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MCAT Organic > Spectroscopy > Flashcards

Spectroscopy Flashcards

1
Q

Spectroscopy

A

Measures the energy differences between the possible states of a molecular system by determining the frequencies of electromagnetic radiation absorbed by molecules

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2
Q

Infrared spectroscopy

A

Measures molecular vibrations, which can be seen as bond stretching or bending
Infrared light passed through sample and absorbance is measured

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3
Q

Wavenumber

A

4000 to 400 cm^-1

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4
Q

What wavelengths do spectroscopy occur?

A

2500 to 25,000 nm

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5
Q

Wavenumber equation

A

1/wavelength

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6
Q

Fingerprint region

A

Above 1500 cm^-1

More complex vibration patterns

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7
Q

What kind of molecules would not exhibit absorption?

A

Ones that don’t experience a change in dople

Same electronegativity or symmetrical molecules

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8
Q

Hydroxyl group absorption

A

Broad OH at 3300 for alcohol

At 3000 for carboxylic acid

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9
Q

Larger wavelength corresponds to what wavenumber?

A

Smaller

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10
Q

Smaller wavelength corresponds to what wavenumber?

A

Larger

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11
Q

Carbonyl group absorption

A

1700 cm^-1 deep

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12
Q

DIfference between NH and OH group

A

NH still at 3300, but sharper peak

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13
Q

Which has a higher wavenumber, bound to alkyne or alkane

A

Alkyne

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14
Q

Percent transmittance

A

Amount of light passed through the sample and reaches detector

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15
Q

Absorbance and percent transmittance equation

A

A=2-log%T

More absorbance with less transmittance

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16
Q

UV Spectroscopy

A

Passing UV light through a sample that is usually dissolved in inert, nonabsorbing solvent, and reading absorbance plotted against wavelength
Tells extent of max absorbance that tells extent of conjugation

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17
Q

More conjugated compounds have what kind of UV spec

A

Lower energy of transition and greater wavelength of max absorbance

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18
Q

What is UV spec most useful for studying

A

Compounds with double bonds or heteroatoms with lps that make conjugated systems

19
Q

UV spec excites what kind of electrons

A

pi electrons or non-bonding electrons

20
Q

What kind of molecules are more easily excited with UV light

A

Smaller energy gap between (HOMO) highest occupied molecular orbital and (LUMO) lowest unoccupied molecular orbitals

21
Q

Conjugated molecules

A

Unhybridized p-orbitals can also be excited by UV light

Conjugation shifts absorption to higher max wavelength and lower frequencies

22
Q

NMR Spec

A

Certain atomic nuclei have magnetic moments oriented at random, in magnetic fields the magnetic moment aligns with or against field

23
Q

a-state

A

Magnetic moments aligned with field (lower energy)

24
Q

B-state

A

Higher energy state when nuclei are irradiated with radiofrequency pulses that match the energy gap between two states and excitation at different frequencies

25
Q

MRI

A

Uses proton NMR to translate into shades of grey (water), lighter (tissue)

26
Q

Plot of NMR axes

A

Frequency v. absorption energy

27
Q

Chemical shift

A

Uses parts per million (ppm) of spectrometer frequency

28
Q

What way does frequency increase on the x-axis

A

To the left (downfield0

29
Q

What is used at 0 ppm to calibrate NMR?

A

TMS tetramethysilane

30
Q

More electronegative elements attached to a carbon causes the hydrogen to shift?

A

Downfield

31
Q

Integration

A

Ratio of peaks corresponds to ratio of protons produced each peak

32
Q

Deshielding

A

Pulling electron density away from surrounding atoms, less it can protect itself from magnetic field and is absorbed at a higher frequency

33
Q

Shielding

A

Electron donating groups shift it upfield

34
Q

Splitting

A

Within three bonds of each other,

35
Q

n +1 rule

A

If a proton has n protons that are n bonds away, it will be split into n + 1 peaks except ones attached to O or N

36
Q

Coupling constant J

A

Magnitude of splitting (in Hertz)

37
Q

Multiplet

A

Greater than 4 shifts

38
Q

Aldehyde ppm

A

9-10 ppm

39
Q

Carboxylic acid ppm

A

10.5- 12 ppm

40
Q

Aromatic ring

A

6-8.5 ppm

41
Q

Which kind of bond is further down field?

A

Sp^2, sp, sp^3

42
Q

Extended conjugation of double bonds causes stretches where compared to normal absorption?

A

Occuring at lower wavenumber

43
Q

Wavenumber is directly proportional to?

A

Frequency bc frequency = c/wavelength

44
Q

Why can’t carbon 12 be used for NMR

A

It has no magnetic moment bc it has an even mass number

MCAT Organic (10 decks)

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