The heat of formation of NCO

Abstract

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

The correlation consistent composite approach (ccCA): An alternative to the Gaussian-n methods

“…[6][7][8][9][10] Other composite methods have been created since the original formulation of the G1 method. Some of these, such as the complete basis set ͑CBS-n͒ model chemistries of Petersson and co-workers, [11][12][13][14] the focal point method of Allen and co-workers [15][16][17][18][19][20] and Császár et al, 21,22 the W1 and W2 methods of Parthiban and Martin 23 and Martin and de Oliveira, 24 and the High Accuracy Extrapolated ab initio Thermochemistry ͑HEAT͒ method of Stanton and co-workers, 25,26 attempt to approach the complete basis set ͑CBS͒/full-configuration interaction ͑FCI͒ limit of smaller systems, consistently obtaining accuracy better than within 0.5 kcal mol −1 of experimental data. Dixon, Feller, and co-workers [27][28][29][30][31][32][33][34][35] used large basis set coupled cluster equilibrium geometries and total energies while using smaller basis sets to perform further electron correlation and scalar relativistic corrections, in order to achieve at least chemical accuracy ͑±1 kcal mol −1 ͒ for enthalpies of formation.…”

Quantitative Computational Thermochemistry of Transition Metal Species

“…Ab initio techniques 1 for the calculation of chemically accurate thermodynamics (i.e., (1-2 kcal mol -1 ) for main group species have been widely investigated, for example, Gaussian-n, 2-5 the complete basis set (CBS-n) methods, [6][7][8][9] the focal point method, [10][11][12][13][14][15][16][17] Wn methods, [18][19][20][21] the HEAT method, 22,23 the composite method of Dixon, Feller, Peterson, and coworkers, [24][25][26][27][28][29][30][31][32] and the newly developed correlation consistent composite approach (ccCA). [33][34][35][36] Yet to our knowledge, no methodical thermochemical analysis of ab initio model chemistries for a series of chemically diverse TM complexes on the scale of the G2 or G3 test sets 37 has been reported, though the most ambitious to date may be represented by the study of 62 TM-containing molecules by Furche and Perdew 38 and used by Truhlar and co-workers.…”

Wave Function and Density Functional Theory Studies of Dihydrogen Complexes

“…The energies computed with wave-function methods were extrapolated to the complete basis set (CBS) limit by considering a cubic interpolation formula [97,98] between cc-pVQZ [93][94][95] and cc-pV5Z [93][94][95] results, except for CCSD(T) calculations that were extrapolated to the CBS limit using a focal point analysis [99,100] (∆CCSD(T) procedure) based on CCSD(T)/cc-pVQZ, MP2/cc-pVQZ, and MP2/CBS results. Such energies can be considered accurate within 1% or ∼0.05 kcal/mol with respect to the true CBS limit [101][102][103] All DFT calculations were performed using the def2-TZVPP basis set [104,105].…”