Donald H. Phillips

On the use of corresponding orbitals in the calculation of nonorthogonal transition moments

Full valence and first‐order CI wave functions are invariant with respect to unitary transformations among the valence orbitals. We exploit this degree of freedom and show that by transforming the valence orbitals into a corresponding orbital basis, nonorthogonal transition moment calculations become an easily managed task. Sample full valence calculations on several states of O+2 and OF are also presented.

The lower electronic states of ClOO: A computational investigation

Eight doublet and eight quartet states of ClOO were investigated by ab initio CI techniques. The potential energy surfaces of the four lowest energy doublet states of both A″ and A′ symmetry indicate that only the 1 2A″ state is bound. In contrast to the model provided by the HO2 radical, all of the excited doublet states investigated were repulsive with respect to dissociation to Cl+O2 and metastable or bound with respect to dissociation to ClO+O. The transitions to the excited states investigated span the visible and near UV spectral regions, but the transition moments indicate that they are very weak. Since the photolysis products are the same as those of the rapid thermal dissociation, photolysis is not expected to be an important atmospheric process. The soft bending potential for the 1 2A′ state and the shape of the 1 4A″ state in the entrance channel of the ClO+O → Cl+O2 reaction provide a qualitative explanation for the underprediction of the low temperature reaction rate by previous trajectory ca...

Luminescence from Aromatic Polymers, Monomers, and Dimers under High‐Energy Electron Excitation

The luminescence spectra of various polymers and solid‐model compounds induced by 1‐MeV electrons are reported and interpreted. The presence of enhanced triplet emission and absence of singlet emission from poly(p‐xylene), poly(monochloro‐p‐xylene), and the model compound dibenzyl are interpreted on the basis of a molecular resonance model. Some triplet excimer and other long‐wavelength (450–550‐mμ) emission were also observed in these spectra. The spectra of the monomers p‐xylene and 2‐chloro‐p‐xylene consisted primarily of long‐wavelength emission (400–600 mμ), while the spectra of frozen toluene and durene exhibited both neutral molecule singlet and the long‐wavelength emission. Benzene exhibited only singlet, singlet excimer, triplet, and triplet excimer emission. The polymers poly(ethylene‐2,6‐naphthalene‐dicarboxylate), poly(ethyleneterephthalate), and polysulfone emitted primarily from singlet and singlet excimer states. The model compounds for these polymers, dimethyl‐2,6‐naphthalenedicarboxylate,...

Spin projection of single‐determinant wavefunctions

Using the method of expansion in terms of the natural orbitals of charge density, we have rederived the components of the one‐ and two‐particle density matrices that result upon spin projection of a single‐determinant wavefunction. We have corrected several errors in the unrestricted two‐particle density matrix elements given by Harriman and co‐workers, pointed out the necessity of correcting the phases of the natural orbitals, and generalized the theory to the extent necessary to treat molecules having unequal numbers of electrons and basis orbitals. The theory has been applied to several of the types of cases for which the use of the projected UHF method is desirable: (a) small hydrocarbon radicals and singly charged ions, (b) hydrocarbon ions with pseudodegenerate ground states, and (c) a postulated photoproduct of dipyridyl, which had previously been predicted to be a triplet. The UHF wavefunctions of several of the small hydrocarbon radicals and monoions have only limited spin contamination by higher...

Gaseous NH4Cl revisited: A computational investigation of the potential surface and properties

An investigation of the potential surface and properties of the 1A1 ground state of gaseous NH4Cl is presented. The calculations, which utilize a ’’good’’ basis set and include investigation of the valence correlation effects, result in a description of gaseous NH4Cl as a weakly bound complex with NH3 and HCl structures essentially equivalent to those of the isolated fragments. The charge distributions within the fragments are modified in the complex, however, and there is a small amount of charge transfer. The results are in reasonable agreement with estimate of D00 from high temperature mass spectroscopic investigations and vibrational frequencies from matrix experiments. The equilibrium constant for NH4Cl?NH3+HCl indicates that the homogeneous formation of NH4Cl does not represent a significant chemical sink for HCl in the upper atmosphere.

A computational study of the TiO2 molecule

We have carried out a computational investigation of the ground 1A1 and lowest energy B2 states of the titanium dioxide molecule. The treatment utilized SCF calculations in an extended basis followed by a CI treatment for each geometry. Our ground state geometry agrees well with experiment, while the agreement between the computed vibrational frequency ν1 and the experimental value for a matrix isolated TiO2 is less satisfactory. Population analysis for the ground state indicates less than one excess electron on each oxygen atom. The first excited state has a linear geometry and the singlet and triplet are essentially degenerate.

An ab initio investigation of possible intermediates in the reaction of the hydroxyl and hydroperoxyl radicals

Ab initio quantum chemical techniques have been used to investigate covalently‐bonded and hydrogen‐bonded species that may be important intermediates in the reaction of hydroxyl and hydroperoxyl radicals. Stable structures of both types were identified. Basis sets of polarized double‐zeta quality and large scale configuration interaction wave functions have been utilized. Based upon electronic energies, the covalently‐bonded HOOOH species is found to be 26.4 kcal/mol more stable than the OH and HO2 radicals. Similarly, the hydrogen‐bonded HO‐‐‐HO2 species is found to have an electronic energy 4.7 kcal/mol below that of the component radicals, after correction is made for the basis set superposition error. The hydrogen‐bonded form is found to be planar, to possess one relatively ‘‘normal’’ hydrogen bond, and to have lowest energy 3A’ and 1A’ states that are essentially degenerate. The 1A‘ and 3A‘ excited states produced by rotation of the unpaired OH electron into the molecular plane are found to be very s...

Spin contamination in unrestricted Hartree‐Fock calculations

We have treated several pi‐electron doublets by both the unrestricted Hartree‐Fock method and the restricted approximation of Longuet‐Higgins and Pople. For two of the molecules, which contain oxygen heteroatoms, serious spin contamination occurs in the UHF wavefunctions and this negates the usefulness of the single annihilation procedure that is often used for spin‐density calculations. The restricted approximation provides a convenient alternative, especially because configuration interaction is easily taken into account.

Half-projected Hartree-Fock calculations on several small molecules

The half‐projected Hartree–Fock (HPHF) method is examined with respect to its ability to obtain molecular correlation, describe molecular potential energy surfaces, and provide a one particle basis for more elaborate treatments. The equivalence, aside from questions of efficiency, of two different HPHF algorithms is demonstrated. The results of calculations on H2O, C2, N2, and CH2 indicate that the performance of the HPHF method, with spin projection in appropriate cases, is roughly equivalent to limited MCSCF treatments. In particular, (1) a small but important fraction of the correlation energy, (2) qualitatively correct potential energy surfaces, and (3) good one particle orbital bases are obtained.

Approximate spin projection of three‐component UHF wavefunctions: The states of the pentachlorocyclopentadienyl cation and the croconate dianion, C5O52−

The approximate spin projection method of Amos et al. is extended to handle UHF wavefunctions having three significant components of differing multiplicity. An expression is given for the energy after single annihilation which differs from that of Amos and Hall. The new expression reproduces the results obtained from a previous exact calculation for which the weights and energies of the components are known. The extended approximate projection method is applied to the pi‐electron UHF wavefunctions for the ground states of the pentachlorocyclopentadienyl cation and the croconate dianion, C5O52−. The results indicate a triplet ground state for the former and a singlet ground state for the latter, in agreement with experimental ESR susceptibility measurements for these molecular ions. C5Cl5+ cannot be treated by restricted Hartree‐Fock theory due to its open‐shell ground state. Incorrect results are obtained for the croconate dianion if restricted Hartree‐Fock theory and singly excited configuration interact...