Implementación de buenas prácticas de operación de las puertas del secador

bond; (b) isotropic reorientational diffusion. Model (a) fails to fit the IQNS spectra, whereas model (b) provides an acceptable fit to the IQNS spectra in the tem perature range investigated (245 K - 300 K) for the high-tem perature phase. The isotropic reorient ational diffusion is considerably slower than the 3-fold jumps of the CH3 group about the S i-C H 3 bond.

5.1

In tr o d u c tio n

Solid tetrakis(trimethylsilyl)silane (Si[Si(CH3)3]4; TTMSS; Figure 5.1) is known

[92] to undergo a phase transition at Tt ~ 238 K - 241 K. Above this tem ­ perature the m aterial behaves as a plastic crystal (see Chapter 1). Recently,

tem perature dependent high-resolution solid state and ^^Si NMR spectro­ scopic investigations of TTMSS have been reported [74], and have provided new insights into the dynamic properties of this m aterial. First, the results from these studies will be summarized. The phases above and below the phase transition tem perature Tt will hereafter be referred to as the high tem perature (HT) phase and the low tem perature (LT) phase respectively.

In the E T phase, the high-resolution solid state NMR spectrum contains a single narrow peak, implying that all CH3 carbons are equivalent on the NMR timescale. This can be interpreted on the basis of rapid molecular motion (rapid with respect to the NMR timescale) allowing all nuclei to experience the same average environment on the timescale of the measurement. Rapid isotropic motion of each molecule about a fixed centre of mass is consistent with this observation. On lowering the tem perature below 233 K, the NMR spectrum changes from one peak to two peaks with 3:1 intensity ratio. On further decreasing the tem perature, the spectrum develops into a set of three peaks with 1:2:1 intensity ratio at 182 K and into four peaks

M olecular D ynam ics of TTM SS in the Solid S tate 109

w ith in te n s ity ra tio b elow 172 K,

(a) CPK- (b) Line-representation

F ig u r e 5 .1 : Molecular structure of a single molecule of tetrakis(trim ethyl- silyl)silane. The globular nature of the molecule is particularly clear in the C PK -representation to the left. The line-drawing to the right shows the bonding within the molecule.

T h e b ig b -r e so lu tio n solid s ta te ^^Si N M R re su lts are in good ag ree­

m e n t w ith th e o b serv atio n s from N M R . S p ecifica lly, in th e H T p h ase,

th e b ig b -r e so lu tio n solid s ta te ^®Si N M R sp ectru m c o n ta in s a sin g le p eak for

th e S i(C H3 ) 3 groups and a sin g le peak for th e cen tral silic o n a to m . On en terin g

th e LT p h a se, th e sign al d ue to th e S i(C H3 ) 3 groups b e c o m e s tw o p eak s w ith 3:1 in te n sity ratio.

B ased on th ese and ^^Si N M R resu lts, th e fo llo w in g ty p e s of m o tio n

w ere p rop osed [74] for th e LT p h ase o f T T M SS :

A : R o ta tio n o f th e w h o le m o lecu le ab ou t a fixed a x is c o in c id e n t w ith o n e

M olecu lar D yn am ics of T T M SS in th e Solid S ta te 110 B: Rotation of each Si(CH3 ) 3 group about the relevant Si-Si(CH3 ) 3 bond.

Rotation of each CH3 group about the relevant Si-CH3 bond was assumed

to be rapid on the experimental timescale at all tem peratures studied.

The generation of two peaks with 3:1 intensity ratio in the and ^^Si NMR spectra upon entering the LT phase suggests th at one Si(CH3 ) 3 group

(denoted type [a]) becomes crystallographically inequivalent from the other three Si(CH3 ) 3 groups (denoted type [b]). Thus, although the crystal struc­

ture of TTMSS in the LT phase is not yet known, the and ^®Si NMR results suggest th at the TTMSS molecule may lie on a crystallographic 3-fold symme­ try axis which is coincident (on average) with the Si[c]-Si[a] bond (where Si[c] denotes the central Si atom). In terms of the dynamic properties, it is clear th at rapid type B rotation of the type [b] Si(CH3 ) 3 groups is required in order

for all nine CH3 carbons of the type [b] Si(CH3 ) 3 groups to become equivalent.

The spectral changes occurring from ca. 208 K to 152 K can be understood completely in terms of type B rotation of the type [b] Si( €1 1 3 ) 3 groups becom­

ing progressively hindered with decreasing tem perature. It is possible th at the molecule may be rotating about the axis coincident with the Si [c]-Si [a] bond in the LT phase. However, although the occurrence of this motion is consistent with the available evidence, interpretation of the NMR spectra does not require th at there is rapid motion of this type in the LT phase.

Solid state NMR studies of a natural abundance sample of TTMSS have also been reported [93, 94]. The NMR spectrum of the high tem ­ perature phase comprises a single narrow line (with a linewidth of ca, 1 kHz),

and is consistent with rapid isotropic reorientation of the TTMSS molecules. However, a dynamic model comprising rapid 4-site, 90° jum ps about the 2-fold axes of the tetrahedron formed by the silicon atoms of the Si(CH3 ) 3 groups,

together with rapid rotation of the Si(CH3 ) 3 groups about the Si-Si(CH3 ) 3

M olecular D y n a m ics o f T T M S S in th e Solid S ta te 111

consistent with this observation. Thus, these different dynamic models for the HT phase cannot be distinguished on the basis of the NMR results alone.

Clearly the solid state NMR results described above have provided inter­ esting insights into the dynamic properties of solid TTMSS, although several aspects remain to be understood in more depth.

In this chapter, the application of incoherent quasielastic neutron scat­ tering (IQNS) spectroscopy to extend our understanding of the dynamic be­ haviour of solid TTMSS is described. In this regard, it is im portant to em­ phasize the complementarity between IQNS spectroscopy and the solid state NMR techniques described above, in terms of the contrasting characteristic timescales of these techniques. It is also relevant to recall th at incoherent neu­ tron scattering for TTMSS is dominated by scattering from the nuclei (see Section 2.3).

5.2

E x p e r im e n ta l

TTMSS was obtained commercially and used as purchased. Since TTMSS is hygroscopic, the sample was ground to a fine powder in a glove box under a dry atmosphere. The sample was loaded into the sample container inside the glove box.

IQNS spectra were recorded on the backscattering spectrom eter IRIS at the ISIS neutron spallation source (Rutherford Appleton Laboratory, Didcot, England). The instrum ental resolution was approximately 15 fxeV (full width at half maxim um height), the neutron wavelength was Aq = 6 .6 Â , using the graphite analysers PG(002). The corresponding experim ental timescale is of the order of r < 500 x 1 0“ ^^5.

Spectra were recorded with the polycrystalline sample of TTMSS in a square, flat-plate aluminium container, the plane of which was oriented at 150° with respect to the incident neutron beam (Figure 5.2).

M olecu lar D y n a m ics o f T T M S S in th e Solid S ta te 112

n

150