Gas-phase Cl+ Affinities Of Pyridines Determined By The Kinetic Method Using Multiple-stage (ms3) Mass Spectrometry

The relative gas-phase halogen cation affinities of a group of substituted pyridines have been ordered, and absolute Cl+ affinity values have been estimated. The Cl+-bound dimer of two pyridines is generated in an ion/molecule reaction using mass-selected Cl-C≡O+ as the chlorinating agent, and its competitive fragmentations to yield the Cl+-pyridine monomers are monitored by multiple-stage (MS3) experiments. These data yield approximate Cl+ affinities which show an excellent linear correlation with literature proton affinity (PA) values. The relationship Cl+ affinity (kcal/mol) = 0.83PA - 42.5 between the two affinities is derived, and both slope and intercept are rationalized in terms of the greater polarizability of Cl+ ion. While proton affinities are unaffected by hindrance near the bonding site in the corresponding proton-bound dimers, the affinities for the larger Cl+ ion are significantly decreased by intramolecular steric effects in those Cl+-bound dimers which involve ortho-substituted pyridines. Electronic effects are separated from steric effects by comparing the fragmentation of the Cl+- and H+-bound dimers composed of a hindered and an unhindered pyridine. In this way, ortho substituents are ordered in terms of the magnitudes of their steric effects. The intramolecular steric effects of ortho substituents, defined here as a gas-phase steric parameter Sk, are found to increase, not only with the size of the substituent but also as the Cl+ affinity of the pyridine increases, due to shortening of the N-Cl+ bond. The Sk values are found also to fall in the same order as the corresponding S0 steric parameters obtained by solution kinetic measurements. Exceptions occur for 2-methoxypyridine and quinoline, where an additional, through-space electronic interaction between the electron-rich substituent and Cl+ is proposed. The methodology used to order Cl+ affinities can be extended to Br+ and I+ affinities, and, in the cases examined, the magnitude of the steric effect falls in the order Br+ > I+ ≃ Cl+ ≫ H+. The intramolecular steric effect in the I+-bound dimers is reduced because of the long N-I bond. The quality of the data obtained is such that it is possible to predict with an estimated uncertainty of 2 kcal/mol Cl+ affinities for compounds which were not examined. To check further on the experimental data and predictions, semiempirical AMI molecular orbital calculations are used to estimate absolute values of Cl+ affinities. An excellent correlation is obtained between the experimental values and the AM1 Cl+ affinities of unhindered pyridines. The calculation overestimates the Cl+ affinities of the hindered pyridines, and this confirms that steric, not electronic, effects are responsible for the decreases observed in the Cl+ affinities of ortho-substituted pyridines. Ab initio MP2/6-31G(d,p)//6-31G(d,p) molecular orbital calculations are used to confirm that Cl+ addition to pyridine occurs at the nitrogen and that the lowest energy structure of the Cl+-bound dimer is the N-Cl+-N-bound species.11662457246

Prospects for a soft x-ray FEL powered by a relativistic-klystron high-gradient accelerator (RK-HGA)

We present here the concept of x-ray FELs using high gain, single-pass amplifiers with electron beams accelerated in high gradient structures powered by relativistic klystrons. Other authors have also considered x-ray FELs; the unique aspect of this paper is the use of high gradient acceleration. One of the authors has previously presented preliminary studies on this concept. The intent in this paper is to display the results of a top level design study on a high gain FEL, to present its sensitivity to a variety of fabrication and tuning errors, to discuss several mechanisms for increasing gain yet more, and to present explicitly the output characteristics of such an FEL. The philosophy of the design study is to find a plausible operating point which employs existing or nearly existing state-of-the-art technologies while minimizing the accelerator and wiggler lengths. The notion is to distribute the technical risk as evenly as possible over the several technologies so that each must advance only slightly in order to make this design feasible. This study entailed no systematic investigation of possible costs so that, for example, the sole criterion for balancing the trade-off between beam energy and wiggler length is that the two components have comparable lengths. 20 refs., 10 figs., 1 tab

Recirculating induction accelerator as a low-cost driver for heavy ion fusion

As a fusion driver, a heavy ion accelerator offers the advantages of efficient target coupling, high reliability, and long stand-off focusing. While the projected cost of conventional heavy ion fusion (HIF) drivers based on multiple beam induction linacs are quite competitive with other inertial driver options, a driver solution which reduces the cost by a factor of two or more will make the case for HIF truly compelling. The recirculating induction accelerator has the potential of large cost reductions. For this reason, an intensive study of the recirculator concept was performed by a team from LLNL and LBL over the past year. We have constructed a concrete point design example of a 4 MJ driver with a projected efficiency of 35% and projected cost of less than 500 million dollars. A detailed report of our findings during this year of intensive studies has been recently completed. 3 refs., 2 figs., 2 tabs