Solvent Effect on the Potential Energy Surfaces of the Fâ +

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Solvent Effect on the Potential Energy Surfaces of F+ CHCHBr Reaction Lopamudra Satpathy, Prabhat K. Sahu, Pradipta Behera, and Bijay K. Mishra J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02687 • Publication Date (Web): 16 Jun 2018 Downloaded from http://pubs.acs.org on June 17, 2018

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The Journal of Physical Chemistry

Solvent Effect on the Potential Energy Surfaces of F–+ CH3CH2Br Reaction

Lopamudra Satpathya, Prabhat K. Sahub, Pradipta K. Beheraa and Bijay K. Mishraa* a

Centre of Studies in Surface Science and Technology, School of Chemistry, Sambalpur University, Jyoti Vihar – 768 019, India b

Computational Modeling Research Laboratory, School of Chemistry Sambalpur University, Jyoti Vihar – 768 019, India Email: [email protected]

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ABSTRACT Although substantial works have been undertaken on reaction pathways involved in basepromoted elimination reactions and bimolecular nucleophilic substitution reaction of F- on CH3CH2X (X=Cl, Br, I), the effect of solvents with varying dielectric constants on the stereochemistry of each of the reaction species involved across the reaction profile have not yet been clearly understood. The present investigation reports the effect of solvents on the potential energy surfaces (PES) and structures of the species appearing in the reaction pathway of F- with bromoethane. The PESs in gas phase have been computed at MP2 level and CCSD(T) level. The performance of several hybrid density functional, such as B3LYP, M06, M06L, BHandH, X3LYP, M05, M05-2X, M06-2X have also been investigated towards describing the elimination and nucleophilic substitution reactions. With respect to MAE values and to make the computation cost effective, we have explored the implicit continuum solvent model, CPCM in solvents like cyclohexane, methanol acetonitrile, dimethyl sulphoxide and water. The reactant complexes proceed through the subsequent steps to produce fluoroethane as the substitution product and ethylene as one of the elimination products. For elimination reaction both syn and anti elimination have been explored. The calculated relatives energies values which are negative in gas phase are found to be positive in polar solvents since the point charge in the separated reactants are more stabilized than the dispersed charge in the transient complex, which has also been analysed through NBO analysis.


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1. INTRODUCTION In recent years, organic reaction on halo alkanes is drawing enormous research interest to exploring E2 and SN2 mechanisms.1-20 E2 is a one- step process involving simultaneous removal of a β-hydrogen and halide group in the haloalkane with the formation of a double bond. 1-7 Bimolecular nucleophilic substitution (SN2) reaction is a concerted process of bond formation and bond cleavage at the α-carbon. The reaction, initially leads to the formation of a transient complex, which follows a series of equilibria, moves to a higher energetic transition state and then suffers a sharp decrease in energy to another binuclear complex, finally leading to the formation of product.8-20 Works have been undertaken on the stereochemistry of base-promoted elimination reactions and bimolecular nucleophilic substitution reactions by different group of researchers. The investigations on the reactions of haloethanes with various nucleophiles revealed that both first and second row nucleophiles favour SN2 mechanism while E2 reactions are favoured by first row nucleophiles only.21 Recently, from an experimental analysis and theoretical calculation using MP2/aug-cc-pVDZ level of theory, Carrascosa et al. showed that the degree of methyl substitution in haloalkane substrates does not have significant change in the SN2 and E2 mechanism.22 In a reaction of n-floropropane with OMs- in ethylene glycol, the reaction barrier of E2 was found to be higher when compared to SN2 reactions.23 The study on the reactivity of ethyl chloride with a series of nucleophile (F, Cl, Br, HO, HS, HSe, NH2, PH2, AsH2, CH3, SiH3, and GeH3) revealed a systematic periodic trend for the E2 and SN2 reactions.24 The increase in electronegativity was found to stabilize the transition states significantly leading to change in the barrier height of E2 and SN2 reactions. In a reaction of F− on CH3CH2 I and CH3CH2 Cl in gas phase some more reaction pathways were proposed which may lead to Walden inverted and/or retention products, and/or anti- and syn-eliminated products.25-26 The computed 3 ACS Paragon Plus Environment


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reaction barrier for the reaction pathway of F- with bromoethane in gas phase is found to be in the order anti-E2  SN2  syn-E2. However, the study on the solvent effect on each of these species appearing across the reaction pathway for F- with bromoethane is not clear enough.27 Earlier attempts have been made to investigate the effect of solvents on the reaction profile of SN2 reactions by using several multi-level QM/MM approaches.28-31 Solvation of the neutral substrate and charged nucleophile affects the reaction profile with a variation of PESs as well as the structures of the complexes involved in the reaction process. The double well potential profile of SN2 reaction in gaseous phase changes to a unimodal one.32 The solvent effect on the reaction mechanism of substitution reaction on halomethane substrates have been studied extensively. In addition to normal backside attack for SN2 reaction, a double inversion mechanism significantly contributed by aqueous phase has been proposed by Liu et al.28 In this work we have used implicit solvent model to analyze the detail on the PESs and structures of the complexes appeared in the reaction pathways of fluoride with bromoethane. The study will also enlighten the prevalence of the structural changes on the reactant complexes and transition states due to the change in the dielectric environment. The computational investigations of this work may help the experimental organic synthesis in halo alkanes towards understanding the reaction mechanisms involved in detail, as such reactions mostly take place in solvent medium.

2. COMPUTATIONAL DETAILS The PESs of F– + CH3CH2Br in gas phase have been computed at MP233- 34 level and CCSD(T)35 level. Although the high-level ab initio approach is satisfactory in terms of accuracy and reliability, to make the computations cost effective, the performance of several hybrid density functionals: such as, B3LYP36, M0637, M06L38, BHandH39, X3LYP40-41, M0542,M05-


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2X,43 M06-2X37 have also been used towards describing the above elimination and nucleophilic substitution reactions. It has also been noted that DFT calculations usually underestimate the activation energies.44-51 Earlier report reveals that B3LYP fails to locate the transitions states associated with the anti-E2 elimination.25,52 Guner et al.53 have also reported that OLYP and O3LYP provide comparable results to B3LYP. Truhlar et al. have analysed the performance of various hybrid density functionals towards investigating the barrier heights and found that M06 and M06-2X perform the best.54 In this context, we have computed the mean absolute error (MAE) for all these considered hybrid density functionals with regard to CCSD(T) in terms of accuracy with which the PESs have been explored (Tables S1 and S2 Supporting information). It has also been noted earlier that, hybrid density functionals perform better as compared to GGA and meta-GGA density functionals in computing the barriers for the X– + CH3 CH2X (X=F, Cl) reaction, with respect to mean absolute error (MAE) values; and in specific M06 as density functional performs the best followed by M06-L and M05 with MAE values.55 The calculated MAE values (Table S2 Supporting information) in our model reaction also show M06 as most reliable hybrid density functional. Moreover, M05, M05-2X and M06-2X level of theories fail to locate all the binuclear transient complexes in the reaction profile. In our subsequent computations, we therefore considered M06/aug-cc-pVDZ level of theory for the PES of F– + CH3CH2Br in different solvents. The implicit continuum solvent model, i.e. conductor-like polarizable continuum model (CPCM)56-58 has been used to explore the potential energy surfaces for E2 and SN2 reactions of F– + CH3CH2Br. The geometry optimization, NBO analysis and the Gibb's free energy of solvation for the separate reactants59,60 binuclear transient complexes, transition state structures and products have been computed by



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using GAUSSIAN 09 (ref 61) program package. All the transition states have been confirmed by the existence of imaginary frequency through vibrational analysis along the reaction coordinates.

3 RESULTS AND DISCUSSION 3(a) Reaction leading to Walden inverted product and ethylene 3 (a) (i) Reaction in gas phase: The computed PESs at CCSD(T)/ aug-cc-pVDZ for the reaction of F - + CH3CH2Br in gas phase are shown in Figure 1. The energies of all the species appearing in the reaction profile are relative to the reactants and reported in kcal mole -1. The analysis of PES in the reaction pathway of F- on bromoethane reveals step wise processes i.e. (i) the separated reactants - bromoethane and fluoride- form a less energetic reactant complex, (ii) the complex leverages to higher energetic transition state, (iii) which leads to a product-bromide complex and finally (iv) the complex breakdown to form the substituted or eliminated product(s). The reaction profiles for the reaction of F- on chloroethane26 and iodoethane25 are observed to be similar in steps involved, based on reactant and product complexes, and transition states leading to products. A comparative analysis has been presented vide in fra.


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TS5 20.1

Anti-E2 Syn-E2 Ret-SN2 Bs-SN2 Di-SN2

TS4 22.0

0.0 +

Relative energy kcal mol-1

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 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TS3 -8.0 M2 -19.3 M1 -19.5

TS2 -14.8

TS1 -16.6

+

+

P2 -24.2

M4 -39.5

+

+

M6 -44.0 M5 -49.4

P1 -40.1

M3 -50.4

Figure 1. Schematic presentation of reaction energy profiles (kcal mol-1) for E2 and SN2 reactions of F−+ CH3CH2Br at CCSD(T)/aug-cc-PVDZ.

The computed reaction energies with respect to the SN2 and E2 processes for bromoethane and F- are found to be −40.0 kcal mol-1 and −24.2 kcal mol-1 respectively, which are in between the values obtained for chloro- (-33.2 and -19.3 kcal mol-1 ref 27) and iodoethane (-44.0 and -26.2 kcal mol-1ref 26) respectively. Though the substituted product is thermodynamically more favorable than the eliminated one, the analysis of the PES of all these complexes involved in the reaction profile is required to know in details to unveil the involved reaction mechanism. The presence of F- within a specific reaction domain of the bromoethane leads to the formation of complexes giving rise to two local minima M1 and M2 due to direct interaction (DI) and hydrogen bond interaction (HBI) respectively. The C-F inter atomic distance in the 7 ACS Paragon Plus Environment


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optimized structure of M1 is found to be 2.655 Å (Figure 2) with a relative energy of M1 to be -19.5 kcal mol-1 while in the optimized structure of M2 the C-F distance is 2.729 Å (Figure 2) and the relative energy is -19.3 kcal mol-1. These complexes (M1 and M2) transform to the higher energetic transition state, which follows subsequent steps to produce fluoroethane as the substitution product and ethylene as one of the elimination products. Szabo and Czako62 have also reported similar type of transient complexes M1 and M2 with respective relative energies, -15.6 kcal mol-1 for DI and -16.9 kcal mol-1for HBI for the reaction of F- with chloromethane. Similar trend in energy has also been reported in fluorination of chloroethane26 with respective M1 (-18.1 kcal mol-1 for DI) and M2 (-17.8 kcal mol-1 for HBI), whereas for the fluorination of iodoethane,25 a reverse trend with -19.6 kcal mol-1 for M1 and -20.1 kcal mol-1 for M2 has been reported. Figure 1 shows that the DI transient complex (M1) experiences an energy barrier of 2.9 kcal mol-1 (TS1) to form the product complex (M3) with an relative energy of -50.4 kcal mol-1, which further leads to the formation of fluoroethane and bromide ion. The energy barriers for the reactant complex to the product complex were reported to be 6.6 kcal mol-1(ref 25) and 3.2 kcal mol-1(ref 24) for the attack of F- on chloro- and iodoethane and the corresponding product complexes have the energies of -43.7 kcal mol-1 and -54.0 kcal mol-1 respectively.

169.0o 6 2.04

1.130 o

4 11 0.

109.3o

158.0o

2.655 2.0

30 A

166.5o

6 2.01

1.648

1.526

1.519 o

1.443 1.519

3.544 84.0o

1.104

M1

M2

36.8o

132.0o 2.877

M3


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0.963

166.7o

2.136

0.939 2.219 72.2o

2.219 72.2o

2.819

2.405 149.6o

121.0o

2.909

2.909

M4

2.244

TS1

166.0o

43 2.0

117.1o 114.3o

1.454

o

179.3

1.237

M5

1.668

116.5o

101.4o

o 1.515 101.1

161.2

120.5o

2.309

2.135

o

1.140

80.2o

2.530

2.007 1.508

1.331

1.535

TS2

1.975 93.3 174.1

TS3

78 1 .9

1.509

1.4 18

109.5o

1.100

o

TS4

111.0o

1.810

1.522

o

1.527

109.5o

o

0.992

TS5

109.3

1.103

CH3CH2Br

CH3CH2F



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121.4o 1.096 0.924

1.354

180.0o

0.962

H-F

C2H4

2.147

FH---Br-

Figure 2. Gas phase optimized structures of the transient complexes, transition states, productcomplexes and products at CCSD(T)/aug-cc-PVDZ. The bond lengths are in A°.

The computed bond lengths (Å) and bond angles (degrees) at CCSD(T)/aug-cc-PVDZ, relevant to understand the reaction mechanisms have been indicated in Figure 2. It has also been noted that the transient complex M2 leads to two different product complexes (M5, M6) through respective energy barrier (TS3) followed by the formation of ethylene as the syn eliminated product (Scheme 1). The elimination reaction as per scheme 1 involves the elimination of (HF, Br)- from M1 with an energy barrier of 4.7 kcal mol-1 (TS2) to lead to product complex (M4) with an energy of -39.5 kcal mol-1 which ultimately gives rise to the formation of P2 with an expense of 15.3 kcal mol-1 as the anti-eliminated product. Further M2 surmounting an energy barrier of 11.3 kcal mol-1 (TS3) leads to the formation of product complex (M5) and then to M6 for a syn-elimination. The extra energy in the barrier height for the syn-conformation may be attributed to the conformational change from the staggered to the eclipse form by C-C bond rotation. The elimination with syn-conformed energy barrier suffers a great fall in energy to the corresponding M5 with -49.4 kcal mol-1. This complex (M5) has more attuned charge dispersion on the (F--H---Br)

–

which is coplanar with ethylene as compared to the corresponding anti-

complex (M4), where both the HF and Br- reside in different planes. In this case the leaving group is coplanar while the FH is perpendicular to the ethylene plane. 10 ACS Paragon Plus Environment

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Scheme-1. Schematic diagram for syn and anti eliminated product 3 (a) (i) Solvent Effect on PES: The implicit continuum solvent model, i.e. conductor-like polarizable continuum model (CPCM)56-58 at M06/aug-cc-pVDZ has been computed to explore E2 and SN2 mechanism for the PES of F– + CH3CH2Br in solvents like cyclohexane (ɛ = 2.02), methanol (ɛ =33.0), acetonitrile (ɛ = 38.0), DMSO (ɛ = 46.7) and water (ɛ = 80.1) and the corresponding energy diagrams are presented in Supporting information Figure S1. Within the QM approach63 solvent effects can be explained by using continuum models. However, to include the dynamic effect, one can extend to explicit solvent models either with molecular dynamics simulation or with suitable QM/MM hybrid approach,63 which is beyond the scope of our present investigation. The geometries of different species involved in the reaction profiles have been presented in Supporting Information Figure S2 and Table S3. Table S4 and Figure S1 reveal that the reactant complexes, transition states and product complexes along with the corresponding products are stabilized due to change in the solvent polarity as compared to the gas phase. The major contribution to the stability is from the ion-polar interaction during the solvation of the fluoride ion. With increase in polarity (dielectric constant) of the solvents the solvation energy of F- increases. The computed energies of the different species along the reaction profiles, in cyclohexane is tabulated in Table S4 and found to be of similar trend with respect to the gas phase. However, 11 ACS Paragon Plus Environment


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there is a significant increase in the C-F distance (3.250 Å) in cyclohexane, as compared to that in gas phase (2.655 Å). Again, the decrease in F-C-C= 62.70 and H...F distance (1.825 Å) in cyclohexane medium indicates an increase in the intra-complex hydrogen bonding interaction. This decrease in distance also favors the formation of TS2 over the TS1 and leads to the increase in the eliminated product. Similar observation has also been reported earlier by Sun and DiMagno.64 It has also been noted that the C-F distance in other solvents like DMSO, acetonitrile, methanol and water are 3.616 Å, 3.704 Å, 3.610 Å and 3.626 Å respectively. With increase in polarity and hydrogen bonding ability in the solvents, the geometrical structure of the complex changes accordingly (Table S5). With increase in solvent polarity (acetonitrile: 2.064 Å, DMSO: 2.078 Å, methanol: 2.073 Å, water: 2.087 Å), the H...F distance increases and thus the possibility of cleavage of intra-molecular hydrogen-bond increases. The calculated relative energies for the M1 complex with respect to the reactant in all different solvents are tabulated in Table 1. Table 1 shows that the relative energies values are positive in polar solvents since the point charge in the separated reactants are more stabilized by the polar solvent, than the dispersed charge in the transient complex. This proposition gets supports from the NBO analysis.


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Table-1 Calculated relative energies, EM1 and EM2 for M1 and M2 in different solvents in kcal mol-1

For anti-E2

Dielectric constant()

Relative energies EM1

EM2

gas phase

1.0

-19.5

-19.3

cyclohexane

2.02

-3.11

-3.28

methanol

33.0

5.16

4.13

acetonitrile

38.0

5.27

4.07

DMSO

46.7

5.25

4.15

water

80.1

5.22

4.23

Table 1 reveals that EM2 is less energetic than E M1 in gas phase, while it is opposite in presence of solvents. The deviation in energy (E M1 ~ EM2) in cyclohexane medium is found to be 0.17 kcal mol-1 while for polar solvents the deviation is within 1-2 kcal mol-1. It suggests that the charge in M1 is relatively more localized or less dispersed (where F- links to C) as compared to M2 (F- links to H). Further the change in energy with regard to the change in solvent polarity is in the range of 17.0 – 33.43 kcal mol-1. In the gas phase, TS1, TS2 and TS3 are less energetic than that of the reactant (ETS being negative) while in solvents the ETS becomes positive. With increase in dielectric constant of the solvent, the computed energies of the transient complexes increase sharply and then level up (Supporting Figure 3). Bogdanov and MacMahon65 have also observed similar result for the chlorination of chloromethane, which has also been supported by experimental results.66 13 ACS Paragon Plus Environment


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The computed reaction profiles indicate that the SN2 substitution product with Walden inversion is generated from the product complex, M3. The calculated energy values of M3 vary significantly (-50.4 kcal mol-1 in gas phase to -15.97 kcal mol-1 in aqueous phase), which may be attributed to the differential polarization of charge in M3. Again, the change in solvation energy varies from 9.78 kcal mol-1 in cyclohexane medium to 18.93 kcal mol-1 in water medium. The differential charge polarization of the halides due to solvation results in significant structural variation in M1. The Br-C-F of M1 in gas phase is found to be 190.9 o convexity, while in solvent medium it is 175.45 o (cyclohexane), 172.05 o (DMSO), 172.06o (methanol), 171.99o (water) 169.03o (acetonitrile) concavity with respect to the methyl group. The CH...F distance is minimum in cyclohexane and maximum in water (2.087 Å) indicating a weakening of H...F interaction due to increase in polarity of the solvent. We have carried out several calculations to optimize the saddle points on the IRC obtained by changing the bond lengths and angles of the TS1 in different solvents and the optimized structure of the TS1 is found to be unaltered, though there is a significant change in the energy (-16.6 kcal mol-1 in gas phase while 16.83 kcal mol-1in water medium) due to the change in the energy of the solvated reactant. Kuechler and York67 have rationalized the variation in solvent assisted change in energy among reactant/product and the transition states by considering an increased solvent-excluded space for the transition states compared to the reactant/product. Further, the transition states have less dipole moments due to some symmetric characteristics when compared to the reactant/product and their ion-dipole complexes, which may subside the effect of solvent on the transition state structure. The dispersion of charge in the transition state results in the decrease of the dipole moment. The difference of NBO charges on 14 ACS Paragon Plus Environment

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Cl and F is 0.349 for the transition state, while 0.732 for the reactant complex and 0.523 for the product complex. A large difference in the charge with flexibility in the bond may perturb the structure in presence of solvent, but as the transition state is more rigid compared to the reactant/product complexes (having more localized charge on the F or Br), the effect of solvent on its structure will be less. To have a molecular concept on the solvent effect on the TS1, we have calculated the energies and analyzed the structures in presence of water molecule by considering many body interaction. The optimized structures of the saddle points with up to two water molecules have the energies in-between the gas phase and that calculated in presence of bulk water (Figure 3). In a recent paper, Laloo et al.32 have also noted a systematic energy variation due to increase in solvation i.e. from gas phase to microsolvated to bulk solvation. The optimized transition states are confirmed from the imaginary frequencies: -345.01, -167.10, -70.67 and -444.75 cm-1 for zero, one, two and many water systems respectively. Adamovic and Gordon have carried a systematic calculation on Cl- +nH2O + CH3Br → CH3Cl + Br- + nH2O by using various functionals. They have calculated the energy of the central barrier with 4 water molecules and found that the structure of the TS is similar with that as in the gas phase.



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Figure 3. Plot of energy of the TS1-(H2O)n complex vs. number of water molecule (n) The HBI interaction leads to the hydrogen bonded reaction complex (M2), where solvent plays a vital role in structure modification. The hydrogen bond interaction decreases with increasing polarity of the medium and thus the H...F distances increase with increasing dielectric constant of the solvent (Table S5 supporting information). Further, the F - is found to be localized differentially with respect to the CH2Br cone of bromoethane (Figure 4). In gas phase it remains 13.1o (H-C-F) and in cyclohexane 12.8 o outside the cone while in methanol it remains almost at the surface of the cone (H-C-F= 0.7o). In more polar solvents like acetonitrile, DMSO and water, F- resides inside the cone. The C-F distance increases in the same trend to take care of the electron density in the CH2Br cone. Inside the cone, the C-F distance is more than 3.00 Å while outside the cone it is less than that. However, the energy of M2 is more than the reactant in polar solvents while it is less in gas phase and cyclohexane.


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Figure 4. Structure of M2 describing the CH2Br cone for the approach of F-

In the reaction profile for ethylene as the elimination product, M1 moves to TS2 with an energy barrier of 4.7 kcal mol-1 which is required for the change of position of F - from C-H to C-H. In nonpolar solvent like cyclohexane, this transfer requires energy of 4.8 kcal mol-1 while in polar solvents it requires 10.8-12.1 kcal mol-1. The geometry of TS2 is a transition between a tetrahedral and planar system, experiencing a shortening of C-C bond from 1.526 to 1.454 Å in M2. There is no significant effect of the solvent on the geometry of the TS2. The synelimination pathway witnesses a transition state (TS3) different from that of anti-elimination pathway (TS2) with respect to the orientation of both nucleophile and leaving group. In gas phase, the TS3 is 6.8 kcal mol-1 more energetic than that from TS2. This difference may be due to the C-C bond rotation during the transformation of M2 to TS3. Further, the interatomic distance of C-F is less and C-Br is more in TS3 than TS2. The change in solvent polarity does not have any effect on the structure of TS3 except there is an elongation of CH...F bond distance up to 4.54 Å. Similarly the product complex M4, descended from TS2 does not suffer any change except there is an increase in the distance of Br from the ethylene.



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The product complex, M5 appearing in the anti elimination pathway has the ethylene unit parallel to the F...H...Br complex. In solvents, the geometry of the complex with respect to the interaction of ethylene unit and the [F...H...Br]- changes significantly having CHBr = 130.6 in water medium. The F...H...Br complex moves linearly in the same plane of ethylene unit to an angular distance of 29.6o. Accordingly the C...Br distances are 3.876 and 4.475 Å in gas phase while 4.012 and 4.082 Å in water medium (Figure 5). In F...H...Br complex also the H...Br distance shortens to almost 4.0 Å in presence of solvents. These changes increase the energy from -49.4 kcal mol-1 in gas phase to -13.6 kcal mol-1 in water medium. In solvent medium, there is no much change in geometry of F...H...Br except the interatomic distance of H.....Br, which elongates from 2.148 Å in gas phase to 2.292 Å in water. Due to differential solvation of F...H...Br and ethylene separately, the energy of M6 increases from -44.0 in gas phase to -20.0 kcal mol-1in water medium.

Figure 5. Shifting of F…H…Br complex in M5 due to water solvation.


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3 (b) Reaction leading to racemic and retention product; (i) Reactions in gas phase: It is well known that the SN2 reaction is characterized by the Walden inverted product, which can be visualized by considering a chiral reaction centre. However, with appropriate orientation of the nucleophile and leaving group in different species (transition states and transient complexes/product complexes) in the reaction profile, retention and racemic products can also be obtained. The complex M2 (Figure 1) leads to the formation of the non inverted or retention product following the transition state (TS4) and the product complex (M3). TS4 is a highly energetic transient species (ETS4 22.0 kcal mol-1) mostly due to the steric interaction and electrostatic repulsion. The possible structural changes in M2 are due to the movement of F - in both the direction of αC-H bond moving towards βC-H engendering to TS2 and moving away from βC-H gives rise to TS4. The highly energetic TS4 descends down to the product complex M3 by releasing an energy of 72.4 kcal mol-1. The three species M2, TS4 and M3 have almost similar geometry with respect to hydrocarbon skeleton. The significant structural changes leading to drastic difference in energy are due to the orientations of attacking species and the leaving group. The corresponding F-CBr are 120o, 80o and 166o with an interatomic separation between F and Br of 4.162 Å, 2.972 Å and 4.959 Å in M2, TS4 and M3 respectively. Recently, Szabo and Czako62 have also reported a double inversion mechanism, where the reaction starts with an abstraction-induced inversion leading to a transition state, which further recedes to the product complex of Walden inversion through a structure of TS1. In the reaction process, F- drags a proton from the αC and converts it to a prochiral centre by changing



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the tetrahedral centre to a relatively planar one. Thus, the attack of F- to αC may lead to an inverted or retention product. For such product to form, the TS energy for the reaction of F - on chloroethane has been computed as 18.0 kcal mol-1(ref 25). We have also obtained a similar transition state with a calculated energy of 20.1 kcal mol-1, which is lower than that of the transition state developed from same side attack. This transition state leads to the Walden inverted product complex by releasing around 70.5 kcal mol-1. (ii) Reactions in solvent phase: The solvated TS4 is found to be more energetic as compared to gas phase, which may be attributed to the decrease in energy of the reactant due to the solvation of the halides. The analysis of the geometry of TS4 and M3 in both gas phase and solvent medium reveals that (i) the halide ions are pulled away from αC by solvent to a distance of 3.0-5.0 Å, (ii) though the FαC-Br (=80-82o) is not changed significantly, the H-αC-F is squeezed in the solvent medium (=70o in gas phase and 50o in water medium) and (iii) deviates more from that of the reaction complex; the H-αC-F being 106o -107o in M3 in both gas phase and water medium. In the double inversion mechanism the transition state (TS5) has a prochiral centre and the solvent polarity does not have much change on the structure of the transition state. However, there is a shortening of αC...H-F bond and the interatomic distance of F and Br due to solvation. These structural variations increase the energy of the TS5 significantly in presence of solvent. 3 (c) NBO Charge analysis: The charge distribution in the molecule is also responsible for guiding the reaction pathways in any bimolecular reaction. We have also analyzed the NBO charge distribution in the reactant, product, transient complexes and the transition states involved in the reaction pathway in different solvents and are presented in supporting information Figure 3. The solvent polarity 20 ACS Paragon Plus Environment

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does not affect much on the overall charge distribution in the bromoethane, except on a βH (staggered to Br) and αC and Br. The absolute charge increases in F and βH while decreases in αC with increase in polarity of the solvent. Due to less ionizability of the product (fluoroethane), compared to bromoethane, the solvent polarity does not have any effect on the charge distribution. The charge on αC is in the range of -0.472 to - 0.489 in bromoethane while it is +0.057 - +0.062 in fluoroethane. The charge distribution changes significantly for M1 as compared to that of the reactant species. Moreover, major changes have also been observed for αC and β-H staggered to bromine, which may be ascribed to the approach of F- to the αC in a specific orientation. The interaction of F - with the β-H has increased the charge on β-H from 0.233 to 0.326 indicating the possibility of a nonconventional hydrogen bonding. This type of hydrogen bonding is more prominent in case of M2, where F - links to αH leading to a significant increase of charge from 0.258 in the reactant to 0.364 in the product. The presence of polar solvent weakens the hydrogen bonding which is reflected in the decrease of the charge. 4. Conclusion The present investigation includes the implicit continuum solvent model, CPCM on reaction pathways involved in base-promoted elimination reactions and bimolecular nucleophilic substitution reaction of F- on CH3CH2Br. Effort has been made to understand the effect of solvents on the stereochemistry of each of the reaction species involved across the reaction profiles. The potential energy surfaces (PES) of F– + CH3CH2Br in gas phase have been computed at CCSD(T) level but to save computational time, the performance of several hybrid density functionals: B3LYP, M06, M06L, BHandH, X3LYP, M05, M05-2X, M06-2X have also been explored with respect to their MAE values. All the solvent phase PESs have been determined at M06/aug-cc-pVDZ. Continuum models can be used to describe solvent effects 21 ACS Paragon Plus Environment


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within the QM approach; however, to include the dynamic effect, one can extend to explicit solvent models either with molecular dynamics simulation or with suitable QM/MM hybrid approach. Basing on the transient complexes and transition states, the computed gas phase reaction profiles are found to be qualitatively similar as obtained for the fluorination reaction on chloroethane26 and iodoethane25. The computed reaction energies with respect to the SN2 and E2 processes for bromo substrate are found to be in between the values obtained for chloro substrates26 and iodo substrates25. The reactant complexes (M1 and M2) proceed for the formation of the transition states and following subsequent steps produce fluoroethane as the substitution product and ethylene as one of the elimination products. Besides, syn or anti elimination has also been explained through M2 leading to different product complexes (M4, M5, M6) surmounting respective energy barriers (TS2, TS3). In solvent medium, the major contribution to the stability of the reactant complexes, transition states and product complexes are due to change in the solvent polarity and the ionpolar interaction during the solvation of the fluoride ion. However, due to very low dielectric constant, in cyclohexane medium, the intra-complex hydrogen bonding interaction increases and hence favors the formation of TS2 over the TS1 and leads to the increase in eliminated product. Similar observation has also been reported earlier by Sun and DiMagno.64 With increase in polarity and hydrogen bonding ability in the solvents, the geometrical structure of the complexes changes accordingly and the possibility of intra-molecular hydrogen-bond breaking also increases. The calculated relative energies values are found to be positive in polar solvents since


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the point charge in the separated reactants are more stabilized by the polar solvent, than the dispersed charge in the transient complexes, which has also been confirmed subsequently by NBO analysis. With increase in dielectric constant of the solvent, the computed energies of the transient complexes and the TSs increase sharply and then level up for polar solvents, which is in line with Bogdanov and MacMahon65 observation for the chlorination of chloromethane and experimental results.66 In the double inversion mechanism the transition state (TS5) has a prochiral centre and the solvent polarity does not have much change on the structure of the transition state. However, there is a shortening of αC...H-F bond and interatomic distances of F and Br due to solvation. It will be of interest to study the dynamics of elimination reactions and bimolecular nucleophilic substitution reaction of F- on CH3CH2Br by extending to explicit solvent models and such dynamics simulations are currently undergoing. SUPPORTING INFORMATION The Supporting Information includes potential energy surfaces of different transient species calculated by using different functionals in gas phase and in different solvents; bond lengths, bond angles and NBO charges in these species. ACKNOWLEDGEMENT LS and BKM acknowledge the financial assistance of University Grants Commission, New Delhi through Basic Science Research and Emeritus Fellowships respectively. The authors thank Prof. A. N. Panda Department of Chemistry, Indian Institute of Technology, Guwahati, India for providing some computational facilities.



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Solvent Effect on the Potential Energy Surfaces of F –+ CH3CH 2Br Reaction Lopamudra Satpathya, Prabhat K. Sahu b, Pradipta K. Beheraa and Bijay K. Mishraa* a

Centre of Studies in Surface Science and Technology, School of Chemistry, Sambalpur University, Jyoti Vihar – 768 019, India b

Computational Modeling Research Laboratory School of Chemistry,

Sambalpur University, Jyoti Vihar – 768 019, India

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Solvent Effect on the Potential Energy Surfaces of the Fâ +

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