AB initio SCF LCAO MO study of the HOH…Cl− hydrogen bond

Ab initio SCF LCAO MO calculations for the [H₂O…Cl]^? complex have been performed. The energy of the linear hydrogen bond has been found to be lower than the energy of the bifurcated one. The difference of the energies is about 3 kcal/mole. The calculated equilibrium distance between the oxygen and chlorine atoms equals 5.75 au. The interaction energy of the chlorine anion and the rigid water molecule amounts to ?19 kcal/mole. The optimization of the OH bond length in the complex (linear hydrogen bond) leads to an interaction energy of ?19.5 kcal/mole (the experimental value equals ?13.1 kcal/mole). As a result of the hydrogen bond formation the OH bond length increases by 0.08 au.     

Molecular orbital theory of the hydrogen bond. VIII. Hydrogen bonding in H2OH2CO in relaxed singlet and triplet n → π* states

Ab initio SCF CI calculations with a minimal STO-3G basis set have been performed on the hydrogen bonded dimers in which H₂O is the proton donor to H₂CO in its relaxed singlet and triplet n→π^* states. Two dimers which are easily interconverted are found in the singet n→π^* state with hydrogen bond energies of 1.82 and 1.71 kcal/mole. The equilibrium dimer in the triplet state has a hydrogen bond energy of 2.97 kcal/mole. In both states, hydrogen bond formation occurs at the carbon atom. The structures of the dimers and the nature of the intermolecular surfaces in the regions of hydrogen bond formation are examined. Electron densities and distributions are also discussed.     

SCF LCAO MO studies on the hydration of ions: The system F− · 2H2O

The energy surface of the dihydrated fluoride anion (F·2H₂O)^–1 is studied for a number of different geometry points near the equilibrium structure within the SCF LCAO MO framework, using an extended gaussian basis set to approximate the molecular wavefunctions. For the first and second hydration step of the fluoride anion the corresponding hydration energies are calculated to beB ₁ ^scf =24.1 kcal/mole andB ₂ ^SCF =20.8 kcal/mole (experimental measurements: 23.3 kcal/mole and 16.6 kcal/mole, respectively). The hydration energies and equilibrium bond distances obtained for the dihydrated fluoride anion (F·2H₂O)^– are compared with those found for the monohydrate (FHOH)^– and with corresponding results of the dihydrated lithium cation (Li · 2H₂O)⁺. The system (F·2H₂O)^– is taken as a very simple model to discuss some basic features of the hydration process of small ions and to study the influence of a negative ion on an adjacent hydrogen bond.We would like to thank our technical staff for valuable help in carrying out these calculations.     

A simple,self-consistent electrostatic model for quantitative prediction of the activation energies of four-center reactions. II.*

A simple electrostatic model of point dipoles is used which permits direct calculation of the activation energies for the addition of the molecules H₂O, H₂S, H₃N, and H₃P to olefins. These calculated values agree with the known experimental data to within ±2 kcal/mole on the average. It was found that the best fit could be obtained with a polar transition state that corresponded to a reduction in bond order from 1 to ½ for the bond-breaking coordinates and an increase in bond order from 0 to 0.18 for the bond-forming coordinates. The replacement of a hydrogen atom of the species H₂O, H₂S, H₃N, or H₃P by a polarizable methyl group is expected to stabilize the charge on the central atoms. The following stabilization energies for the pairs H₂O? CH₃OH, H₂S? CH₃SH, H₃N? CH₃NH₂, H₃P? CH₃PH₂ were calculated: ?4.8 kcal/mole, ?0.7 kcal/mole, ?1.9 kcal/mole, ?0.8 kcal/mole, respectively.     

Correlation effects on energy curves for proton transfer. The cation [H5O2]+

Potential curves for proton transfer in [H₅O₂]⁺ and for the dissociation of one OH bond in [H₃O]⁺ were calculated by both ab initio and semi-empirical LCAO MO SCF CI methods. The energy barrier of the symmetric double minimum potential in [H₅O₂]⁺ is very sensitive to electron correlation. At an OO distance of 2.74 Å it decreases from the HF value of 9.5 kcal/mole to about 7.0 kcal/mole. The results of the semi-empirical calculations agree well with the ab initio data as long as only relative effects are regarded. The partitioning of correlation energy into contributions of individual electron pairs is very similar for proton transfer in [H₅O₂]⁺ and for the dissociation of one OH bond in [H₃O]⁺. In this example the proton transfer appears as a superposition of two “contracted ionic dissociation” processes. An interpretation of the behaviour of correlation during these processes is presented.     

Estimation of O-H bond dissociation energies in alcohols and acids from kinetic data

The O-H bond dissociation energies (D _O-H) in five alcohols and six acids have been determined from experimental data (rate constants of radical reactions). The ratio of the rate constants of the reactions R¹O˙+RH→R¹OH+R˙ and R ⁱ O˙+RH→R ⁱ OH+R˙ and the intersecting parabolas method are used in the estimation procedure. The D _O-H values are used to calculate the activation energies and rate constants for hydrogen abstraction from 2-methylbutane, butene-1, and cumene by alkoxyl and carboxyl radicals. The geometric parameters of the transition state are calculated for these reactions.     

Interaction energy and the shift in OH stretch frequency on hydrogen bonding for the H2O → H2O,CH3OH → H2O,and H2O → CH3OH dimers

The ability to use calculated OH frequencies to assign experimentally observed peaks in hydrogen bonded systems hinges on the accuracy of the calculation. Here we test the ability of several commonly employed model chemistries—HF, MP2, and several density functionals paired with the 6‐31+G(d) and 6‐311++G(d,p) basis sets—to calculate the interaction energy (D_e) and shift in OH stretch fundamental frequency on dimerization (δ(ν)) for the H₂O → H₂O, CH₃OH → H₂O, and H₂O → CH₃OH dimers (where for X → Y, X is the hydrogen bond donor and Y the acceptor). We quantify the error in D_e and δ(ν) by comparison to experiment and high level calculation and, using a simple model, evaluate how error in D_e propagates to δ(ν). We find that B3LYP and MPWB1K perform best of the density functional methods studied, that their accuracy in calculating δ(ν) is ≈ 30–50 cm^?1 and that correcting for error in D_e does little to heighten agreement between the calculated and experimental δ(ν). Accuracy of calculated δ(ν) is also shown to vary as a function of hydrogen bond donor: while the PBE and TPSS functionals perform best in the calculation of δ(ν) for the CH₃OH → H₂O dimer their performance is relatively poor in describing H₂O → H₂O and H₂O → CH₃OH. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Gaseous phosphorus compounds : Part V. The dissociation energy and heat of formation of carbon monophosphide

The chemical equilibrium CP(g) + P(g) ? P₂(g) + C(g) has been studied by means of the Knudsen effusion technique combined with mass spectrometric analysis of the vapor. The enthalpy of reaction, ΔH^O₂₉₈₋ was determined as 6.3 ± 4.0 kcal/mole. Combined with the literature value for the dissociation energy of P₂, the dissociation energy of gaseous carbon monophosphide was calculated as D^O₂₉₈ = 123.2 ± 4.0 kcal/mole or D^O_O = 122.1 ± 4.0 kcal/mole. The corresponding value for the standard heat of formation is ΔH^O_f.298 = 127.5±4.5 kcal/mole. This compares with the selected JANAF value of 111.7 ± 23.1 kcal/mole.     

Determinations of bond energies by time-of-flight single-collision chemiluminescence

A pulsed beam of metastable atoms traverses a scattering chamber filled with oxidant gas at low pressures (beam + gas arrangement); the resulting chemiluminescence is spectroscopically resolved as a function of time to yield a time-of-flight (TOF) spectrum for different internal states. From this data, the initial relative translational energy distribution is derived for the reactants that populate the excited internal state observed. Lower bounds are placed on the barium halide (BaX) dissociation energies, using the reactions Ba(³D) + X₂ → BaX* + X, where X = Br, I. Arguments are presented to show that these lower bounds represent measurements of the true bond energies: it is concluded that D₀⁰(BaBr) = 85.8 ± 2 kcal/mole and D₀⁰(BaI) = 72.9 ± 2 kcal/mole. The present work corrects previous determinations of bond energies from single-collision chemiluminescent studies which were in error because of unrecognized metastable contamination in the high-temperature atomic beam.     

Ab Initio Calculations of Structure and Energetics of the Hypermetalated Hydrogen Oxide Molecules: Li2NaOH and LiNa2OH

We made ab initio electronic calculations of the structure and energetics of mixed hypermetalated hydrogen oxides, Li₂NaOH and LiNa₂OH. There exist five equilibrium geometries for each complex. In all levels of calculation the global minimum structure for Li₂NaOH has C_2v symmetry and a large distance between sodium and oxygen, 4.24 Å (MP2/6-31G*). The dissociation energies to all possible products were also calculated. Li₂NaOH → Na + Li₂OH δH = +25.33 kcal/mol (at MP4/6-311++G**//6-31G* + ZPE scaled by 0.9). All other dissociation processes are highly endothermic. Similar procedures were applied to LiNa₂OH. The global minimum structure for LiNa₂OH belongs to point group C_s. It is also endothermic to all possible dissociation paths. LiNa₂OH →Na + LiNaOH δH = +12.72 kcal/mol (at MP4/6-311++G*//6-31G* + ZPE scaled by 0.9). The nuclear repulsion energy is crucial in energetics of the structures. The distribution of electron density and bonding properties for these equilibrium structures were analyzed.     

The enumeration of reaction pathways using Burnside's lemma

A simplified method for the calculation of molecular correlation energies, developed previously, is applied to the study of the potential energy hypersurface related to the reaction O₂+H→HO₂→OH+O. The main parameters that characterize the topology of the surface have been determined: the equilibrium geometries and vibrational frequencies of HO₂ and of its dissociation products, the dissociation energies, the energy gap between the dissociation limits, the geometry of an activated complex, and the corresponding potential barrier height. The particular importance of including the correlation energy is discussed for each of the calculated parameters. In spite of the minimal computational effort required, the results are in good agreement with the experimental data on the one hand, and with the results proceeding from elaborate variational calculations on the other hand. Moreover the controverted existence of a potential barrier (0.19 eV) along the O₂+H→ HO₂ recombination path is discussed.     

The energy of the N→O bond in 3-nitroisoxazoline N-oxide

The enthalpy of combustion of 3-nitroisoxazoline has been determined as ΔH _c ^298.15 =?414±0.3 kcal/mole and that of 3-nitroisoxazoline N-oxide as ΔH _c ^298.15 =?406.6±0.5 kcal/mole. From the values for the heats of combustion and evaporation, the standard enthalpies of formation have been calculated and the energy of the N→O bond has been evaluated at 64±3 kcal/mole.     

Beam-gas chemiluminescent reactions of Eu and Sm with O3, N2O,NO2 and F2

Studies are made of the visible chemiluminescence resulting from the reaction of an atomic beam of samarium or europium with O₃, N₂O, NO₂ and F₂ under single-collision conditions (～10^?4 torr). The spectra obtained for SmO, EuO, SmF, and EuF are considerably more extensive than previously observed. The variation of the chemiluminescent intensity with metal flux and with oxidant flux is investigated, and it's concluded that the reactions are bimolecular. From the short wavelength curoff of the chemiluminescent spectra, the following lower bounds to the ground state dissociation energies are obtained: D⁰₀(SmO) > 135.5 +- 0.7 kcal/mole, D⁰₀(EuO) > 131.4 ± 0.7 kcal/mole, D⁰₀(SmF) > 123.6 ± 2.1 kcal/mole, and D⁰₀(EuF) > 129.6 ± 2.1 kcal/mole. Using the Clausius-Clapeyron equation, the latent heats of sublimation are found to be ΔH₁₀₅₂ (Eu) = 42.3 ± 0.7 kcal/mole for europium and ΔH₁₀₈₄(Sm) = 47.9 ± 0.7 kcal/mole for samarium. Total phenomena- logical cross sections are determined for metal atom removal. Relative photon yields per product molecule are calculated from the integrated chemiluminescent spectra and it is found that Sm + F₂ → SmF^* + F is the brightest reaction. The comparison of the photon yields under single-collision conditions with those at several torr shows that energy transfer collisons play an important role in the mechanism for chemiluminescence at the higher pressures. A simple model is presented which explains the larger photon yields of the Sm reactions compared to the Eu reactions in terms of the greater number of electronic states correlating with the reactants in the case of samarium.     

Conformational analysis of intramolecular bonded amino alcohols : The conformational free energies of some intramolecular OH ⋯ N hydrogen bonds

Equilibrium positions between intramolecular OH ? N hydrogen bonded and free OH forms of some 3-piperidinols, decahydroisoquinolinols, a decahydroquinolinol, lupinine and N-methyl-3-piperidinemethanol have been determined from dilute solution IR spectral data at 33°. Conformational free energies of the H-bonds (ΔG°_OH?N, attractive) have been calculated. The results suggest a linear relationship between the apparent value of ΔG°_OH?N, as defined by the method of calculation, and the strength of the OH ? N bond expressed as Δν, within the limits of 0·5 ± 0·2kcal/mole per 100 cm^?1, from Δν 90 to 350 cm^?1. For cis-decahydroisoquinoline (N-Me or N-H) systems, a 0·4 kcal/mole difference has been calculated between the two possible ring-fused conformations, in favor of the so-called steroid form. For the corresponding cis-decahydroqumoline equilibrium, a 0·8 kcal/mole difference has been calculated, in favor of the nonsteroid form.     

Ab initio study of the O4 molecule

SCF-CI calculations were done on tetratomic oxygen complexes at various geometries. The results point to the existence of a metastable covalent molecule O₄ completely different from the van der Waals structure (O₂)₂ detected experimentally. At its equilibrium geometry, the O₄ molecule is a quasi-square (r(OO) ≈ 1.4 Å), slightly twisted out of plane, corresponding to the symmetry group D_2d. The activation energy of the reaction O₄(¹A_g) → 20₂(X ³Σ^?_g) is found to be ≈ 15 kcal/mole, that of the inverse reaction, ≈ 75 kcal/mole.     

Very-low-pressure pyrolysis of nitroso- and pentafluoronitrosobenzene C?NO bond dissociation energies

The high-pressure absolute rate constants for the decomposition of nitrosobenzene and pentafluoronitrosobenzene were determined using the very-low-pressure pyrolysis (VLPP) technique. Bond dissociation energies of DH⁰(C₆H₅? NO) = 51.5 ± 1 kcal/mole and DH⁰ (C₆F₅? NO) = 50.5 ± 1 kcal/mole could be deduced if the radical combination rate constant is set at log k_r(M^?1·sec^?1) = 10.0 ± 0.5 for both systems and the activation energy for combination is taken as 0 kcal/mole at 298°K. δH_f⁰(C₆H₅NO), δH_f⁰(C₆F₅NO), and δH_f⁰(C₆F₅) could be estimated from our kinetic data and group additivity. The values are 48.1 ± 1, –160 ± 2, and – 130.9 ± 2 kcal/mole, respectively. C–X bond dissociation energies of several perfluorinated phenyl compounds, DH⁰(C₆F₅–X), were obtained from the reported values of δH_f⁰(C₆F₅X) and our estimated δH_f⁰(C₆F₅) [X = H, CH₃, NO, Cl, F, CF₃, I, and OH].     

Bestimmung der Dissoziationsenergien der gasförmigen Moleküle CuGe,AgGe und AuGe

The dissociation energies of the gaseous molecules CuGe, AgGe, AuGe, Ge₂, and Cu₂ have been determined by mass spectrometric investigations of the vapour phases above the liquid alloys Ge?Cu, Ge?Ag, and Ge?Au. The evaluation according to the third-law method leads to the following values for the dissociation energies:D ₀°(CuGe)=49,0±5 kcal/mole;D ₀°(AgGe)=40,8±5 kcal/mole;D ₀°(AuGe)=65,3±3,5 kcal/Mol;D ₀°(Ge₂)=64,5±5 kcal/Mol;D ₀°(Cu₂)=64,5±5 kcal/Mol.     

Experimental determination of the dissociation energies D0 and De of H2O…HF

The zero-point and equilibrium dissociation energies of the hydrogen-bonded dimer H₂O…HF have been determined as D₀ = 34.3(3) kJmol⁻¹ and D_e = 42.9 (8) kJmol⁻¹ from absolute intensities of rotational transitions in an equilibrium mixture of H₂O, HF and H₂O…HF.     

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). I. A theoretical study

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following the excitation of high OH stretch overtones is studied by quasi-classical molecular dynamics calculations using a global potential energy surface (PES) fitted to ab initio calculations. The PES includes CH(2)OH and CH(3)O minima, dissociation products, and all relevant barriers. Its analysis shows that the transition states for OH bond fission and isomerization are both very close in energy to the excited vibrational levels reached in recent experiments and involve significant geometry changes relative to the CH(2)OH equilibrium structure. The energies of key stationary points are refined using high-level electronic structure calculations. Vibrational energies and wavefunctions are computed by coupled anharmonic vibrational calculations. They show that high OH-stretch overtones are mixed with other modes. Consequently, trajectory calculations carried out at energies about ~3000 cm(-1) above the barriers reveal that despite initial excitation of the OH stretch, the direct OH bond fission is relatively slow (10 ps) and a considerable fraction of the radicals undergoes isomerization to the methoxy radical. The computed dissociation energies are: D(0)(CH(2)OH → CH(2)O + H) = 10,188 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,167 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,787 cm(-1). All are in excellent agreement with the experimental results. For CH(2)OH, the barriers for the direct OH bond fission and isomerization are: 14,205 and 13,839 cm(-1), respectively.