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ACD/Labs Blog

Arvin Moser
Coupling constants from a 1D NMR spectrum can be very useful to a chemist when trying to characterize a structure. However, there are cases where the perceived coupling constants can be misleading and thus wrong. The 1H NMR spectrum below shows several multiplets for a major component tryptophan and a minor component. For tryptophan, the...

Arvin Moser
Originally published in 2008, the underlying essence of a structure elucidation process is to structurally distinguish an unknown from a set of possible isomers. This is evident by the number of possible isomers for a given molecular formula. The chart below divides isomers into three groups: Structural/Constitutional/Regio, Spin and Stereo/Spatial isomers. Wikipedia links are included...

Arvin Moser
Structure Elucidator Tutorial Freeware: a free software package at the undergraduate and graduate levels This manuscript covers various exercises of using a free CASE (computer-assisted structure elucidation) software application “Structure Elucidator”. The purpose of these exercises is to use elucidation software to quickly explore potential candidates given a set of NMR data and a molecular...

Arvin Moser
There are many variants of 1H-13C HSQC experiments that can used for elucidating unknown molecules. In particular, band-selective HSQC data can assist with the data interpretation of correlations with regions exhibiting ambiguous assignments.1 For the following unknown compound, two 1H -13C HSQC spectra were collected. Both datasets (bsgHSQC and gHSQC) were processed with the same...

Arvin Moser
In collaboration with Sequoia Sciences Inc., we developed a strategy for the dereplication of a complete or a partial structure using 1H NMR, 1H–13C HSQC and 1H–1H COSY spectral data, a molecular formula composition range and structural fragments against a massive database of about 22 million compounds is considered. The work was based on 18...

Arvin Moser
From 2012 to 2015, I had the privilege to be involved in two collaborations aimed at elucidating 3 novel unknown compounds. The goals of both projects were to solve the three unknowns: two natural products and a synthetic product. I used a Computer-Assisted Structure Elucidation (CASE) software to explore thousands of isomers to find the...

Arvin Moser
An HSQC-DEPT experiment shows one-bond correlations. The CH and CH3 groups are generally phased positive (red) and the CH2 groups are phased negative (blue). For the following known compound, the 1H -13C HSQC-DEPT spectrum (collected at 150 Hz coupling constant) below shows a negative correlation (blue) at 2.72 and 61.1 ppm with a 1H integral...

Arvin Moser
A key step in being a successful elucidator is to understand the principles behind the experiment. Below is one such case. For the following known compound, the 1H -13C HSQC-DEPT (or multiplicity-edited HSQC) spectrum (collected at 150 Hz coupling constant) below shows a negative correlation (blue) at 2.72 and 61.1 ppm with a 1H integral...

Arvin Moser
An HMBC experiment correlates atoms over a longer range from 2-4 bonds. However, it can also exhibit correlations for 1-bond separation. For a pure, unknown compound, the 1H -13C HMBC spectrum below shows three correlations to the carbon at 42 ppm. The correlations at 3.50 and 4.05 ppm correlate the 1H signal at 3.75 ppm...

Arvin Moser
An HMBC experiment can be a key part in solving for an unknown compound. In this puzzle, ‘extra’ correlations add an extra degree of difficulty to its interpretation. For a pure, unknown compound, the 1H -13C HMBC spectrum below shows three correlations to the carbon at 42 ppm. Can you interpret how the correlations at...