Comprehensive high resolution study of the MSiH4(M=28,29,30) tetradecad stretching bands: Appearance and applications of the isotopic substitution effect in molecules of spherical symmetry.

A set of new relations between different spectroscopic parameters of the high symmetry XY4 spherical top molecules is derived on the basis of the general isotopic substitution theory, and a comprehensive high accurate analysis of the five stretching bands (the bands ν1+ν3(F2), 2ν3(F2), 2ν3(E), 2ν3(A1), and 2ν1(A1); the three latter ones are forbidden in absorption) of the tetradecad of the SiH4 molecule is made. The high resolution spectra of MSiH4(M=28,29,30) in their natural abundance were recorded with a Bruker IFS125 HR Fourier transform infrared spectrometer at the Technische Universität Braunschweig, Germany with an optical resolution of 0.003 cm-1 and theoretically analyzed (in this case, for the first time both for all five bands of the 29SiH4 and 30SiH4 species, and for 2ν3(E), 2ν1(A1) bands of the 28SiH4 one). The number of 2713 transitions of the 28SiH4 species (which is more than fourteen times higher compared to analogous studies carried out earlier) belonging to all five ro-vibrational bands of the tetradecad (which are 272, 1897, 15, 369, and 160 transitions of the ν1+ν3(F2), 2ν1(A1), 2ν3(A1), 2ν3(F2), and 2ν3(E) bands) were assigned with the values of quantum number Jmin/Jmax = 5/22, 1/24, 14/20, 4/24, and 6/23, respectively. The analysis of the obtained experimental data was made on the basis of the Hamiltonian model which takes into account all effects and resonance interactions which are possible in such molecular system. The weighted fit of the 2713 transitions led to a set of 40 rotational, centrifugal distortion, tetrahedral splitting, and resonance interaction parameters which reproduce the initial experimental data with an accuracy of drms=4.11×10-4 cm-1. An analogous analysis was made for the two other isotopologues, 29SiH4 and 30SiH4, of silane. In this case, the initial values of the most important spectroscopic parameters of both species were theoretically estimated on the basis of results of the isotopic substitution theory. As the result of the analysis, two sets of the 12 spectroscopic parameters were obtained from the weighted fits for the 29SiH4 and 30SiH4 molecules which reproduce the 461 and 375 initial ro-vibrational transitions with the drms of 3.91×10-4 and 4.9710-4 cm-1.

Improved Theory of the Effective Dipole Moments and Absolute Line Strengths of the XY2 Asymmetric Top Molecules in the X2B1 Doublet Electronic States.

A new effective dipole moment model for the XY2 (C2v-symmetry) molecule in a doublet electronic state is derived that includes (as special cases) all currently known models of effective dipole moments for such types of molecules, and allows us to take into account the influence of spin-rotation interactions on the effective dipole moment operator that were not considered in the preceding studies. Necessary for the analysis of absolute line strengths, the matrix elements of this dipole moment operator are derived. A comparison with the previous analog models is made and discussed. The efficiency of the obtained results is illustrated, which have been applied to a set of the "forbidden" ΔKa=2 transitions of the ν3 band of the OClO free radical molecule.

Effective Dipole Moment Model for Axially Symmetric C3v Molecules: Application to the Precise Study of Absolute Line Strengths of the ν6 Fundamental of CH335Cl.

The effective dipole moment model for molecules of axial C3v symmetry is derived on the basis of the symmetry properties of a molecule which, on the one hand, is of the same order of efficiency (but much simpler and clearer in applications) as the analogous models derived on the basis of the irreducible tensorial sets theory, and, on the other hand, mathematically more correct in comparison with concepts like the Herman-Walles function used in the models. As an application of the general results obtained, we discuss high-resolution infrared spectra of CH335Cl, recorded with the Zürich prototype ZP2001 (Bruker IFS125 HR) Fourier transform infrared spectrometer at a resolution of 0.001 cm-1 and analyzed in the region of 880-1190 cm-1 (ν6 bending fundamental centered at ν0 = 1018.070790 cm-1). Absolute strengths of more than 2800 transitions (2081 lines) were obtained from the fit of their shapes both with Voigt and Hartmann-Tran profiles, and parameters of the effective dipole moment of the ν6 band were determined by the computer code SYMTOMLIST (SYMmetric TOp Molecules: LIne STrengths), created on the basis of a derived theoretical model. As the first step of the analysis of the experimental data, assignments of the recorded lines were made. A total of 5124 transitions with Jmax = 68, Kmax = 21 were assigned to the ν6 band. The weighted fit of 2077 upper energy values obtained from the experimentally recorded transitions was made with a Hamiltonian which takes into account different types of ro-vibrational effects in doubly degenerate vibrational states of the C3v-symmetric molecule. As the result, a set of 25 fitted parameters was obtained which reproduces the initial 2077 upper "experimental" ro-vibrational energy values with a root mean square deviation drms=4.7×10-5 cm-1. At the second step of the analysis, the computer code SYMTOMLIST was used for determination of the parameters of the derived effective dipole moment model. Six effective dipole moment parameters were obtained from the weighted fit procedure which reproduces absolute experimental strengths of the 2804 initial experimental transitions with a relative drms=3.4%.

High resolution spectroscopy of asymmetric top molecules in nonsinglet electronic states: the ν3 fundamental of chlorine dioxide (16O35Cl16O) free radical in the X2B1 electronic ground state.

Highly resolved spectra of the 16O35Cl16O isotopologue of chlorine dioxide were recorded with a Bruker IFS 125HR Fourier transform infrared spectrometer in the region of the ν3 band. The analysis was made in the frame of the spin-rotational effective Hamiltonian (in A-reduction and Ir-representation) taking into account spin-rotational coupling operators up to the sixth order and the corresponding reduction of the Hamiltonian. The mathematical description of the ro-vibrational spectra was implemented to the specially created computer program ROVDES. Under the present experimental conditions, we were able to assign more than 5200 spin-rotational transitions to the ν3 band. This number is 2.4 times higher compared to the previous studies available from the literature. The vibrational ground state parameters were improved, and the 2220 upper spin-rotation-vibration energy levels were determined and used as initial data in the inverse spectroscopic problem with the derived effective spin-rotational Hamiltonian. A total of 37 fitted parameters were determined (22 rotational and centrifugal parameters and 15 parameters of spin-rotation coupling). The appearance of strong Coriolis resonance interactions between the (001) and (100) vibrational states in the sets of (001)[N,Ka = 9], (001)[N,Ka = 17], (001)[N,Ka = 18] and (001)[N,Ka = 19] spin-rotation-vibration levels was experimentally observed for the first time and explained. The drms = 1.4 × 10-4 cm-1 reproduction of the initial "experimental" upper ro-vibrational energy values was achieved which is considerably better compared to the use of parameters from a previous study ([J. Ortigoso et al., J. Mol. Spectrosc., 1992, 155, 25-43]).

High resolution ro-vibrational analysis of molecules in doublet electronic states: the ν1 fundamental of chlorine dioxide (16O35Cl16O) in the X2B1 electronic ground state.

We report the spectrum of the ν1 fundamental of chlorine dioxide centered in the infrared atmospheric window at 945.592 cm-1 measured with essentially Doppler limited resolution at an instrumental line width of 0.001 cm-1 using the Zürich prototype ZP2001 Bruker IFS 125 HR Fourier transform infrared spectrometer. The ro-vibrational line analysis is carried out with an improved effective Hamiltonian and a newly developed computer code ROVDES for the ro-vibrational spectra of open-shell free radical molecules including spin-rotation interactions. Accurate values of rotational, centrifugal and spin-rotational parameters were determined for 16O35Cl16O in the vibronic ground state X2B1 from more than 3500 ground state combination differences. The 7239 assigned transitions for the ν1 fundamental with Nmax = 76 and Kmaxa = 26 provide a set of 32 accurate effective Hamiltonian parameters for the ν1 fundamental (v1v2v3) = (100) (21 rotational and centrifugal distortion parameters and 11 spin-rotational interaction parameters). This effective Hamiltonian (A - reduction and Ir - representation) reproduces 1703 upper state energies from the experiment with a root-mean-square deviation drms = 1.67 × 10-4 cm-1 and the 7239 transition wavenumbers with drms = 3.45 × 10-4 cm-1. Our results provide a considerable improvement over previous results with which we compare and should provide a benchmark for theoretical studies with applications to atmospheric spectroscopy and laser chemistry, which are discussed in relation to our spectra.

On the method of precise abundance determination of isotopologues in a gas mixture.

A method is presented which allows one to derive partial pressures of isotopologue molecules in a gaseous mixture under the conditions of rapid isotope exchange. For this purpose, isotopic relations between effective dipole moment parameters of a "parent" molecule and its related isotopically substituted species are derived on the basis of the general isotopic substitution theory. The efficiency of the method is illustrated. The result was derived for the fundamental bands and is valid for any asymmetric top molecule. The discussed general consideration offers the possibility to obtain analogous results both for any overtone as well as combinational bands of any asymmetric top molecule, and (with minor corrections) for any symmetric and/or spherical top molecule. The validity and efficiency of the general results are confirmed by comparison of the general results obtained in the present paper with the experimental results for the H2O/HDO molecules with a deviation of 3 to 4%.

Isotope effects on the resonance interactions and vibrational quantum dynamics of fluoroform 12,13CHF3.

We report a comparison of the analysis of the low energy spectrum of 13CHF3 and 12CHF3 from the THz (FIR) range to the ν1 fundamental at high resolution (δ[small nu, Greek, tilde] < 0.001 cm-1 or otherwise Doppler limited) on the basis of FTIR spectra taken both with ordinary light sources and with the synchrotron radiation from the Swiss Light Source. Several vibrational levels are accurately determined including, in particular, the 2ν4 CH-bending overtone and the ν1 CH-stretching fundamental of 13CHF3. Comparison of experimental results with those from accurate full dimensional vibrational calculations allows for a study of the time-dependent quantum dynamics of intramolecular vibrational redistribution (IVR) in the CH chromophore both on short time scales (fs) and longer time scales (ps) when coupling to the lower frequency modes becomes important and where the 12C/13C isotope effects are very large.