Joseph C. Bopp

Vibrational predissociation spectroscopy of the (H2O)6–21− clusters in the OH stretching region: Evolution of the excess electron-binding signature into the intermediate cluster size regime

We report vibrational predissociation spectra of the (H2O)n- cluster ions in the OH stretching region to determine whether the spectral signature of the electron-binding motif identified in the smaller clusters [Hammer et al. Science 306, 675 (2004)] continues to be important in the intermediate size regime (n = 7-21). This signature consists of a redshifted doublet that dominates the OH stretching region, and has been traced primarily to the excitation of a single water molecule residing in a double H-bond acceptor (AA) binding site, oriented with both of its H atoms pointing toward the excess electron cloud. Strong absorption near the characteristic AA doublet is found to persist in the spectra of the larger clusters, but the pattern evolves into a broadened triplet around n = 11. A single free OH feature associated with dangling hydrogen atoms on the cluster surface is observed to emerge for n > or = 15, in sharp contrast to the multiplet pattern of unbonded OH stretches displayed by the H+(H2O)n clusters throughout the n = 2-29 range. We also explore the vibration-electronic coupling associated with normal-mode displacements of the AA molecule that most strongly interact with the excess electron. Specifically, electronic structure calculations on the hexamer anion indicate that displacement along the -OH2 symmetric stretching mode dramatically distorts the excess electron cloud, thus accounting for the anomalously large oscillator strength of the AA water stretching vibrations. We also discuss these vibronic interactions in the context of a possible relaxation mechanism for the excited electronic states involving the excess electron.

Argon cluster-mediated isolation and vibrational spectra of peroxy and nominally D3h isomers of CO3− and NO3−

Vibrational predissociation spectra are reported for two isomeric forms of the gas-phase ions, CO(3)(-) and NO(3)(-). The peroxy forms, (OOCO(-) and OONO(-)) were isolated using an Ar-mediated synthetic scheme involving exchange of CO and NO for the more weakly bound Ar ligands in O(2)(-)Ar(m) clusters, while the forms based on a central heteroatom (CO(3)(-) and NO(3)(-)) were generated by electron impact on CO(2) and HNO(3) vapor. The simple two-band spectrum of OOCO(-) indicates that it is best described as the O(2)(-) x CO ion-molecule complex, whereas the covalently bound CO(3)(-) form yields a much more complicated vibrational spectrum with bands extending out to 4000 cm(-1). In contrast, the NO(3)(-) ion yields a simple spectrum with only one transition as expected for the antisymmetric NO stretching fundamental of a species with D(3h) structure. The spectrum of the peroxynitrite isomer, OONO(-), displays intermediate complexity that can be largely understood in the context of fundamentals associated with its cis and trans structures previously characterized in an Ar matrix.

Structural characterization of (C2H2)1–6+ cluster ions by vibrational predissociation spectroscopy

Vibrational predissociation spectra are reported for the cationic acetylene clusters, (C(2)H(2))(n) (+), n=1-6, in the region of the C-H stretching fundamentals. For n=1 and 2, predissociation could only be observed for the Ar-tagged clusters. These were prepared by charge-transfer collisions of Ar(k) (+) with C(2)H(2) to create C(2)H(2) (+)Ar(m) clusters, which were then converted into larger members of the (C(2)H(2))(n) (+)Ar series by sequential addition of acetylene molecules. The (C(2)H(2))(2) (+)Ar spectrum indicates that this species is predominantly present as the cyclobutadiene cation. Although mobility measurements on the electron-impact-generated (C(2)H(2))(3) (+) ion indicated that it primarily occurs as the benzene cation, [P. O. Momoh, J. Am. Chem. Soc. 128, 12408 (2006)] photofragmentation of (C(2)H(2))(3) (+)Ar in the C-H stretching region is dominated by the loss of C(2)H(2) in addition to the weakly bound Ar atom. This suggests that the dominant n=3 species formed by sequential addition of C(2)H(2) is based on a covalently bound C(4)H(4) (+) core ion. Interestingly, the spectrum of this core C(4)H(4) (+) species is different from that found for the cyclobutadiene cation, displaying instead a new band pattern that is retained in the higher (C(2)H(2))(3-6) (+) clusters. Multiple isomers are clearly involved, as yet another pattern of bands is recovered when the (C(2)H(2))(3) (+)Ar action spectrum is recorded in the (minor) Ar loss fragmentation channel. One of these features does appear in the location of the single band characteristic of the Ar-tagged benzene cation reported earlier [Phys. Chem. Chem. Phys. 4, 24 (2002)], supporting a scenario where the benzene cation is one of the isomers present. We then compare the Ar predissociation results with (C(2)H(2))(n) (+) spectra obtained when the ions are prepared by electron impact ionization of neutral acetylene clusters. The photofragmentation behavior and vibrational spectra indicate that the dominant species formed in this way also occur with a covalently bound C(4)H(4) (+) core. There are absorptions, however, which are consistent with a minor contribution from (C(2)H(2))(n) (+) clusters based on the benzene cation.

Experimental and theoretical investigation of electron attachment to SF5Cl

Thermal electron attachment to SF(5)Cl has been studied with the flowing afterglow Langmuir probe technique. The rate coefficient is moderate, 4.8(+/-1.2)x10(-8) cm(3) s(-1), and invariant with temperature over the temperature range of 300-550 K. The reaction is dissociative, forming mainly SF(5) (-)+Cl. Minor yields of Cl(-) and FCl(-) were also found. The yields of the minor channels increase slightly with temperature. Statistical unimolecular rate modeling is employed to elucidate the character of the dissociation pathways and to support the assumption that the dissociations involve the formation of metastable anionic SF(5)Cl(-).

Determination of the CO3− bond strength via the resonant two-photon photodissociation threshold: Electronic and vibrational spectroscopy of CO3−∙Arn

We use a two-laser pump-probe technique coupled with messenger atom tagging to determine the bond energy of O(-) to CO(2) in the CO(3) (-) ion, a prevalent species in the upper atmosphere. In this technique, the argon-tagged ion is first electronically excited using a visible laser, then irradiated with a tunable near-infrared beam across the CO(2)...O(-) dissociation threshold while O(-) products are monitored. This method yields a bond energy of 2.79+/-0.05 eV, which is about 0.5 eV higher than previously reported. Combining this with the well-known heats of formation of O(-) and CO(2), 105.6 and -393.1 kJmol, respectively [Thermodynamic Properties of Individual Substances, edited by L. V. Gurvich, I. V. Veyts, and C. B. Alcock (Hemisphere, New York, 1989), Vol. 1 and CODATA Thermodynamic Tables, edited by O. Garvin, V. B. Parker, and J. H. J. White (Hemisphere, New York, 1987)], yields the CO(3) (-) heat of formation: DeltaH(0) (0)=-556.7+/-4.8 kJmol. The one-photon (i.e., linear) infrared and electronic spectra of CO(3) (-) are also presented and compared to those obtained previously. The one-photon electronic spectrum is nearly identical to two-photon spectra, implying that argon does not significantly perturb the ion or its symmetry. The infrared spectrum is drastically different than that obtained in an argon matrix, however, indicating that the ion is likely distorted in the matrix environment.

Theoretical and infrared spectroscopic investigation of the O2−∙benzene and O4−∙benzene complexes

The infrared spectra of the O(2) (-).benzene and O(4) (-).benzene complexes are determined by means of Ar predissociation spectroscopy. Several transitions due to CH stretch fundamentals and various combination bands are observed in the 2700-3100 cm(-1) region. The experimental results are interpreted with the aid of electronic structure calculations. A comparison of the calculated and experimental spectra reveals that the spectrum of O(2) (-).benzene most likely arises from an isomer where the superoxide molecule binds preferentially to one CH group of benzene. In contrast, the spectrum of O(4) (-).benzene yields a CH pattern remarkably similar to that displayed by the C(2nu) X(-).benzene (X=halogen) complexes, consistent with a structure with two CH groups equally involved in the bonding. The lower energy vibrational fundamental transitions of the O(4) (-) anion are recovered with a slight redshift in the O(4) (-).benzene spectrum, establishing that this charge-delocalized dimer ion retains its identity upon complexation.

Electron Affinity of trans-2-C4F8 from Electron Attachment−Detachment Kinetics†

Electron attachment and detachment kinetics of 2-C(4)F(8) were studied over the temperature range 298-487 K with a flowing-afterglow Langmuir-probe apparatus. Only parent anions were formed in the attachment process throughout this temperature range. At the highest temperatures, thermal electron detachment of the parent anions is important. Analysis of the 2-C(4)F(8) gas showed an 82/18 mixture of trans/cis isomers. The kinetic data at the higher temperatures were used to determine the electron affinity EA(trans-2-C(4)F(8)) = 0.79 +/- 0.06 eV after making some reasonable assumptions. The same quantity was calculated using the G3(MP2) compound method, yielding 0.74 eV. The kinetic data were not sufficient to establish a reliable value for EA(cis-2-C(4)F(8)), but G3(MP2) calculations give a value 0.017 eV greater than that for trans-2-C(4)F(8). MP2 and density functional theory were used to study the structural properties of the neutral and anion isomers.