Our key observation is that the high-frequency C -H stretches are far separated from the C -D counterparts in the receptor binding pocket environment and at physiological temperature. This is clearly visible when the all-hydrogen and all-deuterium isotopologues' computed IR spectra are overlaid (Fig. ). The C -H stretches lie between 3020 and 3120 cm -1 , while the C -D stretches range from 2200 to 2310 cm -1 . Each C -H mode is 800-840 cm -1 higher in energy than its corresponding C -D stretching mode, almost four times larger than kT = 215 cm -1 , the thermal energy available at 310 K. Note that the modes' fluctuations in the binding pocket are also much smaller than kT , roughly 10-20 cm -1 (Table ). The number of these stretching modes in intermediate isotopologues corresponds with their isotopic composition: C 2 H n D 5 -n COO - has n modes above 3000 cm -1 and 5 -n around 2200-2350 cm -1 . It is worth pointing out the two carbonyl/carboxyl (C --O) modes around 1400-1600 cm -1 , which are by far the most infrared-active peaks in the propionate spectrum. They are shifted to slightly lower frequencies in the olfactory receptor binding pocket relative to the gas phase. This peak differs slightly between the fully hydrogenated and fully deuterated isotopologues, with C 2 D 5 COO - having slightly lower frequencies (but higher intensities) than C 2 H 5 COO -. The differences are only about 10 cm -1 , however, on the order of the fluctuations each isotopologue sees over time (and substantially less than kT ). This is true even though the shorter C --O peak has substantial H/D elemental character, as Table : Details of the labeled peaks in Fig. . Frequency and intensity are given as mean ± standard deviation. C -D stretches are labeled C -H for economy of space. Elemental compositions are projections onto each atom type; they sum to 1, up to rounding error. can be seen in the last line of Table . It is therefore unlikely that the C --O mode plays a part in distinguishing the odors of propionate isotopologues, although it is possible that it has a role in detecting propionate in general. Some modes of lower wavenumber shift strongly upon isotopic subsitution, but the spectrum is fairly crowded below 1500 cm -1 and these low-energy vibrations are more variable over time than their high-energy counterparts; we do not expect them to allow olfactory discrimination between isotopologues.