DFT-GIAO calculation of properties of 19F NMR and stability study of environmentally relevant perfluoroalkylsulfonamides (PFASAmide)

Abstract

Compostos orgânicos perfluorados (POCs), tais como perfluoroctanossulfonato (PFOS) e perfluoralquilsulfonamida (PFASA), são compostos que, recentemente, têm atraído considerável atenção mundial, devido à sua alta persistência e ampla distribuição no ambiente. Entre os métodos espectroscópicos usados para estudar PFASA, ressonância magnética nuclear de 19 F (RMN 19 F) é muito eficiente devido à sua habilidade para determinar concentrações de PFASA em amostras biológicas e para medir poluição em amostras de água. Por esta razão, um estudo teórico das propriedades de RMN 19 F foi realizado. Neste estudo, determinamos a constante de blindagem (σ) para diferentes núcleos de flúor das 18 moléculas em estudo, usando teoria do funcional de densidade (DFT) e o método GIAO no nível B3PW91/6-31+G(d,p). Os cálculos de σ foram realizados no vácuo e na presença de solvente. Os valores dos deslocamentos químicos (d) foram também calculados num nível diferente da teoria. Os melhores resultados foram obtidos com o nível de cálculo DFT-GIAO/B3PW91/6-31+G(d,p), considerando solventes como dimetilsulfóxido (DMSO), clorofórmio (CHCl 3), acetona (CH 3 COCH 3) e metanol (CH 3 OH). Os resultados foram interpretados em termos de dureza calculada no nível DFT/B3PW91/6-31+G(d, p). O comportamento da dureza foi maior nas moléculas de quatro carbonos PFASA do que nas de oito carbonos. Isto explica, teoricamente, a resistência de quatro carbonos PFASA para serem transformados em perfluorobutanossulfonato (PFBS). Perfluorinated organic compounds (POCs), such as perfluorooctanesulfonate (PFOS) and perfluoroalkylsulfonamide (PFASA) are compounds that have recently attracted considerable attention worldwide because of its high persistence and wide distribution in the environment. Among the spectroscopic methods used to study the PFASA, 19 F nuclear magnetic resonance (NMR 19 F) is very effective, due to its ability to determine concentrations of PFASA in biological samples and measure pollution in water samples. For this reason, a theoretical study of the properties of 19 F NMR was performed. In this study we have determined the shielding constant (σ) for different fluorine nucleus of the 18 molecules under study, using density functional theory (DFT) and GIAO method with the B3PW91/6-31+G(d,p) level of calculation. The σ calculations were made at vacuum and in presence of a solvent. The values of chemical shifts (d), were also calculated in a different level of theory. The best results were obtained with the level of calculation DFT-GIAO/B3PW91/6-31+G(d,p) by considering the solvent such as dimethylsulfoxide (DMSO), chloroform (CHCl 3), acetone (CH 3 COCH 3) and methanol (CH 3 OH). The results were interpreted in terms of calculated hardness at DFT/B3PW91/6-31+G(d, p) level. The behaviour of the hardness was higher in the molecules of four carbons PFASA than eight carbons. This explain theoretically resistance of four carbons PFAS to be transformed into perfluorobutanesulfonate (PFBS).

A theoretical and experimental study of the NMR spectra of 4,5,6,7-tetrafluorobenzazoles with special stress on PCM calculations of chemical shifts

“…Other authors have compared different basis sets agreeing that the 6-311++G(d, p) is a convenient one for 19 F NMR chemical shifts. [53][54][55] -DFT-PCM improves the gas phase results and, combined with small clusters (trimers), proved a good compromise for calculating solid-state chemical shifts. This should be useful for identification of organic compounds using 19 F NMR fingerprints.…”

“…‐The present work confirms that DFT/GIAO/6‐311++G(d,p) calculations afford reliable chemical shifts when combined with robust regression equations relating σ and δ values. Other authors have compared different basis sets agreeing that the 6‐311++G(d,p) is a convenient one for 19 F NMR chemical shifts …”

Computational protocol for predicting 19F NMR chemical shifts for PFAS and connection to PFAS structure

“…All quantum mechanical calculations were performed using Gaussian 16 RevC.01 29 software package. Geometries were optimized using the long‐range corrected (LC) ωB97XD 30,31 functional, or Becke three‐parameter Lee‐Yang‐Parr (B3LYP) functional, and 6–31+ G ( d , p ) basis set in the gas phase 20,32–34 unless otherwise noted. The 19 F NMR isotropic shielding tensors () were calculated using the Gauge‐Independent Atomic Orbital (GIAO) methodology.…”

“…Computational methods can be used to obtain 19 F NMR signatures of PFAS families in the absence of standards, but an accurate protocol must be developed. Previous papers in literature have computationally studied various fluorinated aromatic molecules in the gas phase, however, PFAS also contain oxygen heteroatoms (such as in GenX™) and anionic, cationic, and zwitterionic moieties which are not present in previous studies 19–22 . Previous work by Tantillo et al has used computational methods to predict NMR shifts of different organic molecules, namely regarding proton and carbon NMR.…”

“…Previous papers in literature have computationally studied various fluorinated aromatic molecules in the gas phase, however, PFAS also contain oxygen heteroatoms (such as in GenX™) and anionic, cationic, and zwitterionic moieties which are not present in previous studies. [19][20][21][22] Previous work by Tantillo et al has used computational methods to predict NMR shifts of different organic molecules, namely regarding proton and carbon NMR. However, given the broad range of fluorine chemical shifts (0 to 400 ppm), fluorine's high electronegativity and low polarizability, and the ability of PFAS to adopt helical structures, we believe that 19 F NMR is the most suitable method to probe these structures.…”