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

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

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

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