Shapes & Structures 1
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Formula for formal charge?
Formal charge = Nv - Ne where: Nv = no. valence electrons associated with atom if hypothetically neutral, = group no. for groups 1-12, or group no. - 10 for groups 13-18 Ne = no. e- associated with atom (assuming e- are shared equally between bonding atoms): one per bonding pair, 2 per lone pair
why do EN atoms increase shift?
An electronegative atom makes a nucleus resonate at a higher frequency by deshielding it: • E- create a magnetic field opposing the applied one • EN atom attracts bonding e- → lower e- density around magnetic nucleus → weaker local magnetic field → stronger net magnetic field • EN atom deshields nucleus • Larger energy difference between spin states • Higher resonating frequency → higher shift
For 13C and proton NMR, the reference compound is tetramethylsilane (TMS). Why is this used?
Inert Only one 13C / 1H environment → one peak Si less EN than C → Cs have higher e- density → more shielded → smaller ΔE between spin states → lower resonating frequency → less shifted (positioned to the right), out the way
why is deuterium used as an NMR solvet?
Deuterium is used (e.g. CD2Cl2, deuterated chloroform) since it’s not spin-active, so doesn’t interfere with the solute spectrum.
Carbon NMR shift region for sp3?
0-50 ppm
Carbon NMR shift region for sp3 carbons with EN groups attached?
50-100 ppm
Carbon NMR shift region for triple bonded carbons?
Around 80 ppm
Carbon NMR shift region for sp2 carbons: i.e. double bonds, benzene?
100-150 ppm (benzene higher)
Carbon NMR shift region for acid derivatives: esters, acyl chlorides, amides, acid anhydrides?
160-170 ppm
Carbon NMR shift region for carbonyls: ketones, aldehydes?
around 180-200 ppm
which of aldehydes and ketones have higher carbon shift?
ketones
which broad categories do the following shift regions indicate? • 0-50 ppm • 50-100 ppm • 100-150 ppm • 150-200 ppm
• sp3 carbons • sp3 carbons with EN groups attached, also double bonds • sp2 (trig planar), e.g. double bonds/benzene • sp2 with EN groups attached, e.g. ketones
word for • coupling over 2 bonds? • over 3 bonds?
• 2 = geminal (twins) • 3 = vicinal
Attached proton test (APT): Peaks from carbons with an __ number of protons attached point one way (same as deuterated solvent, since ?)
Peaks from carbons with an even number of protons attached point one way (same as deuterated solvent, since it has 0 Hs & 0 is even) Peaks from carbons with an odd number of protons attached point other way
for a 400 MHz instrument, how many MHz represent 1 ppm?
400
Coupling constants – order these in terms of relative size: • Geminal, tetrahedral • Geminal, across double bond • Vicinal, tetrahedral, same environment • Vicinal, trans
• Vicinal, trans: ~15 Hz • Geminal, tetrahedral ~13 Hz • Vicinal, cis: ~10 Hz • Vicinal, tetrahedral, same environment ~ 3 Hz • Geminal, across double bond 0-0.3 Hz
Coupling constants: give rough values for each: • Vicinal, trans • Geminal, tetrahedral • Vicinal, cis • Vicinal, tetrahedral, same environment • Geminal, across double bond
• Vicinal, trans: ~15 Hz • Geminal, tetrahedral ~13 Hz • Vicinal, cis: ~10 Hz • Vicinal, tetrahedral, same environment ~ 3 Hz • Geminal, across double bond 0-0.3 Hz
roofing indicates what?
Protons in similar environments –> smaller shift difference in Hz between coupling nuclei → stronger roofing effect (until singlet seen)
Range of shifts of protons attached to carbon?
0-14 ppm
Shifts of protons attached to carbon: What does proton shift = 0.5 ppm indicate?
on 3-membered ring
Shifts of protons attached to carbon: What does proton shift = 1.0 ppm indicate?
methyl
Shifts of protons attached to carbon: What does proton shift = 1.5 ppm indicate?
-CH2- (alkyl)
Shifts of protons attached to carbon: What does proton shift = 2.5 ppm indicate?
Most attached to double/triple bond: • next to benzene • next to C=O • next to C=C • -N-CH2- (next to N in amine) • -C≡C-H (terminal alkyne)
Shifts of protons attached to carbon: What does proton shift = 3.5 ppm indicate?
• -CONH–CH2- (next to N in amide) • next to alcohol • next to ether