I am writing a short story, and am trying to find a gas with the following properties:

  • It must have a very distinctive odor
  • It must be impossible for it to exist at room temperature under normal conditions
  • It must be toxic to humans

How large a pool of possible substances are there that meet these requirements? Is there a particular family or class of gases where I should be focusing my attention?

Some additional clarification (apologies for not including this when I first wrote the question):

The scenario I am writing is Cthulhu-based. The idea is that this substance is discovered where it shouldn't be able to exist. This is where the second criteria comes from. They will collect a sample of this stuff, analyse it and realise that its mere existence is impossible in the circumstances they found it. A completely hypothetical example might be a substance that spontaneously combusts in earth's atmosphere, or that immediately reacts in some other way with a gas present in air. This would mean that it cannot exist where the protagonists discover it, yet it does.

The toxicity is just intended to add some jeopardy to the situation

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    $\begingroup$ Your second and third requirements contradict each other. If it does not exist at room temperature, how can we get in contact with it, let alone get poisoned by it? Also, what's that with being pyrophoric? Pyrophoric compounds exist at room (and higher) temperatures just fine, as long as you don't expose them to air. $\endgroup$ – Ivan Neretin Dec 15 '16 at 10:35
  • $\begingroup$ Would something like chlorine gas work? Though it is perfectly stable when pure, it doesn't exist naturally ("in normal conditions") because in short order it finds something to react with and turns into other compounds. It fits the two other conditions, too. $\endgroup$ – Nicolau Saker Neto Dec 15 '16 at 11:32
  • $\begingroup$ Does non-existance also apply if the compound in question is a liquid at SATP? $\endgroup$ – Linear Christmas Dec 15 '16 at 15:37
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    $\begingroup$ Does the edit I've just done clarify it at all? $\endgroup$ – Wibbs Dec 15 '16 at 15:59
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    $\begingroup$ @Wibbs After reading your edit: You might want to consider using a substance that contains a radioactive primordial nuclide that should not be able to exist today in such amounts because of its short half-life (i.e. it should be “extinct”, the found sample could even be described as “older than Earth”). The toxic effects could also be a result of the radioactivity, or just be caused by ordinary chemical properties of the substance. $\endgroup$ – user7951 Dec 15 '16 at 16:20

As Ivan Neretin already explained in his comment, you can't have it all.

Anyway, cyanogen, $\ce{N#C-C#N}$ would be a nice candidate. It is colourless, but highly toxic (H330). If it's cold enough, that is below -21 °C, you can even have a pond of it ;-)

In addition, it adds a historical space touch to the story. Back in 1910, when the return of Halley's comet was expected, cyanogen was spectroscopically identified in the comet's tail and the New York Times titled COMET'S POISONEOUS TAIL ;-).

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  • $\begingroup$ Out of curiosity: Where (or how) did you find the hazard statement H330 for cyanogen? According to the official Annex VI of Regulation (EC) No 1272/2008 (CLP Regulation), cyanogen is only H331. I am aware, however, that many companies report H330. $\endgroup$ – user7951 Dec 15 '16 at 13:13
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    $\begingroup$ @Loong I just checked it again to make sure that I didn't fatfinger it. The GESTIS Substance database really reports H330. $\endgroup$ – Klaus-Dieter Warzecha Dec 15 '16 at 14:28

As already mentioned in other comments, your requirements are contradictory. You cannot have a gas that is non-existent at normal conditions but is also toxic to humans and has a distinctive odour at the same conditions.

Depending on when do you want the gas to present (e.g. to be toxic to humans or to be recognizable by its odour) and when do you want it to be absent (e.g. to be harmless or to be undetectable), you might need to introduce a change (like a switch) in your ambient conditions.

If you want the compound to be toxic first and non-existent later, you may consider very volatile compounds (gases, or liquids with a high vapour pressure), which are rapidly dispersed, e.g. by the wind. Thus, you can obtain a toxic concentration of the gas first and also can achieve concentrations below the detection limit later. Several chemical warfare agents are classified as non-persistent agents (i.e. they are likely to be used to kill and incapacitate rather than to deny access to ground) and thus suit this purpose. A typical example for such non-persistent agents is phosgene. It also has a distinctive odour, which is described as resembling freshly-cut hay or grass.

You may also consider a compound that is sensitive to heat and/or sunlight. Thus you could obtain a toxic concentration under dark or cold conditions (e.g. at night) and decompose the gas later using heat and/or sunlight (e.g. after sunrise). However, I don’t know any typical example compound that fulfils these requirements, is also sufficiently volatile and toxic, and has a characteristic odour.

If you want the compound to be absent first and toxic later, you may consider taking advantage of the different vapour pressures of compounds in dependence of the temperature. A typical example for this is the use of sulfur mustard (mustard gas) in winter, e.g. contamination of snow. Sulfur mustard freezes at about 14 °C and therefore detection systems (including the human nose) relying on detection of vapour can fail. If contaminated snow is carried (e.g. sticking to the boots of persons) into buildings, vehicles, or other warm places, you can achieve a toxic concentration in the air. The odour of sulfur mustard is described as resembling garlic, mustard, or leeks; however, the smell is actually caused by impurities of the technical product.

If you want the compound to be absent first, you may also consider using a chemical reaction to generate the toxic compound only when you need it. A typical example is sarin. Modern binary sarin weapons do not contain any sarin; they actually contain methylphosphonic difluoride, isopropyl alcohol (propan-2-ol), isopropylamine (propan-2-amine), and another compound (which is fortunately missing in most cookbooks that are circulating in the internet, and I prefer to ignore the details here, too). The compounds are mixed when the weapon is used (e.g. upon impact of the warhead) and react to the actual sarin. Pure sarin is odorless; however, the smell of impure sarin is described as resembling mustard or burned rubber. Furthermore, the characteristic smell of the precursors would be present (e.g. the fishy, ammoniacal odour of isopropylamine).

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Perhaps trioxygen $\ce{O3}$ is a good fit.

It must have a very distinctive odor.

Ozone has a strong, sharp smell. Highly irritable. It should be detectable to humans at concentrations of $0.01\ \mu\mathrm{mol}\ \ce{O3}$ per one mole of air. By mass in dry air, that comprises about $1.66\cdot10^{-6}\%$ or $16.6\ \mathrm{ppb}$.

It must be impossible for it to exist at room temperature under normal conditions.

The higher the concentration of ozone, the less stable it is. Even with still air and no humidity, half-life of ozone is about $25$ hours. As humidity, air flow, and temperature increase, ozone's stability decreases.

It must be toxic to humans.

No worries here. Lowest lethal concentration $\mathrm{LC_{Lo}}$ for humans is $50\ \mathrm{ppm}$ for $30\ \mathrm{min}$. At lower concentrations, it foremost harmes one's respiratory system but has also been shown to increase chances of heart attack and premature death. Trioxygen will explode if the concentration is too high.

So depending on your compassion for the protagonists, vary the concentration accordingly. Probably it is 'good to have' some $\ce{25\ \mathrm{ppm}}$. Thus there is real danger; but should not be lethal.

The idea is that this substance is discovered where it shouldn't be able to exist.

So in a long-closed room, ozone should have mostly decomposed to ordinary dioxygen.

$$\ce{O3->O + O2}$$ $$\tag{very quick}\ce{O + O -> O2}$$

They will collect a sample of this stuff, analyse it and realise that its mere existence is impossible in the circumstances they found it.

Being the smart (assumingly) characters they are, they immediately know that ozone can qualitatively be detected by the reaction

$$\ce{2KI + O3 + H2O->I2 + 2KOH + O2}.$$

(A paper is dipped in a solution of potassium iodide $\ce{KI}$, or the entire solution is exposed to the atmosphere containing ozone.)

Iodine $\ce{I2}$ colours the paper or solution dark brownish.

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