block 2 - spectroscopy and structure determination Flashcards
molecular spectroscopy
IR, UV-VIS NMR (NOT MS)
depend on interaction of molecules with radiation of specific energy
spectroscopy = study of interaction of electromagnetic radiation with matter
electromagnetic spectrum
high energy cosmic waves to low energy radio waves
energy DECREASES with WAVELENGTH - inversely proportional to wavelength (e = hc/wavelength), energy INCREASES with FREQUENCY - directly proportional to frequency (e = hv)
fundamental equations of electromagnetic radiation
E = energy c = velocity of light (constant) h = Planck's constant v = frequency λ = wavelength v (with line on top) = wavenumbers 1. E = hv 2. c = vλ 3. E = hc / λ 4. v (with line on top) = 1 / λ
basis of molecular spectroscopy (energy levels)
when energy of electromagnetic radiation put onto a sample EXACTLY corresponds to the energy DIFFERENCE between 2 molecular energy levels, then molecule can ABSORB the energy - ground state –> excited state (release energy eg. as heat) –> go back down
transition occur when energy of electromagnetic radiation = E2 - E1 (∆E) therefore energy absorption at ∆E = hv
only one frequency of radiation will be applicable to particular transition
what is the spectrum of a compound in molecular spectroscopy
graph of fraction of radiation ABSORBED or TRANSMITTED by sample vs the WAVELENGTH, FREQUENCY or WAVENUMEBER of the light impinging on the sample
mass spectrometry (graph, how it works, what it determines, limitations etc.)
allows determination of mass of individual ions derived from compounds in gas phase
molecule bombarded with high energy electron beam
M (molecule) –> M+•(–> fragmentation –> other positive ions + neutral fragments) (becomes ionised - 1 e- removed so becomes positive, and other e- is unpaired) + e-
detects charged molecules
plot of relative abundance vs m/z (m = mass of ion, z = charge of ion)
look at mass that is HEAVIEST (M+• = ion of highest m/z - z = 1)
limitations = not able to distinguish isomers, diff molecular formulas (with same molar mass)
IR spec (graph, transmittance, benefits, limitations etc)
units used = wavenumber cm-1 (wavenumber equation)
vibrational excitation
absorption of IR radiation occurs when radiation freq. matches freq of bond vibration
transmittance (%, y) and wavenumber (x)
transmittance = how much energy is RELEASED, what’s left over
low transmittance = mol has absorbed most, little left over
relative masses of atoms+ bond strength determines position of IR absorption (how much absorbed)
BENEFITS = rapid identification of functional groups; “fingerprint” - no two compounds have identical spectra
LIMITATIONS = compounds w same fg –> similar spectra (eg isomers); no indication of no. of fg’s
UV-VIS & energy of e-, spectrum
UV-VIS raises e- in some molecules (π e-) from LOWER energy bonding (or NON-BONDING) molecular orbitals –> HIGHER energy ANTI-BONDING molecular orbitals
EXPLANATION of non-bonding, MOs, ANTI-BONDING (not examinable)
- MOs = 2 e- in bond between 2 atoms –> BONDING MOs (2 atomic orbitals = MO)
- ANTIBONDING = every atom w/ BONDING MO has ANTIBONDING MOS (on the side for C-C bond for eg 0C-C0 0 = antibonding)
- UV-VIS radiation –> one e- jumps from BONDING mo to ANTIBONDING mo –> bond between C-C (for eg.) gets longer and weaker (e- not as close to bond) - if BOTH e- jump to antibonding mo the molecule falls apart (no e- in bonding mo)
- NON-BONDING = O , N have lone pairs - can jump to antibonding orbitals too
antibonding has * next to it eg π*
UV-VIS SPECTRUM = plot of ABSORBANCE (y axis) vs WAVELENGTH (x axis, nm). WAVELENGTH at which MAX ABSORPTION = λmax
conjugation
when there is π bond, sigma bond, π bond
NON conjugated π bonds in UV-VIS (absorption, why)
C=C - ≈ 170nm - 180nm due to a π to π* transition
C=O (or N) - 170nm - 180nm also, but EXTRA absorption at ≈280nm due to n (non-bonding) to π* transition
note - n - π* requires less energy (as n is in between π and π), therefore appears at a larger nm (shorter bump, after the larger π-π transition before it)
conjugated systems in UV-VIS
∆E for e- excitement is smaller - absorption is observed at a LONGER wavelength than for non-conjugated systems
any molecule w/ conjugated double bonds = absorption at >200nm
the greater the number of double bonds in conjugation, the higher nm they will appear at - ∆E (energy gap) is smaller and energy inversely proportional to wavelength
* conjugation has NO RELATION with ε
UV-VIS and Beer’s law
for a particular compound at a specified λ - ABSORBANCE is PROPORTIONAL to CONCENTRATION and PATH LENGTH
the more effective a molecule is at absorbing light at that wavelength, the greater the absorbance
A = ε b c
A = absorption
ε = molar absorptivity, L mol-1 cm-1
b = path length, cm
c = concentration, mol L-1
uses of Beers law
- determine conc. in solution of UV-VIS absorber of known structure - find out λmax, sub into law if absorptivity is known at λmax
- make calibration plot for solutions (dilutions) at a specific wavelength, then at SAME wavelength determine absorbance for solution of UNKNOWN conc. –> read off calibration plot
UV-VIS, MS and IR all have what limitation?
give NO INFO w/ respect to HYDROCARBON SKELETON
NMR technique, nuclei and shielding
info of C-H framework of molecules
nuclei have nuclear spin - random, when magnetic field applied –> nuclei align either PARALLEL (with - LOWER energy) or ANTIPARALLEL (against) mf
radio frequency –> energy absorbed and NUCLEAR SPIN FLIP from low energy (p) –> high energy (anti-p)
energy of spin flip depends on shielding of nucleus
shielding = when e- are shared equally –> lower ppm as nucleus doesn’t feel effect of radiation as much
deshielding = when one (more en) atom pulls e- close to itself –> higher ppm other nucleus is deshielded and feels more of the radiation
