with CH_2XCH_2• radicals. The origin of these differences is attributed to differences in the environment of the radical center. The calculated structures of the CF_2ICF_2• radical were used in interpreting a recent experimental observation (Cao et al. Proc. Natl. Acad. Sci. 1999, 96, 338) and are compared with quantitative results from a new experiment (Ihee et al. Science 2001, 291, 458) using the ultrafast electron diffraction technique

CF_2XCF_2X and CF_2XCF_2• Radicals (X = Cl, Br, I): Ab Initio and DFT Studies and Comparison with Experiments

1,2-dihalotetrafluoroethanes (CF_2XCF_2X, X = I, Br and Cl) and halotetrafluoroethyl radicals (CF_2XCF_2•, X = I, Br, and Cl) have been studied by ab initio molecular-orbital techniques using restricted Hartree−Fock and Density functional theory (DFT-B3PW91). For the optimized HF geometries, we carried out local MP2 calculations to account for electron correlation effects. Each CF_2XCF_2X molecule and CF_2XCF_2• radical has two conformational minima (anti and gauche) and two rotational transition structures in the rotational energy surface along the C−C bond. The rotational barriers of the  radicals are smaller than those of the parent molecules due to the absence of the nonbonded interaction between two halogen atoms. In contrast, the conformational energy difference between two stable rotamers (anti and gauche) of each radical is larger than that in the corresponding parent molecules. This stabilizing effect on the anti conformers of the radicals is rationalized in terms of hyperconjugation between the radical center and the σ^*(C−X) molecular orbital. The dissociation energies for breaking the first and second C−X bonds of CF_2XCF_2X were also calculated and compared with available experimental data. The CF_2XCF_2• radicals show dramatically different behavior compared with haloethyl radicals (CH_2XCH_2•). The CF_2XCF_2• radical has two minima and two saddle points, whereas CH_2XCH_2• radical has only one minimum and one saddle point in the rotational energy surface. In addition, the bridged structures are not stable for CF_2XCF_2• radicals in contrast with CH_2XCH_2• radicals. The origin of these differences is attributed to differences in the environment of the radical center. The calculated structures of the CF_2ICF_2• radical were used in interpreting a recent experimental observation (Cao et al. Proc. Natl. Acad. Sci. 1999, 96, 338) and are compared with quantitative results from a new experiment (Ihee et al. Science 2001, 291, 458) using the ultrafast electron diffraction technique

Structural dynamics probed by X-ray pulses from synchrotrons and XFELs

This review focuses on how short X-ray pulses from synchrotrons and XFELs can be used to track light-induced structural changes in molecular complexes and proteins via the pump–probe method. The upgrade of the European Synchrotron Radiation Facility to a diffraction-limited storage ring, based on the seven-bend achromat lattice, and how it might boost future pump–probe experiments are described. We discuss some of the first X-ray experiments to achieve 100 ps time resolution, including the dissociation and in-cage recombination of diatomic molecules, as probed by wide-angle X-ray scattering, and the 3D filming of ligand transport in myoglobin, as probed by Laue diffraction. Finally, the use of femtosecond XFEL pulses to investigate primary chemical reactions, bond breakage and bond formation, isomerisation and electron transfer are discussed

100 ps time-resolved solution scattering utilizing a wide-bandwidth X-ray beam from multilayer optics

A new method of time-resolved solution scattering utilizing X-ray multilayer optics is presented

Ring closing reaction in diarylethene captured by femtosecond electron crystallography

The photoinduced ring-closing reaction in diarylethene, which serves as a model system for understanding reactive crossings through conical intersections, was directly observed with atomic resolution using femtosecond electron diffraction. Complementary ab initio calculations were also performed. Immediately following photoexcitation, subpicosecond structural changes associated with the formation of an open-ring excited-state intermediate were resolved. The key motion is the rotation of the thiophene rings, which significantly decreases the distance between the reactive carbon atoms prior to ring closing. Subsequently, on the few picosecond time scale, localized torsional motions of the carbon atoms lead to the formation of the closed-ring photoproduct. These direct observations of the molecular motions driving an organic chemical reaction were only made possible through the development of an ultrabright electron source to capture the atomic motions within the limited number of sampling frames and the low data acquisition rate dictated by the intrinsically poor thermal conductivity and limited photoreversibility of organic materials