Laser-Initiated Shuttling of a Water Molecule Between H-Bonding Sites

Abstract

The two-step laser excitation scheme of stimulated emission pumping (SEP) induces shifts of a single water molecule between two remote hydrogen bonding sites on trans-formanilide. This reaction can be initiated by selective excitation of either isomer (CO-bound or NH-bound) with different SEP excitation wavelengths. Energy (E) thresholds for isomerization in both directions have been measured [796 wave numbers </= E(CO-->NH) </= 988 wave numbers and 750 wave numbers </= E(NH-->CO) </= 988 wave numbers], and the energy difference DE between the CO-bound and NH-bound isomers was extracted (-238 wave numbers </= DE </= +192 wave numbers).

Isolated Neutral Peptides

“…One of the striking features shared by the majority of the peptides studied is the small number of conformations observed in the expansion [107,139], despite the significant, if not large, number of energetically low-lying calculated conformations. This clearly points to important conformational relaxation effects during the expansion caused by collisions with the carrier gas, in line with the spectacular energycontrolled isomerisation experiments [47,71,72,119,180]. Laser-desorbed species are often coupled to an expansion of argon because of its capability to remove efficiently the large internal energy resulting from the vaporisation process and therefore eventually achieve a better vibrational cooling.…”

“…Yet, experimental techniques exist to measure conformational distribution [47], and even conformational energetics [149]. Even though conformational distributions have indeed been measured for a few peptides [47,71,72,125,126], conformational energetics have been measured only for slightly simpler systems [149,180,181]. Conclusions about conformational energetics from the majority of experiments have thus been limited so far to identifying major and minor conformations of the distribution.…”

“…Beyond the characterisation of the hydration sites of model peptide bonds, whose structure [15], energetics [17] and barriers between them [180] have been determined, the approach is especially appealing for flexible molecules, such as peptides, for which dramatic effects can be anticipated, even for a modest environment. Theoretically predicted some time ago [196], experimental illustrations of these effects have been published only quite recently.…”

Controlled Hydrogen‐Bond Breaking in a Rotaxane by Discrete Solvation

“…In the expansion, a distribution of rotaxane-methanol clusters formed that were probed with IR spectroscopy using an IR-UV ion-dip approach. [18][19][20] Methanol can act as a hydrogenbond donor and acceptor, and may therefore form hydrogen bonds with either the free C=O groups of the macrocycle or compete with the intramolecular hydrogen bonds between thread and macrocycle. Microsolvation [21][22][23][24] thus enables a quasi continuous tuning of the hydrogen-bond interactions between thread and macrocycle to be achieved.…”

Shaping of a Conformationally Flexible Molecular Structure for Spectroscopy

“…High-resolution electronic excitation spectroscopy can provide unprecedented levels of experimental detail with regards to molecular potential energy surfaces, structure, and dynamics. [1][2][3] The power of this technique stems from the high sensitivity of excitation spectra to molecular shape (i.e. conformation) and the very narrow line widths which can be resolved with ideal substrates in such experiments.…”