ELECTRIC DISCHARGE VS. EXCIMER LASER PHOTOLYSIS: A COMPARISON OF TWO METHODS USED TO PREPARE UNSTABLE MOLECULES FOR FOURIER TRANSFORM MICROWAVE SPECTROSCOPIC STUDY

Author Institution: Laboratorium f\""ur Physikaliche Chemie, Eidgen\""ussische Technische HochschuteAlthough the use of an electric discharge has proved to be effective in the preparation of unstable species for spectroscopic study, this method has the disadvantage of being a ``black magic'' approach which completely lacks selectivity. If the molecule of interest happens to be prepared in a high enough yield that its spectrum is strong enough to be measured, then this technique has the advantage of being simple, relatively inexpensive, and easy to implement; however, if the spectra are weak or even un-observable because too few molecules are being generated, then the high selectivity of a laser photolysis preparation method is preferred. We are currently in the process of coupling an excimer laser to our existing pulsed jet cavity Fourier Transform microwave spectrometer with the hope that this will complement our electric discharge nozzle in allowing us to prepare unstable molecules, free radicals, and even ions for high resolution spectroscopic study. Details on some studies carried out using our electric apparatus will be discussed, and a to-date report on the state of our experiments involving the excimer laser will be given

MICROWAVE SPECTRUM OF THE XE-PROPYNE VAN DER WAALS COMPLEX

$^{a}$Present address: Department of Chemistry, University of Alberta, Edmonton, Alta, Canada, T6G, 2G2 $^{b}$ T. A. Blake et al, J. Chem. Phys. 98, p. 6031 (1993) $^{c}$ R. O. Watts, T. A. Blake, P. Muino and D. F. Eggers| private communicationAuthor Institution: Department of Chemistry, University of British Columbia; Department of Chemistry, University of washingtonThe microwave spectrum of the Xe-propyne Van der Waals has been recorded between 4-22 GHz, using a newly-developed pulsed molecular beam Fourier transform microwave spectrometer incorporating automated frequency scanning. The spectra of five isotopomers, involving $^{129}$Xe, $^{131}$Xe, $^{134}$Xe, and $^{135}$Xe, were observed in natural abundance. The geometry of Xe-propyne is T-shaped, comparable to that of Ar-$propyne^{b}$. Hindered internal rotation of the methyl group in propyne splits the levels into A and E species. The A species transitions could be analysed using predicious from an on-going analysis of the infrared $spectrum^{c}$. The microwave spectrum was analysed using a rotation-internal rotation Hamiltonian. The barrier to internal $V_{3}$ was found to be $\sim$ 22$cm^{-1}$, comparable to that of Ar-propyne

Pnicogen bonded complexes of PO2X (X = F, Cl) with nitrogen bases

An ab initio MP2/aug′-cc-pVTZ study has been carried out on complexes formed between PO2X (X = F and Cl) as the Lewis acids and a series of nitrogen bases ZN, including NH3, H2Cî - NH, NH2F, NP, NCH, NCF, NF3, and N2. Binding energies of these complexes vary from -10 to -150 kJ/mol, and P - N distances from 1.88 to 2.72 Å. Complexes ZN:PO2F have stronger P ...N bonds and shorter P - N distances than the corresponding complexes ZN:PO2Cl. Charge transfer from the N lone pair through the π-hole to the P - X and P - O σ* orbitals leads to stabilization of these complexes, although charge-transfer energies can be evaluated only for complexes with binding energies less than -71 kJ/mol. Complexation of PO 2X with the strongest bases leads to P···N bonds with a significant degree of covalency, and P - N distances that approach the P - N distances in the molecules PO2NC and PO2NH 2. In these complexes, the PO2X molecules distort from planarity. Changes in 31P absolute chemical shieldings upon complexation do not correlate with changes in charges on P, although they do correlate with the binding energies of the complexes. EOM-CCSD spin-spin coupling constants 1pJ(P - N) are dominated by the Fermi-contact term, which is an excellent approximation to total J. 1pJ(P - N) values are small at long distances, increase as the distance decreases, but then decrease at short P - N distances. At the shortest distances, values of 1pJ(P - N) approach 1J(P - N) for the molecules PO 2NC and PO2NH2. © 2013 American Chemical Society.Peer Reviewe