Microwave measurements, calculations, and analysis for the gas phase ammonia-formic acid dimer

The gas-phase doubly hydrogen-bonded formic acid-ammonia complex was obtained by heating a sample of ammonium formate in neon. High-level DFT and MP2 calculations with various basis sets were performed and barriers for proton tunneling and internal rotation were determined. The microwave spectrum was measured in the 7–16 GHz frequency range using a Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometer. Double resonance transitions were observed near 20 GHz. Rotational transitions were measured and fitted for two vibrational states to obtain the rotational constants and quadrupole coupling constants. The rotational constants were determined to have the following values: A = 12017.0(2), B = 4337.331(2), and C = 3227.279(2) for the 0⁺ state and A = 12017.0, B = 4302.02(1), and C = 3238.161(1) for the 0⁻ state and have been fit to 16 transitions and 9 transitions respectively. Each state was fit to a rigid model showing quadrupole splitting and compared with computations.