Ionization potentials of silenes (R′2Si CR2).: Experiment and theory

We have measured the photoelectron (PE) spectra of two new silenes: the kinetically stable long-lived silene t-BuMe2Si(Me3Si)Si=Ad (3) and a transient silene which was generated in the-gas phase by pyrolysis of the corresponding precursor. Previously, PE spectra of only two transient silenes have been reported. The ionization potential UP) of 3 is 6.9 eV, 2 eV lower than that of H2Si=CH2. Quantum-mechanical calculations reproduce quite well the experimental IPs. The trends in IP1 are well-reproduced by calculations of the silene's HOMO, even at the HF/6-31G(d)//HF/6-31G(d) level of theory. However, the correct absolute IPs are reproduced quantitatively (to within 0.1-0.2 eV) only at a higher theoretical level, such as ROVGF/6-311+G(2df,p), MP4/6-311+G(2df,p), or B3LYP/6-311+G(2df,p). Lower level ab initio calculations, such as ROVGF/3-21G(d) and ROVGF/6-31G(d), reproduce the trends in the IPs. The theoretical and experimental data show that the effect of substituents on the HOMO energy of silenes is slightly smaller than in the corresponding alkenes. Disilyl substitution at the doubly bonded Si atom and dimethyl substitution at the doubly bonded C atom raise the HOMO energy, by 0.6 and 0.8 eV, respectively.