Related Experiment Video
Updated: Jul 14, 2026
09:35
Attaching Biological Probes to Silica Optical Biosensors Using Silane Coupling Agents
Published on: May 1, 2012
2J(29Si-O-29Si) coupling constants in oligosiloxanes.
1Institute of Chemical Process Fundamentals of the ASCR, v.v.i, 16502 Prague 6, Czech Republic.
Magnetic Resonance in Chemistry : MRC
|June 15, 2007
Summary
Spin-spin couplings between silicon-29 nuclei in siloxanes were determined. These small couplings are influenced by molecular branching and electronegative substituents on the silicon-oxygen-silicon structure.
Area of Science:
- Nuclear Magnetic Resonance Spectroscopy
- Inorganic Chemistry
- Materials Science
Background:
- Siloxanes are silicon-oxygen polymers with diverse applications.
- Understanding nuclear spin-spin coupling provides insights into molecular structure.
- Silicon-29 (29Si) NMR is a valuable tool for characterizing siloxane materials.
Purpose of the Study:
- To determine the absolute values of spin-spin couplings between 29Si nuclei in siloxanes.
- To investigate the relationship between coupling constants and molecular structure.
- To establish structure-property correlations in siloxane systems.
Main Methods:
- Nuclear Magnetic Resonance (NMR) spectroscopy was employed.
- 29Si NMR experiments were conducted to measure spin-spin coupling constants.
- Computational analysis was used to interpret the observed couplings.
Main Results:
- The spin-spin couplings, denoted as (2)J((29)Si-O-(29)Si), were found to be small, ranging from 0 to 5 Hz.
- Coupling constants showed a dependence on the degree of branching within the siloxane structure.
- The number of electronegative substituents on the Si-O-Si moiety also influenced the coupling values.
Conclusions:
- The determined spin-spin coupling constants provide a quantitative measure of electronic interactions between 29Si nuclei.
- Molecular structure, specifically branching and substituent effects, significantly impacts these couplings.
- This study enhances the understanding of structure-property relationships in siloxanes through 29Si NMR.
Related Concept Videos
01:17Spin–Spin Coupling: One-Bond Coupling
Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
01:08Spin–Spin Coupling Constant: Overview
In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must have a...
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must have a...
01:20Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)
Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
01:22Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)
Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the involved orbitals. The...
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the involved orbitals. The...
01:27¹H NMR: Long-Range Coupling
The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene π orbitals.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene π orbitals.
01:13¹H NMR: Complex Splitting
A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.