Conformational stability, r0 structural parameters, barriers to internal rotation, vibrational assignments and ab initio calculations of c-C3H5GeH2CH3

Abstract

a b s t r a c tThe infrared spectra (3100-40 cm À1 ) of gaseous and amorphous solid and Raman spectra (3200-20 cm À1 ) of the liquid at various temperatures for methylgermylcyclopropane, c-C 3 H 5 GeH 2 CH 3 , have been obtained. Additionally, variable temperature (À55 to À100°C) studies of the infrared spectra of the sample dissolved in xenon have been recorded. From these spectral data, two conformers have been identified with one the cis (syn) form where the methyl group is over the three-membered ring and the other the gauche form. By utilizing six conformer pairs of the vibrational bands the enthalpy difference of the sample dissolved in xenon has been determined to be 43 ± 11 cm À1 (0.51 ± 0.13 kJ mol À1 ) with the gauche form the more stable conformer. It is estimated that there is approximately 30 ± 3% of the cis form present at ambient temperature. It was not possible to achieve crystallization of the compound by annealing it, and both conformers were present at all temperatures. The Ge-H distances of 1.531 and 1.533 Å for the gauche conformer have been determined from their stretching frequencies. By utilizing the microwave rotational constants of the gauche conformer for five isotopomers ( 70 Ge, 72 Ge, 73 Ge, 74 Ge, 76 Ge) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, the adjusted r 0 structural parameters have been obtained. The heavy atom distances (Å) are: (GeC 2 ) = 1.925(5); (C 2 C 4 ) = 1.517(3); (C 2 C 5 ) = 1.519(3); (C 4 C 5 ) = 1.502(3); (GeC 6 ) = 1.947(5) and the angles (°) are: \CGeC = 110.6(5); \GeC 2 C 4 = 120.1(5); \GeC 2 C 5 = 119.2(5). For the cis form very small differences of 0.003 and 0.001 Å from those of the gauche form are predicted for the GeC bonds whereas the other distances are predicted to be the same or differ at the most by 0.002 Å. A complete vibrational assignment is given for both the conformers. To support the vibrational assignments, normal coordinate calculations with scaled force constants from MP2(full)/6-31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values and infrared band contours. Barriers to internal rotation have been predicted. The results are discussed and compared to the corresponding properties of some similar molecules.

Rotational spectrum of five isotopologues of cyclopropylmethylgermane: Ab initio calculations, barrier to internal rotation and nuclear quadrupole coupling constants for the 73Ge nucleus

“…Ab initio calculations carried out in this work and in previous IR studies on CMG [4] predict that the gauche form is the more stable of the cis and gauche conformers (with an enthalpy difference of 0.51 (13) kJ mol À1 ), so it can be expected that the gauche conformer would be most intense in our supersonic expansion. This would also be consistent with our previous observations for CMS [2].…”

“…This is smaller than the estimate of this parameter obtained from analysis of the liquid phase IR spectrum of 5.89 kJ mol À1 (which utilized a vibrational frequency of 145 cm À1 for the methyl group torsion [4]) although it is expected that the microwave determination obtained directly from the internal rotation splittings will be a more reliable value. V 3 in CMG is about 71% of the Table 2 Spectroscopic constants for the gauche conformer of cyclopropylmethylgermane.…”

“…This instrument has been described previously [6] and is based upon the University of Kiel design [7]. A CMG sample was synthesized [4] at the College of Charleston and its identity confirmed by 1 H and 13 C NMR and by IR and Raman vibrational assignments, as discussed further in [4]. Samples for the microwave spectroscopic studies were prepared at EIU by the condensation of small quantities of CMG vapor into a 1L bulb at liquid nitrogen temperatures.…”

“…V 3 in CMG is about 71% of the Table 2 Spectroscopic constants for the gauche conformer of cyclopropylmethylgermane. Rotational constants, centrifugal distortion constants, and internal rotation parameters are tabulated; nuclear quadrupole coupling constants for the 73 Ge species are listed in Table 4 value obtained for the silicon analog, CMS (6.671(9) kJ mol À1 ), by similar analysis of the internal rotation splittings [2]; in the case of CMS, use of the IR torsional frequency [4] We can also compare this microwave barrier in CMG with a reasonable (although crude) ab initio estimate of this parameter of 4.2 kJ mol À1 (Table 2), computed by taking an energy difference between the staggered and the eclipsed conformation of the methyl group.…”

“…A study outlining an infrared analysis of CMG in the gaseous and amorphous solid state, Raman studies of the liquid, and variable temperature IR studies in liquid xenon has been published separately [4]. That work also utilized experimental rotational constants from the present work, in combination with ab initio results, to obtain adjusted r 0 structural parameters for the gauche conformer of this molecule.…”