A Computational Study on 1-silaallene and 2-silaallene

The structural data and vibrational frequencies of 1and 2-silaallenes have been studied computationally using the Gaussian 03 suite of programs. All of 15 normal modes were assigned to one of six types of motion (symmetrical stretching, antisymmetrical stretching, scissoring, rocking, wagging, twisting) determined by a group of quantum chemical analysis. Predicted geometric features, vibrational frequencies, and infrared intensities are also reported herein.


Introduction
Compounds with multiple bonds to silicon have attracted more and more interest since their first isolation in 1981, due to their unique properties compared to carbon analogs [1].
Although chemists have made and afford to synthesize compound which include a cumulenic double bond to silicon [2], the first 1-silaallene was synthesized and isolated as a stable compound by West and co-workers in 1993. It was stabilized by an extremely large steric hindrance around the Si=C=C moiety and characterized by X-ray crystallography, revealing that it is slightly bent (173.5 o ) in contrast to the carbon analogue allene, which is linear [3].
Interpretation of an experimental IR spectrum of silaallenes is a difficult task due to the problems encountered in their synthesis and isolation [1][2][3][4][5]. Literature survey also reveals that to the best of our knowledge no HF/DFT vibrational frequency calculations and corresponding normal mode assignments of 1-and 2-silaallenes without substituents have been reported to this date. Hence, we would like to present the geometric parameters, vibrational frequencies and characteristic normal mode frequencies of 1-and 2-silaallenes.

Computational methods
The computations were performed using the Gaussian 03 program package [6]. First of all, both structures (1-silaallene and 2-silaallene) were optimized at B3LYP/6-31G(d) level which is very successful in modeling for allenes [7,8,9]. The vibrational frequencies of 1-and 2-silaallene were calculated at the DFT (B3LYP) levels of theory with 6-31+G(d,p) and 6-31+G(d,p) basis sets. The computational method helped us to determine the normal modes of 1-and 2-silaallenes. Agreement among the method is a useful indicator that the vibrational modes have been correctly assigned for silaallenes. Each motion (symmetrical stretching, antisymmetrical stretching, scissoring, rocking, wagging, twisting) of normal modes were interpreted by means of visual inspection with help of GaussView program [11,12].

Result and discussion
The optimized structures of 1-and 2-silaallenes are shown in Figure 1  We can see easily from the results, the procedure cause to trustable estimations. However, as a result of calculations which are several levels of theory, there are a bit difference on the data of vibrational frequencies and IR intensities on the same band. For instance, the calculated frequency of C=Si=C bend (V 1 ) of 2-silaallene is 49.0 cm -1 at B3LYP/6-31G(d), whereas it is 78.6 cm -1 at B3LYP/6-31+G(d,p). In addition, the increasing in the intensity of the C=Si=C bend for 2-silaallene can be easily seen from Table 2.

Figure 2 and 3 present a view of the normal modes of 1-and 2-silaallenes using
GaussView 3.0 program [12]. The arrows on the figures show dipole derivative unit vector (in red color) and displacement vectors (in blue color). Table 3 presents the calculated geometric parameters for 1-and 2-silaallenes at B3LYP/6-31G(d) and B3LYP/6-31+G(d,p) levels.

Conclusions
The normal mode geometries and corresponding vibrational frequencies in Cs symmetry were studied theoretically using the Gaussian 03W set of quantum chemistry codes.
All normal modes were successfully determined in accordance with six of motion (symmetrical stretching, antisymmetrical stretching, scissoring, rocking, wagging, twisting) with help of a group of theoretical analysis. In addition to that infrared intensities are reported in this study. These findings would be helpful for further studies of 1-and 2-silaallenes.