I will be on the road again for the next 2 weeks with more business travel. This time, I will be touring a variety of Universities and Colleges in Texas and Ohio. Unfortunately for the P2C2E readers, the blog will be on hiatus for the next 2 weeks. Posts will resume the following week.
Characteristic isotopic patterns in MS can assist the elucidator in revealing the presence or absence of atoms. For the Br, Cl, S and Si atoms, a good approach is to examine the peak intensity of the A+2 signal. The respective contributions by the isotopes 81Br, 37Cl, 34S and 30Si are approximately 49.3, 24.3, 4.2 and 3.1%, respectively.
Below are the molecular ion regions for two EI mass spectra for the same unknown compound. The top MS was collected at 13 eV and the bottom was collected at 70 eV. A list of m/z and intensities are also provided. Given the carbon count to be 10 atoms, are any of the following atoms Br, Cl, S and/or Si present?
A special thanks goes to Scott Van Bramer for allowing me to use the data.
Since I just returned home from a forensic conference (SOFT), the blog P2C2E will be on hiatus for this week from October 18 to October 22. Posts will resume the following week.
There are two approaches to deciding which scenario best fits the NMR data in the puzzle. A structure elucidator can provide evidence that one scenario is more probable than an other and/or eliminate one scenario on the grounds of insufficient/contradictory data to support it.
For the following 1H NMR spectrum, if the integral of the doublet at 7.42 ppm equates to 1 H atom from a single structure, then the 1H signals at 7.58-7.59 ppm integrating to 2.57 (and assuming all signals are accounted for) relate to overlapping signals (probably a doublet overlapping with a singlet) from a mixture of compounds at varying concentrations.
Another approach, as Adolfo has nicely provided, the magnitude of the strong coupling demonstrated by the signals at 7.58-7.59 ppm indicates a nearby partner on the left side circa 7.70 ppm. Since this appears not to be the case, then by process of elimination, the signals at 7.58-7.59 ppm relate to two overlapping multiplets.
The goal of this puzzle is to distinguish between strong coupling and peak overlap.
For the following 1H NMR spectrum, are the 1H signals at 7.58-7.59 ppm exhibiting strong coupling or are they two overlapping multiplets or both?
With intense solvent signals present on a spectrum, a smaller signal(s) can easily be missed. If 2D NMR data is available, then this extra information can assist in clarifying whether a small signal(s) is obscured by larger signals.
On the 1H-13C HMBC below, the correlations for CDCl3/CHCl3 (due to 1J coupling responses and more) are more intense in comparison to the weak correlation at approximately 6.9 and 77.3 ppm. In this case, the weak correlation is attributed to a quaternary carbon obscured by a set of intense solvent signals.
‘How many signals are present?’ is such a simple question, and yet, it is a fundamental question to an elucidator. Mistaken a signal or overlook one and the elucidator can run the risk of wasting time and effort.
The 13C [1H] NMR spectrum below shows 3 discernible signals that are attributed to the solvent CDCl3. In addition, one can begin to speculate on weaker signals; there might a fourth signal at ~77.2 ppm (most likely due to residual CHCl3), possibly a fifth at ~77.5 ppm, perhaps a sixth one at ~77.3 ppm and maybe more.
The next step is to examine additional data and verify whether the ‘weak’ signals are real or not. This can include:
1. comparing the weak signals to other structural signals,
2. applying deconvolution/peak fitting to this region,
3. checking 2D NMR data,
4. acquiring data in a different solvent,
5. modifying the acquisition parameters to exclude the solvent or increase S/N, etc.
