Chemical Physics Letters 598 (2014) 75–80 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett A comparative study on interaction capacity of CO2 with the >S@O and >S@S groups in some doubly methylated and halogenated derivatives of CH3SOCH3 and CH3SSCH3 Vo Thuy Phuong a, Nguyen Thi Thu Trang b,c, Vien Vo a, Nguyen Tien Trung a,⇑ a b c Faculty of Chemistry, and Laboratory of Computational Chemistry, Quy Nhon University, Quy Nhon, Viet Nam Faculty of Science, Hai Phong University, Hai Phong, Viet Nam Faculty of Chemistry, Ha Noi National University of Education, Ha Noi, Viet Nam a r t i c l e i n f o Article history: Received 28 November 2013 In final form March 2014 Available online 12 March 2014 a b s t r a c t Interactions of CO2 with CH3SZCHX2 (Z@O, S; X@H, CH3, F, Cl, Br) induce significantly stable complexes with interaction energies from À13.7 to À16.4 kJ molÀ1 (MP2/aug-cc-pVTZ//MP2/6-311++G(2d,2p)) Remarkably, some stable shapes of CH3SZCH3Á Á ÁCO2 are revealed for the first time Substitution of two H atoms in a CH3 of CH3SZCH3 by two X alike groups makes CH3SZCHX2Á Á ÁCO2 more stable than CH3SZCH3Á Á ÁCO2, and their stability increases in the order F < Cl < Br < CH3 The >S@O is stronger than the >S@S in interacting with CO2, and they both can be valuable candidates in the design of CO2-philic materials and in the findings of materials to adsorb CO2 Ó 2014 Elsevier B.V All rights reserved Introduction Supercritical fluid technology is considered as an attractive option for separation of fine chemicals from liquid solvents, and supercritical carbon dioxide (scCO2) has become of interest as a promising alterative to organic solvents for extractions, separations, chemical reactions, and material processes [1–4] ScCO2 is convenient to use as it possesses a lot of desirable properties Nevertheless, the limitation in the applications of scCO2 is its restrictive capacity of solvation for polar and high molecular weight compounds It is required to unravel the factors for controlling the solubility of compounds in scCO2 and to design CO2-philic materials in order to enhance more applications of scCO2 A large number of experimental and theoretical studies on solute–solvent interactions have been performed to gain understanding on solubility and structures of solutes in scCO2 [5–14] In general, naked or substituted hydrocarbons along with compounds functionalized by hydroxyl, carbonyl, thiocarbonyl, carboxyl and amide groups have been paid much attention as CO2-philic compounds [7,8,11–16] The obtained results showed that the carbonyl and thiocarbonyl compounds have presented a higher stability, as compared to other functionalized ones, when they interact with CO2 This durability has been assigned to a main contribution of the >C@ZÁ Á ÁC (Z@O, S) Lewis acid–base interaction and/or an additional cooperation of the C–HÁ Á ÁO hydrogen bonded interaction, except for a crucial role of ⇑ Corresponding author Fax: +84 563846089 E-mail address: nguyentientrung@qnu.edu.vn (N.T Trung) http://dx.doi.org/10.1016/j.cplett.2014.03.005 0009-2614/Ó 2014 Elsevier B.V All rights reserved the O–HÁ Á ÁO hydrogen bond predominating over the >C@OÁ Á ÁC Lewis acid–base interaction for the HCOOHÁ Á ÁCO2 complex in our previous study [14] Nevertheless, the role of the C–HÁ Á ÁO hydrogen bond in increasing soluble capacity of compounds in scCO2 remains in debate In addition, the finding of a specific scheme that can rationalize the origin of blue shifting hydrogen bond is still an objective of both theoretical and experimental works despite the fact that in previous studies several rationalizations have been offered [17–21] It is more appropriate if one considers the origin of blue shifting hydrogen bond based on inherent properties of isolated isomers that are proton donors and proton acceptors [11,21] Dimethyl sulfoxide (DMSO) is often used in biological and physicochemical studies, and is a common solvent in supercritical antisolvent processes [22–24].Many important applications have been obtained such as micronization of pharmaceutical compounds, polymers, catalysts, superconductors and coloring materials [25] The phase equilibrium between the components including solute, solvent and sometimes a cosolvent plays an important role in the proper technological choice for the micronization process [26] Hence, the experimental investigations into the phase equilibria of DMSO with CO2, with both CO2 and H2O were performed [27] A detailed study on the interaction of DMSO with H2O was reported in ref [23] There is hardly any information relating to the complex between DMSO and CO2 except what mentioned in ref [28] The authors suggested that DMSO interacts strongly with CO2, and the complex strength is contributed by a >S@OÁ Á ÁC (CO2) Lewis acid– base interaction and two C–HÁ Á ÁO hydrogen bonded interactions However, a thorough theoretical investigation into existence and 76 V.T Phuong et al / Chemical Physics Letters 598 (2014) 75–80 role of interactions of DMSO with CO2 at the molecular level has not been put forth yet On the other hand, the interaction of dimethyl thiosulfoxide (CH3SSCH3) with CO2 has not yet been investigated although the CH3SSCH3 was synthesized experimentally [29] and discussed theoretically [30] To the best of our knowledge, a comparative study on the interaction capacity of >S@O and >S@S functionalized compounds including CH3SOCH3 and CH3SSCH3, and their doubly methylated and halogenated derivatives (denoted by CH3CZCHX2, with X@CH3, F, Cl, Br; Z@O, S), with CO2 has not been reported in the literature More remarkably, our objective in this work is also to have a closer look at the origin of the C–HÁÁÁO hydrogen bond based on different polarization of C–H covalent bond acting as the proton donor in the isolated monomer Computational methods Geometry optimizations for monomers and complexes of CH3 SZCHX2 (X@H, CH3, F, Cl, Br; and Z@O, S) and CO2 were carried out at MP2/6-311++G(2d,2p) Harmonic vibrational frequencies at the same level of theory were determined to ensure that the optimized structures were all energy minima on potential energy surface, and to estimate zero-point energy (ZPE) To avoid vibrational couplings between the CH3 stretching modes of CH3SZCH3, CH3SZCH(CH3)2 (Z@O, S), the harmonic frequencies in these monomers and relevant complexes were calculated by means of the deuterium isotope effect Single point energy calculations were done in all cases using MP2/aug-cc-pVTZ//MP2/6-311++G(2d,2p) and in some specific cases using CCSD(T)/6-311++G(3df,2pd)//MP2/6-311++G(2d,2p) for test purposes Basis set superposition errors (BSSE) were calculated by using the counterpoise method of Boys and Bernadi [31] The interaction energies were obtained as the difference in total energy between each complex and the sum of isolated monomers, corrected for ZPE only (DE) or for both ZPE and BSSE (DE⁄) All of the calculations were carried out using the GAUSSIAN 09 program [32] Topological parameters of the complexes were estimated by AIM2000 software [33] based on Bader’s Atoms in Molecules theory [34,35] Finally, the electronic properties of the monomers and complexes were examined through a natural bond orbital (NBO) analysis using GENNBO 5.G program [36] at the MP2/6-311++G(2d,2p) level Result and discussion 3.1 Interactions of CO2 with CH3SZCH3 (Z@O, S) Three stable shapes of the optimized structures of complexes CH3SZCH3Á Á ÁCO2 (Z@O, S) at MP2/6-311++G(2d,2p) are presented in Figure 1, which are denoted hereafter by T1, T2 and T3 Their topological geometries are shown in Figure S1 of Supplementary information (SI) The selected parameters including intermolecular distance, electron density (q(r)) and Laplacian (r2(q(r))) of bond critical points (BCP) are gathered in Table For test purpose, interaction energies of complexes at two different levels of theory are also given in the Table Generally, all OÁÁÁC (CO2), SÁÁÁC (CO2), HÁÁÁO (CO2) and SÁÁÁO (CO2) contact distances are close to or smaller than the sums of van der Waals radii of two relevant atoms (3.22 Å for OÁÁÁC, 3.50 Å for SÁÁÁC, 2.72 Å for HÁÁÁO and 3.32 Å for SÁÁÁO) In addition, the q(r) and r2(q(r)) values of bond critical points of ZÁ Á ÁC, OÁ Á ÁS and OÁ Á ÁH intermolecular contacts fall within the critical limit for formation of non-covalent interactions (0.002–0.035 au for q(r) and 0.02–0.15 au for r2(q(r))) [37] Accordingly, these intermolecular contacts are the Lewis acid–base, chalcogen–chalcogen and hydrogen bonded interactions in the relevant complexes, respectively In particular, the strength of the T1 and T3 shapes is contributed by both the S@ZÁ Á ÁC (CO2) Lewis acid–base and C–HÁ Á ÁO (CO2) hydrogen bonded interactions, while the contributions to the strength of T2 shape arise from the S@ZÁ Á ÁC (CO2) Lewis acid–base and OÁ Á ÁS@Z chalcogen–chalcogen interactions (cf Figure 1) The obtained results point out that there is a slight difference of the interaction energies in two levels of theory applied Thus, the interaction energies of complexes examined range from À13.8 to À17.2 kJ molÀ1 and À9.8 to À14.4 kJ molÀ1 (at MP2/aug-ccpVTZ//MP2/6-311++G(2d,2p)), and À13.6 to À17.7 kJ molÀ1 and À9.6 to À14.5 kJ molÀ1 (at CCSD(T)/6-311++G(3df,2pd)//MP2/ 6-311++G(2d,2p)) for only ZPE correction and both ZPE and BSSE corrections, respectively (cf Table 1) The results indicate that the formed complexes are significantly stable, and more stable than the complexes of the >C@O and >C@S functionalized compounds and CO2 reported in refs [11,14,28] This presents a stronger interaction of CO2 with the >S@O and >S@S counterparts relative to the >C@O and >C@S ones The reason for this is that the O and S atoms in the >C@O and >C@S groups are sp2-hybridized making their lone pairs in plane, while both of them in the> S@O and >S@S groups have a higher p-character hybridization Unlike in the carbonyl and thiocarbonyl compounds, the S–Z– C–O (Z@O, S) dihedrals is indeed nonzero (cf Figure 1) The strength of the CH3SZCH3Á Á ÁCO2 (Z@O, S) complexes decreases in the order of T1 = T3 > T2, and the CH3SOCH3Á Á ÁCO2 complexes are more stable than the corresponding CH3SSCH3 Á Á ÁCO2 ones Both the larger proton affinity (PA) of 907.1 kJ molÀ1 at S site and the smaller deprotonation enthalpy (DPE) of 1578.4 kJ molÀ1 of C–H bond for CH3SSCH3 should be more Figure The stable shapes of complexes between CH3SZCH3 (Z@O, S) and CO2 77 V.T Phuong et al / Chemical Physics Letters 598 (2014) 75–80 Table Some selected parameters of the CH3SZCH3Á Á ÁCO2 complexes (interaction energies in kJ.mol-1, contact distances in Å, electron density and Laplacian in au) Structures DE a DE b DE⁄a DE⁄b R1 or R3 R2 q(ZÁ Á ÁC) or q(OÁ Á ÁS) q(OÁ Á ÁH) r2(q(ZÁ Á ÁC) or q(OÁ Á ÁS)) r2(q(OÁ Á ÁH)) a b Z@O Z@S T1 T2 T3 T1 T2 T3 À17.2 À17.6 À14.4 À14.5 2.63 2.77 0.0119 0.0064 0.0468 0.0228 À14.3 À14.8 À10.9 À11.3 3.49 2.69 0.0143 0.0139 0.0556 0.0359 À17.4 À17.7 À13.7 À14.0 2.65 2.70 0.0140 0.0062 0.0536 0.0209 À17.1 À16.8 À14.2 À13.5 2.59 3.33 0.0079 0.0073 0.0272 0.0258 À13.8 À13.6 À9.8 À9.6 3.37 3.30 0.0085 0.0059 0.0296 0.0224 À16.9 À16.4 À13.2 À12.0 2.54 3.30 0.0085 0.0075 0.0291 0.0248 Taken from MP2/aug-cc-pVTZ//MP2/6-311++G(2d,2p) Taken from CCSD(T)/6-311++G(3df,2pd)//MP2/6-311++G(2d,2p) advantageous to durable enhancement of complexes CH3SSCH3 Á Á ÁCO2 relative to CH3SOCH3Á Á ÁCO2 (PA at O site being 900.1 kJ molÀ1 and DPE of C–H bond being 1610.1 kJ molÀ1 at CCSD(T)/ 6-311++G(3df,2pd)//MP2/6-311++G(2d,2p)) However, a reverse tendency of the strength is observed (cf Table 1) The larger magnitude in strength of the CH3SOCH3Á Á ÁCO2 ones compared to the CH3SSCH3Á Á ÁCO2 ones might be due to a larger contribution of attractive electrostatic interaction to the overall interaction energy Thus, as shown in Table 1, each R2 value, and R1 and R3 values are smaller and larger, respectively, for CH3SOCH3Á Á ÁCO2 than for CH3SSCH3Á Á ÁCO2 This result suggests a stronger interaction of CO2 with the >S@O moiety compared to the >S@S moiety This trend is different from the reported results on substitution of O atom in >C@O by S atom (>C@S) in the carbonyl compounds interacting with CO2 [11], in which the former is weaker than the latter Remarkably, it should be emphasized that the two stable T2 and T3 structures of CH3SOCH3Á Á ÁCO2, and the three stable shapes of CH3SSCH3Á Á ÁCO2 are revealed for the first time For the CH3SOCH3Á Á ÁCO2 complexes, the strength of T3 is close to that of T1 reported by Wallen et al [28] Thus, the interaction energies of T1 in this work are À14.4 kJ molÀ1 at MP2/aug-cc-pVTZ//MP2/6-311++G(2d,2p) and À14.5 kJ molÀ1 at CCSD(T)/6-311++G(3df,2pd)//MP2/6-311++G(2d,2p), which are appropriate to the value of À14.3 kJ molÀ1 at MP2/aug-cc-pVDZ// MP2/6-31+G(d) reported in ref [28] In summary, the CH3SZCH3Á Á ÁCO2 (Z@O, S) complexes are in general stabilized by the Lewis acid–base, chalcogen–chalcogen and hydrogen bonded interactions, nevertheless a crucial role contributing to the overall stabilization energy should be suggested to be the Lewis acid–base interaction 3.2 Interactions of CO2 with CH3SZCHX2 (X@H, CH3, F, Cl, Br; Z@O, S) Apart from the most stable T1 shape in the CH3SZCH3Á Á ÁCO2 complexes and the demands of evaluating the role of Lewis acid– base and hydrogen bonded interactions in stabilizing the complexes as well as pursuing the issue of the C–HÁ Á ÁO blue shifting hydrogen bond based on various polarity of the C–H covalent bond, we replaced two H atoms in a CH3 group of CH3SZCH3 by two CH3, F, Cl and Br alike groups, and investigated the effects of gas phase basicity at Z site and of polarity of the C–H bond in the isolated monomers on the strength of complexes of CO2 and CH3SZCHX2 (X@CH3, F, Cl, Br; Z@O, S) The stable shapes of the F, Cl and Br derivatives and CO2 are virtually similar to the T1 shape, while a slight difference in geometry is observed for CH3SZCH(CH3)3Á Á ÁCO2 All of them are presented in Figure 2, and some selected geometric parameters of CH3SZCHX2Á Á ÁCO2 are gathered in Table S1 of SI All OÁÁÁC (CO2), SÁÁÁC (CO2) and HÁÁÁO (CO2) contact distances are smaller than the sums of van der Waals radii of two relevant atoms (3.22 Å for OÁÁÁC, 3.50 Å for SÁÁÁC and 2.72 Å for HÁÁÁO) They are indeed in the ranges of 2.76–2.80 Å for OÁÁÁC, 3.33–3.40 Å for OÁÁÁC, and 2.30–2.63 Å for OÁÁÁH contacts (cf Table S1) Consequently, these interactions are the Lewis acid–base type and the hydrogen bond The evidence for the interactions is also based on the deviation of the carbon atom of CO2 from sp-hybridization (a3 < 180°) The AIM analyses performed to lend further support for the presence of interactions and their contribution to complex strength are presented in Figure S2 and Table of SI All q(r) and r2(q(r)) values of BCPs in the examined complexes belong to the limitation criteria for the formation of weak intermolecular interactions [37] The a1 values are larger for CH3SSCHX2Á Á ÁCO2 than for CH3SOCHX2 Á Á ÁCO2, indicating the stronger C–HÁ Á ÁO hydrogen bonded interaction for the former than the latter On the contrary, the Lewis acid–base interaction is stronger for CH3SSCHX2Á Á ÁCO2 than for CH3SOCHX2Á Á ÁCO2, which arises from the smaller a2 values of ca 20° for the former Thus, as shown in Table S1, intermolecular contact distances also confirm this point The interaction energies, proton affinities and deprotonation enthalpies in the monomers and the complexes CH3SSCHX2Á Á ÁCO2 are tabulated in Table The interaction energies are significantly negative, implying the very stable complexes of CO2 and CH3SSCHX2 They are indeed from À14.4 to À16.4 kJ molÀ1 and from À13.7 to À15.5 kJ molÀ1 (including ZPE and BSSE) for CH3SOCHX2Á Á ÁCO2 and CH3SSCHX2Á Á ÁCO2, respectively In general, the CH3SOCHX2Á Á ÁCO2 complexes are more stable than the CH3SSCHX2Á Á ÁCO2 complexes This firmly indicates that the >S@O, as compared to the >S@S, has a stronger interaction with CO2, which originates from a contribution of attractive electrostatic interaction larger for the former than for the latter in stabilizing the complexes examined For the CH3SOCHX2Á Á ÁCO2 complexes, the strength is enhanced in the order of X from H via F to Cl to Br and finally to CH3 (cf Table 2) Accordingly, the substitution of two H atoms in a CH3 group of CH3SOCH3 by two X alike groups makes the formed complexes more stable, as compared to CH3SOCH3Á Á ÁCO2 The replacement also leads to a slight enhancement of stability of the CH3SSCHX2 Á Á ÁCO2 complexes in the sequence from F, H, Cl, Br to CH3 (cf Table 2) Coming back to the estimated values of PA at the O and S sites and DPE of the C–H bond involved in hydrogen bond for the isolated monomers, one can see that the gas phase basicity at the O and S sites increases from F via Cl to Br to H and to CH3, and the polarity of the C–H bonds decreases in the sequence from Br, Cl, H, F to CH3 Accordingly, the overall stabilization energy for the CH3SZCHX2Á Á ÁCO2 complexes is contributed by a main role of the >S@ZÁ Á ÁC interaction and an additional cooperation of the C–HÁ Á ÁO hydrogen bond, in which an enhanced contribution of the hydrogen bond should be suggested for the complexes from 78 V.T Phuong et al / Chemical Physics Letters 598 (2014) 75–80 Figure The stable shapes of interactions of CH3SZCH(CH3)2 and CH3SZCHX2 (X@H, F, Cl, Br; Z@O, S) with CO2 at MP2/6-311++G(2d,2p) Table Interaction energies using MP2/aug-cc-pVTZ//MP2/6-311++G(2d,2p), and proton affinities (PA) at the O and S sites and deprotonation enthalpies (DPE) of the C–H bonds involved in hydrogen bond for the isolated monomers using CCSD(T)/6-311++G(3df,2pd)//MP2/6-311++G(2d,2p) (all in kJ molÀ1) Z@O a Z@S X H CH3 F Cl Br H CH3 F Cl Br DE ⁄ PA DPEa À14.4 900.1 1610.1 À16.4 904.8 1711.9 À14.7 876.2 1619.5 À15.0 876.7 1560.8 À16.3 884.9 1540.6 À14.2 907.1 1578.4 À15.5 911.0 1704.7 À13.7 883.8 1606.0 À14.3 891.0 1540.4 À15.4 896.0 1522.0 Single point energies of CH3SZCX2 anions calculated at the respective geometry of isolated monomer without optimization H via F to Cl and finally to Br derivative In short, the obtained results indicate that the >S@O and >S@S counterparts should be valuable candidates in the design of CO2-philic materials and in the findings of materials to adsorb CO2 in the near future Interactions of CO2 with CH3SZCHX2 cause the length changes of the C–H bond involved in hydrogen bond, and its stretching frequency, and the results are listed in Table All values indicate that the C–HÁ Á ÁO interaction in all the examined complexes belongs to the blue shifting hydrogen bond Following complexation, a contraction of the C–H bond length and an increase in its corresponding stretching frequency are indeed observed in all complexes, which are in the range of 0.5–2.0 mÅ and of 8.3–27.8 cmÀ1, respectively The C–H bond length is shortened by 0.5 mÅ for CH3SOCH3 Á Á ÁCO2 at MP2/6-311++G(2d,2p), comparable to the reported value of by 0.3 mÅ at MP2/6-31 + G(d) by Wallen et al [28] Increasing magnitude of the C–H bond length contraction and its stretching frequency blue shift for each CH3SZCHX2Á Á ÁCO2 is in the order of X from Br via Cl and to F This trend is consistent with a decrease of the C–H polarization in the CH3SZCHX2 monomers In other words, the smaller the polarity of the C–H bond involved in hydrogen bond is, the larger the contraction and the stretching frequency blue shift of the C–H bond as a result of complexation are, and vice versa Nevertheless, there is a different tendency in the changes of the C–H bond length and its stretching frequency for CH3SZCHX2 Á Á ÁCO2, with X@H, CH3, and Z@O, S (cf Table 3) Therefore, it might be mentioned that the origin of blue shift hydrogen bond should be slightly affected by the complex shape and the neighbouring intermolecular interactions, besides the crucial dependence on the polarity of covalent bond acting as the isolated proton donor NBO analyses are applied to support for the evidence of the interactions and the origin of the C–HÁ Á ÁO hydrogen bond upon complexation, and the typical results are tabulated in Table All positive values of EDT (electron density transfer) imply a stronger transfer of electron density from CH3SZCHX2 to CO2 In other words, the electron transfer interaction from the n(Z) lone pairs to the p⁄(C@O) orbital dominates rather than the electron transfer from the n(O) lone pairs to r⁄(C–H) orbital in the complex stabilization The EDT values are larger for CH3SOCHX2Á Á ÁCO2 than for CH3SSCHX2Á Á ÁCO2, indicating that the >S@OÁ Á ÁC interaction is more stable than the >S@SÁ Á ÁC interaction The values of intermolecular hyperconjugation energies transferring electron density from the n(Z) to the p⁄(C@O) (denoted by Einter(n(Z10) ? p⁄(C11@O13))), Table The variation of the C5–H6 bond length (Dr, mÅ), its stretching frequencies (Dm, cmÀ1) at MP2/6-311++G(2d,2p) Z@O ⁄ Z@S X H CH3 F Cl Br H CH3 F Cl Br Dr Dm À0.5 8.3 À1.2 (À0.2) 17.6 (1.6) À2.0 27.8 À1.1 19.2 À1.0 17.5 À0.5 9.1 À1.2 (À0.4) 17.4 (1.8) À1.6 24.6 À1.3 22.6 À1.0 18.7 The values given in brackets for the C7–H14 covalent bond 79 V.T Phuong et al / Chemical Physics Letters 598 (2014) 75–80 Table NBO analyses of the CH3SZCHX2Á Á ÁCO2 complexes at MP2/6-311++G(2d,2p) Z@O Z@S a X EDT/electron Dr⁄(C5–H6) 103/electron D%s (C5) electron E(n(O12) ? r⁄(C5–H6)) kJ molÀ1 E(n(Z10) ? r⁄(C11@O13)) kJ molÀ1 H CH3 5.3 4.1 3.1 3.2 3.0 0.4 0.4 0.2a 0.6 0.6 0.7 1.34 0.46 0.11a 2.64 3.1 3.14 14.1 3.31 F Cl Br 0.2 0.3 0.3a À1.3 À1.4 À1.1 H CH3 5.1 3.2 2.0 1.6 1.3 0.4 0.4 0.3a 0.5 0.7 0.9 2.51 0.62 0.21a 5.19 6.61 7.91 6.7 1.37 F Cl Br 0.7 0.2 0.2a À0.8 À1.3 À1.0 11.7 11.6 11.8 6.15 5.52 6.4 For C7–H14 covalent bond and from the n(O) to the r⁄(C–H) (denoted by Einter(n(O12) ? r⁄(C5–H6))) listed in Table indeed confirm this observation Hence, the corresponding intermolecular distances of >S@OÁ Á ÁC for CH3SOCHX2Á Á ÁCO2 are shorter than those of >S@SÁ Á ÁC for CH3SSCHX2Á Á ÁCO2, while a shorter contact distances of OÁ Á ÁH are obtained for the latters relative to the formers (cf Table S1) Upon complexation, a small increase in s-character percentage of the C hybrid orbitals is observed for all complexes They are in the range of ca 0.2–0.7% Such a gain in s-character partly contributes to a contraction of the C–H bond lengths However, there are different variations of electron density in the r⁄(C–H) orbitals In particular, a decrease of electron density in the r⁄(C–H) orbitals by ca 0.0008–0.0014 electron is obtained for CH3SZCHX2Á Á ÁCO2 (X@F, Cl, Br), while an increase of electron density by ca 0.0002– 0.0007 electron is predicted for CH3SZCHX2Á Á ÁCO2 (X@H, CH3) As a consequence, a contraction of the C–H bond involved in hydrogen bond along with a blue shift of its stretching frequency for CH3SZCHX2Á Á ÁCO2 (X@F, Cl, Br) arises from both a decrease of the r⁄(C–H) electron density and an increase in the s-character percentage of the C hybrid orbital On the other hand, for CH3SZCHX2Á Á ÁCO2 (X@H, CH3), a C–H bond length contraction and its stretching frequency blue shift are determined by an increase in the s-character percentage of the C hybrid orbital overriding an increase in the occupation of the r⁄(C–H) orbitals Concluding remarks Acknowledgments This research is funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 104.03-2012.12 NTT and VV also thanks Katholieke Universiteit Leuven for extending computational facilities through the VLIR project ZEIN2012Z129 Appendix A Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2014.03 005 References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] Interactions of CO2 with CH3SOCH3 and CH3SSCH3 induce three quite stable shapes with interaction energies from À9.6 to À14.5 kJ molÀ1 for both ZPE and BSSE corrections at CCSD(T)/6311++G(3df,2pd)//MP2/6-311++G(2d,2p) Remarkably, two quite stable shapes of the CH3SZCH3Á Á ÁCO2 (Z@O, S) complexes are revealed for the first time The interaction energies of the CH3SZCHX2Á Á ÁCO2 complexes range from À13.7 to À16.4 kJ molÀ1 for both ZPE and BSSE corrections (MP2/aug-cc-pVTZ//MP2/ 6-311++G(2d,2p)) Their strength is mainly determined by the > S@ZÁ Á ÁC Lewis acid–base interaction, and an additional contribution of the C–HÁ Á ÁO hydrogen bonded interaction with an enhanced role in the sequence from H to F to Cl to Br derivative The CH3SOCHX2Á Á ÁCO2 complexes are more stable than the CH3SSCHX2Á Á ÁCO2 complexes, which result from a large contribution of attractive electrostatic interaction of the >S@O relative to the >S@S to the overall stabilization energy The substitution of two H atoms in a CH3 group of CH3SZCH3 by two F, Cl, Br and CH3 alike groups makes the CH3SZCHX2Á Á 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