HCP <-> CPH isomerization: Caught in the act

In this overview we discuss the vibrational spectrum of phosphaethyne, HCP, in its electronic ground state, as revealed by complementary experimental and theoretical examinations. The main focus is the evolution of specific s pectral patterns from the bottom of the potential well up to excitation ene rgies of approximately 25,000 cm(-1), where large-amplitude, isomerization- type motion from H-CP to CP-H is prominent. Distinct structural and dynamic al changes, caused by an abrupt transformation from essentially HC bonding to mainly PH bonding, set in around 13,000 cm(-1). They reflect saddle-node bifurcations in the classical phase space-a phenomenon well known in the n onlinear dynamics literature-and result in characteristic patterns in the s pectrum and the quantum-number dependence of the vibrational fine-structure constants. Two polar opposites are employed to elucidate the spectral patt erns: the exact solution of the Schrodinger equation, using an accurate pot ential energy surface and an effective or resonance Hamiltonian (expressed in a harmonic oscillator basis set and block diagonalized into polyads), wh ich is defined by parameters adjusted to fit either the measured or the cal culated vibrational energies. The combination of both approaches-together w ith classical mechanics and semiclassical analyses-provides a detailed spec troscopic picture of the breaking of one bond and the formation of a new on e.