HNMR or Proton NMR

Overview:

HNMR or Proton NMR is addressed in a series of topics including an overview, the number and position of signals, utilization of peak areas, splitting of signals, the coupling constant, and reading the NMR.

The basics of instrumentation are introduced in the overview with a description of the magnetic field requirements, the radio frequency needed, and the impact on a nucleus sensitive to these requirements. The concept of alignment of the nucleus with or against an applied magnetic field is considered, as is the impact of radio frequency of 60 mHz or 100mHz on that alignment. Terms such as shielded and deshielded are introduced as is the standard, tetramethyl silane.

Since protons with identical environments will absorb at the same magnetic field strength, but protons with different environments absorb at different field strengths, the concept of number of expected signals is developed. Students are led through a series of compounds to help them predict when the proton environments will differ.

The impact of electronegative groups on the chemical shift is introduced to help the student understand where signals are expected to appear. Proximity to an electronegative group and the relative extent of deshielding are correlated. The deshielding effects of aromatic and unsaturated structures are also correlated with position of signals.

To determine the number of protons represented by the various signals, it is necessary to develop the information to be gained from the area under the respective peaks. Spectral examples frequently include the electronic integrator and it is important that the way in which relative numbers of protons can be determined from these integrator signals be described.

Splitting patterns are addressed extensively and the reason for the N+1 number of peaks explained. Students learn how to predict the number of protons represented by each set of peaks and to logically determine which groups are adjacent. The concept of the coupling constant, J, is addressed, particularly from its use in determining cis and trans orientations and to identify signals from groups generating splitting patterns of neighboring groups.

A series of five solved problems help the student recognize classic patterns such as the isopropyl group, as well as a few specific functional groups. Splitting and position of signals are shown for disubstitution patterns for benzene. How to recognize overlapping signals is mentioned and signal symmetry stressed. The way in which spectra combination such as IR and NMR can be used for identification of unknowns is used in several examples.   The classes of compounds addressed prior to the NMR study are included in this unit.  Future classes studied each contain a section of spectral analysis.