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H’NMR Spectrum for Caffeine
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Empirical Formula: C8H10N4O2
Molecular Weight: 194.1906
Nominal Mass: 194 Da
Average Mass: 194.1906 Da
Monoisotopic Mass: 194.080376 Da

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The HNMR spectrum for caffeine is clearly in D2O. There is a strong singlet at 4.8. Therefore, no protons on hydroxyl or carboxyl groups will be visible in the HNMNR spectrum.

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This methyl group is bound to nitrogen that is adjacent to two carbonyl groups, one on each side. This structure will show a singlet at approximately 3.2ppm. The hydrogen’s are bound to a carbon that is bound to a nitrogen, which would fall in the 3-4 range of protons bound to a carbon that is bound to a halogen or oxygen. Counting two to three bond lengths away, there are no hydrogen’s that can couple with the protons on the methyl group. Following the N+1 rule, the peak would be a singlet.

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The single hydrogen on the cyclopentene structure will be a singlet at approximately 7.9ppm. The proton is bound to a carbon that is double bound to nitrogen. Normally, vinyl protons fall in the 5-6 range. However, since the carbon is double bound to nitrogen, it may account for the higher ppm in this particular peak. There is no proton that this specific hydrogen can couple with. After counting two to three bond lengths away, there is no other proton. Using the N+1 rule, this would be a singlet. This specific hydrogen will show a singlet at 7.9ppm.

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The methyl group that is bound to the nitrogen in the cyclopentene structure will show a singlet at approximately 3.9ppm. The range for a proton bound to a carbon that is bound to a halogen or oxygen is 3-4. However, this particular methyl group is bound to nitrogen that is adjacent to a vinyl group on each side. This particular methyl group has a higher ppm number than the two other methyl groups. Vinyl groups fall in the 5-6 range and this would raise the ppm number of this particular methyl group. Again, counting two to three bond lengths away, there is no proton to couple with. Using the N+1 rule, this would be a singlet. Therefore, this would be a singlet at 3.9ppm.

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The final methyl group bound to the nitrogen is adjacent one carbonyl group and one vinyl group. This peak is a singlet at 3.4ppm. The range for a proton bound to a carbon that is bound to a halogen or oxygen is 3-4. The range for a vinyl group is 5-6. This peak is slightly higher than the methyl that is adjacent to two carbonyl groups because this specific methyl group is adjacent to one vinyl group. Counting two to three bond lengths away, there is no other proton for coupling. Therefore, using the N+1 rule, this would be a singlet.