Toward subchemical accuracy in computational thermochemistry: Focal point analysis of the heat of formation of NCO and [H,N,C,O] isomers

Abstract

In continuing pursuit of thermochemical accuracy to the level of 0.1 kcal mol(-1), the heats of formation of NCO, HNCO, HOCN, HCNO, and HONC have been rigorously determined using state-of-the-art ab initio electronic structure theory, including conventional coupled cluster methods [coupled cluster singles and doubles (CCSD), CCSD with perturbative triples (CCSD(T)), and full coupled cluster through triple excitations (CCSDT)] with large basis sets, conjoined in cases with explicitly correlated MP2-R12/A computations. Limits of valence and all-electron correlation energies were extrapolated via focal point analysis using correlation consistent basis sets of the form cc-pVXZ (X=2-6) and cc-pCVXZ (X=2-5), respectively. In order to reach subchemical accuracy targets, core correlation, spin-orbit coupling, special relativity, the diagonal Born-Oppenheimer correction, and anharmonicity in zero-point vibrational energies were accounted for. Various coupled cluster schemes for partially including connected quadruple excitations were also explored, although none of these approaches gave reliable improvements over CCSDT theory. Based on numerous, independent thermochemical paths, each designed to balance residual ab initio errors, our final proposals are DeltaH(f,0) ( composite function )(NCO)=+30.5, DeltaH(f,0) ( composite function )(HNCO)=-27.6, DeltaH(f,0) ( composite function )(HOCN)=-3.1, DeltaH(f,0) ( composite function )(HCNO)=+40.9, and DeltaH(f,0) ( composite function )(HONC)=+56.3 kcal mol(-1). The internal consistency and convergence behavior of the data suggests accuracies of +/-0.2 kcal mol(-1) in these predictions, except perhaps in the HCNO case. However, the possibility of somewhat larger systematic errors cannot be excluded, and the need for CCSDTQ [full coupled cluster through quadruple excitations] computations to eliminate remaining uncertainties is apparent.

The puzzling behavior of HNCO isomers in molecular clouds

“…In our preceding paper (Marcelino et al 2009), we considered exothermic neutral-neutral, ion-molecule, and dissociative recombination reactions to study the chemistry of HNCO, HOCN, and HCNO. Here, we reinvestigated the chemical processes and introduced the additional isomer isofulminic acid (HONC), which is the most energetic stable isomer of isocyanic acid, at an energy calculated to be 84.1 kcal/mol above that of HNCO (Schuurman et al 2004), and whose rotational spectrum has recently been obtained in the laboratory (Mladenović et al 2009). To preserve the consistency of the chemical network, we considered new ions arising from the protonation of NCO and CNO, which may be progenitors of the four different CHNO isomers: i.e., HNCO + , HOCN + , HCNO + , and HONC + .…”

“…These isomers lie at increasingly higher energy than HNCO according to quantum calculations (24.7, 70.7, and 84.1 kcal mol −1 , respectively; see Schuurman et al 2004). Since there is not an obvious common precursor for them, their relative abundances in molecular clouds might provide some clues concerning the respective contributions of gas-phase and grain processes to the chemistry.…”

Differential adsorption of CHON isomers at interstellar grain surfaces

“…The first systematic calculations using density functional theory (DFT) in the B3LYP formulation showed HOCN, HCNO, and HONC energetically higher than HNCO by 28.7, 67.9, and 87.1 kcal/mol, respectively (Mebel et al 1996). The most accurate ab initio calculations using full coupled cluster through triple excitations (CCSDT) with large basis sets refined these energy differences to subchemical accuracy as 24.65, 70.67, and 84.12 kcal/mol, respectively (Schuurman et al 2004). …”

Laboratory Detection of Thiocyanic Acid HSCN

“…In light of recent work, the close agreement between the HNCS/HNCO and cosmic S/O ratios may be fortuitous. The recent astronomical detection of two high-lying CHNO isomers-HOCN (Brünken et al 2009a(Brünken et al , 2009b, calculated to lie 25 kcal mol −1 higher in energy than HNCO, and HCNO (Marcelino et al 2009), a surprising 71 kcal mol −1 higher in energy (Schuurman et al 2004) the formation of the energetic isomers in these two related families of molecules.…”