11 Spectroscopy Flashcards
infrared (IR) spectroscopy
measures molecular vibrations
bond stretching, bending, combinations of different vibrational modes
determine what bonds exist within molecule (infer functional groups)
IR wavenumber
4000 to 400 cm^-1
IR fingerprint region
1500 to 400 cm^-1
more complex vibrational patterns
specific absorbance patterns characteristic of each individual molecule
IR vibration must result in change of bond dipole moment
molecules that don’t result in change in dipole?
atoms of same electronegativity
symmetrical molecules
IR hydroxyl (O-H) absorption peak
3300 cm^-1 (alcohols)
3000 cm^-1 (carboxylic acid)
broad (wide) peak
IR carbonyl (C=O) absorption peak
1700 cm^-1
sharp (deep) peak
IR N-H absorption peak
3300 cm^-1
sharp (deep) peak
ultraviolet (UV) spectroscopy
absorbance = electronic transitions between orbitals
wavelength of max absorbance extent of conjugation within conjugated systems
increased conjugation decreases energy of transition increases wavelength of maximum absorbance
nuclear magnetic resonance (NMR) spectroscopy
MCAT = 1H (proton) NMR
certain atomic nuclei have magnetic moments that are oriented at random
when placed in magnetic field, moments align with/against field
NMR alpha state
magnetic moments align with field (lower energy)
NMR beta state
irradiation excited some lower-energy nuclei to higher state (higher energy)
TMS
NMR calibration standard (reference peak)
Marks 0 ppm
Skip when counting peaks
proton (1H) NMR graph
0 to 10 ppm downfield from TMS
each distinct set of nuclei give rise to a separate peak
(if multiple protons are chemically equivalent, they yield same peak)
height of each peak is proportional to # of protons it contains
(integration)
NMR: deshielding
EWG’s pull from surrounding atoms (deshield downfield)
proton cannot shield itself from applied magnetic field (reading further downfield)
*everything is more deshielded than TMS
NMR: shielding
EDG’s supply electron density
shield proton (reading further upfield)
NMR: spin-spin coupling (splitting)
2 protons on adjacent atoms (close proximity) that are not magnetically identical
(within 3 bonds of each other)
NMR: n+1 rule
if proton has “n” protons that are 3 bonds away, it will be split into “n+1” peaks; equally spaced; identical intensity
DO NOT INCLUDE protons attached to O or N
NMR: coupling constant (J)
magnitude of splitting
NMR: peaks that have more than 4 shifts
multiplet
NMR: sp3-hybridized carbons (H) chemical shift range
0-3 ppm
NMR: sp2-hybridized carbons (H) chemical shift range
4.6-6.0 ppm
NMR: sp-hybridized carbons (H) chemical shift range
2.0-5.0 ppm
NMR: aldehyde hydrogens chemical shift range
9-10 ppm
NMR: carboxylic acid hydrogens chemical shift range
10.5-12 ppm
