Deep-Ultraviolet Photoexcitation of Aqueous Urea Forms Carbamic Acid/Carbamate in Less Than One Picosecond.

Urea is believed to have been essential to the synthesis of prebiotic nucleotides and thereby the RNA or DNA of the first lifeforms. Models suggesting that life began in wet-dry cycles around shallow aquatic ponds imply that reactants such as urea were exposed to deep ultraviolet irradiation from the young sun. Detrimental photodissociation of urea induced by deep UV excitation potentially challenges these models. We here follow the primary deep ultraviolet photochemistry of aqueous urea. The data show that urea is barely excited at 200 nm due to weak ultraviolet absorption. The likelihood of photodissociation is further reduced by strong intra-molecular coupling of the CN and CO stretch vibrations accompanied by an efficient dissipation of the excitation energy to the surrounding water molecules mitigated by urea-water hydrogen bonds. We find that 54±5 % of the excited urea molecules dissociate. Reactions between the photoproducts and surrounding solvent molecules form carbamic acid or the carbamate anions within 0.6 ps. The molecules that do not dissociate return to the electronic ground state in 2 ps. Interestingly, the photodissociation processes of urea in the aqueous phase is different from earlier reported reactions observed following the VUV photolysis of urea in noble gas matrices and highlight the potential influence of water on the prebiotic photochemistry.

Ambiguities in Decomposing Molecular Polarizability into Atomic Charge Flow and Induced Dipole Contributions.

The molecular dipole polarizability can be decomposed into components corresponding to the charge flow between atoms and changes in atomic dipole moments. Such decompositions are recognized to depend on how atoms are defined within a molecule, as, for example, by Hirshfeld, iterative Stockholder, or quantum topology partitioning of the electron density. For some of these, however, there are significant differences between the numerical results obtained by analytical response methods and finite field calculations. We show that this difference is due to analytical response methods accounting for (only) the change in electron density by a perturbation, while finite field methods may also include a component corresponding to a perturbation-dependent change in the definition of an atom within a molecule. For some atom-in-molecule definitions, such as the iterative Hirshfeld, iterative Stockholder, and quantum topology methods, the latter effect significantly increases the charge flow component. The decomposition of molecular polarizability into atomic charge flow and induced dipole components thus depends on whether the atom-in-molecule definition is taken to be perturbation-dependent.

Cluster perturbation theory IX: Perturbation series for the coupled cluster singles and doubles ground state energy.

In this paper, we develop and analyze a number of perturbation series that target the coupled cluster singles and doubles (CCSD) ground state energy. We show how classical Møller-Plesset perturbation theory series can be restructured to target the CCSD energy based on a reference CCS calculation and how the corresponding cluster perturbation series differs from the classical Møller-Plesset perturbation series. Subsequently, we reformulate these series using the coupled cluster Lagrangian framework to obtain series, where fourth and fifth order energies are determined only using parameters through second order. To test the methods, we perform a series of test calculations on molecular photoswitches of both total energies and reaction energies. We find that the fifth order reaction energies are of CCSD quality and that they are of comparable accuracy to state-of-the-art approximations to the CCSD energy based on local pair natural orbitals. The advantage of the present approach over local correlation methods is the absence of user defined threshold parameters for neglecting or approximating contributions to the correlation energy. Fixed threshold parameters lead to discontinuous energy surfaces, although this effect is often small enough to be ignored, but the present approach has a differentiable energy that will facilitate derivation and implementation of gradients and higher derivatives. A further advantage is that the calculation of the perturbation correction is non-iterative and can, therefore, be calculated in parallel, leading to a short time-to-solution.

A Cost-Benefit Analysis of Alternative Management Strategies for Red Deer in Denmark.

In this paper, we conduct a cost-benefit analysis (CBA) of five alternative management strategies for red deer in Denmark: free harvest, trophy hunting, maximum harvest and two cases for natural demographic population compositions. To capture the outcome under each strategy we use a biological sex- and age-structured population model. The net benefit function includes meat values, recreational values, browsing damage costs and traffic damage costs and these values and costs are assumed to differ for the various sex and age classes of red deer. We show that the maximum harvest strategy leads to a reasonably high positive total net benefit, while the free harvest strategy yields a small positive net benefit. On the other hand, the trophy hunting strategy generates a high negative net benefit, while small negative net benefits are obtained under the two strategies for natural demographic population compositions.

Peptide Bond of Aqueous Dipeptides Is Resilient to Deep Ultraviolet Irradiation.

The susceptibility of aqueous dipeptides to photodissociation by deep ultraviolet irradiation is studied by femtosecond spectroscopy supported by density functional theory calculations. The primary photodynamics of the aqueous dipeptides of glycyl-glycine (gly-gly), alalyl-alanine (ala-ala), and glycyl-alanine (gly-ala) show that upon photoexcitation at a wavelength of 200 nm, about 10% of the excited dipeptides dissociate by decarboxylation within 100 ps, while the rest of the dipeptides return to their native ground state. Accordingly, the vast majority of the excited dipeptides withstand the deep ultraviolet excitation. In those relatively few cases, where excitation leads to dissociation, the measurements show that deep ultraviolet irradiation breaks the Cα-C bond rather than the peptide bond. The peptide bond is thereby left intact, and the decarboxylated dipeptide moiety is open to subsequent reactions. The experiments indicate that the low photodissociation yield and in particular the resilience of the peptide bond to dissociation are due to rapid internal conversion from the excited state to the ground state, followed by efficient vibrational relaxation facilitated by intramolecular coupling among the carbonate and amide modes. Thus, the entire process of internal conversion and vibrational relaxation to thermal equilibrium on the dipeptide ground state occurs on a time scale of less than 2 ps.

Nonadiabatic Laser-Induced Alignment Dynamics of Molecules on a Surface.

We demonstrate that a sodium dimer, Na_{2}(1^{3}Σ_{u}^{+}), residing on the surface of a helium nanodroplet, can be set into rotation by a nonresonant 1.0 ps infrared laser pulse. The time-dependent degree of alignment measured, exhibits a periodic, gradually decreasing structure that deviates qualitatively from that expected for gas-phase dimers. Comparison to alignment dynamics calculated from the time-dependent rotational Schrödinger equation shows that the deviation is due to the alignment dependent interaction between the dimer and the droplet surface. This interaction confines the dimer to the tangential plane of the droplet surface at the point where it resides and is the reason that the observed alignment dynamics is also well described by a 2D quantum rotor model.

The primary photolysis of aqueous carbonate di-anions.

We study the primary photolysis dynamics of aqueous carbonate, CO32-(aq), and hydrogen carbonate, HCO3-(aq), when they are excited at λ = 200 nm. The photolysis is recorded with sub-picosecond time resolution using UV pump-Vis probe and UV pump-IR probe transient absorption spectroscopy and interpreted with the aid of density functional theory calculations. When CO32- is excited via single photon absorption at λ = 200 nm, Φ(t = 20 ps) = 82 ± 5% of the excited di-anions either detach an electron or dissociate. The electron detachment takes place from the excited state in t < 1 ps and forms ground state CO3˙- and eaq-. Dissociation occurs from both the electronic ground and excited states of CO32-. Dissociation from the CO32- excited state is assisted by water molecules and forms CO2˙-, OH˙ and OH-. The dissociation occurs both directly from the Franck-Condon region in t < 1 ps and indirectly with a time constant of τ = 13.9 ± 0.5 ps as the excited state relaxes. Dissociation of vibrationally excited CO32- molecules in the electronic ground state is also assisted by water molecules and forms CO2 and two OH- anions. The dissociation and subsequent vibrational relaxation of CO2 occur with a time constant of τ = 10.2 ± 0.5 ps. The residual 1 - Φ(t = 20 ps) = 18 ± 5% of the excited CO32- di-anions return by internal conversion to the equilibrated CO32- ground state with a time constant of τ = 4.0 ± 0.4 ps. The extinction coefficient of aqueous hydrogen carbonate HCO3-(aq) at λ = 200 nm is an order of magnitude smaller than that of carbonate, so even though the hydrogen carbonate anions dominate the carbonate di-anions in the hydrogen carbonate solution, the primary photolysis of hydrogen carbonate is obscured by the photo-products of carbonate. Hence, we are unable to assess the primary photolysis of hydrogen carbonate. However, the weak one-photon absorption facilitates two-photon ionization of water, which forms hydronium, H3O+, cations. The sudden increase in the acidity induced by two-photon ionization protonates the ground state hydrogen carbonate molecules, thus offering a rare spectroscopic glimpse of aqueous carbonic acid.

Basis Set Extrapolation of Vibrational Frequencies.

We investigate three different approaches for extrapolating harmonic vibrational frequencies to the complete basis set limit, by direct extrapolation of the frequencies, by calculation of frequencies based on extrapolated Hessians, or based on the Hessian from optimization of the extrapolated energy surface. For regular molecules, the three extrapolation procedures yield essentially identical results, but for loosely bound complexes, the frequencies derived from extrapolated Hessians yield unpredictable behavior. None of the basis set extrapolations, however, provide any significant improvement over the results upon which the extrapolation is based.

The oxygen-organic molecule photosystem: revisiting the past, recalibrating the present, and redefining the future.

Perturbation by a neighboring molecule M appreciably alters the properties of both the ground and excited states of molecular oxygen, as reflected in a variety of photophysical phenomena. In this article, we build upon the ~ 100 year history of work in this field, illustrating how the M-O2 system continues to challenge the scientific community, facilitating better insight into fundamental tenets of chemistry and physics.

Using atomic charges to model molecular polarization.

We review different models for introducing electric polarization in force fields, with special focus on methods where polarization is modelled at the atomic charge level. While electric polarization has been included in several force fields, the common approach has been to focus on atomic dipole polarizability. Several approaches allow modelling electric polarization by using charge-flow between charge sites instead, but this has been less exploited, despite that atomic charges and charge-flow is expected to be more important than atomic dipoles and dipole polarizability. A number of challenges are required to be solved for charge-flow models to be incorporated into polarizable force fields, for example how to parameterize the models and how to make them computational efficient.

The primary photo-dissociation dynamics of lactic acid: decarboxylation as CO2 and CO2˙.

We study the primary photolysis dynamics of lactic acid induced by excitation at λ = 200 nm with the aim of elucidating how simple aqueous carboxyl acids react to the deep ultraviolet exposure on the prebiotic Earth. UV-IR transient absorption spectroscopy shows a photolysis quantum yield of Φ(100 ps) = 100 ± 5%. The primary products are CO2, CO2˙- and their counter products CH3CHOH˙ and CH3CHOH-. DFT calculations suggest that the dissociation takes place from the strongly acidic nπ* excited state. Dehydroxylation of lactic acid is not observed.

The primary photo-dissociation dynamics of aqueous formamide and dimethylformamide.

We study the primary dissociation dynamics of aqueous formamide (HCONH2) and dimethylformamide (HCON(CH3)2) induced by photo-excitation at λ = 200 nm. The photolysis is recorded with sub-picosecond time resolution by UV pump-IR probe transient absorption spectroscopy. Formamide dissociates with a quantum yield of Φ(t = 20 ps) = 0.30 ± 0.05, t = 20 ps after the excitation. The rest of the excited formamide molecules return to the ground state within t = 1 ps and vibrationally relax towards equilibrium in t ≈ 10 ps. The only product observed is NH3. NH3 is produced with a yield of Φ(NH3) = 0.23 ± 0.10 on a timescale of τ = 3 ± 1 ps and likely constitutes the dominating product. The CO counter product to NH3 is not observed. Dimethylformamide is photolysed with a quantum yield of Φ(t = 30 ps) = 0.29 ± 0.05, t = 30 ps after the excitation. The photolysis of dimethylformamide produces CO on a time scale of τ ≈ 30 ps. The data indicate that dimethylamine and the N(CH3)2 radical are likely photoproducts.

Geometry Dependence of Spin-Orbit Coupling in Complexes of Molecular Oxygen with Atoms, H2, or Organic Molecules.

Studies of the interactions between molecular oxygen and a perturbing species, such as an organic solvent, have been an active research area for at least 70 years. In particular, interaction with a neighboring molecule or atom may perturb the electronic states of oxygen to such an extent that the O2(a1Δg) → O2(X3Σg-) transition, formally forbidden as an electric dipole process, achieves significant transition probability. We present a computational study of how the geometry of complexes consisting of molecular oxygen and different perturbing species influences the magnitude of spin-orbit coupling that facilitates the O2(a1Δg) → O2(X3Σg-) transition. We rationalize our results using a model based on orbital interactions: a non-zero spin-orbit coupling matrix element results from asymmetric transfer of charge to or from the 1πg orbitals on oxygen. Our results indicate that the atoms in a perturbing species closest to oxygen are responsible for the majority of the spin-orbit interactions, suggesting that large systems can be simplified appreciably. Furthermore, we infer and confirm that an estimate of the spin-orbit coupling matrix element can be obtained from the magnitude of the induced energy splitting of oxygen's 1πg orbitals. These results should provide further momentum in the long-standing issue of understanding phenomena that influence the O2(a1Δg) → O2(X3Σg-) transition.

The primary photolysis dynamics of oxalate in aqueous solution: decarboxylation.

We study the primary reaction dynamics of aqueous oxalate following photo-excitation of the nO → πCO* transition at λ = 200 nm. After the excitation, some of the oxalate molecules return to the electronic ground state on two very different time scales: a fast component of τ = 1.1 ± 0.5 ps comprising 40% of the excited molecules and a much slower component of τ = 0.28 ± 0.05 ns accounting for 15% of the excited molecules. The remaining 45% of the excited molecules do not return to the ground state during the first 500 ps, because they either detach an electron, dissociate or stay excited for hundreds of picoseconds. Dissociation and electron detachment of oxalate predominantly produces CO2 molecules with only minor yields of CO2˙- radical anions. The CO2 formation is accompanied by the ejection of electrons.

The complex between molecular oxygen and an organic molecule: modeling optical transitions to the intermolecular charge-transfer state.

The collision complex between the ground electronic state of an organic molecule, M, and ground state oxygen, O2(X3Σg-), can absorb light to produce an intermolecular charge transfer (CT) state, often represented simply as the M radical cation, M+˙, paired with the superoxide radical anion, O2-˙. Aspects of this transition have been the subject of numerous studies for ∼70 years, many of which address fundamental concepts in chemistry and physics. We now examine the extent to which the combination of Molecular Dynamics simulations and electronic structure response methods can model transitions to the toluene-O2 CT state. To account for the experimental spectra, we consider (a) the distribution of toluene-O2 geometries that contribute to the transitions, (b) a quantitative description of intermolecular CT, and (c) oxygen-induced local transitions in toluene that complement the CT transitions, specifically transitions that populate toluene triplet states. We find that the latter oxygen-induced local transitions play a prominent role on the long wavelength side of the spectrum commonly attributed to the intermolecular CT transition. Our calculations provide a new perspective on the seminal discussion between R. S. Mulliken and D. F. Evans on the nature of O2-dependent transitions in organic molecules, and bode well for modeling transitions to excited states with CT character in noncovalent weakly-bonded molecular complexes.

The primary photo-dissociation dynamics of lactate in aqueous solution: decarboxylation prevents dehydroxylation.

We study the primary photolysis dynamics of aqueous lactate induced by photo-excitation at λ = 200 nm. Our calculations indicate that both decarboxylation and dehydroxylation are energetically possible, but decarboxylation is favoured dynamically. UV pump - IR probe transient absorption spectroscopy shows that the photolysis is dominated by decarboxylation, whereas dehydroxylation is not observed. Analysis of the transient IR spectrum suggests that photo-dissociation of lactate primarily produces CO2 and CH3CHOH- through the lowest singlet excited state of lactate, which has a lifetime of τ = 11 ps. UV pump - VIS probe transient absorption spectroscopy of electrons from the dissociating lactate anion indicates that the anionic electron from the CO2˙- fragment is transferred to the CH3CHOH˙ counter radical during the decarboxylation process, and CO2˙- is consequently only observed as a minor photo-product. The photo-dissociation quantum yield after the full decay of the excited state is Φ(100ps) = 38 ± 5%.

Exploring pathways of NO and H2S signaling in metabolic depression: The case of anoxic turtles.

In contrast to most vertebrates, freshwater turtles of the genera Trachemys and Chrysemys survive total oxygen deprivation for long periods of time. This remarkable tolerance makes them ideal August Krogh's model animals to study adaptions to survive oxygen deprivation. The gasotransmitters nitric oxide (NO) and hydrogen sulfide (H2S) and their metabolic derivatives are central in regulating the physiological responses to oxygen deprivation. Here, we explore the role of these signaling molecules in the anoxia tolerance of the freshwater turtle, including metabolic suppression and protection against oxidative damage with oxygen deprivation. We describe the interaction of NO and H2S with protein thiols and specifically how this regulates the function of central metabolic enzymes. These interactions contribute both to metabolic suppression and to prevent oxidative damage with oxygen deprivation. Furthermore, NO and H2S interact with ferrous and ferric heme iron, respectively, which affects the activity of central heme proteins. In turtles, these interactions contribute to regulate oxygen consumption in the mitochondria, as well as vascular tone and blood flow during oxygen deprivation. The versatile biological effects of NO and H2S underscore the importance of these volatile signaling molecules in the remarkable tolerance of freshwater turtles to oxygen deprivation.

Effects of temperature on acid-base regulation, gill ventilation and air breathing in the clown knifefish, Chitala ornata.

Chitala ornata is a facultative air-breathing fish, which at low temperatures shows an arterial PCO2  (PaCO2 ) level only slightly elevated above that of water breathers. By holding fish with in-dwelling catheters at temperatures from 25 to 36°C and measuring blood gasses, we show that this animal follows the ubiquitous poikilotherm pattern of reducing arterial pH with increasing temperature. Surprisingly, the temperature increase caused an elevation of PaCO2  from 5 to 12 mmHg while the plasma bicarbonate concentration remained constant at around 8 mmol l-1 The temperature increase also gave rise to a larger fractional increase in air breathing than in gill ventilation frequency. These findings suggest that air breathing, and hence the partitioning of gas exchange, is to some extent regulated by acid-base status in air-breathing fish and that these bimodal breathers will be increasingly likely to adopt respiratory pH control as temperature rises, providing an interesting avenue for future research.

The primary photo-dissociation dynamics of amino acids in aqueous solution: breaking the Cα-bond.

We report a study of the primary photo-dissociation dynamics of aqueous alanine, isoleucine and proline by 200 nm UV pump-IR probe transient absorption spectroscopy. Photo-dissociation of the three amino acids predominantly results in decarboxylation, and 38 ± 7% of the excited alanine, 35 ± 10% of the excited isoleucine and 47 ± 10% of the excited proline zwitterions remain dissociated 100 picoseconds after the excitation. The decarboxylation occurs from a transient intermediate with a lifetime of ∼20 picoseconds to which we assign the excited state of the amino acids based on comparison of the measured and calculated IR spectra, and calculated excited state energy surfaces.

Gas-phase action and fluorescence spectroscopy of mass-selected fluorescein monoanions and two derivatives.

Gaseous fluorescein monoanions are weakly fluorescent; they display a broad fluorescence spectrum and a large Stokes shift. This contrasts with the situation in aqueous solution. One explanation of the intriguing behavior in vacuo is based on internal proton transfer from the pendant carboxyphenyl group to one of the xanthene oxygens in the excited state; another that rotation of the carboxyphenyl group relative to the xanthene leads to a partial charge transfer from one chromophore (xanthene) to the other (carboxyphenyl) when the π orbitals start to overlap. To shed light on the mechanism at play, we synthesized two fluorescein derivatives where the carboxylic acid group is replaced with either an ester or a tertiary amide functionality and explored their gas-phase ion fluorescence using the home-built LUminescence iNstrument in Aarhus (LUNA) setup. Results on the fluorescein methyl ester that has no acidic proton clearly disprove the former explanation: The spectrum remains broad, and the band center (at 605 nm) is shifted even more to the red than that of fluorescein (590 nm). Experiments on the other variant that contains a piperidino amide are also in favor of the second explanation as here the piperidino already causes the dihedral angle between the planes defining the xanthene and the benzene ring to be less than 90° in the ground state (i.e., 63°), according to density functional theory calculations. As a result of the closer similarity between the ground-state and excited-state structures, the fluorescence spectrum is narrower than those of the other two ions, and the band maximum is further to the blue (575 nm). In accordance with a more delocalized ground state of the amide derivative, action spectra associated with photoinduced dissociation recorded at another setup show that the absorption-band maximum for the amide derivative is redshifted compared to that of fluorescein (538 nm vs. 525 nm).