spectroscopy Flashcards
(22 cards)
what can mass spectra be used to identify
the molecular mass or an organic compound and to gain further info about the structure.
what’s the molecular ion and what do mass spec do w it
web an organic compound is placed in the mass spectrometer it loses an electron and forms a positive ion
the mass spec detects the mass to charge ratio of molecular ion which gives molecular mass of compound. so the m+ peak has to be located.
cause of the m+1 peak
Some carbon is present as the carbon-13 isotope.
what’s fragmentation
What causes the other peaks in a mass spectrum?
The molecular ions break down into smaller pieces called fragments.
fragment ions
How can a mass spectrum differentiate between structural isomers?
They will break apart differently and thus will have different mass spectra.][
what does the simplest fragmentation do
breaks a molecular ion into 2 species- a positively charged fragment ion and a radical.
positive ions formed w be detected but uncharged radicals won’t.
when determingin fragments n that
the bottom number is the mass no so the one iftrhest in right would be the molecular ion peal,m+ and the little one next t it is m+1 and the mass should equal the whole fragment
same w other peaks it should count for each tiny lil fragment.
what can m+1 peak help do
a spectrum of an unknown compound has an M peak of height 74mm and an m=1 peak of height 4.5mm.how many C atoms are there in the ionic compound?
help identify the number of carbon atoms present in the molecules of an organic compound.
no of c atoms=height of m+1 peak/height of m peak x 100.
4.5/74 x 100=6.08
organic compound has 6 c atoms
for propan1ol do the formation of the molecular ion
ch3ch2ch2oh—ch3ch2ch2oh+ + e-
and that’s the m+ we see on mass spec thing
show how a fragment ion and a radical can be formed
M+e −→M⁺+e
M⁺→Fragmention(X⁺)+Neutralradical(*Y)
do pg255 qs q3
what’s infrared sepctrocscopy used for and how to use it
identifying functional groups present in organic molecules
1)sample under inv put under IR spec
2)a beam of IR radiation in the range 200-4—cm is passed thru sample
3)molecule absorbs some of ir frequencies n emerging beam of radiations analysed to identify the frequencies that have been absorbed by sample.
4)the ir spectrometer is usually connected to a computer that plots a graph of transmittance against wavenumber.
What do peaks help us predict?
The bond types in the molecules.
what’s the fingerprint region
contains unique peaks which can be used to identify the particular molecule under investigation either w computer software or comparing to booklets of spectra
found under 1500cm
applications for infra red spectroscopy
many pollutants can be identified by their ir spectral fingerprints.
remote sensors analyse ir spectra of vehicle emissions to detect n measure co,co2 and hc’s in busy places to monitor localised pollution.
ir based breathalysers pass a beam. of ir radiation through the captured breath in the sample chamber n detect the IR absorbance of the compounds in the breath.
the characteristics bonds present in the ethanol are detected.the more ir radiation absorbed the higher the reading n the more ethanol in the breath
a typical sequence for identification
elemental analysis-use of % composition data to determine empirical formula
mass spec- use of molecular ion from a mass spec to determine the molecular mass:use pf fragment ions to identify sections of a molecule.
IR spec- use of absorption peaks from an IR spectrum to identify bonds n functional groups present in the molecule.
do q3 p261
Why can covalent bonds in molecules absorb infrared (IR) radiation?
Atoms in molecules are joined by covalent bonds, which possess energy n vibrate naturally about a central point. These vibrations inc w increasing temperature, meaning the atoms are in constant motion. Covalent bonds can absorb IR radiation, which causes them to bend n stretch more intensely by increasing the energy of their vibrations.
what are the 2 vibrations
stretch-rhythmic movement about the lines between the atoms so that the distance between the two atomic centres increases and decreases.(change in bond length)
bend- results in a change of bond angle
what does the amount a bond stretches/bends on depends on:
the mass of the atoms in the bond-heavier atoms vibrate more slowly that the lighter atoms.
the strength of the bond-stronger bonds vibrate faster than weaker bonds.
Why do chemists use wavenumber in IR spectroscopy, and what is its typical range?
Any bond can absorb radiation that has the same frequency as its natural vibration frequency. These frequency values are v large, so chemists use a more convenient scale called wavenumber, which is proportional to frequency. The vibrations of most bonds are observed in the IR wavenumber range of 200 cm⁻¹ to 4000 cm⁻¹.
how do atmospheric gases and IR radiation interact
Most of the Sun’s visible n IR radiation passes through the atmosph n is absorbed by Earth’s surface, which then re-emits it as longer wavelength IR radiation.
GG absorb longer wavelength (IR) radiation re-emitted from the Earth’s surface. This radiation has same frequency as the natural vibrational frequencies of the bonds in CO₂, CH₄, and H₂O molecules. The bonds absorb the energy n vibrate more, then re-emit the energy as IR radiation, which increases the temperature of the atmosphere near the Earth’s surface.
