Electronic excitation and ionization spectra have been widely used in various fields of molecular sciences and technologies. These spectroscopies provide useful information about the electronic structures of molecules. The electronic-structure theory is sometimes necessary to understand and identify the electronic states involved in the spectra. Symmetry adapted cluster-configuration interaction (SAC-CI) method[1-3] is a reliable method for studying the electronic structure of atoms and molecules in their ground, excited, ionized and electron-attached states. Spectroscopy is one of the most suitable targets of the SAC-CI method [4,5]. This method has been applied to a wide variety of chemistry; electronic excitation spectroscopy of valence, Rydberg and inner-core states, ionization (photo-electron) spectroscopy in outer-valence, inner-valence and core regions, molecular spectroscopies including multi-electron processes [4,5].
Outer and inner valence ionizations spectra were studied using the SAC-CI general-R method. The theoretical spectrum satisfactorily reproduces the outer- and inner-valence regions of the spectrum. The exponential generation algorithm followed by perturbation selection was shown to be useful in the generation of small and yet effective higher-excitation operators for the SAC-CI general-R method [6,7]. This approach was extended to a multi-reference SAC theory, MEG4/EX-MEG4, and applied to the ionization spectrum of ozone [8].
Theoretical electronic spectrum of TcO4- was calculated with the SAC-CI method. Calculated spectrum is in good agreement with the experimental one. The observed peaks are assigned, and some new peaks were predicted in the higher energy region. The difference and the similarity between the electronic spectra of TcO4- and MnO4- are clarified.
Electronic excitation and ionization spectra of free base porphin are well reproduced and new assignments for the B, N, L, and M bands were proposed (see Figure below). The present result shows that the four-orbital model is strongly perturbed in the B and N bands by the excitation from the low lying 5-th MO. Latter, theoretical investigations extended to various porphyrin compounds, heme[11,12], metal porphyrins[13,14], phthalocyanine[15], porphyrin dimers[14], azaporphyrins[16,17], chlorophylls[18], and other tetrapyroles[19].
[1]H. Nakatsuji, Chem. Phys. Letters, 59 (1978) 362.
[2]H. Nakatsuji, Chem. Phys. Letters, 67 (1979) 329.
[3]H. Nakatsuji, Chem. Phys. Letters, 67 (1979) 334.
[4]SAC-CI Method Applied to Molecular Spectroscopy, M. Ehara, J. Hasegawa, H. Nakatsuji in "Theory and Applications of Computational Chemistry: The First 40 Years, A Volume of Technical and Historical Perspectives", Ed. by C. E. Dykstra, G. Frenking, K. S. Kim, and G. E. Scuseria, pp. 1099-1141, (Elsevier, Oxford, 2005).
[5]Exploring Photo-Biology and Bio-Spectroscopy with the SAC-CI (Symmetry-Adapted Cluster-Configuration Interaction) Method, J. Hasegawa and H. Nakatsuji, in Radiation Induced Molecular Phenomena in Nucleic Acid: A Comprehensive Theoretical and Experimental Analysis, Ed., by M. Shukla and J. Leszczynsk, pp. 93-123, (Springer, 2008).
[6]Theoretical Study on the Ionized States of Ethylene by the SAC-CI (general-R) Method, J. Hasegawa, M. Ehara, and H. Nakatsuji, Chem. Phys., 230(1), 23-30 (1998).
[7]The Outer Valence Ionization Energies of Thiazyl Cyanide, P. Tomasello, J. Hasegawa, and H. Nakatsuji, Europhys. Lett., 41(6), 611-616 (1998).
[8]Excited and ionized states of ozone studied by the MEG (multi-exponentially generated) / EX(excited)-MEG method, Y. Ohtsuka, J. Hasegawa, and H. Nakatsuji, Chem. Phys. 332, 262-270 (2007).
[9]Theoretical Study on the Electronic Spectrum of TcO4-, J. Hasegawa, K. Toyota, M. Hada, H Nakai and H. Nakatsuji, Theoret. Chim. Acta, 92, 351-359 (1995).
[10]Excited and Ionized States of Free Base Porphin Studied by the SAC-CI Method, H. Nakatsuji, J. Hasegawa, and M. Hada, J. Chem. Phys., 104(6), 2321-2329 (1996).
[11]Ground and Excited States of Oxyheme: SAC/SAC-CI Study, H. Nakatsuji, J. Hasegawa, H. Ueda, and M. Hada, Chem. Phys. Lett., 250(3,4), 379-386 (1996).
[12]Ground and Excited States of Carboxyheme: A SAC/SAC-CI Study, H. Nakatsuji, Y. Tokita, J. Hasegawa, M. Hada, Chem. Phys. Lett., 256(1,2), 220-228 (1996).
[13]Ground and Excited States of Mg Porphin Studied by the SAC/SAC-CI Method, J. Hasegawa, M. Hada, M. Nonoguchi, and H. Nakatsuji, Chem. Phys. Lett., 250(2), 159-164 (1996).
[14]Ground and Excited States of Linked and Fused Zinc Porphyrin Dimers: SAC-CI Study, T. Miyahara. H. Nakatsuji, and J. Hasegawa A. Osuka, N. Aratani, and A. Tsuda, J. Chem. Phys., 117, 11196-11207 (2002).
[15] Excited States of Free Base Phthalocyanine Studied by the SAC-CI Method, K. Toyota, J. Hasegawa, and H. Nakatsuji, J. Phys. Chem., 101(4), 446-451 (1997).
[16]SAC-CI Study of the Excited States of Free Base Tetrazaporphin, K. Toyota, J. Hasegawa, and H. Nakatsuji, Chem. Phys. Lett., 250(5,6), 437-442 (1996).
[17]Aza-substitution Effect on the Q-band Excitations of Free-base Porphin, Chlorin, and Bacteriochlorin: SAC-CI Theoretical Study, J. Hasegawa, T. Kimura, and H. Nakatsuji, J. Por. Phtha., 9, 305 (2005).
[18]Theoretical Study of the Excited States of Chlorin, Bacteriochlorin, Pheophytin a, and Chlorophyll a by the SAC/SAC-CI Method, J. Hasegawa, Y. Ozeki, K. Ohkawa, M. Hada, and H. Nakatsuji, J. Phys. Chem. B, 102 (7), 1320-1326 (1998).
[19]Excited States of Porphyrin Isomers and Porphycene Derivatives: A SAC-CI Study, J. Hasegawa, K. Takata, T. Miyahara, S. Neya, M. J. Frisch, and H. Nakatsuji, J. Phys. Chem. A, 109(14), 3187-3200 (2005).