Electronic Spectra of Large Molecules

Theoretische Organische Chemie, Organisch-Chemisches Institut der Universitdt Munster, Corrensstra/ e 40, 

Electronic spectra arise from transitions between electronic states of different quantum numbers induced by electromagnetic radiation with ultraviolet or visible (UV/vis) light. The term electronic spectra implies the Born-Oppenheimer (BO) picture of molecules where the electronic and nuclear degrees of freedom are separated. Similarly, the description of the spectra in terms of particular electronic states is valid solely in a small region of the 

Reviews in Computational Chemistry, Volume 20 edited by Kenny B. Lipkowitz, Raima Latter, and Thomas R. Cundari ISBN 0-471-44525-8 Copyright 2004 John Wiley Sons, Inc. 

Excited states are very important in quantum chemistry. Obviously, they are the basic quantities of interest when electronic spectra are considered. Furthermore, because the excited states form a complete basis of the Hamiltonian, all second-order properties such as polarizabilities (van der Waals forces), NMR chemical shifts, ESR g-tensors, or optical rotations of chiral molecules can be calculated quite accurately by sum-over-(excited) state expressions. It should also be clear that any attempt to model photochemical reactions must be preceded by a careful examination of the electronic spectra of the reactants and products in order to deduce the electronic character of the states involved. 

This chapter provides a comprehensive overview of the current status of computational chemistry to describe electronic spectra. The focus is predominantly on larger molecular systems under medium-to-low resolution conditions. A quantitative description of the high-resolution spectra of diatomic to four-atom molecules requires special treatments for vibrational and relativistic 

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Stefan Grimme, Calculation of the Electronic Spectra of Large Molecules. 

The electronic spectra of large molecules, particularly when continuous in nature, do not always allow the confident prediction of spin orbit states... 

Grimme S (2004) Calculation of the electronic spectra of large molecules. In Lipkowitz KB, Larter R, Cundari TR (eds). Reviews in computational chemistry, vol 20. Wiley, Hoboken, pp 153-218... 

Among the numerous intrinsic causes of line broadening in the electronic spectra of large molecules, electronic relaxation (widely called radiationless transitions) is the most important and has received the greatest attention in recent times. 

CNDO/S, CNDO/FK, CNDO/BW, INDO/1, INDO/2, INDO/S and SINDOl. These methods are rarely used in modem computational chemistry, mainly because the modified methods described below usually perform better. Exceptions are INDO based methods, such as SlNDOl and INDO/S. SINDO Symmetric orthogonalized INDO) employes the INDO approximations described above, but not the ZDO approximation for the overlap matrix. The INDO/S method (INDO parameterized for Spectmscopy) is especially designed for calculating electronic spectra of large molecules or systems involving heavy atoms. 

The examples presented in the preceeding sections have illustrated that the present FSGO-based Cl methods are very effective in providing a qualitative or semi-quantitative resolution of the electronic spectra of large molecules. Calculations on molecules containing 200 to 500 or more electrons may be performed with reasonable demands on computer resources. The methods are widely applicable, and the results provide for a useful interplay between theory and experiment in interpreting electronic spectra and understanding spectra-structure correlations. 

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