Experimental Observation of the Nature of Weak Chemical Bonds in Labile Compounds

Abstract

and crystallize them. Fortunately, developments in synthetic strategies, especially for a concept of kinetic protection by bulky substituents, [1] have enabled us to sufficiently stabilize transient species with weak chemical bonds, in order to produce single crystals for X-ray diffraction studies. Using such molecules, the nature of the chemical bonds and orbital distributions have been characterized based on experimentally obtained EDDs, focusing on valence electrons as well as the crystal and molecular structures.The properties of chemical bonds, as well as intra-and interatomic interactions, arise from the valence electrons. Information on the electron configurations and EDDs in materials is essential for achieving a better understanding of the nature of the chemical bonds, atomic interactions, and the properties of materials. The EDDs of crystalline materials, including those of valence electrons, can be experimentally derived from X-ray diffraction data. X-rays are scattered by all the electrons in crystals. Consequently, the intensities of the reflections impart information on the distribution of the valence electrons, as well as that of the core electrons. Therefore, the valence EDD and the electronic structure of crystals can be derived by an appropriate analysis of the total EDD.Theoretical calculations are commonly used to obtain electronic structures and visualize the orbitals and EDDs in molecules. However, calculated molecular structures are frequently different to those in the solid state, owing to the problems that arise when thousands of intra-and intermolecular interactions, which occur in the solid state, are introduced into the calculations. In particular, labile compounds, which are kinetically stabilized by bulky substituents, are subject to numerous intraand interatomic forces in the crystals. Consequently, the molecules in crystals occasionally adopt structures that achieve local energy minima, which are difficult to realize via theoretical calculations.The advantage of using the X-ray diffraction method over theoretical calculations for deriving the EDDs of molecules carrying bulky substituents is that all of the interatomic interactions in a crystal structure are inherently incorporated into the resulting EDDs. In this context, experimental X-ray EDD analysis is an excellent method for understanding the electronic structures of crystalline materials.In 1978, Hanssen and Coppens introduced the use of spherical harmonic functions for modeling aspherical parts of EDDs, a method known as multipole expansion. In this method, the electron populations of each multipole and radial expansioncontraction parameters are employed to describe the aspherical Accurate single-crystal X-ray diffraction data afford a total electron density distribution for crystalline materials by employing an aspherical atomic model with comparable accuracy to that of theoretical calculations. Chemical bonds and intermolecular interactions in the crystalline state are characterized based on the electron density distribut...

Dynamics in Catalytic Asymmetric Diastereoconvergent (3 + 2) Cycloadditions with Isomerizable Nitrones and α-Keto Ester Enolates

“…A consideration of the reaction sequence of the formal (3 + 2) cycloaddition, comprising addition and cyclization, led to two queries: (1) whether our bifunctional activation mechanism , can be applied to synthesize other diastereomers of the (3 + 2) adduct and (2) if the enantio- and diastereoselectivity in the reaction with two isomerizable substrates can be explained by the Curtin–Hammett principle . The aforementioned model was supported by various spectroscopic analyses of the Ni­(II) complex in the ground state, on the basis of the electron density distribution (EDD) analysis − of the Ni­(II)-diamine-acetate complex (CCDC 1482741) . However, a computational validation of the TSs has not been systematically conducted.…”

Long Carbon–Carbon Bonding beyond 2 Å in Tris(9-fluorenylidene)methane

“…X-ray crystal structure analysis revealed that the two fluorenyl rings in 1 perpendicularly face each other (the bond angle θ C2−C1−C3 = 89. 19(3)°), instead of having a propeller-like structure (Figure 2b), and a distant C−C interatomic contact was found between the carbon atoms (C2 and C3 in Figure 2b) at the 9-positions of the facing fluorenyl rings. On the basis of single-crystal X-ray analysis, the C−C distance was determined to be 2.0415(5) Å at 90 K, and this distance was well-reproduced (2.0478 Å) by the density functional theory (DFT) calculation at the B3LYP-D3/6-311+G** level of theory (see Table S1 for a DFT benchmark study).…”

“…To gain deeper insight into the electronic interaction between C2 and C3, we performed electron density distribution (EDD) analysis to visualize the distribution of valence electrons in the crystalline state. 19 The single-crystal X-ray diffraction data of 1 were obtained up to [sin θ/λ] max = 1.11 Å −1 using Mo Kα radiation at 90 K. Figure 2c,d shows the static model deformation density maps 20,21 of 1 (90 K) on the C1−C2−C3 plane and the cross section of the C1−C2−C3 plane through the skeletal carbons, respectively. A concentration of electron density (peak height ≈ +0.03 e Å −3 , green circle in Figure 2c,d) is apparently recognized between C2 and C3, indicating orbital overlap that allows the sharing of electrons in the C2−C3 region.…”

Nature of a Long Carbon–Carbon Bond Beyond 2.0 Å

“…Considering that the parent fluorenyl radical possesses a large spin density of >0.5 at the 9position 17 , an appreciable electron-electron interaction appears to be present between C2 and C3. To gain deeper insight into the electronic interaction between C2 and C3, we performed electron density distribution (EDD) analysis to visualise the distribution of valence electrons in the crystalline state 18 . The single-crystal X-ray diffraction data of 1 were obtained up to [sinθ / λ]max = 1.11 Å −1 using Mo Kα radiation at 90 K. Figures 1c and 1d show the static model deformation density maps 19,20 which was estimated from the singlet-triplet vertical excitation energy (ΔES-T) by a B3LYP-D3/6-311+G** calculation of 1 (90 K), reached up to 138 kJ mol −1 .…”