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関連する概念動画

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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水素が少ない超輝かしい恒星爆発.

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
まとめ
この要約は機械生成です。

典型的なタイプIaのイベントよりも10倍も明るく輝く,新しい種類の明るい超新星が発見されました. これらの水素のない爆発は,有意な紫外線を放射し,高い赤偏移で観測できます.

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科学分野:

  • 天文学 天文学
  • 天体物理学 天体物理学
  • コスモロジー・コスモロジーとは

背景:

  • 超新星とは,膨大な量のエネルギーを放出する恒星爆発です.
  • 既知の超新星は,放射性崩壊,爆発ショック,または環恒星物質の相互作用によって動いています.
  • 以前のモデルでは,ある特定の輝く超新星現象を説明できませんでした.

研究 の 目的:

  • 発光超新星の新しいクラスの発見と特性を報告する.
  • これらの異常な恒星爆発のエネルギー源と特徴を説明するために.
  • この新しいクラスのメンバーとして,以前説明できなかった出来事を識別する.

主な方法:

  • 4つの新しい輝く超新星を発見し,観測した.
  • これまでに説明できなかった2つの超新星事件 (SN 2005apとSCP 06F6) の再分析.
  • スーパーノヴァの明るさ,スペクトル特性 (水素欠乏),紫外線流,および遅い時間の崩壊率の分析.

主要な成果:

  • タイプIア超新星より約10倍明るく,新しいクラスの超新星が特定されました.
  • これらの超新星には水素が欠け,強い紫外線放射が広がり,放射能と矛盾する崩壊率を示しています.
  • 特性により,無水素物質からの放射線は,大きな半径 (∼10^15cm) で高速で膨張することを示唆しています.

結論:

  • 観測された性質は,既知のメカニズムとは異なる,明るい超新星のための新しいモデルを必要とします.
  • これらの現象は,高赤偏移 (z > 4) まで観測可能であり,初期の宇宙に関する新しい探査機を提供します.
  • この発見は,恒星爆発現象とその多様性についての理解を広げています.