How long are the ends of polyene chains?

In this work we study conjugation in all-trans polyene chains H[Single Bond](HC[Double Bond]CH)(n)[Single Bond]H with a view to establishing the length scale for the interaction between conjugated double bonds. As a polyene oligomer is made longer, bond length alternation between formal carbon-carbon single and double bonds diminishes toward the middle of the chain, eventually reaching a constant value characteristic of an "infinite" chain. However those bonds near the end of the chain continue to be influenced by the end, even in the long-chain limit. We have determined optimized geometries for polyene oligomers with up to n=11 repeat units at the MP2/cc-pVTZ level. At this length the central-most bonds are almost converged to the long chain limit, for which we estimate R(C[Double Bond]C)=1.3652 A and R(C[Single Bond]C)=1.4238 A. In contrast, the endmost double bond has a length of 1.3442 A and the endmost single bond has a length of 1.4425 A. We find that a given bond is significantly influenced by conjugation paths through up to six neighboring conjugated double bonds. End effects can also be monitored by examining the energy increment per added monomer as the oligomer length is increased. This analysis also indicates that significant conjugation effects extend out through approximately six neighboring double bonds. From the energy per monomer of the longest chains we extract a value of about 8 kcal/mol for the extra stabilization energy per monomer due to conjugation in long chains.

Preference for an ion-neutral complex-mediated pathway over a five-membered-ring H shift in the isomerization of CH3O (+)HCH 2CH 2 (·) to CH 3CH 2CH 2OH (+·) by Ab initio theory.

Ab initio theory is used to explore whether the path from CH3OH(+)CH2CH 2 (·) (1) to CH3CH2CH2OH(+·) (5) goes by way of a conventional 1,4-H shift to form ·CH2OH(+)CH2CH3 (2), or via the ion-neutral complex-mediated H transfer [CH3OHCH2=CH2](+·) (3) → [CH3CH 2 (·) CH2OH(+)] (4). Five levels of theory all place the highest energy point in the complex-mediated reaction 3 → 4 slightly below that for the 1,4-H shift 1 → 2, but both routes appear energetically feasible near the threshold for the dissociation of 1 to CH3CH2 + CH2=OH(+). Thus, 1 may take both paths to 5. It is concluded that when both a conventional and a complex-mediated pathway seem plausible in a given system, the latter should be considered to be as likely as the former. Ab initio descriptions of other species involved in the isomerization of 1 to 2 also are presented.

The effect of ion size on rate of dissociation: RRKM calculations on model large polypeptide ions.

The larger an ion is, the less likely it is to decompose on mass spectrornetry time scales at given critical and internal energies. This is an obstacle to obtaining structural information on large molecules by mass spectrometry. We performed RRKM calculations on model ions with masses from 0.27 kDa to 102.4 kDa to explore what such calculations predict regarding this limitation. According to the calculations, it is impractical to add enough energy to fragment very large ions unless the decomposition has a low critical energy. It is suggested that ion-molecule reactions that are either very low in their critical energies or exothermic may be a feasible approach to fragmenting ionized macromolecules.

Size effects in ion-neutral complex-mediated alkane eliminations from ionized aliphatic ethers.

The effects of the size of the ionic and neutral partners on ion-neutral complex-mediated alkane eliminations from ionized aliphatic ethers were determined by obtaining metastable decomposition spectra and photoionization ionization efficiency curves. Increasing the size of the ionic partner decreases the competitiveness of alkane elimination with alkyl loss. This is attributed to decreasing attraction between the partners with increasing distance between the neutral partner and the center of charge in the associated ion. Increasing the size of the neutral partner lowers the threshold for alkane elimination relative to that for simple dissociation when the first threshold is above ΔHf(products). This is attributed to increasing attraction between the partners with increasing polarizability of the radical in the complex. Adding a CH2 to the radical in a complex seems to increase the attraction between the partners by about 24 kJ mol(-1).

PAVE LOW III: interior lighting reconfiguration for night lighting and night vision goggle compatibility.

The PAVE LOW III aircraft is a modified HH-53H helicopter that has a low altitude--below 30.48 m (100 ft)--night/day rescue mission. The desired night flying configuration is for the pilot to wear night vision goggles (NVGs) to fly the aircraft while the copilot, without NVGs, observes the video display and monitors the aircraft instruments. The problems of NVG incompatibility in the cockpit were successfully countered using several light control techniques. The light control modifications were evaluated on the ground in the PAVE LOW III helicopter at Kirtland AFB in April, 1980, by PAVE LOW instructor pilots. The evaluation results were extremely positive.