with great predictive power. Similarly shaped molecules with different molecular vibrations have different smells (metallocene experiment and deuterium replacement of molecular hydrogen). In the metallocene experiment, Turin observes that while ferrocene and nickelocene have nearly the same molecular sandwich structures, they possess distinct odors. He suggests that "because of the change in size and mass, different metal atoms give different frequencies for those vibrations that involve the metal atoms," an observation which is compatible with the vibration theory. However it has been noted that, in contrast to ferrocene, nickelocene rapidly decomposes in air and the cycloalkene odor observed for nickelocene, but not for ferrocene, could simply reflect decomposition of nickelocene giving trace amounts of hydrocarbons such as cyclopentadiene. The challenge regarding smell of molecules with similar structures 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, with studies showing that the human musk receptor OR5AN1 responds identically to deuterated and non-deuterated musks and with single-neuron comparison of the olfactory receptor response to deuterated and nondeuterated odorants. Differently shaped molecules with similar molecular vibrations have similar smells (replacement of carbon double bonds by sulfur atoms and the disparate shaped amber odorants). Hiding functional groups does not hide the group's characteristic odor. However this is not always the case, since ortho-substituted arylisonitriles and thiophenols have far less offensive odors than the parent compounds. Very small molecules of similar shape, which seem most likely to be confused by a shape-based system, have extremely distinctive odors, such as hydrogen sulfide. However, it has been suggested that metals such as Cu(I) may be associated with a metallo-receptor site in olfaction for strong-smelling volatiles which are also good metal-coordinating ligands, such as thiols. This hypothesis was confirmed in the specific cases of thiol-responsive mouse and human olfactory receptors. It is claimed that odor descriptions in the olfaction literature correlate more strongly with their vibrational frequencies than with their molecular shape. See also Odotope theory Vibration theory of olfaction References Further reading Olfactory system 1949 introductions Theories