A Study of the Atmospherically Important Reactions between Dimethyl

Article pubs.acs.org/JPCA

A Study of the Atmospherically Important Reactions between Dimethyl Selenide (DMSe) and Molecular Halogens (X2 = Cl2, Br2, and I2) with ab initio Calculations Lydia Rhyman,† Nerina Armata,‡ Ponnadurai Ramasami,*,† and John M. Dyke*,‡ †

Computational Chemistry Group, Department of Chemistry, University of Mauritius, Réduit, Mauritius School of Chemistry, University of Southampton, Southampton, UK SO17 1BJ

‡

S Supporting Information *

ABSTRACT: The atmospherically relevant reactions between dimethyl selenide (DMSe) and the molecular halogens (X2 = Cl2, Br2, and I2) have been studied with ab initio calculations at the MP2/aug-cc-pVDZ level of theory. Geometry optimization calculations showed that the reactions proceed from the reagents to the products (CH3SeCH2X + HX) via three minima, a van der Waals adduct (DMSe:X2), a covalently bound intermediate (DMSeX2), and a product-like complex (CH3SeCH2X:HX). The computed potential energy surfaces are used to predict what molecular species are likely to be observed in spectroscopic experiments such as gas-phase photoelectron spectroscopy and infrared matrix isolation spectroscopy. It is concluded that, for the reactions of DMSe with Cl2 and Br2, the covalent intermediate should be seen in spectroscopic experiments, whereas, in the DMSe + I2 reaction, the van der Waals adduct DMSe:I2 should be observed. Comparison is made with previous related calculations and experiments on dimethyl sulfide (DMS) with molecular halogens. The relevance of the results to atmospheric chemistry is discussed. The DMSeX2 and DMSe:X2 intermediates are likely to be reservoirs of molecular halogens in the atmosphere which will lead on photolysis to ozone depletion.

1.0. INTRODUCTION Oxidation reactions of ethers are important in atmospheric chemistry and in automobile fuel combustion where ethers are used as additives.1,2 A number of studies have been made on dimethyl ether (DME, CH3OCH3) oxidation reactions, partly because DME can be viewed as the prototypical ether.3−9 The reaction between DME and molecular chlorine is expected to be slow, and its room temperature rate coefficient does not appear to have been measured. In contrast, the room temperature rate coefficient of the reaction of the sulfur analogue of DME, DMS (dimethyl sulfide, CH3SCH3), with molecular chlorine has been measured.10 DMS results from the decay of ocean phytoplankton in the marine boundary layer and is known to be the major natural source of sulfur in the atmosphere.11,12 It plays an important role in climate regulation, as its oxidation leads to aerosol production and cloud formation.11 The main oxidants of DMS in the atmosphere are the hydroxyl radical (OH), the nitrate radical (NO3), atomic and molecular halogens, and other halogencontaining reactive species.13−16 Recently, using a flow-tube interfaced to a photoelectron spectrometer, we measured the rate coefficient at room temperature for the DMS + Cl2 reaction and studied the DMS + Cl2, DMS + Br2, DMS + I2, and DMS + ICl reactions using photoelectron spectroscopy, infrared matrix isolation spectroscopy, and electronic structure calculations.10,17−19 © 2012 American Chemical Society

Dimethyl selenide (CH 3SeCH3, DMSe) is the most abundant gaseous selenium species in marine environments.20 Laboratory and field research has shown that microorganisms naturally present in contaminated soil and water transform selenium compounds into DMSe, which is liberated to the atmosphere. Volatile organoselenium compounds, such as DMSe, emitted into the environment from biogenic and anthropogenic sources can undergo photolysis, wet and dry deposition, adsorption on particulates, and reaction with atmospheric oxidants such as OH, NO3, and O3.21−26 Such processes represent pathways for biogeochemical cycling and global redistribution of selenium in the atmosphere.27,28 The room temperature rate coefficients of DMSe with OH, O3, and NO3 have been measured by Atkinson and co-workers.29 When these were combined with estimated average atmospheric concentrations of OH, O3, and NO3, the atmospheric lifetimes of DMSe from these reactions were estimated as 2.7 h, 5.8 h, and 5 min, respectively, with the OH and O3 reactions being important during the day and the NO3 reaction being important at night. However, based on known reactions of DMS, reactions of DMSe with molecular halogens are also expected to be important oxidation reactions in the Received: March 22, 2012 Revised: May 22, 2012 Published: May 23, 2012 5595

dx.doi.org/10.1021/jp302750a | J. Phys. Chem. A 2012, 116, 5595−5603

The Journal of Physical Chemistry A

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Figure 1. Relative electronic energy diagram, excluding zero-point vibrational energies, for the reaction DMSe + Cl2.

programs.35 The computational approach used to study the reactions of DMSe with molecular halogens, X2, was based on that used for the analogous reactions of DMS with halogens.10,17−19 The computations were carried out at the Möller−Plesset (MP2)36 level using the aug-cc-pVDZ basis set37,38 for all atoms except for iodine. For complexes containing I2, two sets of calculations were performed: one with the effective core potential (ECP) I-atom basis set, LANL2DZ,39−41 and the other with the more flexible atomic iodine ECP basis set, aug-cc-VDZ-PP, that we used for DMS + I2 and DMS + ICl calculations.19 Harmonic vibrational analysis was performed to verify each minimum on the potential energy surface. Every stationary point identified was characterized by the number of negative eigenvalues of its Hessian matrix: 0 for a minimum and 1 for a transition state. The imaginary frequencies of the located transition states exhibit the expected motion which was assigned by means of visual inspection and animation using the CYLVIEW program.42 Further, the intrinsic reaction coordinate (IRC)43,44 path was traced, at the same level of theory, to ensure that the transition states (TSs) led to the expected reactants and products. For the optimized geometries obtained for the van der Waals complexes, DMSe:X2, and the covalent complexes, DMSeX2, and using the same basis sets as used for the geometry optimizations, vertical transition energies were calculated with time dependent density functional theory (TDDFT) with the range-separated hybrid exchange correlation functional CAMB3LYP.45−48 This functional has different amounts of Hartree− Fock and Becke (B88) exchange at short and long range. It was designed to improve on the B3LYP description of long-range phenomena, such as charge-transfer excitation or polarizability, and is known to be reliable for calculating vertical excitation energies for small molecules to within ca. 0.25 eV.45−48 The results of these TDDFT calculations were very similar with both I basis sets, and only the results with the aug-cc-VDZ-PP I basis set are reported.

atmosphere. It is known that molecular chlorine is present in the atmosphere from algae decomposition and from anthropogenic activities,30 and that molecular bromine is released into the atmosphere from natural sources, both at significant partial pressures.31 It has also been demonstrated in field measurements that moderate concentrations of molecular iodine can occur in the marine boundary layer possibly arising from microalgae at low tide, and frozen acidified sea salt solutions containing halides have been identified as atmospheric sources of molecular halogens, notably I2 and Br2, and interhalogens.32−34 In our recent work on the reactions of DMS with Cl2, Br2, and I2,10,17−19 for each reaction, key minima on the potential energy surface were shown to be the reagents, a van der Waals complex DMS:X2, a covalent intermediate DMSX2, and the products CH3SCH2X + HX. The relative energies of these minima and transition states which interconnect them were determined by electronic structure calculations. In this present paper, this work is extended to study the reactions of DMSe with the molecular halogens (X2 = Cl2, Br2, and I2). For each reaction, the aim is to use ab initio electronic structure calculations to determine the relative energy of the stationary points on the reaction surface. This information will be used to determine the reaction mechanism, to make a comparison with the results obtained for the corresponding DMS + X2 reactions studied earlier, and to predict what species will be observed in future spectroscopic studies of these reactions such as gas-phase photoelectron spectroscopy and infrared matrix isolation experiments. It is also proposed to compute infrared spectra of the identified reaction intermediates for future experimental study. The implications of the results to atmospheric chemistry will then be considered.

2.0. COMPUTATIONAL METHODOLOGY Full geometry optimization of reactants, intermediates, and transition states was performed with the Gaussian 09 suite of 5596



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Figure 2. Relative electronic energy diagram, excluding zero-point vibrational energies, for the reaction DMSe + Br2.

Figure 3. Relative electronic energy diagram, excluding zero-point vibrational energies, for the reaction DMSe + I2 (for iodine, the aug-cc-pVDZ−PP basis set has been used) (with the I LANL2DZ basis set, the energies of the minima relative to the reagents are −14.2, −20.5, −10.7, and −3.6 kcal·mol−1; TS1 and TS2 are computed at +7.4 and +17.1 kcal·mol−1).

3.0. RESULTS AND DISCUSSION

the reagents, a reagent-like complex termed a van der Waals complex (DMSe:X2), a covalent intermediate (DMSeX2), a product-like complex (CH3SeCH2X:HX), and the final products (CH3SeCH2X + HX). Figures 1−3 display the schematic energy profiles for these reactions along with the computed relative energies and the transition states connecting

3.1. Computed Relative Energies of Minima and Transition States for the DMSe + X2 Reactions. In general, for a DMSe + X2 reaction, the ab initio computations revealed the following minima on the potential energy surface: 5597



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thermoneutral with a reaction energy of +1.2 kcal·mol−1 (with the aug-cc-pVDZ-PP I basis; with the LANL2DZ I basis set, the reaction energy is −3.6 kcal·mol−1). On the basis of the experimental results obtained for the reactions DMS + X2 (with X = Cl, Br and I), it is expected that the van der Waals form will be observed in infrared matrix isolation experiments and gasphase photoelectron experiments. It is unlikely that the covalent complex will be observed experimentally as the barrier to interconversion of the van der Waals structure to the covalent complex is probably too high at +8.1 kcal·mol−1 (TS1 relative energy; see Figure 3), although this may be reduced somewhat if a multireference calculation method is used to calculate relative energies. In summary, the covalent intermediate is expected to be observed experimentally for the DMSe + Cl2 and DMSe + Br2 reactions, whereas for the DMSe + I2 reaction the van der Waals structure will be observed. 3.2. Geometrical Parameters. Table 1 summarizes the main structural parameters of the computed minimum energy geometries for the DMSe:X2 van der Waals adducts and the DMSeX2 covalent complexes, and makes a comparison with the corresponding computed parameters for the corresponding DMS:X2 and DMSX2 compounds. Figure 4 presents the minimum energy geometries of the van der Waals DMSe:X2 adducts. It can be seen that there is an increase in the Se−X2 bond distance from Cl2 to I2 with a concomitant decrease in the C−Se−C bond angle. The computed equilibrium bond lengths of the isolated X2 molecules at the MP2 level, 2.039, 2.323, and 2.719 Å for Cl2, Br2, and I2, respectively, compare favorably with available experimental values (Cl2, 1.988 Å; Br2, 2.281 Å; I2, 2.666 Å49). An increase of the bond lengths of the X2 moiety in the adduct is observed, as was found in the DMS:X2 van der Waals adducts (see Table 1). In the covalent structures, the Se atom is covalently bound to two X atoms and two methyl groups, as illustrated in Figure 5. In the van der Waals adducts, on going from Cl to I, the Se−X and Se−C bond lengths increase by about 0.400 Å and 0.004 Å, respectively, while the corresponding C−Se−C bond angle decreases by about 1.0°. The X−Se−X and X−Se−C bond angles increase when going from Cl to Br to I. Similar changes are observed in the DMSX2 covalent complexes (see Table 1 and Figures 4 and 5). No spectroscopic studies have been made of the dimethylselenium dihalide compounds in the gas phase which have yielded structural information. However, dimethylseleniun difluoride is known to be a liquid which freezes at approximately 11 °C at atmospheric pressure, and NMR studies have shown it to be a covalent structure of the type shown in Figure 5.50 Single crystals have also been made of the dimethylselenium dichloride, dibromide, and diodide by reacting Me2Se with SOCl2, Br2, and I2, respectively, in diethyl ether.51 X-ray crystallographic studies of the crystals obtained, which were white, yellow, and red, respectively, revealed that dimethylselenium dichloride and dibromide have covalent structures of the DMSeX2 type shown in Figure 5, and dimethylselenium diiodide has a van der Waals structure of the type shown in Figure 4.51 These structural types are exactly as expected in section 3.1, from the computed potential energy diagrams in Figures 1−3. The structural parameters obtained from these Xray crystallographic studies for the covalent complexes DMSeCl2 and DMSeBr2 and the van der Waals complex DMSe:I2 have been included in Table 1 for comparison with the computed MP2 structural parameters in this work, and as

the minima. The reactions proceed from the reagents to the van der Waals complex and then to the covalent complex via a transition state (TS1). The covalent intermediate then leads to the final products via another transition state (TS2). The reaction DMSe + Cl2 is spontaneous as the formation of the covalent complex is barrier-less (ΔE = −10.0 kcal mol−1, ΔH⧧ = −9.9 kcal mol−1, and ΔS⧧ = −37.4 cal K−1 mol−1, for the change between the reagents and TS1). The final products are also expected to be formed easily as the covalent complex decomposes over another barrier, which is lower than the reagents, of −3.8 kcal mol−1 to form the products, and there is enough internal energy in the reaction system to overcome this barrier. On comparing the DMSe + Cl2 and DMS + Cl2 reactions, both are exothermic (−34.5 and −35.0 kcal·mol−1 at the MP2 level, respectively) to give the final products (CH3SeCH2Cl + HCl and CH3SCH2Cl + HCl, respectively), but the covalent and van der Waals complexes and TS1 and TS2 are lower in energy in the DMSe case. It is particularly notable that the covalent complex is significantly lower in the DMSe case relative to the DMS case (−41.6 and −24.6 kcal·mol−1, respectively, see Figure 1), and in the DMS case, the transition states, TS1 and TS2, are both at a positive energy, +6.0 and +2.9 kcal·mol−1, relative to the reagents, whereas in the DMSe case these transition states are lower than the reagent energies. On the basis of the experimental results obtained for the DMS + Cl2 reaction, it is likely that, for the DMSe + Cl2 reaction, the covalent complex and the final products will be observed both in matrix isolation infrared spectroscopy and gas-phase photoelectron spectroscopy experiments. It is also very unlikely that the van der Waals complex will be observed in infrared matrix isolation experiments because, although it lies −11.5 kcal·mol−1 below the reagents, the barrier to conversion to the lower energy covalent intermediate (which lies at −41.6 kcal·mol−1 relative to the reagents) is low at −1.5 kcal·mol−1. In the case of DMSe + Br2, the covalent structure DMSeBr2 is the lowest minimum on the potential energy surface, as was the case for the DMSe + Cl2 reaction (see Figure 2). Formation of the covalent structure is again expected to be rapid as the process is barrier-less (ΔE = −1.5 kcal mol−1, ΔH⧧ = −1.3 kcal mol−1, and ΔS⧧ = −37.8 cal K−1 mol−1, between the reagents and TS1). The overall reaction is exothermic (−17.8 kcal·mol−1 to form CH3SeCH2Br + HBr), but there is a significant barrier (TS2) for conversion of the covalent structure into the product-like complex (+7.1 kcal·mol−1 relative to the reagents). Compared to the DMS + Br2 reaction, for which only the van der Waals complex was observed experimentally by infrared matrix isolation spectroscopy because of a high TS1 (+18.7 kcal·mol−1) to convert to the covalent form, the covalent form is expected to be produced from DMSe + Br2 and observed in both infrared matrix isolation and gas-phase photoelectron spectroscopy experiments. As is the case for the DMS + X2 reactions, for the DMSe + X2 reactions, the energies of TS1 and TS2 increase relative to the reagents on going from Cl2 to Br2 to I2, and for DMSe + I2, the energies of TS1 and TS2 are both positive relative to the reagents (at +8.1 and +18.4 kcal·mol−1 with the I aug-cc-pVDZPP basis set; see Figure 3). The covalent structure is again the most stable minimum on the potential surface (at −19.5 kcal·mol−1), but the van der Waals minimum is at −15.8 kcal·mol−1 with a higher barrier for interconversion to the covalent structure than for X2 = Cl2 and Br2. The overall DMSe + I2 reaction to give CH3SeCH2I and HI is approximately 5598



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Table 1. Structural Parameters Computed at the MP2 Level for the Minimum Energy Geometries of (i) the DMSe:X2 van der Waals Complex and (ii) the DMSeX2 Covalent Complexa (a) X2 = Cl2 (i) van der Waals complex C(1)−Se C(2)−Se Se−Cl(1) Cl(1)−Cl(2) C(1)−Se− C(2) C(1)−Se− Cl(1) C(2)−Se− Cl(1) Se−Cl(1)− Cl(2)

(ii) covalent complex

exptl ref 51; covalent structure

Bond Lengths (Å) (1.820)b 1.946 (1.828)b (1.820) 1.946 (1.828) (2.552) 2.397 (2.332) (2.190) Angles (deg) 96.7 (98.9) 98.5 (100.3)

1.946 1.946 2.562 2.258

not given not given 2.349, 2.408 not given

Figure 4. Selected geometrical parameters for the van der Waals complex, DMSe:X2 (Ia: LANL2DZ and Ib: aug-cc-pVDZ+PP basis sets were used for iodine).

98.6

91.5 (93.4)

88.0 (90.8)

89.5

91.5 (93.4)

90.7 (90.8)

99.0

174.0 (174.5) (b) X2 = Br2 (i) van der Waals complex

(ii) covalent complex

exptl ref 51; covalent structure

Bond Lengths (Å) (1.815)b 1.949 (1.930)b (1.815) 1.949 (1.930) (2.724) 2.568 (2.510) (2.436)

C(1)−Se C(2)−Se Se−Br(1) Br(1)− Br(2)

1.948 1.948 2.735 2.481

C(1)−Se− C(2) C(1)−S− Br(1) C(2)−S− Br(1) S−Br(1)− Br(2)

96.5 (99.1)

Angles (deg) 98.5 (100.0)

not given not given 2.546, 2.551 not given

91.6 (95.4)

88.3 (91.8)

91.3

91.6 (95.4)

91.2 (91.9)

91.7

174.0 (175.1)

Figure 5. Selected geometrical parameters for the covalent complex, DMSe-X2 (Ia: LANL2DZ and Ib: aug-cc-pVDZ+PP basis sets were used for iodine).

98.0

and there is little evidence of extended solid-state structures. In the case of the dimethylselenium dichloride, more interaction of the DMSeCl2 unit with the rest of the crystal structure is evident, and as a result, this should be borne in mind when making comparisons between the experimental DMSeCl2 structural parameters from X-ray single crystal studies and the results of isolated molecule, MP2 calculations performed in this work. It is also interesting to compare the reaction of DMSe with atomic and molecular chlorine, as both reactions involve the formation of a covalent intermediate: DMSeCl and DMSeCl2. The DMSeCl intermediate forms on addition of Cl to the Se atom.52 The Se−Cl bond is longer in DMSeCl (2.73 Å at the B3LYP/6-311++G(d,p)level52) than in DMSeCl2 (2.562 Å, this work), while the C−Se−C bond angle is smaller in DMSeCl (98.1° at the B3LYP/6-311++G(d,p) level50) than in DMSeCl2 (98.5°, this work). The same trends are observed on comparing the structures of the covalent intermediates of DMS with atomic and molecular chlorine computed at the UMP2/DZP and MP2/aug-cc-PVDZ levels, respectively.10,53 3.3. Computed Infrared Spectra, Electronic Excitation Energies, and Atmospheric Relevance. The computed infrared spectra of the van der Waals adducts DMSe:X2 and the covalent complexes DMSeX2 are shown in Figures 6−8 with descriptions of the most intense absorptions given in Tables 2−4. As was found to be the case for the DMS + X2 reactions studied by infrared matrix isolation spectroscopy, significant differences in the spectra are observed below 310 cm−1, as this is where the Se−X stretching absorptions occur. However, this

177.7 (c) X2 = I2

exptl ref 51; van der Waals structure C(1)−Se C(2)−Se Se−I(1) I(1)−I(2)

C(1)−Se− C(2) C(1)−Se− I(1) C(2)−Se− I(1) Se−I(1)− I(2)

(i) van der Waals complex

Bond Lengths (Å) not given 1.950,c 1.949d (1.816)b not given 1.950,c 1.949d (1.816) 2.768 3.019,c 2.953d (2.969) 2.916 2.788,c 2.853d (2.791) Angles (deg) 99.0 95.9,c 96.2d (99.0) 94.8 90.8,c 91.9d (96.4) 97.6 90.8,c 91.9d (96.4) 174.3 173.3,c 173.8d (175.1)

(ii) covalent complex 1.952,c 1.953d (1.832)b 1.952,c 1.953d (1.832) 2.801,c 2.802d (2.772)

96.6,c 96.9d (99.3) 89.7,c 89.4d (93.0) 91.4,c 91.7d (93.0)

a

See Figures 4 and 5 for the atom numbering used. bValues in brackets are the corresponding DMS:X2 and DMSX2 values.18,19 cLANL2DZ I basis set used. daug-cc-pVDZ-PP basis set used.

can be seen in general, good agreement is obtained. For the dimethylselenium dibromide and diodide crystal structures, the DMSeBr2 and DMSe:I2 units are discrete units in the crystal 5599



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Table 2. Computed Wavenumbers and Intensities for the Main Absorptions in the IR Spectra of DMSe:Cl2 and DMSeCl2 DMSe:Cl2 (intensity/km mol−1) 151 (16) 178 (71) 294 (326) 932 (14) 991 (14) 1311 (25) 1453 (14) 3072 (10)

Figure 6. Simulated spectra of the van der Waals structure DMSe:Cl2 (upper) and the covalent structure DMSeCl2 (lower) (the spectral bands in the 500−3500 cm−1 region have been increased by ×10).

assignment CH3 rocking Se−Cl stretching Cl−Cl stretching CH3 rocking CH3 rocking CH3 deformation H−C−H bending C−H stretching

DMSeCl2 (intensity/km mol−1)

assignment

270 (99) 301 (210)

CH3 rocking Se−Cl stretching

948 (15)

CH3 twisting

995 (5) 1306 (3) 1451 (10)

CH3 rocking CH3 deformation H−C−H bending

3091 (0)

C−H symmetric stretching C−H asymmetric stretching

3235 (1)

Table 3. Computed Wavenumbers and Intensities for the Main Absorptions in the IR Spectra of DMSe:Br2 and DMSeBr2 DMSe:Br2 (intensity/km mol−1) 122 (25) 153 (39) 212 (104) 933 (19) 989 (14) 1312 (26) 1453 (11) 3073 (12)

Figure 7. Simulated spectra of the van der Waals structure DMSe:Br2 (upper) and the covalent structure DMSeBr2 (lower) (the spectral bands in the 500−3500 cm−1 region for DMSe:Br2 have been increased by ×4 and for DMSeBr2 by ×10).

assignment CH3 rocking Se−Br stretching Br−Br stretching H−C−H bending CH3 rocking CH3 deformation H−C−H bending C−H stretching

DMSeBr2 (intensity/km mol−1)

assignment

230 (199) 262 (14)

Se−Br stretching CH3 rocking

941 (1)

CH3 twisting

995 (4)

CH3 rocking

1304 (5) 1443 (10)

CH3 deformation H−C−H bending

3089 (0)


3232 (0)

Table 4. Computed Wavenumbers and Intensities for the Main Absorptions in the IR Spectra of DMSe:I2 and DMSeI2 (with the aug-cc-pVDZ+PP Basis Set Used for Iodine) DMSe:I2 (intensity/km mol−1) 102 (13) 161 (19) 172 (25) 935 (28) 989 (13) 1313 (21) 1455 (7) 3071 (15)

Figure 8. Simulated spectra of the van der Waals structure DMSe:I2 (upper) and the covalent structure DMSeI2 (lower) (the spectral bands in the 500−3500 cm−1 region for DMSeI2 have been increased by ×5; for iodine, the aug-cc-pVDZ+PP basis set was used).

assignment Se−I stretching CH3 rocking I−I stretching CH3 rocking CH3 rocking CH3 deformation H−C−H bending C−H stretching

DMSeI2 (intensity/km mol−1)

assignment

204 (123)

Se−I stretching

253 (30) 929 (2) 994 (3) 1299 (7) 1441 (15)

CH3 rocking CH3 twisting CH3 rocking CH3 deformation H−C−H bending

3084 (0.41)


3213 (0.08)

proved to be a difficult region in which to acquire good quality infrared spectra in the DMS + X2 matrix studies. Probably the spectral region to concentrate on experimentally, where good quality spectra can be obtained in infrared matrix isolation experiments, to distinguish between van der Waals and covalent structures from the DMSe + X2 reactions is the region 1500− 5600



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750 cm−1. This is the region where CH3 rocking and deformation modes in DMSe:X2 and DMSeX2 occur (see Tables 2−4). It is expected that when matrix isolation infrared spectra are recorded for the intermediates obtained from the DMSe + X2 reactions that the relative band intensities obtained in this spectral region, combined with the results of the electronic structure calculations shown in Figures 6−8, will be able to determine whether the intermediate observed is the van der Waals adduct or the covalent complex, as was achieved previously for the DMS + X2 reactions.10,17−19 As stated earlier, based on the schematic potential energy diagrams shown in Figures 1−3, it is expected that the covalent complex will be observed from the DMSe + Cl2 and DMSe + Br2 reactions and the van der Waals complex will be observed from the DMSe + I2 reaction. Some infrared spectra of the DMSe + X2 complexes have been recorded previously as pressed pellets of the crystals and in nujol mulls.54,55 Unfortunately, no spectra were presented in these studies with only band maxima and qualitative relative band intensities (strong, medium, weak) being listed. However, this information, on comparison with Figures 6−8, does appear to be consistent with a covalent structure for the product from the DMSe2 + Cl2 reaction and a van der Waals structure from the DMSe + I2 reaction. For the complex made from the DMSe + Br2 reaction, the relative band intensities listed in the two studies do not agree and hence it is not possible to decide on which structural type is present. Clearly, a more detailed study of these complexes is required by infrared spectroscopy. Computed TDDFT vertical singlet−singlet transition energies from the ground state of the van der Waals adducts DMSe:X2 and the covalent complexes DMSeX2 are shown in Tables 5 and 6, respectively. For the van der Waals adducts, the first transition is weak (expected at 384, 418, and 474 nm for X2 = Cl2, Br2, and I2), and the most intense transition, the third transition for DMSe:Cl2 and the fifth transition for DMSe:Br2 and DMSe:I2, is expected at 258, 262, and 277 nm for X2 = Cl2, Br2, and I2, respectively (see Table 5). The only available experimental electronic absorption spectrum for the DMSe2 + I2 reaction was obtained in carbon tetrachloride solution in the 350−550 nm region.56 This shows a broad band centered at 440 nm (2.82 eV), which probably corresponds to the first electronic transition of the DMSe:I2 van der Waals adduct (computed vertical position by TDDFT calculations 474 nm, 2.62 eV). For the covalent complexes, DMSeX2, the first transition is again quite weak (expected at 255, 306, and 396 nm for X2 = Cl2, Br2, and I2, respectively), and the most intense transition is the sixth transition for DMSeCl2 (expected at 204 nm) and the fifth transition for DMSeBr2 and DMSeI2 (expected at 239 and 295 nm, respectively). If DMSe and one of the molecular halogens, Cl2, Br2, or I2, are deposited from the atmosphere onto cold surfaces such as ice surfaces or aerosols, then, on the basis of the results of this work, the van der Waals adduct DMSe:I2 or one of the covalent complexes, DMSeCl2 and DMSeBr2, will be formed. On warming, DMSe and the molecular halogen will be produced as well as the complex and the reaction products CH3SeCH2X + HX in the gas phase. These complexes could therefore act as reservoirs for diatomic halogens in the atmosphere which on photodissociation by radiation from the sun will give halogen atoms which will lead to ozone depletion. For DMSe:I2, both the first transition, computed by TDDFT at 474 nm, and the most intense transition, computed at 277 nm, are expected from the calculations to correspond to a DMSe to I2 electron

Table 5. van der Waals complexes - TDDFT Computed Vertical Singlet−Singlet Transition Energies for the van der Waals Complexes DMSe:Cl2, DMS:Br2, and DMS:I2 Obtained with the CAM-B3LYP Functional (a) DMSe:Cl2

transition 1 2 3 4 5 6



main excitation HOMO = 43, LUMO = 44 42 41 43 40 39 43

→ → → → → →

44 44 44 44 44 45


→ → → → → →

62 62 62 62 62 63

vertical transition energy (eV) 3.225 3.236 4.801 5.098 5.115 5.184 (b) DMSe:Br2 vertical transition energy (eV) 2.965 2.973 4.641 4.682 4.730 5.170 (c) DMSe:I2

wavelength (nm)

oscillator strength, f

384.4 383.0 258.2 243.2 242.4 239.1

0.0012 0.0001 1.1221 0.0479 0.0000 0.0015

wavelength (nm)


418.2 417.0 267.1 264.8 262.1 239.8

0.0012 0.0004 0.3446 0.0002 0.8838 0.0014

main excitation HOMO = 51, LUMO = 52

vertical transition energy (eV)

wavelength (nm)


→ → → → → →

2.617 2.621 4.139 4.144 4.474 5.087

473.8 473.1 299.5 299.2 277.1 243.7

0.0014 0.0008 0.0038 0.0001 1.1684 0.0013

51 50 48 47 49 49

52 52 52 52 52 54

transfer into the lowest I2 empty orbital, which is antibonding. This will weaken the I−I bond in the adduct. Solar actinic flux values are expected to be significant in the 310−480 nm region,57 and hence only the first two transitions, calculated at 474 and 473 nm, in DMSe:I2 are expected to be excited by radiation from the sun, as the other transitions occur at lower wavelength than 310 nm. Hence, exposure to solar radiation could lead to photodissociation of I2 in the DMSe:I2 complex which will lead to production of atomic iodine and enhanced ozone depletion. For DMSeCl2 and DMSeBr2, the first and most intense transitions correspond to transitions between orbitals delocalized over the halogen and sulfur centers which give rise to weakening of the Se−X bonds. However, in both complexes, the first transition occurs at lower wavelength than 310 nm and hence is outside the solar actinic region. Hence, excitation by radiation from the sun will not be significant in these cases. Nevertheless, DMSeCl2 and DMSeBr2 on frozen surfaces will act as halogen reservoirs, as on warming the molecular halogen will be released to the atmosphere.

4.0. CONCLUSIONS The reactions of Cl2, Br2, and I2 with DMSe have been studied with electronic structure calculations. The results indicate that the DMSe + I2 reaction is expected to give the van der Waals adduct DMSe:I2, the DMSe + Br2 reaction is expected to give the covalent complex DMSeBr2, and the DMSe + Cl2 reaction is expected to give the covalent complex DMSeCl2 as well as the final products CH3SeCH2Cl + HCl. A comparison of the 5601



Article

edges funding from the University of Mauritius. The authors also acknowledge support from the UK National Service in Computational Chemistry (NSCCS).

Table 6. Covalent Complexes - TDDFT Computed Vertical Singlet−Singlet Transition Energies for the Covalent Complexes DMSeCl2, DMSBr2, and DMSI2 Obtained with the CAM-B3LYP Functional

■

(a) DMSeCl2





→ → → → → →

44 44 44 44 45 44


→ → → → → →

62 62 62 62 62 63

vertical transition energy (eV) 4.859 5.132 5.226 5.227 5.957 6.0831 (b) DMSeBr2 vertical transition energy (eV) 4.048 4.197 4.223 4.239 5.176 5.508 (c) DMSeI2

wavelength (nm)


255.2 241.6 237.2 237.2 208.1 203.8

0.0614 0.0000 0.0032 0.0029 0.1112 0.6263

wavelength (nm)


306.2 295.4 293.6 292.5 239.5 225.1

0.0506 0.0000 0.0017 0.0019 0.9447 0.0019

main excitation HOMO = 51, LUMO = 52

vertical transition energy (eV)

wavelength (nm)


→ → → → → →

3.132 3.187 3.203 3.315 4.207 4.886

395.8 388.9 387.0 385.7 294.7 253.8

0.0359 0.0006 0.0009 0.0000 1.1645 0.0025

47 50 49 48 51 51

52 52 52 52 52 53

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results with the results of ab initio calculations performed at the same level, as well as with relevant experimental information, for the corresponding DMS + X2 reactions has also been made. The importance of the stability and electronic spectroscopy of the complexes DMSeX2 and DMSe:X2 to atmospheric chemistry has been considered, notably their possible relevance to halogen to catalyzed ozone depletion.

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ASSOCIATED CONTENT

S Supporting Information *

MP2/aug-cc-pVDZ computed total energies, the unique TS imaginary frequency and Cartesian coordinates of the stationary points involved in the reactions of DMSe with X2 (X = Cl, Br, and I). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.R.); [email protected] (J.M.D.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.A. thanks the EU Early Stage Research Training Network (SEARCHERS) for financial support, and J.M.D. thanks the Leverhulme Trust for an Emeritus Fellowship. P.R. acknowl5602



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A Study of the Atmospherically Important Reactions between Dimethyl

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