Betty B. Coussens

The molecular structure of the urea molecule: is the minimum energy structure planar?

Abstract The structure of the free urea molecule was investigated by both ab initio and semiempirical methods. The minimum-energy structure is found to be non-planar. Although the heavy atom unit is planar, the hybridisation of the nitrogen atoms is sp3 rather than sp2, with the hydrogen out-of-plane torsional angles being close to 13° and 150°, respectively. This finding may be of importance with respect to the three-dimensional structure of amino resin networks. A comparison with some related molecules, i.e. formamide, acetamide and tetramethylurea, is provided.

The molecular structure of hydrazine and melamine: rotational barriers and hybridisation

Abstract Both hydrazine and melamine molecular structures were studied by ab initio quantum mechanical techniques; melamine was also studied by force field and semi-empirical methods. All calculations involved full geometry optimisation. The state of hybridisation and therewith the conformation of the amine groups in both systems are discussed. In addition, for melamine the rotationalenergy barrier of the amine groups was investigated. Comparison with available experimental data is provided where possible.

Contrasting behaviour of hydrogen fluoride and hydrogen chloride in the formation of weak complexes with methane

Abstract A theoretical study of weak molecular complexes of methane interacting with hydrogen fluoride and hydrogen chloride has been carried out. Geometries were optimized at the MP2/6-311G (d,p) level of MO theory while binding energies were determined at MP4/6-311 + + G (d,p). Calculated results show the formation of a non-conventional (HCH 3 …HCl) complex in which methane acts as a proton acceptor (as in the earlier-discussed (HCH 3 …HCN) system) versus a more conventional (H 3 CH…FH) dimer in which methane plays the role of a proton donor. These results are in agreement with those derived from both vibrational and rotational spectrometries. An attempt to rationalize this contrasting behaviour of HF and HCl in complex formation with methane is presented in the framework of an electrostatic model. The non-conventional structure of (CH 4 , HCl) or (CH 4 , HCN) is attributable to the anisotropy of the short-range forces which allows a closer approach in (HCH 3 …HX) than in (H 3 CH…XH).

Calculated properties of the weak complexes between methane and hydrogen cyanide

Abstract An ab initio study of molecular complexes including methane and hydrogen cyanide has been carried out at different levels of theory. Geometries were optimized at MP2/6-311G(d, p) while relative energies were determined at MP n /6-311 + + G( x d, y p). Calculated results confirm the formation of a weak and non-conventional complex having the HCH 3 …HCN structure in which methane acts as a proton acceptor . The complexation energy is about −4 kJ/mol (1 kJ/mol larger than that of the more conventional H 3 CH…NCH dimer). The polarization seems to be an important factor responsible for the stabilization of HCH 3 …HCN. The D- and 14 N-nuclear quadrupole coupling constants in the complexes were also calculated and discussed.

On balanced basis sets in ab initio calculations of transition metal complexes

Abstract In transition metal complexes, the problem of selecting a balanced basis set is particularly important, since one may have to combine atoms of the first row and atoms of the third or fourth row of the periodic system of the elements. A critical evaluation is made of the different existing criteria for basis set balance, and a new proposal is made to use Δ E N as a quality label. Δ E is the energy difference between the numerical Hartree—Fock energy of a given atom, and the energy calculated by using the particular basis set under consideration; N is the number of electrons in the atom.

On quality tests for Hartree-Fock-Roothaan SCF wavefunctions

Abstract We question the suggestion of Sordo and Pueyo that δ is a superior criterion to determine the quality of a given basis set; δ is the sum of the absolute errors (with respect to the Hartree-Fock results) of the one- and the two-electron contributions to the total energy. We show that the δ values published by Sordo and Pueyo are not always reliable because they are based on results for basis sets that are not completely optimized. Furthermore, to the extent that there is a discrepancy between δ and ΔE , no convincing arguments could be found in favour of the δ criterion. ΔE seems to be equally adequate and is much easier to use; in designing well-balanced basis sets for polyatomic molecules, ΔE / N is probably preferable.