Gas Phase Values Research Articles

Insight into Hydrogen Abstractions by Nitrate Radical: Structural, Solvent Effects, and Evidence for a Polar Transition State.

The role of a polarized transition state and solvent effects on nitrate radical reactions was examined with a previously under-reported class of substrates, ethers, for their atmospheric implications. Absolute rate constants for hydrogen abstraction from a series of alcohols, ethers, and alkanes by nitrate radical have been measured in acetonitrile, water, and mixtures of these two solvents. Across all of these classes of compounds, using a modified form of the Evans-Polanyi relationship, it is demonstrated that the observed structure/reactivity trends can be reconciled by considering the number of abstractable hydrogens, strength of the C-H bond, and ionization potential (IP) of the substrate. Hydrogen abstractions by nitrate radical occur with low selectivity and are characterized by an early transition state (α ≈ 0.3). The dependence of the rate constant on IP suggests a polar transition state with some degree (<10%) of charge transfer. These conclusions stand for reactions conducted in solution (CH3CN and H2O) as well as gas-phase values. Because of this polar transition state, the rate constants increase going from the gas phase to a polar solvent, and the magnitude of the increase is consistent with Kirkwood theory.

Cold Traps of Hypervolatiles in the Protosolar Nebula at the Origin of the Peculiar Composition of Comet C/2016 R2 (PanSTARRS)

Abstract Recent observations of the long-period comet C/2016 R2 (PanSTARRS; hereafter R2) indicate an unusually high N2/CO abundance ratio, typically larger than ∼0.05, and at least 2–3 times higher than the one measured in 67P/Churyumov–Gerasimenko. Another striking compositional feature of this comet is its heavy depletion in H2O (H2O/CO ∼ 0.32%), compared to other comets. Here we investigate the formation circumstances of a generic comet whose composition reproduces these two key features. We first envisage the possibility that this comet agglomerated from clathrates, but we find that such a scenario does not explain the observed low water abundance. We then alternatively investigate the possibility that the building blocks of R2 agglomerated from grains and pebbles made of pure condensates via the use of a disk model describing the radial transport of volatiles. We show that N2/CO ratios reproducing the value estimated in this comet can be found in grains condensed in the vicinity of the CO and N2 ice lines. Moreover, high CO/H2O ratios (&gt;100 times the initial gas-phase value) can be found in grains condensed in the vicinity of the CO ice line. If the building blocks of a comet assembled from such grains, they should present N2/CO and CO/H2O ratios consistent with the measurements made in R2’s coma. Our scenario indicates that R2 formed in a colder environment than the other comets that share more usual compositions. Our model also explains the unusual composition of the interstellar comet 2l/Borisov.

Electronic Spectroscopy of for Astrochemical Consideration

Abstract The electronic spectrum of the endohedral fullerene observed by messenger spectroscopy in a cryogenic ion trap is presented. The role played by the messenger tag in the adopted experimental method is evaluated by recording spectra of with n = 1–4. The results indicate a linear shift of ∼0.7 Å in the wavelengths allowing accurate gas phase values to be reported. The presence of the helium inside the cage shifts the absorption bands by 2–3 Å toward shorter wavelengths compared to . The magnitude of this displacement will enable searches for the spectral signatures of this fullerene analogue in interstellar environments by absorption spectroscopy. The implications for potential astronomical detection are discussed.

Correlations in hydrochloride drugs with diverse pharmacological activities. Role of N-H…Cl bonds

In this work, the structural and vibrational properties of sixteen hydrochloride/hydrobromide drugs with different pharmacological activities have been compared and analysed in order to find some correlations among their properties and, mainly elucidate the role of N-H•••Cl bonds in them. Here, the properties of ten alkaloids: tropane, gramine, morphine, cocaine, methadone, naloxone, heroin and scopolamine as hydrochloride and hydrobromide including, the psychotropic 2-CB agent; three antihistaminic: diphenhydramine, cyclizine and promethazine and; three antihypertensive tolazoline, clonidine and guanfacine agents have been evaluated. All properties were predicted in gas phase and aqueous solution by using the hybrid B3LYP/6-31G* method and the same were evaluated in functions of their molecular weights. Here, stabilization and solvation energies, dipole moments and volumes in both media, atomic MK charges and bond N-H and N+Cl- lengths, N-H stretching modes of N-HCl bonds of hydrochloride and their cationic species and, frontier orbitals together with global electrophilicity and nucleophilicity descriptors were compared for those sixteen drugs. The results have shown that bonds N+Cl- lengths of all hydrochloride species are higher in solution, as compared with the values in gas phase. Hydrochloride species of alkaloids and antihistaminic agents in both media present higher positive MK values on the N atoms of N-HCl bonds while the species related to antihypertensive agents show higher negative MK values on the N atoms or low positive values. The species of guanfacine presents the higher number of donors and acceptors groups, higher dipole moment value in solution, low bond N-H lengths, higher negative charge on the N atom of N-HCl bond and, higher global electrophilicity index. Hydrochloride species of scopolamine and heroin present the more negative solvation energies while tolazoline the lower value. Hydrochloride species of 2-CB and diphendramine show the higher expansions volumes in solution while the species of naloxone, scopolamine and cocaine evidence volumes contractions in this medium. These studies show that the knowledge of hydration degrees, that is, the number of water molecules that hydrate the hydrochloride species are essential to understand the hydration process of these species in relation to the differences observed in solvation energies, volume variations and dipole moment values.

Elusive Cyanoform: Computational Probing Its Stability and Reactivity with Accurate Ab Initio Methods.

We have applied the CCSD(T)-F12a/cc-pVTZ-F12//CCSD(T)/cc-pVTZ level of theory to calculate energies for 22 reactions pertinent to the stability and reactivity of hardly isolable cyanoform (HC(CN)3). A number of exothermic processes has been indicated, especially the hydration. In the predicted mechanism for the gas-phase hydration of cyanoform, the H2O addition to the C≡N bond corresponds to a rate-limiting step, which is aided by an extra molecule of water. Also, for the cyanoform dihydrate (H2NC(OH)C(CN)CONH2) product, the experimentally identified compound, the more stable planar isomer exhibits intramolecular O-H···O═C (not N-H···O═C) H-bonding. Our calculated structures, binding energies, and NBO data for [HC(CN)3]n (n = 2,4) clusters suggest that the non-conventional C-H···N H-bonds contribute to their stability. Among the surveyed structures of the C≡N group incorporating products of reactions examined, the CCSD(T)/cc-pVTZ molecular parameters of cyanocarbons C2(CN)4, C2(CN)6, and C6(CN)6 can be regarded as the most accurate gas-phase values up-to-date.

Thermodynamics of Metal Carbonates and Bicarbonates and Their Hydrates for Mg, Ca, Fe, and Cd Relevant to Mineral Energetics.

The heats of formation of the carbonate, bicarbonate, and bicarbonate/hydroxide metal complexes, including hydrates of Mg2+, Ca2+, Fe2+, and Cd2+, and the oxides, dichlorides, and dihydroxides are predicted from atomization energies using correlated molecular orbital theory at the CCSD(T) level extrapolated to the complete basis set limit following the Feller-Peterson-Dixon (FPD) approach. Using the calculated gas phase values and the available experimental solid-state values, we predicted the cohesive energies of selective minerals. The gas phase decomposition energies of MO, CO2, and H2O follow the order Mg ≈ Ca > Cd ≈ Fe and correlate with the hardness of the metal +2 ions. Gas phase hydration energies show that the order is Mg > Fe > Ca ≈ Cd. There are a number of bulk hydrated Mg and Ca complexes that occur as minerals but there are few if any for Fe and Cd, suggesting that a number of factors are important in determining the stability of the bulk mineral hydrates. The FPD heats of formation were used to benchmark a range of density functional theory exchange-correlation functionals, including those commonly used in solid-state mineral calculations. None of the functionals provided chemical accuracy agreement (±1 kcal/mol) with the FPD results. The best agreement to the FPD results is predicted for ωB97X and ωB97X-D functionals with an average unsigned error of 10 kcal/mol. The worst functionals are PW91, BP86, and PBE with average unsigned errors of 32-36 kcal/mol.

State-of-the-art computations of dipole moments using analytic gradients of high-level density-fitted coupled-cluster methods with focal-point approximations.

Using the analytic derivatives approach, dipole moments of high-level density-fitted coupled-cluster (CC) methods, such as coupled-cluster singles and doubles (CCSD), and coupled-cluster singles and doubles with perturbative triples [CCSD(T)], are presented. To obtain the high accuracy results, the computed dipole moments are extrapolated to the complete basis set (CBS) limits applying focal-point approximations. Dipole moments of the CC methods considered are compared with the experimental gas-phase values, as well as with the common DFT functionals, such as B3LYP, BP86, M06-2X, and BLYP. For all test sets considered, the CCSD(T) method provides substantial improvements over Hartree-Fock (HF), by 0.076-0.213 D, and its mean absolute errors are lower than 0.06 D. Furthermore, our results indicate that even though the performances of the common DFT functionals considered are significantly better than that of HF, their results are not comparable with the CC methods. Our results demonstrate that the CCSD(T)/CBS level of theory provides highly-accurate dipole moments, and its quality approaching the experimental results. © 2019 Wiley Periodicals, Inc.

Adsorption and coadsorption of single and multiple natural organic matter on Ag (1 1 1) surface: A DFT-D study

The nature of the interaction of low molecular weight natural organic matter (NOM) with the Ag (1 1 1) surface is of crucial importance in the environment. The low molecular weight organics used in this study are formic acid (FA), acetic acid (AA1) and ascorbic acid (AA2). In this study, we think of a realistic environment where single, multiple or even a mixture of NOM’s can attach on one Ag (1 1 1) surface. Such critical information is relevant in order to understand the behaviour of engineered nanoparticles (ENPs) when they get into the environment. To bridge this gap, we investigate the adsorption and co-adsorption properties of NOM’s on Ag (1 1 1) surface using dispersion-corrected density functional theory (DFT-D) in the gas phase and water as a solvent. Throughout this paper, the number behind the letter represents the number of molecules i.e. nFA, nAA1, nAA2 (n = 1, 2, 3, 4). The results of the calculated adsorption energy suggest that the interaction of 4FA, 2AA1 and 2AA2 molecules with Ag (1 1 1) surface is the strongest with the most negative values (−6.54 and −3.84 eV) in both gas phase and COSMO respectively which reveals that is the most stable system. The global reactivity descriptors in the gas phase and water as a solvent were calculated.

Intact Transition Epitope Mapping - Thermodynamic Weak-force Order (ITEM - TWO)

We have developed an electrospray mass spectrometry method which is capable to determine antibody affinity in a gas phase experiment. A solution with the immune complex is electrosprayed and multiply charged ions are translated into the gas phase. Then, the intact immune-complex ions are separated from unbound peptide ions. Increasing the voltage difference in a collision cell results in collision induced dissociation of the immune-complex by which bound peptide ions are released. When analyzing a peptide mixture, measuring the mass of the complex-released peptide ions identifies which of the peptides contains the epitope. A step-wise increase in the collision cell voltage difference changes the intensity ratios of the surviving immune complex ions, the released peptide ions, and the antibody ions. From all the ions´ normalized intensity ratios are deduced the thermodynamic quasi equilibrium dissociation constants (KDm0g#) from which are calculated the apparent gas phase Gibbs energies of activation over temperature (ΔGm0g#T). The order of the apparent gas phase dissociation constants of four antibody – epitope peptide pairs matched well with those obtained from in-solution measurements. The determined gas phase values for antibody affinities are independent from the source of the investigated peptides and from the applied instrument. Data are available via ProteomeXchange with identifier PXD016024. SignificanceITEM - TWO enables rapid epitope mapping and determination of apparent dissociation energies of immune complexes with minimal in-solution handling. Mixing of antibody and antigen peptide solutions initiates immune complex formation in solution. Epitope binding strengths are determined in the gas phase after electrospraying the antibody / antigen peptide mixtures and mass spectrometric analysis of immune complexes under different collision induced dissociation conditions. Since the order of binding strengths in the gas phase is the same as that in solution, ITEM – TWO characterizes two most important antibody properties, specificity and affinity.

Formation of phenoxy-type Environmental Persistent Free Radicals (EPFRs) from dissociative adsorption of phenol on Cu/Fe and their partial oxides

The interplay of phenolic molecules with 3d transition metals, such as Fe and Cu, and their oxide surfaces, provide important fingerprints for environmental burdens associated with thermal recycling of e-waste and subsequent generation of notorious dioxins compounds and phenoxy-type Environmental Persistent Free Radicals (EPFRs). DRIFTS and EPR measurements established a strong interaction of the phenol molecule with transition metal oxides via synthesis of phenolic- and catecholic-type EPFRs intermediates. In this contribution, we comparatively examined the dissociative adsorption of a phenol molecule, as the simplest model for phenolic-type compounds, on Cu and Fe surfaces and their partially oxidized configurations through accurate density functional theory (DFT) studies. The underlying aim is to elucidate the specific underpinning mechanism forming phenoxy- or phenolate-type EFPRs. Simulated results show that, the phenol molecule undergoes fission of its hydroxyl’s O–H bond via accessible activation energies. These values are lower by 46.5–74.1% when compared with the analogous gas phase value. Physisorbed molecules of phenol incur very low binding energies in the range of −2.1 to −5.5 over clean Cu/Fe and their oxides surfaces. Molecular attributes based on charge transfer and geometrical features are in accord with the very weak interaction in physisorbed states. Thermo-kinetic parameters established over the temperature region of 300 and 1000 K, exhibit a lower activation energy for scission of phenolic’s O–H bonds over the oxide surfaces in reference to their pure surfaces (24.7 and 43.0 kcal mol−1vs 38.4 and 47.0 kcal mol−1).

Characterization of potassium (2-phenylacetyl) trifluoroborate salt by using the UV–Visible, FT-IR and FT-Raman spectra

The potassium 2-phenylacetyl-trifluoroborate (PTFB) salt has been experimentally characterized by using FT-IR, FT-Raman and ultraviolet–visible spectroscopies while its structures were theoretically determined in gas phase and in aqueous solution by using B3LYP/6–311++G** method. The studies in solution were performed at the same level of theory with the integral equation formalism variant polarised continuum method (IEFPCM) and the universal solvation model. The comparisons among the predicted IR, Raman and UV–Visible spectra have evidenced good concordance with the corresponding experimental ones. PTFB presents a solvation energy of −87.64 kJ/mol, a value lower than the observed for 5-hydroxypentanoyl-trifluoroborate (HTFB) (−103.73 kJ/mol) and 2-isonicotinoyl-trifluoroborate (ITFB) (−95.05 kJ/mol) and higher than 3-furoyl-trifluoroborate (FTFB) (−84.72 kJ/mol). Evidently, the phenyl ring in PTFB decreases the solubility of this salt, as compared with hydroxypentanoyl chain in HTFB, furoyl ring in FTFB and pyridine ring in IFTB. The bond orders studies support the ionic characteristics of K⋯B bonds while the NBO studies reveal a lower stability of PTFB in aqueous solution, as compared with the value in gas phase probably due to the low solvation energy of PTFB in this medium. The four interactions observed for PTFB in gas phase by the AIM analyses support the high stability of PTFB in gas phase. The gap values show that PTFB is most reactive in gas phase than in aqueous solution while the comparisons with other salts containing pyridine ring instead phenyl one reveal that reactivity increase when in the 5 position of pyridine ring is incorporates a Br atom in the structure of salt. Hence, the following reactivity order it is observed in gas phase: Br-ITFB > ITFB > PTFB changing in solution to: ITFB > Br-ITFB > PTFB. In solution, probably the salt is most reactive due to the most negative nucleophilicity index and higher electrophilicity index. The harmonic force fields, force constants and complete assignments of all vibration normal modes for PTFB in both media for first time are reported.

FT-IR, FT-Raman and UV-visible spectra of motrilin acetogenin isolated from Annona cherimolia

Annonaceous acetogenin (ACG), motrilin was experimentally isolated and characterized by using the FT-IR and FT-Raman spectra in the solid state and by UV-visible spectrum in methanol solution. The main bands observed in the vibrational spectra were assigned combining their predicted spectra with hybrid functional B3LYP/6-31G* calculations. The structural, electronic and topological properties were predicted for motrilin in gas phase and in methanol solution. The corrected solvation energy by ZPVE and by total non-electrostatic terms reveals for motrilin in methanol solution a value of −147.54 kJ/mol. The NPA charges have evidenced higher changes on the atoms of motrilin in methanol solution than the values in gas phase while the studies of electrostatic potentials have revealed strongest nucleophilic regions on the two O atoms of lactone ring while electrophilic sites on the H atoms of OH groups. Both NBO and AIM studies support the high stabilities of motrilin, especially in methanol solution probably due to the solute-solvent association phenomenom, as suggested by the expansion of volume in solution and by the H bond formation in this medium predicted by the AIM program. A higher reactivity of motrilin in methanol solution was evidenced by using the frontier orbitals while both electrophilicity and nucleophilicity indexes show values for motrilin similar to those observed in the antimicrobial thione agent. The predicted IR, Raman and UV-visible spectra has evidenced reasonable concordance with the corresponding experimental ones.

The role of spin-orbit coupling in the optical spectroscopy of atomic sodium isolated in solid xenon

Molecular dynamics calculations, based on the diatomics-in-molecules method, have been used to probe the manifestations of spin-orbit (SO) coupling in the experimental absorption bands of atomic sodium isolated in solid xenon. Inclusion of SO coupling of –320 cm−1 in spectral simulations of the 3 p 2 P ← 3 s 2 S transition leads to unequal band spacings which very closely match the asymmetrical bandshape observed for blue single vacancy (SV) site occupancy. This SO value, extracted in a previous MCD study, reveals the dramatic change in the effective SO coupling constant of the Na atom (from the gas phase value of +17 cm−1) in solid Xe when it is close to the 12 xenon atoms in the first surrounding sphere. In contrast, the symmetrical three-fold split band of the red tetra vacancy (TV) site in Na/Xe is not affected nearly as much by SO coupling. This reflects a greatly reduced “external heavy atom” effect when the 24 Xe atoms surrounding the Na atom in TV are located at greater distances. The contrasting behavior of sodium in the SV and TV sites suggests a strong dependence of the SO coupling strength on the Na–Xe distance.Molecular dynamics calculations, based on the diatomics-in-molecules method, have been used to probe the manifestations of spin-orbit (SO) coupling in the experimental absorption bands of atomic sodium isolated in solid xenon. Inclusion of SO coupling of –320 cm−1 in spectral simulations of the 3 p 2 P ← 3 s 2 S transition leads to unequal band spacings which very closely match the asymmetrical bandshape observed for blue single vacancy (SV) site occupancy. This SO value, extracted in a previous MCD study, reveals the dramatic change in the effective SO coupling constant of the Na atom (from the gas phase value of +17 cm−1) in solid Xe when it is close to the 12 xenon atoms in the first surrounding sphere. In contrast, the symmetrical three-fold split band of the red tetra vacancy (TV) site in Na/Xe is not affected nearly as much by SO coupling. This reflects a greatly reduced “external heavy atom” effect when the 24 Xe atoms surrounding the Na atom in TV are located at greater distances. The contra...

Thermodynamic of solvation, solute – Solvent electron transfer and ionization potential of BSCAPE molecule and its UV–vis spectra in aqueous solution

The molecular system 2-Phenylethyl (2E)-3-(1-benzenesulfonyl-4,5-dihydroxyphenyl) acrylate (BSCAPE) is a phenolic acid that covers a large spectrum of biological properties. The investigations of solvation and oxidation processes of BSCAPE molecule by computational means were the challenge of this present work. Water was required for solvation throughout the work. The explicit H2O were sequentially added to form the complexes BSCAPE(H2O)n=0-11. The discrete – continuum model was at the heart of this work. DFT and TD-DFT both associated to the continuum model SMD were required. Hence, the structures, the solvation energies, the energies of solute – solvent electron transfer (SSET), the ionisation potential (IP), and the UV–vis spectra were studied. It comes out that, the structure of the CAPE part included in BSCAPE agrees well with the available experimental values of CAPE but with a minor influence due to the presence of benzensulfonyl group. The enthalpy and free energy of solvation increase linearly with nH2O. The global reactivity indexes were assessed to appreciate the oxidation of BSCAPE. The latter quality was strongly assessed by the enthalpy and free energy of SSET and IP. The SSET potential increase with nH2O and the size of water clusters. The values 723.16 and 711.62 kJ/mol were found for enthalpy and free energy of IP respectively. Then in aqueous solution, the results fall down and upon addition of nH2O, they approach gas phase value for 11H2O and still are not stabilized. Therefore, the resistance to oxidation starts to raise at this level. Elsewhere, the UV–vis spectra of BSCAPE present four important peaks about 279.3, 234.8, 208.4 and 199.4 nm in gaseous state. The excitation shifts to the red as the number of H2O increase. Their oscillator strengths also increase with solvation.

Solvent effects on the spectroscopic properties of Damascone derivatives: A sequential Monte Carlo/Quantum Mechanics study

The Sequential Monte Carlo/Quantum Mechanics approach was used for investigate the solvent effect on the solute-solvent structure and spectroscopic properties of Damascone derivatives, Carvone, Perillaldehyde and Menthadiene in water solution. Our in-water results have shown that the dipole moment increase of the compounds are between 36% and 84% when compared with the gas phase values and a solvatochromic shift between −20 and 14 nm whereas the magnetic properties studied have indicated an NMR shift close to 10 ppm for some carbon atoms and greater than 140 ppm for oxygen atoms.

The Kohn-Sham electronic density of states of liquid HCN: Tuning a long-range corrected exchange-correlation functional for predicting electron binding energies

Abstract The electron binding energies and Kohn-Sham electronic density of states of liquid HCN were calculated by combining Born-Oppenheimer molecular dynamics with electron propagator theory and density functional theory. A modified long-range corrected Perdew-Burke-Ernzerhof exchange-correlation functional (LC-wPBE), where the parameter w was tuned for reproducing the electron binding energies of the HCN monomer and small aggregates was applied in the liquid state calculations. The average HOMO energy of HCN is red-shifted by ∼ 0.8 eV relative to its gas-phase value. The vertical electron affinity of liquid HCN is estimated as 1.08 ± 0.38 eV.

Incremental NH stretching downshift through stepwise nitrogen complexation of pyrrole: a combined jet expansion and matrix isolation study.

Aggregates of pyrrole with nitrogen are studied by Fourier transform infrared spectroscopy in supersonic jet expansions as well as in neon, argon and nitrogen cryomatrices. The NH stretching vibration undergoes a significant downshift upon switching from isolated gas phase conditions to bulk nitrogen matrices, which can be reconstructed incrementally by stepwise cluster formation with an increasing number of nitrogen molecules both in supersonic expansions and neon or argon matrices. The modelling of the bulk matrix shift by finite cluster theory remains an interesting challenge. Self-aggregation of pyrrole also yields the first spectra of the homodimer and -trimer in a neon matrix, showing particularly small (up to 10 cm-1) deviations from the isolated gas phase values.

Reaction pathways for HCN on transition metal surfaces.

The adsorption and decomposition of HCN on the Pd(111) and Ru(001) surfaces have been studied with reflection absorption infrared spectroscopy and density functional theory calculations. The results are compared to earlier studies of HCN adsorption on the Pt(111) and Cu(100) surfaces. In all cases the initial adsorption at low temperatures gives rise to a ν(C-H) stretch peak at ∼3300 cm-1, which is very close to the gas phase value indicating that the triple CN bond is retained for the adsorbed molecule. When the Pd(111) surface is heated to room temperature, the HCN is converted to the aminocarbyne species, CNH2, which was also observed on the Pt(111) surface. DFT calculations confirm the high stability of CNH2 on Pd(111), and suggest a bi-molecular mechanism for its formation. When HCN on Cu(100) is heated, it desorbs without reaction. In contrast, no stable intermediates are detected on Ru(001) as the surface is heated, indicating that HCN decomposes completely to atomic species.

Time-of-day-dependent behavior of surficial lunar hydroxyl/water: Observations and modeling

In this study we determine the depth of the absorption band around 3 µm wavelength, which indicates the presence of OH/H2O in a thin surficial layer of the lunar regolith, for 18 lunar highland regions observed by the Moon Mineralogy Mapper (M3) instrument at 4–8 different local times of day. For removing the thermal emission component, a physically motivated thermal equilibrium based method is used, which also takes into account the roughness of the regolith surface. We propose a continuity equation based model of the time-of-day-dependent column densities of surficial atomic hydrogen (H) and hydroxyl (OH). The considered source processes for H are implantation of solar wind protons and photolysis of OH, and for OH the reaction H+O→OH. Sink processes are diffusive loss for H and OH, and photolysis for OH. Sputtering of H and OH is found to be negligible in comparison to the other sink processes. Additionally, we suggest a similar differential equation based model to describe the time-of-day-dependent behavior of micrometeoroid-delivered OH and H2O. The observed 3 µm band depth values indicate that the surficial OH/H2O does not vanish even at local midday at low latitudes, while the model predicts nearly complete removal of surficial OH/H2O at midday. This apparent contradiction between model and observations is reconciled by adding to the model a region-specific “offset” OH component which is assumed to be stable against diffusive loss and photolysis and is therefore interpreted as a strongly bounded OH component. Meteoroid bombardment is found to be negligible in comparison with the solar wind source of OH. Fitting the model to the observed 3 µm band depth values allows for estimating the H activation energy, the OH photolysis time, region-specific values of the offset OH component, and the proportionality factor between OH column density and 3 µm band depth. The observed time-of-day-dependent behavior of the 3 µm band depth at low and high latitudes can be explained convincingly by the modeled source and sink processes. The best-fit OH photolysis time is much shorter than the lunar day and corresponds to 1–3 times the gas-phase value, which indicates that photolysis is a mechanism of high relevance for the behavior of solar wind induced surficial OH. The surface roughness assumed in the M3 data analysis does not have a major influence on the modeling results. The strongly bounded OH component is nearly latitude-independent for low latitudes but decreases sharply for high latitudes. As a possible mode of origin, we suggest slow diffusion of solar wind induced OH to depths inaccessible for ultraviolet photons and into binding states of higher energy, counteracted by sputtering.

Born-Oppenheimer molecular dynamics, hydrogen bond interactions and magnetic properties of liquid hydrogen cyanide

Magnetic properties are a very sensitive probe of hydrogen bond interactions. In this work, the magnetic shielding constants of liquid hydrogen cyanide (HCN) were investigated through a combined approach, where quantum mechanical calculations are carried out by using configurations generated by Born-Oppenheimer molecular dynamics (BOMD). Following the trends observed in small HCN clusters, the results for liquid HCN show that the magnetic shielding constant σ(15N) is increased by 10.5 ± 12 ppm relative to its gas phase value, which is in very good agreement with experiment (10 ppm). The shielding of the N atom in the liquid phase of HCN is in contrast to what is observed in liquid ammonia, where the N atom is deshielded relative to the gas phase. By adopting a natural chemical shielding (NCS) analysis, it is shown that the σd(15N) diamagnetic shielding constant of HCN is not changed when we move from the gas to the liquid phase. Moreover, the results strongly indicate that the gas-to-liquid chemical shift of the 15N atom is essentially determined by the difference between the nitrogen lone-pair (LP) orbital paramagnetic contribution to σp(15N). The importance of coupling NCS to BOMD configurations for a better understanding of the relationship between hydrogen bonding and the somewhat anomalous shielding of the 15N atom in liquid HCN is stressed.