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 ANEWLY DEVELOPED PAIR-POTENTIAL

We performed a Monte Carlo simulation study on liquid formic acid empl oying a recently developed five-site pair-potential. We found that sim ilarly to the crystalline phase, besides the strong O-H ... O interact ions, 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-ene rgy distribution function, and the statistical examination of the hydr ogen-bonding network. Although, the C-H ... O bonds are weaker than th e 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 a cceptor molecule. On the other hand, the hydrogen bond donor molecule appears to rotate freely around the hydrogen bond. Interestingly enoug h, the acceptor atom both in the O-H ... O and in the C-H ... O hydrog en bonds is the carbonyl oxygen, the participation of the hydroxylic o xygen 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 t wo O-H ... O hydrogen bonds, is minor (7% of the molecules) in the liq uid. The liquid structure can be best described as a two-level hydroge n-bonding network. The liquid phase consists mainly of small, often br anching 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 rese mblance to the infinite hydrogen-bonded chains of the crystal containi ng both O-H ... O and C-H ... O hydrogen bonds. The experimental C ... O separation of the crystal structure and recent results of high-qual ity ab initio calculations, however, correlate well with the position of the first hydrogen-bonding peak of the carbon-carbonyl oxygen parti al pair correlation function of the liquid simulation.