A comparative study on interaction capacity of CO2 CPL 2014

This durability has been assigned to a main contribution of the >C@Z C Z@O, S Lewis acid–base interaction and/or an additional cooperation of the C–H O hydrogen bonded interaction, exc

A comparative study on interaction capacity of CO 2 with the >S @O

derivatives of CH 3 SOCH 3 and CH 3 SSCH 3

Vo Thuy Phuonga, Nguyen Thi Thu Trangb,c, Vien Voa, Nguyen Tien Trunga,⇑

a

Faculty of Chemistry, and Laboratory of Computational Chemistry, Quy Nhon University, Quy Nhon, Viet Nam

b

Faculty of Science, Hai Phong University, Hai Phong, Viet Nam

c

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 4 March 2014

Available online 12 March 2014

a b s t r a c t

Interactions of CO2with CH3SZCHX2(Z@O, S; X@H, CH3, F, Cl, Br) induce significantly stable complexes with interaction energies from 13.7 to 16.4 kJ mol1 (MP2/aug-cc-pVTZ//MP2/6-311++G(2d,2p)) Remarkably, some stable shapes of CH3SZCH3  CO2are revealed for the first time Substitution of two

H atoms in a CH3of CH3SZCH3 by two X alike groups makes CH3SZCHX2  CO2 more stable than

CH3SZCH3  CO2, 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

1 Introduction

Supercritical fluid technology is considered as an attractive

op-tion for separaop-tion of fine chemicals from liquid solvents, and

supercritical carbon dioxide (scCO2) has become of interest as a

promising alterative to organic solvents for extractions,

separa-tions, chemical reacsepara-tions, and material processes[1–4] ScCO2is

convenient to use as it possesses a lot of desirable properties

Nev-ertheless, the limitation in the applications of scCO2is its restrictive

capacity of solvation for polar and high molecular weight

com-pounds It is required to unravel the factors for controlling the

sol-ubility of compounds in scCO2and 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

struc-tures 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

functional-ized ones, when they interact with CO2 This durability has been

assigned to a main contribution of the >C@Z  C (Z@O, S) Lewis

acid–base interaction and/or an additional cooperation of the

C–H  O hydrogen bonded interaction, except for a crucial role of

the O–H  O hydrogen bond predominating over the >C@O  C Le-wis acid–base interaction for the HCOOH  CO2complex in our pre-vious study[14] Nevertheless, the role of the C–H  O hydrogen bond in increasing soluble capacity of compounds in scCO2remains

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 of-fered[17–21] It is more appropriate if one considers the origin of blue shifting hydrogen bond based on inherent properties of iso-lated isomers that are proton donors and proton acceptors[11,21] Dimethyl sulfoxide (DMSO) is often used in biological and phys-icochemical studies, and is a common solvent in supercritical anti-solvent 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 CO2and 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 CO2except what mentioned in ref.[28] The authors suggested that DMSO interacts strongly with CO2, and the complex strength is contributed by a >S@O  C (CO2) Lewis acid– base interaction and two C–H  O hydrogen bonded interactions However, a thorough theoretical investigation into existence and

http://dx.doi.org/10.1016/j.cplett.2014.03.005

0009-2614/Ó 2014 Elsevier B.V All rights reserved.

⇑ Corresponding author Fax: +84 563846089.

E-mail address: nguyentientrung@qnu.edu.vn (N.T Trung).

Contents lists available atScienceDirect

Chemical Physics Letters

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c p l e t t

role of interactions of DMSO with CO2at the molecular level has not

been put forth yet On the other hand, the interaction of dimethyl

thiosulfoxide (CH3SSCH3) with CO2has not yet been investigated

although the CH3SSCH3was synthesized experimentally[29]and

discussed theoretically[30] To the best of our knowledge, a

com-parative study on the interaction capacity of >S@O and >S@S

func-tionalized 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 CO2has 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–HO

hydro-gen bond based on different polarization of C–H covalent bond

act-ing as the proton donor in the isolated monomer

2 Computational methods

Geometry optimizations for monomers and complexes of CH3

SZCHX2(X@H, CH3, F, Cl, Br; and Z@O, S) and CO2were 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 CH3stretching 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

ef-fect 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

calcu-lated by using the counterpoise method of Boys and Bernadi[31]

The interaction energies were obtained as the difference in total

en-ergy between each complex and the sum of isolated monomers,

cor-rected for ZPE only (DE) or for both ZPE and BSSE (DE⁄) All of the

calculations were carried out using the GAUSSIAN09 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

com-plexes were examined through a natural bond orbital (NBO) analysis

using GENNBO 5.G program[36]at the MP2/6-311++G(2d,2p) level

3 Result and discussion

3.1 Interactions of CO2with CH3SZCH3(Z@O, S)

Three stable shapes of the optimized structures of complexes

CH3SZCH3  CO2(Z@O, S) at MP2/6-311++G(2d,2p) are presented

inFigure 1, which are denoted hereafter by T1, T2 and T3 Their topological geometries are shown inFigure S1of 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 inTable 1 For test purpose, inter-action energies of complexes at two different levels of theory are also given in theTable 1 Generally, all OC (CO2), SC (CO2), HO (CO2) and SO (CO2) contact distances are close to or smaller than the sums of van der Waals radii of two relevant atoms (3.22 Å for OC, 3.50 Å for SC, 2.72 Å for HO and 3.32 Å for SO) In addi-tion, theq(r) andr2(q(r)) values of bond critical points of Z  C, O  S and O  H 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–chal-cogen and hydrogen bonded interactions in the relevant com-plexes, respectively In particular, the strength of the T1 and T3 shapes is contributed by both the S@Z  C (CO2) Lewis acid–base and C–H  O (CO2) hydrogen bonded interactions, while the contri-butions to the strength of T2 shape arise from the S@Z  C (CO2) Lewis acid–base and O  S@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 13.8 to

17.2 kJ mol1 and 9.8 to 14.4 kJ mol1 (at MP2/aug-cc-pVTZ//MP2/6-311++G(2d,2p)), and 13.6 to 17.7 kJ mol1 and

9.6 to 14.5 kJ mol1 (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 com-pounds and CO2reported in refs.[11,14,28] This presents a stron-ger 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 -hybrid-ized 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 CH3SZCH3  CO2 (Z@O, S) complexes decreases in the order of T1 = T3 > T2, and the CH3SOCH3  CO2

complexes are more stable than the corresponding CH3SSCH3

  CO2ones Both the larger proton affinity (PA) of 907.1 kJ mol1

at S site and the smaller deprotonation enthalpy (DPE) of 1578.4 kJ mol1 of C–H bond for CH3SSCH3 should be more

Figure 1 The stable shapes of complexes between CH SZCH (Z@O, S) and CO

advantageous to durable enhancement of complexes CH3SSCH3

  CO2relative to CH3SOCH3  CO2(PA at O site being 900.1 kJ mol1

and DPE of C–H bond being 1610.1 kJ mol1 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 CH3SOCH3  CO2 ones compared to

the CH3SSCH3  CO2ones might be due to a larger contribution of

attractive electrostatic interaction to the overall interaction energy

Thus, as shown inTable 1, each R2 value, and R1 and R3 values are

smaller and larger, respectively, for CH3SOCH3  CO2 than for

CH3SSCH3  CO2 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

CH3SOCH3  CO2, and the three stable shapes of CH3SSCH3  CO2are

revealed for the first time For the CH3SOCH3  CO2complexes, 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 14.4 kJ mol1

at MP2/aug-cc-pVTZ//MP2/6-311++G(2d,2p) and 14.5 kJ mol1

at CCSD(T)/6-311++G(3df,2pd)//MP2/6-311++G(2d,2p), which are

appropriate to the value of 14.3 kJ mol1at MP2/aug-cc-pVDZ//

MP2/6-31+G(d) reported in ref.[28]

In summary, the CH3SZCH3  CO2 (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 CO2with CH3SZCHX2(X@H, CH3, F, Cl, Br; Z@O, S)

Apart from the most stable T1 shape in the CH3SZCH3  CO2

complexes and the demands of evaluating the role of Lewis acid–

base and hydrogen bonded interactions in stabilizing the

com-plexes as well as pursuing the issue of the C–H  O blue shifting

hydrogen bond based on various polarity of the C–H covalent bond,

we replaced two H atoms in a CH3group of CH3SZCH3by 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 CO2and CH3SZCHX2

(X@CH3, F, Cl, Br; Z@O, S) The stable shapes of the F, Cl and Br

derivatives and CO2are virtually similar to the T1 shape, while a

slight difference in geometry is observed for CH3SZCH(CH3)3  CO2

All of them are presented inFigure 2, and some selected geometric

parameters of CH3SZCHX2  CO2are gathered in Table S1 of SI

All OC (CO2), SC (CO2) and HO (CO2) contact distances are

smaller than the sums of van der Waals radii of two relevant atoms

(3.22 Å for OC, 3.50 Å for SC and 2.72 Å for HO) They are in-deed in the ranges of 2.76–2.80 Å for OC, 3.33–3.40 Å for OC, and 2.30–2.63 Å for OH 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 devia-tion of the carbon atom of CO2from sp-hybridization (a3< 180°) The AIM analyses performed to lend further support for the pres-ence of interactions and their contribution to complex strength are presented inFigure S2andTable 2of SI Allq(r) andr2(q(r)) values of BCPs in the examined complexes belong to the limitation criteria for the formation of weak intermolecular interactions[37] Thea1values are larger for CH3SSCHX2  CO2than for CH3SOCHX2

  CO2, indicating the stronger C–H  O hydrogen bonded interac-tion for the former than the latter On the contrary, the Lewis acid–base interaction is stronger for CH3SSCHX2  CO2 than for

CH3SOCHX2  CO2, which arises from the smallera2values of ca 20° for the former Thus, as shown inTable S1, intermolecular con-tact distances also confirm this point

The interaction energies, proton affinities and deprotonation enthalpies in the monomers and the complexes CH3SSCHX2  CO2

are tabulated inTable 2 The interaction energies are significantly negative, implying the very stable complexes of CO2 and

CH3SSCHX2 They are indeed from 14.4 to 16.4 kJ mol1 and from 13.7 to 15.5 kJ mol1 (including ZPE and BSSE) for

CH3SOCHX2  CO2and CH3SSCHX2  CO2, respectively In general, the CH3SOCHX2  CO2 complexes are more stable than the

CH3SSCHX2  CO2complexes 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 CH3SOCHX2  CO2complexes, the strength is enhanced

in the order of X from H via F to Cl to Br and finally to CH3(cf Ta-ble 2) Accordingly, the substitution of two H atoms in a CH3group

of CH3SOCH3by two X alike groups makes the formed complexes more stable, as compared to CH3SOCH3  CO2 The replacement also leads to a slight enhancement of stability of the CH3SSCHX2

  CO2 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 CH3SZCHX2  CO2complexes is contributed by a main role of the >S@Z  C interaction and an additional cooperation of the C–H  O hydrogen bond, in which an enhanced contribution of the hydrogen bond should be suggested for the complexes from

Table 1

Some selected parameters of the CH 3 SZCH 3   CO 2 complexes (interaction energies in kJ.mol -1

, contact distances in Å, electron density and Laplacian in au).

DE a

DE b

DE ⁄ a

DE ⁄ b

q(Z  C) orq(O  S) 0.0119 0.0143 0.0140 0.0079 0.0085 0.0085

r2

(q(Z  C) orq(O  S)) 0.0468 0.0556 0.0536 0.0272 0.0296 0.0291

a

Taken from MP2/aug-cc-pVTZ//MP2/6-311++G(2d,2p).

b

Taken from CCSD(T)/6-311++G(3df,2pd)//MP2/6-311++G(2d,2p).

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

can-didates in the design of CO2-philic materials and in the findings of

materials to adsorb CO2in the near future

Interactions of CO2with CH3SZCHX2cause the length changes of

the C–H bond involved in hydrogen bond, and its stretching

fre-quency, and the results are listed inTable 3 All values indicate that

the C–H  O interaction in all the examined complexes belongs to

the blue shifting hydrogen bond Following complexation, a

con-traction of the C–H bond length and an increase in its

correspond-ing stretchcorrespond-ing frequency are indeed observed in all complexes,

which are in the range of 0.5–2.0 mÅ and of 8.3–27.8 cm1,

respec-tively The C–H bond length is shortened by 0.5 mÅ for CH3SOCH3

  CO2at 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 CH3SZCHX2  CO2is 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

hydro-gen 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

  CO2, 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 inter-molecular 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–H  O hydrogen bond upon complexation, and the typical results are tabulated inTable 4 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 thep⁄(C@O) orbital dominates rather than the electron transfer from the n(O) lone pairs tor⁄(C–H) orbital in the complex stabil-ization The EDT values are larger for CH3SOCHX2  CO2than for

CH3SSCHX2  CO2, indicating that the >S@O  C interaction is more stable than the >S@S  C 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))),

Figure 2 The stable shapes of interactions of CH 3 SZCH(CH 3 ) 2 and CH 3 SZCHX 2 (X@H, F, Cl, Br; Z@O, S) with CO 2 at MP2/6-311++G(2d,2p).

Table 2

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 mol 1 ).

DE ⁄

14.4 16.4 14.7 15.0 16.3 14.2 15.5 13.7 14.3 15.4

PA 900.1 904.8 876.2 876.7 884.9 907.1 911.0 883.8 891.0 896.0 DPE a 1610.1 1711.9 1619.5 1560.8 1540.6 1578.4 1704.7 1606.0 1540.4 1522.0

a

Single point energies of CH 3 SZCX 2 anions calculated at the respective geometry of isolated monomer without optimization.

Table 3

The variation of the C5–H6 bond length (Dr, mÅ), its stretching frequencies (Dm, cm 1

) at MP2/6-311++G(2d,2p).

Dr 0.5 1.2 (0.2) 2.0 1.1 1.0 0.5 1.2 (0.4) 1.6 1.3 1.0

Dm 8.3 17.6 (1.6) 27.8 19.2 17.5 9.1 17.4 (1.8) 24.6 22.6 18.7

⁄

and from the n(O) to the r⁄(C–H) (denoted by Einter(n(O12) ?

r⁄(C5–H6))) listed in Table 4 indeed confirm this observation

Hence, the corresponding intermolecular distances of >S@O  C

for CH3SOCHX2  CO2 are shorter than those of >S@S  C for

CH3SSCHX2  CO2, while a shorter contact distances of O  H 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

contrib-utes to a contraction of the C–H bond lengths However, there are

different variations of electron density in ther⁄(C–H) orbitals In

particular, a decrease of electron density in ther⁄(C–H) orbitals

by ca 0.0008–0.0014 electron is obtained for CH3SZCHX2  CO2

(X@F, Cl, Br), while an increase of electron density by ca 0.0002–

0.0007 electron is predicted for CH3SZCHX2  CO2(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

CH3SZCHX2  CO2(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

CH3SZCHX2  CO2 (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

over-riding an increase in the occupation of ther⁄(C–H) orbitals

4 Concluding remarks

Interactions of CO2with CH3SOCH3and CH3SSCH3induce three

quite stable shapes with interaction energies from 9.6 to

14.5 kJ mol1 for both ZPE and BSSE corrections at

CCSD(T)/6-311++G(3df,2pd)//MP2/6-311++G(2d,2p) Remarkably, two quite

stable shapes of the CH3SZCH3  CO2 (Z@O, S) complexes are

revealed for the first time The interaction energies of the

CH3SZCHX2  CO2complexes range from 13.7 to 16.4 kJ mol1

for both ZPE and BSSE corrections (MP2/aug-cc-pVTZ//MP2/

6-311++G(2d,2p)) Their strength is mainly determined by the

> S@Z  C Lewis acid–base interaction, and an additional

contribu-tion of the C–H  O hydrogen bonded interaccontribu-tion with an enhanced

role in the sequence from H to F to Cl to Br derivative The

CH3SOCHX2  CO2 complexes are more stable than the

CH3SSCHX2  CO2complexes, which result from a large

contribu-tion of attractive electrostatic interaccontribu-tion of the >S@O relative to

the >S@S to the overall stabilization energy The substitution of

two H atoms in a CH3group of CH3SZCH3 by two F, Cl, Br and

CH3 alike groups makes the CH3SZCHX2  CO2 complexes more

stable, as compared to the CH3SZCH3  CO2 complexes, in going

from F via Cl, Br and finally to CH derivative

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 Uni-versiteit 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

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Table 4

NBO analyses of the CH 3 SZCHX 2   CO 2 complexes at MP2/6-311++G(2d,2p).

X EDT/electron Dr⁄

(C5–H6) D%s (C5) E(n(O12) ?r⁄

(C5–H6)) E(n(Z10) ?r⁄ (C11@O13)) 10 3

/electron electron kJ mol 1

kJ mol 1

0.3 a

0.2 a

0.11 a

0.2 a

0.3 a

0.21 a

a For C7–H14 covalent bond.

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Bielefeld, Germany, 2000

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