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Orgo > Ch. 11: Spectroscopy > Flashcards

Ch. 11: Spectroscopy Flashcards

1
Q

defn + func: Infrared (IR spectroscopy)

A

measures molecular vibrations, which can be as bond stretching, bending, or combinations of different vibrational modes

by determining what bonds exist in a molecule, we hope to infer the functional groups in the molecule

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2
Q

how do you record any IR spectrum?

A

infrared light is passed through a sample and the absorbance is measured

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3
Q

what wavelength does the infrared light range range from to?

A

700 nm to 1 mm

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4
Q

what is the useful range of absorptions for spectroscopy?

A

2500 to 25000 nm

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5
Q

func + range for spectroscopy: wavenumber

equation in comparison to frequency

A

cm^-1, an analog of frequency that we use on an IR spectrum

range corresponding to 2500 to 25000 nm is 4000 to 400 cm-1

frequency –> c/wavelength
wavenumber – 1/wavelength

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6
Q

what happens when light of these wavenumbers is absorbed?

A

the molecules enter excited vibrational states

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7
Q

what are the 6 types of molecular vibration states measured by IR spectroscopy?

A
  1. symmetric bend
  2. asymmetric bend
  3. symmetric stretch
  4. asymmetric stretch
  5. twisting
  6. folding
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8
Q

cause + range + char + use: fingerprint region

A

caused by the motion of the molecule as a whole

1500 to 400 cm^-1 range

more complex vibrational patterns

can be used to identify a substance

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9
Q

why is the fingerprint region called that?

A

because the specific absorbance pattern is characteristic of each individual molecule

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10
Q

what must occur for an absorption to be recorded?

A

the vibration must result in a change in the bond dipole moment, meaning that molecules that do not experience a change in dipole moment, such as those composed of atoms with the same electronegativity or molecules that are symmetrical, do not exhibit absorption

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11
Q

why do symmetric stretches not show up in IR spectra?

A

because they involve no net change in dipole movement

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12
Q

what are the 3 IR absorption peakrs you need memorized for the MCAT?

A
  1. hydroxyl group O-H – broad, wide peak around 3300 for alcohols, 3000 for carboxylic acids
  2. carbonyl C=O – absorbs around 1750 – sharp (deep) peak
  3. N-H bonds – around 3300 – sharp peak
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13
Q

why is the wavenumber for a OH group in a carboxylic acid lower than that of an alcohol?

A

the carbonyl of a carboxylic acid pulls some of the electron density out of the O-H bond, shifting the absorption to a lower wavenumber

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14
Q

what happens to the absorption frequency as we add more bonds between carbon atoms?

A

the absorption frequency increases

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15
Q

does the bond between any atom and H always have a relatively high or low absorption frequency?

A

high

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16
Q

what is the range of wavenumbers that is in the scope of the MCAT? does this include the fingerprint region?

A

1400 - 4000

fingerprint region is lower, and not included

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17
Q

defn: transmittance

A

the amount of light that passes through the sample and reaches the detector

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18
Q

what are the axes for IR spectra plots?

A

% transmittance (y) vs. wavenumber (x)

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19
Q

are the maximum absorptions at the top of mountains or bottom of valleys on the spectrum?

A

bottom of valleys

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20
Q

how are UV spectra obtained?

A

by passing ultraviolet light through a sample that is usually dissolved in an inert, nonabsorbing solvent, and recording the absorbance

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21
Q

what are the axes of UV spectra?

A

absorbance vs. wavelength

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22
Q

what causes the absorbance of UV spectra?

A

caused by electronic transitions between orbitals

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23
Q

what do we get from UV spectroscopy?

A

the wavelength of max absorbance, which tells us the extent of conjugation within conjugated systems

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24
Q

what does it mean the more conjugated the compound?

A

the more conjugated the compound, the lower the energy of the transition, and the greater the wavelength of maximum absorbance

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25
Q

what types of compounds is UV spectroscopy most useful for?

A

compounds containing double bonds or heteroatoms with lone pairs that create conjugated systems

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26
Q

why does UV spectroscopy work?

A

because molecules with pi-electrons or nonbonding electrons can be excited by UV light to higher-energy antibonding orbitals

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27
Q

defn: HOMO, LUMO

A

HOMO = highest occupied molecular orbital

LUMO = lowest unoccupied molecular orbital

28
Q

what is true about molecules that have a low energy gap between HOMO and LUMO?

A

these molecules are more easily excited and can absorb longer wavelengths (lower frequencies) with lower energy

29
Q

what impact does conjugation have on the absorption spectrum?

A

conjugation shifts the absorption spectrum, resulting in higher max wavelengths (lower frequencies)

larger conjugated molecules may even absorb light in the visible range, leading to color

30
Q

why is UV spectroscopy sometimes called UV-Vis spectroscopy?

A

because the technique can also be used at visible wavelengths

31
Q

benzene has 3 broad absorbances, what do these mark and what are their values?

A

they mark the energy level transitions

found at 180, 200, and 255 nm

32
Q

what does NMR spectroscopy stand for?

A

nuclear magnetic resonance spectroscopy

33
Q

what is the basic idea behind NMR spectroscopy?

A

certain atomic nuclei have magnetic moments that are oriented at random

when such nuclei are placed in a magnetic field, their magnetic moments tend to align either with or against the direction of this applied field

34
Q

defn: alpha-state

A

lower energy

nuclei with magnetic moments that are aligned with the applied magnetic field are in this state

35
Q

defn: beta-state

A

higher energy

nuclei can be irradiated with radiofrequency pulses that match the energy gap between the two states, which will excite some lower-energy nuclei into this state from the alpha-state

36
Q

the absorption of radiation leads to exciting nuclei from the alpha-state to the beta-state at different frequencies, what does this depend on?

A

the atom’s magnetic environment

37
Q

are nuclear magnetic moments of atoms affected by nearby atoms that also possess magnetic moments?

A

yes

38
Q

defn + functionality: MRI

A

magnetic resonance imaging

a noninvasive diagnostic tool that uses proton NMR

multiple cross-sectional scans of the patient’s body are taken and the various chemical shifts of absorbing protons are translated into specific shades of grey

39
Q

what are the axes of a typical NMR spectrum?

A

frequency vs. absorption of energy

40
Q

why is there a need for a standardized method of plotting NMR spectra?

A

because different NMR spectrometers operate at different magnetic field strengths

41
Q

defn: chemical shift

A

an arbitrary variable with units of parts per million (ppm) of spectrometer frequency

42
Q

how does chemical shift lie on the NMR spectrum plot?

A

it is on the x-axis and it increases toward the left (referred to as downfield)

43
Q

func: tetramethylsilane (TMS)

A

used as the calibration standard to mark 0 ppm

we use this to make sure that we know just how far downfield compounds are

44
Q

should you include the TMS peak when counting peaks?

A

no, skip it! its a reference peak

45
Q

what is NMR most commonly used to study? what else can it study?

A

most common: 1H nuclei (protons)

else: any atom possessing a nuclear spin (with an odd atomic number, odd mass number, or both)

46
Q

where do most hydrogen nuclei come into resonance on NMR spectrum?

A

0 to 10 ppm downfield from TMS

47
Q

does each distinct set of nuclei give rise to a separate peak?

A

yes

48
Q

what happens if multiple protons are chemically equivalent? what does chemically equivalent mean? + example

A

chemically equivalent = having the same magnetic environment

example: hydrogens on a methyl group –> it would not be possible to describe each of these 3 H’s as distinct from each other because they rotate freely in space

they will lead to the same peak

49
Q

so how do we differentiate how many protons correspond to a peak?

A

the height of each peak is proportional to the number of protons it contains

50
Q

defn: integration

A

the area under the peaks

51
Q

what results in a more downfield reading on an NMR spectrum?

A

the more a proton’s electron density is pulled away, the less it can shield itself from the applied magnetic field, the further downfield is the reading

52
Q

defn: deshielding

A

pulling electron density away from the surrounding atoms

53
Q

by the same reasoning as electron-withdrawing groups causing a downfield reading, what causes an upfield reading?

A

if we had an electron-donating group, it would help shield the 1H nuclei, and give it a position further upfield

54
Q

why is TMS used as the reference peak in NMR?

A

everything else in proton NMR will be more deshielded than TMS

55
Q

in NMR, what are the two things that represent a single group of equivalent protons?

A
  1. each peak
  2. each group of peaks that are part of a multiplet
56
Q

when does splitting occur? + aka

A

aka: spin-spin coupling

when we have 2 protons in such close proximity to each other (within 3 bonds of each other, hydrogens on 2 adjacent atoms) that are not magnetically identical

57
Q

explain what happens when we have protons in such close proximity to each other that splitting occurs (5)

A
  1. the magnetic environment of Ha can be affected by Hb and vice versa
  2. at any given time, Ha can experience two different magnetic environments because Hb can be in either the alpha or the beta state
  3. the different states of Hb influence the nucleus of Ha causing slight upfield and downfield shifts
  4. there is approximately a 50% chance that Hb will be in either of the two states, so the resulting absorption is a doublet
  5. Ha and Hb will both appear as doublets because each one is coupled with one other H
58
Q

char: doublet

A

2 peaks of identical intensity, equally spaced around the true chemical shift of Ha

59
Q

how do we determine the number of peaks present (as doublets, triplets, etc)? what does the splitting of a peak represent?

A

the n+ 1 rule

if a proton has n protons that are 3 bonds away, it will be split into n + 1 peaks

splitting = the number of adjacent hydrogens

60
Q

what is the one caveat to the n + 1 rule?

A

do NOT include protons attached to oxygen or nitrogen

61
Q

defn + unit: coupling constant, J

A

unit: hertz

the magnitude of the splitting

62
Q

we have a proton surrounded by a shield of electrons, what happens as we add electronegative atoms or have resonance structures that pul electrons away from the proton?

A

we Deshield and move Downfield

63
Q

what are the 3 unique frequencies that correspond to a triplet?

A

alpha-alpha

alpha-beta/beta-alpha

beta-beta

64
Q

what is the center of a triplet?

A

the true chemical shift

65
Q

defn: multiplet

A

peaks that have more than four shifts

66
Q

what 3 things is proton NMR good for?

A
  1. determining the relative number of protons and their relative chemical environments
  2. showing how many adjacent protons there are by splitting patterns
  3. inferring certain functional groups
67
Q

what are the 6 chemical shift values downfield from TMS that you should know for the mcat?

A
  1. deshielded aldehyde = 9-10 ppm
  2. deshielded carboxylic acid = 10.5-12 ppm
  3. aromatics/hydrogen of an aromatic ring = 6-8.5 ppm
  4. alkyl groups/hydrogens on sp3-hybridized carbons = 0.0-3.0 ppm (higher if electron-withdrawing groups are present)
  5. alkenes/hydrogens on sp2 hybridized carbons = 4.6-6.0 ppm
  6. alkynes/hydrogens on sp hybridized carbons = 2-3 ppm

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