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Related Concept Videos

¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
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¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)01:20

¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)

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When proton-coupled carbon-13 spectra are simplified by a broadband proton decoupling technique, structural information about the coupled protons is lost. Distortionless enhancement by polarization transfer (DEPT) is a technique that provides information on the number of hydrogens attached to each carbon in a molecule. While the DEPT experiment utilizes complex pulse sequences, the pulse delay and flip angle are specifically manipulated. The resulting signals have different phases depending on...
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¹³C NMR: ¹H–¹³C Decoupling01:04

¹³C NMR: ¹H–¹³C Decoupling

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The probability of having two carbon-13 atoms next to each other is negligible because of the low natural abundance of carbon-13. Consequently, peak splitting due to carbon-carbon spin-spin coupling is not observed in spectra. However, protons up to three sigma bonds away split the carbon signal according to the n+1 rule, resulting in complicated spectra.
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NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

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The position of the absorption signal of a sample is reported relative to the position of the signal of tetramethylsilane (TMS), which is added as an internal reference while recording spectra. The difference between the absorption frequencies of the sample and TMS (in Hz) is divided by the spectrometer operating frequency (in MHz) to obtain a dimensionless quantity called the chemical shift. It is reported on the δ (delta) scale and expressed in parts per million.
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In the AX proton spin system, proton A can sense the two spin states of a coupled proton X, resulting in a doublet NMR signal with two peaks of equal (1:1) intensity. When proton A is coupled to two equivalent protons (AX2 spin system), the spin states of each X can be aligned with or against the external field, creating three possible scenarios. This results in a 1:2:1  triplet signal, where the central peak corresponds to the chemical shift of A and is twice as large or intense as the...
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Molecular Spectroscopy: Absorption and Emission01:14

Molecular Spectroscopy: Absorption and Emission

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Molecules possess discrete energy levels called quantum states. Unlike atoms, which have simpler energy levels, molecules possess additional rotational and vibrational energy levels.  Each energy level is separated by an energy gap, with the gaps between adjacent electronic, vibrational, and rotational levels varying significantly. The three types of energy levels in a diatomic molecule are shown in Figure 1.
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Tip-Enhanced Sum Frequency Generation for Molecular Vibrational Nanospectroscopy.

Atsunori Sakurai1,2,3, Shota Takahashi1, Tatsuto Mochizuki1,2

  • 1Institute for Molecular Science, National Institutes of Natural Sciences, Okazaki, Aichi 444-8585, Japan.

Nano Letters
|April 10, 2025
PubMed
Summary

Researchers developed tip-enhanced SFG (TE-SFG) spectroscopy using a scanning tunneling microscope to overcome the diffraction limit. This technique achieves nanoscale resolution for surface molecular studies under ambient conditions.

Keywords:
molecular vibrationnonlinear opticsscanning near-field optical microscopysum frequency generationtip-enhanced spectroscopy

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Area of Science:

  • Nonlinear optics
  • Surface science
  • Spectroscopy

Background:

  • Vibrational sum frequency generation (SFG) is crucial for surface molecular analysis.
  • Far-field SFG is limited by the diffraction limit, hindering nanoscale investigations.
  • Inhomogeneous structures smaller than the wavelength of light remain poorly resolved.

Purpose of the Study:

  • To develop a method for surpassing the diffraction limit in SFG spectroscopy.
  • To enable nanoscale vibrational spectroscopy of surface-adsorbed molecules.
  • To investigate molecular orientation and structure at the nanoscale.

Main Methods:

  • Development of a tip-enhanced SFG (TE-SFG) system integrated with a scanning tunneling microscope.
  • Detection of vibrational TE-SFG signals from molecules adsorbed on a gold substrate.
  • Utilizing interferometric SFG spectra for phase analysis and molecular orientation determination.

Main Results:

  • Successful detection of vibrational TE-SFG signals under ambient conditions.
  • Confirmation that the TE-SFG signal originates from a highly localized region (tip-sample gap).
  • Demonstration of nanoscale spatial resolution, overcoming far-field diffraction limits.

Conclusions:

  • Tip-enhanced SFG spectroscopy provides a novel platform for nonlinear optical nanospectroscopy.
  • This technique allows for the investigation of surface molecular systems beyond the diffraction limit.
  • TE-SFG opens new avenues for studying molecular details in inhomogeneous surface structures.