Gary E. Douberly

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)).

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.

Automation of an "Aculight" continuous-wave optical parametric oscillator.

We report the automation of a continuous-wave, singly resonant, optical parametric oscillator (Lockheed-Martin Aculight ARGOS 2400-SF-15). This commercially available optical parametric oscillator (OPO) is capable of producing >1 W of continuously tunable idler output between 2.2 and 4.6 μm. An algorithm based on the feedback from a high accuracy wavemeter is implemented to synchronize three separate OPO tuning elements; the translation of a fan-out type periodically poled lithium niobate crystal, the rotation of an intracavity etalon, and the continuous tuning of the pump and idler wavelengths via piezoelectric strain of the tunable fiber pump laser. This allows for several hundred wavenumbers of efficient, automatic, continuous tuning of the idler wave. Continuous feedback from the wavemeter limits the absolute frequency accuracy to ±20 MHz. The broad, automatic tuning of the OPO is demonstrated via its implementation as a probe laser for the infrared action spectroscopy of methanol solvated in helium nanodroplets. LabVIEW virtual instruments for the automation of this OPO laser system are reported, along with detailed schematics of the associated hardware developed at the University of Georgia.

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.

Infrared Spectroscopy of (HCl)m(H2O)n Clusters in Helium Nanodroplets: Definitive Assignments in the HCl Stretch Region

Infrared spectra in the HCl stretch region (2600-2900 cm(-1)) are presented for small, mixed (HCl)(m)(H(2)O)(n) clusters solvated in helium nanodroplets. Sharp bands associated with the Cl-H...Cl stretch vibrations in m:n = 1:1, 2:1, 2:2, and 3:1 clusters are superimposed on a broad background that increases in intensity as larger clusters are grown within the droplets. The broad background is determined to be partially due to mixed clusters with m > 3 and n > 2. The sharp bands are assigned to specific cluster compositions, m:n, via pick-up pressure dependence studies and optically selected mass spectrometry. Orientation of the clusters is achieved with the application of a large electric field to the laser/droplet beam interaction region. The intensity of each band is measured as a function of the applied field strength. Simulations of this electric field dependence based on ab initio calculations of permanent dipole moments and vibrational transition moment angles leads to definitive structural assignments for each sharp band. The 2:1 complex is cyclic, and a band associated with the 2:2 cluster is determined to arise from the nonalternating cyclic structure.

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 HOOO and DOOO in 4He nanodroplets

The HOOO hydridotrioxygen radical and its deuterated analog (DOOO) have been isolated in helium nanodroplets following the in situ association reaction between OH and O(2). The infrared spectrum in the 3500-3700 cm(-1) region reveals bands that are assigned to the ν(1) (OH stretch) fundamental and ν(1) + ν(6) (OH stretch plus torsion) combination band of the trans-HOOO isomer. The helium droplet spectrum is assigned on the basis of a detailed comparison to the infrared spectrum of HOOO produced in the gas phase [E. L. Derro, T. D. Sechler, C. Murray, and M. I. Lester, J. Chem. Phys. 128, 244313 (2008)]. Despite the characteristic low temperature and rapid cooling of helium nanodroplets, there is no evidence for the formation of a weakly bound OH-O(2) van der Waals complex, which implies the absence of a kinetically significant barrier in the entrance channel of the reaction. There is also no spectroscopic evidence for the formation of cis-HOOO, which is predicted by theory to be nearly isoenergetic to the trans isomer. Under conditions that favor the introduction of multiple O(2) molecules to the droplets, bands associated with larger H∕DOOO-(O(2))(n) clusters are observed shifted ~1-10 cm(-1) to the red of the trans-H∕DOOO ν(1) bands.

Infrared laser spectroscopy of the CH3OO radical formed from the reaction of CH3 and O2 within a helium nanodroplet.

Helium nanodroplet isolation and infrared laser spectroscopy are used to investigate the CH(3) + O(2) reaction. Helium nanodroplets are doped with methyl radicals that are generated in an effusive pyrolysis source. Downstream from the introduction of CH(3), the droplets are doped with O(2) from a gas pick-up cell. The CH(3) + O(2) reaction therefore occurs between sequentially picked-up and presumably cold CH(3) and O(2) reactants. The reaction is known to lead barrierlessly to the methyl peroxy radical, CH(3)OO. The ~30 kcal/mol bond energy is dissipated by helium atom evaporation, and the infrared spectrum in the CH stretch region reveals a large abundance of droplets containing the cold, helium solvated CH(3)OO radical. The CH(3)OO infrared spectrum is assigned on the basis of comparisons to high-level ab initio calculations and to the gas phase band origins and rotational constants.

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...

Anomalous Λ-doubling in the infrared spectrum of the hydroxyl radical in helium nanodroplets.

The X(2)Π3/2 hydroxyl (OH) radical has been isolated in superfluid (4)He nanodroplets and probed with infrared laser depletion spectroscopy. From an analysis of the Stark spectrum of the Q(3/2) transition, the Λ-doublet splittings are determined to be 0.198(3) and 0.369(2) cm(-1) in the ground and first excited vibrational states, respectively. These splittings are 3.6 and 7.2 times larger than their respective gas phase values. A factor of 1.6 increase in the Q(1/2) Λ-doublet splitting was previously reported for the He solvated X(2)Π1/2 NO radical [von Haeften, K.; Metzelthin, A.; Rudolph, S.; Staemmler, V.; Havenith, M. Phys. Rev. Lett. 2005, 95, 215301]. A simple model is presented that reproduces the observed Λ-doublet splittings in He-solvated OH and NO. The model assumes a realistic parity dependence of the rotors effective moment of inertia and predicts a factor of 3.6 increase in the OH ground state (J = 3/2) Λ-doubling when the B0(e) and B0(f) rotational constants differ by less than one percent.