The effect of substituents on the first transition in the visible spectra of silanediyls (silylenes)

Abstract

Ab initio calculations show that the first transition in the visible spectra of silanediyls (silylenes), RSiR', is due to an n(Si) + 3p(Si) excitation and that this absorption is shifted to shorter wavelengths by n-donors and to longer wavelengths by bulky substituents; the calculations for Me2Si are more consistent with a h, , , .of 450 nm than with 350 nm.

Matrix‐isolation and electronic structure of vinyl‐substituted silylenes and their complexes with bases [1]

“…The n(Si)-3p(Si) transition bands for 1 and 2, which were found at 505 and 475 nm, respectively, redshifted significant ly relative to that for dimethylsilylene, in disagreement with a recent ab initio theoretical prediction by Apeloig and Karni [7]; they have predicted that a vinyl group serves as a rr donor substituent on silylene to cause a substantial blue shift of the n(Si)-3p(Si) transition. Although the theory also predicted a red shift of the n(Si)-3p(Si) transition of silylenes when the apex angle increases, the apex angles for 1 and 2 estimated by the ab initio calculations were even smaller than the angle for dirnethylsilylene [ 161; geometrical parameters for 1, 2 , and dimethylsilylene fully optimized with the D95, and 3-21G basis sets are shown in Figure i!.…”

“…As photochemical precursors for vinyl-substituted silylenes,2,5-bis(methylene)-1 ,I-bis(trimethylsily1)-1-silacyclopentane (3), 2-methylene-1 ,1-bis(tri-methylsily1)-1-silacyclopentane (4), 2,2-divinyiheptamethyltrisilane ( 5 ) , 1 ,I-dimethyl-4,4bis(trimethylsily1)-1,4-disila-2,5-cyclohexadiene (6), and 1,l -dimethyl-4,4-bis(phenyldimethylsilyl)-1,4disila-2,5-cyclohexadiene (7) were investigated. Figure 1 shows the UV spectra of 3-5 in hexane.…”

“…450 nm [3a]. Although there was a controversy concerning the absorption spectra of dimethylsilylene [4], recent extensive spectroscopic studies by using matrix isolation [3,51, laser flash photolysis [6], and theoretical calculations [7,8] have confirmed that the 450 nm absorption is ascribed to the n(Si)-3p(Si) transition of singlet dimethylsilylene. Intriguing experimental targets in silylene chemistry involve (1) generation and characterization of silylenes having a triplet ground state and (2) synthesis and isolation of stable singlet silylenes as pure substances.…”

The positions of λmax for some trimethylsilyl-substituted silylenes

“…The discussion begins with the electronic effects due to both the trimethylsilyl group and the porbital of the ipso-carbon of the phenyl substituent. At the heart of the issue is the large red shift of silyl-substituted silylenes predicted by Apeloig and Karni [11]. Our general understanding of this matter draws from the work of Liebman and coworkers who explicitly address electronic effects for the substituents on the divalent atom.…”

“…Spectral observation of the n ~ p transition for a wide variety of silylenes frozen in low-temperature organic glasses, supported by chemical trapping studies, has allowed identification of divalent silicon intermediates from the photolysis of polysilanes [10]. Nonetheless, in some instances the assignment of these absorptions to silylenes has not been obvious and theoretical support of the original experimental observations has proved to be helpful [11]. A particularly interesting prediction of theory is that silyl-substituted silylenes should display an absorption that is significantly shifted to longer wavelength.…”