Energy flow in hydrogen-bonded microclusters

Multiple hydrogen bonds are thought to play important role in facilitating photo-induced charge and electronic energy transfer processes, particularly in Biology. For unambiguous demonstrations of the effects of multiple hydrogen bonds in chemical dynamics, an important requirement is to work on clean systems. We have been using the dimers of carboxylic acids as models of many of the doubly hydrogen bonded natural systems as shown here.

Doubly H-bonded DNA base pair (top) and structural model (below)

A comparison of the binding energies of selected H-bonded dimmers are presented here.

The data shows that energetically the H-bonds in carboxylic acid dimers are almost equivalent to the well-characterized dimers between two molecules of water, phenol and hydrogen fluoride. However, the unusual characteristics of the doubly H-bonded dimers are revealed in both the IR and electronic spectra. A few examples of our recently measured systems are presented.

The spectrum shown here is the fluorescence excitation spectrum of benzoic acid dimer. The most unusual feature is the appearance of an intense low-frequency band at 57 cm-1 above the S₁←S₀ origin band. However, no progression corresponding to this transition is visible in the spectrum.

Nandi and Chakraborty JCP, 120, 8521 (2004)

A comparison between the fluorescence excitation spectrum and dispersed fluorescence spectrum from the S₁ origin of benzoic acid dimer is shown here. It shows a loss of mirror symmetry relation between the two spectra, particularly region between the two spectra.

Nandi and Chakraborty JCP, 120, 8521 (2004)

The comparison between the DF spectrum of the 57 cm-1 band with that of the electronicorigin shows that the former originates due to extensive vibrational mixing of the intramolecular modes with the doubly H-bonded intermolecular modes of the dimer in S₁.

Nandi and Chakraborty JCP, 120, 8521 (2004)

Hydrogen bond mediated rotor-ring coupling in acetic acid benzoic acid mixed dimer

It is believed over a long time that methyl rotor plays vital role to modulate the dynamics of electronically excited states. We show here that doubly H-bonded interface behaves as an efficient conduit to transmit the rotor effect to promote IVR of the phenyl ring. A comparison of fluorescence excitation and dispersed fluorescence spectra of following two mixed dimers are presented.

Fluorescence excitation spectra of the two mixed dimers. The vibronic structures in S₁ of both species are identical. The bands labeled by asterisks are the homodimers of BA

The comparison between the DF spectra for 6b₀¹ excitations of two dimers shows that methyl group induced extensive IVR in the Acetic acid-Benzoic acid mixed dimer, whereas the spectrum of the formic acid-benzoic acid dimer exhibits quite regular behavior.