equipped with a pulsed molecular bean1 source and a threshold photoelectron spectrometer that used standard ZEKE pulsedfield ionization (ZEKE-PFI) electron optics.Within the delayed-field-ionizatio~l and electron-collection veeion of the saectrom-

A new scheme for the detection of ions produced by delayed pulsed field ionization of long-lived high-n Rydberg states (ZEKE Rydberg states) is presented. The equivalence of this new ion detection scheme with high resolution zero kinetic energy photoelectron (ZEKE) spectroscopy is proven by the simultaneous measurement of electrons and ions originating from the same ZEKE Rydberg states, taking into account the effects of pulsed and static low electric fields on the ZEKE Rydberg states. Using this ion detection, a rotationally resolved mass analyzed threshold ionization (MATI) spectrum of benzene with the same resolution (the rotational structure of the benzene cation) as the simultaneously recorded ZEKE photoelectron spectrum has been obtained.

High-resolution ZEKE (zero kinetic energy photoelectron) spectroscopy is a tool to study molecular photionisation processes with rotational resolution. Rotationally resolved ZEKE spectra for several molecules (NO, H,S and benzene) are discussed in detail and a qualitative model for t h e investigation of t h e state-selective ionisation dynamics is employed. This compound-state model provides a simple approach for t h e qualitative understanding of rotationally resolved ionisation processes and t h e underlying symmetry selection rules. The description is extended to the interpretation of t h e ZEKE spectra of benzene and C,, asymmetric rotor molecules (H,O, H,S). From these considerations the rotational selection rules that govern t h e ZEKE spectra of these and similar molecules can be predicted. A comparison with recent experimental results on H,S and new measurements on benzene demonstrates t h e importance of electronic symmetry selection rules in ZEKE spectroscopy.

Fig. 5. X-ray structure of the reference compound 14 ("half of the tweezers"). The carboxyl groups are not separated by a spacer and form a dimer, as expected [14]. by hydrogen bondingThe 1 : f composition of the host-guest complexes of 12 and the high association constants suggest that the guest molecules are bound, as planned,[31 by hydrogen bonds in the noncollapsible molecular niche between the two functional groups. Remarkably, in earlier investigations of host compounds for nucleobases, for example tri-and bis(catechol) macrocycles,['Ob-dl the same order of bonding strengths was found for this type of guest.In conclusion, these tweezer molecules, the first constructed from cyclophanes, are strongly binding and selective hosts. The synthesis of other molecular tweezers of this type with various spacers, other functional groups, and with more than two convergent fixed functions appears to be worthwhile. Experimental Procedure 11:Compound 10 (121 mg, 0.35mmol) and copper(1) iodide (100mg, 0.52 mmol) were dissolved in degassed triethylamine (30 mL) under inert gas and heated at 80°C for 6 h. After the reaction mixture had cooled, the solvent was distilled off under vacuum and the residue purified by chromatography ( R , = 0.76; S O , , particle size 60-100 pm, dichloromethane/petroleum ether 40-60 (1 : 1)) to give colorless crystals (70 mg, 58%). 12/13. Compound 11 (69 mg, 0.1 mmol) was dissolved in anhydrous pyridine (30 mL). treated with anhydrous lithium iodide (403 mg, 3 mmol), and heated at reflux for 8 d. The solvent was distilled off under vacuum and the residue taken up in trichloromethane (20 mL) and 2~ HCI (20 mL) and extracted repeatedly with trichloromethane. The combined organic phases were washed three times with 6N HCI and dried over Na,SO,. The solvent was removed under vacuum and the residue purified by flashchromatography (SiO,, particle size 30-60 pm, trichloromethane/methanol (50: 1)). Fraction 1 (R, = 0.45): 36 mg (53%) monocarboxylic acid 13; fraction 2 ( R , = 0.22); 18 mg, biscarboxylic acid 12.

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