Jan Urban

Dissociative electron attachment to gas phase valine: a combined experimental and theoretical study.

Using a crossed electron/molecule beam technique the dissociative electron attachment (DEA) to gas phase L-valine, (CH(3))(2)CHCH(NH(2))COOH, is studied by means of mass spectrometric detection of the product anions. Additionally, ab initio calculations of the structures and energies of the anions and neutral fragments have been carried out at G2MP2 and B3LYP levels. Valine and the previously studied aliphatic amino acids glycine and alanine exhibit several common features due to the fact that at low electron energies the formation of the precursor ion can be characterized by occupation of the pi* orbital of the carboxyl group. The dominant negative ion (M-H)(-) (m/Z=116) is observed at electron energies of 1.12 eV. This ion is the dominant reaction product at electron energies below 5 eV. Additional fragment ions with m/Z=100, 72, 56, 45, 26, and 17 are observed both through the low lying pi* and through higher lying resonances at about 5.5 and 8.0-9.0 eV, which are characterized as core excited resonances. According to the threshold energies calculated here, rearrangements play a significant role in the formation of DEA fragments observed from valine at subexcitation energies.

Ion–Molecule Reactions in Helium Nanodroplets Doped with C60 and Water Clusters†

Helium nanodroplets, which contain some 10 to 10 atoms, provide an avenue for important new experiments. In a lowpressure environment, the droplets cool within microseconds to 0.37 K by evaporation of weakly bound helium atoms. Molecules captured in collisions with a superfluid droplet will quickly aggregate in the droplet s interior into novel, often metastable structures. These “personal nanocryostats” may be used to explore chemical reactions. For example, M ller et al. reported that ionization initiates complete hydrolysis of cesium–water complexes within the droplets. Herein we report ion–molecule reactions between C60 and small water clusters. Water is an integral part of biomolecular organization; its bioactivity can be further understood by characterization of its function at the C60–H2O interface. [4] C60 is hydrophobic; its hard-core radius of 0.5 nm is close to the crossover point beyond which the breakage of hydrogen bonds becomes unavoidable. Molecular dynamics simulations show that fullerenes strongly bind to single and doublestrand DNA; addition of hydrated C60 to drinking water has been found to mitigate damage of ethanol to brain cells of rats without causing any adverse biological effects. In the present work, helium droplets were doped with C60 and water, and subsequently ionized by electron impact ionization. The interpretation of experimental results was aided by ab initio Hartree–Fock calculations. Two observations stand out: Firstly, the weak interaction between neutral C60 and water extends to the cationic system. Desorption of entire water clusters rather than evaporative loss of water molecules occurs for certain water cluster sizes. Secondly, C60OH + is a major product ion. We postulate that this ion results from doubly charged [C60(H2O)] 2+ intermediates that form by charge transfer from a primary He ion. The existence of doubly charged intermediates in doped helium droplets and their role in subsequent ion–molecule reactions has so far been ignored; these intermediates provide a compelling rationale for previous observations of hydrogen loss from clusters of organic molecules and biomolecules. We first summarize results obtained by ionization of helium droplets doped with water (either H2O or D2O) but no C60 . In agreement with a previous report, [11] electron impact ionization results in a prominent series of protonated water cluster ions. Unprotonated water cluster ions are observed with a 10 % abundance relative to the protonated cluster ions. Unprotonated ions are not observable upon electron impact or multiphoton ionization of bare water clusters, but they occur if water clusters are complexed with heavy rare-gas atoms. These trends are well understood—the ground state of (H2O)2 + corresponds to the proton-transferred isomer OH–H3O , and its dissociation to OH + H3O + is energetically much more facile than dissociation to H2O + H2O . Direct ab initio dynamics studies of water clusters show that vertical ionization is followed by one or more barrierless proton transfer reactions within 100 fs; solvent reorganization leads to a highly excited cluster ion and ejection of the OH radical within sub-picoseconds; enough energy remains for the evaporation of several more water molecules. The composition of cluster ions changes drastically when helium droplets are co-doped with C60 . The most prominent ion series in Figure 1 arises from C60(D2O)n , n = 0, 1, 2. Dehydrogenated ions, that is, ions with the stoichiometry C60(D2O)n 1OD + are also observed, while the abundance of protonated ions is weak. For a quantitative analysis, we fitted the distribution of C60 isotopologues by sets of four Gaussians with fixed ratios of amplitudes computed from the 1.11 % natural abundance of [*] Prof. Dr. O. Echt Department of Physics, University of New Hampshire Durham, NH 03824 (USA) Fax: (+ 1)603-862-2998 E-mail: olof.echt@unh.edu Homepage: http://www.physics.unh.edu/

Molecular structures, vibrational spectra and rotational barriers of C2H6, Si2H6, SiGeH6, and Ge2H6—experiment andtheory in harmony

Abstract The staggered and eclipsed conformers of the title compounds were optimized at the HF, MP2, B3LYP, and CISD levels of theory utilizing polarized triple-ζ basis sets; CCSD(T) single points were computed for the CISD structures. All experimental geometrical parameters are reproduced well at the correlated levels. The rotational barriers are better described with larger basis sets, and require sophisticated electron correlation treatments (CISD or CCSD) for quantitative agreement with experiment. B3LYP rotational constants are generally somewhat too small due to the incomplete inclusion of higher-order perturbations. The vibrational frequencies computed using MP2 and B3LYP, agree well with experiment after the application of scaling factors of 0.945 and 0.98, respectively.

Absolute Configuration of Beer′s Bitter Compounds†

The science and art of making beer, likely the oldest liquid fermented by humans, stretches over millennia. Production typically involves boiling beer wort together with hops, which acts as a natural preservative,1 but the generated iso-α-acids are known to be prone to decomposition,2, 3 and consequently, more stable reduced hops extracts, such as the tetrahydro-iso-α-acids, have been developed. These latter compounds are separately produced and frequently added to beer to achieve a consistent level of bitter taste. Scheme 1 gives an overview of the iso-α-acids formed by heat-induced isomerization. Scheme 1 Hops bitter acids. Humulones: R=isobutyl, cohumulones: R=isopropyl, adhumulones: R=sec-butyl. Herein, we determined the absolute configuration of several cis and trans iso-α-acids by X-ray crystallography. We show how we unequivocally assigned the chiral center in (−)-humulone to be (6S) and give absolute structures for several of its derivatives, most of which contradict the general perception circulating through the literature since 1970.4 Typically, the predominant α-acid is humulone, a phloroglucinol derivative with two prenyl groups and one isovaleryl group as side chain. The process of isomerization involves contraction of the six-membered α-acid ring (through acyloin rearrangement) to form the five-membered iso-α-acid ring with two chiral centers,5 resulting in cis and trans diastereomers. Photo-induced isomerization is stereospecific and can be used to produce pure trans iso-α-acids. The isomerization process has been recognized for over 80 years,6–8 yet the absolute configuration of carbon atoms 4 and 5 of the iso-α-acids (Scheme 1) have remained speculative. For the most predominant member of the iso-α-acid family, isohumulone, the cis and trans isomerization products, differing at carbon atom 4, were described preliminarily almost 50 years ago (Scheme 1).9 Ultimately, the absolute configuration of (4R, 5S) for (+)-cis-isohumulone was inferred according to Horeau’s method of partial decoupling. The absolute configuration of (−)-tetrahydrohumulone was inferred using the Cotton effect, which relates spectral details in an optical rotary dispersion curve to the configuration of a molecule, among other things.4, 10 However, contradictive and predominantly unsupported or inconclusive configurational assignments are reported as well.11–13 Claims that beer and the bittering acids found in beer are beneficial when consumed in moderation have accumulated over time, including positive effects on diabetes,14–16 forms of cancer,17–20 and inflammation,21 and even linking reduced iso-α-acid derivatives to weight loss.22–24 Some of these derivatives affect one illness,21, 25–28 whereas others, differing only in the configuration of carbon atoms 4 and 5 of the iso-α-acids (Scheme 1), are ineffective.29 In addition, it was discovered that different grades of bitterness can be related to opposite enantiomers of tetrahydro-iso-α-acids.30 However, the degree of bitterness of the isomerized bittering acids has yet to be related to a specific structure–function relationship after decades of confusion over the configuration of iso-α-acids, and in general, specifics of the isomerization process need to be resolved. Moreover, preservation of configuration during thermal isomerization has in the past been assigned to the ring carbon atom with the 3-methylbutyl side chain (see Scheme 1, C5 of the iso-α-acids, derived from C4 of the α-acids),6, 7, 10 which was opposed by other reports,11, 31, 32 thus demonstrating the state of confusion about the underlying hops chemistry. In order to unequivocally derive the absolute configuration of the iso-α-acids, we sought to use X-ray diffraction on a salt of (+)-cis-isohumulone or a suitable derivative and record the optical rotation of the compounds to ensure accurate literature comparison. Please note that the isomerization process results in a multitude of very similar compounds, which need to be separated from each other, purified, and unambiguously characterized, requiring an elaborate procedure (for details see the Supporting Information). As crystal salts we chose those containing either a heavy atom (to enable anomalous X-ray scattering for adequate phase resolution to define the configuration of a structure) or a compound of known absolute configuration to serve as an internal reference. Empirically, the vast number of useful salt combinations was greatly reduced by the inability to grow X-ray quality crystals. The initial success in elucidating the absolute configuration occurred with a purified ‘tetrahydro’ derivative (compound 1, (+)-cis-tetrahydroisohumulone), synthesized from (+)-cis-isohumulone through heterogeneous catalytic hydrogenation.29 The absolute configuration, which was determined by X-ray analysis on a potassium salt (+)-1 a), indicates that the configuration of compound 1 (4S, 5R) is opposite to the originally reported configuration of the unsaturated precursor (+)-cis-isohumulone.10 Considering that the inversion of both asymmetric centers (C4 and C5 in Figure 1) is unlikely to occur during mild catalytic hydrogenation, the initial assignment found in the literature would appear to be incorrect. In an attempt to verify this observation, additional crystallization experiments were undertaken with compounds related to 1, resulting in the (−)-cinchonidine crystal salts of (+)-cis-tetrahydroisocohumulone (2), (+)-cis-tetrahydroisoadhumulone (3), and (+)-cis-isohumulone (4; Table 1, salts 2 b, 3 b, 4 b). Fig 1 Structural details of (+)-cis-tetrahydroisocohumulone (2). The conserved stereocenter during isomerization is C4, while cis and trans iso-α-acids differ stereochemically at C5, which is nonchiral in the precursor α-humulone molecule. Related ... Table 1 Crystallized compounds suitable for X-ray structure determinations and details on measurement of specific rotation. As expected, the absolute configurations of these compounds were consistent with the one we determined for compound 1 (4S, 5R). For comparison, we added the unnatural enantiomer of 1 to this series, (−)-cis-tetrahydroisohumulone ((−)-1), characterized here as (4R, 5S). Encouraged by these findings, we focused on the stereochemical assignment of the α-acid precursor (−)-humulone. Although neither a potassium nor (−)-cinchonidine salt were amenable to crystallization, we discovered that (−)-humulone (5) formed a small and weakly diffracting crystal (5 c) with trans-(1R, 2R)-(−)-diaminocyclohexane, thus enabling determination of the configuration of (−)-humulone as (6S). This finding is consistent with our initial assignment of (+)-cis-tetrahydroisohumulone, which again is opposite to the configuration found in most references. Finally, we were able to crystallize (−)-trans-tetrahydroisohumulone (6) with (+)-cinchonine (salt 6 d). The resulting structure has unequivocally a (4S, 5S) configuration, and anomalous X-ray diffraction (enabled through the presence of chloroform solvent molecules) confirmed the result. Table 1 summarizes all findings. In light of these discoveries, the isomerization of humulone to isohumulones proceeds with a net retention of configuration from the tertiary alcohol in (6S)-(−)-humulone to the α-hydroxy ketone (C4) in (4S, 5R)-(+)-cis-isohumulone, while configuration at C5 (Figure 1) is not determined. These results are in contrast with the proposed isomerization mechanism found in most reports, which assume that the conserved stereocenter is on C5, while cis and trans differ stereochemically at C4. Considering that the oxygen atoms possess negative charges during the isomerization, one might imagine the chelation of two vicinal oxygen atoms to a divalent cation, a process that is known to accelerate the rate of isomerization, while limiting decomposition. Excessive beer consumption cannot be recommended to propagate good health, but it has been demonstrated that isolated humulones and their derivatives can be prescribed with documented health benefits.21 The absence of correct stereochemical assignment for these compounds has prevented verification of the actual species responsible for biological activity. Now that the stereochemistry for these compounds has been confirmed and methods have been developed to substantiate the configuration of new entities in this series (see the Supporting Information), future work on their biological activities should be greatly accelerated. The utility of X-ray diffraction as the ultimate and preferred method to obtain unequivocal answers regarding absolute stereochemical questions (such as those above) has once more been demonstrated. One question remains: to what extent can one trust those assignments derived indirectly through Horeau’s method of partial decoupling and the Cotton Effect in optical rotary dispersion, now that we have discovered a case where these methods have failed the scientific community?

Ionization of doped helium nanodroplets: Complexes of C60 with water clusters

Water clusters are known to undergo an autoprotonation reaction upon ionization by photons or electron impact, resulting in the formation of (H(2)O)(n)H(3)O(+). Ejection of OH cannot be quenched by near-threshold ionization; it is only partly quenched when clusters are complexed with inert gas atoms. Mass spectra recorded by electron ionization of water-doped helium droplets show that the helium matrix also fails to quench OH loss. The situation changes drastically when helium droplets are codoped with C(60). Charged C(60)-water complexes are predominantly unprotonated; C(60)(H(2)O)(4)(+) and (C(60))(2)(H(2)O)(4)(+) appear with enhanced abundance. Another intense ion series is due to C(60)(H(2)O)(n)OH(+); dehydrogenation is proposed to be initiated by charge transfer between the primary He(+) ion and C(60). The resulting electronically excited C(60)(+*) leads to the formation of a doubly charged C(60)-water complex either via emission of an Auger electron from C(60)(+*), or internal Penning ionization of the attached water complex, followed by charge separation within {C(60)(H(2)O)(n)}(2+). This mechanism would also explain previous observations of dehydrogenation reactions in doped helium droplets. Mass-analyzed ion kinetic energy scans reveal spontaneous (unimolecular) dissociation of C(60)(H(2)O)(n)(+). In addition to the loss of single water molecules, a prominent reaction channel yields bare C(60)(+) for sizes n=3, 4, or 6. Ab initio Hartree-Fock calculations for C(60)-water complexes reveal negligible charge transfer within neutral complexes. Cationic complexes are well described as water clusters weakly bound to C(60)(+). For n=3, 4, or 6, fissionlike desorption of the entire water complex from C(60)(H(2)O)(n)(+) energetically competes with the evaporation of a single water molecule.

Single-root multireference Brillouin-Wigner coupled-cluster theory. Rotational barrier of the N2H2 molecule

Recently developed single-root multireference Brillouin–Wigner coupled-cluster (MR BWCC) theory, which deals with one state at a time while employing a multiconfigurational reference wave function, is applied to study the rotational barrier of the N2H2 molecule. The method represents a brand new coupled-cluster (CC) approach to quasi-degenerate problems which combines merits of two approaches: the single-reference CC method in a nondegenerate case and the Hilbert space MR CC method in quasi-degenerate case. The method is able to switch itself from a nondegenerate to a fully degenerate case in a continuous manner, thus providing smooth potential energy surfaces. Moreover, in contrast to the Hilbert space MR CC theory, it does not contain the so-called coupling terms and in a highly nondegenerate case it reduces to a standard single-reference CC method. In order to better judge the abilitiesof our new approach, we study the rotation barrier of the N2H2 molecule at the CCSD level and the results are compared...

Shellvation of the ammonium cation by molecular hydrogen: a theoretical study

Abstract The results of the theoretical study of NH4+(H2)n (n=1–8) clusters are presented. Two shells for ligand binding which are characterized by the vertex or face binding of H2 to the NH4+ tetrahedron were determined. The main difference in the nature of interactions observed in the structurally different complexes comes from the electrostatic interactions. The dispersion energies are found to be less dependent on the geometry of the complex or on the charge of the core. The properties investigated such as dissociation energy and enthalpy, electronic density distribution and stretching vibrations of H2 are similar for molecules occupying the same shell and are significantly different when neighboring shells are compared.

Electron impact ionization of furanose alcohols

Electron impact ionization of the gas phase 3-furanol, tetrahydro (3-hydroxytetrahydrofuran, 3HTHF) and 2-furanmethanol, tetrahydro (alpha-tetrahydrofurfuryl alcohol, THFA) molecules has been studied both experimentally and theoretically. The electron induced positive ion formation has been investigated experimentally using a crossed electron/neutral beams technique in combination with a quadrupole mass spectrometry. The mass spectra of both molecules have been determined at the incident electron energy of 70 eV. The ionization efficiency curves for each parent cation and a number of fragment cations have been measured near the threshold, and the corresponding appearance energies have been derived using an iterative fitting procedure based on the Wannier threshold law, taking into account the incident electron energy resolution. The appearance energies of the parent cations were experimentally determined to be (9.620+/-0.058) eV for (C(4)H(8)O(2)(+)/3HTHF) and (9.43+/-0.12) eV for (C(5)H(10)O(2)(+)/THFA), which are in a good agreement with G3MP2 calculated results: 9.480 and 9.419 eV, respectively. The most abundant cations in the mass spectra were determined to be 57 amu for 3HTHF and 71 amu for THFA, with the corresponding experimentally determined appearance energies of (10.22+/-0.10) eV and (9.574+/-0.062) eV, respectively. With the help of the energies calculated at B3LYP and G3MP2 levels of theory, the possible fragmentation patterns were discussed.

Ionization energy studies for ozone and OClO monomers and dimers

Electron impact ionization cross sections measured close to threshold are reported for both the monomers and dimers of ozone and OClO using a new high resolution electron impact apparatus. The present appearance energies AE(O3+/O3)=12.70±0.02 eV, AE (OClO+/OClO)=10.55±0.02 and AE(ClO+/OClO=13.37±0.03 eV derived from the measured ionization cross sections are in excellent agreement with the vertical threshold values determined for these ions by high resolution PES and PIMS photoionization studies. The corresponding appearance energies determined for the dimer ions, 10.10±0.3 eV for (O3)2+ and 9.87±0.2 eV for (OClO)2+, are both red shifted with respect to the monomer case. The bond energy (0.70–0.3+0.5) eV of (OClO)2+ estimated from these data is similar to that of other dimer ions, whereas the bond energy of (O3–O3+) with (2.55−0.4+0.6) eV is rather large suggesting an unusual structure for the cationic ozone dimer ion. Based on quantum chemical calculations on various levels we are led to the conclusion t...

Ionization energies of argon clusters: A combined experimental and theoretical study

We have measured appearance energies of Ar(n)+, n<or=30, by electron impact of gas phase clusters. Quantum-chemical calculations have been performed to determine the adiabatic and vertical ionization energies of argon clusters up to n=4 and 6, respectively. The experimental appearance energy of the dimer ion approaches, under suitable cluster source conditions, the adiabatic ionization energy. The agreement with values obtained by photoionization and threshold photoelectron-photoion coincidence (TPEPICO) spectra demonstrates that autoionizing Rydberg states are accessible by electron impact. Appearance energies of larger clusters, though, exceed the TPEPICO values by about 0.5 eV.