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¹H NMR Spectral Problems

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Index:

How to Manipulate Spectra

How to Expand Spectra

How to Measure Coupling Constants

Tips on Solving Proton Spectra

Peak Multiplicity

How to Manipulate Spectra:

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 then 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. 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 and 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 the mouse button and drag the cursor to the right side of the peak(s) and down. Release button. You should see four peaks, a quartet. This procedure can be repeated before refreshing to the original spectrum.

How to Measure Coupling Constants:

Expand the downfield signal in the spectrum. You can do the expansion as many times as you choose. You should see a quartet. Place the cursor in the center of the quartet. The chemical shift is δ 4.103. (The value appears in red in the upper right hand corner. Values may vary depending on the exact placement of the cursor.) Place the cursor over the left hand peak (δ 4.145). Double click. A vertical red line appears with the distance to the nearest peak (7.0 Hz). Measure the chemical shift of the nearest peak (δ 4.116). The difference in chemical shift of the two peaks is 0.029 ppm. Since the spectrum was recorded at 250 MHz, the coupling constant J = 250 Hz/ppm x 0.029 ppm = 7.25 Hz. The values by the two methods are quite close. Expand the high field signal. Is it coupled to the quartet?

Tips on Solving ¹H NMR Spectra:

Determine the Degree of Unsaturation of the compound from the molecular formula.
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 always 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) M	Description	Pattern
0	1	singlet (s)	1
1	2	doublet (d)	1:1
2	3	triplet (t)	1:2:1
3	4	quartet (q)	1:3:3:1
4	5	quintet (qnt)	1:4:6:4:1
7	6	sextet	1:5:10:10::5:1
6	7	septet	1:6:15:20:15:6:1
7	8	octet	1:7:21:35:35:21:7:1
8	9	nonet	1:8:29:56:70:56:29:1