A theoretical simulation of the photoelectron spectrum for the ground state of methoxy radical is reported based on the quasidiabatic model Hamiltonian originally proposed by Köppel, Domcke, and Cederbaum. The parameters in the model Hamiltonian have been obtained from ab initio coupled-cluster calculations. The linear and quadratic force constants have been calculated using equation-of-motion coupled-cluster ionization potential method with the singles, doubles, and triples (EOMIP-CCSDT) truncation scheme together with atomic natural orbital basis sets of triple-zeta quality (ANO1). The cubic and quartic force constants have been obtained from EOMIP-CCSD calculations with ANO basis sets of double-zeta quality (ANO0), and the spin-orbit coupling constant has been computed at the EOMIP-CCSD/pCVTZ level. The nuclear Schroedinger equation has been solved using the Lanzcos algorithm to obtain vibronic energy levels as well as the corresponding intensities. The simulated spectrum compares favorably with the recent high-resolution slow electron velocity-map imaging experiment for vibronic levels up to 2000 cm⁻¹.

        The ground X̃²E vibrations of the methoxy radical have intrigued both experimentalists and theorists alike due to the presence of a conical intersection at the C_3v molecular geometry. This conical intersection causes methoxy's vibrational spectrum to be strongly influenced by Jahn-Teller coupling which leads to large amplitude vibrations and extensive mixing of the two lowest electronic states. The spectrum is further complicated due to spin-orbit and Fermi couplings. The standard diabatic normal mode quantum numbers are poor labels due to this vibronic mixing.
        Using the potential energy force field and calculated spectra of the methoxy radical by Nagesh and Sibert¹ as a starting point, we look to develop a method for assigning states to a spectrum with vibronic coupling. We simplify the analysis by considering only the lowest two e modes of methoxy (the rock and the bend). When we include first-order Jahn-Teller coupling between these two modes in a new zero-order Hamiltonian, we are able to use an expanded version of the linear Jahn-Teller quantum numbers to assign the states.² This zeroth order representation is nontrivial; therefore, we study the properties of its eigenstates using correlation diagrams with respect to the strength of the Jahn-Teller coupling constant.

¹

Nagesh, J.; Sibert, E. L. J. Phys. Chem. A 2012, 116, 3846-3855.

²

Barckholtz, T. A.; Miller, T. A. Int. Revs. in Phys. Chem. 1998, 17, 435-524.

The HO₃ radical is of great interest in both atmospheric and astrophysical chemistry. However, its molecular structure has not been fully characterized by previous spectral studies. Microwave spectral studies on the trans-HO₃ conformer did not access higher K_a levels due to their limited frequency range. As a result, several centrifugal distortion constants could not be determined. We have therefore conducted spectroscopy of HO₃ in the millimeter and submillimeter ranges, from 70 to 450 GHz, under the guidance of a new fast sweep technique we developed for line searching. Large frequency shifts, primarily due to a large ∆_K centrifugal distortion constant, are observed compared to the spectral extrapolation from previous microwave studies. In addition, new spectral branches have been detected. The measured lines and preliminary spectral analysis will be presented, and the implications of these results will be discussed.

The Stark effect is considered for polyatomic open shell complexes that exhibit partially quenched electronic angular momentum. Specifically, a zero-field model Hamiltonian is employed that accounts for the partial quenching of electronic orbital angular momentum in hydroxyl radical containing molecular complexes.M. D. Marshall and M. I. Lester, J. Chem. Phys. 121, 3019 (2004).^,G. E. Douberly, P. L. Raston, T. Liang, and M. D. Marshall, J. Chem. Phys. in presspherical tensor operator formalism is employed to derive matrix elements of the Stark Hamiltonian in a parity conserving, Hund′s case (a) basis for the most general case, in which the permanent dipole moment has projections on all three inertial axes of the system. Ro−vibrational transition intensities are derived, again for the most general case; namely, the laser polarization is projected onto axes parallel and perpendicular to the Stark electric field, and the transition dipole moment vector is projected onto all three inertial axes in the molecular frame. The model discussed here is compared to experimental spectra of OH−(C_2H_2), OH−(C_2H_4), and OH−(H_2 G. E. Douberly, P. L. Raston, T. Liang, and M. D. Marshall, J. Chem. Phys. in pressS

OH···(D₂O)_N complexes are assembled in He droplets via the sequential pickup of D₂O molecules and the hydroxyl radical, which is formed via the pyrolytic decomposition of tert-butyl hydroperoxide. Bands due to clusters as large as N=4 are observed. Ro-vibrational spectroscopy of the binary complex reveals a vibrationally averaged C_2v structure. The effect of partially quenched electronic angular momentum in the complex is partially resolved in the rotational fine structure associated with the ν₁ OH stretch. Stark spectroscopy of this band reveals a permanent electric dipole moment for the binary complex equal to 3.70(5) Debye. OH stretch bands in larger clusters do not exhibit rotational fine structure; however, polarization spectroscopy of the OH···(D₂O)₂ complex, when compared to predictions from ab initio computations, reveals two nearly isoenergetic isomers, both of which resemble the cyclic water trimer. Lower frequency OH stretch bands are assigned to cyclic tetramer and cyclic pentamer clusters on the basis of D₂O pressure dependence and ab initio frequency computations.

The n-propyl and i-propyl radicals were generated in the gas phase via pyrolysis of n-butyl nitrite (CH₃(CH₂)₃ONO) and i-butyl nitrite (CH₃CH(CH₃)CH₂ONO) precursors, respectively. Nascent radicals were promptly solvated by a beam of He nanodroplets, and the infrared spectra of the radicals were recorded in the C-H stretching region. In addition to three vibrations of n-propyl previously measured in an Ar matrix,Pacansky, et. al., J. Phys. Chem. 1977, 81, 2149.e observe many unreported bands between 2800 and 3150 cm⁻¹, which we attribute to propyl radicals. The C-H stretching modes observed above 2960 cm⁻¹ for both radicals are in excellent agreement with anharmonic frequencies computed using VPT2. Between 2800 and 2960 cm⁻¹, however, the spectra of n-propyl and i-propyl radicals become quite congested and difficult to assign due to the presence of multiple anharmonic resonances. Computations reveal the likely origin of the spectral congestion to be strong coupling between the high frequency C-H stretching modes and a lower frequency torsional motion, which modulates quite substantially a through-space hyperconjugation interaction.

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Footnotes:

Pacansky, et. al., J. Phys. Chem. 1977, 81, 2149.w

The technique of corona excited supersonic jet expansion coupled with a pinhole-type glass nozzle was applied to vibronic spectroscopy of jet-cooled chlorofluorobenzyl radicals for the vibronic assignments and measurements of electronic energies of the D₁ → D₀ transition. The vibronic emission spectra were recorded with a long-path monochromator in the visible region. The 2,3-, 2,4-,C. S. Huh, Y. W. Yoon, and S. K. Lee, J. Chem. Phys. 136, 174306 (2012).nd 2,5-S. Y. Chae, Y. W. Yoon, and S. K. Lee, Chem. Phys. Lett. 612, 134 (2014).hlorofluorobenzyl radicals were generated by corona discharge of corresponding precursor molecules, chlorofluorotoluenes seeded in a large amount of helium carrier gas. The emission spectra show the vibronic bands originating from two benzyl-type radicals, chlorofluorobenzyl and fluorobenzyl benzyl radicals, in which fluorobenzyl radicals were obtained by displacement of Cl by H atom produced by the dissociation of methyl C-H bond. From an analysis of the spectra observed, we could determine the electronic energies in D₁ → D₀ transition and vibrational mode frequencies at the D₀ state of chlorofluorobenzyl radicals which show the origin band of the electronic transition to be shifted to red region, comparing with the parental benzyl radical. The red-shift is highly sensitive to the number, position, and kind of substituents in chlorofluorobenzyl radicals. From the quantitative analysis of the red-shift, it has been found that the additivity rule, discovered recently by Lee group predicts the observation very well. In addition, the negligible contribution of the substituent at the 4-position, the nodal point of the Hückel’s molecular orbital theory, can be well describes by the disconnection of substituent from molecular plane of the benzene ring available for delocalized π electrons. In this presentation, I will discuss the spectroscopic observation of new chlorofluorobenzyl radicals and substituent effect on electronic transition energy which is useful for identification of isomeric substituted benzyl radicals.

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Footnotes:

C. S. Huh, Y. W. Yoon, and S. K. Lee, J. Chem. Phys. 136, 174306 (2012).a S. Y. Chae, Y. W. Yoon, and S. K. Lee, Chem. Phys. Lett. 612, 134 (2014).c

Combustion processes involve complex chemistry including pathways leading to polyaromatic hydrocarbons (PAHs) from small molecule precursors. Resonance stabilized radicals (RSRs) likely play an important role in the pathways to PAHs due to their unusual stability. Benzyl radical is a prototypical RSR that is stabilized by conjugation with the phenyl ring. Earlier work on α-methyl benzyl radical showed perturbations to the spectroscopy due to a hindered methyl rotor.Kidwell, N. M.; Reilly, N. J.; Nebgen, B.; Mehta-Hurt, D. N.; Hoehn, R. D.; Kokkin, D. L.; McCarthy, M. C.; Slipchenko, L. V.; Zwier, T. S. The Journal of Physical Chemistry A 2013, 117, 13465.f the alkyl chain is lengthened then multiple conformations become possible. This talk will discuss the jet-cooled spectroscopy of α-ethyl benzyl radical and α-propyl benzyl radical produced from the discharge of 1-phenyl propanol and 1-phenyl butanol respectively. Electronic spectra were obtained via resonant two-photon ionization, and IR spectra were obtained by resonant ion-dip infrared spectroscopy.

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Footnotes:

Kidwell, N. M.; Reilly, N. J.; Nebgen, B.; Mehta-Hurt, D. N.; Hoehn, R. D.; Kokkin, D. L.; McCarthy, M. C.; Slipchenko, L. V.; Zwier, T. S. The Journal of Physical Chemistry A 2013, 117, 13465.I

The vibronic structure of the Ã−X̃ electronic spectrum of NO₃ has been observed using both room-temperature and jet-cooled samples. A recent analysis of this structure is consistent with the Jahn-Teller effect (JTE) in the e′ ν₃ vibrational mode (N-O stretch) being quite strong while the JTE in the e′ ν₄ mode (O-N-O) bend) is rather weak. Electronic structure calculations qualitatively predict these results but the calculated magnitude of the JTE is quantitatively inconsistent with the spectral analysis. Rotationally resolved spectra have been obtained for over a dozen vibronic bands of the Ã−X̃ electronic transition in NO₃. An analysis of these spectra should provide considerably more experimental information about the JTE in the à state of NO₃ as the rotational structure should be quite sensitive to the geometric distortion of the molecule due to the JTE. This talk will focus upon the parallel bands, which terminate on à state levels of a^′′₁ vibronic symmetry, which were the subject of a preliminary analysis reported at this meeting in 2014. We have now recorded the rotational structure of over a half-dozen parallel bands and have completed analysis on the 3¹₀ and 3¹₀ 4¹₀ transitions with several other bands being reasonably well understood. Two general conclusions emerge from this work. (i) All the spectral bands show evidence of perturbations which can reasonably be assumed to result from interactions of the observed à state levels with high vibrational levels of the X̃ state. The perturbations range from severe in some bands to quite modest in others. (ii) Analyses of observed spectra, insofar as the perturbations permit, have all been performed with an oblate symmetric top model including only additional spin-rotation effects. This result is, of course, consistent with an effective, undistorted geometry for NO₃ of D_3h symmetry on the rotational timescale.

The vibronic structure of the NO₃ radical in the à state has been the subject of considerable research in our group and others worldwide. Recently we have collected high resolution, rotationally resolved cavity-ringdown spectra of a number of the vibronic bands terminating on levels of the òE^′′ state. Parallel bands to non-degenerate levels of a₁^′′ vibronic symmetry in the à state, can mostly be satisfactorily fit using an oblate symmetric top Hamiltonian including the effects of spin rotation. The perpendicular bands, to levels of e′ symmetry, are not as satisfactorily described using this Hamiltonian. In particular, the rotational structure of the e′ levels has more transitions than the oblate top model predicts. For this reason we have developed a new rovibronic Hamiltonian capable of analyzing the vibronically degenerate levels. This Hamiltonian is based upon a D_3h configuration for NO₃ corresponding to rotation of an oblate symmetric top. Terms corresponding to coriolis, spin-rotation, spin-orbit, and Jahn-Teller distortions are then added. The simulations of the e′ bands using this model show generally better agreement with the high resolution spectra. Our preliminary analysis indicates only modest effects on the rotational structure due to Jahn-Teller distortion. Details of the analysis of the e′ bands, particularly 2¹₀, will be presented.

In recent years, sophisticated diabatic Hamiltonians have been developed in order to understand the low-energy vibronic level structure of the nitrate radical (NO₃), which exhibits strong coupling between the ~X and doubly degenerate ~B states. Previous studies have reproduced the observed vibronic level positions up to 2000 cm⁻¹ above the zero-point level, yet the rotational structure has remained uninvestigated with ab initio methods. In this talk, we present calculations of the N ≥ 0 rovibronic structure of low-lying vibronic states of NO₃, in which complicated rovibrational and Coriolis interactions have been observed. Our results include calculations using both adiabatic and diabatic Hamiltonians, enabling a direct comparison between the two. We discuss extensions of our treatment to include spin-orbit and spin-rotation effects.

We have measured dispersed fluorescence ( DF ) spectra from the single vibronic levels ( SVL's ) of the B̃ ²E′ state of jet cooled ¹⁴NO₃ and ¹⁵NO₃, and found a new vibronic band around the ν₁ fundamentalM. Fukushima and T. Ishiwata, paper WJ03, ISMS2013, and paper MI17, ISMS2014. This new band has two characteristics; (1) inverse isotope shift, and (2) unexpectedly strong intensity, i.e. comparable with that of the ν₁ fundamental. We concluded on the basis of the isotope effect that the terminated ( lower ) vibrational level of the new vibronic band should have vibrationally a₁′ symmetry, and assigned to the third over-tone of the ν₄ asymmetric (e′) mode, 3 ν₄ (a₁′). We also assigned a weaker band at about 160 cm⁻¹ above the new band to one terminated to 3 ν₄ (a₂′). The 3 ν₄ (a₁′) and (a₂′) levels are ones with l = ±3. Hirota proposed new vibronic coupling mechanismE. Hirota, J. Mol. Spectrosc., in press.hich suggests that degenerate vibrational modes can induce electronic orbital angular momentum ( L ) even in non-degenerate electronic states. We interpret this as a sort of break-down of the Born-Oppenheimer approximation, and think that ±l induces ±―Λ, where ―Λ expresses the pseudo-L; for the present system, one of the components of the third over-tone level, | Λ = 0; v₄ = 3, l = +3 〉, can have contributions of | ―Λ = −1; v₄ = 3, l = +2 〉 and | −2; 3, +1 〉. Under this interpretation, it is expected that there is sixth-order vibronic coupling, (q₊³Q₋³ + q₋³Q₊³), between | 0; 3, +3 〉 and | 0; 3, −3 〉. The sixth-order coupling is weaker than the Renner-Teller term ( the fourth-order term, (q₊²Q₋² + q₋²Q₊²) ), but stronger than the eighth-order term, (q₊⁴Q₋⁴ + q₋⁴Q₊⁴). It is well known in linear molecules that the former shows huge separation, comparable with vibrational frequency, among the vibronic levels of Π electronic states, and the latter shows considerable splitting,  ∼ 10 cm⁻¹, at ∆ electronic states. Consequently, the  ∼ 160 cm⁻¹ splitting at v₄ = 3 is attributed to the sixth-order interaction. The relatively strong intensity for the band to 3 ν₄ (a₁′) can be interpreted as a part of the huge 0-0 band intensity, because the 3 ν₄ (a₁′) level, | 0; 3, ±3 〉, can connect with the vibrationless level, | 0; 0, 0 〉. 3 ν₄ (a₁′) has two-fold intensity because of the vibrational wavefunction, | 0; 3, +3 〉+ | 0; 3, −3 〉, while negligible intensity is expected for 3 ν₄ (a₂′) with | 0; 3, +3 〉− | 0; 3, −3 〉 due to the cancellation. To confirm these interpretations, experiments on rotationally resolved spectra are underway.

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Footnotes:

M. Fukushima and T. Ishiwata, paper WJ03, ISMS2013, and paper MI17, ISMS2014.. E. Hirota, J. Mol. Spectrosc., in press.w

High-resolution fluorescence excitation spectra of ¹⁴NO₃ radical were intermittently recorded in the region 15860 cm⁻¹ to 16050 cm⁻¹ corresponding to the transitions to the vibrationally excited states of the B²E^′ state. Well-separated rotational lines were found to disappear as the vibrational energy increases. The 16050 cm⁻¹ region is almost unstructured even in the high-resolution measurement, and its rotational analysis is almost impossible. The rotational assignment of the 15870 cm⁻¹ region is possible and it has been undertaken by the ground state combination differences and the Zeeman effect observation.

The oxidation by the hydroxyl (OH) radical is one of the most widely studied reactions because of its central role in chemistry, biology, organic synthesis, and photocatalysis in aqueous environments, wastewater treatment, and numerous other chemical processes. Although the redox potential of OH is very high, direct electron transfer (ET) is rarely observed. If it happens, it mostly proceeds through the formation of elusive OH adduct intermediate which facilitates ET and formation of hydroxide anion. Using time resolved resonance Raman technique we structurally characterized variety of OH adducts to sulfur containing organic compounds, halide ions as well as some metal cations. The bond between oxygen of OH radical and the atom of oxidized molecule differs depending on the nature of solute that OH radical reacts with. For most of sulfur containing organics, as well as halide and pseudo-halide ions, our observation suggested that this bond has two-center three-electron character. For several metal aqua ions studied, the nature of the bond depends on type of the cation being oxidized. Discussion on spectral parameters of all studied hydroxyl radical adducts as well as the role solvent plays in their stabilization will be presented.