Daniel P. Zaleski

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.

LABORATORY AND TENTATIVE INTERSTELLAR DETECTION OF TRANS-METHYL FORMATE USING THE PUBLICLY AVAILABLE GREEN BANK TELESCOPE PRIMOS SURVEY

The rotational spectrum of the higher-energy trans conformational isomer of methyl formate has been assigned for the first time using several pulsed-jet Fourier transform microwave spectrometers in the 6-60 GHz frequency range. This species has also been sought toward the Sagittarius B2(N) molecular cloud using the publicly available PRIMOS survey from the Green Bank Telescope. We detect seven absorption features in the survey that coincide with laboratory transitions of trans-methyl formate, from which we derive a column density of 3.1 (+2.6, -1.2) \times 10^13 cm-2 and a rotational temperature of 7.6 \pm 1.5 K. This excitation temperature is significantly lower than that of the more stable cis conformer in the same source but is consistent with that of other complex molecular species recently detected in Sgr B2(N). The difference in the rotational temperatures of the two conformers suggests that they have different spatial distributions in this source. As the abundance of trans-methyl formate is far higher than would be expected if the cis and trans conformers are in thermodynamic equilibrium, processes that could preferentially form trans-methyl formate in this region are discussed. We also discuss measurements that could be performed to make this detection more certain. This manuscript demonstrates how publicly available broadband radio astronomical surveys of chemically rich molecular clouds can be used in conjunction with laboratory rotational spectroscopy to search for new molecules in the interstellar medium.

Hydrogen Bond Cooperativity and the Three‐Dimensional Structures of Water Nonamers and Decamers

Broadband rotational spectroscopy of water clusters produced in a pulsed molecular jet expansion has been used to determine the oxygen atom geometry in three isomers of the nonamer and two isomers of the decamer. The isomers for each cluster size have the same nominal geometry but differ in the arrangement of their hydrogen bond networks. The nearest neighbor OO distances show a characteristic pattern for each hydrogen bond network isomer that is caused by three-body effects that produce cooperative hydrogen bonding. The observed structures are the lowest energy cluster geometries identified by quantum chemistry and the experimental and theoretical OO distances are in good agreement. The cooperativity effects revealed by the hydrogen bond OO distance variations are shown to be consistent with a simple model for hydrogen bonding in water that takes into account the cooperative and anticooperative bonding effects of nearby water molecules.

DETECTION OF E-CYANOMETHANIMINE TOWARD SAGITTARIUS B2(N) IN THE GREEN BANK TELESCOPE PRIMOS SURVEY

The detection of E-cyanomethanimine (E-HNCHCN) toward Sagittarius B2(N) is made by comparing the publicly available Green Bank Telescope (GBT) PRIMOS survey spectra to laboratory rotational spectra from a reaction product screening experiment. The experiment uses broadband molecular rotational spectroscopy to monitor the reaction products produced in an electric discharge source using a gas mixture of NH3 and CH3CN. Several transition frequency coincidences between the reaction product screening spectra and previously unassigned interstellar rotational transitions in the PRIMOS survey have been assigned to E-cyanomethanimine. A total of eight molecular rotational transitions of this molecule between 9 and 50?GHz are observed with the GBT. E-cyanomethanimine, often called the HCN dimer, is an important molecule in prebiotic chemistry because it is a chemical intermediate in proposed synthetic routes of adenine, one of the two purine nucleobases found in DNA and RNA. New analyses of the rotational spectra of both E-cyanomethanimine and Z-cyanomethanimine that incorporate previous millimeter-wave measurements are also reported.

Microwave measurements of proton tunneling and structural parameters for the propiolic acid–formic acid dimer

Microwave spectra of the propiolic acid-formic acid doubly hydrogen bonded complex were measured in the 1 GHz to 21 GHz range using four different Fourier transform spectrometers. Rotational spectra for seven isotopologues were obtained. For the parent isotopologue, a total of 138 a-dipole transitions and 28 b-dipole transitions were measured for which the a-dipole transitions exhibited splittings of a few MHz into pairs of lines and the b-type dipole transitions were split by ~580 MHz. The transitions assigned to this complex were fit to obtain rotational and distortion constants for both tunneling levels: A(0+) = 6005.289(8), B(0+) = 930.553(8), C(0+) = 803.9948(6) MHz, Δ(0+)(J) = 0.075(1), Δ(0+)(JK) = 0.71(1), and δ(0+)(j) = -0.010(1) kHz and A(0-) = 6005.275(8), B(0-) = 930.546(8), C(0-) = 803.9907(5) MHz, Δ(0-)(J) = 0.076(1), Δ(0-)(JK) = 0.70(2), and δ(0-)(j) = -0.008(1) kHz. Double resonance experiments were used on some transitions to verify assignments and to obtain splittings for cases when the b-dipole transitions were difficult to measure. The experimental difference in energy between the two tunneling states is 291.428(5) MHz for proton-proton exchange and 3.35(2) MHz for the deuterium-deuterium exchange. The vibration-rotation coupling constant between the two levels, F(ab), is 120.7(2) MHz for the proton-proton exchange. With one deuterium atom substituted in either of the hydrogen-bonding protons, the tunneling splittings were not observed for a-dipole transitions, supporting the assignment of the splitting to the concerted proton tunneling motion. The spectra were obtained using three Flygare-Balle type spectrometers and one chirped-pulse machine at the University of Virginia. Rotational constants and centrifugal distortion constants were obtained for HCOOH···HOOCCCH, H(13)COOH···HOOCCCH, HCOOD···HOOCCCH, HCOOH···DOOCCCH, HCOOD···DOOCCCH, DCOOH···HOOCCCH, and DCOOD···HOOCCCH. High-level ab initio calculations provided initial rotational constants for the complex, structural parameters, and some details of the proton tunneling potential energy surface. A least squares fit to the isotopic data reveals a planar structure that is slightly asymmetric in the OH distances. The formic OH···O propiolic hydrogen bond length is 1.8 Å and the propiolic OH···O formic hydrogen bond length is 1.6 Å, for the equilibrium configuration. The magnitude of the dipole moment was experimentally determined to be 1.95(3) × 10(-30) C m (0.584(8) D) for the 0(+) states and 1.92(5) × 10(-30) C m (0.576(14) D) for the 0(-) states.

Spatial Distributions and Interstellar Reaction Processes

Methyl formate presents a challenge for the conventional chemical mechanisms assumed to guide interstellar organic chemistry. Previous studies of potential formation pathways for methyl formate in interstellar clouds ruled out gas-phase chemistry as a major production route, and more recent chemical kinetics models indicate that it may form efficiently from radical-radical chemistry on ice surfaces. Yet, recent chemical imaging studies of methyl formate and molecules potentially related to its formation suggest that it may form through previously unexplored gas-phase chemistry. Motivated by these findings, two new gas-phase ion-molecule formation routes are proposed and characterized using electronic structure theory with conformational specificity. The proposed reactions, acid-catalyzed Fisher esterification and methyl cation transfer, both produce the less stable trans-conformational isomer of protonated methyl formate in relatively high abundance under the kinetically controlled conditions relevant to interstellar chemistry. Gas-phase neutral methyl formate can be produced from its protonated counterpart through either a dissociative electron recombination reaction or a proton transfer reaction to a molecule with larger proton affinity. Retention (or partial retention) of the conformation in these neutralization reactions would yield trans-methyl formate in an abundance that exceeds predictions under thermodynamic equilibrium at typical interstellar temperatures of ≤100 K. For this reason, this conformer may prove to be an excellent probe of gas-phase chemistry in interstellar clouds. Motivated by new theoretical predictions, the rotational spectrum of trans-methyl formate has been measured for the first time in the laboratory, and seven lines have now been detected in the interstellar medium using the publicly available PRIMOS survey from the NRAO Green Bank Telescope.

Water–Water and Water–Solute Interactions in Microsolvated Organic Complexes†

A structural study of microsolvated clusters of β-propiolactone (BPL) formed in a pulsed molecular jet expansion is presented. The rotational spectra of BPL-(H2O)n (n=1-5) adducts have been analyzed by broadband microwave spectroscopy. Unambiguous identification of the structures has been achieved using isotopic substitution and experimental measurements of the cluster dipole moment. The observed structures are discussed in terms of the different intermolecular interactions between water molecules and between water and BPL, which include n-π* interactions involving the lone pairs of electrons on water oxygen atoms and the antibonding orbital of the BPL carbonyl group. The changes induced in the structures of the water hydrogen-bonding network by complexation to BPL indicate that water clusters adopt specific configurations to maximize their links to solute molecules.

Probing the CH⋅⋅⋅π Weak Hydrogen Bond in Anesthetic Binding: The Sevoflurane–Benzene Cluster

Cooperativity between weak hydrogen bonds can be revealed in molecular clusters isolated in the gas phase. Here we examine the structure, internal dynamics, and origin of the weak intermolecular forces between sevoflurane and a benzene molecule, using multi-isotopic broadband rotational spectra. This heterodimer is held together by a primary C-H⋅⋅⋅π hydrogen bond, assisted by multiple weak C-H⋅⋅⋅F interactions. The multiple nonbonding forces hinder the internal rotation of benzene around the isopropyl C-H bond in sevoflurane, producing detectable quantum tunneling effects in the rotational spectrum.

Molecular polymorphism: microwave spectra, equilibrium structures, and an astronomical investigation of the HNCS isomeric family

The rotational spectra of thioisocyanic acid (HNCS), and its three energetic isomers (HSCN, HCNS, and HSNC) have been observed at high spectral resolution by a combination of chirped-pulse and Fabry-Pérot Fourier-transform microwave spectroscopy between 6 and 40 GHz in a pulsed-jet discharge expansion. Two isomers, thiofulminic acid (HCNS) and isothiofulminic acid (HSNC), calculated here to be 35-37 kcal mol(-1) less stable than the ground state isomer HNCS, have been detected for the first time. Precise rotational, centrifugal distortion, and nitrogen hyperfine coupling constants have been determined for the normal and rare isotopic species of both molecules; all are in good agreement with theoretical predictions obtained at the coupled cluster level of theory. On the basis of isotopic spectroscopy, precise molecular structures have been derived for all four isomers by correcting experimental rotational constants for the effects of rotation-vibration interaction calculated theoretically. Formation and isomerization pathways have also been investigated; the high abundance of HSCN relative to ground state HNCS, and the detection of strong lines of SH using CH3CN and H2S, suggest that HSCN is preferentially produced by the radical-radical reaction HS + CN. A radio astronomical search for HSCN and its isomers has been undertaken toward the high-mass star-forming region Sgr B2(N) in the Galactic Center with the 100 m Green Bank Telescope. While we find clear evidence for HSCN, only a tentative detection of HNCS is proposed, and there is no indication of HCNS or HSNC at the same rms noise level. HSCN, and tentatively HNCS, displays clear deviations from a single-excitation temperature model, suggesting weak masing may be occurring in some transitions in this source.

Distortions of ethyne when complexed with a cuprous or argentous halide

A new molecule C2H2···CuF has been synthesized in the gas phase by means of the reaction of laser-ablated metallic copper with a pulse of gas consisting of a dilute mixture of ethyne and sulfur hexafluoride in argon.A new molecule C2H2CuF has been synthesized in the gas phase by means of the reaction of laser-ablated metallic copper with a pulse of gas consisting of a dilute mixture of ethyne and sulfur hexafluoride in argon. The ground-state rotational spectrum was detected by two types of Fourier-transform microwave spectroscopy, namely that conducted in a microwave Fabry-Perot cavity and the chirped-pulse broadband technique. The spectroscopic constants of the six isotopologues (12)C2H2(63)Cu(19)F, (12)C2H2(65)Cu(19)F, (13)C2H2(63)Cu(19)F, (13)C2H2(65)Cu(19)F, (12)C2D2(63)Cu(19)F and (12)C2D2(65)Cu(19)F were determined and interpreted to show that the molecule has a planar, T-shaped geometry belonging to the molecular point group C2v, with CuF forming the stem of the T. Quantitative interpretation reveals that the ethyne molecule is distorted when subsumed into the complex in such manner that the C[triple bond, length as m-dash]C bond lengthens (by δr) and the two H atoms cease to be collinear with the C[triple bond, length as m-dash]C internuclear line. The H atoms move symmetrically away from the approaching Cu atom of CuF, to increase each *[triple bond, length as m-dash]C-H angle by δA = 14.65(2)°, from 180° to 194.65(2)°. Ab initio calculations at the explicitly-correlated level of theory CCSD(T)(F12*)/aug-cc-pVTZ lead to good agreement with the experimental geometry. It is shown that similar distortions δr and δA, similarly determined, for four complexes C2H2MX (M = Cu or Ag; X = F, Cl or CCH) are approximately linearly related to the energies De for the dissociation process C2H2MX = C2H2 + MX.