NMR study and AIM analysis for assignment of the two Z- and E-isomers in phosphorane containing a 2-thiazoline-2-thiol

3.1 Calculations

A facile synthesis of the reaction between triphenylphosphine 1, dialkyl acetylendicarboxylates 2 and 2-thiazoline-2-thiol 3 (as a SH-heterocyclic compound) has been earlier reported (Maghsoodlou et al., 2005) for the generation of phosphorus ylides 4(a, c) involving two geometrical isomers such as Z- and E-isomers. The reaction is shown in Fig. 1. For the assignment of the two Z- and E-isomers as a minor or major form in phosphorus ylides 4(a, c) containing a 2-thiazoline-2-thiol, first the Z- and E-isomers were optimized for all ylide structures at HF/6-31G(d,p) level of theory by Gaussian98 package program (Frisch et al., 1998). The relative stabilization energies for both the geometrical isomers have been calculated at HF/6-31G(d,p) and B3LYP/6-311++G(d,p) levels. Atoms in molecules (AIM) and natural population analysis (NPA) methods and also CHelpG keyword at HF/6-31G(d,p) level of theory have been employed in for the calculation of charge on the atoms order to gain a better understanding of most geometrical parameters in both the E-4(a, c) and the Z-4(a, c) of phosphorus ylides. The numbers of critical points and intramolecular hydrogen bonds have been recognized as well as the charge of atoms that was constructed on the Z- and E-isomers. The results altogether reveal the effective factors on stability of Z- and E-ylide isomers. The relative stabilization energies for the two Z-4(a, c) and E-4(a, c) isomers (see Figs. 2 and 3) are reported in Table 1, as can be seen, the Z-4a and the Z-4c isomers are more stable than the E-4a and the E-4c forms (0.33 and 1.82 kcal/mol, respectively) at B3LYP level.

Close

Figure 1 (i) The reaction between triphenylphosphine 1, dialkyl acetylenedicarboxylate 2 (2a or 2c) and 2-thiazoline-2-thiol 3 for generation of stable phosphorus ylides 4 (4a or 4c). (j) The two Z-4a and E-4a rotational isomers (minor and major, respectively) of ylide 4a.

Export to PPT

Close

Figure 2 (i) Intramolecular hydrogen bonds (dotted lines) in the two E-4a and Z-4a geometrical isomers of stable ylide 4a. (j) A part of molecular graphs, including intermolecular hydrogen bond at critical points (BCPs) for the two E-4a and Z-4a geometrical isomers of stable ylide 4a.

Export to PPT

Close

Figure 3 (i) Intramolecular hydrogen bonds (dotted lines) in the two E-4c and Z-4c geometrical isomers of stable ylide 4c. (j) A part of molecular graphs, including intermolecular hydrogen bond at critical points (BCPs) for the two E-4c and Z-4c geometrical isomers of stable ylide 4c.

Export to PPT

Further investigation was undertaken in order to determine more effective factors on the stability of the two Z- and E-isomers, on the basis of AIM calculations (Bader, 1990) at HF/6-31G(d,p) level of theory by the AIM2000 program package (Biegler König et al., 2001). In recent years, AIM theory has often been applied in the analysis of H-bonds. In this theory, the topological properties of the electron density distribution are derived from the gradient vector field of the electron density ρ(r) and on the Laplacian of the electron density ∇²ρ(r). The Laplacian of the electron density, ∇²ρ(r), identifies regions of space wherein the electronic charge is locally depleted [∇²ρ(r) > 0] or built up [∇²ρ(r) < 0] Bader, 1990.

Two interacting atoms in a molecule form a critical point in the electron density, where ∇ρ(r) = 0 is called the bond critical point (BCP). The values of the charge density and its Laplacian at these critical points give useful information regarding the strength of the H-bonds (Biegler König et al., 2001). The ranges of ρ(r) and ∇²ρ(r) are $0.002''–''0.035\text{e}/{\text{a}}_0^3$ and $0.024''–''0.139\text{e}/{\text{a}}_0^5$ , respectively, if H-bonds exist (Grabowski, 2001). The AIM calculation indicates intramolecular hydrogen bonds and critical points (H-BCP) for the two Z-4(a, c) and E-4(a, c) isomers. Intramolecular H-BCPs with a part of molecular graphs for the two rotational isomers are shown in Figs. 2 and 3. Most important rotational parameters involving some H-bonds (bond lengths and their relevant bond angles) are reported in Table 2. The electron density ρ(r), Laplacian of electron density ∇²ρ(r), and energy density −H(r) are also reported in Tables 3 and 4. A negative total energy density at the BCP reflects a dominance of potential energy density, which is the consequence of accumulated stabilizing electronic charge (Arnold and Oldfield, 2000). Herein, the number of hydrogen bonds in both categories (E-4a and Z-4a) and (E-4c and Z-4c) are 8 and 8 and also 15 and 13, respectively. The values of ρ(r) and ∇²ρ(r) are in the ranges 0.006–0.017 and $0.006''–''0.017\text{e}/{\text{a}}_0^3$ , 0.002–0.017 and $0.002''–''0.015\text{e}/{\text{a}}_0^3$ , 0.021–0.058 and $0.023''–''0.058\text{e}/{\text{a}}_0^5$ and 0.007–0.062 and $0.008''–''0.057\text{e}/{\text{a}}_0^5$ , respectively. In addition the Hamiltonian −H(r) is in the range 7.1–18.9 and 7.1–17.9 au and 5.1–18.2 and 4.8–17.7 au (see Tables 3 and 4).

These HBs show ∇²ρ(r) > 0 and H(r) < 0, which according to classification of Rozas et al. (2000) are medium−strength hydrogen bonds. In both ylides the dipole moment for the two E-4a and E-4c isomers (5.14 and 6.13 D, respectively) are smaller than the two Z-4a and Z-4c isomers (7.92 and 9.86 D, respectively) and the value of −H_tot (=H(r)) for the two E-4a and E-4c isomers (107.8 and 184.3 au, respectively) are larger than the two Z-4a and Z-4c isomers (98.6 and 153.6 au, respectively). These differences in the most important geometrical parameters of the E-4(a, c), with respect to the Z-4(a, c), involving a fair difference in dipole moment and a considerable difference in −H(r) and also approximately same number of hydrogen bonds, taken altogether, make a stability on these isomers in comparison with the Z-4(a, c) forms (the results are summarized in Table 5).

On the basis of theoretical calculations (Table 1), both the Z-4a and Z-4c have a slight stability with respect to the two E-4a and E-4c (0.33 and 1.82 kcal/mol) isomers and seem to be different from the results of predictable properties of the most important geometrical parameters (Table 5). Perhaps, this slightly different behavior is relevant to the huge structures of the two ylides 4(a, c) involving four large atoms such as the two sulfurs, one phosphorus, one nitrogen and four oxygens and also the very large number of other atoms (C and H). This point, made a limitation in application of basis set higher than B3LYP/6-311++G(d,p) in a higher performance for more accurate calculations. Nevertheless, the results that are shown in Table 5 (involving a considerable difference in total Hamiltonian (−H_tot) and a fair difference in dipole moment, taken altogetheras two dominant factors of stability on the two E-4a and E-4c) are compatible with the experimental results from the ¹H, ¹³C and ³¹P NMR spectroscopy which indicate the two isomers of Z-4a and E-4a with experimental abundance percentage of 75% for E-4a (as a major form) and also only a lone isomer of 4c (E-4c). Moreover, the total number of hydrogen bonds in the two Z-4c and E-4c (13, 15) are more than two in the Z-4a and E-4a (8, 8). This leads to a large rigidity in these geometrical isomers in comparison with the two Z-4a and E-4a forms. The rigidity of the two molecular structures by the very large intramolecular hydrogen bond accompanied by more steric factor of the bulky tert-buthyl groups (compare with dimethyl groups the structures in both the E-4a and Z-4a, Figs. 2 and 3) within the structures (E-4c and Z-4c forms) makes a good opportunity for enhancement of energy barrier and also explanation of abundance percentage of the these two isomers. Hence, interconversion process between the two E-4c and Z-4c isomers needs to pass through very high restricted barrier energy, particular in solution media. For this reason it is possible to see only a single isomer as a lone isomer of 4c (E-4c). On the contrary, interconversion process for both the E-4a and Z-4a geometrical isomers passes through a considerably low energy barrier, which leads to a plausible observation of the two E-4a and Z-4a (see Fig. 1, j).

Also, the charge on different atoms which are calculated by AIM and NPA methods and also CHelpG keyword at HF/6-31G(d,p) level are reported in Table 6 for the two Z- and E-isomers of ylides 4a and 4c. There is a good agreement between the results in three methods.

Furthermore, the individual chemical shifts have been characterized by NMR calculations at the mentioned level for the two major E-4(a, c) and minor Z-4(a, c) geometrical isomers. The total spin–spin coupling constant is the sum of four components: the paramagnetic spin–orbit (PSO), diamagnetic spin–orbit (DSO), Fermi-contact (FC), and spin-dipole (SD) terms. The value of chemical shifts (δ) and coupling constants (J_x–y) is reported in Tables 7–10. As can be seen there is a good agreement between both the experimental (Maghsoodlou et al., 2005) and theoretical chemical shifts (δ) and coupling constants (J_x–y). In the present work, molecular structures of ylides 4(a, c) involving four large atoms such as two sulfur, one phosphorus, one nitrogen and four oxygen atoms are huge molecules with the very large number of other atoms (C, H), this point makes a limitation for employment of basis set higher than B3LYP/6-311++G(d,p) in a higher performance for more accurate calculations. This limitation causes a small difference between both the experimental and theoretical coupling constants in some functional groups.