Dibutyl Sulfide Research Articles

Desulfurization of raw naphtha cuts using hybrid systems based on acoustic cavitation and advanced oxidation processes (AOPs)

A combination of dual-frequency acoustic cavitation (acoustic cavitation) and UV assisted advanced oxidation processes (AOPs) reaction system was developed for desulfurization of raw naphtha used to produce aviation fuels. Various types of oxidants in hybrid systems including hydrogen peroxide, acetic acid, acetone, air and ozone were compared. At optimum oxidant to sulfur compounds molar ratio (rox) of 5.0, the hybrid process of acoustic cavitation /acetic acid/hydrogen peroxide (acoustic cavitation/peracetic acid) was the most effective treatment process with synergistic index of 7.55 and desulfurization efficiency of 100% (highest reaction rate constant of 0.1337 min-1) for all sulfur compounds present in naphtha within the short reaction time of 30 min. At rox of 0.5 and 1.0, the synergy coefficients of 3.77 and 5.41 corresponding to the desulfurization efficiencies of 93.07 and 95.10% were obtained in 180 min, respectively. Comparatively, at the rox of 5.0, the efficiency of acoustic cavitation combined with hydrogen peroxide alone and acetic acid alone was 63.68 and 47.36%, respectively. Alternative treatment process of acoustic cavitation /UV/acetone/water can be used for the complete desulfurization (100%) of specific sulphur compounds including dibutyl sulfide, di-tert butyl disulfide, 1,4-butanedithiol and benzyl thiol considering the optimum acetone to water ratio of 2:1. All acoustic cavitation-based treatment systems followed the pseudo first order kinetic model. The most expensive process was ozone-aided acoustic cavitation with treatment costs exceeding 100 USD m-3. The peracetic acid-based system was the most economic process with a treatment cost of 15 USD/m3 with electric energy consumption of 100.17 kWh/m3.

Effects of fragrance compounds on growth of the silkworm Bombyx mori.

Due to the contamination and biological toxicity of some fragrance compounds, the environmental and ecological problems of such compounds have attracted more and more attention. However, studies of the toxicity of fragrance compounds for insects have been limited. The toxicity of 48 fragrance compounds for the silkworm Bombyx mori were investigated in this study. All of the fragrance compounds examined had no acute toxicity for B. mori larvae, but eight of them (menthol, maltol, musk xylene, musk tibeten, dibutyl sulfide, nerolidol, ethyl vanillin, and α-amylcinnamaldehyde) exhibited chronic and lethal toxicity with LC50 values from 20 to 120 µM. In a long-term feeding study, musk tibeten, nerolidol, and musk xylene showed significant growth regulatory activity. They were also extremely harmful to the cocooning of B. mori, resulting in small, thin, and loose cocoons. Two important insect hormones, namely, juvenile hormone (JH) and 20-hydroxyecdysone (20-E), were quantified in hemolymph following chronic exposure to musk tibeten, nerolidol, and musk xylene, respectively. Musk tibeten significantly increased JH titer and decreased the 20-E titer in hemolymph, and musk xylene had a significant inhibitory effect on JH titer and increased 20-E titer. Although nerolidol had no effect on hormone levels, exogenous JH mimic nerolidol increased the physiological effects of JH and significantly slowed the growth rate of B. mori larvae. The results showed that these fragrance compounds could interfere with the insect endocrine system, leading to death and abnormal growth. The risk to insects of residual fragrance compounds in the environment is worthy of attention.

Diagnosing complex odor problems occurring in micro-polluted source water: Primary approach and application.

The odor problems in river-type micro-polluted water matrixes are complicated compared to those in lakes and reservoirs. For example, the TY River in Jiangsu Province has been associated with complex odors, whereas the specific odor compounds were not clear. In this paper, a comprehensive study on characterizing the odors and odorants in source water from the TY River was conducted. Six odor types, including earthy, marshy, fishy, woody, medicinal, and chemical odors, were detected for the first time; correspondingly, thirty-three odor-causing compounds were identified. By means of evaluating odor activity values and reconstituting the identified odorants, 95, 93, 92, 90, 89 and 88% of the earthy, marshy, fishy, woody, medicinal and chemical odors in the source waters could be clarified. Geosmin and 2-methylisoborneol were associated with earthy odor, while amyl sulfide, dibutyl sulfide, propyl sulfide, dimethyl disulfide, dimethyl trisulfide and indole were related to marshy odor. The major woody and fishy odor compounds were vanillin, geraniol, β-cyclocitral and 2,4-decadienal, 2-octenal, respectively. Medicinal and chemical odors were mainly caused by 2-chlorophenol, 4-bromophenol, 2,6-dichlorophenol and naphthalene, and 1,4-dichlorobenzene, respectively. This is the first study in which six odor types and thirty-three odorants were identified simultaneously in a river-type micro-polluted water source, which can offer a reference for odor management in drinking water treatment plants.

Occurrence of swampy/septic odor and possible odorants in source and finished drinking water of major cities across China

Swampy/septic odors are one of the most important odor types in drinking water. However, few studies have specifically focused on it compared to the extensive reported musty/earthy odor problems, even though the former is much more offensive. In this study, an investigation covering the odor characteristics, algal distribution and possible odorants contributing to swampy/septic odor, including dimethyl disulfide (DMDS), dimethyl trisulfide (DMTS), diisopropyl sulfide (DIPS), dipropyl sulfide (DPS), dibutyl sulfide (DBS), 2-methylisoborneol (2-MIB) and geosmin (GSM), was performed in source and finished water of 56 drinking water treatment plants (DWTPs) in 31 cities across China. While the musty/earthy and swampy/septic odors were dominant odor descriptors, the river source water exhibited a higher proportion of swampy/septic odor (38.5%) compared to much higher detection rate of musty/earthy odor (50.0%) in the lake/reservoir source water. The occurrence of swampy/septic odor, which was much easier to remove by conventional drinking water treatment processes compared to musty/earthy odors, was decreased by 62.9% and 46.3% in river and lake/reservoir source water respectively. Statistical analysis showed that thioethers might be responsible for the swampy/septic odor in source water (R2 = 0.75, p < 0.05). Specifically, two thioethers, DMDS and DMTS were detected, and other thioethers were not found in all water samples. DMDS was predominant with a maximum odor activity value (OAV) of 2.0 in source water and 1.3 in finished water. The distribution of the thioethers exhibited a marked regional characteristics with higher concentrations being detected in the east and south parts of China. The high concentrations of thioethers in lake/reservoir source water samples could be partly interpreted as the bloom of the cyanobacteria. This study provides basic information for swampy/septic odor occurrence in drinking water and will be helpful for further water quality management in water industry in China.

Green, inexpensive, and fast conversion of sulfides to sulfoxides by multiusable Mo(VI) macrocyclic Schiff base complex supported on Fe3O4 nanoparticles in solvent-free conditions

In the present study, the macrocyclic-based Mo(VI) Schiff base complex was harbored on Fe 3 O 4 nanoparticles and characterized by X-ray powder diffraction, scanning electron microscope, energy-dispersive X-ray spectroscopy, infrared spectroscopy, transmission electron microscopy, vibrating sample magnetometry, diffuse reflectance spectra, and atomic absorption spectroscopy. Separable nanocatalyst was tested under solvent-free conditions for the oxidation of methyl phenyl sulfide, diphenyl sulfide, benzyl phenyl sulfide, dipropyl sulfide, dibutyl sulfide, dimethyl sulfide, bis (4-hydroxyphenyl) sulfide, diallyl sulfide, and benzothiophene using H 2 O 2 (30% in water) as green oxidant. This catalyst is very efficient for thioanisole oxidation with 100% conversion in 3 min. We were able to separate the nanocatalyst magnetically using external magnetic field and to apply the catalyst at least six consecutive times without a significant decrease in conversion. Remarkable and excellent turnover frequency of the catalyst was obtained to oxidize the thioanisole (526,000 h −1 ), dimethyl sulfide (526,000 h −1 ), diallyl sulfide (526,000 h −1 ), dibutyl sulfide (521,000 h −1 ), and dipropyl sulfide (500,000 h −1 ). The prepared nanocatalyst has been beneficial in catalytic activity, selectivity, reaction time, and reusability with easy separation.

Mechanism and Kinetic Model for Autocatalysis in Liquid–Liquid System: Oxidation of Dibutyl Sulfide with Aqueous Hydrogen Peroxide

The oxidation of dibutyl sulfide with aqueous hydrogen peroxide as a liquid–liquid reaction was investigated. The autocatalysis, solubility of H2O2 in organic phase, and effects of temperature, stirring speed, initial organic DBSO concentration, and initial aqueous H2O2 concentration were studied. Solvent effect of dibutyl sulfoxide was proposed for liquid–liquid autocatalysis. The intrinsic reaction was considered as the determining step, and all the other steps were considered as equilibrium processes. Considering interfacial reaction and dynamic equilibrium of hydrogen peroxide between the two phases, the reaction was divided into exterior and interior stages. Exterior and interior mechanisms were proposed for the corresponding stages, and kinetic models were established. The parameters of kinetic model were estimated with the experimental data, and the activation energies of exterior and interior reaction were 30.62 and 73.50 kJ/mol. The validity of the kinetic models with estimated parameters was stu...

Sulfur release and its transformation behavior of sulfur-containing model compounds during pyrolysis under CO2 atmosphere

Sulfur release behavior of selected sulfur-containing compounds, tetradecyl mercaptan, dibutyl sulfide, phenyl sulfide, 2-methyl thiophene, benzo-thiophene and dibenzo-thiophene, were investigated by pyrolysis coupled with mass spectrometer (Py-MS) and pyrolysis connected with gas chromatogram (Py-GC) under CO2 atmosphere. It was found that the order of sulfur stability is tetradecyl mercaptan<dibutyl sulfide<phenyl sulfide<2-methyl thiophene<benzo thiophene<dibenzo thiophene under CO2 atmosphere. The decomposition temperature of each sulfur compound is lower under CO2 atmosphere than that under inert atmosphere during pyrolysis, which indicates that CO2 can help breaking C-S bonds. For tetradecyl mercaptan and dibutyl sulfide, CO2 atmosphere can promote the release of H2S, COS and SO2 in the gas phase, especially with the sharp increase of the COS amount. However, for phenyl sulfide, dibenzo thiophene, 2-methyl thiophene and benzo thiophene, COS release amount under CO2 atmosphere is much higher than that under Ar atmosphere, while the release amount of H2S and SO2 under CO2 atmosphere is lower than that under Ar atmosphere. COS release of these thiophene compounds increases with the temperature increasing up to 850°C under CO2 atmosphere, meanwhile CO release of these three sulfur-containing compounds are also increasing. This testifies that the COS formation is related to the CO formation at higher temperatures.

Green oxidation of sulfides in solvent-free condition by reusable novel Mo(VI) complex anchored on magnetite as a high-efficiency nanocatalyst with eco-friendly aqueous H2O2

In this research work, a novel molybdenum N4-type Schiff base complex anchored covalently on magnetite nanoparticles and characterized perfectly using physico-chemical techniques and it was applied for oxidation of sulfides. The effects of solvent, reaction temperature and amount of catalysts by 30% aqueous H2O2 as environmental friendly oxidant on oxidation catalytic process were optimized. The best results in solvent-free condition was obtained. The catalyst was tested for oxidation of thioanisole, Diphenyl sulfide, Benzyl phenyl sulfide, dipropyl sulfide, dibutyl sulfide, dimethyl sulfide, bis(4-hydroxyphenyl) sulfide, diallyl sulfide and benzothiophene. The presented nanocatalyst is so effective for selective oxidation of thioanisole, dimethyl sulfide, dibutyl sulfide and diallyl sulfide with a >99% conversion, and excellent turnover frequency (93,000h−1<) in shortest time (5min). The magnetic nanocatalyst was easily recovered by using an external magnetic field and employed at least six successive times without significant decreasing in conversion.

Surface single-molecule dynamics controlled by entropy at low temperatures

Configuration transitions of individual molecules and atoms on surfaces are traditionally described using an Arrhenius equation with energy barrier and pre-exponential factor (attempt rate) parameters. Characteristic parameters can vary even for identical systems, and pre-exponential factors sometimes differ by orders of magnitude. Using low-temperature scanning tunnelling microscopy (STM) to measure an individual dibutyl sulfide molecule on Au(111), we show that the differences arise when the relative position of tip apex and molecule changes by a fraction of the molecule size. Altering the tip position on that scale modifies the transition’s barrier and attempt rate in a highly correlated fashion, which results in a single-molecular enthalpy-entropy compensation. Conversely, appropriately positioning the STM tip allows selecting the operating point on the compensation line and modifying the transition rates. The results highlight the need to consider entropy in transition rates of single molecules, even at low temperatures.

Effect of irradiation intensity on rate and selectivity of photosensitized oxidation of dibutyl sulfide over C70 fullerene

Photoinitiated oxidation of dibutyl sulfide (DBS) with air oxygen in a toluene solution of C70 fullerene during irradiation with visible light at a wavelength of 500–700 nm results in the formation of dibutyl sulfone as a main detected product, dibutyl sulfoxide, and butanal. A change in the intensity of radiation incident on the reaction mixture from 2.2 to 44 mW/cm2 insignificantly increases the quantum efficiency of the DBS consumption from 4.5 to 5.9%. The selectivity for sulfoxide increases during the process, while selectivity for sulfone decreases.

Impact of branching on the supramolecular assembly of thioethers on Au(111).

Alkanethiolate monolayers are one of the most comprehensively studied self-assembled systems due to their ease of preparation, their ability to be functionalized, and the opportunity to control their thickness perpendicular to the surface. However, these systems suffer from degradation due to oxidation and defects caused by surface etching and adsorbate rotational boundaries. Thioethers offer a potential alternative to thiols that overcome some of these issues and allow dimensional control of self-assembly parallel to the surface. Thioethers have found uses in surface modification of nanoparticles, and chiral thioethers tethered to catalytically active surfaces have been shown to enable enantioselective hydrogenation. However, the effect of structural, chemical, and chiral modifications of the alkyl chains of thioethers on their self-assembly has remained largely unstudied. To elucidate how molecular structure, particularly alkyl branching and chirality, affects molecular self-assembly, we compare four related thioethers, including two pairs of structural isomers. The self-assembly of structural isomers N-butyl methyl sulfide and tert-butyl methyl sulfide was studied with high resolution scanning tunneling microscopy (STM); our results indicate that both molecules form highly ordered arrays despite the bulky tert-butyl group. We also investigated the effect of intrinsic chirality in the alkyl tails on the adsorption and self-assembly of butyl sec-butyl sulfide (BSBS) with STM and density functional theory and contrast our results to its structural isomer, dibutyl sulfide. Calculations provide the relative stability of the four stereoisomers of BSBS and STM imaging reveals two prominent monomer forms. Interestingly, the racemic mixture of BSBS is the only thioether we have examined to date that does not form highly ordered arrays; we postulate that this is due to weak enantiospecific intermolecular interactions that lead to the formation of energetically similar but structurally different assemblies. Furthermore, we studied all of the molecules in their monomeric molecular rotor form, and the surface-adsorbed chirality of the three asymmetric thioethers is distinguishable in STM images.

Sulfur removal and release behaviors of sulfur-containing model compounds during pyrolysis under inert atmosphere by TG-MS connected with Py-GC

Sulfur containing model compounds, tetradecyl mercaptan, dibutyl sulfide, phenyl sulfide, 2-methyl thiophene, benzothiophene and dibenzothiophene, were selected to investigate their sulfur removal and release behaviors during pyrolysis under inert atmosphere by thermo-gravimetric analyzer with mass spectrometer (TG-MS) and pyrolysis connected with gas chromatogram (Py-GC). It was found that the order of sulfur removal was tetradecyl mercaptan > dibutyl sulfide > 2-methyl thiophene > benzo thiophene > phenyl sulfide > dibenzothiophene. Except for phenylsulfide, this rule is contrary to the decomposition temperature order of the sulfur functional groups. SO2 evolution was detected by MS and GC for all those model compounds and COS evolution was also found except for phenylsulfide and dibenzothiophene; while H2S evolution was measured only for tetradecyl mercaptan, dibutyl sulfide and 2-methyl thiophene. However, SO2 content was much higher than H2S and COS in pyrolysis gas for each model compound, which may be caused by that indigenous hydrogen was much less than indigenous oxygen under inert atmosphere, when activated carbon was used as carrier. Thus, most of sulfur radicals can connect with indigenous oxygen and release in the form of SO2. For phenyl sulfide, benzothiophene and dibenzothiophene, as their indigenous hydrogen was not enough to react with sulfur radicals, no H2S was detected during pyrolysis under inert atmosphere, while SO2 was found and its content was very high in pyrolysis gas.

Sulfur release and transformation behaviors of sulfur-containing model compounds during pyrolysis under oxidative atmosphere

In this study, the sulfur release and transformation behaviors of tetradecyl mercaptan, dibutyl sulfide, phenylsulfide, 2-methylthiophene, benzothiophene and dibenzothiophene during pyrolysis were investigated by thermo-gravimetric analyzer coupled with mass spectrometer (TG–MS). The order of sulfurs release is dibutyl sulfide>2-methylthiophene>tetradecyl mercaptan>benzothiophene>phenylsulfide>dibenzothiophene. The decomposition temperatures of all of the sulfur compounds are lower during pyrolysis under an oxidative atmosphere than that under inert and hydrogen atmospheres, which indicates that oxygen can break C–S bonds more easily than C–C bonds. The SO2 evolution of most sulfur model compounds except 2-methylthiophene and dibenzothiophene is derived mainly by decomposition of their transformation compounds rather than their native forms. Most sulfur compounds can be removed under an oxidative atmosphere with little loss of carbon.

Oxide thin films (ZnO, TeO2, SnO2, and TiO2) based surface acoustic wave (SAW) E-nose for the detection of chemical warfare agents

Chemical warfare agents (CWA) are deadly chemicals even at low levels of concentration. In the present work, an electronic-nose (E-nose) comprising of an array of four surface acoustic wave (SAW) sensors having different chemical sensitive coatings of ZnO, TeO2, SnO2 and TiO2 is prepared for the detection of CWA. The oxide thin films are coated on the surface of SAW devices by rf sputtering technique. ZnO and SnO2 thin films are poly-crystalline with fine surface morphology whereas TeO2 and TiO2 thin films are amorphous. The E-nose shows good sensitivity toward the four simulants (dimethyl methylphosphonate (DMMP), dibutyl sulfide (DBS), chloroethyl phenyl sulfide (CEPS) and diethyl chlorophosphate (DECP)) of CWA allowing sub ppm level detection possible. Different simulants give distinct characteristic pattern when passed through the surface of E-nose. Using principal component analysis (PCA), the four different simulants of CWA are well separated from each other. Even with the incorporation of interferants (petrol, diesel, kerosene, VOCs and water vapors) in the PCA study, all four simulants are well separated showing the capability of the prepared SAW E-nose to classify simulants of chemical warfare agents efficiently.

Dithiolane‐Directed Tandem Oxidation/1,2‐Benzyl Migration of Tetramic Acids under Ambient Conditions

AbstractReactions directly involving triplet oxygen are an important issue in organic and biological chemistry. We now report that exposure of tetramic acids in the open air (molecular oxygen) at room temperature can lead to the formation of succinimide derivatives in high to excellent yields via tandem oxidation/1,2‐benzyl migration. In this reaction, the dithiolane moiety plays a crucial role and succinimides are produced either in the presence or absence of dibutyl sulfide as reductant.

Adsorption, Assembly, and Dynamics of Dibutyl Sulfide on Au{111}

Alkanethiol (RSH) monolayers are by far the most extensively studied surface self-assembly systems due to their robustness and the opportunity to control their assembly in the dimension perpendicular to the surface. Thioethers (RSR′), a similar class of molecule, remain largely unstudied to date but may offer a similar level of assembly control parallel to the surface. Here we report the self-assembly of dibutyl sulfide, a symmetric thioether species, on a Au{111} surface using scanning tunneling microscopy. As with thiols, dibutyl sulfide forms well-ordered monolayers, but due to the slightly weaker molecule−metal bond, the coverage and temperature-dependent behavior is very different than that of alkanethiols. Adsorption is sensitive to the different regions of the Au{111} herringbone reconstruction. Dibutyl sulfide lies parallel to the surface and forms well-ordered chains in domains that preferentially bind first in fcc regions, then hcp, and finally on soliton walls. The sulfide−Au interaction is strong enough to disrupt the native herringbone reconstruction of Au; however, unlike thiols, dibutyl sulfide adsorption does not result in etch pit formation. A monolayer of dibutyl sulfide has a very low defect density as compared to thiol−SAM based systems. This low defect density hints at the possibility of using a thioether moiety as a basis for a self-assembled system free of typical defects like etch pits, which allow attack and degradation of the monolayer. Upon annealing the surface with a high molecular coverage to 575 K, dibutyl sulfide desorbs molecularly from the less energetically favorable regions of the surface first. At this reduced coverage the system returns to an intermediate-coverage structure, thereby demonstrating the reversibility of the assembly. Elevating the temperature further causes the entire monolayer to desorb, and the original herringbone structure of Au returns. We postulate that this reversibility, coupled with the high rate of concerted rearrangements, allows this system to reach a very high level of order.

10.1007/s11494-008-2009-1

Antiwear and extreme-pressure properties of 3-alkyl substituted thietanes (3-butyl-and 3-hexylthietane) taken as an admixture to vaseline oil were examined. A comparison of these properties with those of dibutyl sulfide and didodecyl sulfides, as well as dibutyl and didodecyl disulfides, showed that, although 3-alkylthietanes do not exhibit wear-preventing activity, they are much superior to dialkyl sulfide and rank somewhat below the disulfides in extreme-pressure performance.

Experimental and Theoretical Studies of the Reaction of the OH Radical with Alkyl Sulfides: 3. Kinetics and Mechanism of the OH Initiated Oxidation of Dimethyl, Dipropyl, and Dibutyl Sulfides: Reactivity Trends in the Alkyl Sulfides and Development of a Predictive Expression for the Reaction of OH with DMS

A pulsed laser photolysis-pulsed laser-induced fluorescence technique has been employed to measure rate coefficients for the OH-initiated oxidation of dimethyl sulfide (DMS), its deuterated analog (DMS-d(6)), dipropyl sulfide (DPS), and dibutyl sulfide (DBS). Effective rate coefficients have been measured as a function of the partial pressure of O(2) over the temperature range of 240-295 K and at 200 and 600 Torr total pressure. We report the first observations of an O(2) enhancement in the effective rate coefficients for the reactions of OH with DPS and DBS. All observations are consistent with oxidation proceeding via a two-channel oxidation mechanism involving abstraction and addition channels. Structures and thermochemistry of the DPSOH and DBSOH adducts were calculated. Calculated bond strengths of adducts increase with alkyl substitution but are comparable to that of the DMSOH adduct and are consistent with experimental observations. Reactivity trends across the series of alkyl sulfide (C(2)-C(8)) reactions are analyzed. All reactions proceed via a two-channel mechanism involving either an H-atom abstraction or the formation of an OH adduct that can then react with O(2). Measurements presented in this work, in conjunction with previous measurements, have been used to develop a predictive expression for the OH-initiated oxidation of DMS. This expression is based on the elementary rate coefficients in the two-channel mechanism. The expression can calculate the effective rate coefficient for the reaction of OH with DMS over the range of 200-300 K, 0-760 Torr, and 0-100% partial pressure of O(2). This expression expands on previously published work but is applicable to DMS oxidation throughout the troposphere.

Engineering Dislocation Networks for the Directed Assembly of Two-Dimensional Rotor Arrays

Two-dimensional arrays of molecular rotors may provide entirely new approaches to signal processing, sensing, the development of dielectrics, and energy modulation, all based on rotational motions in the molecular adlayers. To this end, we have engineered a bimetallic surface system with a regular array of dislocations and studied the adsorption of a molecular rotor, dibutyl sulfide. Because of a size difference between the atoms, a single layer of Ag deposited onto Cu{111} reconstructs the Cu surface into a regular hexagonal array of hexagonally close-packed domains with an average spacing of 2.6 ± 0.1 nm surrounded by face-centered cubic close-packed areas. Our data demonstrates that the affinity of adsorbates for these different domains can be used to spatially control single molecule adsorption; individual dibutyl sulfide rotors have been arranged in a hexagonal pattern via their binding preference for hcp sites in a manner analogous to placing cogs on a pegboard.

A Quantitative Single-Molecule Study of Thioether Molecular Rotors

This paper describes a fundamental, single-molecule study of the motion of a set of thioethers supported on Au surfaces. Thioethers constitute a simple, robust system with which molecular rotation can be actuated both thermally and mechanically. Low-temperature scanning tunneling microscopy allowed the measurement of the rotation of individual molecules as a function of temperature and the quantification of both the energetic barrier and pre-exponential factor of the motion. The results suggest that movement of the second CH(2) group from the S atom over the surface is responsible for the barrier. Through a series of single-molecule manipulation experiments, we have switched the rotation on and off reversibly by moving the molecules toward or away from one another. Arrhenius plots for individual dibutyl sulfide molecules reveal that the torsional barrier to rotation is approximately 1.2 kJ/mol, in good agreement with the temperature at which the molecule's appearance changes from a linear to a hexagonal shape in the STM images. The thioether backbone constitutes an excellent test bed for studying the details of molecular rotation at the single-molecule level.