Gas-phase reaction rate constants for the reaction of silylene, SiH2, with deuterated methanol, CD3OD, have been determined over the temperature range 294−423 K and at total pressures over the range 100−800 Torr of the inert bath gas, Ar. Rate constants have also been measured for the reaction of SiH2 with CH3OH at 294 K over the range 100−800 Torr, also with Ar. Rate constants for the reaction of SiH2 with H2O over the range 50−200 Torr in Ar have been determined at 294 K. The second-order rate constants are pressure-dependent up to the maximum pressures investigated. For CD3OD, for which temperature-dependent data have been obtained, the rate constants decrease with increasing temperature, indicating that the reaction proceeds via the formation of a complex. At the highest temperature studied (423 K), the experimental decay curves indicate the system approaching equilibrium, providing direct experimental evidence for the formation of the complex. Analysis of the 423 K decay curves provides an experimental determination of the equilibrium constant, K eq, and a value for the dissociation energy of the complex of 83.0 ± 1.3 kJ mol-1. The Rice−Ramsperger−Kassel−Marcus (RRKM)/master equation modeling gives a dissociation energy for the SiH2−CD3OD complex of 83.7 kJ mol-1. Ab initio calculations, performed at the MP2/6-311+G** level of theory, give a value of 75.4 kJ mol-1, in reasonable agreement with this value. The RRKM/master equation modeling for SiH2 + CD3OD, when adjusted to account for the changes arising from deuteration, reproduces the behavior observed for SiH2 + CH3OH. The high-pressure limit predictions of the RRKM/master equation modeling are quite unusual and may indicate unusual pressure and temperature behavior in weakly bonded systems.

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We demonstrate the power of high resolution, two dimensional laser induced fluorescence (2D-LIF) spectroscopy for observing rovibronic transitions of polyatomic molecules. The technique involves scanning a tunable laser over absorption features in the electronic spectrum while monitoring a segment, in our case 100 cm−1 wide, of the dispersed fluorescence spectrum. 2D-LIF images separate features that overlap in the usual laser induced fluorescence spectrum. The technique is illustrated by application to the S1–S0 transition in fluorobenzene. Images of room temperature samples show that overlap of rotational contours by sequence band structure is minimized with 2D-LIF allowing a much larger range of rotational transitions to be observed and high precision rotational constants to be extracted. A significant advantage of 2D-LIF imaging is that the rotational contours separate into their constituent branches and these can be targeted to determine the three rotational constants individually. The rotational constants determined are an order of magnitude more precise than those extracted from the analysis of the rotational contour and we find the previously determined values to be in error by as much as 5% [G. H. Kirby, Mol. Phys. 19, 289 (1970)10.1080/00268977000101291]. Comparison with earlier ab initio calculations of the S0 and S1 geometries [I. Pugliesi, N. M. Tonge, and M. C. R. Cockett, J. Chem. Phys. 129, 104303 (2008)10.1063/1.2970092] reveals that the CCSD/6–311G** and RI-CC2/def2-TZVPP levels of theory predict the rotational constants, and hence geometries, with comparable accuracy. Two ground state Fermi resonances were identified by the distinctive patterns that such resonances produce in the images. 2D-LIF imaging is demonstrated to be a sensitive method capable of detecting weak spectral features, particularly those that are otherwise hidden beneath stronger bands. The sensitivity is demonstrated by observation of the three isotopomers of fluorobenzene-d1 in natural abundance in an image taken for a supersonically cooled sample. The ability to separate some of the 13C isotopomers in natural abundance is also demonstrated. The equipment required to perform 2D-LIF imaging with sufficient resolution to resolve the rotational features of large polyatomics is available from commercial suppliers.