Event

Prof. Robert W. Field
Massachusetts Institute of Technology (MIT)

Tuesday, April 1st, 2025
12:00 PM
Faculty Conference Room
Chem 73 Building

Broken Patterns of Broken Patterns

Assignment of a spectrum is based on discovery of patterns in the spectrum. Such patterns reflect the existence of a rigorously or approximately conserved structural feature of the molecule.  Nature is simultaneously cruel and kind. Some patterns are born to be broken. Sometimes the pattern is parametrically broken (e.g. centrifugal distortion), requiring only an additional fit parameter. Sometimes the pattern-breaking is catastrophic (e.g. an accidental near degeneracy of vibrational levels of two electronic states): a “perturbation” or, in German, “Störung.” A parameter, an off-diagonal element of an effective Hamiltonian matrix, which expresses the nature and strength of the perturbation, permits recovery of the simple pattern. However, a super-pattern can exist among the set of observed perturbation parameters: a “pattern-of-broken-patterns.” This pattern-of-broken-patterns has been used to determine the vibrational quantum numbers of an otherwise unobservable perturbing state.  An N-atom polyatomic molecule has 3N-6 vibrational normal modes and the polyad model expresses all of the “accidental-but-on-purpose” anharmonic interactions among these normal modes. This is another example of a pattern-of-broken-patterns. When a polyatomic molecule potential energy surface supports more than one isomeric minimum, a sudden disruption of the polyads can occur, forming a broken-pattern-of-broken-patterns. Nature has kindly provided a way to observe the energy and geometric structure of the transition state between two isomers.