The XâCÂ·Â·Â·Ï (X = F, Cl, Br, CN) Carbon Bond - ACS Publications

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The X-C•••# (X=F/Cl/Br/CN) Carbon Bond Devendra Mani, and Elangannan Arunan J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp507849g • Publication Date (Web): 07 Oct 2014 Downloaded from http://pubs.acs.org on October 10, 2014

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The Journal of Physical Chemistry

The X-C•••π (X=F/Cl/Br/CN) Carbon Bond Devendra Mania,b and Elangannan Arunana a. Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, India-56001. b. Current Address: Department of Physical Chemistry II, Ruhr-Universität Bochum, Universitätsstraße 150, D-44780 Bochum, Germany. *Corresponding author Devendra Mani Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, India-560012 Email: [email protected] or [email protected] Contact: +49-15213417867

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ABSTRACT

High level ab initio calculations have been used to study the interactions between CH3 group of CH3X (X=F/Cl/Br/CN) molecules and π-electrons. These interactions are important due to the abundance of both the CH3 groups and πelectrons in biological systems. Complexes between C2H4/C2H2 and CH3X molecules have been used as model systems. Various theoretical methods like atoms in molecules theory, reduced density gradient analysis and natural bond orbital analysis have been used to discern these interactions. These analyses show that the interaction of the π-electrons with the CH3X molecules leads to the formation of X-C•••π carbon bonds. Similar complexes with other tetrel molecules, SiH3X and GeH3X, have also been considered.

KEYWORDS Hydrogen bonding, intermolecular interactions, non-covalent interactions, van der Waals interactions, Atoms in molecules analysis, Reduced density gradient analysis.


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1. INTRODUCTION Non-covalent interactions have been one of the central themes of investigations, both experimentally and theoretically, for the last few decades.1 These investigations have resulted in a better understanding of some already well known non-covalent interactions like hydrogen bonding and halogen bonding. These have also resulted in the identification of several new non-covalent interactions.2-4 Ever growing knowledge of a vast number of acceptors and donors, which can participate in the hydrogen and halogen bonding, prompted IUPAC to redefine these important interactions.5-7 Studies on the other recently identified interactions, like pnicogen bonds,3,8-10 chalcogen bonds,2,11-13 carbon bonds, a sub-set of tetrel bonds4,14-18etc., have also been gaining momentum and now much more is known about these newer interactions than ever before. Experimental and computational studies often complement each other. One such study guided us to the discovery of carbon bond. The discovery was an offshoot of our investigations on the Ar•••Propargyl alcohol complex by microwave spectroscopy and quantum chemical calculations.19 Global minimum for the complex could be observed experimentally and it exhibits Ar•••H-O and Ar•••π interactions. However, theory predicts another secondary minimum for the complex, which shows an unusual Ar•••C bond. Analysis of this secondary minimum structure eventually led us to the carbon bonding interactions. Carbon bond is little more than one year old now. There have been many studies on carbon bonding since it was first proposed. Different types of carbon bonds are now known in literature.4,16,20-22 Moreover, carbon bond has also been confirmed experimentally using X-ray crystallography.14



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The carbon bond can be represented as X-C•••Y, where X-C is the carbon bond donor fragment and Y is the carbon bond acceptor. X is any atom or group which is more electronegative than carbon, such as OH/F/Cl/Br/NO2/NF2 etc., and Y is an electron rich centre, like a lone pair of electrons.22 Carbon bonding with lone pair of electrons as acceptors has been studied in detail.4,22 Carbon bonding with radicals (single electron donor) has also been reported in literature.16 However, there has been no study yet in which such interactions with π-electrons were considered. Since π-electrons are well known hydrogen,1,23-25 halogen26 as well as pnicogen bond27 acceptors, the natural question is, ‘can

π-electrons act as carbon bond acceptors?' Moreover, both the methyl groups as well as π-electrons are present in abundance in biological systems and drive many processes which are responsible for the very existence of life. The methyl and other hydrophobic hydrocarbon moieties are well known to be responsible for physiological phenomena like protein folding. Similarly, the π-π stacking interactions are the main factors for the stability of the double helical structure of DNA. Even though both the CH3 moieties and π-electrons are present in biological systems, interactions between them remain somewhat unexplored till date. Is there any interaction between the methyl group and π-electrons? Do X-C•••π carbon bonds, like the X-H•••π hydrogen bonds, exist? In this manuscript, we have tried to answer these questions by considering complexes of several CH3X (X=F/Cl/Br/CN) molecules with potential π-electron donors: ethylene and acetylene. Interactions similar to carbon bonding are known for the other atoms of group IV (Si and Ge) as well. Though these interactions were known for long,28,29 these have recently


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been named as tetrel bonds.15,18 We have also considered the interactions between SiH3X/GeH3X and acetylene/ethylene in this work. Results from high level ab initio quantum chemical calculations are presented in the upcoming sections. Other analyses like atoms in molecules (AIM), reduced density gradient (RDG) and natural bond orbital (NBO) have also been performed to gain more insights into the nature of these interactions. 2. COMPUTATIONAL DETAILS All the calculations were performed using Gaussian 09 software suite.30 The ab initio Moller-Plesset second order perturbation theory (MP2), which is considered suitable to take electron correlation into account, was used to perform geometry optimization calculations. Large basis sets such as Pople’s triple zeta basis set 6-311+G(3df,2p) and Dunning’s correlation consistent augmented triple zeta basis set Aug-cc-pVTZ were used to perform these calculations. Harmonic frequency calculations were performed to confirm that the optimized geometries are minima on the potential energy hypersurface as well as to obtain the zero-point vibrational energy. Binding energies for the complexes were obtained using the supermolecular approach in which sum of the energies of the constituting monomers is subtracted from the total energy of the complex. The binding energies obtained in this way were corrected for the basis set superposition error (BSSE) using counterpoise method31 which is inherent in Gaussian 09. The binding energies were further corrected for the zero-point vibrational energies and 'BSSE+zero-point corrected' binding energies were obtained. To get a better estimate of the binding energies, single point counterpoise calculations were also performed at the CCSDT/Aug-cc-pVTZ level. These calculations were performed on the optimized geometries at



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MP2/Aug-cc-pVTZ level. Atoms in molecules calculations were done using the AIMAll program.32 Natural bond orbital analysis (NBO) was performed using NBO 6.0 software.33 Electrostatic potential (ESP) analysis and reduced density gradient analysis were performed using Multiwfn software.34 The ESP surfaces were plotted using GaussView software.35 Wave functions for these analyses were extracted from the corresponding Gaussian calculations. 3. RESULTS AND DISCUSSIONS 3.1. Geometry optimization, bonding parameters and binding energies Electrostatic potential for a typical CH3X molecule is shown in Figure 1. In these molecules, the presence of the more electronegative X moiety polarizes the C-X bond and due to this there exists a region of positive electrostatic potential, termed σ-hole, in the opposite direction.29,36 The electrostatic potential extremum points at 0.001 a.u. electron density surface were located using Multiwfn software.34 The calculations show a presence of a maximum point at the face centre of CH3 group for all the molecules. The ESP values at the surface maximum point (Vmax) for different CH3X molecules are also given in Figure 1. Initial geometries, for the optimization of the C2H4/C2H2•••CH3X complexes, were taken in such a way that the sigma hole of the CH3X molecule faces π-electron density of ethylene/acetylene. All the geometries were confirmed to be minima by obtaining real harmonic frequencies for all the normal modes. Geometrical parameters for the expected XC•••π interactions are given in Table 1. Frequency shifts of the C-X stretching vibration upon complex formation are also given.


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Figure 1. Electrostatic potential for a typical CH3X molecule, mapped on the 0.001 a.u. surface of electron density. Color code bar is shown on the right hand side. The ESP values at surface maxima at the CH3 face centre are also given. Table 1. Structural and spectroscopic parameters for the optimized geometries of the C2H4/C2H2•••CH3X complexes at MP2/6-311+G(3df,2p) and MP2/Aug-cc-pVTZ levels. The distances are in angstrom, angles in degree and frequencies in cm-1.

MP2/6-311+G(3df,2p)

MP2/Aug-cc-pVTZ

Complex

R(C...πcentre)

∆r (C-X)*

∠ (X-C...πcentre)

∆ν (C-X)*

R(C...πcentre)

∆r(C-X)*

∠(X-C...πcenter)

∆ν (C-X)*

C2H4•••CH3F

3.4253

0.0025

179.7˚

-9.7

3.4004

0.0023

179.8˚

-9.8

C2H2•••CH3F

3.4375

0.0023

176.3˚

-8.5

3.3931

0.0022

175.9˚

-8.8

C2H4••• CH3Cl

3.4740

0.0015

179.0˚

-5.1

3.4512

0.0018

179.9˚

-5.8

C2H2••• CH3Cl

3.4830

0.0010

176.1˚

-3.3

3.4408

0.0016

174.4˚

-4.1

C2H4••• CH3Br

3.4799

0.0021

179.1˚

-4.1

3.4239

0.0016

179.3˚

-3.9

C2H2••• CH3Br

3.4686

0.0016

176.1˚

-2.9

3.4180

0.0013

176.6˚

-2.8

C2H4••• CH3CN

3.5619

0.0002

178.7˚

-1.6

3.5610

0.0001

178.7˚

-2.0

C2H2••• CH3CN 3.5546 0.0002 171.3˚ -0.5 3.5328 0.0001 171.0˚ -1.5 *∆r(C-X) is the change in the C-X bond length upon complex formation. Positive values represent bond elongation upon complex formation. Negative values of the C-X frequency change, ∆ν(C-X), represent red shifts in these frequencies upon complex formation.

The C•••π-centre distances (R) are shorter at the MP2/Aug-cc-pVTZ level as compared to the MP2/6-311+G(3df,2p) level. However, at both the levels, the C•••π-centre distances are 7 ACS Paragon Plus Environment


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much longer for the CH3CN complexes than for the other CH3X complexes. Such longer contacts have earlier been observed in the case of hydrogen bonded X•••H-CN and halogen bonded X•••X’-CN (X’=Cl/Br) complexes as well.37 Why the π-moieties are not able to come as close to the carbon σ-hole of CH3CN as in the case of other CH3X complexes? Is the carbon atom more covered in CH3CN molecule than the other CH3X molecules under consideration which hinders the approach of the other molecules? To know this we decided to look at the X-C-H angle in the different CH3X molecules. This angle is 108.7, 108.3, 107.9 and 109.9 for CH3F, CH3Cl, CH3Br and CH3CN molecules respectively. The larger value for CH3CN molecule suggests that the carbon atom is more buckled in CH3CN than the other CH3X molecules. Therefore, other interacting molecules can not approach the carbon atom of CH3CN as close as it is possible in case of other CH3X molecules. To explore it further, we calculated the non-bonding radii of the carbon atom in the CH3X molecules. The non-bonded radius for the carbon atom in CH3F, CH3Cl, CH3Br and CH3CN are 1.756 Å, 1.805 Å, 1.826 Å and 1.824 Å respectively. These radii were calculated as the minimum distance between the carbon atom and the 0.001 a.u. electron density surface of the corresponding CH3X molecule using the Multiwfn34 software. The non-bonded radius of C in CH3CN is longer than that in CH3F by 0.068 Å which explains, to a certain extent, the observance of longer C•••π contacts in the CH3CN complexes as compared to the CH3F complexes. However, the non-bonded radius alone does not explain why the contacts in CH3CN complexes are longer than CH3Br complexes even though the non-covalent radius of C in CH3Br is longer than that in CH3CN. This can be explained on taking the X-C-H angle, which is larger for CH3CN than CH3Br, also into account as discussed before. Thus, we can say that the larger X-C-H angle, which makes the carbon atom less approachable,


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and larger non-bonded radius of carbon in CH3CN are collectively responsible for the larger intermolecular contacts observed in CH3CN complexes than in other CH3X complexes. In all the complexes, the C-X bond distance increases upon complex formation, though the increase is very less in the case of the complexes with CH3CN. All the complexes show a red shift in C-X stretching frequency upon complex formation at both the levels of theory. Binding energies for all the complexes at MP2/6-311+G(3df,2p), MP2/Aug-cc-pVTZ and CCSD-T/Aug-cc-pVTZ levels are given in Table 2. Table 2. BSSE corrected (∆EBSSE) and BSSE as well as Zero-point corrected (∆E(BSSE+ZPC)) binding energies for the C2H4/C2H2•••CH3X complexes at different levels of theory. The values are in kJ.mol-1.

Complex

MP2/6-311+G(3df,2p)

MP2/Aug-cc-pVTZ

CCSD-T /Aug-cc-pVTZ

∆EBSSE

∆E(BSSE+ZPC)

∆EBSSE

∆E(BSSE+ZPC)

∆EBSSE

C2H4•••CH3F

-4.8

-3.0

-5.4

-3.6

-5.1

C2H2•••CH3F

-4.6

-3.1

-5.2

-3.7

-4.7

C2H4•••CH3Cl

-4.8

-3.3

-5.3

-3.8

-4.7

C2H2•••CH3Cl

-4.5

-3.4

-5.1

-3.9

-4.4

C2H4•••CH3Br

-4.7

-3.3

-5.2

-3.8

-4.5

C2H2•••CH3Br

-4.4

-3.4

-5.0

-3.9

-4.2

C2H4•••CH3CN

-4.9

-3.5

-5.3

-4.0

-5.0

C2H2•••CH3CN

-4.9

-3.9

-5.3

-4.3

-5.0

The complexes are more stable at the MP2/Aug-cc-pVTZ level as compared to the MP2/6311+G(3df,2p) level as is clear from the binding energy values. This is in accordance with the shorter C•••π distances observed at the MP2/Aug-cc-pVTZ level, Table 1. The binding energy pattern for these complexes is not exactly the same as the pattern of the ESP values at the CH3 face centre of the bond donor CH3X molecule. It is clear that the electrostatic interactions are not enough to explain the interactions in these complexes and 9 ACS Paragon Plus Environment


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there is much more to it.

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This is not surprising, as both the acceptors, ethylene and

acetylene, have zero dipole moment and hence the dominant dipole-dipole interaction is absent in these complexes. The acceptors do have a quadrupole moment. Hence, quadrupole-dipole, induction, dispersion and exchange-correlation interactions must contribute to the binding energy. To understand these interactions better, we decided to find out the correlation energy contribution towards the stability of these complexes. The MøllerPlesset second order perturbation theory (MP2) recovers good amount of correlation energy when used with a large basis set. For a particular geometry, difference between the MP2 energy with a particular basis set and the Hartree-Fock energy at the same basis set can be taken as the correlation energy contribution. The energy obtained in this way is by no means the complete correlation contribution however it does represent a large fraction of it. For all the complexes under consideration, the correlation energy contribution was calculated in this way. The energy values at the HF and MP2 levels for the complexes as well as the constituting monomer units were extracted from the geometry optimization calculations at MP2/Aug-cc-pVTZ level. These energies were used to calculate the binding energies at the HF level and the MP2 level using the earlier mentioned supermolecular approach. The values are given in Table 3. It should be noted that these values have not been corrected for the basis set super position error. For the sake of comparison, values for the carbon-bonded geometries of H2O•••CH3F and H3N•••CH3F, in which interactions are with lone-pair of electrons4 have also been included.


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Table 3. Correlation energy contributions for the C2H4/C2H2•••CH3X complexes, optimized at MP2/Aug-ccpVTZ level, are listed. The energy values are in kJ.mol-1.

Complex

∆E(HF)

∆E(MP2)

C2H4•••CH3F

2.6 2.0 2.7 2.1 3.5 2.7 1.2 0.7 -3.9 -3.8

-6.9 -6.3 -6.8 -6.2 -7.8 -7.0 -6.6 -6.4 -8.4 -9.0

C2H2••• CH3F C2H4••• CH3Cl C2H2••• CH3Cl C2H4••• CH3Br C2H2••• CH3Br C2H4•••CH3CN C2H2•••CH3CN H2O•••CH3F H3N•••CH3F

Correlation energy [∆E(MP2)- ∆E(HF)] -9.5 -8.3 -9.5 -8.4 -11.2 -9.8 -7.8 -7.1 -4.5 -4.5

Positive sign of the binding energy means that energy of the complex is more than the sum of energies of the constituting units i.e. the complex is not bound. It is clear that none of the complexes with C2H4/C2H2 are bound according to the HF energies. It should be noted that the HF energies are for the geometries optimized at the MP2 level of theory. However, all the complexes are bound according to the MP2 energy values. For the H2O•••CH3F and H3N•••CH3F complexes, even the HF binding energies are negative and show them bonded. Among the C2H4/C2H2•••CH3X complexes, complexes with CH3CN are the most stable ones considering only the HF binding energies. This is in accordance to the maximum ESP value for the CH3CN σ-hole. It should be noted that this is even after the presence of much longer C•••π contacts in the CH3CN complexes. The calculated correlation energy contribution is the most in the case of CH3Br complexes and the least in the case of CH3CN complexes. Maximum correlation contribution in CH3Br complexes is due to the bigger size of the bromine atom. The least value of the correlation contribution for the CH3CN complexes is



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again due to longer C•••π contacts. It is clear that in the case of these complexes, HartreeFock theory is not suitable. One must use at least MP2 level to include the dispersion contributions, which play a significant role in the stability of these complexes. The values for the H2O•••CH3F and H3N•••CH3F complexes imply that electrostatic interactions dominate much more in these complexes than in the C2H4/C2H2•••CH3X complexes. This is not surprising since both H2O and NH3 have finite dipole moments along the direction of the intermolecular bond. Moreover, in these complexes, only one atom, O or N, effectively interacts with the CH3 face and thus, the dispersion contribution is much less. In contrast to this, neither C2H4 nor C2H2 have a net dipole moment and their interaction with CH3X is quadrupole-dipole type of interaction. Moreover, in these systems, all the atoms of the C2H4/C2H2 molecule are in close proximity to the CH3 face which results in more dispersion contribution.


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3.2. Atoms in molecules analysis (AIM) Atoms in molecules (AIM) theoretical analysis was performed on all the complexes. Initially, wave functions of the optimized geometries at MP2/Aug-cc-pVTZ level were used for these analyses. However, an undesired non-nuclear attractor (NNA) point was found to be present at the π-bond centre of acetylene in all the C2H2•••CH3X complexes. Such an NNA was also present in the case of monomer C2H2 while using the wave function obtained from the MP2/Aug-cc-pVTZ level. In such cases, the calculations were performed by putting a dummy atom (zero charge, zero atomic number and zero mass) at the coordinates of the NNA. Further, to see the effect of change of basis sets, calculations were also performed using wave functions of the optimized geometries at MP2/6-311+G(3df,2p) level. With these wave functions no NNA was found to be present. Such basis set artifacts of the AIM theory are not new and have been noticed earlier also.38 The electron density topologies resulting from the AIM analysis are shown in Figure 2a-h. The topologies are shown for the calculations using MP2/6-311+G(3df,2p) level wave functions.



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Figure 2. Electron density topologies for the C2H4/C2H2•••CH3X complexes are shown. Bond critical points are shown with green dots.

In each of the complexes, an intermolecular bond critical point is present. An intermolecular bond path connects this bond critical point to carbon atom of the CH3X molecule and one of the carbons of the C2H4/C2H2 molecule. The bond path becomes curved near the π-electron


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density and finally ends up into one of the carbon atoms of C2H4/C2H2. Such curved bond paths are common in case of the complexes involving π-electron density and have been reported earlier in the case of many hydrogen bonded complexes in which C2H4/C2H2 act as π-electron donors.37,39 Values for the electron density (ρ) and Laplacian of electron density (∇2ρ) at the intermolecular bond critical points are given in Table 4. Electron density values are within the range defined for the C-H•••O hydrogen bonds.40 Laplacian of electron density values are positive for all the complexes. It suggests that these interactions are closed shell interactions. However, Cremer and Kraka have shown that only the sign of the Laplacian is not sufficient to distinguish between closed shell and shared shell interactions.41 They suggested that other properties, like potential (V) and kinetic (G) electron energy densities, should also be considered for such classifications. Espinosa et al. proposed that for shared shell interactions |V|/G ratio should be more than 1 and for closed shell interactions it should be less than 1.42 Another criterion was proposed by Sosa and coworkers which is based on the eigenvalues of the Hessian matrix at the BCP.43 According to this criterion, |λ1|/λ3, the ratio of first and third eigenvalues of the Hessian matrix, should be greater than 1 for the shared shell interactions. Shahi and Arunan have recently pointed out that this ratio is less than 0.25 for the closed shell interactions.37 These ratios are also given in Table 4 for all the complexes under consideration. The values were calculated at the intermolecular bond critical point. It can be seen that for all the complexes |V|/G ratio is less than 1 and |λ1|/λ3 ratio is less than 0.25. This confirms that the intermolecular interactions present in these complexes are closed shell interactions.



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Table 4. Values of the different properties,* resulting from AIM analysis, at the intermolecular bond critical point are listed. All values are in atomic units. MP2/6-311+G(3df,2p) Complex

MP2/Aug-cc-pVTZ

ρ

∇2ρ

|V|/G

|λ1|/λ3

ρ

∇2ρ

|V|/G

|λ1|/λ3

C2H4•••CH3F

0.0046

+0.0168

0.27

0.06

0.0048

+0.0183

0.26

0.07

C2H2••• CH3F

0.0044

+0.0158

0.28

0.06

0.0048

+0.0188

0.26

0.07

C2H4••• CH3Cl

0.0045

+0.0159

0.28

0.06

0.0047

+0.0173

0.27

0.06

C2H2••• CH3Cl

0.0043

+0.0151

0.28

0.05

0.0046

+0.0176

0.26

0.07

C2H4••• CH3Br

0.0046

+0.0156

0.29

0.05

0.005

+0.018

0.28

0.06

C2H2••• CH3Br

0.0045

+0.0158

0.28

0.06

0.0048

+0.0185

0.26

0.07

C2H4•••CH3CN

0.0041

+0.0142

0.29

0.05

0.004

+0.0147

0.27

0.05

C2H2•••CH3CN

0.0040

+0.0143

0.28

0.05

0.0042

+0.0159

0.26

0.06

*ρ and ∇2ρ are electron density and Laplacian of electron density, respectively. V and G are potential and kinetic electron energy densities and λ1 and λ2 are the first and third eigenvalues of the Hessian matrix respectively. All values are at the intermolecular bond critical point.

3.3. Reduced density gradient analysis Reduced density gradient analysis has become increasingly popular in the last few years to identify non-covalent interactions.44 The analysis was performed using Multiwfn software.34 Results of this analysis, for all the complexes under consideration, are shown in Figure 3a-h. The figure shows reduced density gradient surface (shown in green) between the two moieties. Presence of this surface, in the intermolecular region, indicates that there is an interaction between the two moieties. In the same figure, plots of reduced density gradient [s= 1/(2(3π2)1/3)|∇ρ|/ρ4/3] vs. electron density multiplied by the sign of the second eigenvalue (sign(λ2)ρ) of the electron density Hessian, are also shown. For all the complexes, these plots show two spikes. The spike on the right hand side with positive value of λ2 indicates a steric interaction, present between the two moieties. The steric interaction arises due to presence of the CH3 hydrogens. The spike on the left side with negative value of λ2 indicates the presence of non-covalent interactions between the two moieties.


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Figure 3. Surfaces of reduced density gradient (s), in the intermolecular region, are shown in green. Plots of the reduced density gradient (s) versus the electron density multiplied by the sign of the second Hessian eigenvalue [sign (λ2)ρ] are also shown. All the values are given in atomic units.



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3.4. Natural bond orbital (NBO) analysis To have an idea of molecular orbitals involved in the intermolecular interaction, NBO analysis was performed on all the complexes optimized at MP2/Aug-cc-pVTZ level. The results from the second order perturbation analysis of Fock matrix in NBO basis are given in Table 5. Table 5. Second order perturbation energy, E(2), corresponding to the charge transfer from π (C-C) orbital to σ* (CX) orbital. The values are in kJ.mol-1.

Complexes

E(2) [π(C-C) to σ*(C-X)]

C2H4•••CH3F

3.3

C2H2••• CH3F

2.5

C2H4••• CH3Cl

3.5

C2H2••• CH3Cl

2.4

C2H4••• CH3Br

4.3

C2H2••• CH3Br

2.8

C2H4•••CH3CN

1.5

C2H2•••CH3CN

1.2

Second order perturbation energies, given in the table, imply significant stabilization due to the charge transfer from the π-bonds to the C-X antibonding orbital. This is similar to the charge transfer from the lone pair orbitals to the H-X antibonding orbital observed in the case of hydrogen bonding. The analysis shows no significant stabilization due to charge transfer between other orbitals of the two moieties. The values are much lesser for the CH3CN complexes, which is in accordance to the lesser binding energies and longer intermolecular distances observed for these complexes. 3.5. Interactions with SiH3X and GeH3X molecules Interactions similar to the carbon bond are known for the other Group 14 atoms, silicon and germanium, as well. Such interactions with Group 14 elements have been named as tetrel 18 ACS Paragon Plus Environment

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bonds in the literature,15,18 with carbon bonding being the most important subset of it. We have considered the interactions between C2H4/C2H2 and SiH3X/GeH3X (X=F/Cl/Br/CN) molecules also. The ESP values at the surface maxima (Vmax) corresponding to SiH3 face centre and GeH3 face centre of SiH3X and GeH3X molecules are given in Table 6. Wave functions of the optimized geometries at MP2/Aug-cc-pVTZ level were used for these calculations. The Si and Ge atoms are more polarizable than carbon. Therefore, the σ-hole in the case of SiH3X/GeH3X molecules is more prominent than that for the corresponding CH3X molecules. It is also reflected in higher Vmax values for these molecules than CH3X molecules, Figure 1 and Table 6. Table 6. The ESP values corresponding to the surface maxima (Vmax) at SiH3/GeH3 face centre of SiH3X/GeH3X molecules. The values are in kJ.mol-1.

Molecule SiH3F SiH3Cl SiH3Br SiH3CN

Vmax 176.5 165.2 160.9 196.5

Molecule GeH3F GeH3Cl GeH3Br GeH3CN

Vmax 200.4 177.7 169.9 194.3

Initial geometries for the optimization were taken similar to those taken for the C2H4/C2H2•••CH3X complexes. The optimized geometries were confirmed to be minima by frequency

calculations.

Interaction

energies

for

the

C2H4/C2H2•••SiH3X

and

C2H4/C2H2•••GeH3X complexes are given in Table 7. Frequency calculations for C2H4•••GeH3CN complex could not be completed at MP2/Aug-cc-pVTZ level. Therefore, its ∆E(BSSE+ZPC) value at MP2/Aug-cc-pVTZ level is missing from the table. 19 ACS Paragon Plus Environment


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Table 7. Interaction energies for the C2H4/C2H2•••SiH3X and C2H4/C2H2•••GeH3X complexes at different levels of theory are given. All the values are in kJ.mol-1.

MP2/6-311+G(3df,2p)

MP2/Aug-cc-pVTZ

CCSD-T/Aug-cc-pVTZ

Complex

∆EBSSE

∆E(BSSE+ZPC)

∆EBSSE

∆E(BSSE+ZPC)

∆EBSSE

C2H4•••SiH3F

-9.7

-5.8

-11.0

-7.1

-10.0

C2H2•••SiH3F

-8.3

-5.8

-9.6

-7.1

-8.7

C2H4•••SiH3Cl

-9.1

-5.9

-10.5

-7.1

-9.3

C2H2•••SiH3Cl

-7.9

-5.8

-9.3

-7.1

-8.2

C2H4••• SiH3Br

-9.1

-6.1

-10.3

-7.0

-9.0

C2H2••• SiH3Br

-7.9

-6.0

-9.2

-7.0

-7.9

C2H4•••SiH3CN

-9.3

-6.4

-10.2

-7.2

-9.4

C2H2•••SiH3CN

-8.3

-6.3

-9.4

-7.4

-8.6

C2H4•••GeH3F

-11.7

-8.1

-11.7

-8.0

-10.4

C2H2•••GeH3F

-10.1

-7.8

-10.6

-8.2

-9.4

C2H4•••GeH3Cl

-10.2

-7.2

-10.8

-7.5

-9.4

C2H2•••GeH3Cl

-8.9

-7.0

-9.8

-7.7

-8.5

C2H4•••GeH3Br

-10.0

-7.2

-10.3

-7.1

-8.8

C2H2•••GeH3Br

-8.7

-7.0

-9.5

-7.4

-8.1

C2H4•••GeH3CN

-9.4

-6.8

-9.7

-

-8.9

C2H2•••GeH3CN

-8.5

-6.7

-9.2

-7.4

-8.3

The higher binding energies for the SiH3X/GeH3X complexes than the CH3X complexes are in agreement with the higher Vmax values for SiH3X/GeH3X molecules than the corresponding CH3X molecules. At MP2/6-311+G(3df,2p) level each of the GeH3X complexes is more stable than the corresponding SiH3X complex. At the MP2/Aug-cc-pVTZ level each of the GeH3X complexes except GeH3CN complexes is more stable than the corresponding SiH3X complex. At this level the SiH3CN complexes are slightly more stable than the GeH3CN complexes which are in agreement with the ESP values given in Table 6. 20 ACS Paragon Plus Environment

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The AIM analysis shows that the interactions are more complicated in the case of SiH3X complexes. It shows the presence of one or two intermolecular bond paths which do not go to the silicon atom rather connect one or two hydrogens of the SiH3 moiety to one or both the carbons of the C2H4/C2H2. Only in the case of SiH3CN complexes and C2H2•••SiH3Br complex, bond path connects to the silicon atom. However, the interactions with GeH3X are similar to those for CH3X molecules and the intermolecular bond path connects to the Ge atom in the case of all the complexes. Electron density topologies for C2H4•••SiH3F and C2H4•••GeH3F complexes are shown in Figure 4a and 4b respectively.

Figure 4. Electron density topologies for (a) C2H4•••SiH3F and (b) C2H4•••GeH3F complexes. Bond critical points are shown in green and ring critical point in red.

Electron density topologies for all the complexes of SiH3X and GeH3X molecules are given in the supporting information. The ρ and ∇2ρ values corresponding to the intermolecular BCPs are also given. These values are consistent with the other parameters discussed above. 4. CONCLUDING REMARKS The results from high level ab initio calculations confirm the interaction between C2H4/C2H2 and CH3X molecules. In these complexes, C2H4/C2H2 molecule acts as the π-electron donor (Lewis base) and CH3X molecule acts as the electron acceptor (Lewis acid). The study of 21 ACS Paragon Plus Environment


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these complexes points out that there is a specific interaction between the CH3 face and the π-electrons, provided the CH3 moiety is attached to an atom or a group which is more electron withdrawing than carbon. Results from the AIM analysis reveal that the interaction is with the carbon of the CH3X molecule. Frequency shifts on complex formation, second order perturbation analysis by NBO theory along with the AIM analysis suggest that these interactions are X-C•••π carbon bonds. Here the π-electrons are carbon bond acceptors. Considering the fact that both the π-electrons and methyl moieties are present in biological systems, these interactions might be important in living systems. Interactions between other tetrel molecules SiH3X/GeH3X were also considered. In the case of the SiH3X complexes, the electron density topologies are more complicated than those in the case of CH3X/GeH3X complexes and the interaction is not directly with the silicon atom in most of these complexes. The GeH3X complexes are similar to the CH3X complexes in terms of electron density topologies and show direct interaction with the germanium atom leading to the formation of a tetrel bond. 5. SUPPORTING INFORMATION AVAILABLE Electron density topologies for the C2H4/C2H2•••SiH3X and C2H4/C2H2•••GeH3X complexes are given in the Supporting information, Figure S1. The corresponding ρ and ∇2ρ values are given in Table S1.The second order perturbation energies, E(2), are given in Table S2 for these complexes. In Table S3 to Table S5, the cartesian coordinates are given for all the complexes under consideration. This material is available free of charge via the Internet at http://pubs.acs.org.


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ACKNOWLEDGMENTS DM thanks CSIR India for a research fellowship. EA acknowledges funding from the IndoFrench Centre for Promotion of Advanced Scientific Research. We thank SERC-IISc for providing excellent computational facilities. We thank the Director of the Indian Institute of Science, Bangalore for financial supports at crucial junctures.



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TOC graphics

Interaction of the sigma hole of CH3X molecules with π-electrons leads to the formation of XC•••π 'carbon bond'.


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