Millimeter-wave and high-resolution FTIR spectroscopy of : the ground and states
Introduction
Fluorosilanes form a homologous series of molecules for which we can expect systematic effects. The present rotational and rovibrational spectroscopic work on has been initiated in order to fill an existing gap, after [1], [2], [3], [4], [5], and [6], [7] have been extensively studied. All these molecules are of interest in the search for new laser sources in the far infrared region because both the SiF stretching and HSiH bending modes fall into the range of the laser [8], [9]. Moreover, , as well as the other fluorosilanes are of interest for electronic or optical devices, as they are used in the production of some optical materials and semi-conductors [10].
is a prolate asymmetric rotor that has been the least studied of the fluorosilanes and only very few studies have been reported on this molecule [11], [12], [13]. No rovibrational state-resolved high-resolution work has been published at all. Only a few low-resolution infrared studies are available, the most recent one being of 1964 [13]. Furthermore, some spectroscopic constants have been determined in some rotational studies of the ground and the states. These are, however, restricted to transitions with small rotational quantum numbers, i.e., J smaller than 50 and smaller than 22, observed in the 8–40 GHz range [12].
In the frame of systematic studies of fluorosilanes, the present contribution reports on first investigations of by studying rotational transitions of the ground and first excited states, extending rotational spectroscopy to higher frequencies and performing high-resolution infrared measurements. These are also important to define overtone and combination levels that may perturb other vibrational states.
Here we report on rotational transitions that have been measured between 350 and 471 GHz, and on infrared spectra recorded in the 250– region in which the fundamental band is located. The rotational analysis of the ground state, and of the state based on simultaneous rotational and rovibrational studies, will be presented.
Accurate quartic and sextic experimental centrifugal distortion constants were determined. Experimental rotation–vibration interaction constants were obtained for the state. These parameters were also calculated using a new ab initio anharmonic force field, as we have previously done also for [14]. The first goal is to check the accuracy which may be achieved with the ab initio method. The second goal is to determine a semi-experimental equilibrium structure using the ground state rotational constants and the rotation–vibration interaction constants deduced from the cubic ab initio force field. This method is indeed known to be generally reliable [15]. Furthermore, in the particular case of difluorosilane, it seems rather difficult to obtain a purely experimental equilibrium structure because several vibrational states are in resonance in the as well as in the isotopic species. Moreover, is inactive in the infrared.
Experimental details will be reported in Section 2 while the analysis of the spectra will be described in Section 3. Thereafter the models used, and the results of fitting the experimental data obtained, will be described in Section 4. A conclusion and an outlook will be given at the end of the contribution, which is the first reporting on our ongoing high-resolution spectroscopic studies of .
Section snippets
Material
Silanes are reactive molecules that require precautions during their syntheses, which usually have to be performed in dry nitrogen atmosphere, or in a greaseless vacuum line. The synthesis of starting from involves three steps according to Eqs. (1), (2), (3) [16]:The isolation of the reaction products according to Eq. (1) [17] was achieved by first separating the salts from the rest of the mixture by
Structure and vibration modes of
is a prolate asymmetric top molecule with symmetry (see Fig. 1). Its asymmetry parameter is . reveals nine different vibration modes (see Table 1). The different infrared active species and give rise to b-, c-, and a-type bands, respectively; the vibration is inactive in the infrared. There are no bands with hybrid rotational structure allowed for symmetry. The band, reaching the lowest excited vibrational state, corresponds to the symmetric scissors
General aspects
All experimental line positions have been analysed using Pickett’s program [26] both for Watson’s A- and S-reductions of the Hamiltonian in representation [27]. The ground state and the state were fitted independently.
The Hamiltonians used for the analysis of the spectra including octic centrifugal distortion terms are given by Eqs. (4), (5) [25], where , are the projections of the angular momentum P on the three principal axes of the molecule:
A-reduction
Methods of computation
The ab initio force field was calculated at the correlated level of second-order Møller–Plesset perturbation theory [28] using the Gaussian 94 program [29]. The correlation-consistent polarized valence basis sets (cc-pVnZ) [30], [31] were used throughout. For the H and C atoms, the cc-pVTZ basis set was used, whereas for Si it was replaced by the larger cc-pVQZ+1 one. This latter basis set is derived from the cc-pVQZ one by adding a single set of d functions whose exponent equals the highest d
Equilibrium structure
The theoretical rotation–vibration interaction constants deduced from the ab initio cubic force field of the previous section were combined with the experimental ground state rotational constants of and [12] to yield the semi-experimental equilibrium rotational constants of Table 6. The equilibrium structure was calculated from a non-weighted least-squares fit of the equilibrium moments of inertia and is given in Table 7. It is compared to the approximate equilibrium structure
Conclusions
We report a study of difluorosilane, in which for the first time infrared and MMW transitions have been measured, assigned, analysed, and combined with existing rotational data, for the ground state and state. The band centre has been accurately determined and higher order centrifugal distortion constants have been obtained from the merged rotational and rovibrational data for both states.
High level ab initio calculations using MP2 methods with a large basis set of at least triple zeta
Acknowledgments
This work was supported by the European Community (Research Training Network number HPRN-CT-2000-00022) and the PICS 599 project. We thank Drs L. Margulès and H. Beckers for assistance and Dr. J. Breidung for many helpful discussions.
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