John F. Olsen

Cyanogen isocyanate and dicyanoether: ab initio studies of geometries and electronic structures

Abstract Complete geometry optimizations, employing a minimal STO-3G basis set, have been applied to the recently-prepared cyanogen isocyanate [NCNCO] and to the isomeric dicyanoether [NCOCN]. Cyanogen isocyanate is found to be a rather flexible molecule with the computed barrier to inversion about the central nitrogen being ~5 k cal mol−1. In addition, the inversion motion is found to be coupled to the bending of the NCN and OCN linkages away from colinearity. On the other hand, dicyanoether is predicted to be a fairly rigid molecule, with no important inversion motions. Both molecules are predicted to have planar trans bent equilibrium structures similar to that found for the simpler HNCO-HOCN isomers. Cyanogen isocyanate is predicted to be the more stable isomer. Electronic structures of these molecules are discussed in the light of the results of a Mulliken population analysis.

The question of an open-ring form of cyclopropanone

Abstract The semi-empirical lcao-mo-scf theory has been applied to both the closed-ring and open-ring structures of cyclopropanone. Three main geometrical configurations have been considered, namely the (0,0), (0,90), and (90,90) forms, with the numbers in parentheses referring to the angles between the planes of the CH 2 group and the CCC plane. The ground state of the compound is found to be the (90,90) form, with a small value (62.5°) of the unique CCC angle. Thus the compound is best described as a closed-ring ketone, in agreement with recent experimental data. The open-ring form, which is found to be quite high in energy by these calculations, should probably not be considered as a major intermediate in the cycloaddition reactions of the compound. The interconversion of the two forms through a disrotatory motion involves a barrier, which occurs around the (30,30) form. Protonated cyclopropanone is also predicted to resist opening, as is the lowest triplet state of the compound ( 3 A 2 ), which loosely corresponds to an n → π * transition.

Electronic structures and geometries of N-O-F compounds: Fluorinated hydroxylamines

Abstract STO-3G and 4-31G calculations have been carried out on hydroxylamine and its fluorinated derivatives with the hope of shedding some light on the electronic structures of these interesting molecules. The geometries of the hydroxylamines have been optimized using both computational methods and the resulting geometries predicted from the two methods are compared. We have also computed the atomization energies, the bond separation energies, the hydrogenation energies, the heats of formation and the isomerization energies. Whereas STO-3G theory is adequate for hydroxylamine, the indtroduction of fluorines in the molecule necessitates the use of a more flexi ble basis set (4-31G) for adequate description.

A comparative ab-initio molecular orbital study of ammonia oxide and trifluoramine oxide

Abstract Geometry optimizations using various basis sets in the LCAO-SCF-MO method have been applied to F 3 NO and H 3 NO. Making use of the electronic wave-functions bonding is discussed in terms of donation from the oxygen lone pair into the N-F(H)o* orbitals and d-type orbitals on the nitrogen. Participation of d orbitals in the bonding is of modest importance for F 3 NO but not H 3 NO at least as far as the overlap population analysis is concerned.

Ab initio studies on several species of the formula X3YO and X3YOH

Ab initio molecular orbital calculations using both minimal and extended basis sets have been applied to two isoelectronic sets of molecules. One set corresponds to the 18 electron species H3NO, H3CO− and H3COH while the second set contains the 42 electron fluorinated molecules F3NO, F3CO− and F3COH. The geometries of these molecules have been optimized, using both the minimal STO-3G and the extended 4-31G basis sets. These comparative calculations reveal that the 4-31G basis produced structural parameters in much better agreement with experiment. The effect of includingd-orbitals in the basis set was also investigated. For the fluorinated oxides it has been found that the optimized 4-31G structures were only slightly altered by the addition ofd-orbitals. For H3NO, on the other hand, the inclusion ofd-orbitals considerably shortens the N-O bond distance. Both H3NO and CF3OH, which are unknown experimentally, are theoretically predicted to be capable of existence. The electronic structures of these molecules have also been examined using electronic partitioning according to the Mulliken scheme.

AB-initio studies of N-O-F compounds. Internal rotation in hydroxylamine and its fluorinated derivatives

Abstract Ab-initio molecular orbital theory at both the minimal and extended basis set levels have been applied to the study of internal rotation in hydroxylamine and its fluorinated derivatives. The computed energies are analyzed in terms of a Fourier-type expansion of the potential function. The total potential function V(φ)can be dissected into onefold (V1), twofold (V2) and threefold (V3) components and plots of these components together with V(φ) are given for each of the molecules studied herein. Additionally geometry optimizations have been carried out as a function of the internal rotation angle φ (φ = : NOX dihedral angle) for H2NOH and F2NOF. For H2NOH geometry optimizations are found to be less important than for F2NOF. In general the fluorinated hydroxylamines prefer a trans -conformation (φ = 180°) while hydroxylamine itself adopts the cis -conformation (φ = 0°) largely as a result of a lower dipole interaction (V1 term) in the cis -conformation.

Conformational isomers of 1-fluoro-2-propanol

Abstract The conformational energies of 1-fluoro-2-propanol have been studied using ab initio molecular orbital theory employing minimal (STO-3G) and extended (4-31G) basis sets. The calculations favor the internally H-bonded conformations (I and III). A recent microwave study of the molecule has only detected conformation I, suggesting that conformation III is at least 0.75 kcal/mol higher in energy. Partial geometry optimizations at both levels of theory suggest that the two gauche conformations (I and III) are rather similar energetically and should be experimentally detectable. The conformational energies are analyzed in terms of a Fourier-type expansion of the potential function. The barrier to rotation of the methyl group has also been computed.

Ab initio studies of the conformers of 2-propene-1-imine

Abstract Ab initio molecular orbital calculations employing both the minimal STO-3G and the split-valence 4-31G basis sets have been applied to the four planar conformers of 2-propene-1-imine. Initial calculations revealed that geometry optimizations were needed and partial geometry optimizations using the STO-3G method have been performed. Contrary to experiment, however, the STO-3G favors the wrong conformer (TS) as the lower energy structure. 4-31G calculations, on the other hand, are in reasonable agreement with the microwave study of the compound which predicts the TA conformer as the lower energy conformer with the TS conformer being 0.9 kcal mole −1 higher in energy. The most significant geometric variation in going from the TA to the TS conformer is an opening up of the CCN angle by about 5°. A comparison of the cis—trans rotamers of 1,3-butadiene, glyoxal and acrolein with 2-propene-1-imine is also made. The 4-31G calculations indicate that the TA — CA energy difference for 2-propene-1-imine is about 2 kcal mole −1 while the corresponding value for the TS — CS pair is 2.8 kcal mole −1 .

AB initio molecular orbital studies of CF3O2H, CF3O2F and CF2(OF)2

Abstract Ab initio calculations employing an extended 4-31G basis set have been applied to the highly fluorinated molecules, CF 3 O 2 H, CF 3 O 2 F and CF 2 (OF) 2 . Partial geometry optimizations have also been carried out on these molecules allowing a comparison between theory and the recently completed gas-phase electron diffraction results. The O-O bond distance in CF 3 O 2 H is found to be longer (by 0.02 A) than the corresponding bond in CF 3 O 2 F while the CO bond is found to be shorter (by 0.02 A) in CF 3 O 2 H. The OF bond in CF 3 O 2 F is found to be longer (by 0.03–0.04 A) than the corresponding bond in CF 3 OF or F 2 O. Torsional barriers have been computed for CF 3 O 2 H and CF 3 O 2 F with the aid of Fourier analysis of the potential curves. CF 3 O 2 H is found to have a torsional potential about the peroxide bond rather similar to that found for H 2 O 2 while in CF 3 O 2 F the cis and trans barriers are predicted to be much larger (14.6 and 8.4 kcal mol −1 , respectively). The threefold barrier to rotation of the CF 3 group in CF 3 O 2 F is predicted to be 4.4 kcal mol −1 . Various conformations of CF 2 (OF) 2 have also been studied with conformations consistent with the operation of the gauche -effect being most stable. Bond separation energies and molecular properties have also been computed for these molecules.

Vinyl isocyanate and vinylcyano ether: ab initio studies of geometries and conformations

Abstract Ab initio molecular orbital calculations using the minimal STO-3G basis set have been carried out on vinyl isocyanate. Preliminary calculations employing experimental geometric parameters indicate the existence of two stable conformations: cis and trans , with the latter being of lower energy. Only after geometry optimizations have been carried out is the cis conformer found to be more stable. However, the energy difference is computed to be rather small, namely 0.08 kcal mole −1 . Similar results are found for vinylcyano ether using both the minimal STO-3G basis and the split-valence 4-31G basis. The cis -vinylcyano ether optimized STO-3G structure is computed to be greater than 5 kcal mole −1 more stable than the corresponding optimized cis -vinyl isocyanate.