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

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

Shapes & Structures 1 Flashcards by Katya Bungay-Hill

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

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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

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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

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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.

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Carbon NMR shift region for sp³?

0-50 ppm

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Carbon NMR shift region for sp3 carbons with EN groups attached?

50-100 ppm

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Carbon NMR shift region for triple bonded carbons?

Around 80 ppm

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Carbon NMR shift region for sp2 carbons: i.e. double bonds, benzene?

100-150 ppm (benzene higher)

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Carbon NMR shift region for acid derivatives: esters, acyl chlorides, amides, acid anhydrides?

160-170 ppm

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Carbon NMR shift region for carbonyls: ketones, aldehydes?

around 180-200 ppm

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which of aldehydes and ketones have higher carbon shift?

ketones

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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

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word for

coupling over 2 bonds?
over 3 bonds?

2 = geminal (twins)
3 = vicinal

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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

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for a 400 MHz instrument, how many MHz represent 1 ppm?

400

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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

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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

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roofing indicates what?

Protons in similar environments –> smaller shift difference in Hz between coupling nuclei → stronger roofing effect (until singlet seen)

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Range of shifts of protons attached to carbon?

0-14 ppm

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Shifts of protons attached to carbon:

What does proton shift = 0.5 ppm indicate?

on 3-membered ring

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Shifts of protons attached to carbon:

What does proton shift = 1.0 ppm indicate?

methyl

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Shifts of protons attached to carbon:

What does proton shift = 1.5 ppm indicate?

-CH₂- (alkyl)

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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)

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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

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Shifts of protons attached to carbon: What does proton shift = 4 ppm indicate?

* -COO-CH2- (next to ester) * Cl-CH2- (next to chlorine)

Shifts of protons attached to carbon: What does proton shift around 5.5 ppm indicate?

alkene / double bond

Shifts of protons attached to carbon: What does proton shift around 7.5 ppm indicate?

**on** benzene

Shifts of protons attached to carbon: What does proton shift = 8 ppm indicate?

-O-CO-H (formate/methanoate ester)

Shifts of protons attached to carbon: What does proton shift = 10 ppm indicate?

-CO-H (aldehyde)

Proton shift for 3-membered ring?

0.5 ppm

Proton shift for * methyl? * alkyl?

Methyl 1 ppm Alkyl 1.5 ppm

Proton shift for * aklyl next to benzene? * on benzene?

* next to = 2.5 ppm * on = 7.5, varies

Proton shift for * aklyl next to double bond, -C=C-CH2- * on double bond?

* next to = 2.5 ppm * on = 5.5, varies

Proton shift for -N-CH2- (next to N in amine)?

2.5 ppm

Proton shift for -C≡C-H (terminal alkyne)

2.5 ppm

Proton shift for -CONH–CH2- (next to amide)

3.5 ppm

proton shift for -O-CH2- (next to alcohol/ether)

3.5 ppm

proton shift for -COO-CH2- (next to ester)

4 ppm

proton shift for Cl-CH2- (attached to chlorine)

4 ppm

proton shift for -O-CO-H (formate/methanoate ester)

8 ppm

proton shift for -CO-H (aldehyde)

10 ppm

Why do protons on aromatic rings have greater shifts (~7.5) than alkene protons (5.5), & why does the proton on a terminal alkyne have such a low shift (~2.5)?

Due to local magnetic fields set up by e- in π-systems

why do shifts of exchangeable protons attached to electronegative elements vary so much?

Shifts may take a range of values, since they depend on amount of H bonding, which depends on: * Compound * Solvent * Sample concentration * Temperature

how would you distinguish peaks of exchangeable protons attached to electronegative elements?

* Broader than other signals * Exchangeable protons don’t couple (swapped protons may be spin up or down, so nearby protons experience an average spin state) * D2O shake test * Shake sample, prepared in deuterated solvent, with heavy water * All NH and OH protons are exchanged for deuterons from solvent → ND & OD; protons end up in HOD molecules * Broad peaks disappear since deuterium resonates at a different frequency to normal protons

For exchangeable protons attached to electronegative elements, how could you remove broad signals & observe coupling?

prepare a dilute solution in a dry solvent

proton shift in water?

1.5 ppm, broad

proton shift in amines?

1-3.5 ppm, broad

proton shift in alcohols?

1-3.5 ppm, broad

proton shift in * Ar-NH2 * Ar-OH

* 3.5-5.5 ppm, broad * 5.5-9 ppm, broad

proton shift in HOD?

4.5 ppm, broad

proton shift in amides -CONH-

6-11 ppm, broad

proton shift in carb acids?

**confirm** broad, probs high around 10

what does a broad proton peak around 1-3.5 ppm indicate?

* amine * alcohol * water

what does a broad proton peak around 6-11 ppm indicate?

amide | (maybe carb acid)

why does deuterated chloroform have a quintet, in ratio 1:2:3:2:1?

Show that a high wavenumber indicates high-energy vibration.

Wavenumber, ṽ, = 1/λ. Proportional to frequency since: f = c/λ = c(1/λ) So f = cṽ So f ∝ ṽ ΔE = hf, so frequency of absorption ∝ change in energy of vibrational transition. So a high wavenumber indicates high-energy vibration.

give formula for reduced mass show how it can be simplified if one atom is heavier

The reduced mass, μ, accounts for mass of both atoms: μ = m1m2/(m1 + m2) When m1 \> m2, μ ~ m1m2/m1 = m2 i.e. vibrations of heavier atom are negligible compared to that of lighter atom

what is Raman spec used for?

Used to determine frequencies for symmetrical molecules whose vibrations aren’t IR-active. Analyses frequencies scattered by a sample (as opposed to absorbed). For diatomic molecules: * Homonuclear diatomics are only Raman-active * Heteronuclear diatomics are both IR-active & Raman-active

IR: name the species responsible for peaks in the following regions: * 0-1500 cm^-1(fingerprint) * 1500-2000 * 2000-2500 * 2500-4000

* 0-1500 = X-Y single bonds plus pther vibrational modes * 1500-2000 = double bonds * 2000-2500 = triple bonds * 2500-4000 = X-H single bonds

IR: What peaks are seen in the double bond region, 1500-2000 cm-1?

C=O, variable C=C, weak Benzene: several peaks, medium NO2

IR: range of C=O peaks?

1640-1820 cm-1

IR: range of C=C peaks?

~1635-1690 cm-1, weak

IR: range of benzene peaks?

several peaks ~1625-1450 cm-1, medium

IR: peaks for NO2?

Symmetric stretch: ~1350 cm-1 Asymmetric stretch: ~1530 cm-1

C≡C peak?

~2100-2250 cm-1, weak

C≡N peak?

~2250 cm-1, strong

OH peak -- with and without H bonding?

H bonding: ~3300 cm-1, broad No H bonding (due to steric bulk): ~3600, sharp

Amine NH2 IR frequencies?

Symmetric ~ 3300 cm-1 Asymmetric ~3400 cm-

IR: C-H peak range?

2900-3200 cm-1

IR ethyne peak? (one case of C-H stretch which is useful)

~3300 cm-1, strong & sharp

what could a sharp peak at 3300 cm-1 be?

* ethyne (strong) * Amine NH * OH, no H bonding (sterics)

what could a peak at 2250 cm-1 be? if strong, likely to be\_\_, if weak \_\_?

C≡C (weak) C≡N (strong)

IR peaks 1350-1530 could be?

* Benzene: several peaks ~1625-1450 cm-1, medium * NO2 group: * Symmetric stretch: ~1350 cm-1 Asymmetric stretch: ~1530 cm-1

IR peak at 1640 could be?

C=O (varied) or C=C (weaker)

ketone IR frequency?

1715 cm-1

compare the IR peaks of the C=O in * amides * acid chlorides * carboxylic acids/ esters

frequency: acid chloride \> carb acid / ester \> amide * acid chloride ~1790 * ester ~ 1745 * carb acid ~ 1730 * amide ~ 1660 * *N donates e- density (lone pair is weakly e–-donating into pi-system → 2 e- shared over 3 atoms → weakens C=O pi bond)* * *Oxygen is intermediate between N & Cl: more donating than Cl since lone pair interacts, but more withdrawing than N since more EN* * *Cl withdraws e- density (lone pair doesn’t interact with carbonyl since wrong shell)*

how do the IR peaks of C=O on rings compare to normal ketone?

normal ketone ~1715. ring C=O _higher_ peaks Smaller ring → stretching frequency increased by 30-35 cm-1: * 5-membered ~ 1745 * 4-membered ~ 1780 * 3-membered ~ 1815 *Since:* * *During a vibration, as bonds compress, C experiences resistance from adjacent Cs* * *Fewer-membered ring → smaller bond angle → bonds compressed more → more resistance → requires more energy → higher stretching frequency*