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Related Concept Videos

Hydrogen Bonds00:26

Hydrogen Bonds

134.7K
Hydrogen bonds are weak attractions between atoms that have formed other chemical bonds. One of these atoms is electronegative, like oxygen, and has a partial negative charge. The other is a hydrogen atom that has bonded with another electronegative atom and has a partial positive charge.
Hydrogen Bonds Control the World!
Because hydrogen has very weak electronegativity when it binds with a strongly electronegative atom, such as oxygen or nitrogen, electrons in the bond are unequally shared....
134.7K
Hydrogen Bonds01:04

Hydrogen Bonds

15.2K
A hydrogen bond is formed when a weakly positive hydrogen atom already bonded to one electronegative atom (for example, the oxygen in the water molecule) is attracted to another electronegative atom from another polar molecule, such as water (H2O), hydrogen fluoride (HF), or ammonia (NH3). The huge electronegativity difference between the H atom (2.1) and the atom to which it is bonded (4.0 for an F atom, 3.5 for an O atom, or 3.0 for an N atom), combined with the very small size of an H atom...
15.2K
Reduction of Alkenes: Catalytic Hydrogenation02:13

Reduction of Alkenes: Catalytic Hydrogenation

14.2K
Alkenes undergo reduction by the addition of molecular hydrogen to give alkanes. Because the process generally occurs in the presence of a transition-metal catalyst, the reaction is called catalytic hydrogenation.
Metals like palladium, platinum, and nickel are commonly used in their solid forms — fine powder on an inert surface. As these catalysts remain insoluble in the reaction mixture, they are referred to as heterogeneous catalysts.
The hydrogenation process takes place on the...
14.2K
Reduction of Benzene to Cyclohexane: Catalytic Hydrogenation01:28

Reduction of Benzene to Cyclohexane: Catalytic Hydrogenation

5.9K
Unlike the easy catalytic hydrogenation of an alkene double bond, hydrogenation of a benzene double bond under similar reaction conditions does not take place easily. For example, in the reduction of stilbene, the benzene ring remains unaffected while the alkene bond gets reduced. Hydrogenation of an alkene double bond is exothermic and a favorable process. In contrast, to hydrogenate the first unsaturated bond of benzene, an energy input is needed; that is, the process is endothermic. This is...
5.9K
IR Spectrum Peak Broadening: Hydrogen Bonding01:23

IR Spectrum Peak Broadening: Hydrogen Bonding

1.9K
The vibrational frequency of a bond is directly proportional to its bond strength. As a result, stronger bonds vibrate at higher frequencies, while weaker bonds vibrate at lower frequencies. The stretching vibration of the strong O–H bond in alcohols and phenols (very dilute solution or gas phase) appears as a sharp peak at 3600–3650 cm−1.
However, the extent of hydrogen bonding influences the observed stretching frequency and band broadening. Intermolecular or intramolecular...
1.9K
Reduction of Alkenes: Asymmetric Catalytic Hydrogenation02:17

Reduction of Alkenes: Asymmetric Catalytic Hydrogenation

3.9K
Catalytic hydrogenation of alkenes is a transition-metal catalyzed reduction of the double bond using molecular hydrogen to give alkanes. The mode of hydrogen addition follows syn stereochemistry.
The metal catalyst used can be either heterogeneous or homogeneous. When hydrogenation of an alkene generates a chiral center, a pair of enantiomeric products is expected to form. However, an enantiomeric excess of one of the products can be facilitated using an enantioselective reaction or an...
3.9K

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In Situ High Pressure Hydrogen Tribological Testing of Common Polymer Materials Used in the Hydrogen Delivery Infrastructure
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In Situ High Pressure Hydrogen Tribological Testing of Common Polymer Materials Used in the Hydrogen Delivery Infrastructure

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Hydrogen-poor superluminous stellar explosions.

R M Quimby1, S R Kulkarni, M M Kasliwal

  • 1Cahill Center for Astrophysics 249-17, California Institute of Technology, Pasadena, California 91125, USA. quimby@astro.caltech.edu

Nature
|June 10, 2011
PubMed
Summary
This summary is machine-generated.

A new class of luminous supernovae, ten times brighter than typical Type Ia events, has been discovered. These hydrogen-free explosions emit significant ultraviolet light and are observable at high redshifts.

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Area of Science:

  • Astronomy
  • Astrophysics
  • Cosmology

Background:

  • Supernovae are stellar explosions releasing vast amounts of energy.
  • Known supernovae are powered by radioactive decay, explosion shocks, or circumstellar material interaction.
  • Previous models fail to explain certain luminous supernova events.

Purpose of the Study:

  • To report the discovery and properties of a novel class of luminous supernovae.
  • To explain the energy source and characteristics of these unusual stellar explosions.
  • To identify previously unexplained events as members of this new class.

Main Methods:

  • Discovery and observation of four new luminous supernovae.
  • Re-analysis of two previously unexplained supernova events (SN 2005ap and SCP 06F6).
  • Analysis of supernova brightness, spectral properties (lack of hydrogen), ultraviolet flux, and late-time decay rates.

Main Results:

  • Identified a new class of supernovae approximately ten times brighter than Type Ia supernovae.
  • These supernovae lack hydrogen, exhibit strong, extended ultraviolet emission, and show decay rates inconsistent with radioactivity.
  • Properties suggest radiation from hydrogen-free material expanding at high speeds over large radii (∼10^15 cm).

Conclusions:

  • The observed properties necessitate a new model for luminous supernovae, distinct from known mechanisms.
  • These events are observable out to high redshifts (z > 4), offering new probes of the early universe.
  • The findings expand our understanding of stellar explosion phenomena and their diversity.