Results and Discussion Substituent Effects

The most important factor limiting the accuracy of Av(7tg is the influence of the medium. Even at dilute concentrations of pyridines in heptane, there is a clustering of polar pyridine molecules around the polar complexes, causing a shift of the band of the complex to short wavelengths. The concentration dependence of Av(7tg cr ) can be minimized by using as low a pyridine concentration as allowed by an accurate measurement of the visible band maximum. 

From a ap database [20], many new pATbi2 values can now be estimated by means of Equations 7.49 and 7.50. 

11 Mass Spectrometric Determination of the Gas-Phase Lithium Cation Basicity of Dimethyl Sulfoxide and Methyl Phenyl Sulfoxide hy the Kinetic Method 

As explained in Section 6.3, Eourier transform ion cyclotron resonance mass spectrometry (FnCR-MS) and high-pressure mass spectrometry (HPMS) are the methods of choice for 

The proposed experiment consists in determining the LiCB of dimethyl sulfoxide (DMSO) and methyl phenyl sulfoxide (MPSO) from the measurement of the intensity ratios of the ions [L —Li+] and [L —Li+], /[L —Li+]//[L —Li+], involving five ligands L. Among them, three amides [A-methylformamide (MFA), NAf-dimethylformamide (DMF) and AA -dimethylacetamide (DMA)] are considered as references with known LiCBs. The unknown LiCBs of the two sulfoxides are determined from a plot of ln(/[L —Li+]//[L —Li+]) against known thermodynamic LiCB values. As explained in Section 6.3, the ratio of ion intensities is equal to the ratio of the unimolecular rate constants of the dissociation reactions of [L —M+—L ] into [L —Li+] and [L —Li+]. Hence ln(/[L —Li+]//[L —Li+]) may be called a relative lithium cation kinetic basicity . Remember that, under specified assumptions. 

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