Generation and Spectroscopic Identification of the Thiuram Radical

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Generation and Spectroscopic Identification of the Thiuram Radical (CH3)2NCS2 Published as part of The Journal of Physical Chemistry virtual special issue “Leo Radom Festschrift”. Artur Mardyukov,* Felix Keul, and Peter R. Schreiner Institute of Organic Chemistry, Justus Liebig University, Heinrich-Buff-Ring 17, 35392 Giessen, Germany

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S Supporting Information *

ABSTRACT: We report the first preparation, matrix-isolation, and IR and UV/vis spectroscopic characterization of the thiuram radical that is a highly important species for many industrial processes. The thiuram radical was prepared by thermal dissociation of tetramethylthiuram disulfide and was identified by matching its spectroscopic data with density functional theory [UB3LYP/6-311++G(3df,3pd)] computations. The title compound proved to be highly photolabile, and irradiation with light at λ = 623 nm affords a hitherto unknown carbamodithioic acid, N-(methyl)-N-methyl radical, as characterized by IR and UV/vis spectroscopy in low-temperature matrices.

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HCS2• has been studied by photoelectron and infrared spectroscopy.28,29 Notwithstanding its relevance in a variety of processes, 1 has not yet been isolated or characterized by infrared (IR) spectroscopy. Radical 1 was generated as a transient species in solution by photolysis of 2 and investigated by time-resolved spectroscopy.30 The transient UV/vis absorption spectrum of 1 exhibits a broad transition at around 600 nm, which could be used for kinetic measurements.30 According to G3X(MP2) computations,31 the homolytic cleavage of the S−S bond requires an activation enthalpy of only 35 kcal mol−1, which is attributed to the remarkable stability of 1, which is a σ-radical with the unpaired electron being distributed equally between the two sulfur atoms and a four-centered delocalized πsystem.19 Despite these important kinetic and theoretical studies, to the best of our knowledge, no reports have appeared so far regarding the IR and UV/vis spectroscopic identification of 1. Hence, it is tempting to isolate 1 at cryogenic temperatures with the goal to characterize these species spectroscopically and study their (photo)chemistry. Recently, we reported the synthesis and spectroscopic characterization of the previously elusive phenylthiyl14 and phenylselenyl32 radicals under matrix isolation conditions either by thermolysis or photolysis of the corresponding diphenyl disulfide and diphenyl diselenide, respectively. We assumed that owing to its relatively weak S−S bond in 2, it readily undergoes S−S bond homolysis, which leads to the formation of elusive thiuram radical 1. Herein, we report the

INTRODUCTION Sulfur-centered radicals are fascinating reactive intermediates that are highly relevant to various applications including organic synthesis,1,2 chain transfer reactions in radical polymerizations,3,4 the vulcanization of rubber,5,6 cross-linking of polymers,7,8 and many biological9,10 as well as atmospheric processes. 11 A very important group of organosulfur compounds are disulfides representing useful precursors for the generation of free sulfur-centered radicals that can be activated either thermally or photochemically.12−14 Because disulfides are used in such a broad variety of applications, it is important to determine how they can form selectively and to identify the factors that control their reactivity. Tetramethylthiuram disulfide (TMTD, 2) is the simplest thiuram disulfide, and it is one of the most important accelerators for the vulcanization of rubber;15 it is also a very widely used fungicide (“Thiram”).16 Compound 2 is usually applied together with zinc oxide, and the function of the latter in this context has been extensively studied experimentally and theoretically.17−21 Moreover, 2 is widely used as the initiator in radical (photo)polymerization processes.22−24 The thermal fragmentation of 2 at 130−150 °C produces various radical species with general formula Me2NCSn (n = 1−4); these have been detected by electron spin-resonance (ESR) spectroscopy.25 The signal triplet at g = 2.05 with a peak separation of ca. 15 G was assigned to radical 1 (Me2NCS2•),26 the S-centered radical derived from dimethylcarbamodithioic acid. Interestingly, a weak signal was recorded at g = 2.05, which was attributed to an unknown carbon radical.26 In analogy, the iPr2NCS2• radical was generated by thermolysis of the related tetraisopropyl thiuramdisulfide, which was detected by a single line at g = 2.015 in the ESR spectrum.27 The parent radical © 2019 American Chemical Society

Received: April 9, 2019 Revised: May 20, 2019 Published: May 22, 2019 4937

DOI: 10.1021/acs.jpca.9b03307 J. Phys. Chem. A 2019, 123, 4937−4941

Article

The Journal of Physical Chemistry A first IR and UV/vis spectroscopic characterization of 1 together with its hitherto unexplored photochemistry (Scheme 1). Scheme 1. Thiuram Radical (1) Generated from Tetramethylthiuram Disulfide (“Thiram”, TMTD, 2) through Pyrolysis and Trapping in an Argon Matrixa

Subsequent irradiation with light at λ = 623 nm led to rearrangement to the novel radical 3. a

Figure 1. IR spectra showing the product of pyrolysis of 2 in argon matrix with subsequent trapping in an argon matrix at 10 K. (a) IR spectrum of 1 computed at UB3LYP/6-311++G(3df,3pd) (unscaled). (b) IR difference spectra showing the photochemistry of 1 after irradiation with λ = 312 nm in argon at 20 K. Downward bands assigned to 1 after 20 min irradiation time. (c) IR spectrum of 1-d6 computed at UB3LYP/6-311++G(3df,3pd) (unscaled). (d) IR difference spectra showing the photochemistry of 1-d6 after irradiation with λ = 312 nm in argon at 10 K. Bands pointing downward assigned to 1-d6 disappear appear after 20 min irradiation time.

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METHODS Matrix Apparatus Design. For the matrix isolation studies, we used an APD Cryogenics HC-2 cryostat with a closed-cycle refrigerator system, equipped with an inner CsI window for IR measurements. Spectra were recorded with a Bruker IFS 55 FT-IR spectrometer with a spectral range of 4500−400 cm−1 and a resolution of 0.7 cm−1 and UV/vis spectra were recorded with a JASCO V-670 spectrophotometer equipped with an sapphire windows. Matrices were generated by codeposition of 2 (evaporated at 150 °C from a storage bulb) with a large excess of argon (typically 60−120 mbar from a 2000 mL storage bulb) on the surface of the matrix window at 10 K (20 K). A high-pressure mercury lamp (HBO 200, Osram) with a monochromator (Bausch & Lomb) was used for irradiation. Computations. All geometries were optimized and characterized as minima or transition structures by means of analytical harmonic vibrational frequency computations at the B3LYP/6-311++G(3df,3pd) level of theory.33−35 All computations were performed with the Gaussian16 program.36

agreement with computed shifts of 370 and 336 cm−1, respectively. The good agreement between the computed (UB3LYP/6-311++G(3df,3pd)) and experimental frequencies of the 1 and 1-d6 isotopologues underline the successful preparation of 1 (Figure 1 and Table S1). The UV/vis spectrum of matrix-isolated 1 exhibits two strong absorption bands at λmax = 241 and 263 (Figure 2, solid

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RESULTS AND DISCUSSION We prepared 1 through flash vacuum pyrolysis (FVP) of 2 at 600 °C, and subsequent irradiation at λ = 312 nm in an Ar matrix at 10 K led to a slow photoreaction. After 20 min, all of the characteristic IR and UV bands of 1 disappeared, and quite a number of new IR bands were recorded. In the FVP experiments, 1 is produced by the cleavage of the S−S bond in 2. The UB3LYP/6-311++G(3df,3pd) (unscaled) computed spectrum of 1 reproduces the experimental spectrum obtained after FVP experiments well. The most intense bands due to the C−N stretch, CH3 out-of-plane deformation, and CH3 wagging modes were observed at 1541, 1404, and 1148 cm−1, in good agreement with the computed values (1563, 1437, and 1166 cm−1) (Figure 1 and Figure S1). With the help of the computations, the additional IR bands of low intensity could be assigned to 1 (Table S1). The good agreement between the experimental and computed spectra is taken as evidence for the successful preparation of 1. To confirm these assignments, we also prepared 1-d6 from 2d12, which leads to characteristic isotopic shifts. For example, the IR band at 1541 cm−1 shows a strong red shift of 66 cm−1 and can thus be attributed to the C−N stretch (computed at 1490 cm−1) of 1, in good agreement with a computed shift of 73 cm−1. Other strong bands at 1409 and 1148 cm−1 show large isotope shifts of 351 and 327 cm−1, also in good

Figure 2. Solid line: UV/vis spectrum of 1 isolated at 10 K in Ar. Dashed line: UV/vis spectrum of 3 at 10 K; the photochemistry of 1 after irradiation at λ = 623 nm in argon at 10 K. Inset: computed [TD-UB3LYP/6-311++G(3df,3pd)] electronic transitions for 1 and 3.

line) and a weak absorption band λmax = 598 nm, and it is in good agreement with its computed UV/vis spectrum using time-dependent density functional theory (TD-DFT); TDUB3LYP/6-311++G(3df,3pd) computations exhibit two strong transitions at 224 nm (f = 0.11) and 256 nm ( f = 0.073) as well as a moderate transition at 551 nm (f = 0.018) (Figure 2 solid line). According to the orbitals involved in the electronic excitations, the weak absorption band in the visible region at 598 nm is an n−π* transition, while the strong band 4938

DOI: 10.1021/acs.jpca.9b03307 J. Phys. Chem. A 2019, 123, 4937−4941

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The Journal of Physical Chemistry A

Figure 3. (a) Selected bond lengths (Å) and angles of 1 and 3 at the UB3LYP/6-311++G(3df,3pd) level. (b) Computed spin densities of 1 and 3 at the UB3LYP/6-311++G(3df,3pd) level. (c) Resonance structures for 1.

Figure 4. Potential energy profile (ΔH0) in kcal mol−1 of the reaction of radical 1 at the UB3LYP/6-311++G(3df,3pd) level of theory.

at 263 nm corresponds to a π−π* transition (Figure S2, Supporting Information). Matrices containing 1 are blue due to a broad absorption with a maximum around 598 nm. This finding is in accordance with the broad band in the visible region observed in timeresolved experiments.30 Irradiation of this band (argon, 10 K, λ = 623 nm) rapidly results in the disappearance of all bands assigned to 1 and formation of a new set of IR bands at 1378, 1300, and 1164 cm−1 (Figure S3). The major constituent of the photolysis products of 1 was identified as a carbamodithioic acid, N-(methyl)-N-methyl radical (3), the C-centered radical derived from dimethylcarbamodithioic acid. To confirm these assignments, we also performed photochemical experiments with 1-d6. In particular, we found a small isotope red-shift for the band at 1300 cm−1 (expt, 7 cm−1; calc, 13 cm−1) for the NCH deformation in 3. The experimental red-shift of 81 cm−1 (calc, 69 cm−1) was observed for the CH3 deformation vibration (Figure S4). The IR frequencies, intensities, and isotopic shifts of two isotopologs (3 and 3d6) closely match the computed data (Table S2). The photolysis of 1 was also followed by UV/vis spectroscopy. In line with the IR experiments, the transition bands at 241, 263, and 598 nm of 1 disappear upon irradiation (λ = 623 nm), and simultaneously the formation of transitions at 278 nm (strong), 373 nm (moderate), and 595 nm (weak, broad) (Figure 2, dashed line) appears. All bands of 3 correlate well with the values of the electronic excitations at 271 and 287 nm (f = 0.052 and 0.026), 378 nm (f = 0.087), and 476 nm (f = 0.003) computed at TD-UB3LYP/6-311++G(3df,3pd). According to UB3LYP/6-311++G(3df,3pd) computations, 1 displays a C2v point group with a 2B2 electronic ground state, which corresponds to a σ-radical with conjugated C−S bonds (1.696 Å)32 and a relatively short C−N bond (1.333 A)38

(Figure 3). We also computed the α and β spin densities at the UB3LYP/6-311++G(3df,3pd) level of theory. According to computations, the odd electron is localized almost exclusively on the two sulfur atoms with computed values of 0.538. Naturally, the SOMO is a linear combination of the two sulfur lone pair orbitals. The stabilization of 1 apparently arises from the delocalization of the π-orbitals over the 4-center NCS2 framework as well as the delocalization of the spin density; this is also apparent from the computed high dipole moment of 4.75 D for 1. The NPA atomic charges of 1 are −0.402 at nitrogen and −0.391 at the sulfur atoms, and the central carbon atom carries a charge of +0.574. In contrast to 1, the spin density in 3 is largely localized on the carbon (0.740) and sulfur atoms (0.272) (Figure 3). Furthermore, we undertook a detailed computational analysis to shed some light into the mechanism of the formation of 3 starting from radical 1. Although it is not clear if this is a photochemical or a hot-ground-state reaction, only ground-state reactions could be investigated at UB3LYP/6311++G(3df,3pd). We suggest that radical 1, formed by S−S homolysis of 2, can decompose through several pathways. The first reaction path implies the intramolecular hydrogen atom transfer from the methyl group to a sulfur atom in 1 to produce radical 3 via TS1 (Figure 4). The barrier is 30.2 kcal mol−1 (including the zero-point vibrational energy correction, ZPVE, denoted as ΔH0). The fact that we did not observe characteristic peaks for 3 directly after FVP suggests that the energy is insufficient in the pyrolysis zone for 1 to undergo [1,4]H-shift to give 3. Subsequent C−N bond fission to the thiyl radical (4) and methanimine (5) is accompanied by an activation barrier (TS2) of 49.8 kcal mol−1. Finally, the pyrolysis of starting material 2 at 600 °C also yielded infrared signals that were clearly identifiable as carbon disulfide (CS2), 4939

DOI: 10.1021/acs.jpca.9b03307 J. Phys. Chem. A 2019, 123, 4937−4941

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which is the dissociation product of 1 to CS2 and the dimethyl amine radical (CH3)2N• (6). The transition structure TS3 for this dissociation is associated with a sizable barrier of 37.4 kcal mol−1. Previous studies have argued that either the S−S bond or one of the C−S single bonds of 2 breaks on heating, which would give either two (CH3)2NCS2• radicals or (CH3)2NCS• together with (CH3 ) 2 NCS 3 •. 26 According to previous computational studies, the homolytic dissociation energies of the S−S bond and of one of the C−S single bonds of 1 differ considerably, clearly favoring the former process.18,19 The thermal decomposition of 2 has been reported to yield eventually CS2, tetramethylthiurea (TMTU), and elemental sulfur.26 However, in our FVP experiments, we did not observe TMTU as a product of thermal decomposition of 2.

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AUTHOR INFORMATION

Corresponding Author

*(A.M.) E-mail: [email protected]. de. ORCID

Artur Mardyukov: 0000-0003-3908-6967 Peter R. Schreiner: 0000-0002-3608-5515 Author Contributions

The experimental and theoretical work was carried out by A.M. and F.K. The work was discussed and the manuscript written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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CONCLUSIONS

ACKNOWLEDGMENTS This work was supported by the Justus Liebig University.

We report the first identification and isolation of thiuram radical 1 via gas phase pyrolysis of tetramethylthiuram disulfide 2 at 600 °C and subsequent trapping in argon matrices at 10 K, and characterized it by IR and UV/vis spectroscopy as well as with DFT computations. We observed that 1 is photolabile, and upon photolysis with light at λ = 623 nm, it rearranges to the hitherto unknown carbon-centered radical 3. The formation of 3 from 1 is also supported by isotopic labeling experiments using 1-d6. The experimentally observed IR spectra are consistent with the spectra computed at the UB3LYP/6-311++G(3df,3pd). The stability of 1 is readily reflected in the S−S bond dissociation enthalpy of 2.39,40 The computed value of 35 kcal mol−1 is one of the lowest among various known disulfide-containing compounds.19,39 Radical 1 is resonance stabilized, as evident from the C−S bond lengths of 1.696 Å, intermediate between the typical length of a C−S single bond (1.803 Å) and a CS double bond (1.584 Å).37 The C−N bond (1.333 Å) is also slightly shorter than that in the corresponding dimer (1.350 Å), suggesting an increased πelectron delocalization into the NCS2 framework. Moreover, a hypothetical isodesmic reaction of 1 with hydrogen sulfide (H2S) at 298.15 K gives a large reaction enthalpy (ΔH298) at UB3LYP/6-311++G(3df,3pd) of +20.4 kcal mol−1 shown in eq 1.

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Knowledge of the key properties of these radicals provides valuable insights into the complex mechanisms of their formation and decomposition reactions, and we expect that these observations are also of importance for the understanding of radical reactions in subsequent (photo)polymerization processes.

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Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.9b03307. IR spectra, IR tables, Cartesian coordinates, absolute energies of all optimized geometries, and experimental procedures (PDF) 4940

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DOI: 10.1021/acs.jpca.9b03307 J. Phys. Chem. A 2019, 123, 4937−4941