Proton affinity methanol

The formation of either the radical cation M,+ or the protonated [M + H]+ molecule, or both together, will depend on the relative ionization energies or proton affinities of the sample molecules and the solvent components. Concerning the solvent, the charge exchange is favoured for solvents with low proton affinity (water, chloroform, cyclohexane, etc.), while solvents with higher proton affinities (methanol, acetonitrile, etc.) will favour proton transfer. 

Methanol is used exclusively for protonation. Because of its medium proton affinity, methanol allows a broad spectrum of classes of compounds to be determined. It is therefore suitable for a preliminary Cl measurement of compounds not previously investigated. The medium proton affinity does not give any pronounced selectivity. However, substances with predominantly alkyl character remain transparent. Fragments have low intensities. 

In many cases, a protonated molecular ion (M - - H)+ is the only ion observed in a thermospray spectrum but if ammonium acetate buffer is used, depending upon the relative proton affinities of the species present, an ammonium adduct (M - - NH4)+ may be the predominant ion. In addition, clusters may be formed with components of the mobile phase. Although the thermospray ionization process involves less energy than conventional Cl, and very little intense fragmentation is usually observed, the presence of ions due to the elimination of small molecules, e.g. water, methanol and ketene, is not unknown. These latter ions are usually of relatively low intensity when compared to the protonated or... 

Most reported triazine LC applications are reversed-phase utilizing C-8 and C-18 analytical columns, but there are also a few normal-phase (NH2,CN) and ion-exchange (SCX) applications. The columns used range from 5 to 25-cm length and from 2 to 4.6-mm i.d., depending on the specific application. In general, the mobile phases employed for reversed-phase applications consist of various methanol and/or acetonitrile combinations in water. The ionization efficiency of methanol and acetonitrile for atmospheric pressure chemical ionization (APcI) applications were compared, and based on methanol s lower proton affinity, the authors speculated that more compounds could be ionized in the positive ion mode when using methanol than acetonitrile in the mobile phase. 

The data points in Fig. 6 show a slow decrease of Cq for aluminosilicate zeolites with increasing proton affinity of the probe molecule. Experimental data of methanol were not included in the illustration because of possible proton hopping which... 

Calculations at the 6-3IG level indicate that in the gas phase, 2//-l,2,3-triazole is more stable than 1//-1,2,3-triazole by about 4.5 kcal moC. In solution, the IH isomer becomes the more stable species because the large difference in dipole moments favors the more polar tautomer. The triazolium ion (75) is predicted to be more stable than (76) by about 13.5 kcal mol <89Mi40i-0i>. 2//-1,2,3-Triazole represents more than 99.9% of the equilibrium mixture in the gas phase. However, the ab initio calculated proton affinity of 1//-benzotriazole is 10.2 kcal mol larger than that of 2//-benzotriazole, which is consistent with ICR measurements (1-methylbenzotriazole is 10 kcal mol more basic than 2-methylbenzotriazole). Measurements of enthalpies of solution, vaporization, sublimation and solvation in water, methanol and DMSO confirm the predominance of the IH tautomer in solution <89JA7348>. The energy difference between the tautomers of 1,2,3-triazole has also been estimated at the 6-31G (MP2)//3-21G level including zero-point effects. The... 

Most studies of proton transfer in aromatic molecules concern phenol (Abe et al. 1982a,b,c Fuke and Kaya 1983 Gonohe et al. 1985 Jouvet et al. 1990 Lipert and Colson 1988 Mikami et al. 1987, 1988 Oikawa et al. 1983 Solgadi et al 1988 Steadman and Syage 1990 Syage 1990 Syage and Steadman 1991) or 1-naphthol (Cheshnovsky and Leutwyler 1985, 1988 Knochenmuss et al. 1988 Knochenmuss and Leutwyler 1989). These compounds can be associated with various proton acceptors ammonia, water, methanol, monoethylamine, piperidine, etc., differing essentially by their different gas phase proton affinities. 

A correlation can be made between the gas phase proton affinities (PA) of the B clusters which are strongly dependent on their sizes and the propensity of the AH-B clusters to undergo proton transfer in the excited state. These proton affinities of clusters (water, methanol, ammonia, and piperidine) which are estimated (e.g., Knochenmuss and Leutwyler 1989) or deduced from experiment (Bisling et al. 1987 Ceyer et al. 1979 Kamke et al. 1988) are reported in Figure 4-16. 

Figure 4-16. Gas phase proton affinities (PA in kcal mol-1) of B clusters versus /n (B = piperidine, ammonia, methanol, and water) (from Knochenmuss and Leutwyler 1989). The threshold proton affinity corresponds to the energetic limit for which excited state proton transfer occurs for 1-naphthol in small clusters.

For pro tic solvents with larger dielectric constants and stronger basicity, the La and 1Lb states are inverted and relaxation from xLb to xLa takes places but there is no proton transfer to the solvent. The fluorescence is then due to the 1LB state with a small Stokes shift. The intermediate sized water clusters (n = 10-20) belong in this category. The clusters with methanol for any size n < 10 (due to a weak basicity or a small dielectric constant) follow this mechanism. From the evaluated proton affinities (see Figure 4-16), it can be seen that for n k 10 molecules of methanol (PA 243 kcal mol-1 which corresponds to the limit for proton transfer evidence in 1-naphthol complexes with piperidine or ammonia), a proton transfer should be observed. The absence of such a transfer can be related to a cluster structure effect. 

From these data it can be pointed out that for a given size of the clusters the proton affinity of water is smaller than for the other solvents consequently, for fluorobenzene/methanol or para-difluorobenzene/water systems, a proton affinity of 205/215 kcal mol-1 seems to be the limit of the reaction process (it is reached for two molecules of methanol and three molecules of water). 

In RPLC-APCl-MS, where the mobile phase consists of a mixture of water and methanol or acetonitrile, and eventually a buffer, the formation of protonated water clusters can be considered as a starting point in a series of even-electron ion-molecule reactions. The protonated water clusters transfer their proton to any species in the gas mixture with a higher proton affinity (Table 6.1). The mass spectrum of acetonitrile (MeCN)-water mixture shows protonated MeCN-water clusters, [(MeCN), (HjO) + H]", with /w-values of 1-3, and -values of 0-1. The addition of aimnonium acetate to MeCN-water results in the observation of mixed solvent clusters, e.g., [(MeCN), + and [(MeCN) , (HjO) +... 

Selective synthesis of 2-methyl naphthalene has been studied over HZSM-5, HZSM-11, HSABO-11, HZBS-11, Zn-ZSM-11, Ga-ZSM-11, HY and HZSM-20 type zeolites. The nature of the naphthalene-methanol interaction has been investigated in order to elucidate the reaction mechanism. According to the data obtained by FT-IR, TPD of naphthalene, ionization potential and proton affinity of different aromatic rings, zeolites with medium pores and with sites of medium or high acid strength are necessary for the reaction. The results seem to be consistent with the Rideal type mechanism. 

Arad et al. (1990) simulated the reaction sequence of papain by constructing several enzyme-substrate models with molecular mechanics and following reaction paths with semiempirical quantum mechanics. AMBER force field (Weiner et al., 1986a) was employed for the construction. AMI (Dewar et al, 1985) results for proton affinities of the modeled molecules were compared to 4-31G and to experiments. AMI underestimates the proton affinities of methanethiol and of imidazole but overestimates the proton affinity of methanol. However, the proton transfer reactions from methanol to imidazole and from methanethiol to imidazole are overestimated by only 6 and 11 kcal/mol, respectively, and PT from imidazolium to formamide is underestimated by 6 kcal/mol. 

This fact can be attributed to its lower number of substituents and consequently lower proton affinity in comparison with the other two triazines. In contrast, it has been reported that when filament-off or thermospray ionization is employed [M + H]+ is the base peak for different chloroatrazines (21,6) similarly as when DLI LC-MS was used (13). Such a difference in the relative abundance of the different adducts in the mass spectrum between filament-on and filament-off has been previously observed for other groups of pesticides (22,23). For ohlorotriazines a [M + 60]+-ion was the base peak using an eluent of methanol-water and ammonium... 

Proton affinity (PA) acetaldehyde, 123 acetone, 123 acrolein, 123 butenone, 123 dimethyl ether, 123 dimethylacrolein, 123 formaldehyde, 123 methanol, 123 methyl acetate, 123 methyl acrylate, 123 (Aj-mcthylacrolein, 123 oxetane, 123 table of, 123 tetrahydrofuran, 123 water, 123... 

In the case of methanol, a similar intermediacy of protonated methanol may be suggested, although we have not detected this species spectroscopically in the in-situ pulse experiments (the lower proton affinity of methanol may mean that it is protonated only above 200°C, and immediately eliminates f O). 

It is known from mass spectrometry that the proton affinity (gas phase ) of many alkenes is much higher than that of water, methanol or DME (ref. 2), so there will hardly be any protonated water, methanol or DME in the zeolite when there are alkenes present, but there will be carbeniun ions. Table 1. 

The proton affinities (Table 22.1) of methanol (754 kJ moTi) and water (691 kJ moTi) are significantly lower than that of ammonia (854 kJ mol i) and whether or not these molecules are protonated in H-zeolites has created lively debates in the literature. 

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