Allen M. Ricks

Infrared Spectroscopy of Gas Phase Benzenium Ions: Protonated Benzene and Protonated Toluene, from 750 to 3400 cm -1

Gas phase C 6H 7 (+) and C 7H 9 (+) ions are studied with infrared photodissociation spectroscopy (IRPD) and the method of rare gas tagging. The ions are produced in a pulsed electric discharge supersonic expansion source from benzene or toluene precursors. We observe exclusively the formation of either the C 2 v benzenium ion (protonated benzene) or the para isomer of the toluenium ion (protonated toluene). The infrared spectral signatures associated with each ion are established between 750 and 3400 cm (-1). Comparing the gas phase spectrum of the benzenium ion to the spectrum obtained in a superacid matrix [ Perkampus, H. H.; Baumgarten, E. Angew. Chem. Int. Ed. 1964, 3, 776 ], we find that the C 2 v structure of the gas phase species is minimally affected by the matrix environment. An intense band near 1610 cm (-1) is observed for both ions and is indicative of the allylic pi-electron density associated with the six membered ring in these systems. This spectral signature, also observed for alkyl substituted benzenium ions and protonated naphthalene, compares favorably with the interstellar, unidentified infrared emission band near 6.2 microm (1613 cm (-1)).

Seven-Coordinate Homoleptic Metal Carbonyls in the Gas Phase

Gas-phase metal carbonyl cations of the vanadium-group metals (V(+), Nb(+), Ta(+)) were produced in a molecular beam by laser vaporization and then mass-analyzed and size-selected in a time-of-flight spectrometer and studied with IR laser photodissociation spectroscopy in the carbonyl-stretching region. The abundances in the mass spectra, the fragmentation patterns, and the IR spectra provided a combined approach that revealed the coordination numbers in these systems. Although seven-coordinate structures would have 18 electrons in each case, V(CO)(6)(+) was found to be formed rather than V(CO)(7)(+). Nb(+) formed both six- and seven-coordinate species, while Ta(+) formed only the Ta(CO)(7)(+) complex. Density functional theory computations were used to predict the IR spectra for these systems, which are dramatically different for the six- and seven-coordinate structures and in excellent agreement with the measurements. V(CO)(6)(+) and Nb(CO)(6)(+) have structures slightly distorted from octahedral, while Nb(CO)(7)(+) and Ta(CO)(7)(+) have C(3v) capped octahedral structures.

Infrared Spectroscopy of Extreme Coordination: The Carbonyls of U+ and UO2+

Uranium and uranium dioxide carbonyl cations produced by laser vaporization are studied with mass-selected ion infrared spectroscopy in the C-O stretching region. Dissociation patterns, spectra, and quantum chemical calculations establish that the fully coordinated ions are U(CO)(8)(+) and UO(2)(CO)(5)(+), with D(4d) square antiprism and D(5h) pentagonal bipyramid structures. Back-bonding in U(CO)(8)(+) causes a red-shifted CO stretch, but back-donation is inefficient for UO(2)(CO)(5)(+), producing a blue-shifted CO stretch characteristic of nonclassical carbonyls.

Communications: Infrared spectroscopy of gas phase C3H3+ ions: The cyclopropenyl and propargyl cations

C(3)H(3)(+) ions produced with a pulsed discharge source and cooled in a supersonic beam are studied with infrared laser photodissociation spectroscopy in the 800-4000 cm(-1) region using the rare gas tagging method. Vibrational bands in the C-H stretching and fingerprint regions confirm the presence of both the cyclopropenyl and propargyl cations. Because there is a high barrier separating these two structures, they are presumed to be produced by different routes in the plasma chemistry; their relative abundance can be adjusted by varying the ion source conditions. Prominent features for the cyclopropenyl species include the asymmetric carbon stretch (nu(5)) at 1293 cm(-1) and the asymmetric C-H stretch (nu(4)) at 3182 cm(-1), whereas propargyl has the CH(2) scissors (nu(4)) at 1445, the C-C triple bond stretch (nu(3)) at 2077 and three C-H stretches (nu(2), nu(9), and nu(1)) at 3004, 3093, and 3238 cm(-1). Density functional theory computations of vibrational spectra for the two isomeric ions with and without the argon tag reproduce the experimental features qualitatively; according to theory the tag atom only perturbs the spectra slightly. Although these data confirm the accepted structural pictures of the cyclopropenyl and propargyl cations, close agreement between theoretical predictions and the measured vibrational band positions and intensities cannot be obtained. Band intensities are influenced by the energy dependence and dynamics of photodissociation, but there appear to be fundamental problems in computed band positions independent of the level of theory employed. These new data provide infrared signatures in the fingerprint region for these prototypical carbocations that may aid in their astrophysical detection.

Structure of Protonated Carbon Dioxide Clusters : Infrared Photodissociation Spectroscopy and ab Initio Calculations

The infrared photodissociation spectra (IRPD) in the 700 to 4000 cm(-1) region are reported for H+ (CO2)n clusters (n = 1-4) and their complexes with argon. Weakly bound Ar atoms are attached to each complex upon cluster formation in a pulsed electric discharge/supersonic expansion cluster source. An expanded IRPD spectrum of the H+ (CO2)Ar complex, previously reported in the 2600-3000 cm(-1) range [Dopfer, O.; Olkhov, R.V.; Roth, D.; Maier, J.P. Chem. Phys. Lett. 1998, 296, 585-591] reveals new vibrational resonances. For n = 2 to 4, the vibrational resonances involving the motion of the proton are observed in the 750 to 1500 cm(-1) region of the spectrum, and by comparison to the predictions of theory, the structure of the small clusters are revealed. The monomer species has a nonlinear structure, with the proton binding to the lone pair of an oxygen. In the dimer, this nonlinear configuration is preserved, with the two CO2 units in a trans configuration about the central proton. Upon formation of the trimer, the core CO2 dimer ion undergoes a rearrangement, producing a structure with near C2v symmetry, which is preserved upon successive CO2 solvation. While the higher frequency asymmetric CO2 stretch vibrations are unaffected by the presence of the weakly attached Ar atom, the dynamics of the shared proton motions are substantially altered, largely due to the reduction in symmetry of each complex. For n = 2 to 4, the perturbation due to Ar leads to blue shifts of proton stretching vibrations that involve motion of the proton mostly parallel to the O-H+-O axis of the core ion. Moreover, proton stretching motions perpendicular to this axis exhibit smaller shifts, largely to the red. Ab initio (MP2) calculations of the structures, complexation energies, and harmonic vibrational frequencies are also presented, which support the assignments of the experimental spectra.

Mid- and Far-IR spectra of H5+ and D5+ compared to the predictions of anharmonic theory

H5(+) is the smallest proton-bound dimer. As such, its potential energy surface and spectroscopy are highly complex, with extreme anharmonicity and vibrational state mixing; this system provides an important benchmark for modern theoretical methods. Unfortunately, previous measurements covered only the higher-frequency region of the infrared spectrum. Here, spectra for H5(+) and D5(+) are extended to the mid- and far-IR, where the fundamental of the proton stretch and its combinations with other low-frequency vibrations are expected. Ions in a supersonic molecular beam are mass-selected and studied with multiple-photon dissociation spectroscopy using the FELIX free electron laser. A transition at 379 cm(-1) is assigned tentatively to the fundamental of the proton stretch of H5(+), and bands throughout the 300-2200 cm(-1) region are assigned to combinations of this mode with bending and torsional vibrations. Coupled vibrational calculations, using ab initio potential and dipole moment surfaces, account for the highly anharmonic nature of these complexes.

Infrared spectroscopy of gas phase C3H5+ : The allyl and 2-propenyl cations

C3H5+ cations are probed with infrared photodissociation spectroscopy in the 800-3500 cm(-1) region using the method of rare gas tagging. The ions and their complexes with Ar or N2 are produced in a pulsed electric discharge supersonic expansion cluster source. Two structural isomers are characterized, namely, the allyl (CH2CHCH2+) and 2-propenyl (CH3CCH2+) cations. The infrared spectrum of the allyl cation confirms previous theoretical and condensed phase studies of the C(2nu) charge delocalized, resonance-stabilized structure. The 2-propenyl cation spectrum is consistent with a C(s) symmetry structure having a nearly linear CCC backbone and a hyperconjugatively stabilizing methyl group.

Infrared spectroscopy of the protonated nitrogen dimer: The complexity of shared proton vibrations

The proton-bridged dimers of nitrogen, e.g., N2–H+–N2 and N2–D+–N2, are produced in a pulsed-discharge supersonic nozzle source, mass selected in a reflectron time-of-flight spectrometer, and studied with infrared photodissociation spectroscopy using the method of messenger atom tagging with argon. Both complexes are studied from 700–4000 cm−1. These spectra reproduce the high frequency vibrations seen previously but discover many new vibrational bands, particularly those in the region of the shared proton modes. Because of the linear structure of the core ions, simple vibrational spectra are expected containing only the antisymmetric N–N stretch and two lower frequency modes corresponding to proton stretching and bending motions. However, many additional bands are detected corresponding to various combination bands in this system activated by anharmonic couplings of the proton motions. The anharmonic coupling is stronger for the H+ system than it is for the D+ system. Using anharmonic proton vibrations c...

IR Spectroscopy of Gas Phase V(CO2)n+ Clusters: Solvation-Induced Electron Transfer and Activation of CO2

Ion-molecule complexes of vanadium and CO2, i.e., V(CO2)n(+), produced by laser vaporization are mass selected and studied with infrared laser photodissociation spectroscopy. Vibrational bands for the smaller clusters (n < 7) are consistent with CO2 ligands bound to the metal cation via electrostatic interactions and/or attaching as inert species in the second coordination sphere. All IR bands for these complexes are consistent with intact CO2 molecules weakly perturbed by cation binding. However, multiple new IR bands occur only in larger complexes (n ≥ 7), indicating the formation of an intracluster reaction product whose nominal mass is the same as that of V(CO2)n(+) complexes. Computational studies and the comparison of predicted spectra for different possible reaction products allow identification of an oxalate-type C2O4 anion species in the cluster. The activation of CO2 producing this product occurs via a solvation-induced metal→ligand electron transfer reaction.

The structure of protonated acetone and its dimer: infrared photodissociation spectroscopy from 800 to 4000 cm−1

The infrared spectra of protonated acetone and the proton bound acetone dimer are obtained revealing vibrational resonances associated with the shared proton motions, which are in agreement with the predictions from ab initio, MP2, harmonic frequency calculations.