Lecture 12 - NMR Spectroscopy Flashcards
NMR and how it differs to MS & IR:
probes molecular structure in greater detail than IR or MS
what can be determined from NMR spectroscopy:
a complete molecular structure can oftentimes be determined by NMR alone
NMR spectroscopy has revolutionized organic chemistry since its introduction in the 1950’s
Before the advent of NMR, structure elucidation may have taken months (or years!)
NMR is based on:
nuclear excitation
nuclei can only be observed when:
nuclei posses the magnetic property of spin
what nuclei are of the most important spin:
1H & 13C
natural abundance of - 1H, 13C & 12C:
1H - 99.9%
13C - 1.10%
12C - 98.90%
what is the large TMS shift at the end of the NMR spectra?
the large TMS shift is a standard control molecule to compare to
in 1H-NMR the absorptions detect:
the protons in the molecule
the numbers on the ppm scale (delta) and the frequency are:
proportional - related by the operating frequency of the NMR spectrometer, Vo (in MHz)
peaks are referred to as:
resonances, absorptions or lines - all are used and accepted as correct
the position of the adsorption is called:
the chemical shift (ppm)
___________ is added to each sample as an internal reference (d 0.0 ppm)
tetramethylsilane (TMS)
TMS properties:
TMS has a strong absorption, is chemically inert and can be easily removed (volatile)
organic compounds generate a separate resonance for:
each chemically non-equivalent set of nuclei
the chemical shift is determined by:
the nature of nearby groups (the chemical environment) in a predictable way
the size (area under, or integration) of a peak is proportional to:
the number of contributing protons
what can also be determined from the 1H-NMR spectra?
the number of protons on adjacent carbons can also be determined
nuclear spin causes the nucleus to behave like:
a tiny magnet
the tiny magnet can either:
align with the field or go against the field
the 1H (and 13C) nucleus can have one of two:
spin states (quantum numbers +½ or -½)
(YH) gyromagnetic ratio:
constant that is different for different nuclei
how do the tiny nuclear magnets align with a magnetic field?
initially the nuclei exist as randomly oriented spins of equal energy → then you add a magnetic field → before the lower energy state has an excess of spins, however after radiation + energy you have half -1/2 spins and half +1/2 spins
what actually is the absorption?
this absorption is nuclear magnetic resonance and is detected by an NMR spectrometer
the local magnetic field sensed by a proton is different than the applied magnetic field, what is this due to?
due to the electrons around the proton which oppose the external field
reduction of the local field is called:
shielding, hence the electronegativity of nearby atoms will affect the shielding around a proton
one of the most important factors in NMR:
electronegativities of nearby groups
proton chemical shift is increased by:
•Increasing electronegativity
•Increasing number
•Decreasing distance
•or nearby electronegative groups
how will alkyl substitution affect chemical shift?
the more R groups surrounding a proton, the higher the ppm delta values (chemical shift)
Protons have different chemical shifts when they are in:
different chemical environments
predicting chemical shift non-equivalence is usually the same as:
predicting chemical non-equivalence
chemically equivalent protons have:
identical chemical shifts
Groups are constitutionally equivalent when:
they have the same connectivity relationship
In general, constitutionally nonequivalent groups are:
chemically nonequivalent
why might Constitutionally equivalent groups may be chemically nonequivalent?
Chemical equivalency between two groups depends on their stereochemical relationship
Equivalency is revealed by:
a substitution test
substitution tests to look for equivalency:
(1) substitute each constitutionally equivalent group with a fictitious group and compare
(2) if identical molecules are obtained, the groups
are homotopic and are chemically equivalent
(3) If enantiomers are obtained,
the groups are enantiotopic
(4) If diastereomers are obtained, the groups are diastereotopic
enantiotopic groups are:
chemically nonequivalent toward chiral reagents, but are chemically equivalent to achiral reagents
Diastereotopic groups are:
chemically nonequivalent under all conditions
