Microwave Spectra, Structure, and the Aromatic Character of 1-Chloroborepin.

High resolution microwave spectra for the somewhat unstable compound 1-chloroborepin were measured in the 5-10 GHz range using a pulsed beam Fourier transform microwave spectrometer. Transitions were assigned and measured for three isotopologues, which include the most abundant isotopologue, 11B35Cl, and the less abundant 10B35Cl and 11B37Cl isotopologues. The molecular parameters (MHz) determined for the 11B35Cl isotopologue are A = 3490.905(35), B = 1159.38520(79), C = 870.59492(56), 1.5χaa (11B) = -0.220(22), 0.25(χbb - χcc) (11B) = -1.5300(99), 1.5χaa (35Cl) = -54.572(33), and 0.25(χbb - χcc) (35Cl) = 4.7740(79). The inertial defect is calculated to be Δ = -0.174 amu Å2 from the experimental rotational constants, indicating a planar structure with some out of plane vibrational motion. An extended Townes-Dailey analysis was performed on the 11B and 35Cl nuclei to determine the electron occupations in the valence hybridized orbitals using the experimental quadrupole coupling strengths. From the analysis it was determined that Cl is sharing some electron density with the empty p-orbital on B. The B-Cl bond length determined from the data is 1.798(1) Å, and the B-C bond lengths are 1.533(10) Å. The structural parameters and electronic structure properties of 1-chloroborepin are consistent with an aromatic boron-containing molecule.

Microwave spectra, molecular structure, and aromatic character of 4a,8a-azaboranaphthalene.

The microwave spectra for seven unique isotopologues of 4a,8a-azaboranaphthalene [hereafter referred to as BN-naphthalene] were measured using a pulsed-beam Fourier transform microwave spectrometer. Spectra were obtained for the normal isotopologues with (10)B, (11)B, and all unique single (13)C and the (15)N isotopologue (with (11)B), in natural abundance. The rotational, centrifugal distortion and quadrupole coupling constants determined for the (11)B(14)N isotopologue are A = 3042.712 75(43) MHz, B = 1202.706 57(35) MHz, C = 862.220 13(35) MHz, DJ = 0.06(1) kHz, 1.5χaa ((14)N) = 2.5781(61) MHz, 0.25(χbb - χcc) ((14)N) = - 0.1185(17) MHz, 1.5χaa (11B) = - 3.9221(75) MHz, and 0.25(χbb - χcc) ((11)B) = - 0.9069(24) MHz. The experimental inertial defect is Δ = - 0.159 amu Å(2), which is consistent with a planar structure for the molecule. The B-N bond length from the experimentally determined structure is 1.47 Å, which indicates π-bonding character between the B and N. The measured quadrupole coupling strengths provide important and useful information about the bonding, orbital occupancy, and aromatic character for this aromatic molecule. Extended Townes-Dailey analyses were used to determine the B and N electron sp(2)-hybridized and p-orbital occupations. These results are compared with electron orbital occupations from the natural bond orbital option in theoretical calculations. From the analyses, it was determined that BN-naphthalene has aromatic character similar to that of other N-containing aromatics. The results are compared with similar results for B-N bonding in 1,2-dihydro-1,2-azaborine and BN-cyclohexene. Accurate and precise structural parameters were obtained from the microwave measurements on seven isotopologues and from high-level G09 calculations.

Microwave measurements of the tropolone-formic acid doubly hydrogen bonded dimer.

The microwave spectrum was measured for the doubly hydrogen bonded dimer formed between tropolone and formic acid. The predicted symmetry of this dimer was C2v(M), and it was expected that the concerted proton tunneling motion would be observed. After measuring 25 a- and b-type rotational transitions, no splittings which could be associated with a concerted double proton tunneling motion were observed. The calculated barrier to the proton tunneling motion is near 15,000 cm(-1), which would likely make the tunneling frequencies too small to observe in the microwave spectra. The rotational and centrifugal distortion constants determined from the measured transitions were A = 2180.7186(98) MHz, B = 470.873 90(25) MHz, C = 387.689 84(22) MHz, DJ = 0.0100(14) kHz, DJK = 0.102(28) kHz, and DK = 13.2(81) kHz. The B3LYP/aug-cc-pVTZ calculated rotational constants were within 1% of the experimentally determined values.

Microwave spectra and structure of the cyclopropanecarboxylic acid-formic acid dimer.

The rotational spectrum of the cyclopropanecarboxylic acid-formic acid doubly hydrogen bonded dimer has been measured in the 4-11 GHz region using a Flygare-Balle type pulsed-beam Fourier transform microwave spectrometer. Rotational transitions were measured for the parent, four unique singly substituted (13)C isotopologues, and a singly deuterated isotopologue. Splittings due to a possible concerted double proton tunneling motion were not observed. Rotational constants (A, B, and C) and centrifugal distortion constants (DJ and DJK) were determined from the measured transitions for the dimer. The values of the rotational (in MHz) and centrifugal distortion constants (in kHz) for the parent isotopologue are A = 4045.4193(16), B = 740.583 80(14), C = 658.567 60(23), DJ = 0.0499(16), and DJK = 0.108(14). A partial gas phase structure of the dimer was derived from the rotational constants of the measured isotopologues, previous structural work on each monomer units and results of the calculations.

Microwave Spectrum for a Second Higher Energy Conformer of Cyclopropanecarboxylic Acid and Determination of the Gas Phase Structure of the Ground State.

Microwave spectra for a higher-energy conformer of cyclopropanecarboxylic acid (CPCA) were measured using a Flygare-Balle-type pulsed-beam Fourier transform microwave spectrometer. The rotational constants (in megahertz) and centrifugal distortion constants (in kilohertz) for this higher-energy conformer are A = 7452.3132(57), B = 2789.8602(43), C = 2415.0725(40), DJ = 0.29(53), and DJK = 2.5(12). Differences between rotational constants for this excited-state conformation and the ground state are primarily due to the acidic OH bond moving from a position cis relative to the cyclopropyl group about the C1-C9 bond to the more stable trans conformation. Calculations indicate that the relative abundance of the higher-energy state should be 15% to 17% at room temperature, but the observed relative abundance for the supersonic expansion conditions is about 1%. The measurements of rotational transitions for the trans form of CPCA were extended to include all of the unique (13)C singly substituted positions. These measurements, along with previously measured transitions of the parent and -OD isotopologues, were used to determine a best-fit gas-phase structure.

Gas phase measurements of mono-fluoro-benzoic acids and the dimer of 3-fluoro-benzoic acid.

The microwave spectrum of the mono-fluoro-benzoic acids, 2-fluoro-, 3-fluoro-, and 4-fluoro-benzoic acid have been measured in the frequency range of 4-14 GHz using a pulsed beam Fourier transform microwave spectrometer. Measured rotational transition lines were assigned and fit using a rigid rotor Hamiltonian. Assignments were made for 3 conformers of 2-fluorobenzoic acid, 2 conformers of 3-fluorobenzoic acid, and 1 conformer of 4-fluorobenzoic acid. Additionally, the gas phase homodimer of 3-fluorobenzoic acid was detected, and the spectra showed evidence of proton tunneling. Experimental rotational constants are A(0(+)) = 1151.8(5), B(0(+)) = 100.3(5), C(0(+)) = 87.64(3) MHz and A(0(-)) = 1152.2(5), B(0(-)) = 100.7(5), C(0(-)) = 88.85(3) MHz for the two ground vibrational states split by the proton tunneling motion. The tunneling splitting (ΔE) is approximately 560 MHz. This homodimer appears to be the largest carboxylic acid dimer observed with F-T microwave spectroscopy.

Microwave measurements of the spectra and molecular structure for the monoenolic tautomer of 1,2- cyclohexanedione.

The microwave spectrum for the monoenolic tautomer of 1,2-cyclohexanedione was measured in the 4-14 GHz regime using a pulsed-beam Fourier transform (PBFT), Flygare-Balle-type microwave spectrometer. The molecular structure and moments of inertia were initially calculated using Gaussian 09 using MP2 and 6-311++G** basis sets, and these calculations were used to predict the rotational constants and microwave spectra. Rotational transition frequencies were measured and used to determine rotational constants (A, B, and C) and centrifugal distortion constants (D(J) and D(K)). The rotational constants for the parent isotopologue, one singly substituted deuterium and six singly substituted (13)C isotopologues, were used in a least-squares fit to determine gas-phase structural parameters for this molecule. All hydrogen atoms were held fixed to the calculated positions, as well as the carbon atoms at positions 1 and 10 and the oxygen atoms at positions 6 and 7. The rotational constants for the parent isotopologue are A = 3161.6006(12), B = 2101.5426(3), and C = 1320.7976(4) MHz. The distortion constants obtained from the fit are D(J) = 0.0436 and D(K) = 0.436 kHz. Structural parameters from the MP2 calculations are in fair agreement with the measured parameters.