Resonance structures precursor molecules

The chemistry of the so-called calicene 23 and its stable derivatives has recently been reviewed [14, 15], as has that of pentafulvalene 24 and its derivatives [14]. In view of the possible stabilization of at least the cationic moieties in the zwitterionic resonance structures 23 1 and 24 +, it would certainly be interesting to prepare the percydopropylated hydrocarbons 25 and 26. The easily accessible tetracydopropylcydopentadiene 14 [13] appears to be an appropriate precursor for these target molecules (Scheme 5). 

Tandem mass spectrometry (MS/MS) is a method for obtaining sequence and structural information by measurement of the mass-to-charge ratios of ionized molecules before and after dissociation reactions within a mass spectrometer which consists essentially of two mass spectrometers in tandem. In the first step, precursor ions are selected for further fragmentation by energy impact and interaction with a collision gas. The generated product ions can be analyzed by a second scan step. MS/MS measurements of peptides can be performed using electrospray or matrix-assisted laser desorption/ionization in combination with triple quadruple, ion trap, quadrupole-TOF (time-of-flight), TOF-TOF or ion cyclotron resonance MS. Tandem... 

The h NMR spectrum provides some evidence for aromatic stabilization in 5. The important number is the chemical shift for the ring C-H protons, which falls at 6.75 ppm. This is significantly deshielded compared with the same protons in the precursor 4, 5.73 ppm, or the corresponding dihydride (LSiH2), 6.00 ppm. Theoretical calculations also support the idea that 5 is an aromatic molecule, with aromatic resonance energy of 12 s kcal mol . Particularly convincing, however, is a comparison of the properties of 5 with those of its saturated analog [22]. Silylene 13 was made by a reaction sequence similar to that for 5 in Eq. 4 and Eq. 5, except that for the final step it was necessary to use the dibromo rather than the dichloro compound (Eq. 13). For 13 an X-ray crystal structure could be determined it is shown in Fig. 5. 

---