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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. 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 spectrometer. The two nonequivalent N-atoms exhibit distinct quadrupole coupling. The rotational, centrifugal distortion and quadrupole coupling constants determined from the spectra have 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), 1.5χaa (N2) = 0.381(2), 0.25(χbb-χcc)(N2) =-0.0324(2), and D J = 0.002145(5) MHz. The experimental inertial defect, ∆ =-0.243 amu Å 2 , is consistent with a planar structure.

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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 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.

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