Torsional Barriers in Aromatic Molecular Clusters as Probe of the Electronic Properties of the Chromophore

Abstract

We present a computer program that is capable of fitting n-fold torsional barriers Vn (n = 2-6) and torsional constants F simultaneously to high- and low-resolution spectroscopic data of different isotopomeric internal rotors. The program has been utilized to fit independently barriers and torsional constants for both electronic states of several aromatic clusters. The constant F of the ammonia moiety in the phenol-ammonia cluster is shown to decrease upon electronic excitation, thus imaging the formation of a hydrogen-bonded complex between the phenoxy radical and the NH4 radical in the excited state. In contrast, for the naphthol-ammonia 1:1 clusters no change of F of ammonia could be found. For phenol-methanol cluster we found a decrease of F upon excitation which points to a stronger hydrogen bond between phenol and methanol in the excited state. A strong reduction of the torsional barrier upon excitation points to the formation of a methoxonium radical in a similar photoreaction as in phenol-ammonia cluster. For the phenol-water system we postulate the same mechanism, a photoreaction, which leads to a translocated hydrogen atom in the S1 state what can be deduced from the change of the torsional constant upon electronic excitation.

Flickering dipoles in the gas phase: Structures, internal dynamics, and dipole moments of β-naphthol-H2O in its ground and excited electronic states

“…The derived barrier heights from the high resolution spectrum of phenol-H 2 O, 4 using the 1D analysis, are V 2 = 177 and V 2 = 127 cm −1 . Jacoby and Schmitt 35 have suggested that the decrease in V 2 upon excitation observed in phenol-H 2 O arises from the loss of the second C − H · · · OH 2 HB. This hypothesis appears consistent with the data presented here on cis-2HNW (V 2 = 180 cm −1 ), as well as a subtle photoinduced change in α (α = 18.5, α = 30.6 • ) that may result from a less favorable secondary interaction between H 2 O and the naphthalene ring.…”

“…The two HB lengths in Fig. 1 are predicted by theory to be 1.89 and 2.74 Å, respectively Jacoby and Schmitt 35 have developed a program (called HTORFIT) in which "best fit" values of F and V n for 1:1 complexes are determined using all available vibrationally and rotationally resolved experimental data. From a global fit of all experimental data, they find that V 2 = 175.4 (13) cm −1 and V 2 = 109.1 (2) cm −1 for phenol-H 2 O.…”

“…the findings from the trans-1HN-NH 3 (trans-1HNA) and trans-2HNA complexes studied using HTORFIT, 35 as well as cis-1HNA and cis-2HNA studied by high resolution spectroscopy. 36 This shows that the axis of motion relative to water does not change when the photon is absorbed.…”

The structure of 4-methylphenol and its water cluster revealed by rotationally resolved UV spectroscopy using a genetic algorithm approach

“…In two recent publications 3,28 we showed how a combined fit to high-resolution rovibronic data and lowresolution torsional transitions can be used to improve the determination of the barriers and torsional constants in both electronic states. A, B, C, V 3 , V 6 , F, and were fitted for each electronic state to the rotationally resolved origin and to the torsional transitions from Ref.…”

Probing the Acidity of p‐Substituted Phenols in the Excited State: Electronic Spectroscopy of the p‐Cyanophenol–Water Cluster

“…The theory of coupling of the two‐fold internal rotation of the water moiety to the overall rotation of the cluster has been elaborated in detail in refs. 1 and 29 and will not be repeated here. The upper trace of Figure 1 shows the experimental spectrum, the following trace the complete simulation using the parameters from Table 1.…”