Chirped-pulse Fourier Transform Microwave Spectroscopy Research Articles

Structural Changes Induced by Quinones: High-Resolution Microwave Study of 1,4-Naphthoquinone.

1,4‐Naphthoquinone (1,4‐NQ) is an important product of naphthalene oxidation, and it appears as a motif in many biologically active compounds. We have investigated the structure of 1,4‐NQ using chirped‐pulse Fourier transform microwave spectroscopy and quantum chemistry calculations. The rotational spectra of the parent species, and its 13C and 18O isotopologues were observed in natural abundance, and their spectroscopic parameters were obtained. This allowed the determination of the substitution r s, mass‐weighted r m and semi‐experimental r e SE structures of 1,4‐NQ. The obtained structural parameters show that the quinone moiety mainly changes the structure of the benzene ring where it is inserted, modifying the C−C bonds to having predominantly single or double bond character. Furthermore, the molecular electrostatic surface potential reveals that the quinone ring becomes electron deficient while the benzene ring remains a nucleophile. The most electrophilic areas are the hydrogens attached to the double bond in the quinone ring. Knowledge of the nucleophilic and electrophilic areas in 1,4‐NQ will help understanding its behaviour interacting with other molecules and guide modifications to tune its properties.

A novel Ka-band chirped-pulse spectrometer used in the determination of pressure broadening coefficients of astrochemical molecules

A novel chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer has been constructed to cover the Ka-band (26.5 GHz-40 GHz) for use in the CRESUCHIRP project, which aims to study the branching ratios of reactions at low temperatures using the chirped-pulse in uniform flow technique. The design takes advantage of recent developments in radio-frequency components, notably, high-frequency, high-power solid-state amplifiers. The spectrometer had a flatness of 5.5 dB across the spectral range, produced harmonic signals below -20 dBc, and the recorded signal scaled well to 6 × 106 averages. The new spectrometer was used to determine pressure broadening coefficients with a helium collider at room temperature for three molecules relevant to astrochemistry, applying the Voigt function to fit the magnitude of the Fourier-transformed data in the frequency domain. The pressure broadening coefficient for carbonyl sulfide was determined to be (2.45 ± 0.02) MHz mbar-1 at room temperature, which agreed well with previous measurements. Pressure broadening coefficients were also determined for multiple transitions of vinyl cyanide and benzonitrile. Additionally, the spectrometer was coupled with a cold, uniform flow from a Laval nozzle. The spectrum of vinyl cyanide was recorded in the flow, and its rotational temperature was determined to be (24 ± 11) K. This temperature agreed with a prediction of the composite temperature of the system through simulations of the experimental environment coupled with calculations of the solution to the optical Bloch equations. These results pave the way for future quantitative studies in low-temperature and high-pressure environments using CP-FTMW spectroscopy.

Investigation of a polar form of fluoroethylene dimer, (C2H3F)2, by microwave spectroscopy

The structure of fluoroethylene dimer ((C2H3F)2, (FE)2) has been determined using chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy. Although the most stable structure was expected to be nonpolar, a planar, polar configuration was observed with sufficient intensity to measure spectra for four unique 13C substituted isotopologues in natural abundance. This allowed a least-squares fit of the structural parameters of the dimer to observed moments of inertia. The resulting structure has a T-shaped arrangement of CC bonds, similar to the recently studied FE⋯1,1-difluoroethylene complex. Both dimers contain a cyclic arrangement of CH⋯F contacts between the H–C–F end of one monomer and the H–CC–F side of the second monomer. ωB97X-D/6-31+G(d,p), MP2/6–311++G(2d,2p) and MP2/aug-cc-pVDZ calculations give the expected nonpolar end-to-end structure, ranging from 9 cm−1 (DFT) to 63 cm−1 and 57 cm−1 (MP2/6–311++G(2d,2p) and aug-cc-pVDZ, respectively) more stable than the experimentally observed orientation. The ωB97X-D/6-31+G(d,p) calculations also indicate a nonplanar, polar configuration that is over 60 cm−1 more favorable than the nonpolar structure; however, MP2 calculations predict significantly higher energy, and spectroscopic results provide no evidence of that structure’s existence. The rotational constants of the observed T-shaped (FE)2 structure are A = 6582.8323(12) MHz, B = 1256.6203(4) MHz, C = 1060.3474(3) MHz, and the dipole moment components, determined via Stark effect measurements, are μa = 0.683 (3) D, μb = 0.301 (5) D, μtot = 0.746 (5) D.

Bifunctional Hydrogen Bonding of Imidazole with Water Explored by Rotational Spectroscopy and DFT Calculations

Laser vaporization of imidazole in the presence of an argon buffer gas has allowed the generation and isolation of two isomers of an imidazole monohydrate complex, denoted herein as imid···H2O and H2O···imid, within a gas sample undergoing supersonic expansion. Imidazole and water are respectively proton-accepting and proton-donating in imid···H2O, but these roles are reversed in the H2O···imid complex. Both isomers have been characterized by chirped-pulse Fourier transform microwave spectroscopy between 7.0 and 18.5 GHz. The ground-state rotational spectra of four isotopologues of imid···H2O and three isotopologues of H2O···imid have been measured. All spectra have been assigned and fitted to determine rotational (A0, B0, C0), centrifugal distortion (DJ, DJK), and nuclear quadrupole coupling constants (χaa(N1), [χbb(N1) - χcc(N1)], χaa(N3), and [χbb(N3) - χcc(N3)]). Structural parameters (r0 and rs) have been accurately determined from measured rotational constants for each isomer. The imid···H2O complex contains a nonlinear hydrogen bond (∠(O-Hb···N3) = 172.1(26)° in the experimentally determined, r0 geometry) between the pyridinic nitrogen of imidazole and a hydrogen atom of H2O. The DFT calculations find that the H2O···imid complex also contains a nonlinear hydrogen bond between the oxygen atom of water and the hydrogen attached to the pyrrolic nitrogen of imidazole (∠(O···H1-N1) = 174.7°). Two states observed in the spectrum of H2O···imid, assigned as 0- and 0+ states, confirm that large amplitude motions occur on the time scale of the molecular rotation. Density functional theory has been performed to characterize these large amplitude motions.

Structures and internal dynamics of diphenylether and its aggregates with water.

We report on a detailed multi-spectroscopic analysis of the structures and internal dynamics of diphenylether and its aggregates with up to three water molecules by employing molecular beam experiments. The application of stimulated Raman/UV and IR/UV double resonance methods as well as chirped-pulse Fourier transform microwave spectroscopy in combination with quantum-chemical computations yield the energetically preferred monomer and cluster geometries. Furthermore, the complex internal dynamics of the diphenylether monomer and the one-water clusters are analysed. In the cluster with three water molecules, water forms a cyclic structure similar to the isolated water trimer. The interactions ruling the structures of the higher-order water clusters are a combination of the ones identified for the two monohydrate isomers, with dispersion being a decisive contribution for systems that have a delicate energetic balance between different hydrogen-bonded arrangements of similar energy.

Sulfur hydrogen bonding and internal dynamics in the monohydrates of thenyl mercaptan and thenyl alcohol.

The monohydrates of thenyl alcohol and thenyl mercaptan have been probed in a supersonic jet expansion using chirped-pulse and Fabry-Perot Fourier-transform microwave spectroscopy. The rotational spectra revealed a single isomer for each of the dimers. The thenyl alcohol hydrate is stabilized by an O-HOw hydrogen bond between the alcohol and water, with water acting as a proton acceptor and additionally engaging in an Ow-Hπ interaction with the thenyl ring. Conversely, water behaves as a proton donor in the thenyl mercaptan hydrate, linking to the thiol group though an Ow-HS hydrogen bond and secondary Ow-Hπ interactions with the ring. In both dimers water retains internal mobility, as tunneling doublings in the spectrum confirm an internal rotation motion of water inside the cluster. The experimental results have been complemented with density-functional-theory molecular orbital calculations, binding energy decomposition and a topological analysis of the electronic density, providing a comparative description of the effects of hydrogen bonding of water to the alcohol and thiol groups in the dimers, relevant to understand hydrogen bonding to sulfur centers.

Barriers to internal rotation in methylimidazole isomers determined by rotational spectroscopy.

The rotational spectra of N-, 2-, 4-, and 5-methylimidazole are reported and analyzed. Liquid N-methylimidazole was vaporized from a reservoir, and each of 2-, 4-, and 5-methylimidazole was laser-vaporized from a solid target prior to mixing with argon buffer gas and undergoing supersonic expansion from a pulsed nozzle. The spectra were recorded by chirped-pulse Fourier transform microwave spectroscopy in the 7.0-18.5 GHz frequency range. Rotational constants, A0, B0, and C0, centrifugal distortion constants, DJ, DJK, DK, d1, and d2, and nuclear quadrupole coupling constants of nitrogen atoms, χaa(N1), χbb(N1) - χcc(N1), χaa(N3), and χbb(N3) - χcc(N3), are determined from experimentally measured transition frequencies. Data recorded for isotopologues containing 13C or 15N are used to determine the rs coordinates of all heavy atoms in N-, 2-, and 4-methylimidazole. The results allow fitting of parameters in the Hamiltonian that describes internal rotation of the CH3 group about its local C3 axis. The V3 terms in the periodic potential energy functions that describe the internal rotation in N-, 2-, 4-, and 5-methylimidazole are determined to be 185.104(11), 122.7529(38), 317.20(14), and 386.001(19) cm-1, respectively, by the internal axis method. The experiments are supported by density functional theory calculations. Observed variations in barrier height are explained with reference to the symmetry of overlap between a π-like orbital on the CH3 group and π-orbitals on the aromatic ring.

Rotational Spectrum, Tunneling Motions, and Intramolecular Potential Barriers in Benzyl Mercaptan.

The rotational spectrum of benzyl mercaptan (parent and four isotopologues) has been assigned in a supersonic jet expansion using chirped-pulse Fourier transform microwave spectroscopy. The spectrum is characterized by torsional tunneling doublings, strongly perturbed by Coriolis interactions. The experimental rotational constants reveal that the sulfur atom is located above the ring plane in a gauche conformation. The torsion dihedral θ0 = φ (SCα-C1C2) is approximately 74°, according to a flexible molecular model calculation reproducing the energy separation (ΔE01 ∼ 2180.4 MHz) between the first two torsional substates. The global minimum configuration is 4-fold degenerate, corresponding to potential minima with θ0 ≈ ±74° and ±(180-74)°. The four equivalent minima are separated by potential barriers at θ = ±90°, 0°, or 180°. The tunneling splittings are caused by the potential barrier at θ = ± 90°, while the barriers at torsions of 0° or 180° are too large to generate detectable splittings. The tunnelling barrier has been determined as 248 cm-1, similar to the value obtained with high-level MP2 ab initio calculations (259 cm-1), but smaller than in benzyl alcohol (280 cm-1).

Molecular Structure of 1-Isocyano-1-silacyclopent-3-ene: A Combined Microwave Spectral and Theoretical Study.

The microwave spectrum of 1-isocyano-1-silacyclopent-3-ene has been obtained from broad-band chirped-pulse Fourier transform microwave spectroscopy. The rotational constants (RCs) for the standard abundant isotopic species are A = 3328.4182(23), B = 1017.69404(53), and C = 1012.33297(58) MHz. The symmetric quartic centrifugal distortion constants, using the Ir representation in CS symmetry for ΔJ,ΔJK, ΔK, and δJ, have been evaluated; similarly, the 14N nuclear quadrupole coupling has been determined. Several singly substituted isotopologues observed in natural abundance enabled most of the heavy atom substructure to be determined. The five-membered ring is close to planar, but the orientation of the isocyanate unit, derived from the N13CO spectrum, unexpectedly lies above the ring center in a cis C2,5-Si-N═C conformation. Our initial equilibrium structural searches led to a trans orientation of the C2,5-Si-N═C unit, i.e., bending away from the ring. When the cis conformation was applied, the final equilibrium structure, assuming CS symmetry, gave RC values of 3221.3 ( A), 1037.0 ( B), and 1031.3 ( C) MHz, very close to the MW values. This enabled the full-equilibrium structure to be determined with confidence. The principal bond lengths were 1.7157 (Si-N), 1.8696 (Si-C), 1.1998 (N═C), and 1.1737 (C═O) Å, with angles 163.3 (Si-N═C), 178.1 (N═C═O), 96.5 (C-Si-C), and 118.7° (C-C═C), respectively. The extensive widening of the SiNC angle is particularly notable; the SiNCO unit has a trans dihedral angle. The cis orientation implies a (weak) attractive force between the ring and isocyanate groups by a through-space interaction. An atoms in molecule study, where the local minima of electron density are determined, fails to disclose the exact nature of the interaction; however, a highly polarized skeleton was obtained. A systematic theoretical study of the Si-N═C angle potential energy surface (PES) relative to the ring gave a very shallow double minimum with the barrier being less than 1 cm-1; a polynomial fit to the surface shows major contributions of both harmonic and quartic components. A similar study of the XSiN angle, where X is at the ring center, also gave a PES with considerable quartic character.

The 1-Naphthol Dimer and Its Surprising Preference for π-π Stacking over Hydrogen Bonding.

Using chirped-pulse Fourier transform microwave spectroscopy, we have experimentally identified the most abundant dimer of an alcohol analogue of naphthalene, 1-naphthol. The 1-naphthol dimer features a V-shaped, partially overlapping π-π stacked structure with no canonical hydrogen bonds between the subunits. This structural assignment is in contradiction to an earlier study of the 1-naphthol dimer using UV-IR dip double resonance spectroscopy in the O-H stretch region, which assigns a π-stacked but also canonically hydrogen-bonded structure. We use an improved theoretical analysis to resolve this discrepancy and show that the new, V-shaped structure is also consistent with the previously measured UV-IR data. These new computational and spectroscopic results shed light on the complicated nature of evaluating energetics and structures for larger, dispersion-bound systems.

Microwave spectra and nuclear quadrupole structure of the NH3-N2 van der Waals complex and its deuterated isotopologues.

The microwave spectrum of the NH3-N2 van der Waals complex has been observed in a supersonic molecular jet expansion via broadband (2-8 GHz) chirped-pulse Fourier-transform microwave spectroscopy. Two pure rotational R(0) transitions (J = 1 - 0) with different hyperfine structure patterns were detected. One transition belongs to the (ortho)-NH3-(ortho)-N2 nuclear spin isomer in the ground K = 0 state reported earlier [G. T. Fraser et al., J. Chem. Phys. 84, 2472 (1986)], while another one is assigned to the (para)-NH3-(para)-N2 spin isomer in the K = 0 state not reported before (K is the projection of the total angular momentum J on the intermolecular axis). The complicated hyperfine structure arising from three quadrupole 14N nuclei of NH3-N2 was resolved for both transitions, and the quadrupole coupling constants associated with the NH3 and N2 subunits were precisely determined for the first time. These constants provided the dynamical information about the angular orientation of ammonia and nitrogen indicating that the average angle between the C 3 axis of NH3 and the N2 axis is about 66°. The average van der Waals bond lengths are slightly different for (ortho)-NH3-(ortho)-N2 and (para)-NH3-(para)-N2 and amount to 3.678 Å and 3.732 Å, respectively. Similar results for the deuterated isotopologues, ND3-N2, NHD2-N2, and NH2D-N2, and their nuclear spin isomers were also obtained thus confirming and extending the analysis for the parent NH3-N2 complex.

The Structural Signs of Sweetness in Artificial Sweeteners: A Rotational Study of Sorbitol and Dulcitol

A gas-phase study on the artificial sweeteners sorbitol and dulcitol has been carried out for the first time by using a combination of chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy and laser ablation (LA). The isolation conditions provided by the supersonic expansion reveal the intrinsic conformational structures of these sweeteners. The three and five observed conformers for sorbitol and dulcitol, respectively, are stabilized by networks of cooperative intramolecular hydrogen bonds between vicinal hydroxyl groups in clockwise or counterclockwise arrangements. Suitable places in the structure of seven out of eight conformers identified for both polyalcohols meet the requirements of the glucophore proposed by Shallenberger and Acree's molecular theory of sweet taste. Present results provide the first linkage between sweetness and structure in sugar alcohols.

Rotational spectra and nuclear quadrupole coupling constants of 4- and 2-iodoimidazole

Pure rotational spectra of 4-iodoimidazole and 2-iodoimidazole have been measured and assigned to determine rotational, centrifugal distortion and nuclear quadrupole coupling constants. Samples of each isomer were laser-vaporised from the surface of a solid target rod using the focussed pulse of a Nd:YAG laser prior to cooling via supersonic co-expansion with argon buffer gas. Microwave spectra were recorded by chirped-pulse Fourier transform microwave spectroscopy between 2.0 and 18.5 GHz. Inertial defects of 4-iodoimidazole and 2-iodoimidazole were determined to be 0.0906(1) a.m.u. Å2 and 0.0814(2) a.m.u. Å2 respectively, consistent with rigid planar geometries for these molecules. The evaluated nuclear quadrupole coupling constants of iodine atoms imply that approximately 1% of an electron is transferred from the iodine lone pair into aromatic π orbitals when iodine is substituted for a hydrogen atom in either the 4- or 2-position. Results are compared with data for other halogenated aromatic molecules and discussed in context of the extent of conjugation across the range.

Multiplexed characterization of complex gas-phase mixtures combining chirped-pulse Fourier transform microwave spectroscopy and VUV photoionization time-of-flight mass spectrometry.

We report details of the design and operation of a single apparatus that combines Chirped-Pulse Fourier Transform Microwave (CP-FTMW) spectroscopy with vacuum ultraviolet (VUV) photoionization Time-of-Flight Mass Spectrometry (TOFMS). The supersonic expansion used for cooling samples is interrogated first by passing through the region between two microwave horns capable of broadband excitation and detection in the 2-18 GHz frequency region of the microwave. After passing through this region, the expansion is skimmed to form a molecular beam, before being probed with 118 nm (10.5 eV) single-photon VUV photoionization in a linear time-of-flight mass spectrometer. The two detection schemes are powerfully complementary to one another. CP-FTMW detects all components with significant permanent dipole moments. Rotational transitions provide high-resolution structural data. VUV TOFMS provides a gentle and general method for ionizing all components of a gas phase mixture with ionization thresholds below 10.5 eV, providing their molecular formulae. The advantages, complementarity, and limitations of the combined methods are illustrated through results on two gas-phase mixtures made up of (i) three furanic compounds, two of which are structural isomers of one another, and (ii) the effluent from a flash pyrolysis source with o-guaiacol as the precursor.

Towards microsolvation of fluorocarbons by CO2: Two isomers of fluoroethylene-(CO2)2 observed using chirped-pulse Fourier-transform microwave spectroscopy

Understanding structural changes during microsolvation of a solute by increasing numbers of CO2 molecules provides a path towards improved models of supercritical CO2 solvent properties. Microwave spectroscopic observations of fluoroethylene (FE) microsolvation by CO2 give two FE-(CO2)2 isomers, each with fragments similar to previously studied planar FE-CO2 dimers. In each trimer, a second CO2 is located above the FE plane, in a position not observed for FE-CO2. Current work on FE/CO2 mixtures focuses on deconvoluting interleaved spectra in the microwave scan, aiming to identify FE-(CO2)3 and other clusters, with patterns from at least five additional cluster spectra already identified.

Microsolvated complexes of ibuprofen as revealed by high-resolution rotational spectroscopy.

Hydrogen-bonded complexes between ibuprofen and water generated in a supersonic expansion were characterized using chirped-pulse Fourier transform microwave spectroscopy in the 2-8 GHz frequency range. Four spectra were observed allowing the determination of their rotational parameters. Comparison with quantum-chemical calculations led to their identification as the lowest energy 1 : 1 ibuprofen-water complexes. These correspond to the complexes between water and the four different conformers of ibuprofen previously detected in the gas phase, owing to their similar stabilization energies and abundances. Water seems to not change the conformational distribution of ibuprofen.

Electron-withdrawing effects on the molecular structure of 2- and 3-nitrobenzonitrile revealed by broadband rotational spectroscopy and their comparison with 4-nitrobenzonitrile.

The rotational spectra of 2- and 3-nitrobenzonitrile were recorded via chirped-pulse Fourier transform microwave spectroscopy in the frequency range of 2-8 GHz. These molecules each display large dipole moments, making them viable candidates for deceleration and trapping experiments with AC-electric fields. For both molecules, the main isotopologues and all isotopologues of the respective 13C-, 15N-, 18O-monosubstituted species in their natural abundance were assigned. These assignments allowed for the structural determination of 2- and 3-nitrobenzonitrile via Kraitchman's equations as well as a mass-dependent least-squares fitting approach. The experimentally determined structural parameters are then compared to those obtained from quantum-chemical calculations for these two molecules and 4-nitrobenzonitrile. Structural changes caused by steric interaction and competition for the electron density of the phenyl ring highlight how these strong electron-withdrawing substituents affect one another according to their respective positions on the phenyl ring.

Molecular geometries and other properties of H2O⋯AgI and H3N⋯AgI as characterised by rotational spectroscopy and ab initio calculations.

The rotational spectra of H3N⋯AgI and H2O⋯AgI have been recorded between 6.5 and 18.5 GHz by chirped-pulse Fourier-transform microwave spectroscopy. The complexes were generated through laser vaporisation of a solid target of silver or silver iodide in the presence of an argon gas pulse containing a low concentration of the Lewis base. The gaseous sample subsequently undergoes supersonic expansion which results in cooling of rotational and vibrational motions such that weakly bound complexes can form within the expanding gas jet. Spectroscopic parameters have been determined for eight isotopologues of H3N⋯AgI and six isotopologues of H2O⋯AgI. Rotational constants, B0; centrifugal distortion constants, DJ, DJK or ΔJ, ΔJK; and the nuclear quadrupole coupling constants, χaa(I) and χbb(I) - χcc(I) are reported. H3N⋯AgI is shown to adopt a geometry that has C3v symmetry. The geometry of H2O⋯AgI is Cs at equilibrium but with a low barrier to inversion such that the vibrational wavefunction for the v = 0 state has C2v symmetry. Trends in the nuclear quadrupole coupling constant of the iodine nucleus, χaa(I), of L⋯AgI complexes are examined, where L is varied across the series (L = Ar, H3N, H2O, H2S, H3P, or CO). The results of experiments are reported alongside those of ab initio calculations at the CCSD(T)(F12*)/AVXZ level (X = T, Q).

Halogen bonding properties of 4-iodopyrazole and 4-bromopyrazole explored by rotational spectroscopy and ab initio calculations.

The combination of halogen- and hydrogen-bonding capabilities possessed by 4-bromopyrazole and 4-iodopyrazole has led to them being described as "magic bullets" for biochemical structure determination. Laser vaporisation was used to introduce each of these 4-halopyrazoles into an argon gas sample undergoing supersonic expansion prior to the recording of the rotational spectra of these molecules by chirped-pulse Fourier transform microwave spectroscopy. Data were obtained for four isotopologues of 4-bromopyrazole and two isotopologues of 4-iodopyrazole. Isotopic substitutions were achieved at the hydrogens attached to the pyrrolic nitrogen atoms of both 4-halopyrazoles and at the bromine atom of 4-bromopyrazole. The experimentally determined nuclear quadrupole coupling constants, χaa(X) and χbb(X)-χcc(X), of the halogen atoms (where X is the halogen atom) of each molecule are compared with the results of the ab initio calculations and those for a range of other halogen-containing molecules. It is concluded that each of 4-bromopyrazole and 4-iodopyrazole will form halogen bonds that are broadly comparable in strength to those formed by CH3X and CF3X.

Structural determination and population transfer of 4-nitroanisole by broadband microwave spectroscopy and tailored microwave pulses.

The rotational spectrum of 4-nitroanisole was recorded via chirped-pulse Fourier transform microwave spectroscopy in the frequency range of 2-8 GHz. The spectra of the parent molecule and all of its 13C-, 15N-, and 18O-monosubstituted species in their natural abundance were assigned, and the molecular structure was determined using Kraitchman's equations as well as a least-square fitting approach. 4-nitroanisole has a large dipole moment of 6.15 D along the inertial a-axis and a smaller dipole moment of 0.78 D along the b-axis. The large dipole moment component makes this molecule a potential candidate for deceleration experiments using static electric fields or electromagnetic radiation. Using tailored microwave pulses, we investigate the possibility of transferring population between the rotational states of 4-nitroanisole. Such a technique could be applied to selectively increase the population for specific rotational states of interest, which are then accessible for further, more advanced experiments, such as deceleration.