Chemical properties of isotopes

Question

I've always heard that different isotopes of the same element have exactly the same chemical properties.

But for example, I've read that some compounds which have hydrogen atoms interchanged by deuterium become odorless (this is one of the arguments for the vibrational theory of olfaction).

A 2001 study by Haffenden et al. showed humans able to distinguish benzaldehyde from its deuterated version. In addition, tests with animals have shown fish and insects able to distinguish isotopes by smell

So, are there any situations in which neutrons do matter (besides nuclear decays)?

Addendum 2013/10/28

Another curious effect is that water is slightly blue, due to vibration. But heavy water is not.

Heavy water is colorless because all of its corresponding vibrational transitions are shifted to lower energy by the increase in isotope mass. For example the H2O band at 760 nm is shifted to approximately 1000 nm in D2O.

(Left: tube if filled with (light) water. Right: empty tube.)

Answer 1

Short answer: nuclear neutrons greatly affect chemistry involving hydrogen.

Longer answer

Hydrogen is the lightest element, and deuterium is just less than twice as heavy. This makes a pretty massive (pun intended) difference to any vibrational modes involving moving hydrogen/deuterium. This means that, for example, the deuterated equivalents of weak acids are less acidic, and any reaction involving the loss of a proton will be significantly slower with the deuterated version.

Since helium is pretty unreactive, and by the time you're at lithium an extra neutron is only increasing the mass by about 14%, the effects on reaction energetics are much, much smaller for anything else.

I'd suggest looking up the "Kinetic Isotope Effect" for more information and more complex effects the mass change on substituting isotopes can cause to chemical activity.

As Eric Brown said in his answer, different isotopes may also have very different spectroscopic properties if the spectroscopic technique depends either on vibrational or nuclear properties.

As a minor curiosity, I'm given to understand this also means that heavy water is mildly hazardous to your health, since deuterated water and deuterated analogues of various vital chemicals have a nasty tendency to react mostly irreversibly or very slowly with vital enzymes that are "tuned" for hydrogen.

Answer 2

I think isotopic analogues have mostly the same bulk physical properties.

• There are some striking differences, e.g. water and heavy water (D2O) have perceptibly different boiling points.

• There are many spectroscopies where isotopic substitution has a dramatic effect on the spectrum: including vibrational spectroscopies (IR/Raman), and nuclear spin (NMR, EPR, Mossbauer?).

• Rates of reaction can be substantially altered by using isotopic substitution.

But I think there is some confusion here, as far as smell. It is when you change chiral centers from $\ce{R -> S}$ or $\ce{S->R}$, thereby generating enantiomers and/or diastereomers, that you begin to affect things like smell. It is profound in, e.g. menthol.

Diastereomers can have drastically different properties. Enantiomers may interact differently with olfactory sensors, because our bodies (amino acids, sugars) have an intrinsic chirality.

Answer 3

My first idea when I read this question was: the masses are different, so everything that is affected by the mass of the ion or molecule must be different.

Diffusion and dissociation also come to mind. I did a quick search and found this: http://www.mendeley.com/research/secondary-deuterium-isotope-effects-acidity-glycine/

Answer 4

Lithium 6 and 7 have different electrolysis properties such that given an appropriate membrane - Lithium 7 can be enriched. Common is the mercury process. Alternatively this process.

http://jolisfukyu.tokai-sc.jaea.go.jp/fukyu/mirai-en/2012/4_10.html