Michael R. Peterson

Ab initio study of rotational isomerism in vinylcyclopropane

Abstract The geometry of vinylcyclopropane has been completely optimized at each critical point by analytic gradient (force) methods at the minimal STO-3G and the split-valence 3-21G basis set levels. The geometries obtained for the various critical points have been used to generate potential energy curves for vinyl group rotation within the rigid rotor approximation. Comparison of these curves clearly demonstrates the importance of complete geometry optimization. The potential energy curve for vinyl group rotation, generated with the s-trans STO-3G optimized geometry, predicts secondary gauche minima which are an artifact of the rigid rotor approximation. With complete geometry optimization along the curve, the STO- 3G basis set computations predict only s-trans and s-cis minima. In contrast, the complete optimizations with the 3-21G basis set, in agreement with experiment, predict a three-fold rotational contour with two equivalent gauche minima. These minima lie 6.86 kJ mol−1 above th e s-trans minimum. The computed barrier to rotation for the s-trans → gauche interconversion is 13.3 kJ mol−1. The electric dipole moment computed with the 3-21G basis for the s-trans 3-21G optimized geometry is 0.446 D or about 10% less than the experimental value.

Conformations of triplet carbonyl compounds: Formaldehyde, acetaldehyde, propionaldehyde and acetone

Abstract A conformational study on the lowest triplet states of formaldehyde, acetaldehyde, propionaldehyde and acetone has been done using a minimal basis set, within the unrestricted Hartree—Fock framework. For the C 3 H 6 O species, the energy hypersurfaces ( E θ 1 , θ 2 , θ 3 ) were generated, where energy is a function of the methyl rotations (θ 1 , θ 2 ) and Cue5f8O out-of-plane bending for acetone, and a function of methyl rotation (θ 1 ), C 2 H 5 ue5f8C rotation (θ 2 ) and CHO out-of-plane deformation (θ 3 ) for propionaldehyde. The analysis of the hypersurface equations revealed the location and relative energies of the critical points (minima, first and second order saddle points as well as maxima): the barriers to inversion at the carbonyl group were 2.7 kcal mol −1 for acetone and 4.2 kcal mol −1 for propionaldehyde. Partial geometry optimization reduced these barriers to 2.5 and 2.4 kcal mol −1 respectively. For comparison, both the pyramidal minimum and planar saddle point for the inversion of triplet formaldehyde and acetaldehyde were totally optimized; the resultant barriers were 2.0 kcal mol −1 and 2.3 kcal mol −1 , respectively. The barrier to rotation about the bond to the α-carbon was 1.1 kcal mol −1 for pyramidal acetone, 1.0 for acetaldehyde and ranged from 0.8 to 1.8 kcal mol −1 for the various propionaldehyde conformers.

Theoretical studies of C2H4O isomers: Part 1. Acetaldehyde ground, triplet n → π∗ and triplet n → 3s electronic states

Abstract The ground, triplet n → π* and triplet Rydberg n → 3s states of acetaldehyde have been studied at both the SCF and CI levels. The minimal STO-3G, split valence 3-21G and polarized 6-31G* basis sets have been used in conjunction with both spin unrestricted (UHF) and restricted (RHF) wavefunctions. The CI calculations included all single and double excitations from the SCF reference configuration, excluding three core and the six highest-lying virtual orbitals, using the 3-21G basis set. The Rydberg calculations used the 3-21G basis, augmented by a diffuse s function, plus CI. The agreement between theory and experiment is very good where corresponding data exist. The ground-state methyl rotation barrier is calculated to be 4.3 kJ mol−1 at the 6-31G*/SCF level and 5.2 kJ mol−1 at the 3-21G/CI level, with the syn conformer lowest. The triplet n → π* state is pyramidal, and lies 264 kJ mol−1 above the S0 state while the vertical excitation energy is 362 kJ mol−1. The main effect of either an RHF versus a UHF wavefunction, a larger basis, or CI, is to increase barrier and excitation energies somewhat. The Cue5f8O bond length is reduced below the experimentally determined value at the 6-31G* level. The preferred Rydberg state conformation is syn, in agreement with the recent experimental result. The methyl rotation barrier is computed to be 9.0 kJ mol−1 and the excitation energy 595 kJ mol−1 (602 kJ mol−1 vertical).

Ab initio study of rotational isomerism in cyclopropanecarboxaldehyde

Abstract The geometry of cyclopropanecarboxaldehyde has been optimized completely at each critical point in the torsional potential curve for—CHO group rotation by analytic gradient (force) methods at the STO—3G, 3—21G and 6—31G* basis set levels. In agreement with the available gas-phase electron-diffraction and microwave spectroscopy results, minima in the torsional potential energy curves are located only at the cis and trans conformations. The STO—3G basis computations place the cis 1.4 kJ mol −1 above the trans conformer and predict the barrier for the trans → cis rotation to be 13.8 kJ mol −1 at θ = 99.3° The computations with the 3–21G and 6–31G* basis sets predict (i) that the cis is more stable than the trans rotamer by 7.0 and 1.2 kJ mol −1 , respectively, (ii) that the cis → trans rotational barrier is 31.7 and 24.3 kJ mol −1 , respectively and (iii) that the transition state is located at θ = 101.5 and 99.9°, respectively. The geometrical parameters obtained are compared with the corresponding values for vinylcyclopropane, acetaldehyde and glyoxal. Evidence is found for important geometry changes due to the attraction between the oxygen and adjacent ring hydrogen atoms in the cis -cyclopropanecarboxaldehyde rotamer. Electric dipole moments were computed with the optimized geometries at each critical point. Those obtained with the STO—3G basis are unsatisfactory, being less than 65% of the experimental values. In contrast, the dipole moments computed with the 3–21G and 6–31G* basis sets (which are identical within 0.02 D for the same rotamer) are larger than the experimental values by less than 8%.

Structures of C3H6O species in the triplet state: Methyloxirane, propanal, acetone, methyl vinyl ether and propenol

Abstract The geometries of thirteen triplet C 3 H 6 O species, plus methyloxirane and propene in the ground state, have been completely optimized by analytic gradient (force) techniques at both the minimal STO-3G and split valence 3-21G basis set levels. Single calculations using the 6-31G* polarized basis set were performed for each 3-21G optimum structure. With the exception of bond angles at oxygen and the pyramidality at carbon radical centers, the geometries obtained with the two basis sets differ little. Except for the propene +O( 3 P ) atom, the addition of d orbitals has little effect on the relative energies compared to the 3-21G energy differences (Δ E ), but both were markedly different from the STO-3G results. Whereas the latter Δ E values covered a range of almost 40 kcal mol −1 , this range was reduced to 24.5 and 22.4 kcal mol −1 at the 3-21G and 6-31G − * levels, respectively.

Protonation of phosphoric amides. Molecular orbital calculations on phosphinamide, H2P(O)NH2, and its protonated forms

Ab initio SCF molecular orbital calculations were carried out on the amide H2P(O)NH2(IV) and its N-protonated H2P(O)NH3+(V), and O-protonated H2P(OH)NH2+(VI) forms using optimized geometries. The total energies as well as bond lengths and angles were obtained for each individual structure. Conjugate acid (VI) was found more stable than (V). The preferred conformations of (VI) involve the syn- and anti-periplanar arrangement of the nitrogen lone pair and the POH group. In order to investigate the possibility of the intramolecular hydrogen bonding in (VI) a potential energy surface involving OH torsion and inversion at nitrogen was generated.The results obtained are related to the reactivity of phosphoric amides under acidic conditions.