OMCTS

While evidence for hydration forces date back to early work on clays [1], the understanding of these solvent-induced forces was revolutionized by Horn and Israelachvili using the modem surface force apparatus. Here, for the first time, one had a direct measurement of the oscillatory forces between crossed mica cylinders immersed in a solvent, octamethylcyclotetrasiloxane (OMCTS) [67]. 

Fig. VI-7. The force between two crossed mica cylinders in dry OMCTS. The cylinder radii R were about 1 cm. The dashed lines show the presumed, experimentally inaccessible, transition between a repulsive maximum and an attractive minimum. (From Ref. 68.)...

This stick-slip cycle, observed for all types of film compounds ranging from long-chain (e.g., hexadecane) to spheroidal [e.g., octamethylcyclotetra-siloxane (OMCTS)] hydrocarbons [21], has been attributed by Gee et al. [30] to the formation of solid-like films that pin the walls together (region of sticking) and must be made to flow plastically in order for the walls to slip. This suggests that the structure of the walls induces the formation of a solid film when the walls are properly registered and that this film melts when... 

To illustrate the relationship between the microscopic structure and experimentally accessible information, we compute pseudo-experimental solvation-force curves F h)/R [see Eq. (22)] as they would be determined in SEA experiments from computer-simulation data for T z [see Eqs. (93), (94), (97)]. Numerical values indicated by an asterisk are given in the customary dimensionless (i.e., reduced) units (see [33,75,78] for definitions in various model systems). Results are correlated with the microscopic structure of a thin film confined between plane parallel substrates separated by a distance = h. Here the focus is specifically on a simple fluid in which the interaction between a pair of film molecules is governed by the Lennard-Jones (12,6) potential [33,58,59,77,79-84]. A confined simple fluid serves as a suitable model for approximately spherical OMCTS molecules confined... 

Hartle et al. have also observed the transition point in their experiment as shown in Fig. 5 [45]. The film thickness of octamethylcyclotetrasiloxane (OMCTS) exhibits a devia-... 

Because EHD film thickness is determined by the viscosity of the fluid in the contact inlet [46], it is obvious that the viscosity of OMCTS remains at the bulk value down to approximately 0.1 m/s. However, below this speed the discretization of both central and minimum film thicknesses can be observed. The central film thickness begins to deviate from the theory at about 10 nm and the interval of the discretization is approximately 2 nm. If the molecular diameter of OMCTS that is about 1 nm is taken into account, it corresponds to approximately two molecular layers. 

Figure 29 shows a comparison of experimental results with calculation results obtained for cyclohexane. The discretization of film thickness is again observed as in the OMCTS results, and the interval is 0.5-0.6 nm, which is roughly the same as the molecular diameter of cyclohexane. 

It is considered that the calculation described in this section agrees well with the experiments for the liquids that have a strong solvation force such as OMCTS and cyclohexane. It may be more difficult to apply this theory to the liquids that have a molecular shape far from spherical and exhibit weak solvation force. 

Another remarkable feature of thin film rheology to be discussed here is the quantized" property of molecularly thin films. It has been reported [8,24] that measured normal forces between two mica surfaces across molecularly thin films exhibit oscillations between attraction and repulsion with an amplitude in exponential growth and a periodicity approximately equal to the dimension of the confined molecules. Thus, the normal force is quantized, depending on the thickness of the confined films. The quantized property in normal force results from an ordering structure of the confined liquid, known as the layering, that molecules are packed in thin films layer by layer, as revealed by computer simulations (see Fig. 12 in Section 3.4). The quantized property appears also in friction measurements. Friction forces between smooth mica surfaces separated by three layers of the liquid octamethylcyclotetrasiloxane (OMCTS), for example, were measured as a function of time [24]. Results show that friction increased to higher values in a quantized way when the number of layers falls from n = 3 to n = 2 and then to M = 1. 

There is evidence to suggest that the yield stress of thin hlms grows with the time of experiments, over a remarkably long duration—minutes to hours, depending on the liquid involved. Figure 9 gives the critical shear stress of OMCTS, measured by Alsten and Granick [26], as a function of experiment time. The yield stress on the hrst measurement was 3.5 MPa, comparable to the result presented in Ref. [8], but this value nearly tripled over a 10-min interval and then became stabilized as the time went on. This observation provides a possible explanation for the phenomenon that static friction increases with contact time. 

Fig. 9—Shear stress of confined OMCTS versus experiment time [23].

As already discussed in Section 2.2, crystalline dimethylsilanediol 53 can be prepared by hydrolysis from hexamethylcyclotrisilazane 51, from dimethoxydimethyl-silane [40], and from octamethylcyclotetrasilazane (OMCTS) 52. The most simple preparation of 53 is, however, controlled hydrolysis of dimethyldichlorosilane 48 in the presence of (NH4)2C03 or triethylamine [41]. Likewise, hydrolysis of hexam-ethylcyclotrisiloxane 54 and of octamethylcyclotetrasiloxane 55 eventually gives rise to dimethylsilanediol 53. In all these reactions the intermediacy of the very reactive dimethylsilanone 110 has been assumed, which can be generated by pyrolytic [42, 43] and chemical methods [44—46] and which cyclizes or polymerizes much more rapidly, e.g. in contact with traces of alkali from ordinary laboratory or even Pyrex glassware [40, 47] to 54, 55, and 56 than trimethylsilanol 4 polymerizes to hexamethyldisiloxane 7. Compound 111 is readily converted into dimethylsilanone 110 and MesSil 17 [46] (Scheme 3.6). 

Silylation of 2-pyridone 245 with octamethyltetrasilazane (OMCTS) probably leads to the activated dimer 246 (Section 4.2.3). Finally it should be mentioned... 

Free carboxylic acids such as benzoic acid, phenylacetic acid, or 4-hydroxyben-zoic acid 297 are converted on heating with HMDS 2 or OMCTS 52, via their N,0-bis(silylated) amides such as 22a, into nitriles such as 298 [99, 100] (Scheme 4.38). 

Thus we hope that these O-silylations-activations with the readily available HMDS (MesSiNHSiMes), TCS (MesSiCl), dimethyldichlorosilane (Me2SiCl2), hexa-methylcyclotrisilazane (HNSiMe2)s, OMCTS (HNSiMe2)4, tetra(alkoxy) silane (Si(OR)4) or sihcon tetrachloride (SiCL ), most of which can also effect the transient protection of any present hydroxyl group, and the subsequent or concomitant reaction with nucleophiles accompanied by formation of silylated water as HMDSO (MesSiOSiMes), (OSiMe2)n or Si02 will be applied more often in the fu-... 

The identification of the structures responsible for D1 and D2 and their stabilities toward hydrolysis are further supported by investigations of the hydrolysis behavior of the corresponding isolated ring molecules, octamethylcyclotetrasiloxane (OMCT) and hexamethylcyclotrisiloxane (HMCT) (51.). OMCT is stable in aqueous environments, whereas HMCT hydrolyzes with a pseudo-first order rate constant 3.8 ( 0.4) x 10 3 min 1 (tt, 2 — 3.0 hours). This latter value is comparable to the rate constant for D2 hydrolysis, 5.2 ( 0.5) x 10 3 min"1 (t1/2 - 2.2 hours) and is 75x greater than the rate constant describing hydrolysis of unstrained, conventional a-Si02 (as estimated by extrapolation of the data in reference (52.). 

We have carried out DFT (B3LYP/6-31G(d)) calculations (the basis set comprises 312 cGTOs) in order to establish the energetic order of the different possible isomers of (Me2Si-NH)4, OMCTS (Fig. 15). At the local minima on the potential energy surfaces, the Hessian matrices were computed. Harmonic vibrational frequencies were used to calculate the zero-point vibration-corrected energetics. (Results are collected in Table I and Fig. 16.)... 

Relative Energies of the Different Isomers and Conformers of OMCTS (B3LYP/6-31G(d)... 

Fig. 16. Energy diagram of the neutral isomers A-D of OMCTS. The energies refer to the most stable isomer A and include zero-point energy contributions.

In single crystals of OMCTS,53 two conformers - the chair-(Al) and the boat-form (A2) - are found (see Fig. 15). The calculated most stable twist conformation A appears only in the unsymmetrical [(Me3C)2Si-NH-SiMe2NH]2. 

In addition to the three conformers A, A1 and A2 of the eight-membered-ring compounds, OMCTS can exist in the following structures a six-membered (B) and a symmetrical (C) and a non-symmetrical (D) four-membered ring with exocyclic SiMe2NH2 groups. 

Isomer D with the exocyclic SiMe2-NH-SiMe2-NH2 chain exhibits an almost planar four-membered ring (sum of angles 179°). The species is by far the least stable isomer of OMCTS discussed in this work. 

Structural Isomers of the Mono-Anion of OMCTS, DFT Calculations... 

The most likely isomeric mono-anions of OMCTS are shown in Scheme 28. 

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