Temperature-Dependent Photoelectron Spectroscopy of Methyl Benzoate Anions: Observation of Steric Effect in o-Methyl Benzoate

Probing Dipole-Bound States Using Photodetachment Spectroscopy and Resonant Photoelectron Imaging of Cryogenically Cooled Anions

Molecular anions with polar neutral cores can support highly diffuse dipole-bound states below their detachment thresholds due to the long-range charge-dipole interaction. Such nonvalence states constitute a special class of excited electronic states for anions and were observed in early photodetachment experiments to measure the electron affinities of organic radicals. Recent experimental advances, in particular, the ability to create cold anions using a cryogenically cooled Paul trap, have allowed the investigation of dipole-bound excited states at a new level. For the first time, the zero-point level of dipole-bound excited states can be observed via resonant two-photon detachment, and resonant photoelectron spectroscopy can be performed via the above-threshold vibrational levels (Feshbach resonances) of the dipole-bound states. This Perspective describes recent progress in the investigation of dipole-bound states in the authors' lab using an electrospray photoelectron spectroscopy apparatus equipped with a cryogenically cooled Paul trap and high-resolution photoelectron imaging.

Structures and Chemical Rearrangements of Benzoate Derivatives Following Gas Phase Decarboxylation

Decarboxylation of carboxylate ions in the gas phase provides a useful window into the chemistry displayed by these reactive carbanion intermediates. Here, we explore the species generated by decarboxylation of two benzoate derivatives: 2-formylbenzoate (2FBA) and 2-benzoylbenzoate (2BBA). The nascent product anions are transferred to a cryogenic ion trap where they are cooled to ∼15 K and analyzed by their pattern of vibrational bands obtained with IR photodissociation spectroscopy of weakly bound H₂ molecules. The structures of the quenched species are then determined by comparison of these spectra with those predicted by electronic structure calculations for local minima on the potential energy surface. The 2-phenide carbanion generated by decarboxylation of 2FBA occurs in two isomeric forms that differ in the orientation of the formyl group, both of which yield a very large (∼110 cm^-1) redshift in the stretching frequency of the H₂ molecule attached to the anionic carbon center. Although calculated to be a local minimum, the analogous 2-phenide species could not be isolated upon decarboxylation of 2BBA. Rather, the anionic product adopts a ring-closed structure, indicating efficient nucleophilic attack on the pendant phenyl group by the nascent phenide. The barrier for ring closing is evaluated with electronic structure calculations.

Photoelectron photofragment coincidence spectroscopy of aromatic carboxylates: benzoate and p-coumarate

Photoelectron-photofragment coincidence spectroscopy was used to study the dissociation dynamics of the conjugate bases of benzoic acid and p-coumaric acid. Upon photodetachment at 266 nm (4.66 eV) both aromatic carboxylates undergo decarboxylation, as well as the formation of stable carboxyl radicals. The key energetics are computed using high-level electronic structure methods. The dissociation dynamics of benzoate were dominated by a two-body DPD channel resulting in CO₂ + C₆H₅ + e^-, with a very small amount of stable C₆H₅CO₂ showing that the radical ground state is stable and the excited states are dissociative. For p-coumarate (p-CA^-) the dominant channel is photodetachment resulting in a stable radical and a photoelectron with electron kinetic energy (eKE) <2 eV. We also observed a minor two-body dissociative photodetachment (DPD) channel resulting in CO₂ + HOC₆H₄CHCH + e^-, characterized by eKE <0.8 eV. Evidence was also found for a three-body ionic photodissociation channel producing HOC₆H₅ + HCC^- + CO₂. The ion beam contained both the phenolate and carboxylate isomers of p-CA^-, but DPD only occurred from the carboxylate form. For both species DPD is seen from the first and second excited states of the radical, where vibrational excitation is required for decarboxylation from the first excited radical state.

Photoelectron photofragment coincidence spectroscopy of carboxylates

Photoelectron–photofragment coincidence (PPC) spectroscopy is a powerful technique for studying the decarboxylation dynamics of carboxyl radicals. Measurement of photoelectron and photofragment kinetic energies in coincidence provides a kinematically complete measure of the dissociative photodetachment (DPD) dynamics of carboxylate anions. PPC spectroscopy studies of methanoate, ethanoate, propanoate, 2-butenoate, benzoate, p-coumarate and the oxalate monoanion are reviewed. All of the systems studied undergo decarboxylation via a two-body DPD channel i.e., driven by the thermodynamic stability of CO₂. Additionally, decarboxylation is observed via a three-body ionic photodissociation channel for p-coumarate. In some cases photodetachment also results in a stable carboxyl radical (RCO₂). The branching ratio for DPD, the threshold detachment energy and the peak of the kinetic energy release spectrum are compared for different carboxylates, as a probe of the character of the potential energy landscape in the Franck–Condon region.

Photoelectron photofragment coincidence spectroscopy studies of a range of carboxylate anions are reviewed, revealing details of the decarboxylation dynamics of carboxyl radicals.

Molecular Specificity and Proton Transfer Mechanisms in Aerosol Prenucleation Clusters Relevant to New Particle Formation

Atmospheric aerosol particles influence the Earth's radiative energy balance and cloud properties, thus impacting the air quality, human health, and Earth's climate change. Because of the important scientific and overarching practical implications of aerosols, the past two decades have seen extensive research efforts, with emphasis on the chemical compositions and underlying mechanisms of aerosol formation. It has been recognized that new particle formation (NPF) contributes up to 50% of atmospheric aerosols. Nowadays, the general consensus is that NPF proceeds via two distinct stages: the nucleation from gaseous precursors to form critical nuclei of sub-1-2 nm size, and the subsequent growth into large particles. However, a fundamental understanding of both the NPF process and molecular-level characterization of the critical size aerosol clusters is still largely missing, hampering the efforts in developing reliable and predictive aerosol nucleation and climate models.Both field measurements and laboratory experiments have gathered convincing evidence about the importance of volatile organic compounds (VOCs) in enhancing the nucleation and growth of aerosol particles. Numerous and abundant small clusters composed of sulfuric acid or bisulfate ion and organic molecules have been shown to exist in ∼2 nm sized aerosol particles. In particular, kinetic studies indicated the formation of clusters with one H₂SO₄ and one or two organics being the rate-limiting step.This Account discusses our effort in developing an integrated approach, which involves the laboratory cluster synthesis via electrospray ionization, size and composition analysis via mass spectrometry, photoelectron spectroscopic characterization, and quantum mechanics based theoretical modeling, to investigate the structures, energetics, and thermodynamics of the aerosol prenucleation clusters relevant to NPF. We have been focusing on the clusters formed between H₂SO₄ or HSO₄^- and the organics from oxidation of both biogenic and anthropogenic emissions. We illustrated the significant thermodynamic advantage by involving organic acids in the formation and growth of aerosol clusters. We revealed that the functional groups in the organics play critical roles in promoting NPF process. The enhanced roles were quantified explicitly for specific functional groups, establishing a Molecular Scale that ranks highly hierarchic intermolecular interactions critical to aerosol formation. The different cluster formation pathways, probably mimicking the various polluted industrial environments, that involve cis-pinonic and cis-pinic acids were unveiled as well. Furthermore, one intriguing fundamental phenomenon on the unusual protonation pattern, which violates the gas-phase acidity (proton affinity) prediction, was discovered to be common in sulfuric acid-organic clusters. The mechanism underlying the phenomenon has been rationalized by employing the temperature-dependent experiments of sulfuric acid-formate/halide model clusters, which could explain the high stability of the sulfuric acid containing aerosol clusters. Our work provides critical molecular-level information to shed light on the initial steps of nucleation of common atmospheric precursors and benchmarks critical data for large-scale theoretical modeling to further address problems of environmental interest.

Dissociative photodetachment vs. photodissociation of aromatic carboxylates: the benzoate and naphthoate anions

Photodetachment leads to a stable radical and to dissociation. Both processes are characterized by the kinetic energy release of the neutral particles.

H-bond and dipole-dipole interactions between water and COO functional group in methyl benzoate derivatives: Substituent and heteroatom effects

Interactions of COOCH₃ functional group of methyl benzoate (MB) and its derivatives can substantially be affected by substituents and heteroatoms. The substituent and heteroatom effects on the intermolecular interactions in the complexes of MB and its derivatives with the water molecule have been investigated by the quantum mechanical calculations in the present study. Electron donating (ED)/electron withdrawing (EW) substituents increase/decrease the binding energy (ΔE_bin) between mentioned functional group and hydrogen bond donor, and the N heteroatom in the six membered ring weakens the effect. The effects of the N heteroatoms depend on their positions in the ring, and the effects of substituents depend on the number and the positions of heteroatoms. Some relations were obtained between the dipole moment of carbonyl group, results obtained from the natural bond orbital (NBO) and the atoms in molecules (AIM) analysis, binding energy and Hammett constants of substituents. The results can be used to predict the factors affect the intermolecular interactions, and to improve the ligand-protein bindings in biological systems.

Development of a low-temperature photoelectron spectroscopy instrument using an electrospray ion source and a cryogenically controlled ion trap

The ability to control ion temperatures is critical for gas phase spectroscopy and has been a challenge in chemical physics. A low-temperature photoelectron spectroscopy instrument has been developed for the investigation of complex anions in the gas phase, including multiply charged anions, solvated species, and biological molecules. The new apparatus consists of an electrospray ionization source, a three dimensional (3D) Paul trap for ion accumulation and cooling, a time-of-flight mass spectrometer, and a magnetic-bottle photoelectron analyzer. A key feature of the new instrument is the capability to cool and tune ion temperatures from 10 to 350 K in the 3D Paul trap, which is attached to the cold head of a closed cycle helium refrigerator. Ion cooling is accomplished in the Paul trap via collisions with a background gas and has been demonstrated by observation of complete elimination of vibrational hot bands in photoelectron spectra of various anions ranging from small molecules to complex species. Further evidence of ion cooling is shown by the observation of H2-physisorbed anions at low temperatures. Cold anions result in better resolved photoelectron spectra due to the elimination of vibrational hot bands and yield more accurate energetic and spectroscopic information. Temperature-dependent studies are made possible for weakly bonded molecular and solvated clusters, allowing thermodynamic information to be obtained.

Lithium monoxide anion: a ground-state triplet with the strongest base to date

Lithium monoxide anion (LiO(-)) has been generated in the gas phase and is found to be a stronger base than methyl anion (CH(3)(-)). This makes LiO(-) the strongest base currently known, and it will be a challenge to produce a singly charged or multiply charged anion that is more basic. The experimental acidity of lithium hydroxide is DeltaH degrees (acid) = 425.7 +/- 6.1 kcal.mol(-1) (1 kcal = 4.184 kJ) and, when combined with results of high-level computations, leads to our best estimate for the acidity of 426 +/- 2 kcal.mol(-1).

Thermochemical Studies of Benzoylnitrene Radical Anion: The N−H Bond Dissociation Energy in Benzamide in the Gas Phase

The thermochemical properties of benzoylnitrene radical anion, C6H5CON-, were determined by using a combination of energy-resolved collision-induced dissociation (CID) and proton affinity bracketing. Benzoylnitrene radical anion dissociates upon CID to give NCO- and phenyl radical with a dissociation enthalpy of 0.85 +/- 0.09 eV, which is used to derive an enthalpy of formation of 33 +/- 9 kJ/mol for the nitrene radical anion. Bracketing studies with the anion indicate a proton affinity of 1453 +/- 10 kJ/mol, indicating that the acidity of benzamidyl radical, C6H5CONH, is between those of benzamide and benzoic acid. Combining the measurements gives an enthalpy of formation for benzamidyl radical of 110 +/- 14 kJ/mol and a homolytic N-H bond dissociation energy in benzamide of 429 +/- 14 kJ/mol. Additional thermochemical properties obtained include the electron affinity of benzamidyl radical, the hydrogen atom affinity of benzoylnitrene radical anion, and the oxygen anion affinity of benzonitrile.

Electronic structures and electron detachment energies of halogen substituted acetate anions, XCH2COO− (X=F,Cl,Br)

The electronic structures and the halogen inductive effects on the acetate anion were investigated in XCH2COO- (X=F,Cl,Br) by photoelectron spectroscopy (PES) and ab initio calculations. The PES spectra indicated that the electron binding energies increased in the order of F<Cl<Br, contradictory to the known electron affinities of the halogen atoms. The measured adiabatic detachment energies (ADEs) are 3.80, 3.93, and 3.97 eV and the vertical detachment energies (VDEs) are 3.96, 4.10, and 4.13 eV for the F-, Cl-, and Br-substituted species, respectively. Structures of these anions and their neutral species were obtained by full geometry optimizations at the CCSD(T)/aug-cc-pVDZ level of theory, and final energies were calculated at the CCSD(T)/aug-cc-pVTZ level. The calculated ADEs (3.76, 3.88, and 3.91 eV for F, Cl, Br, respectively) and VDEs (4.14, 4.29, and 4.32 eV, respectively) are in good agreement with the corresponding experimental results. Theoretical analysis shows that the increase of ADE/VDE from F to Cl to Br is related to that the matching of the p orbital energy of X with the COO- group is better for Br than that of Cl and F. For comparison, additional calculations were carried out to include halogen substituted ethanol, XCH2CH2OH. Similar trend on electron binding energies was also found. In contrast, the ionization potentials (IPs) of both XCH2COOH and XCH2CH2OH decrease in the order of F>Cl>Br. These systematic changes of detachment energy and IPs were explained by examining the charge redistributions upon detaching electrons.

Observation of Cysteine Thiolate and-S···H−O Intermolecular Hydrogen Bond

The cysteine anion was produced in the gas phase by electrospray ionization and investigated by photoelectron spectroscopy at low temperature (70 K). The cysteine anion was found to exhibit the thiolate form [-SCH2CH(NH2)CO2H], rather than the expected carboxylate form [HSCH2CH(NH2)CO2-]. This observation was confirmed by two control experiments, that is, methyl cysteine [CH3SCH2CH(NH2)CO2-] and cysteine methyl ester [-SCH2CH(NH2)CO2CH3]. The electron binding energy of [-SCH2CH(NH2)CO2H] was measured to be about 0.7 eV blue-shifted relative to [-SCH2CH(NH2)CO2CH3] due to the formation of an intramolecular -S-...HO2C- hydrogen bond in the cysteine thiolate. Theoretical calculations at the CCSD(T)/6-311++G(2df,p) and B3LYP/6-311++G(2df,p) levels were carried out to estimate the strength of this intramolecular -S-...HO2C- hydrogen bond. Combining experimental measurements and theoretical calculations yielded an estimated value of 16.4 +/- 2.0 kcal/mol for the -S-...HO2C- intramolecular hydrogen-bond strength.

Low-temperature Photoelectron Spectroscopy of Aliphatic Dicarboxylate Monoanions, HO2C(CH2)nCO2- (n = 1−10): Hydrogen Bond Induced Cyclization and Strain Energies

Photoelectron spectra of singly charged dicarboxylate anions HO(2)C(CH(2))(n)CO(2)(-) (n = 1-10) are obtained at two different temperatures (300 and 70 K) at 193 nm. The electron binding energies of these species are observed to be much higher than the singly charged monocarboxylate anions, suggesting that the singly charged dicarboxylate anions are cyclic due to strong intramolecular hydrogen bonding between the terminal -CO(2)H and -CO(2)(-) groups. The measured electron binding energies are observed to depend on the chain length, reflecting the different -CO(2)H...(-)O(2)C- hydrogen bonding strength as a result of strain in the cyclic conformation. A minimum binding energy is found at n = 5, indicating that its intramolecular hydrogen bond is the weakest. At 70 K, all spectra are blue shifted relative to the room-temperature spectra with the maximum binding energy shift occurring at n = 5. These observations suggest that the cyclic conformation of HO(2)C(CH(2))(5)CO(2)(-) (a ten-membered ring) is the most strained among the 10 anions. The present study shows that the -CO(2)H...(-)O(2)C- hydrogen bonding strength is different among the 10 anions and it is very sensitive to the strain in the cyclic conformations.

Determination of the Electron Affinity of the Acetyloxyl Radical (CH3COO) by Low-Temperature Anion Photoelectron Spectroscopy and ab Initio Calculations

The electronic structure and electron affinity of the acetyloxyl radical (CH3COO) were investigated by low-temperature anion photoelectron spectroscopy and ab initio calculations. Photoelectron spectra of the acetate anion (CH3COO-) were obtained at two photon energies (355 and 266 nm) and under three different temperatures (300, 70, and 20 K) with use of a new low-temperature ion-trap photoelectron spectroscopy apparatus. In contrast to a featureless spectrum at 300 K, a well-resolved vibrational progression corresponding to the OCO bending mode was observed at low temperatures in the 355 nm spectrum, yielding an accurate electron affinity for the acetyloxyl radical as 3.250 +/- 0.010 eV. This experimental result is supported by ab initio calculations, which also indicate three low-lying electronic states observed in the 266 nm spectrum. The calculations suggest a 19 degrees decrease of the OCO angle upon detaching an electron from acetate, consistent with the vibrational progression observed experimentally.