Problem of the Month: October 2020

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1 1 1 11 1 Integral 102 104 106 108 110 112 116 118 120 122 124 13

C

7.8 7.6 7.4 7.2 1 7.0 6.8 6.6

H

ppm 7.30 7.25 7.20 1 2 0 1 2 2 1 2 4 1 1 1

The HSQC looks a bit crowded due to the inset enlarged area. This was necessary due to the close proximity of two proton

multiplets. Carefully identify the six cross peaks (all are positive).

H,C HSQC @ 600.52/151.02 MHz 102.65 111.07 119.87 120.78 122.03 124.17 enlarged part of the HSQC

Problem of the Month:

October 2020

Deduce the structure of a compound with the molecular formula C₈H₇N and try to assign all

chemical shifts and coupling constants

Special challenge!

If you inspect the broad proton signal at about 8.08/8.13 ppm very closely, you see two low-intensity

„bumps“ at about 8.04 and 8.23 ppm. Try to explain!

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8.4 8.3 8.2 8.1 8.0 7.9 7.8 7.7 7.6 1 7.4 7.3 7.2 7.1 7.0 6.9 6.8 6.7 6.6 ppm

H

4 6 4 7 .0 3 4 6 4 6 .1 0 4 6 4 5 .08 4 6 4 4 .1 1 4 6 3 9 .1 5 4 6 3 8 .2 1 4 6 3 7 .1 5 4 6 3 6 .2 0 H z 3 9 8 2 .4 7 3 9 8 1 .5 1 3 9 8 0 .4 6 3 9 7 9 .3 7 3 9 7 8 .3 2 3 9 7 7 .24 3 9 7 6 .2 4 4 3 3 1 .9 9 4 3 3 0 .9 8 4 3 2 4 .9 7 4 3 2 4 .0 2 4 3 2 3 .0 7 4 3 1 7 .0 8 4 3 1 6 .0 4 H z

1

1 1 1

1

Integral 1_{H NMR spectrum} recorded at 600.52 MHz Please note:

Some of the proton multiplets have been enlarged here. For the assignment of the enlarged multiplets, please refer to their

colours.

October 2020

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4 4 6 8 .1 7 4 4 6 7 .2 4 4 4 6 6 .3 0 4 4 6 5 .3 7 4 4 6 0 .0 4 4 4 5 9 .0 9 4 4 5 8 .1 5 4 4 5 7 .2 1 H z 4 3 4 1 .8 4 4 3 3 9 .4 7 4 3 3 8 .6 2 4 3 3 6 .2 0 H z 4 3 7 5 .4 3 4 3 7 4 .1 9 4 3 6 8 .42 4 3 6 7 .2 5 4 3 6 6 .0 5 4 3 6 0 .2 7 4 3 5 9 .0 4 H z 8.4 8.3 8.2 8.1 8.0 7.9 7.8 7.7 7.6 1 7.4 7.3 7.2 7.1 7.0 6.9 6.8 6.7 6.6 ppm

1

Integral 1_{H NMR spectrum} recorded at 600.52 MHz Please note:

Some other proton multiplets have been enlarged here. For the assignment of the enlarged multiplets, once again please refer to their colours.

October 2020

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8.00 8.05 8.10 8.15 8.20 8.25 8.30 4 1 2 0 .2 6 4 1 1 9 .4 3 4 1 1 7 .3 3 4 1 1 5 .7 6 4 1 1 4 .9 5 H z 4 0 2 2 .7 3 4 0 2 1 .2 3 4 0 2 0 .4 4 4 0 1 9 .0 3 4 0 1 8 .2 6 H z 1_{H NMR spectrum} recorded at 500.14 MHz Broad signal at 8.08 ppm (600.52 MHz) in detail

Usually one ignores the marked "impurities". It is not trivial to record these signals, but they offer helpful pieces of information.

(There is a slight difference in the chemical shift compared to the spectrum given in the previous slides. This is the spectrum of a separate sample with slight differences in concentration and temperature. And it’s in a different magnetic field)

October 2020

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6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0 1

H

8.0 7.8 7.6 7.4 1 7.2 7.0 6.8 6.6 ppm

H

H,H TOCSY @ 600.52 MHz using 10ms mixing time Sometimes a TOCSY with an

unusually short mixing time is a good substitute for a COSY.

This is especially true, if you want to exclude cross peaks due to small (four and five bond) coupling constants in unsaturated spin systems, i.e. aromatics.

In such cases a TOCSY with a short mixing time only shows correlations between protons separated by two or three bonds (assuming other homonuclear coupling

constants are not abnormally large).

October 2020

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7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.2 7.3 7.4 7.5 7.6 7.7 7.8 1

H

7.3 7.2 7.1 7.0 6.9 6.8 6.7 7.3 7.2 7.1 7.0 6.9 6.8 6.7

Due to the previously mentioned proximity of two proton multiplets the TOCSY is also a bit crowded.

Fortunately it’s possible to separate two spin systems almost perfectly. Almost …

enlarged TOCSY sections

October 2020

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4 9 0 8 .0 4 8 11 .1 4 4 6 7 .4 4 4 5 2 .2 4 3 4 8 .5 4 3 2 7 .6 3 9 7 8 .9 H z 122 123 124 125 126 15

N

8.4 8.2 8.0 7.8 7.61 7.4 7.2 7.0 6.8 6.6 ppm

H

A not very often recorded spectrum…

Due to the limited number of nitrogen atoms within the molecular formula, the number of peaks inside the HMBC is rather limited.

Nevertheless this HMBC is one very helpful key to get an unambiguous constitution.

Especially have a look at the area around 8.1 ppm (1_H).

The cross peak with a proton chemical shift of about 7.2 ppm is a little bit ambiguous. It’s very helpful to get reliable values for this shifts using the peak labels in the slice.

H,N HMBC @ 600.52/60.86 MHz

The only slice from this HMBC

October 2020

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100 105 110 115 120 125 130 135 13

C

7.8 7.6 7.4 7.2 1 7.0 6.8 6.6

H

ppm H,C HMBC @ 600.52/151.02 MHz Often the HMBC contains a

crazy amount of pieces for the structural puzzle.

Even in the case of simple compounds it looks like a part of the milky way.

Fortunately at this stage you should already have a lot of information about the

structure coming form other measurements.

Only a few pieces of

information from this HMBC should be necessary to finish the solution of this problem.

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127 128 129 130 131 133 134 135 136 137 13

C

7.8 7.7 7.6 7.5 1 7.4 7.3 7.2

H

ppm part of H,C HMBC @ 600.52/151.02 MHz

Because it is not possible for you to zoom through the whole HMBC looking for the key pieces of information, the essential part is presented here in expanded form. This selection of the key part of the HMBC spectrum simplifies the solution by

limiting the number of correlations that you have to consider. In the real world, you would have to make this selection yourself!

The vertical grey lines should help you to assign the correct cross peaks to the

adjacent proton multiplets around 7.2 ppm.

October 2020

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6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0 8.2 8.4 ppm 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0 8.2 8.4 1_H,1_{H NOESY spectrum} recorded at 500.14 MHz

The NOESY looks almost as crowded as the HMBC.

But don’t worry. You only need to be concerned with the

correlations that are

complementary to those you have already seen in the TOCSY, and they are well separated.

October 2020

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Contributions

Measurements Christine Schwarz, Wolfgang Eisenreich, Rainer Haeßner Spectrometer time TU Munich Discussions and native English language support Alan M. Kenwright Nils Schlörer Compilation Rainer Haeßner