The laboratory detection of gas-phase IZnCH₃ (X¹A₁), using millimeter-wave direct absorption methods, was reported previously. This work has been extended by the measurement of the pure rotational spectrum of several isotopolgues: I⁶⁴ZnCH₃, I⁶⁶ZnCH₃, I⁶⁴ZnCD₃, and I⁶⁴Zn¹³CH₃. These species were all created by the reaction of zinc vapor with CH₃I, CD₃I, or ¹³CH₃I in the presence of a DC discharge. The zinc isotopolgues were observed in natural abundance. Rotational transitions in the range 256−293 GHz (J = 109 ← 108 to J = 132 ← 131, for K = 0 to 6) have been recorded for each species. From these measurements, an r₀ structure has been determined. This structure was found to be in good agreement with previous DFT calculations. Interestingly, the 110.2^° Zn – C – H bond angle of IZnCH₃ is identical to that of the hydrogen substituted zinc insertion complex HZnCH₃ (X¹A₁). These data are further evidence that IZnCH₃ is not created by the generation of free radical fragments, but by the direct insertion of atomic zinc into the C – I bond of iodomethane.

The pure rotational spectrum of methylzinc chloride, ClZnCH₃ (X¹A₁), has been recorded in the gas phase using direct absorption spectroscopic techniques. ClZnCH₃ was synthesized by the reaction of zinc vapor, generated in a Broida-type oven, with chloromethane in the presence of a DC discharge. Rotational transitions of the main isotopologue, ³⁵Cl⁶⁴ZnCH₃, were measured in the frequency range of 260−297 GHz. The presence of clear K-ladder structure (K = 0−6) indicates that the species is a symmetric top with C_3V symmetry. ClZnCH₃ appears to be formed by the oxidative addition of atomic zinc to chloromethane. Searches for the ³⁷Cl, ⁶⁶Zn, ¹³C, and D substituted isotopologues are currently in progress. The geometries of ClZnCH₃ and IZnCH₃ will be compared with reactivity trends for halogen-substituted organometallic reagents.

Fourier transform microwave spectroscopy in combination with double-resonance techniques has been used to detect the rotational spectra of all five singly-substituted isotopic species of H₂C=O–O, the simplest Criegee intermediate. By correcting the rotational constants of these species and those of four others previously reported by Nakajima and Endo (J. Chem. Phys. 39, 101103, 2013) for zero-point vibrational motion calculated theoretically, a highly precise equilibrium structure is reported for this important atmospheric intermediate. In contrast to the production method employed by most other groups, which has emphasized the use of halogenated percursors, we find that H₂C=O–O is produced in good yield and fairly selectively by passing a mixture of methane and excess molecular oxygen through an electrical discharge. For this reason H₂C=O–O may be produced in the direct vicinity of atmospheric lightning.

HO₃ is a radical species of atmospheric and astrophysical chemical importance. While the microwave spectrum of this radical is known, its rotational spectrum has not been measured at higher frequencies. Studies of HO₃ in the millimeter and submillimeter ranges would provide the information needed to further refine its structural determination, as well as provide additional spectroscopic guidance for observational searches. In order to study the rotational spectrum of HO₃ at higher frequencies, we have coupled a pulsed supersonic expansion discharge source with a multipass direct absorption millimeter and submillimeter spectrometer. Initial experiments focused on the optimization of the HO₂ radical, which has a known rotational spectrum and is the intermediate species in the production of HO₃. Searches for lines of HO₃ are currently underway. The spectrometer design, results of the HO₂ studies, and initial results for HO₃ will be presented.

The rotational spectra for hydrazoic acid (HN₃), its isotopologues, and its vibrational satellites have been reexamined using millimeter-wave rotational spectroscopy in the range of 240-360 GHz. Treating sodium azide (NaN₃) or the commercially available singly ¹⁵N-labeled NaN₃ with phosphoric acid or deuterated phosphoric acid yielded 6 different isotopologues. From these samples, we were also able to observe all of the isotopologues containing one additional ¹⁵N at natural abundance. In total, we assigned rotational transitions to 14 different species; only H¹⁵N₃ and D¹⁵N₃ were not accessible. With the large number of rotational constants determined for these isotopologues, an excellent equilibrium structure determination was performed with CFOUR's xrefit routine. This structure shows a bent azide sub-unit, and is in excellent agreement with the geometry optimization performed at the CCSD(T)/ANO2 level of theory. The Coriolis perturbation of the ground and first two vibrationally excited states of HN₃ will also be discussed.

By means of Fourier transform microwave spectroscopy of a supersonic jet, the pure rotational spectra of two second-row analogs to isocyanic acid, HNCO, have been observed for the first time. The phosphorus and silicon analogs HPCO and HNSiO, respectively, were observed by their fundamental a-type rotational transitions (μ_a(HPCO)= 0.45 D, μ_a(HNSiO)= 2.10 D) in the centimeter wave range from 10 to 32 GHz through discharges of appropriate precursor gases highly diluted in neon. Spectroscopic searches and identification were based on predictions from high-level quantum-chemical calculations at the CCSD(T) level of theory in combination with large basis sets. Excellent agreement between experimental and calculated molecular parameters is found. In case of HPCO, the ¹³C isotopic species was also observed. Since both the stem compound HNCO and its sulphur analog HNCS are known to be present in space, and because also a sizable number of phosphorus and silicon-bearing species were detected there, both compounds are plausible targets for future radio astronomical searches using sensitive radio astronomical instrumentation.

There are several practical algorithms for building molecular geometries based on bond lengths, bond angles, and torsional angles. There seem to be few discussions of the effect changing one angle has on the remaining bond angles depending upon local symmetry. For example, in methane, CH₄, the H-C-H bond angles are all tetrahedral, i.e., α = 109.4712206... deg. If one considers CH₃F, a molecule with C_3v symmetry, how are the H-C-F bond angles related to the H-C-H bond angles? This study derives the bond angle relationships for a 4-bonded central atom such as a saturated C atom. For a 4-bonded central atom (6 bond angles) the possible local point group symmetries are T_d(0), D_2d(1), C_3v(1), C_2v(1), D₂(2), C₂(3), C_s(3), and C₁(4). The numbers in parentheses are the degrees of freedom, i.e., the number of angles which can be assigned arbitrary values with the remaining angles fixed by symmetry. Analytical formulas relating the bond angles for each of the eight possible symmetries are derived. Also, formulas have been derived for the five possible symmetries of a planar 4-bonded atom, D_4h(0), D_2h(1), C_2v(pendant, 1), C_2v(trapezoid, 2), and C_s(3); the three possible structures of planar 3-bonded atoms, (D_3h(0), C_2v (1), and C_s(2); the three possible symmetries of pyramidal 3-bonded atoms, C_3v(1), C_s(2), and C₁ (3); and the trivial case of 2-bonded atoms, D_∞h(0) and C_2v(bent 1). There are also six distinct 4-bonded central atom structures with all the bonds directed into a hemisphere, C_4v(1), C_2v(2), C₂(3), C_s(trapezoid 3), C_s(pendant 3), and C₁(4), geometries rarely seen in molecules.

Figure The microwave spectrum of the molecule bromoperfluoroacetone has been measured on a newly constructed CP-FTMW spectrometer along with a FTMW spectrometer relocated from Oxford University to Missouri S&T. Rotational constants, centrifugal distortion parameters, and nuclear quadrupole coupling constants will be discussed. Comparisons to the previously studied halogen analogues perfluoroacetone¹ and chloroperfluoroacetone² will be discussed.







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¹J.-U. Grabow, N. Heineking, and W. Stahl, Z. Naturforsch. 46a (1991) 229. ²G. Kadiwar, C. T. Dewberry, G. S. Grubbs II and S. A. Cooke, Talk RH11, 65^th International Symposium on Molecular Spectroscopy (2010).

Perfluorobutyric acid (PFBA) is highly soluble in water and is a molecule of environmental importance. Rotational spectra of PFBA and its monohydrate were studied using a broadband chirped pulse and a narrow band cavity based Fourier transform microwave spectrometers and high level ab initio calculations. Extensive conformational search was performed for both the acid and its monohydrate at the MP2/6-311++G(2d,p) level of theory. Two and three conformers were predicted for PFBA and its monohydrate, respectively. One set of rotational transitions of PFBA and its mono-hydrate in each case was observed and assigned. Based on the broadband spectra obtained, one can confidently conclude that only one dominate conformer exists in each case. The orientation of the hydroxyl group in PFBA was determined using isotopic analysis. Comparison of the observed transition intensities and the calculated electric dipole moment components allowed one to identify the most stable monohydrate conformation which takes on the insertion hydrogen-bonding topology. Comparison to the shorter chain analogues, i.e. trifluoroacetic acid, perfluoropropionic acid, and their monohydrates, was made to elucidate the general trend in their conformational preference and binding topologies.

For use in determining the semiexperimental structure of trans-hexatriene, its 3-d₁ isotopologue has been synthesized and the high-resolution (0.0015 cm⁻¹) IR spectrum has been recorded. The rotational structure in four C-type bands has been analyzed. These bands are for ν₂₆ at 997.4, ν₂₈ at 908.8, ν₂₉ at 902.2, and ν₃₂ at 678.6 cm⁻¹, which are all out-of-plane modes. Ground state rotational constants are A₀ = 0.7952226(8), B₀ = 0.0446149(7), and C₀ = 0.0422661(4) cm⁻¹. The inertial defect is –0.2009 amu Ų, which confirms planarity for this molecular species. The ultimate intent of this investigation is to evaluate the degree to which the “C=C” bonds are lengthened and the sp²–sp² “C–C” bonds are shortened in comparison with localized bonds and with butadiene.

Phenyl (C₆H₅) is the prototypical aryl radical, and is thought to be a critical intermediate for soot formation in combustion environments and a precursor to the formation of PAHs in space. Despite the fundamental importance of this species, its molecular structure has never been measured experimentally. We have measured the rotational spectra of C₆H₅ and all of its singly-substituted D and ¹³C isotopologues in the 170-190 and 250-270 GHz bands in a positive column discharge of dilute benzene (or C₆H₅D/¹³CC₅H₆) in Ar. The experimentally determined rotational constants were combined with zero point vibrational and electronic corrections calculated at the CCSD(T) level of theory to derive the semi-experimental equilibrium structure (r_e^emp).

In order to determine an accurate equilibrium structure for the equatorial form of piperidine, C ₅H₁₁N, microwave transitions and ground state rotational constants are reported for the ¹³C and ¹⁵N isotopologues measured in natural abundance. These rotational constants along with those of the parent and ND species were used together with vibration-rotation constants obtained from a cubic force fields calculated at the B3LYP/cc-pVTZ and MP2/cc-pVTZ levels of theory to derive a semiexperimental (SE) equilibrium structure. However, the rotational constants of the CD deuterated species are not available, and there are many small Cartesian coordinates. Furthermore, piperidine is an oblate top. Quite generally, there is a large rotation of the principal axis system upon isotopic substitution in oblate tops that may considerably reduce the accuracy of the SE structure. For these reasons, the mixed estimation method was used. In this method, internal coordinates from good-quality quantum chemical calculations (with appropriate uncertainties) are fitted simultaneously with moments of inertia of the full set of isotopologues. In order to have predicates as accurate as possible, the geometry was optimized at the MP2 and CCSD(T) levels of theory using Dunning basis sets up to quadruple-zeta quality. This combination of experimental rotational constants and high-level ab initio calculations allowed us to determine an accurate equilibrium structure.

The ground state rotational spectra of benzonitrile (BN) and pentafluorobenzonitrile (PFBN) were investigated using Balle-Flygare Fourier transform microwave (FTMW) spectroscopy in the region between 5 and 25 GHz. In addition to the parent species, transitions due to the five ¹³C isotopologues were measured in natural abundance and used to calculate relevant geometric parameters of the ring backbone of the two compounds. The experimental results compare well with the equilibrium structure determined via ab initio theory (MP2/6-311++G(2d,2p)). To better understand the effect of fluorination on the ring moiety, the electronic environments around the nitrogen atom in both species were compared through a Townes-Dailey-type analysis of the observed ¹⁴N quadrupole hyperfine structure.

Several monoterpenes and terpenoids are biogenic volatile organic compounds which are emitted in the atmosphere, and react with OH, O₃, NO_x, etc. to give rise to several oxydation and degradation products. Spectroscopic information on these atmospheric species are still very scarse. Meanwhile we have demonstrated that combining quantum calculations to microwave spectroscopy led to the unambiguous characterization of the most stable conformers for perillaldehyde, ¹ limonene and carvone. ² This information can be used to subsequently model accurately the vibrational signature for atmospheric purposes. ³ We have recorded the pure rotational spectra of α-pinene and β-pinene (C₁₀H₁₆), and of nopinone (C₉H₁₄O), using the MB-FTMW spectrometer of Lille, in the 2-20 GHz range at temperatures varying between 340 and 380 K. For these three bicyclic molecules only one conformer can be observed, and the rotational structure was observed up to J, K_a = 8, 3 ; 8, 4 ; 8, 5, respectively. All the spectra were modeled with a semi-rigid rotor Hamiltonian and fitted to obtain a rms value better than 5 kHz using a-, b- and c- type transitions. All the experimental results were supported by several quantum calculations performed at different levels of theory (DFT and ab initio). In particular no experimental evidence of internal rotation motion was found (methyl groups), in good agreement with the calculated barriers. Support from the French Laboratoire d'Excellence CaPPA (Chemical and Physical Properties of the Atmosphere) through contract ANR-10-LABX-0005 of the Programme d'Investissements d'Avenir is acknowledged.
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¹J.-R. Aviles Moreno, F. Partal Urena, J.-J. Lopez Gonzalez and T. R. Huet, Chem. Phys. Lett. 473 (2009) 17 ²J.-R. Aviles Moreno, T. R. Huet, F. Partal Urena, J.-J. Lopez Gonzalez, Struct. Chem. 24 (2013) 1163 ³T. R. Huet, J.-R. Aviles Moreno, O. Pirali, M. Tudorie, F. Partal Urena, J.-J. Lopez Gonzalez, JQSRT. 113 (2012) 1261