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, D_J = 0.00040(4) MHz, D_JK = 0.0018(2) MHz, C_cc = 0.183(6) MHz, and eQq_cc= -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 eQq_aa= 44.9(47) MHz and 0.25(eQq_bb - eQq_cc) = 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.