Date of Award
Doctor of Philosophy (PhD)
Qianli R. Chu
The quantification of a molecule’s conformational rigidity was achieved via development of a new measure called the Rigidity Factor, Rgf. This value is a unit interval wherein molecules with corresponding Rgf values closer to 1 are rigid and values closer to 0 are more flexible. The value is determined as a ratio between the molecule’s Van der Waals volume, VW, and it’s Van der Waals potential volume, VWpot. The VWpot was defined as the minimum total volume occupied by a rotamer ensemble of the molecule which has been aligned according to the RMSD for all heavy atoms to achieve a minimum volume. The rotamer ensembles were constructed using CREST (a tool in the xtb suite of programs) or Gromacs. The volumes were calculated using a custom Python script written for this project which uses a Monte Carlo volume approximation method. The script was written to take advantage of parallel computing architecture when present.The Rgf was calculated for several series of similarly structured molecules. These series include straight-chain alkanes, cycloalkanes, diols, diacids, and polymer repeat groups. The values for each series fit the expected trend of longer backbones having a greater flexibility and thus lower Rgf. It was observed that rotamer ensembles which represented a molecule with the capability of intramolecular hydrogen bonding, these rotamers dominate the ensemble. This gives a higher Rgf than expected and is more similar to a cyclic isomer of similar length. In one case, the repeat unit for polybutylene succinate, PBS, the H-bond donors were replaced with methyl groups. This destroyed intramolecular H-bond potential and the Rgf of the molecule more closely resembled the expected value. Conformational entropy, Sconf, for each compound was calculated using CREST. These values were plotted against Rgf for each compound and found to have a general correlation, i.e. structures with lower Rgf have higher Sconf. Several compounds were identified which had a higher-than-expected Sconf despite having a very high Rgf. It was theorized that degenerate rotamers born from the rotation of methyl groups provide this entropy. Since these structures are spatially and volumetrically equivalent, this rotation is not reflected in Rgf calculations. A series of new cyclobutane-containing, difunctional monomers (CBDx) was developed for the intention of being used in polymeric materials. Two polymers were synthesized using two different CBDxs and were shown to be colorless and very lightweight. The monomers were built on a maleic anhydride skeletal core and each contain a central cyclobutane moiety created in the key [2+2]photocycloaddition step between two maleic anhydride or imide olefins. This key step was achieved using low power, fluorescent UV-A light sources. By using these sources, the syntheses were much more efficient and safer than traditional photochemical reactions which often necessitate the use of specialized glassware to mitigate the dangers of the broad-spectrum UV bulbs employed. A poly(cyclobutanebutanebisimide), or PCBBI-1, was synthesized using a cyclobutane-containing diester (CBDE-6) and 1,7-heptandiol. The polymer was confirmed via NMR and HRMS of the soluble oligomeric components. The bulk material had a foamy texture with a very low density. The polymer was colorless. A second polymer, PCBBI-2, was synthesized using a cyclobutane-containing diol (CBDO-2) and adipic acid. The extremely poor solubility of the material precluded NMR or HRMS analysis. Polymerization was assumed based on physical characteristics of bulk material which was extremely similar to PCBBI-1. A classic [2+2] photocycloaddition reaction between maleic anhydride and dissolved ethylene gas was improved on the measures of safety, efficiency, and reduced complexity. This was achieved by using a UV source which targeted the absorption of the maleic anhydride starting material. In doing so, the use of high-power UV photoreaction bulbs can be avoided. This eliminates the need for specialized cooling glassware thus reducing apparatus complexity. An isolated yield of 70% was achieved via vacuum distillation after 5 days reaction time. A similar reaction using acetylene gas was performed but did not provide the same high efficiency. However, the reaction was a success as a crystal structure of the desired product was collected for the first time. Additionally, a new polymorph of a side product of the reaction, biscyclopropanedicarboxylic anhydride, CPDAn-1, was collected. In total, 21 compounds are presented here. Aside from maleimide and its photo dimer, all of these compounds are either not present or virtually ignored in literature for various reasons as discussed. Eleven of these compounds are novel and initial characterizations are presented herein. Further characterizations are provided for the remainder. Characterization includes 10 initial crystal structures reported for both novel and extant species.
Robertson, Joseph Earl Mcgilligan, "Quantification Of Rigidity And Synthesis Of Cyclobutane Bisimide Building Blocks" (2023). Theses and Dissertations. 5696.