Calculated molecular properties and microwave spectrum analysis for formamidinium formate

Formamidinium formate occurs in the gas phase as a doubly hydrogen-bonded dimer. New high-level calculations on formamidinium formate using various methods and basis sets are described. The calculated rotational constants and quadrupole coupling constants given here are reasonably consistent between the different methods and in reasonably good agreement with values obtained from the new analysis of the microwave data. In the revised fit 38 measured transitions are fit with a standard deviation of 30 kHz. The revised rotational, centrifugal distortion and quadrupole coupling constants determined from the spectra have values: A = 5890.39(8), B = 2148.731(1), C = 1575.0958(8), 1.5 χaa (N1) = 2.254(24), 0.25(χbb - χcc)(N1) = 1.747(12), 1.5χaa (N2) = -3.134(29), 0.25(χbb - χcc)(N2) = 0.553(10) MHz. The calculated rotational constants which agree best with experiment are from an M11 /def2-QZVPP calculation: A = 5785.29, B = 2143.25and C = 1563.88 MHz. The calculated quadrupole coupling constants which agree best with experiment are from a PBE0 aug-cc-pVQZ (uncontracted) calculation: 1.5 χaa (N1) = 1.78, 0.25(χbb - χcc)(N1) = 1.49, 1.5χaa (N2) = -3.41), 0.25(χbb - χcc)(N2) = 0.51 MHz. The calculated rotational constants agree within 2%, and quadrupole coupling constants within 11% or better with the experimental values.