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CHEM1603 CH > Section B > Flashcards

Section B Flashcards

1
Q

What is the equation for refractive index?

A

refractive index n = c/v

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2
Q

What is the speed of light to 4s.f.?

A

2.998 x 10^8m/s

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3
Q

What is the equation for c?

A

c = lambda . f (wavelength x frequency)

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4
Q

What is the difference in wavelength between red and blue light?

A

~300nm (red ~ 700nm, blue ~ 400nm)

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5
Q

What receptors in our eyes detect colour?

A

Cone receptors, photorhodopsin molceules

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6
Q

What colours are in a rainbow? (sing the song with me)

A

FUCKING SING IT YOU PRICK

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7
Q

What interacts in UV/Vis spectroscopy?

A

Electrons are raised energy levels

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8
Q

What does IR spectroscopy interact with?

A

Vibrations of whole molecules

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9
Q

What does NMR interact with?

A

spin flip transitions of atomic nuclei - magnetic part of light

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10
Q

What is another term for the ‘pilot wave’ released by a particle?

A

A photon

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11
Q

What is the units for Max Planck’s constant?

A

Js (Joule seconds)

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12
Q

What is Max Planck’s constant (4s.f.)?

A

6.626 x 10^-34Js

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13
Q

What is the equation for light energy E?

A

E = hc/lambda = hf (f can be funny v)

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14
Q

Electron transitions in the IR range correspond to tightly/weakly bound electrons?

A

Weakly bound electrons, IR has longer wavelength = less energy

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15
Q

What transitions is UV/vis spectroscopy used to measure in aromatic organic molecules?

A

pi-pi* transitions

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16
Q

What else is UV/vis used to measure (4 things)?

A

Carbon chain length, C=O/C=C, presence of chromophores, transition metals

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17
Q

What is a chromophore?

A

Light absorbing species

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18
Q

What gives rise to absorption bands in UV/vis?

A

Electrons jumping between molecular orbitals

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19
Q

How do you calculate wavenumber from wavelength and what are it’s units?

A

1/lambda, 1/cm

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20
Q

What region of the spectrum does the vibrational frequency of molecules correspond to?

A

Infrared region, fluctuating molecular electric field which resonates with electric field of light wave = absorption at vibrational frequency

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21
Q

In NMR, what does irradiation with light cause nuclei to do?

A

causes spins of nuclei to flip

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22
Q

What does the energy of the spin flip depend on (3)?

A

size of external field, type of nucleus, local environment

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23
Q

What happens when spin nuclei are place in a magnetic field?

A

The spins of the nuclei line up with the field direction

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24
Q

What region of the spectrum do spin flip energies occur in?

A

Radiowave (long wavelength), 1-500MHz

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25
Q

Between which wavelengths do electronic transitions occur in UV/vis?

A

200-700nm

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26
Q

What is Raman spectroscopy?

A

observes shifts in scattered light from laser due to molecular vibrations

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27
Q

How do you find absolute peak intensity?

A

Measuring absorbance values over several concentrations, use beer-lambert to find ε

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28
Q

What can you find using relative peak intensities in UV/vis?

A

find out what kind of electronic transitions are involved, find bonds

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29
Q

Which of carbon’s orbitals are used by other molecules in bonding?

A

2s, 2px, 2py, 2pz -> these are hybridised

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30
Q

Which hydrogen orbitals are used by other molecules in bonding?

A

1s

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31
Q

What is the symbol for a nonbonding orbital?

A

n

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32
Q

What arrangement are sp3 orbitals in alkanes?

A

Tetrahedral (s + px + py + pz)

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33
Q

What interactions are there between molecules in an sp2 hybridisation bond?

A

sp2 forms a double bond, there is one pi bond between the px’s and py’s of each atom (one pi bond = when both ends of p orbital overlap) and one sigma bond

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34
Q

Which bonds are stronger pi or sigma?

A

Sigma bonds are stronger, pi bonds form after sigma bonds

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35
Q

How many areas of electron shell overlap are there in a pi bond?

A

2 at top and bottom areas of pi orbitals, these equate to one bond

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36
Q

What is the respective s and p character of sp3 hybrid orbitals?

A

25% s character, 75% p character

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37
Q

What is the respective s and p character of sp hybrid orbitals?

A

50% s character, 50% p character

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38
Q

What are the relative lobe sizes of an sp3 hybrid orbital?

A

Contain a big lobe and a little lobe (little lobe usually ignored)

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39
Q

What type of overlap of orbitals occurs in a sigma bond?

A

Head on overlap, overlap indirection of ‘travel’

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40
Q

What is the length of a carbon-carbon bond?

A

1.54Å

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41
Q

What interactions are there between molecules in an sp hybridisation bond?

A

sp hybridisation forms a triple bond, there are 2 pi bonds and 1 sigma bond

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42
Q

What wavelengths do transitions between pi & pi* orbitals occur between?

A

200-300nm in simple organic molecules

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43
Q

What happens to the wavelength of the pi-pi* transition when carbon chain length of conjugated hydrocarbons increases?

A

wavelength becomes longer, by 30nm per extra C=C bond

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44
Q

What is a conjugated hydrocarbon?

A

A hydrocarbon containing alternating single and double carbon bonds e.g. -C=C-C=C- etc

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45
Q

What is an example of a common conjugated/polyunsaturated hydrocarbon?

A

Beta Carotene, 11 conjugated bonds, absorption in 480-510nm range (blue), causes beta carotene to appear orange

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46
Q

What important vitamin is beta carotene a precursor to?

A

Vitamin A, important for growth,infection resistance, night vision

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47
Q

What common molecule contains a polyunsaturated ring system?

A

Chlorophyll, absorbs in 400-450nm range, green-yellow light reflected

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48
Q

Using the beer-lambert law, what can be discerned from the molar extinction co.?

A

Helps work out electronic structure and bonding

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49
Q

If there is no UV/vis spectra, what type of molecule is it likely to be?

A

A fully saturated hydrocarbon (no double bonds)

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50
Q

Where in the electromagnetic spectrum do small unsaturated molecules have their absorption peaks?

A

UV region

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51
Q

Which groups provide the strongest absorption bands for pi-pi* transitions?

A

Aromatic groups, and unsaturated hydrocarbons

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52
Q

What transition occurs in molecules containing C, O, N, S and Cl?

A

n-pi* transitions, in the 200-400nm range

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53
Q

What is a vibrational mode?

A

The different well-defined patterns of vibration which occur around equilibrium positions in atoms in molecules

54
Q

What is the equation for Hooke’s Law?

A

F = -kx (k = force constant/bond strength)

55
Q

How do we calculate the reduced mass (mu)?

A

Use relative masses of both atoms: m1.m2/(m1+m2)

56
Q

What is the point of calculating reduced mass?

A

Allows a 2 body to problem to be solved as 1 body problem

57
Q

How do we calculate v0?

A

v0 = vibrational frequency = (1/[2pi]).sqrt(k/mu0)

58
Q

What are the units of reduced mass?

A

kg

59
Q

What are the units of k?

A

N/m or kg/s

60
Q

How do we convert Hz (1/s) to wavenumber (1/cm)?

A

Hz/c (c= speed of light in cm/s 3 x 10^10)

61
Q

What equation can be used to quickly calculate wavenumber?

A

v0 (1/cm) ~ 130.sqrt(k/mu)

62
Q

What is changed by isotopes?

A

Changing of reduced mass due to extra neutron, leads to different vibrational frequency

63
Q

What is the isotope substitution formula?

A

(v0 heavy)/(v0light) = sqrt[(mu light)/(mu heavy)]

64
Q

What are the names of the 3 vibrational modes?

A

Symmetric stretching, bending, asymmetric stretching

65
Q

Which occurs at a higher frequency symmetric or asymmetric stretching?

A

Asymmetric stretching

66
Q

Do bending vibrations occur at a lower or higher frequency than stretching?

A

bending occurs at a ~50% LOWER frequency than stretching

67
Q

Does H-bonding lower or increase OH stretching effects?

A

Lowers OH stretching frequency

68
Q

What is the equation for absorbance (involving logs)?

A

Absorbance = log(I0/It), useful for quantitative studies of IR spectra

69
Q

What type of molecule has no dipole moment?

A

Homonuclear molecules (molecules consisting of only one element)

70
Q

What type of molecule does have a dipole moment?

A

Heteronuclear molecules e.g. HCl, so have IR absorption spectrum

71
Q

What is the relationship between dipole moment change and IR absorption intensity?

A

If dipole moment change is large than IR absorption intensity is strong

72
Q

Which vibrational modes are IR active?

A

Bending and asymmetric stretching are IR active, symmetric stretching is IR INactive

73
Q

What provides the source of infrared spectra in IR spectroscopy?

A

Heated Globar, SiC (ceramic)

74
Q

How did ‘old’ IR machines create spectra?

A

Scanned through all wavelengths, building up a spectra

75
Q

How do current FTIR machines create spectra?

A

Use Fourier Transform mathematical techniques

76
Q

Is it necessary to run a ‘blank’ to calculate background effects in FTIR?

A

No, FTIR spectroscopy calculates this first

77
Q

What are the ways samples of different phase are run in an FTIR?

A

Solid phase: mixed with Nujol gel, pressed into KBr disc, Liquid phase: run between 2 KBr plates, Gas phase: placed in Gas cell with KBr windows

78
Q

What is another name for absorption peaks in IR spectroscopy?

A

transmission minima

79
Q

What groups would be present if there was a sharp peak 2940-2860cm^-1 region?

A

CH2 or CH3 groups, this area is characteristic of a C-H bond

80
Q

What type of bond (including vibrational mode) do peaks between 2940-2860cm^-1 suggest?

A

Symmetric/Asymmetric C-H stretching in saturated hydrocarbons

81
Q

What 6 vibrational modes occur between C and H atoms?

A

Symmetric stretching, Asymmetric stretching, Symmetric in plane bending, asymmetric bending, out of plane bending (wag), twisting/torsion

82
Q

What wavenumber do unsaturated C-H bonds (alkenes, aromatics, alkynes) occur?

A

greater than 3000cm^-1, higher than C-H in saturated hydrocarbons, alkyne group even higher ~3310cm^-1

83
Q

Other than alkyne bonds, what other bonds could be present ?

A

O-H or N-H bonds occupy a similar frequency

84
Q

What is the difference in O-H bond environment if you see a sharp O-H bond peak or a broad one?

A

Sharp peak at 3650-3700 suggests O-H is non-bonded, Broad peak at 3600-2500 suggests O-H is bonded e.g COOH

85
Q

What effect does intermolecular H-bonding have on OH stretch?

A

H-bonding causes OH stretch to occur at a lower frequency

86
Q

What is the difference in peak between N-H and O-H bonding?

A

N-H bonds usually occur at a lower frequency and are usually a sharper peak

87
Q

What would a peak in the range 1800-1600cm^-1 suggest?

A

A carbonyl group, C=O, position determines strength of C=O bond

88
Q

Do IR peak positions change depending on protein conformation?

A

Yes, they change slightly if alpha helix or beta sheet

89
Q

What happens to the C-O bonding frequency in pure ethanol?

A

Single bond C-O stretch occurs at a lower frequency ~1150cm^-1

90
Q

In NMR, if electron spin aligned against an external magnetic field, is it in the unstable or stable configuration?

A

Unstable configuration, high energy state

91
Q

What is the symbol for an external magnetic field in NMR?

A

B0

92
Q

What is the Larmor frequency (vL)?

A

the frequency at which the nucleus precesses around the B0 direction (like a spinning top about to fall when it wobble)

93
Q

What is the gyromagnetic ratio (gamma)?

A

Constant which determines Larmor frequency - different for each nucleus/isotope, varies with B0 also

94
Q

Does the gyromagnetic ratio change with isotopes of the same element?

A

YES IT FUCKING DOES BARRY YOU INSOLENT PRICK

95
Q

Other than isotope, what can cause the gyromagnetic ratio to vary?

A

The strength of the external magnetic field, B0

96
Q

What is the typical strength of the external magnetic field used in NMR?

A

4-10Tesla

97
Q

What range does the Larmor Frequency occur in?

A

Radiowave frequency (10-1000MHz), so NMR is otherwise known as Radiowave spectroscopy

98
Q

What particles contribute to the spin of a nucleus?

A

Protons and Neutrons

99
Q

What is the equation used to find the number of quantised spin states?

A

2 I + 1 (when I is total nuclear spin quantum number)

100
Q

What is the equation used to find the difference in energy between spin states?

A

ΔE = (gamma.B0.h)/2pi (gamma=nuclear gyromagnetic ratio, h=planck’s constant)

101
Q

What is the equation for ΔE using Larmor frequency?

A

ΔE = h.vL

102
Q

What surrounds the superconducting magnet in an NMR machine?

A

Liquid nitrogen (77K) and liquid helium (4.4K)

103
Q

What causes the spins of the nucleus to flip in NMR?

A

electromagnetic signal sent by a radiotransmitter provides enough energy

104
Q

What frequency must we ‘tune’ the radio receiver to?

A

Larmor Frequency (vL)

105
Q

What compound is added to calibrate the NMR machine?

A

Tetramethylsilane (Si(CH3)4))

106
Q

What is done to ensure all the chemical environments are averaged in the magnetic field?

A

Sample is spun

107
Q

What is the difference when NMR is undertaken on solids?

A

Speed of spinning is much increased (15-20kHz)

108
Q

What is shielding?

A

An effect caused by electron clouds which create a small magnetic field due to their circulation, this magnetic field opposes the external magnetic field

109
Q

What is the equation for the effective magnetic field (Beff)?

A

B eff = B0.(1-rho) where rho = shielding constant

110
Q

Shielding in NMR causes a chemical shift in which direction?

A

Chemical shift values are usually smaller when shielded

111
Q

If an adjacent species to a proton is electron donating what would happen to the NMR frequency?

A

shielding electron density around proton would increase, so rho (shielding constant) would increase, Beff decreases, NMR frequency decreases

112
Q

What will be the electron density around the O-H protons in methanol be?

A

Electron density would be decreased compared to CH3 protons, as O is more electronegative than C, so draws more electron density to O nucleus and away from H nucleus, shielding smaller, higher NMR frequency

113
Q

What does ‘magnetically equivalent’ mean in terms of NMR?

A

Same chemical shift values, e.g. all protons in CH3 are magnetically equivalent

114
Q

Which is a better electron withdrawing group H or CH3?

A

solitary H is a better electron withdrawing group (more electronegative) than CH3, so a proton attached to a C next to a CH3 group will be better shielded and have a lower NMR frequency

115
Q

What is the equation for delta (ppm)?

A

[(vsample - vreference)/vreference] x 10^6 = ppm, shift values relative to reference frequency

116
Q

What is the chemical standard used in NMR?

A

TMS (tetramethylsilane) -> (CH3)4Si

117
Q

Why is TMS as good standard?

A

unreactive, liquid (mixes w/ solvents), single NMR peak, peak at lower freq. (Si is v. electron donating)

118
Q

Are most proton peaks at higher or lower delta than TMS?

A

Most are higher, as shielding effect is weaker than in TMS (due to v. electron donating Si)

119
Q

Roughly what ppm do CH3/CH2 groups have in saturated hydrocarbons?

A

~1-4ppm, close to TMS peak, very highly shielded by electrons

120
Q

Roughly what ppm do OH groups have?

A

~2-5ppm, less shielded due to electronegativity of O drawing electron density away from H

121
Q

What 2 groups have a similar ppm range, but one shows a more broad peak?

A

O-H and N-H, N-H has a broader peak due to interaction of N14/N15 nuclear spins

122
Q

Roughly what ppm do COOH/COH groups have?

A

~9-10ppm as they are highly deshielded, small shielding constant

123
Q

What aspect of aromatic compounds causes high shift values (8-9ppm)?

A

Electrons travelling around ring create own magnetic flux (‘ring current’), which adds to B0 (reduces shielding) = greater shift values

124
Q

Where in an aromatic compound are there likely to be protons with a negative shift?

A

On the inside of certain compounds, ring current opposes B0 e.g. annulene

125
Q

What is changed in a high resolution NMR?

A

Stronger magnetic field used, allows splitting patterns to be analysed

126
Q

How does a multiplet pattern arise?

A

protons on adjacent C, diff. environments, when one proton undergoes resonance & flips spin, other proton could be either in up or down position, changes environment

127
Q

What is the magnitude of coupling constant (J)?

A

the strength of the interaction between two coupling protons

128
Q

Why do triplet patterns form?

A

2 equivalent protons, so 3 possible combinations of spins: uu, 2ud, dd (middle peak = 2x as high because 2x as common)

129
Q

What else can cause peak splitting?

A

Interactions with other NMR active nuclei

130
Q

Why is MRI able to ‘see’ soft tissues?

A

MRI observes relaxation time of protons in H2O molecules, these times vary depending on the mobility of water -> shows density of tissue

131
Q

Why is fMRI different to MRI?

A

fMRI observes spin state of Iron (Fe) in oxygenated haemoglobin, able to map brain activity in real time

CHEM1603 CH (6 decks)

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