Temperature coalescence

Nl pyrazoles NMR Coalescence temperature (°C) Activation energy (kJmoP ) Ref. 

Finally from the logarithmic form of the Eyrlng equation, the free enthalpy of activation, AG, of rotation of the dimethylamino group at the coalescence temperature (318 K) can be calculated ... 

Rree energy barriers, AG calculated at coalescence temperature of signals H-3, H-5, or Me-3, Me-5. 

Ring inversions of 1H- and 4//-azepines between their two stable boat forms have been studied extensively by HNMR spectroscopy.37,38-40-76 82,85 A coalescence temperature of — 55 7 C and a AG value for ring inversion of 42.7 kJ mol have been determined for the two conformers 10 A and 10 B of A-phenyl-3//-azepin-2-amine.82... 

However, the relative stabilities of azepine conformers are highly dependent on the nature of the ring substituents, and some substantial inversion energy barriers have been noted, e.g. dimethyl 2,7-dimethyl-3//-azepine-4,6-dicarboxylate [57.3 kJ - mol-coalescence temperature (Tc) 25 5°C],76 isochalciporone (26)(49.4kJ mol-1 Tc 2 + 1 C),40 and 2,4,6,7-tetraphenyl-3/7-azepine (68.1 kJ mol-1 Tt 80°C).37 Ring-inversion activation energies of similar magnitudes have been determined for 4//-azepines.83 85... 

The only other C5(i-Pr)5 complex, Mo(Pr5Cp)(CO)3CH3 (30) also shows fluxional iso-propyl groups at 100 °C, but below the coalescence temperature two isomers in the ratio 4 3 are obeserved according to, 3C NMR spectra. They are... 

A Relaxation time measurement in the solid (Al) in solution (A2). B Mechanical spectroscopy. C Variable-temperature NMR spectroscopy (coalescence temperature measurement). D Variable-temperature EPR spectroscopy... 

The relatively complex form of Eq. (11-37) leads to some approximate solutions that were widely used before the modem computer solutions were available. One of these was based on the coalescence temperature. This is the point at which two separate resonances are no longer observable. It can usually be measured to 0.2 °C. The rate constant in either direction at this temperature is... 

Chain reactions, 181 branching, 189 initiation step, 182 propagation steps, 182 rate laws for, 188 termination step, 182 well-behaved, 187 Chemical mechanism, 9 Chemical relaxation, 255-260 Coalescence temperature, 262 Col, 170... 

Although 1- and/or 4-acylated hydroquinoxalines might not be considered as regular quinoxalone ketones, they are included here for pragmatic reasons. Examination of the NMR chemical shifts for H5 and H8 in l,4-diacyl-l,2,3,4-tetrahydroquinoxalines (197, R = Me, Et, Ph, etc.) indicated that the endo-exo conformation (shown) predominated in most such compounds at ambient temperatures. " However, this was disturbed in l,4-dibenzoyl-6,7-dimethyl-l,2,3,4-tetra-hydroquinoxaline (198), which showed a predominance of the exo-exo conformation (shown) below the coalescence temperature. ... 

Homoleptic complexes have been obtained also with tetrakis(l-pyrazolyl)borates, e.g., [ B(pz)4 2Cd] and [ B(3-Mepz)4 2Cd] (both P2 jc, Z = 2) in both compounds structure analyses the ligands have been shown to coordinate trihapto, i.e., with one pz ring free. In both cases Cd has a distorted octahedral environment, with averaged structural data very similar to those for the tris(l-pyrazolylhydridoborate complexes.201 Variable-temperature 3H NMR studies of these and of mixed complexes with tris- and bis(l-pyrazolylhydridoborates indicate fluxional behavior (coalescence temperatures and barriers for the dynamic processes are given). 

Table 4 shows the NMR coalescence temperatures for this process for a number of ligands. In addition to this data it was shown that the exchange rates for the ligands acetonitrile, dimethylsulfoxide, anddiphenyl-... 

Table 4. Coalescence temperatures for several six coordinate methyl cobaloximes...

When the data in this table are plotted, the graph shown in Fig. 33 is obtained. From this one can calculate a pKa of 2.85 for displacement of benzimidazole in D2O. In addition, since room temperature is above the coalescence temperature, it is possible to set a lower limit on the exchange rate between coordinated and uncoordinated benzimidazole of 3.1 X 102 sec-1. From Fig. 33 one can, by extrapolation, calculate the C(20)-methyl resonance of the base-on and "base-off forms to be 0.41 and 1.05 respectively. These numbers can be used, with the assumption of fast exchange, to determine the relative amounts of "base-on and base-off" species from the observed C(20)-chemical shift for any arbitrary sample. Such information would be useful, for instance, when investigating the displacement of benzimidazole by other Lewis bases. Thus for the simple case of benzimidazole displacement we have shown that NMR provides a method for studying the molecular conformation of vitamin B12. 

Solvent DK Av(COCH3)(Hz Tc CCJ (coalescence temperature AG ( kcal, mole... 

The di-tert. butyl substituted calicene 230 was calculated to possess considerable non-bonding (steric) interactions in the planar geometry184. The relief of strain when going to a perpendicular transition state is reflected by the low coalescence temperature of tert. butyl signals found on temperature-dependent H-NMR spectroscopy187. ... 

H NMR of pyrrolophane 30 shows two singlets for SiMe groups that do not coalesce in the temperature range of -80 to +110°C, in contrast to furanophane 28, which shows rapid anti/syn isomerization down to -78°C, and thiophenophane 29, with a coalescence temperature of 15°C. The rigidity of 30 is obviously due to the steric bulk of the 2V-methyl groups. 

At higher temperatures kinetic line broadening and coalescence is observed (Fig. 8). Upon warming above the coalescence temperature of about 110 °C four lines for the aromatic methine carbons with decreasing line width are observable until decomposition takes place at about -70 °C (Fig. 8). From line shape analysis the energy barrier for the isomerization process is obtained as AG11 — 7.5kcal/mol.101... 

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