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Videos de Conceptos Relacionados

¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
¹H NMR: Pople Notation01:09

¹H NMR: Pople Notation

The Pople nomenclature system classifies spin systems based on the difference between their chemical shifts. Coupled spins are denoted by capital letters with subscripts indicating the number of equivalent nuclei. When the coupled nuclei have well-separated chemical shifts, they are assigned letters that are far apart in the alphabet, such as A and X. When the difference in chemical shifts is small, coupled nuclei are named using adjacent letters of the alphabet (AB, MN, or XY).
A proton...
¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR01:15

¹H NMR of Conformationally Flexible Molecules: Variable-Temperature NMR

The axial and equatorial protons in cyclohexane can be distinguished by performing a variable-temperature NMR experiment. In this process, except for one proton, the remaining eleven protons are replaced by deuterium. The deuterium substitution avoids the possible peak splitting caused by the spin-spin coupling between the adjacent protons. The remaining proton flips between the axial and equatorial positions.
¹³C NMR: Distortionless Enhancement by Polarization Transfer (DEPT)01:20

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

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...
¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
Mass Spectrometry of Amines01:15

Mass Spectrometry of Amines

In mass spectroscopy, amines undergo fragmentation to give parent ions with odd molecule weights. This observed mass spectrum follows the nitrogen rule; a molecule with an odd number of nitrogen atoms produces a molecular ion with an odd molecular weight. Amines undergo fragmentation through α cleavage, producing nitrogen-containing cations—iminium ions—and alkyl radicals. Mass spectra of aromatic and cyclic aliphatic amines exhibit strong molecular ion peaks, but acyclic aliphatic amines show...

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MALDI-ToF MS Method for the Characterization of Synthetic Polymers with Varying Dispersity and End Groups
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Análisis de polímero universal por (1) RMN H utilizando grupos finales trimetilsililo complementarios.

Michael Päch1, Daniel Zehm, Maik Lange

  • 1Fraunhofer Institute of Applied Polymer Research, Geiselbergstrasse 69, D-14476 Potsdam-Golm, Germany.

Journal of the American Chemical Society
|June 10, 2010
PubMed
Resumen

Los nuevos agentes de transferencia de cadena degenerativa con marcadores de trimetilsililo (TMS) permiten el análisis universal de polímeros utilizando la espectroscopia de RMN (1) H. Estos agentes proporcionan una masa molar precisa y la determinación de grupos finales para la polimerización radical controlada, simplificando la caracterización del polímero.

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Área de la Ciencia:

  • Química de Polímeros La Química de Polímeros es la química de los polímeros.
  • Síntesis orgánica La síntesis orgánica.
  • Química analítica Química analítica es la que

Sus antecedentes:

  • Las técnicas de polimerización radical controlada como la polimerización de transferencia de cadena de adición-fragmentación reversible (RAFT) son esenciales para sintetizar polímeros con propiedades definidas.
  • La caracterización precisa de la masa molar del polímero y la funcionalidad del grupo final es crucial para comprender el comportamiento y las aplicaciones del polímero.
  • Los métodos analíticos convencionales pueden ser un desafío para determinar simultáneamente estos parámetros, especialmente para diversas estructuras de polímeros.

Objetivo del estudio:

  • Diseñar y sintetizar nuevos agentes de transferencia de cadena degenerativa para la polimerización radical controlada.
  • Demostrar la utilidad de estos nuevos agentes para el análisis universal de polímeros a través de la espectroscopia de RMN (1) H.
  • Para permitir una fácil determinación de las masas molares absolutas y el contenido de grupos finales en polímeros.

Principales métodos:

  • Diseño y síntesis de agentes de transferencia de cadena degenerativa de doble etiqueta con marcadores trimetilsililo (TMS) complementarios.
  • Aplicación de estos agentes en la polimerización de transferencia de cadena de fragmentación por adición reversible (RAFT) de varios monómeros (acrilato de n-butilo, estireno, inimeros).
  • Análisis de los polímeros resultantes utilizando espectroscopia convencional de RMN (1) H para determinar la masa molar y el contenido del grupo final.

Principales resultados:

  • Se obtuvieron polímeros con bajas polidispersidades y altas masas molares utilizando los nuevos agentes RAFT.
  • Los espectros de RMN (1) H de rutina permitieron la determinación precisa de las masas molares de hasta 10(5) g/mol y el contenido del grupo final (>95%).
  • Los marcadores complementarios de grupos finales de TMS revelaron diferencias dependientes del disolvente en la idoneidad de la polimerización RAFT.

Conclusiones:

  • Los agentes RAFT recientemente desarrollados son potentes herramientas para el análisis universal de polímeros por (1) H NMR espectroscopia.
  • Estos agentes facilitan la determinación de las masas molares absolutas y el contenido de grupos finales, cruciales para la síntesis de polímeros de alta calidad como los telecélicos y los copolímeros de bloque.
  • Los resultados preliminares sugieren una aplicación potencial en sistemas análogos de polimerización radical por transferencia atómica (ATRP).