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What are the characteristics of 1H NMR spectrum, and what they represent (brief)?
- Number of signals: number of electrons with different electronic environments
- Chemical shift: based on electronic environment of protons
- Integration: number of protons giving rise to the signal
- multiplicity: Number of neighboring protons
What are the rules regarding number of signals in an NMR
- Number of signals indicates the number of protons with a different electronic environment
- If protons are homotopic or enantiotopic they are equivalent (a single signal) - can be interchanged by rotation or reflection
- The 3 protons of a CH3 group are always equivalent
- The 2 protons of a CH2 group are equivalent if the compound has no chirality centers
- The 2 protons of a CH2 group are not equivalent if the compound has chirality centers
Values to memorize regarding induction and other chemical shifts.
- Methyl (CH3): ~.9ppm
- Methylene (CH2): ~1.2ppm
- Methine (CH): ~1.7
- Higher electronegativity = greater shift
- Beta protons affected 1/5 as much as alpha protons
- Oxygen of alcohol or ether (ROH, ROR): +2.5
- Oxygen of ester (R(C=O)OR): +3
- Carbonyl group (C=O): +1
- Aromatic methyl (arene-CH3): ~2.5
- Alkyl halide (R2-XH): ~2-4
- Aryl (arene-H): ~6.5-8
- Aledhyde (R(C=O)H): ~10
- Carboxylic acid (RCOOH): ~12
What is the process for determining integration?
- Measure the step curve, or the area given
- Divide all areas by the smallest, this gives the relative number
- Match the relative number to the given molecular formula by multiplying all by same integer.
Describe the methods behind coupling
- Singlet, doublet, triplet - based on nonequivalent neighboring protons
- Splitting is most common 2-3 sigma bonds apart, nothing highter than 3 sigma bonds apart will split
- Intensity of the peaks follows the pyramid
Coupling constant and familiar patterns
- J value: the distance between peaks of a splitting pattern. Will be equal between protons that split eachother.
- ethyl group pattern: triplet integration 3 upfield from quartet integration 2
- isopropyl group pattern: doublet integration 6 upfield from septet integration 1
- tert-buytl group pattern: singlet with integration 9
- When a proton has two different kinds of nonequivalent neighboring protons
- Example - if Hb is split by 3 Ha and 2 Hc...
- if Jab >> Jbc the signal will appear as a quartet of triplets
- If Jbc >> Jab the signal will appear as a triplet of quartets
How to analyze and interpet NMR spectra
- inspect the molecular formula - calculate the degrees unsaturation. 4+ = aromatic ring
- consider the number of signals and their integration (clues about symmetry)
- analyze each signal (chemical shift, integration, multiplicity) and then draw fragments consistent with each signal.
- assemble the fragments into a molecular structure
All 13C NMR data
- Requires more sample
- Only number of signals and chemical shift is considered
- 220 - 0 ppm
- Each signal is C w/ unique electronic environment
- 0-50: R2-C-R2
- 50-100: C=C, C-N, C-O
- 100-150: C=C, aromatic-R
- 150+: C=O
- DEPT-90: only CH signals
- DEPT-135: CH3 and CH give + signals, CH2 give - signals