Microwave spectra, molecular structure and aromatic character of BN-Naphthalene (4A,8A-Azaboranaphthalene)

\begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.36]{BN-nap1.eps} \end{wrapfigure} The microwave spectra for seven unique isotopologues of BN-naphthalene (4a,8a-azaboranaphthalene) were measured using a pulsed-beam Fourier transform microwave spectrometer. Spectra were obtained for the normal isotopologues with $^{10}$B, $^{11}$B, all unique single $^{13}$C and the $^{15}$N isotopologue, in natural abundance. The rotational, centrifugal distortion and quadrupole coupling constants determined for the $^{11}$B$^{14}$N isotopologue are A = 3042.7128(4) MHz, B = 1202.7066(4) MHz, C = 862.2201(4) MHz, D$_{J}$ = 0.06(1) kHz, 1.5 eQq$_{aa}$ ($^{14}$N) = 2.578(6) MHz, 0.25(eQq$_{bb}$- eQq$_{cc}$) ($^{14}$N) = -0.119(2) MHz, 1.5 eQq$_{aa}$ ($^{11}$B) = -3.922(8) MHz, and 0.25(eQq$_{bb}$- eQq$_{cc}$) ($^{11}$B) = -0.907(2) MHz. The experimental inertial defect is $\Delta$ = -0.159 amu \AA$^{2}$, which is consistent with a planar structure. The B-N bond length is 1.47 \AA, indicating $\pi$-bonding character. The results are compared with similar results for B-N bonding in 1,2-dihydro-1,2-azaborine and BN-cyclohexene

Identification and characterization of 1,2-BN cyclohexene using microwave spectroscopy

\begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.3]{newH10b.eps} \end{wrapfigure} 1,2-BN Cyclohexene was produced from 1,2-BN Cyclohexane through the loss of H$_2$ and characterized and identified using a pulsed-beam Fourier-transform microwave spectrometer. The first microwave spectra for 1,2-$^{10}$BN Cyclohexene 1,2-$^{11}$BN Cyclohexene have been measured in the frequency range of 5.5-12.5 GHz, providing accurate rotational constants and nitrogen and boron quadrupole coupling strengths for two isotopologues. High-level ab initio calculations provided rotational constants and quadrupole coupling strengths for the precursor 1,2-BN Cyclohexane (C$_4$H$_{12}$BN) and 1,2-BN Cyclohexene(C$_4$H$_{10}$BN). Calculated molecular properties for 1,2-BN Cyclohexene are in very good agreement with measured parameters. Calculated parameters for the starting material, 1,2-BN Cyclohexane do not agree with the experimental data. Rotational constants for 1,2-$^{11}$BN Cyclohexene are A = 4702.058(2) MHz, B = 4360.334(1) MHz and C = 2494.407(1) MHz. The inertial defect is $\Delta$$_0$ = -20.78 amu-\AA$^2$ clearly indicating a nonplanar structure. These microwave experiments show that heating the initial compound, 1,2-BN Cyclohexane, to 60 C in a 1 atm neon stream results in the loss of H$_2$ and conversion to 1,2-BN Cyclohexene. This appears to be the first characterization of the 1,2-BN Cyclohexene monomer

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 13C 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 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

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

MEASUREMENT OF MAGNETIC SUSCEPTIBILITY ANISTROPY AND MOLECULAR QUADRUPOLE MOMENT IN H2COH_{2}CO

Author Institution: Department of Chemistry, Massachusetts Institute of TechnologyThe molecular Zeeman effect was observed in $H_{2}CO$ using a molecular beam maser in a high magnetic field. $\Delta M=0$ or $\Delta M=\pm 1$ transitions were observed by using TE or TM mode cavities. The resolution was 5 kHz and magnetic fields up to 17 kG were used. The $l_{10}\rightarrow l_{11}$ transition was studied. The magnetic susceptibilities were accurately measured and values for the molecular quadrupole moments and other quantities were determined from these measurements

MICROWAVE MEASUREMENTS OF BENZENE C - C BOND LENGTH ALTERNATION IN BENZENE CHROMIUM TRICARBONYL

Author Institution: The University of Arizona, Tucson, AZ 85721The microwave rotational spectrum of $1,2-D_{2}$ -benzene chromium tricarbonyl was measured using a Flygare-Balle type pulsed-beam spectrometer. This spectrum contains two sets of lines, due to two different structural isomers of this complex. These isomers are believed to be due to reduction of the benzene symmetry due to interactions with the $Cr(CO)_{3}$ moiety which result in C - C bond length differences of $0.016 {\AA}$

MICROWAVE MEASUREMENTS OF STRUCTURE CHANGES FOR LIGAND MOLECULES BOUND TO TRANSITION METALS

Supported by THE NATIONAL SCIENCE FOUNDATIONAuthor Institution: Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721Precise values for structural parameters for transition metal complexes have been obtained from high-resolution PBFT microwave measurements. The changes in structural parameters for the small organic molecule ligands are relatively large and well-determined. Results for C$_2$H$_4$-Os-(CO)$_4$, C$_2$H$_4$-Fe-(CO)$_4$, C$_2$H$_2$-Re-(O)$_2$CH$_3$, C$_6$H$_6$-Cr-(CO)$_3$ and C$_4$H$_6$-Fe-(CO)$_3$ will be discussed and compared. For the Ethylene Osmium Tetracarbonyl complex, the experimental ethylene C-C bond length is 1.432 \AA, which falls between the free ethylene value of 1.339 \AA { } and the ethane value of 1.534 \AA. The angle between the plane of the CH$_2$ group and the extended ethylene C-C bond (out-of-plane angle) is 26\,^irc}. Ethylene structural changes are larger for the Os complex than for the similar Fe complex. For Acetylene Methyl dioxoRhenium, the C-C bond length is increased by 0.08 \AA { } to 1.29 \AA. The H-C-C interbond angles are reduced from 180\,^irc} to 146\,^irc}, and 147\,^irc}. The planar, D_6_h structure of free benzene is changed to a C_3_v structure with alternating C-C bond lengths due to interaction with Cr-(CO)$_3$ in the complex. The structural changes are small but significant, since the benzene reactivity is changed. For Butadiene Iron Tricarbonyl, the terminal CH$_2$ groups are rotated by 28\,^irc} out of the butadiene plane and the CH$_2$ plane is folded away from the butadiene C1-C2 axis by 27\,^irc} in a direction away from the iron atom. Free butadiene has a trans planar conformation, much different from the distorted cis conformation in the complex. These structural changes are usually accompanied by significant changes in reactivity, which has proved useful for transition metal catalysts and metal containing enzymes