STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS — V. AB INITIO CONFIGURATION INTERACTION CALCULATIONS ON THE GROUND AND LOWER EXCITED SINGLET AND TRIPLET STATES OF ETHYL CHLOROPHYLLIDE a AND ETHYL PHEOPHORBIDE a

Abstract— Ab initio configuration interaction wavefunctions and energies are reported for the ground state and many low-lying excited singlet and triplet states of ethyl pheophorbide a (Et-Pheo a) and ethyl chlorophyllide a (Et-Chl a), and are employed in an analysis of the electronic absorption spectra of these systems. In both molecules the visible spectrum is found to consist of transitions to the two lowest-lying ¹(π, π*) states, S₁ and S₂. The configurational compositions of S₁ and S₂ in both molecules are similar, and are described qualitatively in terms of a four-orbital model. The S₁← S₀ transition in each case is predicted to be intense, and is largely in-plane y-polarized, while the S₂← S₀ transition is predicted to be extremely weak and in-plane polarized. The orientation of the S₂← S₀ transition dipole is not conclusively established in the present calculations. The Soret band in both molecules is composed of transitions to no less than ten states (S_3-S₁₂ in Et-Chl a and S₃-S₇S₉-S₁₂. and S₁₄ in Et-Pheo a), which exhibit primarily (π, π*) character. The configurational compositions of these states are generally a complex mixture of excitations from both occupied macrocyclic π molecular orbitals and occupied orbitals with electron density in the cyclopen-tanone ring and the carbomethoxy chain. No clear correspondences are evident between respective Soret states of the two systems. Transitions to these states are generally intense and display a variety of in-plane polarizations. Two additional Soret states of Et-Pheo a, S₈ and S₁₃, exhibit primarily (n. π*) character. S₈ is characterized by excitations from u and non-bonding regions of the carbomethoxy chain, while S₁₃ is described by n →π* excitations involving the nitrogen atom of ring II. No corresponding (n, π*) states were found for Et-Chl a. In both molecules the lowest two triplet states, T₁ and T₂, are found to lie lower in energy than S₁. while T, and S₁ are approximately degenerate. The configurational compositions of T₁-T₄ of both molecules are nearly identical, and may be described by a four-orbital model. However, the compositions of T₁-T₄ differ sharply from those of S₁ and S₂. A number of higher-lying ³(π, π*) states of both molecules (T₅-T₁₃ in Et-Chi a and T₈-T₉, T₁₁-T₁₃ in Et-Pheo a) are found to have energies similar to the singlet Soret states, relative to S₀. They are characterized by a complex mixture of configurations which do not include significant contributions involving the four-orbital model. In addition, two ³(n, π*) states of Et-Pheo a, T₁₀ and T₁₄, are found, which are somewhat analogous to S₈ and S₁₃. Additional data presented include the charge distributions and molecular dipole moments of the S₀. S₁, and T₁ states of both molecules, as well as energies and oscillator strengths of computed S_n←S₁ and T_n←₁ transitions.     

Application of the shape group method to conformational processes: Shape and conjugation changes in the conformers of 2-phenyl pyrimidine

The shape group method (SGM) and the associated (a,b)-parameter maps provide a detailed shape characterization of molecular charge distributions. This method is applied to the study of the variations of shape and conjugation of conformers of 2-phenyl pyrimidine in their electronic ground state. Within the SGM framework, the method of (a,b)-parameter maps provides a concise, nonvisual, algorithmic technique for shape characterization of molecules with fixed nuclear geometries. Moreover, shape codes derived from the (a,b)-parameter maps afford a practical means for efficiently storing the shape properties of molecules in an electronic database. The shape codes of two or more charge distributions can be compared directly, and numerical measures of molecular shape similarity can be computed using a technique that is simple, fast, and inexpensive, especially in relation to direct, pairwise comparisons of electronic charge densities. The quantitative and automated nature of the method suggests applications in the field of computer-aided molecular design. In this study, the method is used for the first time to determine detailed numerical shape codes and shape similarity measures for a nontrivial conformational problem involving changes in energy and in conjugation. Numerical shape similarity measures of eight conformers of 2-phenyl pyrimidine are determined and correlated with variations in conformational energy and conjugation. The competing effects of steric repulsion and conjugation lead to important correlations between conformational energy and shape. © 1995 John Wiley & Sons, Inc.     

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

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 ${}^1\text{(π, π}{}^1\text{)}$ 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 ${}^1\text{(π, π}{}^1\text{)}$ 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 ${}^1\text{(π, π}{}^1\text{)}$ states lie at the high energy edge of the Soret band and introduce a measure of asymmetry into the band envelope. Two ${}^1\text{(n, π}{}^1\text{)}$ 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 ${}^1\text{(π, π}{}^1\text{)}$ 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 ${}^1\text{(π, π}{}^1\text{)}$ states constitute the bulk of the intensity in the Soret band envelope. In distinction to chlorin, the moderately intense ${}^1\text{(π, π}{}^1\text{)}$ state at the low energy edge of the band envelope is x-polarized. Two intense ${}^1\text{(π, π}{}^1\text{)}$ 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 ${}^1\text{(π, π}{}^1\text{)}$ 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 ${}^1\text{(n, π}{}^1\text{)}$ 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 ${}^3\text{(π, π}{}^1\text{)}$ states which lie below S₁ and a single ${}^3\text{(π, π}{}^1\text{)}$ which lies slightly above S₂. All four of the low-lying ${}^3\text{(π, π}{}^1\text{)}$ 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 ${}^3\text{(π, π}{}^1\text{)}$ states are clustered in the same energetic region as the comparable ${}^1\text{(π, π}{}^1\text{)}$ 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.     

EXPERIMENTAL and THEORETICAL STUDIES OF SCHIFF BASE CHLOROPHYLLS

Abstract— A protonated Schiff base of Ni (II)-pyrochlorophyll a has been synthesized which exhibits a reversible bathochromic shift of 504 cm^-1 relative to Ni (II)-pyrochlorophyll a. The magnitude of this shift lies between those observed for P700 and P680, the photoactive pigments of photosystems I and II in plants. Cyclic voltammetric measurements show that the protonated Schiff base is about 0.2 V more difficult to oxidize than its unprotonated form. These results suggest that a protonated Schiff base may be a better model for P680 than, as was originally assumed, for P700. In addition, the results of solvent and counterion effect studies show that microenvironmental perturbations in the neighborhood of the protonated Schiff base moiety are unlikely to induce further spectral shifts. Ab initio quantum mechanical calculations show a small hypsochromic shift rather than the observed bathochromic one, and the reasons for this discrepancy are discussed.     

STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS—IX. AB INITIO QUANTUM MECHANICAL CHARACTERIZATION OF THE ELECTRONIC STRUCTURE AND SPECTRUM OF THE BACTERIOCHLOROPHYLLIDE a ANION RADICAL

Abstract— Ab initio configuration interaction wavefunctions and energies are reported for 19 doublet states of the anion radical of ethyl bacteriochlorophyllide a (Et-BChl a˙), and are employed in a resolution of the electronic absorption spectrum, as well as in a comparison with a previously reported study of the electronic states and spectrum of the anion radical of ethyl bacteriopheophorbide a (Et-BPheo a˙). The lowest two excited doublet states, D₁ and D₂, of Et-BChl a˙ are found to be approximately degenerate and are predicted to contribute to the experimentally observed absorption at 10000 cm^?1. In contrast, the D₂←D₀ transition in Et-BPheo a˙ is predicted to contribute to the 11000 cm^?1 absorption, while the D₁←D₀ transition appears at approximately 8600 cm^?1 with a low oscillator strength (f= 0.002). The prominent visible absorption at ～15700 cm^?1 in both molecules is found to be due to the D₄← D₀ transition. Another difference between the predicted spectra of the two molecules appears in the low-energy shoulder of the Soret band. Here, two intense transitions, D₁₀←D₀ and D₁₁←D₀, are predicted for Et-BChl a˙, as opposed to three fairly intense transitions, D₇←D₀, D₈←D₀ and D₉←D₀, for Et-BPheo a˙, differences which may provide a means of distinguishing between the two molecules using resonance Raman spectroscopy. The remainder of the Soret band of Et-BChl a˙ above 26000 cm^?1 consists of a number of closely-spaced transitions to states D₁₂←D₁₈. The intense transitions D₁₂←D₀, D₁₃←D₀, D₁₄←D₀ are predicted to contribute to the Soret maximum near 30000 cm^?1. The ground state spin densities of the two molecules are similar, with the minor difference of somewhat less spin density located on the methine carbon atoms of Et-BChl a˙ compared with Et-BPheo a˙.     

Ab initio quantum-mechanical characterization of the electronic states of anthraquinone,quinizarin, and 1,4-diamino anthraquinone

The ground and excited singlet and triplet states of 9,10-anthraquinone and its 1,4-dihydroxy and 1,4-diamino derivatives are investigated by ab initio configuration interaction calculations, using a floating Gaussian basis. For anthraquinone the calculated electronic absorption spectrum is consistent with previous experimental and theoretical results. The possible mechanisms of intersystem crossing and intensity borrowing in phosphorescence are discussed in terms of first- and second-order spin-orbit and vibronic perturbations of the computed singlet and triplet states. The calculated spectra of the 1,4-disubstituted derivatives are more complex than previously suggested from low-resolution polarized absorption studies. The principal effect of 1,4-substitution by electron donating groups is shown to be a powerful conjugative effect which significantly modifies selected molecular orbitals of anthraquinone; the resultant effect on the absorption spectra is the creation of two new substituent-induced π → π* transitions, one of which corresponds to the intense visible absorption band of these systems.     

STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS—VIII. AB INITIO QUANTUM MECHANICAL CHARACTERIZATION OF THE ELECTRONIC STRUCTURE AND SPECTRUM OF THE BACTERIOPHEOPHORBIDE a ANION RADICAL

Ab initio configuration interaction wavefunctions and energies are reported for 16 doublet states of the anion radical of ethyl bacteriopheophorbide a (Et-BPheo a^-˙), and are employed in an analysis of the electronic absorption spectrum. The lowest excited doublet state D₁ is predicted to lie 8601 cm^-1 above the ground state D₀; the D₁← D₀ transition is nearly forbidden, with a computed oscillator strength f= 0.002. The visible absorption spectrum is shown to consist of transitions to three ²(π, π*) states, D₂, D₃, and D₄. The D₄← D₀ transition (y-polarized, f= 0.91) appears to account for observed intense absorption at 15 800 cm^-1. The Soret band of Et-BPheo a^-˙ is shown to consist of transitions to several ²(π,π*) states, D₇-D₁₅. Transitions of particularly high intensity include D₇← D₀ (y-polarized, f= 0.72), D₁₀← D₀ (y-polarized, f= 1.1), D₁₂← D₀ (xy-polarized, f= 0.86) and D₁₅← D₀ (y-polarized, f= 0.83). Spin density data and plots are used to describe and compare the general features of the unpaired spin distributions in D₀ and D₁, which are in reasonable agreement with other reported calculated values and available experimental data for D₀.     

Cumulative and discrete similarity analysis of electrostatic potentials and fields

A new index suitable for computing molecular similarity based upon the similarity of molecular properties such as electrostatic potentials or electrostatic fields is presented in two forms. For one form of the present index, general conditions are established for which a linear measure of similarity is obtained. An illustrative example is provided in which the electrostatic field and electrostatic potential of guanine obtained from different wave functions are compared. © 1993 John Wiley & Sons, Inc.     

Nature and location of excited charge-transfer states in the porphine-magnesium porphine dimer

The ground and excited states of a cofacial porphine-magnesium porphine dimer with a ring separation of 5.35 Å are investigated by ab initio configuration interaction calculations, using a floating gaussian basis. A pair of charge-transfer states are found ≈23000 cm^?1 above the ground state, but are lowered by ≈7400 cm^?1 upon coordination of the Mg atom with chloride ion.     

A General Analysis of Field-Based Molecular Similarity Indices

A formalism is presented that incorporates the entirety of all field-based molecular similarity indices of general form S _ij = _ij /h( _ii , _jj ), where the numerator is given by the inner product or overlap of field functions F _i and F _j corresponding to the ith and jth molecules, respectively, and the denominator is given by a suitable mean function of the self-similarities _ii and _jj . This family of similarity indices includes the index initially introduced by Carbó nearly twenty years ago, where h( _ii , _jj ) is taken to be the geometric mean of _ii and _jj , and the well-known indices due to Hodgkin and Richards, and Petke, where h( _ii , _jj ) is taken to be the arithmetic mean and maximum of _ii and _jj , respectively. Two new indices based upon the harmonic mean and minimum of _ii and _jj are also defined, and it is demonstrated that the entire set of field-based similarity indices can be generated from a one-parameter family of functions, called generalized means, through proper choice of the parameter value and suitable limiting procedures. Ordering and rigorous bounds for all of the indices are described as well as a number of inter-relationships among the indices. The generalization of field-based similarity indices, coupled with the relationships among indices that have been developed in the present work, place the basic theory of these indices on a more unified and mathematically rigorous footing that provides a foundation for a better understanding of the quantitative aspects of field-based molecular similarity.