Ultrafast Production of NiCO and Ni Following 197 nm Photodissociation of Nickel Tetracarbonyl.

Herein, we report on the ultrafast photodissociation of nickel tetracarbonyl-a prototypical metal-ligand model system-at 197 nm. Using mid-infrared transient absorption spectroscopy to probe the bound C≡O stretching modes, we find evidence for the picosecond time scale production of highly vibronically excited nickel dicarbonyl and nickel monocarbonyl, in marked contrast with a prior investigation at 193 nm. Further spectral evolution with a 50 ps time constant suggests an additional dissociation step; the absence of any corresponding growth in signal strongly indicates the production of bare Ni, a heretofore unreported product from single-photon excitation of nickel tetracarbonyl.

Excited state electronic structure of dimethyl disulfide involved in photodissociation at ∼200 nm.

Dimethyl disulfide (DMDS), one of the smallest organic molecules with an S-S bond, serves as a model system for understanding photofragmentation in polypeptides and proteins. Prior studies of DMDS photodissociation excited at ∼266 nm and ∼248 nm have elucidated the mechanisms of S-S and C-S bond cleavage, which involve the lowest excited electronic states S and S. Far less is known about the dissociation mechanisms and electronic structure of relevant excited states of DMDS excited at ∼200 nm.

Site-Specific Photochemistry along a Protonated Peptide Scaffold.

We present a detailed study of the time-dependent photophysics and photochemistry of a known conformation of the two protonated pentapeptides Leu-enkephalin (Tyrosine-Glycine-Glycine-Phenylalanine-Leucine, YGGFL) and its chromophore-swapped analogue FGGYL, carried out under cryo-cooled conditions in the gas phase. Using ultraviolet-infrared (UV-IR) double resonance, we record excited state IR spectra as a function of time delay between UV and IR pulses. We identify unique Tyr OH stretch transitions due to the S state and the vibrationally excited triplet state(s) formed by intersystem crossing, T(v).

Matrix-formation dynamics dictate methyl nitrite conformer abundance.

Methyl nitrite has two stable conformational isomers resulting from rotation about the primary C-O-N-O dihedral angle: cis-CH3ONO and trans-CH3ONO, with cis being more stable by ∼5 kJ/mol. The barrier to rotational interconversion (∼45 kJ/mol) is too large for isomerization to occur under ambient conditions. This paper presents evidence of a change in conformer abundance when dilute CH3ONO is deposited onto a cold substrate; the relative population of the freshly deposited cis conformer is seen to increase compared to its gas-phase abundance, measured by in situ infrared spectroscopy.

Ground-State Orbital Analysis Predicts S Charge Transfer in Donor-Acceptor Materials.

Donor-acceptor (D-A) materials can exhibit a wide range of unique photophysical properties with applications in next-generation optoelectronics. Electronic structure calculations of D-A dimers are often employed to predict the properties of D-A materials. One of the most important D-A dimer quantities is the degree of charge transfer (DCT) in the S state, which correlates with properties such as fluorescence lifetimes and intersystem crossing rates in D-A materials.

Comparing the structures and photophysical properties of two charge transfer co-crystals.

Organic co-crystals have emerged as a promising class of semiconductors for next-generation optoelectronic devices due to their unique photophysical properties. This paper presents a joint experimental-theoretical study comparing the crystal structure, spectroscopy, and electronic structure of two charge transfer co-crystals. Reported herein is a novel co-crystal Npe:TCNQ, formed from 4-(1-naphthylvinyl)pyridine (Npe) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecular self-assembly.

Excited-State Dynamics during Primary C-I Homolysis in Acetyl Iodide Revealed by Ultrafast Core-Level Spectroscopy.

In typical carbonyl-containing molecules, bond dissociation events follow initial excitation to states. However, in acetyl iodide, the iodine atom gives rise to electronic states with mixed and character, leading to complex excited-state dynamics, ultimately resulting in dissociation. Using ultrafast extreme ultraviolet (XUV) transient absorption spectroscopy and quantum chemical calculations, we present an investigation of the primary photodissociation dynamics of acetyl iodide via time-resolved spectroscopy of core-to-valence transitions of the I atom after 266 nm excitation.

Effects of Microhydration on the Mechanisms of Hydrolysis and Cl Substitution in Reactions of N O and Seawater.

The reaction of N O at atmospheric interfaces has recently received considerable attention due to its importance in atmospheric chemistry. N O reacts preferentially with Cl to form ClNO /NO (Cl substitution), but can also react with H O to form 2HNO (hydrolysis). In this paper, we explore these competing reactions in a theoretical study of the clusters N O /Cl /nH O (n=2-5), resulting in the identification of three reaction motifs.

Ultrafast infrared transient absorption spectroscopy of gas-phase Ni(CO) photodissociation at 261 nm.

We employ ultrafast mid-infrared transient absorption spectroscopy to probe the rapid loss of carbonyl ligands from gas-phase nickel tetracarbonyl following ultraviolet photoexcitation at 261 nm. Here, nickel tetracarbonyl undergoes prompt dissociation to produce nickel tricarbonyl in a singlet excited state; this electronically excited tricarbonyl loses another CO group over tens of picoseconds. Our results also suggest the presence of a parallel, concerted dissociation mechanism to produce nickel dicarbonyl in a triplet excited state, which likely dissociates to nickel monocarbonyl.

Hydration and Hydrogen Bond Order of Octadecanoic Acid and Octadecanol Films on Water at 21 and 1 °C.

The temperature-dependent hydration structure of long-chain fatty acids and alcohols at air-water interfaces has great significance in the fundamental interactions underlying ice nucleation in the atmosphere. We present an integrated theoretical and experimental study of the temperature-dependent vibrational structure and electric field character of the immediate hydration shells of fatty alcohol and acid headgroups. We use a combination of surface-sensitive infrared reflection-absorption spectroscopy (IRRAS), surface potentiometry, and molecular dynamics simulations to elucidate detailed molecular structures of the octadecanoic acid and octadecanol (stearic acid and stearyl alcohol) headgroup hydration shells at room temperature and near freezing.

Ultraviolet photodissociation of gas-phase iron pentacarbonyl probed with ultrafast infrared spectroscopy.

It is well known that ultraviolet photoexcitation of iron pentacarbonyl results in rapid loss of carbonyl ligands leading to the formation of coordinatively unsaturated iron carbonyl compounds. We employ ultrafast mid-infrared transient absorption spectroscopy to probe the photodissociation dynamics of gas-phase iron pentacarbonyl following ultraviolet excitation at 265 and 199 nm. After photoexcitation at 265 nm, our results show evidence for sequential dissociation of iron pentacarbonyl to form iron tricarbonyl via a short-lived iron tetracarbonyl intermediate.

Gas-Phase Optical Detection of 3-Ethynylcyclopentenyl: A Resonance-Stabilized CH Radical with an Embedded 1-Vinylpropargyl Chromophore.

The 3-ethynylcyclopentenyl radical (3ecpr) has been identified as the carrier of an electronic spectrum with origin at 21792 cm using resonant ionization and laser-induced fluorescence spectroscopies. The radical was first detected in a toluene discharge and is most efficiently produced from 1,6-heptadiyne. Overwhelming spectroscopic and chemical evidence support our diagnosis: (1) the observed (6.

Correction: Isomer-specific cryogenic ion vibrational spectroscopy of the D tagged Cs(HNO)(HO) complexes: ion-driven enhancement of the acidic H-bond to water.

Correction for 'Isomer-specific cryogenic ion vibrational spectroscopy of the D2 tagged Cs+(HNO3)(H2O)n=0-2 complexes: ion-driven enhancement of the acidic H-bond to water' by Sayoni Mitra et al., Phys. Chem.

Isomer-specific cryogenic ion vibrational spectroscopy of the D tagged Cs(HNO)(HO) complexes: ion-driven enhancement of the acidic H-bond to water.

We report how the binary HNO3(H2O) interaction is modified upon complexation with a nearby Cs+ ion. Isomer-selective IR photodissociation spectra of the D2-tagged, ternary Cs+(HNO3)H2O cation confirms that two structural isomers are generated in the cryogenic ion source. In one of these, both HNO3 and H2O are directly coordinated to the ion, while in the other, the water molecule is attached to the OH group of the acid, which in turn binds to Cs+ with its -NO2 group.

Mechanisms and competition of halide substitution and hydrolysis in reactions of NO with seawater.

S2-type halide substitution and hydrolysis are two of the most ubiquitous reactions in chemistry. The interplay between these processes is fundamental in atmospheric chemistry through reactions of NO and seawater. NO plays a major role in regulating levels of O, OH, NO , and CH.

Ion reactions in atmospherically-relevant clusters: mechanisms, dynamics and spectroscopic signatures.

Reactions of nitrogen oxides with seawater are of major atmospheric importance, but microscopic understanding of these processes is still largely unavailable. In this paper we explore models of reactions of N2O4 with ions in water in order to provide molecular-level understanding of the processes. Presented here are studies of N2O4 interacting with two ions, SO42- and Cl-, in small water clusters.

UV photofragmentation dynamics of acetaldehyde cations prepared by single-photon VUV ionization.

Acetaldehyde cations (CHCHO) were prepared using single-photon vacuum ultraviolet ionization of CHCHO in a molecular beam and the fragmentation dynamics explored over the photolysis wavelength range 390-210 nm using velocity-map ion imaging and photofragment yield (PHOFY) spectroscopy. Four fragmentation channels are characterized: CHCHO→ CHO + H (I), CHCHO→ HCO + CH (II), CHCHO→ CH + HCO (III), CHCHO→ CH + CO (IV). Channels (I), (II), and (IV) are observed across the full photolysis wavelength range while channel (III) is observed only at λ < 317 nm.

Double Photodetachment of F·HO: Experimental and Theoretical Studies of [F·HO].

Double photodetachment of the cluster F·HO in a strong laser field is explored in a combined experimental-theoretical study. Products are observed experimentally by coincidence photofragment imaging following double ionization by intense laser pulses. Theoretically, equation of motion coupled cluster calculations (EOM-CC), suitable for modeling strong correlation effects in the electronic wave function, shed light on the Franck-Condon region, and ab initio molecular dynamics simulations also performed using EOM-CC methods reveal the fragmentation dynamics in time on the lowest-lying singlet and triplet states of [F·HO].

Communication: Thermal unimolecular decomposition of syn-CHCHOO: A kinetic study.

The thermal decomposition of syn-ethanal-oxide (syn-CHCHOO) through vinyl hydrogen peroxide (VHP) leading to hydroxyl radical is characterized using a modification of the HEAT thermochemical protocol. The isomerization step of syn-CHCHOO to VHP via a 1,4 H-shift, which involves a moderate barrier of 72 kJ/mol, is found to be rate determining. A two-dimensional master equation approach, in combination with semi-classical transition state theory, is employed to calculate the time evolution of various species as well as to obtain phenomenological rate coefficients.

Chirped-Pulse Fourier Transform Microwave Spectroscopy Coupled with a Flash Pyrolysis Microreactor: Structural Determination of the Reactive Intermediate Cyclopentadienone.

Chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW) is combined with a flash pyrolysis (hyperthermal) microreactor as a novel method to investigate the molecular structure of cyclopentadienone (C5H4═O), a key reactive intermediate in biomass decomposition and aromatic oxidation. Samples of C5H4═O were generated cleanly from the pyrolysis of o-phenylene sulfite and cooled in a supersonic expansion. The (13)C isotopic species were observed in natural abundance in both C5H4═O and in C5D4═O samples, allowing precise measurement of the heavy atom positions in C5H4═O.