Analytical techniques in supramolecular chemistry Flashcards
1D NMR
Can determine Ka, stoichiometry, kinetics and thermodynamic functions e.g. vtNMR
Conc. range 10^-5 to 10^-1 M
Ka range 10^1 to 10^6
Evaluation of binding constant using 1D NMR
If the exchange of complexed and uncomplexed guests is slow on the NMR timescale
However… most host-guest equilibria are fast on the (relatively slow) NMR spectroscopic timescale
This means the chemical shift observed for a particular resonance that is sensitive to the complexation reaction is a weighted average between the chemical shift of the free and bound species
NMR titration experiment
Small aliquots of guest are added to a solution of host of known concentration and the NMR spectrum of the sample measured as a function of guest conc/host-guest ratio
The shape of the titration curve (plot of change in chemical shift vs added [guest]) gives quantitative information about the binding constant
Job plot
A plot of [complex] vs. [host]/([host]+[guest])
Obtained from monitoring the changing concentration of host-guest complex in samples
Gives information about the stoichiometry
i.e. 1:1 complex will have peak at 0.5
2:1 complex will have peak at 0.66
UV-Vis
Can determine Ka and stoichiometry
Stoichiometry determined via Job plot or mole-ratio plot
Conc. range 10^-7 to 10^-1 M (more sensitive than 1D NMR with a broader conc. range, but a chromophore is required)
Ka range 10^1 to 10^7
Plot generated from UV-Vis
Absorbance intensity vs. [guest] added to a solution of known [host]
Can observe isobestic points
Isobestic point
Where the observed absorption intensity is constant throughout the titration
Provides good evidence for the conversion of free host into the complex without the involvement of significant intermediate species
Fluorescence titrations
Can determine Ka and stoichiometry
Conc. range 10^-6 to 10^-3 (also more sensitive than NMR)
Ka range 10^2 to 10^8
2 methods of fluorescence titration
Turn on fluorescence: host and guest are non-fluorescent but host-guest complex is
Turn off fluorescence: host and/or guest fluorescent, host-guest complex less fluorescent (i.e. there is a quenching of fluorescence)
Circular dichroism
Can determine Ka, stoichiometry, chirality information and conformation
Conc range 10^-6 to 10^-3 M
Ka range 10^2 to 10^6
Used to investigate the secondary structure of proteins
= difference in absorption of left-handed circularly polarised light and right-hand circularly polarised light - occurs when a molecule contains one or more chiral chromophores
ITC
Isothermal Titration Calorimetry
Can determine Ka, stoichiometry, kinetic information and thermodynamic functions
Conc. range 10^-8 to 10^-2 M
Ka range 10^2 to 10^7
Involves the measurement of the heat (enthalpy) evolved from a carefully insulated sample as a function of added guest or host concentration
Gradient of ITC curve
Can be fitted the determine the binding constant and hence DeltaGcomplex
Integration of area under ITC plot
Gives complexation enthalpy (DeltaHcomplex)
ITC can give a measurement of…
…all thermodynamic parameters of the system
2D NMR
e. g. NOE (nuclear Overhauser effect) - can be used to determine structure/geometry of host-guest interaction
e. g DOSY (diffusion ordered spectroscopy) can be used to separate the NMR signals of different species according to their diffusion coefficient. Spherical molecules, use Stokes-Einstein equation to calculate molecular volume
