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I am encountering a strange error with the following molecule:

ClearAll[mol];
mol = Molecule[
    "C=c1[nH]nc(C(=O)N2CCN(C(=O)CCCOC)CC2)c1=CC=CC",
    IncludeHydrogens -> All
];

MoleculePlot[mol, {{27}, {28}}]

Molecule

The orange hydrogen atom has the index 27, and the blue one the index 28.

Now I try to evaluate the following two expressions:

mol // MoleculeModify[{"DeleteAtom", 27}]
mol // MoleculeModify[{"DeleteAtom", 28}]

In the first case, I get back a molecule. But in the second case, I do not. I simply get back the unevaluated expression.

DeleteAtom expressions

I wanted to see whether it was the bond with the nitrogen atom that was causing the problem. Evidently, it was not.

ClearAll[nh3];
nh3 = Molecule["ammonia", IncludeHydrogens -> All];
nh3 // MoleculeModify[{"DeleteAtom", 2}]

DeleteAtom for NH3

Question. Why does deleting atom 28 in mol not work? I could find nothing in the documentation of MoleculeModify to indicate that deleting that hydrogen should not work.

EDIT

Mathematica Version 13.0.1, Windows 11 Pro

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4
  • $\begingroup$ is bug, will fix $\endgroup$
    – Jason B.
    Commented Mar 2, 2022 at 15:52
  • $\begingroup$ As a workaround, when trying to delete a hydrogen atom you can increment the unpaired electron count of the neighbor atom (this has to be done anyway or the hydrogen atom will be re-added to fill the valence). MoleculeModify[mol, {"SetUnpairedElectronCount", 3 -> 1}] works for this molecule. $\endgroup$
    – Jason B.
    Commented Mar 2, 2022 at 15:58
  • $\begingroup$ @JasonB. Just to confirm: Is this workaround needed only when deleting hydrogen atoms? $\endgroup$ Commented Mar 2, 2022 at 19:10
  • $\begingroup$ The only time the neighbor atom's unpaired electron count is incremented is when deleting a hydrogen atom, when you are explicitly creating a radical. $\endgroup$
    – Jason B.
    Commented Mar 2, 2022 at 19:25

1 Answer 1

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This bug was fixed in version 13.1:

In[1]:= mol = Molecule["C=c1[nH]nc(C(=O)N2CCN(C(=O)CCCOC)CC2)c1=CC=CC"];                                      

In[2]:= mol // MoleculeModify[{"DeleteAtom", 27}]                                                             

Out[2]= Molecule[<C18H25N4O3>]

In[3]:= mol // MoleculeModify[{"DeleteAtom", 28}]                                                             

Out[3]= Molecule[<C18H25N4O3>]

In[4]:= $Version                                                                                              

Out[4]= 13.1.0 for Mac OS X x86 (64-bit) (June 16, 2022)
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