The infrared spectrum of pyrazine in its metastable triplet state. A free-electron-laser study

Abstract

Vibrational transitions of pyrazine in the lowest triplet state have been observed as transient changes in the intensity of the phosphorescence induced by the free-electron laser FELIX. All seven fundamental infrared-active modes in the range of 250 to 1600 cm−1 have been detected and all vibrational frequencies are found to be considerably lower than the corresponding ones in the ground state.

Ab Initio and Density-Functional Calculations of the Vibrational Structure of the Singlet and Triplet Excited States of Pyrazine

“…Tomer et al have tentatively assigned an observed line in the phosphorescence excitation spectrum observed at 800 cm -1 as the overtone band 11 1 0 16b 1 0 , suggesting 563 cm -1 for ν 11 . This frequency coincides with a very weak peak found by Kok et al at 565 cm -1 . Like ν 16b , ν 11 is of b 3u symmetry.…”

“…An asterisk indicates that the potential-energy surface is not a continuous function of that particular mode; i indicates an imaginary frequency, i.e., a transition state. b From Fischer, 17 except for those described in footnote c. c Our assignment of an observed 30 it a localized minimum-energy structure is found. The coordinate change was projected onto the CASSCF normal modes and found to be 2.2Q zpt , and this point is indicated in Figure 2.…”

“…This mode has not been experimentally assigned for T 1 ( 3 B 3u ) although it is observed 6,50,51 at 636 cm -1 for S 1 ( 1 B 3u ). The B3LYP analytical frequency for the spin-contaminated 27 "average" S 1 -T 1 pair is 646 cm -1 , and so a crude estimate of the actual frequency in T 1 would be 636 + 2 × (667 -646) ) 678 cm -1 ; an infrared active mode has been observed 30 in T 1 at 690 cm -1 , and we have assigned this frequency to ν 12 in Table 4. Densityfunctional methods may thus prove very useful in the interpretation of excited-state molecular spectra, and new functionals are being developed for this purpose.…”

“…Recently, Kok et al 30 observed the infrared absorption spectrum of pyrazine T 1 as the transient change in the phosphorescence spectrum (in dodecane matrix at 1.3 K) induced by infrared laser excitation. The spectrum appears among that of the S 0 ground state and that of the matrix and hence requires careful analysis.…”

“…In this work we consider primarily the vibrational structure of the lowest triplet state of pyrazine, T 1 ( 3 B 3u ). For this state, 8 of the 24 vibrational frequencies have been experimentally assigned by Fischer, and various other possible vibration frequencies have been suggested; , we concentrate on the theoretical prediction of the eight assigned frequencies. Note that this state is selected for detailed study in preference to S 1 ( 1 B 3u ) as we can apply a larger range of theoretical techniques to the triplet state, and as much is already known 11 about S 1 and its low-lying conical intersection with S 2 ( 1 B 2u ).…”