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Abstract
Large-amplitude molecular motions which occur during isomerization can cause significant
changes in electronic structure. These variations in electronic properties can be
used to identify vibrationally-excited eigenstates which are localized along the potential
energy surface. This work demonstrates that nuclear quadrupole hyperfine interactions
can be used as a diagnostic marker of progress along the isomerization path in both
the HC14N/H14NC and DC15N/D15NC chemical systems. Ab initio calculations at the CCSD(T)/cc-pCVQZ
level indicate that the hyperfine interaction is extremely sensitive to the chemical
bonding of the quadrupolar 14N nucleus and can therefore be used to determine in which
potential well the vibrational wavefunction is localized. A natural bonding orbital
analysis along the isomerization path further demonstrates that hyperfine interactions
arise from the asphericity of the electron density at the quadrupolar nucleus. Using
the CCSD(T) potential surface, the quadrupole coupling constants of highly-excited
vibrational states are computed from a one-dimensional internal coordinate path Hamiltonian.
The excellent agreement between ab initio calculations and recent measurements demonstrates
that nuclear quadrupole hyperfine structure can be used as a diagnostic tool for characterizing
localized HCN and HNC vibrational states.