Hands, feet, shoes, screws are not superposable upon their mirror images. Mirror image yet different objects are enantiomorphs. Handedness is chirality if mirror-reflected along one axis, parity if true along all three axes. Put a left then right shoe on your left foot. Achiral socks (superposable mirror images) do not detect left feet.
Protein amino acids (less achiral glycine) are all left-handed. All natural chiral sugars are right-handed. l-Carvone is spearmint, d-carvone is caraway. Pharmacology is roughly 1/3 chiral drugs. Enantiomorphs might be inactive, naughty (dextromethorphan cough syrup vs. levomethorphan opiate), or lethal.
Friedrich Hund in 1927 showed a chiral molecule is not left- or right-handed. Eigenstates are not enantiomorphs, they are even and odd superpositions. Enantiomorphs are energetically indistinguishable, the molecules' Hamiltonian is parity-invariant (commutes with reflection in any plane).
Quantum mechanics says optical isomers do not exist. Somebody is demonstrably wrong. Got experiment?
Ammonia's even- and odd-parity ground states satisfy Hund's paradox. Room temperature mixed states were molecular beamed through a hexapole cylindrical electrostatic field. It scattered even-parity (negative Stark effect) and focused odd-parity (positive Stark effect) molecules. A single species filling a low loss cavity lased as the population thermalized, 23.6944955 and 23.6893348 GHz, or 21.34 nano-eV of difference. A carbon-carbon single bond is 3.6 eV.
The quantum Zeno ("watched pot") effect maintains spearmint chewing gum. l-Carvone persists, "seen" to be chiral by huge numbers of interactions/second with surrounding medium. In vacuum it is ceaselessly probed by its own electromagnetic field.
Folks calculated a modest 10-5 torr vacuum with local temperature below 10°K allows chiral dideuterium disulfide, DSSD (MW = 68.158, 91.3° dihedral angle), chiral gauche rotamer to quantum racemize. Resolved chiral DSSD cannot test theory. Chiral rotamers racemize with a 32 kJ/mole S-S rotation barrier, spinning even though snuggling absolute zero.
Hard vacuum supersonic expansion of cold resolved HDC=C=CHD (mp = 137°K, MW = 42.076, 90° dihedral angle) into a 10-5 torr 10°K molecular beam is mundane; 0.27°K obtains. Flow through volume over time, trap on a cryogenic plate. Will recovered HDC=C=CHD be resolved or racemized? Chiral gauche rotamers racemize with a 205 kJ/mole C=C=C rotation barrier. Chemistry expects no paradox.
Hard vacuum ends decoherence by absence of collisions. The electronic quantum Zeno effect also has a kill switch. Quantum or classical behavior obtains from local dissipation. The divider is h-bar, the quantum of action, in ohms: h/4(pi)e2 or 2054.118 ohms. Run resolved chiral molecules in vacuum through a small shiny box to avoid radiative dissipation and all other environmental coupling.
C60, MW = 720.642, as a molecular beam de Broglie wave diffracted through a grating. C60 approaches a classical body given internal degrees of freedom and their environmental couplings. A small rigid chiral molecule with ferocious optical rotation facilitates easy detection of Hund or no Hund.
Norbornenone, MW = 108.138, has 1146° rotation for (1S,2S)-bicyclo[2.2.l]hept-5-en-2-one. High yield enantioselective synthesis with Cu(II)-bisoxazoline complexes is polished with full optical resolution. Aliphatic analog resolved norbornanone (norcamphor) has 29.8° rotation.
Homochiral molecules arrive at a grating, are diffracted as they pass through, and are collected on the far side. Is diffraction even-parity chemistry or odd-parity Hund? (Mirror plane normal to the grating and parallel to slit length.)
Chirality does not first order couple to a grating. Chirality perturbs electronic properties (polarizability, dipole moment). Diffraction gratings can couple to external fields. Gold metallized gratings - shiny boxes - have strong couplings through surface plasmons, as in surface enhanced Raman spectroscopy.
Chemistry versus quantum mechanics then plays. Refrigerated resolved chiral norbornenone is hard vacuum supersonically expanded into a deeply cryogenic molecular beam. Collect on a cryogenic plate downstream. Did it racemize for lack of collisions? Said beam is diffracted through a gilded grating and collected by a cryogenic plate one the other side. Did it racemize for lack of dissipation, too?
Chemistry or quantum mechanics is wrong. Chemists learn early on
to trust Pyrex not chalkboards.
 Physics Today 62(9) 16 (2009) "Month-long calculation
resolves 82-year old quantum paradox"
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