Sulfur-33 NQR investigation of the electric-field-gradient tensor in an organosulfur compound

1 Introduction

Nuclear quadrupole resonance (NQR) spectroscopy is one of the standard chemical analysis techniques in chemistry [1], [2]. The NQR, which arises from the interactions of the electric field gradient (EFG) with the quadrupole moment of the target nuclei (I>1/2), gives rich information on chemical properties, regardless if a sample status is crystalline, powdery or amorphous.

Analysis of the bonding of sulfur is highly important for the understanding of the physical characteristics of rubber, since this element is mainly involved in cross-linked structures. The nature of the cross-linking has remained unresolved yet for a long time despite its importance. This may be because conventional analytical methods, such as X-ray diffraction and electron microscopy, are less suitable for amorphous materials. Sulfur contains a nuclear magnetic resonance (NMR)/NQR active nucleus, ³³S (I=3/2, natural abundance=0.75%, γ=2.055×10⁷ rad T⁻¹ s⁻¹, Q=−5.5×10²⁶ m⁻²), so that NQR and solid-state NMR spectroscopy are expected to allow an application to the structural analysis of cross-linked rubber.

There have been a large number of reports on solid-state ³³S NMR of inorganic compounds mainly with ionic bonding [3], [4], [5]. Since the molecular symmetry is generally high in inorganic molecules, the magnitude of quadrupolar interactions, which are roughly proportional to the spectral width of the NMR spectra, tends to be small, i.e. the experimental conditions are not critical. On the other hand, there have been very few reports on the solid-state ³³S NMR of organosulfur compounds with covalent bonding, in which the magnitude of quadrupolar interactions tends to be larger. Recently, O’Dell and Moudrakovski [6] reported the ³³S NMR results for ³³S-enriched α-S₈ (with covalent bonding) using the wideband uniform rate smooth truncation–quadrupolar Carr-Purcell-Mei boom-Gill (WURST-QCPMG) techniques. Our group also presented ³³S NMR parameters of an organosulfur compound, ³³S-enriched diphenyl disulfide, using a low magnetic field [7].

From the viewpoint of experimental conditions, NQR techniques are much easier to apply than solid-state NMR spectroscopic investigations since the former requires no external magnetic fields. As for ³³S NQR experiments of compounds with covalent bonds, there was a literature on ³³S NQR of α-S₈ in 1953 [8]. In principle, however, the asymmetry parameter of the ³³S EFG tensor cannot be obtained from the NQR experiments of spin-3/2 nuclei. Moreover, the low natural abundance of ³³S made it very difficult to perform ³³S NQR experiments of organosulfur compounds. As a result, there have been few reports on ³³S NQR experiments of organosulfur compounds. In this paper, we describe the synthesis of an ³³S-enriched organosulfur compound, [³³S]-S-4-phenyl 4-toluenethiosulfonate (see Scheme 1), and present the results of ³³S NQR experiments, as a benchmark for future applications of the structural analysis of cross-linked rubber. The 2D zero-field nutation NQR technique [9], [10] was applied to determine the asymmetry parameter, and quantum chemical calculations were carried out to obtain the ³³S EFG tensor orientation.

Scheme 1:

Synthetic route of ³³S-enriched S-4-phenyl 4-toluenethiosulfonate.

2 Experimental

Sample: Sulfur-³³S 99 at%, purchased from Merck (Kenilworth, NJ, USA), was used without further purification. As shown in Scheme 1 [11], [³³S]-S-4-phenyl 4-toluenethiosulfonate was obtained from [³³S]-diphenyl disulfide according to a published procedure [7]. The details are as follows. To a mixture of [³³S]-diphenyl disulfide (82.5 mg, 0.38 mmol), sodium p-toluenesulfinate (132.2 mg, 0.74 mmol), and NH₄BF₄ (53.2 mg, 0.51 mmol) in DMA (0.75 mL) and H₂O (0.25 mL) were added CuI (12.2 mg, 0.063 mmol) and Phen·H₂O (8.8 mg, 0.049 mmol). The mixture was stirred at T=30°C for 21 h in air using a balloon to safely monitor the reaction. After the residue (white powder attached to the wall of the flask) was removed by dissolution in CHCl₃ not to lose any of the ³³S-enriched sample, the solution was washed with H₂O and saturated sodium chloride and dried over anhydrous magnesium sulfate. Chromatography on silica gel (CHCl₃-hexane=1:1, v/v) caused [³³S]-S-4-phenyl 4-toluenethiosulfonate (33 mg, 0.12 mmol, 33%) to be a white solid (melting point: 76.4−77.2°C). The ¹H NMR and Fourier-transform infrared spectroscopy (FT-IR) spectra of [³³S]-S-4-phenyl 4-toluenethiosulfonate are given in the Supporting information available online.

NQR measurement: Approximately 30 mg of [³³S]-S-4-phenyl 4-toluenethiosulfonate was packed into a polytetrafluoroethylene tube (3.0 mm outside diameter), and both ends were sealed with an epoxy-based adhesive. The homemade NMR probe, in which the sample tube was wounded by 14-turn copper wire, was inserted to the cryogen-free cryostat. Temperatures were changed from T=300 to 110 K, and the system was held for more than 10 min to allow it to reach thermal equilibrium when the temperature changed. All the NQR experiments were performed by radioRadioProcessor™ and PulseBlasterDDS-II-300 boards (SpinCore Technologies. Inc., Gainesville, FL, USA) with pulsed NMR amplifiers of 300 W (model 3900C-12, AMT/Herlay, Lancaster, PA, USA) or 1 kW (model 3446, AMT/Herlay, Lancaster, PA, USA), pre-amplifier (model N141-3028A, Thamway Co., Ltd., Shizuoka, Japan) and NMR duplexer (model N120-3066H, Thamway Co., Shizuoka, Ltd., Japan). The standard QCPMG sequence [12] was used to detect ³³S NQR signals with the following experimental conditions: π/2 pulse width=3.0 μs, number of echo=10, acquisition points for each echo=100, recycle delay=1.0 s, accumulation number=640 and spectral width=1 MHz. The 2D nutation NQR sequence [9], [10] was performed with the following parameters: pulse increments of 2.0 μs, irradiation frequency=23.26 MHz, scan number t₂=1400, inter-pulse delay time=40 μs and number of t₁ slices=43. After all the time domain signals had been Fourier transformed with phasing in f₂ dimension, Fourier transformation was carried out in f₁ dimension with zero-filling to 512 points and absolute-value display. The f₂ domain corresponds to a pure zero-field or NQR/NMR spectrum, while projection of the f₁ domain to a nutation NQR spectrum. All NQR spectra were processed by Delta software (JEOL. Inc., Tokyo, Japan).

Quantum chemical calculations: All the quantum chemical calculations for ³³S EFG tensors were performed at the B3LYP/6-311++G** and cc-pVTZ level using the Gaussian03W program package [13] on a laptop PC. Q=−6.78 fm² [14] was employed in the ³³S EFG calculations. The EFG results calculated by the two basis sets were averaged out. To the best of our knowledge, the crystal structure of S-4-phenyl 4-toluenethiosulfonate has not been reported so that the structure was optimized with RHF/6-311G. In the obtained structure, the two aryl rings were found to be twisted so that the torsion angle of C-S-S-C was 70.2° (see Fig. 4).

3 Results and discussion

In general, the observed NQR frequency or quadrupole frequency, ν_Q, is expressed by

with

and

where C_Q and η_Q are the quadrupole coupling constant and the quadrupole asymmetry parameter, respectively, and V_ii (i=X, Y and Z) is one of the three principal components of the EFG tensor (|V_XX|<|V_YY|<|V_ZZ|). Traditionally, C_Q and η_Q are frequently used rather than ν_Q among the NMR spectroscopy community, and, unfortunately, such EFG tensor parameters cannot be directly obtained from ν_Q. As mentioned already, the 2D nutation NQR technique [9], [10] was therefore applied in the present work.

Figure 1 shows a typical ³³S NQR or zero-field NMR spectrum of [³³S]-S-4-phenyl 4-toluenethiosulfonate, acquired at T=150 K. A single sharp peak was observed, indicating that there are no inequivalent sites in its crystal structure. The quadrupolar frequencies depended on the observed temperatures. Figure 2 shows the temperature dependence of the quadrupolar frequencies of [³³S]-S-4-phenyl 4-toluenethiosulfonate. With the decrease in temperature, the quadrupolar frequencies tend to increase, which is consistent with the observations reported in the literature [1], [2]. Note that no signals could be observed below T=110 K under the present experimental conditions (the recycle delay was set to 1.0 s). This may be because T₁ relaxation times drastically increase below 110 K.

Fig. 1:

A typical ³³S NQR spectrum of [³³S]-S-4-phenyl 4-toluenethiosulfonate, acquired at T=150 K.

Fig. 2:

Plot of the temperature dependence of the quadrupolar frequencies of [³³S]-S-4-phenyl 4-toluenethiosulfonate.

Figure 3 shows the ³³S nutation echo NQR spectrum of [³³S]-S-4-phenyl 4-toluenethiosulfonate, acquired at T=150 K. The nutation NQR spectrum corresponds to a powder pattern with three sharp singularities, denoted as ν₁, ν₂ and ν₃ (ν₁<ν₂<ν₃), and these frequencies can be used to calculate η_Q in a straightforward way using

Fig. 3:

³³S NQR nutation echo spectrum of [³³S]-S-4-phenyl 4-toluenethiosulfonate, observed at T=150 K. Two singularities are expressed by arrows, from which C_Q=42.1 MHz and η_Q=0.80(9) could be obtained.

The positions of the singularities (ν₂ and ν₃) of the present compound are given in Fig. 3, from which the asymmetry parameter could be obtained to be 0.80(9). As a result, the ³³S C_Q value at T=150 K was found to be 42.3 MHz, which is much larger than those for inorganic compounds or those with ionic bonds (C_Q values of less than 3 MHz). For reference, the spectral width of the present compound, with the assumption of the observation at 11.7 T, is expected to be approximately 40 MHz, and that of the central transition approximately 10 MHz. It may be very difficult or nearly impossible to observe solid-state ³³S NMR spectra of organosulfur compounds or ones with covalent bonds at moderate magnetic fields.

In general, it is difficult to obtain information on the EFG tensor orientation from NQR/solid-state NMR spectroscopic experiments. Recently, ab initio calculations are recognized as a reliable and powerful tool to determine EFG tensor orientations with respect to the molecular frame. The results of the calculations presented here for the ³³S EFG tensors yielded the following parameters: C_Q=42.9 MHz and η_Q=0.79, in reasonable agreement with the above experimental results. The calculated ³³S EFG tensor orientations with respect to the molecular frame of S-4-phenyl 4-toluenethiosulfonate, with the atomic labeling, is shown in Fig. 4. The largest EFG tensor component, V_ZZ, was found to be approximately perpendicular to the phenyl ring with the torsion (dihedral) angle C5-C4-S7-V_ZZ at 113°, and the smallest component, V_XX, approximately 48° off the C4-S7 bond. The orientation is consistent with the previously calculated result for diphenyl disulfide [7].

Fig. 4:

Molecular structure of the calculated model for [³³S]-S-4-phenyl 4-toluenethiosulfonate with the atomic labeling used in this study.

The present work has demonstrated that C_Q and η_Q values of organosulfur compounds and related compounds with covalent bonds can be obtained from quadrupolar frequencies. There is no doubt that the ³³S EFG tensor shows a close correlation with the torsion (dihedral) angles of disulfide bonding, and we believe that the present method should be useful for future investigations of rubber cross-linked structures by sulfur.

4 Summary

The ³³S NQR frequencies of an ³³S-enriched organosulfur compound, S-4-phenyl 4-toluenethiosulfonate, were observed in the range of 22.96–23.31 MHz at temperatures between T=110 and 300 K. A single sharp peak was obtained at all the temperatures. At 150 K, the 2D nutation echo method was applied, and the C_Q values and the asymmetry parameter were found to be 42.3 MHz and 0.80(9), respectively. Quantum chemical calculations were performed to derive the ³³S EFG tensor orientation with respect to the molecular frame. The results agree with data obtained previously for diphenyl disulfide.

5 Supporting information

The ¹H NMR and FT-IR spectra of [³³S]-S-4-phenyl 4-toluenethiosulfonate are given as supplementary material available online (DOI: 10.1515/znb-2019-0003).