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Related Concept Videos

Heterogeneous Catalysis01:22

Heterogeneous Catalysis

Heterogeneous catalysis involves a catalyst in a different phase from the reactants. It is a process where the catalyst and the reactants are in distinct phases, typically solid and gas or liquid.Most heterogeneous catalysts are metals, metal oxides, or acids. The list includes transition metals like iron (Fe), cobalt (Co), nickel (Ni), palladium (Pd), platinum (Pt), chromium (Cr), manganese (Mn), tungsten (W), silver (Ag), and copper (Cu). These metals possess partially vacant d orbitals that...
¹H NMR: Long-Range Coupling01:27

¹H 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.
Spin–Spin Coupling Constant: Overview01:08

Spin–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...
Spin–Spin Coupling: One-Bond Coupling01:17

Spin–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,...
Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

Spin–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...
Valence Bond Theory and Hybridized Orbitals02:38

Valence Bond Theory and Hybridized Orbitals

According to valence bond theory, a covalent bond results when: (1) an orbital on one atom overlaps an orbital on a second atom, and (2) the single electrons in each orbital combine to form an electron pair. The strength of a covalent bond depends on the extent of overlap of the orbitals involved. Maximum overlap is possible when the orbitals overlap on a direct line between the two nuclei.
A σ bond (single bond in a Lewis structure) is a covalent bond in which the electron density is...

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Updated: Jun 25, 2026

Attaching Biological Probes to Silica Optical Biosensors Using Silane Coupling Agents
09:35

Attaching Biological Probes to Silica Optical Biosensors Using Silane Coupling Agents

Published on: May 1, 2012

Understanding the molecule-surface chemical coupling in SERS.

Seth M Morton1, Lasse Jensen

  • 1The Pennsylvania State University, Department of Chemistry, 104 Chemistry Building, University Park, Pennsylvania 16802, USA.

Journal of the American Chemical Society
|March 4, 2009
PubMed
Summary
This summary is machine-generated.

Chemical enhancement in surface-enhanced Raman scattering (SERS) depends on the energy difference between metal and molecule energy levels, not charge transfer. This finding aids in designing molecules for stronger SERS chemical enhancements.

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Area of Science:

  • Physical Chemistry
  • Computational Chemistry
  • Materials Science

Background:

  • Surface-Enhanced Raman Scattering (SERS) is a powerful technique for molecular detection.
  • Chemical enhancement in SERS arises from molecule-surface chemical coupling.
  • Understanding this mechanism is crucial for optimizing SERS sensitivity.

Purpose of the Study:

  • To elucidate the mechanism of chemical enhancement in SERS.
  • To investigate the role of electronic structure in SERS chemical enhancement.
  • To provide a framework for designing molecules with enhanced SERS signals.

Main Methods:

  • Time-dependent density functional theory (TD-DFT) calculations were employed.
  • Systematic study of meta- and para-substituted pyridines interacting with a silver cluster (Ag(20)).
  • Analysis of electronic properties, including HOMO-LUMO energy levels and charge transfer.

Main Results:

  • Chemical enhancement is primarily governed by the energy difference between the metal's HOMO and the molecule's LUMO.
  • Contrary to expectations, increased charge transfer did not correlate with higher enhancement.
  • A scaling relationship for enhancement was proposed: (omega(X)/omega(e))(4).
  • The trend was validated with substituted benzenethiols and varying silver cluster sizes.

Conclusions:

  • The energy gap between the molecule's HOMO-LUMO levels is a key factor for strong SERS chemical enhancement.
  • Molecules with stabilized HOMO-LUMO gaps, particularly those accepting pi-backbonding, are predicted to exhibit strong chemical enhancement.
  • This study offers a design principle for novel molecules with superior SERS performance.