The Effect Of Benzyl Group Of Tus Strengthened The Intermolecular Interactions Toward An Organophosphonate
1129 WordsApr 12, 20175 Pages
The 1H-NMR titration experiment revealed that the benzyl group of TUs strengthened the intermolecular interactions toward an organophosphonate (Figure 3). It has been known that the chemical shift of thiourea N–H changes according to the degree of hydrogen bond with an acceptor (i.e., DMMP), and its association constant (Ka) can be obtained by non-linear regression analysis (Figure 3a).18 The 1H-NMR spectra of TU 1, 2, 4, or 8 in chloroform-d (~100 mM) indicate that the chemical shift of the N–H protons shifted downfield with increasing amount of DMMP up to 80 equivalents, confirming the formation of the TU-DMMP complex through hydrogen bonding (Figure 3b). In the 1H-NMR titration experiment, proton peak shifting shows deferent shapes…show more content…
To support this hypothesis, the TU’s chemical shift changes were measured by varying its concentration in chloroform-d (the Supporting Information). Indeed, the presence of a N-phenyl group induced self-aggregation to a greater extent; the degree of peak shifts (self-aggregation) appeared in the order of TU 8 > 1 > 4 > 2, corresponding to the hydrogen-bond donor strength. Thus we may conclude that the incorporation of a phenyl group render a TU more acidic or strong in hydrogen bond and also promotes more self-aggregation. Figure 3. Calculated structure between 4 and DMMP by hydrogen bonding and CH-π interaction.
Although the decreased interaction of TUs 1 and 8 with DMMP than TUs 2 and 4, respectively, can be explained by the phenyl’s adverse effect, it is still not clear why N-benzyl-substituted TU 4 is better than unsubstituted TU 2 (or TU 8 better than TU 1). Since a N-benzyl-substituted TU has a similar donor strength as that of N-alkyl-substituted TU, we suspect that a secondary interaction may exist between TU 4-DMMP in order to have a larger association constant. To gain further insight into the binding mechanism between N-benzyl-substituted TU 4 and DMMP, density functional theory (DFT) calculation was performed using B3LYP functional with empirical dispersion of Grimme (B3LYP-D3BJ) and d3 basis set employing the Gaussian0920 suite of programs. As shown in Figure 4, the DFT calculation revealed the possible existence of CH-π interactions from