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 S1 state and the vibrationally excited triplet state(s) formed by intersystem crossing, Tn(v). Photofragment mass spectra are recorded out of the S1 origin and following UV-IR double resonance. Several competing site-specific fragmentation pathways are discovered involving peptide backbone cleavage, Tyr side chain loss, and N-terminal NH3 loss mediated by electron transfer. In YGGFL, IR excitation in the S1 state promotes electron transfer (ET) from the aromatic ring to the N-terminal R-NH3+ group leading to loss of neutral NH3. This product channel is missing in FGGYL due to the larger distance for ET from Y(4) to NH3+. Selective loss of the Tyr side chain occurs out of an excited state process following UV excitation and is further enhanced by IR excitation in S1 and Tn(v) states of both YGGFL and FGGYL. Finally, IR excitation in the S1 or Tn(v) states fragments the peptide backbone exclusively at amide(4), producing the b4 cation. We postulate that this selective fragmentation results from intersystem crossing to produce vibrationally excited triplets with enough energy to launch the proton along a proton conduit present in the known starting structure.
Photochemical Processes , Protons , Spectrophotometry, Infrared , Peptides/chemistry , Enkephalin, Leucine/chemistry
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 â¼5 kJ/mol. The barrier to rotational interconversion (â¼45 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. We observe abundance changes depending on the identity of the bath gas (N2, Ar, and Xe) and deposition angle. The observations indicate that the surface properties of the growing matrix influence conformer abundance-contrary to the widely held assumption that conformer abundance in matrices reflects gas-phase abundance. We posit that differences in the angle-dependent host-gas deposition dynamics affect the growing surfaces, causing changes in conformer abundances. Quantum chemistry calculations of the binding energies between CH3ONO and a single bath-gas component reveal that significant energetic stabilization is not observed in 1:1 complexes of N2:CH3ONO, Ar:CH3ONO, or Xe:CH3ONO. From our results, we conclude that the growing surface plays a significant role in trapping cis-CH3ONO more effectively than trans-CH3ONO, likely because cis-CH3ONO is more compact. Taken together, the observations highlight the necessity for careful characterization of conformers in matrix-isolated systems, emphasizing a need for further study into the deposition dynamics and surface structure of chemically inert matrices.
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 S1 state, which correlates with properties such as fluorescence lifetimes and intersystem crossing rates in D-A materials. While predictive metrics of the S1 DCT generally require an excited-state quantum chemistry calculation, presented here is a novel metric that predicts S1 DCT solely with ground-state orbital analysis. This metric quantifies the similarity of the orbitals between a dimer complex and its monomer components. A linear relationship is found between this similarity metric and the S1 DCT, calculated using a data set of 31 D-A dimers. Best practices for integrating this novel orbital structure-function relationship into high-throughput screening methods are discussed.
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) via molecular self-assembly. This work also presents a revised study of the co-crystal composed of Npe and 1,2,4,5-tetracyanobenzene (TCNB) molecules, Npe:TCNB, herein reported with a higher-symmetry (monoclinic) crystal structure than previously published. Npe:TCNB and Npe:TCNQ dimer clusters are used as theoretical model systems for the co-crystals; the geometries of the dimers are compared to geometries of the extended solids, which are computed with periodic boundary conditions density functional theory. UV-Vis absorption spectra of the dimers are computed with time-dependent density functional theory and compared to experimental UV-Vis diffuse reflectance spectra. Both Npe:TCNB and Npe:TCNQ are found to exhibit neutral character in the S0 state and ionic character in the S1 state. The high degree of charge transfer in the S1 state of both Npe:TCNB and Npe:TCNQ is rationalized by analyzing the changes in orbital localization associated with the S1 transitions.
In typical carbonyl-containing molecules, bond dissociation events follow initial excitation to nπCâO* states. However, in acetyl iodide, the iodine atom gives rise to electronic states with mixed nπCâO* and nσC-I* 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. The probed I 4d-to-valence transitions show features that evolve on sub-100-fs time scales, reporting on excited-state wavepacket evolution during dissociation. These features subsequently evolve to yield spectral signatures corresponding to free iodine atoms in their spin-orbit ground and excited states with a branching ratio of 1.1:1 following dissociation of the C-I bond. Calculations of the valence excitation spectrum via equation-of-motion coupled cluster with single and double substitutions (EOM-CCSD) show that initial excited states are of spin-mixed character. From the initially pumped spin-mixed state, we use a combination of time-dependent density functional theory (TDDFT)-driven nonadiabatic ab initio molecular dynamics and EOM-CCSD calculations of the N4,5 edge to reveal a sharp inflection point in the transient XUV signal that corresponds to rapid C-I homolysis. By examining the molecular orbitals involved in the core-level excitations at and around this inflection point, we are able to piece together a detailed picture of C-I bond photolysis in which d â σ* transitions give way to d â p excitations as the bond dissociates. We also report theoretical predictions of short-lived, weak 4d â 5d transitions in acetyl iodide, validated by weak bleaching in the experimental transient XUV spectra. This joint experimental-theoretical effort has thus unraveled the detailed electronic structure and dynamics of a strongly spin-orbit coupled system.
The reaction of N2 O5 at atmospheric interfaces has recently received considerable attention due to its importance in atmospheric chemistry. N2 O5 reacts preferentially with Cl- to form ClNO2 /NO3 - (Cl- substitution), but can also react with H2 O to form 2HNO3 (hydrolysis). In this paper, we explore these competing reactions in a theoretical study of the clusters N2 O5 /Cl- /nH2 O (n=2-5), resulting in the identification of three reaction motifs. First, we uncovered an SN 2-type Cl- substitution reaction of N2 O5 that occurs very quickly due to low barriers to reaction. Second, we found a low-lying pathway to hydrolysis via a ClNO2 intermediate (two-step hydrolysis). Finally, we found a direct hydrolysis pathway where H2 O attacks N2 O5 (one-step hydrolysis). We find that Cl- substitution is the fastest reaction in every cluster. Between one-step and two-step hydrolysis, we find that one-step hydrolysis barriers are lower, making two-step hydrolysis (via ClNO2 intermediate) likely only when concentrations of Cl- are high.
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. Mechanisms for the formation of these photoproducts in multiple electronic excited states are theoretically predicted with one-dimensional cuts through the potential energy surfaces and computation of spin-orbit coupling constants using equation of motion coupled cluster methods (EOM-CC) and coupled cluster theory with single and double excitations (CCSD). Bond dissociation energies are calculated with CCSD, and anharmonic frequencies of ground and excited state species are computed using density functional theory (DFT) and time-dependent density functional theory (TD-DFT).
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 ab initio 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. In experiments, the alcohol at high surface concentration exhibits the largest surface potential; yet we observe a strengthening of the hydrogen-bonding for the solvating water molecules near freezing for both the alcohol and the fatty acid IRRAS experiments. Results reveal that the hydration shells for both compounds screen their polar headgroup dipole moments reducing the surface potential at low surface coverages; at higher surface coverage, the polar headgroups become dehydrated, which reduces the screening, correlating to higher observed surface potential values. Lowering the temperature promotes tighter chain packing and an increase in surface potential. IRRAS reveals that the intra- and intermolecular vibrational coupling mechanisms are highly sensitive to changes in temperature. We find that intramolecular coupling dominates the vibrational relaxation pathways for interfacial water determined by comparing the H2O and the HOD spectra. Using ab initio molecular dynamics (AIMD) calculations on cluster systems of propanol + 6H2O and propionic acid + 10H2O, a spectral decomposition scheme was used to correlate the OH stretching motion with the IRRAS spectral features, revealing the effects of intra- and intermolecular coupling on the spectra. Spectra calculated with AIMD reproduce the red shift and increase in intensity observed in experimental spectra corresponding to the OH stretching region of the first solvation shell. These findings suggest that intra- and intermolecular vibrational couplings strongly impact the OH stretching region at fatty acid and fatty alcohol water interfaces. Overall, results are consistent with ice templating behavior for both the fatty acid and the alcohol, yet the surface potential signature is strongest for the fatty alcohol. These findings develop a better understanding of the complex surface potential and spectral signatures involved in ice templating.
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. Photodissociation at 199 nm results in the prompt production of Fe(CO)3 within 0.25 ps via several energetically accessible pathways. An additional 15 ps time constant extracted from the data is tentatively assigned to intersystem crossing to the triplet manifold of iron tricarbonyl or iron dicarbonyl. Mechanisms for formation of iron tetracarbonyl, iron tricarbonyl, and iron dicarbonyl are proposed and theoretically validated with one-dimensional cuts through the potential energy surface as well as bond dissociation energies. Ground state calculations are computed at the CCSD(T) level of theory and excited states are computed with EOM-EE-CCSD(dT).
We report the water-mediated charge separation of nitric acid upon incorporation into size-selected Cs+·(HNO3)(H2O)n=0-11 clusters at 20 K. Dramatic spectral changes are observed in the n = 7-9 range that are traced to the formation of many isomeric structures associated with intermediate transfer of the acidic proton to the water network. This transfer is complete by n = 10, which exhibits much simpler vibrational band patterns consistent with those expected for a tricoordinated hydronium ion (the Eigen motif) along with the NO stretching bands predicted for a hydrated NO3- anion that is directly complexed to the Cs+ cation. Theoretical analysis of the n = 10 spectrum indicates that the dissociated ions adopt a solvent-separated ion-pair configuration such that the Cs+ and H3O+ cations flank the NO3- anion in a microhydrated salt bridge. This charge separation motif is evidently assisted by the electrostatic stabilization of the product NO3-/H3O+ ion pair by the proximal metal ion.
The 3-ethynylcyclopentenyl radical (3ecpr) has been identified as the carrier of an electronic spectrum with origin at 21792 cm-1 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 (6.93 eV) and calculated (CCSD(T)/pVQZ) adiabatic ionization energies are the same; (2) the origin band rotational contour can be well simulated with calculated rotational constants; (3) convincing vibrational assignments can be made using computed frequencies; and (4) the same spectrum was observed in a discharge of 1-ethynylcyclopentanol, which contains the 3ecpr carbon framework. The π-chromophore is essentially that of trans-1-vinylpropargyl, a highly resonance-stabilized C5H5 radical that persists in conditions relevant to both combustion and circumstellar atmospheres. We suggest that 3ecpr may be a similarly important radical warranting inclusion in models of C7H7 chemistry. It is the second C7H7 isomer with a five-membered ring yet to be detected, the other being vinylcyclopentadienyl, a species crucially involved in a recently proposed mechanism of soot formation (Science, 2018, 361, 6406, 997-1000). We argue that 3ecpr should be a significant product of H addition to ethynylcyclopentadiene (C7H6), a known product of benzyl decomposition. Further, it is plausible that 3ecpr is the unidentified C7H7 product of sequential addition of acetylene to propargyl (J. Phys. Chem. Lett., 2015, 6, 20, 4153-4158) in which 1-vinylpropargyl is an intermediate. As such, the nC2H2 + C3H3 cascade could represent a facile synthesis of a substituted five-membered ring in flames and stellar outflows.
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. Chem. Phys., 2020, DOI: 10.1039/c9cp06689f.
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. The acidic OH stretching fundamental in the latter isomer displays a â¼300 cm-1 red-shift relative to that in the neutral H-bonded van der Waals complex, HNO3(H2O). This behavior is analyzed with the aid of electronic structure calculations and discussed in the context of the increased effective acidity of HNO3 in the presence of the cation.
SN2-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 N2O5 and seawater. N2O5 plays a major role in regulating levels of O3, OH, NO x , and CH4. While the reactions of N2O5 and seawater are of central importance, little is known about their mechanisms. Of interest is the activation of Cl in seawater by the formation of gaseous ClNO2, which occurs despite the fact that hydrolysis (to HNO3) is energetically more favorable. We determine key features of the reaction landscape that account for this behavior in a theoretical study of the cluster N2O5/Cl-/H2O. This was carried out using ab initio molecular dynamics to determine reaction pathways, structures, and time scales. While hydrolysis of N2O5 occurs in the absence of Cl-, results here reveal that a low-lying pathway featuring halide substitution intermediates enhances hydrolysis.
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. Reactions of the asymmetric conformer of N2O4 with SO42- ions in water clusters are studied via ab initio molecular dynamics (AIMD) simulations in order to unravel the microscopic mechanism of the processes and predict the timescales of different steps. Spectroscopic signatures of the reaction are proposed. The mechanisms of chloride substitution and hydrolysis of symmetric and asymmetric N2O4 are explored via intrinsic reaction coordinate (IRC) calculations. Spectroscopic calculations for relevant species suggest possible experimental signatures for the processes. The results of these model ion-N2O4 reactions in water throw light on the molecular-level mechanisms of the reactions of nitrogen oxides with seawater.
Acetaldehyde cations (CH3CHO+) were prepared using single-photon vacuum ultraviolet ionization of CH3CHO 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: CH3CHO+â C2H3O+ + H (I), CH3CHO+â HCO+ + CH3 (II), CH3CHO+â CH3+ + HCO (III), CH3CHO+â CH4+ + 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. Maximum fragment ion yields are obtained at â¼250 nm. Ion images were recorded over the range 316-228 nm, which corresponds to initial excitation to the B[combining tilde]2A' and C[combining tilde]2A' states of CH3CHO+. The speed and angular distributions are distinctly different for each detected ion and show evidence of both statistical and dynamical fragmentation pathways. At longer wavelengths, fragmentation via channel (I) leads to modest translational energies (ET), consistent with dissociation over a small barrier and production of highly internally excited CH3CO+. Additional components with EINT greater than the CH3CO+ secondary dissociation threshold appear at shorter wavelengths and are assigned to fragmentation products of vinyl alcohol cation or oxirane cation formed by isomerization of energized CH3CHO+. The ET distribution observed for channel (III) products peaks at zero but is notably colder than that predicted by phase space theory, particularly at longer photolysis wavelengths. The colder-than-statistical ET distributions are attributed to contributions from secondary fragmentation of energized CH3CO+ formed via channel (I), which are attenuated by CH3CHO+ isomerization at shorter wavelengths. Fragmentation via channels (II) and (IV) results in qualitatively similar outcomes, with evidence of isotropic statistical components at low-ET and anisotropic components due to excited state dynamics at higher ET.
