Temperature-dependent ways of proton transfer - A benchmark study on cyclic HF oligomers

Taking the example of vibrational-assisted concerted proton exchange in cyc lic (HF)(n) (n = 4, 5) oligomers, we demonstrate that proton transfer occur s in three different ways, depending on the temperature. At high temperatur es (>400 K) mainly overbarrier transitions take place. To predict the react ion rate, the barrier height needs to be known, at least at chemical accura cy. At intermediate temperatures (200-400 K) additionally an accurate knowl edge of the barrier width is important, as the protons mainly tunnel throug h the barrier near its top. The adiabatic tunneling correction can be used to predict reaction dynamics, as the vibrational state does not change duri ng the reaction. At low temperatures (<200 K) the slow skeletal modes are f rozen and only fast hydrogenic movement takes place. For this reason vibrat ional adiabaticity is lost and a much wider region of the potential surface called reaction swath is crossed during the reaction. predictions of the r esulting exchange dynamics require the potential on the swath to be calcula ted accurately. In the zero-temperature limit these nonadiabatic tunneling paths solely determine the exchange reaction and cause spectroscopically me asurable tunneling splittings, which can, therefore, be estimated reliably in the framework of transition-state theory from accurate calculations of e nergies on the reaction swath. All the above findings arise just from the f act that a light atom is transferred between two heavy atoms. Therefore, tw o crossover temperatures of proton transfer should qualitatively exist in a ll systems containing hydrogen bonds.