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 Mn quadrupole coupling parameters. The rotational constants, the manganese (Mn) nuclear quadrupole coupling constant, the centrifugal distortion constants, and the spin-rotation constant were determined for the symmetric CHMn(CO) and have the following values: = = 793.153(3) MHz, = 0.00040(4) MHz, = 0.0018(2) MHz, = 0.183(6) MHz, and eQq= -87.4(3) MHz. Rotational constants and Mn quadruple coupling constants were determined for the isomeric acyl-CHC(O)Mn(CO) and have the following values: = 839.96(4) MHz, = 774.20(7) MHz, = 625.63(1) MHz, and 1.5 eQq= 44.9(47) MHz and 0.25(eQq - eQq) = 11.9(12) MHz. The measured rotational constants from the isomeric acyl-CHC(O)Mn(CO) 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 , , and .