Noel V. Riggs

A theoretical approach to molecular conformational analysis

The application of ab initio molecular orbital theory to the study of molecular conformational analysis is discussed. Examples presented include methyl rotational barriers, internal rotation in 1,2-dihalogenoethanes, cis-trans isomerism in 1,2-dihalogenoethylenes, rotational barriers in substituted acetones and conformational preferences in substituted hydrazines.

An ab initio study of the stationary structures of the major gas-phase tautomer of adenine

Abstract The equilibrium structure of the major gas-phase tautomer of adenine is confirmed as being essentially planar, and it is shown that a reported second minimum is, in fact, one of a pair of transition structures for internal rotation of the amino group about the C–NH 2 bond. Barrier heights for such rotation are reported at various levels up to MP2 / 6-31G* // 3-21G (H 2 N*) and prove to be much greater than the corresponding values for aniline.

Molecular structure and spectroscopic properties of carbodiimide (HNCNH)

Abstract Ab initio molecular orbital calculations, including geometry optimizations up to the level of MP3/6-31G**, have been carried out for carbodiimide (HNCNH). The best predicted structure for carbodiimide, which includes empirical adjustments to some geometrical parameters, has r (NH) = 1.010 A, r (CN) = 1.224 A, ∠ HNC = 119.7°, ∠ NCN= 170.8° and ∠HN…#NH=89.6°. The calculated HNC angle shows a large variation between different levels of theory. Similar behaviour is found to occur for the related molecules, isocyanic acid (HNCO) and ketenimine (HNCCH 2 ), but not for methanimine (HNCH 2 ). The experimental observation that carbodiimide is one of the most nearly accidentally symmetric top molecules known has been confirmed. Stereomutation in carbodiimide is predicted to occur via internal rotation rather than pure inversion, but substantial (≈ 13°) opening of the HNC bond angle accompanies the torsional motion. The vibrational frequencies for carbodiimide have been calculated at the HF/6–31 G* level and empirically adjusted to yield predicted experimental values.

An ab initio molecular orbital study of the C3H+3 potential energy surface

Abstract The C 3 H + 3 potential energy surface was restudied using moderately high level ab initio molecular orbital calculations. Full geometry optimization was performed for equilibrium structures and saddle points at the Hartree-Fock level with the 4-31G and 6-31G ∗ basis sets and, in most cases, at the correlated level MP2/6-31G ∗ ; improved energy comparisons were obtained at the correlated level MP4/6-31G ∗∗ . Four C 3 H + 3 equilibrium structures were characterized and their heats of formation calculated. In ascending energy order they are the cyclopropenylium ion, the propargylium ion, 1-propen-3-ylidynylium ion, and the 1-propynylium ion. In addition, four saddle points were located. Two of these correspond to transition structures for internal rotation of the methylene group in the 1-propen-3-ylidynylium ion along alternative pathways, and a third is the transition structure for 1,2-methyl migration in the 1-propynylium ion; a second possible transition structure for the methyl migration has been characterized as a second-order saddle point. These computational results are compared with those from previous theoretical investigations and with those from a recent mass spectroscopic study in which all four of the above-mentioned equilibrium species were observed.

The formamidine–formic acid dimer: a theoretical examination of its equilibrium structure and of the double-proton-transfer process

Ab initio molecular-orbital calculations with moderately-sized basis sets and incorporating electron correlation predict that the formamidine–formic acid dimer has a doubly-hydrogen-bonded equilibrium structure of C1 symmetry and that double-proton transfer takes place via an ion-pair structure of C2v symmetry in a stepwise process.

Trithia[1.1.1]Propellane

Abstract Trithia[1.1.1]propellane 2 is predicted to be a tightly-bound molecule which should be experimentally accessible. It is characterized by an intrabridgehead C-C bond of length 1.575A, a heat of formation (ΔHl298) of 542 kJ mol−1, and vibrational spectra with frequencies all lying below 900 cm−1. The synthesis of a key precursor to 2 is currently underway.

Ab initio calculations on the structure of Covington's sesquicarbonate ion

Ab initio calculations suggest that the sesquicarbonate ion, H3C2O6–, may well exist but has a planar structure (6) held together by two hydrogen bonds instead of the ‘sandwich’ structure (1) or (2) proposed by Covington; the planar structures lies 182 kJ mol–1 below the sum of the energies of bicarbonate ion and (Z,Z)-carbonic acid.

A planar hydrazine: bi(1,2,3-azadiboriridin-1-yl)

Both 1-amino-1,2,3-azadiboriridine and bi(1,2,3-azadiboriridin-1-yl) are found by ab initio molecular orbital calculations to prefer conformations with coplanar lone-pair orbital axes on the adjacent nitrogen atoms; the latter molecule is predicted to prefer a fully coplanar (D2h) conformation.