Rotational spectroscopic study of hydrogen cyanide embedded in small 4He clusters

Understanding (coupled) large amplitude motions: the interplay of microwave spectroscopy, spectral modeling, and quantum chemistry

A large variety of molecules contain large amplitude motions (LAMs), inter alia internal rotation and inversion tunneling, resulting in tunneling splittings in their rotational spectrum. We will present the modern strategy to study LAMs using a combination of molecular jet Fourier transform microwave spectroscopy, spectral modeling, and quantum chemical calculations to characterize such systems by the analysis of their rotational spectra. This interplay is particularly successful in decoding complex spectra revealing LAMs and providing reference data for fundamental physics, astrochemistry, atmospheric/environmental chemistry and analytics, or fundamental researches in physical chemistry. Addressing experimental key aspects, a brief presentation on the two most popular types of state-of-the-art Fourier transform microwave spectrometer technology, i.e., pulsed supersonic jet expansion–based spectrometers employing narrow-band pulse or broad-band chirp excitation, will be given first. Secondly, the use of quantum chemistry as a supporting tool for rotational spectroscopy will be discussed with emphasis on conformational analysis. Several computer codes for fitting rotational spectra exhibiting fine structure arising from LAMs are discussed with their advantages and drawbacks. Furthermore, a number of examples will provide an overview on the wealth of information that can be drawn from the rotational spectra, leading to new insights into the molecular structure and dynamics. The focus will be on the interpretation of potential barriers and how LAMs can act as sensors within molecules to help us understand the molecular behavior in the laboratory and nature.

Competition Between Intra- and Intermolecular Hydrogen Bonding: o-Anisic Acid⋅⋅⋅Formic Acid Heterodimer

Four conformers of the heterodimer o-anisic acid-formic acid, formed in a supersonic expansion, have been probed by Fourier transform microwave spectroscopy. Two of these forms have the typical double intermolecular hydrogen-bond cyclic structure. The other two show the o-anisic acid moiety bearing a trans-COOH arrangement supported by an intramolecular O-H⋅⋅⋅O bond to the neighbor methoxy group. In these conformers, formic acid interacts with o-anisic acid mainly through an intermolecular O-H⋅⋅⋅O hydrogen bond either to the O-H or to the C=O moieties, reinforced by other weak interactions. Surprisingly, the most abundant conformer in the supersonic expansion is the complex in which the o-anisic acid is in trans arrangement with the formic acid interacting with the O-H group. Such a trans-COOH arrangement in which the intramolecular hydrogen bond dominates over the usually observed double intermolecular hydrogen bond interaction has never been observed previously in an acid-acid dimer.

A microwave spectroscopic and ab initio study of keto–enol tautomerism and isomerism in the cyclohexanone–water complex

Experimental structure and keto–enol conversion barrier of cyclohexanone–water from microwave spectroscopy and ab initio calculations.

A Direct Link from the Gas to the Condensed Phase: A Rotational Spectroscopic Study of 2,2,2-Trifluoroethanol Trimers

Rotational spectra of the three most stable conformers (I, II, III) of the ternary 2,2,2-trifluoroethanol (TFE) cluster were measured using Fourier transform microwave spectrometers, and unambiguously assigned with the aid of ab initio calculations. The most stable conformer, I, contains one trans-TFE subunit which is unstable in its isolated gas phase form. The study offers new insights into how the trans conformation is stabilized in TFE clusters of increasing size, and eventually becomes a dominant conformation in the liquid phase. A detailed analysis shows that while O-H⋅⋅⋅O-H and O-H⋅⋅⋅F-C hydrogen bonds are the most significant attractive interactions which stabilize all three conformers, the main driving force for the stability of I over III, which has all gauche-TFE subunits, is the attractive interaction of C-F⋅⋅⋅F-C contact pairs. A new type of three-point F⋅⋅⋅F⋅⋅⋅F attractive contact interaction is also identified.

Constructing accurate interaction potentials to describe the microsolvation of protonated methane by helium atoms

An ab initio-derived interaction potential is derived and used in path integral Monte Carlo simulations to investigate stationary-point structures of CH₅⁺ microsolvated by up to four helium atoms.

Tunnelling and barrier-less motions in the 2-fluoroethanol–water complex: a rotational spectroscopic and ab initio study

Rotational spectrum of 2-fluoroethanol–water reveals interesting water and methyl internal rotation tunneling and barrier-less motions in the hydrogen-bonded complex.

Rotational spectroscopy of the methyl glycidate–water complex: conformation and water and methyl rotor tunnelling motions

Rotational transitions of methyl glycidate–water exhibit relatively large water tunnelling splittings, a surprise considering that water is quite tightly bound.

Unusual H-Bond Topology and Bifurcated H-bonds in the 2-Fluoroethanol Trimer

By using a combination of rotational spectroscopy and ab initio calculations, an unusual H-bond topology was revealed for the 2-fluoroethanol trimer. The trimer exhibits a strong heterochiral preference and adopts an open OH⋅⋅⋅OH H-bond topology while utilizing two types of bifurcated H-bonds involving organic fluorine. This is in stark contrast to the cyclic OH⋅⋅⋅OH H-bond topology adopted by trimers of water and other simple alcohols. The strengths of different H-bonds in the trimer were analyzed by using the quantum theory of atoms in molecules. The study showcases a remarkable example of a chirality-induced switch in H-bond topology in a simple transient chiral fluoroalcohol. It provides important insight into the H-bond topologies of small fluoroalcohol aggregates, which are proposed to play a key role in protein folding and in enantioselective reactions and separations where fluoroalcohols serve as a (co)solvent.

Microwave spectroscopy of the seeded binary and ternary clusters CO-(pH2)2, CO-pH2-He, CO-HD, and CO-(oD2)(N=1,2)

We report the Fourier transform microwave spectra of the a-type J = 1-0 transitions of the binary and ternary CO-(pH2)2, CO-pH2-He, CO-HD, and CO-(oD2)N=1,2 clusters. In addition to the normal isotopologue of CO for all clusters, we observed the transitions of the minor isotopologues, (13)C(16)O, (12)C(18)O, and (13)C(18)O, for CO-(pH2)2 and CO-pH2-He. All transitions lie within 335 MHz of the experimentally or theoretically predicted values. In comparison to previously reported infrared spectra [Moroni et al., J. Chem. Phys. 122, 094314 (2005)], we are able to tentatively determine the vibrational shift for CO-pH2-He, in addition to its b-type J = 1-0 transition frequency. The a-type frequency of CO-pH2-He is similar to that of CO-He2 [Surin et al., Phys. Rev. Lett. 101, 233401 (2008)], suggesting that the pH2 molecule has a strong localizing effect on the He density. Perturbation theory analysis of CO-oD2 reveals that it is approximately T-shaped, with an anisotropy of the intermolecular potential amounting to ∼9 cm(-1).

Chirality Synchronization in Trifluoroethanol Dimer Revisited: The Missing Heterochiral Dimer

Chirality self-recognition in the dimer of transient chiral 2,2,2-trifluoroethanol (TFE) is studied using chirped pulse and cavity-based Fourier transform microwave spectroscopy with the aid of ab initio calculations. The broad-band and extreme high-resolution capabilities enable us to assign rotational spectra of the most stable homo- and heterochiral dimers and analyze their structural and dynamical properties in detail. A strong preference for the homochiral over the heterochiral diastereomers is observed. The current study unambiguously identifies the structure of the most stable homochiral dimer and supports the identification by the previous low-resolution infrared study. More importantly, it also indisputably detects the so far elusive, most stable heterochiral dimer.

Microscopic superfluidity in 4He clusters stirred by a rotating impurity molecule

The effective moment of inertia of a CO impurity molecule in 4HeN and p-(H2)N solvent clusters initially increases with N but then commences a nonclassical decrease at N=4 (4He) or N=6 (p-H2). This suggests molecule-solvent decoupling and a transition to microscopic superfluidity. However, the quantum decoupling mechanism has not been elucidated. To understand the decoupling mechanism, a one-dimensional model is introduced in which the 4He atoms are confined to a ring. This model captures the physics and shows that decoupling happens primarily because of bosonic solvent-solvent repulsion. Quantum Monte Carlo and basis set calculations suggest that the system can be modeled as a stirred Tonks-Girardeau gas. This allows the N-particle time-dependent Schrödinger equation to be solved directly. Computations of the integrated particle current reveal a threshold for stirring and current generation, indicative of superfluidity.

Probing the Superfluid Response of para-Hydrogen with a Sulfur Dioxide Dopant

We recently presented the first attempt at using an asymmetric top molecule (para-water) to probe the superfluidity of nanoclusters (of para-hydrogen) [ Zeng , T. ; Li , H. ; Roy , P.-N. J. Phys. Chem. Lett. 2013 , 4 , 18 - 22 ]. Unfortunately, para-water could not be used to probe the para-hydrogen superfluid response. We now report a theoretical simulation of sulfur dioxide rotating in para-hydrogen clusters and show that this asymmetric top can serve as a genuine probe of superfluidity. With this probe, we predict that as few as four para-hydrogen molecules are enough to form a superfluid cluster, the smallest superfluid system to date. We also propose the concept of "exchange superfluid fraction" as a more precise measurement. New superfluid scenarios brought about by an asymmetric top dopant and potential experimental measurements are discussed.