Solid State Isomerisation

Weigh out accurately two portions of about 180 mg of the c/5-isomer. Heat one at 110 C for 2 hours. Cool and quickly dissolve in 50 cm of water and measure the absorbance in a 1 cm cell at the previously determined visible wavelengths of maximum absorption at intervals eg 5, 20, 40, 60 minutes and plot each set of readings against time, t ing zero time when the solution was prepared. Extrapolate to zero time and estimate the initial absorbances. Dissolve the sample which was not heated in 50 cm water and carry out the same measurements. The two runs should be carried out at the same room temperature unless a thermostatted spectrophotometer cell is available. In that case the same thermostat temperature is used. Compare the two sets of results. Repeat using the finely ground tram-isomer instead of the c/s-complex. Comment on your results. 

Prepare KBr discs (using purest KBr previously dried and kept in a desiccator) from a mixture of each of the three solid complexes and KBr ( 1 10). Record the infrared spectra of the three prepared solids in the range 400-4000 cm and tabulate the peaks obtained. Compare the spectra and assign the peaks. 

Three compounds of the empirical formula CrCl3.6H20 are known. This is an example of hydration isomerism where water can be either as a ligand in the inner co-ordination 

1 Purification of the Commercial [CrCl2(H20)4)C1.2H20 The dark green commercially available solid is thought to be the rra/u-isomer. The purification procedure described below is based on a paper by W.B.Guenther J.J.Stuart in J. Tennesse Acad.Sci., 35 (1960) 244. 

Grind the commercial product under pure acetone. Replace the acetone by another aliquot of acetone by decantation. Filter through a sintered glass funnel, wash with acetone until the filtrate is colourless. Then wash well with dry ether. Suck dry and store in a dessicator over solid NaOH in contact with its saturated solution. 

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Trans-to-cis-[somer isat on of solutions of transition metal complexes such as the inert complexes of Cr(III), usually proceeds at a conveniently slow rate. Since the electronic spectrum of the /ran -isomer contains bands of lower intensity than those of the cis-isomer, the reactions can be followed by spectrophotometry. Solid state isomerisation may be similarly studied. 

The solid state isomerisation is clearly demonstrated by the absorption spectra of the solutions after various heating times or by their ir spectra. 

The JV-phosphino-l-azadiene 66 undergoes cycloaddition with DMAD in ether at -20 °C to form the bridged structure 67 <96AG(E)896>. The new compound is thermally unstable and isomerises to the alternative bridged structure 68 at 25 °C. X-ray analysis data have been interpreted to suggest that the isomer 69 co-exists in the solid state. The same ring system is also formed by protonation at the sp carbon atom neighbouring phosphorus in 67 (Scheme 15). 

The product of solid-state grinding exhibits no evidence of complex 4, which would be expected to result following solution-mediated redox isomerisation of the starting components. Solid-state grinding preparation of quinhydrone complexes was later pursued by Guarrera et al. [19]. 

Dicumyl peroxide vulcanisation of high-cis BR and NR samples at different peroxide levels were investigated by solid-state 13C NMR [42], As the peroxide level increased, decreased signal intensity and peak broadening was observed in the main chain peaks in both BR and NR due to decreased segmental motion. In addition, cis-trans isomerisation was observed during the vulcanisation process in both rubbers. New peaks at 35 and 44 ppm were observed in the BR spectra while peaks at 14.9, 21.4, 30.6, 37.5, and 45.0... 

Network structure and reaction mechanisms in high pressure vulcanisation (HPV) and peroxide vulcanisation of BR was studied by 13C solid-state NMR [43]. Different samples of polybutadiene (51% trans, 38% cis, and 11 % vinyl) were peroxide cured with dicumyl peroxide on a silica carrier and by the HPV conditions of 250 °C and 293 MPa. The 13C NMR spectra from peroxide and HPV cures were compared to a control samples heated to 250 °C for 6 minutes under atmospheric pressure. Although no new isolated strong peaks were detected in either the peroxide or HPV vulcanisations, small increases in both spectra were observed at 29.5, 36.0, 46.5, and 48.0 ppm. These peaks compare favourably with calculated shifts from structures that arise from main chain radical addition to the pendent vinyl groups. These assignments are further reinforced by the observation that the vinyl carbon concentration is substantially reduced during vulcanisation in both peroxide and HPV curing. Two peaks at 39.5 and 42.5 ppm appear only in the peroxide spectrum. Cis-trans isomerisation was absent in both cures. 

Solid-state 13C NMR was used to characterise the structure produced in gamma irradiated NR [45]. Cis-trans isomerisation increases from 5.1% at 40 Mrad to 10.3% with 161 Mrad. Peaks in the NMR spectra were observed during irradiation due to the formation of a quaternary carbon (44 ppm) and its attached methyl group (21 ppm), a methine carbon (38 ppm), and a g-carbon of a cis isomeric unit adjacent to a trans isomeric unit (30.1 ppm). 

Ultra-violet light irradiation of cinnamoyl derivatives in the solid state produces truxinates resulting from 2+2 cycloaddition. In solution the dimerisation usually does not compete with rapid cis-trans isomerisation. However, it has now been shown that intramolecular 2+2 cycloaddition to give truxinates occurs in solution... 

Unstabilised a-iodoalkyl ylides (1) have been prepared and used in Wittig reactions to give 1-iodoalkenes with (Z)-stereoselectivity. The P-fluoro-substituted ylides (3), containing a chiral alkoxy ligand, are formed as 1 1 mixtures of diastereoisomers from the corresponding alkoxydifluorophosphoranes (2). However, (3) can be epimerised to predominately (>95% in one case) one diastereomer by treatment with lithium fluoride. The first carbodiphosphoranes (4) and (6) containing a P-H bond have been reported. Compound (4) is thermally stable in the solid state but in solution at room temperature both (4) and (6) slowly isomerise to the ylides (5) and (7), respectively. The structures of all these products are supported by n.m.r. data. 

Trans-PA in the pristine (i.e. insulating) state is known to be obtained from a solid-state thermal isomerisation of cis-PA synthesized in various ways [21], whose discussion is beyond the scope of this paper. The mechanism of thermal isomerisation is not yet known. Time-dependent FTIR vibrational spectra have been recorded at time intervals of two seconds a set of spectra is reported in Fig. 1 for an experiment at 145 C [22,23]. 

The stability of the 1,3,2,4-dithiadiazolyl radical is extremely sensitive to substituent and is susceptible to both thermal and photochemical isomerisation to the thermodynamically more stable 1,2,3,5-dithiadiazolyl radical (Section 12.1.1.1) via a bimolecular rearrangement." In a number of other instances the radicals appear indefinitely stable in dilute solution, but decomposition appears to occur at high concentrations. As a consequence few 1,3,2,4-dithiadiazolyl radicals have been isolated in the solid state. Of these the phenyl-ene-bridged fe/5(l,3,2,4-dithiadiazolyl) adopts a n -n dimeric structure in the solid state" while the mixed l,2,3,5-/l,3,2,4-fcw(dithiadiazolyl) has recently been reported (Figure 8)." This too adopts a n n bonding mode," although the dimers exhibit an unexpected behaviour with two sets of n -n interactions between 2 and 3 rather than the symmetric 2/2 and 3/3 dimer. 

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