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The bifurcate chalcogen bond: Some theoretical observations
Journal of Molecular Structure: THEOCHEM 916 (2009) 135–138
Contents lists available at ScienceDirect
Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem
The bifurcate chalcogen bond: Some theoretical observations Yu Zhang, Weizhou Wang * College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, China
a r t i c l e
i n f o
Article history: Received 17 August 2009 Received in revised form 11 September 2009 Accepted 11 September 2009 Available online 16 September 2009
a b s t r a c t The bifurcate S(SS) Y (Y = F , Cl , Br , N or O) interactions have been investigated theoretically at the MP2/aug-cc-pVDZ theory level. The strength of the strongest bifurcate chalcogen bond studied was found to be about 35 kcal/mol, larger than that of any known halogen bond. Employing the natural bond orbital analysis, the nature of the bond-length change and the corresponding spectral shift upon bifurcate chalcogen bond formation was also studied. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Bifurcate chalcogen bond Bond-length change Spectral shift NBO
1. Introduction Very recently, a sister noncovalent bond to halogen bond, termed chalcogen bond, was defined [1]. It was found that many properties of the chalcogen bond are analogous to those of the halogen bond [1]. Let us add here that extensive studies have shown that the properties of the halogen bond are also very much like those of the hydrogen bond [2]. This means that the nature of the three types of molecular interactions is almost the same. However, unlike the halogen atom and the hydrogen atom, the chalcogen atom can have two single bonds with other atoms, which results in the structures of the chalcogen bond may be different from those of the halogen bond and the hydrogen bond. For instance, the unique bifurcate S(SS) Y interactions (Fig. 1) were frequently observed in organic crystals [3]. A search of the Cambridge Structural Database [4,5] (CSD, version 5.28, plus 3 updates) showed that there are 315 crystal structures that contain such kind of configuration. Then, an interesting and important question is whether these unique structures can have some unique properties. In this paper, in order to answer this question, we selected 1,2,4-dithiazolidine-3,5dione (DtsNH) [6–8] as the chalcogen donor and F , Cl , Br , NH3, H2O and HCHO as the chalcogen acceptors to study the bifurcate S(SS) Y (Y is electron donor) interactions in detail theoretically. Since it is not the purpose of this paper to systematically explore the potential-energy surfaces of the model com-
* Corresponding author. Fax: +86 379 65523821. E-mail address: [email protected] (W. Wang). 0166-1280/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2009.09.021
plexes DtsNH F , DtsNH Cl , DtsNH Br , DtsNH NH3, DtsNH H2O and DtsNH HCHO, we only concentrate on the structure of the complex with the bifurcate S(SS) Y interaction no matter whether it is a minimum or a transition state.
2. Computational details Using the Gaussian 03 program package [9], structures were fully optimized and characterized by frequency computations at the second-order Møller–Plesset (MP2) theory level with the Dunning’s correlation consistent basis set aug-cc-pVDZ. Interaction energies and electrostatic potentials were also calculated at the MP2/aug-cc-pVDZ level of theory. The basis set superposition error (BSSE) was eliminated by the standard counterpoise (CP) correction method of Boys and Bernardi [10]. In order to check the reliability of the MP2/aug-cc-pVDZ calculations, we also calculated the C–S bond-length change and interaction energy of the complex DtsNH Cl (Fig. 2) at the MP2/aug-cc-pVTZ and MP2/aug-cc-pVQZ levels of theory, respectively. The interaction energies equal to 20.25, 22.38, and 23.34 kcal/mol and the C–S bonds contract about 0.0133, 0.0127, and 0.0123 Å at the MP2/aug-cc-pVDZ, MP2/aug-cc-pVTZ, and MP2/aug-cc-pVQZ levels of theory, respectively. Here, we can see the accuracy of the MP2/aug-cc-pVDZ calculation is pretty good. The properties of the chalcogen-bonded complexes were further investigated employing the natural bond orbital (NBO) theory of Weinhold and co-workers [11]. NBO analysis used the MP2-optimized structures, the Hartree–Fock (HF) densities, and the built-in subroutines of the Gaussian 03 program.
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As shown in Table 1, all the C–S, S–S, and N–H bonds of the six chalcogen-bonded complexes are contracted upon the complexes formation. In each complex, the contraction of the C–S and S–S bonds is more outstanding than that of the N–H bond. The strongest electron donor, F , causes a maximum contraction of the C–S, S–S, or N–H bond. The corresponding changes of the harmonic vibrational frequencies are also shown in Table 1. The frequency analysis reveals the blue-shifting character of all the C–S, S–S, and N–H bonds of the six chalcogen-bonded complexes. This is consistent with the commonly accepted view on the bond-length change frequency shift correlation upon molecule complexation, that is, bond elongation means red shift and bond contraction indicates blue shift. The existence of the blue-shifting hydrogen bond has been verified theoretically and experimentally [12–14]. Similarly, the blue-shifting lithium bond and the blue-shifting halogen bond were also proposed [15–17]. Here, the formation of the C–SY chalcogen bond leads to C–S bond shortening and to a blue shift of the C–S IR stretching frequency. So the C–SY chalcogen bonds reported in this paper can be classified as the blue-shifting chalcogen bond. Again, this shows that the properties of the three types of molecular interactions are very similar. It is also noticed from Table 1 that only the structure of
S Y S (Y: electron donor) Fig. 1. The bifurcate S(SS) Y interactions.
3. Results and discussion 3.1. Geometries, vibrational frequencies and interaction energies Changes of the bond-length and the corresponding stretching frequency, number of imaginary frequencies, and CP-corrected interaction energies of the six complexes which contain the bifurcate S(SS) Y interactions were listed in Table 1. The structures of DtsNH and the complexes can be seen from Fig. 2 along with the values of some selected bond lengths and distances.
O S
2.3807 S
2.1216
N
H
1.7806 Cl
2.3807 S 1.7806
1.7942
S
3.0149
2.1046 N H 1.0179
F
1.0193
S
O
O
1.7942
1.7809
-
DtsNH···Cl
-
O S
3.1865
2.1180 N H 1.0182
3.1865 S 1.7822
O
O
1.7822
Br
H
1.0180
S
3.0149
DtsNH···F
DtsNH
N
2.1159
O
O
1.7809
HH N H
O
3.0889
1.7902
S
2.1160 N H 1.0191
3.0935 S 1.7900
O
-
DtsNH···Br
DtsNH···NH3
O
O
1.7898
1.7899
3.0346 H H O
S
2.1204 3.0294
3.0315
H N
H
O
1.0190
S
1.7897
H
S N
2.1200
H
1.0189
3.0315 S 1.7898
O
O
DtsNH···H2O
DtsNH···HCHO
Fig. 2. Optimized structures of DtsNH and the complexes of DtsNH with F , Cl , Br , NH3, H2O, and HCHO, respectively, at the MP2/aug-cc-pVDZ theory level. Bond lengths or distances indicated are in Å.
Table 1 Changes of the bond-length (Dr, Å) and the corresponding stretching frequency (Dt, cm 1), number of imaginary frequencies (Nimg), and CP-corrected interaction energies (DECP , kcal/mol) of the six chalcogen-bonded complexes at the MP2/aug-cc-pVDZ theory levela. Complex DtsNH F DtsNH Cl DtsNH Br DtsNH NH3 DtsNH H2O DtsNH HCHO a
Dr(C–S) 0.0136 0.0133 0.0120 0.0042 0.0043 0.0044
Dt(C–S) +6.4 +8.1 +6.7 +5.8 +4.8 +4.6
Dr(S–S) 0.0170 0.0057 0.0035 0.0056 0.0012 0.0016
Dt(S–S) +19.9 +10.9 +8.8 +9.2 +4.4 +4.3
Dr(N–H) 0.0015 0.0013 0.0011 0.0002 0.0004 0.0004
For each property X, DX represents the difference of the property between the complex and the free molecule.
Dt(N–H)
Nimg
+8.2 +8.5 +6.3 +1.5 +3.9 +4.3
0 0 0 0 0 1
DECP 34.34 20.25 17.70 4.56 3.52 3.35
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Y. Zhang, W. Wang / Journal of Molecular Structure: THEOCHEM 916 (2009) 135–138
DtsNH HCHO is a transition state (only one imaginary frequency) and other structures are true minima (all real frequencies) on their respective potential-energy surfaces. It can be clearly seen from Table 1 that the calculated CP-corrected interaction energies of the six chalcogen-bonded complexes are in the range of 3.00 to 35.00 kcal/mol. The strongest interaction energy ( 34.34 kcal/mol) was observed between DtsNH and F , larger than that of any known halogen bond [16–20]. The strong bifurcate S(SS) Y interactions indicate their possible important roles in the crystal growth and design. Fig. 3 is the molecular electrostatic potential map of DtsNH. The electronegative atoms N and O withdraw electron density from the hetero ring and the two S atoms. So the MEP values all become positive around the S–S bond. Evidently, the positive values of the MEP around the S–S bond are responsible for the existence of the bifurcate S(SS) Y interactions and the electrostatic energy represents the dominant attractive contribution.
3.2. Nature of the bond-length change upon complex formation In a recent paper, the nature of the X–Hal bond-length change upon the X–Hal Z complexes (Hal = Cl, Br, I; Z = region of high electron density) formation has been revisited theoretically [17]. The results have clearly shown that the X–Hal bond-length change upon halogen-bonded complex formation is determined mainly by the electrostatic attractive interaction and the charge transfer interaction [17]. In the case of strongly polar bond, the electrostatic interaction always causes bond elongation while in the case of weakly polar bond it causes bond contraction. The charge transfer interaction generally results in the X–Hal bond elongation; either it is a more polar bond or it is a less polar bond. The net bond-length change is determined by the balance between the electrostatic attractive interaction and the charge transfer interaction. In this paper, we will apply this simple ‘‘electrostatic interaction plus charge transfer interaction” explanation to the chalcogen-bonded complexes and check if this explanation is suitable for the bondlength change upon the bifurcate chalcogen bond formation. For a better understanding of the nature of the bond-length change upon chalcogen-bonded complex formation, NBO analysis was carried out at the MP2/aug-cc-pVDZ theory level. Some significant donor-acceptor orbital interactions and their second-order perturbation stabilization energies are summarized in Table 2. The second-order perturbation stabilization energy, DE2, will allow us quantitatively evaluating the charge transfer between different orbitals. From the molecular electrostatic potential map of DtsNH (Fig. 3), we can see that the C–S, S–S, and N–H bonds are all of the less polar bonds. According to the ‘‘electrostatic interaction plus charge transfer interaction” explanation, on the one hand, all these bonds will be contracted in the negative electric field region of the electron donor Y and, on the other hand, the charge
Table 2 Some significant donor-acceptor orbital interactions and their second-order perturbation stabilization energies (DE2, kcal/mol) of the six chalcogen-bonded complexesa. Complex
Donor
Acceptor
Interaction
DE 2
DtsNH F
LP(1) LP(3) LP(4) LP(3)
F F F F
BD*(1) BD*(1) BD*(1) BD*(1)
C–S C–S C–S S–S
n ? r* n ? r* n ? r* n ? r*
1.51 3.24 10.22 2.12
DtsNH Cl
LP(1) LP(3) LP(4) LP(3)
Cl Cl Cl Cl
BD*(1) BD*(1) BD*(1) BD*(1)
C–S C–S C–S S–S
n ? r* n ? r* n ? r* n ? r*
0.29 1.33 6.00 0.29
DtsNH Br
LP(1) LP(3) LP(4) LP(3)
Br Br Br Br
BD*(1) BD*(1) BD*(1) BD*(1)
C–S C–S C–S S–S
n ? r* n ? r* n ? r* n ? r*
0.20 1.09 5.36 0.17
DtsNH NH3
LP(1) N
BD*(1) C–S
n ? r*
2.09
DtsNH H2O
LP(1) O LP(2) O LP(1) O
BD*(1) C–S BD*(1) C–S BD*(1) S–S
n ? r* n ? r* n ? r*
0.18 1.06 0.08
DtsNH HCHO
LP(1) O LP(2) O LP(2) O
BD*(1) C–S BD*(1) C–S BD*(1) S–S
n ? r* n ? r* n ? r*
0.74 0.31 0.08
a BD* denotes the formally empty antibonding orbital and LP denotes the occupied lone pair.
transfer from the lone pair orbital(s) of Y to the C–S, S–S, and N–H antibonding r* orbitals will cause them to be elongated. As can be seen from Table 2, there is no charge transfer from the lone pair orbital(s) of Y to the N–H antibonding r* orbital because of the long distance and little charge transfer from the lone pair orbital(s) of Y to the S–S antibonding r* orbital due to the unfavorable orbital overlap. So it is not difficult to understand the net contraction of the N–H and S–S bonds. In fact, the bond-length changes of S–S and N–H bonds here are two very good examples to show the role of the electrostatic attractive interaction in the bond-length change upon molecule complexation. For the C–S bonds, we can see from Table 2 that the charge transfer from the lone pair orbital(s) of Y to the C–S antibonding r* orbital is considerable. The largest charge transfer occurs between the lone electron pairs of F and the C–S antibonding r* orbital (the DE2 is 10.22 kcal/mol). Why do the C–S bonds grow consistently shorter although there is such a large amount of charge being transferred into the C–S antibonding r* orbital? This can be explained by employing the electrostatic attractive interaction: it is the stronger electrostatic attractive interaction that causes the C–S bond in these complexes to be contracted. Evidently, the ‘‘electrostatic interaction plus charge transfer interaction” explanation still works for the bond-length change upon the bifurcate chalcogen bond formation. In comparing the stabilization energy terms in Table 2 with the corresponding interaction energy terms in Table 1, it is found that the values of the second-order perturbation stabilization energies are much smaller than those of the corresponding interaction energies. This indicates that the consideration of only the charge transfer interaction is not sufficient in describing the ground-state stabilization of the bifurcate S(SS) Y chalcogen bond and the electrostatic effect plays a more important role. 4. Conclusions
Fig. 3. MP2/aug-cc-pVDZ molecular electrostatic potential for 1,2,4-dithiazolidine3,5-dione. The black region represents the positive part of the electrostatic potential, and the white region represents the negative part of the electrostatic potential.
The geometries, vibrational frequencies and interaction energies of the bifurcate S(SS) Y (Y = F , Cl , Br , N or O) halogen bonds have been studied at the MP2/aug-cc-pVDZ theory level. The results shows that the bifurcate S(SS) Y halogen bonds studied here are dominant mainly by the electrostatic attractive interaction. It was also found that the C–S, S–S, and N–H bonds are all contracted upon the bifurcate halogen bonds formation,
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companied by the corresponding blue shifts of the stretching frequencies. The bond-length changes of the C–S, S–S, and N–H bonds can be explained by the simple ‘‘electrostatic interaction plus charge transfer interaction” explanation proposed previously for the halogen bond [17]. All these indicate that the properties of the bifurcate chalcogen bond are still similar to those of the halogen bond and the hydrogen bond. Acknowledgments The author is grateful to Prof. Anmin Tian and Prof. Ning-Bew Wong for continuous help. This work was supported by a grant from Luoyang Normal University. References [1] W. Wang, B. Ji, Y. Zhang, J. Phys. Chem. A 113 (2009) 8132. [2] P. Metrangolo, G. Resnati, Science 321 (2008) 918. [3] M. Iwaoka, S. Takemoto, S. Tomoda, J. Am. Chem. Soc. 124 (2002) 10613.
[4] F.H. Allen, Acta Crystallogr. Sect. B 58 (2002) 380. [5] F.H. Allen, W.D.S. Motherwell, Acta Crystallogr. Sect. B 58 (2002) 407. [6] Q. Xu, K. Musier-Forsyth, R.P. Hammer, G. Barany, Nucleic Acids Res. 24 (1996) 1602. [7] L. Chen, T.R. Thompson, R.P. Hammer, G. Barany, J. Org. Chem. 61 (1996) 6639. [8] M.J. Barany, R.P. Hammer, R.B. Merrifield, G. Barany, J. Am. Chem. Soc. 127 (2005) 508. [9] M.J. Frisch et al., Gaussian 03, Revision C.02, Gaussian Inc., Wallingford, CT, 2004. [10] S.F. Boys, F. Bernardi, Mol. Phys. 19 (1970) 553. [11] A.E. Reed, L.A. Curitss, F. Weinhold, Chem. Rev. 88 (1988) 899. [12] P. Hobza, V. Spirko, H.L. Selzle, E.W. Schlag, J. Phys. Chem. A 102 (1998) 2501. [13] P. Hobza, Z. Havlas, Chem. Rev. 100 (2000) 4253. [14] B.J. van der Veken, W.A. Herrebout, R. Szostak, D.N. Shchepkin, Z. Havlas, P. Hobza, J. Am. Chem. Soc. 123 (2001) 12290. [15] Y. Feng, L. Liu, J.T. Wang, X.S. Li, Q.X. Guo, Chem. Commun. (2004) 88. [16] W. Wang, N.B. Wong, W. Zheng, A. Tian, J. Phys. Chem. A 108 (2004) 1799. [17] W. Wang, P. Hobza, J. Phys. Chem. A 112 (2008) 4114. [18] A. Karpfen, Struct. Bond. 126 (2008) 1. [19] Y.H. Wang, Y.X. Lu, J.W. Zou, Q.S. Yu, Int. J. Quantum Chem. 108 (2008) 90. [20] K.E. Riley, J.S. Murray, P. Politzer, M.C. Concha, P. Hobza, J. Chem. Theory Comput. 5 (2009) 155.
Contents lists available at ScienceDirect
Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem
The bifurcate chalcogen bond: Some theoretical observations Yu Zhang, Weizhou Wang * College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, China
a r t i c l e
i n f o
Article history: Received 17 August 2009 Received in revised form 11 September 2009 Accepted 11 September 2009 Available online 16 September 2009
a b s t r a c t The bifurcate S(SS) Y (Y = F , Cl , Br , N or O) interactions have been investigated theoretically at the MP2/aug-cc-pVDZ theory level. The strength of the strongest bifurcate chalcogen bond studied was found to be about 35 kcal/mol, larger than that of any known halogen bond. Employing the natural bond orbital analysis, the nature of the bond-length change and the corresponding spectral shift upon bifurcate chalcogen bond formation was also studied. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Bifurcate chalcogen bond Bond-length change Spectral shift NBO
1. Introduction Very recently, a sister noncovalent bond to halogen bond, termed chalcogen bond, was defined [1]. It was found that many properties of the chalcogen bond are analogous to those of the halogen bond [1]. Let us add here that extensive studies have shown that the properties of the halogen bond are also very much like those of the hydrogen bond [2]. This means that the nature of the three types of molecular interactions is almost the same. However, unlike the halogen atom and the hydrogen atom, the chalcogen atom can have two single bonds with other atoms, which results in the structures of the chalcogen bond may be different from those of the halogen bond and the hydrogen bond. For instance, the unique bifurcate S(SS) Y interactions (Fig. 1) were frequently observed in organic crystals [3]. A search of the Cambridge Structural Database [4,5] (CSD, version 5.28, plus 3 updates) showed that there are 315 crystal structures that contain such kind of configuration. Then, an interesting and important question is whether these unique structures can have some unique properties. In this paper, in order to answer this question, we selected 1,2,4-dithiazolidine-3,5dione (DtsNH) [6–8] as the chalcogen donor and F , Cl , Br , NH3, H2O and HCHO as the chalcogen acceptors to study the bifurcate S(SS) Y (Y is electron donor) interactions in detail theoretically. Since it is not the purpose of this paper to systematically explore the potential-energy surfaces of the model com-
* Corresponding author. Fax: +86 379 65523821. E-mail address: [email protected] (W. Wang). 0166-1280/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2009.09.021
plexes DtsNH F , DtsNH Cl , DtsNH Br , DtsNH NH3, DtsNH H2O and DtsNH HCHO, we only concentrate on the structure of the complex with the bifurcate S(SS) Y interaction no matter whether it is a minimum or a transition state.
2. Computational details Using the Gaussian 03 program package [9], structures were fully optimized and characterized by frequency computations at the second-order Møller–Plesset (MP2) theory level with the Dunning’s correlation consistent basis set aug-cc-pVDZ. Interaction energies and electrostatic potentials were also calculated at the MP2/aug-cc-pVDZ level of theory. The basis set superposition error (BSSE) was eliminated by the standard counterpoise (CP) correction method of Boys and Bernardi [10]. In order to check the reliability of the MP2/aug-cc-pVDZ calculations, we also calculated the C–S bond-length change and interaction energy of the complex DtsNH Cl (Fig. 2) at the MP2/aug-cc-pVTZ and MP2/aug-cc-pVQZ levels of theory, respectively. The interaction energies equal to 20.25, 22.38, and 23.34 kcal/mol and the C–S bonds contract about 0.0133, 0.0127, and 0.0123 Å at the MP2/aug-cc-pVDZ, MP2/aug-cc-pVTZ, and MP2/aug-cc-pVQZ levels of theory, respectively. Here, we can see the accuracy of the MP2/aug-cc-pVDZ calculation is pretty good. The properties of the chalcogen-bonded complexes were further investigated employing the natural bond orbital (NBO) theory of Weinhold and co-workers [11]. NBO analysis used the MP2-optimized structures, the Hartree–Fock (HF) densities, and the built-in subroutines of the Gaussian 03 program.
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As shown in Table 1, all the C–S, S–S, and N–H bonds of the six chalcogen-bonded complexes are contracted upon the complexes formation. In each complex, the contraction of the C–S and S–S bonds is more outstanding than that of the N–H bond. The strongest electron donor, F , causes a maximum contraction of the C–S, S–S, or N–H bond. The corresponding changes of the harmonic vibrational frequencies are also shown in Table 1. The frequency analysis reveals the blue-shifting character of all the C–S, S–S, and N–H bonds of the six chalcogen-bonded complexes. This is consistent with the commonly accepted view on the bond-length change frequency shift correlation upon molecule complexation, that is, bond elongation means red shift and bond contraction indicates blue shift. The existence of the blue-shifting hydrogen bond has been verified theoretically and experimentally [12–14]. Similarly, the blue-shifting lithium bond and the blue-shifting halogen bond were also proposed [15–17]. Here, the formation of the C–SY chalcogen bond leads to C–S bond shortening and to a blue shift of the C–S IR stretching frequency. So the C–SY chalcogen bonds reported in this paper can be classified as the blue-shifting chalcogen bond. Again, this shows that the properties of the three types of molecular interactions are very similar. It is also noticed from Table 1 that only the structure of
S Y S (Y: electron donor) Fig. 1. The bifurcate S(SS) Y interactions.
3. Results and discussion 3.1. Geometries, vibrational frequencies and interaction energies Changes of the bond-length and the corresponding stretching frequency, number of imaginary frequencies, and CP-corrected interaction energies of the six complexes which contain the bifurcate S(SS) Y interactions were listed in Table 1. The structures of DtsNH and the complexes can be seen from Fig. 2 along with the values of some selected bond lengths and distances.
O S
2.3807 S
2.1216
N
H
1.7806 Cl
2.3807 S 1.7806
1.7942
S
3.0149
2.1046 N H 1.0179
F
1.0193
S
O
O
1.7942
1.7809
-
DtsNH···Cl
-
O S
3.1865
2.1180 N H 1.0182
3.1865 S 1.7822
O
O
1.7822
Br
H
1.0180
S
3.0149
DtsNH···F
DtsNH
N
2.1159
O
O
1.7809
HH N H
O
3.0889
1.7902
S
2.1160 N H 1.0191
3.0935 S 1.7900
O
-
DtsNH···Br
DtsNH···NH3
O
O
1.7898
1.7899
3.0346 H H O
S
2.1204 3.0294
3.0315
H N
H
O
1.0190
S
1.7897
H
S N
2.1200
H
1.0189
3.0315 S 1.7898
O
O
DtsNH···H2O
DtsNH···HCHO
Fig. 2. Optimized structures of DtsNH and the complexes of DtsNH with F , Cl , Br , NH3, H2O, and HCHO, respectively, at the MP2/aug-cc-pVDZ theory level. Bond lengths or distances indicated are in Å.
Table 1 Changes of the bond-length (Dr, Å) and the corresponding stretching frequency (Dt, cm 1), number of imaginary frequencies (Nimg), and CP-corrected interaction energies (DECP , kcal/mol) of the six chalcogen-bonded complexes at the MP2/aug-cc-pVDZ theory levela. Complex DtsNH F DtsNH Cl DtsNH Br DtsNH NH3 DtsNH H2O DtsNH HCHO a
Dr(C–S) 0.0136 0.0133 0.0120 0.0042 0.0043 0.0044
Dt(C–S) +6.4 +8.1 +6.7 +5.8 +4.8 +4.6
Dr(S–S) 0.0170 0.0057 0.0035 0.0056 0.0012 0.0016
Dt(S–S) +19.9 +10.9 +8.8 +9.2 +4.4 +4.3
Dr(N–H) 0.0015 0.0013 0.0011 0.0002 0.0004 0.0004
For each property X, DX represents the difference of the property between the complex and the free molecule.
Dt(N–H)
Nimg
+8.2 +8.5 +6.3 +1.5 +3.9 +4.3
0 0 0 0 0 1
DECP 34.34 20.25 17.70 4.56 3.52 3.35
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Y. Zhang, W. Wang / Journal of Molecular Structure: THEOCHEM 916 (2009) 135–138
DtsNH HCHO is a transition state (only one imaginary frequency) and other structures are true minima (all real frequencies) on their respective potential-energy surfaces. It can be clearly seen from Table 1 that the calculated CP-corrected interaction energies of the six chalcogen-bonded complexes are in the range of 3.00 to 35.00 kcal/mol. The strongest interaction energy ( 34.34 kcal/mol) was observed between DtsNH and F , larger than that of any known halogen bond [16–20]. The strong bifurcate S(SS) Y interactions indicate their possible important roles in the crystal growth and design. Fig. 3 is the molecular electrostatic potential map of DtsNH. The electronegative atoms N and O withdraw electron density from the hetero ring and the two S atoms. So the MEP values all become positive around the S–S bond. Evidently, the positive values of the MEP around the S–S bond are responsible for the existence of the bifurcate S(SS) Y interactions and the electrostatic energy represents the dominant attractive contribution.
3.2. Nature of the bond-length change upon complex formation In a recent paper, the nature of the X–Hal bond-length change upon the X–Hal Z complexes (Hal = Cl, Br, I; Z = region of high electron density) formation has been revisited theoretically [17]. The results have clearly shown that the X–Hal bond-length change upon halogen-bonded complex formation is determined mainly by the electrostatic attractive interaction and the charge transfer interaction [17]. In the case of strongly polar bond, the electrostatic interaction always causes bond elongation while in the case of weakly polar bond it causes bond contraction. The charge transfer interaction generally results in the X–Hal bond elongation; either it is a more polar bond or it is a less polar bond. The net bond-length change is determined by the balance between the electrostatic attractive interaction and the charge transfer interaction. In this paper, we will apply this simple ‘‘electrostatic interaction plus charge transfer interaction” explanation to the chalcogen-bonded complexes and check if this explanation is suitable for the bondlength change upon the bifurcate chalcogen bond formation. For a better understanding of the nature of the bond-length change upon chalcogen-bonded complex formation, NBO analysis was carried out at the MP2/aug-cc-pVDZ theory level. Some significant donor-acceptor orbital interactions and their second-order perturbation stabilization energies are summarized in Table 2. The second-order perturbation stabilization energy, DE2, will allow us quantitatively evaluating the charge transfer between different orbitals. From the molecular electrostatic potential map of DtsNH (Fig. 3), we can see that the C–S, S–S, and N–H bonds are all of the less polar bonds. According to the ‘‘electrostatic interaction plus charge transfer interaction” explanation, on the one hand, all these bonds will be contracted in the negative electric field region of the electron donor Y and, on the other hand, the charge
Table 2 Some significant donor-acceptor orbital interactions and their second-order perturbation stabilization energies (DE2, kcal/mol) of the six chalcogen-bonded complexesa. Complex
Donor
Acceptor
Interaction
DE 2
DtsNH F
LP(1) LP(3) LP(4) LP(3)
F F F F
BD*(1) BD*(1) BD*(1) BD*(1)
C–S C–S C–S S–S
n ? r* n ? r* n ? r* n ? r*
1.51 3.24 10.22 2.12
DtsNH Cl
LP(1) LP(3) LP(4) LP(3)
Cl Cl Cl Cl
BD*(1) BD*(1) BD*(1) BD*(1)
C–S C–S C–S S–S
n ? r* n ? r* n ? r* n ? r*
0.29 1.33 6.00 0.29
DtsNH Br
LP(1) LP(3) LP(4) LP(3)
Br Br Br Br
BD*(1) BD*(1) BD*(1) BD*(1)
C–S C–S C–S S–S
n ? r* n ? r* n ? r* n ? r*
0.20 1.09 5.36 0.17
DtsNH NH3
LP(1) N
BD*(1) C–S
n ? r*
2.09
DtsNH H2O
LP(1) O LP(2) O LP(1) O
BD*(1) C–S BD*(1) C–S BD*(1) S–S
n ? r* n ? r* n ? r*
0.18 1.06 0.08
DtsNH HCHO
LP(1) O LP(2) O LP(2) O
BD*(1) C–S BD*(1) C–S BD*(1) S–S
n ? r* n ? r* n ? r*
0.74 0.31 0.08
a BD* denotes the formally empty antibonding orbital and LP denotes the occupied lone pair.
transfer from the lone pair orbital(s) of Y to the C–S, S–S, and N–H antibonding r* orbitals will cause them to be elongated. As can be seen from Table 2, there is no charge transfer from the lone pair orbital(s) of Y to the N–H antibonding r* orbital because of the long distance and little charge transfer from the lone pair orbital(s) of Y to the S–S antibonding r* orbital due to the unfavorable orbital overlap. So it is not difficult to understand the net contraction of the N–H and S–S bonds. In fact, the bond-length changes of S–S and N–H bonds here are two very good examples to show the role of the electrostatic attractive interaction in the bond-length change upon molecule complexation. For the C–S bonds, we can see from Table 2 that the charge transfer from the lone pair orbital(s) of Y to the C–S antibonding r* orbital is considerable. The largest charge transfer occurs between the lone electron pairs of F and the C–S antibonding r* orbital (the DE2 is 10.22 kcal/mol). Why do the C–S bonds grow consistently shorter although there is such a large amount of charge being transferred into the C–S antibonding r* orbital? This can be explained by employing the electrostatic attractive interaction: it is the stronger electrostatic attractive interaction that causes the C–S bond in these complexes to be contracted. Evidently, the ‘‘electrostatic interaction plus charge transfer interaction” explanation still works for the bond-length change upon the bifurcate chalcogen bond formation. In comparing the stabilization energy terms in Table 2 with the corresponding interaction energy terms in Table 1, it is found that the values of the second-order perturbation stabilization energies are much smaller than those of the corresponding interaction energies. This indicates that the consideration of only the charge transfer interaction is not sufficient in describing the ground-state stabilization of the bifurcate S(SS) Y chalcogen bond and the electrostatic effect plays a more important role. 4. Conclusions
Fig. 3. MP2/aug-cc-pVDZ molecular electrostatic potential for 1,2,4-dithiazolidine3,5-dione. The black region represents the positive part of the electrostatic potential, and the white region represents the negative part of the electrostatic potential.
The geometries, vibrational frequencies and interaction energies of the bifurcate S(SS) Y (Y = F , Cl , Br , N or O) halogen bonds have been studied at the MP2/aug-cc-pVDZ theory level. The results shows that the bifurcate S(SS) Y halogen bonds studied here are dominant mainly by the electrostatic attractive interaction. It was also found that the C–S, S–S, and N–H bonds are all contracted upon the bifurcate halogen bonds formation,
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companied by the corresponding blue shifts of the stretching frequencies. The bond-length changes of the C–S, S–S, and N–H bonds can be explained by the simple ‘‘electrostatic interaction plus charge transfer interaction” explanation proposed previously for the halogen bond [17]. All these indicate that the properties of the bifurcate chalcogen bond are still similar to those of the halogen bond and the hydrogen bond. Acknowledgments The author is grateful to Prof. Anmin Tian and Prof. Ning-Bew Wong for continuous help. This work was supported by a grant from Luoyang Normal University. References [1] W. Wang, B. Ji, Y. Zhang, J. Phys. Chem. A 113 (2009) 8132. [2] P. Metrangolo, G. Resnati, Science 321 (2008) 918. [3] M. Iwaoka, S. Takemoto, S. Tomoda, J. Am. Chem. Soc. 124 (2002) 10613.
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