NMR Flashcards
What are the 2 types of NMR?
MRI used for diagnostic imaging of soft tissue & NMR spectroscopy used to identify compounds and for characterisation
What are some other uses of NMR and MRI?
in medicinal chemistry to determine exact structure of unknown drug metabolites, to study 3D structures of enzyme AS and to determine conformation of both free and bound drugs. Used in formulation science to determine aggregation properties of drugs, for evaluating rates of drug degradation and estimating rates of release of drugs from controlled release systems. Also in pharmacokinetics for determination of metabolic products in urine and plasma
Do all atoms possess magnetic properties?
Any atom with odd number of neutrons or protons has magnetic properties. Because protons have a positive charge, certain nuclei with an odd mass are spinning and nucleus generates magnetic field along spinning axis
What are the basic concepts of NMR?
A nucleus with ½ spin can adopt two possible states: with or against field. NMR spectroscopy measures the absorption of radio frequency causing changes in nuclear spin orientation. Only occurs when a sample is in a strong magnetic field. Different nuclei absorb EMR at different frequencies and in an NMR experiment you detect relaxation back to equilibrium. Sample only generates signal at a particular combination of radio frequency and magnetic field strength to satisfy resonance conditions for Vo equation
What happens during a Fourier transformation experiment?
During experiment, sample is irradiated with a short pulse of radiofrequency radiation, causing excitation of all nuclei in sample. Nuclei relax back to lower energy state, releasing a pulse of EMR. NMR detects this radiation received from sample. Results in an FID plot of signal intensity against time. FT converts spectrum from units of time to units of frequency.
There is a linear relationship between radio frequency and what?
magnetic field strength
How do you increase magnetic resonance sensitivity?
By increasing the magnetic field strength, low sensitivity would mean you need lots of sample.
What are the 5 things to consider when interpreting NMR spectra?
number of peaks, area under each peak, positions of peak, the shapes of the peaks
What does the number of peaks correspond to?
the number of non-equivalent protons
What does the position of signal indicate?
the EM environment of the protons which produced this signal
What is the area under the signal (integral) proportional to?
number of protons responsible for signal
The shape of the signal depends on what?
whether the proton is exchangeable or non-exchangeable
What type of field is induced by electrons surrounding the nucleus?
local magnetic field
What is a chemical shift?
The variation of resonance frequency (Vo) with chemical environment of the nucleus. Measured in ppm
What causes shielding and de-shielding
shielded protons are located upfield (on the right) and have a lower resonance frequency, an electron donating group attached to the atom increases the electron density surrounding the nucleus. A deshielded proton will be located more downfield (left) and have a higher resonance frequency, as an electron withdrawing group attached to the atom results in decrease of electron density surrounding the nucleus
