The acidity of strong and superstrong Brønsted acids, an outreach for the “limits of growth”: a quantum chemical study
Date
2011-08-03
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Abstract
Käesoleva uurimistöö eesmärgiks oli arvutuslikult uurida mono- ja polüasendatud erinevate Brønstedi hapete derivaatide happelisi ja superhappelisi omadusi ning elektronstuktuuri. Selleks kasutati arvutusmeetodit DFT B3LYP 6-311+G** tasemel, multilineaarset regressioonanalüüsi asendusrühmade sigma konstantide abil ning homodesmiliste isodesmiliste reaktsioonide meetodit.
Töös käsitletakse nelja erinevat closo-karboraani struktuuri tüüpi, mis baseeruvad ühenditel: CB4H5H, CB5H6H, CB11H12H and CB21H22H. Neile derivaatidele omased gaasifaasilised happelisused algavad väärtusest 319.6 kcalmol-1 ning ulatuvad allapoole happelisuse kasvu “maagilist” piiri 200 kcalmol-1 ja hõlmavad 42 erinevat ühendit. Uuritakse ka F ja CF3 asendajate mõju Kekulé benseeni ja fenooli valentsisomeeride stabiilsusele ja happelisusele. Benseeni ja fenooli valentsisomeeride derivaatide happelisused asuvad vahemikus 394.7 kcalmol-1 ja 288.5 kcalmol-1. Töö käigus disainiti mõned tugeivamatest orgaanilistest CH ja OH hapetest. Töö kolmandas osas on peamine tähelepanu monoasendatud aniliinide happelisustel. Arvutusi sooritati kokku ca 60 derivaadi jaoks. Monoasendatud aniliinide gaasifaasiliste happelisuste skaala ulatuseks saadi peaaegu 50 kcalmol-1 ning leiti, et arvutustulemused on väga heas kooskõlas eksperimendiga. Samuti uuriti mainitud ühendite pKa väärtusi DMSO’s kasutades COSMO-RS programmide süsteemi. Leiti, et COSMO-RS võimaldab rahuldavalt ennustada aniliinide happelisuste trende DMSO keskonnas.
The goal of the present work was to investigate computationally the electronic structure and superacidic properties of the mono- and poly-substituted derivatives of different classes of Brønsted acids. The work has three parts: carboranes, Kekulé benzene’s valence isomeres (prismane, benzvalene, Dewar benzene), and anilines. The substituent effects of the derivatives of carboranes and anilines were analyzed in terms of multilinear regression analysis using the substituent sigma constants (σF, σR-, σα). In case of derivatives of the valence isomers of Kekulé benzene and phenol as well as the anilines using homodesmotic isodesmic reactions approach the substituent effects were analyzed in the respective anionic and neutral forms. The first part, focusing on carborane acids, considers four different cage-types of closo-monocarboranes based on: CB4H5H, CB5H6H, CB11H12H and CB21H22H. The intrinsic gas-phase acidities of 42 derivatives of these systems calculated at DFT B3LYP 6-311+G** level of theory span from 319.6 kcalmol-1 down to less than the groundbreaking 200 kcalmol-1, thus, extending the “limits of growth” of acidity of neutral acids. The second part disserts the effects of F and CF3 substituents on the stability and acidity of the derivatives of the valence isomers of Kekulé benzene and phenol. The acidities of these derivatives range between 394.7 kcalmol-1 and 288.5 kcalmol-1. In the course of this work some of the strongest organic CH and OH acids were calculated. In the third part the main attention is on the acidities of monosubstituted anilines. The intrinsic gas-phase acidities of ca. 60 monosubstituted aniline derivatives were calculated (with acidity span of almost 50 kcalmol-1) and found to be in excellent correlation with the experimental results. Also the results of COSMO-RS method for calculating pKa values in DMSO were considered and found to be useful in predicting pKa trends of aniline acids.
The goal of the present work was to investigate computationally the electronic structure and superacidic properties of the mono- and poly-substituted derivatives of different classes of Brønsted acids. The work has three parts: carboranes, Kekulé benzene’s valence isomeres (prismane, benzvalene, Dewar benzene), and anilines. The substituent effects of the derivatives of carboranes and anilines were analyzed in terms of multilinear regression analysis using the substituent sigma constants (σF, σR-, σα). In case of derivatives of the valence isomers of Kekulé benzene and phenol as well as the anilines using homodesmotic isodesmic reactions approach the substituent effects were analyzed in the respective anionic and neutral forms. The first part, focusing on carborane acids, considers four different cage-types of closo-monocarboranes based on: CB4H5H, CB5H6H, CB11H12H and CB21H22H. The intrinsic gas-phase acidities of 42 derivatives of these systems calculated at DFT B3LYP 6-311+G** level of theory span from 319.6 kcalmol-1 down to less than the groundbreaking 200 kcalmol-1, thus, extending the “limits of growth” of acidity of neutral acids. The second part disserts the effects of F and CF3 substituents on the stability and acidity of the derivatives of the valence isomers of Kekulé benzene and phenol. The acidities of these derivatives range between 394.7 kcalmol-1 and 288.5 kcalmol-1. In the course of this work some of the strongest organic CH and OH acids were calculated. In the third part the main attention is on the acidities of monosubstituted anilines. The intrinsic gas-phase acidities of ca. 60 monosubstituted aniline derivatives were calculated (with acidity span of almost 50 kcalmol-1) and found to be in excellent correlation with the experimental results. Also the results of COSMO-RS method for calculating pKa values in DMSO were considered and found to be useful in predicting pKa trends of aniline acids.
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Keywords
bronstedi happed, derivaadid (keem.), keemilised omadused, elektronstruktuur, arvutuskeemia, Bronsted acids, derivatives (chem.), electronic structure, computational chemistry