Interpreting a 1H-1H COSY spectrum

June 18, 2008

by Arvin Moser, Team Manager, Application Scientists, ACD/Labs

A component of structure elucidation involves the capability to interpret spectral data of an unknown compound. The interpretation of the data generally leads into a wide range of structural possibilities for the unknown. The goal of the elucidator is to narrow down the structural possibilities to a minimum set of fragments. Taking these fragments, the elucidator can piece together the parts that make structural sense.

Below is a portion of a 1H-1H COSY-45 spectrum for an unknown compound. The COSY data exhibits 5 correlations: 3 diagonal and 2 off-diagonal. The 2 off-diagonal correlations at (1.45, 2.36) and (2.36, 1.45) ppm indicate 4 possible 1H-C-1H structural configurations between the protons at 1.45 and 2.36 ppm. The issue is compounded by the fact that for each off-diagonal correlation, there are 4 possibilities to consider when trying to build a fragment(s). Keep in mind that COSY data by itself may not constitute adequate information to narrow down the possibilities. However, noting the multiple possibilities will ensure that nothing is overlooked.

Note: the intensity of the off-diagonal correlations, judged by the number of contours in respect to the diagonal correlations, may provide a clue in eliminating some of the possibilities. However, this is dependent on how the data is collected and/or on the dihedral angles of the coupled protons.