Electric Dipole Moments of Nanosolvated Acid Molecules in Water Clusters

Abstract

The electric dipole moments of (H 2 O) n DCl (n=3-9) clusters have been measured by the beam deflection method. Reflecting the (dynamical) charge distribution within the system, the dipole moment contributes information about the microscopic structure of nanoscale solvation.The addition of a DCl molecule to a water cluster results in a strongly enhanced susceptibility.There is evidence for a noticeable rise in the dipole moment occurring at n≈5-6. This size is consistent with predictions for the onset of ionic dissociation. Additionally, a molecular dynamics model suggests that even with a nominally bound impurity an enhanced dipole moment can arise due to the thermal and zero point motion of the proton and the water molecules. The experimental measurements and the calculations draw attention to the importance of fluctuations in defining the polarity of water-based nanoclusters, and generally to the essential role played by motional effects in determining the response of fluxional nanoscale systems under realistic conditions. 2 INTRODUCTION.Water clusters are convenient experimental platforms for the study of the microscopic physics and chemistry of solvation. By monitoring cluster properties as a function of the number of water molecules, it is possible to follow the step-by-step progression of intermolecular interactions [1], following the transition from isolated molecule to bulk By virtue of having a large fraction of their molecules located near the surface, clusters can serve as surrogates for important processes occurring on aerosols and hydrometeors [2,3]. Composed of only a finite number of constituents, they often serve as test beds for theoretical methods and models.Small water clusters doped with an acid molecule -here, for concreteness, hydrogen chloride -have been the subject of a great number of theoretical papers and a sizeable (but regrettably much smaller) number of experimental ones. Hydrogen chloride readily dissociates into H 3 O + and Cl -and the dissociation also takes place in HCl hydrates [4][5][6][7][8], on the ice surface [9][10][11][12] or on larger water nanoparticles [13][14][15] and protonated water cluster ions [16,17]. One inquiry that persistently threads its way through this subject is: what is the minimum quantity of water molecules, (H 2 O) n , required to dissociate the acid [18]? According to calculations, the acidically dissociated structure is supported starting from n=4; most of the theoretical approaches agree that at this size the dissociated form represents the global minimum on the potential energy surface [18][19][20][21][22][23][24]. Experimentally, for neutral clusters the question has been probed by laser spectroscopy (see, e.g., [25][26][27][28]) but finding a conclusive spectroscopic signature of the onset of dissociation in a nanocluster is not straightforward.As a matter of fact, the challenge is not just experimental but conceptual. The free water clusters in natural and laboratory environments generally exist at temperatures appreciably above absolute zero ...

Cold physics and chemistry: Collisions, ionization and reactions inside helium nanodroplets close to zero K

“…Unfortunately, clear observation of ionically dissociated HCl(H2O)n complexes in HNDs remains elusive. Electric dipole moments for DCl(H2O)n (n = 3-9) have, however, been measured via beam deflection [540]. While sharp IR bands are observed for relatively small complexes, broad underlying features become evident as the dopant partial pressures increase.…”

Generating Excess Protons in Microsolvated Acid Clusters under Ambient Conditions: An Issue of Configurational Entropy versus Internal Energy

“…In stark contrast to all this previous research, we focus here on the temperature sensitivity of this reaction, having in mind acid‐catalyzed processes at the typical environmental and physiological conditions of about 300 K. Indeed, there is some evidence in the literature for the stabilization of molecular HCl and recombination of salt ions in small water clusters simply by raising the temperature. However, whereas at room temperature, acid dissociation, excess protons and proton transfer have been extensively investigated in bulk water, the same cannot be said when it comes to limiting the amount of water to only a few molecules.…”

Electronic Structure, Optical Properties and Quantum Chemical Investigation on Synthesized Coumarin Derivative in Liquid Media for Optoelectronic Devices

“…Nevertheless, the solvatochromic method is simpler and accurate compared above mention methods and also it gives a straight measure of the distribution of electrons in the molecule, photons causing excitation of electrons in a molecule resulting the charge distribution takes place which leads to a conformational change in the excited state [22][23][24]. Many of the researchers have intensively reported for the determination of ground and excited state dipole moments experimentally and theoretically, in different types of organic fluorescent compounds like indoles, purines, thiadiazole Nile red dyes, phloroglucinol, quinolones, coumarins, excite dyes, laurdan, badan, benzimidazolone, chalcones [25][26][27][28] etc.…”