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91 terms

Kaplan MCAT OChem Ch. 13: Spectroscopy

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spectroscopy
identify new unknown compound
spectroscopy
measures energy diff. between possible states of molecular system by determining frequencies of electromagnetic radiation (light) absorbed by the molecules
spectroscopy possible states
quantized energy levels asso. w/ diff. types of molecular motion like:

molecular rotation

vibration of bonds

nuclear spin transition

electron absorption
spectroscopyq
diff types measure diff. molecular prop => identify presence of specific func groups
spectroscopy advantages
small sample need and can be reused (except mass spec)
Ir spec
measures molecular vibrations, seen as bond stretching, bending, or combo of diff. vib modes.
abs. of IR light
wavelengths of 3000 to 30,000 nm
ir spec
represented on graph we use wavenumber (analog of freq) => 3,500 to 300 cm^-1
Ir spec
light of freq/wavenumbers are absorbed => molecules enter excited vib state
Bond stretching (sym or asym, ir spec)
involves largest change in energy => observed in highest freq. of 4000 to 1500 cm^-1
bending vibrations
observed in lower freq region of 1500 to 400 cm^-1
four types of vibrations
sym and asym bend

sym/asym stretch
Ir spec
can be vibrations that combo bending, stretching and rotating
ir spec
for absorption to be recorded => vibration must result in change in bond dipole moment (sym bonds = silent)
ir spec
to get -> pass IR light (4000 to 400 cm^-1) through sample=> record abs pattern
ir spec
plotted percent transmittance verus. freq.
percent transmittance
equal abs - 1 (max abs at bottom valleys of spectrum)
alkanes abs freq
2800-3000, (C-H)

1200 (C-C)
alkenes
3080-3140 (=C-H)

1645 (C=C)
alkynes
2200 (C≡C)

3300 (≡C-H)
aromatic
2900-3100 (C-H)

1,475-1625 (C-C)
alcohols
3100-3500 (O-H) (broad)
ethers
1,050-1150 (C=O)
aldehydes
2700-2900 (O)C-H

1725-1750 (C=O)
ketones
1700-1750 (C=O)
acids
1700-1750 (C-O)

2900-3300 O-H (broad)
amines
3100-3500 (N-H) (sharp)
infrared spec
used to ID func. groups

most important are alcohols and carbonyls
IRC
info from freq between 1400 and 4000 cm^-1 (anything lower doesn't matter)
NMR
based on fact that nuclei have magnetic moments that are oriented at random
NMR
when nuclei placed in magnetic field => this tend to align either w/ or against direction of applied force => nuclei then irradiated w/ radio freq. pulses that match energy gap between two states => excite some lower-energy nuclei into β-state => abs of this radiation leads to excitation at diff. frequencies, depending on atom's magnetic environment
α -state
lower energy, nuclei whose mag. moments are aligned w/ filed
β-state
those whose mag are aligned against field; higher energy
NMR
nuclear magnetic moments of each atom are affected by nearby atoms that also possess magnetic moments, hence a compound may contain many nuclei that resonate at diff. frequencies, producing a complex spectrum
NMR
plot of freq. vs. absorption of energy during resonance
NMR
standarized method of plotting is chemical shift
chemical shift
parts per million of spec freq
chemical shift
increases towards the left (downfield)

TMS = 0 ppm (reference peak)
has magnetic momenet
nuclei w/ odd mass or odd atomic numbers, or both, will have this when placed in magnetic field
H NMR
each distinct set of nuclei gives rise to a separate peak
H NMR
if multiple identical nuclei in relatively identical locations => magnetically equivalent => all resonate at same frequency
H NMR
greater number of protons => taller peak
deshielding (H NMR)
atoms pull electron density away from surrounding atoms => less can shield itself from apparent magnetic field => downfield
shield (H NMR)
EDG helps to do this => further upfield
coupling (H NMR)
when two protons are in close proximity (two C's away, not magnetically identical => this occurs
doublet, triplets, and multiplets
number of peaks that follow n + 1 rule

n = number of protons
coupling constant, J
magnitude of this splitting, measured in Hertz
splitting of peaks
represents number of adjacent hydrogens
Proton NMR
1. number of protons and relative chem environments
2. shows how many adjacent protons by splitting patterns
3. certain func groups
RCH3
0.9
RCH2
1.25
R3CH
1.5
2CH5CH
4.6-6.0
2C; CH
2.0-3.0
Ar2H
6.0-8.5
2CHX
2.0-4.5
2CHOH/2CHOR
3.4-4.0
RCHO
9.0-10.0
RCHCO2
2.0-2.5
2CHCOOH/2CHCOOR
2.0-2.6
2CHOH2CH2OH
1.0-5.5
ArOH
4-12
2COOH
10.5-12.0
2NH2
1.0-5.0
NMR Spec
1. # of nonequivalent nuclei, determined from number of peaks
2. mag. environment of nucleus, determined by chemical shift
3. relative numbers of nuclei, determined by integrating peaks areas in H NMR
4. number of neighboring nuclei, determining by splitting pattern
13C NMR
signal occurs 0 210 chem shift downfield from carbon peak of TMS
13C NMR
1. large sample size need (x50 mg)
2. coupling generally not observed
13C NMR
however, couplng is observed between C atoms and protons that are directly attached
(spin decoupling) 13C NMR
ability to record a spectrum w/o coupling of adjacent protons
(spin decoupling) 13C NMR
produces a spectrum of singlets. each corresponding to a separate, magnetically equivalent carbon atom
13NMR
number of 13C resonances will gove us carbon count in molecule
UV spec
study compounds w/ DBs and/or hetero atoms w/ lone pairs that create conjugated systems
UV spec
obtained by passing UV light through sample (usually dissolved in inert, nonabsorbing solvent) => absorbance plotted against wavelength
UV spec
absorbance cause by electronic transitions between orbitals
UV spec
info we get is wavelength of max absorbance => tells extent of conjugation w/in conjugated systems, as well as other structural and compositional info
UV spec
more conjugated system -> lower the energy of transitions
mass spec: basic theory
destructive tech since destroys compound and can't reuse
mass spec: basic theory
high-speed beam of electrons to ionize the sample (eject electron) => particle acc. to put charged particles in flight => a magnetic field to deflect acc. cationic frag => detector records number of particles of each mass that exits deflector area
molecular radical-cation (M+)
initially formed ion in mass spec, resulting from a single electron being removed from a molecule of sample
molecular radical-cation (M+)
unstable species => decompses rapidly into cationic frag and rad. frag.
mass spec: basic theory
since many molecules in sample, this is composed of many lines, each corresponding to speciifc mass/charge ratio (m/z)
mass spec: basic theory
mass/charge - horizontal

relative abundance of various cationic fragments - vertical axis
characteristics of mass spec = tallest peak (highest intensity)
belongs to most common ion called the base peak and is assigned relative abundance value of 100 percent
characteristics of mass spec = tallest peak (highest intensity) = highest m/z ratio (peak furtherest to right)
is generally molecular ion peak or M+(parent ion peak)
molecular ion peak or M+(parent ion peak)
since one electron missing:
1. charge value is usually 1 and the m/z ratio usually can be read as mass of fragment itself
mass spec application
frag pattern compounds => ID or distinguish certain compounds/clues to compound's structure by way of molecular mass
mass spec application
shows weight of molecule w/ a diff frag missing
carbonyls
sharp peak at 1,700 cm^-1
hydroxides
broad peak at 3300 cm^-1
amines
sharper peaks at 3300 and 3400 cm^-1 for primary amines; secondary amines have one peak.
mass spec.
m/z(really just mass) vs. intensity