Si—OH Stretching Vibrations

A homoleptic bulky a,y-diketonate yttrium complex (fod = 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionate) was immobihzed on MCM-41.280 ( s = 1140m g, Vp = 0.93 cm g, dp = 2.7 nm) and a monopodaUy anchored surface species 6 has been proposed (Scheme 12.5). As suggested by FTIR (strong band for the Si-OH stretch vibration) as well as metal and carbon analysis (circa 3.4 wt% Y, fod/Y circa 2) only around half of the silanol population has been consumed [110]. 

The band observed at 3745 cm"1 is similar in frequency to that of silanol groups in silica, and Angell and Schaffer (147) attributed it to a Si-OH-stretching vibration. No structural position was assigned with certainty, although it possibly arises from siliceous material occluded within the zeolite structure. It has also been ascribed to Si-OH groups terminating the zeolite framework. 

The hydronium exchanged synthetic mordenite does have a band, as shown in spectrum 10 in Figure 1. The iron substituted mordenite samples (spectra 8 and 9) do not show the presence of the band it was "removed" as a result of the substitution reaction. An absorption band at 950 cm- is normally attributed to an Si-OH stretch vibration (14, 15), and is typically observed in some acid or hydrothermally treated zeolites. 

Fig. 7.14. IR spectra (a) of an Si-H groups on the silica surface, correlation diagram (b) between EPR constants (tfiSo(29Sia)) for surface silyl radicals and frequencies of Si-H stretching vibrations of the products of their hydrogenation, and (c) the Si-H group absorption band in H2-loaded silica glass (4) [55], and surface groups =Si-H (1), >Si(H)(OH) (2), and >Si(H)(H) (3) [54].

For the sake of comparison, the diffuse reflectance IR spectrum of amorphous silica was also studied The latter showed bands at 3745,4550 and 7325 cm which were clearly indicative of the fundamental OH stretching vibration of silanol groups, i.e., 9[Si(OH)], its combination with a bending OH mode, i.e.. 

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