NI transition

Ab initio MO calculations have been carried out for two carbocation-generating reactions the 6 nI reaction of protonated 1-phenylethanol (H2O leaving group) and the acid-catalysed hydration of styrene. Optimizations were done at the MP2/6-31G level. The 6 nI transition state lies half way between the reactant and the product with respect to the bond lengths, charge distribution, and secondary deuterium isotope effects. 

As a matter of fact, it has been suggested that the presence of free bipyridine is required to allow the formation of a mixed complex in which the Zn11 species and the a-arylnickel compound are close to each other (Scheme 12). This process, which occurs at the potential of the Nin/Ni° transition, has been proposed to give an account of the arylzinc... 

Why microwave resonance techniques are so attractive for the spectroscopy of Rydberg states becomes clear when we estimate how much microwave power is required to drive the transitions. To drive the electric dipole ml— ni transition A... 

A third resonance method which has been used to measure the intervals of alkali and Ba Rydberg atoms is delayed field ionization which takes advantage of the increase of the lifetime with L The method used by Safinya et al.15 in the study of Cs nf rch and ni —> ni transitions is typical. Atoms are excited to the ni states in a manner similar to the one used by Fredriksson et al.,1 except that pulsed... 

In other respects, the cyano group coordinated on various tervalents (Co, Rh, Ir, Ru) or divalents (Pt, Pd, Ni) transition metals can undergo nucleophilic addition in the coordination sphere with modification of the nitrogen ligand which remains coordinated on the metal (refs. 23, 24, 25). 

The conversion of the allylic enantioface is most likely to proceed via the tx riHCO-octadienediyl-Ni" transition state TSefc[2] (Fig. 5). In accordance with evidence provided from both experimental [17,18] and theoretical [19] studies, allylic isomerization is most Ukely to occur via the ix, ri (C )-octa-dienediyl-Ni" transition state TSiso[3], which constitutes the internal rotation of the vinyl group around the formal C -C single bond (Fig. 5). The explicit investigation of monomer-assisted reaction paths revealed that additional ethylene or butadiene does not accelerate either allyhc isomerization or enantioface conversion by coordinative stabilization of the corresponding transition state. 

An accurate determination of a and c in the low-pressure region is crucially important. To avoid the effect of the well-known anomaly in the immediate vicinity of the NI transition point, the values estimated in the stable region are extrapolated to the phase boundaries. Thermal pressure coefficients can thus be calculated from the relation... 

Similar discordance in a, c, and y at the NI interphase is also observed for 5CB (Figure 7.14d-f), where the analysis has been carried out on the PVT data collected at 0.1 and 80 MPa. Of the extensive PVT measurements on nCB, the only ones reported were those above room temperature. Since 5CB exhibits the NI transition at 308 K under 0.1 MPa, a reliable estimate of the nematic properties is difficult for the ordinary pressure, and the results are shown only for the isotropic state. The NI transition temperature shifts toward higher temperatures with pressure. At 80 MPa, the midpoint... 

It is customary to elucidate the entropy change (A5 Ni)y by correcting the latent entropy observed under an ordinary pressure (A5 ni)f for the contribution due to the volume change ASy. The formula given in Mandelkern, [1964, 2002] Mandelkern s books [1964, 2002] may be applicable to the NI transition as well ... 

Rg.5 Schematic representation of the crystal-nematic (CN) and nematic-isotropic (NI) transitions, illustrated for a trimer liquid-crystal system. The conformational distribution of the spacer remains quite stable in the nematic phase, and is termed nematic conformation... 

The values of y were similarly obtained for dimer CBA- (n = 9,10) and trimer CBA-Tn (n = 9,10). These compounds exhibit the nematic LC phase over a limited temperature range, hampering an accurate estimation of y by the extrapolation from this phase. Accordingly the y values were estimated by method 1 only from higher-temperature phases i.e., y i values are estimated from the isotropic phase, and ycN values from the nematic phase [95]. The ytr values thus derived are all accommodated in Tables 2 and 3, respectively, for the NI and CN transitions. Thermal pressure coefficients of monomer liquid crystals such as 4-cyano-4 -alkylbiphenyls ( CB) and 4-cyano-4 -alkoxybiphenyls ( OCB) are available in the article by OrwoU et al. [112]. The y values applicable to the NI transition of these compounds are cited in Table 4 for comparison. As shown in these tables, use of the volume change A Vtr at the transition (column 4) leads to the estimate of the volume-dependent entropy ASy (column 5) according to Eq. 3. 

As comparison of Tables 2 and 4 indicates, the changes in volume and entropy at the NI transition obtained for the mainchain dimer and trimer liquid crystals are much larger than those reported for conventional monomer liquid crystals [112]. In Tables 2 and 3, the constant-volume transition entropies (AStr)v are expressed in terms of joules per mole per kelvin. The conformational entropy changes estimated on the basis of the NMR quadrupolar splitting data observed in the LC state are as follows = 13.3... 

At the NI transition, an orientation-dependent term such as AS " " must play a role [26]. In practice, however, the contribution from this source seems to be comparatively small by inference from those of the monomer liquid crystals [112] (cf. Table 4). In treating the transition entropy of real systems, contributions from the so-called communal entropy as well as other residual entropies are often considered by introducing an extra term ASj in Eq. 1 [15,17,34,35,120] ... 

Had we taken higher order terms of r into account, the divergence would disappear. The Curie-type order parameter relaxation has been studied on a typical nematic (5CB), see Fig. 6.16. The measurements have been made using a pulse pyroelectric technique [18]. As the nematic-isotropic transition in 5CB is weak first order, it clearly demonstrates some features of the softening the relaxation time of the orientational order parameter on the nematic side of the NI transition increases five times. 

With the axial-aryl chair conformation clearly established for all cases of alkaloids in the mesembrine subgroup previously studied, it was surprising to find that the hydrochloride of (—)-mesembrine exists in the solid state with the cyclohexanone ring in a twist-boat conformation, as indicated in 17 (24). A study of the CD of mesembrine hydrochloride in H2O was undertaken in an attempt to seek a possible correlation between the conformation of the hydrochloride in the solid state revealed by the X-ray structure analysis and the solution conformation. Unfortunately, the CD spectrum of mesranbrine hydrochloride in H2O shows the absence of an n ni transition associated... 

If temperature in the LL model is increased to T = 1.2, in a bulk sample the isotropic phase is stable — recall that the nematic-isotropic (NI) transition takes place at T j = 1.1232 [16]. However, like in PDLC droplets, also in the vicinity of polymer fibers one should expect... 

The last issue considered in this Chapter will be pretransitional ordering in a nematic sample with dispersed polymer networks above the NI transition T j = 1.1232). As seen already in studying a regular fiber array, some surface-induced paranematic ordering may persist in the vicinity of fibers. This ordering can be detected by optical means [2] alternatively, it can be detected also by NMR [23]. 

T. Curves in Fig. 14 axe similar to the corresponding experimental ones plotted in Ref. [2]. Note, however, that the increase of I/Iq on approaching from above is not as abrupt as in Ref. [2]. This may be because in the present simulation the NI transition was not approached as closely as in the experiment. Further, in the present study I/Iq is well-behaved also because the effective birefringence is not necessarily small — the polymer network is rather dense and C 1 does not always hold (e.g., for — 1) — and any significant variation of A is saturated in the I sin (A4>/2) dependence. For the same reason, the curve for in = 1 is not monotonous close to the NI transition. With... 

Nickel (Ni) Transition metal element with atomic number 28, forming the NP ion in water. 

In Table II we list some thermodynamic properties connected with phase transitions. The enthalpy changes for the NI transition are one order of magnitude smaller than the heat of melting, but still indicating a... 

Pressure influences the dielectric permittivity of liquid crystalline phases considerably. For some LCs that form antiparallel dimers, the observation of a small maximum in s(T) at 1 atm above the clearing temperature is well established. In recent high-pressure investigations a similar effect was found, that is, a maximum in e(p) at T = const in the isotropic phase in the neighbourhood of the NI transition. 

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