Isotopic dependence of intramolecular and intermolecular vibrational couplings in cooperative hydrogen bond networks: singly hydrated phenyl-α-D-mannopyranoside as a case study.

Hydrogen bonding (HB) is associated with frequency shifts, spectral broadening and intensity variation of the vibrational bands of the donor stretching modes. This is true in all systems, from the most basic molecular models, to more complex ones, and biological molecules. In the gas phase, the latter can be either fully isolated, with only intramolecular HB, or micro-solvated. The conformations of such systems are stabilized by networks of intramolecular and intermolecular HB where the donor groups can be coupled. This has been well-identified in the case of singly hydrated monosaccharides and in particular for phenyl-α-D-mannopyranoside, where the addition of a single water molecule reduces the number of observed conformations to a unique one, stabilized by such a cooperative network of intramolecular and intermolecular HB. In the present study we have re-examined this prototypical system to scrutinize subtle effects of isotopic substitution in the solvent molecule. Besides the obvious isotopic shift, coupling between intramolecular modes of sugar and water is observed, promoted by the intermolecular HB. The systematic substitution of water with heavy water, or methanol, also allowed the decomposition of the relation between HB strength and frequency shift.