Fourier Transform Microwave Spectra Research Articles

Structure determination of 2,5-difluorophenol by microwave spectroscopy

The Fourier transform microwave spectrum of 2,5-difluorophenol has been obtained in the centimeter wave range, revealing only one conformer stabilized by an intramolecular interaction where the hydrogen atom of the OH group is in a syn orientation to the fluorine atom at the 2-ring position. The heavy atom backbone structure was obtained from the rotational constants of the 13C and 18O isotopologues whose spectra were measured in natural abundance. The spectrum of the OD isotopologue obtained by deuterium enrichment was also measured, and the nuclear quadrupole hyperfine structures arising from deuterium were analyzed. The semi-experimental equilibrium structure (reSE) was determined by correcting the experimental rotational constants with the vibration-rotation interaction constants obtained via an anharmonic force field. Quantum chemical calculations at various levels of theory were used to support results from the experiments.

Exploring the conformational landscape through rotational spectroscopy and computational modelling: The tunneling dynamics in 2,6-diethylphenol

Phenol and some of its derivatives exhibit interesting tunneling motions consisting of two groups of transitions separated by a few hundred MHz. Recently, one of its derivatives, 2,6-di-tert-butylphenol, has shown additional hyperfine tunneling components, the origin of which remains unclear. In this work, another member of the family, 2,6-diethylphenol, has been investigated through its rotational spectrum. The jet-cooled broadband chirped-pulse Fourier transform microwave spectra in the 2–8 GHz frequency region revealed the presence of two conformers. The comparison with the equilibrium structure obtained by computational calculations at the B3LYP-D3(BJ)/Def2-TZVP level validates the structural determination and the orientation of the lateral ethyl groups. Additional observation of all the singly-substituted 13C isotopologues for the most stable ones allowed the determination of the substitution structure by means of the Kraitchman equations. Both conformers exhibited tunneling that was reproduced using an advanced 1D model, which provides an estimate of the barrier height for both conformers.

A Rotational Study of 2-tert-Butylphenol and Its 1 : 1 Argon Complex.

The chirped-pulse Fourier Transform microwave spectrum of 2-tert-butylphenol, an industrial intermediate for the production of antioxidants, has been investigated in the 2-8 GHz frequency range. The spectral analysis has allowed obtaining precise structural information on the most stable conformer and its complex with argon. The conformation of the monomer reveals that the hydroxyl group is coplanar with the ring but points in the opposite direction to the tert-butyl group, reducing steric interactions. In the tert-butyl group one methyl group is coplanar and the other two are symmetrically staggered respect to the ring. The complex shows the rare gas sitting above the aromatic ring. Interestingly, neither the monomer nor the complex exhibit large-amplitude hydroxyl torsion motions, previously observed in 2,6-disubstituted phenols such as 2,6-di-tert-butylphenol or propofol. The experimental results are supported by computational calculations, validating the molecular structure. Additionally, symmetry-adapted perturbation theory has allowed determining the van der Waals intermolecular interaction energy of the complex.

Reinvestigation of the Fourier transform microwave spectrum of N-ethylacetamide: Pseudo a-type transitions

We observed ten pseudo a-type transitions in the E state of N-ethylacetamide (NEAA) using a Fourier transform microwave spectrometer. We found triplets caused by the coupling between the two methyl groups for the observed pseudo a-type transitions. Also, we succeeded in observing similar triplets for the 5 a-type Q-branch transitions, 20 b-type and 7 c-type transitions of the E state of NEAA. These observed triplets allowed us to determinate the potential barrier V3 to internal rotation of the ethyl-methyl group to be 1078 (12) cm−1. In addition, though the observed intensities of the c-type transitions in the A state were weak, the c-type transition in the E state were observed to be as intense as the b-type transitions of the E state. We explained in this paper the abnormal intensities of the c-type transitions and pseudo a-type transitions in the E state of NEAA.

Probing intra- and inter-molecular interactions through rotational spectroscopy: The case of the odorant 2'-aminoacetophenone and its 1:1 water and neon complexes.

The chirped-pulse Fourier transform microwave spectrum of 2'-aminoacetophenone, an aromatic chemical species with odorant properties, has been recorded in the 2-8GHz frequency range and analyzed, obtaining precise information on the structure of the monomer and its neon and water complexes. The conformation of the monomer is determined by the formation of a resonance-assisted hydrogen bond (RAHB) between the carbonyl and amino groups, which leads to the formation of a bicyclic-like aromatic structure. Accordingly, the cycle formed by the non-covalent bond is preferred to the phenyl ring as the interaction site for neon. In the 1:1 complex, water lies in the molecular plane and forms a strong hydrogen bond with the carbonyl group coupled to an ancillary interaction with the methyl group, leaving the intramolecular RAHB unchanged. The experimental findings are supported by atoms in molecules and symmetry-adapted perturbation theory, which allowed for determining the hydrogen bond and intermolecular interaction energies, respectively.

Rotational spectroscopy of the large saturated dinitriles hexanedinitrile and heptanedinitrile

Dinitriles with a saturated hydrocarbon skeleton and a –C≡N group at each end can have large electric dipole moments. Their formation can be related to highly reactive radicals such as CH2CN, C2N, or CN. Thus, these saturated dinitriles are potential candidates to be observed in the interstellar medium. In this work, two members of this family, hexanedinitrile and heptanedinitrile, have been investigated through their rotational spectra. The jet–cooled broadband chirped–pulse Fourier transform microwave spectra of both molecules were measured in the 2–8 GHz frequency region. Three and six conformers of hexanedinitrile and heptanedinitrile, respectively, were detected and assigned based on the rotational and quadrupole coupling constants.

Gas-phase spectroscopic identification of the chlorovinyl radical.

Fourier transform microwave spectra for two isomers of the chlorine substituted vinyl radical have been observed in the 4-52 GHz frequency region. The observed radicals (2A') have been generated using electric discharges of diluted dichloro derivatives of ethylene as molecular precursors. Fine and hyperfine components observed for each rotational transition are fully assigned in the present study to two isotopologues (35Cl and 37Cl), and precise molecular constants are determined for both radicals.

Fine and hyperfine coupling constants of the cis-β-cyanovinyl radical, HCCHCN.

A Fourier-transform microwave spectrum of the cis-β-cyanovinyl radical is re-measured for the Ka = 0 ladder of the a-type transitions up to 30 GHz and the 212-111 transition at 19.85 GHz. Four b-type transitions are also observed using a MW-MW double-resonance technique. Fine and hyperfine components observed for each rotational transition are fully assigned in the present study, and the precise molecular constants are determined for the radical. From the comparisons of the hyperfine coupling constants with those of the vinyl radicals, it is concluded that the substitution of one of the β-hydrogens by the cyano group has little effect on the electronic structure of the vinyl radical.

Dipole forbidden, nuclear electric quadrupole allowed transitions and chirality: The broadband microwave spectrum and structure of 2-bromo-1,1,1,2-tetrafluoroethane

The chirped pulse Fourier transform microwave spectrum of the chiral molecule 2-bromo-1,1,1,2-tetrafluoroethane has been observed in the 6–18 GHz region of the electromagnetic spectrum and is reported. Six isotopologues, including 13C isotopic substitution at each carbon atom in both the 79Br and 81Br parent species have been observed in natural abundance. All rotational constants, quartic centrifugal distortion contants, and nuclear quadrupole coupling tensor components have been experimentally determined and have been reported. In addition, spin-rotation components Maa, Mbb, and Mcc are also determined and reported. The theoretical structure of the species has been determined and reported. The C–C–Br backbone structure has been determined experimentally implementing both Kraitchman single and Rudolph double substitution analysis, the later being explained in the text. 3440 transitions total have been observed with 45 transitions being electric dipole forbidden. These dipole forbidden transitions have been identified as they are planned to be used in microwave three-wave mixing experiments in order to better understand their role, if any, in determining chirality. The determined nuclear electric quadrupole coupling tensor has been diagonalized and compared to similar small alkane species in order to make inferences on the effects of fluorination on the C–Br bond.

Local and global approaches to treat the torsional barriers of 4-methylacetophenone using microwave spectroscopy.

The Fourier transform microwave spectrum of 4-methylacetophenone recorded from 8 GHz to 18 GHz under jet-cooled conditions has revealed large tunneling splittings arising from a low barrier to internal rotation of the ring methyl group and small splittings from a high torsional barrier of the acetyl methyl group. The large splittings are especially challenging to model, while the small splittings are difficult to analyze due to the resolution limit of 120 kHz. The combination of two methyl groups undergoing internal rotations caused each rotational transition to split into five torsional species, which were resolved and fitted using a modified version of the XIAM code and the newly developed ntop code to a root-mean-square deviation close to measurement accuracy, providing an estimate of the V3 potential barriers of about 22 cm-1 and 584-588 cm-1 for the ring and the acetyl methyl groups, respectively. The assignment was aided by separately fitting the five torsional species using odd-power order operators. Only one conformer in which all heavy atoms are located on a symmetry plane could be identified in the spectrum, in agreement with results from conformation analysis using quantum chemical calculations.

Coupled large amplitude motions: The effects of two methyl internal rotations and 14N quadrupole coupling in 4,5-dimethylthiazole investigated by microwave spectroscopy

The molecular jet Fourier-transform microwave spectrum of 4,5-dimethylthiazole has been recorded between 2.0 and 26.5 GHz, revealing torsional splittings arising from two inequivalent methyl internal rotations with relatively low hindering barriers and nitrogen quadrupole hyperfine structures. Two global fits with 1009 hyperfine components of 97 rotational transitions involving 315 torsional lines were performed using the program XIAM and BELGI-Cs-2Tops-hyperfine, an extended version of the BELGI-Cs-2Tops code which includes the effect of the 14N quadrupole coupling, giving a root-mean-square deviation of 399.8 kHz and 4.2 kHz, respectively. Compared to the monomethyl substituted thiazole derivatives, the barriers to internal rotation are drastically lower. This is also in contrast to chemical intuition which suggests high barriers due to steric hindrance. Because of the strong interaction between the methyl groups, strong top-top couplings in both the potential energy and kinetic parts of the Hamiltonian were observed.

Carboxylic Acids, Reactivity with Alcohols and Clustering with Esters: A Rotational Study of Formic Acid-Isopropylformate.

The rotational spectrum of the 1:1 complex formic acid-isopropylformate (FA-IPF) has been first observed when trying to assign the pulsed jet Fourier transform microwave (FTMW) spectrum of the adduct formic acid-2-propanol, by expanding a binary mixture of HCOOH and 2-propanol in He. The strong FTMW spectrum of isopropylformate, formed by the esterification reaction, was observed instead. However, when HCOOH was in excess in the binary mixture, it was possible to observe and assign the rotational spectrum of FA-IPF. Later on a much intense spectrum of FA-IPF was obtained, when combining FA with IPF. Finally, the spectra of five isotopologues of the most stable isomer of formic acid-isopropylformate have been observed by means of rotational spectroscopy in supersonic expansion. Some of them, HCOOH-(CH3)2CHOOCD and HCOOH-(CH3)2CDOOCH have been synthesized in the MW cavity by using DCOOH or (CH3)2CDOH as precursors in the esterification process. In the observed isomer of the complex, the two subunits are linked to each other by a standard O-H···O and a weak C-H···O hydrogen bond. The dissociation energy has been estimated to be 34.1 kJ·mol-1.

The microwave spectrum and molecular structure of 3,3-difluoro-1,2-epoxypropane and its complex with the argon atom

As a small, chiral molecule with a strong and simple microwave rotational spectrum, 3,3-difluoro-1,2-epoxypropane is a promising candidate as a chiral tag for conversion of enantiomeric analytes into spectroscopically distinct diastereomers via formation of a non-covalently bound complex. The Fourier transform microwave spectra of this species and its complex with the argon atom are obtained from 5.1 to 21.0 GHz, and their structures determined. Not surprisingly, these structures for both the monomer and the argon complex show strong similarities with those previously determined for 3,3,3-trifluoro-1,2-epoxypropane and 3,3,3-trifluoro-1,2-epoxypropane–argon, but are different from those of the analogous propylene oxide species. This suggests that fluoromethyl and methyl substitution affect the bonding in the epoxy moiety and intermolecular interactions in distinctly different ways.

Fourier transform microwave spectroscopy of the SiCl+ ion

Fourier transform microwave spectra for the J = 1 ← 0 and 2 ← 1 rotational transitions of the SiCl+ ion were observed for two isotopologues (35Cl and 37Cl) in the ground and the first excited vibrational states of the ground 1Σ+ electronic state. Thanks to the high resolution of the FTMW spectrometer, hyperfine structures due to the quadrupole moment of the chlorine nucleus and the nuclear spin-rotation interaction were fully resolved. The observed FTMW spectra were combined with previously reported MMW and diode laser spectra in an analysis to determine the mass-independent Dunham coefficients Uk,l as well as a mass scaling parameter Δ01Cl=-0.856(30). The equilibrium bond length of SiCl+ determined is re=1.9439729(10) Å and the nuclear quadrupole coupling constant of Si35Cl+ is eQqe=-11.8788(23) MHz.

Examining transition metal hydrosulfides: The pure rotational spectrum of ZnSH (X̃2A').

The pure rotational spectrum of the ZnSH (X̃2A') radical has been measured using millimeter-wave direct absorption and Fourier transform microwave (FTMW) methods across the frequency range 18-468 GHz. This work is the first gas-phase detection of ZnSH by any spectroscopic technique. Spectra of the 66ZnSH, 68ZnSH, and 64ZnSD isotopologues were also recorded. In the mm-wave study, ZnSH was synthesized in a DC discharge by the reaction of zinc vapor, generated by a Broida-type oven, with H2S; for FTMW measurements, the radical was made in a supersonic jet expansion by the same reactants but utilizing a discharge-assisted laser ablation source. Between 7 and 9 rotational transitions were recorded for each isotopologue. Asymmetry components with Ka = 0 through 6 were typically measured in the mm-wave region, each split into spin-rotation doublets. In the FTMW spectra, hyperfine interactions were also resolved, arising from the hydrogen or deuterium nuclear spins of I = 1/2 or I = 1, respectively. The data were analyzed using an asymmetric top Hamiltonian, and rotational, spin-rotation, and magnetic hyperfine parameters were determined for ZnSH, as well as the quadrupole coupling constant for ZnSD. The observed spectra clearly indicate that ZnSH has a bent geometry. The rm(1) structure was determined to be rZn-S = 2.213(5) Å, rS-H = 1.351(3) Å, and θZn-S-H = 90.6(1)°, suggesting that the bonding occurs primarily through sulfur p orbitals, analogous to H2S. The hyperfine constants indicate that the unpaired electron in ZnSH primarily resides on the zinc nucleus.

The effect of internal rotation in p-methyl anisole studied by microwave spectroscopy

The Fourier transform microwave spectrum of p-methyl anisole, CH3C6H4OCH3, was measured in the frequency range from 2 to 26.5GHz under molecular jet conditions. The conformational analysis yielded only one stable conformer, in which all heavy atoms are co-planar, and which was identified after analyzing the spectrum by comparison with the results from quantum chemical calculations. The barrier of the V3 potential of the ring methyl rotor was found to be 49.6370(1) cm−1, and was compared with that found in other para-substituted toluenes as well as in o-methyl anisole. A comparison between two theoretical approaches treating internal rotations, the rho axis method (program BELGI-Cs) and combined axis method (program XIAM), was also performed.

Microwave spectroscopy of carbonyl sulfide isotopologues solvated with 2–5 para-hydrogen molecules

We report high resolution Fourier transform microwave spectra of (pH2)N-OC32S and (pH2)N-OC34S clusters in the size range from N=2 to 5. Observation of the J=1–0 and J=2–1 transitions allowed for determination of the rotational (B) and quartic distortion (D) constants for each N. Comparison with theory (Paesani et al., 2003) reveals that the predicted B values are of good quality (all within 100MHz of the actual values), while the predicted D values are an order of magnitude too high. Results from linear molecule Kraitchman analyses for clusters with N≤5 are consistent with theoretical calculations which suggest that the initial pH2 density accumulates in a donut ring about the carbon-oxygen bond.

Structural elucidation of 2-fluorothiophenol from Fourier transform microwave spectra and ab initio calculations

Pure rotational transitions corresponding to the ground vibrational state of 2-fluorothiophenol (2FTPh) were recorded via Fourier transform microwave (FTMW) spectroscopy in the range of 4–26 GHz. The observed transitions were assigned to two planar conformers in which the SH bond is directed toward (cis-2FTPh) or away from (trans-2FTPh) the fluorine substituent with the former predicted to lie 3.86 kJ/mol lower in energy from ab initio calculations (MP2/6-311++G(2d,2p)). The rotational constants determined from the spectral analysis were used to derive effective ground state (r0) structures of the lower energy cis-2FTPh conformer using spectra of the corresponding 13C and 34S isotopologues which were observed in natural abundance. Geometry optimization at the MP2/6-311++G(2d,2p) level provided the equilibrium (re) structure which is in close agreement with the experimentally-derived geometry. Comparison with results from natural bond orbital (NBO) calculations provide evidence of a weak intramolecular interaction between the lone pair on sulfur and the CF moiety in cis-2FTPh that is not found in trans-2FTPh or thiophenol (TPh). This interaction stabilizes the cis-2FTPh conformer by 2.18 kJ/mol.

Rotational Spectroscopic Study of Quantum Solvation in Isotopologic (pH2)N–CO Clusters

We report the Fourier transform microwave spectra of (pH2)N-13C16O, (pH2)N-12C18O, and (pH2)N-13C18O (N ≤ 8) clusters. We find that the frequencies of the a-type J = 1-0 transitions decrease to a minimum at N = 6 and then rapidly increase up to at least N = 8; this is similar to what was previously reported for (pH2)N-12C16O, for which the turnaround was found to correlate with an increase in the superfluid fraction of the pH2 component of the clusters [ Raston Phys. Rev. Lett. 2012 , 108 , 253402 ]. The data suggest that the turnaround in the transition frequency marks an abrupt decrease in the anisotropy of the potential (i.e., in going from N = 6 → 7 → 8), as evidenced from the isotopologic differences rapidly evolving from end-over-end to free-rotor character. Structurally, a more quantitative analysis of the anisotropy was hindered by the lack of accurate frequencies in the b-type series, and a simple Kraitchman analysis yielded unphysical results. In addition to comparing the transition frequencies of the different isotopologic clusters, we provide here more comprehensive details and further discussion of the initial measurements made on (pH2)N-12C16O.

The role of amino acid side chains in stabilizing dipeptides: the laser ablation Fourier transform microwave spectrum of Ac-Val-NH2.

The steric effects imposed by the isopropyl group of valine in the conformational stabilization of the capped dipeptide N-acetyl-l-valinamide (Ac-Val-NH2) have been studied by laser ablation molecular beam Fourier transform microwave (LA-MB-FTMW) spectroscopy. The rotational and quadrupole coupling constants of the two 14N nuclei determined in this work show that this dipeptide exists as a mixture of C7 and C5 conformers in the supersonic expansion. The conformers are stabilized by a C[double bond, length as m-dash]OH-N intramolecular hydrogen bond closing a seven- or a five-membered ring, respectively. The observation of both conformers is in good agreement with previous results on the related dipeptides containing different residues, confirming that the polarity/non-polarity of the side chains of the amino acid is responsible for the conformational locking/unlocking. The voluminous isopropyl group is not able to prevent the less stable C5 conformer from forming but it destabilizes the C[double bond, length as m-dash]OH-N interaction.