Methanol dimer dissociation

Using high intensity (especially pulsed) lasers it is possible to dissociate the methanol dimer by two-photon excitation. Again a two-peak dissociation spectrum about twice as broad as the one that is found for pure excitation is observed without essential shifts. Embarrassing is the fact that also beams seeded in He show this two-peak spectrum (only slightly broadened) [11], in contrast to the observations discussed in section 2, where optothermal detection yielded very broad banded dissociation spectra (FWHM of 40 cm ) after absorption of a single photon. To explain this discrepancy we have to keep in mind that the detection efficiency E is for the dissociated dimers -caused by one or two-photon transitions as it may be - is different from that of excited dimers Eex- for optothermal detectors (see Table 1). 

T. D. Frigden, L. MacAleese, T. B. McMahon, J. Lemaire, and P. Maitre, Gas phase infrared multiple photon dissociation spectra of methanol, ethanol and propanol proton bound dimers, protonated propanol and the propanol/water proton bound dimer. Phys. Chem. Chem. Phys. 8, 955 966 (2006). 

Although the quinolizinium ion (1), like naphthalene, does not undergo photodimerization, its linear benzo derivative, the acridizinium ion, like anthracene, does so readily (Scheme 27) (57JOC1740). The photodimer dissociates when heated in ethanol. It has been reported that both the dimerization and dissociation in methanol are light-catalyzed and that the quantum yields for the two reactions are 0.23 and 0.49 respectively (78JPR739). 

It is pertinent at this point to refer briefly to the sources of quinone methides, though these have been reviewed (B-74M122400). The general approach used in chroman syntheses involves the thermal elimination of HX from an -substituted phenol. Commonly the eliminated molecules are water, methanol or dimethylamine (287 X = OH, OMe, NMe2, respectively). However, these methods are not entirely suitable because the eliminated molecules may promote side reactions. In the case of 1,2-naphthoquinone 1-methide, the thermal dissociation of the spirodimer (288) is a better source than the other methods. Its formation represents another example of dimerization by a [4+2]-cycloaddition, since it is prepared by heating l-dimethylaminomethyl-2-naphthol in dodecane or xylene with careful exclusion of moisture (73JCS(P1)120,81CJC2223). 

Trivalent yttrium and lanthanide metals, except for promethium, have been complexed to octaethylporphyrin by heating at 210 °C in an imidazole melt.17 The complexes obtained as hydroxides, Mm(OEP)(OH), are unstable in acidic media. As the charge radius ratio rule predicts, the early lanthanide metalloporphyrins, MIU(OEP)(OH) (M = La, Ce, PR, Nd), are demetallated during purification, and the middle series (M = Sm, Eu, Gd, Tb, Dy) in 1 % acetic acid in methanol, while the last five (M = Ho, Er, Tm, Yb, Lu) survive in 2% acetic acid in methanol but are dissociated in dilute hydrochloric acid. The Mnl(OEP)(OH) appears to coordinate more than one equivalent of pyridine and piperidine, and dimerizes in noncoordinating solvents such as benzene and dichloromethane at 10 4 M concentration. The dimer is considered to be a di-p-hydroxo-bridged species, different from the p-oxo dimer, Scin(OEP) 20 (Scheme 6). 

Minkwitz and Schneider54 have been able to obtain protonated methanol in the form of hexafluorometalate salts (MeOH2+MF6, M = As, Sb), and these were characterized by X-ray diffraction. Protonated propanol and proton-bound dimers of methanol, ethanol, and propanol with the proton shared between two alcohol molecules (5) have also been characterized by infrared multiphoton dissociation... 

Studying the ESPT of hydroxy aromatic sulfonates, Huppert and co-workers [40-44] suggested an alternative model based on the geminate proton-anion recombination, governed by diffusive motion. The analysis was carried out by using Debye-Smoluchowskii-type diffusion equations. Their ESPT studies in water-methanol mixtures showed that solvent effects in the dissociation rate coefficient are equal to the effects in the dissociation equilibrium constant [45], 4-Hydroxy-1-naphthalenesulphonate in a water-propanol mixture as the solvent system has been found to behave somewhat differently than water-methanol or water-ethanol media, with a possible role of a water dimer [46,47],... 

Huisken, F., Kulcke, A., Laush, C., and Lisy,. 1. M., Dissociation of small methanol clusters after excitation of the O—H stretch vibration at 2.7 p, J. Chem. Phys. 95, 3924-3929 (1991). Bleiber, A., and Sauer, J., The vibrational frequency of the donor OH group in the H-bonded dimers of water, methanol and silanol. Ab initio calculations including anharmonicities, Chem. Phys. Lett. 238, 243-252 (1995). 

Other examples of dissociative ionization, but at lower excitation energies, are shown for the clusters of methanol and methylchloride in figure 10.2 (Booze and Baer, 1992). Both of these dimer ions were produced by photoionization at the lowest possible energies consistent with a measurable signal level. Yet, dissociative ionization is apparently the dominant process as is evident from the broad TOF peaks (see chapter 5). Only the small sharp TOF peaks in figure 10.2 are the result of nondissociative... 

In more polar media, conductometric, spectrophotometric and P NMR spectroscopic studies reveal the presence of an alternative dissociation pathway for [RuCli(PPh3)3], namely loss of Cl" to produce Ru" cations (equations 99 and 100) although unpublished work indicates that the triply-bridged chloride dimer [Ru2Cl4(DMA)(PPh3)4] is present. Synthesis of pure [RuBr2(PPh3)3] by reaction of methanolic RuClj xHjO /LiBr solutions with PPhj has also been... 

However, with unhindered alcohols (in carbon tetrachloride) such as methanol and ethanol, there is a competition between the dissociation pathway and a pathway involving direct attack of the alcohol on the dimer (Eqs. 4.24, 4.25). The reaction when carried out under pseudounimolecular conditions (with excess ROH) at several different concentrations, and plotted versus [ROH], linear plots are realized. The rate constants for the dissociation pathways k ) and the direct attack pathway (k ) can be evaluated from these plots. 

---