The concept behind active thermochemical tables (ATcT) is presented. As opposed to traditional sequential thermochemistry, ATcT provides reliable, accurate, and internally consistent thermochemistry by utilizing the thermochemical network (TN) approach. This involves, inter alia, a statistical analysis of thermochemically relevant determinations that define the TN, made possible by redundancies in the TN, such as competing measurements and alternate network pathways that interrelate the various chemical species. The statistical analysis produces a self-consistent TN, from which the optimal thermochemical values are obtained by simultaneous solution in error-weighted space, thus allowing optimal use of all of the knowledge present in the TN. ATcT offers a number of additional features that are not present nor possible in the traditional approach. With ATcT, new knowledge can be painlessly propagated through all affected thermochemical values. ATcT also allows hypothesis testing and evaluation, as well as discovery of weak links in the TN. The latter provides pointers to new experimental or theoretical determinations that will most efficiently improve the underlying thermochemical body of knowledge. The ATcT approach is illustrated by providing improved thermochemistry for several key thermochemical species.

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In a recent letter (J. Phys. Chem. A, 2001, 105,1), we argued that, although all major thermochemical tables recommend a value of ∆H°f 0 (OH) based on a spectroscopic approach, the correct value is 0.5 kcal/mol lower as determined from an ion cycle. In this paper, we expand upon and augment both the experimental and theoretical arguments presented in the letter. In particular, three separate experiments (mass-selected photoionization measurements, pulsed-field-ionization photoelectron spectroscopy measurements, and photoelectron-photoion coincidence measurements) utilizing the positive ion cycle to derive the O-H bond energy are shown to converge to a consensus value of the appearance energy AE 0 (OH(18.116 2 ( 0.003 0 eV). With the most accurate currently available zero kinetic energy photoionization value for the ionization energy IE(OH) ) 104989 ( 2 cm -1 , corroborated by a number of photoelectron measurements, this leads to D 0 (H-OH) ) 41128 ( 24 cm -1 ) 117.59 ( 0.07 kcal/mol. This corresponds to ∆H f0 (OH) ) 8.85 ( 0.07 kcal/mol and implies D 0 (OH) ) 35593 ( 24 cm -1 ) 101.76 ( 0.07 kcal/mol. These results are completely supported by the most sophisticated theoretical calculations ever performed on the H x O system, CCSD(T)/aug-cc-pVnZ, n ) Q, 5, 6, and 7, extrapolated to the CBS limit and including corrections for core-valence effects, scalar relativistic effects, incomplete correlation recovery, and diagonal Born-Oppenheimer corrections. These calculations have an estimated theoretical error of e0.2 kcal/mol based on basis set convergence properties. They reproduce the experimental results for dissociation energies, atomization energies, and ionization energies for the H x O system to within 0.0-0.2 kcal/mol. In contrast, the previously accepted values of the two successive bond dissociation energies of water differ from the current values by 0.5 kcal/mol. These values were derived from the spectroscopic determinations of D 0 (OH) using a very short Birge-Sponer extrapolation on OH/OD A 1 Σ + . However, on the basis of a calculation of the A state potential energy curve (with a multireference single and double excitation wave function and an augcc-pV5Z basis set) and an exhaustive reanalyzis of the original measured data on both the A and B states of OH, the Birge-Sponer extrapolation can be demonstrated to significantly underestimate the bond dissociation energy, although only the last vibrational level was not observed experimentally. The recommended values of this paper affect a large number of other thermochemical quantities which directly or indirectly rely on or refer to D 0 (H-OH), D 0 (OH), or ∆H°f(OH). This is illustrated by an analysis of several reaction enthalpies, deprotonation enthalpies, and proton affinities.

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Ab initio POL–CI calculations, augmented by a dispersion term, are used to predict the potential energy surface for the reaction Cl+HCl. The saddle point is found to be nonlinear. The surface is represented by a rotated-Morse-oscillator spline fit for collinear geometries plus an analytic bend potential. Variational transition state theory calculations, based on a linear reference path, are carried out, and they yield much smaller rate constants than conventional transition state theory, confirming that earlier similar results for this heavy–light–heavy mass combination were consequences of the small skew angle and were not artifacts of the more approximate potential energy surfaces used in those studies. Transmission coefficients are calculated using approximations valid for large-reaction-path curvature and the potential along the reference path is scaled so that the calculated rate constant agrees with experiment. The resulting surface is used to compute the H/D kinetic isotope effect which is in qualitative agreement with experiment.

There are two experimental approaches to determining ∆H f0 °(OH), which produce values of this key thermodynamic quantity that differ by >0.5 kcal/mol. The apparent uncertainty of the positive ion cycle approach resides in the measurement of the appearance energy of OH + from H 2 O, while the uncertainty of the spectroscopic approach resides in the determination of the dissociation energy of OH(A 2 Σ + ). In this note we present an independent experimental determination of the appearance energy that confirms the accuracy and enhances the precision of the existing positive ion cycle value for ∆H f0 °(OH). We also present electronic structure calculations of the OH(A 2 Σ + ) potential energy curve, which suggest that the extrapolation method used to obtain the spectroscopic dissociation energy is in error. Finally, we present the largest ab initio electronic structure calculations ever performed for ∆H f0 °(OH) that have an apparent uncertainty much less than 0.5 kcal/mol and support only the positive ion cycle value. Although all major thermochemical tables recommend a value of ∆H f0 °(OH) based on the spectroscopic approach, the correct value is that of the positive ion cycle, ∆H f0 °(OH) ) 8.83 ( 0.09 kcal/mol, D 0 (H-OH) ) 117.57 ( 0.09 kcal/mol, and D 0 (OH) ) 101.79 ( 0.09 kcal/mol.

The third-order reaction, H + O2 + M → HO2 + M, has been measured near the low-pressure limit at room temperature for M = He, Ne, Ar, Kr, O2, N2, and H2O and over an extended range of temperatures in a shock tube for M = Ar, O2, and N2. In all cases, H atoms were produced by the laser photolysis of NH3 and detected by atomic resonance absorption spectroscopy. The measurements are consistent with the available experimental record and, in particular, confirm the exceptionally high recombination rate constant when M = H2O. The standard theoretical analysis is applied to this entire experimental record to derive the value of the average energy change per collision, −ΔE all. The resulting −ΔE all values are sensible for all M but H2O. The problem with H2O motivates a change in the standard theoretical analysis that both rationalizes the behavior of H2O and also quantitatively changes the derived −ΔE all values for the other species of M. These changes involve three modifications of the standard treatment:  (1) explicit temperature dependence in the number of active rotational degrees of freedom contributing to the HO2* state density, (2) the replacement of Lennard-Jones potential for the HO2* + M interaction with an electrostatic + dispersion potential, and (3) the calculation of the collision rate between HO2* + M by a free rotor model for “complex formation” between the M and HO2*. The optimized values of −ΔE all that are produced from this new analysis have the following characteristics:  (1) the value of −ΔE all is the same for all rare gases, and (2) −ΔE all for di- and polyatomic molecules are enhanced relative to the rare gas atoms. This work supports the conclusions of previous trajectory studies that collision rates between activated complexes and bath gases are often underestimated while −ΔE all derived from recombination kinetics measurements are often overestimated.

Directly measured low-pressure thermal HCO dissociation rate constants and isotope effects are presented for the first time. The temperature range of the measurements is 637-832 K. A theoretical model developed in the preceding paper is found to be highly consistent with these results and with all available H + CO thermal addition rate constant measurements. The calculations are used to extend the measured dissociation rate constant to combustion temperatures. The calculated low-pressure dissociation rate constant k¡ in various buffer gases is accurately represented from 300 to 3000 K by At,(Ar) = 3.09 X 10-7 7•°/* , A:,(He) = 3.80 X 10"7r1e"171/-Rr, A:,(N2) = 3.07 X lO^rV170/*7; and A:,(H2) = 5.79 X lO^r'e"17,0/*7", where kx is in cm3 4/(molecule s) and R is in kcal/(mol deg). The calculations suggest that {AE)lot, the average energy transfer between metastable HCO* and the buffer gas, varies between -40 and -50 cm"1 for buffer gases N2, H2, He, and Ar.