Ultraviolet photoelectron (PE) spectroscopic experiments were carried out on cyclopentadienyl cobalt dicarbonyl CpCo(CO)2 (I) using a pyrolysis inlet system. Above a certain temperature this molecule loses a carbonyl group, resulting in the 16-electron molecule CpCoCO (II). By subtracting the PE spectrum of (I) from the mixed experimental spectrum, the PES of (II) was obtained. Adiabatic and vertical ionization energies (IEs) were also determined. By using a thermodynamic cycle, which included the appearance energy of CpCoCO+ determined in an earlier threshold photoelectron photoion coincidence (TPEPICO) experiment, the dissociation energy of CpCoCO···CO was obtained to be 1.91 ± 0.05 eV. Quantum-chemical calculations were also carried out to help the reassignment of the photoelectron spectrum of (I) and to help establish a reliable value of the adiabatic IE of (II).

The electron transmission and dissociative electron attachment spectra of the 1-chloroalkyl benzene derivatives, C(6)H(5)(CH(2))(3)Cl and C(6)H(5)(CH(2))(4)Cl, and of the sulfur and silicon derivatives, C(6)H(5)SCH(2)Cl, C(6)H(5)Si(CH(3))(2)CH(2)Cl and C(6)H(5)CH(2)Si(CH(3))(2)CH(2)Cl, are presented for the first time. The relative Cl(-) fragment anion currents generated by electron attachment to the benzene pi* LUMO are measured in the series C(6)H(5)(CH(2))(n)Cl, with n = 1-4, and in the heteroatomic compounds. The Cl(-) yield reflects the rate of intramolecular electron transfer between the pi-system and the remote chlorine atom, which in turn depends on the extent of through-bond coupling between the localized pi* and sigma*(Cl-C) orbitals. In compounds C(6)H(5)(CH(2))(n)Cl the Cl(-) current rapidly decreases with increasing length of the saturated chain. This decrease is significantly attenuated when a carbon atom of the alkyl skeleton is replaced with a third-row heteroatom. This greater ability to promote through-bond coupling between the pi* and sigma*(Cl-C) orbitals is attributed to the sizably lower energy of the empty sigma*(S-C) and sigma*(Si-C) orbitals with respect to the sigma*(C-C) orbitals. In the sulfur derivative the increase of the Cl(-) current is larger than in the silicon analogue. In this case, however, other negative fragments are observed, due to dissociation of the S-C bonds.

The photoelectron-photoion coincidence (PEPICO) technique has been used to investigate the unimolecular decomposition of the (CH3).$i2+ molecular ion. The absolute rates of C3H9Si+ and C5H15Si2+ ion formation were measured as a function of the internal energy by analyzing the ion time-of-flight distribution. The results are compared to the rates predicted by the statistical theory (RRKM/QET). The two dissociation channels are in competition with each other, and their observed onsets are subject to a considerable kinetic shift which is taken into account in evaluating the thermochemical dissociation limits. The rate data show that methyl loss is associated with a tighter transition state than the complex producing C3H9Si+ ions. The AHHfo298 (in kJ/mol) for the following species were measured: Me3Si (-49); Me& (-226.2); Me3Si+ (629.7); Me2Si2+ (423). A Si-Si bond energy of 265 kJ/mol is derived from the heat-of-formation data.The trimethylsilyl ion is particularly important in the mass spectrometry of methylsilanes, polymethylsilanes, and siloxanes as well as silylated compounds.' This ion has been investigated by several authors, and its thermochemistry has been the subject (1) Litzow, M. R.; Spalding, T. R. In

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