Non-natural amino acid NAL parameter when it is in N-terminal and C-terminal of a peptide?

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Hi!
I have a peptide which has nonstandard amino acid NAL at the N-terminal and C-terminal, and I want to use CHARMM36 force field to generate it’s parameters, there are no dihedral parameters composed by NAL and N-terminal or C-terminal in the force field; However, when the NAL in the middle of the peptide, it has parameters. So, why does the NAL have no parameters when it is at the N and C ends?