Ch. 11: Spectroscopy Flashcards
defn + func: Infrared (IR spectroscopy)
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
how do you record any IR spectrum?
infrared light is passed through a sample and the absorbance is measured
what wavelength does the infrared light range range from to?
700 nm to 1 mm
what is the useful range of absorptions for spectroscopy?
2500 to 25000 nm
func + range for spectroscopy: wavenumber
equation in comparison to frequency
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
what happens when light of these wavenumbers is absorbed?
the molecules enter excited vibrational states
what are the 6 types of molecular vibration states measured by IR spectroscopy?
- symmetric bend
- asymmetric bend
- symmetric stretch
- asymmetric stretch
- twisting
- folding
cause + range + char + use: fingerprint region
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
why is the fingerprint region called that?
because the specific absorbance pattern is characteristic of each individual molecule
what must occur for an absorption to be recorded?
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
why do symmetric stretches not show up in IR spectra?
because they involve no net change in dipole movement
what are the 3 IR absorption peakrs you need memorized for the MCAT?
- hydroxyl group O-H – broad, wide peak around 3300 for alcohols, 3000 for carboxylic acids
- carbonyl C=O – absorbs around 1750 – sharp (deep) peak
- N-H bonds – around 3300 – sharp peak
why is the wavenumber for a OH group in a carboxylic acid lower than that of an alcohol?
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
what happens to the absorption frequency as we add more bonds between carbon atoms?
the absorption frequency increases
does the bond between any atom and H always have a relatively high or low absorption frequency?
high
what is the range of wavenumbers that is in the scope of the MCAT? does this include the fingerprint region?
1400 - 4000
fingerprint region is lower, and not included
defn: transmittance
the amount of light that passes through the sample and reaches the detector
what are the axes for IR spectra plots?
% transmittance (y) vs. wavenumber (x)
are the maximum absorptions at the top of mountains or bottom of valleys on the spectrum?
bottom of valleys
how are UV spectra obtained?
by passing ultraviolet light through a sample that is usually dissolved in an inert, nonabsorbing solvent, and recording the absorbance
what are the axes of UV spectra?
absorbance vs. wavelength
what causes the absorbance of UV spectra?
caused by electronic transitions between orbitals
what do we get from UV spectroscopy?
the wavelength of max absorbance, which tells us the extent of conjugation within conjugated systems
what does it mean the more conjugated the compound?
the more conjugated the compound, the lower the energy of the transition, and the greater the wavelength of maximum absorbance
what types of compounds is UV spectroscopy most useful for?
compounds containing double bonds or heteroatoms with lone pairs that create conjugated systems
why does UV spectroscopy work?
because molecules with pi-electrons or nonbonding electrons can be excited by UV light to higher-energy antibonding orbitals
defn: HOMO, LUMO
HOMO = highest occupied molecular orbital
LUMO = lowest unoccupied molecular orbital
what is true about molecules that have a low energy gap between HOMO and LUMO?
these molecules are more easily excited and can absorb longer wavelengths (lower frequencies) with lower energy
what impact does conjugation have on the absorption spectrum?
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
why is UV spectroscopy sometimes called UV-Vis spectroscopy?
because the technique can also be used at visible wavelengths
benzene has 3 broad absorbances, what do these mark and what are their values?
they mark the energy level transitions
found at 180, 200, and 255 nm
what does NMR spectroscopy stand for?
nuclear magnetic resonance spectroscopy
what is the basic idea behind NMR spectroscopy?
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
defn: alpha-state
lower energy
nuclei with magnetic moments that are aligned with the applied magnetic field are in this state
defn: beta-state
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
the absorption of radiation leads to exciting nuclei from the alpha-state to the beta-state at different frequencies, what does this depend on?
the atom’s magnetic environment
are nuclear magnetic moments of atoms affected by nearby atoms that also possess magnetic moments?
yes
defn + functionality: MRI
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
what are the axes of a typical NMR spectrum?
frequency vs. absorption of energy
why is there a need for a standardized method of plotting NMR spectra?
because different NMR spectrometers operate at different magnetic field strengths
defn: chemical shift
an arbitrary variable with units of parts per million (ppm) of spectrometer frequency
how does chemical shift lie on the NMR spectrum plot?
it is on the x-axis and it increases toward the left (referred to as downfield)
func: tetramethylsilane (TMS)
used as the calibration standard to mark 0 ppm
we use this to make sure that we know just how far downfield compounds are
should you include the TMS peak when counting peaks?
no, skip it! its a reference peak
what is NMR most commonly used to study? what else can it study?
most common: 1H nuclei (protons)
else: any atom possessing a nuclear spin (with an odd atomic number, odd mass number, or both)
where do most hydrogen nuclei come into resonance on NMR spectrum?
0 to 10 ppm downfield from TMS
does each distinct set of nuclei give rise to a separate peak?
yes
what happens if multiple protons are chemically equivalent? what does chemically equivalent mean? + example
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
so how do we differentiate how many protons correspond to a peak?
the height of each peak is proportional to the number of protons it contains
defn: integration
the area under the peaks
what results in a more downfield reading on an NMR spectrum?
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
defn: deshielding
pulling electron density away from the surrounding atoms
by the same reasoning as electron-withdrawing groups causing a downfield reading, what causes an upfield reading?
if we had an electron-donating group, it would help shield the 1H nuclei, and give it a position further upfield
why is TMS used as the reference peak in NMR?
everything else in proton NMR will be more deshielded than TMS
in NMR, what are the two things that represent a single group of equivalent protons?
- each peak
- each group of peaks that are part of a multiplet
when does splitting occur? + aka
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
explain what happens when we have protons in such close proximity to each other that splitting occurs (5)
- the magnetic environment of Ha can be affected by Hb and vice versa
- at any given time, Ha can experience two different magnetic environments because Hb can be in either the alpha or the beta state
- the different states of Hb influence the nucleus of Ha causing slight upfield and downfield shifts
- there is approximately a 50% chance that Hb will be in either of the two states, so the resulting absorption is a doublet
- Ha and Hb will both appear as doublets because each one is coupled with one other H
char: doublet
2 peaks of identical intensity, equally spaced around the true chemical shift of Ha
how do we determine the number of peaks present (as doublets, triplets, etc)? what does the splitting of a peak represent?
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
what is the one caveat to the n + 1 rule?
do NOT include protons attached to oxygen or nitrogen
defn + unit: coupling constant, J
unit: hertz
the magnitude of the splitting
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?
we Deshield and move Downfield
what are the 3 unique frequencies that correspond to a triplet?
alpha-alpha
alpha-beta/beta-alpha
beta-beta
what is the center of a triplet?
the true chemical shift
defn: multiplet
peaks that have more than four shifts
what 3 things is proton NMR good for?
- determining the relative number of protons and their relative chemical environments
- showing how many adjacent protons there are by splitting patterns
- inferring certain functional groups
what are the 6 chemical shift values downfield from TMS that you should know for the mcat?
- deshielded aldehyde = 9-10 ppm
- deshielded carboxylic acid = 10.5-12 ppm
- aromatics/hydrogen of an aromatic ring = 6-8.5 ppm
- alkyl groups/hydrogens on sp3-hybridized carbons = 0.0-3.0 ppm (higher if electron-withdrawing groups are present)
- alkenes/hydrogens on sp2 hybridized carbons = 4.6-6.0 ppm
- alkynes/hydrogens on sp hybridized carbons = 2-3 ppm
