Role of the C-H center dot center dot center dot O hydrogen bonds in liquids: A Monte Carlo simulation study of liquid formic acid using a newly developed pair-potential

We performed a Monte Carlo simulation study on liquid formic acid employing a recently developed five-site pair-potential. We found that similarly to the crystalline phase, besides the strong O-H ... O interactions, C-H ... O hydrogen bonds are also present in the liquid and play an important role in the hydrogen-bonding structure. The presence of C-H ... O hydrogen bonds is manifest in the results of the structural analysis based on the partial pair correlation functions, the pair-energy distribution function, and the statistical examination of the hydrogen-bonding network. Although, the C-H ... O bonds are weaker than the O-H ... O interactions and their geometry is somewhat more distorted, the basic geometric preferences are the same for all hydrogen bonds: the X-H ... O (X = C,O) bonds tend to be linear in the plane of the acceptor molecule. On the other hand, the hydrogen bond donor molecule appears to rotate freely around the hydrogen bond. Interestingly enough, the acceptor atom both in the O-H ... O and in the C-H ... O hydrogen bonds is the carbonyl oxygen, the participation of the hydroxylic oxygen as hydrogen bond acceptor being negligible. The liquid structure of formic acid is significantly different from that in the gas phase. The role of the cyclic dimers, characteristic of the gas phase with two O-H ... O hydrogen bonds, is minor (7% of the molecules) in the liquid. The liquid structure can be best described as a two-level hydrogen-bonding network. The liquid phase consists mainly of small, often branching oligomers held together by O-H ... O hydrogen bonds. The small hydrogen-bonding oligomers are also connected to each other by weaker C-H O interactions forming space-filling networks of hydrogen bonds. The two-level hydrogen-bonding network of the liquid shows little resemblance to the infinite hydrogen-bonded chains of the crystal containing both O-H ... O and C-H ... O hydrogen bonds. The experimental C ... O separation of the crystal structure and recent results of high-quality ab initio calculations, however, correlate well with the position of the first hydrogen-bonding peak of the carbon-carbonyl oxygen partial pair correlation function of the liquid simulation.