S. M. Sickafoose

Microwave spectrum of the 15N16O-15N16O dimer

The microwave transitions for the (15NO)2 dimer were reinvestigated following a report by McKellar et al. (1995, Molecular Physics, 86, 273) of a discrepancy between a previously measured microwave transition frequency and the corresponding frequency calculated from their molecular constants. The present microwave measurements confirm the previous observation of a rotational transition at 11159•297(4) MHz, but this transition is apparently not due to the cis N—N bonded NO dimer, since a mixture of 15NO in argon gas is required for observation of the 11159•297 MHz transition. A new doublet was located at 11128•434(4) MHz and this transition is now assigned to the 303 ← 212 transition of the 15N—15NO dimer, in agreement with predictions from the rotational constants of McKellar et al. This transition can be observed with either argon or neon as the expansion gas. New structural parameters obtained from the microwave data are in excellent agreement with those based on the FTIR data.

MEASUREMENTS OF THE MICROWAVE SPECTRUM AND STRUCTURAL PARAMETERS FOR BENZENE CHROMIUM TRICARBONYL

Microwave spectra for four isotopomers of benzene chromium tricarbonyl were measured in the 4–17 GHz range using a Flygare–Balle type pulsed beam spectrometer. Rotational constants obtained are B(52Cr)=732.8886(6) MHz and B(53Cr)=732.8966(3) MHz. Asymmetric top spectra were observed for a single 13C substitution on the benzene ring giving B(13C–bz)=729.9606(3) and C(13C–bz)=727.9024(2) MHz. For a single 13C substitution on one of the carbonyl carbons B(13CO)=731.9036(8) and C(13CO)=729.1657(8) MHz. Since no effects of possible internal rotation were observed on the 13C asymmetric top spectra, we can place a lower limit on the V6 potential for internal rotation of V6≳4.0 THz (=1.6 kJ/mole). The centrifugal distortion constants are small, DJ=0.05 kHz and DJK=−0.05 kHz, indicating a fairly rigid structure. The 53Cr quadrupole coupling strength is low, eqQ(53Cr)=−12.11(1) MHz, indicating a near octahedral charge distribution around the Cr atom. Structural parameters obtained are the center of the benzene chro...

Measurements of the microwave spectrum, Re–H bond length, and Re quadrupole coupling for HRe(CO)5

Rotational transition frequencies for rhenium pentacarbonyl hydride were measured in the 4–10 GHz range using a Flygare–Balle type microwave spectrometer. The rotational constants and Re nuclear quadrupole coupling constants for the four isotopomers, (1) H187Re(CO)5, (2) H185Re(CO)5, (3) D187Re(CO)5, and (4) D185Re(CO)5, were obtained from the spectra. For the most common isotopomer, B(1)=818.5464(2) MHz and eq Q(187Re)=−900.13(3) MHz. The Re–H bond length (r0) determined by fitting the rotational constants is 1.80(1) A. Although the Re atom is located at a site of near‐octahedral symmetry, the quadrupole coupling is large due to the large Re nuclear moments. A 2.7% increase in Re quadrupole coupling was observed for D‐substituted isotopomers, giving a rather large isotope effect on the quadrupole coupling. The Cax–Re−Ceq angle is 96(1)°, when all Re–C–O angles are constrained to 180°.

MICROWAVE SPECTRA AND QUADRUPOLE COUPLING MEASUREMENTS FOR METHYL RHENIUM TRIOXIDE

Abstract Microwave rotational transitions for J′ ← J = 1 ← 0 and 2 ← 1 were measured in the 6–14 GHz range for methyl rhenium trioxide using a Flygare-Balle type, pulsed-beam spectrometer. The rotational constants for the most abundant isotopomers are B( 187 Re ) = 3466.964(2) MHz and B( 185 Re ) = 3467.049(3) MHz . The quadrupole coupling strengths are eQq( 187 Re ) = 716.55(2) MHz and eQq( 185 Re ) = 757.19(3) MHz . Transitions were also observed for 13C isotopomers and 18O isotopomers. The value for the ReC bond length obtained from a Kraitchman analysis is R( ReC ) = 2.080 A . The rhenium quadrupole coupling strengths are about 20% smaller than those obtained for HRe(CO)5.

MICROWAVE MEASUREMENTS OF THE GAS-PHASE MOLECULAR STRUCTURE OF COBALT TETRACARBONYL HYDRIDE

Microwave rotational spectra for six isotopomers of HCo(CO)4 were measured using a Flygare–Balle type spectrometer. Transitions were measured in the 4–10 GHz range. The data were analyzed to obtain rotational constants, 59Co quadrupole coupling strengths, distortion constants and spin–rotation interaction strengths. The common isotopomer is clearly a symmetric top, with C3v symmetry. For the common isotopomer B=1131.7212(9) MHz and eQq(59Co)=116.62(3) MHz. The rotational constants were used to determine the complete, three‐dimensional structure of this complex. The measured H–Co bond length of 1.52(2) A is significantly shorter than the previous value from electron diffraction work. Other structural parameters for this complex are reported and discussed.

The microwave spectrum and MnH bond length for HMn(CO)5

Abstract The microwave rotational spectrum for the symmetric top, manganese pentacarbonylhydride, was measured in the 5–11 GHz range using a pulsed-beam, Fourier transform microwave spectrometer. For HMn(CO) 5 we obtained A = B = 907.9841 (2) MHz and eqQ (MN) = −144.22(2) MHz. For DMn(CO) 5 , A = B = 902.7682(3) MHz and eqQ (Mn) = −45.24(4) MHz. Only K = 0 and K = 4 transitions were observed, confirming the expected C 4v symmetry. With other structural parameters constrained to previously reported values, r 0 (MnH) = 1.64(4) A. The C ax MnC eq angle derived from the data is approximately 97°C, if MnCO angles are constrained to 180°C.