Role of Nonvalence States in the Ultrafast Dynamics of Isolated Anions.

Nonvalence states of neutral molecules (Rydberg states) play important roles in nonadiabatic dynamics of excited states. In anions, such nonadiabatic transitions between nonvalence and valence states have been much less explored even though they are believed to play important roles in electron capture and excited state dynamics of anions. The aim of this Feature Article is to provide an overview of recent experimental observations, based on time-resolved photoelectron imaging, of valence to nonvalence and nonvalence to valence transitions in anions and to demonstrate that such dynamics may be commonplace in the excited state dynamics of molecular anions and cluster anions.

Spectroscopic Determination of an Anion-π Bond Strength.

The anion-π bond has emerged as an important nonvalence interaction in supramolecular and biological structure. Although recognized as a strong noncovalent interaction, driven by electrostatic charge-quadrupole moment and correlation interactions, benchmark experimental and computational studies on the intrinsic anion-π bond strength are scarce. Here, we present a gas-phase photoelectron spectroscopic study on the archetypical iodide-hexafluorobenzene anion-π bonded complex. In combination with high-level electronic structure calculations, the anion-π bond strength is found to be 0.53 eV (51 kJ mol-1). The interaction arises for a large part from correlation forces (∼40%), with electrostatic quadrupole-anion and polarization making up most of the remainder.

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.

Evidence of Electron Capture of an Outgoing Photoelectron Wave by a Nonvalence State in (C6F6) n.

Frequency-resolved photoelectron spectra are presented for (C6F6) n- with n = 1-5 that show that C6F6- is solvated by neutral C6F6 molecules. Direct photodetachment channels of C6F6- are observed for all n, leaving the neutral in the S0 ground state or triplet states, T1 and T2. For n ≥ 2, an additional indirect electron loss channel is observed when the triplet-state channels open. This indirect emission appears to arise from the electron capture of the outgoing photoelectron s-wave by a neutral solvent molecule through an anion nonvalence state. The same process is not observed for the S0 detachment channel because the outgoing electron wave is predominantly a p-wave. Our results show that anion nonvalence states can act as electron-accepting states in cluster environments and can be viewed as precursor states for diffuse states of liquid C6F6.

Ultrafast dynamics of low-energy electron attachment via a non-valence correlation-bound state.

The primary electron-attachment process in electron-driven chemistry represents one of the most fundamental chemical transformations with wide-ranging importance in science and technology. However, the mechanistic detail of the seemingly simple reaction of an electron and a neutral molecule to form an anion remains poorly understood, particularly at very low electron energies. Here, time-resolved photoelectron imaging was used to probe the electron-attachment process to a non-polar molecule using time-resolved methods. An initially populated diffuse non-valence state of the anion that is bound by correlation forces evolves coherently in ∼30 fs into a valence state of the anion. The extreme efficiency with which the correlation-bound state serves as a doorway state for low-energy electron attachment explains a number of electron-driven processes, such as anion formation in the interstellar medium and electron attachment to fullerenes.

Molecular mapping of five soybean genes involved in male-sterility, female-sterility.

In soybean, asynaptic and desynaptic mutants lead to abnormal meiosis and fertility reduction. Several male-sterile, female-sterile mutants have been identified and studied in soybean, however, some of these mutants have not been mapped to locations on soybean chromosomes. The objectives of this study were to molecularly map five male-sterile, female-sterile genes (st2, st4, st5, st6, and st7) in soybean and compare the map locations of these genes with already mapped sterility genes. Microsatellite markers were used in bulked segregant analyses to locate all five male-sterile, female-sterile genes to soybean chromosomes, and markers from the corresponding chromosomes were used on F2 populations to generate genetic linkage maps. The st2, st4, st5, st6, and st7 genes were located on molecular linkage group (MLG) B1 (chromosome 11), MLG D1a (chromosome 01), MLG F (chromosome 13), MLG B2 (chromosome 14), and D1b (chromosome 02), respectively. The st2, st4, st5, st6, and st7 genes were flanked to 10.3 (∼ 399 kb), 6.3 (∼ 164 kb), 3.9 (∼ 11.8 Mb), 11.0 (∼ 409 kb), and 5.3 cM (∼ 224 kb), and the flanked regions contained 57, 17, 362, 52, and 17 predicted genes, respectively. Future characterization of candidate genes should facilitate identification of the male- and female-fertility genes, which may provide vital insights on structure and function of genes involved in the reproductive pathway in soybean.

Segregation distortion in a region containing a male-sterility, female-sterility locus in soybean.

In diploid segregation, each alternative allele has a 50% chance of being passed on to the offspring. Mutations in genes involved in the process of meiotic division or early stages of reproductive cell development can affect allele frequency in the gametes. In addition, competition among gametes and differential survival rates of gametes can lead to segregation distortion. In a recent transformation study, a male-sterile, female-sterile (MSFS) mutant was identified in the soybean cultivar, Williams. The mutant in heterozygous condition segregated 3 fertile:1 sterile in the progeny confirming monogenic inheritance. To map the lesion, we generated an F(2) mapping population by crossing the mutant (in heterozygous condition) with Minsoy (PI 27890). The F(2) progeny showed strong segregation distortion against the MSFS phenotype. The objectives of our study were to molecularly map the gene responsible for sterility in the soybean genome, to determine if the MSFS gene is a result of T-DNA insertion during Agrobacterium-mediated transformation, and to map the region that showed distorted segregation. The fertility/sterility locus was mapped to molecular linkage group (MLG) D1a (chromosome Gm01) using bulked segregant analysis. The closest marker, Satt531, mapped 9.4cM from the gene. Cloning of insertion sites for T-DNA in the mutant plants revealed that there are two copies of T-DNA in the genome. Physical locations of these insertion sites do not correlate with the map location of the MSFS gene, suggesting that MSFS mutation may not be associated with T-DNA insertions. Segregation distortion was most extreme at or around the st_A06-2/6 locus suggesting that sterility and segregation distortion are tightly linked attributes. Our results cue that the distorted segregation may be due to a gamete elimination system.