On the relative stability of two noradrenaline conformers

The relative stability of the two noradrenaline conformers AG1a and GG1a has been computed using MP2 and df-LMP2 (density-fitting local MP2) with basis sets up to aug-cc-pVQZ. For all method/basis-set combinations, the AG1a/GG1a energy gap is very small. Different basis set series show different convergence trends: for example, basis set enlargement favours AG1a in the MP2/cc-pVnZ series, but favours GG1a in the MP2/aug-cc-pVnZ and df-LMP2/cc-pVnZ series. The small difference in the AG1a and GG1a OH → N stretch frequency, predicted by DFT, practically disappears at the MP2 level of theory, thereby rendering the infrared spectrum unusable to distinguish between the two conformers.