Dehydrative Cyclocondensation Mechanisms of Hydrogen

Jun 22, 2012 - C(CH3)3, CF3, CCl3) and the mechanisms of their dehydrative cyclocondensation to the respective sulfinothioic acid (H−. (S O)−S−H...
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Dehydrative Cyclocondensation Mechanisms of Hydrogen Thioperoxide and of Alkanesulfenic Acids Fillmore Freeman,*,† An Bui,† Lauren Dinh,† and Warren J. Hehre†,‡ †

Department of Chemistry, University of California, Irvine, Irvine, California 92697, United States Wavefunction, Inc., Irvine, California 92612, United States

‡

S Supporting Information *

ABSTRACT: Structural features of hydrogen thioperoxide (oxadisulfane, H−S−O−H) and of alkanesulfenic acids (R− S−O−H; R = CH3, CH2CH3, CH2CH2CH3, CH(CH3)2, C(CH3)3, CF3, CCl3) and the mechanisms of their dehydrative cyclocondensation to the respective sulfinothioic acid (H− (SO)−S−H) and alkyl alkanethiosulfinates (R−(SO)− S−R) have been studied using coupled cluster theory with single and double and perturbative triple excitations [CCSD(T)] and quadratic configuration interaction with single and double and perturbative triple excitations [QCISD(T)] with the cc-pVDZ basis set and also using second-order Møller-Plesset perturbation theory (MP2) and the hybrid density functionals B3LYP, B3PW91, and PBE1PBE with the 6-311+G(d,p) basis set. The concerted cyclodehydration mechanisms include cyclic five-center transition states with relatively long distance sulfur−sulfur bonding interactions. Attractive and repulsive nonbonding interactions are predicted in the sulfenic acids, transition states, and thiosulfinates. In the alkyl alkanethiosulfinates attractive cyclic C−H----OS nonbonding interactions are predicted. CCSD(T) and QCISD(T) predict similar values for the relative energies and CCSD(T) predicts the barrier to the cyclocondensation of H− S−O−H to sulfinothioic acid (H−(SO)−S−H) to be 41.8 kcal/mol, and barriers in the range of 37.5 to 39.6 kcal/mol are predicted for the condensation of alkanesulfenic acids to alkyl alkanethiosulfinates.

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INTRODUCTION Compounds of sulfur play important roles in atmospheric chemistry, biochemistry, combustion chemistry, industrial chemistry, and many other areas of science. Of strong current interest is the chemistry and properties of the elusive unstable sulfenic acids (R−SOH),1−25 particularly in biological systems. Recent studies are increasingly showing the important roles of cysteine (Cys), which is the only amino acid with a reactive sulfur moiety, cysteine sulfenic acid, and cysteine thiosulfinate in proteins. Although acyclic sulfenic acids can be generated in situ, it appears there are no reports of the isolation of these highly reactive species. Owing in part to this instability, relatively little is known about the chemistry of sulfenic acids although they and their derivatives are useful reaction intermediates and are found in living systems and in natural products including garlic and onion.5−10 Hydrogen thioperoxide (1, oxadisulfane),17−20 methanesulfenic acid (3),10,12,13 2-propanesulfenic acid (9),11 and 2methyl-2-propanesulfenic acid (11, eq 1)11,13,14,16 have been generated in flash vapor pyrolysis (FVP) studies and have been trapped with activated alkenes and alkynes. Presumably, an important contributing factor to the instability of sulfenic acids is their facile exothermic dehydrative self-condensation through sulfenic acid dimers to the corresponding thiosulfinates (R− (SO)−S−R). Thiosulfinates, which are known to have health-promoting properties, are thermally labile, easily oxidized to vicinal disulfoxides and to thiosulfonates, show a © XXXX American Chemical Society

wide range of bioactivity, and undergo a variety of reactions including sulfine formation and facile disproportionation to the corresponding disulfides and thiosulfonates (eq 2).10,15 An example is ethyl ethanethiosulfinate (6) that is easily oxidized, is a precursor for thioethanal and for ethanesulfenic acid (hydroxysulfanylethane, 5),24,25 and is a thiol protecting group.15

There does not appear to be any computational or theoretical studies of the mechanism of the cyclocondensation of sulfenic acids to thiosulfinates, and there appear to be only two experimental studies of the mechanism of their conversion Received: March 14, 2012 Revised: May 9, 2012

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moments (μ) in debyes (D), vibrational frequencies in wavenumbers (cm−1), infrared intensities in km/mol, and entropy in entropy units (eu, cal/(mol-Kelvin). Atom numbering and nonbonding interactions (dashed lines) are shown for the B3LYP/6-311+G(d,p) optimized structures in the Figures.

to thiosulfinates. The self-condensation of 2-methyl-2-propanesulfenic acid (11) to 2-methylpropyl 2-methylpropanethiosulfinate (12) in benzene has been reported to be a first order reaction that is catalyzed by acid or by base.14 In the other study in aqueous ethanenitrile, which presumably inhibits intermolecular hydrogen bonding between two molecules of (11), second order kinetic behavior was observed, and it was suggested that the mechanism involves nucleophilic attack of the free sulfenic acid at the backside of the S−O bond of the protonated sulfenic acid.14 We have investigated the electronic effects, inductive effects, nonbonding interactions, and steric effects in (1), (3), (5), propanesulfenic acid (7), (9), (11), 1,1,1-trifluoromethyl 1,1,1-trifluoromethanesulfenic acid (13), and 1,1,1-trichloromethyl 1,1,1-trichloromethanesulfenic acid (15) at high levels of modern electronic theory to better understand their ground state structures, diagnostic infrared frequencies, and the structures of the transition states involved in condensation reactions to sulfinothioic acid (2) and to their respective alkyl alkanethiosulfinates (4, 6, 8, 10, 12, 14, 16).

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RESULTS AND DISCUSSION Although hydrogen thioperoxide (1) and sulfenic acids can exist as tautomers (R−SOH ⇌ R−S(O)H), the sulfenyl

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Figure 1. Atom numbering and selected nonbonding interactions in the B3LYP/6-311+G(d,p) geometry optimized structures of hydrogen thioperoxide (1), TSl, and sulfinothioic acid (2).

COMPUTATIONAL AND THEORETICAL METHODS Calculations were carried out with the Gaussian26,27 and Spartan28−30 computational programs using the frozen-core approximation. Initial approaches to locating the lowest energy conformers and rotamers of the organosulfur compounds included conformational searches using the Monte Carlo algorithm at various levels of theory. Potential energy profile plots with constrained internal rotation about torsion angles from 0 to 360° in 30° increments were performed at different levels of theory on all reactants and products. Several levels of theory were employed because at a given level some of the conformers are isoenergetic and the lowest energy conformer from one level of theory does not uniformly translate into the lowest energy conformer at another level. The structures of the lowest energy conformers were confirmed by calculations at several high levels of electronic theory. No constraints were imposed on the structures in the equilibrium geometry calculations or in the transition state optimizations. Equilibrium geometry and frequency calculations were carried out using second-order Møller-Plesset perturbation theory (MP2),31 B3LYP,32 B3PW91,33 and PBE1PBE34,35 with the 6-311+G(d,p) basis set.29 Energy calculations were carried out on B3LYP/6-311+G(d,p) geometry optimized structures using coupled cluster theory with single and double and perturbative triple excitations [CCSD(T)]36,37 and the quadratic configuration interaction method with single and double and perturbative triple excitations [QCISD(T)]38,39 with the Dunning’s correlation consistent polarized valence basis set cc-pVDZ.40 Relative energies (Erel) calculations include electronic energy plus the zero point vibrational energy (ZPVE). Vibrational frequency analyses were carried out to assess the nature of the stationary points and to obtain zero point vibrational energies. The characteristics of local minima and transition states were verified by establishing that the matrices of the energy second derivatives have either zero or one negative eigenvalue, respectively. The computed vibrational frequencies in the discussion and in the tables are not scaled.41,42 Throughout the article bond angles and dihedral (torsion) angles are given in degrees, bond lengths and nonbonded distances in angstroms (Å), total energies in atomic units (au), and relative energies in kcal/mol. The natural population analysis (NPA) atomic charges are given in electrons,43 dipole

tautomer (R−SOH) is predicted to be significantly lower in energy than the corresponding sulfinyl tautomer. B3LYP, B3PW91, and PBE1PBE predict similar structural features for the lowest energy sulfenyl tautomers of (1) and of alkanesulfenic acids (3, 5, 7, 9, 11, 13, 15) as well as for sulfinothioic acid (2) and for the corresponding thiosulfinates (4, 6, 8, 10, 12, 14, 16, eq 1). Hydrogen thioperoxide (1) has been characterized as a nonplanar molecule with a skew chain structure analogous to hydrogen peroxide (HOOH) and disulfane (HSSH). Potential energy profile plots indicated that the gauche form of hydrogen thioperoxide (1, torsion angle = τ = H−S−O−H = 94°) is the lowest energy conformer. Optimized geometrical parameters for the lowest energy conformer of (1) are shown in Figure 1. The predicted parameters are in very good agreement with other calculations at comparable and higher levels of theory and with experimental studies.17−20,44−50 Although the Lewis model for (1) predicts an S−O single bond and no formal charges for the sulfur and oxygen atoms, NPA predicts atomic charges of 0.250 and −0.808 for S and O, respectively (Table SI 1, Supporting Information). There is an attractive nonbonding interaction in (1) between hydrogen and oxygen and a repulsive nonbonding interaction between the sulfur atom and the electropositive hydrogen. The predicted dipole moment and vibrational frequencies for (1), which are in agreement with experimental studies and with previous calculations, are shown in Table 1.17−20,44−50 Hydrogen thioperoxide (1) shows a strong S−O−H bending (deformation) vibration at 1169 cm−1, strong symmetric and asymmetric S−H stretching vibrations at 2331 and 2322 cm−1, respectively, and a strong O−H stretch at 3801 cm−1. Owing to the presence of two lone pairs of electrons on the sulfur atom, the sulfenyl tautomer of sulfenic acids can react as sulfur nucleophiles. In addition, the polar nature of the sulfur− oxygen bond in sulfenic acids allows the sulfur atom to behave as an electrophilic center. Thus, the interaction between two sulfenic acid molecules lead to the cyclic transition state for cyclodehydration. The five member cyclic hydrogen bonded transition state TS1 for the self-condensation of hydrogen thioperoxide (1) to sulfinothioic acid (2) is shown in Figure 1. B

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Table 1. Dipole Moments (μ) and Infrared Vibrations and Intensities for Hydrogen Thioperoxide (1) and Alkanesulfenic Acids (3, 5, 7, 9, 11, 13, 15) μ

Ra,b c

S−C

H CH3 CH2CH3 CH2CH2CH3 C(CH3)2 C(CH3)3 CF3

1 3 5 7 9 11 13

1.9 2.2 2.2 2.1 2.3 2.3 1.9

CCl3

15

1.5

S−O 720 (53) 722 (67)g 714 (63) 713 (66) 713(64)j 718 (73)m 749 (14) 750 (48) 735 (36) 755 (146)

682 (4) 635(0) 626 (0) 592 (0) 578 (0) 749 (14) 750 (48) 755 (146) 773 (109)

δS−O−H

O−H d

3801 3794 3795 3793 3800 3800 3778

e

(75) (65)h (67) (68) (64)k (66)n (89)

1169 1149 1147 1148 1142 1144 1185

3762 (78)

(37)f (36)i (36) (34) (39)l (40)o (23)

1173 (46)

B3LYP/6-311+G(d,p). bcm−1, relative intensities are in parentheses. cSymmetric S−H 2331(153). Asymmetric S−H 2322 (107). dExpt 763 in argon matrix, ref 17a. eExpt 3626, in the gas phase, ref 17. fExpt 1176 in argon matrix, ref 17a. gExpt 767, ref 11. hExpt 3612, ref 10. iExpt 1161, ref 11. jExpt 764, ref 11. kExpt 3611, ref 11. lExpt 1157, ref 11. mExpt 762, ref 11. nExpt 3610, ref 11. oExpt 1155, ref 11. a

Table 2. Activation Barriers to Cyclocondensation of Hydrogen Thioperoxide (1) and of Sulfenic Acids (3, 5, 7, 9, 11, 13, 15) ΔE⧧a

a

level of theory

1

3

5

7

9

11

13

15

B3LYPb B3PW91b PBEIPBEb CCSD(T)c,d QCISD(T)c,d

37.6 37.8 37.5 41.8 41.4

32.6 32.6 32.3 39.2 38.9

37.3 37.2 36.3 40.0 39.4

34.0 34.2 33.9 39.6 39.6

34.1 33.9 33.4 38.7 39.0

35.0 34.7 33.8 37.8 38.7

35.4 35.4 34.9 39.7 39.3

33.1 32.6 31.9 37.6 37.3

ΔE⧧ = Etransition state − E2(sulfenic acid). b6-311+G(d,p) basis set. ccc-pVDZ basis set. dB3LYP/6-311+G(d,p) optimized structure.

Table 3. Dipole Moments (μ) and Infrared Stretching Vibrations and Intensities for Sulfinothioic Acid (2), Alkyl Alkanethiosulfinates (4, 6, 8, 10, 12, 14, 16), and Dimethyl Disulfide (17) μ

Ra,b c

H CH3 CH2CH3 CH2CH2CH3 C(CH3)2 C(CH3)3 CF3 CCl3 CH3SSCH3

2 4 4a 6 8 10 12 14 16 17

2.8 2.9 2.8 3.0 2.7 3.1 3.3 1.6 1.9 2.1

S1−C3 646 (19) 642 (13) 630 (23) 598 (6) 598 (4) 535 (2) 441 (37) 678 (39) 677(2)e

S2−C3′

S1−S2

693 (2) 696 (2) 645 (4) 630 (3) 620 (8) 566 (8) 475(7) 747 (108) 679 (0)f

405 (59) 411 (30) 425 (45) 394 (22) 477(38) 469 (41) 488 (28) 427 (24) 423 (35) 471 (0)

S1O 1098 1064 1082 1051 1076 1060 1058 1092 1147

(193) (143)d (182) (96) (62) (113) (150) (149) (157)

B3LYP/6-311+G(d,p). bcm−1, relative intensities are in parentheses. cS1−H 2364 (71); S2−H 2672 (1). dSee also refs 20 and 62. eAsymmetric. Symmetric.

a f

functionals systematically predict similar values of ΔE⧧ but lower values than those computed by CCSD(T) and QCISD(T). The first computational studies on sulfinothioic acid (2) using HF52,53 were followed by studies using MP2,20,54,55 and B3LYP.56 Owing to a large number of allotropes sulfur has unusually large variations in its bonding properties. Sulfur− sulfur single bonds are extraordinary flexible and have bond lengths that vary from 1.7 to 3.0 Å and bond angles between 90 and 180°.20 Sulfinothioic acid (2) contains the polar SO bond and its S−S bond is longer (2.207 Å) than that in dihydrogen disulfide (HSSH, 2.055 Å) which is considered a normal sulfur−sulfur single bond, and shorter than that in TS1 (2.710 Å). Although the sulfur oxygen double bond in sulfoxides and in other sulfinyl derivatives is often represented as SO with 10 electrons on the hypervalent sulfur atom, it is also represented in the Lewis model as an ylide structure with a

Intrinsic reaction coordinate (IRC) calculations connect TS1 with the reactant (1) and with the product sulfinothioic acid (2).51 Along the reaction path from (1) to TS1 the S2−O2 bond distance is increasing (breaking), the S1−H bond is stretching (elongated) as hydrogen is leaving sulfur to participate in the formation of the water molecule, the S1−S2 bond is forming, and the S1−O1 bond distance is decreasing as double bond formation is occurring. It is seen in TS1 that the atomic charge on the sulfur atom (S1) that is becoming the sulfinyl sulfur in the product (2) has significantly increased positive charge relative to ground state (1) whereas the sulfur atom (S2) that is becoming the sulfenyl sulfur in the product (2) has decreased positive charge relative to (1) (Table SI 1, Supporting Information). Table 2 shows the activation barriers (ΔE⧧) for the self-condensation of hydrogen thioperoxide (1) and for cyclocondensation of alkanesulfenic acids (3, 5, 7, 9, 11, 13, 15). It is seen in Table 2 that the three hybrid density C

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dimethyl disulfide (17) with B3LYP giving C−S and S−S bond lengths of 1.834 and 2.093 Å, respectively, and a C−S−S−C torsion angle of 87.7°. These results are in good agreement with experimental results57 and with other computational studies.3,20,55 The calculated infrared spectrum of (17) shows asymmetric C−S, symmetric C−S, and S−S stretching vibrations at 677, 679, and 471 cm−1, respectively (Table 3). Figure 2 shows the potential energy profile plot for methanesulfenic acid (3) and Figure 3 shows selected geometry parameters and nonbonding interactions for (3), TS3, and methyl methanethiosulfinate (4). The predicted O−S−C bond angle and C−S−O−H torsion angle for (3) are 100.4° and 94.2°, respectively, and its predicted geometrical parameters are in good agreement with coupled FVP-microwave spectroscopic studies10 and photoelectron spectroscopy (PES) results.8 Repulsive nonbonding interactions between electropositive sulfur and the electropositive hydrogen atoms in (3) are seen in Figure 2 (Table SI 1, Supporting Information). The limited available experimental data and computational studies show that sulfenic acids generally have characteristic diagnostic infrared vibrations (Table 1). Although coupling among fundamental modes occur, selected computed and experimental diagnostic infrared vibrations for methanesulfenic acid (3) are shown in Table 1. The geometry of a sulfenic acid is expected to undergo significant changes from ground state to the transition state for cyclocondensation. Methanesulfenic acid (3), as hydrogen thioperoxide (1), goes through a five member cyclic transition state (TS3) on its path to thiosulfinate (4, Figure 3). Along the reaction path from (3) to TS3 the S2−O2 bond distance is increasing (breaking), the O1−H bond is stretching as it leaving oxygen to form the water molecule, the S1−S2 bond is forming, the S2−C2 bond is lengthening, and the S1−O1 bond distance is decreasing as double bond formation is occurring. It is seen in TS3 that the atomic charge on the sulfur atom (S1) that is becoming the sulfinyl sulfur in product (4) has significantly increased positive charge relative to ground state

Figure 2. B3LYP/6-311+G(d,p) potential energy profile rotation about the SI−O2 bond (τ = H−O2−S1−C3) in methanesulfenic acid (3).

highly polarized sulfur oxygen single bond with formal charges of +1 and −1 on S and O, respectively. NPA predicts atomic charges of +0.910 and −0.860 for the S and O atoms, respectively, in sulfinothioic acid (2). The H−S−S−H and H− S−SO torsion angles in (2) are −163.6° and 89.0°, respectively. The predicted and experimental dipole moment and S−H, S−S, SO stretching vibrational frequencies for (2) are shown in Table 3. Dimethyl disulfide (dimethyl disulfane, CH3S−SCH3, 17) was optimized using MP2, B3LYP, B3PW91, and PBE1PBE with the 6-311+G(d,p) basis set to obtain additional information concerning the geometrical features in organosulfur compounds. The four levels of theory predict similar geometric parameters for the lowest energy conformer of

Figure 3. Atom numbering and selected nonbonding interactions in the B3LYP/6-311+G(d,p) geometry optimized structures of methanesulfenic acid (3), TS3, and methyl methanethiosulfinate (4, 4a). D

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Figure 4. B3LYP/6-311+G(d,p) geometry optimized structures of ethanesulfenic acid (5), TS5, and ethyl ethanethiosulfinate (6).

Figure 5. B3LYP/6-311+G(d,p) geometry optimized structures of propanesulfenic acid (7), TS7, and propyl propanethiosulfinate (8). Figure 6. Atom numbering and selected nonbonding interactions in the B3LYP/6-311+G(d,p) geometry optimized structures of 2propanesulfenic acid (9), TS9, and 2-propyl 2-propanethiosulfinate (10).

(3) whereas the sulfur atom (S2) that is becoming the sulfenyl sulfur in product (4) has decreased positive charge relative to (3). (Table SI 1, Supporting Information). It is seen in Table 2 that the barrier to the self-condensation of (3) is smaller than that for (1). The most stable conformations of alkyl alkanethiosulfinates have similar structural features. The bonds are staggered in the minimum energy forms and the geometry at the sulfinyl sulfur atoms is trigonal pyramidal. Two low energy conformers (4, 4a), which are close in energy, were located for methyl methanethiosulfinate (Figure 3). Although the three DFT functionals and MP2 with various basis sets give mixed results concerning the relative energies of the two low energy rotamers

of (4, τ = C3−S1−S2−C3′ = 80.7°) and (4a, τ = C3−S1−S2− C3′ = 175.9°), CCSD(T) and QCISD(T) predict (4) to be 2.3 kcal/mol lower in energy than (4a). In contrast to the structure of (4), the crystal structure of 4-methylphenyl 4-methylbenzenethiosulfinate (4-CH3C6H4S−S(O)C6H4CH3-4, 18) has a C−S−S−C torsion angle of 174°,58 which is closer to that of structure (4a) than to (4). In addition, the observed average bond lengths in (18) involving sulfur atoms are S−C 1.776, S− O 1.457, and S−S 2.108 Å. E

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Figure 8. Atom numbering and nonbonding interactions in the B3LYP/6-311+G(d,p) geometry optimized structures of 1,1,1trifluoromethanesulfenic acid (13), TS13, and 1,1,1-trifluoromethyl 1,1,1-trifluoromethanethiosulfinate (14).

excellent agreement with the experimentally observed values (Table 3). Figure 4 shows the optimized structures for ethanesulfenic acid (5), TS5, and ethyl ethanethiosulfinate (6), and Figure 5 shows the structures for propanesulfenic acid (7), TS7, and propyl propanethiosulfinate (8). It is seen in the Figures 4 and 5 that all of the structures have numerous nonbonding interactions. The respective S−O and O−H bond lengths and the C−S−O−H torsion angles in alkanesulfenic acids (3), (5), and (7) are essentially the same, and the S−C bonds lengthen as the sizes of the alkyl group increase. Table 1 shows that the S−C, S−O, and O−H stretching frequencies and the S−O−H bending vibrations are similar for the linear alkanesulfenic acids (3), (5), and (7). It is also seen in Figures 4 and 5 that the bond angles and bond lengths around the sulfur atoms in (4) and (5) undergo significant structural changes during the cyclocondensation reactions as described above for (1) and (3). The developing long sulfur−sulfur interactions (bond) distances in TS3, TS5, and TS7 are similar as are their activation barriers (Table 2). In the alkyl alkanethiosulfinates (4), (6), and (8) the respective S−O and S−S bond lengths are about the same, and the S1− C3 bonds are longer than the corresponding S2−C3′ bonds. The H−S−S−H torsion angle in sulfinothioic acid (2) is 89.0° and the C−S−S−C torsion angles in (4), (6), and (8) are 80.7°, −197.9°, and −165.2°, respectively. Although there is coupling between the S−C and SO vibrations, diagnostic infrared absorptions for thiosulfinates (4), (6), and (8) are shown in Table 3.

Figure 7. Atom numbering, nonbonding interactions, and B3LYP/6311+G(d,p) geometry optimized structures of 2-methyl-2-propanesulfenic acid (11), TS11, and 2-methyl-2-propyl 2-methyl-2-propanesulfinothioic acid (12).

The S1−C3 bond in (4) is longer than its S2−C3′ bond, and the S−S bond in (4) is longer than that in dimethyl disulfide (17) and shorter than that in (18). This is consistent with the observation that sulfur−sulfur single bonds involving one sulfur atom of coordination number larger than two are longer (weaker) than the average S−S bond length of 2.050 Å. The sum of the bond angles about the sulfinyl sulfur atom in (4) is 312° which is comparable to the value for dimethyl sulfoxide (311°)59 and consistent with trigonal pyramidal geometry. The B3LYP/6-311+G(d,p) density functional theory (DFT) calculated dipole moment for methyl methanethiosulfinate (4) is lower than that calculated at the MP2/6-31G(d) level20 and that observed in benzene (3.14 D).60 Sulfoxides and thiosulfinates show strong SO stretching vibrations in the 1100 to 1030 cm−1 region.61 Hydrogen bonding lowers the SO absorption in sulfoxides while electron withdrawing substituents increase the frequency. The SO stretching frequency for (4) has been observed at 1075 cm−1 in CHCl3 and at 1097 cm−1 in CCl4,62 and the calculated vibrations of 1064 cm−1 for (4) and 1082 cm−1 for (4a) are in F

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groups in the transition states. Interestingly, CCSD(T) predicts the activation barriers for the cyclocondensation of branched (9) and (11) to be similar to those for the linear alkanesulfenic acids (3), (5), and (7). It has been suggested that increasing steric factors would increase the barriers to thiosulfinate formation and lead to greater stability for sulfenic acids with bulky sterically congested substituents (Table 2). In the alkyl alkanethiosulfinates (10) and (12) the respective S−O and S− S bond lengths are about the same, and the S1−C3 bonds are longer than the corresponding S2−C3′ bonds. The C−S−S−C torsion angles in (10) and (12) are −168.3 and 154.0°, respectively. Diagnostic infrared absorptions for thiosulfinates (10) and (12) are shown in Table 3. Replacing the hydrogens on the methyl group in methanesulfenic acid (3) with halogens leads to shortening of the S−O bonds and elongation of the C−S bonds in 1,1,1trifluoromethanesulfenic acid (13) and in 1,1,1-trichloromethanesulfenic acid (15, Figures 8, 9) relative to (3). The predicted C−S−O−H torsion angles in (13) and (15) are 87.8° and 85.5°, respectively, and the respective C−S−S−C torsion angles are 138.0° and 139.1°. There are attractive nonbonding interactions among the fluorine atoms and sulfur atoms in (13), TS13, and 1,1,1-trifluoromethyl 1,1,1-trifluoromethanethiosulfinate (14) 63,64 as well as a repulsive interaction between oxygen and one of the fluorine atoms (2.996 Å) in (13). Interestingly, with the three density functionals B3LYP, B3PW91, and PBE1BE, the Mulliken and APT population analyses predict negative atomic charges on the chlorine atoms in 1,1,1-trichloromethanesulfenic acid (15), TS15, and 1,1,1-trichloromethyl 1,1,1-trichloromethanesulfinate (16) while NPA predicts positive atomic charges on the chlorine atoms (Table SI 4, Supporting Information). The activation barriers and the mechanism for the condensation of (13) and (15) are similar to those described above for the other alkanesulfenic acids. Although similar S−S bond lengths are predicted for thiosulfinates (4), (14), and (16), the electron withdrawing trifluoromethyl and trichloromethyl groups lead to elongated C−S bonds and shorter S O bonds in (14) and (16) relative to (4). The thiosulfinates have shorter S2−C3′ bonds than the corresponding S1−C3 bonds. As can be seen in Table 1, the O−H stretching vibration and S−O−H bending vibration are reasonable for (13) and (15), but the S−C and S−O frequencies are strongly coupled and may be considered only in a qualitative way. Halogenated hydrocarbons are known to have strong absorptions over a wide region in the infrared. In addition, when several halogens are attached to the same carbon atom, the absorption pattern may become more complex and more intense owing to strong carbon−halogen stretching vibrations.

Figure 9. Atom numbering and nonbonding interactions in the B3LYP/6-311+G(d,p) geometry optimized structures of 1,1,1trichloromethanesulfenic acid (15), TS15, and 1,1,1-trichloromethyl 1,1,1-trichloromethanethiosulfinate (16).

Figure 6 shows the optimized structures for 2-propanesulfenic acid (9), TS9, and 2-propyl 2-propanethiosulfinate (10), and Figure 7 shows the structures for 2-methyl 2propanesulfenic acid (11), TS11, and 2-methyl-2-propyl 2propanethiosulfinate (12). As with sulfenic acids (3), (5), and (7), it is seen in Figures 6 and 7 that all of the structures have many nonbonding interactions. Other nonbonding interactions for (12) not shown in Figure 7 are S1----C4H 2.915 Å, S1---CH 2.867 Å, and S1----C6H 2.870 Å. The respective S−O and O−H bond lengths and the C−S−O−H torsion angles in the branched chain alkanesulfenic acids (9) and (11) are essentially the same. As the sizes of the alkyl groups increase in the alkanesulfenic acids (3, 5, 7) there is a corresponding increase in the C−S bond lengths (Figures 3, 4, 5). The S−O and O−H bond lengths in the alkanesulfenic acids are essentially the same irrespective of the substituent. Table 1 shows that the S−C stretching frequencies for the alkanesulfenic acids decrease as the sizes of the alkyl groups increase and that the S−O and O− H stretching frequencies and the S−O−H bending vibrations are similar for sulfenic acids (3), (5), (7), (9), and (11). It is seen in Figures 6 and 7 that the bond angles and bond lengths around the sulfur atoms in (9) and (11) also undergo significant structural changes during the cyclocondensation reactions (vide supra). The developing sulfur−sulfur bond distances in TS9 and TS11 are similar and longer than that for TS3 owing in part to large distance between the two alkyl

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CONCLUSIONS The highly reactive and unstable sulfenyl tautomers of hydrogen thioperoxide (1) and seven alkanesulfenic acids undergo self-condensation through cyclic hydrogen bonded five member transition states to form the corresponding sulfinothioic acid (2) and alkyl alkanethiosulfinates. The predicted activation barrier for the cyclocondensation of (1) is 41.8 kcal/ mol, and the barriers for the alkanesulfenic acids range from 37.5 to 39.6 kcal/mol. In the self-condensation mechanism one molecule of sulfenic acid acts as an electrophile and one molecule acts as an electrophile. The S−S bonds in the thiosulfinates are shorter than the S−S bonds in the G

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corresponding transition states. The alkyl alkanethiosulfinates have longer S2−C3′ bonds than the corresponding S1−C3 bonds. A plethora of attractive and repulsive nonbonding interactions are predicted in the sulfenic acids, transition states, and thiosulfinates. In the alkyl alkanethiosulfinates attractive cyclic five member C−H----OS nonbonding interactions are predicted. Characteristic diagnostic infrared vibrations (S−C, S−O, O−H, S−O−H, S−S, SO) for alkanesulfenic acids and alkyl alkanethiosulfinates have been computed (Tables 1 and 3).

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ASSOCIATED CONTENT

S Supporting Information *

Tables of atomic charges and optimized geometrical parameters of the hydrogen thioperoxide, alkanesulfenic acids, transition states, and alkyl alkanethiosulfinates are available. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: ff[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Science Foundation (NSF CHE-0840513) for support of the University of California, Irvine Greenplanet computing cluster.

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