Modelling of atactic and isotactic polymethacrylic acid with emphasis on dependence of structure on tacticity and dielectric constant of the solvent

V magistrskem delu sem se ukvarjal z molekulskim modeliranjem polimetakrilne kisline (PMA) s pomočjo semiempiričnih metod. Neionizirana ataktična in izotaktična oblika PMA se v raztopini obnašata različno, obnašanje je močno odvisno tudi od topila. Pokazal sem, da se strukturi verig obeh oblik v raztopini bistveno razlikujeta. Izotaktična oblika tvori dve ravnotežni strukturi, vijačno in polkrožno verigo. Ataktična oblika na drugi strani tvori le polkrožne verige, ki pri daljših verigah prehajajo v obliko zanke. Primerjava solvatacijskih energij obeh oblik je pokazala, da je izotaktična oblika bistveno slabše solvatirana, na kar kaže tudi njena netopnost v vodi. Primerjal sem tudi strukturo in solvatiranost obeh verig v vodi in dimetilsulfoksidu (DMSO), vendar rezultati niso pokazali pričakovanih razlik. Strukture in solvatacijske energije so bile zelo podobne, kljub temu, da se izotaktična oblika v neionizirani obliki v vodi ne topi, medtem ko je v DMSO topna. Poleg tega sem v obeh topilih poskusil oceniti tudi interakcijsko energijo in geometrijo dveh krajših fragmentov PMA. Izkazalo se je, da so interakcijske energije splošno po absolutni vrednosti večje za izotaktično obliko, s čimer tudi lahko razložimo veliko tendenco po medmolekulski asociaciji in slabšo topnost izotaktične oblike PMA, saj je manj karboksilnih skupin na voljo za tvorbo vodikovih vezi s topilom. To vodi do slabše solvatacije v topilih, ki lahko tvorijo vodikove vezi s PMA.In the following work I focused on the molecular modelling of polymethacrylic acid (PMA) with semiempirical methods. Completely protonated atactic and isotactic forms of PMA act very differently when dissolved in solution. The structure of chains of both forms of PMA differ greatly. While isotactic form can be present either in a helical or semicircular form, atactic form is only present in a semicircular form. Comparison of solvation energies shows that isotactic form is not solvated as well as the atactic form. This is in agreement with the experimental data. Next, I compared the structure and solvation of PMA in water and dimethyl sulfoxide (DMSO). The results for water and DMSO were very similar which is not what was expected, since isotactic form of PMA dissolves in DMSO, while it is insoluble in water. Lastly, I compared energies of interaction for dimers of both forms of PMA. The results showed greater interaction between two chains of isotactic form of PMA through potential hydrogen bonds when compared to the atactic form. This is in agreement with the experimental data, since less carboxylic groups are available for the formation of hydrogen bonds with the solvent, which in turn leads to less favourable solvation in solvents like water