Ab initio spectroscopy and thermochemistry of the platinum hydride ions, PtH+ and PtH.

Rovibrational levels of low-lying electronic states of the gas-phase, diatomic molecules, PtH+ and PtH-, are computed on potential-energy functions obtained by using a hybrid spin-orbit configuration-interaction procedure. PtH- has a well-separated Σ0++1 ground state, while the first two electronic states of PtH+ (Σ0++1 and 3Δ3) are nearly degenerate. Combining the experimental photoelectron (PE) spectra of PtH- with theoretical photodetachment spectroscopy leads to an improved value for the electron affinity of PtH, EA(PtH) = (1.617 ± 0.015) eV. When PtH- is a product of photodissociation of PtHCO2-, its PE spectrum is broad because of rotational excitation. Temperature-dependent thermodynamic functions and thermochemistry of dissociation are computed from the theoretical energy levels. Previously published energetic quantities for PtH+ and PtH- are revised. The ground 1Σ+ term of PtH+ is not well described using single-reference theory.

Ab initio spectroscopy and thermochemistry of diatomic platinum hydride, PtH.

Rovibrational levels of low-lying electronic states of the diatomic molecule PtH are computed using non-relativistic wavefunction methods and a relativistic core pseudopotential. Dynamical electron correlation is treated at the coupled-cluster with single and double excitations and a perturbative estimate of triple excitations level, with basis-set extrapolation. Spin-orbit coupling is treated by configuration interaction in a basis of multireference configuration interaction states. The results compare favorably with available experimental data, especially for low-lying electronic states. For the yet-unobserved first excited state, Ω = 1/2, we predict constants including Te = (2036 ± 300) cm-1 and ΔG1/2 = (2252.5 ± 8) cm-1. Temperature-dependent thermodynamic functions, and thermochemistry of dissociation, are computed from the spectroscopic data. The ideal-gas enthalpy of formation is ΔfH298.15o(PtH) = (449.1 ± 4.5) kJ mol-1 (uncertainties expanded by k = 2). The experimental data are reinterpreted, using a somewhat speculative procedure, to yield the bond length Re = (1.5199 ± 0.0006) Å.

Theoretical dissociation, ionization, and spin-orbit energetics of the diatomic platinum hydrides PtH, PtH+, and PtH.

Spin-orbit configuration interaction (SO-CI) and coupled-cluster [CCSDT(Q)] theoretical methods are combined to evaluate zero-temperature thermochemical properties of PtH, PtH+, and PtH-. We obtain vibrational zero-point energies and spin-orbit stabilization energies, which lead to predictions for observable quantities: ionization energy IE(PtH) = (9.44 ± 0.07) eV; electron affinity EA(PtH) = (1.65 ± 0.04) eV; and dissociation energies D0(Pt-H) = (329.6 ± 3.9) kJ mol-1, D0(Pt+-H) = (279.3 ± 5.7) kJ mol-1, and D0(Pt--H) = (285.0 ± 2.4) kJ mol-1 (uncertainty coverage factor = 2). Compared with available experiments, our value of D0(Pt+-H) is higher by (8 ± 8) kJ mol-1, EA(PtH) is higher by 0.15 eV, and D0(Pt-H) is lower than the upper bound by (2 ± 4) kJ mol-1.

Polarizability of atomic Pt, Pt+, and Pt.

Electrostatic properties are important for understanding and modeling many phenomena, such as the adsorption of a catalytic metal upon an oxide support. The charge transfer between the metal and the support can lead to positive or negative charges on the metal. Here, the static dipole polarizability is computed for atomic platinum in charge states 0, +1, and -1 in several low-lying electronic terms and levels. Core pseudopotentials are used along with coupled-cluster theory. The best results are estimates for the coupled-cluster CCSDTQ/q-aug-cc-pwCV∞Z-PP values for atomic terms, combined with compositional data from spin-orbit configuration interaction. The polarizability of the anion Pt- is especially challenging for the theory with wildly varying results from different coupled-cluster perturbative approximations such as CCSD(T). For atomic mercury (Hg), selected as a nearby experimental value, our polarizability volume is larger than experiment by 0.8 bohrs3 (or 0.12 × 10-30 m3). For the ground level of neutral platinum, Pt(3D3), we find α0 = (41.2 ± 1.1) bohrs3 or (6.10 ± 0.16) × 10-30 m3. A handful of density functional theory methods are tested and found generally within 10% of our best values.

Multireaction Approach to Quantum Thermochemistry.

Gas-phase enthalpies of formation are frequently obtained from electronic-structure calculations. For a compound containing elements that are not permanent gases, experimental data are needed to connect the ab initio energies with the elements in their standard states. Usually only one chemical reaction, such as atomization, is used to make this connection. Using a single reaction has the supposed advantage of producing a single value, but it remains challenging to estimate the associated uncertainty. Here, we propose using several chemical reactions instead of one. This yields several values for the target enthalpy of formation. The relative reliability of each reaction is estimated and used for weighted averaging of the corresponding enthalpies of formation. The resulting weighted mean is an estimate for the target quantity, and the standard deviation of the weighted mean is a reasonable estimate for the associated standard uncertainty. Two implementations are explored here and applied predictively to dihydrolevoglucosenone (cyrene), which has been proposed as a "green" polar aprotic solvent.

Ab Initio Computation of Energy Deposition During Electron Ionization of Molecules.

The dissociative ionization of molecules under electron impact forms the basis for analytical mass spectrometry of volatile compounds. It is also important in other situations, notably plasmas. Although qualitative theory for mass spectrometry was developed long ago, progress toward predictive theory has been slow. A major obstacle has been ignorance of the amount of energy deposited in the molecular ion, prior to its fragmentation. Here, we consider the Binary-Encounter Bethe (BEB) theory, which was originally developed for predicting total ionization cross sections. The energy deposition function constructed from BEB molecular-orbital cross sections compares well with the two comparable (e, 2e + ion) experimental measurements. When combined with experimental breakdown data from photoelectron-photoion-coincidence measurements, the BEB energy deposition function successfully reproduces library mass spectra for all but one of the six molecules studied here. This indicates that BEB molecular-orbital cross sections are physically meaningful and are useful for modeling the energy deposition during electron ionization of molecules.

N-Protonated Isomers and Coulombic Barriers to Dissociation of Doubly Protonated Ala8Arg.

Collision-induced dissociation (or tandem mass spectrometry, MS/MS) of a protonated peptide results in a spectrum of fragment ions that is useful for inferring amino acid sequence. This is now commonplace and a foundation of proteomics. The underlying chemical and physical processes are believed to be those familiar from physical organic chemistry and chemical kinetics. However, first-principles predictions remain intractable because of the conflicting necessities for high accuracy (to achieve qualitatively correct kinetics) and computational speed (to compensate for the high cost of reliable calculations on such large molecules). To make progress, shortcuts are needed. Inspired by the popular mobile proton model, we have previously proposed a simplified theoretical model in which the gas-phase fragmentation pattern of protonated peptides reflects the relative stabilities of N-protonated isomers, thus avoiding the need for transition-state information. For singly protonated Ala n (n = 3-11), the resulting predictions were in qualitative agreement with the results from low-energy MS/MS experiments. Here, the comparison is extended to a model tryptic peptide, doubly protonated Ala8Arg. This is of interest because doubly protonated tryptic peptides are the most important in proteomics. In comparison with experimental results, our model seriously overpredicts the degree of backbone fragmentation at N9. We offer an improved model that corrects this deficiency. The principal change is to include Coulombic barriers, which hinder the separation of the product cations from each other. Coulombic barriers may be equally important in MS/MS of all multiply charged peptide ions. Graphical Abstract ᅟ.

Partial Ionization Cross Sections of Organic Molecules.

Partial ionization cross sections are the absolute yields of specific ions from an electron-molecule collision. They are necessary for modeling plasmas and for determining the sensitivity of mass spectrometers, among other applications. One mass-spectrometric application is estimating the abundance of organic compounds on Mars, as sampled by the rover Curiosity. This is a report of semitheoretical data obtained for a collection of organic molecules identified as possible biomarkers in this exotic context.

Semi-empirical estimation of ion-specific cross sections in electron ionization of molecules.

Partial ionization cross sections are the absolute yields of specific ions from an electron-molecule collision. They are necessary for modeling plasmas and determining the sensitivity of mass spectrometers, among other applications. They can be predicted semi-empirically when experimental data are available for channel-specific oscillator strengths. However, such data are seldom available because they are obtained using specialized apparatus. Here, an alternative semi-empirical method is proposed that exploits experimental data obtained using ordinary mass spectrometers, as corrected for mass discrimination. Data are presented for an incident electron energy of 70 eV.

Thermochemistry of HO2 + HO2 → H2O4: Does HO2 Dimerization Affect Laboratory Studies?

Self-reaction is an important sink for the hydroperoxy radical (HO2) in the atmosphere. It has been suggested (Denis, P. A.; Ornellas, F. R. J. Phys. Chem. A, 2009, 113 (2), 499-506) that the minor product hydrogen tetroxide (HO4H) may act as a reservoir of HO2. Here, we compute the thermochemistry of HO2 self-reactions to determine if either HO4H or the cyclic hydrogen-bound dimer (HO2)2 can act as reservoirs. We computed electronic energies using coupled-cluster calculations in the complete basis set limit, CCSD(T)/CBS[45]//CCSD(T)/cc-pVTZ. Our model chemistry includes corrections for vibrational anharmonicity in the zero-point energy and vibrational partition functions, core-valence correlation, scalar relativistic effects, diagonal Born-Oppenheimer, spin-orbit splitting, and higher-order corrections. We compute the Gibbs energy of dimerization to be (-20.1 ± 1.6) kJ/mol at 298.15 K (2σ uncertainty), and (-32.3 ± 1.5) kJ/mol at 220 K. For atmospherically relevant [HO2] = 10(8) molecules per cm(3), our thermochemistry indicates that dimerization will be negligible, and thus H2O4 species are atmospherically unimportant. Under conditions used in laboratory experiments ([HO2] > 10(12) molecules per cm(3), 220 K), H2O4 formation may be significant. We compute two absorption spectra that could be used for laboratory detection of HO4H: the OH stretch overtone (near-IR) and electronic (UV) spectra.

High-resolution, vacuum-ultraviolet absorption spectrum of boron trifluoride.

In the course of investigations of thermal neutron detection based on mixtures of (10)BF3 with other gases, knowledge was required of the photoabsorption cross sections of (10)BF3 for wavelengths between 135 and 205 nm. Large discrepancies in the values reported in existing literature led to the absolute measurements reported in this communication. The measurements were made at the SURF III Synchrotron Ultraviolet Radiation Facility at the National Institute of Standards and Technology. The measured absorption cross sections vary from 10(-20) cm(2) at 135 nm to less than 10(-21) cm(2) in the region from 165 to 205 nm. Three previously unreported absorption features with resolvable structure were found in the regions 135-145 nm, 150-165 nm, and 190-205 nm. Quantum mechanical calculations, using the TD-B3LYP/aug-cc-pVDZ variant of time-dependent density functional theory implemented in Gaussian 09, suggest that the observed absorption features arise from symmetry-changing adiabatic transitions.

Facile Smiles-type rearrangement in radical cations of N-acyl arylsulfonamides and analogs.

RATIONALE: N-Alkylation of sulfonylbenzamides was reported recently to cause a dramatic and surprising change in electron ionization mass spectrometry (EIMS), leading to a closed-shell base peak. Only an incomplete, speculative mechanism was available at that time. The fragmentation mechanism is determined in the present work and set in the context of related compounds. METHODS: Candidate reaction mechanisms were evaluated theoretically using modest density-functional calculations. The fragmentation mechanism with the lowest barriers was identified and one of its implications tested successfully by experimental (18) O-isotopic substitution. RESULTS: The amide oxygen atom attacks the arylsulfonyl group at the ipso position (Smiles-type rearrangement), displacing a molecule of SO2 . The resulting carboximidate radical cation has a weak C-O bond that breaks easily. The incipient aryloxyl radical abstracts a proton from the amide nitrogen to form the dominant product ion, but if the molecule is N-alkylated this cannot occur. Instead, the neutral aryloxyl radical is lost and a closed-shell, N-alkyl nitrilium ion is the major product. CONCLUSIONS: The Smiles-type ion fragmentation mechanism is facile for the title compounds, despite the necessity for carbonyl oxygen to serve as a nucleophile. This rearrangement probably occurs in many of the mass spectra reported for structurally similar compounds, in which the nucleophile may be a thione, arylthio, imine, methylene, or methine moiety.

Aminoxyl (nitroxyl) radicals in the early decomposition of the nitramine RDX.

The explosive nitramine RDX (1,3,5-trinitrohexahydro-s-triazine) is thought to decompose largely by homolytic N-N bond cleavage, among other possible initiation reactions. Density-functional theory (DFT) calculations indicate that the resulting secondary aminyl (R2N·) radical can abstract an oxygen atom from NO2 or from a neighboring nitramine molecule, producing an aminoxyl (R2NO·) radical. Persistent aminoxyl radicals have been detected in electron-spin resonance (ESR) experiments and are consistent with autocatalytic "red oils" reported in the experimental literature. When the O-atom donor is a nitramine, a nitrosamine is formed along with the aminoxyl radical. Reactions of aminoxyl radicals can lead readily to the "oxy-s-triazine" product (as the s-triazine N-oxide) observed mass-spectrometrically by Behrens and co-workers. In addition to forming aminoxyl radicals, the initial aminyl radical can catalyze loss of HONO from RDX.

Anharmonic Vibrational Frequency Calculations Are Not Worthwhile for Small Basis Sets.

Anharmonic calculations using vibrational perturbation theory are known to provide near-spectroscopic accuracy when combined with high-level ab initio potential energy functions. However, performance with economical, popular electronic structure methods is less well characterized. We compare the accuracy of harmonic and anharmonic predictions from Hartree-Fock, second-order perturbation, and density functional theories combined with 6-31G(d) and 6-31+G(d,p) basis sets. As expected, anharmonic frequencies are closer than harmonic frequencies to experimental fundamentals. However, common practice is to correct harmonic predictions using multiplicative scaling. The surprising conclusion is that scaled anharmonic calculations are no more accurate than scaled harmonic calculations for the basis sets we used. The data used are from the Computational Chemistry Comparison and Benchmark Database (CCCBDB), maintained by the National Institute of Standards and Technology, which includes more than 3939 independent vibrations for 358 molecules.

Gas-phase energetics of thorium fluorides and their ions.

Gas-phase thermochemistry for neutral ThF(n) and cations ThF(n)(+) (n = 1-4) is obtained from large-basis CCSD(T) calculations, with a small-core pseudopotential on thorium. Electronic partition functions are computed with the help of relativistic MRCI calculations. Geometries, vibrational spectra, electronic fine structure, and ion appearance energies are tabulated. These results support the experimental results by Lau, Brittain, and Hildenbrand for the neutral species, except for ThF. The ion thermochemistry is presented here for the first time.

Tryptic y(++) fragment ion distributions are guided by Coulombic repulsion.

Ideal tryptic peptides contain only a single basic residue, located at the C-terminus. Collisional fragmentation of their doubly- or triply-protonated ions generates doubly-charged y(++) fragment ions with modest intensities. The size distribution of the y(++) fragments, when averaged over many spectra, corresponds closely to the expectations from charge-directed backbone cleavage and a Coulomb-Boltzmann distribution of mobile protons. This observation should be helpful in developing mechanistic models for y(++) formation.

N-Protonated isomers as gateways to peptide ion fragmentation.

According to the popular "mobile proton model" for peptide ion fragmentation in tandem mass spectrometry, peptide bond cleavage is typically preceded by intramolecular proton transfer from basic sites to an amide nitrogen in the backbone. If the intrinsic barrier to dissociation is the same for all backbone sites, the fragmentation propensity at each amide bond should reflect the stability of the corresponding N-protonated isomer. This hypothesis was tested by using ab initio and force-field computations on several polyalanines and Leu-enkephalin. The results agree acceptably with experimental reports, supporting the hypothesis. It was found that backbone N-protonation is most favorable near the C-terminus. The preference for C-terminal N-protonation, which is stronger for longer polyalanines, may be understood in terms of the well known "helix macrodipole" in the corresponding helical conformations. The opposite stability trend is found for peptides constrained to be linear, which is initially surprising but turns out to be consistent with the reversed direction of the macrodipole in the linear conformation.

Symmetry numbers for rigid, flexible, and fluxional molecules: theory and applications.

The use of molecular simulations and ab initio calculations to predict thermodynamic properties of molecules has become routine. Such methods rely upon an accurate representation of the molecular partition function or configurational integral, which in turn often includes a rotational symmetry number. However, the reason for including the symmetry number is unclear to many practitioners, and there is also a need for a general prescription for evaluating the symmetry numbers of flexible molecules, i.e., for molecules with thermally active internal degrees of freedom, such as internal rotors. Surprisingly, we have been unable to find any complete and convincing explanations of these important issues in textbooks or the journal literature. The present paper aims to explain why symmetry numbers are needed and how their values should be determined. Both classical and quantum approaches are provided.

Thermochemistry of ammonium nitrate, NH4NO3, in the gas phase.

Hildenbrand and co-workers have shown recently that the vapor above solid ammonium nitrate includes molecules of NH4NO3, not only NH3 and HNO3 as previously believed. Their measurements led to thermochemical values that imply an enthalpy change of D298 = 98 ± 9 kJ mol⁻¹ for the gas-phase dissociation of ammonium nitrate into NH3 and HNO3. Using updated spectroscopic information for the partition function leads to the revised value of D298 = 78 ± 21 kJ mol⁻¹ (accompanying paper in this journal, Hildenbrand, D. L., Lau, K. H., and Chandra, D. J. Phys. Chem. B 2010, DOI: 10.1021/jp105773q). In contrast, high-level ab initio calculations, detailed in the present report, predict a dissociation enthalpy half as large as the original result, 50 ± 3 kJ mol⁻¹. These are frozen-core CCSD(T) calculations extrapolated to the limiting basis set aug-cc-pV∞Z using an anharmonic vibrational partition function and a variational treatment of the NH3 rotor. The corresponding enthalpy of formation is Δ(f)H298°(NH4NO3,g) = −230.6 ± 3 kJ mol⁻¹. The origin of the disagreement with experiment remains unexplained.

Scaling Factors and Uncertainties for ab Initio Anharmonic Vibrational Frequencies.

To predict the vibrational spectra of molecules, ab initio calculations are often used to compute harmonic frequencies, which are usually scaled by empirical factors as an approximate correction for errors in the force constants and for anharmonic effects. Anharmonic computations of fundamental frequencies are becoming increasingly popular. We report scaling factors, along with their associated uncertainties, for anharmonic (second-order perturbation theory) predictions from HF, MP2, and B3LYP calculations using the 6-31G(d) and 6-31+G(d,p) basis sets. Different scaling factors are appropriate for low- and high-frequency vibrations. The method of analysis is based upon the Guide to the Expression of Uncertainty in Measurement, published by the International Organization for Standardization (ISO). The data used are from the Computational Chemistry Comparison and Benchmark Database (CCCBDB), maintained by the National Institute of Standards and Technology, which includes more than 3939 independent vibrations for 358 molecules.