R0 Structures Research Articles

The gas-phase structure determination of α-pinene oxide: An endo-cyclic epoxide of atmospheric interest.

The gas-phase rotational spectra of α-pinene oxide have been recorded using a chirped-pulse Fourier transform microwave spectrometer in the 6-18GHz frequency range. The parent species and all heavy atom isotopologues (13C and 18O) have been observed in their natural abundance. The experimental rotational constants of all isotopic species observed have been determined and used to obtain the substitution (rs) and the effective (r0) structures of the most stable conformer of α-pinene oxide. Calculations using the density functional theories B3LYP, M06-2X, and MN15-L and the abinitio method MP2 level of theory were carried out to check their performance against experimental results. The structure of the heavy atom's skeleton of α-pinene oxide has been compared to that of α-pinene and has shown that epoxidation does not overly affect the structure of the bicycle, validating its robustness. Furthermore, the structural features have been compared to those of other bicyclic molecules, such as nopinone and β-pinene.

The Shapes of Sulfonamides: A Rotational Spectroscopy Study.

Benzenesulfonamides are a class of molecules of extreme interest in the biochemical field because many of them are active against a variety of diseases. In this work, the pharmacophoric group benzensulfonamide, its derivatives para-toluensulfonamide and ortho-toluensulfonamide, and the bioactive molecule sulfanilamide, were investigated using rotational spectroscopy to determine their conformations and the influence of different substituents on their structures. For all species, the hyperfine structure due to the 14N atom was analyzed, and this provided crucial information for the unambiguous identification of the observed conformation of all molecules. In addition, for ortho-toluensulfonamide, the vibration–rotation hyperfine structure related to the methyl torsion was analyzed, and the methyl group rotation barrier was determined. For benzensulfonamide, partial rS and r0 structures were established from the experimental rotational constants of the parent and two deuterated isotopic species. In all compounds except ortho-toluensulfonamide, the amino group of the sulfonamide group lies perpendicular to the benzene plane with the aminic hydrogens eclipsing the oxygen atoms. In ortho-toluensulfonamide, where weak attractive interactions occur between the nitrogen lone pair and the methyl hydrogen atoms, the amino group lies in a gauche orientation, retaining the eclipsed configuration with respect to the SO2 frame. A comparison of the geometrical arrangements found in the PDB database allowed us to understand that the bioactive conformations are different from those found in isolated conditions. The conformations within the receptor are reached with an energy cost, which is balanced by the interactions established in the receptor.

Accurate equilibrium structures of linear triatomic molecules from a combined theoretical–experimental method: The protonated nitrogen molecule, HN[formula omitted

This article examines several strategies which can be used to derive molecular equilibrium structure from experimental data and extends our previously suggested method to HN2+. Full-dimensional rovibrational calculations are carried out for eight isotopologues of HN2+ using two ab initio potential energy representations. Rovibrational energies computed for rotational angular momenta J as high as 15 in both parities are used to derive theoretical vibration–rotation corrections to ground-state rotation constants B0. These results are combined with the available experimentally derived B0 values to determine the r0, rα, and re structures of the ion. Higher-order corrections beyond the vibration–rotation α constants are found to be essential for the structure determination. Our analysis is supported by graphical representations, illustrating the anticorrelation of the structural parameters. The r0 structure is rationalized in terms of a structure projected onto the principal a axis and examined for the isoelectronic series of linear triatomic molecules HCO+, HCN, HN2+, HNC, and HOC+.

Conformers of Allyl Isothiocyanate: A Combined Microwave Spectroscopy and Computational Study.

The pure rotational spectrum of allyl isothiocyanate (CH2=CHCH2-NCS) was collected from 4 to 26 GHz using Fourier transform microwave (FTMW) spectroscopy. Its analysis revealed the presence of two conformers that arise due to variation in the CCCN and CCNC dihedral angles. The observed spectrum is consistent with the accompanying potential energy surfaces derived using quantum chemical calculations at the B3LYP-D3(BJ) and MP2 levels of theory. Together, this experimental and theoretical study unequivocally identifies a new conformer (I) as the global minimum geometry. The spectral assignment of this new conformer is verified by the observation of transitions consistent with its 34S, 13C, and 15N isotopologues and with the characteristic 14N quadrupole hyperfine patterns. For conformer I, the substitution (rs) and effective ground state (r0) structures were derived and reveal contributions from a large amplitude motion in the CCNC angle. The remaining geometric parameters compare well with the equilibrium structure (re) from B3LYP-D3(BJ)/cc-pVQZ calculations. The derived CNC bond angle of 152.6(3)° for conformer I of allyl-NCS is found to be ∼15° larger than that of allyl-NCO (137.5(4)°), which is in line with a change in the hybridization at nitrogen from an orbital with more ∼sp character in allyl-NCS to ∼sp1.5 in allyl-NCO as determined via natural bond orbital analyses.

Supersonic jet microwave rotational spectrum of 2,3-difluorophenol

The microwave rotational spectrum of the aromatic 2,3-difluorophenol (2,3-DFP) has been recorded and analyzed in the frequency range of 5–25 GHz using a pulsed-jet Fourier transform microwave spectrometer. Rotational transitions were measured for the parent, the 18O, and all six 13C singly substituted isotopologues in natural abundance as well as the deuterium enriched 2HO-species. In addition to the rotational and quartic centrifugal distortion parameters, the quadrupole coupling constants of the deuterium species were determined. The rotational constants for the parent species are obtained as A = 2313.37516(14) MHz, B = 1797.935693(62) MHz, C = 1011.695171(36) MHz. For the deuterated species, the 2H quadrupole coupling constants are χaa = −0.11693(80) MHz and (χbb−χcc) = 0.4112(14) MHz. The partial rs substitution structure as well as a semi-experimental r0 structure was determined from the rotational constants of the observed isotopologues. Both rs and r0 structures agree well with each other and with MP2/6-311++g(2d, 2p) and B3LYP/6-311++g(2d, 2p) predicted equilibrium structures with the biggest deviation in heavy atom bond lengths being 0.008 Å.

Conformational landscape of the weakly bound difluoromethane–1,1-difluoroethane dimer explored by rotational spectroscopy and quantum chemical calculations

The rotational spectra of three isomers of the difluoromethane–1,1-difluoroethane dimer were measured and assigned by using Fourier transform microwave spectroscopy. Experimental results, ab initio calculations and quantum theory of atoms in molecules (QTAIM) analysis confirm that the two subunits in the observed isomers are held together by three weak CH⋯FC interactions. Rotational spectra of all the 13C isotopologues were also observed for all the three isomers. The experimental data leads to precise determination of the substitution (rs) and effective (r0) structures of the observed isomers. The Symmetry-adapted perturbation theory (SAPT) analysis demonstrates that electrostatic and dispersion terms play a major role in determining the stabilities of the complex. The relative populations of the three observed isomers in the supersonic expansion in helium were estimated as NI/NII/NIII ≈ 5/2/6 from relative intensity measurements.

Fourier Transform Microwave Spectroscopic and ab Initio Study of the Rotamers of 2-Fluorobenzaldehyde and 3-Fluorobenzaldehyde.

The rotational spectra of 2-fluorobenzaldehyde (2-FBD) and 3-fluorobenzaldehyde (3-FBD) were recorded using Fourier transform microwave (FTMW) spectroscopy from 4 to 26 GHz. Two planar rotamers were observed for each species that correspond to structures in which the carbonyl bond is directed toward (O-cis) or away from (O-trans) the C1-C2 bond. Observation of transitions due to heavy atom isotopes (13C, 18O) in natural abundance allowed derivation of the ground state effective (r0) structures and mass dependence (rm) structures for the lowest energy rotamer of 2-FBD (O-trans) and both rotamers of 3-FBD, which compare favorably with ab initio estimates of the equilibrium (re) geometries at the MP2/aug-cc-pVTZ level. The resultant parameters are consistent with the introduction of bond length alternation in the benzene ring, which is dependent on the orientation of the aldehyde group. Careful study of the experimental structure and results of natural bond orbital (NBO) analysis do not support the presence of intramolecular hydrogen bonding as the source of its stabilization of O-trans 2-FBD over its cis counterpart. Furthermore, calculations of the interconversion pathways between rotamers suggest that despite being 9.39 kJ/mol higher in energy, the O-cis 2-FBD moiety is metastable in the molecular beam, which has allowed the observation of its microwave spectrum for the first time.

The gas phase structure of α-pinene, a main biogenic volatile organic compound.

The gas phase structure of the bicyclic atmospheric aerosol precursor α-pinene was investigated employing a combination of quantum chemical calculation and Fourier transform microwave spectroscopy coupled to a supersonic jet expansion. The very weak rotational spectra of the parent species and all singly substituted 13C in natural abundance have been identified, from 2 to 20 GHz, and fitted to Watson's Hamiltonian model. The rotational constants were used together with geometrical parameters from density functional theory and ab initio calculations to determine the rs, r0, and rm(1) structures of the skeleton, without any structural assumption in the fit concerning the heavy atoms. The double C=C bond was found to belong to a quasiplanar skeleton structure containing 6 carbon atoms. Comparison with solid phase structure is reported. The significant differences of α-pinene in gas phase and other gas phase bicyclic monoterpene structures (β-pinene, nopinone, myrtenal, and bicyclo[3.1.1]heptane) are discussed.

Oxygen-18 Isotopic Studies of HOOO and DOOO.

Owing to questions that still persist regarding the length of the O-H and central O-O bond, and large-amplitude torsional motion of trans hydridotrioxygen HOOO, a weakly bound complex between OH and O2, new 18O isotopic measurements of HOOO and DOOO were undertaken using Fourier transform microwave and microwave-millimeter-wave double resonance techniques. Rotational lines from three new 18O species of DOOO (D18OOO, DO18O18O, and D18O18O18O) were detected, along with the two singly substituted 18O isotopic species of HOOO (HO18OO and HOO18O) that were not measured in the previous isotopic investigation. From a least-squares fit, spectroscopic constants, including the three rotational constants, were precisely determined for all five species. The inertial defect of DOOO and its 18O species is uniformly negative: of order -0.04 amu Å2, regardless of the number or location of the 18O atoms, in contrast to that found for HOOO or its 18O isotopic species. A reanalysis of the molecular structure was performed using either normal HOOO and its four singly substituted isotopic species, the new DOOO data, or all the isotopic species (10 in total). The differences between the purely experimental (r0) structures are generally quite small, of order ±0.01 Å for the bond lengths and ±1° for the bond angle. The length of the O-H bond remains unrealistically short compared to free OH, and the central O-O bond length is consistently very close to 1.68 Å. On the basis of the effective O-H bond length derived from the experimental structure, the average displacement of the large amplitude torsional motion from planarity is estimated to be ∼22°.

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 spectra and conformer geometries of 2-fluorophenol and 3-fluorophenol

The ground states of 2-fluorophenol (2FPh) and 3-fluorophenol (3FPh) and their 13C isotopologues were investigated using Fourier transform microwave (FTMW) spectroscopy in the range of 6–26GHz. Two planar conformers were observed for 3FPh, corresponding to cis and trans orientations of OH relative to F, while only the cis conformer of 2FPh was detected. The rotational constants determined were used to derive substitution (rs) and effective ground state (r0) structures of the lowest energy conformer of each compound: cis-2FPh and trans-3FPh. Geometry optimization at the MP2/6-311++G(2d,2p) level provided equilibrium (re) structures which are in close agreement with the experimental parameters. The derived structures along with results from natural bond orbital (NBO) calculations provide evidence of an intramolecular interaction in cis-2FPh between the fluorine atom and OH moiety that stabilizes this conformer by 0.73kcal/mol.

Structure of fenchone by broadband rotational spectroscopy.

The bicyclic terpenoid fenchone (C10H16O, 1,3,3-trimethylbicyclo[2.2.1]heptan-2-one) has been investigated by chirped pulse Fourier transform microwave spectroscopy in the 2-8 GHz frequency region. The parent species and all heavy atom isotopologues have been observed in their natural abundance. The experimental rotational constants of all isotopic species observed have been determined and used to obtain the substitution (rs) and effective (r0) structures of fenchone. Calculations at the B3LYP, M06-2X, and MP2 levels of theory with different basis sets were carried out to check their performance against experimental results. The structure of fenchone has been compared with those of norbornane (bicyclo[2.2.1]heptane) and the norbornane derivatives camphor (1,7,7-trimethylbicyclo[2.2.1]heptan-2-one) and camphene (3,3-dimethyl-2-methylenebicyclo[2.2.1]heptane), both with substituents at C2. The structure of fenchone is remarkably similar to those of camphor and camphene. Comparison with camphor allows identification of changes in ∠CCC angles due to the different position of the methyl groups. All norbornane derivatives display similar structural changes with respect to norbornane. These changes mainly affect the bond lengths and angles of the six-membered rings, indicating that the substituent at C2 drives structural adjustments to minimise ring strain after its introduction.

Potential energy surface of fluoroxene: experiment and theory.

The potential energy surface (PES) of the general anesthetic fluoroxene (2,2,2-trifluoroethyl vinyl ether) was probed in a supersonic jet expansion using broadband chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy and theoretical calculations. The PES is dominated by a single conformation, as other stable minima are shown to kinetically relax in the expansion to the global minimum. Consistently, the rotational spectrum reveals a single conformation. Fluoroxene adopts a CS heavy-atom planar skeleton structure in the gas phase, with a cis-trans conformation (cis for the CH2=CH-O-CH2- and trans for the =CH-O-CH2-CF3 part). The sensitivity of a recently-built CP-FTMW spectrometer at the UPV/EHU is demonstrated by the detection of five isotopologues of fluoroxene in natural abundance, corresponding to the (13)C and (18)O monosubstituted species. The rS and r0 structures were determined and are in good agreement with theoretical predictions using the MP2, B3LYP and M06-2X methods.

Microwave spectroscopy of AgCCH and AuCCH in the [formula omitted] states

The rotational spectra of AgCCH and AuCCH were measured using Fourier-transform microwave (FTMW) and source-modulated millimeter-wave spectrometers. For the FTMW measurements, AgCCH and AuCCH were generated in supersonic jets using the pulse-discharge reaction of laser-ablated metal atoms with HCCH diluted with Ar. For the millimeter-wave measurements, these molecules were generated in a free-space cell by sputtering of metal sheets. Rotational transitions were measured for 107AgCCH, 109AgCCH, 107AgCCD, 109AgCCD, AuCCH, and AuCCD. The r0 structures of both molecules were obtained from the rotational constants. The quadrupole parameters of the Au and D nuclei were compared with those of other metal monoacetylides.

Heavy atom nitroxyl radicals. II: Spectroscopic detection of H2As=O, the prototypical arsenyl free radical

The previously unknown arsenyl (H(2)AsO) free radical has been identified in the gas phase through a combination of laser-induced fluorescence and single vibronic level emission spectroscopy in a supersonic expansion. Three isotopologues, H(2)AsO, HDAsO, and D(2)AsO have been detected as products of an electric discharge in mixtures of arsine or deuterated arsines, CO(2), and argon. The observed spectra are assigned as due to the B (2)A(')-X (2)A(') electronic transition in which an electron in the ground state pi orbital is promoted to the pi( *) orbital. Rotational analysis of high-resolution spectra proves that the radical is nonplanar in both electronic states with the following r(0) structures: r(")(As-H)=1.513(4) A, r(")(As-O)=1.672(1) A, theta(")(HAsH)=101.8(4) degrees , ground state out-of-plane angle=63.1 degrees ; r(')(As-H)=1.525(10) A, r(')(As-O)=1.806(3) A, theta(')(HAsH)=93.4(10) degrees , and excited state out-of-plane angle=70.7 degrees . Small hyperfine splittings in the spectra have enabled the determination of the arsenic Fermi contact parameter in both states. The results of our ab initio studies of the ground and excited state of this radical (see immediately preceding paper) are in good agreement with the spectroscopic analysis.

Gas‐Phase Structure of N,N‐Dimethylglycine

Three conformers of the neutral amino acid N,N-dimethylglycine [(CH3)2NCH2COOH] were detected in a supersonic expansion by a combination of laser ablation (LA) and molecular-beam Fourier transform microwave (MB-FTMW) spectroscopy. A bifurcated methyl-to-carbonyl (C--HO==C) weak intramolecular hydrogen bond might stabilise the most stable conformer of C(s) symmetry. The second most stable conformer of C1 symmetry has a hydrogen bond between the hydroxyl group and the lone pair at the nitrogen atom (NH--O). The r(s) and r0 structures were derived for this conformer from the rotational data for the parent and six minor 13C, 15N and OD isotopomers. A third conformer exhibits a cis-carboxyl functional group and C1 symmetry. Ab initio MP2/6-311++G(d,p) predictions of the spectroscopic parameters were useful in analysing the spectra. In particular, the agreement of the nuclear quadrupole coupling constants with those calculated was conclusive in identifying the different conformers.

Experimental and ab Initio Equilibrium Structure and Harmonic Force Field of 1,2,5-Oxadiazole

The equilibrium structure of 1,2,5-oxadiazole has been calculated ab initio at the CCSD(T) level using a polarized valence quadruple ζ basis set. The harmonic force field has also been calculated at the MP2/cc-pVTZ, B3LYP/6-311++G(3df, 2pd), and B3LYP/cc-pVQZ levels. These force fields have been subsequently scaled and further refined by fitting them to the experimental values of the vibrational fundamentals of three isotopomers and the centrifugal distortion constants of the parent molecule. The specific refinement of those scaled force constants particularly sensitive to the experimental data set was decisive for obtaining a more reliable harmonic potential. The resulting force fields are presented and used, together with the ground state rotational constants, to calculate an rz structure. The experimental r0, rs, and rm structures have also been determined. The different results have been compared and it is concluded that the ab initio structure is a good approximation of the equilibrium structure. It is also shown that the magnetic correction is not negligible, particularly for the inertial defect. Another interesting conclusion is that the anharmonicity of the C–H stretching might be unusually small.

Coupled-cluster characterization of the ground and excited states of the CH2N and CH2P radicals

High-level coupled-cluster theory with large basis sets was used to determine the optimized geometries and harmonic vibrational frequencies for the ground and low-lying excited electronic states of the CH2N and CH2P radicals. Additionally, isotropic hyperfine splitting constants were determined for the C2v CH2X, trans-HCXH and cis-HCXH (where X=N and P) isomers as a gauge of the delocalization of the unpaired electron. The geometrical parameters of X̃ 2B2 CH2X, the trans-HCXH and cis-HCXH conformers, and the first three excited states are reported at the coupled-cluster level with single, double, and perturbatively applied triple excitations [CCSD(T)] using Dunning’s correlation consistent polarized valence quadruple-ζ basis set (cc-pVQZ). The C2v structures on the ground state surface are predicted to lie 9.3 and 13.5 kcal/mol lower than the trans- and cis-isomers, respectively, for CH2N and 28.1 and 30.0 kcal/mol, respectively, for CH2P. There are many other important properties of CH2N and CH2P which are not known from experiment. The geometrical parameters of the CH2N ground state [r(C–N)=1.2462 Å, r(C–H)=1.0921 Å, and θ(HCH)=119.4°] and the CH2P ground state [r(C–P)=1.6583 Å, r(C–H)=1.0842 Å, and θ(HCH)=118.9°] agree well with the C–N and C–P bond distances of the r0 structures derived from microwave data, although notable differences were observed in the C–H bond distance and HCH bond angle. This research resolves an earlier discrepancy between theory and experiment for the ground state C–P distance in CH2P. The Fermi contact terms for 1H, 13C, 14N, and 31P were determined at CCSD(T) level of theory with the cc-pVTZ and cc-pVQZ basis sets and are in reasonable agreement with the experimental values with a maximum deviation of 26 MHz for CH2N and 11 MHz for CH2P. The excited states of CH2N are predicted to lie 33 000 cm−1 (Ã 2B1), 36 000 cm−1 (B̃ 2A′), and 38 000 cm−1 (C̃ 2A1) above the ground state, and the excited states of CH2P to lie approximately 21 000 cm−1 (Ã 2A′), 26 000 cm−1 (B̃ 2B1), and 33 000 cm−1 (C̃ 2A1) above the ground state.

Noble Gas−Metal Chemical Bonds. Microwave Spectra, Geometries, and Nuclear Quadrupole Coupling Constants of Ar−AuCl and Kr−AuCl

The pure rotational spectra of Ar−AuCl and Kr−AuCl have been measured using a pulsed-jet cavity Fourier transform microwave spectrometer. Both complexes have been found to be linear and are relatively rigid in their ground vibrational states. The noble gas−gold stretching frequencies have been estimated to be 198 and 161 cm-1 for Ar−AuCl and Kr−AuCl, respectively. From the isotopic data obtained, r0 structures have been calculated for both Ar−AuCl and Kr−AuCl, while a partial substitution (rs) structure has been obtained for Kr−AuCl. The Ar−Au distance has been found to be 2.47 Å, while the Kr−Au distance is 2.52 Å. Ab initio calculations have been performed at the MP2 level of theory on both complexes to obtain geometries, vibrational frequencies, and dissociation energies. The dissociation energies for Ar−AuCl and Kr−AuCl have been estimated to be 47 and 71 kJ mol-1, respectively. The nuclear quadrupole coupling constant of Au has been found to change significantly on complex formation (to −259.8 MHz in Ar−AuCl, and −349.9 MHz in Kr−AuCl) from its value in the monomer unit (+9.6 MHz in AuCl), which is consistent with extensive charge rearrangement on formation of the complexes. This, in conjunction with the sizable dissociation energies, indicates that the Ar−Au and Kr−Au bonds are weakly covalent.

The detection of the free radical FO (X 2Π3/2) by submillimeter-wave spectroscopy

Pure rotational transitions of the free radical FO in its X 2Π3/2 ground electronic state have been detected by millimeter-wave spectroscopy. Four transitions, which are all magnetic dipole in character, were measured in the frequency region from 219 to 408 GHz. The radical was produced by a well-known efficient reaction between ozone and fluorine atoms. The high sensitivity available in the submillimeter-wave region and a very efficient production method were essential for the detection of rotational transitions in this radical, which many other people have attempted in the past. No electric dipole transitions have been detected in this experiment. The precision of parameters associated with the rotational motion of FO has been improved considerably by fitting these lines together with previous data. Moreover, the parity-dependent nuclear spin rotation term CI′ has been determined for the first time. The r0 and re structures of FO have been revised accordingly.