Philosophy to Chemistry to Elucidation

Logic Puzzle #42: How to Convert to PPM for NMR and MS Data?

Many analytical data rely on common calculations. This puzzle covers one such common calculation.

NMR and MS data were collected on two unknown compounds. The TOF-MS data shows the [M+H]+ at m/z 300.0000 and 600.000 at an error of 0.001 Da. The 500 MHz 1H NMR spectrum shows two chemical shifts at 1500.00 and 3000.00 Hz referenced to TMS at 0.00 Hz.

Convert the MS error and NMR shifts to PPM.

Logic Puzzle #41: Using MS TOF data to Determine the Molecular Formula … Solution

Working on poorly collected data is not fun. It can be frustrating and time consuming.

A good LC-MS+ TOF dataset is consistent across a peak and exhibits a mass measurement accuracy of < 5 ppm. The TOF data below shows a significant variation across the peak m/z 205.136 to 205.076 to 205.096 (range m/z 205.106 +/- 0.030 Da or 146 ppm). This variation can inhibit the determination of a concrete molecular formula and increase false positives. The variation can be a result of bad calibrations or issues with the flight tube.

Thank you Richard L. for your expertise in this matter.

 

Logic Puzzle #41: Using MS TOF data to Determine the Molecular Formula

On the surface, both good data and poor data offer information about an unknown compound. The challenge is to differentiate between what is good data and what is poor data. The purpose of this puzzle is to examine some poor data.

From an LC-MS+ TOF dataset, an unknown compound elutes at 2.86 minutes and spans 14 scans (see data below). At retention times 2.85 min. (scan 5 of 14), 2.86 min (scan 6 of 14) and 2.88 min. (scan 8 of 14), the molecular ion ([M+H]⁺) appears at m/z 205.136, 205.076 and 205.096, respectively.

Is the TOF data consistent across the peak? Is the TOF data suitable for narrowing down a MF? Is the data good or poor?

Logic Puzzle #40: What Fragment is This? … Solution

Logical reasoning and pattern recognition form the basis for making a pattern-to-fragment connection.

For an unknown, the 1H NMR spectrum below shows 5 multiplets in the aromatic region. The multiplet at 7.57 ppm shows two overlapping multiplets: a broad signal overlapping a 'doublet' (noted as d*).

Based on the multiplets' tilt/lean and integrals, a 1,2-disubstituted benzene ring can be inferred from the multiplets at 7.11, 7.17, 7.34 and 7.57(d*) ppm. The multiplet at 6.96 ppm is a doublet with a small coupling. The small coupling can come from an adjacent NH group. If this is the case, then the broad signal at 7.57 ppm may be attributed to the NH group.

A good candidate for the unknown is an indole group (IUPAC: 1H-indol-3-yl).

Thank you Serge and Walter for your comments.

Logic Puzzle #40: What Fragment is This?

An experienced elucidator can recognize spectral patterns and use this information to piece together a fragment. This puzzle examines one such pattern-to-fragment connection.

A 1H NMR spectrum below shows the aromatic region for an unknown compound? Can you identify the fragment that would produce this pattern?

5th Year Anniversary … Hurray!

February 1, 2013 marked the fifth year anniversary of the weblog P2C2E.

Readership is increasing nicely. Thank you readers.

 

 

The statistics are based on results reported from Google Feedburner and Typepad.

Logic Puzzle #39: Coincidental Overlap or Not? … Solution

Quaternary carbons can be tricky in identifying. Keep your options open to ensure the bigger picture is not missed.

The expanded region of the 13C NMR spectrum shows 4 signals for an unknown compound. The signal at 172 ppm represents 1 or 2 amide group(s). The higher intensity of the signal at 172 ppm in relation to the other quaternary carbon signals suggests the presence of more than 1 carbon. However, additional experiments are needed to confirm this.

1. Collect a quantitative 13C NMR spectrum,
2. Recollect the 13C NMR with C6D6 or other solvent added to try to resolve the carbons,
3. Collect a 1H-13C HMBC and count the number of correlations associated to the signal at 172 ppm,
4. Collect MS data and relate it to the carbon atom count,
5. Collect 13C NMR at different temperatures,
6. Etc.

Logic Puzzle #39: Coincidental Overlap or Not?

Spectral data is just data until you can see the bigger picture. This puzzle looks at a scenario of quaternary carbons.

For an unknown compound, a standard 13C NMR spectrum (500 MHz CDCl3) was collected. The expanded region below shows 4 signals. Is the signal at 172 ppm attributed to 1 or 2 carbons (from an amide group(s)) if the remaining signals represent 1 carbon atom each? How can we test this?

Logic Puzzle #38: Count This … Solution

Yes, basic counting is important in solving an unknown. How did you stack up with this puzzle?

The 13C NMR spectrum below shows 7 signals in total, 3 intense signals and 4 weak signals. The 4 weak signals follow a quartet pattern produced by a CF3 group. The unknown contains a minimum of 4 C atoms.

BONUS You get 1 extra point for indicating that the unknown also contains a minimum of 3 F atoms.

Thank you Walter, Inamul and Adolph for your comments.

 

Blog on Hiatus for the Holidays

As the holidays approach, I will be home resting with my family. The blog Philosophy to Chemistry to Elucidation will be on hiatus from December 20^th to January 6^th.

I want to wish the readers of P2C2E a very Happy Holidays. Stay tuned for the solution to the Logic Puzzle # 38 in the New Year.