Rigid or floppy water-containing dipole-bound dimer anions

We here report a comparative experimental and theoretical study of dipole-bound electron attachment on four polar hydrogen-bonded dimers containing one water molecule (water-water, ammonia-water, phenol-water and pyridine-water). When the water molecule is the proton acceptor in the neutral complex, with a trans-linear hydrogen bond (water-water and phenol-water), it is shown that the equilibrium geometry of the dipole-bound anion tends to a cis-linear hydrogen bond for which the total dipole moment is larger. On the contrary, when the water molecule is the proton donor to a nitrogen atom (ammonia-water and pyridine-water), dipole-bound electron attachment does not lead to subsequent modifications of the complex geometry.     

Valence and dipole-bound anions of the most stable tautomers of guanine

Anionic states of guanine, which is the only nucleic acid base of which the anions have not yet been studied in either photoelectron spectroscopic (PES) or Rydberg electron transfer (RET) experiments, have been characterized for the four most stable tautomers of neutral guanine using a broad spectrum of electronic structure methods from the density functional theory, with the B3LYP exchange-correlation functional, to the coupled-cluster method, with single, double, and perturbative triple excitations. Both valence and dipole-bound anionic states were addressed. We identified some of the difficulties facing future PES or RET experiments on the anion of guanine. Even if guanine is successfully transferred to the gas phase without thermal decomposition, it is critical to have the canonical amino-oxo (G) and both amino-hydroxy (GH and GHN7H) tautomers in the beam, not only the most stable, a noncanonical, amino-oxo tautomer (GN7H), as the latter does not support an adiabatically bound anionic state. We also suggested a scheme for enrichment of gas-phase guanine with the canonical tautomer, which is not the most stable in the gas phase, but which is of main interest due to its biological relevance. The tautomers G, GN7H, and GHN7H support vertically bound valence anionic states with the CCSD(T) value of vertical detachment energy of +0.58, +0.21, and +0.39 eV, respectively. These anionic states are, however, adiabatically unbound and thus metastable. The vertical electronic stability of these valence anionic states is accompanied by serious "buckling" of the molecular skeleton. The G and GHN7H tautomers support dipole-bound states with the CCSD(T) values of adiabatic electron affinity of 65 and 36 meV, respectively. A contribution from higher-than-second-order correlation terms represents, respectively, 48 and 68% of the total vertical electron detachment energy determined at the CCSD(T) level.     

Multiple Fano resonances in monolayer hexagonal non-close-packed metallic shells

In this study, we first numerically investigate the appearance and properties of multiple Fano resonances in two-dimensional hexagonal non-close-packed arrays of symmetric metallic shells. The coexistence of broad sphere-like plasmon modes formed from the near-field interaction between the individual sphere plasmons and substantially narrower void plasmon modes supported by the inner surface of the individual shell resonant over the same range of energies can produce such Fano resonances. In particular, void and sphere-like plasmon modes of different angular momentum could directly interact without the need of symmetry breaking in the structure. A cost-effective colloidal crystal templating method is utilized to prepare the arrays of the metallic shells with small openings. The effect of the symmetry breaking on the Fano resonances in metallic cup arrays is experimentally and numerically investigated. Further tunability on the Fano resonances is gained by changing the size of the inner dielectric core, hence changing the moment of the void plasmon modes and consequently the resonance frequency. By adopting the polymer dielectric core with gain materials, our study may offer realizable experimental opportunities towards subwavelength low threshold plasmonic lasing.     

Vibrational resonances in infrared spectra of uracils

Two regions in the i.r. spectra of uracils are analyzed: (a) the NH stretching region in crystals, and (b) the CO stretching region in matrix-isolated monomers. In these regions there is clear evidence of resonances leading to energy splittings and re-distributions of intensifies. These are: (a) a harmonic resonance between the NH stretching in hydrogen-bonded groups and the CH stretching, and (b) an anharmonic (Fermi) resonance between the CO stretching and a combination band involving the N₃H and CO bending vibrations. A theoretical interpretation of the resonances is presented.     

Theoretical prediction of new dipole-bound singlet states for anions of interstellar interest

Anions that exhibit dipole-bound singlet states have been proposed as a potential class of molecules that may be identified in the interstellar medium. Using high-level coupled cluster theory, we have computed the dipole moments, electron binding energies, and excited states of 14 neutral radicals and their corresponding closed-shell anions. We have calibrated our methods against experimental data for CH(2)CN(-) and CH(2)CHO(-) and demonstrated that coupled cluster theory can closely reproduce experimental dipole moments, electron binding energies, and excitation energies. Using these same methods, we predict the existence of dipole-bound excited states for six of the 14 previously unknown anions, including CH(2)SiN(-), SiH(2)CN(-), CH(2)SiHO(-), SiN(-), CCOH(-), and HCCO(-). In addition, we predict the existence of a valence-bound excited state of CH(2)SiN(-) with an excitation wavelength near 589 nm.     

Vibrational Feshbach resonances in uracil and thymine

Sharp peaks in the dissociative electron attachment (DEA) cross sections of uracil and thymine at energies below 3 eV are assigned to vibrational Feshbach resonances (VFRs) arising from coupling between the dipole bound state and the temporary anion state associated with occupation of the lowest sigma* orbital. Three distinct vibrational modes are identified, and their presence as VFRs is consistent with the amplitudes and bonding characteristics of the sigma* orbital wave function. A deconvolution method is also employed to yield higher effective energy resolution in the DEA spectra. The site dependence of DEA cross sections is evaluated using methyl substituted uracil and thymine to block H atom loss selectively. Implications for the broader issue of DNA damage are briefly discussed.     

Vibrational anharmonicity and Fermi resonances in CHF2Cl

The infrared spectrum of CHF₂Cl has been recorded between 15 000 and 350 cm⁻¹. The Fermi resonance between levels involving ν₄ and 2ν₆ is analysed in bands extending from 800 cm⁻¹ to 7000 cm⁻¹ leading to a best value of k₄₆₆ = ± 14.98 cm⁻¹. In conjunction with the recent results of Amrein, Dubal and Quack, Molec. Phys. 56,727 (1985); estimates are reported for 38 out of 45 possible x_ij constants. A variation in the relative intensity of the two Q branches associated with ν₁, on cooling the gas cell, indicates that a hot band contributes to the upper branch at 3024.55 cm⁻¹. However, other evidence suggests that the latter arises also from the combination ν₂ + ν₇ + ν₉, in a very weak, close resonance with ν₁ at 3021.27 cm⁻. A number of anomalous band contours are reported.     

Vibrational analysis of chlorinated methylphosphonic acid and its anions

The vibrational spectra (IR and Raman) of CH₂ClPO₃H₂ and of its anions in solutions of H₂O and D₂O are reported. The IR spectra of the solid dibasic sodium and potassium salts, the solid normal and O-deuterated monobasic sodium and potassium salts and of the solid normal and O-deuterated acid are discussed. Symmetry and internal stretching force constants, stretch-stretch interactions and potential energy distributions are obtained from normal coordinate analysis of CH₂C1PO₃H₂, CH₂ClPO₃H^? and CH₂C1PO₃^2?, and the calculated and observed frequencies for O-deuterated acid and monobasic anion are compared.     

Valence- and dipole-bound anions of the thymine-water complex: ab initio characterization of the potential energy surfaces

The potential energy surfaces of the neutral and anionic thymine-water complexes are investigated using high-level ab initio calculations. Both dipole-bound (DB) and valence-bound (VB) anionic forms are considered. Four minima and three first-order stationary points are located, and binding energies are computed. All minima, for both anions, are found to be vertically and adiabatically stable. The binding energies are much higher for valence-bound than for dipole-bound anions. Adiabatic electron affinities are in the 66-287 meV range for VB anions and the 4-60 meV range for DB anions, and vertical detachment energies are in the 698-977 meV and 10-70 meV range for VB and DB anions, respectively. For cases where literature data are available, the computed values are in good agreement with previous experimental and theoretical studies. It is observed that electron attachment modifies the shape of the potential energy surfaces of the systems, especially for the valence-bound anions. Moreover, for both anions the size of the energy barrier between the two lowest energy minima is strongly reduced, rendering the coexistence of different structures more probable.     

Vibrational spectroscopy and dynamics of small anions in ionic liquid solutions

Fourier-transform infrared (FTIR) and time-resolved IR spectroscopies have been used to study vibrational band positions, vibrational energy relaxation (VER) rates, and reorientation times of anions in several ionic liquid (IL) solutions. The ILs primarily investigated are based on the 1-butyl-2,3-dimethylimidazolium ([BM(2)IM]) cation with thiocyanate (NCS-), dicyanamide (N(CN)2-), and tetrafluoroborate (BF4-) anions. Spectroscopic studies are carried out near 2000 cm-1 for the C[Triple Bond]N stretching bands of NCS- and N(CN)2- as the IL anion as well as for NCS-, N(CN)2-, and azide (N3-) anions dissolved in [BM2IM][BF4]. The VER studies of N(CN)2- are reported for the first time. VER of N3-, NCS-, and N(CN)2- is measured in normal solvents, such as N-methylformamide, to compare with the IL solutions. The spectral shifts and VER rates of the anions in IL solution are quite similar to those in polar aprotic, conventional organic solvents, i.e., dimethylsulfoxide, and significantly different than those in methanol, in which there is hydrogen bonding. Similar studies were also carried out for the anions in another IL, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), in which the C2 hydrogen is present. The results for the anions are similar to those in the [BM2IM] containing ILs, in which the C2 hydrogen is methyl substituted. This suggests that substituting this hydrogen has, at most, a minor effect on the degree of hydrogen bonding in the anion-IL solvation interaction based on the infrared spectra and dynamics.     

Dynamic role of the correlation effect revealed in the exceptionally slow autodetachment rates of the vibrational Feshbach resonances in the dipole-bound state

Real-time autodetachment dynamics of the loosely bound excess electron from the vibrational Feshbach resonances of the dipole-bound states (DBS) of 4-bromophonoxide (4-BrPhO⁻) and 4-chlorophenoxide (4-ClPhO⁻) anions have been thoroughly investigated. The state-specific autodetachment rate measurements obtained by the picosecond time-resolved pump-probe method on the cryogenically cooled anions exhibit an exceptionally long lifetime (τ) of ∼823 ± 156 ps for the 11′¹ vibrational mode of the 4-BrPhO⁻ DBS. Strong mode-dependency in the wide dynamic range has also been found, giving τ ∼ 5.3 ps for the 10′¹ mode, for instance. Though it is nontrivial to get the state-specific rates for the 4-ClPhO⁻ DBS, the average autodetachment lifetime of the 19′¹20′¹/11′¹ mode has been estimated to be ∼548 ± 108 ps. Observation of these exceptionally slow autodetachment rates of vibrational Feshbach resonances strongly indicates that the correlation effect may play a significant role in the DBS photodetachment dynamics. Fermi''s golden rule has been invoked so that the correlation effect is taken into account in the form of the interaction between the charge and the induced dipole where the latter is given by the polarizable counterparts of the electron-rich halogenated compound and the diffuse non-valence electron. This report suggests that one may measure, from the real-time autodetachment dynamics, the extent of the correlation effect contribution to the stabilization and/or dynamics of the excess non-valence electron among many different types of long-range interactions of the DBS.

Exceptionally slow autodetachment dynamics of the vibrational Feshbach resonances found in the dipole-bound state of 4-bromophonoxide (4-BrPhO⁻) or 4-chlorophenoxide (4-ClPhO⁻) anions reveals the associated dynamic role of the correlation effect.     

Vibrational dynamics of some anions in aqueous solutions from isotropic Raman scattering

From the I_VV and I_VH Raman spectra of a totally symmetric vibration, isotropic scattering profiles have been obtained for several anions in dilute aqueous solutions at various temperatures. The Raman spectrometer was connected to an A/D converter and a small computer, which allowed multiple scanning, data accumulation, and numerical processing of these profiles with good precision over a frequency range of about 20 times their full width at half peak height.The theoretical analysis of the isotropic Raman profiles was accomplished by assuming vibrational phase relaxation based on a MARKOV-GAUSS mechanism. Fitting procedures to the experimental data permitted the determination of the vibrational frequency distribution and its modulation time. Temperature effects are discussed in relation to the distribution and motional characteristics of the immediate environment of water molecules of the anions.     

Molecular rotations and dipole-bound state lifetimes

A computational model based on classical molecular rotation provides insight into the observability of dipole-bound states. The observability is related to the lifetime of the state prior to rotational autodetachment of the electron. The model tracks an ensemble of dipole-bound states. Their motion in space is integrated as a function of time, which provides a means to analyze the lifetimes of the dipole-bound states. The results are generally in good agreement with experimental data. Some exceptions show the limitations of the model but also provide insight into the autodetachment mechanism.     

Calculation of the photodetachment cross sections of the HCN- and HNC- dipole-bound anions as described by a one-electron Drude model

The Drude model for treating the interaction of excess electrons with polar molecules is extended to calculate continuum functions and to evaluate photodetachment cross sections. The approach is applied to calculate the cross sections for photodetachment of dipole-bound electrons from HCN(-) and HNC(-). In addition, an adiabatic model separating the angular and radial degrees of freedom of the excess electron is introduced and shown to account in a qualitative manner for the cross sections.     

Critical conditions for stable dipole-bound dianions

We present finite size scaling calculations of the critical parameters for binding two electrons to a finite linear dipole field. This approach gives very accurate results for the critical parameters by using a systematic expansion in a finite basis set. A complete ground state stability diagram for the dipole-bound dianion is obtained using accurate variational and finite size scaling calculations. We also study the near threshold behavior of the ground state energy by calculating its critical exponent.     

Metastable anions of dinitrobenzene: resonances for electron attachment and kinetic energy release

Attachment of free, low-energy electrons to dinitrobenzene (DNB) in the gas phase leads to DNB(-) as well as several fragment anions. DNB(-), (DNB-H)(-), (DNB-NO)(-), (DNB-2NO)(-), and (DNB-NO(2))(-) are found to undergo metastable (unimolecular) dissociation. A rich pattern of resonances in the yield of these metastable reactions versus electron energy is observed; some resonances are highly isomer-specific. Most metastable reactions are accompanied by large average kinetic energy releases (KER) that range from 0.5 to 1.32 eV, typical of complex rearrangement reactions, but (1,3-DNB-H)(-) features a resonance with a KER of only 0.06 eV for loss of NO. (1,3-DNB-NO)(-) offers a rare example of a sequential metastable reaction, namely, loss of NO followed by loss of CO to yield C(5)H(4)O(-) with a large KER of 1.32 eV. The G4(MP2) method is applied to compute adiabatic electron affinities and reaction energies for several of the observed metastable channels.     

Vibrational analysis of methylphosphonic acid and its anions: II. Normal coordinate analysis

Normal coordinate analysis of methylphosphonic acid and its anions, is reported. Symmetry force constants were obtained using the method of Becher and Mattes. The fundamental vibrations are described by the potential energy distribution. An approximate method for the calculation of the errors on the symmetry force constants, due to the uncertainties on the vibrational frequencies, is presented. A discussion on the prediction of the CD₃ rocking mode is given.     

Vibrational analysis of chlorinated methylphosphonic acids: Part 2. Vibrational analysis of CHCl2PO3H2 and its anions

The vibrational spectra (IR and Raman) of CHCl₂PO₃H₂ and its anions in H₂O and D₂O solutions are reported. The IR spectra of the solid dibasic sodium and potassium salts, the solid normal and O-deuterated monobasic sodium and potassium salt and the solid normal and O-deuterated acid are discussed. The principal results of the normal coordinate analysis of the compounds are tabulated.