Acetonitrile ionization effect

Variation of Ionization Effects of Acetonitrile in 50 50 Methanol/Water (2 1)... 

In contrast to intermolecular reactions intramolecular substitutions may offer higher diastereoselectivities but again, complete lack of stereoselectivity is often encountered. Allylsi-lanes can serve as precursors for the stoichiometric preparation of 7c-allylpalladium(II) complexes12. If a stereogenic center is present in the molecule, diastereomeric complexes are formed, whose ratio can be determined by H NMR. Ionization of the dimeric complex 3 with triphenylphosphane gives a zwitterion which cyclizes in a 3-exo-trig mode to the vinyl cyclopropane derivative 4 with unknown diastereomeric ratio. For the cyclization step, dimethylsul-foxide and acetonitrile arc effective solvents, whereas with tetrahydrofuran and methylene chloride no reaction is observed. 

The rate of this isomerization is increased by a- or y-alkyl substituents on the allylic system. The relative rates of isomerization of allyl, y-methylallyl and a-methylallyl thionbenzoates at 100°C are 1, 3,1, and 35 in cyclohexane and 1, 5.5, and 52 in acetonitrile These effects are some two powers of ten smaller than substituent effects on ionization of allylic chlorides (ref. 98, p. 786),... 

Electrospray ionization (ESI) recently developed in connection with LC/MS is superior to FAB in the soft ionizations of labile high molecular weight compounds, and has the advantage of producing predominant pseudo-molecular ions such as [M+H]+ or [M+Na]" for the ionic minute compounds. The ionization effect in ESI-MS depends on whether the solvent contains acetic acid, formic acid or trifluoroacetic acid etc. This technique with direct injection has been successfully applied to hydrolyzable tannins up to tetramers when 50% aqueous MeOH or 50% aqueous acetonitrile containing a small amount of ACONH4 was used as a solvent. In this condition, the observed molecular ion species for hydrolyzable tannins are an [M+NH4] with high sensitivity. 

Fontes tt al. [224,225 addressed the acid—base effects of the zeolites on enzymes in nonaqueous media by looking at how these materials affected the catalytic activity of cross-linked subtilisin microcrystals in supercritical fluids (C02, ethane) and in polar and nonpolar organic solvents (acetonitrile, hexane) at controlled water activity (aw). They were interested in how immobilization of subtilisin on zeolite could affected its ionization state and hence their catalytic performances. Transesterification activity of substilisin supported on NaA zeolite is improved up to 10-fold and 100-fold when performed under low aw values in supercritical-C02 and supercritical-ethane respectively. The increase is also observed when increasing the amount of zeolite due not only to a dehydrating effect but also to a cation exchange process between the surface proton of the enzyme and the sodium ions of the zeolite. The resulting basic form of the enzyme enhances the catalytic activity. In organic solvent the activity was even more enhanced than in sc-hexane, 10-fold and 20-fold for acetonitrile and hexane, respectively, probably due to a difference in the solubility of the acid byproduct. 

The van der Waals volume can be related to the hydrophobicity of the solutes, and retention of molecular compounds can be predicted from their van der Waals volumes, 7r-energy, and hydrogen-bonding energy effects [72-74], It should be noted that the isomeric effect of substituents cannot be predicted with good precision because this is not simply related to Hammett s a or Taft s other hand, the hydrophobicity is related to enthalpy [75], Retention times of non-ionizable compounds were measured in 70 and 80% acetonitrile/water mixtures on an octadecyl-bonded silica gel at 25-60°C and the enthalpy values obtained from these measurements. 

Similarly to the low chemical reactivity of (simple) alkylsilanes devoid of functional groups, the electrochemical reactivity of simple alkylsilanes is quite low. Klingler and Kochi measured the oxidation potentials of tetraalkyl derivatives of group-14-metal compounds by using cyclic voltammetry3. These compounds exhibit an irreversible anodic peak in acetonitrile. The oxidation potential (7 p) decreases in the order of Si>Ge>Sn>Pb as illustrated in Table 1. This order is the same as that of the gas-phase ionization potentials (7p). The absence of steric effects on the correlation of Ev with 7p indicates that the electron transfer should take place by an outer-sphere mechanism. Since tetraalkylsilane has an extremely high oxidation potential (>2.5 V), it is generally difficult to oxidize such alkylsilanes anodically. 

A selective, sensitive, and rapid hydrophilic interaction liquid chromatography with electrospray ionization tandem mass spectrometry was developed for the determination of donepezil in human plasma [32], Donepezil was twice extracted from human plasma using methyl-ferf-butyl ether at basic pH. The analytes were separated on an Atlantis HILIC Silica column with the mobile phase of acetonitrile ammonium formate (50 mM, pH 4.0) (85 15, v/v) and detected by tandem mass spectrometry in the selective reaction monitoring mode. The calibration curve was linear (r = 0.9994) over the concentration range of 0.10-50.0 ng/ ml and the lower limit of quantification was 0.1 ng/ml using 200 /d plasma sample. The CV and relative error for intra- and inter-assay at four quality control levels were 2.7% to 10.5% and —10.0% to 0.0%, respectively. There was no matrix effect for donepezil and cisapride. The present method was successfully applied to the pharmacokinetic study of donepezil after oral dose of donepezil hydrochloride (10 mg tablet) to male healthy volunteers. 

Suppressing matrix effects on ionization of ipratropium (21 % suppression) were described by Ariffin and Anderson when analysing SPE extracts of whole blood by gradient RP-LC-ESI MS/MS [54], In contrast, only slight ion suppression was described by John et al. for simultaneous measuring of cocaine (4 %), homatropine (4 %), ipratropium (4 %),, S-hyoscyaminc (3 %), littorine (1 %), A-butyl-scopolamine (1 %), and scopolamine (10 %) by gradient RP-LC-ESI MS/MS [50], In this study plasma samples were precipitated with acetonitrile and the supernatant was further diluted prior to injection. 

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