Synthesis and vibrational properties of trifluoromethyl trifluoromethanethiosulfonate and comparison with covalent sulfonates

Vibrational Spectroscopy 59 (2012) 40–46

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Synthesis and vibrational properties of trifluoromethyl trifluoromethanethiosulfonate and comparison with covalent sulfonates M.E. Defonsi Lestard a , L.A. Ramos b , M.E. Tuttolomondo a , S.E. Ulic b,c , A. Ben Altabef a,∗ a

INQUINOA - CONICET, Instituto de Química Física, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, San Lorenzo 456, T4000CAN, Tucumán, Argentina CEQUINOR, Departamento de Química, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, C. C. 962 1900, La Plata, Argentina c Departamento de Ciencias Básicas, Universidad Nacional de Luján, Rutas 5 y 7, 6700, Luján, Buenos Aires, Argentina b

a r t i c l e

i n f o

Article history: Received 24 June 2011 Received in revised form 20 December 2011 Accepted 22 December 2011 Available online 30 December 2011 Keywords: Quantum chemical calculations Internal rotational barrier Fourier-type expansion Natural bond orbital analysis Vibrational spectroscopy

a b s t r a c t Trifluoromethyl trifluoromethanethiosulfonate, CF3 SO2 SCF3 was characterized by 13 C NMR, 19 F NMR, and vibrational spectroscopy. Infrared spectra of CF3 SO2 SCF3 were obtained for the gaseous and liquid phases, while the Raman spectrum was recorded for the liquid phase. The experimental data were complemented with quantum chemical calculations. Both experimental and theoretical data indicate that only one conformer, gauche, is possible by rotating around the S–S bond. This conformational preference was studied using the total energy scheme and natural bond orbital partition scheme. These results evidence that electron delocalization and especially LP Y(Y = S, O) → ␴* S(6) O(4,5) and LP O(4,5) → ␴* S(6) Y interactions play an interesting role in the reactivity of oxoesters and thioesters. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Thiosulfonates (CX3 SO2 S R) are important in biochemistry and organic chemistry [1], besides, their structural features and conformational preferences are interesting because some of them have applications in cancer research and act as antiviral agents. Methyl methanethiosulfonate (MMTS) show its antioxidant activity against lipid peroxidation, resulting in possible preventive agents for hepatic neoplasia when administered in conjunction with Phenobarbital [1]. Moreover, many organosulfur compounds have activity against viruses. The antiviral activity of compounds can be modulated by the introduction of organosulfur groups in their structure. Levin et al. [2] studied the antiviral activity and immune modulator properties of substances containing the group SO2 S, in relation to its ability to prevent the reproduction of some viruses. On the other hand, the cysteine residue plays a key role in biological systems as the main stabilizer of the tertiary structure of proteins [3], by means of intramolecular disulfide bonds. As R-methanethiosulfonate (MTS) compounds are useful reagents to detect the thiol group present in cysteine residues of proteins by forming disulfide bonds [4] (see Scheme 1), these are

∗ Corresponding author. Tel.: +54 381 4311044; fax: +54 381 4248169. E-mail address: [email protected] (A. Ben Altabef). 0924-2031/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2011.12.014

appropriate agents to characterize the structure and function of proteins. In addition, the CF3 SO2 S R group can interact with different intracellular proteins such as enzymes or receptors [5] (see Scheme 1). The structural and conformational properties of CH3 SO2 SCH3 [6] (MMTS) were previously studied in this laboratory. The gas-phase electron diffraction analyses of MMTS result in gauche structures with a CSSC dihedral angle of 80.1◦ . Because of the extensive and varied field of application of this compound, we have extended our investigation to the CF3 SO2 SCF3 molecule, to obtain information about the conformational and vibrational properties. CF3 SO2 SCF3 was characterized by 13 C and 19 F NMR, infrared and Raman spectroscopies. These experimental data were complemented by quantum chemical calculation to obtain an optimized molecular structure and a scaled quantum mechanical force field. In addition, the barrier to internal rotation about the S S bond was calculated using an assortment of computational approaches and fitted to the six-fold Fourier-type expansion. This methodology allowed to analyze the nature of the potential function and to assess the preferred conformation of the molecule. The study has been completed by natural bond orbital (NBO) analysis to determine the presence of hyperconjugative interactions, which would favor one conformation over another. The experimental and theoretical results for CF3 SO2 SCF3 were compared with those previously reported for CH3 SO2 SCH3 [6], and CF3 SO2 OCF3 [7], which preferably adopt gauche conformation.

M.E. Defonsi Lestard et al. / Vibrational Spectroscopy 59 (2012) 40–46

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O

O Protein

SH + RS

S

CH3

Protein

S

S

R + CH3

S

SH

O

O MTS Reagent

Sulfinic Acid

Scheme 1. R-methanethiosulfonate (MTS) as characterization reagent for S H detection in proteins.

2. Materials and methods Trifluoromethyl trifluoromethanethiosulfonate, CF3 SO2 SCF3 , was obtained by reaction of CF3 SCl and (CF3 SO2 )2 Zn, based on the synthesis proposed by Haszeldine and Kidd [8] with some modifications. (CF3 SO2 )2 Zn (3 mmol) was placed in a reaction vessel together with CF3 SCl (7 mmol). They reacted for 5 days at room temperature with vigorous agitation. The products were separated by trap to trap distillations, keeping traps at −40, −95 and −196 ◦ C. CF3 SO2 SCF3 and CF3 SO2 CF3 were retained in the −95 ◦ C trap. They were separated by several distillations through traps held at −60, −95 and −196 ◦ C. Pure CF3 SO2 SCF3 was isolated as a colorless liquid in the −60 ◦ C trap. 2.1. Instrumentation 2.1.1. General procedure Volatile materials were manipulated in a glass vacuum line equipped with two capacitance pressure gauges (280E Transducer, Setra, MA, USA), three U-traps and valves with PTFE stems (Young, London, U.K.). IR spectra of gas samples were recorded with an infrared cell placed in the spectrometer sample compartment that allows monitoring the course of the reaction and the purification process. The purified compound was stored in flame-sealed glass ampoules under liquid nitrogen in a Dewar vessel. They were opened with an ampoule key in the vacuum line, an appropriate amount was used for the characterization, and they were flamesealed again. 2.1.2. Infrared and Raman spectroscopy Infrared spectra of CF3 SO2 SCF3 in the gas and liquid phases were recorded with a resolution of 2 cm−1 in the 4000–400 cm−1 range at room temperature using a LUMEX Infra LUM FT-02 spectrometer. An IR glass cell with 200 mm optical path length and 0.5 mm thick Si windows were used to obtain gas phase spectra. AgCl windows were employed to obtain the infrared spectra of the liquid. Raman spectra of the liquid at room temperature were obtained using a Bruker IFS 66 spectrometer (spectral resolution 4 cm−1 ). The 1064 nm radiation line of an Nd/YAG laser was used for excitation. The liquid sample was handled in flame-sealed capillaries (4 mm o.d.). 2.1.3. NMR spectra For 13 C and 19 F NMR measurements, a pure sample was flamesealed in thin-walled 4 mm o.d. tubes and placed into 5 mm NMR tubes. The NMR spectra were recorded in a Varian, Mercury Plus 200 spectrometer. The samples were measured at room temperature using CDCl3 as internal lock and reference.

6-311+G(d) basis set. Likewise, the ab initio Moller-Plesset second order perturbations method (MP2) [18] along with the 6-311+G(d) was used to optimize molecular structures. The conditions of local minima in the total energy surface were evaluated by the presence of all real harmonic vibrational frequencies. The potential energies associated with the CSSC dihedral angle were calculated at the B3LYP, mPW1PW91 and MP2 levels using 6-31G(d), 6-311G(d) and 6-311+G(d) basis sets. In such calculations the torsion angle was frozen, whereas all other parameters were relaxed. The total energy curve was scanned in 10◦ steps using default convergence criteria as implemented in the Gaussian program [19]. Natural bond orbital (NBO) calculations were performed at the B3LYP/6-311+G(d) level using the NBO 3.0 code [20]. The harmonic force field in Cartesian coordinates calculated at the B3LYP/6-31G(d) level was transformed to a set of natural internal (local symmetry) coordinates via the B matrix [21], which was obtained using a standard program. Subsequently, an SQM force field was obtained using the scheme of Pulay et al. [22], in which the diagonal force constants are multiplied by scale factors fi , fj , . . . and the corresponding interaction constants are multiplied by (fi ·fj )1/2 , adjusting the scale factors to reproduce the experimental wavenumbers as well as possible. No empirical correction of the theoretical geometry was used. The potential energy distribution was then calculated with the resulting SQM force field. The scaled force field for the gauche conformer and determination of the potential-energy distribution were performed with the FCARTP program [23]. The atomic displacements for each vibrational mode, given by the Gaussian 03 program, were used to visualize the nature of the molecular vibrations, and were graphically represented using the GaussView program [24]. 3.1. Prediction of Raman intensities The Raman activities (SRa) calculated with Gaussian 03 program converted to relative Raman intensities (IRa) using the following relationship derived from the intensity theory of Raman scattering [25] Ii =

f (0 − i )4 Si i [1 − exp (−hci /kT )]

(1)

where 0 is the laser exciting wavenumber in cm−1 (in this work, we have used the excitation wavenumber 0 = 9398.5 cm−1 , which corresponds to the wavelength of 1064 nm of a Nd:YAG laser), i the vibrational wavenumber of the ith normal mode (cm−1 ), while Si is the Raman scattering activity of the normal mode i . f (is a constant equal to 10−12 ) is a suitably chosen common normalization factor for all peak intensities. 4. Results and discussion

3. Computational details 4.1. Physical properties and spectroscopic characterization The optimized geometry was obtained for CF3 SO2 SCF3 using the density functional theory (DFT) with the B3LYP [9–11] hybrid functional in conjunction with the standard split-valence basis sets 6-31G(d), 6-311G(d,p) and 6-311+G(d) [12–16]. The influence of the functional on the optimized geometries as well as on their energies was tested by using the functional mMPW1PW91 [17] with the

CF3 SO2 SCF3 is a colorless liquid at room temperature and presents the characteristic odor of sulfonates molecules. The 19 F NMR spectrum exhibits two singlet signals at −38.0 and −78.7 ppm which correspond to the fluorine atoms of the SCF3 and CF3 SO2 groups, respectively. The 13 C NMR spectrum shows two signals, a

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Fig. 1. Vibrational spectra of CF3 SO2 SCF3 between 1500 and 500 cm−1 : (upper trace) SQM calculated; (medium trace) gas phase (resolution 2 cm−1 , path length 10 cm−1 , pressure 3 Torr); (lower trace) Raman spectrum of liquid at 2 cm−1 resolution. The calculated spectrum is obtained from the theoretical frequencies (B3LYP/6-31G(d)) calculated from the scales force field.

quartet signal located at ı = 119.7 ppm (1 J(C F) = 327.1 Hz), which corresponds to the C atom of the SCF3 group while the second signal, a quartet located at ı = 126.6 ppm (1 J(C F) = 314.8 Hz), is attributed to the carbon of the CF3 SO2 group. This is in agreement with the results derived from quantum chemical calculations (B3LYP/6311+G(d)) (ıcalc CF3 S = 127.24 (q); ıcalc CF3 SO2 = 137.4 (q)) and with other CF3 containing compounds [26–28]. Additional evidences of the identity of CF3 SO2 SCF3 are given by the IR (gas, liquid) and Raman (liquid) spectra (see Figs. 1 and 2 and Table 1). The intense and characteristic bands in the IR (gas) spectra located at 1422 cm−1 and 1100 cm−1 , attributed to the SO2 group, and the medium band at 619 cm−1 assigned to the S S bond, agree with the expected structure of this compound (Table S1).

Since no theoretical or experimental structure is available for CF3 SO2 SCF3 , the first objective was to obtain the most stable molecular conformation. The search for secondary minima in the potential energy surface was carried out through a series of calculations varying the dihedral angle (CSSC) in steps of 10◦ and keeping it fixed, while the other parameters were adjusted using the levels of calculations mentioned in the details of computation, and the results are depicted in Fig. 3. The obtained conformations characterized by local minima are: the gauche, with lower energy and C1 symmetry, corresponding to two symmetrically equivalent minima (enantiomers) and a maximum denoted as anti with Cs

Fig. 2. Raman spectra of CF3 SO2 SCF3 between 600 and 100 cm−1 : (upper trace) SQM calculated; (lower trace) Raman spectrum of liquid at 2 cm−1 resolution.

Fig. 3. Torsional potential curves around the S S bond in CF3 SO2 SCF3 at different approximation levels.

4.2. Structural results

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Table 1 Experimental, calculated frequencies (cm−1 ), intensities and assignments of the fundamental vibrational modes of CF3 SO2 SCF3 . Mode

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 RMSD [cm−1 ]

Experimental

Calculated

Approximated description

IRa (gas)

IRa (liq)

Ramanb (liq)

SQMc

B3LYP/6-31G

IR intensityd

Raman intensitye

1422 s 1236 vs

1410 s 1225 vs

1418 (15) 1230 sh

1205 s 1190 s

1187 vs

1434 1245 1237 1206 1200 1181 1110 1095 772 766 614 555 551 542 533 518 473 401 361 325 313 268 248 236 186 126 108 67 47 33 7.8

1371 1284 1277 1244 1229 1185 1104 1094 764 759 597 556 544 540 535 510 462 394 354 324 305 261 243 228 197 125 116 68 45 35 19.1

221.3 186.9 189.1 217.7 258.1 58.7 381.7 297.1 3.7 30.5 189.7 2.7 0.7 52.5 5.7 48.7 1.2 1.2 2.4 0.0 0.7 1.8 5.1 0.4 2.2 1.1 0.7 0.3 0.2 0.0

3.8 0.8 0.8 0.4 0.5 9.2 4.6 7.5 12.2 7.8 4.0 4.3 4.5 7.2 3.5 10.2 13.3 37.9 24.6 6.1 28.0 36.2 100.0 35.3 0.7 4.4 2.8 1.0 0.0 0.0

1122 s 1100 vs 764 w 619 m 565 sh 554 w 532 w 512 vw 464 vw 418 vw

1210 (24)

1113 vs 1084 vs 764 m 756 sh 615 s 572 sh 562 sh 552m 529 m

1179 (13) 1112 (22) 1084 (13) 763 (80) 613 (12) 572 sh 562 (13) 551 (17) 527 (50) 509 (6) 467 (24) 414 (50) 363 (30) 324 sh 316 (48) 279 (33) 257 (100) 239 (28) 197 (6) 133 (29) 104 (5) 75

a SO2 a CF3 (SO2 ) a CF3 (SO2 ) a CF3 (S S) a CF3 (S S) s CF3 (SO2 ) s CF3 (S S) s SO2 ıs CF3 (SO2 ) ıs CF3 (S S) S S ␦a CF3 (S S) ıa CF3 (SO2 ) ıa CF3 (SO2 ) ıa CF3 (S S) ı SO2  C S(S) ω SO2  SO2  CF3 (S S)  C S(SO2 )  CF3 (SO2 )  CF3 (S S) tw SO2 ␳ CF3 (SO2 ) ı C S(O2 ) S ı S(O2 ) S C S S  CF3 (S)  CF3 (SO2 )

a

Band intensities: vs, very strong; s, strong; m, medium strong; w, weak; vw, very week and sh, shoulder. Relative intensities in parentheses. c From scaled quantum mechanics force field (see text for further definition). Scale factors for the force field of CF3 SO2 SCF3 : as (SO2 ), (C SO2 ), (C S) (S S): 1.10; as (CF3 ): 0.93; ı(CF3 ); (CF3 ); ıC S(O2 ) S, s (CF3 ), s (SO2 ): 1.0; ı(SO2 ); (SO2 ); tw(SO2 ); ω(SO2 ): 1.10; ˛ S(O2 ) S C: 0.83; (S–S); (CF3 ): 0.89. d Units are in km mol−1 . e Relative intensity from Eq. (1). b

symmetry (see Table S2). The gauche conformation, with a CSSC dihedral angle of about 92.9◦ is represented in Fig. 4, whereas the calculated geometrical parameters are reported in Table S3. The predicted most stable conformer agrees with the results obtained previously for the CH3 SO2 SCH3 [6] and with the experimental or theoretical conformations obtained for related molecules (CF3 SO2 OCF3 [7], CF3 SO2 OCCl3 [29], CF3 SO2 OCH2 CH3 [30], and CCl3 SO2 OCH2 CF3 [31]). In fact, in all cases a gauche conformation with dihedral angles (C S O/S C) of 92–130◦ were predicted for the most stable structure. As found for the related compound, CH3 SO2 SCH3 [6] the inclusion of extra polarization functions (beyond a single d function)

is necessary to predict accurately the bond lengths in this type of molecules. A basis set size test, using the B3LYP functional, shows differences in bond distances of less than 2.3 pm. The most sensitive parameter to this orbital description is the S S bond, which results shortened by 5 pm when replacing the 6-311G(d) basis set by 6311G(3df). All bonds involving the sulfur atom were shortened around 1 pm, while other bond lengths remain unchanged. In the case of the bond angles, the largest variation is 1.7◦ for the S S C angle when comparing MP2 and DFT methods. The largest differences found for the bond angles is 0.6◦ for C S S, while the C S S C dihedral angle shows values within the range of 91.8–95.6◦ . 4.3. Internal barrier decomposition schemes The study of the nature of the internal rotational barrier around of the S-S bond, in terms of hyperconjugative, steric and electrostatic interactions, gives information about the stability of the different conformers. The total energy surface for this torsion angle was calculated in the range of 0–180◦ in 10◦ steps, relaxing all geometrical parameters except the one to be scanned. The energy profiles were fitted to a sixth-order Fourier expansion (2), where: N, the symmetry number, is equal to 1. No contributions of zero-point energy were taken into account.

Fig. 4. Optimized molecular structure of gauche-CF3 SO2 SCF3 (C1 symmetry) calculated at B3LYP/6-311G+(d) level.

V () =

6  1 i=1

2

ViN (1 − cos iN)

(2)

44

M.E. Defonsi Lestard et al. / Vibrational Spectroscopy 59 (2012) 40–46

Fig. 5. Fourier decomposition of the potential function V() for CF3 SO2 SCF3 calculated using the B3LYP method with the 6-311+G(d) basis set.

Decomposition of the total energy function and the analysis of the different terms Vi have previously been described as a simple way to analyze the stabilization of different conformations in molecular systems [32–36]. Fig. 5 shows the Fourier decomposition of the total energy function calculated at the B3LYP level of theory with the 6-311+G(d) basis set. V1 and V2 are the main contributions to the rotational barrier, where V2 > V1 > V3 , V4 , V5 and V6 are less significant when deconvoluting the potential-energy curve. V2 is usually associated with conjugative and hyperconjugative effects that have a periodicity of 180◦ . V1 usually accounts for interactions between local dipoles and for steric interactions (electrostatic effect); this value is large and positive showing that there is a strong preference for the 180◦ (anti) geometry over the 0◦ (syn) one. The remaining terms involve steric and electrostatic interactions (see Table 2). The curves in Fig. 5 show how V2 is the same form of potential energy curve, which indicates that V2 accounts for the conjugative and hyperconjugative effects that favor the gauche conformer stabilization. In this case, Table 2 shows that the magnitude and the sign of V1 and V2 are predominant in the majority of the sulfonates, with a predominance of V1 for oxosulfonates and V2 for thiosulfonates. In oxo- and thio-sulfonates, the change of H atom by F atom produces a diminution of V1 and a slight increase of V2 contributions. This decrease of V1 is related to the small change of the dipole moment between the gauche and the anti form. Whereas for the methyl sulfonates the change of the dipole moment is very pronounced as the V1 values (Fig. S1). As shown in Fig. S1, the overall dipole moment (l) stabilizes the anti conformer of the four covalent sulfonates. The dipole moment as a function of the torsion angle shows the same trend as V1 , which indicates that V1 accounts for the repulsive non-bonding interactions that favor the stabilization of the anti conformer. Taking into account the V1 values and the energy of the barrier, the change in

the dipole moment is much smaller for the CF3 SO2 OCF3 molecule than for the other three sulfonates and the conformational energy difference should be also smaller. 4.4. NBO analysis The role of hyperconjugative interactions in the stabilization of the gauche conformer has been assessed using NBO analysis, where the hyperconjugation represents the transfer of an electron between a lone pair or bonding and nonbonding orbitals. Table 3 contains the main hyperconjugative interactions for the gauche conformer of the CF3 SO2 SCF3 and related molecules (CH3 SO2 SCH3 and CF3 SO2 OCF3 ). Taking into account the electronegativity change of the R group, the transferred electronic charge is greater in CF3 SO2 SCF3 (839.68 kJ mol−1 ) than in CH3 SO2 SCH3 (590.00 kJ mol−1 ). This fact is mainly due to the different interaction between the following orbitals: LP X(X = F, H) → ␴* C S, LP X(X = F, H) → ␴* S S, LP O(4,5) → ␴* S S and to a lesser extent to LP S (2) → ␴* C X(7,8,9) and LP S (2) → ␴* C(1) S(6) (Table 3). Thus, the calculated C(1) S(6), C(3) S(2) and S S bond lengths are greater for CF3 SO2 SCF3 than for CH3 SO2 SCH3 , which agrees with the lower occupancy of the ␴ and the highest occupancy of the ␴* of these bonds and consequently a lower bond order (Wiberg bond index) in CF3 SO2 SCF3 than in CH3 SO2 SCH3 (Table 4). The greater thermodynamic stability of an oxoester can be attributed to the higher electronic delocalization or resonance of a lone pair of the bridging O with the SO2 group. This may also Table 3 Relevant hyperconjugative interactions (kJ mol−1 ) for CF3 SO2 SCF3 , CH3 SO2 SCH3 and CF3 SO2 OCF3 calculated at the B3LYP/6-311 G(3df) level. CF3 SO2 SCF3 LP S/O(2) → ␴* C X (7,8,9) LP S/O(2) → ␴* C(1) S(6) LP S/O(2) → ␴* S(6) O(4,5) LPO(4) → ␴* S(6) O(5) a

Table 2 Fourier expansion parameters (kJ mol−1 ) for CF3 SO2 SCF3 and related molecules.

V1 V2 V3 V4 V5 V6 a b

CF3 SO2 SCF3 a

CF3 SO2 OCF3 a

CH3 SO2 SCH3 b

CH3 SO2 OCH3 a

−15.26 −22.18 −3.13 −1.81 −0.89 −1.01

−22.39 −13.69 −1.15 −1.38 −0.53 −0.69

−17.78 −18.66 −8.99 −2.89 0.13 −0.75

−28.06 −13.61 −7.66 −0.22 0.63 −0.37

B3LYP/6-311+G(d). Ref. [6].

b

LPO(5) → ␴* S(6) O(4)

 LP O(4,5) → ␴* S S/O  LP X(7,10) → ␴* S S/O  LP X(7,8,9) → ␴* C(3) S/O(2) 

LP X(10,11,12) → ␴* C(1) S(6)

a b

62.57 19.31 11.07 99.69

CH3 SO2 SCH3

CF3 SO2 OCF3

36.65 14.30 12.96 101.68

116.0 20.11 30.52 81.64

97.77

96.01

93.63

267.31

249.00

316.40

6.94

–

106.14

144.63

–

145.3

148.89

–

153.0

LP denotes electron lone pair in the atom (for atom numbering see Fig. 3). X = F, H.

M.E. Defonsi Lestard et al. / Vibrational Spectroscopy 59 (2012) 40–46 Table 4 C S and S S bond distances (pm), electron occupancy of the natural bond orbitals and Wiberg index of CF3 SO2 SCF3 and CH3 SO2 SCH3 at the B3LYP/6-311G(3df) level of theory. CF3 SO2 SCF3

CH3 SO2 SCH3

rS S Occupancy ␴ S S Occupancy ␴* S S Wiberg index

211.9 1.9504 0.3038 0.788

211.1 (207.5)a 1.9563 0.2898 0.8029

r C(1) S(6) Occupancy ␴ S(6) C(1) Occupancy ␴* S(6) C(1) Wiberg index

188.8 1.956 0.323 0.699

179.0(178.5) 1.977 0.175 0.863

r S(2) C(3) Occupancy ␴ S(2) C(3) Occupancy ␴* S(2) C(3) Wiberg index

184.8 1.980 0.1137 0.9713

182.0 (182.2) 1.987 0.013 1.030

a

Experimental value between parentheses.

rationalizate the lower reactivity of the oxoesters than thioesters, toward most nucleophiles. This behavior agrees with the increase of the electronic charge transferred in LP Y(Y = S, O) → ␴* S(6) O(4,5) and LP O(4,5) → ␴* S(6) Y interactions in CF3 SO2 OCF3 and in CF3 SO2 SCF3 . 4.5. Vibrational study CF3 SO2 SCF3 exhibits C1 symmetry and therefore its 30 normal modes of vibration are active in the IR and Raman. Representative vibrational spectra appear in Figs. 1 and 2 together with the observed frequencies and calculated information in Table 1. Besides, the proposed assignment of the vibrational modes given in Table 1 is based on the calculated frequencies, intensities and by comparison with reported information for related molecules. The B3LYP calculations reproduced well the experimental vibrational frequencies with root-mean-square-deviation (RMSD) values of 19.1, 16.0 and 22.7 cm−1 employing 6-31G(d), 6-311G(d) and 6311+G(d) basis sets, respectively. The first and second basis sets are in better agreement, and 6-31G(d) was used in the vibrational analysis. The most intense IR (gas) absorptions appear in the 1500–1000 cm−1 region, as expected for the SO2 and CF3 groups of the molecule. In fact, the bands observed at 1422 and 1100 cm−1 are assigned to the antisymmetric and symmetric stretching modes of the SO2 vibrations, although the last one contains some other contributions. Similar modes reported in the 1357–1467 cm−1 and 1140–1157 cm−1 regions, respectively, for related molecules (CF3 SO2 R, R = OCF3 , SCH3 , OCH2 CH3 , OCCl3 , OCH3 ) [6,7,29,30,37], support the present assignment. The band located at 1179 cm−1 in the Raman spectra of the liquid was assigned to the CF3 symmetric stretching, while the band at 1236 cm−1 (IR gas) would correspond to both antisymmetric stretching absorptions. Besides, the band at 1122 cm−1 in the IR (gas) spectra is attributed to the symmetric stretching of the CF3 group bonded to the S atom and the bands at 1205 and 1190 cm−1 to the antisymmetric stretching. The S–S stretching mode appears as a medium intense band located at 619 cm−1 in the IR spectra (gas) according to what was predicted by calculations and consistent with observations for different disulfides (XS SX) [38–41]. However, this mode appears at higher frequencies following the electronegativity trend of the X group [42] (Table S1). At lower wavenumbers, the deformations and torsional modes were mainly observed in the Raman spectra of the liquid. The medium intense band at 509 cm−1 is assigned to the SO2 bending mode, in agreement with related SO2 containing compounds. The remaining bands corresponding to the movements of the whole SO2

45

group appear at relatively low frequencies: SO2 wagging, 414 cm−1 (483–628 cm−1 ); SO2 rocking, 363 cm−1 (359–463 cm−1 ) and SO2 twisting, 239 cm−1 (303–351 cm−1 ), which are also supported by computed and previous results (between parentheses). The 316 cm−1 band shows a strong contribution of the C S (SO2 ) stretching, which was reported at 320 cm−1 in CF3 SO2 OCF3 . The band observed at 467 cm−1 was assigned to the C S(S) stretching, which appears around 450 cm−1 in CF3 SSX molecules. The C S(O2 ) S bending appears at 133 cm−1 in the Raman spectrum, while the weak band at 104 cm−1 was assigned to the C S SO2 bending mode. Besides, the molecule presents three torsional modes, but they were not observed due to their very low predicted intensities. 4.6. Calculation of force constants The force field in Cartesian coordinates, generated by the Gaussian program, was transformed to a set of non-redundant natural coordinates described in Table S4. Such coordinates take into account the local symmetry around the carbon atoms and follow the proposals of Fogarasi et al. [43]. The resulting force field was subsequently scaled using the scheme proposed by Pulay et al. [44] and the initial scale factors were defined using the unity for all modes, as shown in Table S5. These scale factors were subsequently refined by the nonlinear least-squares procedure to fit the experimental frequencies. The refined scale factors corresponding to each force constant are shown in Table S5, while the resulting frequencies, RMSD final value, and the potential-energy distribution are presented in Table S6. It can be seen that only half of the modes have a contribution ≥ 50% of a single coordinate, whereas other modes describe very complex vibrations in which several coordinates are involved. The calculated S S bond is 1.6 pm higher in CF3 SO2 SCF3 than in CH3 SO2 SCH3 and its force constant increases from 1.94 to 2.23. However, the vibrational mode of the S S stretching undergoes a shift to higher frequencies in the fluorinated compound. This unusual behavior of S S bond can be explained by the high electronegativity of the CF3 group, which seems to enhance the contribution of the electronic charge of the S (CF3 S) atom in the S S bond increasing its covalent character, and consequently a higher vibrational frequency in the CF3 SO2 SCF3 molecule (see Table S1). The SQM force field was used to calculate the internal force constants (Table S7), which are in good agreement with the obtained values for CH3 SO2 SCH3 and CF3 SO2 OCF3 . 5. Conclusions The optimized molecular geometry and the lowest energy conformation of CF3 SO2 SCF3 have been calculated using MP2 and DFT techniques with different basis sets. The structural results predict a gauche conformation (C1 symmetry) as the most stable form. The decomposition of the potential-energy function as a Fourier expansion and the study of different terms (Vi ) are useful to analyze the relative stabilities of different conformations of molecular systems. V2 and V1 are the main terms of the Fourier expansion. It can be concluded that the hyperconjugative effects favor the C1 orientation. The major stability of the gauche conformer is interpreted mainly by the higher delocalization contributions than the electronic and steric effects. These results evidence that electron delocalization and especially LP Y(Y = S, O) → ␴* S (6) O(4,5) and LP O(4,5) → ␴* S(6) Y interactions play a interesting role in the reactivity of oxoesters and thioesters. Infrared and Raman spectra of CF3 SO2 SCF3 show bands assignable to 27 of the 30 expected normal modes of vibration. Using the observed wavenumbers, it was possible to scale the theoretical force field; the resulting SQM force field was applied to

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calculate the potential energy distribution, which reveals the physical nature of the molecular vibrations and force constants in terms of internal coordinates. Acknowledgements The authors thank Consejo de Investigaciones de la Universidad Nacional de Tucumán (CIUNT), Consejo Nacional de Investigaciones Científicas y Técnicas, PIP 0629 (CONICET), Universidad Nacional de La Plata (UNLP) and Departamento de Ciencias Básicas de la Universidad Nacional de Luján (UNLu) for financial support. S.E.U. and L.A.R. especially thank Deutscher Akademischer Austauschdienst Germany (DAAD) for the FTIR spectrometer grant and financial support.

[12] [13] [14] [15] [16] [17] [18] [19]

Appendix A. Supplementary data [20]

The supporting information includes: S S bond lengths and stretching mode frequencies of CF3 SO2 SCF3 and related molecules (Table S1). Total and relative differences energies for C1 and Cs forms at different level of theory (Table S2). Geometrical parameters for CF3 SO2 SCF3 at different levels of theory (Table S3). Definition of natural internal coordinates for CF3 SO2 SCF3 (Table S4). Scale factors for the force field of CF3 SO2 SCF3 (Table S5). Observed and calculated frequencies and potential energy distribution for CF3 SO2 SCF3 (Table S6). Force constants in internal (valence) coordinates for CF3 SO2 SCF3 and related molecules (Table S7). Dipole moment () as a function of the torsion angle CSXC (X = S, O) for CF3 SO2 OCF3 , CF3 SO2 SCF3 , CH3 SO2 OCH3 , CH3 SO2 SCH3 at the B3LYP/6-311+G(d) level (Fig. S1). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.vibspec.2011.12.014. References [1] S. Sugie, K. Okamoto, M. Ohnishi, H. Makita, T. Kawamori, T. Watanabe, T. Tanaka, Y. Nakamura, Y.K. Nakamura, I. Tomita, H. Mori, Jpn. J. Cancer Res. 88 (1997) 5. [2] Y.A. Levin, O.K. Pozdeev, N.A. Shvink, S.V. Andreev, M.S. Skorobogatova, Pharm. Chem. J. 792 (1991) 25. [3] L. Stryer, Biochemistry, Freeman W.H. & Co., New York, 1988. [4] K. Nakamura, T. Matsuo, K. Shimo, Y. Nakamura, I. Tomita, Biosci. Biotechnol. Biochem. 60 (1996) 1439. [5] P. Jeschke, Pest Manage. Sci. 66 (2010) 10. ˜ E.L. Varetti, S.A. Hayes, D.A. Wann, [6] M.E. Tuttolomondo, A. Navarro, T.P.E. Pena, H.E. Robertson, D.W.H. Rankin, A. Ben Altabef, J. Phys. Chem. A 111 (2007) 9952. ˜ [7] M.E. Tuttolomondo, P.E. Arganaraz, E.L. Varetti, S.A. Hayes, D.A. Wann, H.E. Robertson, D.W.H. Rankin, Eur. J. Inorg. Chem. (2007) 1381. [8] R.N. Haszeldine, J.M. Kidd, J. Chem. Soc. (1955) 2901. [9] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [10] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [11] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996), Erratum: Phys. Rev. Lett. 78 (1997) 1396.

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