Temperature dependence, dynamic

Temperature-dependent (dynamic) NMR studies are suited to the study of processes with rate constants between 10 and 10 s Some applications are shown in Table 2.13 and in problems 13 and 14. 

Titanium imido complexes supported by amidinate ligands form an interesting and well-investigated class of early transition metal amidinato complexes. Metathetical reactions between the readily accessible titanium imide precursors Ti( = NR)Cl2(py)3 with lithium amidinates according to Scheme 84 afforded either terminal or bridging imido complexes depending on the steiic bulk of the amidinate anion. In solution, the mononuclear bis(pyridine) adducts exist in temperature-dependent, dynamic equilibrium with their mono(pyiidine) homologs and free pyridine. 

Fig. 19. 90.4-MHz 13C MAS spectra of benzene-l,C6 on zeolite HY, showing the temperature-dependent dynamics of benzene inside zeolite HY. Note that benzene is not protonated by zeolite HY.

Supplementary support for the interpretation of the temperature-dependent dynamic H NMR spectra of 33 is presented by additional studies of (A,A,A,A)/(A,A,A,A)-[(EtNH3)4n Mg4(L12)6 ] (34). In 33 and 34, the methylene protons of the ligands exhibit identical VT NMR spectra. Moreover, the diastereotopic methylene protons (magenta) of the ethyl ammonium counterions of 34 display similar temperature-dependent coalescence as the ligand vinylether methylene protons (green). This is due to the fact that, even in solution, the ethyl ammonium groups are fixed to the tripodal calix-like surfaces of the [Mg4(L12)6]4 scaffold and therefore the methylene protons are in a chiral environment and display diastereotopicity. 

Also, external parameters influencing the crystal field effects (besides the temperature-dependent dynamical behaviour) such as pressure or external electric fields have to be taken into account. 

The following section contains a more detailed treatment of the theory behind the nonresonant spectroscopy of liquids. This will be followed by a description of the experimental implementation and data analysis techniques for a common OKE scheme, optical-heterodyne-detected Raman-induced Kerr-effect spectroscopy (22). We will then discuss the application of this technique to the study of the temperature-dependent dynamics of simple liquids composed of symmetric-top molecules. 

From the point of view of the solvent influenee, there are three features of an electron spin resonance (ESR) speetrum of interest for an organic radical measured in solution the gf-factor of the radical, the isotropie hyperfine splitting (HFS) constant a of any nucleus with nonzero spin in the moleeule, and the widths of the various lines in the spectrum [2, 183-186, 390]. The g -faetor determines the magnetic field at which the unpaired electron of the free radieal will resonate at the fixed frequency of the ESR spectrometer (usually 9.5 GHz). The isotropie HFS constants are related to the distribution of the Ti-electron spin density (also ealled spin population) of r-radicals. Line-width effects are correlated with temperature-dependent dynamic processes such as internal rotations and electron-transfer reaetions. Some reviews on organic radicals in solution are given in reference [390]. 

Hill, unpublished data) involving dioxaphospholane 14 [prepared from the bis(transoxyphosphoranylation) of diol 1 with DTPP] do not corroborate this slow equilibration between oxyphosphonium ions U and V and dioxaphospholane 14 in benzoic acid in THF solvent. In fact, at -78°C the addition of benzoic acid to dioxaphospholane 14 in THF solvent gives the regioisomeric benzoates directly and the isomeric oxyphosphonium ions U and V are not observed by ip NMR spectroscopy. However, in dichloromethane solvent ions U and V are actually observable and do undergo a temperature dependent, dynamic exchange with dioxaphospholane 14 and benzoic acid (Mathieu-Pelta, I., The University of North Carolina at Chapel Hill, unpublished data). 

The electropositivity of the boron atom makes C-3 obviously the most nucleophilic atom. However, there is one example where the C-5 position takes part in dynamic processes. 2-Methyl-bis(triethylsilyl)-l,2-azaborolyllithium shows temperature-dependent dynamic behavior in solution. At — 100°C three different states can be trapped in which the carbon-bonded TMS group substitutes positions 3, 4, and 5 <88CB1873>. Compound (a) in Scheme 1 is the longest-lived one, as subsequent substitution reactions give products exclusively with the TMS group at C-3. Thus, with TMS-Cl the... 

Apart from only some few cases, in which the exchange of free and complexed guests is slow enough on the NMR time scale in order to observe different signals in each case, usually coalescence is found. This has been used in temperature dependent dynamic NMR sectroscopy, e.g. by Cram, to determine activation entropies... 

Summary Employing temperature-dependent dynamic NMR spectroscopy we have been able to measure both the activation enthalpy and entropy of inversion processes. Moreover we have performed a systematic study of the influence of the substituent groups, the nature of the cation, and solvent effects on the height of the inversion barrier. 

Vogt J (2007) Tensor LEED study of the temperature dependent dynamics of the NaCl(lOO) single crystal surface. Phys Rev 875 125-423... 

Further examples for real-time studies, which are becoming increasingly more important, target (surface) glass-transition temperatures (225-228), and thermotropic transitions in liquid crystalline polymers both at high and low temperatures (299-301). The analysis of temperature-dependent dynamical-mechanical surface properties has already been mentioned in the section under Surface Mechanical Properties (229-231,270-273). 

Here, only the athermal version of the DLL model has been described. It has been shown elsewhere [11 13] that the model is able to represent the temperature dependent dynamics of systems as well. This, however, requires additional assumptions concerning free volume distribution and related potential barriers for displacements of individual elements. It has been demonstrated [11-13] that in such a version the model can describe a broad class of temperature dependencies of relaxation rates in supercooled liquids with the variety of behavior ranging between the extremes described by the free volume model on one side and the Arrhenius model on the other side. 

In crystalline solid polymers, not only do the chains vibrate in the normal manner which we associate with infrared or Raman modes, but it is also possible for the all-tew.s section of a chain such as polyethylene to undergo an accordion type of motion. The element which is involved in the motion is constrained by the size of the crystal lamellae described in Chapter 6. The motion is not a relaxation and it gives rise to a resonance observed in the far infrared or Raman spectrum. Typically, crystalline polyethylene will possess a vibration band at about 120 cm" which is associated with this accordion motion. The precise position depends on the length of the lamellae, the shorter lamellae having a higher frequency of resonance. This collective vibration of the chains is quantised, and is a p/jowow. These well defined acoustic vibrations are very important in understanding the temperature dependent dynamic behaviour of crystalline solid polymers. 

F. W. Deeg, S. R. Greenfield, J. J. Stankus, V. J. Newell, M. D. Fayer, Nonhydrodynamic molecular motions in a complex liquid temperature dependent dynamics in pentylcyanobiphenyl, J. Chem. Phys. 1990, 93, 3503. 

The complex balance of Coulomb, van der Waals, and hydrogen bond interactions is reflected in the viscosity of a medium. The temperature-dependent dynamic viscosity measurements showed no linear correlation. However, although the dynamic viscosity is a non-molar quantity (as it is a measure of the inner friction between neighboring liquid layers moving with different velocities), it depends linearly on the molar composition of the mixture. The only exception in our current work (Fig. 11) is the mixture [C2mim][0Ac]/[C2mim][CF3C02]. 

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