NMR Spectroscopy Flashcards
2 kinds of NMR focused on
1H (proton) and 13C
NMR experiment
- apply external magnetic field to make all spins either parallel or anti-parallel to magnetic field; more spins in lower energy state
- high energy light pulse -> excites low energy electrons -> photon emits as e- falls back -> detected
what happens to spins as magnetic field increased
larger difference between parallel and anti-parallel spins
standard reference for NMR
TMS; at 0 ppm
what do the number of signals tell you
the number of different environments in the molecule
what does the chemical shift tell you
type of environment H atom is in; downfield shift as deshielding occurs
downfield shift
increase ppm; C bonded to electroneg. atom which takes electron density = deshielding
*less electron density around carbon bonded to H
upfield shift
decrease in ppm value; impact of electron shielding
* more electron density around carbon bonded to H
what do the integration values tell you
number of H in the environment
1 = -CH
2 = -CH2
3 = -CH3
what do coupling patterns tell you
how many hydrogens in close proximity (in diff environments)
N+1 rule
predicts number of peaks for any given proton (coupling)
what does coupling constant show you
stereochem/ isomers
what is n in the n+1 rule
n= number of protons on adjacent carbon (s)
how are ratios for coupling determined
pascals triangel
ex.
triplet: 1:2:1
quartet: 1:3:3:1
Curphy Morrison parameters
used to calculate chemical shifts based on sub groups;
standard shift + (sum of alpha shifts) + (sum of beta shifts)
how does symmetry in molecules impact peaks
since environments are identical, there are less peaks but they are more intense
steps for predicting structure of molecule
1) look at formula and calculate unsaturated units
2) think about possible functional groups (use IR)
3) think about connectivity (use NMR)
4) confirm with mass spec (draw out fragmentation)
how to calculate saturation unit
U= [ 2C +2 + N - H - X]/2
H-NMR coupling with long carbon chains
*N+1 rule does not work; as chain extents growth of peaks representing each environment
*second order effects increase
diastereotopic protons
2 protons very close in chemical shift AND coupled with one another
*N+1 rule does not apply; second order effects
*diastereomers
how is 13C-NMR different from proton NMR
-no coupling; because only 1% of all C is 13 C
- more sensitive to small changes in molecular environment
how does the solvent CDCl3 show up on C-NMR
1:1:1 triplet between 8 and 75 ppm
impact of H- decoupling
enhance total intensity of signal
primary, secondary, etc carbons ppm
highest ppm -> lowest ppm =
4 > 3> 2> 1
impact of planes of symmetry on NMR spectra
simplify the spectra; less signals overall BUT signal is intensified if equivalent environment on other side of plane of symmetry
Vinyl group
alkene function group with one of the carbons having 2H and the other having 1 H and 1 R group
alkene magnetic field
protons of C=C group experience enhanced magnetic field -> higher ppm
*5.8 plus or minus 1.5 ppm
Vinyl group coupling
trans H, cis H, geminal H (on same carbon)
vinyl group coupling intensity
strongest -> weakest coupling
trans > cis > gem
coupling trees
diagrams to interpret splitting pattern shown in NMR spectra peaks; strongest coupling pattern (trans> cis > gem) splits first -> other coupling exhibited
Curphy Morrison Parameters
use Curphy Morrison Parameters for H- NMR constants;
standard value for group (-CH3, vinyl group, etc) + value for coupled functional group (cis/trans/gem function group)
H-NMR aromatic ppm
7.5 plus or minus 1 downward ppm shift; conj pi system creates magnetic feild
benzene ring coupling
strongest -> weakest
ortho> meta> para
ortho = sub groups on neighboring carbons; A and B
meta = sub groups on non neighboring carbons; A and A’
C-NMR: what does the height of carbon peak indicate
number of H attached to it; taller the peak, more saturated the carbon
ipso carbon
carbon with no hydrogens attached > small signal
