Flashcards
further 1H nmr
Flashcards do conjunto (24)
- 1When there is free rotation in a molecule what is the approximate value of 3JHH?7 Hz
- 2What is 3JHH a function of and what does 7 Hz represent?It is a function of the dihedral angle between protons (φ), 7 Hz represents an average value of 3JHH across all values of φ.
- 3What is the Karplus Curve? Give a description of its shape.A plot of 3JHH against φ.
Approximately cosinusoidal in shape.
Maxima at φ = 0 and 180 degrees
Minimum at φ = 90
Maximum is higher at 180 than 0. - 4Where do we see fixed values of φ in molecules what are their J valuesAlkenes (E/Z isomers)
Z –φ = 0 degrees and 3J = 8-12 Hz
E – φ = 180 degrees and 3J = 12-18 Hz
In rings: - 5What effect do π bonds have on the size of 3J and why?3J is larger in π bonds because they assist in transmitting spin-spin information from one nucleus to another.
- 6How can we understand the phenomenon of 3JHH being a function of φ?Involves scalar coupling through bonds, therefore the maximum orbital overlap will result in maximum spin-spin coupling and the largest value of 3JHH.
This occurs with an antiperiplanar arrangement where φ = 180 degrees
- Orbitals at 90 degrees to each other cannot overlap, hence 3JHH = 0 and φ = 90 degrees.
- 7For a benzene ring what are the coupling constants for the protons at the ortho, para and meta positions?Ortho (3J) = 8-9 Hz
Meta (4J) = 1-2 Hz
Para (5J) = 0-1 Hz - 8What’s the arrangement of protons that gives surprisingly large values of 4J and why?In W-coupling 4J = 7-8 Hz. Large due to arrangement of C-H σ and C-C σ* orbitals leading to efficient coupling over 4 bonds.
- 9What is the coupling called for protons that are not equivalent e.g. Ha & Hb on a ring or a double bond and what are their constants?Geminal coupling.
For sp3 carbon atoms (in a ring) 2J is usually large – 12-15 Hz
For sp2 carbon atoms (in a double bond) 2J is much smaller – 1-2 Hz - 10What is the term for two groups on adjacent carbons?Vicinal
- 11What is the cis 3 bond coupling constant?3J = 8-12 Hz
- 12What is the trans 3 bond coupling constant?3J = 12-18 Hz
- 13What are the coupling constants of the inequivalent protons in this rigid ring system:Ha and Hb couple to each other by 2 bond coupling – sp3 C, 2J = 10-15 Hz
Ha and Hb both couple to Hx by 3 bond coupling, small value due to dihedral angles (almost at 90 degrees) – 3J = 2-3 Hz
- 14What is the outcome of two protons coupling to Ha with the same coupling constant?Jab = Jac, the signal is split into a doublet then spit once more to form a triplet by consequence of adding the splitting together.
- 15What’s the outcome of three protons coupling to Ha with the same coupling constant?Jab = Jac = Jad, quartet.
- 16What’s the outcome of two protons coupling to Ha with different coupling constants?Jab ≠ Jac, therefore doublet of doublets is observed.
- 17What’s the outcome of three protons coupling to Ha one with a different coupling constant and two with the same?Jab ≠ Jac = Jad, a doublet of triplets is observed.
- 18What type of pattern is produced when there are two large coupling constants and one small one?Jab = Jac ≠ Jad, triplet of doublets.
- 19Why does roofing occur in NMR signals?Due to strong coupling, the difference in chemical shift between signals is relatively small and the coupling constant is relatively large – causing distortion in the signals becoming more pronounced with closer chemical shifts or with larger values of J.
- 20What is an AB system?When two protons have very similar chemical environments and so the difference in chemical shift is small. AB systems show strong coupling.
- 21What is an AX system?When two protons have very dissimilar chemical environments and so the difference in chemical shift is large. AX systems do not show strong coupling.
- 22What can be implemented to reduce strong coupling effects?Use of a stronger magnetic field gives a clearer spectrum.
- 23What must two protons have to be magnetically equivalent?The same geometric relationship with their coupling partners.
- 24What are some cases where protons are chemically equivalent but magnetically inequivalent?Para-substituted benzene, ortho substituted benzene, 1,1 difluoroethene.
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- Chemistry (sas) FlashCards sobre further 1H nmr, criado por Casey Champion em 23-04-2023.
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- Casey Champion
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