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ACD/Labs Blog

Arvin Moser
I would like to begin the new year with a weblog that summarizes the components of an elucidation process. Each component, shown as a Venn-like diagram below, can blend in with the next until a conclusion is reached. The goal for the elucidator is to exhaust all aspects of each possible component thus ensuring that...

Arvin Moser
When peak picking a 2D NMR experiment, past weblogs have advocated zooming in on correlations especially in cases dealing with ambiguity. Depending on the data collection parameters, a 1H-13C HMBC experiment can contain paired 1J coupling responses. Without careful scrutiny of the data, these extra responses can be misinterpreted as long-range correlations (2J or longer). The...

Arvin Moser
Many organic chemists employ Mass spectrometry (MS) as a convenient verification tool for their product in a synthetic reaction. Derivatization such as adding a protective (or protecting) group can often be detected by MS. The EI mass spectrum for tert-butyl 3-aminopiperidine-1-carboxylate is shown below. The ‘terminal’ atoms belonging to the protective group, tert-Butyloxycarbonyl (BOC or...

Arvin Moser
Infrared (IR) spectrometry is an excellent diagnostic tool for identifying, or lack of, a carbonyl functional group(s). Carbonyl stretching vibration absorbs between 1900-1600 cm-1—a region where few other functional groups absorb. In addition, the carbonyl vibration is typically intense and thus easy to spot. The gas phase FT-IR spectra, shown below, represent five carbonyl bands...

Arvin Moser
Infrared (IR) spectrometry can serve as a simple method to gather information on the presence of OH. Characteristic OH absorptions occur around the ranges of 3700-3200 cm-1 (OH stretching) and 1200-1000 cm-1 (OH bending). In the case of hydrogen-bonded OH, the band in the region 3700-3200 cm-1 generally appears broad and sometimes can go unnoticed....

Arvin Moser
Like bromine, compounds that contain chlorine atoms have a distinct ion pattern on a mass spectrum. The A+2 peak for a monochlorinated compound will be at almost one-third the intensity to the 35Cl peak due to the presence of 37Cl isotope. A compound with two chlorine atoms will show distinct A+2 and A+4 peaks with...

Arvin Moser
For synthetic reactions where rearrangement, derivatization, or cyclization has occurred, a common task is to compare the NMR spectra between the starting material and the product. The similar peaks indicate a structural region where change has not occurred whereas the unique peaks indicate a region where change has occurred. Arguably, this becomes a peak mapping...

Arvin Moser
Carbon peaks that overlap on an 1H -13C HMBC experiment can be tricky to deal with especially when additional experiments do not help to clarify the situation. A good approach is to keep note of any high correlation counts for a carbon resonance, and subsequently, treat the carbon resonance as possibly multiple carbons with coincidental...

Arvin Moser
When the incorrect number of directly-bonded protons are assigned to carbons, the elucidator is left with extra protons. (This can happen in situations with a highly-crowded region on a 1H NMR spectrum.) Where possible, tallying the expected number of exchangeable protons can serve as a warning flag that something is amiss. The following carbons, shown...

Arvin Moser
The 1H-13C HMQC, HSQC, DEPT-HSQC, HSQC-TOCSY and HETCOR experiments offer the elucidator information on the proton-carbon connectivity. The interpretation process comes down to 3 basic assignments: the correlation belongs to a methyl, methylene or methine carbon. A methyl or methine carbon exhibits at most a single correlation between the 1H and 13C axes. A methylene...