Make the aliphatic imine rule more closely match the definition in the paper

[greglandrum]

The rule for imines in the paper from Sitzmann et al that we use for tautomer enumeration looks like this:

Our rules for this are:

        std::make_tuple(std::string("aliphatic imine f"),
                        std::string("[CX4R{0-2}!H0]-[Cz1]=[NX2]"), std::string(""),
                        std::string("")),
        std::make_tuple(std::string("aliphatic imine r"),
                        std::string("[NX3!H0]-[C;z{1-2}]=[CX3]"), std::string(""),
                        std::string("")),

The result is that our rules match this structure:

and transform it to this one, which is aromatic:

The back-transform doesn't apply to the aromatic structure, so we have an asymmetry.

The original reaction SMARTS for Rule 3:

[#1,a:5][NX2:1]=[Cz1:2][CX4R{0-2}:3][#1:4]>>[#1,a:5] [NX3:1]([#1:4])[Cz1,Cz2:2]=[C:3]

should not match this molecule at all since they require that the only non-H substituent on the N atom is an aromatic atom.

This PR adjusts the SMARTS definitions for aliphatic imines in order to reflect this requirement.

This is clearly a bug fix, but since it may have a non-trivial impact on the number of tautomers generated I'm not sure if we want to do it in a patch release. Maybe it's better to hold it for 2022.09.1; what do you think?

Thanks to @rachelnwalker for pointing this one out.

[ricrogz]

Sounds good to me. But I'm interested in @rachelnwalker's opinion, since she has actually hit this.

[rachelnwalker]

Yes this looks good to me -- thanks @greglandrum!

[ptosco]

LGTM. I am in favor of including the bug fix in the next patch release if you decide to.