Vibrational spectroscopy and intramolecular energy transfer in isocyanic acid (HNCO)

Room temperature photoacoustic spectra in the region of the first through the fourth overtones (2 nu(1) to 5 nu(1)) and free-jet action spectra of the second through the fourth overtones (3 nu(1) to 5 nu(1)) of the N-H stretching vibration permit analysis of the vibrational and rotational structure of HNCO. The analysis identifies the strong intramolecular couplings that control the early stages of intramolecular vibrational energy redistribution (IVR) and gives the interaction matrix elements between the zero-order N-H stretching states and the other zero-order states with which they interact. The experimentally determined couplings and zero-order state separations are consistent with ab initio calculations of East, Johnson, and Allen [J. Chem. Phys. 98, 1299 (1993)], and comparison with the calculation identifies the coupled states and likely interactions. The states most strongly coupled to the pure N-H stretching zero-order states are ones with a quantum of N-H stretching excitation (nu(1)) replaced by different combinations of N-C-O asymmetric or symmetric stretching excitation (nu(2) or nu(3)) and trans-bending excitation (nu(4)). The two strongest couplings of the n nu(1) state are to the states (n-1) nu(1) + nu(2) + nu(4) and (n-1) nu(1) + nu(3) + 2 nu(4), and sequential couplings through a series of low order resonances potentially play a role. The analysis shows that if the pure N-H stretch zero-order state were excited, energy would initially flow out of that mode into the strongly coupled mode in 100 fs to 700 fs, depending on the level of initial excitation. (C) 1999 American Institute of Physics. [S0021-9606(99)01120-4].