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NST Chemistry > Shapes + Structures of Molecules I > Flashcards

Shapes + Structures of Molecules I Flashcards

Michaelmas

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1
Q

acetyl fragment

A

CH3C(=O)-

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2
Q

toluene

A

benzene w methyl branch

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3
Q

chloroform

A

CHCl3

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4
Q

X ray crystallography is used to / shows

A

determine structure of crystals
bond lengths and angles
positions of atoms
packing of molecules

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5
Q

outline how x-ray crystallography works

A

beam of X-ray focused on crystal diffracts due to interactions with electrons
diffraction pattern analysed through electron density maps

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6
Q

why do H not show up on electron density maps?

A

H have very little e- density, do not significantly diffract x-rays in crystallography

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7
Q

disadvantages of x ray crystallography

A

good quality crystals needed
may be difficult to locate H’s
in solutions, crystals often have different structure

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8
Q

mass spectrometry is used to

A

find the weight of a molecule

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9
Q

mass spec method

A

sample vaporised, ionised
ions focused onto detector using mag/elec fields

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10
Q

two methods of ionising the sample for mass spec

A
  1. knock e- off by firing high energy e- at vapour
  2. electrospray: charged aerosol, ion attaches to molecule
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11
Q

IR spectroscopy is used to ___ by measuring _____

A

identify functional groups present in molecules
by measuring changes in molecular vibrational energy levels

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12
Q

molecules always possess what three types of energy?

A

translational, vibrational, rotational
all are quantised

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13
Q

transitions in what type of energy correspond with infrared radiation?

A

vibrational transitions

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14
Q

relationship between wavenumber and energy for transition

A

proportional as E=hv
higher wavenumber, more energy needed

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15
Q

frequency of a vibration depends on:

A
  1. mass 2. stiffness
    faster freq = stiffer spring, lighter mass
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16
Q

reduced mass

A

in a diatomic, both atoms vibrate
µ = m1m2 / m1+m2
for m1»m2, µ=m2 (esp for H)

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17
Q

fingerprint region

A

500-1500 (RHS)

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18
Q

double bonds region

A

1500-2000

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19
Q

triple bonds region

A

2000-2500

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20
Q

X-H single bonds region

A

2500+

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21
Q

there is only absorption of IR if…

A

there is a significant change in dipole moment, ie asymmetric stretch

which is why N2 in air doesn’t give rise to peaks, symmetric

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22
Q

define Raman spectroscopy

A

for homonuclear diatomics (symmetric, doesn’t absorb IR)
looks at frequency of light scattered by a sample
(heteronuclear diatomics are also Raman active)

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23
Q

N-H absorption

A

about 3300 cm-1

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24
Q

-NH2 group absorption feature, explain

A

usually two peaks (3300, 3400 ish)
due to the sym / asymm N-H stretches respectively

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25
Q

why is the O-H stretch often broad?

A

H bonding between molecules possible, results in many slightly different strengths of O-H bonds

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26
Q

C triple bond N and C peaks

A

C-N > C-C due to greater dipole
C-N 2250
C-C 2100-2250

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27
Q

C=C peak

A

1635-1690, weak

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28
Q

C=C peaks for benzene ring

A

multiple peaks between 1625-1450
medium-weak

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29
Q

-NO2 group IR spec

A

TWO peaks (sym/asym)
1350 (fingerprint), 1530

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30
Q

C=O peak

A

in KETONE, 1715
strengthen C=O bond –> higher
weaken –> lower
(“stiffer” spring = higher freq)

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31
Q

what causes strengthening / weakening of the C=O bond

A

EWG (eg. Cl) strengthen
EDG (eg. NH2) weaken
(dipole)

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32
Q

acyl chloride IR

A

1750-1830 cm-1
EWG strengthens C=O, higher than 1715

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33
Q

amide IR

A

1640-1690 cm-1
EDG weakens C=O, lower than 1715

34
Q

carboxylic acid IR

A

1730 cm-1
weakly EWG

35
Q

ester IR

A

1745 cm-1
weakly EWG

36
Q

aldehyde IR

A

1730 cm-1
bc ketone (1715) alkyl groups weakly EDG compared to H of ald

37
Q

acid anhydride IR

A

TWO stretches (Sym/asym)
BUT sym 1820 > asym 1750
(vs asym>tym for NH2 / NO2)

38
Q

how does conjugation affect the frequency of the C=O bond?

A

LOWERS base freq by 20-30 cm-1
as e- density spread over system

39
Q

how does ring size affect freq of C=O

A

smaller ring, higher freq
35 cm-1 step

40
Q

what does NMR measure?

A

transitions between energy levels of nuclear spin

41
Q

how does NMR work?

A

radio waves of appropriate frequency cause transitions in energy levels of nuclei in a strong magnetic field

42
Q

what is nuclear spin quantum number and how does it relate to energy levels?

A

I = 0, 1/2, 1, 3/2, …
nucleus spin I gives rise to 2I+1 different energy levels when placed in mag field, given by -ve to +ve m(I) in integer steps
eg. I=1, m(I)= -1,0,1
I=3/2, m(I)= -3/2, -1/2, 1/2, 3/2

43
Q

rules for I based on nuclei mass number

A

odd mass # = 1/2-integral spin
odd #prot and neutron = integral
even #prot and neutron = 0

44
Q

energy difference between spin states depends on

A

strength of mag field, both supplied and local (e-dens, other nuclei)
the nucleus itself

45
Q

how does electron density impact the strength of the local magnetic field`

A

electrons move to oppose the mag field
more e- = more shielding, lower freq

e-neg atoms DESHIELD other atoms by withdrawing e-dens, higher freq

46
Q

chemical shift scale NMR

A

ppm, relative to a reference compound
shift = diff in freq / ref freq

47
Q

standard reference compound used for NMR

A

TMS, tetramethylsilane
inert, one signal, away from other peaks (to the right)

48
Q

C NMR shift for sp3 C’s

A

0-100 ppm
more e-withdrawing (e-neg) groups attached –> higher freq

49
Q

C NMR shift for sp2 C’s

A

100-200 ppm
more EWG, higher shift

50
Q

C NMR shift for sp C triple bond C

A

70-80 ppm

51
Q

what groups have shift around 200ppm?

A

ketones - just over
aldehydes - just under

52
Q

what groups have shift around 160-170ppm?

A

carboxylic acids, esters, acyl chlorides, amides, acid anhydrides

note - LOWER shift than ald/ket despite more EWG attached! unexpected

53
Q

how can quaternary C’s be identified in C NMR?

A

much smaller line

54
Q

how does coupling between atoms work?

A

if neighbouring atom also has spin states, a through bond interaction

up reinforces mag field experienced by the C nuclei –> slightly higher freq
down reduces –> slightly lower
=> doublet

55
Q

why is C-C coupling not seen in C NMR?

A

low abundance of spin active C-13

tiny chance of C-13 - C-13 coupling lost in the noise
could see if artificially enriched

56
Q

when are splittings due to couplings of spin active nuclei NOT seen?

A

when the nuclei are EQUIVALENT (often symmetric)
they still interact but the effects are not seen in splitting

57
Q

how is a triplet splitting formed?

A

C couples to two other identical nuclei with spins

58
Q

general rule splitting for coupling to multiple nuclei

A

coupling to n equivalent nuclei with spin I
splits into 2nI+1 lines

for I=1/2, pascals triangle

59
Q

define equivalent nuclei (for coupling)

A

not only the same atom but also interchangeable positions!
axial and equatorial are DIFFERENT for T-shape molecules

60
Q

how is C NMR decoupled from protons?

A

by broadband proton decoupling - irradiating over a range of frequencies so all protons rapidly interconvert spin states, couplings to C average 0

61
Q

what is the APT?

A

Attached Proton Test
run on NMR spectrometer

C with even# and odd# protons attached point up/down
determine by solvent peak (CDCl3=0, even)

62
Q

what are satellites in NMR?

A

some isotopes with spin that have low natural abundance
this portion couples –> doublet (satellites), the rest appears as a singlet

looks like a triplet (if the proportion is high enough) but really is a singlet w satellites

63
Q

define vicinal coupling

A

the main difference between C NMR coupling and H NMR
Coupling between protons easily seen separated by three bonds (vicinal) or more

64
Q

H NMR shift scale

A

approx 0-14ppm (NOT the same scale as the CNMR ofc!)

65
Q

vicinal coupling constant rough value

A

about 7 Hz

66
Q

vicinal coupling constant value across C=C

A

cis about 8Hz
trans about 15Hz

67
Q

shift on protons on three-membered rings

A

unusually low, close to 0ppm

68
Q

proton shift for H on C attached to double bonded C (C=C, C=O, Ar ring)

A

around 2.5

69
Q

shift of H on C attached to N

A

around 2.5

70
Q

shift on C attached to amide (N side)

A

3.5 ish
C=O increases shift of H on C attached to N by 1

71
Q

shift of H on C attached to O

A

around 3.5

72
Q

shift of H on C attached to ester (O side)

A

around 4.5
C=O increases shift of H on C attached to O by 1

73
Q

shift of H on C attached to Cl

A

3.7-4ish

74
Q

H on C=C

A

4.5-6 ppm

75
Q

shift of H on benzene ring

A

6-9ppm

76
Q

shift of terminal H on aldehyde

A

9.5-10.5 ppm

77
Q

shift of H on formate ester

A

8 ppm (a bit counterintuitive that this shift is lower than terminal H on aldehyde given there’s another O)

78
Q

feature of proton shifts of H attached to non-C atoms

A

they tend to be a lot broader than other signals but exact position is difficult to guess

79
Q

D2O shake

A

identifying N-H and O-H protons by shaking NMR sample prepared in CDCl3 with D2O, which exchange these H for D
signals of exchangeable protons disappear

80
Q

define relaxation in spin states

A

rapid interconversion of spin states, most rapid in nuclei with spins greater than 1/2
also the reason why coupling to spins greater than 1/2 is not observed (eg. Cl, I=3/2)

81
Q

Deuterium spin states

A

I=1
hence three spin states
solvent CDCl3 split into triplet

NST Chemistry (16 decks)

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