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In their paper, Lang, Cezar, Adamowicz, and Pedersen show, for the first time, that molecular structure indeed emerges from the fully quantum-mechanical, non-Born-Oppenheimer molecular wave function as a manifestation of strong statistical correlation between nuclear positions, thus shedding new light on the perhaps most fundamental concept in chemistry.
The structure of a molecule is key to understanding chemistry. Molecules such as the water molecule H2O or the famous double-helix structure of DNA are known to most people. Chemical formulas like H2O indicate the atoms that combine to form the molecule, here two hydrogen atoms and one oxygen atom. Atoms, in turn, are composed of an atomic nucleus and exactly as many electrons as required to keep the atom electrically neutral. Nuclei and electrons, however, are quantum-mechanical particles whose positions cannot be exactly determined according to Heisenberg’s uncertainty relation. So, if atoms are quantum-mechanical, how come chemists can speak confidently about the structure of a molecule, which is defined as the position of the constituent atoms relative to each other?
The answer is the Born-Oppenheimer approximation (yes, that Oppenheimer!), which states that the nuclei are so heavy, and therefore sluggish, that we may consider them essentially immobile relative to the ultrafast motion of the electrons. In this way, molecular structure can be defined as the relative positions of the nuclei that yield the lowest-energy state where the repulsive and attractive forces between the charged particles are in perfect equilibrium. While mathematically sound, the Born-Oppenheimer approximation is still just that – an approximation.
Date of publication: 10th January 2024.
Read the paper “Quantum Definition of Molecular Structure” by L. Lang, H. M. Cezar, L. Adamowicz, and T. B. Pedersen in J. Am. Chem. Soc. 146, 1760-1764 (2024).