documented smell of borane compounds is sulfurous, though these molecules contain no sulfur. He proposes to explain this by the similarity in frequency between the vibration of the B-H bond and the S-H bond. However, it has been pointed out that for o-carborane, which has a very strong B−H stretch at 2575 cm−1, the "onion-like odor of crude commercial o-carborane is replaced by a pleasant camphoraceous odor on careful purification, reflecting the method for commercial preparation of o-carborane from reactions promoted by onion-smelling diethyl sulfide, which is removed on purification." Consistency with physics Biophysical simulations published in Physical Review Letters in 2006 suggest that Turin's proposal is viable from a physics standpoint. However, Block et al. in their 2015 paper in Proceedings of the National Academy of Sciences indicate that their theoretical analysis shows that "the proposed electron transfer mechanism of the vibrational frequencies of odorants could be easily suppressed by quantum effects of nonodorant molecular vibrational modes". Correlating odor to vibration A 2004 paper published in the journal Organic Biomolecular Chemistry by Takane and Mitchell shows that odor descriptions in the olfaction literature correlate with EVA descriptors, which loosely correspond to the vibrational spectrum, better than with descriptors based on the two dimensional connectivity of the molecule. The study did not consider molecular shape. Lack of antagonists Turin points out that traditional lock-and-key receptor interactions deal with agonists, which increase the receptor's time spent in the active state, and antagonists, which increase the time spent in the inactive state. In other words, some ligands tend to turn the receptor on and some tend to turn it off. As an argument against the traditional lock-and-key theory of smell, very few olfactory antagonists have been found. In 2004, a Japanese research group published that an oxidation product of isoeugenol is able to antagonize, or prevent, mice olfactory receptor response to isoeugenol. Additional challenges to the docking theory of olfaction Similarly shaped molecules with different molecular vibrations have different smells (metallocene experiment and deuterium replacement of molecular hydrogen). However this challenge is contrary to the results obtained with silicon analogues of bourgeonal and lilial, which despite their differences in molecular vibrations have similar smells and similarly activate the most responsive human receptor, hOR17-4, and with studies showing that the human musk receptor OR5AN1 responds identically to deuterated and non-deuterated musks. In the metallocene experiment, Turin observes that while ferrocene and nickelocene have