Nuclear Quadrupole Hyperfine Structure in HC14N/H14NC and DC15N/D15NC Isomerization: A Diagnostic Tool for Characterizing Vibrational Localization

摘要

Large-amplitude molecular motions which occur during isomerization can causesignificant changes in electronic structure. These variations in electronicproperties can be used to identify vibrationally-excited eigenstates which arelocalized along the potential energy surface. This work demonstrates thatnuclear quadrupole hyperfine interactions can be used as a diagnostic marker ofprogress along the isomerization path in both the HC14N/H14NC and DC15N/D15NCchemical systems. Ab initio calculations at the CCSD(T)/cc-pCVQZ level indicatethat the hyperfine interaction is extremely sensitive to the chemical bondingof the quadrupolar 14N nucleus and can therefore be used to determine in whichpotential well the vibrational wavefunction is localized. A natural bondingorbital analysis along the isomerization path further demonstrates thathyperfine interactions arise from the asphericity of the electron density atthe quadrupolar nucleus. Using the CCSD(T) potential surface, the quadrupolecoupling constants of highly-excited vibrational states are computed from aone-dimensional internal coordinate path Hamiltonian. The excellent agreementbetween ab initio calculations and recent measurements demonstrates thatnuclear quadrupole hyperfine structure can be used as a diagnostic tool forcharacterizing localized HCN and HNC vibrational states.