Leo Radom

Extension of Gaussian-2 (G2) theory to molecules containing third-row atoms K and Ca

Gaussian-2 (G2) theory has been extended to molecules containing the third-row nontransition elements K and Ca. Basis sets compatible with those used in G2 theory for molecules containing first- and second-row atoms, as well as the third-row elements Ga–Kr, have been derived. As for Ga–Kr, first-order spin–orbit corrections are included explicitly in the G2 energies of K- and Ca-containing atoms and molecules. In contrast to the procedure for Ga–Kr, the 3s and 3p orbitals need to be included in the correlation space for K- and Ca-containing molecules. The geometries obtained with the new basis sets are in satisfactory agreement with experiment. Assessment of the thermochemical predictions is limited because of the sparsity of accurate experimental data for molecules containing K and Ca. Ionization energies are generally in good agreement with experimental values, as are the atomization energies for K-containing molecules. However, there are larger differences between theoretical and experimental atomizati...

Extension of Gaussian-1 (G1) theory to bromine-containing molecules

Bromine bases suitable for computing G1 energies of bromine‐containing molecules have been derived. Our recommended procedure for calculating such energies adopts the G1 steps previously introduced by Pople and co‐workers with the following modifications: (i) second‐order Mo/ller–Plesset (MP2) geometry optimizations use the polarized, split‐valence SV4P basis set for bromine along with 6‐31G(d) for first‐ and second‐row atoms; (ii) fourth‐order Mo/ller–Plesset (MP4) and QCISD(T) energies are calculated with our new bromine bases along with supplemented 6‐311G and McLean–Chandler (MC) 6‐311G bases for first‐ and second‐row atoms; and (iii) bromine atomic spin–orbit corrections are explicitly taken into account. G1 energies have been calculated for a selection of simple processes involving bromine‐containing molecules. The results obtained are within 0.1 eV of experiment except for the ionization energy of Br2, where the inclusion of molecular spin–orbit corrections is necessary to achieve 0.1 eV accuracy.

Extension of Gaussian‐2 (G2) theory to bromine‐ and iodine‐containing molecules: Use of effective core potentials

Basis sets have been developed for carrying out G2 calculations on bromine‐ and iodine‐containing molecules using all‐electron (AE) calculations and quasirelativistic energy‐adjusted spin–orbit‐averaged seven‐valence–electron effective core potentials (ECPs). Our recommended procedure for calculating G2[ECP] energies for such systems involves the standard G2 steps introduced by Pople and co‐workers, together with the following modifications: (i) second‐order Mo/ller–Plesset (MP2) geometry optimizations use polarized split‐valence [31,31,1] basis sets for bromine and iodine together with 6‐31G(d) for first‐ and second‐row atoms; (ii) single‐point higher‐level energies are calculated for these geometries using our new supplemented bromine and iodine valence basis sets along with supplemented 6‐311G and McLean–Chandler 6‐311G bases for first‐ and second‐row atoms, respectively; and (iii) first‐order spin–orbit corrections are explicitly taken into account. An assessment of the results obtained using such a p...

G3-RAD and G3X-RAD: Modified Gaussian-3 (G3) and Gaussian-3X (G3X) procedures for radical thermochemistry

The authors gratefully acknowledge generous allocations of computing time on the Compaq Alphaserver of the National Facility of the Australian Partnership for Advanced Computing, Australian National University Supercomputer Facility, and the support of the Australian Research Council.

Transition structures for the interchange of hydrogen atoms within the water dimer

High levels of ab initio molecular orbital theory were used to examine rearrangement processes in the water dimer corresponding to the interchange of various hydrogen atoms. Our most reliable calculations involve MP4/6‐311+G(2df,2p) energy evaluations at MP2/6‐311+G(d,p) optimized structures. The lowest energy rearrangement pathway corresponds to the interchange of hydrogen atoms of the acceptor molecule within the Cs water dimer structure (1). This proceeds via a transition structure of C1 symmetry (2) and requires an energy of 0.59 kcal mol−1. The interchange of donor and acceptor molecules can be achieved via a transition structure with Ci symmetry (4) and requires an energy of 0.87 kcal mol−1. Finally, the interchange of hydrogen atoms of the donor molecule, via a C2v transition structure (9), requires 1.88 kcal mol−1. The rearrangements via 2 and 4 lead to complete scrambling of hydrogen atoms within the individual H2O moieties at a cost of 0.87 kcal mol−1; the transition structure 9 is not necessary...

The application of ab initio molecular orbital theory to the anomeric effect. A comparison of theoretical predictions and experimental data on conformations and bond lengths in some pyranoses and methyl pyranosides

Abstract Ab initio molecular orbital calculations on methanediol have been used to predict the favored orientations and interatomic distances of the C-O-C-O-R portion in pyranoses. The results found for methanediol suggest, for the sugars, favored conformations that are consistent with the observed anomeric and exo -anomeric effects. The calculations also show that shortenings of the C-O bond of the order of 0.01 to 0.04 A, relative to methanol, are to be expected, and that the bond lengths have a strong conformational dependence. A comparison with the experimental data from the X-ray crystal-structure determinations, both for conformational angles and bond lengths, of eighteen pyranoses and methyl pyranosides shows agreement with the theory that is surprisingly good when consideration is taken of the experimental errors, the limitations of the theoretical model, and the expected differences in the structures of the crystal and the isolated molecule.

AB INITIO STATISTICAL THERMODYNAMICAL MODELS FOR THE COMPUTATION OF THIRD-LAW ENTROPIES

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−1 K−1 for molecules with no internal rotations. For molecules with internal rotations, errors of up to 1.8 J mol−1 K−1 per internal rotation are observed. Our E2 procedure, which treats each individual internal rotation explicitly with a simpl...

An assessment of theoretical procedures for the calculation of reliable free radical thermochemistry: A recommended new procedure

The ability to predict reliable thermochemical properties of molecules and ions has led to an ever increasing application of ab initio molecular orbital theory. Methods such as G2 theory have been shown to generally give accurate heats of formation (ΔfH) for closed-shell molecules and ions. Open-shell systems have been less thoroughly examined to date and the present paper attempts to redress this situation through a detailed assessment of the performance of a variety of levels of theory in calculating Δf H values for free radicals. Representatives of three families of theoretical procedures have been studied: the infinite basis set extrapolation techniques of Martin, the CBS procedures of Petersson et al., and the G2 methods of Pople et al. Among the specific influences investigated are choice of geometry, zero-point vibrational energy, high level electron correlation treatment and basis set size. We recommend a new procedure called CBS-RAD for the treatment of free radicals. CBS-RAD is a modification of...

The weakly exothermic rearrangement of methoxy radical (CH3O⋅) to the hydroxymethyl radical (CH2OH⋅)

Although the CH3O⋅ and CH2OH⋅ radicals have long been considered critical intermediates in combustion and atmospheric processes, only very recently has the potential significance of the isomerization CH3O⋅→CH2OH⋅ been appreciated. This isomerization and related aspects of the CH3O⋅/CH2OH⋅ potential surface have been studied here using nonempirical molecular electronic structure theory with moderately large basis sets and with incorporation of electron correlation. The vibrational frequencies of CH3O⋅, CH2OH⋅ and seven other stationary points on the potential energy hypersurface have been predicted, both to compare with results from spectroscopy and to provide estimates of zero‐point vibrational corrections. In general, there is reasonable agreement with those vibrational frequencies of CH3O⋅ and CH2OH⋅ which are known from experiment. Our ab initio calculations predict that CH3O⋅ lies 5.0 kcal mol−1 higher in energy than CH2OH⋅ with a barrier to rearrangement to CH2OH⋅ of 36.0 kcal mol−1. Rearrangement of...

Gaussian‐2 (G2) theory: Reduced basis set requirements

Two variations of G2(MP2) theory which employ smaller basis sets in evaluating the quadratic configuration interaction [QCISD(T)] component of the energy are presented. The first, G2(MP2,SVP), uses the split‐valence plus polarization (SVP) 6‐31G(d) basis, while the second, G2(MP2,SV), uses the split‐valence (SV) 6‐31G basis. The methods are evaluated on the basis of results for the set of 125 systems used for testing G2 theory. The mean absolute deviation of G2(MP2,SVP) results from experimental values is 1.63 kcal mol−1 compared with 1.58 and 1.21 kcal mol−1 for G2(MP2) and G2, respectively. The G2(MP2,SVP) method thus provides results which are generally very similar in quality to those obtained from G2(MP2) but at considerably reduced computational expense. On the other hand, the mean absolute deviation of G2(MP2,SV) results from experiment is substantially larger (2.13 kcal mol−1). The G2(MP2,SV) method exceeds the 2 kcal mol−1 target accuracy of G2 theory for an unacceptably large number of comparisons.