Second hyperpolarizability of trithiapentalene

ELSEVIER

Synthetic

Metals 102 (1999) 1543

Secondhyperpolarizability of trithiapentalene M. Nakano, S. Yamada, S. Kiribayashi

and K. Yamaguchi

Department of Chemistry, Faculty of Science, Osaka University, Osaka. 560, Japan Abstract Trithiapentalene has gotten much attention since its unique ground-state structure. We calculate the yof trithiapentalene by using a density functional method. The three-sulfur region is found to mainly contribute to the y Keywords

Ab initio quantum chemical methods and calculations, Density functional calculations,

1. Introduction The electronic and molecular structure of trithiapentalene (‘ITP) have been investigated for several decades because of the unusual three-coordinated structure around the sulfur atoms. There were two types of structures proposed as shown in Fig. l(a). The resonance structure 1 indicates the single-bond no-bond resonance, while the structure 2 corresponds to valence tautomerization between the two alternate forms. Recent studies elucidated that the TTP can be a q, symmetrical equilibrium structure. From electron-correlation calculations, the ground-state potential is found to be a very flat U-shaped function. Judging from the sensitive structure dependences of y [ 11, the unique electronic structure of TTP has a possibility of providing an interesting feature of its longitudinal r We investigate the y for TTP by using the DF method, i.e., B3LYP method with an extended basis set (6-31G*+pd [2]). The spatial contribution is elucidated by using the hyperpolarizability density analysis [2]. 2. Calculated

system and calculation

Nolinear optical methods

C-C-H region is found to be negative. Since the K electron contributions of x-conjugated systems are usually found to be positive in sign, this difference in sign of the n electron contributions in S-S-S and H-C-C-C-C-C-H regions is unique. The electron-rich three-center sulfur atom region is found to significantly contribute to the y Judging from the unique bond nature in the S-S-S region, the feature of the electron distribution in this region is predicted to be sensitively influenced by slight chemical modifications of this molecule. Therefore, these related compounds are expected to have remarkable dependences of yon the structure changes. Ca)

1

SW-ST

2

sw*sm

(b)

method

The geometry of TTP optimized by B3LYP using 6-31 lG** is shown in Fig. l(b). The B3LYP calculations are performed by using GAUSSIAN 94 program package. The static ydensity can be expressed by a third-derivative of the electron density ( p:(r) ) with respect to the applied electric fields [2]. We consider a pair of localized p2 (r). The arrow drawn from positive (white region) to negative (black region) p:(r) shows the sign of the contribution determined by the relative spatial configuration between two pt (r) ‘s. The sign of the contribution becomes positive when the direction of the arrow coincides with the positive direction of the coordinate system.

Fig.1. (a) Resonance (1) and tautomerization (2) structures of lTP. (b) Geometry of ‘ITP optimized by B3LYP using 6-3 1lG**.

Fig. 2. ydensity plots of lTP on the plane located at 1 a.u. above the molecular plane. B3LYP using 6-31G*+pd is employed.

3 Results and discussion

4. References

Figure 2 shows the y values and y densty plots for TTP calculated by B3LYP using 6-31G*+pd. The yvalue at 6-31G*+pd is found to be positive in sign. The n electron contribution in the S-S-S region is found to be positive, while that in the H-C-C-C-

[l] M.Nakano, S. Kiribayashi, S. Yamada, 1. Shigemoto, K. Yamaguchi, Chem. Phys. Lett., 262 66 (1996). [2] M.Nakano, I. Shigemoto, S. Yamada and K. Yamaguchi, J. Chem. Phys. 103 4175 (1995)

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0 1999

Elsevier

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