105 Lecture 2 – Flashcards
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Unlock answers| in between which regions would you find the infra red spectrum? |
| in between visible and micrwave regions of the electro magnetic spectrum |
| which sort of changes is the IR region associated with? |
associated with changes in Vibrational states of molecules
photons in mid-IR range have energies 10-19 - 10-20 J wavelengths 2.5 -25 um wavenumbers 4000 -400 cm-1
E =hc/? E =hcv- |
| how do you work out absorbance using intensity of incident beam (I0) and intensity of emergent beam? (I) |
Absorbance = Log intensity of incindent beam/intensity of emergent beam absorbance = Log I0/I |
| which wave number range do routing IR spectra cover? |
4000 cm-1 - 400 cm-1 high energy to low energy |
| in an IR spectrum what is routinely plotted on the x axis and what is found on the y axis? |
X axis you would fine the wavenumber (cm-1) y axis you would find % transmittance |
| what can IR be used for? |
confirming the presence of certain functional groups in an organic compound Every molecular compund has a unique IR Spectrum that can serve as a 'fingerprint'
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true or false? it is possible to identify an unknown from its IR spectrum alone? |
False
it is rarely possible to identify an unknown from IR alone. Supporting evidence from other techniques is often required like NMR or MS |
| in IR which functional group would have a strong transmittance between 1750-1600cm-1? |
| carbonyl (-c=o) group |
| in IR which group would have a very strong transmittance at ~3600 cm-1 |
| O-H group |
| in IR a transmittance at 3000cm-1 suggests which group? |
| C-H group |
| in IR a transmittance at ~3400cm-1 suggests which group? |
| N-H group |
| carbonyl groups c=o are found strongly at which wavenumber range? |
| 1750-1600 cm-1 |
at which range would you find: N-H groups C-H groups O-H groups |
N-H ~3400 cm-1 C-H 3000 cm-1 O-H 3600 cm-1 |
Sample prep: liquid/ gasoue samples placed in cells with windows made from metal halide plates
why are the liquid or gaseous samples in IR place in cels with windows made from metal halide plates (KBr, NaCl) and not glass? |
| glass is not transparent in the IR region |
| how are solid samples prepared for IR? |
Mixed with KBr, powdered and pressed into a solid disk.
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| what is Nujol? when ground up with solid if forms which product that is spread on the KBr plates? |
| Nujol is a hydrocarbon oil. it can be ground with solids to make 'mull,' a paste which is spread between KBr plates |
how long does it take an FT-IR spectrometer to produce the IR spectrum?
what happens to the background spectrum caused by air? |
it takes minutes to produce
the background spectrum is automatically subtracted by the on board software |
| do molecules vibrate even at absolute zero? |
yes they do!
a vibrating bond between atoms can be linked to a spring in perpetual oscillation
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| in general, a molecule with N atoms has how many distinct vibrational modes? |
a molecule with N atoms has 3N-6 distinct vibrational mode
some bonds are stretches, others are bending motions each mode vibrates in its ground vibrational state at ordinary temperatures
it cant lose this minimal vibrational energy nor accept additional random amounts of vibrational energy |
| each ......... mode in a ........ has its own set of discrete vibrational ........ levels. only the first ...... vibrational state of each mode is accesible by IR photon absorbtion. Transitions to more ...... lying vibrational energy levels are........... |
vibrational, molecule, energy, excited, high, forbidden
each vibrational mode in a molecule has its own set of discrete vibrational energy levels. only the first excited vibrational state of each mode is accesible by IR photon absorbtion. Transitions to more high lying vibrational energy levels are forbidden |
| what do active vibrational mode show up as on iR spectroscopy? |
| show as absorbtion bands |
| methanal has formula H2CO.. how many vibrational modes does it have? |
6
working out...remember 3N - 6 = how many distinct vibrational levels there are for the molucule.
methanl has 4 atoms thus N=4
3 x 4 = 12-6 = 6 =) |
| for which range of the electromagnetic spectrum is absorbtion spectrum usually used? what do we use for IR spectrum? |
UV/visible range uses absorbtion spectrum
for IR Spectroscopy transmittance is usually used. |
| define transmittance |
| the % of incident light passing through a sample |
| what is IR spec used for? |
for identifying the presence of certain functional groups.
though it is rarely possible to identify an unknown based on IR alone..NMR and MS data would also be required. |
| in sample preparation for IR spec, what is the name of the special head used on solids and liquids? |
ATR head - attenuated total reflectance head
here, powder or liquid film is held on surface of an internally reflecting crystal eg diamond or ZnSe |
| what are the 6 vibrational modes for mathanal. the modes are all IR active so how many peaks would you expect so see in the iR Spec of methanal? |
symmetric C-H stretch asymmetric C-H stretch C=O stretch CH2 bend HCO bend Out of plane bend
since all 6 are IR active one would expect to see 6 peaks on the IR spec of Methanal |
for most organic compounds we dont attempt to assign every band because they are complex. instead we look for the presence or absense of bond stretches due to particular groups. what are the wave numbers of the following groups? O-H, N-H, (sp2) C-H, (sp3)C-H, C=O, C?N
|
O-H ~3600 cm-1 N-H ~ 3400 cm-1
(sp2) C-H ~3100-3000 cm-1 (sp3) C-H ~3000 -2850 cm-1
C=O ~1750-1600 cm-1 C?N ~ 2250 cm-1 |
True or false? Bending modes occur at lower wavenumbers than stretching modes? |
True
it takes less energy to bend than to stretch bonds. |
| what determines the position of an IR band? |
the energy seperation of relevant vibrational levels determines the postion of the IR band.
this depends directly on the bond strength and inversely on the masses of the atoms involved |
from a bond stretch the equation from quantum mechanics is [image] |
V- = wavenumber of the IR band k = the force constant ( this is a measure of how ‘stiff’ or strong the bond is) m = the reduced mass of the atoms |
[image]
from this equation which two key factors emerge? |
very strong bonds particularly mutiple bonds absorb at relatively high wavenumbers > 1500 cm-1
Bonds involving light atoms and in particular X-H bonds absorb at high wave numbers > 2500 cm-1 |
how do you work out the reduced mass (µ) of a bond between atoms of mass m1 and m2?
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m1 x m2 / m1 + m2
eg C-C m1 = 12 m2 =12
(12 x 12)/ 12+ 12 = 144/ 24 = 6 |
| if a bond is more polar how will this affect its IR band? |
it will have a more INTENSE IR absorbtion
eg a carbonyl band C=O is always more intense than C-H or C-C. this is because C=O is more polar compared to the other two. |
| when a polar bond vibrates what is created? |
an oscillating field is created that can 'couple' with the oscillating electric field of a photon with suitable energy and give a mechanism for suitable absorbtion.
in contrast, non-polar bonds create little or no electric field as they vibrate. this means a passing photon, even if it has the corect energy has little chance of being absorbed thus the corresponding IR band will be weak or even absent |
| do perfectly non-polar vibrations give IR bands? |
no they do not.
bond stretching of perfectly non-polar has no effect on the molecules dipole.
eg the bond stretches in O2, N2 are all IR inactive
|
| its generally convenient to divide IR Spec into two parts. Name these parts and their wavenumbers. |
1. The Group Frequency Region is the high energy region with wavenumbers between 4000-1500 cm-1
2. The Fingerprint Region is the lower energy region with wavenumbers between 1500- 700 cm-1 |
| in which region does absorbtion give the clearest evidence of presence or absence of functional groups? |
The group frequency region
several common functional groups have their bond stretches here eg, O-H, N-H, C-H, C=O, C?N
their postion is insensitive to the rest of the molecule's structure |
| what is the fingerprint region (below 1500 cm-1) mainly used for? |
used to compare spectrums of unknown to established compounds in a library database. If two samples have identical spectra then they are likely to be of the same composition
skeletal C-C, C-O, C-N stretching and C-H bending all occur here. their postions and frequencies may vary from compund to compound due to local effects within the molecule |
| bonds to hydrogens eg, X-H stretches are relevant in IF spectra. which wavenumbers are these found in? |
| 3700 -2500cm-1 |
| triple bonds in IF specta such as C?C C?N are found in which wavenumber range? |
| 2300-2000 cm-1 |
| double bonds in IF Spectra are found in which range? |
| 1800-1600 cm-1 |
| in IF spectra band assignment, what are the 3 relevant ranges in broad terms? |
X-H stretches 3700-2500 cm-1 triple bonds 2300-2000 cm-1 double bonds 1800-1600 cm-1 |
in broad terms wavenumbers between 3700-2500 cm-1 would signify the presence of which molecules?
|
| Bonds to hydrogens |
in broad terms wavenumbers between 2300 - 2000 cm-1 would signify the presence of which bonds?
|
| triple bonds |
in broad terms wavenumbers between 1800 - 1600 cm-1 would signify the presence of which bonds? |
| double bonds |
| where do single bonds (not to H) tend to have their stretches? |
below 1400 cm-1 in the fingerprintprint zone
the bending modes of highest frequency are for O-H and N-H groups and occur at ~1600 -1500 cm-1
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the positions of O-H and N-H stretches are sensitive to which type of bonding?
what affect does this have on O-H and N-H bonds? |
hydrogen bonding.
H bonding lengthens and weaknes covalent the covalent O-H and N-H bond hence lowers the energy at which it is absorbed
Non H bonded (N-H 3400 Sharp) (O-H 3600 Sharp) H bonded (N-H 3300 Broad) (O-H 3200 Broad) |
