ochem week 8 part 1

Overview of NMR Spectroscopy

• NMR (Nuclear Magnetic Resonance) spectroscopy is a powerful analytical technique used to determine the structure of organic compounds.

• It provides detailed information about molecular connectivity and the environment around specific atoms.

• NMR is considered complex due to the depth of information it provides.

Importance of Functional Groups in Spectroscopy

• Specific bonds, like O-H bonds, can be identified through IR (Infrared) spectroscopy, indicating the presence of functional groups.

• NMR helps determine the connectivity of carbon atoms and provides insights into the molecular structure beyond functional groups alone.

Key Concepts in NMR Operability

• Nuclear Spin: Certain nuclei (e.g., hydrogen, carbon-13) possess spin, leading to the generation of a magnetic moment.

• Magnetic Field Alignment: Spinning charged nuclei align with an external magnetic field, either parallel (lower energy) or antiparallel (higher energy).

• Energy Absorption: When exposed to radiofrequency radiation, the nuclei can absorb energy to flip their spin state, which is detected in the NMR spectrum.

Electronic Environment and Chemical Shielding

• The local electronic environment around a nucleus affects its magnetic shielding:

  • Shielded Nuclei: Electrons surrounding a nucleus reduce the external magnetic field's effect, resulting in a higher field (upfield) signal.

  • Deshielded Nuclei: When electronegative atoms are nearby, they pull electron density away, causing a lower field (downfield) signal.

• The x-axis of an NMR spectrum is measured in PPM (parts per million).

Spectral Patterns in Carbon NMR

• Carbon atoms display varying chemical shifts depending on their electronic environment:

  • Typical Ranges:

    • Carbonyl Carbon (C=O): 160-220 ppm

    • Alkene (C=C): ~100-150 ppm

    • Aromatic Carbons: ~100-160 ppm

    • Sp3 Carbons (C-C): ~0-70 ppm

• Peaks are interpreted based on the type and number of neighboring atoms, bond types, and hybridization states.

Chemically Equivalent Carbons

• Chemically equivalent carbons will appear at the same position on an NMR spectrum due to similar electronic environments.

• Recognizing symmetry in molecular structure helps predict equivalent carbon signals.

• For example, in a symmetrical molecule, there might be fewer peaks in the NMR spectrum than there are carbons.

The DEPT Experiment

d- DEPT (Distortionless Enhancement by Polarization Transfer): This NMR experiment helps differentiate between different types of carbon environments based on the number of attached hydrogens:

• Methine (C-H): Peaks corresponding to one hydrogen appear positively.

• Methylene (C-H₂): Peaks corresponding to two hydrogens appear negatively.

• Methyl (C-H₃): Peaks corresponding to three hydrogens appear positively.

• Quaternary Carbons (C): Does not appear on DEPT spectra.

Practical Implications of NMR

• Quality of the instrument affects the clarity of NMR spectra; higher resolution instruments yield clearer results.

• Running NMR does not alter the sample, allowing for analysis without loss of material.

• Techniques like DEPT allow researchers to glean more information from a spectrum, guiding structure elucidation in complex molecules.

Sample Questions for Understanding NMR

• Identify which carbon is more downfield based on its connectivity to electronegative atoms.

• Discuss how symmetry in a chemical structure affects expected NMR peaks.

ochem week 8 part 1

NMR - How it works • Nuclear magnetic resonance works by measuring the energy associated with nuclear spin under an applied magnetic field at a given resonance frequency • Nuclei will spin parallel to an applied magnetic field (low energy) until they are hit with an appropriate amount of energy to allow them to flip to the antiparallel (higher energy) state. • Different nuclei require different amounts of energy Strength of applied,

NMR - How it works Nuclei require different amounts of energy depending on: •the element (different setting on the instrument, no overlap) • the instrument (stronger magnetic field requires a larger amount of energy, giving a more precise measurement) •electronic environment around the nucleus (where it is in the molecule will affect where related peaks appear in the spectra) Atoms with an odd number of protons or neutrons will spin and be magnetically active, but will have different resonance frequencies Ex. 12C does not spin, but the isotope 13C does, so only 13C isotopes are able to be detected using NMR Ex. 1H and 2H both spin, but at different frequencies, so when looking at 1H, any 2H atoms are effectively invisible.

electronic shielded- by the electronic environment, the more the electrons come together the more the nucleus is shielded

if it is double bonded, with less hydrogens on a carbon, molecules move away andcarhon has a higher bpm

chemically equivalent carbon

down field has a higher chemical shift (electrons spend more time somewhere else than on our carbon)

unique carbons will have different distances and/or different connections, and its sp ranking

chemically equivalent carbons must have the same substituents and same distance

when they are chemically equivalent, but still diffferent compared to a seperate atom, they are counted BOTH with 1 number

can only mesh two carbons into one number,

there's often a plane of symmetry that can share a peak

cant have any extra carbons in graph of peaks- there is less, but not more

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DEPT 13C NMR

dept determines amount of hydrogens on a carbon

NOT THE SAME AS PRIMARY- QUATERNARY. THIS IS BASED ON HYDROGEN COUNT

DEPT 13C NMR • One of the characteristics that can impact the electronic environment of a carbon is how many hydrogens it is attached to. Carbon with 3 H (CH3) - methyl • Carbon with 2 H (CH2) - methylene Carbon with 1 H (CH) - methine Carbon with 0 H - quaternary (different than 4° with alkanes) This is different than if a carbon is primary (1°), secondary (2°), tertiary (3°), or quaternary (4°). These carbons can't be differentiated on a typical 13C NMR, but can be if a series of different spectra are collected (typically referred to as DEPT-90 and DEPT 135) • DEPT allows for connecting specific peaks with specific carbon atoms, which can be useful for determining complex structures

DEPT-90 shouws CH

meso compound is opitcally inactive