Chapter 11: Spectroscopy and Chapter 12: Separations and Purifications Flashcards
What does spectroscopy measure?
The energy differences between the possible states of a molecular system
How does spectroscopy work?
Determining the frequencies of EM radiation absorbed by the molecules
What does infrared (IR) spectroscopy measure?
Molecular vibrations
How is an IR spectrum recorded?
IR light is passed through a sample, and the absorbance is measured
By determining what bonds exist within a molecule, we hope to infer what?
The functional groups in the molecule
The useful absorptions for spectroscopy occur at wavelengths of what?
2500 to 25,000 nm
On an IR spectrum, we use an analog of frequency called ____________.
wavenumber
When light of these wavenumbers is absorbed, what happens to the molecules?
They enter excited vibrational states.
What are 4 types of vibration that can occur?
- symmetric bend
- asymmetric bend
- symmetric stretch
- asymmetric stretch
More complex vibration patterns, caused by the motion of the molecule as a whole, can be seen in what wavenumber range? This is called the ________________ ________ because the specific absorbance pattern is characteristic of each individual molecule.
1500 to 400 cm-1 range; the fingerprint region
For an absorption to be recorded, the vibration must result in a change in the ____________ ____________ ____________.
bond dipole moment
Which molecules do not have recordable absorptions?
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
The hydroxyl group absorbs with a ________ peak at around one of two frequencies: ________ cm-1 for alcohols, and ________ cm-1 for carboxylic acids.
broad; 3300, 3000
The carbonyl of a carboxylic acid pulls some of the electron density out of the OH bond, shifting the absorption to a ____________ wavenumber.
lower
The carbonyl absorbs around ________ cm-1 with a ________ peak.
1700; sharp
The bond between any atom and hydrogen always has a relatively ________ absorption frequency. As we add more bonds between carbon atoms, the absorption frequency ____________.
high; increases
N-H bonds are in the same region as O-H bonds at ________ cm-1 but have a ________ peak instead of a ________ one.
3300; sharp instead of broad
IR spectra are plotted as percent ________________, the amount of light that passes through the sample and reaches the detector, vs. wavenumber.
transmittance
UV spectra are obtained by passing ________ light through a sample that is usually dissolved in an inert, nonabsorbing solvent, and recording the ____________.
absorbance
UV spectra plot what?
absorbance against wavelength
The most significant piece of info we get from UV spectroscopy is the ____________ of ____________ ____________, which tells us what?
wavelength of maximum absorbance; the extent of conjugation within conjugated systems
The more conjugated a compound is, the ________ the energy of the transition and the ________ the wavelength of maximum absorbance.
lower; greater
Molecules with a lower energy gap between ________ occupied molecular orbital and ________ unoccupied molecular orbital are more easily ________ and can absorb ________ wavelengths (lower frequencies) with lower energy.
highest; lowest; excited; longer
high yield
UV spectroscopy is most useful for studying compounds containing ________ bonds or heteroatoms with ________ ________ that create conjugated systems.
double; lone pairs
____________ molecules, or molecules with ____________ p-orbitals, can also be excited by UV light.
conjugated; unhybridized
Conjugation shifts the absorption spectrum, resulting in ________ maximum wavelengths.
higher
Nuclear magnetic resonance (NMR) spectroscopy is based on the fact that certain atomic nuclei have ____________ ____________ that are oriented at random.
magnetic moments
When certain atomic nuclei are placed in a magnetic field, their magnetic moments tend to align either with or against the direction of this applied field.
Nuclei with magnetic moments that are aligned with the field are said to be in the ____-state (lower energy).
alpha
Nuclei in the alpha state can be irradiated with radiofrequency pulses that match the energy gap between the two states, which will excite some lower-energy nuclei into the ____-state (high energy).
beta
What does NMR plot?
frequency vs. absorption of energy
NMR uses an arbitrary variable called ____________ ________ with units of parts per million of spectrometer frequency.
chemical shift
The chemical shift is plotted on the x-axis, and it ________ toward the left (referred to as ________.
increases; downfield
To make sure that we know just how far downfield compounds are, we use ________ as the calibration standard to mark 0 ppm; when counting peaks, make sure to skip its peak.
TMS
NMR is most commonly used to study what?
1H nuclei (protons)
Any atom with a ________ ________ (an odd atomic number, odd mass number, or both) can be studied with NMR.
nuclear spin
MCAT only tests 1H NMR
Most hydrogen (1H) nuclei come into resonance ____ to ____ ppm downfield from TMS.
0 to 10
Each distinct set of nuclei gives rise to a ____________ peak.
separate
This means that if multiple protons are ____________ ____________, having the same magnetic environment, they will lead to the same peak.
chemically equivalent
