Kenneth B. Wiberg

Application of the pople-santry-segal CNDO method to the cyclopropylcarbinyl and cyclobutyl cation and to bicyclobutane☆

Abstract The CNDO method has been applied to the cyclopropylcarbinyl and cyclobutyl cations, and has given results which are in very good accord with experimental data. A cross-ring interaction is calculated to be of importance with cyclobutyl derivatives, and agrees with the large difference in rate observed with equatorial and axial leaving groups. Some properties of bicyclobutane as well as the relative energies for some models of the activated complex for the thermal rearrangement of bicyclobutane have also been calculated and compared with experimental data. The CNDO method appears to have considerable promise in the investigation of the organic chemical phenomena.

Comparison of atomic charges derived via different procedures

Atomic charges were obtained from ab initio molecular orbital calculations using a variety of procedures to compare them and assess their utility. Two procedures based on the molecular orbitals were examined, the Mulliken population analysis and the Weinhold–Reed Natural Population Analysis. Two procedures using the charge density distribution were included; the Hirshfeld procedure and Baders Atoms in Molecules method. Charges also were derived by fitting the electrostatic potential (CHELPG) and making use of the atomic polar tensors (GAPT). The procedures were first examined for basis set independence, and then applied to a group of hydrocarbons. The dipole moments for these molecules were computed from the various atomic charges and compared to the total SCF dipole moments. This was followed by an examination of a series of substituted methanes, simple hydrides, and a group of typical organic compounds such as carbonyl derivatives, nitriles, and nitro compounds. In some cases, the ability of the charges to reproduce electrostatic potentials was examined.

Hartree–Fock second derivatives and electric field properties in a solvent reaction field: Theory and application

A compact formalism for the second and third derivatives of the Hartree–Fock energy in the presence of an Onsager solvent reaction field is presented. All three standard algorithms (MO, AO, and direct) are extended to include the reaction field in a unified way. Predictions of the infrared spectrum of formaldehyde in a variety of solvents and of solvent‐induced shifts in carbonyl stretching frequencies are presented along with the results of new measurements. As for the gas‐phase case, analytical second derivatives are far more efficient than numerical ones. The reaction field provides very good predictions of solvent effects at negligible computational cost.

A time-dependent density functional theory study of the electronically excited states of formaldehyde, acetaldehyde and acetone

Abstract The electronic transition energies for formaldehyde, acetaldehyde and acetone were calculated via time-dependent density functional theory using a series of hybrid density functionals. The B3P86 functional was found to give the best agreement with the experimental values. The effect of basis set size on the calculated transition energies also was examined.

Properties of atoms in molecules: Dipole moments and transferability of properties

This paper uses the theory of atoms in molecules to investigate the origin of molecular dipole moments. The dipole moment is given by a sum over the net charge and first moment of every atom in a molecule. The first term leads to a charge transfer contribution μc, the second to an atomic polarization contribution μa. It is shown that both terms are, in general, of equal importance in determining both the static molecular dipole moment and the moment induced by a nuclear displacement. Models which imploy only point charges and corresponding bond moments which follow rigidly the nuclear framework, i.e., models which approximate μc and ignore μa, are shown to lead to results that are incompatible with the changes that are found to occur in a molecular charge distribution during a nuclear vibration. The dipole moment is shown to be another group property that is transferable between molecules in the normal hydrocarbons, This property, along with the net charge, the energy, the correlation energy (expressed as...

Electronic Transition Energies: A Study of the Performance of a Large Range of Single Reference Density Functional and Wave Function Methods on Valence and Rydberg States Compared to Experiment

This work reports a comparison among wave function and DFT single reference methods for vertical electronic transition energy calculations toward singlet states, valence and Rydberg in nature. A series of 11 small organic molecules are used as test cases, where accurate experimental data in gas phase are available. We compared CIS, RPA, CIS(D), EOM-CCSD, and 28 multipurpose density functionals of the type LSDA, GGA, M-GGA, H-GGA, HM-GGA and with separated short and long-range exchange. The list of functionals is obviously not complete, but it spans more than 20 years of DFT development and includes functionals which are commonly used in the computation of a variety of molecular properties. Large differences in the results were found between the various functionals. The aim of this work is therefore to shed some light on the performance of the plethora of functionals available and compare them with some traditional wave function based methods on a molecular property of large interest as the transition energy.

A vibrational study of cyclohexane and some of its isotopic derivatives-III. A vibrational analysis of cyclohexane, cyclohexane-d12, cyclohexane-1,1,4,4-d4 and cyclohexane-1,1,2,2,4,4,5,5-d8

Abstract A vibrational analysis has been carried out on cyclohexane, cyclohexane-d12, cyclohexane-1, 1,4,4-d4 and cyclohexane-1,1,2,2,4,4,5,6-d8 utilizing the set of 60 internal coordinates (all 18 bond stretches, all 36 valence angle bends and six CCCC torsional bends) that is compatible with the Snyder and Schachtschneider transferable valence force field for saturated hydrocarbons. Twenty-three of these valence force constants can be defined in the cyclohexane system; all 23 parameters were varied independently to fit 133 assigned fundamental frequencies using the method of least squares and giving a final RMS deviation of 14.68 cm−1. The resulting data are compared with those determined from the original Snyder and Schachtschneider values of the 23 force constants. A numerical method has been devised for determining independent linear combinations of valence force constants in each symmetry block; this permits the selection of an independent subset of valence force constants from a dependent or redundant set when redundant internal coordinates are included.

Microwave Spectrum and Structure of Bicyclo[1.1.0]butane

The microwave spectra of four isotopic species of bicyclo[1.1.0] butane have been investigated and the rotational constants of each species have been determined. Combining these data with those of the previously studied normal isotopic species, a complete set of structural parameters have been found. These include C1–C3 = 1.497 A, C3–C4 = 1.498 A, C3–C6 = 1.071 A, C–H (methylene) = 1.093 A, ∠C1C3H6 = 128°22′, dihedral angle, α, between cyclopropane rings = 121°40′, ∠HCH = 115°34′. These results are discussed with regard to the electronic structure of the molecule.

Basis set effects on calculated geometries: 6-311++G** vs. aug-cc-pVDZ.

The effect of basis sets on MP2 and CCSD calculated geometries has been investigated for a series of small organic molecules with two to four nonhydrogen atoms. Whereas 6‐311++G** usually leads to satisfactory structures, this was not true with aug‐cc‐pVDZ, although it uses more basis functions than the former set. In addition, the smaller 6‐311++G** invariably leads to lower calculated energies than aug‐cc‐pVDZ. With a given basis set, CCSD gives more satisfactory calculated geometries than MP2, but with a much greater computation time. For the compounds in this report, B3LYP/6‐311++G** is quite effective in giving satisfactory calculated geometries at a relatively small computational cost.

Oscillator Strength: How Does TDDFT Compare to EOM-CCSD?

In this work, we compare a large variety of density functionals against the equation of motion coupled cluster singles and doubles (EOM-CCSD) method for the calculation of oscillator strengths. Valence and Rydberg states are considered for a test set composed of 11 small organic molecules. In our previous work, the same systems and methods were tested against experimental results for the excitation energies. The results from this investigation confirm our previous findings, i.e., that there is a large difference between the functionals. For the oscillator strength, the average best agreement with EOM-CCSD is provided by CAM-B3LYP followed by LC-ωPBE and, to a lesser extent, B3P86 and LC-BLYP.