Science in China Series B: Chemistry © 2008 SCIENCE IN CHINA PRESS Springer Sci China Ser B-Chem | Dec. 2008 | vol. 51 | no. 12 | 1166-1173 www.scichina.com chem.scichina.com www.springerlink.com Theoretical study on S 1 ( 1 B 3u ) state electronic structure and absorption spectrum of pyrazine HE RongXing 1 , ZHU ChaoYuan 2† , CHIN Chih-Hao 3 & LIN Sheng-Hsien 2,3 1 College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China; 2 Department of Applied Chemistry, Institute of Molecular Science and Center for Interdisciplinary Molecular Science, National Chiao-Tung University, Hsinchu 300, China; 3 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, China Making use of a set of quantum chemistry methods, the harmonic potential surfaces of the ground state (S 0 ( 1 A g )) and the first (S 1 ( 1 B 3u )) excited state of pyrazine are investigated, and the electronic structures of the two states are characterized. In the present study, the conventional quantum mechanical method, taking account of the Born-Oppenheimer adiabatic approximation, is adopted to simulate the absorp- tion spectrum of S 1 ( 1 B 3u ) state of pyrazine. The assignment of main vibronic transitions is made for S 1 ( 1 B 3u ) state. It is found that the spectral profile is mainly described by the Franck-Condon progression of totally symmetric mode ν 6a . For the five totally symmetric modes, the present calculations show that the frequency differences between the ground and the S 1 ( 1 B 3u ) state are small. Therefore the displaced harmonic oscillator approximation along with Franck-Condon transition is used to simulate S 1 ( 1 B 3u ) absorption spectra. The distortion effect due to the so-called quadratic coupling is demonstrated to be unimportant for the absorption spectrum, except the coupling mode ν 10a . The calculated S 1 ( 1 B 3u ) ab- sorption spectrum is in reasonable agreement with the experimental spectra. pyrazine, absorption spectrum, quantum mechanical method, electronic structure 1 Introduction The electronic structures and absorption spectra of pyra- zine have attracted much attention over the past several decades as it is one of the especially significant mole- cules in relation to intramolecular electronic relaxa- tion [1,2] . In the experimental studies [3－5] , the absorption spectra and the electronic structures of the low-lying excited state have been determined accurately; the first excited state S 1 ( 1 B 3u ) has an nπ* electronic configuration, and the second excited state S 2 ( 1 B 2u ) has a ππ* feature. The so-called “channel three” problem well studied in the benzene has also been demonstrated in pyrazine [3] . That is, the non-radiative decay of S 2 ( 1 B 2u ) state in- creases sharply when the excitation energy is above a threshold, which makes the two types of lowest singlet excited state have different features of the absorption spectra. In the aspect of the theoretical investigations, the spectroscopies and the radiationless decay dynamics of the S 1 ( 1 B 3u ) and S 2 ( 1 B 2u ) state were explored in great detail by means of several different methods [63－11] . Seidner et al. set up an S 1 -S 2 vibronic-coupling model consisting of four most important vibrational modes (ν 1 , ν 6a , ν 9a and ν 10a ) [6,7,12] . Most recently, Shalashilin et al. [10] reported the quantum simulation of absorption spectrum of pyrazine based on the coupled coherent state (CCS) [13] technique. In the previous theoretical studies, the absorption spectra of pyrazine were calcu- Received May 30, 2008; accepted July 8, 2008 doi: 10.1007/s11426-008-0124-2 † Corresponding author (email: cyzhu@mail.nctu.edu.tw) Supported by Taiwan National Science Council (Grant Nos. NSC 96-2113-M-009- 021 and NSC 96-2811-M- 009-023)