Conformational Stability Studies By Variable Temperature Infrared Spectra Of Rare Gas Solutions And Ab Initio Calculations Of Some Substituted Three Membered Ring Compounds And Ethylene Molecules

The infrared and Raman spectra of a number of amine/hydrazine and cyclopropylmethyl compounds in the gaseous, liquid and solid phase were recorded, along with temperature-dependent infrared spectra of the compounds dissolved in liquefied noble gases. Complete vibrational assignments were proposed for the most stable conformer of each compound and a number of vibrational assignments were made for the less stable conformers, based on group theory, scaled ab initio frequencies, infrared intensities, Raman activities, depolarization ratios, gas-phase infrared band envelopes and spectral comparison between fluid and polycrystalline phases. Thorough normal coordinate analysis was made for each conformer and potential energy distributions were calculated for each normal mode. For ethylamine, the trans conformer was determined to be 54 ± 4 cm−1 more stable than the gauche form. For ethylmethylamine, the Me-trans conformer was determined to be 335 ± 35 and 177 ± 27 cm−1 more stable than the LP-trans and the H-trans conformers, respectively. For allylamine, the Ct conformer was found to be the most stable, the Gg, Gt, and Cg forms were determined to be 92 ± 8, 122 ± 12 and 240 ± 60 cm−1 less stable, respectively. For N-methylpropargyl amine, the LP-trans conformer was determined to be 360 ± 36 cm−1 more stable than the H-trans conformer. For methylhydrazine, the inner conformer was determined to be 323 ± 30 cm−1 more stable than the outer conformer. For 1,1 dimethylhydrazine, only the gauche conformer was identified. For aminomethyl cyclopropane, the gauche-gauche-1 conformer was found toe be the most stable, the Gt, Cg, Gg-2 and Ct conformers were determined to be 109 ± 11, 267 ± 28, 400 ± 40 and 480 ± 48 cm−1 less stable, respectively. For fluoromethylcyclopropane, the gauche conformer was determined to be 262 ± 26 cm−1 more stable than the trans conformer. For cyclopropylmethyl isothiocyanate and isocyanate, the gauche-cis conformer was determined to be 228 ± 23 and 193 ± 19 cm−1 more stable than the cis-trans form, respectively. Adjusted r0 structural parameters were determined for all molecules. Potential functions governing conformational interchange were also determined for all molecules. Potential functions governing conformational interchange were also determined, based on torsional transitions frequencies, conformational enthalpy differences and dihedral angles.

Studies of the structure and conformational stabilities of molecules have become increasingly important due in part to advances in the fields of biochemistry and nanochemistry. Specifically the conformation of organoamines is currently at work in every human alive with this arising from the amino acids in proteins and the substituted bases in DNA and RNA. The importance of conformational stability is further demonstrated in biological systems by enzyme binding where the conformational changes of the protein can change the binding site either enhancing or deactivating the enzyme. These systems however tend to have relatively large molecules with many conformers. Thus in this work we explore the use of multiple spectroscopies to stuy four and five membered rings and amines. Infrared, far-infrared, and Raman spectroscopy to of vapor, liquid, solid, and rare gas solutions to determine vibrational assignments, conformational stabilities, and even some structural parameters. We combine this vibrational approach with ab initio calculations and microwave spectroscopy for greatly improved conformational and structural studies. Also explored are new methodologies of variable temperature Raman spectra of rare gas solutions to improve conformational stuies of polar compounds or compounds with four or more conformers. Additionally chirped-pulse FT microwave spectroscopy has been applied to structural problems to greatly improve the structural parameters.

The physiochemical properties of a molecule of interest are usually obtained from its molecular structure, and it involves the orientation of one part of the molecule about a particular bond with respect to the rest of the molecule. These properties depend mainly upon the three-dimensional arrangement of one part of the molecule in space with respect to the other and, additionally, the way the molecule will react chemically and its available reaction pathways are often critically dependent upon the orientation of the reactant molecule. The energy differences of various conformations generally strongly couple or interact by way of nonbonded interactions which even though are individually too weak to determine, any single geometric feature may nevertheless act together to uniquely determine the spatial structures of large and complicated molecules such as proteins and DNA. Therefore, conformational analysis can lead to significant improvement in the understanding of more complex system. The infrared (3100-40 cm−1) and Raman spectra (3200-20 cm−1) of a number of substituted (thiol, isocyanide, cyanide, acetylchloride, silane, amine, and phosphine) 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 xenon has 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 the isotopomer(s) combined with the structural parameters predicted from the MP2(full)/6 311+G(d,p) calculations, adjusted r0 structural parameters have been obtained for the stable forms of some 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 results will be discussed and compared to the corresponding properties of some analogous molecules wherever possible.

The infrared and Raman spectra of compounds with hydroxyl and amino functional groups, as well as some with cyclic skeletal structures have been recorded of the gas and in condensed phases. Temperature dependent infrared spectra in xenon and krypton solutions were also recorded. A complete vibrational assignment, conformational stability determination and adjusted r0 parameters have been obtained for each of the most stable conformers and in some cases for lesser stable conformers. The results of these investigations were supported by ab initio predicted infrared intensities and depolarization ratios, band contours an conformational stabilities. Normal coordinate calculations were carried out for all of the molecules. From these calculations potential energy distributions for all the fundamentals were predicted for each molecules from which approximate descriptions of the vibrations were ascertained. Cyclobutanes substituted with asymmetric rotor such as hydroxyl and amine groups have four possible conformers. In case of cyclobutanol two of the four possible stable conformers have been confidently identified and their order of stabilities has been experimentally determined whereas three out of the four conformers were experimentally determined for cyclobutylamine. For halogen substituted cyclobutanes only two possible determined for cyclobutylamine. For halogen substituted cyclobutanes only two possible conformers exist and the enthalpy difference between the more stable equatorial conformer and the axial form has been determined for fluoro-, chloro-, and bromocyclobutane. For fluorocyclopentane only a single twist conformer was found which is at a variance to previously reported structures of both envelope-equatorial and envelope-axial conformers or only a single envelope-equatorial conformer. Thus, fluorocyclopentane provides a unique example of pseudorotation for five member rings. However for cyanocyclopentane both the equatorial and axial forms were observed. The conformational stabilities, adjusted R0 structural parameters and other scientific information are discussed and compared to the corresponding properties of some similar molecules based upon their functional groups and molecular structure.

Conformational Analysis: Scope and Present Limitations contains the proceedings of the Brussels International Symposium on Conformational Analysis held in Brussels, Belgium, in September 1969. The papers focus on the theoretical aspects and applications of conformational analysis, such as those concerning the aliphatic and especially the cyclic series. Topics covered include the geometry of five-membered rings; conformational transmission in steroids; conformational aspects of N-quaternization; and applications of nuclear magnetic resonance spectrometry in conformational studies of cyclohexane derivatives. This book is comprised of 20 chapters and begins with a discussion on the conformational aspects of some five-membered ring compounds based mainly on observed (diffraction methods) and calculated torsional angles. The reader is then introduced to nuclear magnetic resonance studies of the conformations and conformational barriers in cyclic molecules; conformational studies of six-membered heterocycles; conformational transmission in steroids; and solvolytic cyclizations involving double bonds. The remaining chapters explore the conformational analysis of methylcyclohexane, cyclohexane systems, and carbonium ions; conformations of membrane-active cyclodepsipeptides; energetics of isomeric transition states and competitive reaction pathways in conformational analysis; and conformational aspects of the reaction of the 1-methylcyclodecane-l,6-diols with acid. This monograph will be of interest to organic chemists.