M. A. Roehrig

The microwave spectrum and structure for the HCCHCO complex

Abstract Rotational transitions were measured for five isotopomers of HCCHCO using a Flygare—Balle type microwave spectrometer. The observed spectrum is consistent with a linear, hydrogen-bonded HCCHCO structure. Measured rotational constants in MHz, are B = 1397.370(1) for HCCOCO, 1394.963(1) for HCCDCO, 1385.142(1) for HCCHCO, 1331.23(1) for DCCHCO and 1329.684(2) for DCCDCO. The measured rotational constants were fit to obtain a center-of-mass separation for HCCHCO of R * c.m. = 5.018(7) A. Distortion constants were obtained and analyzed to obtain a force constant and estimate the dissociation energy.

Microwave measurements and theoretical calculations on the structures of NNO–HCl complexes

Pulsed‐beam Fourier transform microwave spectroscopy was used to measure a and b dipole transitions for the N2O–H35Cl, N2O–H37Cl, N2O–D35Cl, and 15NNO–H35Cl van der Waals complexes. The observed transition frequencies were fit to determine the spectroscopic constants A–DK, B, C, DJ, DJK, eQqaa(Cl), and eQqbb(Cl). The structure of the complex appears to be a planar asymmetric top with a centers‐of‐mass separation Rc.m. ≊ 3.51 A. The angle θ between Rc.m. and the HCl axis is approximately 110°. The angle φ between the N2O axis and Rc.m. is approximately 77°. The structure was fit using a weighted least squares fit to B and C isotopic rotational constants with Rc.m., θ, and φ as the adjustable parameters, and this procedure yielded three local minima with standard deviations less than 5 MHz. Principal axis coordinates for the Cl, H, and terminal N atoms in the complex were determined with single isotopic Kraitchman analysis to aid in the selection of the ‘‘best’’ structure. In a second structural analysis Rc...

Microwave measurements of cobalt and nitrogen quadrupole coupling in Co(CO)3NO

J=2→3, 3→4, 4→5, and 5→6 transitions in the oblate symmetric top molecule cobalt tricarbonyl nitrosyl were measured using a Flygare–Balle type pulsed beam microwave spectrometer. K=0 and K=3 transitions were observed for J=3→4 and 4→5. Hyperfine structure due to 59Co and 14N nuclear quadrupole coupling interactions was well resolved. The measured quadrupole coupling strengths are eQqcc (59Co)=35.14(30) MHz and eQqcc (14N)=−1.59(10). Measured rotation and distortion constants are B0=1042.1590(4) MHz and Dj =0.17(8) kHz. The measured B value is 4% smaller than the B value calculated from electron diffraction data. Spin–rotation and a quadrupole distortion term were also obtained for 59Co.

Microwave measurements of the rotational spectrum for cyclopentadienyl—cobalt dicarbonyl

Abstract Rotational transitions in the 6–18 GHz range were measured for cyclopentadienyl—cobalt dicarbonyl (CpCo(CO) 2 ) using a Flygare-Balle type pulsed beam spectrometer. Splitting and additional transitions due to hindered internal rotation and cobalt quadrupole coupling were observed in the spectrum. Analysis of the spectrum provided the rotational constants A =1625(20), B =1257(2) and C =876(2) MHz. This molecule appears to have a tenfold barrier to rotation of the —Co(CO) 2 fragment about the C 5 axis of the Cp group. The potential energy barrier to internal rotation is V 10 =0.82(20) THz (0.3 kJ/mol). The value of the OCCoCO angle was found to be θ=98(5)°. This and other structure parameters will be compared with electron diffraction results. The 59 Co quadrupole coupling components are eQq aa =12(4) MHz and eQq bb =132(4) MHz.

Microwave measurements of manganese quadrupole coupling in cyclopentadienyl manganese tricarbonyl

Microwave measurements of rotational transitions in cyclopentadienyl manganese tricarbonyl were made using a Flygare–Balle type pulsed beam Fourier transform microwave spectrometer operating in the 4–14 GHz range. Ninety‐six hyperfine transitions were assigned for this prolate symmetric top for the rotational transitions J=2→3, 3→4, 4→5, 5→6, and 6→7. Molecular constants obtained from the analysis of the spectrum are B=828.0333(6) MHz, DJ=0.088(9) kHz, DJK=−0.04(3) kHz, eQqaa(Mn)=68.00(4) MHz, Cbb(Mn)=−5.5(8) kHz. The distortion parameter DJ for CpMn(CO)3 is compared to other DJ values for similar type transition metal complexes.

The microwave spectrum of cyclobutadiene iron tricarbonyl

Abstract The rotational spectrum of cyclobutadiene iron tricarbonyl was measured using a Flygare-Balle-type microwave spectrometer. Twenty a -dipole transitions were measured in the 5–16 GHz range for this prolate symmetric-top molecule. A least-squares fit of the data to a distortable symmetric top Hamiltonian yielded B = 961.9856(8) MHz, D J = 0.184(8) kHz, D JK = 1.20(3) kHz. These results indicate that the vibrationally averaged structure of the cyclobutadiene ring is square and perpendicular to the a -molecular axis.

Structure and quadrupole coupling measurements on ClF3

Seventy‐nine new microwave transitions for 35ClF3 and 37ClF3 in the 6–18 GHz range were measured using a Flygare–Balle‐type spectrometer. Rotational transition frequencies were used to obtain ‘‘effective’’ structure parameters for the ground vibrational state zCl–F (along C2 axis)=1.5985(4) A, rCl–F =1.700 73(5) A and ΘF–Cl–F =87.48(4)°. Analysis of hyperfine structure due to chlorine quadrupole coupling and observed transition frequencies yield the following molecular parameters for 35ClF3: A=13 748.25(1) MHz, B=4611.719(2) MHz, C=3448.629(3) MHz, eQqaa=82.03(3) MHz, and eQqbb=65.35(2) MHz. Molecular parameters obtained for 37ClF3 are: A=13 653.54(1) MHz, B=4611.866(2) MHz, C=3442.719(4) MHz, eQqaa=64.66(4) MHz, and eQqbb=51.53(3) MHz.

Rotational transition and quadrupole coupling measurements on the NNO-HCN complex

Abstract Seven a -dipole and seven b -dipole transitions were measured for one isomer of the NNO-HCN complex using a pulsed-beam, Fourier transform microwave spectrometer. Rotational constants A +Δ K =10326.3(4) MHz, B =2814.32(14) MHz and C =2201.00(12) MHz and distortion constants Δ J =0.014(8) MHz and Δ JK =0.14(6) MHz were obtained by fitting the observed transition frequencies. A quadrupole coupling strength due to the HCN nitrogen of eQq aa =1.97±0.05 MHz was obtained by fitting low- J transitions. Approximate structural parameters are obtained using moments of inertia and quadrupole coupling data.

MICROWAVE MEASUREMENTS OF THE ROTATIONAL SPECTRUM OF BUTADIENE IRON TRICARBONYL

The rotational spectrum of butadiene iron tricarbonyl was measured using a Flygare–Balle type microwave spectrometer. A total of 71 a‐dipole and c‐dipole transitions in the 5–17 GHz range were obtained for this asymmetric top transition metal complex. The ‘‘best fit’’ rotational constants are A=1005.4201(3) MHz, B=958.0408(2) MHz, and C=933.6865(3) MHz. Centrifugal distortion constants were also obtained in the least‐squares fit to the measured transitions. The present measurements indicate an upper limit of 40 kHz for splittings due to internal rotation, which places a lower limit on the barrier height of V3 ≥ 1.2 THz. The measured rotational constants are used to determine CO bond angles and the distances between Fe and the butadiene C atoms.

Microwave measurements and ab initio dynamics of the large amplitude ring puckering motion in 2-sulpholene

Microwave measurements were made on the rotational spectrum of 2‐sulpholene using a modified Flygare–Balle pulsed beam Fourier transform spectrometer. Analysis and calculations provided information on the large amplitude ring puckering vibration of this system. Twelve and six rotational transitions were measured for the v=0 and v=1 states of the ring puckering vibration, respectively. The transitions for each vibrational state were fitted to a Watson’s A reduced Hamiltonian including terms for quartic distortion yielding for v=0 the values B=2125.96(6), C=1983.28(8), ΔJK=0.664(4), ΔK=−0.34(4) MHz, and for v=1 the values A=3995(26), B=2128.3(1), C=1984.6(1), ΔJK=−0.8(1), ΔK=−32(6) MHz. Subsequently, ab initio calculations were performed at the self‐consistent‐field (SCF)/3‐21G*, MP2/6‐31G*, and MP4/6‐31G* levels of theory to determine the barrier to inversion. The MP4/6‐31G* barrier was ΔE=116 cm−1, and can be considered to be the most accurate barrier value calculated in this study. An ab initio potential...