Chalcogen-Bond-Assisted Formation of the N→C Dative Bonds in the Complexes between Chalcogenadiazoles/Chalcogenatriazoles and Fullerene C60

The existence of the N→C dative bonds in the complexes between N-containing molecules and fullerenes have been verified both theoretically and experimentally. However, finding stable N→C dative bonds is still a highly challenging task. In this work, we investigated computationally the N→C dative bonds in the complexes formed by fullerene C60 with 1,2,5-chalcogenadiazoles, 2,1,3-benzochalcogenadiazoles, and 1,2,4,5-chalcogenatriazoles, respectively. It was found that the N→C dative bonds are formed along with the formation of the N–Ch···C (Ch = S, Se, Te) chalcogen bonds. In the gas phase, from S-containing complexes through Se-containing complexes to Te-containing complexes, the intrinsic interaction energies become more and more negative, which indicates that the N–Ch···C chalcogen bonds can facilitate the formation of the N→C dative bonds. The intrinsic interaction energies are compensated by the large deformation energy of fullerene C60. The total interaction energies of Te-containing complexes are negative, while both total interaction energies of the S-containing complexes and Se-containing complexes are positive. This means that the N→C dative bonds in the Te-containing complexes are more easily observed in experiments in comparison with those in the S-containing complexes and Se-containing complexes. This study provides a new theoretical perspective on the experimental observation of the N→C dative bonds in complexes involving fullerenes. Further, the formation of stable N→C dative bonds in the complexes involving fullerenes can significantly change the properties of fullerenes, which will greatly simulate and expand the application range of fullerenes.


Introduction
The "dative bond" is not a new term.In 1999, the International Union of Pure and Applied Chemistry (IUPAC) defined the dative bond as "the coordination bond formed upon interaction between molecular species, one of which serves as a donor and the other as an acceptor of the electron pair to be shared in the complex formed" [1].Unlike covalent bonds, dative bonds have more significant polarity, weaker strength, and longer bond lengths.The distinctive features and nature of dative bonds have been reviewed extensively [2][3][4][5].In a relatively recent review article, Nandi and Kozuch detailed the historical development of dative bonds and provided an outlook on their future advancement [5].Currently, controversies related to dative bonds have become increasingly rare.
The well-known dative bond is the central bond in ammonia borane (H 3 N→BH 3 ).Like the case in H 3 N→BH 3 , the dative bond is always represented by an arrow from the electron donor to the electron acceptor.There are many different kinds of dative bonds [2][3][4][5].Fullerenes are of considerable interest due to their unusual structures and peculiar properties [6].Through combined experimental and theoretical studies, Hobza and colleagues have discovered the existence of N→C dative bonds in the complexes between piperidine and fullerene molecules such as C 60 and C 70 [7][8][9].The charge transfer from  (d, Å), intrinsic interaction energies (∆E INTR , kcal/mol), total interaction energies (∆E, kcal/mol), total dipole moments (µ, Debye) and Mulliken charges (q, e) on N, C 1 , Ch and C 2 of the complexes studied.The Van der Waals radii of S, Se, Te and C are 1.80, 1.90, 2.06 and 1.70 Å, respectively [24].The Ch•••C2 interatomic distances in Table 1 are all smaller than the sum of the radii of two interacting atoms, which provides preliminary support for classifying the Ch•••C2 interactions as chalcogen bonds.The N•••C1 interatomic distances are in the range of 1.466-1.507Å, which is clearly less than the interatomic distance of 1.606 Å for the N→C dative bond in the complex between piperidine and fullerene C60 [7].This means that the covalent character of the N→C dative bonds in this study is significantly enhanced.In Table 1, the intrinsic interaction energies and total interaction energies contain the contributions of all the noncovalent interactions because it is difficult to completely separate N→C dative bonds from other noncovalent interactions.From the S-containing complexes through Secontaining complexes to Te-containing complexes, the intrinsic interaction energies become more and more negative.It is well known that the strength of a chalcogen bond increases in the order of S < Se < Te.Hence, such a result indicates that the N-Ch•••C chalcogen bonds facilitate the formation of the N→C dative bonds.The intrinsic interaction energies are compensated by the large deformation energies of the monomers.Unlike the intrinsic interaction energies, the total interaction energies do not neglect the deformation energies of the monomers.As shown in Table 1, the total interaction energies of Te-containing complexes are negative, while both total interaction energies of the S-containing complexes and Se-containing complexes are positive.These results indicate that the N→C dative bonds in the Te-containing complexes are more easily observed in experiments in comparison with those in the S-containing complexes and Se-containing complexes.
At the PBE0-D3/def2-TZVPP theory level, the calculated total dipole moments of the monomers are in the range of 0-1.78 Debye.Compared to the monomers, the total dipole moments of the complexes significantly increase.This also conforms to the characteristics of dative bond formation.It is very convenient to employ the Mulliken charge for qualitative estimation of possible changes in electron distribution, although it is often criticized that Mulliken charge is arbitrary and heavily basis set dependent.Table 1 summarizes the Mulliken charges on N, C1, Ch and C2.The Mulliken charges on Ch atoms are all positive The Van der Waals radii of S, Se, Te and C are 1.80, 1.90, 2.06 and 1.70 Å, respectively [24].The Ch•••C 2 interatomic distances in Table 1 are all smaller than the sum of the radii of two interacting atoms, which provides preliminary support for classifying the Ch•••C 2 interactions as chalcogen bonds.The N•••C 1 interatomic distances are in the range of 1.466-1.507Å, which is clearly less than the interatomic distance of 1.606 Å for the N→C dative bond in the complex between piperidine and fullerene C 60 [7].This means that the covalent character of the N→C dative bonds in this study is significantly enhanced.In Table 1, the intrinsic interaction energies and total interaction energies contain the contributions of all the noncovalent interactions because it is difficult to completely separate N→C dative bonds from other noncovalent interactions.From the S-containing complexes through Se-containing complexes to Te-containing complexes, the intrinsic interaction energies become more and more negative.It is well known that the strength of a chalcogen bond increases in the order of S < Se < Te.Hence, such a result indicates that the N-Ch•••C chalcogen bonds facilitate the formation of the N→C dative bonds.The intrinsic interaction energies are compensated by the large deformation energies of the monomers.Unlike the intrinsic interaction energies, the total interaction energies do not neglect the deformation energies of the monomers.As shown in Table 1, the total interaction energies of Te-containing complexes are negative, while both total interaction energies of the S-containing complexes and Se-containing complexes are positive.These results indicate that the N→C dative bonds in the Te-containing complexes are more easily observed in experiments in comparison with those in the S-containing complexes and Se-containing complexes.
At the PBE0-D3/def2-TZVPP theory level, the calculated total dipole moments of the monomers are in the range of 0-1.78 Debye.Compared to the monomers, the total dipole moments of the complexes significantly increase.This also conforms to the characteristics of dative bond formation.It is very convenient to employ the Mulliken charge for qualitative estimation of possible changes in electron distribution, although it is often criticized that Mulliken charge is arbitrary and heavily basis set dependent.Table 1 summarizes the Mulliken charges on N, C 1 , Ch and C 2 .The Mulliken charges on Ch atoms are all positive and the Mulliken charges on C 2 atoms are all negative, which once again demonstrates the formation of chalcogen bonds.
In fact, we also calculated the complexes involving the 6:5 bonds of fullerene C 60 at the PBE0-D3/def2-TZVPP theory level.Figure S1 in the Supplementary Materials illustrates the optimized structures and corresponding total interaction energies of the complexes S-2(6:5), Se-1(6:5), Se-2(6:5), Te-1(6:5) and Te-2(6:5).Unlike the results in Figure 1 and Table 1, the expected complex S-1(6:5) in Figure S1 does not exist, and the total interaction energies of the Te-containing complexes in Figure S1 are not negative.In line with our predictions based on the molecular electrostatic potentials of fullerene C 60 , the interactions between 6:5 bonds of fullerene C 60 and N-Ch bonds of chalcogenazoles are much weaker than the interactions between 6:6 bonds of fullerene C 60 and N-Ch bonds of chalcogenazoles.On the other hand, Table S1 in the Supplementary Materials also lists the calculated results for the complexes S-1, S-2, Se-1, Se-2, Te-1 and Te-2 at the low-cost PBE0-D3/6-31G(d) or PBE0-D3/SDD level of theory.Comparing the calculated results of Tables 1 and S1, it can be observed that the low-cost PBE0-D3/6-31G(d) or PBE0-D3/SDD calculations yield qualitatively consistent results with those from high-cost PBE0-D3/def2-TZVPP calculations.Therefore, if the sole purpose is to conduct large-scale computational searches for stable N→C dative bonds in large complexes involving large molecular carbon materials, we recommend utilizing low-cost PBE0-D3/6-31G(d) or PBE0-D3/SDD calculations.

Molecular Electrostatic Potential Maps of Chalcogenadiazoles
So far, there is still one question left unanswered: Why do the complexes O-1 and O-2 in Figure 1 not exist?To address this question, we plotted the electrostatic potential mapped electron density surfaces for 1,2,5-chalcogenadiazoles and 2,1,3-benzochalcogenadiazoles.and the Mulliken charges on C2 atoms are all negative, which once again demonstrates the formation of chalcogen bonds.
In fact, we also calculated the complexes involving the 6:5 bonds of fullerene C60 at the PBE0-D3/def2-TZVPP theory level.Figure S1 in the Supplementary Materials illustrates the optimized structures and corresponding total interaction energies of the complexes S-2(6:5), Se-1(6:5), Se-2(6:5), Te-1(6:5) and Te-2(6:5).Unlike the results in Figure 1 and Table 1, the expected complex S-1(6:5) in Figure S1 does not exist, and the total interaction energies of the Tecontaining complexes in Figure S1 are not negative.In line with our predictions based on the molecular electrostatic potentials of fullerene C60, the interactions between 6:5 bonds of fullerene C60 and N-Ch bonds of chalcogenazoles are much weaker than the interactions between 6:6 bonds of fullerene C60 and N-Ch bonds of chalcogenazoles.On the other hand, Table S1 in the Supplementary Materials also lists the calculated results for the complexes S-1, S-2, Se-1, Se-2, Te-1 and Te-2 at the low-cost PBE0-D3/6-31G(d) or PBE0-D3/SDD level of theory.Comparing the calculated results of Tables 1 and S1, it can be observed that the low-cost PBE0-D3/6-31G(d) or PBE0-D3/SDD calculations yield qualitatively consistent results with those from high-cost PBE0-D3/def2-TZVPP calculations.Therefore, if the sole purpose is to conduct large-scale computational searches for stable N→C dative bonds in large complexes involving large molecular carbon materials, we recommend utilizing low-cost PBE0-D3/6-31G(d) or PBE0-D3/SDD calculations.

Molecular Electrostatic Potential Maps of Chalcogenadiazoles
So far, there is still one question left unanswered: Why do the complexes O-1 and O-2 in Figure 1 not exist?To address this question, we plotted the electrostatic potential mapped electron density surfaces for 1,2,5-chalcogenadiazoles and 2,1,3-benzochalcogenadiazoles. Figure 2 shows that, except for 1,2,5-oxadiazole and 2,1,3-benzooxadiazole, other chalcogenadiazoles all have two σ-holes on the chalcogen atoms.Figure 4 shows the correlation between the most positive electrostatic potentials of σ-holes of 2,1,3-benzochalcogenadiazoles and the intrinsic interaction energies of S-2, Se-2 and Te-2.The correlation coefficients (R-Square) in Figures 3 and 4 are both close to 1, indicating a very strong correlation between V S,max and ∆E INTR .The value of V S,max determines the strength of a chalcogen bond.At the same time, the more stable the complex, the more stable the N→C dative bond.Once again, it proves that the formation of chalcogen bonds determines the formation of N→C dative bonds.
Molecules 2024, 29, x FOR PEER REVIEW 5 of 11 1. Figure 4 shows the correlation between the most positive electrostatic potentials of σholes of 2,1,3-benzochalcogenadiazoles and the intrinsic interaction energies of S-2, Se-2 and Te-2.The correlation coefficients (R-Square) in Figures 3 and 4 are both close to 1, indicating a very strong correlation between VS,max and ∆E INTR .The value of VS,max determines the strength of a chalcogen bond.At the same time, the more stable the complex, the more stable the N→C dative bond.Once again, it proves that the formation of chalcogen bonds determines the formation of N→C dative bonds.

AIM Analyses
As can be seen in Figure 1, according to the bonding situation of the nitrogen atom, it can be determined that a N→C dative bond is formed between the nitrogen atom and the carbon atom.In previous studies, the dative bonds with significant covalent bond

AIM Analyses
As can be seen in Figure 1, according to the bonding situation of the nitrogen atom, it can be determined that a N→C dative bond is formed between the nitrogen atom and the carbon atom.In previous studies, the dative bonds with significant covalent bond

AIM Analyses
As can be seen in Figure 1, according to the bonding situation of the nitrogen atom, it can be determined that a N→C dative bond is formed between the nitrogen atom and the carbon atom.In previous studies, the dative bonds with significant covalent bond Molecules 2024, 29, 2685 6 of 11 components were also referred to as "dative/covalent bonds", "covalent/dative bond" or "covalent dative bonds" [8,10,11,25].Therefore, we will no longer discuss the nature of N→C dative bonds in this section and instead focus on the nature of chalcogen bonds.
The AIM theory was often used to study the nature of the noncovalent bonds [26-30].Figure 5 shows the bond critical points and bond paths for the complexes Se-1 and Se-2.The molecular graphs of S-1 and Te-1 are almost the same as the one of Se-1, and the molecular graphs of S-2 and Te-2 are almost the same as the one of Se-2 (Figure S2).For the sake of clarity, only the bond critical points and bond paths are shown in Figures 5 and S2.The electron density, Laplacian of electron density, eigenvalues of the Hessian of electron density and ellipticity at the Ch•••C 2 bond critical point of each complex are summarized in Table 2.
The AIM theory was often used to study the nature of the noncovalent bonds [26][27][28][29][30]. Figure 5 shows the bond critical points and bond paths for the complexes Se-1 and Se-2.The molecular graphs of S-1 and Te-1 are almost the same as the one of Se-1, and the molecular graphs of S-2 and Te-2 are almost the same as the one of Se-2 (Figure S2).For the sake of clarity, only the bond critical points and bond paths are shown in Figures 5 and  S2.The electron density, Laplacian of electron density, eigenvalues of the Hessian of electron density and ellipticity at the Ch•••C2 bond critical point of each complex are summarized in Table 2.The bond paths between Ch and C2 and the corresponding bond critical points can be clearly seen in the representative Figure 5.This is a necessary condition for the formation of the N-Ch•••C2 chalcogen bond.The electron densities at the bond critical points of the N-Ch•••C2 chalcogen bonds range from 0.0393 to 0.0651 au.In contrast, the electron densities at the bond critical points of hydrogen bonds range from 0.002 to 0.035 au [26,27].The larger electron density means a much stronger noncovalent bond.Here, the N-Ch•••C2 chalcogen bonds should belong to strong chalcogen bonds.The Laplacian of electron  The bond paths between Ch and C 2 and the corresponding bond critical points can be clearly seen in the representative Figure 5.This is a necessary condition for the formation of the N-Ch•••C 2 chalcogen bond.The electron densities at the bond critical points of the N-Ch•••C 2 chalcogen bonds range from 0.0393 to 0.0651 au.In contrast, the electron densities at the bond critical points of hydrogen bonds range from 0.002 to 0.035 au [26,27].The larger electron density means a much stronger noncovalent bond.Here, the N-Ch•••C 2 chalcogen bonds should belong to strong chalcogen bonds.The Laplacian of electron density is the sum of λ 1 , λ 2 and λ 3 .According to AIM theory, the Laplacian of electron density is positive at the bond critical point of a noncovalent bond [31].The values of ▽ 2 ρ b in Table 2 are all positive at the Ch•••C 2 bond critical point of each complex, which proves once again that the Ch•••C 2 contacts are of chalcogen bonds.The ellipticity ε can be calculated using the formula λ 1 /(λ 2 − 1), and it can be used to assess the π character and structural stability of a bond.In Table 2, the values of ε are all very small, which indicates that the N-Ch•••C 2 chalcogen bonds are all very stable.This is consistent with the results of electron densities at the Ch•••C 2 bond critical points.

Complexes between Chalcogenatriazoles and Fullerene C 60
It is meaningful to expand the complexes studied above to include more complexes.First, the fullerene C 60 in the complexes can be replaced by other fullerene molecules or other molecular carbon materials.The results should be very similar.Second, the chalcogenadiazole can be replaced by its derivatives or other similar molecules.Here, we studied the complexes between 1,2,4,5-chalcogenatriazoles and fullerene C 60 .
Figure 6  62 kcal/mol, respectively, whereas the total interaction energies of the complexes N-, N-Se and N-Te are +27.22,+17.44 and +1.21 kcal/mol, respectively.The positive total interaction energies indicate that these three complexes are difficult to form in the gas phase.In fact, the calculated total interaction energy of the complex between piperidine and fullerene C 60 is also positive, but it has been detected by both FTIR spectra and NMR spectroscopy [7].As pointed out above, the influence of the chemical environment around the complexes is also very important and must be considered during the experimental process.Aside from the 1,2,4,5-chalcogenatriazoles, we believe that there should be many similar nitrogen-containing organic compounds that can form stable chalcogen-bond-assisted N→C dative bonds with molecular carbon materials.
Molecules 2024, 29, x FOR PEER REVIEW 7 of 11 density is the sum of λ1, λ2 and λ3.According to AIM theory, the Laplacian of electron density is positive at the bond critical point of a noncovalent bond [31].The values of ▽ 2 ρb in Table 2 are all positive at the Ch•••C2 bond critical point of each complex, which proves once again that the Ch•••C2 contacts are of chalcogen bonds.The ellipticity ε can be calculated using the formula λ1/(λ2 − 1), and it can be used to assess the π character and structural stability of a bond.In Table 2, the values of ε are all very small, which indicates that the N-Ch•••C2 chalcogen bonds are all very stable.This is consistent with the results of electron densities at the Ch•••C2 bond critical points.

Complexes between Chalcogenatriazoles and Fullerene C60
It is meaningful to expand the complexes studied above to include more complexes.First, the fullerene C60 in the complexes can be replaced by other fullerene molecules or other molecular carbon materials.The results should be very similar.Second, the chalcogenadiazole can be replaced by its derivatives or other similar molecules.Here, we studied the complexes between 1,2,4,5-chalcogenatriazoles and fullerene C60.The positive total interaction energies indicate that these three complexes are difficult to form in the gas phase.In fact, the calculated total interaction energy of the complex between piperidine and fullerene C60 is also positive, but it has been detected by both FTIR spectra and NMR spectroscopy [7].As pointed out above, the influence of the chemical environment around the complexes is also very important and must be considered during the experimental process.Aside from the 1,2,4,5-chalcogenatriazoles, we believe that there should be many similar nitrogen-containing organic compounds that can form stable chalcogen-bond-assisted N→C dative bonds with molecular carbon materials.

Materials and Methods
Unless otherwise stated, the structures and properties of all the monomers and complexes were calculated using the PBE0 density functional with Grimme s D3 dispersion correction in conjunction with the def2-TZVPP basis set [32][33][34].The Becke-Johnson damping function and ultrafine integration grid were used in all PBE0-D3/def2-TZVPP

Materials and Methods
Unless otherwise stated, the structures and properties of all the monomers and complexes were calculated using the PBE0 density functional with Grimme's D3 dispersion correction in conjunction with the def2-TZVPP basis set [32][33][34].The Becke-Johnson damping function and ultrafine integration grid were used in all PBE0-D3/def2-TZVPP calculations [35].The basis set superposition error was corrected with the conventional counterpoise method [36].
The calculations of noncovalent systems have always required extreme caution [37,38].Bickelhaupt and colleagues carried out benchmark calculations for the chalcogen bonds with the type of D 2 Ch•••A -(Ch = S, Se; D, A = F, Cl) [38].They found that some density functionals with specific dispersion corrections could give varying results for the chalcogen bonds.In this study, we calculated the complexes at the PBE0-D3/def2-TZVPP theory level.Our previous publications have shown that the PBE0-D3/def2-TZVPP calculations are reliable for the study of noncovalent systems [39,40].Further, benchmark calculations on the dative bonds have been performed by employing a relatively small complex between fullerene C 20 and piperidine [7].The results again proved the reliability of the PBE0-D3/def2-TZVPP calculations for the study of the dative bonds.
It is also highly meaningful to study the reliability of results at low computational costs.Due to the generally large size of molecular carbon material systems, low-cost computations make studying these systems feasible.Using a small basis set 6-31G(d) for the S/Se-containing complexes and the small pseudopotential basis set SDD for the Tecontaining complexes, the structures, harmonic frequencies, and energies of the complexes considered in this study were calculated at the PBE0-D3, PBE0, B3LYP-D3, B3LYP and M06-2X levels of theory, respectively.Let us add here that calculations involving heavy atom systems require the use of pseudopotential basis sets.The frequency calculations show that the structures of the complexes are true minima on their respective potential energy surfaces.For the geometries and energies of the complexes studied, quantitative differences exist between low-cost and high-cost computational results, but qualitative consistency can be achieved.
The fullerene C 60 has two different types of carbon-carbon bonds as follows: the 6:6 bonds between two hexagons [1.401( 10) Å] and the 6:5 bonds between a hexagon and a pentagon  The calculations of noncovalent systems have always required extreme caution [37,38].Bickelhaupt and colleagues carried out benchmark calculations for the chalcogen bonds with the type of D2Ch•••A -(Ch = S, Se; D, A = F, Cl) [38].They found that some density functionals with specific dispersion corrections could give varying results for the chalcogen bonds.In this study, we calculated the complexes at the PBE0-D3/def2-TZVPP theory level.Our previous publications have shown that the PBE0-D3/def2-TZVPP calculations are reliable for the study of noncovalent systems [39,40].Further, benchmark calculations on the dative bonds have been performed by employing a relatively small complex between fullerene C20 and piperidine [7].The results again proved the reliability of the PBE0-D3/def2-TZVPP calculations for the study of the dative bonds.
It is also highly meaningful to study the reliability of results at low computational costs.Due to the generally large size of molecular carbon material systems, low-cost computations make studying these systems feasible.Using a small basis set 6-31G(d) for the S/Se-containing complexes and the small pseudopotential basis set SDD for the Te-containing complexes, the structures, harmonic frequencies, and energies of the complexes considered in this study were calculated at the PBE0-D3, PBE0, B3LYP-D3, B3LYP and M06-2X levels of theory, respectively.Let us add here that calculations involving heavy atom systems require the use of pseudopotential basis sets.The frequency calculations show that the structures of the complexes are true minima on their respective potential energy surfaces.For the geometries and energies of the complexes studied, quantitative differences exist between low-cost and high-cost computational results, but qualitative consistency can be achieved.
The fullerene C60 has two different types of carbon-carbon bonds as follows: the 6:6 bonds between two hexagons [1.401( 10) Å] and the 6:5 bonds between a hexagon and a pentagon   The formation of the N-Ch•••C chalcogen bonds was analyzed employing Bader s "atoms in molecules" (AIM) theory [31].Despite some controversies, the AIM theory remains a very useful tool for analyzing noncovalent interactions.The AIM analyses were performed with the AIM2000 software, version 2.0 [42].The most positive electrostatic The formation of the N-Ch•••C chalcogen bonds was analyzed employing Bader's "atoms in molecules" (AIM) theory [31].Despite some controversies, the AIM theory remains a very useful tool for analyzing noncovalent interactions.The AIM analyses

Figure 2
shows that, except for 1,2,5-oxadiazole and 2,1,3-benzooxadiazole, other chalcogenadiazoles all have two σ-holes on the chalcogen atoms.In 1,2,5-oxadiazole and 2,1,3benzooxadiazole, the electrostatic potentials on the extensions of the N-O bonds are all negative, which means that the N-O•••C chalcogen bonds cannot be formed in the two complexes.When N-O•••C chalcogen bonds are absent, N→C dative bonds are also absent; when N-Ch•••C (Ch = S, Se, Te) chalcogen bonds are present, N→C dative bonds are also present.Clearly, the formation of chalcogen bonds determines whether N→C dative bonds form or not.Molecules 2024, 29, x FOR PEER REVIEW 4 of 11 In 1,2,5-oxadiazole and 2,1,3-benzooxadiazole, the electrostatic potentials on the extensions of the N-O bonds are all negative, which means that the N-O•••C chalcogen bonds cannot be formed in the two complexes.When N-O•••C chalcogen bonds are absent, N→C dative bonds are also absent; when N-Ch•••C (Ch = S, Se, Te) chalcogen bonds are present, N→C dative bonds are also present.Clearly, the formation of chalcogen bonds determines whether N→C dative bonds form or not.

Figure 3
Figure3shows the correlation between the most positive electrostatic potentials of σholes of 1,2,5-chalcogenadiazoles and the intrinsic interaction energies of S-1, Se-1 and Te-

Figure 3
Figure3shows the correlation between the most positive electrostatic potentials of σholes of 1,2,5-chalcogenadiazoles and the intrinsic interaction energies of S-1, Se-1 and Te-1.

Figure 5 .
Figure 5.The bond critical points (small red dots) and bond paths of the complexes Se-1 and Se-2.

Figure 5 .Table 2 .
Figure 5.The bond critical points (small red dots) and bond paths of the complexes Se-1 and Se-2.Table 2.The electron density (ρ b ), Laplacian of electron density (▽ 2 ρ b ), eigenvalues of the Hessian of electron density (λ 1 , λ 2 , λ 3 ) and ellipticity (ε) at the Ch•••C 2 bond critical point of each complex.All the values are given in atomic units.
shows the PBE0-D3/def2-TZVPP optimized structures of the complexes N-S, N-Se and N-Te along with the N•••C 1 and Ch•••C 2 interatomic distances.Similarly, the Ch•••C 2 interatomic distances in Figure 6 are all smaller than the sum of the radii of two interacting atoms, which indicates the formation of the N-Ch•••C 2 chalcogen bonds.At the same time, the bonding situation in the three complexes clearly shows the formation of the N→C dative bonds.The intrinsic interaction energies of the complexes N-S, N-Se and N-Te are −0.11,−18.55 and −43.

Figure 6
shows the PBE0-D3/def2-TZVPP optimized structures of the complexes N--S, N-Se and N-Te along with the N•••C1 and Ch•••C2 interatomic distances.Similarly, the Ch•••C2 interatomic distances in Figure 6 are all smaller than the sum of the radii of two interacting atoms, which indicates the formation of the N-Ch•••C2 chalcogen bonds.At the same time, the bonding situation in the three complexes clearly shows the formation of the N→C dative bonds.The intrinsic interaction energies of the complexes N-S, N-Se and N-Te are −0.11,−18.55 and −43.62 kcal/mol, respectively, whereas the total interaction energies of the complexes N-, N-Se and N-Te are +27.22,+17.44 and +1.21 kcal/mol, respectively.

Figure 6 .
Figure 6.PBE0-D3/def2-TZVPP optimized structures of the complexes N-S, N-Se and N-Te.The red dashed lines represent the chalcogen bonds.The numbers shown are the interatomic distances (Å).The red dashed lines represent chalcogen bonds.

Figure 6 .
Figure 6.PBE0-D3/def2-TZVPP optimized structures of the complexes N-S, N-Se and N-Te.The red dashed lines represent the chalcogen bonds.The numbers shown are the interatomic distances (Å).The red dashed lines represent chalcogen bonds.
Figure 7 shows the electrostatic potential mapped electron density surface of fullerene C 60 .The electrostatic potentials above the 6:6 bonds are more negative than those above 6:5 bonds.The C atoms of fullerene C 60 act as the electron donors upon the formation of the N-Ch•••C chalcogen bonds.Therefore, only the interactions between 6:6 bonds of fullerene C 60 and N-Ch bonds of chalcogenazoles were considered in this study.
Figure 7 shows the electrostatic potential mapped electron density surface of fullerene C60.The electrostatic potentials above the 6:6 bonds are more negative than those above 6:5 bonds.The C atoms of fullerene C60 act as the electron donors upon the formation of the N-Ch•••C chalcogen bonds.Therefore, only the interactions between 6:6 bonds of fullerene C60 and N-Ch bonds of chalcogenazoles were considered in this study.

Figure 7 .
Figure 7.The electrostatic potential mapped electron density surface (isoval = 0.001 au) of fullerene C60.The color bar is in kcal/mol.

Figure 7 .
Figure 7.The electrostatic potential mapped electron density surface (isoval = 0.001 au) of fullerene C 60 .The color bar is in kcal/mol.

Table 2 .
The electron density (ρb), Laplacian of electron density (▽ 2 ρb), eigenvalues of the Hessian of electron density (λ1, λ2, λ3) and ellipticity (ε) at the Ch•••C2 bond critical point of each complex.All the values are given in atomic units.