Potential Energy Surfaces of HN(CH)SX:CO2 for X = F, Cl, NC, CN

Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Potential Energy Surfaces of HN(CH)SX:CO2 for X = F, Cl, NC, CN, CCH, and H: N···C Tetrel Bonds and O···S Chalcogen Bonds Janet E. Del Bene,*,† Ibon Alkorta,*,‡ and Jose ́ Elguero‡ †

Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555, United States Instituto de Química Médica (CSIC), Juan de la Cierva, 3, E-28006 Madrid, Spain

‡

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S Supporting Information *

ABSTRACT: MP2/aug′-cc-pVTZ calculations have been performed in search of complexes, molecules, and transition structures on the HN(CH)SX:CO2 potential energy surfaces, for X = F, Cl, NC, CN, CCH, and H. Complexes stabilized by traditional N···C tetrel bonds and O···S chalcogen bonds exist on all surfaces and are bound relative to the isolated monomers. Molecules stabilized by an N−C covalent bond and an O···S chalcogen bond are found when X = F, Cl, and NC, but only the HN(CH)SF:CO2 molecule is bound. The binding energies of these complexes correlate with the O−S distance but not with the N−C distance. Binding energies of complexes rotated by 90° about the N···C tetrel bond and by 90° about the O···S chalcogen bond provide estimates of these bond energies. Charge-transfer energies across tetrel and chalcogen bonds correlate with the N−C and O−S distances, respectively. As a function of the N−C distance, equation-of-motion coupled cluster singles and doubles spin−spin coupling constants 1tJ(N−C) for complexes and transition structures and 1J(N−C) for molecules describe the evolution of the N···C tetrel bonds in the complexes and transition structures to N−C covalent bonds in the molecules. The O···S chalcogen bond gains some covalency in the transition structures and again in the molecules but does not become a covalent bond.

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INTRODUCTION The importance of noncovalent interactions such as hydrogen bonding on the three-dimensional structures of bioorganic and material systems has been known for many years.1−3 Other aspects of noncovalent interactions, including their influence on reactivity, have been explored more recently.4 One of the more spectacular examples of reactivity involves what are known as frustrated Lewis pairs (FLP).5,6 FLPs are capable of forming two noncovalent interactions simultaneously in reactions that produce new molecules. FLPs such as those depicted in Figure 1a illustrate the reaction with carbon dioxide to form an oxaphoshaborolane 1b.7 This reaction is quite general, and other related systems have been investigated.8−10 In this reaction involving two lone pairs and two sigma holes that are represented in Figure 1a as gray circles, electron donation occurs in a favored circular way. The simultaneous donor−acceptor interactions in these processes benefit from cooperativity,11,12 thereby facilitating both interactions. Similar reactions do not occur in systems in which only one of these interactions is present. The circular pathways illustrated in Figure 1a are called conrotatory in electrocyclic reactions.13 In a cluster of circular hydrogen bonds, they are called homodromic.14 We first investigated such circular pathways in a study of the reaction of methanimidothioic acid (2a, X = H) with CO2 to see if compounds related to 2b (1,2,4-oxathiazol-5-ones) are © XXXX American Chemical Society

formed. This reaction involves the formation of an N···C tetrel bond15−17 and an O···S chalcogen bond18−21 and can lead to molecule 2b with a covalent N−C bond and an O−S bond. Other 1,3-dipolar cycloadditions in which CO2 is the dipolarophile (3a → 3b)22,23 have formal relationships such as described above. There are other systems such as carbenes,24−27 phosphines,28 and guanidines29 that can react with CO2 to form such adducts. Although derivatives of the 5H-1,2,4-oxathiazole ring are known,30,31 systems resembling compound 2b have not been reported. This is probably a consequence of the hypervalent nature of the sulfur atom, which is designated by IUPAC as λ4, indicating that the sulfur atom is involved in four bonds. The reactions of these compounds are either cycloreversion accompanied by the expulsion of carbon dioxide or reactions similar to those of the sydnones and other mesoionic compounds that are related to their dipolar resonance forms.32,33 In the present study, we have investigated the frustrated Lewis pair reactions involving CO2 and molecules HN(CH)SX, for X = F, Cl, NC, CN, CCH, and H. We have searched the potential energy surfaces HN(CH)SX:CO2 for equilibrium Received: May 2, 2019 Revised: July 19, 2019

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DOI: 10.1021/acs.jpca.9b04144 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 1. Reactions involving a circular displacement of electrons.

Table 1. Binding Energies (−ΔE, kJ mol−1), C−N, O2−S, and S−A Distances (R, Å), and N−C−O2 and O2−S−A Angles (∠, deg) for Complexes HN(CH)SX:CO2 X=

−ΔE

R(C−N)

R(O2−S)

R(S−A)

∠N−C−O2a

∠O2−S−Ab

F Cl NC CCH H CN

19.1 17.3 17.2 16.2 16.0 15.9

2.798 2.847 2.855 2.860 2.853 2.878

2.885 3.068 3.007 3.212 3.316 3.128

1.636 2.036 1.664 1.691 1.337 1.695

86 88 87 89 90 88

175 173 174 174 179 174

a

O2 is the oxygen atom that interacts with S, as illustrated in Figure 2. bA is the X atom directly bonded to S.

Therefore, the charge-transfer interactions were computed using the B3LYP functional with the aug′-cc-pVTZ basis set at the MP2/aug′-cc-pVTZ geometries, so that at least some electron correlation would be included. Atoms in Molecules methodology45−48 was used to produce the molecular graphs of complexes, molecules, and transition structures employing the AIMAll49 program. The molecular graph identifies the location of electron density features of interest, including the electron density (ρ) maxima associated with the various nuclei, and saddle points, which correspond to bond critical points (BCPs). The zero gradient line which connects a BCP with two nuclei is the bond path. Coupling constants were evaluated using the equation-ofmotion coupled cluster singles and doubles (EOM-CCSD) method in the configuration interaction-like approximation50,51 with all electrons correlated. For these calculations, the Ahlrichs52 qzp basis set was placed on 13C, 15N, 17O, and 19 F, the qz2p basis set on 33S and 35Cl, and the Dunning ccpVDZ basis on 1H atoms. All terms that contribute to the total coupling constant, namely, the paramagnetic spin orbit (PSO), diamagnetic spin orbit (DSO), Fermi contact (FC), and spin dipole (SD) have been evaluated. The EOM-CCSD calculations were performed using ACES II53 on the HPC cluster Owens at the Ohio Supercomputer Center.

complexes stabilized by N···C tetrel bonds and O···S chalcogen bonds, the equilibrium molecules that are produced by these reactions and the transition structures (TSs) that interconvert the equilibrium structures. In this paper, we discuss their binding energies, geometries, charge-transfer energies, and spin−spin coupling constants across the N···C tetrel bonds and the O···S chalcogen bonds. We also provide an assessment of the individual strengths of the N···C tetrel and O···S chalcogen bonds in the complexes.

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METHODS The structures of the isolated monomers CO2 and HN(CH)SX for X = F, Cl, NC, CN, CCH, and H and the complexes HN(CH)SX:CO2 were optimized at the second-order Møller− Plesset perturbation theory (MP2)34−37 with the aug′-ccpVTZ basis set.38 This basis set was derived from the Dunning aug-cc-pVTZ basis set39,40 by removing diffuse functions from H atoms. Searches were made of the HN(CH)SX:CO2 potential surfaces for both complexes and molecules. When two minima were found on a surface, the transition structure that interconverts these was also optimized. Frequencies were computed to establish that the optimized complexes and molecules correspond to equilibrium structures with no imaginary frequencies, and that transition structures have one imaginary frequency along the coordinate that connects the two equilibrium structures. Optimization and frequency calculations were performed using the Gaussian 16 program.41 The binding energies of the binary complexes, molecules, and the transition structures were computed as −ΔE for the reaction that forms these moieties from the corresponding isolated monomers. The natural bond orbital (NBO) method42 has been used to obtain the stabilizing charge-transfer interactions using the NBO-6 program.43 The NBO method is able to evaluate second-order charge-transfer energies from ground-state orbitals only when these have been generated by a welldefined one-electron effective Hamiltonian operator, such as a Fock or Kohn−Sham operator, but not by the two-electron Hartree−Fock operator.44 Thus, only the self-consistent field orbitals are available for the charge-transfer calculations, but these orbitals do not contain any electron correlation effects.

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RESULTS AND DISCUSSION Structures and Binding Energies of Complexes. Table S1 of the Supporting Information presents the structures, total energies, and molecular graphs of the HN(CH)SX:CO2 complexes for X = F, Cl, NC, CN, CCH, and H. Table 1 presents the binding energies of these complexes and their intermolecular bond distances and angles. HN(CH)SX:CO2 complexes are minima on the potential surfaces and are bound relative to the isolated monomers. Although the binding energies of these complexes are similar, the complex HN(CH)SF:CO2 is the most strongly bound complex with a binding energy of 19 kJ mol−1. This may be attributed to the strong electron-withdrawing ability of F, which makes the S atom a better electron-pair acceptor from O. Moreover, the loss of electron density by O leads to electron polarization in the CO2 molecule from C to O, thereby making the C atom a B


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Figure 2. HN(CH)SF:CO2 (a) complex, (b) transition structure, and (c) molecule. Atom O2 is indicated with an asterisk.

and the N−C distance. In contrast, the binding energies of these complexes have an exponential dependence on the O−S distance, but the correlation coefficient is only 0.782. The intermolecular N−C−O2 and O2−S−A angles in these complexes are consistent with stabilization by an N···C tetrel bond and an O···S chalcogen bond. The CO2 molecule is nearly perpendicular to the N···C bond, with N−C−O2 angles near 90°. Thus, this bond forms as N donates an electron pair to C through its local π-hole. For the chalcogen bond, the O− S−A arrangement is nearly linear since the O2−S−A angle approaches 180°, thereby facilitating electron donation by O2 to S through its σ-hole. It would be interesting to examine the nonadditivity of interaction energies, that is, how the presence of one intermolecular bond in these complexes influences the strength of the other. This is not strictly possible for the HN(CH)SX:CO2 complexes since the monomers HN(CH)SX and CO2 are involved in both bonds simultaneously. However, it is possible to isolate these bonds by rotating either the HN(CH)SX or the OCO molecules. A rotation of O CO by 90° about the N···C bond removes the O···S chalcogen bond while keeping the N···C tetrel bond essentially intact. Similarly, a rotation of the HN(CH)SX molecule by 90° about the O···S bond keeps the chalcogen bond essentially intact while destroying the N···C tetrel bond. It should be noted, however, that the resulting rotated complexes do not give the exact energy of either the tetrel bond or the chalcogen bond in the equilibrium HN(CH)SX:OCO complexes, since by rotating these molecules the electron distribution in the resulting complexes is changed. However, the binding energies of the rotated complexes can give approximate values for each of these bonds in the equilibrium complexes, and some insight into how the presence of one of these bonds influences the strength of the other. Table 2 presents the binding energies of the rotated HN(CH)SX:OCS complexes. The energies of the N···C tetrel bond vary from 8.2 to 12.3 kJ mol−1, whereas the O···S chalcogen bond energies vary from 4.4 to 7.0 kJ mol−1. The binding energies of the rotated complexes that leave the chalcogen bond intact are largest when X = F, Cl, and NC and smallest when X = CCH, H, and CN. This is the same relationship observed for the equilibrium complexes. In contrast, the binding energies of the rotated complexes that leave the N···C tetrel bond intact are quite different. The complexes HN(CH)SX:OCO with X = H and CCH have the greatest binding energies, and those with X = F and NC have the smallest, but the optimized complexes with X = H and CCH have the smallest binding energies and those with X = F and Cl have the greatest binding energies. Hence, there is a reversal in the binding energies in these rotated complexes.

better electron acceptor for the formation of the tetrel bond. The complexes with X = Cl and NC have binding energies of 17 kJ mol−1, whereas those with CCH, H, and CN as substituents have binding energies of about 16 kJ mol−1. The HN(CH)SX:CO2 complexes are stabilized by two intermolecular bonds, an N···C tetrel bond and an O···S chalcogen bond, as illustrated in Figure 2a by HN(CH)SF:CO2. The corresponding transition structure and molecule are shown in Figure 2b and c, respectively. Figure S1 provides the molecular electrostatic potentials of the two isolated molecules which form the HN(CH)SF:CO2 complex, and demonstrates their electrostatic complementarity. The shortest tetrel bond distance is 2.80 Å in the HN(CH)SF:CO2 complex, suggesting that this is the strongest tetrel bond. In the remaining complexes, there is little variation in the N−C distance, which is about 2.85 Å, except for the most weakly bound complex with X = CN, where the N−C distance is 2.88 Å. There is a much greater variation in the O− S distance across the chalcogen bond, which has values of 2.89, 3.07, and 3.01 Å in the most stable complexes with X = F, Cl, and NC, respectively, and then increases to 3.21, 3.32, and 3.13 Å in the complexes with X = CCH, H, and CN, respectively. The N···C bonds are traditional tetrel bonds, and the O···S bonds are traditional chalcogen bonds.54,55 Figure 3 presents plots of the binding energies of these complexes versus the N− C and O−S distances. This plot illustrates the small distance changes in the N···C tetrel bond in this series of complexes and the lack of correlation between the complex binding energies

Figure 3. Binding energies of optimized HN(CN)SX:OCO complexes vs the N−C and O−S distances, and binding energies of the rotated complexes that retain the N···C bond vs the N−C distance, and of the rotated complexes that retain the O···S bond vs the O−S distance. C


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Table 2. Binding Energies of Rotated Complexes, the Sum of the Binding Energies of the Rotated Complexes, and the Binding Energies of the Optimized Complexes (−ΔE, kJ mol−1) X=

−ΔE: N···C bond

−ΔE: O···S bond

−[ΔE(O···S) + ΔE(N···S)]

−ΔE optimized

F Cl NC CCH H CN

8.2 10.7 8.9 11.6 12.3 9.9

7.0 6.1 6.6 5.3 4.4 5.6

15.2 16.8 15.5 16.9 16.7 15.5

19.1 17.3 17.2 16.2 16.0 15.9

Table 3. Binding Energies (−ΔE, kJ mol−1), C−N, O−S, and S−A Distances (R, Å), and N−C-O and O−S−A Angles (∠, deg) for Complexes, Transition Structures, and Molecules HN(CH)SX:CO2 X=

−ΔE

R(C−N)

R(O2−S)

R(S−A)

∠N−C−O2

∠O2−S−A

F complex TS molecule Cl complex TS molecule NC complex TS molecule

19.1 −16.9 14.9 17.3 −24.0 −19.9 17.2 −43.7 −37.6

2.798 1.875 1.455 2.847 1.781 1.470 2.855 1.726 1.499

2.885 2.286 1.941 3.068 2.346 2.006 3.007 2.300 2.055

1.636 1.658 1.723 2.036 2.076 2.177 1.664 1.704 1.772

86 100 107 88 103 107 87 104 107

175 174 174 173 175 175 174 173 172

structures, and molecules. Figure 2b shows the transition structure, and Figure 2c depicts the molecule on the HN(CH)SF:CO2 surface. The relative binding energies of the complexes, transition structures, and molecules are illustrated in Figure 4. The point at 5.0 Å represents the

Relationships between the binding energies of the optimized and the rotated complexes can also be seen in Figure 3. It is evident from this figure that the energies of the tetrel bond in the rotated complexes exhibit greater variation with the N−C distance than observed in the optimized complexes. Moreover, the point for the optimized complex with the shortest N−C distance has the highest binding energy, whereas the point with that distance for the rotated complex has the smallest binding energy. In contrast, in the rotated complexes that leave the O··· S bond intact, the binding energies exhibit an exponential dependence on the O−S distance, with a correlation coefficient of 0.958. This correlation is significantly better than the correlation between the binding energies of the optimized complexes as a function of the O−S distance. Table 2 also reports the sum of the tetrel and chalcogen bond energies for the rotated complexes and the binding energies of the optimized complexes for comparison. To what extent are the binding energies nonadditive with reference to the binding energies of the rotated complexes? Once again, it must be stated that the data are not strictly valid for this comparison. Nevertheless, it is interesting to note that the binding energies of the optimized complexes HN(CH)SX:OCO are synergistic for the complexes with X = F and NC. The binding energies for the complexes with X = Cl and CN are synergistic but by only 0.5 and 0.4 kJ mol−1, respectively. In contrast, the binding energies for the complexes with X = CCH and H are antagonistic but by only 0.7 kJ mol−1. Structures and Binding Energies of Molecules and Transition Structures. There are three HN(CH)SX:CO2 systems that exhibit double minima on their potential surfaces, namely, HN(CH)SF:CO2, HN(CH)SCl:CO2, and HN(CH)S(NC):CO2. These have the strongest electron-withdrawing substituents. The geometries, total energies, and molecular graphs of these molecules and the transition structures which interconvert them to the complexes, are given in Table S2. Table 3 provides their binding energies and a comparison of selected distances and angles for the complexes, transition

Figure 4. Relative energies of complexes, transition structures, and molecules on the HN(CH)SX:CO2 surfaces, for X = F, Cl, and NC. The points at R(N−C) = 5.0 Å represent the noninteracting monomers.

noninteracting monomers. On the HN(CH)SF:CO2 surface, the complex and molecule are bound relative to the isolated monomers, whereas the transition structure is not. The barrier to go from the complex to the molecule is 36 kJ mol−1, whereas the reverse barrier is 32 kJ mol−1. The surfaces for the complexes with X = Cl and NC present a very different picture. On these two surfaces, the molecules exist as true minima with no imaginary frequencies, but they are not bound relative to the isolated monomers. From Figure 4, it is apparent that the barriers separating these molecules from their corresponding complex are small, with values of 4 D


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The Journal of Physical Chemistry A and 6 kJ mol−1 for X = Cl and NC, respectively. Therefore, even at low temperatures, these molecules would not exist. Bond distances and bond angles for the complexes, transition structures, and molecules are also given in Table 3. The C−N distance, which is an intermolecular distance in the complexes, decreases but remains an intermolecular C−N distance in the transition structures as the tetrel bond gains some covalent character. In the molecules, the C−N distance further decreases as the C−N bond becomes a covalent bond. At the same time, the O−S distance, which is an intermolecular distance in the complex, decreases and acquires some covalent character in the transition structure. It further decreases in the molecule and becomes a relatively short intermolecular O···S chalcogen bond in the complexes with additional covalent character. As the C−N and O−S distances decrease, the S−F, S−Cl, and S−N bond lengths increase, an indication that the S−A bonds are weakened in these molecules. Relative to the complex, the N−C−O angle increases in the transition structure and further increases in the molecule, as the CO2 molecule bends to accommodate the formation of the C−N covalent bond, as illustrated in Figure 2. However, the O−S−A bond angle is essentially unchanged since the O···S bond remains a chalcogen bond. We have searched the Cambridge Structural Data (CSD) database56 for structures with the 1,2,4-oxathiazol ring and found only seven structures having O−S distances between 1.66 and 2.11 Å, with an average of 1.87 Å. The reference codes of these structures are given in Table S3 of the Supporting Information. Thus, the computed O2−S distances in the HN(CH)SX:CO2 molecules lie in the upper range of distances found for molecules with 1,2,4-oxathiazol rings in the CSD. Charge-Transfer Energies in Complexes. Charge-transfer interactions can only be evaluated for complexes, since the NBO program does not separate the transition structures and the molecules into the appropriate subunits. Table 4 presents

Figure 5. O2lp → σ*S−A charge-transfer energies vs the O−S distance and Nlp → π*C−O3 charge-transfer energies vs the N−C distance.

Charge transfer also occurs across the N···C tetrel bond. These charge-transfer interactions arise from donation of the lone pair on N to the in-plane local π*C−O3 orbital. The charge-transfer energies vary from 7.3 to 10.0 kJ mol−1 and are reported in Table 4. These energies decrease in a different order, F > Cl ≈ H > CCH ≈ NC > CN. A plot of the Nlp → π*C−O3 charge-transfer energies versus the N−C distance is also given in Figure 5. The exponential trendline has a correlation coefficient of 0.923. Spin−Spin Coupling Constants. The PSO, DSO, FC, and SD components of spin−spin coupling constants for the complexes, transition structures, and molecules HN(CH)SX:CO2 are reported in Table S4. The FC term is equal to within 0.1 Hz to total 1tJ(N−C) in complexes and transition structures and approximates 1J(N−C) to within 1 Hz in the molecules. The FC term approximates 1cJ(O−S) to within 0.5 Hz in complexes and transition structures. The FC term also approximates 1cJ(O−S) in molecules to within 0.5 Hz, except for HN(CH)SCl:CO2, in which case the PSO and SD terms contribute −0.8 and −0.3 Hz, respectively. The FC term is −21.9 Hz, and total 1cJ(O−S) is −23.0 Hz. It should also be noted that the SD term was not computed for the HN(CH)S(CCH):CO2 complex but was set to 0.0 Hz, its value in all of the other complexes. Table 5 reports the values of the total spin−spin coupling constants 1tJ(N−C) for complexes and transition structures and 1J(N−C) for molecules. 1tJ(N−C) is less than 1 Hz in the complexes and increases to about 16 Hz in the transition structures. 1J(N−C) then decreases and becomes negative in the molecules, with values between −5 and −11 Hz, where it describes coupling across a covalent N−C bond. 1cJ(O−S) varies between −1.8 and −8.3 Hz in the complexes and increases in absolute value to about −24 Hz as the O···S bond acquires some covalent character in the transition structures. Subsequently, 1cJ(O−S) decreases in absolute value in the molecules as the O···S bond acquires additional covalent character. The behavior of these two coupling constants can best be demonstrated graphically by plotting their values against the corresponding distances. Figure 6 illustrates the distance dependence of the N−C coupling constants. 1tJ(N− C) values are small and positive in the complexes where they refer to an intermolecular bond and increase dramatically in the transition structures as the N−C distance decreases and the N···C bond acquires some covalent character. In the

Table 4. Charge-Transfer Energies (kJ mol−1) for Complexes HN(CH)SX:CO2 X= F Cl NC CCH H CN

O2lp → σ*S−Aa,b 6.4/4.4 4.4/2.1 4.5/3.1 2.4/1.1 1.8/0.9 2.9/1.7

(10.8) (6.5) (7.6) (3.5) (2.7) (4.6)

Nlp → π*C−O3 10.0 8.7 7.8 7.9 8.5 7.3

There are two O2lp → σ*S−X charge-transfer energies. The sum of these is given in parentheses. bA is the X atom directly bonded to S. a

the charge-transfer interactions and their energies for the complexes HN(CH)SX:CO2. There are two lone pairs of electrons on O2, and two O2lp → σ*S−A charge-transfer interactions across the O···S chalcogen bond are found in the complexes. The first set reported in Table 4 contains the greater charge-transfer energies, which range from 1.8 to 6.4 kJ mol−1, and decreases in the order F > NC ≈ Cl > CN > CCH > H. The second O2lp → σ*S−A charge-transfer energies range from 0.9 to 4.4 kJ mol−1 and decrease in a similar order. The total charge-transfer O2lp → σ*S−A energies for the HN(CH)SX:CO2 complexes are plotted against the O2−S distance in Figure 5. The correlation coefficient of the exponential trendline is 0.987. E


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Table 5. Spin−Spin Coupling Constants 1tJ(N−C) and 1cJ(O−S) for HN(CH)SX:CO2 Complexes and Transition Structures, 1 J(N−C) and 1cJ(O−S) for Molecules (Hz) HN(CH)SX:CO2 X=F Cl NC CCH H CN

1t

J(N−C) complex 0.6 0.5 0.4 0.4 0.4 0.3

1t

1

J(N−C) TS

J(N−C) molecule −11.1 −8.6 −4.5

16.4 16.3 14.7

1c

J(O−S) complex

1c

J(O−S) TS

−8.3 −4.7 −3.7 −2.8 −1.8 −5.8

−25.5 −23.5 −24.2

1c

J(O−S) molecule −18.9 −23.0 −25.9

the HN(CH)SX:CO2 potential energy surfaces, for X = F, Cl, NC, CCH, H, and CN. The results of these calculations support the following statements. 1. Complexes stabilized by traditional N···C tetrel bonds and O···S chalcogen bonds exist on all surfaces, and these are bound relative to the isolated monomers. Molecules stabilized by an N−C covalent bond and an O···S chalcogen bond are found when X = F, Cl, and NC, but only the HN(CH)SF:CO2 molecule is bound relative to the isolated monomers. 2. The binding energies of all of the complexes are similar, ranging from 16 to 19 kJ mol−1. The binding energy of the HN(CH)SF:CO2 molecule is 15 kJ mol−1, and the barrier that separates the complex from the molecule is 36 kJ mol−1.

Figure 6. 1tJ(N−C) for complexes and transition structures and 1 J(N−C) for molecules vs the N−C distance.

3. The binding energies correlate with the O−S distance but not with the N−C distance. Charge-transfer energies across the tetrel bond and across the chalcogen bond do correlate with the N−C and O−S distances, respectively.

1

molecules, the N−C bonds are covalent bonds, and J(N−C) decreases dramatically and becomes negative. The plot of 1c J(O−S) versus the O−S distance in Figure 7 is similar to the

4. Rotated complexes, which preserve one intermolecular bond while destroying the other, provide insight into these bonds individually. The binding energies of complexes which retain the O···S bond correlate with the O−S distance, but those of the complexes which retain the N···C tetrel bond do not correlate with the N−C distance. 5. As a function of the N−C distance, EOM-CCSD spin− spin coupling constants 1tJ(N−C) for complexes and transition structures and 1J(N−C) for molecules describe the evolution of the N···C tetrel bonds in the complexes and transition structures to N−C covalent bonds in the molecules. The O···S chalcogen bond gains some covalency in the transition structures and again in the molecules but does not become a covalent bond.

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Figure 7. 1cJ(O−S) for complexes, transition structures, and molecules vs the O−S distance.

ASSOCIATED CONTENT

S Supporting Information *

plot of 1tJ(N−C), but the points for the molecules are close to those for the transition structures. Although the O···S bond has gained covalency, it is not a typical covalent bond in the molecules. Similar plots for other systems that illustrate the distance dependence of spin−spin coupling constants have previously been related to the changing nature of the intermolecular bonds.57−59

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.9b04144. Geometries, total energies, and molecular graphs for HN(CH)SX:CO2 complexes, molecules, and transition state structures; molecular electrostatic potentials of CO2 and HN(CH)SF; CSD reference codes and S−O distances for systems containing an 1,2,4-oxathiazol ring; components of spin−spin coupling constants 1tJ(N−C) and 1cJ(O−S) for HN(CH)SX:CO2 complexes, molecules, and transition structures (PDF)

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CONCLUSIONS MP2/aug′-cc-pVTZ calculations have been performed in search of complexes, molecules, and transition structures on F


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +1 330-609-5593 (J.E.D.B.). *E-mail: [email protected]. Phone: +34 915622900 (I.A.). ORCID

Janet E. Del Bene: 0000-0002-9037-2822 Ibon Alkorta: 0000-0001-6876-6211 José Elguero: 0000-0002-9213-6858 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out with financial support from the Ministerio de Ciencia, Innovación y Universidades of Spain (PGC2018-094644-B-C22) and Comunidad Autónoma de Madrid (P2018/EMT-4329 AIRTEC-CM). Thanks are also given to the Ohio Supercomputer Center and CTI (CSIC) for their continued computational support.

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H


Potential Energy Surfaces of HN(CH)SX:CO2 for X = F, Cl, NC, CN

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