Facts and hypotheses of molecular chemical tunnelling

Vetting molecular candidates posited for the first diffuse interstellar bands (5780 and 5797 Å): a quantum chemical study

Diffuse interstellar bands (DIBs) comprise over 550 celestial absorption features whose molecular carriers remain largely unidentified or contested. In this study, we present a statistical analysis that identifies two previously overlooked families of strongly correlated lines associated with the original Heger features at 5780 and 5797 Å. Comprehensive UV-vis spectra were computed at several levels of theory (mainly TD-PBE0 and EOM-CCSD with an aug-cc-pVTZ basis set) for the following candidates posited as diffuse interstellar band carriers (in both their neutral and cationic forms): 2-cyclopenten-1-one, 3(2H)-thiophenone, 2(5H)-furanone, 3(2H)-selenophenone, 3-hydroxypropanamide, oxamic acid, lactamide, and glycolamide. Glycolamide is of particular interest since it has recently been detected in microwave (rotational) spectra of the comparatively dense molecular cloud G+0.693-0027. Importantly, the computations reveal that the anions exhibit marginal electron affinities despite producing improbable lines (i.e., with excitations to levels above the ionization threshold) overlapping DIBs, whereas the neutral molecules yield lines shortward of DIBs and possibly linked to the broad 220 nm interstellar feature, and their cations produced too few lines in the DIB domain inspected. Further vetting of candidates awaits the construction of an expansive optical-infrared molecular ion database, which will facilitate concurrent matching to DIBs in the optical (electronic) and their energy differences in the mid-infrared (vibrational), thereby narrowing the parameter space.

Chemical reactivity from the vibrational ground-state level. The role of the tunneling path in the tautomerization of urea and derivatives

Recent developments of low-temperature techniques are providing valuable knowledge about chemical processes that manifest in the quantum regimen. The tunneling effect from the vibrational ground-state is the main mechanism of these reactions, which usually involves the motion or transfer of hydrogen atoms. Theoretical methods can enrich the information supplied by these experimental methods through an insightful analysis of the tunneling process. In this context, canonical variational transition state theory with multidimensional tunneling corrections (CVT/MT) can handle this type of reaction, and it has been applied to several systems within the small-curvature approximation for tunneling (SCT). This method is of proven reliability for polyatomic reactions occurring at room temperature and above, but no tests have been performed to check its performance when only the lowest energy level is populated. In this work, we compare SCT against the least-action tunneling (LAT) method to study the tautomerization and cis-trans interconversion reactions in the enol forms of urea, thiourea, and selenourea. To the best of our knowledge, this is the first time that the LAT method is applied to a polyatomic reaction occurring in the deep-tunneling region. The theoretical results indicate that the reaction mechanisms are controlled by tunneling. The SCT and LAT tautomerization reaction times are in good agreement with the experimental values; however, LAT seems superior to SCT for reactions (tautomerizations) that involve moderate reaction path curvature, whereas the opposite is true for reactions with small curvature (interconversions). These results led us to introduce and recommend the microcanonically optimized tunneling path that selects the tunneling probability as the maximum between the SCT and LAT tunneling probabilities.

Quantum physics meets biology

Quantum physics and biology have long been regarded as unrelated disciplines, describing nature at the inanimate microlevel on the one hand and living species on the other hand. Over the past decades the life sciences have succeeded in providing ever more and refined explanations of macroscopic phenomena that were based on an improved understanding of molecular structures and mechanisms. Simultaneously, quantum physics, originally rooted in a world-view of quantum coherences, entanglement, and other nonclassical effects, has been heading toward systems of increasing complexity. The present perspective article shall serve as a "pedestrian guide" to the growing interconnections between the two fields. We recapitulate the generic and sometimes unintuitive characteristics of quantum physics and point to a number of applications in the life sciences. We discuss our criteria for a future "quantum biology," its current status, recent experimental progress, and also the restrictions that nature imposes on bold extrapolations of quantum theory to macroscopic phenomena.

Effective Rate Constants for the Surface Reaction between Solid Methanol and Deuterium Atoms at 10 K

The surface reactions of CH3OH, CH2DOH, and CHD2OH with cold D atoms at 10 K were investigated using an atomic beam source and FTIR. Methyl-deuterated isotopologues CH2DOH, CHD2OH, and CD3OH were produced by exposure of amorphous solid CH3OH to D atoms at 10 K, and the pseudo-first-order rates for the reactions CH3OH + D --> CH2OH + HD, CH2DOH + D --> CHDOH + HD, and CHD2OH + D --> CD2OH + HD were estimated. The ratios of the reaction rates of the second and third reactions to the first reaction were 0.69 +/- 0.11 and 0.52 +/- 0.14, respectively. The difference in reaction rates is thought to be due to a secondary kinetic isotope effect on the H-abstraction reaction from the methyl side by D atoms.

An intangible energy in the functioning biosystem. II: Useful parallels with circuit theory and with non-linear optics

The argument is developed that a structure and function already exists in selected inanimate systems for an intangible energy dissipating these systems and that, in so doing, this energy exhibits certain properties, readily recognised in the functioning biosystem. The central thesis is that, during dissipation, the structure of the biosystem affords opportunity for an enhanced display of these properties, so that this structure can be rationally recognised as obligatory in the transition, inanimate to animate matter. The systems chosen are those of reactance in linear circuit theory of electronics, and some recent developments in non-linear optics, both of which rely on imaginary or quantal force to display observable effects. Discussion occurs on the fashion which the development of a statistical formalism as a basis for the study of squeezed states of light in these non-linear systems, has, at the same time, overcome a long standing veto on the practical use of quantal energy associated with the Uncertainty Principle of Heisenberg. These ideas are used to vindicate the suggestion that a theoretical basis is presently available for an engineering type approach, toward an intangible force as it exists in the biosystem. The origins and properties of such a force continue to be considered by many as immersed in mysticism.

Handedness, origin of life and evolution

Biological polymers have a preferred chirality ond can replicate themselves. Physical arguments provide insight into which of these unique and apparently related properties evolved first, and by what mechanism.

Recombination of carbon monoxide to ferrous horseradish peroxidase types A and C

The recombination of carbon monoxide to isoenzymes A2 and C of horseradish peroxidase (HRP) was studied as a function of temperature (2 to 320 K) and pH (5 to 8.3) with flash photolysis and infrared difference absorption. At low temperatures three geminate recombination processes are observed. One of these internal processes, denoted by I*, is exponential in time with a rate coefficient that deviates strongly from an Arrhenius behavior below 100 K, implying phonon-assisted tunneling. The two other processes, denoted by I, are non-exponential in time and related to different carbonyl isomers, as shown by the infrared difference spectra. The existence of three internal processes indicates that HRP differs considerably from myoglobin where only one internal process, I, is seen. Moreover, the internal processes in HRP are faster than process I in myoglobin. At 300 K, only one recombination process from the solvent is observed and it is very slow (lambda s approximately 1 s-1 at 1 atm CO (1 atm = 101,325 Pa)), much slower than the corresponding association process in myoglobin. Since process I is fast, but binding from the solvent is slow, the barrier at the heme cannot be responsible for the small association rate. The infrared absorption difference spectra of the amide I/II bands indicate that photolysis and recombination trigger a two-step structural change. The slow recombination rate at 300 K can thus be explained by the large Gibbs energy of the conformational transition that is necessary to let CO move into the heme pocket. The partition coefficient for the CO in the heme pocket and the solvent is extremely small, while bond formation with the heme iron occurs in less than 100 nanoseconds.

Evolution of uphill electron transfer

The evolution of photosynthetic energy storage is considered. The primary event in primordial inorganic or organic photoreceptors was charge separation at the expense of light quantum energy. The subsequent improvement of energy storage was attained by separately channeling electrons and "holes" to prevent back reactions. The anisotropic arrangement of photoreceptors in the primary membrane caused a coupling of photochemical charge separation to subsequent ion dislocation and was a prerequisite of primary photophosphorylation. The gradual improvement of the molecular organization of photoreceptor units resulted in antenna and reaction center development. The "hole" was primary located on a peculiar photoreceptor form and the electron passed by tunneling through the chain of intermediate carriers (chlorophylls and pheophytins); thus long-lived charge separation was achieved. The use of the electrons and the "holes" stored in reaction centers for the functioning of the photosynthetic electron transfer chain was realized by cyclic and non-cyclic pathways when the coupling of two photochemical events became the more perfect mechanism to use water molecule as an ultimate electron donor. The appearance of primitive cells inevitably required the coupling of the solar energy conversion mechanism to the reproduction mechanism which used stored solar energy.