| Water is a widely distributed substance in nature,and it plays an important role in many processes such as the origin of life,climate regulation,and biological metabolism.Although the structure of water molecules is simple,they exhibit many special properties due to the formation of a complex hydrogen bond network between water molecules and the influence of environmental confinement.When water is confined in material pores,around and inside cells,proteins and other biomolecules,the complex interaction with the confined environment will cause the hydrogen bond structure of water produce complex microscopic behaviors under different pressures and temperatures,which will affect the macroscopic state of water.So far,although many studies have been conducted on the properties of water systems in different environments,including transport,there are still many shortcomings in the understanding of the hydrogen bond rotation properties of confined water at the atomic level.Even the rotation model of the low-dimensional hydrogen bond network has not been effectively confirmed,and this will inevitably hinder the development of related exploration and applications.Therefore,this article combines first-principles calculations and empirical force field methods to carry out research work from the following three aspects:(1)The establishment of the hydrogen bond rotation model of confined water;(2)the quantum effect of tunneling during the rotation of the confined water hydrogen bond;(3)Comparing the empirical force field with the quantum first-principles calculation on the rotational properties of the confined water hydrogen bond.It is hoped that these works can play a reference role in basic research on the structure and dynamics of complex water systems,and provide theoretical support for related applications such as artificial nano-water channels.First,by using carbon nanotubes(CNT)as a simplified model of the confinement environment,the problem of hydrogen bond rotation in a quasi-one-dimensional confined water chain was studied,and it was found that the process of turning over the direction of the water chain is actually carried out step by step,rather than a concerted mechanism process as previously thought.This discovery changed the long-standing model understanding of water chain direction flipping.Through the dynamic simulation and corresponding trajectory analysis of the water chain in the CNT based on the empirical force field,we obtained the exponential relationship between the average duration t of the dipole consistency of the water chain along the CNT axisand the number of water molecules n:Log(t)=a×n+b.Based on this,it is further deduced that when the number of water molecules is more than 20,the water chain can maintain a unidirectional flow on the time scale of seconds.By tracing the typical dynamic simulation trajectory of the water chain in the direction flipping process,it is found that the direction flipping of the water chain is realizd by a the step-by-step process of hydrogen bond rotation transfer between adjacent water molecules.In order to further confirm this model,and to deeply explore the physical mechanism that affects the difficulty of hydrogen bond rotation in the water chain,quantum first-principles density functional theory(DFT)calculations have been completed.By constructing the reaction potential energy surface of the direction flipping process of the water chain composed of 2,3 and 4 water molecules in a confined environment,a flipping model consistent with classical dynamics simulation is established.That is,by comparing the corresponding reaction paths of these three confined water chains with different molecular numbers during the direction flipping process,this once again proved that the step-by-step flipping process of the water chain.Not only that,the analysis of the potential energy surface structure also found that as the number of water molecules increases,the energy barriers that the water chain needs to overcome during the flipping process will not only increase in number but also increase in height,making the water chain flipping more difficult.And the calculation of the reaction rate based on the Arrhenius activation energy also shows that the water chain flipping rate decreases with the increase of the number of water molecules.In addition,this study also found that the defect state structures considered in the water chain flipping process actually correspond to the necessary intermediates in the water chain flipping process.The appearance of these structures is an indispensable step in the flipping process.These findings provide a new perspective on the understanding of water transport and other processes in one-dimensional confined channels,and help to understand the propeties of related cell biological channels.Second,on the basis of the established hydrogen bond step-by-step rotation model of quasi-one-dimensional confine water,we further calculated through DFT and found that there are two types of water chain direction flipping paths in(6,6)CNT,and explored the quantum tunneling effect of hydrogen bond rotation in the water chain flipping process,especially for the first time we discovered the important influence of the resonance tunneling effect on this step-by-step rotation process.Through the construction of potential energy surface,we found that there are L-type defect and D-type defect intermediates in the typical two reaction paths of the water chain in CNT,respectively,and the energy difference between the extreme points of the two paths is within 0.06 e V.Based on these two reaction paths,we further calculated the tunneling effect and even the resonant tunneling effect on the two paths of the direction flipping processes of water chain.First,the sequential tunneling results shown that the tunneling effect will dominate the hydrogen bond flipping process of the water chain at temperatures below 84 K.And when an energy of 0.597 e V is provided,the tunneling probability along the reaction path will reach the magnitude value of 10-6.In contrast,the analysis results of the resonance tunneling effect show that due to the existence of quantum coherence,it is only necessary to provide 0.597 e V of energy along the flip direction at the double energy barriers of the reaction path to make the resonance tunneling probability of the water chain close to 1.Moreover,for the two paths containing L-type and D-type defect intermediates,only the energy range is different,but the qualitative trend is the same.This shows that,compared with not considering quantum coherence,resonant tunneling can achieve a higher tunneling probability by providing less energy.This research result provides a necessary reference for realizing the control of the water chain flipping method through quantum control.Third,the empirical force field method is still indispensable for theoretical research on large-scale confined water channel due to its advantages such as low computational resource consumption and fast calculation speed.The previous research has shown that although the quantum theory has obtained a unique tunneling effect,the empirical force field method has indeed obtained a conclusion consistent with the trend of the DFT method under the general confined conditions.However,at present,the tightly confined conditions within the Van der Waals boundary have attracted more and more attention.Therefore,whether the widely used traditional empirical force field method and the developing polarized empirical force field can describe these confinement processes well needs attention.In this part of the work,the applicability of the empirical force field method in the confinement system was discussed through the calculation and comparison of the empirical force field and DFT.Specifically,through theoretical calculation on the hydrogen bond rotation process of water in a quasi-one-dimensional confined space formed by CNTs of different diameters,the hydrogen bond rotation behavior under different confined conditions was explored.Calculation shows that for the general confinement of larger space,both the non-polarized force field and the polarized force field have obtained results that are consistent with that by using the DFT calculation method,which also conforms to the conclusion that the step by step flipping model of hydrogen bond of the water chain that we established in the first part.However,when the size of the confined space is smaller than the sum of the van der Waals radii of the CNT tube wall and water molecules,for this tight confined condition,it is found that no matter it is the general traditional non-polarized force field method or the polarized force field method,qualitatively consistent results with DFT can longer be obtained.The energy decomposition analysis shows that due to the Pauli repulsion effect,the hydrogen bond rotation enhancement phenomenon in the tightly bound region will be promoted,that is,the existence of the quantum effect in the tightly confinement makes the water molecules easy to rotate.However,the results of the empirical force field method show that the tightly confinement is more obstructive to the rotation of water molecules than the general confinement.This shows that the quantum effect caused by this electronic correlation has not been described in the conventional empirical force field method based on the general system fitting.At the same time,it lacks the fitting parameters suitable for the tightly bound system.It needs to be treated with caution in its use in the tightly confinement system,and needs to be improved and developed in the future empirical force field methodology.In summary,through the study of the water hydrogen bond rotation properties in a confined environment,this work revealed that the hydrogen bond rotation of the water chain under quasi-one-dimensional confined conditions follows a step-by-step model;found that the important influence of quantum tunneling,especially resonance tunneling,on this process;And pointed out the qualitative differences between the empirical force field and the quantum first-principles method to describe the hydrogen bond rotation in different confined environments.Our research shows that the unidirectional flow in the water channel can be maintained on the macroscopic time scale of seconds without the need for an external field,which provides a theoretical basis for the design of the artificial water channel.At the same time,the quantum tunneling effect has been proved by us to play an important role in the water chain flipping process.In particular,the consideration of quantum coherence provides new ideas for realizing quantum regulation of confined water transport.Furthermore,by comparing the empirical force field and quantum first-principles calculations on the rotation properties of confined water hydrogen bond,we point out that the quantum effect of electron correlation also has an important influence on the flipping process,which provides a reference for the development direction of the empirical force field method.This research work is of great significance for further understanding the mechanism of the relevant confinement system from the atomic level,and promotes its application in physics,chemistry,materials,electronics,biology and so on. |
PDF Full Text Request