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This interested me because of Melvin Calvin's experiment with photosynthesis where he used radioactive Carbon-14. If a plant used it as a reactant to make glucose, and that Carbon-14 decayed into nitrogen, what would happen to the molecule?

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  • $\begingroup$ I don't think this is biochemistry. This is clearly radiochemistry and should be asked here: chemistry.stackexchange.com/questions/43616/… ;-) $\endgroup$ – inf3rno Jan 12 '16 at 4:30
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    $\begingroup$ I'm voting to close this question as off-topic because it should go to chemistry or physics. It is a very interesting question!! But just offtopic here. $\endgroup$ – AliceD Jan 12 '16 at 6:21
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    $\begingroup$ Carbon likes having 4 bonds, nitrogen likes having 3, though 4 is possible when the nitrogen is positively charged. The molecule would most likely break a bond when the C became an N, but going from C to N requires converting a neutron to a proton by beta decay. The energetic electron ejected during beta decay might disrupt other parts of the molecule on its way out, but I don't really know, so this is a comment. $\endgroup$ – user137 Jan 12 '16 at 7:19
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    $\begingroup$ @user137 I think I found the solution, the decay has far more energy than the chemical bonds can withstand, so the molecule will fall apart. $\endgroup$ – inf3rno Jan 12 '16 at 8:24
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    $\begingroup$ @steveverrill You never heard of atomic force microscopy I assume. $\endgroup$ – inf3rno Jan 13 '16 at 5:24
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I am not experienced in the topic, and could not find any article related to these kind of reactions, but my guess is the following. During the nuclear transmutation the radiocarbon will lose an electron (beta particle), so the nitrogen product will have positive charge. After that there are 2 possible scenarios.

a.) less plausible scenario

The beta particle carries away the energy of the decay, so the molecule will have low energy and it will be able to rearrange.

The electron shells will collapse a little bit, since the nitrogen has smaller nuclear radius. The part after that depends on which carbon we are talking about in the glucose molecule. I would guess the following rearrangements:

  • ${-CH2OH} \rightarrow {-NHOH} + H^+ $
  • ${=CHOH} \rightarrow {=NH} + H^+ $
  • ${-CHO} \rightarrow {-NO} + H^+ $

It is not easier to break an N-H bond compared to the N-C and N-O bonds according to this table, but I think what really matters here is the stability of the final product, and $H^+$ is way more stable than any $^+O*$, $^+C*$, etc. ion.

b.) what really happens

The beta particle carries away the energy of the decay only partially. So the nitrogen will have a lot of energy, which is more than enough to break all of the bonds. According to wikipedia the decay energy is 0.156476 MeV. Let's compare it with the bond energies:

$ 0.156476 MeV = 2.50702189 \cdot 10^{-17} kJ $

${2.51 \cdot 10^{-17} kJ} \cdot {6 \cdot 10^{23} \cdot 1/mol} = 1.51 \cdot 10^7 kJ/mol$

According to our table the covalent bonds have about a few hundred kJ/mol energy, e.g. a C-N bond has 305 kJ/mol energy. So it is very likely that the chemical bonds will break immediately after the decay and the molecule will fall apart.

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    $\begingroup$ My guess is it's hard to study because even the C-14 to N transition is slow on lab scale, so getting enough decay products to do structural analysis is hard. $\endgroup$ – user137 Jan 12 '16 at 8:48

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