This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: In research published in the journal Physical Chemistry Chemical Physics, the electronic structures of organic molecules containing sp2-hybridized carbons in organic solvents were studied using C K-edge X-ray absorption spectroscopy (XAS), termed "Soft-XAS-OS." The C‒Si bond lengths of an arylsilane and silicates, which exhibit different reactivities in the Hiyama cross-coupling reaction, were studied using the C=C π* peaks of phenyl groups in C K-edge XAS spectra and compared with inner-shell quantum chemical calculations combined with molecular dynamics (MD) simulations. The length of a chemical bond at a reaction site is known to influence reactivity.
Single-crystal X-ray diffraction usually determines bond lengths in solid samples and cannot be applied to solutions, where most organic reactions take place. Solution-phase techniques such as NMR, UV-visible and infrared spectroscopy provide valuable insights into chemical structures but are generally unsuitable for evaluating bond lengths. XAS is an element-specific method for studying the electronic structures of organic molecules.
Previous work measured the C and N K-edge XAS spectra of organic molecules in aqueous solutions (Soft-XAS-H2O). No established method applies XAS measurements to organic molecules in organic solvents, where most organic reactions are conducted. The C K-edge XAS spectra of organic molecules in organic solvents (Soft-XAS-OS) were measured using a transmission-type liquid cell, in which the thickness of the liquid layer is precisely controlled from 20 nm to 40 μm.
In the C K-edge XAS spectrum of 100 mM trimethoxyphenylsilane (Ph‒Si(OMe)3) in tetrahydrofuran (THF), the C=C π* peak of the phenyl groups of Ph‒Si(OMe)3 appears below the energy thresholds of the THF solvent. Measurements of the energy thresholds of representative organic solutions and the C=C and C=N π* peaks of organic molecules confirmed that the electronic states of sp2-hybridized carbons can be distinguished from the absorption of organic solvents. To study the mechanism of the Hiyama cross-coupling reaction, a carbon-carbon bond-forming reaction, Ph‒Si(OMe)3, fluorotrimethoxyphenylsilicate (Ph‒Si(OMe)3F) and ethylene-glycol-derived spirosilicate Ph‒Si(OCH2CH2O)2 were prepared, and their reactivities were analyzed.
The different structures of Ph‒Si(OMe)3F were determined using 19F NMR. The C=C π* peak of Ph‒Si(OMe)3 in the C K-edge XAS spectrum can be divided into the ipso, ortho + para, and meta + para carbons of the phenyl group. The ipso C=C π* peak of Ph‒Si(OMe)3 appears at 284.56 eV, while those of Ph‒Si(OCH2CH2O)2 (284.54 eV) and Ph‒Si(OMe)3F (284.50 eV) show shifts to lower energies.
Because changes in C‒Si bond lengths affect the energy shifts of the XAS peaks, C K-edge inner-shell quantum chemical calculations of the arylsilane and silicates in organic solvents were performed using snapshots from the MD simulations. As a result, the energy shifts of the ipso C=C π* peaks can be interpreted in terms of the C(sp2)‒Si bond lengths. In the optimized molecular structures, the C‒Si bond lengths in organic solvents follow the order: Ph‒Si(OMe)3 < Ph‒Si(OCH2CH2O)2 < Ph‒Si(OMe)3F.
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