Dithiocarbamate is a kind of common ligand containing sulfur, which could form complexes with all kinds of transition metals, B,B metals and rare earth. For the wide application in extracting metal ions^[1], lubricant industry^[2] and rubber vulcanization accelerant^[3], these complexes have been more and more studied. What’s more, these compounds could be the precursors for / materials^[4]and effective antidotes for cadmium poisoning^[5].
    The structures of these compounds are various. As far as dithiocarbamate is concerned, it is easy to form dimmer, polymer with B,B metals, whose coordination number is 4 or 5^[6,7]. However, the metal ions in those compounds have not been coordinative saturation, they could form adducts with ligands containing N, P, and the coordination number became 5 or 6^[8]. When nitric heterocycle were introduced, the coordination number , the difference among S-M bonds length and average S-M bond length were increasing; the symmetry and stability of complexes were decreasing; the differences of the chemical environment among sulfur atoms in ligands have been obvious; IR spectra became more complicated. In recent years, the researchers have more and more interests in studying the vibration of C-S, N-C and CSS frame.
    There are many studies reported on dithiocarbamate complexes in experimental, but seldom in theory. In recent work, we have studied Zn₂(S₂CNBuⁿ₂)₂ by quantum chemistry method^[9]. In this paper, zinc complex with dibenzyldithiocarbamate Zn(S₂CNBz₂)₂ 1 and adduct with pyridine Zn(S₂CNBz₂)₂Py 2 (Bz = benzyl, Py = pyridine) have been performed by density functional theory at B3LYP/6-31G* level. Through contrasting between the energies of frontier molecular orbital, charge distribution and components of HOMO orbital, the authors found that after introduced pyridine, the molecular symmetry had been destroyed, and the Zn-S bond length became longer for the weakening of the ZnS₂CN conjugation. The pyridine in the molecular form had tendency to loose. The calculated IR spectra of complexes , which was similar to actual, have been interpreted.

2.2 Natural charges distribution of atoms in the complexes              
Table 3 shows the natural charges distribution of atoms in the complexes. It is indicated from Table 3:
1. Obvious covalent character: The valence of Zn atoms in two complexes are both +2, but the natural charges of Zn atoms, all less than +1, are 0.481 and 0.564, respectively. It demonstrated that obvious covalent bond have been formed between Zn atom and ligand.
2. Natural charges distribution of S₂CNC₂ motion in dithiocarbamate exhibits polarity alternation: Under the effect of polar atoms S, N, the natural charges distribution of S₂CNC₂ motion in dithiocarbamate exhibits polarity alternation along (S,S)-C-N-(C,C). And, in complex 2, the C atoms in pyridine also exhibit polarity alternation along N1-C1-C2-C3-C4-C5.
3. Natural charges distribution incline to equilibration: The absolute values of all atoms' natural charges are below 1. It indicated that natural charges distribution of complexes incline to equilibration because it exhibits polarity alternation.
    When pyridine was introduced, the natural charge of Zn atom has been more positive; natural charges of ligands(DTC) have been more negative; and natural charge of pyridine in complex 2 was positive(see Table 3). It demonstrated that electrons had transferred from Zn to DTC; the covalent character of Zn-S has been decreased, and weakened; Zn-S bond length has been extended, meanwhile C-S has shortened and strengthened. That the natural charge of pyridine in complex 2 is positive indicates that combination between pyridine and Zn is not so stable, and the pyridine in complex 2 would lose under the circumstance^[11]. In addition, the introduced pyridine made the natural charges of S atoms in DTC different, which increased the asymmetry of DTC.

2.3 The bond character in complexes                  
The square sums of all kinds of atom orbital coefficients have been added, and been normalized. The results are the contribution of atom orbital in HOMO orbital or components of HOMO orbital(see Table 4). Fig 2 shows HOMO orbital of complexes 1 and 2.
    HOMO orbital of complexes are π bond which was composed of multitudinous p(p) orbital of S and little p(p) orbital of N, C(see Table 4). HOMO orbital of complex 2 is a little distorted d-p antibonding orbital, and its energy is -0.17701a.u.. HOMO orbital of complex 1 is torsional d-p antibonding orbital. Its energy(-0.21516 a.u.) is lower than complex 2 because of partly overlapping between S atoms and Zn atom(see Fig 2).
    The s orbital population of HOMO orbital in complex 2 is higher than complex 1, which imaged that the symmetry of complex 2 was lower than complex 1. The p(p) orbital of N, C of HOMO in complex 2 is lower than complex 1, and p(p) orbital of S of HOMO in complex 2 is higher than complex 1. It indicated that the introduced pyridine had destroyed conjugate of ZnS₂CN, made electrons congregate on S atoms, which agreed with natural charges distribution. From crystal x-ray analysis, Zn-S,C-N bond length in complex 2 is longer than complex 1, and S-C is shortened and strengthened.

                             Zn(S₂CNBz₂)₂                                                             Zn(S₂CNBz₂)₂Py

Fig. 2 HOMO orbital of the complexes

Table4 The components of HOMO orbital in complexes calculated by B3LYP

Table 5 Average S-Zn bond length in Zinc complexes with dithiocarbamate and their nitric heterocycle adducts (nm)

Table 6 Mainly calculated and experimental IR vibration peak value of complex 1(cm^-1)

values in parentheses have been multiplied correction factor: 0.9613

Fig. 3 The calculated and experimental IR spectra of complex 1
(The calculated frequency will be multiplied by correction factor: 0.9613)

2.5 IR spectra          
Fig 3 illustrates the calculated and experimental IR spectra of complex 1. Table 6 shows the mainly calculated and experimental IR vibration peak value of complex 1. Fig 4 illustrates the calculated and experimental IR spectra of complex 2. Table 7 shows the mainly calculated and experimental IR vibration peak value of complex 2.
    From crystal x-ray analysis, it is known that complex 1 has higher symmetry than complex 2. Two ligands in complex have little difference. The C-N vibration appears in 1549.1 (1489)cm^-1 and 1544.64 (1485) cm^-1(value in parentheses have been multiplied by correction factor: 0.9613^[20]), and the experimental value is 1489 cm^-1 and 1485 cm^-1. It demonstrates that C-N have double bond character, which is in accordance with existing reports. The vibration of appears in 1224.49 (1177)cm^-1 and 1199 (1152) cm^-1, and the experimental value is 1157 cm^-1, which reveals obvious single bond character. It indicates that the electrons have only delocalized in S₂CN. The C-S vibration has little split, appears in 1027.5 (987.7)cm^-1 and 1026.55 (986.8)cm^-1, and experimental value is only one: 987.8cm^-1. It indicates that the chemical circumstances of S atoms in ligands have little difference.


    The symmetry of complex 2 is lower. Two dithiocarbamate ligands have greater difference, and the IR spectra is more complicated. The C-N vibration appears three peaks, 1554.54 (1494) cm^-1, 1531.68 (1472) cm^-1, 1498.57 (1441) cm^-1, and experimental value is 1494 cm^-1, 1470 cm^-1, 1450cm^-1. In two S₂CNC₂ frame of complex 2, the asymmetry of A ( C6-S1: 0.1713nm, C6-S2: 0.1705nm, C14-N2:0.1449nm, C7-N2:0.1471nm ) is higher than B ( C21-S3: 0.1716nm, C21-S4:0.1712nm, C22-N3:0.1475nm, C29-N3:0.1473nm ) and complex 1( C1-S1: 0.1730nm, C1-S2:0.1726nm, C9-N1:0.1473m、C2-N10.1479 ). And the polarization of A( S1: -0.274, S2: -0.186, C6: -0.025, N2:-0.372, C7: -0.252, C14: -0.170) is also higher than B ( S3: -0.186, S4: -0.258, C21: -0.028, N3:-0.350, C22: -0.212, C29: -0.239) and complex 1 (S1: -0.176, S2: -0.172, C1: -0.041, N1:-0.363, C2: -0.230, C9: -0.183). These higher asymmetry and polarization induce split in C-N vibration of A. The vibration also splits, which appears in 1330.39 (1279)cm^-1 and 1221.49 (1174) cm^-1, and experimental value is 1274cm^-1 and 1158cm^-1. It indicates that the electrons have greater delocalized area in A, which induces C14-N2 (0.1449nm) was closer than other three C-N (0.1471nm, 0.1475nm, 0.1473nm ) and splits the vibration.The C-S vibration has also three peaks, 1058.79 (1018) cm^-1, 1040.01 (999.8)cm^-1, 1025.42 (985.7)cm^-1，and the experimental value is 1028 cm^-1, 998cm^-1, 985cm^-1. It demonstrates that the chemical circumstances of S atoms are not the same. From crystal x-ray analysis data, the C-S bond length could be considered as three groups, 0.1716nm, 0.1705nm, 0.1712nm and 0.1713nm; from natural charges distribution, they could also be considered as three groups, -0.274,-0.258,-0.186.

Table 7 Mainly calculated and experimental IR vibration peak value of complex 2(cm^-1)

valuse in parentheses have been multiplied by correction factor: 0.9613

Fig 4 The calculated and experimental IR spectra of complex 2
(The calculated frequency will be multiplied by correction factor: 0.9613)

3 CONCLUSION                  
Introduced nitric hetercycle have increased the coordinate number of zinc complexes with dithiocarbamate, destroyed the conjugate of ZnS₂CN, weakened the Zn-S, extended Zn-S, C-N and strengthened C-S. The complexes are more stable because the occupational molecular orbital of complexes are all negative and lower. It indicates that zinc complexes with dithiocarbamate could react with nitric heterocycle to form adducts. In complex 2, the introduced pyridine has destroyed the symmetry of complex, increased the asymmetry and polarization of S₂CNC₂ in ligands, induced greater difference among S atoms. The IR spectra of complex 2 is more complicated, and the vibrations of C-N,C-S and have all split.

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含氮杂环对二硫代氨基甲酸类金属配合物晶体结构的影响
钟昀¹ 徐志广² 章伟光² 殷霞² 谭民裕³
(¹ 华东交通大学 基础科学学院南昌 330013; ²华南师范大学 化学与环境学院 广州 510631；³ 兰州大学 化学化工学院 兰州 730000)
摘要 用密度泛函方法(DFT)对二硫代氨基甲酸锌配合物Zn(S₂CNBz₂)₂ 1，Zn(S₂CNBz₂)₂Py 2 (Bz=苄基，Py=吡啶) 进行了研究，准确计算并分析两者的红外光谱，通过前线轨道能量、电荷布局、HOMO轨道成分分析和红外光谱对比，结合同类报道，发现含氮杂环配体的引入使得配合物的稳定性、对称性都有所下降，尤其是S-Zn键被明显削弱。以电荷转移、轨道能量变化及价键分析合理地解释该现象。
关键词 密度泛函， 二硫代氨基甲酸，含氮杂环，红外光谱