J. Barrie Peel

Photoelectron spectroscopy of argon clusters: Evidence for an Ar13 ionization chromophore

Ultraviolet photoelectron spectra have been measured for argon clusters prepared in pulsed supersonic expansions of both pure and seeded mixtures of argon. The broad band spectra show structural features which relate to the degree of condensation and hence the mean cluster size obtained at different stagnation pressures. These are interpreted in terms of an ion–core hypothesis with diatomics‐in‐molecules calculations being used to simulate spectral features. The analysis shows that Ar+3 cores are formed in the initial ionization process of very small clusters, with the Ar+13 core becoming dominant for all larger clusters and for the condensed state of argon.

Photoelectron spectrum of methylenimine by spectrum stripping

The He I photoelectron spectrum of methylenimine, CH2NH, has been obtained from the spectra of methylamine pyrolysis products recorded at different temperatures. The time-averaged digitized spectra are relatively noise-free and the spectrum stripping was carried out using an interactive computer program producing visually displayed spectra.

IDENTIFICATION OF DIANIONS OF C84 AND C90 BY ELECTROSPRAY MASS SPECTROMETRY

Abstract We have observed the dianions of the fullerenes C 84 and C 90 in the negative-ion electrospray mass spectrometry (EMS) of specific solutions of a commercial sample of a mixture of higher fullerenes (C 60 to C 96 ) which contains mostly C 84 . The results of semiempirical calculations show that, in contrast to the cases of C 60 and C 70 , these dianions are likely to have strongly positive electron affinities, explaining their stability in a high-vacuum environment. While C 84 2− has recently been observed in the isolated state, this is the first observation of the intrinsically stable C 90 2− .

Photoelectron spectroscopy of the nitrogen dimer (N2)2 and clusters (N2)n: N2 dimer revealed as the chromophore in photoionization of condensed nitrogen

The He i photoelectron spectra of gas‐phase nitrogen dimer and nitrogen clusters have been measured in a pulsed cluster beam. The dimer (N2)2 is characterized by broad bands with vertical ionization energies which are 0.3±0.1 eV lower than for N2 monomer. The bands observed for a mixture of small clusters, estimated to be of average size N=10, are identical to the dimer bands except for further shifts of 0.3 eV to lower ionization energies. The clusters bandwidths and band shapes are virtually the same as measured for thin films of condensed N2, indicating that the nitrogen dimer (N2)2 is the ionization chromophore in each case. This offers support for Haberland’s hypothesis that ionization of any Mn cluster produces the ion M+2Mn−2 provided M is a closed‐shell atom or molecule. The theory of electronic relaxation polarization of the dielectric medium, which explains the gas‐to‐solid ionization energy shifts, is modified for the case of finite clusters and to account for dimer ion formation.

Theoretical study of deprotonated glucopyranosyl disaccharide fragmentation

Molecular orbital calculations were used to investigate the fragmentation of deprotonated glucopyranosyl disaccharides. Based on data from collisional activation and isotopic labeling experiments, fragmentation mechanisms are proposed, with calculated transition states being used to study the energetics of fragmentation. The calculations suggest that deprotonation at the C(2) hydroxyl of the non-reducing ring, following ring opening, may be important for disaccharide fragmentation. It is also shown that the stereochemistry at the 2-position of the non-reducing ring may have a significant effect on disaccharide fragmentation, particularly with regard to determination of the anomeric configuration.

Calculations on the Jahn–Teller configurations of the benzene cation

The analysis of the zero-electron-kinetic-energy photoelectron spectra of benzene has led to a reinvestigation by ab initio methods of the electronic states of the C6H6+ cation resulting from Jahn–Teller distortions on ionization. The calculations involving a range of currently used methodologies all verify that the two cation configurations, 2B2g and 2B3g of D2h symmetry, resulting from removal of an electron from the e1g(π) degenerate MOs of C6H6, comprise a true minimum and a transition state, differing only slightly in energy. These are linked through the in-plane b1g vibration, confirming that b1g is actually a pseudorotational coordinate. Hence C6H6+ exhibits similar structural floppiness to the cations of methane and cyclopropane although with a much smaller barrier to pseudorotation than for these smaller species. These results support the general proposition that such Jahn–Teller distortions associated with molecular ionization (of stable closed-shell hydrocarbon molecules of high symmetry) gener...

A COMPARISON OF HYDROGEN-BONDED AND VAN DER WAALS ISOMERS OF PHENOL..NITROGEN AND PHENOL..CARBON MONOXIDE : AN AB INITIO STUDY

The hydrogen-bonded and van der Waals isomers of phenol⋅⋅nitrogen and phenol⋅⋅carbon monoxide in their neutral electronic (S0) and cation ground state (D0) were studied using ab initio HF/6-31G*, MP2/6-31G*, and B3LYP/6-31G* methods. The hydrogen-bonded isomers have the ligand bound via the hydroxyl group of the phenol ring, while the van der Waals isomers studied have the ligand located above the aromatic ring. For both complexes, the hydrogen-bonded isomer was found to be the most stable form for both the S0 and the D0 states. For phenol⋅⋅carbon monoxide, twice as many isomers as compared to phenol⋅⋅nitrogen were found. The hydrogen-bonded isomer with the carbon end bonded to the hydroxyl group was the most stable structure for both the S0 and the D0 states.

Photoelectron spectroscopy of the NO dimer

He i photoelectron spectra are presented for the NO dimer, (NO)2, and for a mixture of higher clusters, (NO)nn≥3. The NO dimer shows a weak second band associated with a shake‐up transition at a vertical I.E. of 10.8 eV, 1.9 eV above that of the first band. This disagrees with a theoretical estimate of 2.7 eV. The ratio of intensities of these two bands is measured as 1:0.18 in comparison with the theoretical estimate of 1:0.28. The spectrum of the mixture of higher clusters shows the analogous two bands at vertical I.E.s of 8.4 and 10.8 eV, respectively. The band separation of 2.4 eV is close to the value of 2.6 eV observed in the photoemission spectrum of condensed NO.

Ammonia dimer studied by photoelectron spectroscopy

The He i photoelectron spectrum of (NH3)2 is shown to have only a single broad unstructured ionization band near 10 eV. This demonstrates the equivalence or near‐equivalence of the nitrogen lone‐pair orbitals of each NH3 unit in the dimer structure. While theory has generally predicted that (NH3)2 has a classical structure with a near‐linear N‐‐H–N linkage, such a structure would be expected to show two ionization bands separated by about 1.5 eV. However analysis of the observed low ionization energy band suggests that a maximum possible band separation would be only 0.5 eV for ionization from the nitrogen lone‐pair orbitals. While this result concurs with the nonclassical structure for (NH3)2 proposed by Klemperer and co‐workers, it does not exclude the symmetric C2h structure as a likely equilibrium geometry for the dimer. On the contrary, it is argued that the photoelectron spectrum is supportive of the latter structure. Split‐valence basis Hartree–Fock calculations on (NH3)2 at various nonclassical ge...

Relative gas-phase acidities of glucopyranose from molecular orbital calculations

AM1 and ab initio calculations were performed for molecular glucopyranose and its alkoxy anions. Minimum energy conformers were found for both the α- and β-anomers of molecular glucose and each of the corresponding deprotonated species. Cooperative hydrogen bonding towards the charge site was observed for the anions, which is similar to the cooperative counterclockwise hydrogen bonding that occurs with the neutral molecular species. The anomeric hydroxyl was found to be the most acidic hydroxyl group for α-glucopyranose, while the C(4) hydroxyl was the most acidic site for the β-anomer. In all cases the primary hydroxyl at C(6) was the least acidic site. Copyright