How To Draw Structure From Nmr Data

Make up one's mind Construction with Combined Spectra

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At that place are a many ways we tin use NMR spectroscopy to analyse compounds. 1 common application is in determination of an unknown structure. Given the MS, IR, ¹³C and ¹H NMR spectra, what might be the structure of an unknown sample?

It is often easiest to kickoff with the IR spectrum.

identify at least 3 peaks in the IR spectrum. Which peaks seem to tell you the about information near this compound?
don't call up with your head; think with your hands. Write down ideas on the spectrum.
if y'all are working on a formal proof of construction, on a form test or a lab written report, yous may be required to enter your data in a table correlating wavenumber with height assignment:

For example, a pupil might obtain the following IR spectrum.

From that information, she constructs the post-obit table. She might even write this tabular array, by hand, directly on her spectrum. She makes useful notes on the edges, and might even include some guesses, which she later crosses out, just does not erase. She is assisted in this task past consulting an IR table, that suggests what some of these peaks might mean.

Remember:

brand special notation of what atoms are present in the compound: C, H, N, O...
also note your initial ideas about specific functional groups that may be nowadays.
if you are unsure of an assignment, put a question mark abreast it to signal this uncertainty.
some information may demand to be discarded later if it is not consistent with other data.

Look at the ^xiiiC spectrum.

How many unlike carbons are there, based on the number of peaks in the spectrum? This is the first pace in estimating the molecular formula.
Do you accept reason to believe there is symmetry in the structure? In the unabridged compound or just part of it? Adjust the number of carbons you think y'all are dealing with.
As in IR spectroscopy, brainstorm assigning peaks, either on the spectrum or, if required, in a tabular array:

For example, a student might obtain the following ¹³C NMR spectrum:

From that data, she puts together the post-obit table:

Think:

you will be able to assign all peaks in the NMR spectrum, not just a few like in IR.

You can get more data on the formula from the ^oneH NMR spectrum and the mass spectrum.

In the ¹H NMR spectrum, what does the sum of the integrations propose most the number of hydrogens?
At this betoken y'all may accept a formula, C_tenH_yO_z. What would exist the mass of a compound with this formula?
Compare this mass to the mass spectrum. Does it match?
If non, consider whether a common atom (such as oxygen) is missing from your formula, or if there might exist symmetry that you lot have missed.
Does the mass spectrum suggest the presence of a nitrogen (odd molecular weight) or chlorine or bromine (isotope pattern)?

Once you accept the molecular formula, the number of possible structures is automatically express. The number of units of unsaturation can assistance y'all narrow down the possibilities.

The ratio of C : H in a saturated, acyclic hydrocarbon is northward : 2n+two.
Each pair of H missing from the formula corresponds to a multiple bond or a ring.
The presence of oxygen (which is divalent) does not change the C : H ratio.
The presence of halogen (which is monovalent) means there is one H missing.
The presence of a nitrogen (which is trivalent) ways there is an extra H in the formula.
In other words, C₄H₈O has one ring or double bond just similar C_fourH₈, and so do C_fourH₇Br and C₄H₉North.

As in ¹³C NMR, you should be able to assign all peaks in the ¹H NMR spectrum. You may exist able to do so by making notes on the spectrum. If you retrieve you know the structure, you may be able to draw information technology and notation which peak belongs with which proton.

A formal proof of structure might require a table of assignments.

ppm	int	mult	partial construction	assignment

This table demonstrates your ability to read the spectrum. Can you lot decide what ratio of protons is suggested by the integral line? Tin can you decide whether a height is a quartet?
The partial construction column should explain the shift, integration and multiplicity for the superlative in that row. It should non show any other data from elsewhere in the structure. This restriction forces you to demonstrate a thorough understanding of the data in a way that "getting the right respond" does not.
The partial structure cavalcade is best filled in with drawings, not words. The cartoon is a partial structure.
Considering the partial structure will show the protons absorbing at the shift in that row likewise the neighbouring protons, you need to distinguish between them in your picture. Most people circle or underline or make assuming the protons that show up at the shift given in that row.
When finished with the partial structure column, yous should be able to link the partial structures together to make an entire structure in the assignment column.

An instance of a spectrum and its accompanying data table is given below. Here is the spectrum:

Hither is a information tabular array:

Things to note:

This student has used two integration columns instead of merely one.
The first column shows the integral measured from the spectrum. She probably used a ruler.
The 2d column, which she called int(due north), contains a convenient ratio taken from the raw data. This ratio is easier to use in her assignments.
Also note that the meridian at 9.seven ppm does not take a very skilful integral. There is either a "phasing" or a "level & tilt" problem here that tin be corrected using the NMR software, but this is sometimes difficult to do. If she had taken an automatic printout of this integral measurement, she would have gotten a foreign number; in this case, it would be near -five, because the stop of the integral line is lower than the starting time. It clearly isn't a negative number of hydrogens, though. She has instead measured the vertical rise in the integral and recorded that; information technology isn't perfect, but is a off-white estimate in this instance.

How To Draw Structure From Nmr Data,

Source: https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Magnetic_Resonance_Spectroscopies/Nuclear_Magnetic_Resonance/NMR%3A_Structural_Assignment/Determine_Structure_with_Combined_Spectra

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