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Related Concept Videos

Proton (¹H) NMR: Chemical Shift01:07

Proton (¹H) NMR: Chemical Shift

3.5K
Organic molecules primarily contain carbon and hydrogen atoms. While all the hydrogen isotopes are NMR-active, protium or hydrogen-1 is the most abundant. It has a significant energy separation between its nuclear spin states due to its large gyromagnetic ratio. As per Boltzmann's distribution, an increase in the energy separation implies a greater excess population of nuclei available for excitation, resulting in a strong NMR absorption signal.
Absorption signals of all the protium nuclei...
3.5K
NMR Spectroscopy: Chemical Shift Overview01:15

NMR Spectroscopy: Chemical Shift Overview

3.3K
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.
For instance, the proton...
3.3K
Inductive Effects on Chemical Shift: Overview01:27

Inductive Effects on Chemical Shift: Overview

2.2K
The protons in unsubstituted alkanes are strongly shielded with chemical shifts below 1.8 ppm. Methine, methylene, and methyl protons appear at approximately 1.7, 1.2 and 0.7 ppm, while the proton signal from methane appears at 0.23 ppm. An electronegative substituent, such as chlorine, withdraws the electron density from the protons, increasing their chemical shift. Progressive substitution of the hydrogens in methane by chlorine shifts the proton signals increasingly downfield, to 3.05 ppm in...
2.2K
Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

1.4K
In an NMR sample, precise measurement of the absolute absorption frequencies of nuclei is difficult. A standard internal reference compound is added, and the frequency difference between the reference signal and sample signals is measured.
The internal reference compound generally used in NMR spectroscopy is tetramethylsilane (TMS). TMS is preferred because it is chemically inert, soluble in NMR solvents, and easily removable. Also, the highly shielded methyl protons in TMS yield an intense...
1.4K
π Electron Effects on Chemical Shift: Overview01:27

π Electron Effects on Chemical Shift: Overview

1.7K
An applied magnetic field causes loosely bound π-electrons in organic molecules to circulate, producing a local or induced diamagnetic field over a large spatial volume. As the molecules tumble in solution, the field generated by π-electrons in spherical substituents results in a zero net field. However, the net field generated by π-electrons in non-spherical substituents is not zero. The effect of this induced field depends on the orientation of the molecule with respect to B0,...
1.7K
¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons01:03

¹H NMR Chemical Shift Equivalence: Homotopic and Heterotopic Protons

4.2K
Protons in identical electronic environments within a molecule are chemically equivalent and have the same chemical shift. The replacement test is a useful tool to identify chemical equivalence and predict NMR spectra. A substituent replaces each of the protons being examined and the resulting molecules are compared. If the same molecule is obtained, the protons are equivalent or homotopic. Replacement of any hydrogens in ethane by chlorine yields chloroethane because all six protons are...
4.2K

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Methods for Image-based Surveys of Benthic Macroinvertebrates and Their Habitat Exemplified by the Drop Camera Survey for the Atlantic Sea Scallop
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Subtidal Benthic Invertebrates Shifting Northward Along the US Atlantic Coast.

Stephen S Hale1, Henry W Buffum1, John A Kiddon1

  • 1Atlantic Ecology Division, National Health and Environmental, Effects Research Laboratory, Office of Research and Development, US Environmental Protection Agency, Narragansett, RI, USA.

Estuaries and Coasts : Journal of the Estuarine Research Federation
|September 18, 2018
PubMed
Summary

Global climate change is causing marine species to shift northward along the U.S. Atlantic coast. This study reveals significant poleward range shifts in benthic invertebrates, impacting community structure and fisheries.

Keywords:
Benthic invertebratesCarolinian Biogeographic ProvinceClimate changeSpecies’ range shiftsUS Atlantic coastVirginian Biogeographic Province

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Area of Science:

  • Marine ecology
  • Climate change biology
  • Benthic science

Background:

  • Global climate change drives species range shifts globally.
  • Subtidal benthic communities in estuarine and nearshore waters are understudied regarding climate-driven range shifts.
  • Warming ocean temperatures are a key factor influencing marine species distribution.

Purpose of the Study:

  • To investigate range shifts of soft-bottom, benthic invertebrates along the Atlantic coast of the USA over 20 years.
  • To analyze changes in species abundance and distribution in response to documented temperature increases.
  • To assess the impact of these shifts on community composition and biogeographic boundaries.

Main Methods:

  • Utilized 20 years (1990-2010) of occurrence and abundance data for benthic invertebrates.
  • Extracted data from a national coastal assessment program covering two biogeographic provinces (Carolinian and Virginian).
  • Analyzed changes in centers of abundance and range limits in relation to mean water temperature increases.

Main Results:

  • Mean water temperatures increased significantly (1.6°C bottom, 1.7°C surface) over the study period.
  • Of 30 prevalent species, 18 (60%) showed significant northward shifts in their centers of abundance, averaging 181 km.
  • Southern range limits shifted northward for 22 species, leading to an average 25% range contraction.
  • Community composition changed, particularly in southern latitudes, with some species exceeding their biogeographic boundaries.

Conclusions:

  • Benthic invertebrate communities along the U.S. Atlantic coast are responding to climate warming with poleward range shifts.
  • Observed range shifts result in community restructuring and potential range contractions for some species.
  • These changes have significant implications for marine ecosystem functioning, services, and fisheries dependent on benthic prey.