Exchange Of Hydrogen Atoms Research Articles

Possibility of using active secondary charge-exchange particle diagnostics for measuring the magnetic field direction in the plasma of a magnetic fusion reactor

An active particle diagnostic method based on the secondary charge exchange of hydrogen atoms of a probing (diagnostic) beam is proposed for local measurements of the magnetic field direction in the plasma of a thermonuclear fusion reactor. Experiments with new-generation large devices require searching for novel methods for measuring the direction of the total magnetic field in a plasma at different points along the radius of the plasma column. The main idea of the method proposed, which holds great promise for large devices, is outlined. The possibility of using the method on ITER—a large fusion reactor that is now at the design stage—is illustrated by carrying out relevant numerical simulations. The results obtained for one of the main discharge scenarios, with the injection geometry and probing beam energy (100 eV) that are now adopted for the ITER design, show that the method can provide local measurements of the magnetic field direction (the magnetic pitch angle) and of the spatial variations of the field vector. Further analysis has revealed, however, that, from the standpoint of signal intensity and signal-to-noise ratio, it is expedient to increase the energy of the beam atoms to 200–250 keV. With such probing beams, the method ensures a spatial (radial) resolution of about 10 cm in the plasma core during a signal acquisition time of 10 ms. The magnetic pitch angle can be measured with an accuracy of 5 × 10−3 rad. An important advantage of the method proposed is its ability to directly measure the pitch angle of the magnetic field lines.

A Computational Study on Fluxional Behavior of Group 6 and 7 Transition-metal Complexes of Borane–Lewis Base Adducts

Abstract Fluxional behavior of group 6 and 7 metal borane complexes was investigated with use of density functional theory (DFT). Site exchange of BH hydrogen atoms of the borane ligand BH3·L or B2H4·2PMe3 proceeds via a transition state in which the borane ligand interacts with metal with a bidentate fashion. Calculated values of the activation energy were in good agreement with experimentally observed barriers.

Secondary isotope effects in liquid chromatography behaviour of 2H and 3H labelled solutes and solvents

The separation of solutes that differ only in the extent of isotopic substitution of their hydrogen atoms, using either mixtures of isotopically non-modified or perdeuterated solvents as mobile phases, is described. The occurrence of a secondary isotope effect is demonstrated in reversed-phase liquid chromatography, which is independent of the nature of the stationary phase (different octadecyl-bonded silicas, an embedded alkylamide-bonded silica, as well as one polymeric stationary phase were tested), and the water content and the nature of organic modifier of the mobile phase. The separation of 24 structurally different isotopologue pairs (apolar compounds and polar compounds with exchangeable or non-exchangeable hydrogen atoms) is examined using reversed-phase liquid chromatography. It is found that the greater the number of isotopically substituted hydrogen atoms in a given organic solute, the better is the separation of a particular isotopologue pair. The single secondary isotope effect is shown to be dependent on the number of isotopic substitutions. The greater the number of these substitutions, the smaller is the single isotope effect. The single secondary isotope effect is higher for aromatic hydrocarbons than for aliphatic hydrocarbons. A secondary isotope effect is also observed in chiral chromatography and normal-phase liquid chromatography, as well as on changing the nature of the substituting isotope, i.e.: tritium instead of deuterium. Thus, we have demonstrated that the total secondary isotopic effect for hydrogen/tritium is higher than for hydrogen/deuterium. This isotope effect involves only the consequences of changes in interactions due to nuclear motions. Overall this study confirms the predominance of hydrophobic effects in retention processes in reversed-phase liquid chromatography. In reversed-phase liquid chromatography, a secondary isotope effect related to mobile phase composition is also observed. The behaviour of deuterium oxide and water in mobile phases of the same composition (%, w/w) is compared. Independent of the nature of the organic modifier (methanol, acetonitrile or ethanol), the effect of replacing H 2O with 2H 2O in the mobile phase, is an increase in the retention factors and an improvement in the chromatographic resolution of isotopologue pairs. This increase in the resolution is not accompanied by a change in the chromatographic selectivity. The measurement of liquid–liquid extraction coefficients proves that the effect is mainly due to the modification of the phase ratio. In general the effect of 2H-labelled solvents ( 2H 2O and C 2H 3CN) as mobile phase components, compared to their isotopically non-modified isomers, can be rationalized on the basis of their lower polarisabilities. Overall the use of perdeuterated rather than isotopically non-modified solvents as mobile phase components leads to the most efficient separation systems.

Laser Spectroscopic Excitation Function and Reaction Threshold Studies of the H + DCl → HCl + D Gas-Phase Isotope Exchange Reaction

The dynamics of the gas-phase hydrogen atom exchange reaction H + DCl --> HCl + D were studied using the pulsed laser photolysis/laser induced fluorescence "pump-and-probe" method. Laser photolysis of H2S at 222 nm was used to generate nonequilibrium distributions of translationally excited hydrogen atoms at high dilution in a flowing moderator gas (Ar)/reagent (DCl) mixture. H and D atoms were detected with sub-Doppler resolution via Lyman-alpha laser induced fluorescence spectroscopy, which allowed the measurement of the line shapes of the moderated H atom Doppler profiles as well as the concentration of the D atoms produced in the H + DCl --> HCl + D reaction. From the measured H atom Doppler profiles, the time evolution of the initially generated nascent nonequilibrium H atom speed distribution toward its room-temperature thermal equilibrium form was determined. In this way, the excitation function and the reaction threshold (E0 = 0.65 +/- 0.13 eV) for the H + DCl --> HCl + D reaction could be determined from the measured nonequilibrium D atom formation rates and single collision absolute reaction cross-section values of 0.12 +/- 0.04 A2 and 0.45 +/- 0.11 A2 measured at reagent collision energies of 1.0 and 1.4 eV, respectively.

Protein Folding Mechanisms Studied by Time-Resolved Electrospray Mass Spectrometry

Mass spectrometry (MS) owes its remarkable rise as a bioanalytical tool to the development of two “soft” ionization techniques: matrix-assisted laser desorption/ ionization (MALDI) (1,2) and electrospray ionization (ESI) (3). The salient feature of soft ionization is that a wide range of analytes, from small metabolites to very large biomolecular systems, can be transferred intact into the gas phase, thus making them amenable to mass spectrometric analysis. For MALDI-MS, the sample is embedded into a crystalline matrix prior to analysis. For ESI, in contrast, ionization is initiated directly from the liquid phase by spraying analyte solution from the tip of an electrically charged capillary. The small droplets formed by the ESI emitter undergo rapid evaporation and fi ssion, ultimately releasing the analyte as multiply protonated (or deprotonated) species (4).

Solution‐phase deuterium/hydrogen exchange at a specific residue using nozzle‐skimmer and electron capture dissociation mass spectrometry

Information about protein conformation can be obtained with hydrogen/deuterium exchange (HDX) mass spectrometry. The isotopic solution-phase exchange of specific amide hydrogen atoms can be followed using low-vacuum nozzle-skimmer collision-induced dissociation (CID). In this study, the nozzle-skimmer technique was complemented by electron capture dissociation (ECD) Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS). The solution-phase exchange at a specific residue is monitored by comparing isotopic distributions of two consecutive b- or c-type ions. While nozzle-skimmer fragmentation takes place in the low-vacuum region of the mass spectrometer, ECD occurs at ultra-high vacuum within the mass analyzer cell of the FTICR mass spectrometer. The dissociations take place at 10(-4) and 10(-9) mbar, respectively. Low-vacuum nozzle-skimmer fragmentation can result in intramolecular exchange between product ions and solvent molecules in the gas phase. Consequently, the solution-phase information about protein or peptide conformation is lost. It was not possible to monitor isotopic solution-phase exchange at the eighth residue in substance P, (Phe)8, with nozzle-skimmer CID. By using the in-cell ECD fragmentation method, the solution-phase exchange at the (Phe)8 residue was preserved during mass spectrometric analysis. This result shows the complementary aspects of applying fragmentation at low and at high vacuum, when studying isotopic exchange in solution at specific residues using FTICRMS.

A crossed beams study of the reaction of carbon atoms, C(3Pj), with vinyl cyanide, C2H3CN(X1A′)—investigating the formation of cyano propargyl radicals

The chemical dynamics of the reaction of ground state carbon atoms, C(3Pj), with vinyl cyanide, C2H3CN(X 1A'), were examined under single collision conditions at collision energies of 29.9 and 43.9 kJ mol(-1) using the crossed molecular beams approach. The experimental studies were combined with electronic structure calculations on the triplet C4H3N potential energy surface (H. F. Su, R. I. Kaiser, A. H. H. Chang, J. Chem. Phys., 2005, 122, 074320). Our investigations suggest that the reaction follows indirect scattering dynamics via addition of the carbon atom to the carbon-carbon double bond of the vinyl cyanide molecule yielding a cyano cyclopropylidene collision complex. The latter undergoes ring opening to form cis/trans triplet cyano allene which fragments predominantly to the 1-cyano propargyl radical via tight exit transition states; the 3-cyano propargyl isomer was inferred to be formed at least a factor of two less; also, no molecular hydrogen elimination channel was observed experimentally. These results are in agreement with the computational studies predicting solely the existence of a carbon versus hydrogen atom exchange pathway and the dominance of the 1-cyano propargyl radical product. The discovery of the cyano propargyl radical in the reaction of atomic carbon with vinyl cyanide under single collision conditions implies that this molecule can be an important reaction intermediate in combustion flames and also in extraterrestrial environments (cold molecular clouds, circumstellar envelopes of carbon stars) which could lead to the formation of cyano benzene (C6H5CN) upon reaction with a propargyl radical.

Spectroscopy and dynamics of A [2B1] allyl radical

The A [(2)B1] <--X [(2)A2] band system between 380 and 420 nm was observed in a supersonic jet expansion. The allyl radical was found to dissociate following electronic excitation, releasing a hydrogen atom. Monitoring the appearance of the hydrogen atom photoproduct as a function of the excitation laser wavelength, similar spectral features are observed as in earlier absorption experiments. Time- and frequency-resolved photoionization of the hydrogen atom product provides information on the unimolecular dissociation dynamics. The measured dissociation rates and kinetic energy releases of both allyl radical, C3H5, and partially deuterated allyl radical, C3DH4, suggest direct loss of the central hydrogen atom, leading to allene as the major product.

Thermodynamics of open two-phase systems with coherent interfaces: Application to metal–hydrogen systems

We develop a thermodynamic theory for the absorption/desorption of interstitial atoms by a solid crystalline host forming a coherent mixture of two phases with different compositions of interstitial solutes. It is shown that the misfit-generated strain produces a macroscopic energy barrier between the transforming phases. The energy barrier is proportional to the macroscopic volume and thus cannot be surmounted by the thermally assisted nucleation of the new phase. The barrier splits the phase-transition pressure (temperature) into two pressures (temperatures): one during increase of pressure (cooling) and another during decrease of pressure (heating). The difference between these pressures (temperatures) determines the system hysteresis. It is also shown that the chemical potential of interstitials within the two-phase region of the coherent phase diagram is a linear function of composition rather than a constant, as is the case in the absence of coherency strains. We apply the theory to the absorption/desorption of H atoms by a metal that is free to exchange hydrogen atoms with an external source (reservoir). The theory explains the ubiquitous hysteresis in the ‘plateaus’ of the pressure–composition isotherms and the critical pressure–composition point for the disappearance of the hysteresis. The proposed theory is generic in the sense that it is applicable to any open interstitial systems forming coherent hydrides or other interstitial phases in the host lattice of the same symmetry.

Hydrogen—Deuterium Exchange in Bovine Serum Albumin Protein Monitored by Fourier Transform Infrared Spectroscopy, Part I: Structural Studies

The structure of the amide I band of bovine serum albumin (BSA) was determined using an H/D exchange experiment. The difference between the dry and hydrated exchange spectrum revealed the fine structure of the amide I band. The band at 1717 +/- 2 cm(-1) is due to the vibration of the COOH moieties from the protein side chains. Band components at 1682 +/- 2 cm(-1), 1655 +/- 2 cm(-1), and 1637 +/- 2 cm(-1) are assigned to the vibrations of the backbone C=O. These three bands belong to vibrations of three different populations of amide groups differing in the number of established H-bonds. The connectivity between the frequencies of various amide vibrations was determined by two-dimensional generalized correlation spectroscopy and spectral decomposition. About 7% of the whole exchangeable hydrogen atom population (NH, NH2, and OH groups from backbone and side chains) remains unexchanged, and these hydrogen atoms belong mainly to the NH groups, which are H-bonded to specific C=O groups. Moreover, this study concerns the approximately 10% of hydrogen atoms belonging to a particular HN...O=C population with a characteristic amide A frequency at 3290 cm(-1) and an amide I band at 1655 +/- 2 cm(-1), usually attributed to the alpha-helical structure that remains unexchanged. At higher temperature the exchange is more efficient. Upon heating, a further 4% of these NH groups are deuterated. The comparison of the exchange spectrum at higher temperature with the structural changes of the protein at the same temperature implies that the change in overall dynamics of the protein improve the level of exchange.

Kinetics and Thermodynamics of Hydrogen Atom Exchange Reactions in Sterically Hindered Hydroxylamine-Nitroxyl Radical Systems

Rate constants and activation energies were determined for the forward and reverse hydrogen exchange reactions in systems involving a sterically hindered hydroxylamine and a nitroxyl radical of dihydroquinoline, tetrahydroquinoline, diphenylamine, imidazoline, pyrrolidine, or piperidine. The equilibrium constants and heats of these reactions were determined, and the dissociation energies of the NO-H bonds in the hydroxylamines were estimated.

Surface‐enhanced Raman scattering (SERS) of random silver or gold particle arrays on aminodextran‐coated polystyrene beads

AbstractCitrate and/or chemically modified aminodextran (Amdex) SERS bands were detected from silver particle‐Amdex‐polystyrene (PS) beads either as singlets or small clusters, prepared by reaction between aqueous silver nitrate, Amdex‐coated PS beads, and aqueous sodium citrate solution. However, citrate on individual silver or gold particles of the same size was not detected with 633‐nm excitation. SERS of dyes such as rhodamine 6 G was readily observed on individual silver particles or on the gold beads. Three distinct modes of citrate/modified‐Amdex binding to the silver particle‐Amdex‐PS beads were characterized: (1) exchangeable citrate bound through one or more carboxylate groups—citrate could be washed away with H2O or D2O; displaced by successively greater concentrations of R6G; or squeezed out mechanically by applying the pressure of a second coverslip on top of a droplet of bead suspension; (2) nonexchangeable Amdex bound to silver via a primary alcoholate ion of oxidized, glutaraldehyde‐cross‐linked Amdex; and (3) nonexchangeable oxidized Amdex bound to silver via carboxylate group(s) but with H/D exchangeable alcohol hydrogen atom. That oxidation of the glucopyranose residues of Amdex on PS beads had taken place by Ag(I) as silver nitrate in aqueous solution to split out carboxylate groups was first identified from their characteristic vibrational modes in the SERS spectra of colloidal silver suspensions. Minimum amplification factors for (R6G on Ag)/(R6G on Au) with 633‐nm excitation were estimated as 1 × 1015/1 × 1011. An extra SERS enhancement of about 1000‐fold due to multiplicity of metal particles per bead was included. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Probing the Structure of the Infectious Amyloid Form of the Prion-forming Domain of HET-s Using High Resolution Hydrogen/Deuterium Exchange Monitored by Mass Spectrometry

The HET-s prion protein of Podospora anserina represents a valuable model system to study the structural basis of prion propagation. In this system, prion infectivity can be generated in vitro from a recombinant protein. We have previously identified the region of the HET-s protein involved in amyloid formation and prion propagation. Herein, we show that a recombinant peptide corresponding to the C-terminal prion-forming domain of HET-s (residues 218-289) displays infectivity. We used high resolution hydrogen/deuterium exchange analyzed by mass spectrometry to gain insight into the structural organization of this infectious amyloid form of the HET-s-(218-289) protein. Deuterium incorporation was analyzed by ion trap mass spectrometry for 76 peptides generated by pepsin proteolysis of HET-s-(218-289). By taking into account sequence overlaps in these peptides, a resolution ranging from 4-amino acids stretches to a single residue could be achieved. This approach allowed us to define highly protected regions alternating with more accessible segments along the HET-s-(218-289) sequence. The HET-s-(218-289) fibrils are thus likely to be organized as a succession of beta-sheet segments interrupted by short turns or short loops.

Fragmentation of Peptides in MALDI In-Source Decay Mediated by Hydrogen Radicals

In-source decay (ISD) in matrix-assisted laser desorption/ionization (MALDI) shares some similarities with the novel fragmentation technique electron capture dissociation (ECD). In both reactions, the otherwise strong N-C(alpha) bond is cleaved, forming fragment ions of the c and z types, while labile posttranslational modifications are preserved. Therefore, it is tempting to assume that ISD and ECD have some mechanistic aspects in common. Because electrons are present in the MALDI plume, we investigated the previously suggested possibility that ISD is a variation of ECD. However, experiments with peptides with only one site for efficient protonation revealed that ISD is not caused by electron capture. Instead, ICD seems to be induced by hydrogen atoms generated by a photochemical reaction of the matrix. We provide evidence for this reaction by hydrogen/deuterium exchange experiments with peptides containing a minimal number of exchangeable hydrogen atoms. The hydrogen atom model in ECD is indirectly supported by the proposed fragmentation mechanism for ISD, because our data suggest that hydrogen radicals can induce fragmentation by cleavage of the N-C(alpha) bond, independent from their origin.

Experimental Evidence of Differences in the Hydrogen-Bonded Structure of Concentrated Aqueous Solutions Involving Dl- and l-Alanine Molecules

Abstract Time-of-Flight (TOF) and steady state reactor neutron diffraction measurements have been carried out for aqueous 2.5 mol% Dl- and l-alanine solutions in order to investigate the differences in intermolecular hydrogen-bonded structure between solutions involving amino acid molecules with different optical activities. The observed difference functions, Δi(Q) and Δiinter(Q), between observed scattering cross sections for Dl- and l-alanine solutions with a D content of 96.1% exhibit the first peak located at Q = 2 Å−1 followed by oscillatory features extending up to the higher-Q region. The difference distribution function, Δg(r), obtained from the Fourier transform of Δi(Q), clearly indicates negative peaks at r = 2 and 2.5 Å and a positive one at r = 3.5 Å. Partial structure factors, aHH(Q), aXH(Q), and aXX(Q) (X: O, N, C, HM, and HM′, where HM and HM′ denote methyl and methine hydrogen atoms within the alanine molecule, respectively) for 2.5 mol% Dl-alanine solutions are successfully determined from combined analyses of intermolecular interference terms observed for solutions with 96.1, 66.0, and 35.9% exchangeable deuterium content. The nearest neighbor O···Hex and Hex···Hex (Hex: exchangeable hydrogen atom) distances are determined from the least squares fit of the observed partial structure factors, 1.90(1) and 2.48(1) Å, respectively. These values correspond to the positions of negative peaks observed in the present Δg(r) function. The least squares fitting analysis of the observed Δi(Q) revealed that the difference in the coordination number of the nearest neighbor O···Hex and Hex···Hex interactions between the Dl- and l-alanine solutions are −0.031(5) and −0.072(5), respectively. It is concluded that the intermolecular hydrogen bonds among solvent water molecules in the Dl-alanine solution are ca. 2% weaker than those in the l-alanine solution.

Charge exchange of hydrogen atoms on nuclei of multicharged ions

Charge exchange of hydrogen atoms on nuclei of multicharged ions

In vitro metabolism of ethoxidine by human CYP1A1 and rat microsomes: identification of metabolites by high-performance liquid chromatography combined with electrospray tandem mass spectrometry and accurate mass measurements by time-of-flight mass spectrometry.

Ethoxidine (N-methyl-12-ethoxy-2,3,8,9-tetramethoxybenzo[c]phenanthridinium methylsulfonate salt) is a synthetic 2-methoxy-12-ethoxy derivative of the natural alkaloid fagaronine. This new inhibitor of DNA-topoisomerase I is considered as a potential antitumor agent with higher in vitro activity than fagaronine. In order to further improve the efficiency of ethoxidine, its in vitro biotransformation by hepatic monooxygenases and the structures of its metabolites were investigated by high-performance liquid chromatography (HPLC) combined with electrospray ionization tandem mass spectrometry (ESI-MS/MS) and accurate mass measurement by time-of-flight mass spectrometry (TOFMS). When ethoxidine was incubated with BNF-treated rat liver microsomes or with cells expressing different recombinant human cytochrome P450, the same four ethoxidine metabolites (m(1)-m(4)) were detected and were formed exclusively by CYP1A1. The structures of these metabolites were assigned from ESI-MS/MS mass spectra and compared with those of ethoxidine derivatives. Accurate mass measurements of in-source ESI-TOFMS fragment ions exhibited successive neutral losses of C(2)H(4) and CO for ethoxidine and its metabolites. Whereas a 15 Da loss (methyl radical) was observed for the metabolites m(1)-m(4) containing a quaternary ammonium group, a 16 Da loss (methane) was observed for ethoxidine and could have resulted from the presence of two methoxy groups at adjacent positions (C-2 and C-3). The proposed oxidative modifications of ethoxidine were further confirmed by determination of the number of exchangeable hydrogen atoms and by the proposed elemental compositions of the metabolites based on accurate mass measurements by TOFMS. Two major metabolites resulted from O-demethylation of ethoxidine; one was tentatively identified as 12-ethoxyfagaronine (m(3)) and the second as an O-demethylated ethoxidine isomer (m(4)). Two polar metabolites were shown to be O-demethylated (m(1)) and hydroxylated (m(2)) derivatives of 12-ethoxyfagaronine. When 12-ethoxyfagaronine was incubated under the same conditions as ethoxidine, m(2) was formed, thus supporting the proposal that 12-ethoxyfagaronine is the primary oxidative product of ethoxidine.

Unraveling the chemical dynamics of bimolecular reactions of ground state boron atoms, B(2Pj), with acetylene, C2H2(X1Σg+)

The reaction dynamics of atomic boron, B(2P), with acetylene, C2H2(X 1Σg+), were investigated at two collision energies of 12.9 kJ mol−1 and 16.3 kJ mol−1 employing the crossed molecular beams technique. Only the atomic boron versus hydrogen atom exchange pathway was observed. Forward-convolution fitting of the laboratory data at m/z=36 (11BC2H+) shows that the reaction dynamics are indirect, proceed via addition of the boron atom to the carbon–carbon triple bond via a cyclic intermediate, and form the linear HBCC(X 1Σ+) molecule after an atomic hydrogen ejection. The formation of the HBCC(X 1Σ+) isomer under single collision conditions presents the first “clean” gas phase synthesis of this important organo-boron molecule and opens the door to a prospective spectroscopic investigation.

Molecular structure of Hf(BH4)4 investigated by quantum mechanical calculations and gas-phase electron diffraction.

The structure of the gaseous hafnium tetrakis(tetrahydroborate) molecule, Hf(BH4)4, has been investigated by detailed quantum mechanical calculations and by analysis of its gas electron-diffraction (GED) pattern. The ground-state geometry possesses T symmetry with all of the triply-bridged BH4 groups twisted equally about the Hf...B-H axes. Salient structural parameters (ra distances, r angles) deduced from the GED pattern by the SARACEN method were: r(Hf...B) 231.4(2), r(Hf-Hb) 221.5(7), r(B-Hb) 127.6(5), r(B-Ht) 121(1) pm, Hf...B-Hb 69.4(3), Hb-B-Hb 108.4(4), Hb-B-Ht 110.6(3), B...Hf...B-Hb 166(1) degrees. A notable feature is the large magnitude of the Hf...B and Hf-Hb anharmonicity parameters, attributed to the fluxional hydrogen atom exchange process. The properties are compared with those of related tetrahydroborates..

Ab Initio Comparative Study of the Structure and Properties of H2-Porphin and H2-Phthalocyanine. The Electronic Absorption Spectra

The structure and properties of free base porphin (PH2) and phthalocyanine (PcH2) are compared using B3LYP/6-31G(d,p) (5D) and UHF/6-31G(d,p) (5D) calculations. The two molecules show two isomers where the two central hydrogen atoms lie in trans and cis positions. The C2v cis-isomers lie 2840 cm-1 (PH2) and 3183 cm-1 (PcH2) over the most stable D2h trans-geometries. The tautomerization processes are hindered by transition states at 5269 cm-1 (PH2) and 5107 cm-1 (PcH2). The ionization potential and the electron affinity are calculated to be +6.40 and −1.11 eV for PH2 and +6.03 and −1.91 eV for PcH2, from the determination of the cation and anion energies and structures. Harmonic frequencies and electric and nonlinear optical properties are also reported. An interpretation of the electronic absorption spectra is also given for both molecules. For PH2, CASPT2 calculations have been achieved using the 6-31G(d,p) (5D) base set and different active spaces. For PcH2, the spectrum has been interpreted at the CASP...