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Question 1225

Given a certain chemical structure, can it be determined which spectrum of light that molecule will absorb? Are there any known compounds that charge their color or intensity when exposed to electric fields? - GS
While it is possible in principle to calculate the exact spectrum of light that a molecule will absorb, in practice it is normally extremely difficult. It's a matter of complexity—the quantum mechanical equations describing a molecule's electromagnetic structure are easy to write down but extraordinarily difficult to solve, even in approximation. One of the great challenges of atomic and molecular physics and physical chemistry is determining the full quantum mechanical structure of atoms and molecules through calculation alone. Except with small atoms and molecules, it's awfully hard but not impossible. As computers get faster and approximation schemes get better, the calculated spectra of molecules get closer to their experimental values.

As for compounds that change their optical properties while in electric fields, the answer is yes—all compounds exhibit such changes, although they may be undetectably small. However, I can't think of any isolated molecules that change dramatically in normal fields. Still, electric fields can alter the "selection rules"—the symmetry-based laws that often control which optical transitions can or cannot occur. It's possible that a modest electric field will turn on or off import optical transitions in some molecules so that they exhibit large color changes in small fields. Still, I can't think of any useful examples.


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