Chapter 11- Measurement and Data Processing Flashcards
IHD (Index of Hydrogen Deficiency)
Is a measure of “How many molecules of H2” would be needed in ‘Theory’ to convert the molecule to the corresponding saturated, non-cyclic molecule.
What can the IHD be used to determine in a molecule?
The number of rings or multiple bonds (π bond)
What is the IHD number of a:
1) Double bond
2) Triple bond
3) Ring
4) Aromatic ring (Benzene)
1) 1 (1 degree of saturation)
2) 2 (2 degrees of saturation)
3) 1 (1 degree of saturation)
4) 4 (4 degrees of saturation)
Spectroscopy
The way in which the absorption or emission of electromagnetic radiation by substance varies w/ frequency
c (velocity of wave)=?
vλ
→ v = freq.
→ λ = wavelength
E (Energy of electromagnetic radiation) =?
hv
→ h = Planck’s constant (6.626 x 10^-34 Js)
→ v = freq.
Radiowaves: what the energy does + Spectroscopy technique
+ Reverse Nuclear spin on certain atoms when in a strong magnetic field
+ NMR- spectroscopy
→ gives info. abt the chemical environment of atoms (usually H)
Microwaves: what the energy does + Spectroscopy technique
+ Rotates molecules
+ Gives info. on bond length
Infrared: what the energy does + Spectroscopy technique
+ Vibrates (stretches and bends) bonds in molecule
+ IR-spectroscopy
→ info. on bonds present
UV/Visible light: what the energy does + Spectroscopy technique
+ Cause electronic transitions in atoms and molecules
+ UV-vis-spectroscopy
→ info. abt electronic energy levels
X-rays: what the energy does + Spectroscopy technique
+ Produce diffraction patterns as their λ is similar to inter-atomic distances.
+ X-ray crystallography
→ info. on bond lengths and angles
What does too much exposure to the electromagnetic radiations do to make them harmful?
Initiates reactions by ionising atoms and breakig chemical bonds
IR-spectroscopy: what does the infrared wave of light do?
Excite a molecule to a higher vibrational level
What graph does IR-spec produce?
+ transmission against freq. (often expressed as wavenumber= 1/λ) when infrared is passed through the sample
What does the absorption in a particular freq. range indicate?
The presence of bonds in a molecule
Each bonds vibrate at a particular freq. depending on what?
Strength of the bond and masses of the atom
Light atoms vibrate at a?
Higher freq. than heavier atoms
Multiple bonds vibrate at a?
Higher freq. than single bonds
What are the 2 types of vibration possible in IR-spec?
Stretching and bending
Only what kinds of molecules will absorb IR radiation?
Vibrational molecules that result in a change of DIPOLE moment (polarity) of a molecule.
→ O=O and H-H = DON’T absorb IR = cuz non-polar even on stretching (dipole moment = 0)
IR-spectroscopy: Symmetrical stretching mode
In molecules w/ dipole moment of 0
→ e.g CO2 = maintains symmetry and dipole moment = 0 therefore doesn’t absorb IR
IR-spectroscopy: Asymmetrical stretching mode
Stretching and bending modes of CO2 = BOTH absorb IR
IR-spectroscopy: What do presence of H.bonds cause to happen in the absorption?
Broadening of absorption
IR-spectroscopy: Fingerprint region
from 400-1500 cm^-1
→ unique to the compound
Mass spectrometry can be used to determine what?
The MOLAR MASS of a substance and give some info. on the structure of the molecule
Mass spectrometry: How does it work?
+ Gaseous molecules = converted to + ions (cations)
+ which are ACCELERATED into an Electric field and DEFLECTED by Magnetic field
→ so they are separated according to their mass/charge (m/z) ratio
Mass spectrum records what?
The of the fragments of diff. m/z ratio reaching the detector
Mass spectrometry: Why are collisions betw/ particles = v. unlikely?
Because inside the spectrometer it is v. LOW pressure
Mass spectrometry: What does the ion with the greatest mass usually correspond to?
A molecule that has only lost a
→ The MOLECULAR ION
= gives the Relative Molecular Mass (Mr) of the substance
Mass spectrometry: MOLECULAR ION is when?
+ A whole molecule is ionised M+
+ Whole molecule - 1 electron = MOLAR MASS
Mass spectrometry: Why is there always a much smaller peak at 1 greater than the Molecular ion peak?
+ Cuz C contains abt. 1% C13
→ so theirs always that smaller peak greater than the molecular ion peak
Mass spectrometry: What does the excess energy from the impact of the electron forming the Molecular ion cause?
+ Cause the molecules to breakdown into “Fragments”
→ gives rise to “Fragmentation patterns”
Mass spectrometry: What do Fragmentation patterns give clues to?
The structure of the molecule through the mass of the units that’s broken off
Mass spectrometry: What happens when only 1 H breaks off? (Fragmentation patterns)
It gives a peak at one mass # less than the main peak?
→ cause the mass of H = 1 so the substance now weighs 1 less!
What are 2 uses of the Mass Spectra?
1) Radiocarbon dating
→ used to detect the % of C14 present in samples
2) Forensic Science
→ Determine the presence of small amts of drugs and other substances
HNMR and how works?
Hydrogen Nuclear Magnetic Resonance
+ Results from the changes that occur in the nucleus atoms w/ an odd mass #
→ when placed in magnetic field = absorbs radiation of radio freq.
What does the NMR indicate?
Gives info abt. the chemical environment of the H atoms in the molecule
Chemical shift
The position of absorption/ppm
Internal standard for HNMR, and why needed?
TMS (Tetramethylesilane)
→ The freq. of radiation = dependent on the strength of the applied magnetic field = difficult to ensure that it’ll remain constant
+ By adding TMS = can record the absorption relative to TMS = “arbitrary 0”
Why is TMS used as an “internal standard”? (3)
1) Chemically inert
2) Produces a ‘single’ string signal (as it has 12 hydrogens in an identical environment)
3) Cuz of the low electronegativity of Silicon
→ It absorbs radiation of a freq. different from that of MOST other Organic compounds
→ DOESN’T INTERFERE W/ ABSORPTION SIGNALS
The nuclei of the H atoms possess a spin what happens when the electric field applied?
The nuclei align themselves either:
+ with or against the field
→ The nuclei can absorb a photon of energy to move from the lower to the higher spin state
What is the HNMR showed in?
Integration Trace
The more electronegative the chem environment?
The greater the chem shift
What are splitting patterns caused by?
+ When the precise freq. at which the absorption occurs = influenced by the direction of magnetic field of any hydrogens attached to the neighboring C atom of the one producing the signal
of peaks = ?
The number of H on the neighboring C + 1 (n+1)
Name of the peaks
+ Singlet
+ Doublet
+ Triplet
+ Quartet
HNMR in body scans (MRI)
+ water and lipids are the main constituents of the body than contain H atoms
+ Water - lipid ration in parts of body = diff = absrob radio freq differently
+ Radio waves = harmless
+ Used to diagnose cancer, multiple sclerosis and other conditions.
What is X-Ray Crystallography used to identify?
Bond lengths and bond angles of crystalline compounds
Why can X-rays be used to determine the molecular and atomic structure of crystals?
Because the wavelength is similar to the dist. betw/ atoms
What happens when an X-ray is passed through a crystalline solid?
They are scattered in an orderly way by their interactions w/ electrons in the substance
→ Diffraction patter produced
What does the diffraction pattern depend on? (3)
+ angle of incidence of the X-ray (θ)
+ Wavelength of the incidence ray (λ)
+ dist. betw/ atoms (d)
