Prediction Of Bond Dissociation Energies Research Articles

Predicting bond dissociation energies of cyclic hypervalent halogen reagents using DFT calculations and graph attention network model.

Although hypervalent iodine(III) reagents have become staples in organic chemistry, the exploration of their isoelectronic counterparts, namely hypervalent bromine(III) and chlorine(III) reagents, has been relatively limited, partly due to challenges in synthesizing and stabilizing these compounds. In this study, we conduct a thorough examination of both homolytic and heterolytic bond dissociation energies (BDEs) critical for assessing the chemical stability and functional group transfer capability of cyclic hypervalent halogen compounds using density functional theory (DFT) analysis. A moderate linear correlation was observed between the homolytic BDEs across different halogen centers, while a strong linear correlation was noted among the heterolytic BDEs across these centers. Furthermore, we developed a predictive model for both homolytic and heterolytic BDEs of cyclic hypervalent halogen compounds using machine learning algorithms. The results of this study could aid in estimating the chemical stability and functional group transfer capabilities of hypervalent bromine(III) and chlorine(III) reagents, thereby facilitating their development.

High-level ab initio composites: thermochemical bond dissociation energies for vanadium species

Bond dissociation energies (BDE) are among the most sought-after properties in chemistry. Accurate prediction of BDE’s for transition metal species can be daunting for both experiment and computation. Experimental data has been limited and often has large error bars, making the identification of suitable computational methods difficult. Recent advancements in the experimental determination of BDE’s with techniques such as Velocity Map Imaging and 2 Photon Ionisation now provide useful gauges for computational strategies and new methodologies. The vanadium diatomics have been challenging for computational chemistry methods, and, thus, a new experimental gauge enables methods to be reevaluated and developed for these species. Herein, the super-correlation consistent Composite (s-ccCA), a thermochemical scheme centered around CCSD(T)/complete basis set (CBS) limit computations with additional contributions that account for scalar-relativistic effects, and coupled cluster contributions up to quintuple excitations has been considered. The agreement between the s-ccCA scheme and by recent experiments is excellent, demonstrating the utility of the new approach in addressing challenging systems, even those of multireference nature. The longstanding correlation consistent composite approach (ccCA) is also reevaluated for the vanadium species and find that the mean absolute deviation (MAD) is greatly reduced compared to previously used experimental values.

BonDNet: a graph neural network for the prediction of bond dissociation energies for charged molecules.

A broad collection of technologies, including e.g. drug metabolism, biofuel combustion, photochemical decontamination of water, and interfacial passivation in energy production/storage systems rely on chemical processes that involve bond-breaking molecular reactions. In this context, a fundamental thermodynamic property of interest is the bond dissociation energy (BDE) which measures the strength of a chemical bond. Fast and accurate prediction of BDEs for arbitrary molecules would lay the groundwork for data-driven projections of complex reaction cascades and hence a deeper understanding of these critical chemical processes and, ultimately, how to reverse design them. In this paper, we propose a chemically inspired graph neural network machine learning model, BonDNet, for the rapid and accurate prediction of BDEs. BonDNet maps the difference between the molecular representations of the reactants and products to the reaction BDE. Because of the use of this difference representation and the introduction of global features, including molecular charge, it is the first machine learning model capable of predicting both homolytic and heterolytic BDEs for molecules of any charge. To test the model, we have constructed a dataset of both homolytic and heterolytic BDEs for neutral and charged (−1 and +1) molecules. BonDNet achieves a mean absolute error (MAE) of 0.022 eV for unseen test data, significantly below chemical accuracy (0.043 eV). Besides the ability to handle complex bond dissociation reactions that no previous model could consider, BonDNet distinguishes itself even in only predicting homolytic BDEs for neutral molecules; it achieves an MAE of 0.020 eV on the PubChem BDE dataset, a 20% improvement over the previous best performing model. We gain additional insight into the model's predictions by analyzing the patterns in the features representing the molecules and the bond dissociation reactions, which are qualitatively consistent with chemical rules and intuition. BonDNet is just one application of our general approach to representing and learning chemical reactivity, and it could be easily extended to the prediction of other reaction properties in the future.

Predicting Bond Dissociation Energies and Bond Lengths of Coordinatively Unsaturated Vanadium-Ligand Bonds.

Understanding the electronic structure of coordinatively unsaturated transition-metal compounds and predicting their physical properties are of great importance for catalyst design. Bond dissociation energy De and bond length re are two of the fundamental quantities for which good predictions are important for a successful design strategy. In the present work, recent experimentally measured bond energies and bond lengths of VX diatomic molecules (X = C, N, S) are used as a gauge to consider the utility of a number of electronic structure methods. Single-reference methods are one focus because of their efficiency and utility in practical calculations, and multireference configuration interaction (MRCISD) methods and a composite coupled cluster (CCC) method are a second focus because of their potential high accuracy. The comparison is especially challenging because of the large multireference M diagnostics of these molecules, in the range 0.15-0.19. For the single-reference methods, Kohn-Sham density functional theory (KS-DFT) has been tested with a variety of approximate exchange-correlation functionals. Of these, MOHLYP provides the bond dissociation energies in best agreement with experiments, and BLYP provides the bond lengths that are in best agreement with experiments; but by requiring good performance for both the De and re of the vanadium compounds, MOHLYP, MN12-L, MGGA_MS1, MGGA_MS0, O3LYP, and M06-L are the most highly recommended functionals. The CCC calculations include up to connected pentuple excitations for the valence electrons and up to connected quadruple excitations for the core-valence terms; this results in highly accurate dissociation energies and good bond lengths. Averaged over the three molecules, the mean unsigned deviation of CCC bond energies from experimental ones is only 0.4 kcal/mol, demonstrating excellent convergence of theory and experiments.

Structural and thermochemical studies of pyrrolidine borane and piperidine borane by gas electron diffraction and quantum chemical calculations

The gaseous structures, thermochemical properties and dehydrogenation reaction energy profiles of the borane complexes of pyrrolidine and piperidine have been investigated using gas electron diffraction (GED) and state-of-the-art computational methods. These complexes are of interest because of their potential as hydrogen storage materials for future onboard transport applications. A comparative structural and thermochemical analysis revealed structures with a slight difference in the essential B–N bond length, with the piperidine borane having a longer bond even though it has a stronger B–N bond according to predicted bond dissociation energies, a trend common with amine boranes. To identify the most favourable dehydrogenation pathway, BH3-catalysed and BH3-uncatalysed dehydrogenation channels have been explored, where the former has been shown to be the favourable process for both complexes. The energy requirements for the hydrogen release reactions are expected to be minimal as evidenced from the calculated dehydrogenation reaction energies, implying their suitability for onboard chemical hydrogen storage.

DFT benchmark study of the O–O bond dissociation energy in peroxides validated with high-level ab initio calculations

Peroxides play a central role in many chemical and biological processes such as the Fenton reaction. The relevance of these compounds lies in the low stability of the O–O bond which upon dissociation results in radical species able to initiate various chemical or biological processes. In this work, a set of 64 DFT functional-basis set combinations has been validated in terms of their capability to describe bond dissociation energies (BDE) for the O–O bond in a database of 14 ROOH peroxides for which experimental values of BDE are available. Moreover, the electronic contributions to the BDE were obtained for four of the peroxides and the anion $$\hbox {H}_2\hbox {O}_2^-$$ at the CBS limit at CCSD(T) level with Dunning’s basis sets up to triple- $$\zeta$$ quality providing a reference value for the hydrogen peroxide anion as a model. Almost all the functionals considered here yielded mean absolute deviations around $$5.0\hbox { kcal mol}^{-1}$$ . The smallest values were observed for the $$\omega$$ B97 family and the Minnesota M11 functional with a marked basis set dependence. Despite the mean deviation, order relations among BDE experimental values of peroxides were also considered. The $$\omega$$ B97 family was able to reproduce the relations correctly whereas other functionals presented a marked dependence on the chemical nature of the R group. Interestingly, M11 functional did not show a very good agreement with the established order despite its good performance in the mean error. The obtained results support the use of similar validation strategies for proper prediction of BDE or other molecular properties by DFT methods in subsequent related studies.

Accurate Prediction of Bond Dissociation Energies and Barrier Heights for High-Energy Caged Nitro and Nitroamino Compounds Using a Coupled Cluster Theory.

Highly accurate theoretical values of bond energies and activation barriers of primary decomposition reactions are crucial for reliable predictions of thermal decomposition and detonation-related phenomena of energetic materials (EM). However, due to the prohibitive computational cost, high-level ab initio calculations had been impractical for a large number of important EMs, including, e.g., hexanitrohexaazaisowurtzitane (CL-20). In the present work, we obtained accurate bond dissociation energies and the activation barriers for primary decomposition reactions for a series of novel promising caged polynitroamino and polynitro EMs, viz., CL-20, TEX, octanitrocubane (ONC), and hexanitro derivatives of adamantane, using the recently proposed domain-localized pair natural orbitals (DLPNO) modifications of coupled cluster techniques. DLPNO-CCSD(T) allows for routine quadruple-ζ basis set quality coupled cluster calculations for the species comprised of ∼30 non-H atoms. The benchmarks on a number of simpler congeners of CL-20 and ONC revealed that the DLPNO approach does not deteriorate the quality of the quadruple-ζ coupled cluster procedure. With the aid of this technique, the full set of gas-phase primary decomposition reactions for all 9 conformers of CL-20 was considered. For all species studied, C-NO2 or N-NO2 radical decomposition channels dominate over molecular counterparts. The best theoretical results reported in the literature so far, viz., density functional theory energies of nitro group radical elimination in CL-20 and ONC, underestimate the value by ∼10 kcal mol-1. We also present reliable and accurate gas-phase formation enthalpies for CL-20, ONC, and related species. In a more general sense, these results offer a new level of predictive computational kinetics for polynitro-caged energetic materials.

Accurate prediction of bond dissociation energies of large n-alkanes using ONIOM-CCSD(T)/CBS methods

Accurate determination of the bond dissociation energies (BDEs) of large alkanes is desirable but practically impossible due to the expensive cost of high-level ab initio methods. We developed a two-layer ONIOM-CCSD(T)/CBS method which treats the high layer with CCSD(T) method and the low layer with DFT method, respectively. The accuracy of this method was validated by comparing the calculated BDEs of n-hexane with that obtained at the CCSD(T)-F12b/aug-cc-pVTZ level of theory. On this basis, the C-C BDEs of C6-C20 n-alkanes were calculated systematically using the ONIOM [CCSD(T)/CBS(D-T):M06-2x/6-311++G(d,p)] method, showing a good agreement with the data available in the literature.

Carbones as Ligands in Novel Main-Group Compounds E[C(NHC)2 ]2 (E=Be, B+ , C2+ , N3+ , Mg, Al+ , Si2+ , P3+ ): A Theoretical Study.

Quantum chemical calculations of the main-group compounds E[C(NHCMe )2 ]2 (E=Be, B+ , C2+ , N3+ , Mg, Al+ , Si2+ , P3+ ) have been carried out using density functional theory at the BP86/def2-TZVPP and BP86-D3(BJ)/def2-TZVPP levels of theory. The geometry optimization at BP86/def2-TZVPP gives equilibrium structures with two-coordinated species E and bending angles C-E-C between 152.5° (E=Be) and 110.5° (E=Al). Inclusion of dispersion forces at BP86-D3(BJ)/def2-TZVPP yields a three-coordinated beryllium compound Be[C(NHCMe )2 ]2 as the only energy minimum form. Three-coordinated isomers are found besides the two-coordinated energy minima for the boron and carbon cations B[C(NHCMe )2 ]2+ and C[C(NHCMe )2 ]22+ . The three-coordinated form of the boron compound is energetically lower lying than the two-coordinated form, while the opposite trend is calculated for the carbon species. The theoretically predicted bond dissociation energies suggest that all compounds are viable species for experimental studies. The X-ray structure of the benzoannelated homologue of P[C(NHCMe )2 ]23+ that was recently reported by Dordevic et al. agrees quite well with the calculated geometry of the molecule. A detailed bonding analysis using charge and energy decomposition methods shows that the two-coordinated neutral compounds Be[C(NHCMe )2 ]2 and Mg[C(NHCMe )2 ]2 possess strongly positively charged atoms Be and Mg. The carbodicarbene groups C(NHCMe )2 serve as acceptor ligands in the compounds and may be sketched with dative bonds (NHCMe )2 C←E→C(NHCMe )2 (E=Be, Mg). Dative bonds in which the carbones C(NHCMe )2 are donor ligands are suggested for the cations (NHCMe )2 C→E←C(NHCMe )2 (E=B+ , Al+ ). The dications and trications possess electron-sharing bonds in which the bonding situation is best described with the formula [(NHCMe )2 C]+ -E-[C(NHCMe )2 ]+ (E=C, Si, N+ , P+ ).

Predicting Bond Dissociation Energies of Transition-Metal Compounds by Multiconfiguration Pair-Density Functional Theory and Second-Order Perturbation Theory Based on Correlated Participating Orbitals and Separated Pairs.

We study the performance of multiconfiguration pair-density functional theory (MC-PDFT) and multireference perturbation theory for the computation of the bond dissociation energies in 12 transition-metal-containing diatomic molecules and three small transition-metal-containing polyatomic molecules and in two transition-metal dimers. The first step is a multiconfiguration self-consistent-field calculation, for which two choices must be made: (i) the active space and (ii) its partition into subspaces, if the generalized active space formulation is used. In the present work, the active space is chosen systematically by using three correlated-participating-orbitals (CPO) schemes, and the partition is chosen by using the separated-pair (SP) approximation. Our calculations show that MC-PDFT generally has similar accuracy to CASPT2, and the active-space dependence of MC-PDFT is not very great for transition-metal-ligand bond dissociation energies. We also find that the SP approximation works very well, and in particular SP with the fully translated BLYP functional SP-ftBLYP is more accurate than CASPT2. SP greatly reduces the number of configuration state functions relative to CASSCF. For the cases of FeO and NiO with extended-CPO active space, for which complete active space calculations are unaffordable, SP calculations are not only affordable but also of satisfactory accuracy. All of the MC-PDFT results are significantly better than the corresponding results with broken-symmetry spin-unrestricted Kohn-Sham density functional theory. Finally we test a perturbation theory method based on the SP reference and find that it performs slightly worse than CASPT2 calculations, and for most cases of the nominal-CPO active space, the approximate SP perturbation theory calculations are less accurate than the much less expensive SP-PDFT calculations.

Mass Spectrometric Observation of Doubly Charged Alkaline-Earth Argon Ions.

Doubly charged diatomic ions MAr(2+) where M=Mg, Ca, Sr or Ba have been observed by mass spectrometry with an inductively coupled plasma ion source. Abundance ratios are quite high, 0.1 % for MgAr(2+) , 0.4 % for CaAr(2+) , 0.2 % for SrAr(2+) and 0.1 % for BaAr(2+) relative to the corresponding doubly charged atomic ions M(2+) . It is assumed that these molecular ions are formed through reactions of the doubly charged metal ions with neutral argon atoms within the ion source. Bond dissociation energies (D0 ) were calculated and agree well with previously published values. The abundance ratios MAr(+) /M(+) and MAr(2+) /M(2+) generally follow the predicted bond dissociation energies with the exception of MgAr(2+) . Mg(2+) should form the strongest bond with Ar [D0 (MgAr(2+) )=124 to 130 kJ mol(-1) ] but its relative abundance is similar to that of the weakest bound BaAr(2+) (D0 =34 to 42 kJ mol(-1) ). The relative abundances of the various MAr(2+) ions are higher than those expected from an argon plasma at T=6000 K, indicating that collisions during ion extraction reduce the abundance of the MAr(2+) ions relative to the composition in the source. The corresponding singly charged MAr(+) ions are also observed but occur at about three orders of magnitude lower intensity than MAr(2+) .

Theoretical study on interactions of N-heterocyclic carbene with the bare first-row transition metals

The electronic and structural features of the free and substituted N-heterocyclic carbenes (NHCs) and their complexes with the bare first-row transition metals (TMs) have been explored theoretically. The present results indicate that the electron-donating behavior of NHC can be tuned by introduction of the substituent, and the NHC–TM σ-bonding interactions strongly depend on the d-electron configuration of TMs. The B3LYP- and CCSD(T)-predicted bond dissociation energies show a double-peak profile, and the maxima appear in [NHC–V] (26.2 kcal/mol) and [NHC–Ni] (29.7 kcal/mol) with the stable half- and full-filled d configurations, respectively, after accepting the σ-lone pair of NHC. Theoretical calculations on the η2-type structure of [NHC–TM] from π interactions between TM and C=C double bond in NHC are also done by B3LYP-D3. For Zn with d 10, no σ-type structure was located, and all geometry optimization attempts converge to only metastable η6-type configuration, arising from the weak aromatic π interactions. For other TMs, the π interactions between the double C=C bond and TM are responsible for their η2-type structures. The σ- and π-type [NHC–TM] complexes can accommodate additional NHC ligand to form stable [NHC–TM–NHC], and the introduction of second NHC can enhance the bonding interactions between TM and NHCs for most of the bare TMs, showing cooperation effect of the multiple NHC ligands on the TM–NHC bonding.

Prototypical metal–oxo bonds: the reactions of Cr(PF3)6, Fe(PF3)5, and Ni(PF3)4 with oxygen

Terminally bound oxo complexes of late transition metals have been difficult to synthesize or isolate. Their scarcity is in part due to the electronic repulsion between oxo ligands and metal elements with highly occupied d orbitals. This study attempts to provide extra stability to such metal–oxo species. The prototypical metal–oxo trifluorophosphane complexes [(PF3)5CrO, (PF3)4FeO, and (PF3)3NiO] are studied here. The results are compared with the corresponding carbonyl counterparts [(CO)5CrO, (CO)4FeO, and (CO)3NiO]. Predicted bond dissociation energies are 90 (Cr–O), 83 (Fe–O), and 59 (Ni–O) kcal/mol, higher than those of their carbonyl counterparts by around 10 kcal/mol. Consistent with bonding considerations and population analyses, the metal–oxo bonds are strengthened from the carbonyl to the trifluorophosphine complexes. Although the improvement is modest, it proves clearly that strong electron-withdrawing frameworks help in stabilizing metal–oxo complexes. This general idea may be utilized in further studies to seek or even design elusive terminal metal–oxo species and to keep pushing the limits of this area. In addition, improved viabilities are also found for the dioxygen–metal complexes [(PF3)5CrO2, (PF3)4FeO2, and (PF3)3NiO2], compared with their carbonyl counterparts.

Size-extensivity-corrected multireference configuration interaction schemes to accurately predict bond dissociation energies of oxygenated hydrocarbons.

Oxygenated hydrocarbons play important roles in combustion science as renewable fuels and additives, but many details about their combustion chemistry remain poorly understood. Although many methods exist for computing accurate electronic energies of molecules at equilibrium geometries, a consistent description of entire combustion reaction potential energy surfaces (PESs) requires multireference correlated wavefunction theories. Here we use bond dissociation energies (BDEs) as a foundational metric to benchmark methods based on multireference configuration interaction (MRCI) for several classes of oxygenated compounds (alcohols, aldehydes, carboxylic acids, and methyl esters). We compare results from multireference singles and doubles configuration interaction to those utilizing a posteriori and a priori size-extensivity corrections, benchmarked against experiment and coupled cluster theory. We demonstrate that size-extensivity corrections are necessary for chemically accurate BDE predictions even in relatively small molecules and furnish examples of unphysical BDE predictions resulting from using too-small orbital active spaces. We also outline the specific challenges in using MRCI methods for carbonyl-containing compounds. The resulting complete basis set extrapolated, size-extensivity-corrected MRCI scheme produces BDEs generally accurate to within 1 kcal/mol, laying the foundation for this scheme's use on larger molecules and for more complex regions of combustion PESs.

Bonding analysis of trimethylenemethane (TMM) complexes [(CO)3M–TMM] (M = Fe, Ru, Os, Rh+). Absence of expected bond paths

Quantum chemical calculations using gradient-corrected DFT methods (BP86) using effective core potentials and TZP quality basis sets have been carried out for the complexes (CO)3M–TMM (M = Fe, Ru, Os, Rh+; TMM = trimethylenemethane). The calculated equilibrium geometries give trigonal pyramidal structures which are in good agreement with experimental results for the iron and ruthenium species. The TMM ligand adopts a pyramidal coordination of the central carbon atom which has a shorter distance to the metal atom than the terminal carbon atoms. The theoretically predicted bond dissociation energies of the TMM ligand are between 108.0 kcal/mol for the iron complex and 155.1 kcal/mol for the rhodium complex. The trend of the calculated TMM–M bond strength is Fe < Ru < Os < Rh+. The investigation of the electronic structure of the complexes (CO)3M–TMM using MO correlation diagrams and energy partitioning methods clearly shows that the metal atom is mainly bonded to the terminal carbon atoms of the TMM ligand while the bonding to the closer central carbon atom is much weaker. This is further supported by the AIM analysis which shows that the Laplacian distribution ∇2ρ(r) at the terminal carbon atoms possesses local area of charge concentration which points toward the metal atom while the Laplacian distribution at the central carbon atoms is nearly undistorted. In spite of the shape of the Laplacian distribution, there are bond paths between the metal atom and the central carbon atom but not to the terminal carbon atom which comes from the different interatomic distances. This clearly shows that the absence and the existence of a bond path are not reliable criteria for chemical bonding.

Bonding Analysis of Metal−Metal Multiple Bonds in R3M−M′R3(M, M′ = Cr, Mo, W; R = Cl, NMe2)

The bonding situation of homonuclear and heteronuclear metal-metal multiple bonds in R(3)M-M'R(3) (M, M' = Cr, Mo, W; R = Cl, NMe(2)) is investigated by density functional theory (DFT) calculations, with the help of energy decomposition analysis (EDA). The M-M' bond strength increases as M and M' become heavier. The strongest bond is predicted for the 5d-5d tungsten complexes (NMe(2))(3)W-W(NMe(2))(3) (D(e) = 103.6 kcal/mol) and Cl(3)W-WCl(3) (D(e) = 99.8 kcal/mol). Although the heteronuclear molecules with polar M-M' bonds are not known experimentally, the predicted bond dissociation energies of up to 94.1 kcal/mol for (NMe(2))(3)Mo-W(NMe(2))(3) indicate that they are stable enough to be isolated in the condensed phase. The results of the EDA show that the stronger R(3)M-M'R(3) bonds for heavier metal atoms can be ascribed to the larger electrostatic interaction caused by effective attraction between the expanding valence orbitals in one metal atom and the more positively charged nucleus in the other metal atom. The orbital interaction reveal that the covalency of the homonuclear and heteronuclear R(3)M-M'R(3) bonds is due to genuine triple bonds with one σ- and one degenerate π-symmetric component. The metal-metal bonds may be classified as triple bonds where π-bonding is much stronger than σ-bonding; however, the largest attraction comes from the quasiclassical contribution to the metal-metal bonding. The heterodimetallic species show only moderate polarity and their properties and stabilities are intermediate between the corresponding homodimetallic species, a fact which should allow for the experimental isolation of heterodinuclear species. CASPT2 calculations of Cl(3)M-MCl(3) (M = Cr, Mo, W) support the assignment of the molecules as triply bonded systems.

Bond Dissociation Energies of Organophosphorus Compounds: an Assessment of Contemporary Ab Initio Procedures

Thermodynamic properties of phosphorus-containing compounds were investigated using high-level ab initio computations. An extended set of contemporary density functional theory (DFT) procedures was assessed for their ability to accurately predict bond dissociation energies of a set of phosphoranyl radicals. The results of meta- and double-hybrids as well as more recent methods, in particular M05, M05-2X, M06, and M06-2X, were compared with benchmark G3(MP2)-RAD values. Standard heats of formation, entropies, and heat capacities of a set of ten organophosphorus compounds were determined and the low-cost BMK functional was found to provide results consistent with available experimental data. In addition, bond dissociation enthalpies (BDEs) were computed using the BMK, M05-2X, and SCS-ROMP2 procedure. The three methods give the same stability trend. The BDEs of the phosphorus(III) molecules were found to be lower than their phosphorus(V) counterparts. Overall, the following ordering is found: BDE(P-OPh) < BDE(P-CH(3)) < BDE(P-Ph) < BDE(P-OCH(3)).

Determination of apparent decomposition threshold energies of lithium adducts of acylglycerols using tandem mass spectrometry and a novel derived effective reaction path length approach

Apparent decomposition threshold energies for the fragmentation pathways of lithiated acylglycerols were experimentally determined by collisional activation in a quadrupole-hexapole-quadrupole (QhQ) mass spectrometer. A previously developed 'derived effective reaction path length' approach for predicting bond dissociation energies (BDEs) of simple dissociations of electrostatic complexes such as alkali metal adducts (Li+), or halide adducts (Cl(-)) of acylglycerols, was extended to predict covalent bond apparent decomposition threshold energies of lithium adducts of a mono-acylglycerol, a 1,2-diacylglycerol, and a 1,3-diacylglycerol. The ability of the model to treat relatively large ionic systems (e.g. more than 100 atoms) represents a huge advantage of this approach. The model's calculated apparent decomposition threshold energies (Ea) are used in conjunction with the method of energy-resolved mass spectrometry, employing breakdown graphs, to give a more complete quantitative description of the fragmentation processes. Calculated Ea values allowed ranking of the 1,2-diacylglycerol as more reactive than the 1,3-diacylglycerol; the mono-acylglycerol was ranked the least reactive. The method was applied to the low molecular weight product ions generally associated with the hydrocarbon series CnH2n+1+, where two separate pathways are deduced as contributing to the production of the abundant m/z 81 fragment ion. The favored ranking of the neutral losses of fatty acyl substituents for the 1,2-diacylglycerol was determined as: loss of lithium fatty acetate > loss of fatty acid > loss of fatty acyl chain as ketene. For the 1,3-diacylglycerol, the descending order of ease of neutral loss was: loss of fatty acyl ketene > loss of lithium fatty acetate > loss of fatty acid. The results of this study demonstrate that the newly developed method is general in nature, and it can be used for the measurement of covalent bond decomposition threshold energies, as well as for the previously documented electrostatic (noncovalent) bond energies.

Structure and coordinate bonding nature of the rhenium–σ-borane complexes

The electronic and molecular structures of the complexes [(MeC 5H 4)Re(CO) 2(σ-HBcat)] 1, [(MeC 5H 4)Re(CO) 2(σ-HBpin)] 2 and [(MeC 5H 4)Re(CO) 2(σ-HBMe 2)] 3 have been investigated at the DFT BP86/TZ2P levels in order to understand the structures, bonding and energetic of the interactions between a transition metal and a σ-HBR 2 ligand. The nature of the Re–η 2-HBR 2 interactions was analyzed with charge and energy decomposition methods. The Re–B, B–H, Re–H bond distances are longer than that expected for single bond based on covalent radius predictions. The B–Re–H1 bond angles in the complexes [(η 5-MeC 5H 4)Re(CO) 2(η 2-HBR 2)]: 40.8° in 1, 41.5° in 2 and 38.5° in 3 are small and are consistent with B–H bonding. These results are consistent with the description of [(MeC 5H 4)Re(CO) 2(η 2-HBR 2)] as a Re(I) complexes in which both hydrogen and boron of the [HBR 2] ligands have a bonding interaction with the rhenium and B–H bond character is preserved. Upon coordination of [HBR 2], reduction in B–H bond order about 1/2 was calculated. The theoretically predicted bond dissociation energies of the borane ligands are −40.5 kcal/mol for [(η 5-MeC 5H 4)Re(CO) 2(HBcat)], −39.7 kcal/mol for [(η 5-MeC 5H 4)Re(CO) 2(HBpin)] and −43.3 kcal/mol for [(η 5-MeC 5H 4)Re(CO) 2(HBMe 2)]. The [(η 5-MeC 5H 4)Re(CO) 2]–[η 2-H-BR 2] bonding in borane complexes 1– 3 is more than half electrostatic. All three complexes exhibit about 42–44% covalent bonding of the borane ligand to the metal fragment. Indeed the three center-two electron bond in the Re–H–B bridge may be regarded as a “protonated π-bond”. The metal–η 2-H-BR 2 moiety in borane complexes is, therefore, analogous to metal boryl complexes with M–B σ and π bonds with a boron atom lying in the plane defined by the metal and two R substituents.

Accurate Treatment of Energetics and Geometry of Carbon and Hydrocarbon Compounds within Tight-Binding Model.

We show that a simple noniterative tight-binding model can provide reliable estimates of energetics and geometries of molecules with C-C and C-H bonds. The mean absolute error in heats of formation, ∼4.6 kcal/mol, is essentially smaller than those found in previous tight-binding schemes. The internal consistency of the calculated heats of formation enables the reliable prediction of bond dissociation energies and isomerization enthalpies. The model gives accurate molecular geometries of hydrocarbons; the mean absolute errors in bond lengths and bond angles are 0.015 Å and 1.4°, respectively. The calculated vibration frequencies agree reasonably well with experimental values. The method has proven to be transferable to complex carbon and hydrocarbon systems. The good performance of the model and its computational efficiency make it promising for simulations of carbon and hydrocarbon systems.