Search for: All records

New analysis and spectra are reported for the gas-phase ammonia-formic acid complex. Calculations to deter- mine the theoretical barrier to internal rotation were conducted and led to the new internal rotation analysis of the dimer. Using the new analysis and calculations, 12 new lines were measured and assigned and included in the present analysis. This is the !rst internal rotation analysis for this complex. The measurements were made in the 7–22 GHz range using two Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometers. The complex was analyzed as a hindered rotor and 20 A and 16 E state transitions were !t with the XIAM5 program. The rotational constants were determined to have the following values: A = 11970.19(9) MHz, B = 4331.479(4) MHz, and C = 3227.144(4) MHz. Rotational constants, quadrupole coupling constants, and internal rotor parameters were !t to the spectrum. Double resonance was used to verify line assignments and access higher frequencies. The barrier to internal rotation was found to be 195.18(7) cm 1. High level calculations are in good agreement with experimental values. The calculated V3 barrier values range from 168.3 to 212.8 cm 1. 

The microwave spectra for a hydrogen-bonded trans-2 glyoxylic acid–water complex were measured in the 6–16 GHz frequency range using two Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometers. The rotational constants for the dimer were determined to have the following values: A = 9384.2354(31), B = 1707.63973(73), and C = 1447.44879(56) MHz. The hydrogen bonded structures and rotational constants were calculated for the lowest energy dimglyoxylic acid - water using DFT, MP2 and CCSD calculations with various basis sets. The B3LYP/aug-cc-PVQZ-DG3 calculations yielded rotational constants of A = 9393.59, B = 1713.76, and C = 1453.23 MHz, in very good agreement with experimental values. The calculations show two feasible tunneling motions involving hydrogen atoms in this complex. 

The formic acid-ammonia dimer is an important example of a hydrogen-bonded complex in which a double proton transfer can occur. Its microwave spectrum has recently been reported and rotational constants and quadrupole coupling constants were determined. Calculated estimates of the double-well barrier and the internal barriers to rotation were also reported. Here we report a full-dimensional potential energy surface (PES) for this complex, using two closely related Δ-machine learning methods to bring it to the CCSD(T) level of accuracy. The PES dissociates smoothly and accurately. Using a 2d quantum model the ground vibrational-state tunneling splitting is estimted to be less than 10−4 cm−1. The dipole moment along the intrinsic reaction coordinate is calculated along with a Mullikan charge analysis and supports mildly ionic character of the minimum and strongly ionic character at the double-well barrier.