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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.
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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....
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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...
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Reduction of Alkenes: Catalytic Hydrogenation02:13

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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.
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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...
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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.
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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.
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Las explosiones estelares superluminosas pobres en hidrógeno son explosiones estelares superluminosas.

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
Resumen
Este resumen es generado por máquina.

Se ha descubierto una nueva clase de supernovas luminosas, diez veces más brillantes que los eventos típicos de Tipo Ia. Estas explosiones libres de hidrógeno emiten una luz ultravioleta significativa y son observables a altos desplazamientos al rojo.

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Área de la Ciencia:

  • La astronomía es la astronomía.
  • La astrofísica es la astrofísica.
  • Cosmología Cosmología.

Sus antecedentes:

  • Las supernovas son explosiones estelares que liberan grandes cantidades de energía.
  • Las supernovas conocidas son impulsadas por la desintegración radiactiva, los choques de explosión o la interacción del material circunstelar.
  • Los modelos anteriores no logran explicar ciertos eventos luminosos de supernova.

Objetivo del estudio:

  • Para informar sobre el descubrimiento y las propiedades de una nueva clase de supernovas luminosas.
  • Para explicar la fuente de energía y las características de estas inusuales explosiones estelares.
  • Para identificar eventos previamente inexplicables como miembros de esta nueva clase.

Principales métodos:

  • Descubrimiento y observación de cuatro nuevas supernovas luminosas.
  • Reanálisis de dos eventos de supernovas previamente inexplicables (SN 2005ap y SCP 06F6).
  • Análisis del brillo de la supernova, las propiedades espectrales (falta de hidrógeno), el flujo ultravioleta y las tasas de decaimiento tardío.

Principales resultados:

  • Se identificó una nueva clase de supernovas aproximadamente diez veces más brillantes que las supernovas de tipo Ia.
  • Estas supernovas carecen de hidrógeno, exhiben una fuerte emisión ultravioleta extendida y muestran tasas de desintegración inconsistentes con la radioactividad.
  • Las propiedades sugieren que la radiación del material libre de hidrógeno se expande a altas velocidades en grandes radios (∼10 ^ 15 cm).

Conclusiones:

  • Las propiedades observadas requieren un nuevo modelo para las supernovas luminosas, distinto de los mecanismos conocidos.
  • Estos eventos son observables hasta altos desplazamientos al rojo (z > 4), ofreciendo nuevas sondas del universo temprano.
  • Los hallazgos amplían nuestra comprensión de los fenómenos de explosión estelar y su diversidad.