A theoretical study of the isotope effects on the fluorescence excitation spectrum of 5-aminotropolone

Abstract

A combined ab initio+nuclear dynamics study is performed to theoretically analyze the intramolecular H-atom transfer process in 5-aminotropolone in both the ground (S0) and first excited (S1) singlet electronic states. A complete active space self-consistent field (CASSCF) method is used to optimize the geometries. Energies are then corrected through the second order Mo/ller–Plesset perturbation theory. These results are used to build up reduced bidimensional energy surfaces so that the nuclear wave functions for the nuclear motions in both electronic states are obtained. In particular we have analyzed the six isotopomers that result from deuteration of the amino and hydroxy groups of 5-aminotropolone. It is found that for symmetric structures (−OH/−NH2, −OH/−ND2, −OD/−NH2, and −OD/−ND2), the two lowest vibrational levels in both S0 and S1 appear as a quasidegenerated tunneling doublet. The tunneling splitting in S0 is much lower so that the doublet at the origin, seen in the fluorescence excitation spectra of 5-aminotropolone, can be entirely assigned to the S1 state. In agreement with the experimental findings, this splitting greatly diminishes when the transferring hydrogen is substituted by a deuterium, whereas deuteration of the amino group produces only a modest decrease of such a splitting. A quite different result is found for the nonsymmetric isotopically substituted structures (−OH/−NHD and −OD/−NHD), as the isotope induced asymmetry, combined with the high energy barrier in the S0 potential energy surface, leads to a complete localization of the two lowest vibrational wave functions in S0. On the other hand, for S1 the asymmetry and energy barriers are low enough so that an important degree of delocalization of the two lowest vibrational wave functions is found. These results are again in agreement with the presence of an isotope induced quartet in the fluorescence excitation spectra of these species.

Rotationally resolved electronic spectra of 9,10-dihydrophenanthrene. A “floppy” molecule in the gas phase

“…twisting coordinate, evidencing a kind of cooperative motion involving the two coordinates. A similar behavior has been observed in tropolone, [28][29][30][31] where it has been shown that the barrier to intramolecular hydrogen atom transfer from the -OH to the C v O group is substantially reduced by excitation of -C v C-stretching modes in the seven-membered ring.…”

Theoretical Studies of Excited State Proton Transfer in Small Model Systems

“…The computational study of hydrogen bonds and proton transfers in their ground states has been sufficiently extensive that there exists a general understanding of the errors incurred by the use of any particular method. For example, it is widely recognized that enlargements of basis set lead to higher transfer barriers, while electron correlation reduces the barrier. − Moreover, there is every indication that most of the popular means of including correlation, i.e., MP2, MP4, QCISD, coupled cluster, etc., are in fair agreement concerning transfer barriers. − Transfers in excited states, on the other hand, have been studied far less extensively, so that there is no consensus yet as to which methods are most appropriate and accurate. Indeed, this issue is not limited to the proton transfer problem; computational methodology for excited states of molecular systems is generally much less mature than for closed-shell ground electronic states.…”

Theoretical Study of 5-phenyltropolone in the S0 and S1 States

“…Introduction of asymmetry by substituting one D atom into the NH 2 group quenched proton tunneling in the ground state . These experimental findings have been supported by a combined ab initio + nuclear dynamic study . In 5-hydroxytropolone, the two in-plane orientations of the 5-OH substituent produced two isomers, syn and anti, relative to the 2-OH. , It has been postulated that there are two reaction coordinates for the syn-anti photoisomerization; these coordinates involve 2-OH tunneling and 5-OH torsion. − Nishi et al have clarified the coupling as regards the internal rotation of the methyl group and the intramolecular proton transfer. , They showed a drastic decrease in the tunneling splitting of the zero-vibrational level, compared to that of TRN.…”

“…4 These experimental findings have been supported by a combined ab initio + nuclear dynamic study. 9 In 5-hydroxytropolone, the two inplane orientations of the 5-OH substituent produced two isomers, syn and anti, relative to the 2-OH. 5,6 It has been postulated that there are two reaction coordinates for the syn-anti photoisomerization; these coordinates involve 2-OH tunneling and 5-OH torsion.…”

“…It is interesting to know the effects of internal motion of substituents on the proton tunneling of TRN. Many studies have been performed to examine the torsional mode coupling effects of substituents in the proton tunneling of TRN. − In the ground state of 5-aminotropolone, intramolecular proton tunneling between equivalent minima has been reported to be accompanied by an internal rotation of the amino group . Introduction of asymmetry by substituting one D atom into the NH 2 group quenched proton tunneling in the ground state .…”