Insight into the Effects of Electrostatic Potentials on the Conversion

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Article Cite This: ACS Omega 2019, 4, 231−241

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Insight into the Effects of Electrostatic Potentials on the Conversion Mechanism of the Hydrogen-Bonded Complexes and CarbonBonded Complexes: An Ab Initio and Quantum Theory of “Atoms in Molecules” Investigation Runtian Chu,† Yanli Zeng,*,†,‡ Mengyu Liu,† Shijun Zheng,† and Lingpeng Meng*,† †

Institute of Computational Quantum Chemistry, College of Chemistry and Material Science, and ‡National Demonstration Center for Experimental Chemistry Education, Hebei Normal University, Shijiazhuang 050024, P. R. China

ACS Omega 2019.4:231-241. Downloaded from pubs.acs.org by 188.68.1.92 on 01/16/19. For personal use only.

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ABSTRACT: Carbon bond and hydrogen bond are common noncovalent interactions; although recent advances on these interactions have been achieved in both the experimental and computational aspects, little is known about the conversion mechanism between them. Here, MP2 calculations with aug-ccpVDZ basis set (aug-cc-pVDZ-pp for element Sn) were used to optimize the geometric configurations of the hydrogen-bonded complexes MH3F···HCN (M = C, Si, Ge, and Sn), carbonbonded complexes HCN···MH3F (M = C, Si, Ge, and Sn), and transition states; the conversion mechanism between these two types of interactions has been carried out. The molecular electrostatic potential, especially the σ-hole, is directly related to the flatten degree of intrinsic reaction coordinate (IRC) curve. The energy barriers from the hydrogen-bonded complexes to the carbon-bonded complexes are 6.99, 7.73, 10.56, and 13.59 kJ· mol−1. The energy barriers from the carbon-bonded complexes to the hydrogen-bonded complexes are 4.65, 7.81, 9.10, and 13.04 kJ·mol−1. The breakage and formation of the bonds along the reaction paths have been discussed by the topological analysis of electronic density. The energy barriers are obviously related to the width of the structure transition region (STR). For the first derivative curve of IRC energy surface versus reaction coordinate, there is a maximum peak and a minimum peak, reflecting the structural transition states in the ring STRs.

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INTRODUCTION Noncovalent interactions1−5 play a special role in supramolecular chemistry, molecular biology, and materials science.6−10 There are a range of different noncovalent forces and various ways of classifying them, such as halogen bond,10−14 chalcogen bond,15,16 pnicogen bond,17−19 and so on. Specifically, the hydrogen bond is perhaps most widely recognized and analyzed because of its importance in chemical, physical, and biological processes.20−22 The hydrogen bond acceptors are commonly expected to be an electronegative atom, such as O, N, and F atoms.20,23,24 Now, various groups such as fluoromethyl group and fluorinated organic compounds25−27 acting as the hydrogen bond acceptors have attracted more attention. In recent years, the newly discovered noncovalent interaction, carbon bond, has been more widely recognized.28−31 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.32,33 The above two interactions of hydrogen bond and carbon bond are widely studied both in experimental and computational aspects. In previous works, researchers have focused more attention on the characteristics and properties of different noncovalent interactions, such as X−C···π interactions,28 XH3F···HM (X = © 2019 American Chemical Society

C, Si, Ge, and Sn; M = Li, Na, BeH, and MgH) tetrel−hydride interaction,34 XH3F···N3−/OCN−/SCN−(X = C, Si, Ge, and Sn) interactions,35 and so on.36,37 However, studies on further mechanism and structural transformation are seldom.38−40 The atoms in molecules (AIM) theory,41−43 which is rooted in quantum mechanics, has been widely applied to study the chemical bonds and chemical reactions. In our previous works,44−46 some typical isomerization reactions have been studied, investigating the structure changes along the reaction pathways. Two concepts of structure transition region (STR) and structure transition state (STS) were put forward by us. Along the reaction pathway, there are two possible possibilities: one is the T-shaped conflict STS that includes a bond path linking a nuclear and a bond critical point (BCP), the other is the bifurcation-type ring STR enveloped by some bond paths contributing from three or more nuclei. For comparison, we named the traditional transitional state corresponding to the maximum on the reaction pathway as the “energy transition state” (ETS). Received: October 6, 2018 Accepted: December 21, 2018 Published: January 4, 2019 231

DOI: 10.1021/acsomega.8b02669 ACS Omega 2019, 4, 231−241

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In this work, we choose MH3F (M = C, Si, Ge, and Sn) and NCH as an investigated subject. On the basis of an analysis on the positive and negative electrostatic potential (ESP) regions of MH3F (M = C, Si, Ge, and Sn), the hydrogen-bonded complexes of MH3F···HCN (M = C, Si, Ge, and Sn) and the carbon-bonded complexes of HCN···MH3F (M = C, Si, Ge, and Sn) were constructed, to investigate (1) the hydrogenbonded interaction between MH3F and NCH; (2) the carbonbonded interaction between MH3F and NCH; (3) the conversion mechanism between the hydrogen-bonded complexes of MH3F···HCN (M = C, Si, Ge, and Sn) and the carbon-bonded complexes of HCN···MH3F (M = C, Si, Ge, and Sn); (4) the topological structural changes between the conversion processes, (5) the relationship of the reaction barriers and the STRs, and (6) the relationship of the energy surface and STS.

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THEORETICAL METHODS The geometric configurations of hydrogen-bonded complexes, carbon-bonded complexes, and transition states (TSs) formed by MH3F (M = C, Si, Ge, and Sn) with NCH were optimized by aug-cc-pVDZ47,48 basis set (aug-cc-pVDZ-pp for element Sn) with the Møller−Plesset49−51 second-order perturbation (MP2) theory. At the same time, the interaction energy was obtained by basis set superposition error using the counterpoise procedure proposed by Boys and Bernardi’s.52,53 The truthfulness of the TS is determined by the vibration frequency analysis, and the intrinsic reaction coordinates (IRCs)54 are used to verify the relationship between hydrogen-bonded complexes, TSs, and carbon-bonded complexes. In this work, the reaction paths were traced out by the mass-weighted internal coordinate (S), which is more precise than simply using the length of the bond and the angle.55,56 Topological analyses of electron density were used to analyze the breaking, generating, and changing trends of chemical bonds at the key points on the IRC paths, which are carried out with AIMAll program.57 ESPs were obtained and depicted on the 0.001 a.u. (electrons/bohr3) contour of the molecule’s electronic density at the same level after geometry optimization using the wave function analysis surface analysis suite,58,59 which provides a fundamental basis for the conversion mechanism60 between the hydrogen-bonded complexes of MH3F···HCN (M = C, Si, Ge, and Sn) and the carbon-boned complexes of HCN···MH3F (M = C, Si, Ge, and Sn). Geometry optimization of each point along the IRC pathway was carried out at the MP2/aug-cc-pVDZ level using Gaussian 09 program package.61

Figure 1. MEP maps on the 0.001 a.u. (electrons/bohr3) contour of the molecule’s electronic density: (a) CH3F, (b) SiH3F, (c) GeH3F, (d) SnH3F, and (e) NCH. Color ranges, in kcal mol−1: red, more positive than 20; yellow, 10−20; green, −10 to 10; and blue, more negative than −10. Positions of VS,max and VS,min are indicated by black arrows.

Table 1. The Most Positive (VS,max, kcal·mol−1) and Most Negative (VS,min, kcal·mol−1) ESPs of the Monomers CH3F SiH3F GeH3F SnH3F NCH

VS,max

VS,min

24.06 45.02 45.65 60.15 58.51

−27.63 −26.55 −28.34 −40.20 −33.01

F−M bond; these regions (red regions) are called σ-holes, with the most positive ESP (VS,max) appearing at the bottom of the M atom. The VS,max values are 24.06, 45.02, 45.65, and 60.15 kcal·mol−1, increasing gradually according to the atomic number of the main group element M = C, Si, Ge, and Sn. The corresponding ESP values are presented in Table 1. Equilibrium Geometries and Interaction Energies of Hydrogen-Bonded Complexes and Carbon-Bonded Complexes. On the basis of the analysis of the surface ESP of the above molecules, we can predict the binding sites for the formation of the hydrogen-bonded complexes and carbonbonded complexes. As shown in Figure 2, the first way of interacting is the H···F hydrogen-bonded interaction. The hydrogen atom of NCH acts as the hydrogen bond donor, and the F atom of MH3F (M = C, Si, Ge, and Sn), which contains lone pair electrons, acts as the hydrogen bond acceptor. Therefore, the H···N hydrogen-bonded interaction is formed, namely, MH3F (M = C, Si, Ge, and Sn)···HCN. The second way of interacting is the N···M carbon-bonded interaction. The σ-hole outside the M atom of MH3F (M = C, Si, Ge, and Sn) acts as the carbon bond donor, and the nitrogen atom of NCH, containing lone pair electrons with a large negative ESP region,

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RESULTS AND DISCUSSION Molecular ESPs of the Monomers. The molecular ESPs (MEP) of MH3F (M = C, Si, Ge, and Sn) and NCH are displayed in Figure 1. For NCH, the most positive and negative ESP (VS,max and VS,min) exist outside the H atom and the N atom, respectively. For MH3F (M = C, Si, Ge, and Sn), the negative ESP region exists outside the F atom, owing to the strong electronegativity of F. For CH3F, there are two VS,min positions outside two sides of F atom with the same value −27.63 kcal·mol−1. For SiH3F, GeH3F, and SnH3F, the VS,min exists outside the F atom along the extension of M−F bond, with the values −26.55, −28.34, and −40.20 kcal·mol−1, respectively. The outside of M atom (M = C, Si, Ge, and Sn) is surrounded by the positive ESP regions along the extension of 232


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Figure 4. Energy curve of SiH3F···HCN → SiF-TS → HCN···SiH3F reaction.

C, Si, Ge, and Sn) atom. In addition to CH3F···HCN, all of the geometries in this work belong to C3v symmetry. The interaction energies ΔE are collected in Table 2. When MH3F (M = C, Si, Ge, and Sn) and NCH interact to form the hydrogen-bonded complexes and carbon-bonded complexes, the interaction energies ΔE are in accordance with the VS,max of the positive ESP described above. For comparison with the MP2 results, ωB97XD calculations were also carried out with the same level basis set of MP2 and the interaction energies are also listed in Table 2. For the hydrogen-bonded complexes MH3F···HCN (M = C, Si, and Ge) and carbon-bonded complexes of HCN···MH3F (M = C, Si, and Ge), the interaction energies obtained by MP2 method (ΔEa) and ωB97XD method (ΔEb) are close. However, For SnH3F···HCN and HCN···SnH3F, the interaction energies calculated by the ωB97XD method are obviously greater. On the basis of the MP2/aug-cc-pVDZ optimized geometries, the CCSD(T)/aug-cc-pVTZ method was used to carry out only single point energy corrections; the obtained interaction energies (ΔEc) is also listed in Table 2. Compared with MP2 results (ΔEa), CCSD(T)/aug-cc-pVTZ calculated interaction energies are greater. For more insight into the nature of the hydrogen-bonded complexes and the carbon-bonded complexes, energy decomposition analyses were carried out by means of the LMOEDA method62 with the GAMESS program;63 the results are collected in Table S1. Comparing the three attractive terms [electrostatic energy (ES), polarization energy (POL), and dispersion energy (DISP)], the ES contributes the most in both the hydrogen-bonded complexes and the carbon-bonded complexes. Conversion Mechanism between the HydrogenBonded Complexes and Carbon-Bonded Complexes. What kind of conversion mechanism has been experienced between the two types of noncovalent interactions, and what is the relationship between them? In this work, the TSs of the conversion reaction have been found. For each of the optimized TS, frequency analysis is carried out to confirm that there exists one and only one imaginary frequency. Furthermore, the vibration model is analyzed, and the reliability of TS is determined. The relationship between the hydrogen-bonded complexes, the TSs, and the carbon-bonded complexes is obtained through the calculation of IRCs. The optimized geometries of the TSs are given in Figure 3. The energy barriers of the conversion reactions are obtained and displayed in Figure 4, and the relative energies are listed in

Figure 2. Optimized geometries for hydrogen-bonded complexes of MH3F···HCN (M = C, Si, Ge, and Sn) and carbon-bonded complexes of HCN···MH3F (M = C, Si, Ge, and Sn).

Table 2. The Interaction Energies (in kJ·mol−1) for the Complexes CH3F···HCN SiH3F···HCN GeH3F···HCN SnH3F···HCN HCN···CH3F HCN···SiH3F HCN···GeH3F HCN···SnH3F

ΔEa

ΔEb

ΔEc

−11.62 −11.39 −15.59 −19.73 −9.28 −11.47 −14.13 −19.18

−11.14 −11.45 −16.45 −35.18 −6.03 −10.38 −13.70 −32.38

−16.89 −15.78 −22.78 −28.11 −9.61 −16.66 −20.96 −27.72

a

Interaction energies obtained at the MP2/aug-cc-pVDZ level based on MP2/aug-cc-pVDZ optimized geometries. bInteraction energies obtained at the ωB97XD/aug-cc-pVDZ level based on ωB97XD/augcc-pVDZ optimized geometries. cInteraction energies obtained at the CCSD(T)/aug-cc-pVTZ level based on MP2/aug-cc-pVDZ optimized geometries.

Figure 3. Optimized geometries of TSs of MH3F (M = C, Si, Ge, and Sn) with HCN.

acts as the carbon bond acceptor. Therefore, the group IV σhole interaction was constructed. We refer to the group IV σhole interactions as carbon-bonded interaction, that is, HCN··· MH3F (M = C, Si, Ge, and Sn). The most negative ESP (VS,min) of CH3F is on the side of the F atom; the hydrogen-bonded interaction of CH3F···HCN is formed on the side of the F atom. The carbon-bonded interaction is combined at the top of the F atom and M (M = 233


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Table 3. The Energies of the Hydrogen-Bonded Type Complexes, TSs and Carbon-Bonded Type Complexes (in kJ·mol−1) freq/cm−1 CH3F···HCN CF-TS HCN···CH3F SiH3F···HCN SiF-TS HCN···SiH3F GeH3F···HCN GeF-TS HCN···GeH3F SnH3F···HCN SnF-TS HCN···SnH3F

E/(a.u.) −232.5501 −232.5475 −232.5492 −483.6402 −483.6372 −408.6402 −2270.0475 −2270.0434 −2270.0469 −408.0398 −408.0346 −408.0309

−25.53

−32.34

−28.79

−30.53

ΔE1/(kJ·mol−1)

ΔE2/(kJ·mol−1)

6.99

4.65

7.73

7.81

10.56

9.10

13.59

13.04

Figure 5. Charge-transfer diagram between the donor and the acceptor orbitals on the 0.015 isosurface: (a) SiF-TS, (b) GeF-TS, and (c) SnF-TS.

Table 4. Natural Bond Orbital (NBO) Parameters for the TSs (Δ2E in kcal·mol−1, δi and qCT in e) SiF-TS GeF-TS SnF-TS

donor NBO (i)

acceptor NBO (j)

Δ2E

δi

εj − εi

Fij

qCT

N lone pair N lone pair N lone pair

Si−H anti-bond Ge−H anti-bond Sn−H anti-bond

0.07 0.11 0.24

1.98211 1.98188 1.98143

1.21 1.17 1.09

0.008 0.010 0.015

0.00009 0.00014 0.00038

Table 5. Electronic Densities ρb at the Critical Points for the CH3F···HCN → CF-TS → HCN···CH3F IRC Patha S

F−H

CH3F···HCN −15.88 −13.44 −12.54 −12.23 −10.39 −9.78 −8.86 −5.80 −2.74 0.00 +1.82 +4.89 +5.30 +5.91 +8.98 +9.99 HCN···CH3F

0.01438 0.01319

F-BCP(C−H)

F−C

RCP(C−F−C−N)

0.00755 0.00677 0.00371 0.00311

0.00453 0.00351 0.00311

RCP(C−H−N−H)

C−N

H−N

0.00897

0.00460 0.00472 0.00394 0.00514 0.00579 0.00587 0.00583

0.00456 0.00446 0.00448 0.00451 0.00460 0.00473 0.00395 0.00517 0.00582 0.00594 0.00590 0.00642 0.00633 0.00620

a

S: reaction coordinate in unit of (amu)1/2 bohr.

gradually along the increased atomic number of main group elements M = C, Si, Ge, and Sn. Natural Bond Orbital Analysis of the TSs. From Figure 1, compared with the ESP distribution of the Si/Ge/SnH3F surface, there is no strong positive hole outside the F atom of CH3F. In order to understand the role of the ESP in the formation of TS and the energy barriers in the conversion process, the charge transfer and orbitals of the TSs were calculated by the natural bond orbital analysis. Figure 5 shows

Table 3. From the hydrogen-bonded complexes to the carbonbonded complexes, MH3F···HCN → MF-TS → HCN···MH3F (M = C, Si, Ge, and Sn), the reaction energies are 6.99, 7.73, 10.56, and 13.59 kJ·mol−1, respectively. From the carbonbonded complexes to the hydrogen-bonded complexes, HCN···MH3F → MF-TS → MH3F···HCN (M = C, Si, Ge, and Sn), the reaction energies are 4.65, 7.81, 9.10, and 13.04 kJ·mol−1, respectively. The reaction energy barrier increases 234


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Figure 6. Topological analysis of each stagnation for the CH3F···HCN → CF-TS → HCN···CH3F IRC path.

electrostatic interaction. With the increase of atomic number of group IV, it is gradually shown that the lone pair electrons of the N atom in the NCH molecule transfer to the Si/Ge/Sn−H antibond, which can be concluded that the TS Si/Ge/Sn-TS interaction also has partial charge transfer in addition to the electrostatic interaction. The charge-transfer quantities qCT’s are 0.00009e, 0.00014e, 0.00038e. Topological Analyses of Electronic Density on IRC Paths. The quantum theory of “AIM”,41−43 proposed by Bader, has been applied broadly and successfully to study the properties of intra- and intermolecular interactions. Topological analyses for each stagnation point on reaction path can provide valuable information, by studying related information such as electron density at the critical point, the strength, and the properties of the chemical bond, to further analyze the conversion process. Topological Analyses of Electronic Density on Reaction Channels of CH3F···HCN → CF-TS → HCN··· CH3F. Electronic density parameters at the critical points for the transformation path from the hydrogen-bonded complexes to the carbon-bonded complexes formed by CH3F and HCN are given in Table 5. The representative topological graphs for some points along the IRC path are plotted in Figure 6. First of all, the electron density ρb at the critical points of the F···H hydrogen bond formed by F atom of CH3F and the H atom of NCH gradually decreases, indicating that the hydrogen bond becomes weaker and weaker. At the point S = −13.44, there exists a bond path between the F atom and the BCP of the C− N bond, forming a T-shaped structure. After that point, the F··· H hydrogen bond is broken and the F···C bond forms. The ring critical point (RCP) appears in the reaction process of S = −12.23 to S = −9.78, indicating the formation of the C−F− C−N four-membered ring. With the proceeding of the reaction, the electron density ρb at the BCP of the F···C bond becomes smaller and the RCP moves to the center of the ring and then gradually approaches the F···C bond. At the point of S = −9.78, the RCP overlapped with the BCP of F···C bond and the F···C bond is broken and the four-membered ring disappeared. In the region of S = −2.74 to S = +5.91, the RCP appears again and another C−H−N−H four-membered ring exists. The traditional ETS,44 appearing at S = 0.00, is included in this

Figure 7. Structures along IRC path of CH3F···HCN → HCN···CH3F (a) and the first derivative curve (b).

the donor and acceptor orbitals on the 0.015 isosurface. The red region and blue region represent the positive and negative phases of the wave function, respectively. It is observed that the lone pair orbitals of N atom in NCH overlap with the M− H (M = Si, Ge, and Sn) antibond orbitals, and the overlapping regions are increasing along the sequence of M = Si, Ge, and Sn. Table 4 shows the natural bond orbital parameters for the TSs. In the TS CF-TS, the effective value of charge transfer from lone pair electrons of N atom to the C−H antibond is not obtained because the combination of CF-TS is mainly 235


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Figure 8. Topological analysis of each stagnation for the SiH3F···HCN → SiF-TS → HCN···SiH3F IRC path.

Figure 9. Topological analysis of each stagnation for the GeH3F···HCN → GeF-TS → HCN···GeH3F IRC path.

region. At this point, the electron density at the BCP of H···N bond was the smallest and this structure is very unstable. In this process, the C···N bond path becomes more and more bent and begins to expand slowly to the two sides to form H··· N hydrogen bond and then back up again to form a stable C··· N bond. Finally, the stable carbon-bonded complex HCN··· CH3F is formed. The process from S = −2.74 to S = +5.91 is consistent with the change of the ESP around CH3F. Through the observation of the ESP, the positive ESP regions outside three H atoms of CH3F molecule can serve as the electron acceptor, N atom of NCH with lone pair electrons can be used as electron donor, and the H···N hydrogen bond is formed

possibly by the N atom with each of the three H atoms of CH3F. Figure 7 gives the structures along the IRC path of CH3F··· HCN → HCN···CH3F (a) and the first derivative curve of energy versus reaction coordinate (b). For the first derivative curve, there is a maximum peak corresponding to the structure of S = −11.01 and a minimum peak corresponding to the structure of S = +5.91. The points of S = −11.01 and S = +5.91 lie on the deepest positions uphill and downhill the energy curve, reflecting the STSs in the ring STRs. Topological Analyses of Electronic Density on Reaction Channels of Si/GeH3F···HCN → Si/GeF-TS → 236


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Table 6. Electronic Densities ρb at the Critical Points for the SiH3F···HCN → SiF-TS → HCN···SiH3F IRC Patha S

F−H

SiH3F···HCN −20.09 −19.26 −18.70 −17.31 −15.09 −13.15 −9.52 −8.12 −6.16 −3.91 0.00 +4.28 +9.89 +12.69 +13.10 +15.28 HCN···SiH3F

0.01251

F−C

F-BCP(C−N)

0.00800 0.00718 0.00660

RCP(Si−F−C−N)

RCP(Si−F−N)

RCP(Si−H−N−H)

0.00465 0.00474 0.00570 0.00612 0.00655

Si−N

H−N

0.00471 0.00490 0.00543 0.00643 0.00711 0.00662 0.00562

0.00509 0.00600 0.00654

0.00472 0.00426 0.00343 0.00484 0.00680

0.00472 0.00427 0.00346 0.00491 0.00680 0.00861 0.00915 0.01126 0.00983

a


Table 7. Electronic Densities ρb at the Critical Points for the GeH3F···HCN → GeF-TS → HCN···GeH3F IRC Patha S

F−H

GeH3F···HCN −26.79 −22.02 −20.60 −19.62 −18.40 −16.05 −12.36 −7.41 −4.87 −3.34 0.00 +4.72 +7.31 +8.57 +12.27 +19.62 HCN···GeH3F

0.01529

F−C

F-BCP(C−N)

0.01001 0.00805 0.00742

RCP(Ge−F−C-N)

RCP(Ge−F−N)

RCP(Ge−H−N−H)

0.00563 0.00577 0.00590 0.00677 0.00618 0.00625

Ge−N

H−N

0.00566 0.00630 0.00659 0.00663 0.00713 0.00723 0.00823 0.00559 0.00500

0.00602 0.00595 0.00624

0.00482 0.00379 0.00524 0.00579 0.00616

0.00482 0.00381 0.00530 0.00581 0.00618 0.00754 0.01352 0.01060

a


in the region of S = −3.34 to S = +7.31, the structural transition region of a Ge−H−N−H four-membered ring appears. In the above process, the Si/Ge−N bond path becomes more and more bent, expands slowly to the two sides to form H···N hydrogen bonds, and then back up again to form a stable Si/Ge−N bond. These changes are in good agreement with the changes of the ESP around Si/GeH3F. At the point of S = 0.00 (ETS), the electron density at the BCP of H···N bond was the smallest and this structure is very unstable. Therefore, the above-mentioned reaction channels have two fourmembered ring structural transition regions, that is, Si/Ge− F−C−N ring and Si/Ge−H−N−H ring, and a three membered-ring structural transition region, namely, Si/Ge− F−N ring. As the reaction proceeds, a stable carbon-bonded complex is formed, that is, HCN···Si/GeH3F. Figures 10 and 11 show the structures along the IRC path of Si/GeH3F···HCN → HCN···Si/GeH3F (a) and the first

HCN···Si/GeH3F. Figures 8 and 9 display the topological graphs for the key points along the IRC paths of the Si/ GeH3F···HCN → Si/GeF-TS → HCN···Si/GeH3F processes; the corresponding electronic density parameters at the critical points are listed in Tables 6 and 7. The initial changing process of the F···H bond is similar to the F···H bond formed by the above CH3F and NCH; the difference is that the F···C bond path slips along the C−N bond of NCH and changes to the F···N bond; therefore, the Si/Ge−F−C−N four-membered ring changes to the Si/Ge−F−N three-membered ring. As the process goes on, the RCP moves to the center of the ring; when the RCP goes to the BCP of F···N bond, the F−N bond is broken and the three-membered ring disappears. For the SiH3F···HCN → SiF-TS → HCN···SiH3F process, in the region of S = −6.16 to S = +9.89, the structural transition region of a Si−H−N−H four-membered ring exists. For the GeH3F···HCN → GeF-TS → HCN···GeH3F process, 237


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Figure 10. Structures along IRC path of SiH3F···HCN → SiF-TS → HCN···SiH3F (a) and the first derivative curve (b). Figure 11. Structures along IRC path of GeH3F···HCN → GeF-TS → HCN···GeH3F (a) and the first derivative curve (b).

derivative curve of energies versus reaction coordinates (b). For the first derivative curve of SiH3F···HCN → HCN···SiH3F, there is a maximum peak corresponding to the structure of S = −18.42 and a minimum peak corresponding to the structure of S = +9.52. For the first derivative curve of GeH3F···HCN → HCN···GeH3F, there is a maximum peak corresponding to the structure of S = −21.80 and a minimum peak corresponding to the structure of S = +10.26. These points reflect the STSs in the ring STRs. Topological Analyses of Electronic Density on Reaction Channels of SnH3F···HCN → SnF-TS → HCN··· SnH3F. Figure 12 displays the topological graphs for the IRC path of SnH3F···HCN → SnF-TS → HCN···SnH3F, and the corresponding electronic density parameters at the critical points are listed in Table 8. This process is relatively simple compared with the above three reaction channels of C/Si/ GeH3F···HCN → SnF-TS → HCN···SnH3F. First of all, the STR of the Sn−F−C−N four-membered ring is changed to the Sn−F−N three-membered ring structure. As the reaction goes on, the RCP is gradually close to the F···N bond. At the point of S = −21.05, the F···N bond is broken and the Sn−F−N three-membered ring disappears and only the Sn···N bond is left. The electron density at the BCP of Sn···N bond decreases gradually, and the electron density reaches the minimum in the ETS SnF-TS (S = 0.00). Finally, NCH moves to the bottom Sn atom of the SnH3F to form the stable carbon-bonded complex HCN···SnH3F. The SnH3F···HCN → SnF-TS → HCN···SnH3F process does not form the H···N bond and does not experience the Sn−H−N−H structural transition region, which is related to the ESP of SnH3F. Observing the ESP of SnH3F, it can be

Figure 12. Topological analysis of each stagnation for the SnH3F··· HCN → SnF-TS → HCN···SnH3F IRC path.

found that there is a large positive ESP region around the Sn atom. As the reaction continues, the N atom in the NCH could interact with the positive ESP region to form the σ-hole interaction, instead of extending to both sides to form the H− N bond. 238


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Table 8. Electronic Densities ρb at the Critical Points for the SnH3F···HCN → SnF-TS → HCN···SnH3F IRC Patha S

F−H

SnH3F···HCN −33.28 −30.27 −30.01 −29.74 −29.47 −25.87 −21.05 −19.28 −11.43 −3.54 0.00 +5.91 +10.06 +12.87 +16.92 +21.41 HCN···SnH3F

0.0181

F−C

F-BCP(C−N)

F−N

RCP(Sn−F−C−N)

0.0129 0.0104

RCP(Sn−F−N)

0.0080 0.0080 0.0079 0.0081

0.0100 0.0099 0.0093 0.0085 0.0088

0.0079 0.0083 0.0088

Sn−N 0.0088 0.0091 0.0089 0.0092 0.0090 0.0099 0.0105 0.0107 0.0079 0.0054 0.0042 0.0056 0.0065 0.0080 0.0117 0.0169 0.0166

a


Table 9. The Energy Barriers and the Width of the STR M M M M M

= = = =

ΔE1/(kJ·mol−1)

M−F−C−N/M−F−N C Si Ge Sn

S S S S

= = = =

−12.23 −19.26 −26.79 −33.28

→ → → →

−9.78 (2.45) −13.15 (6.11) −16.05 (10.74) −21.05 (12.23)

6.99 7.73 10.56 13.59

and a minimum peak corresponding to the structure of S = +15.01, reflecting the STSs in the ring STRs. Relationship of the Energy Barriers and the Width of the STR. Table 9 summarizes the width of the STRs of M−F− C−N/M−F−N ring. It can be observed that the energy barriers are obviously related to the width of the STR. The larger the energy barriers, the wider the ring STR.

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CONCLUSIONS

(1) On the basis of the analysis of the ESP, there are two ways of MH3F (M = C, Si, Ge, and Sn) interacting with NCH. The first way of combining is the hydrogenbonded interaction, namely, MH3F···HCN (M = C, Si, Ge, and Sn). The second way of combining is the carbon-bonded interaction, that is, HCN···MH3F (M = C, Si, Ge, and Sn). (2) The TSs from the hydrogen-bonded complexes to the carbon-bonded complexes were found, and IRC calculations were carried out to verify the conversion mechanism. The molecular ESP, especially the σ-hole, is directly related to the flatten degree of IRC curve. (3) The breakage and formation of the bonds along the reaction paths have been discussed by the topological analysis of electronic density. The energy barriers are obviously related to the width of the STR. For the first derivative curve of IRC energy surface versus reaction coordinate, there is a maximum peak and a minimum peak, reflecting the STSs in the ring structural transition regions.

Figure 13. Structures along IRC path of SnH3F···HCN → SnF-TS → HCN···SnH3F (a) and the first derivative curve (b).

Figure 13 shows the structures along the IRC path of SnH3F···HCN → HCN···SnH3F (a) and the first derivative curve of energies versus reaction coordinates (b). For the first derivative curve of SnH3F···HCN → HCN···SnH3F, there is a maximum peak corresponding to the structure of S = −24.84 239


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02669. Energy decomposition analysis of the interaction energies; topological characteristics of the BCPs and RCPs for the MH3F···HCN → HCN···MH3F (M = C, Si, Ge, and Sn) IRC paths; and topological analysis of each stagnation for the MH3F···HCN → HCN···MH3F (M = C, Si, Ge, and Sn) IRC paths (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.Z.). *E-mail: [email protected] (L.M.). ORCID

Yanli Zeng: 0000-0002-7141-6172 Lingpeng Meng: 0000-0003-0445-0749 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (Contract nos: 21371045 and 21373075). Thanks are also due to Education Department of Hebei Province of China through innovative hundred talents support program (SLRC2017041).

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Insight into the Effects of Electrostatic Potentials on the Conversion

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