Showing papers by "Wesley D. Allen published in 2004"

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 quadrupl...

Molecular Structure of Proline

Abstract: The molecular structures of the two lowest-energy conformers of proline, Pro-I and Pro-II, have been characterized by ab initio electronic structure computations. An extensive MP2/6-31G* quartic force field for Pro-I, containing 62,835 unique elements in the internal coordinate space, was computed to account for anharmonic vibrational effects, including total zero-point contributions to isotopomeric rotational constants. New re and improved r0 least-squares structural refinements were performed to determine the heavy-atom framework of Pro-I, based on experimentally measured (A. Lesarri, S. Mata, E. J. Cocinero, S. Blanco, J. C. Lopez, J. L. Alonso, Angew. Chem. 2002, 114, 4867; Angew. Chem. Int. Ed. 2002, 41, 4673) rotational constant sets of nine isotopomers and our ab initio data for structural constraints and zero-point vibrational (ZPV) shifts. Without the ab initio constraints, even the extensive set of empirical rotational constants cannot satisfactorily fix the molecular structure of the most stable conformer of proline, a 17-atom molecule with no symmetry. After imposing the ab initio constraints, excellent agreement between theory and experiment is found for the heavy-atom geometric framework, the root-mean-square (rms) residual of the empirical rotational constant fit being cut in half by adding ZPV corrections. The most significant disparity, about 0.07 A, between the empirical and the best ab initio structures, concerns the r(N...H) distance of the intramolecular hydrogen bond. Some of the experimental quartic centrifugal distortion constants assigned to Pro-II have been corrected based on data obtained from a theoretical force field.

Thermochemistry of disputed soot formation intermediates C4H3 and C4H5.

Abstract: Accurate isomeric energy differences and standard enthalpies of formation for disputed intermediates in soot formation, C4H3 and C4H5, have been determined through systematic extrapolations of ab initio energies. Electron correlation has been included through second-order Z-averaged perturbation theory (ZAPT2), and spin-restricted, open-shell coupled-cluster methods through triple excitations [ROCCSD, ROCCSD(T), and ROCCSDT] utilizing the correlation-consistent hierarchy of basis sets, cc-pVXZ (X=D, T, Q, 5, and 6), followed by extrapolations to the complete basis set limit via the focal point method of Allen and co-workers. Reference geometries were fully optimized at the ROCCSD(T) level with a TZ(2d1f,2p1d) basis set. Our analysis finds that the resonance-stabilized i-C4H3 and i-C4H5 isomers lie 11.8 and 10.7 kcal mol−1 below E-n-C4H3 and E-n-C4H5, respectively, several kcal mol−1 (more, less) than reported in recent (diffusion Monte Carlo, B3LYP density-functional) studies. Moreover, in these systems G...

The Electronic Structure and Vibrational Spectrum of trans-HNOO

Abstract: This paper reports the theoretical results of a thorough, state-of-the-art, coupled-cluster, renormalized coupled-cluster, and vibrational study on the molecule imine peroxide, HNOO, in its trans conformation. This molecule is isoelectronic with ozone and presents many of the same difficulties for theory as ozone. We report both the theoretical geometry and the vibrational frequencies, including anharmonic corrections to the computed harmonic vibrational frequencies obtained by calculating the quartic force field at the high levels of coupled cluster theory, including CCSD(T) and its renormalized and completely renormalized extensions and methods including the combined effect of triply and quadruply excited clusters [CCSD(TQf) and CCSDT-3(Qf)]. The motivation behind our study was the disagreement between two previous reports that appeared in the literature on HNOO, both reporting theoretical (harmonic) and experimental (matrix isolation) vibrational spectra of HNOO. Our new theoretical results and our ana...

Low-lying electronic states of FeNC and FeCN: A theoretical journey into isomerization and quartet/sextet competition

Abstract: With several levels of multireference and restricted open-shell single-reference electronic structure theory, optimum structures, relative energetics, and spectroscopic properties of the low-lying 6Δ, 6Π, 4Δ, 4Π, and 4Σ− states of linear FeNC and FeCN have been investigated using five contracted Gaussian basis sets ranging from Fe[10s8p3d], C/N[4s2p1d] to Fe[6s8p6d3f2g1h], C/N[6s5p4d3f2g]. Based on multireference configuration interaction (MRCISD+Q) results with a correlation-consistent polarized valence quadruple-zeta (cc-pVQZ) basis set, appended with core correlation and relativistic corrections, we propose the relative energies: Te(FeNC), 6Δ(0)<6Π (2300 cm−1)<4Δ (2700 cm−1)<4Π (4200 cm−1)<4Σ−; and Te(FeCN), 6Δ(0)<6Π (1800 cm−1)<4Δ (2500 cm−1)<4Π (2900 cm−1)<4Σ−. The 4Δ and 4Π states have massive multireference character, arising mostly from 11σ→12σ promotions, whereas the sextet states are dominated by single electronic configurations. The single-reference CCSDT-3 (coupled cluster singles and doubles with iterative partial triples) method appears to significantly overshoot the stabilization of the quartet states provided by both static and dynamical correlation. The 4,6Δ and 4,6Π states of both isomers are rather ionic, and all have dipole moments near 5 D. On the ground 6Δ surface, FeNC is predicted to lie 0.6 kcal mol−1 below FeCN, and the classical barrier for isocyanide/cyanide isomerization is about 6.5 kcal mol−1. Our data support the recent spectroscopic characterization by Lei and Dagdigian [J. Chem. Phys. 114, 2137 (2000)] of linear 6Δ FeNC as the first experimentally observed transition-metal monoisocyanide. Their assignments for the ground term symbol, isotopomeric rotational constants, and the Fe–N ω3 stretching frequency are confirmed; however, we find rather different structural parameters for 6Δ FeNC:re(Fe–N)=1.940 A and r(N–C)=1.182 A at the cc-pVQZ MRCISD+Q level. Our results also reveal that the observed band of FeNC originating at 27 236 cm−1 should have an analog in FeCN near 23 800 cm−1 of almost equal intensity. Therefore, both thermodynamic stability and absorption intensity factors favor the eventual observation of FeCN via a 6Π←6Δ transition in the near-UV.