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The ν5 antisymmetric stretching vibration of 1Σ+g C9 has been observed using direct infrared diode laser absorption spectroscopy of a pulsed supersonic cluster beam. Twenty-eight rovibrational transitions measured in the region of 2079–2081 cm-1 were assigned to this band. A combined least squares fit of these transitions with previously reported ν6 transitions yielded the following molecular constants for the ν5 band: ν0=2 079.673 58(17) cm-1, B″=0.014 321 4(10) cm-1, and B′=0.014 288 9(10) cm-1. The IR intensity of the ν5 band relative to ν6 was found to be 0.108±0.006. Theoretical predictions for the relative intensities vary widely depending upon the level of theory employed, and the experimental value reported here is in reasonable agreement only with the result obtained from the most sophisticated ab initio calculation considered (CCSD). © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Terahertz VRT laser spectra of four (H[sub 2]O)[sub 2] intermolecular vibrations consisting of 362 transitions have been measured between 87 and 108 cm[sup -1] with ca. 2 MHz precision. The results differ both qualitatively and quantitatively from the predictions of dimer potentials tested. The spectra also reveal an ordering of the intermolecular vibrations which differs dramatically from that predicted by normal mode analysis. Strong coupling is indicated between the low barrier tunneling motions and the intermolecular vibrations as well as among different vibrations. In particular the 102.1 cm[sup -1] (H[sub 2]O)[sub 2] vibration assigned as the acceptor wag (ν[sub 8]) exhibits two types of perturbations. In one of these a component of K[sub a]=1 coupling with a tunneling component of K[sub a]=0 in the 108 cm[sup -1] acceptor twist (ν[sub 11]) vibration. There is also an indication that the 103.1 cm[sup -1] (H[sub 2]O)[sub 2] band assigned as the donor in-plane bend (ν[sub 6]) is coupled to the acceptor wag resulting in a lower of the in-plane bend frequency and a higher acceptor wag frequency. Detailed analysis of the VRT levels confirms the extreme nonrigidity of this complex, indicating that the use of approximate models with reduced dimensionality to calculate its properties are likely to fail. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]