Zbigniew Kisiel

Structures of Cage, Prism, and Book Isomers of Water Hexamer from Broadband Rotational Spectroscopy

Cage, Book, and Prism The array of hydrogen bonds governing the extended structure of liquid water is so intricate that chemists have often sought to understand it by studying simpler clusters. Even so, it has been challenging to get a handle on the preferred arrangement adopted by just six water molecules. Interdependent theoretical and spectroscopic studies have narrowed down the lowest-energy hexamer structures to three isomers—respectively designated the cage, the book, and the prism—but their relative energies remain uncertain. Now, Pérez et al. (p. 897; see the Perspective by Saykally and Wales) have observed all three isomers in a single experiment, using Fourier transform microwave spectroscopy, and were able to establish definitively their energy ordering. Observing three distinct water clusters in the same experiment resolves long-standing questions about their relative stabilities. Theory predicts the water hexamer to be the smallest water cluster with a three-dimensional hydrogen-bonding network as its minimum energy structure. There are several possible low-energy isomers, and calculations with different methods and basis sets assign them different relative stabilities. Previous experimental work has provided evidence for the cage, book, and cyclic isomers, but no experiment has identified multiple coexisting structures. Here, we report that broadband rotational spectroscopy in a pulsed supersonic expansion unambiguously identifies all three isomers; we determined their oxygen framework structures by means of oxygen-18–substituted water (H218O). Relative isomer populations at different expansion conditions establish that the cage isomer is the minimum energy structure. Rotational spectra consistent with predicted heptamer and nonamer structures have also been identified.

Least-squares mass-dependence molecular structures for selected weakly bound intermolecular clusters

Abstract The applicability of the recently proposed method of Watson et al. [J. Mol. Spectrosc. 196 (1999) 102] to the determination of geometries of weakly bound clusters was tested. The method delivers equilibrium–quality structural parameters from highly precise fits to only the ground state rotational constants and very encouraging results were obtained for several families of clusters: Rg–HX, N 2 –HX, H 2 O –HX, and Ar 2 HX . Considerable improvement in standard deviations of least-squares fits of geometry relative to ground state fits was obtained. In cases where corroborating information concerning the values of equilibrium geometrical parameters was available, those were found to be reproduced successfully, in particular for Ar–HF, Ar–HCl, H 2 O –HF, and H 2 O –HCl. The long established large amplitude structural averaging correction for complexed hydrogen halides, based on a quadrupolar averaging angle, was found to be a well-defined term in the parameterisation of vibration–rotation contributions to the ground state moments of inertia. However, it is one of several mutually canceling terms and is not necessarily a dominating term in the vibration–rotation contribution. A computer code for such calculations is made available.

Concerted hydrogen-bond breaking by quantum tunneling in the water hexamer prism

Gear-like rotation by a wobbly water duo The molecules in liquid water move about constantly, but on average they cling to each other through hydrogen bonds, like dancers who keep switching partners. Richardson et al. uncovered a fresh twist in this molecular dance (see the Perspective by Clary). Studying clusters of six molecules each—essentially the smallest three-dimensional water droplets—they observed coupled motion of two different molecules in the cluster. The process breaks two different hydrogen bonds concurrently in a pattern akin to rotating gears. Science, this issue p. 1310; see also p. 1267 Rotational spectroscopy and accompanying theory uncover gearlike joint motion of a pair of water molecules in a cluster. [Also see Perspective by Clary] The nature of the intermolecular forces between water molecules is the same in small hydrogen-bonded clusters as in the bulk. The rotational spectra of the clusters therefore give insight into the intermolecular forces present in liquid water and ice. The water hexamer is the smallest water cluster to support low-energy structures with branched three-dimensional hydrogen-bond networks, rather than cyclic two-dimensional topologies. Here we report measurements of splitting patterns in rotational transitions of the water hexamer prism, and we used quantum simulations to show that they result from geared and antigeared rotations of a pair of water molecules. Unlike previously reported tunneling motions in water clusters, the geared motion involves the concerted breaking of two hydrogen bonds. Similar types of motion may be feasible in interfacial and confined water.

Electric dipole moments of the cyclic trimers (H2O)2HCl and (H2O)2HBr from Stark effects in their rotational spectra

Abstract The electric dipole moments of two weakly bound cyclic trimers, (H 2 O) 2 HCl and (H 2 O) 2 HBr, have been determined from Stark effect measurements in rotational spectra recorded at conditions of supersonic expansion. The experimental results are compared with ab initio and induction calculations and the dipole moments of all three monomers are found to be subject to appreciable enhancement on complexation. The effect on the total dipole moment of each trimer is, however, less marked owing to partial cancellation of the various component moments. A novel electrode system for Stark effect measurements in a cavity Fourier transform microwave spectrometer is also described.

Formation and photostability of N-heterocycles in space I. The effect of nitrogen on the photostability of small aromatic molecules

Nitrogen-containing cyclic organic molecules (N-heterocycles) play important roles in terrestrial biology, for exam- ple as the nucleobases in genetic material. It has previously been shown that nucleobases are unlikely to form and survive in interstellar and circumstellar environments. Also, they were found to be unstable against ultraviolet (UV) radiation. However, nucleobases were detected in carbonaceous meteorites, suggesting their formation and survival is possible outside the Earth. In this study, the nucleobase precursor pyrimidine and the related N-heterocycles pyridine and s-triazine were tested for UV stabil- ity. All three N-heterocycles were found to photolyse rapidly and their stability decreased with an increasing number of nitrogen atoms in the ring. The laboratory results were extrapolated to astronomically relevant environments. In the diffuse interstellar medium (ISM) these N-heterocycles in the gas phase would be destroyed in 10-100 years, while in the Solar System at 1 AU distance from the Sun their lifetime would not extend beyond several hours. The only environment where small N-heterocycles could survive, is in dense clouds. Pyridine and pyrimidine, but not s-triazine, could survive the average lifetime of such a cloud. The regions of circumstellar envelopes where dust attenuates the UV flux, may provide a source for the detection of N-heterocycles. We conclude that these results have important consequences for the detectability of N-heterocycles in astro- nomical environments.

A search for interstellar pyrimidine

We have searched three hot molecular cores for submillimetre emission from the nucleic acid building block pyrimidine. We obtain upper limits to the total pyrimidine (beam-averaged) column densities towards Sgr B2(N), Orion KL and W51 el/e2 of 1.7 × 10 1 4 , 2.4 × 10 1 4 and 3.4 × 10 1 4 cm - 2 , respectively. The associated upper limits to the pyrimidine fractional abundances lie in the range (0.3-3) x 10 - 1 0 . Implications of this result for interstellar organic chemistry, and for the prospects of detecting nitrogen heterocycles in general, are discussed briefly.

Rotational spectra and structures of van der Waals dimers of Ar with a series of fluorocarbons: Ar⋅⋅⋅CH2CHF, Ar⋅⋅⋅CH2CF2, and Ar⋅⋅⋅CHFCF2

Rotational spectra of van der Waals dimers between an argon atom and CH2CHF, CH2CF2, and CHFCF2 have been obtained by pulsed‐supersonic nozzle Fourier transform microwave spectroscopy. Analysis of the derived spectroscopic constants shows that the dimers have structures such that for CH2CHF, CH2CF2, and CHFCF2 the Ar atom is positioned over the FCCH, FCF, and FCCF atomic chains with Ar‐molecular center‐of‐mass distances of 3.62 A, 3.51 A, and 3.56 A, and angles between the Ar–cm axis and molecular planes of 48.2°, 72.9°, and 60.5°, respectively. Structures for the three dimers are also predicted with a simple multisite model which describes the anisotropy of the dispersive interaction; both the Ar acceptor site and the atom–atom distances are satisfactorily reproduced.

Rotational spectra of quinoline and of isoquinoline: spectroscopic constants and electric dipole moments

Abstract Rotational spectra of quinoline and of isoquinoline have been observed in the centimeter- and millimeter-wave regions. The spectra were assigned on the basis of bands formed by high-J transitions, which were measured up to J″⩽128 and ν⩽234 GHz. Complementary measurements were also made on low-J, centimeter-wave spectra observed in supersonic expansion and with fully resolved nuclear quadrupole hyperfine structure. Accurate rotational, centrifugal distortion and hyperfine splitting constants for the ground states of both molecules are reported. The electric dipole moments for the two molecules were also determined from Stark effect measurements and are μa=0.14355(19), μb=2.0146(17), μtot=2.0197(17) D for quinoline, and μa=2.3602(21), μb=0.9051(14), μtot=2.5278(20) D for isoquinoline. The experimental observables were found to be rather accurately predicted by MP2/6-31G** ab initio calculations, and corresponding molecular geometries are also reported.

The rotational spectra, electric dipole moments and molecular structures of anisole and benzaldehyde

The rotational spectra of anisole and of benzaldehyde were investigated in supersonic expansion at frequencies up to 41 GHz, and at room temperature in the millimetre-wave region, from 170 to 330 GHz. Accurate spectroscopic constants for the parent isotopomers in the ground vibrational state and for the first excited torsional state were determined for both molecules. The supersonic expansion spectrum allowed measurement, in natural abundance, of all singly substituted 13C isotopomers, as well as of the 18O isotopomer for both anisole and benzaldehyde. The rotational constants were used to determine the r(s) and the r(m)(1) gas-phase geometries, which are found to be consistent with prediction of bond length alternation in the phenyl ring induced by the asymmetric substituent. Stark measurements were made on the supersonic expansion spectrum resulting in electric dipole moment determination, /mu a/ = 2.9061(22) D, /mu b/ = 1.1883(10) D, /mu tOt/ = 3.1397(24) D for benzaldehyde and /mu a/ = 0.6937(12) D, /mu b/ = 1.0547(8) D, mu tOt = 1.2623(14) D for anisole. During the investigation it was found that use of a carrier gas mixture consisting of 30% Ar in He carries significant advantages for studies of weak lines, and pertinent experimental details are reported.

Pre-reactive complexes in mixtures of water vapour with halogens: characterisation of H2O...ClF and H2O...F2 by a combination of rotational spectroscopy and ab initio calculations.

Complexes H2O...ClF and H2O...F2 were detected by means of their ground-state rotational spectra in mixtures of water vapour with chlorine monofluoride and difluorine, respectively. A fast-mixing nozzle was used in conjunction with a pulsed-jet, Fourier-transform microwave spectrometer to preclude the vigorous chemical reaction that these dihalogen species undergo with water. The ground-state spectra of seven isotopomers (H2 16O...35ClF, H2 16O...ClF, H2 18O...35ClF, D2 16O... 35ClF, D2 16O...37ClF, HDO...35ClF and HDO...37ClF) of the ClF complex and five isotopomers (H2O...F2, H2 18O...F2, D2O...F2, D2 18O...Fi and HDO...F2) of the F2 complex were analysed to yield rotational constants, quartic centrifugal distortion constants and nuclear hyperfine coupling constants. These spectroscopic constants were interpreted with the aid of simple models of the complexes to give effective geometries and intermolecular stretching force constants. Isotopic substitution showed that in each complex the H2O molecule acts as the electron donor and either CIF or F2 acts as the electron acceptor, with nuclei in the order H2O...ClF or H2O...F2. For H2O...ClF, the angle phi between the bisector of the HOH angle and the O...Cl internuclear line has the value 58.9(16)degrees, while the distance r(O...Cl)= 2.6081(23) A. The corresponding quantities for H2O...F2 are phi = 48.5(21)degrees and r(O...Fi) = 2.7480(27) A, where Fi indicates the inner F atom. The potential energy V(phi) as a function of the angle phi was obtained from ab initio calculations at the aug-cc-pVDZ/MP2 level of theory for each complex by carrying out geometry optimisations at fixed values of phi in the range +/-80degrees. The global minimum corresponded to a complex of Cs symmetry with a pyramidal configuration at O in each. The function V(phi) was of the double-minimum type in each case with equilibrium values phie = +/-55.8degrees and +/-40.5degrees for H2O...ClF and H2O...F2, respectively. The barrier at the planar C2v conformation was V0= 174cm(-1) for H2O...ClF and 7cm(-1) for H2O...F2. For the latter complex, the zero-point energy level lies above the top of the barrier.