Anion photoelectron spectroscopy of deprotonatedortho-,meta-, andpara-methylphenol

Spectroscopy and Dynamics of the Dipole-Bound States of ortho-, meta-, and para-Methylphenolate Anions

A photodetachment and photoelectron spectroscopic study by employing a cryogenically cooled ion trap combined with a velocity-map imaging setup has been carried out to unravel the vibrational structures and autodetachment dynamics of the dipole-bound states (DBSs) of o-, m-, and p-methylphenolate anions (o-, m-, and p-CH3PhO-). The electron binding energy of the DBS increases monotonically with the increase of the neutral dipole moment to give respective values of 66 ± 15, 123 ± 18, or 154 ± 14 cm-1 for the o-, m-, or p-isomer. The different electron-donating effects of the methyl moieties in the three geometrically different isomers seem to be reflected in the experiment. Mode-specific DBS dynamics of the o-, m-, and p-CH3PhO- complexes have been interrogated by using picosecond time-resolved photoelectron velocity-map imaging spectroscopy. Autodetachment lifetimes of the DBS vibrational Feshbach resonances have been measured and discussed quantitatively using Fermi's golden rule, especially in comparison with those of the phenoxide anion to get insights into the methyl substitution effect on the electron binding dynamics of the metastable DBS.

Predicted Negative Ion Photoelectron Spectra of 1-, 2-, and 9-Cyanoanthracene Radical Anions and Computed Thermochemical Values of the Three Cyanoanthracene Isomers

In this work, the negative ion photoelectron spectra of 1-, 2-, and 9-cyanoanthracene (anthracenecarbonitrile, ACN) radical anions, obtained via the calculation of Franck-Condon (FC) factors based on a harmonic oscillator model, are reported. The FC calculations utilize harmonic vibrational frequencies and normal mode vectors derived from density functional theory using the B3LYP/6-311++G (2d,2p) basis set. The removal of an electron from the doublet anion allows for the computation of the negative ion photoelectron spectra that represents the neutral ground singlet state (S_o) and the lowest triplet state (T₁) in each of the three ACN molecules. The respective adiabatic electron affinity (EA) values for the S_o state in 1-, 2-, and 9-ACN isomers are calculated to be 1.353, 1.360, and 1.423 eV. The calculated EA of the 9-cyanoanthracene singlet isomer is in close agreement with the previously reported experimental value of 1.27 ± 0.1 eV. Calculations show that the T₁ states in 1-, 2-, and 9-ACN are located 1.656, 1.663, and 1.599 eV above the S_o state. The calculated T₁ negative ion spectra exhibit intense vibrational origins and weak FC activity beyond the origins, indicating little change in geometry following electron detachment from the doublet anionic state. Upon deprotonation, the EA values of the radical isomers increase by ∼400-700 meV, resulting in neutral deprotonated radicals with EAs between 1.740 and 2.220 eV. The calculated site-specific gas-phase acidity values of ACN isomers indicate that ACN molecules are more acidic than benzonitrile.

From Benzonitrile to Dicyanobenzenes: The Effect of an Additional CN Group on the Thermochemistry and Negative Ion Photoelectron Spectra of Dicyanobenzene Radical Anions

The negative ion photoelectron spectra of 1,2-dicyanobenzene (o-DCNB), 1,3-dicyanobenzene (m-DCNB), and 1,4-dicyanobenzene (p-DCNB) radical anions (DCNB·^-), acquired through the computation of Frack-Condon (FC) factors, are presented. The FC calculations utilize harmonic frequencies and normal mode vectors derived from density functional theory at the B3LYP/aug-cc-pVQZ basis set. All the totally symmetric vibrational modes are treated with Duschinsky rotations to yield neutral DCNBs in their singlet (S_o) and lowest triplet (T₁) states, following an electron removal from the doublet anionic ground state. For the S_o state, the adiabatic electron affinities (EAs) for o-, m-, and p-DCNB are 1.179, 1.103, and 1.348 eV. The EAs for the lowest T₁ state in o-, m-, and p-DCNB are 4.151, 4.185, and 4.208 eV, resulting in an S_o-T₁ energy difference (ΔE_ST) of 2.973, 3.082, and 2.860 eV. A vibrational analysis reveals evidence of FC activity involving ring distortion, C-N bending, and ring C═C stretching vibrational progressions in both the S_o and T₁ states. With the detection of cyanonaphthalene (C₁₀H₇CN) and cyanoindene (C₉H₇CN) in the interstellar medium (ISM), our results highlight the extent to which replacing a single hydrogen on an aromatic molecule with a cyano group, C≡N, can alter the vibrational structure of the molecule/radical anion. As such, dicyano-polyaromatic hydrocarbons may be reasonably robust in the ISM, making it appealing to search for them in future interstellar detection missions.

Experimental Observation of the Resonant Doorways to Anion Chemistry: Dynamic Role of Dipole-Bound Feshbach Resonances in Dissociative Electron Attachment

Anion chemical dynamics of autodetachment and fragmentation mediated by the dipole-bound state (DBS) have been thoroughly investigated in a state-specific way by employing the picosecond time-resolved or the nanosecond frequency-resolved spectroscopy combined with the cryogenically cooled ion trap and velocity-map imaging techniques. For the ortho-, meta-, or para-iodophenoxide anion (o-, m-, or p-IPhO^-), the C-I bond rupture occurs via the nonadiabatic transition from the DBS to the nearby valence-bound states (VBS) of the anion where the vibronic coupling into the S₁ (πσ*) state (repulsive along the C-I bond extension coordinate) should be largely responsible. Dynamic details are governed by the isomer-specific nature of the potential energy surfaces in the vicinity of the DBS-VBS curve crossings, as manifested in the huge different chemical reactivity of o-, m-, or p-IPhO^-. It is confirmed here that the C-I bond dissociation is mediated by DBS resonances, providing the foremost evidence that the metastable DBS plays the critical role as the doorway into the anion chemistry especially of the dissociative electron attachment (DEA). The fragmentation channel is dominant when it is mediated by the DBS resonances located below the electron-affinity (EA) threshold, whereas it is kinetically adjusted by the competitive autodetachment when the DBS resonances above EA convey the electron to the valence orbitals. The product yield of the C-I bond cleavage is strongly mode-dependent as the rate of the concomitant autodetachment is much influenced by the characteristics of the individual vibrational modes, paving a new way of the reaction control of the anion chemistry.

Negative Ion Photoelectron Spectra of Deprotonated Benzonitrile Isomers via Computation of Franck-Condon Factors

Negative ion photoelectron spectra of ortho (o-), meta (m-), and para (p-) deprotonated benzonitrile (o-, m-, p-C₆H₄(CN)^-) isomers as well as the associated thermochemical values corresponding to deprotonation at o-, m-, and p-positions in C₆H₅(CN) are presented. Quantum mechanical results based on the density functional theory (DFT) utilizing the aug-cc-pVQZ basis set indicate that the o-, m-, p-C₆H₄(CN)^● radicals have electron affinity values (EAs) of 1.901, 1.778, and 1.789 eV, respectively. The computed Franck-Condon (FC) factors give rise to o-, m-, and p-C₆H₄(CN)^- negative ion spectra with FC active ring distortion vibrational modes with harmonic vibrational frequencies of ∼450, 760, and 1000 cm^-1 as the dominant vibrational progressions. Deprotonation at the o-, m-, and p-positions in C₆H₅(CN) results in calculated gas-phase acidity values (Δ_acidH_298K^o) of 383.9, 385.7, and 385.3 kcal mol^-1, respectively. The calculated Δ_acidH_298K^o is in close agreement with the previously reported high-pressure mass spectrometry experimental value of 383.4.0 ± 4.4 kcal mol^-1. The computed Δ_acidH_298K^o and EAs are utilized to estimate the bond dissociation energy (DH₂₉₈(H-C₆H₄CN)) associated with the formation o-, m-, and p-C₆H₄(CN)^● using the negative ion thermochemical cycle: DH₂₉₈(C₆H₅CN) = Δ_acidH_298K^o (H-C₆H₄(CN) + EA (C₆H₅CN)^● - IP(H). The respective values of DH₂₉₈(H-C₆H₄CN) corresponding to the formation of ortho, meta, and para C₆H₄(CN) radicals are 114.15, 113.11, and 113.51 kcal mol^-1.

Photoelectron Spectroscopy of the Hexafluorobenzene Cluster Anions: (C6F6) n- ( n = 1-5) and I-(C6F6)

Frequency-resolved (2D) photoelectron (PE) spectra of the anionic clusters (C₆F₆) _n^-, for n = 1-5, and time-resolved PE spectra of I^-C₆F₆ are presented using a newly built instrument and supported by electronic structure calculations. From the 2D PE spectra, the vertical detachment energy (VDE) of C₆F₆^- was measured to be 1.60 ± 0.01 eV, and the adiabatic detachment energy (ADE) was ≤0.70 eV. The PE spectra also contain fingerprints of resonance dynamics over certain photon energy ranges, in agreement with the calculations. An action spectrum over the lowest resonance is also presented. The 2D spectra of (C₆F₆) _n^- show that the cluster can be described as C₆F₆^-(C₆F₆) _n-1. The VDE increases linearly (200 ± 20 meV n^-1) due to the stabilizing influence on the anion of the solvating C₆F₆ molecules. For I^-C₆F₆, action spectra of the absorption just below both detachment channels are presented. Time-resolved PE spectra of I^-C₆F₆ excited at 3.10 eV and probed at 1.55 eV reveal a short-lived nonvalence state of C₆F₆^- that coherently evolves into the valence ground state of the anion and induces vibrational motion along a specific buckling coordinate. Electronic structure calculations along the displacement of this mode show that at the extreme buckling angle the probe can access an excited state of the anion that is bound at that geometry but adiabatically unbound. Hence, slow electrons are emitted and show dynamics that predominantly probe the outer-turning point of the motion. A PE spectrum taken at t = 0 contains a vibrational structure assigned to a specific Raman- or IR-active mode of C₆F₆.

Photoelectron spectroscopy and thermochemistry of o-, m-, and p-methylenephenoxide anions

The anionic products following (H + H+) abstraction from o-, m-, and p-methylphenol (cresol) are investigated using flowing afterglow-selected ion flow tube (FA-SIFT) mass spectrometry and anion photoelectron spectroscopy (PES). The PES of the multiple anion isomers formed in this reaction are reported, including those for the most abundant isomers, o-, m- and p-methylenephenoxide distonic radical anions. The electron affinity (EA) of the ground triplet electronic state of neutral m-methylenephenoxyl diradical was measured to be 2.227 ± 0.008 eV. However, the ground singlet electronic states of o- and p-methylenephenoxyl were found to be significantly stabilized by their resonance forms as a substituted cyclohexadienone, resulting in measured EAs of 1.217 ± 0.012 and 1.096 ± 0.007 eV, respectively. Upon electron photodetachment, the resulting neutral molecules were shown to have Franck-Condon active ring distortion vibrational modes with measured frequencies of 570 ± 180 and 450 ± 80 cm-1 for the ortho and para isomers, respectively. Photodetachment to excited electronic states was also investigated for all isomers, where similar vibrational modes were found to be Franck-Condon active, and singlet-triplet splittings are reported. The thermochemistry of these molecules was investigated using FA-SIFT combined with the acid bracketing technique to yield values of 341.4 ± 4.3, 349.1 ± 3.0, and 341.4 ± 4.3 kcal mol-1 for the o-, m-, and p-methylenephenol radicals, respectively. Construction of a thermodynamic cycle allowed for an experimental determination of the bond dissociation energy of the O-H bond of m-methylenephenol radical to be 86 ± 4 kcal mol-1, while this bond is significantly weaker for the ortho and para isomers at 55 ± 5 and 52 ± 5 kcal mol-1, respectively. Additional EAs and vibrational frequencies are reported for several methylphenyloxyl diradical isomers, the negative ions of which are also formed by the reaction of cresol with O-.

Slow Photoelectron Velocity-Map Imaging of Cryogenically Cooled Anions

Slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled anions (cryo-SEVI) is a powerful technique for elucidating the vibrational and electronic structure of neutral radicals, clusters, and reaction transition states. SEVI is a high-resolution variant of anion photoelectron spectroscopy based on photoelectron imaging that yields spectra with energy resolution as high as 1-2 cm^-1. The preparation of cryogenically cold anions largely eliminates hot bands and dramatically narrows the rotational envelopes of spectral features, enabling the acquisition of well-resolved photoelectron spectra for complex and spectroscopically challenging species. We review the basis and history of the SEVI method, including recent experimental developments that have improved its resolution and versatility. We then survey recent SEVI studies to demonstrate the utility of this technique in the spectroscopy of aromatic radicals, metal and metal oxide clusters, nonadiabatic interactions between excited states of small molecules, and transition states of benchmark bimolecular reactions.

Sensitivity of Photoelectron Angular Distributions to Molecular Conformations of Anions

An anion photoelectron imaging study probing the sensitivity of the photoelectron angular distribution (PAD) to conformational changes is presented. The PADs of a series of para-substituted phenolate anions is compared with those calculated using the Dyson orbital formalization. Good agreement was attained for the two observed direct detachment channels of all anions, except for the lowest-energy detachment channel of para-ethyl phenolate for which two conformations exist that yield very different PADs. The conformational freedom leads to an observed PAD that is the incoherent sum of the PADs from all conformers populated under experimental conditions. In contrast, a second detachment channel shows no sensitivity to the conformational flexibility of para-ethyl phenolate. Our results show that PADs can provide detailed information about the electronic structure of the anion and its conformations.

Chromophores of chromophores: a bottom-up Hückel picture of the excited states of photoactive proteins

Many photoactive proteins contain chromophores based on para-substituted phenolate anions which are an essential component of their electronic structure.