STRUCTURE AND VIBRATIONS OF CATECHOL AND CATECHOL-CENTER-DOT-H2O(D2O)IN THE S-0 AND S-1 STATE

The inter- and intramolecular vibrations in the S-0 and S-1 state of c atechol, d2-catechol, catechol(H2O)(1), and d2-catechol (D2O)(1) have been investigated experimentally by resonant two photon ionization (R2 PI), spectral hole burning (SHB), and dispersed fluorescence spectrosc opy (DF). The experimental frequencies are compared to the vibrational frequencies obtained from ab initio normal mode calculations using th e 6-31G(d,p) basis set. In order to get a complete interpretation of t he S-0 state spectra of d2-catechol the strong coupling of the two OD torsional motions has been taken into account. A two-dimensional calcu lation of the torsional eigenvalues based on an ab initio potential [- 31G(d,p) basis] obtained from single point calculations is presented. Due to these calculations all vibrations in the S-0 state can be assig ned. Furthermore a new assignment of the vibrations in the S-1 state o f d2-catechol is given. In the case of catechol (H2O)(1) [d2-catechol( D2O)(1)] different structural isomers are discussed. Using HF ab initi o calculations (including MP2, BSSE, and ZPE corrections) a trans-line ar hydrogen bonding arrangement turns out to be more stable by an amou nt of 840 cm(-1) compared to a cyclic structure which is also a minimu m of the PES. Normal mode calculations have been carried out for both structures and anharmonic corrections are calculated for the tau and b eta(2) mode of the trans-linear arrangement. The prediction of the ab initio calculations is supported by the vibrational transitions observ ed in the spectra of the S-0 and S-1 state, which can be assigned on t he basis of the vibrations calculated for the trans-linear structure. The most important feature of the R2PI spectrum of catechol(H2O)(1) [d 2-catechol(D2O)(1)] is the occurrence of intermolecular vibrations of very low frequencies (14, 37 cm(-1)). These vibrations and the low fre quency torsional modes in the spectra of the S-1 state of the catechol monomer strongly support the assumption that catechol is nonplanar in the S-1 state with respect to the OH groups. Due to this nonplanarity a double minimum potential for the intermolecular rho(1) mode of cate chol(H2O)(1) is postulated. Using this assumption the low frequency vi brations of the R2PI spectra as well as the vibrations observed in the spectra of the S-0 state can be assigned. (C) 1996 American Institute of Physics.