Formic Acid-Ammonia Heterodimer: A New Δ-Machine Learning CCSD(T)-Level Potential Energy Surface Allows Investigation of the Double Proton Transfer.

The formic acid-ammonia dimer is an important example of a hydrogen-bonded complex in which a double proton transfer can occur. Its microwave spectrum has recently been reported and rotational constants and quadrupole coupling constants were determined. Calculated estimates of the double-well barrier and the internal barriers to rotation were also reported. Here, we report a full-dimensional potential energy surface (PES) for this complex, using two closely related Δ-machine learning methods to bring it to the CCSD(T) level of accuracy. The PES dissociates smoothly and accurately. Using a 2d quantum model the ground vibrational-state tunneling splitting is estimated to be less than 10-4 cm-1. The dipole moment along the intrinsic reaction coordinate is calculated along with a Mullikan charge analysis and supports the mildly ionic character of the minimum and strongly ionic character at the double-well barrier.

Microwave Spectra and Theoretical Calculations for Two Structural Isomers of Methylmanganese Pentacarbonyl.

The first microwave rotational spectra for two structural isomers of methylmanganese pentacarbonyl were measured in the 4-9 GHz range using a pulsed-beam Fourier transform microwave spectrometer. The spectra for the two isomers, a symmetric-top structure and an asymmetric-top acyl isomeric structure, were fit to obtain rotational and centrifugal distortion constants and 55Mn quadrupole coupling parameters. The rotational constants, the manganese (55Mn) nuclear quadrupole coupling constant, the centrifugal distortion constants, and the spin-rotation constant were determined for the symmetric CH3Mn(CO)5 and have the following values: A = B = 793.153(3) MHz, DJ = 0.00040(4) MHz, DJK = 0.0018(2) MHz, Ccc = 0.183(6) MHz, and eQqcc= -87.4(3) MHz. Rotational constants and 55Mn quadruple coupling constants were determined for the isomeric acyl-CH3C(O)Mn(CO)4 and have the following values: A = 839.96(4) MHz, B = 774.20(7) MHz, C = 625.63(1) MHz, and 1.5 eQqaa= 44.9(47) MHz and 0.25(eQqbb - eQqcc) = 11.9(12) MHz. The measured rotational constants from the isomeric acyl-CH3C(O)Mn(CO)4 were compared with various theoretical computations. The calculated rotational constants for the dihapto-acyl and the agostic-acyl structures are reasonably close to the experimental values. We note that the calculated dihapto-acyl structure most closely matches the experimental data, as the calculation for the dihapto structure using the B3LYP functional with the aug-cc-pVDZ basis set closely reproduced the experimental values for A, B, and C.

Synthesis, microwave spectra, x-ray structure, and high-level theoretical calculations for formamidinium formate.

An efficient synthesis of formamidinium formate is described. The experimental x-ray structure shows both internal and external H-bonding to surrounding molecules. However, in the gas phase, this compound occurs as a doubly hydrogen bonded dimer between formamidine and formic acid. This doubly hydrogen-bonded structure is quite different from the solid state structure. Microwave spectra were measured in the 6-14 GHz range using a pulsed-beam Fourier transform microwave (MW) spectrometer. The two nonequivalent N-atoms exhibit distinct quadrupole coupling. The rotational, centrifugal distortion, and quadrupole coupling constants determined from the spectra have the following values: A = 5880.05(2), B = 2148.7710(2), C = 1575.23473(13), 1.5 χaa (N1) = 1.715(3), 0.5(χbb-χcc) (N1) = -1.333(4), 1.5 χaa (N2) = 0.381(2), 0.25(χbb-χcc) (N2) = -0.0324(2), and DJ = 0.002145(5) MHz. The experimental inertial defect, Δ = -0.243 amu Å2, is consistent with a planar structure. Accurate and precise rotational constants (A, B, and C), obtained from the MW measurements, were closely reproduced, within 1%-2% of the measured values, with the M11 DFT theoretical calculations. Detailed comparison of the measured and calculated A, B, and C rotational constants confirms the planar doubly hydrogen bonded structure. The calculated nitrogen quadrupole coupling strengths of the monomer are quite different from either of the two nitrogen sites of the dimer. The poor agreement between measured and calculated quadrupole coupling strengths shows that the dimer is not locked in the equilibrium structure but is likely undergoing large amplitude vibrational motion of the hydrogen atoms moving between the N and O atoms involved in the hydrogen bonding.

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.

Measurements of deuterium quadrupole coupling in propiolic acid and fluorobenzenes using pulsed-beam Fourier transform microwave spectrometers.

The pure rotational spectra of deuterated propiolic acids (HCCCOOD and DCCCOOH), 1-fluorobenzene (4-d1), and 1,2-difluorobenzene (4-d1) in their ground states have been measured using two Fourier transform microwave (FTMW) spectrometers at the University of Arizona. For 1-fluorobenzene (4-d1), nine hyperfine lines of three different ΔJ = 0 and 1 transitions were measured to check the synthesis method and resolution. For 1,2-difluorobenzene (4-d1), we obtained 44 hyperfine transitions from 1 to 12 GHz, including 14 different ΔJ = 0, 1 transitions. Deuterium quadrupole coupling constants along the three principal inertia axes were well determined. For deuterated propiolic acids, 37 hyperfine lines of Pro-OD and 59 hyperfine lines of Pro-CD, covering 11 and 12 different ΔJ = - 1, 0, 1 transitions, respectively, were obtained from 5 to 16 GHz. Deuterium quadrupole coupling constants along the three inertia axes were well resolved for Pro-OD. For Pro-CD, only eQq(aa) was determined due to the near coincidence of the CD bond and the least principal inertia axis. Some measurements were made using a newer FTMW spectrometer employing multiple free induction decays as well as background subtraction. For 1-fluorobenzene (4-d1) and 1,2-difluorobenzene (4-d1), a very large-cavity (1.2 m mirror dia.) spectrometer yielded very high resolution (2 kHz) spectra.

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.

Calculations and measurements of the deuterium tunneling frequency in the propiolic acid-formic acid dimer and description of a newly constructed Fourier transform microwave spectrometer.

The concerted proton tunneling frequency for the propiolic acid-formic acid dimer was calculated using a relaxed ab initio double-well potential in the imaginary-frequency mode of the saddle point, and new measurements were made for the deuterated propiolic acid-formic acid (ProOD-FAOD) isotopologue. It is important to have consistent calculated tunneling frequency values between normal and deuterated isotopologues since parameters can be readily adjusted to get good agreement with one isotopologue. High-resolution rotational spectra of deuterated (ProOD-FAOD) dimer were measured using a newly constructed Fourier Transform microwave spectrometer. The new spectrometer has mirror size: 30 cm in diameter with a radius of curvature of 59 cm and is equipped with multiple-FID data collection (5-10 FID's for each gas pulse). For the deuterated (ProOD-FAOD) isotopologue, 45 rotational lines (a type: 34; b type: 11) were measured in the lowest tunneling states range between 6.5 GHz and 15.5 GHz. With the new high-resolution measurements of the tunneling doublets (b-dipole transitions), the double potential well responsible for the deuterium tunneling was depicted much more precisely. The two tunneling states are separated by 3.48 MHz. The rotational constants obtained in this work are quite helpful for further structure analysis as well.

Microwave structure for the propiolic acid-formic acid complex.

New microwave spectra were measured to obtain rotational constants and centrifugal distortion constants for the DCCCOOH···HOOCH and HCCCOOD···DOOCH isotopologues. Rotational transitions were measured in the frequency range of 4.9-15.4 GHz, providing accurate rotational constants, which, combined with previous rotational constants, allowed an improved structural fit for the propiolic acid-formic acid complex. The new structural fit yields reasonably accurate orientations for both the propiolic and formic acid monomers in the complex and more accurate structural parameters describing the hydrogen bonding. The structure is planar, with a positive inertial defect of Δ = 1.33 amu Å(2). The experimental structure exhibits a greater asymmetry for the two hydrogen bond lengths than was obtained from the ab initio mp2 calculations. The best-fit hydrogen bond lengths have an r(O1-H1···O4) of 1.64 Å and an r(O3-H2···O2) of 1.87 Å. The average of the two hydrogen bond lengths is r(av)(exp) = 1.76 Å, in good agreement with r(av)(theory) = 1.72 Å. The center of mass separation of the monomers is R(CM) = 3.864 Å. Other structural parameters from the least-squares fit using the experimental rotational constants are compared with theoretical values. The spectra were obtained using two different pulsed beam Fourier transform microwave spectrometers.

Microwave spectra and gas phase structural parameters for N-hydroxypyridine-2(1H)-thione.

The microwave spectrum for N-hydroxypyridine-2(1H)-thione (pyrithione) was measured in the frequency range 6-18 GHz, providing accurate rotational constants and nitrogen quadrupole coupling strengths for three isotopologues, C(5)H(4)(32)S(14)NOH, C(5)H(4)(32)S(14)NOD, and C(5)H(4)(34)S(14)NOH. Pyrithione was found to be in a higher concentration in the gas phase than the other tautomer, 2-mercaptopyridine-N-oxide (MPO). Microwave spectroscopy is best suited to determine which structure predominates in the gas phase. The measured rotational constants were used to accurately determine the coordinates of the substituted atoms and provided sufficient data to determine some of the important structural parameters for pyrithione, the only tautomer observed in the present work. The spectra were obtained using a pulsed-beam Fourier transform microwave spectrometer, with sufficient resolution to allow accurate measurements of the (14)N nuclear quadrupole hyperfine interactions. Ab initio calculations provided structural parameters and quadrupole coupling strengths that are in very good agreement with measured values. The experimental rotational constants for the parent compound are A = 3212.10(1), B = 1609.328(7), and C = 1072.208(6) MHz, yielding the inertial defect Δ(0) = -0.023 amu·Å(2) for the C(5)H(4)(32)S(14)NOH isotopologue. The observed near zero inertial defect clearly indicates a planar structure. The least-squares fit structural analysis yielded the experimental bond lengths R(O-H) = 0.93(2) Å, R(C-S) = 1.66(2) Å, and angle (N-O-H) = 105(4)° for the ground state structure.

Design, construction, and testing of a large-cavity, 1-10 GHz Flygare-Balle spectrometer.

A large pulsed-beam, Fourier transform microwave spectrometer employing 48 in. diameter mirrors and 35(") (NHS-35) diffusion pump has been constructed at the University of Arizona. The Fabry-Perot-type cavity, using the large mirrors provides Q-values in the 15,000 to 40,000 range. Test spectra were obtained using transverse and coaxial injection of the pulsed-nozzle molecular beams. The measured molecular resonance linewidths were 8 kHz for the transverse injection and 2 kHz for coaxial molecular beam injection. Good signal to noise ratios were obtained for the test signals. Strong lines for butadiene iron tricarbonyl were seen with a single beam pulse (S/N = 5/1). Transitions were measured as low as 900 MHz and some previously unresolved hyperfine structure is now resolved for the butadiene iron tricarbonyl spectra. The spectrometer is operated using a personal computer with LABVIEW programs, with provisions for automatic frequency scanning. The extended, low-frequency range of this spectrometer should make it very useful for making measurements on significantly larger molecules and complexes than have been previously studied. The improved resolution, in the coaxial beam mode, will allow better resolution of hyperfine structure. The large diffusion pump allows a higher beam pulse frequency to compensate for the generally lower sensitivity at lower frequencies.

Microwave measurements of proton tunneling and structural parameters for the propiolic acid-formic acid dimer.

Microwave spectra of the propiolic acid-formic acid doubly hydrogen bonded complex were measured in the 1 GHz to 21 GHz range using four different Fourier transform spectrometers. Rotational spectra for seven isotopologues were obtained. For the parent isotopologue, a total of 138 a-dipole transitions and 28 b-dipole transitions were measured for which the a-dipole transitions exhibited splittings of a few MHz into pairs of lines and the b-type dipole transitions were split by ~580 MHz. The transitions assigned to this complex were fit to obtain rotational and distortion constants for both tunneling levels: A(0+) = 6005.289(8), B(0+) = 930.553(8), C(0+) = 803.9948(6) MHz, Δ(0+)(J) = 0.075(1), Δ(0+)(JK) = 0.71(1), and δ(0+)(j) = -0.010(1) kHz and A(0-) = 6005.275(8), B(0-) = 930.546(8), C(0-) = 803.9907(5) MHz, Δ(0-)(J) = 0.076(1), Δ(0-)(JK) = 0.70(2), and δ(0-)(j) = -0.008(1) kHz. Double resonance experiments were used on some transitions to verify assignments and to obtain splittings for cases when the b-dipole transitions were difficult to measure. The experimental difference in energy between the two tunneling states is 291.428(5) MHz for proton-proton exchange and 3.35(2) MHz for the deuterium-deuterium exchange. The vibration-rotation coupling constant between the two levels, F(ab), is 120.7(2) MHz for the proton-proton exchange. With one deuterium atom substituted in either of the hydrogen-bonding protons, the tunneling splittings were not observed for a-dipole transitions, supporting the assignment of the splitting to the concerted proton tunneling motion. The spectra were obtained using three Flygare-Balle type spectrometers and one chirped-pulse machine at the University of Virginia. Rotational constants and centrifugal distortion constants were obtained for HCOOH···HOOCCCH, H(13)COOH···HOOCCCH, HCOOD···HOOCCCH, HCOOH···DOOCCCH, HCOOD···DOOCCCH, DCOOH···HOOCCCH, and DCOOD···HOOCCCH. High-level ab initio calculations provided initial rotational constants for the complex, structural parameters, and some details of the proton tunneling potential energy surface. A least squares fit to the isotopic data reveals a planar structure that is slightly asymmetric in the OH distances. The formic OH···O propiolic hydrogen bond length is 1.8 Å and the propiolic OH···O formic hydrogen bond length is 1.6 Å, for the equilibrium configuration. The magnitude of the dipole moment was experimentally determined to be 1.95(3) × 10(-30) C m (0.584(8) D) for the 0(+) states and 1.92(5) × 10(-30) C m (0.576(14) D) for the 0(-) states.

Microwave spectrum and structural parameters for the formamide-formic acid dimer.

The rotational spectra for six isotopologues of the complex formed between formamide and formic acid have been measured using a pulsed-beam Fourier transform microwave spectrometer and analyzed to obtain rotational constants and quadrupole coupling parameters. The rotational constants and quadrupole coupling strengths obtained for H (12)COOH-H(2) (14)NCOH are A = 5889.465(2), B = 2148.7409(7), 1575.1234(6), eQq(aa) = 1.014(5), eQq(bb) = 1.99(1), and eQq(cc) = -3.00(1) MHz. Using the 15 rotational constants obtained for the H (13)COOH, HCOOD, DCOOH, and H(2) (15)NCHO isotopologues, key structural parameters were obtained from a least-squares structure fit. Hydrogen bond distances of 1.78 Å for R(O3⋯H1) and 1.79 Å for R(H4⋯O1) were obtained. The "best fit" value for the angle(C-O-H) of formic acid is significantly larger than the monomer value of 106.9° with an optimum value of 121.7(3)°. The complex is nearly planar with inertial defect Δ = -0.158 amu Å(2). The formamide proton is moved out of the molecular plane by 15(3)° for the best fit structure. Density functional theory using B3PW91, HCTH407, and TPSS as well as MP2 and CCSD calculations were performed using 6-311++G(d,p) and the results were compared to experimentally determined parameters.

Communications: Evidence for proton tunneling from the microwave spectrum of the formic acid-propriolic acid dimer.

The microwave spectrum of the formic acid-propriolic acid dimer was measured in the 5-13 GHz range using a pulsed-beam, Fourier transform spectrometer. 22 a-dipole rotational transitions and 3 b-dipole rovibrational transitions were measured for the normal isotopomer. All of these observed transitions were split into doublets by the effects of the concerted tunneling of the two acid protons. The smaller splittings of 1-1.5 MHz for the a-dipole transitions are due to the differences in rotational constants for the upper and lower tunneling states. The b-dipole transitions are rovibrational (combination) transitions with a change in rotational state and tunneling state and provide direct information on the tunneling splittings since these observed splittings are the sum of the tunneling level splittings for the two rotational states involved in the transition. The b-dipole splittings are 55.16(0(00)-1(11)), 58.58(1(01)-2(12)), and 71.24 MHz(2(02)-3(13)). No similar splittings were observed when deuterium was substituted for either or both of the hydrogen bonding protons. For the lower tunneling state (nu(0) (+)), A=5988.7(7), B=927.782(7), and C=803.720(7) MHz. For the upper tunneling state (nu(0) (-)), A=5988(1), B=927.78(1), and C=804.06(1) MHz. Using a simple model with potential function V=ax(4)-bx(2) the splittings could be reproduced reasonably well with a barrier height of H(e)=3800 cm(-1).

Microwave spectrum, structural parameters, and quadrupole coupling for 1,2-dihydro-1,2-azaborine.

The first microwave spectrum for 1,2-dihydro-1,2-azaborine has been measured in the frequency range 7-18 GHz, providing accurate rotational constants and nitrogen and boron quadrupole coupling strengths for three isotopomers, H(6)C(4)(11)B(14)N, H(6)C(4)(10)B(14)N, and H(5)DC(4)(11)B(14)N. The measured rotational constants were used to accurately determine coordinates for the substituted atoms and provide sufficient data to determine most of the important structural parameters for this molecule. The spectra were obtained using a pulsed beam Fourier transform microwave spectrometer, with sufficient resolution to allow accurate measurements of (14)N, (11)B, and (10)B nuclear quadrupole hyperfine interactions. High-level ab initio calculations provided structural parameters and quadrupole coupling strengths that are in very good agreement with measured values. The rotational constants for the parent compound are A = 5657.335(1), B = 5349.2807(5), and C = 2749.1281(4) MHz, yielding the inertial defect Delta(0) = 0.02 amu x A(2) for the ground-state structure. The observed near-zero and positive inertial defect clearly indicates that the molecular structure of 1,2-dihydro-1,2-azaborine is planar. The least-squares fit analysis to determine the azaborine ring structure yielded the experimental bond lengths and 2sigma errors R(B-N) = 1.45(3) A, R(B-C) = 1.51(1) A, and R(N-C) = 1.37(3) A for the ground-state structure. Interbond angles for the ring were also determined. An extended Townes-Dailey population analysis of the boron and nitrogen quadrupole coupling constants provided the valence p-electron occupancy p(c) = 0.3e for boron and p(c) = 1.3e for nitrogen.