Measurement and Spectroscopy Flashcards
What is spectroscopy?
the study of the way that light (EM radiation) and matter interact
Nuclear magnetic resonance
radio waves can cause nuclei in some atoms to change magnetic orientation
it tells us how neighbouring atoms of certain nuclei in a molecule are connected together as well as how many atoms of these types are present in different locations in the molecule
Structure determination from spectroscopic data
- empirical formula
- mass spectrum
- index of hydrogen deficiency (IHD = 0.5(2C + 2 - H - X + N)
- Infra-red (identifies functional groups)
- 1HNMR (identifies number of H’s bonded to other atoms)
Index of hydrogen deficiency
for every two hydrogen atoms fewer than in the alkane with the same number of carbon atoms, there is one double bond or ring present
- O and S do not contribute to IDH
- halogens (X) count as H equivalent
- for any N atom we need to add a H equivalent (IHD increases by 1)
IHD = 0.5 x ( 2c + 2 - h - x + n)
How does mass spectrometry work?
does NOT use EM radiation
the compounds are ionised, then broken into fragments
the ion of the heaviest mass (molecular ion) corresponds to molar mass
in a mass spectrometer, molecules are bombarded with a high energy electron which can remove an electron from the molecules. This leaves the molecules with a positive charge. These molecular ions are then accelerates through magnetic and/or electric fields which, by deflecting them relative to their mass, give information that allows their molecular mass to be calculated
Other ions, produced by fragmentation (breaking of C-C bonds, halogens are also normally lost) of the molecular ion are also produced. Examining these fragments provides clues about the chemical structure.
Mass spectrometry bromine
two isotopes of bromine (79 and 81) exist in almost equal amounts
therefore, when a sample contains bromine there will be two molecular ion peaks produced
Mass spectrometry chlorine
will be two molecular ions (on with Cl35 and Cl 37) however the peak corresponding to the ion containing Cl35 will be three times larger than Cl37
How Infra-red spectroscopy works?
passes a beam of IR radiation through an organic sample and the chemical bonds in the sample are able to absorb some of the wavelengths of IR radiation, with different bonds absorbing different wavelengths
different covalent bonds have different strengths due to the masses of different atoms and electronegativities at either end of the bond. As a result the bonds vibrate at different frequencies.
strong bonds and light elements absorb at lower wavenumbers
weak bonds and heavy elements absorb at high wavenumbers
Various types of vibration (involves a change in dipole moment) is possible e.g. stretching and bending
a bond will absorb radiation of a frequency similar to its vibration (s)
What does IR spectroscopy tell us about organic compounds?
tells us what bonds are present and which are not
Proton nuclear magnetic resonance
all nuclei possess charge and mass
those with either an odd mass number or an odd atomic number also possess spin
meaning they have angular momentum
a nucleus without spin can NOT be detected by NMR
a spinning nucleus such as H behaves as a spinning charge and generates a magnetic field
when it is places in an externally applied field it can align with, or against, the field
the field strength required depends on the environment of the hydrogen
What does NMR tell us?
uses radio waves to flip the protons in the nucleus of H atoms
signals are recorded in units of (ppm) from TMS which is assigned a value of 0
in the HNMR spectrum only hydrogens give absorption signals
the number of absorption signals equals the number of non-identical hydrogen environments
the position of the absorption signal depends on the molecular environment of hydrogen (s)
The relative areas under the signals corresponds to the relative numbers of hydrogens giving the absorption signals
The splitting pattern depends on the number of neighbouring hydrogens
Why is tetramethylsilane (TMS) used as reference signal?
- the Si shifts the peaks away from others
- unreactive
- low b.p, easy to remove
- only one peak (as each H has an environment similar to others)
- soluble (in most organic solvents)
Interpretation of HNMR spectrum - number of absorption signals
H’s in identical molecular environments give only one signal
To see if a hydrogen is chemically different it is necessary to look at the whole structure
Interpretation of HNMR spectrum - position of absorption signal
mostly in the range 0-10 ppm
for H’s bonded to sp3 carbon, the shift will depend on the EN of other atoms bonded to C
the greater the EN, the higher the chemical shift
for H’s bonded to sp2 carbon, the shift will be high
Interpretation of HNMR spectrum - intensity of signals (integration)
the area under a signal is proportional to the number of hydrogen atoms present
it is achieved using an integration device that scans the peaks
by measuring the distance between the integration lines one can work out the simple ratio between various types of H
