Density functional theory investigation of the stereochemistry effects on 1H and 13C NMR chemical shifts of poly(vinyl chloride) oligomers
Introduction
The microstructure of poly(vinyl chloride) (PVC), which is determined by the concentrations and distributions of branches, of internal double bonds, and of the relative configurations of the stereogenic centers, is known to have important effects on the solid-state physical properties and morphologies of the PVC resins but also on their chemical stability. Their identification and characterization have thus been the focus of much research, a large part of it using high resolution NMR spectroscopy. Indeed, the NMR chemical shifts provide detailed information on the structural and electronic properties of polymers [1]. For instance, in the case of PVC, branch structures were investigated from 13C NMR spectra of PVC samples that have been subjected to reductive dechlorination [2], [3] because of the difficulty of a direct NMR examination of PVC. Then, from the structural characterizations, the degree of chain and hydrogen transfer reactions was inferred.
In order to assign the chemical shifts in the experimental spectra as well as to deduce information on the electronic structure of polymers, ab initio calculations turn out to be valuable tools [4]. As recently reviewed, substantial methodological efforts have been made to estimate chemical shifts with high accuracy, accounting therefore for electron correlation, solvent, ro-vibrational, and temperature effects [5], [6]. For instance, the coupled-cluster singles and doubles level including a perturbative treatment of triple excitations [CCSD(T)] enables to attain, for the nuclear magnetic shielding tensors, the quantitative accuracy required for establishing absolute NMR scales for various nuclei as well as for providing benchmark calculations in view of calibrating less expensive theoretical approaches [7]. However, such highly correlated schemes are still nowadays restricted to small compounds. On the other hand, density functional theory (DFT) schemes offer a good compromise between computational cost and reliability and therefore present the potential to assist in assigning experimental spectra. As a matter of illustration, DFT calculations helped in the configuration determination of passiflorin A [8]. Indeed, the errors on the chemical shifts are often systematic in nature and they can be corrected by using a linear scaling procedure [9]. In the case of 1H chemical shifts, Rablen et al. [10] determined the linear scaling parameters for a collection of exchange-correlation (XC) functionals combined with different atomic basis sets and found that the root-mean-square error on the so-predicted chemical shifts in comparison with solution experimental values can be as small as 0.15 ppm. This strategy was recently adopted by one of us to re-interpret the 1H NMR spectra of the E and Z isomers of alkyl phenyl ketone phenylhydrazones [11]. Linear scaling procedures based on DFT calculations have also been validated for predicting the nuclear shieldings of other atoms including 13C, 19F, and 35Cl [12], [13], [14]. In addition to DFT schemes, scaling constants were also deduced for chemical shifts evaluated at the Hartree–Fock level [13], [14].
This work aims at assessing the relationships between the configuration of PVC oligomers and their 1H and 13C NMR chemical shifts through a joint experimental/theoretical investigation. Following the study by Tanuma and collaborators [14], which shows that the scaling coefficients for the 13C chemical shifts deviate substantially from unity for small halogenated alkanes, the first step of this work consists in determining the linear regression parameters for Cl-containing compounds. Then, corrected calculated chemical shifts for stereoisomers of PVC oligomers are compared to NMR experimental data. The chemical shifts are rationalized in terms of the relative configurations of the stereogenic centers as well as their corresponding conformations induced essentially by steric interactions.
Section snippets
Experimental aspects
13C NMR spectra were recorded at 110 °C on a Varian Inova 400 (9.4 T) spectrometer in a dedicated 10 mm carbon probe. The sample was prepared by dissolving 200 mg of PVC in 3 ml of 1,2,4-trichlorobenzene and 1 ml benzene-d6. To reduce the carbon T1C relaxation decay times, 28 mg of the relaxation agent chromium(III)acetylacetonate (20 mM) was added, permitting the acquisition of quantitative spectra by using a delay time of only 5 s. To eliminate NOE effects, the decoupler was only gated on
Computational procedure and calibration step
Standard computational schemes have been adopted. The geometry optimizations have been carried out using the hybrid B3LYP exchange-correlation (XC) functional [15] and the 6-311G(d) basis set [16], [17]. The theoretical NMR chemical shifts have been calculated as the differences of isotropic shielding constants with respect to the reference (TMS) molecule. To evaluate the origin-independent shielding tensors, the GIAO [18], [19] method has been employed at the B3LYP level of approximation with
Results and discussion
Three stereoisomers of 3,5,7-trichlorononane have been considered to investigate the stereochemistry effects on the chemical shifts of the CHCl unit (Fig. 3) whereas for the CH2 unit, six stereoisomers of 2,4,6,8-tetrachlorononane have been studied (Fig. 4). Following the usual notation [31], the configuration of each stereoisomer, which possesses three/four asymmetric or pseudoasymmetric C atoms, is determined by two/three dyads (m for meso and r for racemic). Each stereoisomer can adopt
Conclusions and outlook
Using DFT approaches, 1H and 13C chemical shifts of stereoisomers of PVC oligomers have been simulated and compared to experimental data. Linear relationships between theoretical and experimental chemical shifts, which have been deduced for a set of small compounds containing H, C, and Cl atoms are used to correct the estimates for missing electron correlation and surrounding effects. Simulations are shown to reproduce the experimental trends among the stereoisomers and also, in most cases, to
Acknowledgments
This work was supported from Research Grants from the Belgian Government (IUAP No. P5-03 “Supramolecular Chemistry and Supramolecular Catalysis”). The authors thank P. Denkova for providing NMR chemical shifts of reference compounds. Ph. d’A. and E.B. thank the IUAP Program No. P5-03 for their PhD and postdoctoral grants, respectively. B.C. thanks the Belgian National Fund for Scientific Research for his Senior Research Associate position. J.W. is grateful to the Fund for Scientific
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