Articles you may be interested inToward subchemical accuracy in computational thermochemistry: Focal point analysis of the heat of formation of NCO and [H,N,C,O] isomers The heat of formation of NCO has been determined rigorously by state-of-the-art ab initio electronic structure methods, including M0ller-Plesset perturbation theory from second through fifth order (MP2-MP5) and coupled-cluster and Brueckner methods incorporating various degrees of excitation [CCSD, CCSD(T), BD, BD(T), and BD(TQ)]. Five independent reactions_ were investigated to establish a consis~nt value for AHj-,o(NCO): (a) HNCO(X tA')->HeS) +NCOerr) , (b) HNCO(X tA')->H+ +NCO-, (c) N(4S) +CO-.NCOerr}, (d) HCN+Oep)-.HeS)+NCOerr), and (e) NHe~-)+CO->HeS) +NCOerr). The one-particle basis sets employed in the study were comprised of as many as 377 contracted Gaussian functions and ranged in quality from [4s2pld] to [14s9p6d4f] on the (C,N,O) atoms and from [2s1p]to [8s6p4d] on hydrogen. After the addition of bond additivity corrections evaluated from related reactions of precisely known thermochemistry, all five approaches were found to converge on the value AHj-,o(NCO)=31.4(5) kcal mol-t. Appurtenant refinements were obtained for the heat of formation of isocyanic acid, AHj-,oCHNCO) = -27.5(5) kcal mol-I, and hydrogen cyanide, AHj-,o(HCN) =31.9(5) kcal mol-t. The final proposals for aHf,o(NCO) and LVif,o(HNCO) resolve outstanding discrepancies with experiment and provide updates for thermochemical cycles of relevance to combustion chemistry.4638

Third-law gas-phase statistical entropies are computed for a variety of closed-shell singlet state species using standard formulae based upon canonical partition functions. Molecular parameters are determined ab initio, and sensitivity analyses are performed to determine expected accuracies. Several choices for the canonical partition function are examined for internal rotations. Three general utility procedures for calculating the entropies are developed and designated E1, E2, and E3 in order of increased accuracy. The E1 procedure adheres to the harmonic oscillator approximation for all vibrational degrees of freedom other than for very low barrier internal rotations, these being treated as free rotations, and yields entropies to an accuracy of better than 1 J mol Ϫ1 K Ϫ1 for molecules with no internal rotations. For molecules with internal rotations, errors of up to 1.8 J mol Ϫ1 K Ϫ1 per internal rotation are observed. Our E2 procedure, which treats each individual internal rotation explicitly with a simple cosine potential, yields total entropies to an accuracy of better than 1 J mol Ϫ1 K Ϫ1 for species with zero or one internal rotation, and better than 2 J mol Ϫ1 K Ϫ1 for species with two internal rotation modes. Rotor-rotor coupling is found to contribute on the order of 1 J mol Ϫ1 K Ϫ1 for a third-law entropy. Our E3 procedure takes this into account and, with the aid of new ab initio two-dimensional torsional potential energy surfaces of state-of-the-art accuracy, improves the accuracy of the predicted entropy for species with two internal rotation modes to approximately 1 J mol Ϫ1 K Ϫ1 .

The coupled electronic and vibrational motions governing chemical processes are best viewed from the molecule's point of view-the molecular frame. Measurements made in the laboratory frame often conceal information because of the random orientations the molecule can take. We used a combination of time-resolved photoelectron spectroscopy, multidimensional coincidence imaging spectroscopy, and ab initio computation to trace a complete reactant-to-product pathway-the photodissociation of the nitric oxide dimer-from the molecule's point of view, on the femtosecond time scale. This method revealed an elusive photochemical process involving intermediate electronic configurations.