Zero kinetic energy spectroscopy of hydroquinone-water (1:1) complex: A probe for conformer assignment

Abstract

Zero kinetic energy (ZEKE) photoelectron spectroscopy of the hydroquinone-water (HQW) complex was carried out to characterize its S(1)-S(0) resonantly enhanced multiphoton ionization (REMPI) spectrum in terms of the cis and trans conformers. The ZEKE spectra of the hydroquinone isomers show differences in the Franck-Condon (FC) activity of a few ring modes, viz., modes 15, 9b, and 6b, due to the different symmetries of the two isomers. These modes were used as a "diagnostic tool" to carry out the categorical assignment of the REMPI spectrum of the HQW complex. It was found that the FC activity of these diagnostic modes in the cationic ground state (D(0)) of the water complex is similar as that of the monomer. The two lowest energy transitions in the REMPI spectrum of the water complex, 33,175 and 33,209 cm(-1), were reassigned as the band origins of the cis and trans hydroquinone-water complexes, which is opposite of the previous assignment. The intermolecular stretching mode (sigma) of the complex shows a long progression, up to v(')=4, in the cationic ground state and is strongly coupled to other observed ring modes. The Franck-Condon factors for different members in the progression were calculated using the potential energy surfaces computed ab initio. These agree well with the observed intensity patterns in the progression. The ionization potential of the trans and cis complexes was determined to be 60,071+/-4 and 60,024+/-4 cm(-1), respectively.

Spatial Isolation of Conformational Isomers of Hydroquinone and Its Water Cluster Using the Stark Deflector

“…Therefore, it is quite certain that the 33 192 cm –1 band is the origin band of the cis -HyQ–H 2 O cluster whereas the 33 155 cm –1 band should be due to the trans -HyQ–H 2 O cluster according to our Stark–R2PI spectroscopy. This is consistent with the assignment from ref rather than that from ref even though the latter is more recently reported. Furthermore, low-frequency van der Waals modes associated with the specific conformational cluster isomer could be also identified.…”

“…Later, however, from conformer-specific zero-electron kinetic energy (ZEKE) spectroscopic study of the same system combined with Franck−Condon analyses, the conformational assignment was reversed by the same research group, and thus 33 155 and 33 192 cm −1 bands were assigned as S 1 −S 0 origins of cis-HyQ− H 2 O and trans-HyQ−H 2 O clusters, respectively. 36 This controversy stems from the fact that electrostatic differences of two cluster conformational isomers in the ground and excited states are quite subtle for theoretical calculations to be decisive.…”

“…We would like to emphasize a great advantage of the Stark deflector in terms of conformer-specific spectroscopic assignment as one can identify specific conformers from the extent of deflection of each conformer . Specifically, this allows us to put an end to the dispute about spectral assignment of trans -HyQ–H 2 O or cis -HyQ–H 2 O cluster, which was not unambiguously identified by hole-burning spectroscopic or conformer-specific photoionization studies of the same system. , Even though separation of molecular cluster from monomer in the Stark deflector had been previously demonstrated, we believe that this is the first report for the spatial separation of conformational isomers for molecular clusters.…”

S1-State Decay Dynamics of Benzenediols (Catechol, Resorcinol, and Hydroquinone) and Their 1:1 Water Clusters

“…The 1:1 water clusters with these molecules are also studied. Actually, spectroscopy and dynamics of the excited states of all three dihydroxybenzenes had been much studied to date. ,− For instance, the state-specific S 1 lifetime of catechol was reported to be 7 or 11 ps at the S 1 origin and first quantum of the OH torsional mode, respectively . This is more or less consistent with the results that was independently obtained from the femtosecond (fs) , or picosecond (ps) , time-resolved studies.…”

“…According to the spectroscopic studies by the Wategaonkar group, , the vibronic band located at ∼156 cm –1 above the S 1 origin of the hydroquinone­( cis )·water or hydroquinone­( trans )·water cluster corresponds to the intermolecular hydrogen bond stretching (σ) mode, giving the corresponding S 1 lifetime of ∼2.10 or ∼2.31 ns, respectively (Supporting Information). In contrast to the case of the catechol·water cluster where the σ mode excitation accelerates the S 1 relaxation rate twice, the influence of σ mode excitation seems to be little in both conformers of the hydroquinone·water cluster, implying that the ESHT may not be activated.…”