MECHANISTIC STUDIES OF THE THERMOLYSIS OF TETRANEOPENTYLTITANIUM(IV) .2. SOLID-STATE AND ULTRA-HIGH-VACUUM STUDIES OF THE CHEMICAL-VAPOR-DEPOSITION OF TIC FILMS

The chemical pathway responsible for the conversion of the organotitan ium compound tetraneopentyltitanium to titanium carbide has been studi ed under chemical vapor conditions and on single crystals in ultra-hig h vacuum. For every equivalent of TiNp4 consumed in the deposition pro cess, 3.28 equiv of neopentane and 0.16 equiv of isobutane are produce d; other organic species are also formed but in relatively small amoun ts. About 93% of the carbon and hydrogen originally present in the pre cursor can be accounted for in these products. Thermolysis of the spec ifically deuterated analogue Ti(CD2CMe3)(4) yields a 2.25:1 ratio of n eopentane-d(3) and neopentane-d(2); this result combined with a kineti c isotope effect of 4.9 at 385 K shows unequivocally that the first st ep in the deposition pathway under CVD conditions is alpha-hydrogen ab straction. The alpha-hydrogen abstraction step produces 1 equiv of neo pentane and a titanium alkylidene, which undergoes further alpha- (and eventually gamma-) hydrogen activation processes to generate the seco nd and third equivalents of neopentane. In the last stages of the ther molysis sequence, neopentyl (or neopentyl-derived) organic groups evid ently fragment and generate the carbon atoms that eventually form the titanium carbide phase. Spectroscopic studies with IR and HREELS techn iques have also been carried out in order to provide additional eviden ce about the nature of the species present when single crystal surface s dosed with TiNp4 are heated. A band at 1121 cm(-1) is tentatively as cribed to the v(M=C) band of surface-bound neopentylidene groups.