🇺🇦 nmr_spectroscopy / organic chemistry ⌬

As you know, the presence of 1% of the ¹³C isotope in symmetric molecules can be used to determine their structure by analyzing the multiplicity of the corresponding ¹³C satellites. In particular, the figure shows an example of using the NMR homodecoupling technique to simplify the structure of ¹³C satellites in a symmetric alkene.
#spin #coupling #isomerism

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It’s #NMRweekend ⁉️🧲 Use the poll below 😊. Note: This is an experimental spectrum. The explanation for the challenge will be published in a few days 😉.
#nmrchallenge #nmrquiz

UPD ✅. The correct answer is C: The pyridine protons have a spin coupling of 4.3 Hz, which is only consistent with this structure. In the case of structures A or B, this spin coupling would be approximately ~8 Hz and ~2 Hz, respectively.

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🧲 ᗑ Multiplet of the Week. The CH₂ group is diastereotopic, so there are three different couplings to this particular proton: two from the CH₂-O group and one from the methyl group.
#spin #coupling

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It’s #NMRweekend ⁉️🧲 Use the poll below 😊. Note: This is an experimental spectrum. The explanation for the challenge will be published in a few days 😉.
#quiz #nmrchallenge

UPD ✅. The correct answer is C. This is the only structure that could have a doublet with a 2.3 Hz coupling (7.07 ppm, meta-coupling).

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It’s #NMRweekend ⁉️🧲 Use the poll below 😊. This is an experimental spectrum. The explanation for the challenge will be published in a few days 😉
#nmrchallenge #quiz

UPD ✅ The correct answer is B. There are two signals in the aromatic region, forming a complex AA’BB’ spin system. Structures A and C would have one and three signals in the aromatic region, respectively.

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Yet another extravagant molecule! Enamine LTD chemists are astonishing us with nortricyclene core. Find this Building Block at the Enamine store.
#enaminecompany

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It’s #NMRweekend ⁉️🧲 Use the poll below. The question is about protons of each O-CH₂ group‼️
#nmrchallenge #quiz

UPD ✅ Each O-CH₂- group is enantiotopic. The protons of these groups are diastereotopic. Since I formulated the question slightly inaccurately, let’s assume that there are two correct answers here (B and C).
Why are these protons diastereotopic? Try replacing each of them with a substituent (for example, Cl), and you will see that a pair of diastereomers is formed. Therefore, the corresponding protons are diastereotopic.

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It’s #NMRweekend ⁉️🧲. Use the poll below 😊. This one is relatively tough. Note: This is an experimental spectrum. The explanation for the challenge will be published in a few days 😉.
#nmrchallenge #quiz

✅ UPD: The correct answer is B. As you can see from the ¹H spectrum, each multiplet has four completely different couplings: 10.2, 8.7, 12.1, and 7.4 Hz. This means that all these couplings are either ³J or ⁴J H-F. None of them corresponds to J(HH), which is only possible for structure B (⁵J(HH) ≈ 0 Hz) and not for A (³J ≈ 6–8 Hz) or C (⁴J ≈ 1.5–2 Hz).

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An elegant small cyclobutane with hellish multiplets 🤯. Enamine LTD chemists never fail to impress! Check out new.enaminestore.com for this molecule: EN300-1936715.
#spin #coupling #fluorine

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One more piece of useful information on ¹⁹F NMR of fluorinated aromatics.
#fluorine #spin #coupling

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It’s #NMRweekend ⁉️🧲 Use the poll below 😊. Note: This is an experimental spectrum. The explanation for the challenge will be published in a few days 😉.
#nmrchallenge

✅ UPD: The correct answer is C. The aromatic protons’ splitting pattern rules out structure A. The presence of two chemically equivalent methyl singlets rules out structure B.

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Part V of our “NMR Basics” series: the HMBC experiment. Here, we cover only the most common principles of this experiment, which is far more complex and has many variations.
#HMBC #nmrbasics (use the last tag to find all other parts)

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It’s #NMRweekend ⁉️🧲 Use the poll below 😊. This is an experimental spectrum. The explanation for the challenge will be published in a few days 😉.
#nmrchallenge

✅ UPD: The correct answer is B.
Each molecule is symmetrical and has only one signal for the CH(sp²) proton, which is a singlet in every case. However, the presence of 1% of the ¹³C isotope breaks the symmetry of the molecules, allowing us to analyze the ¹³C satellites.
In our case, the splitting of the ¹³C satellites is ~16 Hz, which is typical E-coupling for alkenes (molecule B). In the case of C, we would observe Z-coupling with J ~12 Hz. For A, there would be no splitting of the ¹³C satellites because both protons are bonded to the same carbon.


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Let’s start 2025 with some useful NMR tricks! ☝️

Here, you can observe the effect of trifluoroacetic acid (TFA) on the ¹H NMR spectrum of an NH-tautomeric heterocycle. Adding TFA significantly narrows all the signals, allowing the multiplicities to be clearly resolved. 🎯

This happens because TFA protonates the nitrogen atoms, suppressing tautomerism. As a result, the interpretation of ¹H NMR spectra, as well as 2D experiments, becomes much simpler and more convenient. 🧪

Follow for more NMR tips and tricks! 😉

#nmrtips #acid #spin #coupling

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Happy New Year, NMR community 🎉🍾
🎄🔬 ¹H-¹³C HSQC Spectrum of a Christmas Tree Extract 🎅✨
This festive HSQC spectrum highlights the diverse range of compounds present in the methanol extract of a Christmas tree.
The spectrum is rich with signals corresponding to primary and secondary metabolites commonly found in Christmas tree, such as:
• Terpenes: The backbone of essential oils, contributing to the tree’s characteristic aroma.
• Lignin-derived aromatics: Building blocks of the tree’s structural integrity.
• Sugars and polyols: Indicators of energy storage and osmoregulation in plant cells.
In the spectrum.
🎄 #HSQC #christmastree

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It’s #NMRweekend ⁉️🧲 Use the poll below 😊. This is an experimental spectrum. The explanation for the challenge will be published in a few days 😉.
#nmrchallenge #quiz

✅ UPD: The correct answer is C. The molecule has three CH₃ groups: CH₃(Ph) at 2.18 ppm, CH₃(CH) as a doublet at 1.36 ppm, and CH₃O at 3.83 ppm. Additionally, the molecule has one acidic signal at 1.50 ppm.

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Did you know that you can observe ¹H-¹⁴N coupling not only in highly symmetrical ammonium salts, but also in isonitriles?
In most compounds containing nitrogen, even including some in which hydrogen is bonded directly to nitrogen, the electric field gradient about the ¹⁴N nucleus is sufficiently strong to decouple either partially or completely the ¹⁴N and ¹H spins through quadrupole relaxation. The fact that splitting is observed with isonitriles indicates that the electric field gradient is unusually small and that the spin-lattice relaxation time of ¹⁴N must be comparable to the coupling constant. The low electric field gradient must be due to the axial symmetry of the electron density near the nitrogen atom. DETAILS: dx.doi.org/10.1063/1.1732106

Here, you can also see a great example of using the HOMOdecoupling technique: we selectively irradiate the signal at ~2.0 ppm to remove its coupling with the signal of interest at 3.37 ppm.
#spin #coupling #nitrogen

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It’s #NMRWeekend‼️ The purpose of this post is not only to present a quiz but also to demonstrate the importance of fluorine decoupling (the spectrum on the right) during the ¹H acquisition of fluorinated aromatics. Use the poll below 👇.
#fluorine #spin #coupling
NOTE: Fluorine spectrum is proton decoupled!

UPD ✅ The correct answer is “C.” It’s the classical spin-coupling pattern for an ortho-substituted phenyl ring: doublet-triplet-triplet-doublet (~7-8 Hz coupling each) plus smaller (~1-2 Hz) couplings. Some of you may have been confused because one of the multiplets overlapped with the chloroform signal.


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Why are paramagnetic samples a challenge for NMR? 🧲🔬
In the presence of paramagnetic impurities, the lifetime of the exited spin states is shortened due to interactions with the fluctuating magnet fields that arise from unpaired electrons. This significantly impact your NMR experiment:
🔹Shorter T₁ and T₂ relaxation times: signal broadening and peaks vanishing.
🔹 Distorted Chemical Shifts: Paramagnetic interactions can shift signals far from their expected positions, complicating analysis.
Because the magnetic moment of an unpaired electron is larger than the nuclear magnetic moment by a factor of about 1000, this mechanism is very effective and paramagnetic compounds can normally not be measured by NMR. Even the presence of trace amounts of oxygen, a paramagnetic molecule, shows the line broadening effect and for a precision of better than 0.1 Hz the sample tubes have to be
degassed on the vacuum line and sealed.
Some common types of paramagnetic contaminants include: Transition Metals: Examples include iron (Fe²⁺/Fe³⁺), manganese (Mn²⁺), cobalt (Co²⁺/Co³⁺), nickel (Ni²⁺), and copper (Cu²⁺). Lanthanides: Such as gadolinium (Gd³⁺), dysprosium (Dy³⁺), and europium (Eu³⁺), often used in NMR as relaxation agents or shift reagents but problematic as contaminants. Free radicals, such as those generated in chemical reactions (e.g., organic radical species), can act as paramagnetic contaminants.

🌌In this ¹H spectrum, you can clearly see how the peaks are broadened and distorted, rendering the data nearly unusable!

💡 NMR of paramagnetic compound has a lot of practical aspects though. See the following links for example:
Link 1
Link 2
(or Google “NMR of paramagnetic compounds”)

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Is there a way to calculate the spin coupling constant between equivalent nuclei❓ Yes, there is ❗️For this purpose, you need the corresponding deuterated isotopomer and a well-recorded ¹H NMR spectrum☝️ Details are in the picture.
#spin #coupling

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It’s #NMRweekend ⁉️🧲 Use the poll below 👇. This is an experimental spectrum. The explanation for the challenge will be published in a few days 😉.
#nmrchallenge #quiz

✅ UPD: The correct answer is A.
1. The aromatic protons’ spin coupling constant of 8.3 Hz eliminates B.
2. The spin coupling patterns of the aliphatic CH₂ groups show that they are all in the same spin system, which eliminates C.

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Five interesting facts about GABA:
1. Key Role as a Neurotransmitter: GABA (Gamma-aminobutyric acid) is the brain’s primary inhibitory neurotransmitter 🧠⚡. It keeps neurons calm by reducing excitability, preventing overstimulation, and promoting balance. ⚖️
2. Diverse Receptors: GABA interacts with two main receptor types: GABA-A 🌀 (fast-acting ion channels) and GABA-B 🕰️ (slower, modulatory receptors). Together, they fine-tune the nervous system’s responses.
3. Not Just in the Brain: While famous for its role in the central nervous system, GABA is also active in the gut and pancreas 🩺🍴, where it regulates insulin secretion and gut motility.
4. Unique Amino Acid: GABA is a non-proteinogenic amino acid 🧪—unlike most amino acids, it doesn’t form part of proteins. Instead, it is synthesized from glutamate by the enzyme glutamic acid decarboxylase (GAD). 🔬
5. A Key to Calmness: Many anti-anxiety medications, like benzodiazepines, work by enhancing GABA’s effects 🛌💊. This makes GABA pathways vital for treating stress, epilepsy, and sleep disorders.
#gaba #natural #products

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The Multiplet of the Week 🧲:
For those who wanted a more complex challenge from me: is this particular proton in a cis or trans configuration relative to the cyclobutane ring?

✅ UPD. Cyclobutane is cis to the proton 2,08 ppm. The explanation is on stories and/or the “posts” tab of the channel.

#nmr #nmrchat #nmrchallenge #spin #coupling #multiplet

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It’s #NMRweekend ⁉️🧲 Use the poll below 😊. This is an experimental spectrum. The explanation for the challenge will be published in a few days 😉.

✅ UPD: The correct answer is B. (1) CH₂ group is bonded to Oxygen (4.11 ppm). (2) Para-substituted Phenyl ring (AA’BB’ spin system).

#nmrchallenge #nmrweekend #quiz

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Caption to the last challenge is updated. Here are some details

It’s #NMRweekend ⁉️🧲 Use the poll below 😊. This is an experimental spectrum. The explanation for the challenge will be published in a few days 😉.
HINT: pay attention to couplings.
#nmrchallenge #quiz

✅ UPD: The correct answer is C.
Structure A doesn’t have singlets in ¹H NMR. Between structures B and C, only C can contain an AB spin system with a ³J coupling constant of 8.6 Hz (check out my old post on H-H spin couplings in pyridine or Google it 😉).


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Menthol molecule. Multiplet of the week❗️This is another example where multiplet analysis based on n+1 first order rules produces an ambiguous 'm' annotation while iterative quantum mechanical spectral analysis is able to extract all the relevant coupling constants in full automation.
The spectrum was recorded at Enamine Ltd. and calculated at NMR Solutions Ltd.
#spin #coupling

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Here’s the ¹H-¹³C HSQC spectrum of a methanol extract from autumn leaves, recorded in DMSO-d₆. 🍁✨ The complex array of signals reflects a fascinating mixture of compounds, including:
🌿 Phenolic compounds: often linked to antioxidant properties.
🌈 Flavonoids: contributing to vibrant colors and potential health benefits.
🍬 Sugars and organic acids: essential for plant energy and defense pathways.
💪 Amino acids: vital building blocks of proteins, likely contributing to metabolic activity and stress responses in the leaves.
🔬 NMR spectroscopy unveils the intricate chemistry behind the beauty of autumn leaves, showcasing the diverse metabolites that drive plant physiology and their roles in nature’s seasonal artistry.
☝️As you can see, using of the HSQC experiment allows significantly simplify the interpretation of the spectrum 🤌
Follow for more natural products NMR 😉
#naturalproducts

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It’s #NMRweekend ⁉️🧲 Use the poll below 😊. Note: This is an experimental spectrum. The explanation for the challenge will be published in a few days 😉.
#nmrchallenge #quiz

✅ UPD: The correct answer is B: meta-coupling between aromatic protons, a phenyl-bonded CH₃ group, and an OH proton at 8.74 ppm.

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Among all the CH and CH₂ protons in menthol, this one is the simplest 😬.
#spin #coupling

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The last #NMRWeekend question requires a detailed answer 😉.
‘A’ is ruled out because of the chemical shift of the Me group. OMe should appear at ~3.5 ppm.
‘B’ is excluded due to the presence of a proton with δ 9.09 ppm, showing only two couplings: 2.3 Hz (⁴J) and 0.9 Hz (⁵J). If it were ‘B,’ each proton would exhibit a ³J coupling with a minimum value of ~4 Hz for an alpha pyridine proton.
The correct answer is ‘C’. Details are provided in the picture.
#nmrchallenge

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It’s #NMRweekend ⁉️🧲 Use the poll below 👌😊. Note: This is an experimental spectrum. The answer will be published here in a few days 😉.
#nmrchallenge

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Another informative example of the solvent effect on ¹H NMR. As you can see, only in D₂O are all 8 aliphatic signals completely resolved, which was the key to solving the cis/trans problem. NOTE: both ¹R and ²R are groups without aliphatic protons.

#nmrtricks #solvent

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It’s #NMRweekend ⁉️🧲 Use the quiz below 😊. Note: This is an experimental spectrum. The explanation for the challenge will be published in a few days 😉.

✅ UPD: The correct answer was B: it’s the only molecule that is chiral and has all four CH₃ groups, as well as CH and CH₂ protons, chemically inequivalent.


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5 interesting facts about Menthol ❗️:
1. Cooling Sensation Mechanism ❄️: Menthol activates the TRPM8 receptor, giving a cooling effect without actually lowering temperature. That’s why it feels so refreshing on the skin and in the mouth!
2. Natural Occurrence and Sources 🌿: Found in peppermint and other mint plants, menthol makes up about 50-60% of peppermint oil, making it a primary natural source.
3. Stereochemistry 🔬: Menthol has multiple stereoisomers, but only one, (-)-menthol, delivers that distinct cooling sensation. Its unique structure is key to how it interacts with our senses.
4. Antimicrobial Properties 🦠: Menthol’s antimicrobial and antifungal qualities make it popular in oral care and hygiene products. It disrupts bacterial growth by penetrating cell membranes.
5. Role in Medicine 💊: Beyond its cool feel, menthol has analgesic effects, making it a go-to ingredient in pain-relief balms. It helps by desensitizing pain receptors on the skin.
#menthol #naturalproducts

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The multiplet of the week!
🧲You may notice the slight broadening of each component of this multiplet. The reason of it ⁴J to Ph protons.

#spin #coupling

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It’s #NMRweekend ⁉️🧲 Use the poll below 😊. Note: This is an experimental spectrum. The explanation for the challenge and the DOI of the original article will be published in a few days 😉.
#nmrchallenge #quiz

✅ UPD: The correct answer is ‘B.’
🔴 In ‘A,’ CH₂ would appear as a singlet, and CH would have a higher chemical shift.
🔴 In ‘C,’ the two CH₃ groups would be chemically inequivalent.
🟢 ‘B’ completely matches the spectrum: CH₂ at a relatively high ppm, CH at a lower ppm, and chemically equivalent CH₃ groups.
(Those are not all differences in spectra)
The DOI of the original article: 10.1002/adsc.201500912


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Happy halloween to everyone! 🎃👻
Here is a WET spectrum (CD₃OD, 600MHz) of methanol extract of pumpkin pulp. As you can see, all regions of the spectrum are fairly crowded with signals. To find out what substances are in pumpkins - visit https://www.compoundchem.com/2016/10/24/pumpkins/
#halloween #pumpkin

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It’s #NMRweekend ⁉️🧲 Today, you’ll use proton-decoupled ¹⁹F data to distinguish the compounds 🫢. Use the quiz below 👇 😊.
#nmrchallenge #quiz

UPD. ‼️The correct answer is “B.”
🔴A: Achiral molecule. Both fluorines appear as a singlet because they are chemically equivalent.
🔴C: Chiral molecule, but since there is a CF₃ group, all three fluorines are homotopic and appear as a singlet.
🟢B: Chiral molecule, which makes both fluorines chemically inequivalent, with a ²J coupling constant of approximately 279 Hz.


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A small, hard-to-synthesize spirocyclic amine developed by Enamine Ltd. chemists, with an elegant 🤌 ¹H NMR spectrum.
#enaminecompany

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Tobacco in the NMR Spotlight 🚬🔬🧲

Check out this ¹H NMR spectrum of a tobacco sample in DMSO-d6! 🎉
The spectrum is packed with signals, including nicotine 🧪, which is easily detected thanks to its characteristic peaks. But it’s not just nicotine—tobacco contains a variety of other organic compounds like alkaloids, sugars, and even small organic acids.

So, how can NMR help in quality control of tobacco products? 📊 It provides a molecular fingerprint of the sample, ensuring product consistency, monitoring for contaminants, and checking for the authenticity of the natural components. 🔍 In this spectrum, every peak tells a story about the different molecules present!

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It’s #NMRweekend ⁉️🧲. Use the quiz below 😊.
#nmrchallenge #quiz

✅ UPD. The correct answer was “C.” Key points:

1. Acidic signals at 9.9 ppm, indicating the presence of a COOH group.
2. Two inequivalent triazole protons, as the molecule is not symmetrical.

P.S. Many were surprised by the chemical shift of the N-bonded CH₂ group (4.40 ppm), but it’s actually quite typical for such methylene groups in triazoles, pyrazoles, and tetrazoles.

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🧲 ᗑ Multiplet of the Week. The interesting thing about this multiplet is that such small ³J couplings (1 Hz) are not typical for a small cyclopropane ring. Usually, ³J values are in the range of 5–10 Hz. The small ³J in this case is apparently a consequence of the presence of the heavy Br atom and two highly electronegative fluorine atoms.
#spin #coupling
Find this molecule at new.enaminestore.com | EN300-1600518

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It’s #NMRweekend ⁉️🧲. Use the quiz below 😊.
#nmrchallenge #quiz

UPD ✅: The right answer was B. This is the only structure which has Methyl bonded to Oxygen (3,88ppm).

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The problem I’ve recently encountered in my NMR practice. ❓The question concerned the mutual positions of a Fluorine atom and an -O-R substituent. ☺️It turned out that a proton-decoupled Fluorine-19 spectrum is sufficient in this particular case. ☝️If the Fluorine is near a CF₃ group, a strong ⁴J coupling is observed, unlike in the other case.
#fluorine #proton #spin #coupling

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It’s #NMRweekend ⁉️🧲. This week’s challenge is about N- and O-alkylation, a common issue in organic chemistry synthesis. To solve the problem, an HSQC spectrum (one bond H-C correlation) was recorded. Use the quiz below 😊.
#nmrchallenge #chemistry #quiz

ANSWER:
The right answer was A
The key - ¹³C chemical shifts of CH₂- 43,5 ppm (bonded to N)
and CH₃- 55,7ppm (bonded to O)

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¹H NMR of Milk 🧲🥛: This is the Proton NMR spectrum of milk without any specific sample preparation. 🔍 However, with modern NMR techniques and methodologies, it is now possible to analyze dozens of metabolites in this product. Milk metabolomics represents a highly promising field of study with the potential to impact primary producers 👩‍🌾, the dairy industry 🧀, and consumers alike 🛒. Furthermore, studies focused on identifying milk metabolomes are crucial for advancing our understanding of mammary gland physiology, food science 🍽️, dairy research, and animal science 🧬. Through insights into mammary gland metabolic activity, milk composition, and quality, NMR-based metabolomics has provided evidence of the significant role milk metabolites play in relation to nutritional value 🥛, technological properties 🧑‍🔬, quality control ✅, and bioactivity 🌱.
👌🤝The spectrum was recorded by Andrii Sviatnenko
ℹ️ For more details find the publication by Sundekilde et al. Metabolites 2013, 3, 204-222
#products #milk #metabolomics

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It’s #NMRweekend ⁉️🧲. Use the quiz below 😊.
PS: In this series of posts, we’ll consider not only ¹H-related quizzes but a wide range of NMR experiments, so stay tuned 😉.
#nmrchallenge #quiz

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Look at this, the Eye of Sauron 😈 — Multiplet of the week. Once again, my software failed to analyze it, but apparently, this signal has four different couplings to each chemically inequivalent nucleus. Find this Building Block at the Enamine store. ID: EN300-1936715.
#spin #coupling

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❗️An interesting molecule I’ve recently encountered in my practice.
🔗The CF₃ groups appear to have both strong ‘through-space’ and more classical ‘through-bond’ spin-spin coupling pathways.
#spin #coupling

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🍀🧪The smallest cycloalkane, cyclopropane, is a basic structure element in a wide range of naturally occurring compounds.
💊Especially, the 1,2,3-trisubstituted cyclopropanes are found widely in natural products and biologically active molecules.
ℹ️🛒Check this 1,2,3-trisubstituted cyclopropane. More datails: https://new.enaminestore.com

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❗️ 5 interesting facts about Riboflavin (Vitamin B2):
1. 💡Fluorescence: Riboflavin glows yellow-green under UV light, making it stand out in biological samples. Its fluorescence comes from its conjugated ring system, which absorbs UV light.
2. ⚙️Cofactor Role: Riboflavin is a precursor for the coenzymes FMN and FAD, which play a key role in cellular energy production, especially in the Krebs cycle and the electron transport chain.
3. ☀️Light Sensitivity: Riboflavin breaks down quickly when exposed to light, particularly UV rays. That’s why foods like milk, rich in riboflavin, are stored in opaque containers to prevent degradation.
4. 🌈Natural Colorant: Riboflavin is a bright yellow compound used as a natural food dye (E101). Its vibrant color adds a yellow tint to many food products and supplements.
5. 🦠Essential for Growth: Riboflavin is vital for cell growth and development. It helps the body break down carbohydrates, proteins, and fats, and is especially important for maintaining healthy skin, eyes, and nervous system function.
#riboflavin #compound

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🧲 Multiplets of the week. Unfortunately, MestReNova failed to accurately analyze these multiplets. Each of the protons in this case has at least 4 strong couplings, plus some long-range ⁴J couplings.
#spin #coupling #proton #fluorine

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ℹ️ In 1D TOCSY, a single peak is selected, and the signal is transferred from it to all J-coupled protons in a stepwise process.
❗️This technique can be particularly useful for separating spectra in mixtures of compounds.
#TOCSY

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Recently, I encountered similar NMR problem in practice.
ℹ️ Found the answer in Breitmaier's book:
‼️Geminal couplings ²J(NH) distinguish the relative configurations of aldoximes/aldimines very clearly. Thus, anti-furan-2-aldoxime(a) shows a considerably larger ²J(NH) coupling than does the syn isomer b.
#nitrogen #coupling

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🧲‼️Reference deconvolution is a technique that able to remove some distortions that affect all peaks in a spectrum in the same (!) way, usually in the case of bad shimming.
🔑 The key idea is finding the distortion function of a reference signal (singlet) followed by the whole *.fid’s correction using this function.
ℹ️ The analysis was made using MNova software

#processing

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🧲☀️ Cinnamic acid derivatives are a good example of why it is necessary to protect your NMR samples from direct sunlight ☝️.
ℹ️ These NMR spectra illustrate the transformation of the trans isomer into the cis isomer when the sample is exposed to open light.
#uv #light

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