Toggle Grid turns the spectrum grid on and off. Reverse Plot flips the spectrum. To expand the spectrum, hold the mouse button down,
sweep over the portion of the spectrum you wish to expand and the
release the button. To reset to the original spectrum, click the
Refresh Spectrum button once. To read
chemical shifts, place the cursor on the peak and click. The
chemical shift and intensity (x-axis, y-axis) will appear in the
upper right hand corner of the spectrum. The blue arrows to the right of the spectrum increase/decrease the spectrum intensity.
How to Expand
Spectra (Zoom):
In the spectrum below, place the cursor to left of the peak at
δ 4.1. Depress mouse button and drag the cursor to the right
side of the peak. Release button. You should see four peaks, a
quartet. This procedure can be repeated before resetting to the
original spectrum.
a
Reverse plot
Toggle Grid
How
to Measure Coupling Constants:
Expand the signal at δ 4.1. You should see a quartet.
Place the cursor over the left peak. The value is δ
4.145. Now measure the shift of the peak to the right of δ
4.1. Its shift is δ 4.118. peak. The difference in
chemical shifts is 0.027 ppm. Since the spectrum was recorded
at 250 MHz, the coupling constant J = 250 Hz/ppm x 0.027 ppm =
6.75 Hz.
Is the number of protons in the spectrum the same or fewer
(half?) than in the molecular formula? If the latter is true, look
for symmetry in the structure.
What is the significance of the chemical shifts of the
signals? Use the chemical shift
chart.
Coupling Patterns
Who is coupled to whom? Check coupling constants.
Are there recognizable patterns? Ethyl, isopropyl,
etc.
Is the coupling pattern due to a non-exchanging -OH or
-NH2?
Impurities
Some spectra may contain impurities. Don't include these
signals in your interpretation. They will not be included in
the integrated area list.
CHCl3 absorbs at ~δ
7.27; Tetramethylsilane (TMS) absorbs at ~δ
0.00
Peak
Multiplicity:
The chart below lists the simplest coupling patterns for an
observed hydrogen given the following conditions. The coupling nuclei
must all have a spin of 1/2, which hydrogen has. The coupling
constants must be equal. Homotopic and enantiotopic protons qualify
for these patterns but diastereotopic protons may or may not qualify
depending upon the instrument's ability (field strength) to resolve
chemical shifts that are very close to one another. Invariably, the
coupling is between hydrogens on vicinal (adjacent) carbons. The
hydrogens need not be all on the same carbon (otherwise, you could
never exceed a quartet). This value of J is ~7 Hz whereas geminal
same carbon (gemini, twin) sp3-sp3 coupling is
~10 Hz. The geminal hydrogens would have to be diastereotopic to have
different chemical shifts and , consequently, be able to couple. The
formula, M = 2SN + 1, is helpful to
determine the multiplicity (M) of a signal independent of the spin
(S) of the coupling nuclei [S = 1/2, 1, etc]. For hydrogen,
where S = 1/2, the formula reduces to M = N +
1. Thus, the multiplicity is aways one greater than the number
of hydrogens that are coupled.
Number of coupled H's (J's equal)
N
Number of peaks for
observed nuclei (multiplicity)