Identifying Peak Overlap on an HMBC Spectrum … Part 2

One of the trickiest parts of interpreting a 1H-13C HMBC is deciding whether a 13C resonance is coinciding with another 13C resonance, i.e. overlapping 13C peaks. A past blog, Part 1, describes a specific case where the possibility of two coinciding 13C resonances can be deciphered based on a high carbon correlation count. Herein, we present a specific case for a 1H-13C HMBC with paired 1J responses to distinguish peak overlap.

The 1H -13C HMBC spectrum below illustrates 2 protons multiplets at 1.89 and 2.37 ppm and 2 carbon resonances at 26.8 and 43.0 ppm. The paired 1J responses at 43.0 ppm indicate a CH2 group with the proton multiplet at 1.89 ppm. Similarly, the carbon at 26.8 ppm shows a paired 1J response with the proton multiplet at 2.37 ppm (CH group). In addition, a single correlation at exactly 2.37, 26.8 ppm indicates a quaternary carbon coinciding with the carbon resonance for the CH group.

The quaternary carbon and CH group are in the vicinity of each other.

Identifying Peak Overlap on an HMBC Spectrum … Part 2

One of the trickiest parts of interpreting a 1H-13C HMBC is deciding whether a 13C resonance is coinciding with another 13C resonance, i.e. overlapping 13C peaks. A past blog, Part 1, describes a specific case where the possibility of two coinciding 13C resonances can be deciphered based on a high carbon correlation count. Herein, we present a specific case for a 1H-13C HMBC with paired 1J responses to distinguish peak overlap.

The 1H -13C HMBC spectrum below illustrates 2 protons multiplets at 1.89 and 2.37 ppm and 2 carbon resonances at 26.8 and 43.0 ppm. The paired 1J responses at 43.0 ppm indicate a CH2 group with the proton multiplet at 1.89 ppm. Similarly, the carbon at 26.8 ppm shows a paired 1J response with the proton multiplet at 2.37 ppm (CH group). In addition, a single correlation at exactly 2.37, 26.8 ppm indicates a quaternary carbon coinciding with the carbon resonance for the CH group.

The quaternary carbon and CH group are in the vicinity of each other.

Sticking out like a Trimethyl Ammonio Group

When dealing with a structure elucidation problem where little or no previous fragment information exists, the number of possibilities can seem endless. Although typically one envisions a neutral compound for the unknown, ionic compounds, such as salts and zwitterions, are also just as likely and thus can add to the complex nature of an elucidation.

The trimethyl ammonio group shown below exhibits an intense ¹H signal generally between 2.8 - 3.2 ppm and a ¹³C signal generally between 39 - 62 ppm.

The 1H NMR spectrum below exhibits an obvious singlet circa 3.1 ppm belonging to an R-N(CH3)₃⁺ group. Compare the difference in chemical shifts to a t-butyl group listed here.

Sticking out like a Trimethyl Ammonio Group

When dealing with a structure elucidation problem where little or no previous fragment information exists, the number of possibilities can seem endless. Although typically one envisions a neutral compound for the unknown, ionic compounds, such as salts and zwitterions, are also just as likely and thus can add to the complex nature of an elucidation.

The trimethyl ammonio group shown below exhibits an intense ¹H signal generally between 2.8 - 3.2 ppm and a ¹³C signal generally between 39 - 62 ppm.

The 1H NMR spectrum below exhibits an obvious singlet circa 3.1 ppm belonging to an R-N(CH3)₃⁺ group. Compare the difference in chemical shifts to a t-butyl group listed here.

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              Arvin M.

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Dear fellow subscribers,

I assure you that your email will not be distributed to any additional source(s) other than what is being used by FeedBurner in delivering the current blog. Should there be any problems in this regard, please feel free to contact me via the comment section below so I may rectify the situation.

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              Arvin M.

Gearing up the Right Thought Process for Solving Problems

The approach behind solving a structure elucidation problem is very similar to attempting a math problem or working on a jigsaw puzzle. This similar mental framework shares a basis, but not limited to, on understanding the basic rules that define the problem, and in some cases the creativity and flexibility to redefine the problem.

When presented with a set of data, an elucidator can recognize that the data is valid and logical, address any problematic areas and explore a strategy to fix or solve it without making too many assumptions. Although various problem solving techniques exist, ultimately, it is experience that goes a long way.

  

The following reference addresses some of the underlying issues described in this blog: J.E. Davidson and R.J. Sternberg. The Psychology of Problem Solving. Cambridge University Press, 2003.