Nature Chemistry 2, 390–393 (2010); published online 28 March 2012; corrected after print 20 June 2014.10.1038/nchem.596

The interactive version of the table shown in Fig. 1 of this Article is now additionally available at http://dx.doi.org/10.6084/m9.figshare.1009575. The online versions of the Article have been amended accordingly.