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Accurate Pt-An bond dissociation enthalpies (BDEs) for PtAnOn+ (An = U, Np, Pu and n = 0-2) and the corresponding enthalpies for the Pt + OAnOn+ substitution reactions have been studied for the first time using an accurate composite coupled cluster approach. Analogous O-AnOn+ bond dissociation enthalpies are also presented. To make the study possible, new correlation consistent basis sets optimized using the all-electron third-order Douglas-Kroll-Hess (DKH3) scalar relativistic Hamiltonian are developed and reported for Pt and Au, with accompanying benchmark calculations of their atomic ionization potentials to demonstrate the effectiveness of the new basis sets. For the charged PtAnOn+ species (n = 1, 2), a low-spin state (LSS) for which the Pt-An σ bond is doubly occupied is studied together with a high-spin state (HSS) obtained by unpairing the σ bond orbital and placing one electron into the An 5f shell. The relative energies of the two spin states have been compared and qualitatively assessed via natural population and natural bond analyses. The enthalpies for the Pt substitution reactions, i.e., Pt + OAnOn+ → PtAnOn+ + O, are calculated to range from about 14-62 kcal/mol, and the Pt-AnOn+ bond dissociation enthalpies range from about 78-149 kcal/mol for the ground electronic states. For the PtAnO+ species, the LSSs were all predicted to be the ground state, whereas the PtAnO2+ molecules all favored the HSSs. The prediction for PtUO2+ is consistent with previous theoretical findings. The natural bond orbital analyses indicate a triple bond between An and O, with a double to quadruple bond between the An and Pt.

Thermochemical and spectroscopic properties for actinyl complexes involving UO22+/1+/0 and NpO22+/1+/0 with N2 and CO, together with the UO2-O2, UO2+-O2, and UO2+-NO complexes, have been studied for the first time using an accurate composite coupled cluster approach. Two general bonding motifs were investigated, end-on (η1) and side-on (η2) relative to the metal center of the actinyls. For end-on CO complexes, both C-coordinated (An-C) and O-coordinated (An-O) structures were examined, with the former always being lower in energy. All of the η1 complexes were calculated to be stable, with dissociation energies ranging from 2 to 36 kcal/mol, except for that of UO2+-O2 (the η1 orientation for UO2+-NO was not amenable to single reference coupled cluster). In agreement with a previous study, the η2 structure for UO2+-O2 was calculated to be relatively strongly bound, by 22.3 kcal/mol in this work. The closely related NO complex, however, had a calculated dissociation energy of just 4.0 kcal/mol. The binding energy of O2 to neutral UO2 in a η2 orientation was calculated to be very strong, 75.4 kcal/mol, and strongly resembled a UO2+(O2-) complex at equilibrium. The N-N and C-O bonds were found to be somewhat activated for all the side-on (η2) neutral An(IV) complexes, with stretching frequencies of N2 or CO being red-shifted by as much as 480 cm-1 with a 0.06 A bond length elongation. Dissociation energies for the η1 complexes are strongly correlated with the extent of electron transfer from ligand to actinyl. The nature of bonding in the actinyl complexes is examined using natural resonance theory (NRT). The correlation between bonding motif and small molecule activation is in agreement with experiments in condensed phases.

Accurate Pt-An bond dissociation enthalpies (BDEs) for PtAnO

We describe the formal algorithm and numerical applications of a novel convex quadratic programming (QP) strategy for performing the variational minimization that underlies natural resonance theory (NRT). The QP algorithm vastly improves the numerical efficiency, thoroughness, and accuracy of variational NRT description, which now allows uniform treatment of all reference structures at the high level of detail previously reserved only for leading "reference" structures, with little or no user guidance. We illustrate overall QPNRT search strategy, program I/O, and numerical results for a specific application to adenine, and we summarize more extended results for a data set of 338 species from throughout the organic, bioorganic, and inorganic domain. The improved QP-based implementation of NRT is a principal feature of the newly released NBO 7.0 program version. © 2019 Wiley Periodicals, Inc.

What is now called "resonance theory" has a long and conflicted history. We first sketch the early roots of resonance theory, its heritage of diverse physics and chemistry conceptions, and its subsequent rise to reigning chemical bonding paradigm of the mid-20th century. We then outline the alternative "natural" pathway to localized Lewis- and resonance-structural conceptions that was initiated in the 1950s, given semi-empirical formulation in the 1970s, recast in ab initio form in the 1980s, and successfully generalized to multi-structural "natural resonance theory" (NRT) form in the 1990s. Although earlier numerical applications were often frustrated by the ineptness of then-available numerical solvers, the NRT variational problem was recently shown to be amenable to highly efficient convex programming methods that yield provably optimal resonance weightings at a small fraction of previous computational costs. Such convexity-based algorithms now allow a full "reboot" of NRT methodology for tackling a broad range of chemical applications, including the many familiar resonance phenomena of organic and biochemistry as well as the still broader range of resonance attraction effects in the inorganic domain. We illustrate these advances for prototype chemical applications, including (i) stable near-equilibrium species, where resonance mixing typically provides only small corrections to a dominant Lewis-structural picture, (ii) reactive transition-state species, where strong resonance mixing of reactant and product bonding patterns is inherent, (iii) coordinative and related supramolecular interactions of the inorganic domain, where sub-integer resonance bond orders are the essential origin of intermolecular attraction, and (iv) exotic long-bonding and metallic delocalization phenomena, where no single "parent" Lewis-structural pattern gains pre-eminent weighting in the overall resonance hybrid.

We describe a practical algorithm for calculating NBO-based "resonance natural bond orbitals" (RNBOs) that can accurately describe the localized bond shifts of a reactive chemical process. Unlike conventional NBOs, the RNBOs bear no fixed relationship to a particular Lewis-structural bonding pattern but derive instead from the natural resonance theory (NRT)-based manifold of all bonding patterns that contribute significantly to resonance mixing (and associated multichannel reactivity) at a chosen point of the potential energy surface. The RNBOs typically retain familiar localized Lewis-structural character for stable near-equilibrium species, yet they freely adopt multicenter character as required to satisfy Pople's prerequisite that no allowed computational basis set should be inherently biased toward a particular nuclear arrangement or bonding pattern. A simple numerical application to intramolecular Claisen rearrangement demonstrates the computational and conceptual advantages of describing reactive bond-shifts with RNBOs rather than other conventional NBO- or MO-based expansion sets.

Gas phase actinyl cation–cation interactions (CCIs) were studied by an accurate composite coupled cluster thermochemical approach for the first time. A number of CCI dimers were constructed from the monomers UO22+, UO2+, NpO22+, NpO2+, PuO2+, and AmO2+. All CCI dimers studied were calculated to be thermodynamically unstable, with dissociation energies ranging from −60 to −90 kcal mol−1, but in many cases kinetic stability was indicated by calculated local minima with well depths as large as ∼15 kcal mol−1. Most of the dimers studied involved a T-shaped geometry, although one side-on dimer, (UO2+)2, was included since it was amenable to coupled cluster methods. In the T-shaped isomers the most stable dimers were calculated to arise when the oxo-group of an An(V) actinyl cation was oriented towards the metal center of an An(VI) actinyl cation. For both mixed-valent An(VI)/An(V) and mono-valent An(V) dimers, the stability as estimated from the depth of the calculated local minimum decreased in the donor series U(V) > Np(V) > Pu(V) > Am(V). These trends correlate well with experimental trends in condensed phase CCIs. A rationale for the bonding in CCIs was investigated by carrying out charge transfer analyses using the natural bond orbital (NBO) method. Augmenting the usual Lewis acid–base explanation, CCIs are the direct result of a competition between charge transfer stabilization, which can be as much as 0.11e or 30.7 kcal mol−1 at equilibrium, and Coulombic repulsive destabilization.

We show how a simple natural orbital-based modification of Karafiloglou’s poly-electron population analysis (PEPA) [generalizing conventional one-electron natural population analysis (NPA)] allows efficient numerical evaluation of Born probabilities (populations) for an unlimited variety of localized electronic excitation patterns. The computational advantages are illustrated by simple numerical applications to CH3NH2 and benzene in the framework of natural bond orbital (NBO)-based “NPEPA” keyword implementation in the forthcoming NBO 7.0 program.

We describe a general approach to computational investigation of reaction pathways with Natural Resonance Theory (NRT), allowing synthetic strategies to incorporate insights from modern quantum chemical descriptors. We show how computed NRT weightings and bond orders provide resonance-type visualization of the electronic sequence and logic of complex bond rearrangements along a reaction pathway. The NRT-based approach is illustrated for a model Claisen rearrangement reaction to demonstrate how specific substituents can be logically selected to alter the transition state barrier in a desired manner.

We reply to the recent comments of Prof. A. J. Stone in this Journal [J. Phys. Chem. A 2017, 121, 1531-1534].

We provide a didactic introduction to 2nd-quantized representation of complex electron-hole (e/h) excitation patterns in general configuration interaction wave functions built from orthonormal local orbitals of natural atomic orbital or natural bond orbital (NBO) type. Such local excitation patterns of chemically oriented basis functions can be related to the resonance concepts of valence bond theory, and quantitative evaluation of the associated excitation probabilities then provides an alternative assessment of resonance "weighting" that may be compared with those of NBO-based natural resonance theory. We illustrate the usefulness of anticommutation relations in deriving Pauli-compliant expressions for allowed excitation patterns, showing how the exciton-like promotions φλ → φν (creating an e/h excitation with h in φλ and e in φν ) impose strict constraints on associated e/h-probabilities (requiring, e.g., that the e-probability for an electron "to be" or "not to be" in φν must be rigorously linked to the complementary h-probabilities in φλ ). Specific examples are presented of the quantum Boolean logic for four or six local spin-orbitals, with emphasis on Natural Poly-Electron Population Analysis (NPEPA) evaluation of VB-type covalent and ionic contributions in conventional 2-center bonding, resonance weightings in 3-center hydrogen bonding, and general characteristics of higher-order m-center bonding motifs for m > 3. Numerical results are presented for methylamine, acrolein, and water dimer to illustrate current NPEPA implementation in the NBO program. © 2019 Wiley Periodicals, Inc.

Natural bond orbital (NBO) analysis is one of many available options for ‘translating’ computational solutions of Schrodinger’s wave equation into the familiar language of chemical bonding concepts. In this Review, we first address the title questions by describing characteristic features that distinguish NBO from alternative analysis methodologies (e.g. of QTAIM or EDA type) and answering criticisms that have been raised in specific chemical applications. We then address the general ‘usefulness’ of NBO analysis in the context of widely accepted philosophical criteria, including (i) broad consistency, both internally and with respect to known experimental data, (ii) multi-faceted predictive capacity, including numerical model predictions of specific properties, general correlative and statistical regression relationships, and ‘risky’ falsifiable predictions of previously unknown chemical phenomena, and (iii) general pedagogical value, promoting organisation, unification, and orderly rationalisation of che...

A project for students in an upper-level course in quantum or computational chemistry is described in which they are introduced to the concepts and applications of a high quality, ab initio treatment of the ground-state potential energy curve (PEC) for H2 and D2. Using a commercial computational chemistry application and a scientific spreadsheet, students learn how to obtain the energy of H2 (D2) employing configuration interaction with single and double excitations (CISD) and an extrapolation technique to estimate the energy in the complete basis set (CBS) limit. They construct the PEC from the CISD/CBS energies obtained at several internuclear separations. Students obtain the equilibrium dissociation energy and internuclear separation, as well as the rovibrational molecular constants, from the regression parameters of a sixth-order polynomial fit to the PEC. Results are in nearly exact agreement with experimental data.

A three-part project for students in physical chemistry, computational chemistry, or independent study is described in which they explore applications of valence bond (VB) and molecular orbital–configuration interaction (MO–CI) treatments of H2. Using a scientific spreadsheet, students construct potential-energy (PE) curves for several states of H2 from the kinetic and potential energies, calculated from closed-form analytical expressions of the ten unique integrals arising from the Born–Oppenheimer Hamiltonian. For this project students use hydrogen 1s basis functions that include a screening parameter. From the calculated PE curves, they find the dissociation energy, De, and equilibrium internuclear distance, Re. In part I students use the Heitler–London (VB) form of the wave function to obtain the PE curves. In part II they optimize the value of the screening parameter to improve the results, and in part III they explore the treatment of H2, using both the simple MO wave function and the application of CI, with and without screening parameter optimization, to obtain the PE curves. Students compare their De and Re results with the experimental values. A set of questions, exercises, and a sample spreadsheet are provided as Supporting Information.

We describe principal features of the newly released version, NBO 6.0, of the natural bond orbital analysis program, that provides novel “link-free” interactivity with host electronic structure systems, improved search algorithms and labeling conventions for a broader range of chemical species, and new analysis options that significantly extend the range of chemical applications. We sketch the motivation and implementation of program changes and describe newer analysis options with illustrative applications. © 2013 Wiley Periodicals, Inc.

An upper-level undergraduate or beginning graduate project is described in which students obtain the Lennard–Jones 6-12 potential parameters for Ne2 and Ar2 from ab initio calculations and use the results to express pairwise interactions between the atoms in clusters containing up to N = 60 atoms. The students use simulated annealing, or the genetic algorithm, to find the globally optimized binding energies and structures of the Ne and Ar clusters. They employ the liquid drop model to extrapolate the cluster binding energies to the solid state and compare the result with the experimental cohesive energy of the rare gas solid. A Windows-based application is provided that allows students to explore the energetic and structural properties of the rare gas clusters. Students encounter the “magic numbers”, for example, N = 13, 55, and others, associated with clusters that have higher-than-expected binding energies arising from enhanced nearest-neighbor interactions. They also estimate the solid density of each element from the size of the model cubic cluster (N = 14) that represents the face-centered unit cell.

Natural bond orbital (NBO) methods encompass a suite of algorithms that enable fundamental bonding concepts to be extracted from Hartree-Fock (HF), Density Functional Theory (DFT), and post-HF computations. NBO terminology and general mathematical formulations for atoms and polyatomic species are presented. NBO analyses of selected molecules that span the periodic table illustrate the deciphering of the molecular wavefunction in terms commonly understood by chemists: Lewis structures, charge, bond order, bond type, hybridization, resonance, donor–acceptor interactions, etc. Upcoming features in the NBO program address ongoing advances in ab initio computing technology and burgeoning demands of its user community by introducing major new methods, keywords, and electronic structure system/NBO communication enhancements. © 2011 John Wiley & Sons, Ltd.

We present experimental infrared spectra and theoretical electronic structure results for the geometry, anharmonic vibrational frequencies, and accurate estimates of the magnitude and the origin of the ring-puckering barrier in C4F8. High-resolution (0.0015 cm-1) spectra of the nu12 and nu13 parallel bands of perfluorocyclobutane (c-C4F8) were recorded for the first time by expanding a 10% c-C4F8 in helium mixture in a supersonic jet. Both bands are observed to be rotationally resolved in a jet with a rotational temperature of 15 K. The nu12 mode has b2 symmetry under D2d that correlates to a2u symmetry under D4h and consequently has +/--- +/- ring-puckering selection rules. A rigid rotor fit of the nu12 band yields the origin at 1292.56031(2) cm-1 with B' = 0.0354137(3) cm-1 and B' ' = 0.0354363(3) cm-1. The nu13 mode is of b2 symmetry under D2d that correlates to b2g under D4h, and in this case, the ring-puckering selection rules are +/--- -/+ . Rotational transitions from the ground and first excited torsional states will be separated by the torsional splitting in the ground and excited vibrational states, and indeed, we observe a splitting of each transition into strong and weak intensity components with a separation of approximately 0.0018 cm-1. The strong and weak sets of transitions were fit separately again using a rigid rotor model to give nu13(strong) = 1240.34858(4) cm-1, B' = 0.0354192(7) cm-1, and B' ' = 0.0354355(7) cm-1 and nu13(weak) = 1240.34674(5) cm-1, B' = 0.0354188(9) cm-1, and B' ' = 0.0354360(7) cm-1. High-level electronic structure calculations at the MP2 and CCSD(T) levels of theory with the family of correlation consistent basis sets of quadruple-zeta quality, developed by Dunning and co-workers, yield best estimates for the vibrationally averaged structural parameters r(C-C) = 1.568 A, r(C-F)alpha = 1.340 A, r(C-F)beta = 1.329 A, alpha(F-C-F) = 110.3 degrees , thetaz(C-C-C) = 89.1 degrees , and delta(C-C-C-C) = 14.6 degrees and rotational constants of A = B = 0.03543 cm-1 and C = 0.02898 cm-1, the latter within 0.00002 cm-1 from the experimentally determined values. Anharmonic vibrational frequencies computed using higher energy derivatives at the MP2 level of theory are all within27 cm-1 (in most cases5 cm-1) from the experimentally measured fundamentals. Our best estimate for the ring-puckering barrier at the CCSD(T)/CBS (complete basis set) limit is 132 cm-1. Analysis of the C4F8 electron density suggests that the puckering barrier arises principally from the sigmaCC--sigmaCF hyperconjugative interactions that are more strongly stabilizing in the puckered than in the planar form. These interactions are, however, somewhat weaker in C4F8 than in C4H8, a fact that is consistent with the smaller barrier in the former (132 cm-1) with respect to the latter (498 cm-1).

Natural energy decomposition analysis (NEDA) is a method for partitioning molecular interaction energies into physically meaningful components, including electrical interaction, charge transfer, and core repulsions. The method is a numerically stable procedure that was originally developed for analyzing Hartree-Fock (HF) wave functions based on the localized orbital description of natural bond orbital analysis. In this work, we extend NEDA to treat charge densities from density functional theory (DFT) calculations, replacing the intermolecular exchange (EX) component of the HF analysis with an exchange-correlation (XC) component. DFT/NEDA is applied to hydrogen bonding interactions and cooperative effects in water clusters. Electrical interactions and charge transfer contribute importantly to hydrogen bonding. Comparison of HF and DFT results reveals that the exchange and correlation effects of DFT slightly enhance the extent of charge transfer and core repulsions in the water clusters. Cooperative stabilization of the cyclic water trimer and tetramer is considered by performing a many-body expansion of the interaction energy. Natural energy decomposition analysis of this expansion suggests that charge transfer is the leading source of cooperative stabilization. Polarization effects have only marginal influence on cooperativity.

The influence of resonance on the acidities of dimethyl sulfide (DMS), dimethyl sulfoxide (DMSO), and dimethyl sulfone (DMSO2) and their group 16 congeners (DMXO(n) for X = Se, Te, Po and n = 0-2) is examined using ab initio methods and the natural bond orbital (NBO) and natural resonance theory (NRT) analyses. Gas-phase acidities are evaluated using B3LYP-optimized geometries with coupled cluster energies and complete basis set extrapolation. The acidity of the DMSO(n) molecules increases with increasing coordination of the central S atom. Acidity also tends to increase when the central atom is substituted by a heavier group 16 atom. NRT analysis reveals significant resonance delocalization in the DMXO(n) molecules and their anions. On deprotonation, the DMXO(n) molecules undergo structural changes that are consistent with changes in the resonance character of the calculated charge densities. However, resonance cannot account for the trends in the deprotonation energies. Whereas the DMX- anions are more strongly resonance stabilized than their parent molecules DMX, the DMXO2(-) anions and DMXO2 molecules are nearly equally resonance stabilized. Thus, there appears to be no extra stabilization of DMXO2(-) compared to that of DMX- that would account for the enhanced acidity of DMXO2 relative to DMX.

Potential energy surfaces are evaluated for H atom and H2 elimination in the gas phase reaction of a Y atom with acetylene, C2H2. Coupled-cluster calculations are performed with extrapolations to the complete basis set limit and zero point energy, core correlation, and spin−orbit corrections. The resulting surfaces reveal that the lowest energy reaction channel leads to H2 elimination, consistent with the YC2 + H2 products observed in crossed molecular beam experiments. This reaction proceeds in three steps: (i) YC2H2 adduct formation, (ii) C−H insertion, and (iii) 1,3-elimination of H2. A higher energy reaction channel leads from the C−H insertion intermediate to the H atom elimination products YC2H + H. Our calculations predict product asymptotes of −7.1 kcal/mol for YC2 + H2 and 15.0 kcal/mol for YC2H + H, energies that differ considerably from those (−18.5 ± 4.3 and 21.5 ± 2.0 kcal/mol, respectively) determined in the beam experiments. Natural bond orbital methods are used to determine how the metal ...

Intrinsic reaction coordinate (IRC) calculations of the torsional potentials of C2H6, CH3CD3, and C2D6 have been carried out at the MP2/6-31++G** level. The C2H6 potential was corrected at the coupled-cluster single double (triple) [CCSD(T)] level with extrapolation to the complete basis set limit (CBS). For CH3CD3 and C2D6, the MP2 potentials were scaled by 0.862 to approximate CCSD(T)/CBS results. The IRC potential for the D3h→D3d relaxation in C2H6 was fit to a two-term Fourier series containing V3 and V6 coefficients for which the barrier height, V3, was set to the CCSD(T)/CBS value (941 cm−1), and V6 was optimized at 6.7 cm−1. Sixfold torsional potentials were constructed from the CCSD(T)/CBS profiles and the resulting eigenvalues were used to calculate the Δn(ν4)=2 transitions, which are compared with experimental assignments. Comparisons are also made with observed IR transitions. Our best estimate of the rotational barrier is 941 cm−1. This value, as well as other high-level ab initio results, is ...

Reaction pathways are identified for Na-, Al-, and Y-induced acetylene (HCCH)−vinylidene (CCH2) rearrangements in the gas phase. Density functional and coupled cluster calculations are performed with basis set extrapolations. The rearrangement barriers decrease from 44.0 kcal/mol in the metal-free reaction to 41.3 (Na), 19.1 (Al), and 16.1 (Y) kcal/mol in the metal-induced reactions. This decrease results from the strengthening of the M−C bonds (Na−C < Al−C < Y−C) in the M(HCCH) and M(CCH2) complexes. Natural bond orbital analysis reveals the metallacyclopropene and metallaallene character of several of the M(HCCH) and M(CCH2) complexes. In addition, analysis of the transition states provides a detailed picture of the redistribution of bonding and nonbonding electrons along the reaction pathway. The metal-free and Na-induced rearrangements proceed via 1,2-hydride shifts, whereas the Al- and Y-induced reactions proceed via 1,2-hydrogen shifts. The latter reactions involve homolytic bond cleavage and format...

The geometry, binding energies, and isotropic hyperfine coupling constants of Gd(III)−ligand interactions in the Gd3+(H2O) and Gd3+(NH3) complexes have been determined by electronic structure metho...

Mass-weighted intrinsic reaction coordinate calculations were carried out for the inversion/bending motion in the ground and lowest excited singlet states of NH 3 and ND 3 . Vibrational eigenstates were obtained from these IRC potentials directly, obviating the need to make assumptions about the coordinate dependence of the reduced mass. The 0 ± inversion splittings for the X state of NH 3 and ND 3 are found to be 1.17 and 0.89 cm −1 , respectively, at the MP4(SDQ)/aug-cc-pVTZ level of theory. CASPT2-refined IRC scans of both the relaxed and vertical A -state surfaces were carried out. The A – X absorption spectra of NH 3 and ND 3 were modeled using the respective Boltzmann-weighted vibrational electronic transition matrix elements.

The ground- and lowest excited-state properties of piperidine vapor are explored with respect to understanding its absorption and fluorescence properties. A ground-state intrinsic reaction coordinate (IRC) calculation was used to model the conformational potential energy surface connecting the equatorial and axial conformers. At the MP2/6-311++G** level of theory, the equatorial conformer is more stable by 310 cm-1 than the axial conformer, and the inversion barrier height is 2033 cm-1. Two transitions in the UV, with origins of 38 707 and 44 070 cm-1 are assigned. The S1 ← S0 transition (fobs ∼ 3.2 × 10-3) is Rydberg in nature, with considerable involvement of all the ring heavy atoms. A vibrational analysis of this transition shows a main progression in 640 cm-1, which is assigned as the N−H out-of-plane bending motion. The CIS-calculated equilibrium geometry of the S1 state indicates considerable distortion of the N atom relative to the Cα atoms. The one-dimensional absorption spectrum is modeled on th...

Structures and incremental binding enthalpies were determined for the M+(H2O)n ionic clusters, M=Cu, Ag, Au; n=1–4 (5 for Cu) using correlated ab initio electronic structure methods. The effects of basis set expansion and high-level correlation recovery were found to be significant, in contrast to alkali and alkaline earth cation/water complexes, where correlation of the d electrons is unimportant. The use of a systematic sequence of one-particle basis sets permitted binding enthalpies in the complete basis set limit to be estimated. Overall, the best theoretical binding enthalpies compared favorably with the available experimental data for copper and silver. No experimental data is available for gold/water clusters. The largest deviation was noted for Ag+(H2O)2, where theory predicts an incremental binding enthalpy of 28 kcal/mol and experiment measures ∼25 kcal/mol. However, the uncertainty associated with one of the two experimental values is quite large (±3 kcal/mol) and almost encompasses the theoretical result. Results were also obtained with the more cost-effective 6-31+G* basis set and calibrated against the estimated complete basis set limits.

Hartree−Fock and second-order perturbation theory methods were used to determine structures and binding enthalpies of complexes formed from a single alkali metal cation (Li+ through Cs+) and one or two 1,2-dimethoxyethane ligands or 12-crown-4. These calculations employed multiple basis sets in order to determine the sensitivity of the results to the completeness in the one-particle basis. The results are compared with recently reported collision-induced dissociation experimental findings. In general, good agreement was found between the experimental and theoretical bond dissociation enthalpies, although for the heavier cations discrepancies of as much as 14 kcal/mol or more were uncovered. Possible reasons for these anomalies are discussed.

The influence of resonance on the structure and rotation barrier of formamide and its S, Se, and Te replacements analogues is examined using the natural bond orbital methods. Calculations are performed at the RHF, B3LYP, and MP2 levels of theory with 6-31+G* basis sets and effective core potentials. At the MP2 level, the rotation barriers increase with the increasing size of the chalcogen, from 17.2 kcal mol-1 for formamide to 21.0 kcal mol-1 for telluroformamide. Natural population analysis and natural resonance theory (NRT) reveal shifts in the charge density that are consistent with the strong resonance stabilization of the equilibrium, planar geometries. NRT provides a simple, quantitative description of the amides as a resonance hybrid consisting primarily of two contributing structures, the parent Lewis structure and a secondary dipolar form. Amide resonance effects strengthen from formamide to telluroformamide as the weight of the dipolar form increases. Polarizability appears to contribute importa...

The structures and binding enthalpies of a single alkali metal cation complexed with up through four dimethyl ether (DME) ligands were obtained with Hartree-Fock wave functions and second-order perturbation theory, with consideration of core/valence correlation and relativistic effects. The basis sets used in this study included diffuse functions on oxygen, in order to minimize undesirable basis set superposition error, and polarization functions on all non-hydrogen atoms. The observed trends in complex formation energy along the sequence of cations are discussed and compared to the available experimental data obtained from collision-induced dissociation measurements. Minimum energy M + (DME)2 geometries are predicted to be linear for the light metal complexes and bent for the heavy metal complexes.

Computations are reported at several theoretical levels for the exchange reaction of group 3 and 4 metal hydrides with hydrogen. At the CISD+Q level weak complexes are formed with an association energy of 0.1−3.0 kcal mol-1. The transition states for the exchange with group 3 transition metals have C2v symmetry and an energy of 8−10 kcal mol-1 relative to the reactants. This barrier is lower than that for exchange of alkali-metal hydrides with hydrogen (15.7−21.7 kcal mol-1) and much lower than for exchange of group 4 transition-metal hydrides (31.5−45.9 kcal mol-1). The transition states of alkali-metal hydrides approximate ion pairs of the alkali-metal cations with H3- ion, whereas the transition-metal cases involve bonding interactions with empty d orbitals.

The collision diameters of a series of 34 atoms and both nonpolar and polar molecules were obtained from ab initio calculations. Molecular volumes, VM, were determined from a Monte Carlo integration of the electron density distribution of the optimized HF/6-31G∗ structures. The isotropic collision diameter, dvol, defined as ( 6V M π ) 1 3 was compared in all cases with the respective Lennard-Jones potential minima positions, σm. An excellent linear correlation between dvol and σm for the 34 species was found, with slope and intercept of 1.025 and −0.07 A, respectively. It is thus suggested that dvol may be used directly as an estimate of the isotropic collision diameter of a species. dvol values for several atoms and radicals, as well as for the electronically excited states of NH3 and acetone, are included. The results show that dvol values for the Rydberg excited states are larger than those of the respective ground states.

We present an ab initio, quantum mechanical study of 18-crown-6 (18c6) and its interaction with the alkaline earth dications Mg2+, Ca2+, Sr2+, Ba2+, and Ru2+. Geometries, binding energies, and binding enthalpies are evaluated at the restricted Hartree−Fock (RHF) and second-order Moller−Plesset perturbation (MP2) levels of theory using the 6-31+G* basis set and relativistic effective core potentials. The affinity of 18c6 for the dications is considerable, ranging from 150−300 kcal mol-1. The cation−18c6 interaction arises principally from the electrostatic interaction of the dication with the nucleophilic ether backbone and from the polarization of the crown ether by the electric field of the dication. Whereas Ba2+ selectivity is observed for 18c6 in aqueous environments, our calculations clearly show that the crown ether in fact binds Mg2+ most strongly in gas phase. Thus, solvation effects appear to strongly influence cation selectivity. Indeed, Ba2+ selectivity is recovered when we consider the competit...