Cyclotron solutions

Photoelectron spectroscopic studies show that the first ionization potential (lone pair electrons) for cyclic amines falls in the order aziridine (9.85 eV) > azetidine (9.04) > pyrrolidine (8.77) >piperidine (8.64), reflecting a decrease in lone pair 5-character in the series. This correlates well with the relative vapour phase basicities determined by ion cyclotron resonance, but not with basicity in aqueous solution, where azetidine (p/iTa 11.29) appears more basic than pyrrolidine (11.27) or piperidine (11.22). Clearly, solvation effects influence basicity (74JA288). 

While a proper solution of the radiative transfer problem in media at B Bqed — 4.4 x 1013 G has necessarily to wait for a description in terms of the Stokes parameters, the search for the proton cyclotron feature in the spectra of AXPs and SGRs begun. Up to now no evidence for the proton line has been found in the thermal components of SGRs and AXPs, although these... 

The major activity in gas-phase studies now depends on the use of modem techniques such as ion cyclotron resonance (ICR). Thus, as already mentioned (Section ELD). Fujio, Mclver and Taft131 measured the gas-phase acidities, relative to phenol, of 38 meta- or para-substituted phenols by the ICR equilibrium constant method, and their results for +R substituents led them to suggest that such substituents in aqueous solution exerted solvation-assisted resonance effects. It was later163 shown by comparison of gas-phase acidities of phenols with acidities of phenols in solution in DMSO that solvation-assisted resonance effects could also occur even when the solvent did not have hydrogen-bond donor properties. Indeed for p-NC>2 and certain other substituents these effects appeared to be larger than in aqueous solution. 

As a starting point for an examination of the mechanisms of gas phase reactions, the Claisen condensation is a multistep reaction that appears to proceed by essentially the same mechanism in the gas phase as in solution, as illustrated in Figure 5. In the gas phase, in cases where this reaction occurs, all that is observed is a disappearance of the enolate reactant and the appearance of P-carbonyl enolate product. The intermediate ions in the mechanism react too rapidly to exist long enough for detection. In the ICR spectrometer, unless an ion exists for at least a millisecond or longer, there are not enough cyclotron cycles to create a detectable signal. Intermediates such as the ones postulated for this reaction, with 10-50... 

Xenon difluoride labelled with positron-emitting F has been prepared by reaction of cyclotron produced [ F]p2 with xenon [88]. This low-yielding method requires high pressure. [ F]Xep2 was also obtained by treating sulfuryl chloride fluoride solutions of Xep2 in fluorinated ethylene propylene vessels with Bronst-... 

General Methods. The instrument that will be used to execute the gas-phase experimental portion of the proposed research is a Finnigan 2001 dual-cell Fourier transform ion cyclotron resonance mass spectrometer (FTMS or FTICR), equipped with both electron impact (FI) and electrospray ionization (FSl). FTMS is a high-resolution, high-sensitivity technique that allows the entrapment and detection of gas-phase species. Gas-phase ions are trapped in a magnetic field, much like a reactant sits in a flask in solution. The instrument is a mass spectrometer therefore, we will often refer to the mass-to-charge (m/z) ratio of ions, which is the method we use to identify species. (M-l) or (M-H) refers to a molecule M that has been deprotonated for example, HjO has an (M-f) ion of m/z 17 (HO ). 

Equations (A9) show that the electron exhibits the expected cyclotron motion in the presence of the magnetic field. However, collisions must also be taken into account. Let N(t) be the number of particles that have not experienced a collision for time t (after some arbitrary beginning time, t = 0). Then it is reasonable to assume that the rate of decrease of N(t) will be given by dN oc —Ndt = —Ndt/1. The solution of this equation is N(t) = N0 exp (—t/ ), where N0 is the total number of particles. It can easily be shown that x is simply the mean time between collisions. The probability of having not experienced a collision in time t is, of course, N(t)/N0 = exp(—t/ ). The... 

In recent years proton transfers in the gas phase have been extensively studied. The gas phase allows measurements on strong acids and strong bases that are not available in solution. The methodology is rather specialized, involving mass spectrometry and its variants such as ion cyclotron resonance (ICR), so the reader is referred to a review.94... 

Fornarini, Matire, and co-workers110 have recently used Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry assaying the multiphoton dissociation behavior (IR-MPD) of the C3H7+ ion. This study has confirmed the conclusions of the computational results discussed above. The IR spectra recorded in solution and in a solid matrix display close resemblance to the spectral characteristics found by the IR-MPD study. Theoretical studies also indicated that the virtually free methyl rotation allows the interconversion of the two enantiomers of the isopropyl cation. 

Gas phase proton affinities of phosphabenzene and arsabenzene have been determined by ion-cyclotron resonance techniques 94>. These confirm the qualitative solution phase data (see Fig. 5). Phosphabenzene (PA = 194.5 kcal/mol) has a proton affinity nearly 30 kcal/mol less than trimethylphosphine and only slightly greater than that of phosphine. Arsabenzene (PA = 188.0 kcal/mol) has a proton affinity 23 kcal/mol less than trimethylarsine. In the case of arsabenzene, protonation occurred on carbon rather than arsenic so the As-basidty may be even lower. By contrast, the proton affinity of pyridine (PA = 218 kcal/mol) is only slightly less than that of trimethylamine (PA = 222 kcal/mol) but considerably larger than ammonia (PA = 202 kcal/mol). 

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