Pulsed-nozzle Fourier Transform Microwave Spectroscopy Research Articles

A strong dependence of the CH3 internal rotation barrier on conformation in thioacetic acid: Microwave measurements and an energy decomposition analysis.

Rotational spectra of thioacetic acid (CH3COSH) have been observed by pulsed-nozzle Fourier transform microwave spectroscopy. Spectroscopic constants are reported for both the syn and anti conformers of the parent species, as well as the 34S and 13C carbonyl isotopologues. Transitions arising from the lowest A and E internal rotor states of the methyl group have been observed and analyzed. Experimental values of the three-fold internal rotation barrier, V3, for the syn and anti conformers of the parent isotopologue are 76.300(12) and 358.056(51) cm-1, respectively, indicating a large effect of the S-H orientation on the CH3 internal rotation potential. M06-2X/6-311+G(d,p) calculations are in good agreement with these results. The block localized energy decomposition method has been applied to understand the origins of this strong dependence of V3 on conformation. The results indicate that π conjugation from the SH to the carbonyl group and steric repulsion between the SH and the methyl group in the anti form are main contributors to the difference.

A Perfluorinated Carboxylic Sulfuric Anhydride: Microwave and Computational Studies of CF3COOSO2OH.

Trifluoroacetic sulfuric anhydride (CF3COOSO2OH, TFASA) and its deuterated isotopologue have been observed by pulsed-nozzle Fourier transform microwave spectroscopy. TFASA was generated in situ in a supersonic expansion from the reaction of CF3COOH or CF3COOD with SO3. The spectrum, which was notably weaker than those of previously studied carboxylic sulfuric anhydrides, is that of a simple asymmetric rotor with no evidence of internal rotation of the CF3 group. Calculations at the M06-2X/6-311++G(3df,3pd) level indicate that the title compound is produced via a mechanism involving a concerted cycloaddition, analogous to that found for other carboxylic sulfuric anhydrides. The calculations further show that the equilibrium orientation of CF3 relative to the C═O bond changes upon formation of the anhydride, indicating that any path connecting the equilibrium structures of CF3COOH and CF3COOSO2OH necessarily includes both cycloaddition and internal rotation. CCSD(T)/complete basis set with double and triple extrapolation [CBS(D-T)] single-point energy calculations at key points on the potential surface indicate that the barrier to form TFASA from a putative CF3COOH···SO3 complex is about 1.2 kcal/mol after zero-point energy corrections. This value is significantly larger than the near-zero or slightly negative barriers previously reported for the reactions of SO3 with nonfluorinated carboxylic acids and likely accounts, at least in part, for the reduced spectral intensity. Thus, TFASA is a somewhat unique addition to the series of carboxylic sulfuric anhydrides studied to date. Theoretical values of certain structural parameters, atomic charges, and vibrational frequencies also support this point of view. Despite the differences, however, this work clearly demonstrates that the reaction RCOOH + SO3 → RCOOSO2OH readily occurs in the gas phase and is not restricted to acids with hydrocarbon R groups.

Observation of Two Conformers of Acrylic Sulfuric Anhydride by Microwave Spectroscopy.

The rotational spectrum of acrylic sulfuric anhydride (CH2═CHCOOSO2OH, AcrSA) has been observed using pulsed-nozzle Fourier transform microwave spectroscopy. The species was produced from the reaction between acrylic acid and sulfur trioxide in a supersonic jet. Spectroscopic constants are reported for both the s-cis- and s-trans-AcrSA conformers of the parent and monodeuterated (OD) isotopologues. Geometries were optimized for both conformers using M06-2X/6-311++G(3df,3pd) methods. Single-point energy calculations at the M06-2X geometries were calculated using the CCSD(T)/complete basis set method with double and triple extrapolation [CBS(D-T)]. Further calculations indicate that the anhydride results from a π2 + π2 + σ2 cycloaddition reaction within the acrylic acid-SO3 complex. Because the C═O double bond of the acrylic acid migrates from one of the COOH oxygens to the other during the reaction, the s-cis form of acrylic acid leads to the s-trans form of the anhydride and vice versa. With zero-point energy corrections applied to the CCSD(T) energies, the s-cis and s-trans forms of CH2═CHCOOSO2OH are 19.0 and 18.8 kcal/mol lower in energy than that of SO3 + their corresponding CH2═CHCOOH precursor conformation. The zero-point-corrected transition state energies for formation of the s-trans and s-cis anhydrides are 0.22 and 0.33 kcal/mol lower than those of the complexes of SO3 with s-cis and s-trans acrylic acid, respectively, indicating that the reaction is essentially barrierless. This system adds to a growing body of examples demonstrating that carboxylic acids readily add to SO3 in the gas phase to produce the corresponding carboxylic sulfuric anhydride.

Microwave spectrum and structure of the 3,5-difluoropyridine⋯CO2 van der Waals complex

The rotational spectrum of the weakly bound complex 3,5-difluoropyridine⋯CO2 has been observed using pulsed-nozzle Fourier transform microwave spectroscopy. Spectroscopic constants are reported for the parent and 13CO2 isotopologues. The data indicate a planar structure in which the nitrogen approaches the carbon of the CO2 with either a C2v or effectively C2v geometry in the ground vibrational state. The N⋯C van der Waals bond distance is 2.8245(16)Å and the oxygen⋯ortho-hydrogen distance is 3.091(2)Å. The N⋯C van der Waals bond length is 0.027(8)Å longer than that previously determined for pyridine–CO2, but is still considerably shorter than the 2.998Å distance in HCN⋯CO2. M06-2X/6-311++G(3df,3pd) calculations place the binding energy of the complex at 4.3kcal/mol (4.1kcal/mol with counterpoise correction). The calculations further indicate that a secondary interaction between the ortho-hydrogens of the ring and the CO2 oxygens account for ∼50% of the total binding energy.

Microwave spectrum, van der Waals bond length, and 131Xe quadrupole coupling constant of Xe–SO3

Nine isotopologues of Xe–SO3 have been observed by pulsed-nozzle Fourier transform microwave spectroscopy. The complex is a symmetric top with a Xe–S van der Waals distance of 3.577(2) Å. The increase in rare gas distance relative to that in Kr–SO3 is equal to the difference in van der Waals radii between Xe and Kr. The 131Xe nuclear quadrupole coupling constant indicates that the electric field gradient at the xenon nucleus is 78% larger than that at the Kr nucleus in Kr–SO3.

Rotational spectrum and structure of the pyridine–CO2 van der Waals complex

The rotational spectrum of the pyridine–CO2 van der Waals complex has been recorded in a cold supersonic jet using pulsed-nozzle Fourier transform microwave spectroscopy. Spectroscopic constants are reported for the parent species and for all four singly-substituted 13C derivatives. The data indicate a planar structure in which the pyridine nitrogen approaches the CO2 carbon with the C2 axis of the pyridine perpendicular to the CO2. No evidence of internal rotation is observed. The zero point vibrationally averaged N⋯C van der Waals distance is 2.7977(64)Å and the O⋯ortho-hydrogen distance is 3.090(6)Å. The 14N nuclear quadrupole coupling constants are consistent with a bending vibrational amplitude of the pyridine moiety of about 9° away from the C2v geometry. The observed structure is in excellent agreement with that previously calculated to correspond to the lowest of several nearly isoenergetic isomers, suggesting that it is, indeed, the global minimum-energy structure. Somewhat surprisingly, the C⋯N van der Waals bond distance is about 0.2Å shorter than those observed in the related systems HCN⋯CO2 and H3N⋯CO2.

Microwave spectrum of (CH 3) 3CCN–SO 3

Eight rotational transitions of the complex (CH 3) 3CCN–SO 3 have been recorded using pulsed-nozzle Fourier transform microwave spectroscopy and a series of ab initio calculations has been performed. The complex is a symmetric top with free or nearly free internal rotation of the SO 3 and (CH 3) 3CCN subunits. The nitrogen–sulfur bond distance is determined to be 2.394(19) Å. Calculations at the MP2/aug-cc-pVTZ level/basis, which are in excellent agreement with the experimental results, give a binding energy of 11.0 kcal/mol relative to (CH 3) 3CCN and SO 3. Physical properties of the system, including N–S bond length, N–S–O angle, binding energy, and the degree of electron transfer (obtained from Townes and Dailey analysis of the 14N nuclear quadrupole coupling constant) are compared with those of similar complexes. The proton affinity of the base is a useful parameter for ordering complexes in the series.

Dimers of Fluorinated Methanes with Carbonyl Sulfide: The Rotational Spectrum and Structure of Difluoromethane−OCS

The pure rotational spectra of four isotopologues of the difluoromethane-carbonyl sulfide dimer have been measured in the 5-15 GHz region with use of pulsed-nozzle Fourier-transform microwave spectroscopy. The complex was determined to possess an ab plane of symmetry with a center of mass separation of 3.41(2) A and dipole moment components mu(a) = 1.1386(18) D, mu(b) = 0.4840(63) D, mu(total) = 1.2372(41) D. Experimental planar moments indicate that the two fluorine atoms straddle the symmetry plane while one of the C-H bonds of the difluoromethane monomer is aligned to interact with the oxygen atom of the OCS molecule. The assignment of the rotational spectrum for this dimer completes the experimental studies of the series of dimers involving fluorinated methanes (HCF(3), H(2)CF(2), and H(3)CF) complexed with OCS and makes possible a comparison of properties within this series.

Microwave spectrum of the OD–OH 2 complex: A strong deuterium isotope effect on angular momentum quenching in the hydroxyl moiety

Microwave spectra of the hydrogen bonded complex 16OD– 16OH 2 have been recorded using pulsed-nozzle Fourier transform microwave spectroscopy. The potential splitting, ρ, which describes the partial quenching of the OD orbital angular momentum upon complexation, is determined to be −142.703173(65) cm −1. Within the spectroscopic model employed, this value implies an energy difference of 202.46 cm −1 between the ground ( 2A′) and first excited ( 2A′′) states of the complex. The observed value of ρ represents a rather large change of 3.85710(11) cm −1 relative to that in the parent complex and implies a 1.30 cm −1 decrease in the 2A′– 2A′′ energy spacing relative to the parent species. Comparison with previous results for the 18OH complex suggests that these changes likely arise from changes in vibrationally averaged geometry upon deuteration. Magnetic hyperfine structure from the deuterium and the water protons is analyzed, as is the nuclear electric quadrupole coupling of the deuterium nucleus. Assuming negligible changes in the axial component of the electric field gradient at the deuterium upon complexation, the deuterium quadrupole coupling constant implies an average angular excursion of the OD bond axis from the vibrationally averaged a-inertial axis of the complex of ∼24°.

Microwave-Based Structure and Four-Dimensional Morphed Intermolecular Potential for HI−CO2

(Microwave spectra of the four isotopologue/isotopomers, HI-(12)C(16)O(2), HI-(12)C(18)O(2), HI-(12)C(18)O(16)O, and HI-(12)C(16)O(18)O, have been recorded using pulsed-nozzle Fourier transform microwave spectroscopy. In the last two isotopomers, the heavy oxygen atom tilted toward and away from the HI moiety, respectively. Only b-type Ka = 1 <-- 0 transitions were observed. Spectral analysis provided molecular parameters including rotational, centrifugal distortion, and quadrupole constants for each isotopomer. Then, a four-dimensional intermolecular energy surface of a HI-CO2 complex was generated, morphing the results of ab initio calculations to reproduce the experimental data. The morphed potential of HI-(12)C(16)O(2) had two equivalent global minima with a well depth of 457(14) cm(-1) characterized by a planar quasi-T-shaped structure with the hydrogen atom tilted toward the CO2 moiety, separated by a barrier of 181(17) cm(-1). Also, a secondary minimum is present with a well depth of 405(14) cm(-1) with a planar quasi-T-shaped structure with the hydrogen atom tilted away from the CO2 moiety. The ground state structure of HI-(12)C(16)O(2) was determined to have a planar quasi-T-shaped geometry with R = 3.7717(1) A, thetaOCI = 82.30(1) degrees , thetaCIH = 71.55(1) degrees . The morphed potential obtained is now available for future studies of the dynamics of photoinitiated reactions of this complex.

Electric dipole moment of sulfuric acid from Fourier transform microwave spectroscopy

The electric dipole moment of H 2SO 4 in the gas phase has been measured using pulsed-nozzle Fourier transform microwave spectroscopy. Ninety one Stark-shifted frequencies at a series of electric field strengths have been recorded for the 1 10 ← 0 00 transition. The total molecular dipole moment ( μ tot = μ c) was determined to be 2.9643(67) D. This is approximately 0.24 D greater than the previous literature value obtained from similar measurements on the 6 43 ← 5 33 transition. Both ab initio and DFT calculations using Dunning’s aug-cc-pVnZ (n = D, T, Q, Q+d) basis sets concur with the present result.

Microwave spectrum of Ne:HBr: structural perspectives on Rg:HX, Rg = Ne, Ar, Kr; X = F, Cl, Br, I

Microwave spectra of ground state 20Ne: 1H 79Br, 20Ne: 1H 81Br, 22Ne: 1H 79Br, 22Ne: 1H 81Br, and their corresponding deuterated isotopomers are obtained using pulsed-nozzle Fourier Transform microwave spectroscopy. The 20Ne: 1H 79Br ground state 〈 P 2 ( cos θ ′ ) 〉 = 0.09025 ( 11 ) is characteristic of a nearly free rotor. The available 20Ne: 1H 79Br, 20Ne: 2H 79Br, 22Ne: 1H 79Br and 22Ne: 2H 79Br data are analyzed and used to determine an initial ground vibrational state morphed potential for the dimer. This resulting intermolecular potential indicates a global minimum Ne:BrH linear isomeric structure with R cm = 3.42 Å. These analyses also provide structural perspectives on the prototypical homologous series Rg:HX (Rg = Ne, Ar, Kr; X = F, Cl, Br, I).

The Ar–HCCCN van der Waals complex: Microwave spectroscopy and coupled cluster calculations

Large-scale coupled cluster calculations have yielded an almost T-shaped equilibrium structure for the Ar–HCCCN van der Waals complex, with an equilibrium dissociation energy of De=221 cm−1. The theoretical predictions are confirmed by pulsed-nozzle Fourier transform microwave spectroscopy in the frequency range from 3 to 26 GHz. Strong b-type and much weaker a-type transitions have been observed, and the N14 nuclear quadrupole hyperfine components of the rotational transitions have been resolved. The spectral analysis has produced rotational, centrifugal distortion, and N14 nuclear quadrupole coupling constants. Structural parameters describing a (vibrationally averaged) T-shaped complex geometry have been derived.

Rotational spectroscopy of the gas phase complex of water and bromine monochloride in the microwave region: Geometry, binding strength and charge transfer

Pulsed-nozzle Fourier transform microwave spectroscopy has been used to measure the ground-state rotational spectra of eight isotopomers H2O···79Br35Cl, H2O···81Br35Cl, H2O···79Br37Cl, H2O···81Br37Cl, D2O···79Br35Cl, D2O···81Br35Cl, HDO···79Br35Cl and HDO···81Br35Cl of a complex formed by water and bromine monochloride. Fitting of measured frequencies yields the spectroscopic constants B0, C0, ΔJ, ΔJK, χaa(X), [χbb(X) − χcc(X)] and Mbb(X) for each of the first six species. For HDO containing isotopomers only K−1 = 0 transitions were observed and hence only (B0 + C0)/2, ΔJ, χaa(X) and Mbb(X) were determinable. The geometry of the complex is shown to have the arrangement O···Br–Cl of the heavy nuclei, with a weak bond formed between O and Br. Analysis shows that the zero-point state of the complex is effectively planar (C2v) with a low potential energy barrier to the motion that interconverts the two equivalent equilibrium conformers of Cs symmetry. The distance r(O···Br) is determined to be 2.7809(3) A and the out-of-plane angle, ϕ = 48.0(2)° in the ground state. The Townes–Dailey model has been used to interpret the halogen nuclear quadrupole coupling constants. This indicates that a fraction of an electron, δi = 0.013, is transferred from O to Br and a fraction, δp = 0.043, is transferred from Br to Cl on complex formation. Comparison between the intermolecular force constants, kσ, of the complexes H2O···BrCl and H2O···HBr indicates that the interaction in the H2O···BrCl complex is of similar strength to that of the hydrogen bond in H2O···HBr.

The microwave spectrum and ground state dynamics of Kr–IH

The structure and ground state dynamics of the atom–diatom interaction between Kr and HI has been investigated by pulsed-nozzle Fourier-transform microwave spectroscopy. The ground state of 84Kr–IH is shown to have R cm=4.0526(1) Å, θ=152.87(1)° while R cm=4.0195(1) Å, θ=158.85(1)° for 84Kr–ID. The potential surface from an ab initio molecular orbital calculation was scaled and shifted to yield a non-linear least-squares fit of the rovibrational state energies to the experimental data. The ground state, Kr–IH isomeric structure is similar to Ar–IH but contrasts with other members of the homologous series Kr–HX (X=F, Cl and Br).

Rotational spectrum of the trimethylphosphine–hydrogen fluoride complex

The rotational spectra of the four isotopomers (CH 3) 3P⋯HF, (CH 3) 3P⋯DF, (CH 3) 2( 13CH 3)P⋯HF and (CD 3) 3P⋯HF of a hydrogen-bonded complex formed by trimethylphosphine and hydrogen fluoride have been observed by pulsed-nozzle Fourier-transform microwave spectroscopy. The H, F nuclear spin–spin coupling constant D HF aa and the D-nuclear quadrupole coupling constant χ D aa were determined for the first two isotopomers. Interpretation of these coupling constants in conjunction with rotational constants and centrifugal distortion constants allows the conclusion that the intermolecular binding is relatively weak ( k σ =17.2(3) N m −1), with only a small lengthening δ r≈0.012 Å of the HF bond on formation of a symmetric-top complex (CH 3) 3P⋯HF having r(P⋯F)=3.120(2) Å.

Density functional study of the H 3NCl 2 system—the importance of Hartree-Fock exchange in density functional methods

Studies on the abilities of different density functional methods applied to charge transfer complexes have been carried out on the ammonia-chlorine, H 3NC1 2, complex. Various non-local exchange-correlation and hybrid functionals have been used in the investigation of the structure and energetic properties of the 1:1 complex. A triple-zeta basis set, added with diffuse functions and multiple sets of polarization functions (e.g. 6-311++G(2df,2p)), has been used throughout the study. Comparison with experimental data from pulsed-nozzle Fourier-transform microwave spectroscopy and also with results from the second-order Møller-Plesset perturbation theory shows that most variants of the density functional theory (DFT) yield the intermolecular distance too short. The agreement is best for the hybrid methods mixing the Hartree-Fock exchange with Slater exchange.

Fourier transform rotational spectrum and molecular structure of vinylcyclopropane

The sensitive method of pulsed-nozzle Fourier-transform microwave spectroscopy has been employed to record selected weaker features of the natural abundance rotational spectrum of vinylcyclopropane. The weak µc type rotational transitions of the common species were observed for the first time. The rotational spectra of all singly substituted 13C isotopic species were also recorded. The resulting rotational constants were used in the first microwave determination of the heavy-atom geometry of vinylcyclopropane and rs, r0 and r* structural parameters were derived. The effect on the rs geometry of corrections for bond shrinkage on isotopic substitution was examined. The reliability of the various structures for this type of molecule is discussed; it is anticipated that the most reliable structure will be that in which the experimental data are viewed with assumptions concerning the angular geometry of the CH bonds based on results of ab initio calculations. In the preferred r* structure r(CC)= 1.346 A, r(C—Cring)= 1.466(3)A and the two ring CC bond lengths are 1.518(3) and 1.505(3)A, adjacent and opposite to the substituted vertex.

Microwave spectra and structure of hydrogen cyanide-boron trifluoride: an almost weakly bound complex

Rotational spectra of the complex HCN-BF 3 have been observed using pulsed-nozzle Fourier transform microwave spectroscopy. Spectra have been observed for the 11 B and 10 B species with both HC 14 N and HC 15 N, and the hyperfine structure has been analyzed. The complex has the expected C 3υ structure, with the nitrogen end of the HCN toward the boron, but the observed BN bond length of 2.473(29) A is notably shorter than that in the related weakly bound systems N≡N-BF 3 and N≡C-C≡N-BF 3 . The out-of-plane distortion of the BF 3 cannot be determined accurately but is probably less than 3 o

Induced dipole moments and intermolecular force fields of rare gas-carbon dioxide complexes studied by Fourier-transform microwave spectroscopy

Pure rotational spectra and their Stark effects of X-CO 2 type complexes (X:Ne, Kr, and Xe) have been studied by pulsed-nozzle Fourier-transform microwave spectroscopy. Structures, intermolecular force fields, and electric dipole moments in the ground state have been determined, which have been discussed in connection with the previously reported results of Ar-CO 2 and Hg-CO 2