Conformational stability of some silanes and amines from temperature dependent Infrared spectra of rare as solutions, r₀ structural parameters, ab initio calculations, and vibrational assignments
Panikar, Savitha Subash
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The infrared (3600-40 cm-1) and Raman (3600-20 cm-1) spectra of numerous molecules containing silanes or amines have been recorded in the gas, liquid and solid phases. Additionally, variable temperature studies of the infrared spectra of the sample dissolved in rare gases have been recorded. After identifying the conformer(s) from these spectral data, relatively complete vibrational assignments were proposed for these form(s). These vibrational assignments were supported by normal coordinate calculations with scaled force constants from MP2(full)/6-31G(d) calculations from which the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values and infrared band contours were predicted. Barriers to internal rotation are also reported for some of the silane molecules. Potential functions governing conformational interchange were also determined, based on torsional transitions frequencies, conformational enthalpy differences and dihedral angles. For methylsilylcyclopropane, the enthalpy difference of the sample dissolved in xenon has been determined to be 98 ± 13 cm-1 (1.17 ± 0.16 kJ mol-1) with gauche the more stable conformer. For ethylenediamine, three of the possible ten conformers have been identified, and the enthalpy differences have been determined among the most stable g′G′g′ conformer and the second stable conformer, g′G′t, to be 64 ± 6 cm-1 (0.77 ± 0.07 kJ mol-1) and the third conformer, gG′g′, to be 210 ± 19 cm-1 (2.51 ± 0.23 kJ mol-1). For 2,2-difluoroethylamine, the enthalpy differences among the most stable Tt conformer and the second stable conformer, Gg, is 83 ± 8 cm-1 (0.99 ± 0.10 kJ mol-1), the third stable conformer, Gt, is 235 ± 11 cm-1 (2.81 ± 0.13 kJ mol-1). For pyrrolidine, the envelope-equatorial conformer is determined to be more stable by 109 ± 11 cm-1 (1.30 ± 0.13 kJ mol-1) than the than the twist form. By utilizing previously determined rotational constants from microwave studies combined with structural parameters predicted from MP2(full)/6-311+G(d,p) calculations, adjusted r0 structures have been obtained for conformers of the various molecules.
Table of Contents
Introduction -- Experimental methods -- Theoretical methods -- Vibrational spectra, r₀ structural parameters, barriers to internal rotation, and ab initio calculations of ClCh2SiH3, Cl2CHSiH3, Clch2SiF, and Cl2CHSiF3 -- Conformational stability, r₀ structural parameters, barriers to internal rotation, vibrational spectra and ab initio calculations of c-C3H5SiH2CH3 -- Infrared and raman spectra, conformational stability and vibrational assignment of 1-chloro-1-silacyclopentane -- Infrared and raman spectra, ab initio calculations, conformational stability and vibrational assignment of 1-bromo-1-silacyclopentane -- Microwave, infrared and raman spectra, r₀ structural parameters, ab initio calculations and vibrational assignment of 1-fluoro-1-silacyclopentane -- Microwave spectrum, r₀ structure, dipole moment, barrier to internal rotation, and ab initio calculations for fluoromethysiland, CH2FSiH3 -- Vibrational spectrum, conformational stability, structural parameters and ab inition calculations of dimethylaminodifluorophosphine -- Conformational stability of ethylenediamine from temperature dependent infrared spectra of liquid xenon solutions, r₀ structural parameters, ab inition calculations, and vibrational assignments -- Conformational and structural studies of 2,2-difluoroethylamine from temperature dependent infrared spectra of xenon solution and ab initio calculations -- Conformational stability from variable temperature infrared spectra of xenon solutions, r₀ structural parameters and vibrational assignments of pyrrlidine -- Infrared spectra, vibrational assignment, ad ab inition calculations for n-bromo-hexafluoro-2-propanamine -- Conclusions -- Appendix, tables