Conformational Behavior and Structural Parameters of Some Cyclic and Chain Molecules

Abstract

The physicochemical properties of a molecule derive from its molecular structure, and that structure often involves the orientation of one part of the molecule about a particular bond with respect to the rest of the molecule. Additionally, the way the molecule will react chemically and its available reaction pathways are often critically dependent upon the architecture of the reactant molecule. The energy differences of various conformations generally result from non-bonded interactions which, even though they are individually too weak to determine any single geometric feature by themselves, may nevertheless act together to uniquely determine the spatial structures of large and complicated molecules such as proteins and even DNA. Therefore, conformational analysis can lead to significant improvements in the understanding of more complex systems. The mid-infrared (3100-400 cm⁻¹) and Raman spectra (3200-20 cm⁻¹) of a number of substituted ring and straight chain molecules were recorded in the gaseous, liquid and solid phases. Additionally, variable temperature studies of the infrared spectra of the sample dissolved in liquid xenon have been carried out. From these spectral data, the possible stable conformers have been identified and the enthalpy differences are given among the various forms for each molecule. By utilizing microwave determined rotational constants for isotopomers, combined with structural parameters predicted from MP2(full)/6-311+G(d,p) calculations, adjusted r0 structural parameters have been obtained for the stable forms of some of the aforementioned molecules. Complete vibrational assignments are proposed for the stable conformers of each molecule. 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. The abovementioned spectroscopic and computational methods have been successfully applied for the determination of the enthalpy difference(s) between two or more conformers and r₀ structural parameters of 2-methylbutane, isocyanocyclopentane, cyanocyclopentane, cyclopropylcyanosilane, and (chloromethyl)fluorosilane. In the study of 2-methylbutane, both the trans and gauche conformers have been identified and the enthalpy difference between conformers has been determined to be 161 ± 5 cm⁻¹ (1.93 ± 0.06 kJ/mol) with the trans conformer the more stable form. The percentage of the gauche conformer is estimated to be 18 ± 1% at ambient temperature. For isocyanocyclopentane, the axial (Ax) and envelope-equatorial (Eq) conformers have been identified. The enthalpy difference between these two conformers has been determined to be 102 ± 10 cm⁻¹ (1.21 ± 0.11 kJ mol⁻¹) with the Ax conformer the more stable form. The percentage of the Eq conformer is estimated to be 38 ± 1% at ambient temperature. Similar to isocyanocyclopentane, the Eq and Ax conformers have been identified for cyanocyclopentane. The enthalpy difference between these two rotamers has been determined to be 55 ± 12 cm⁻¹ (0.66 ± 0.14 kJ/mol) with the Eq conformer the more stable form in this case. The percentage of the Ax conformer is estimated to be 45 ± 1% at ambient temperature. For cyclopropylcyanosilane, the enthalpy difference between the cis and gauche conformers has been determined to be 123 ± 13 cm⁻¹ (1.47 ± 0.16 kJ mol⁻¹) with the cis conformer as the more stable form. Approximately 48 ± 2 % of the cis form is present at ambient temperature. Both trans and gauche conformers are predicted and observed for (chloromethyl)fluorosilane. The enthalpy difference between the trans and gauche conformers in xenon solutions has been determined to be 109 ± 15 cm⁻¹ (1.47 ± 0.16 kJ mol⁻¹) and in krypton solution the enthalpy difference has been determined to be 97 ± 16 cm⁻¹ (1.16 ± 0.19 kJ mol⁻¹) with the trans conformer as the more stable form in both cases. Approximately 46 ± 2 % of the trans form is present at ambient temperature. For all aforementioned molecules, r₀ structural parameters have been determined for heavy-atom bond lengths, bond angles, and dihedral angles. In addition, comparisons with the conformations and r₀ structural parameters of related molecules have been made and are discussed.