Lester L. Shipman

Stereoelectronic properties of photosynthetic and related systems: Ab initio configuration interaction calculations on the ground and lower excited singlet and triplet states of magnesium porphine and porphine

Abstract Ab initio configuration interaction wavefunctions and energies are reported for the ground state and many low-lying singlet and triplet states of magnesium chlorin and chlorin, and are employed in an analysis of the electronic absorption spectra of these systems. In chlorin, the calculated visible spectrum consists of two 1 (π, π ∗ ) states, the lower energy, y-polarized state exhibiting moderate absorption intensity in contrast to the very weak absorption of the higher energy x-polarized state. The configurational composition of both states is well described by the four-orbital model. Five 1 (π, π ∗ ) states are responsible for the Soret band envelope. A moderately intense y-state lies under the low energy edge of the band envelope, while two x-polarized states of moderate and strong intensity, respectively, are responsible for the band maximum. The final two 1 (π, π ∗ ) states lie at the high energy edge of the Soret band and introduce a measure of asymmetry into the band envelope. Two 1 (n, π ∗ ) states of very weak oscillator strength are also found in this region of the spectrum. All the Soret states are of complex configurational composition, and several of the higher lying states contain contributions from doubly excited configurations. The calculated visible spectrum of magnesium chlorin also consists of two 1 (π, π ∗ ) states, with the weakly absorbing x-polarized state lying approximately 200 cm−1 lower in energy than the moderately intense y-polarized state. The configurational composition of both states is well described by the four-orbital model. Four 1 (π, π ∗ ) states constitute the bulk of the intensity in the Soret band envelope. In distinction to chlorin, the moderately intense 1 (π, π ∗ ) state at the low energy edge of the band envelope is x-polarized. Two intense 1 (π, π ∗ ) states of y- and x-polarization, respectively, constitute the band maximum region, and a single x-polarized state of moderately strong intensity can be assigned to the high energy shoulder of the band envelope. Two other weakly absorbing 1 (π, π ∗ ) states are also found in this region, along with another weakly absorbing state of mixed in-plane and out-of-plane polarization. No clearly defined 1 (n, π ∗ ) states are observed. As was the case for chlorin, all the Soret states are of complex configurational composition, and some of the higher energy states contain significant contributions from doubly excited configurations. Chlorin and magnesium chlorin both possess three 3 (π, π ∗ ) states which lie below S1 and a single 3 (π, π ∗ ) which lies slightly above S2. All four of the low-lying 3 (π, π ∗ ) states in each molecule are well described by the four-orbital model, with T1 being essentially a single configuration in each case. The remainder of the 3 (π, π ∗ ) states are clustered in the same energetic region as the comparable 1 (π, π ∗ ) Soret states, with comparably complex configurational compositions. Dipole moments and charge distributions for low-lying singlet and triplet states are also reported, and are used to rationalize chemical reactivity characteristics.

Ab initio calculations on large molecules using molecular fragments. Initial P-matrices for scf calculations

Abstract A new procedure is described and characterized for constructing initial P -matrices for Hartree-Fock SCF calculations that is applicable when basis sets composed of localized orbitals that can be classified as inner shell, lone pair, or bonding, are used.

Ab initio calculations on large molecules using molecular fragments. Characterization of the zwitterion of glycine

The ab initio molecular fragment approach is applied to a characterization study of the ground state of the zwitterion of glycine. Included among the properties studied are the φ — ψ conformational energy surface, the electronic structure, and the magnitude and direction of the dipole moment. The results of the present study are compared to the results of other theoretical and experimental studies.

A new method for the calculation of vibrational force constants. Application to H2

Abstract A new method for the quantum mechanical calculation of vibrational force constants is presented. This method is applied to the calculation of the vibrational force constant of H 2 , using a completely optimized wavefunction constructed from a single gaussian orbital. The value of the force constant obtained using this method is k 0 = 0.422341088751 au (= 6.5754 × 10 5 dyne/cm), compared to the value of k 0 = 0.42234079S380 au (= 6.5754 × 10 5 dyne/cm) obtained using an analytic method, and the experimental value of k e = 0.3692 au (= 5.748 × 10 5 dyne/cm).