Influence of F and Se substitution on the structures, stabilities and nature of the complexes between F₂CSe and HOX (X = F, Cl, Br, and I)

Abstract

Ab initio MP2/aug-cc-pVTZ calculations have been performed to identify local minima on the F₂CSe–HOX (X = F, Cl, Br, and I) potential surface for characterizing the types of interactions that stabilize the complexes found at these minima, and to evaluate their relative stabilization. Four types of structures are found for each complex, except for the HOF one. The Se⋯H hydrogen-bonded complexes (I) are accompanied with a secondary X⋯F interaction. The structure II is jointly connected with a tetrel bond and an X⋯Se interaction. The structures III and IV are stabilized by a chalcogen bond and a halogen bond, respectively. The stabilization of I has little dependence on the nature of the X atom, while II, III, and IV become more stable with the increase of the atomic mass of X. The chalcogen-bonded complexes are least stable, the halogen-bonded complexes are more stable than the tetrel-bonded ones, and the hydrogen-bonded complex is weaker than the halogen-bonded one in the HOI complex but stronger in the other complexes. The formation of these interactions has been understood by means of molecular electrostatic potentials and orbital interactions. The electrostatic energy is dominant in the complexes I and IV, although the polarization and dispersion contributions are also important, while the dispersion energy is comparable with the electrostatic contribution in complexes II and III.

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1. Introduction

The physicochemical and spectroscopic properties of X₂CY (X = H, F, Cl, and Br; Y = O, S, and Se) have attracted a great deal of attention for many decades.^1–3 This is because such studies are beneficial for understanding the effects of chalcogen and halogen substitutions that play a significant role in chemistry and biochemistry.^4,5 For instance, F₂CO and Cl₂CO have been found to be the primary products in the photodecomposition reaction of methinehalide in the presence of oxygen.⁶ Organoselenium chemistry has rapidly developed since selenium was demonstrated to be an important factor in understanding the biological functions of several selenoproteins.⁷ Compounds with a selenocarbonyl group have been considered as compositional units in organic syntheses and as important intermediates involved in the synthesis of selenium-containing molecules.^8–10 Moreover, it was shown that selenoformaldehyde (H₂CSe) and its corresponding dihalogen derivatives are related to the systems of biological importance.^1,11,12

Hypohalous acids (HOX, X = halogen) are powerful oxidizing agents with diverse action sites and significant chemical activities. However, their chemical instability makes it difficult to study their properties experimentally. Thus, theoretical methods have usually been used to study the complexes of hypohalous acids with themselves¹³ and other molecules.^14–18 Hypohalous acids are also important in the field of atmospheric chemistry because they are plentifully formed in the atmosphere by reactions between X and OH radicals. For instance, hypobromous acid has a key function in the catalytic processes in the depletion of stratospheric ozone.¹⁹ More importantly, hypohalous acids are closely related to protein pharmaceutical²⁰ and infectious diseases.²¹ Hypochlorous and hypobromous acids are antimicrobial oxidants produced by innate immune cells.²² Moreover, hypochlorous acid can awaken the enzymatic activity during the oxidation of lysozyme.²³

In this paper, we perform a study of the complexes of F₂CSe and HOX (X = F, Cl, Br, and I) by quantum chemical calculations. The first interest is to investigate whether their structures can be stabilized by a hydrogen bond, halogen bond, chalcogen bond, or tetrel bond. Our second focus is the effect of these interactions on the structures and properties of the complexes. Third, it is urgent to unveil the origin of these interactions. To the best of our knowledge, the complexes of F₂CSe and hypohalous acids have not been investigated both theoretically and experimentally. This research gap pushes us to investigate the structures, properties, and nature of these complexes. We think that this study could be vital to deepen the understanding of the interaction mechanisms among them with potential significance in biology and atmospheric chemistry.

2. Theoretical calculations

The geometries of binary systems were optimized at the MP2 level with the aug-cc-pVTZ basis set for all atoms except the iodine atom. The aug-cc-pVTZ-PP basis set involving pseudo potential was adopted for the iodine atom to account for relativistic effects. Harmonic frequency calculations were computed at the same level to confirm that all the structures correspond to the true minima on the potential energy surfaces. Interaction energies were computed as the difference between the energy of the complex and the sum of the energies of the two optimized monomers. Interaction energies were corrected for basis set superposition error (BSSE) using the counterpoise procedure proposed by Boys and Bernardi.²⁴ All the calculations were carried out with the Gaussian 09 program.²⁵

Molecular electrostatic potentials (MEPs) at the 0.001 electrons per Bohr³ isodensity surfaces were calculated with the Wave Function Analysis-Surface Analysis Suite (WFA-SAS) program²⁶ at the MP2/aug-cc-pVTZ level. The MP2/aug-cc-pVTZ wave functions were applied as inputs to obtain the topological parameters using AIM2000 software²⁷ as well as the electron density shifts plotted using the Multiwfn package.²⁸ NBO calculations²⁹ were performed to analyze orbital interactions at the HF/aug-cc-pVTZ level via the procedures contained within Gaussian 09. The interaction energy was decomposed using the GAMESS program³⁰ with the LMOEDA method³¹ at the MP2/aug-cc-pVTZ level.

3. Results and discussion

3.1. MEPs of F₂CSe and hypohalous acids

It is well known that F₂CSe can act as either a Lewis acid or base when participating in different types of intermolecular interactions.³² To have a good understanding for the interaction sites in F₂CSe, its molecular electrostatic potential (MEP) map is plotted in Fig. 1. Blue and red regions represent the negative and positive MEPs, respectively. Clearly, two red regions are observed in F₂CSe. One red area is perpendicular to the plane composed of the C and two F atoms and the other one is at the outer surface of the Se atom along the C=Se axis. The former belongs to the π-hole, while the latter is called the σ-hole. The π-hole (1.164 eV) displays a larger positive MEP than the σ-hole (0.808 eV). Simultaneously, two negative regions with the value of −0.423 eV are located at both sides of the Se atom within the molecular plane of F₂CSe, corresponding to the lone-pair electrons on the Se atom. Therefore, F₂CSe provides not only the Lewis acid sites (the σ-hole and π-hole) to the Lewis base but also the Lewis base sites to the Lewis acid.

Fig. 1 MEPs of F₂CSe. Color ranges, in eV, are: red, greater than 0.27; yellow, between 0.27 and 0; blue, less than 0.

Fig. 2 is the MEP maps of hypohalous acids. Actually, they have been studied in the previous studies.^18,33 Moreover, two types of positive MEPs are found in HOX. One corresponds to the acidic proton and the other is associated with the σ-hole on the halogen atom, except in the case of F. In addition, the surfaces of the oxygen and halogen atoms exhibit blue areas with negative MEPs. The most positive and negative MEPs in HOX (X = F, Cl, Br, and I) are collected in Table 1. With the increase of the halogen atomic size, the positive MEP on the H atom becomes smaller, whereas that on the σ-hole of the halogen atom is larger. This order is related to the electronegativity of the halogen atom. Moreover, the former value is bigger than the latter one in all hypohalous acid molecules, indicating that the acidic proton is a stronger Lewis acid than the halogen atom. For the same reason, the negative MEP on the oxygen atom becomes more negative with the increase of halogen atomic mass. However, the negative MEP on the halogen atom becomes smaller in the same order. The abovementioned analyses for the MEPs of HOX indicate that these molecules are multi-functional molecules with two Lewis acid sites (H and X) and two Lewis base sites (O and X).

Fig. 2 MEPs of HOX (X = F, Cl, Br, and I). Color ranges, in eV, are: red, greater than 0.27; yellow, between 0.27 and 0; blue, less than 0.

Table 1 The most positive (V_max) and negative (V_min) MEPs in HOX (X = F, Cl, Br, and I). All are in eV

Monomer	Vmax (H)	Vmax (X)	Vmin (X)	Vmin (O)
HOF	2.592	—	−0.675	−0.744
HOCl	2.427	1.035	−0.406	−0.817
HOBr	2.329	1.358	−0.339	−0.868
HOI	2.202	1.716	−0.261	−0.950

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3.2. Geometries and frequency shifts

According to the electrostatic potentials of F₂CSe and HOX, we have obtained four types of complexes for each hypohalous acid, except HOF; for the latter molecule three types of complexes are found. It should be noted that there are other types of structures for the complexes of F₂CSe and HOX as well. However, they are less stable than the structures shown in Fig. 3 or are second-order saddle points.

Fig. 3 Optimized structures of the complexes with distances (Å).

In F₂CSe–HOX-I, the acidic H atom in HOX points to the lone-pair electron on the Se atom in F₂CSe. However, the H–O bond has a small deviation from the molecular plane of F₂CSe. In the initial optimization of F₂CSe–HOX-I, we designed the planar structure of the complex with a hydrogen bond and the X atom on the opposite side of F atom in F₂CSe to reduce the repulsion interaction between the X atom of HOX and the F atom of F₂CSe. Unfortunately, this planar structure is unstable with one imaginary frequency and becomes the nonplanar structure, as shown in Fig. 3, in the re-optimization process. With the increase in the atomic mass of X in HOX (X = Cl, Br, and I), the Se⋯H hydrogen bond shows a longer binding distance and a weaker strength, which is consistent with the positive MEP on the H atom of HOX. However, F₂CSe–HOF-I has the longest binding distance, which is inconsistent with the largest MEP on the H atom of HOF. This abnormal result was also found in the hydrogen-bonded complexes of HOX and some nitrogenated bases (NH₃, N₂, and HCN).³⁴ Interestingly, there is an attractive interaction between the X atom of HOX and the F atom of F₂CSe, characterized with a X⋯F bond critical point (BCP) in Fig. 4. Furthermore, with the increase of the atomic mass of X, the X⋯F interaction is stronger, evidenced by the greater electron density at the X⋯F BCP in Fig. 4. This attractive interaction has an important effect on the nonplanar structure of F₂CSe–HOX-I, although it is weak with a long binding distance and a small electron density. A combination of both the Se⋯H hydrogen bond and the X⋯F interaction is responsible for the stability of the cyclic structure of F₂CSe–HOX-I, characterized with a ring critical point in Fig. 4. The presence of the X⋯F interaction in F₂CSe–HOX-I can be understood with the MEPs of HOX and F₂CSe. One can see from Fig. 1 that both the F atoms of F₂CSe have positive MEPs near the region of the π-hole. As a result, the lone-pair electron of the X atom in HOX is close to the F atom of F₂CSe. Furthermore, with the increase in the atomic mass of X, the deviation of the X–O bond from the molecular plane of F₂CSe becomes larger, as shown in Fig. S1.† The nonplanar structure of F₂CSe–HOX-I is different from the planar structure of the hydrogen-bonded complexes of H₂CO–HOX¹⁸ and H₂CS–HOX,³⁵ where the X atom approaches the H atom of H₂CY (Y = O and S) with a weak H⋯X interaction.

Fig. 4 Molecular graphs of all complexes with bond critical points (red points) and ring critical points (yellow points). The values of electron densities are shown in au.

In F₂CSe–HOX-II, the lone-pair electron on the oxygen atom of HOX attacks the π-hole of F₂CSe, forming a tetrel-bonded complex. The tetrel bond is an attractive interaction between a positive region of MEP adjoined with the IV group atom and a Lewis base.^36–39 The acidic H atom of HOX is located between the two F atoms of F₂CSe, while the X atom deviates from the C=Se axis. As shown in Fig. S1,† this deviation grows with the increase in the atomic mass of X. It is interesting to find a Se⋯X BCP in F₂CSe–HOBr-II and F₂CSe–HOI-II complexes, which can be explained with the MEPs of HOX and F₂CSe. It can be inferred from Fig. 1 that the C=Se bond has positive MEPs in the vertical direction to the molecular plane of F₂CSe. To confirm this Se⋯X interaction, we also performed an orbital interaction analysis for this type of complex. The results show that there is an orbital interaction of Lp_X → BD^*_C–Se, where Lp_X and BD^*_C–Se are the lone-pair orbital of the X atom and the anti-bonding orbital of the C–Se bond, respectively. The corresponding second-order perturbation energy is 0.46 kJ mol⁻¹ in the HOCl complex, 2.88 kJ mol⁻¹ in the HOBr complex and 5.43 kJ mol⁻¹ in the HOI complex. Clearly, the Se⋯X interaction becomes stronger with the increase in the atomic mass of X. Besides, the C⋯O distance decreases in the same growth sequence, showing a consistent change with the negative MEP on the oxygen atom in HOX (Table 1) and the electron density at the C⋯O BCP (Fig. 4). The results indicate that the tetrel bond becomes stronger with the increase in the atomic mass of X.

In F₂CSe–HOX-III, the lone-pair electron of the O atom in HOX is associated with the σ-hole on the Se atom, forming a chalcogen bond. The chalcogen bond is an attractive interaction between the σ-hole on the V group atom and a Lewis base.^40–42 F₂CSe–HOX-III is a planar structure with Cs symmetry, except for the HOI complex. The planar structure of the HOI complex is unstable with one imaginary frequency. Probably, the chalcogen bond also becomes stronger with the increase in the atomic mass of X, confirmed by the shorter Se⋯O distance (Fig. 1), the more negative MEP on the oxygen atom in HOX (Table 1), and the greater electron density at the Se⋯O BCP (Fig. 4). In F₂CSe–HOX-IV, a halogen bond is formed between the σ-hole of the halogen atom and the lone-pair electron on the Se atom for all the complexes, except the HOF complex. This is consistent with the fact that the F atom seldom participates in halogen bonding.⁴³ In the initial optimization of configuration IV, the HOX molecule is in the same plane as the F₂CSe molecule. However, in the final optimized structure, the H–O bond is almost perpendicular to the molecular plane of F₂CSe. This is also different from the planar structure of the halogen-bonded complexes of H₂CO–HOX¹⁸ and H₂CS–HOX,³⁵ where the H atom of HOX is opposite to the H atom of H₂CY (Y = O and S).

Hypohalous acids have been studied both experimentally^44–47 and theoretically.^48–50 Alkorta et al.³⁴ pointed out that the geometries of hypohalous acids at the MP2/aug-cc-pVTZ level are in good agreement with the experimental results. The geometrical parameters of HOX obtained at the MP2/aug-cc-pVTZ level, including bond lengths and H–O–X angles, are also close to the experimental values. For instance, the experimental value of the Br–O bond length in HOBr is 1.828 Å (ref. 44) and the corresponding theoretical value is 1.824 Å.

Table 2 presents the changes of H–O and X–O bond lengths in the complexes. Upon the formation of configurations I, II, and III, the H–O bond is elongated and its elongation is larger in configuration I. However, the X–O bond shows an irregular change in the three configurations. In I, the X–O bond is elongated in the F₂CSe–HOF complex but is contracted in the other complexes. The change of X–O bond length in II and III is very small. In IV, the associated X–O bond is elongated and the distant H–O bond displays a tiny contraction. The elongation of the associated X–O bond is related to the interaction strength.

Table 2 Changes of bond lengths (Δr, Å) and frequency shifts (Δv, cm⁻¹) of H–O and X–O bonds in the complexes at the MP2/aug-cc-pVTZ level

Complexes	ΔrH–O	ΔrX–O	ΔvH–O	ΔvX–O
F2CSe–HOF-I	0.008	0.002	−169.0	1.7
F2CSe–HOCl-I	0.010	−0.003	−197.1	3.4
F2CSe–HOBr-I	0.009	−0.005	−196.3	5.5
F2CSe–HOI-I	0.009	−0.006	−186.1	5.1
F2CSe–HOF-II	0.001	0.000	−9.1	−3.0
F2CSe–HOCl-II	0.001	0.001	−9.0	−1.8
F2CSe–HOBr-II	0.001	−0.000	−8.9	2.0
F2CSe–HOI-II	0.001	−0.000	−9.7	3.4
F2CSe–HOF-III	0.001	−0.001	−5.7	−0.5
F2CSe–HOCl-III	0.001	0.001	−5.7	−1.6
F2CSe–HOBr-III	0.001	0.001	−6.7	−0.5
F2CSe–HOI-III	0.001	0.002	−6.5	−2.0
F2CSe–HOCl-IV	−0.000	0.013	1.8	−39.7
F2CSe–HOBr-IV	−0.000	0.021	3.0	−46.9
F2CSe–HOI-IV	−0.001	0.022	10.8	−37.4

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The frequency shifts of stretching vibrations for the H–O and X–O bonds are also listed in Table 2. In general, these shifts are consistent with the change of the respective bond length. In I, the associated H–O stretching vibration exhibits a large red shift, varying from −169.0 to −197.1 cm⁻¹, while the X–O stretching vibration has a small blue shift. In II and III, a small shift is found for the stretching vibrations of the H–O and X–O bonds. As expected, the associated X–O bond also displays a red shift in IV, although its shift is smaller than that of the H–O bond in I. The distant H–O bond in IV shows a small blue shift, and this phenomenon was also observed in the other halogen-bonded complexes of HOX.^18,35

3.3. Interaction energies

Table 3 presents the interaction energies of the complexes at the MP2/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ levels. The CCSD(T) values are obtained with a single-point energy calculation at the CCSD(T) level on the MP2 geometry optimized structure. Both types of interaction energies are corrected for BSSE. As expected, the MP2 method overestimates the interaction energy relative to the CCSD(T) result. For the same type of complex, this overestimation is expanded with the increase of X atomic mass. The relative deviation is less than 30% in I, II, and III complexes but varies from 35% to 50% in IV. Obviously, the calculation method exerts the smallest effect on the interaction energy of the F₂CSe–HOX-I complex, has a similar effect on the interaction energies of F₂CSe–HOX-II and F₂CSe–HOX-IIII complexes, and imposes the largest influence on the interaction energy of the F₂CSe–HOX-IV complex. However, the changes of interaction energies are almost similar at both the levels of theory. The following discussion is based on the MP2 interaction energies.

Table 3 Interaction energies corrected for BSSE (ΔE, kJ mol⁻¹) at the MP2/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ levels

Complexes	ΔE^MP2	ΔE^CCSD(T)
F2CSe–HOF-I	−14.4	−14.6
F2CSe–HOCl-I	−17.2	−15.5
F2CSe–HOBr-I	−17.4	−15.2
F2CSe–HOI-I	−17.5	−14.7
F2CSe–HOF-II	−7.3	−6.5
F2CSe–HOCl-II	−10.5	−8.5
F2CSe–HOBr-II	−11.6	−9.0
F2CSe–HOI-II	−13.3	−10.2
F2CSe–HOF-III	−4.8	−4.2
F2CSe–HOCl-III	−7.3	−6.0
F2CSe–HOBr-III	−8.3	−6.6
F2CSe–HOI-III	−9.6	−7.6
F2CSe–HOCl-IV	−10.1	−6.8
F2CSe–HOBr-IV	−15.7	−10.7
F2CSe–HOI-IV	−22.1	−16.3

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The relative stability of the complex depends on the nature of the X atom. For the F complexes, F₂CSe–HOF-I is the most stable, followed by F₂CSe–HOF-II, and F₂CSe–HOF-III is the weakest. For the Cl complexes, they become more stable in the order of III < IV ≈ II < I. For the Br complexes, F₂CSe–HOBr-IV is more stable than F₂CSe–HOBr-II, with both of them being less stable than F₂CSe–HOBr-I. For the I complexes, F₂CSe–HOI-IV exhibits larger stability than F₂CSe–HOI-I, and the relative stabilities of II and III are similar to those in the Br counterpart. Clearly, the chalcogen-bonded complexes are the most unstable. The tetrel-bonded complex is more stable than the chalcogen-bonded one, consistent with the positive MEP on the π-hole and σ-hole in F₂CSe. For the tetrel- and chalcogen-bonded complexes, the interaction energy shows a consistent change with the binding distance. Thus, higher interaction energy corresponds to shorter binding distance. Of course, there is little difference in stability for the four structures, thus a competition occurs between them.

It has been demonstrated that HOX can form a hydrogen bond and a halogen bond with H₂CO, and the hydrogen bond is stronger than the halogen bond although their difference in stability is reduced with the increase of X atomic mass.¹⁸ This conclusion still holds in the complexes of HOX and F₂CSe when X is F, Cl, and Br. For HOI, however, the halogen bond is stronger than the hydrogen bond in the F₂CSe complex despite the existence of both a Se⋯H hydrogen bond and a F⋯I interaction in F₂CSe–HOI-I. Actually, the interaction energy of the Se⋯H hydrogen bond is smaller than −17.5 kJ mol⁻¹ if the F⋯I interaction is deleted from F₂CSe–HOI-I. For this reason, the interaction energy of F₂CSe–HOX-I does not accurately reflect the strength of the Se⋯H hydrogen bond, which can be estimated with the binding distance. However, the strength of halogen bond is not measured with the binding distance due to the different halogen atom.

3.4. NBO analysis

To unveil the formation mechanism of these complexes, we performed an NBO analysis for them. Charge transfer and second-order perturbation energy results are given in Table 4. A charge transfer occurs between the two molecules in all the complexes. Generally, the charge transfer shows a consistent change with the interaction energy. This indicates that the charge transfer interaction plays an important role in the formation of these complexes. The magnitude of charge transfer is small in the tetrel- and chalcogen-bonded complexes, showing the minor contribution of charge transfer. The charge transfer in F₂CSe–HOX-I is larger than that of the hydrogen-bonded complex of H₂CO and HOX,¹⁸ although the interaction energy in the former is smaller than that in the latter. The possible reason is attributed to the nature of the Se atom. This atom has a bigger atomic radius and a smaller electronegativity, thus it is easier for it to lose electrons. In addition, the change of charge transfer in F₂CSe–HOX-I is not prominent with the increase of X atomic mass. A similar reason results in a big charge transfer in the halogen-bonded complexes of F₂CSe and HOX. Furthermore, the charge transfer in the halogen-bonded complexes is changed significantly with the increase of X atomic mass. For instance, it varies from 0.0244 e in F₂CSe–HOCl-IV to 0.0808 e in F₂CSe–HOI-IV. A comparative analysis indicates that the value of charge transfer is small for the complexes with Se as the Lewis acid but is large for the complexes with Se as the Lewis base.

Table 4 Charge transfer (CT, e) and second-order perturbation energy (E², kJ mol⁻¹) in the complexes at the HF/aug-cc-pVTZ level^a

Complexes	CT	E1^2	E2^2	E3^2
a E1^2 is the stabilization energy due to the orbital interaction of BDC–Se → BD^*H–O in I, LpO → BD^*C–Se in II and III, BDC–Se → BD^*Cl–O in F2CSe–HOCl-IV, and LpSe → BD^*X–O in other IV. E2^2 denotes the orbital interactions of LpSe → BD^*H–O in I, LpX → BD^*C–Se in II and III, and BDO–X → RY^*Se in IV. E3^2 denotes the orbital interaction of BDO–X → RY^*F in I. The charge transfer is the absolute value of the sum of charge on all the atoms of F2CSe in the complexes.
F2CSe–HOF-I	0.0274	47.6	6.0	0.6
F2CSe–HOCl-I	0.0286	51.1	6.8	2.5
F2CSe–HOBr-I	0.0278	48.9	6.3	2.9
F2CSe–HOI-I	0.0251	0.3	46.3	3.2
F2CSe–HOF-II	0.0054	8.2	—	—
F2CSe–HOCl-II	0.0075	10.2	0.46	—
F2CSe–HOBr-II	0.0083	11.2	2.88	—
F2CSe–HOI-II	0.0094	12.5	5.43	—
F2CSe–HOF-III	0.0031	7.5	—	—
F2CSe–HOCl-III	0.0041	8.7	—	—
F2CSe–HOBr-III	0.0039	9.7	—	—
F2CSe–HOI-III	0.0094	11.4	7.6	—
F2CSe–HOCl-IV	0.0244	28.7	9.3	—
F2CSe–HOBr-IV	0.0544	78.0	9.9	—
F2CSe–HOI-IV	0.0808	117.1	7.2	—

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The F₂CSe–HOX-I complex is analyzed with three orbital interactions of BD_C–Se → BD^*_O–H, Lp_Se → BD^*_O–H and BD_O–X → RY^*_F. The first two orbital interactions correspond to the formation of an Se⋯H hydrogen bond, while the third one is connected with the X⋯F interaction. One can see in Table 5 that the first two orbital interactions are much stronger than the third one. This means that the hydrogen bond has a dominant contribution to the stability of the F₂CSe–HOX-I complex and the X⋯F interaction plays a minor stabilizing role. In the formation of a hydrogen bond, the orbital interaction between the C=Se bond orbital and the H–O anti-bonding orbital is dominant in the complexes of HOX (X = F, Cl, and Br), whereas the orbital interaction between the lone-pair orbital of Se and the H–O anti-bonding orbital is mainly responsible for it in F₂CSe–HOI-I. The former orbital interaction is different from that in the hydrogen-bonded complexes of H₂CO–HOX¹⁸ and H₂CS–HOX,³⁵ where Lp_O(S) → BD^*_O–H is the dominant orbital interaction. With the increase of X atomic mass, the BD_O–X → RY^*_F orbital interaction becomes stronger, indicating a stronger X⋯F interaction.

Table 5 Electrostatic energy (E^ele), polarization energy (E^pol), dispersion energy (E^disp), exchange and repulsion energy (E^ex+rep), and interaction energy (E_int) in the complexes at the MP2/aug-cc-pVTZ level. All are in kJ mol^a

Complexes	E^ele	E^pol	E^disp	E^ex+rep	Eint
a Data in parentheses are the percentage of each term to the sum of the three attractive energies.
F2CSe–HOF-I	−23.9 (48.6%)	−15.5 (31.5%)	−9.8 (19.9%)	34.6	−14.5
F2CSe–HOCl-I	−26.4 (44.1%)	−15.9 (26.6%)	−17.5 (29.3%)	42.4	−17.4
F2CSe–HOBr-I	−27.4 (43.7%)	−15.7 (25.0%)	−19.6 (31.3%)	45.5	−17.3
F2CSe–HOI-I	−28.1 (42.9%)	−15.1 (23.1%)	−22.3 (34.0%)	48.3	−17.1
F2CSe–HOF-II	−10.1 (44.1%)	−2.4 (10.5%)	−10.4 (45.4%)	15.6	−7.2
F2CSe–HOCl-II	−15.5 (45.3%)	−3.7 (10.8%)	−15.0 (43.9%)	24.0	−10.3
F2CSe–HOBr-II	−20.6 (48.1%)	−5.3 (12.4%)	−16.9 (39.5%)	31.4	−11.3
F2CSe–HOI-II	−27.0 (49.5%)	−8.1 (14.8%)	−19.5 (35.7%)	41.9	−12.7
F2CSe–HOF-III	−5.3 (36.3%)	−1.8 (12.3%)	−7.5 (51.4%)	8.4	−6.1
F2CSe–HOCl-III	−8.7 (42.2%)	−2.3 (11.2%)	−9.6 (46.6%)	13.4	−7.3
F2CSe–HOBr-III	−10.7 (43.3%)	−2.9 (11.7%)	−11.1 (44.9%)	16.4	−8.2
F2CSe–HOI-III	−14.2 (45.4%)	−4.0 (12.8%)	−13.1 (41.9%)	21.9	−9.4
F2CSe–HOCl-IV	−26.8 (44.7%)	−13.3 (22.2%)	−19.9 (33.2%)	49.5	−10.4
F2CSe–HOBr-IV	−49.4 (47.9%)	−26.7 (25.9%)	−27.0 (26.2%)	87.1	−15.9
F2CSe–HOI-IV	−60.7 (46.5%)	−41.3 (31.6%)	−28.5 (21.8%)	108.4	−22.0

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Both the types of orbital interactions (Lp_O → BD^*_C–Se and Lp_X → BD^*_C–Se) have been analyzed for the F₂CSe–HOX-II complex. The former orbital interaction is related to the formation of a tetrel bond, while the latter one is involved in the X⋯Se interaction. Obviously, the F₂CSe–HOX-II complex is mainly stabilized by the tetrel bond, and the secondary X⋯Se interaction has a small contribution to its conformation. Likely, the Lp_O → BD^*_C–Se orbital interaction is a main orbital interaction in the chalcogen-bonded complexes. For the nonplanar F₂CSe–HOI-III complex, another orbital interaction of Lp_I → BD^*_C–Se also has a comparable contribution to its stability with the Lp_O → BD^*_C–Se orbital interaction. The abovementioned orbital interactions confirm the weak interactions in II and III.

There are two main orbital interactions in the halogen-bonded complexes. The dominant orbital interaction is BD_C–Se → BD^*_Cl–O in F₂CSe–HOCl-IV and Lp_Se → BD^*_X–O in F₂CSe–HOBr-IV and F₂CSe–HOI-IV, depending on the nature of HOX. The former orbital interaction is like that in F₂CSe–HOX-I (X = F, Cl, and Br). The corresponding perturbation energies are all larger than the energies of the BD_O–X → RY^*_Se orbital interactions. The large perturbation energy is consistent with the big charge transfer in the halogen-bonded complexes.

3.5. Electron density shifts

It has been demonstrated that total electron density maps can accurately determine electron density shifts,⁵¹ which is useful for detecting noncovalent interactions, particularly in complicated systems. The shifts that occur in all the complexes are plotted in Fig. 5, where red and blue regions represent increased and decreased electron densities, respectively. These maps were generated by comparing the electron density in the complex to the sum of the electron densities of the isolated subsystems frozen in the optimized structure of the complex.

Fig. 5 Electron density shifts maps of all complexes. Red and blue lines represent the increased and decreased electron densities, respectively. Contours are shown at the 0.0012 au level.

In general, there is a commonality in all these complexes. A red increase occurs on the electron donor atom such as the Se lone pair in the hydrogen and halogen bonds as well as the O lone pair in the tetrel and chalcogen bonds. Simultaneously, a blue area of charge loss is found on the acidic proton in the hydrogen bond, the π-hole in the tetrel bond, the σ-hole on the Se atom in the chalcogen bond, and the σ-hole on the halogen atom in the halogen bond. Moreover, the magnitudes of these areas can be used to estimate the strengths of the same type interactions. For instance, with the increase in the atomic mass of X, the contour near the σ-hole on the Se atom in the chalcogen bond becomes denser, consistent with the change of interaction strength. It is also found that the maps are more complicated with the increase of interaction strength and the presence of another secondary interaction.

3.6. Energy decomposition analysis

To unveil the nature of interactions in the complexes, the interaction energies are decomposed into three attractive terms: electrostatic (E^ele), polarization (E^pol), and dispersion (E^disp), which are gathered in Table 5. One can see that the electrostatic energy plays an important role in stabilizing the complexes, except for some weak complexes, where the dispersion energy is more important than the electrostatic energy or has a comparable contribution with the electrostatic energy. In the tetrel- and chalcogen-bonded complexes, the contribution of the polarization energy is much smaller than that from the electrostatic and dispersion energies.

With the increase of X atomic mass in complex I, the contribution of electrostatic and polarization energies is decreased, while that of dispersion energy is increased. The electrostatic energy has a consistent change with the positive electrostatic potential on the H atom of HOX (Table 1), indicating that the electrostatic energy is mainly from the Se⋯H hydrogen bond. The change of dispersion energy is consistent with the strength of the X⋯F interaction, showing the chief contribution of the X⋯F interaction to the dispersion energy. Moreover, the contribution of dispersion energy shows a more prominent change than that of electrostatic energy. The reverse change of both electrostatic energy and dispersion energy is responsible for the small difference of the interaction energy in different I complexes.

With the increase of X atomic mass in IV, the contribution of polarization energy increases, whereas that of dispersion energy is reduced, and both of them are different from those in I. Although the values of the three attractive terms are increased in IV with the increase of X atomic mass, the relative contribution of electrostatic energy is changed a little and the relative contribution of polarization and dispersion energies has a prominent change. Moreover, the relative contribution of polarization energy has a consistent change with the interaction energy. As a result, the polarization energy has a large contribution to the strength of the halogen bond when the halogen atom is varied.

4. Conclusions

The complexes of F₂CSe and hypohalous acid HOX (X = F, Cl, Br, and I) have been studied to identify local minima on the complex potential surface for characterizing the types of interactions to evaluate their stabilization and unveil their formation mechanism. These calculations support the following conclusions.

(1) Four equilibrium structures have been found on the F₂CSe–HOX (X = F, Cl, Br, and I) potential surface, except for F₂CSe–HOF, in which three structures are found. These complexes are mainly stabilized by a Se⋯H hydrogen bond (I), a π⋯O tetrel bond (II), a Se⋯O chalcogen bond (III), and a Se⋯X halogen bond (IV). A secondary X⋯F interaction coexists with the hydrogen bond in I and a Se⋯X interaction is also present in most II complexes. Both secondary interactions in I and II become stronger with the increase of X atomic mass.

(2) Complex I is the most stable for HOX (X = F, Cl, and Br), showing a little dependence on the nature of X due to the coexistence of the Se⋯H hydrogen bond and the X⋯F interaction. For HOI, complex IV is more stable than complex I. With the increase of X atomic mass, complexes II, III and IV become more stable. The chalcogen bond is weakest, and the tetrel bond is stronger than the chalcogen bond.

(3) In I, the associated H–O bond is elongated and shows a large red shift, while the X–O has a small change. In IV, the X–O bond is also elongated but has a small red shift, while the distant H–O bond displays a small blue shift. The changes of H–O and X–O bonds are negligible in II and III.

(4) In I, the BD_C–Se → BD^*_H–O orbital interaction is dominant in the hydrogen bond of HOX (X = F, Cl, and Br), while the Lp_Se → BD^*_H–O one plays a main role in the hydrogen bond of HOI. The Lp_O → BD^*_C–Se orbital interaction has a major contribution to the formation of tetrel and chalcogen bonds, although the Lp_I → BD^*_C–Se one is also important in F₂CSe–HOI-III. For the halogen bond, the BD_C–Se → BD^*_Cl–O orbital interaction is dominant for HOCl, while the Lp_Se → BD^*_X–O one is principal for HOBr and HOI.

(5) The electrostatic energy is dominant in the Se⋯H hydrogen bond, while the X⋯F interaction in I is dominated by the dispersion energy. The dispersion energy is comparable with the electrostatic energy in the tetrel and chalcogen bonds. The electrostatic energy is also the principal attractive force in the halogen bond, but the polarization energy has major responsibility for the change of its interaction energy with HOX.

Acknowledgements

This work was supported by the Outstanding Youth Natural Science Foundation of Shandong Province (JQ201006), the Key Project of Natural Science Foundation of Shandong Province of China (ZR2013HZ004), and the Graduate Innovation Foundation of Yantai University (YJSZ201509).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra08034g

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