I do not know if a dimer of $\ce{N2O5}$ (dinitrogen pentaoxide) exist, and if its formula is $\ce{N4O10}$. Can you explain me that please? I do not know where to search for this information.
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10$\begingroup$ It is a big universe. There has to be at least one such "molecule" floating around in deep space somewhere. You certainly couldn't make a pressurized tank of it. $\endgroup$ – MaxW Nov 16 '15 at 23:05
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PubChem database N4O10 (molecular formula) search shows that there is no known compound with this formula.
While $\ce{NO}$ (“mononitrogen monoxide”) and/or $\ce{NO2}$ (“mononitrogen dioxide”), having odd number of electrons, will dimerize or combine to form
$\ce{N2O2{: } 2NO -> ON-NO}$
$\ce{N2O4{: } 2NO2 -> O2N-NO2}$
$\ce{N2O3{: } NO + NO2 -> ON-NO2}$
(note the $\ce{N-N}$ bond),
$\ce{N2O5}$ is formed by the condensation of nitric acid $\ce{HNO3}$:
$\ce{ 2 { }O2N-OH ->[-H2O] O2N-O-NO2 }$
(note the $\ce{N-O-N}$ bond), thus can be considered nitric acid anhydride. And unlike polyphosphoric acid formation from $\ce{H3PO4}$, there are no remaining hydroxy groups for further condensation.
With non-standard formal approach, $\ce{N2O5}$ already is a dimer of hypothetical franctional-oxygen-atom-count “mononitrogen(V) oxide” $\ce{NO_{2.5}}$.
