NMR is a Powerful analytical technique used to characterize organic molecules by identifying carbon-hydrogen frameworks within molecules. This file elaborate following sub topics of NMR spectroscopy. Oversampling and its importance Digital filtration purpose of digital filtration Decoupling for X nuclei and NMR spectra Excitation modes in NMR Inverse detection and its superiority Data processing data acquisition spectrum generation

2. Hajira Mahmood
Sub topics of NMR
spectroscopy
1. Oversampling
2. Digital filtration
3. Decoupling for
X nuclei
4. Excitation modes
5. Inverse detection
6. Data processing

3. Oversampling in NMR
Sampling faster than required by the sampling theorem
• effectively increases the dynamic range by a factor of n
• prevents certain sources of noise that are NOT band-limited to same extent as
the systematic NMR signals from being aliased into the spectral window.
• Especially used to alleviate problem while studying large proteins to know
whether typical 16-bit analog-to-digital converter (ADC) has sufficient
dynamic range to digitize the sum of signals as well as a small signal of
interest, even in the presence of excellent solvent suppression.
• Noise in an NMR spectrum consists of analog receiver noise, analog noise
from the analog-to-digital converter components and "quantization noise" due
to the limited precision of vertical digitization decreases by oversampling.

4. Diagrammatic representation of oversampling

5. Digital filtration
• In signal processing, digital filter is a system that performs mathematical
operations on a sample discrete-time signal to reduce or to enhance certain
aspects of that signal.
• largely used in signal processing and differs from an analog filter, which is an
electronic circuit working with continuous signals.
• Expensive compared to analog ones, but they can turn many impractical or
impossible designs into possibilities..0
• Digital filters are used for two general purposes
1. For the Separation of signals that have been combined
2. For the restoration of signals that have been distorted in some way.
Analog (electronic) filters can be used for these same tasks; however, digital filters
can achieve far superior results.

6. Purpose to use digital filters
• A digital filter contains an analog-to-digital converter (ADC), which samples
the signal coming in as input, a microprocessor and some other components for
storing filter coefficients and data.
• There is also a digital-to-analog converter that is present just before the output
stage.
• The software that runs on the microprocessor implements a digital filter by
acting on a number from the ADC and perform mathematical operations.
• can perform several effects such as amplification & delay on the sample
signal

7. Spin decoupling
Special technique used in NMR spectroscopy to avoid splitting of signals by
eliminating partially of fully observed coupling.
Involves irradiation of proton with sufficiently intense radio frequency energy,
so it prevents the coupling with neighboring proton & gives spectral line as a
singlet.
Helps to determine structure of chemical compounds
Used to simplify a complex NMR spectrum
Possible to irradiate each coupled protons in molecule to produce less complex
spectrum.
It causes multiples to collapse to a doublet or singlet to give easily interrupt
spectra
Important phenomenon used to enhance spectral signals to get clear distinctions
between certain signals

8. Coupling & decoupling effect

9. Excitation modes in NMR
• Sample is irradiated with radiofrequency waves of appropriate
frequency
• will excite transitions from the ground to the excited state due to the
interaction of magnetic dipole with the oscillating magnetic field
component of the electromagnetic radiation.
• This excitation field must be orthogonal to the direction of the magnetic
dipoles to generate transitions of the nuclear spin state.

10. Inverse detection
• This technique become increasingly popular due to software improvements of
spectrometers from one hand and to the increase of sensitivity of the detection
of a X nucleus by the indirect way.
• In fact, it solves the problem of low concentrations.
• Moreover, it allows to reach the NMR parameters such as chemical shifts,
coupling constants & relaxation time spin lattice for nuclei that are impossible
to study by the direct detection.
• arrows figure the magnetization transfer from proton towards 13C and then
from 13C towards the proton.

11. Excitation modes

12. Data Acquisition
NMR experiments generally require the co-addition of a number of the nuclear spin
response.
Radiofrequency pulse and receiver phases plays role in it.
The data are recorded as a set of complex numbers which sample NMR signals as a
function of time.
Early Fourier transform spectrometers had limited word length and required some
scaling of the data before co addition, but this is no longer necessary.
The use of DSP gives better filtration of unwanted noise and signals, better effective
ADC resolution, and less baseline distortion.
In two-dimensional (2D) NMR a series of FIDs is acquired using a pulse sequence.
3D and 4D NMR extend the principle to two and three evolution periods respectively,
but time constraints limit the number of samples in each dimension more severely.
Also increase the pressure to use non-uniform sampling of the signal. A very effective
alternative way is to use methods where high resolution is not required

13. Spectrum Generation
 To make the raw experimental data interpretable they must be converted into a
frequency domain spectrum. The classical, and commonest, method is discrete
Fourier transformation. This is a linear operation. the information content of the
data is unchanged here.
In Fourier processing a weighting function is usually applied to the time
domain data before transformation. After weighting and any zero-filling, the
fast Fourier transform (FFT) algorithm is used to produce the frequency
spectrum. The resultant spectrum consists of a set of complex numbers
sampling.
In 2D NMR, weighting and zero-filling are carried out on the FIDs as normal,
but after Fourier transformation the data matrix is transposed to give a set of
‘inter ferograms.

14. •