仪器分析实验 (5).pdf

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1、See discussions,stats,and author profiles for this publication at:https:/ spectroscopy of neutral water clusters at finite temperature:Evidence for a noncyclic pentamerArticleinProceedings of the National Academy of Sciences June 2020DOI:10.1073/pnas.2000601117CITATIONS15READS22717 authors,including

2、:Some of the authors of this publication are also working on these related projects:X-beam molecular reaction dynamics and photodissociation dynamics View projectStudy on the mechanism of CO2 activation reduction reaction.View projectBingbing ZhangDalian Institute of Chemical Physics21 PUBLICATIONS1

3、82 CITATIONSSEE PROFILEYong YuDalian Institute of Chemical Physics19 PUBLICATIONS153 CITATIONSSEE PROFILEYangyang ZhangTsinghua University8 PUBLICATIONS106 CITATIONSSEE PROFILEShukang JiangShanghaiTech University7 PUBLICATIONS82 CITATIONSSEE PROFILEAll content following this page was uploaded by Yan

4、gyang Zhang on 25 July 2020.The user has requested enhancement of the downloaded file.Infrared spectroscopy of neutral water clusters at finitetemperature:Evidence for a noncyclic pentamerBingbing Zhanga,1,Yong Yua,b,1,Yang-Yang Zhangc,1,Shukang Jianga,b,Qinming Lia,b,Han-Shi Huc,Gang Lia,Zhi Zhaoa,

5、Chong Wanga,Hua Xiea,Weiqing Zhanga,Dongxu Daia,Guorong Wua,Dong H.Zhanga,Ling Jianga,2,Jun Li(李 隽)c,d,2,and Xueming Yanga,d,2aState Key Laboratory of Molecular Reaction Dynamics,Dalian Institute of Chemical Physics,Chinese Academy of Sciences,116023 Dalian,China;bUniversityof Chinese Academy of Sci

6、ences,100049 Beijing,China;cDepartment of Chemistry,Key Laboratory of Organic Optoelectronics and Molecular Engineeringof the Ministry of Education,Tsinghua University,100084 Beijing,China;anddDepartment of Chemistry,College of Science,Southern University of Scienceand Technology,518055 Shenzhen,Chi

7、naEdited by Robert W.Field,Massachusetts Institute of Technology,Cambridge,MA,and approved May 14,2020(received for review January 11,2020)Infrared spectroscopic study of neutral water clusters is crucial tounderstanding of the hydrogen-bonding networks in liquid waterand ice.Here we report infrared

8、 spectra of size-selected neutralwater clusters,(H2O)n(n=36),in the OH stretching vibrationregion,based on threshold photoionization using a tunable vac-uum ultraviolet free-electron laser.Distinct OH stretch vibrationalfundamentals observed in the 3,5003,600-cm1region of(H2O)5provide unique spectra

9、l signatures for the formation of a noncyclicpentamer,which coexists with the global-minimum cyclic structurepreviously identified in the gas phase.The main features of in-frared spectra of the pentamer and hexamer,(H2O)n(n=5 and6),span the entire OH stretching band of liquid water,suggestingthat th

10、ey start to exhibit the richness and diversity of hydrogen-bonding networks in bulk water.water cluster|infrared spectroscopy|hydrogen bonding|free-electron laserAs one of the most important matters on the earth,water andits interactions with other substances are essential in humanlife.However,under

11、standing the structure of liquid water and itshydrogen-bonding networks remains a grand challenge.Spectro-scopic investigation of gas-phase water clusters and its transition tobulk liquid water is critical in understanding the structures andproperties of condensed-phase water(13).Here,we present ate

12、chnique for infrared(IR)spectra of size-selected neutral waterclusters based on threshold photoionization using a tunable vac-uum ultraviolet free-electron laser.It is found that noncyclic three-dimensional(3D)structure of water clusters begins to exist alreadyat the pentamer with low finite tempera

13、ture.On the one hand,cationic or anionic forms of water clustershave been extensively investigated because of relatively easy sizeselection and detection.For the protonated water clusters,H+(H2O)n,small-sized clusters prefer chain-or treelike net-works,closed net motifs are formed at n=10,and hydrog

14、en-bonding networks evolve into closed cage structures at n=21(46).It was found that the Eigen H3O+(H2O)3 and Zundel(H2OH+OH2)motifs play important roles in the proton ac-commodation in water(411).Experimental studies of anionicwater clusters were devoted to understanding the nature of theexcess ele

15、ctron in water(1216).These studies provide impor-tant knowledge on the structures and dynamics of the ionic waterclusters.On the other hand,hydrogen-bonding network struc-tures in neutral water clusters are substantially different fromthose in ionic water clusters.Therefore,it is more relevant toinv

16、estigate the hydrogen-bonding networks of confinement-freeneutral water clusters to gain deeper insights into the evolutionof hydrogen-bonding networks from small clusters to liquid wa-ter.Unfortunately,it is even more challenging experimentally toinvestigate size-specific neutral water clusters due

17、 to difficult sizeselection for neutral clusters in general.Over the past several decades,various experimental techniqueswere developed to study the neutral water clusters in the mid-IRand far-IR regions.The gas-phase mid-IR spectrum of water di-mer in the OH stretching region was initiated by Lee a

18、nd co-workers(17,18)and its rotational resolved spectra were thenstudied by Miller and coworkers(19),using IR predissociationspectroscopic methods.Saykally and coworkers carried out thefar-IR absorption spectroscopy of neutral water clusters of dif-ferent sizes,which provided detailed information on

19、 the structureand tunneling of these clusters(2024).IR spectroscopy of size-selected neutral water clusters was also studied by Huisken et al.,in which size selection was achieved by the rare-gas scatteringmethod,followed by electron impact ionization(25).Vibrationalspectra of neutral water hexamer

20、were studied by Johnson andcoworkers using argon-mediated,population-modulated electronattachment spectroscopy(26).IR-UV double-resonance spec-troscopy of size-selected neutral water clusters attached by abenzene molecule was investigated by Zwier and coworkers(27,28),and similar study on water clus

21、ters attached by a sodium atomwas also performed(29).In 2012,Pate and coworkers identifiedthe cage,prism,and book isomers of the water hexamer using thebroadband microwave spectroscopy(30).Subsequently,concertedSignificanceSpectroscopic investigation of neutral water clusters is criticalin understan

22、ding the structures and properties of condensed-phase water.Because of the difficulty for size selection ofneutral clusters in general,infrared spectroscopic study ofstructural evolution of confinement-free,neutral water clustershas been a grand challenge.In this work,we exploit recentlydeveloped,hy

23、brid instruments that integrate infrared spec-troscopy with a tunable vacuum ultraviolet free-electron laserto capture structural evolution of water clusters.Strikingspectral change in the OH stretch region is observed fromwater tetramer to penta-and hexamer,due to appearance ofthree-dimensional hyd

24、rogen-bonding networks.The techniquehas the potential to obtain the infrared spectra of size-selectedneutral clusters and to access their rich structural landscape.Author contributions:L.J.,J.L.,and X.Y.designed research;B.Z.,Y.Y.,Y.-Y.Z.,S.J.,Q.L.,G.L.,Z.Z.,C.W.,H.X.,W.Z.,D.D.,G.W.,D.H.Z.,L.J.,J.L.

25、,and X.Y.performed research;H.-S.H.,D.H.Z.,L.J.,J.L.,and X.Y.analyzed data;and L.J.,J.L.,and X.Y.wrote the paper.The authors declare no competing interest.This article is a PNAS Direct Submission.Published under the PNAS license.1B.Z.,Y.Y.,and Y.-Y.Z.contributed equally to this work.2To whom corresp

26、ondence may be addressed.Email:,or .This article contains supporting information online at https:/www.pnas.org/lookup/suppl/doi:10.1073/pnas.2000601117/-/DCSupplemental.www.pnas.org/cgi/doi/10.1073/pnas.2000601117PNAS Latest Articles|1 of 6CHEMISTRYDownloaded at EAST CAROLINA UNIVERSITY on June 15,2

27、020 quantum-tunneling-induced hydrogen-bond breaking in the neu-tral prism hexamer was investigated(31).Along with significantadvancesintheoreticalcalculations(3236),thesestudiesprovided great insights into the hydrogen bonding structures anddynamics of neutral water clusters.As pointed out by Clary

28、(3),water molecules in liquid are heldtogether by an ever-changing network of hydrogen bonds,whichare 3D in nature.From previous studies,it is known that thewater trimer,tetramer,and pentamer all have cyclic minimum-energy structures with all oxygen atoms in a two-dimensional(2D)plane,while the wate

29、r hexamer has noncylic 3D struc-tures(3436).Clearly there are two important issues:when doesthe 2D-to-3D structural change occur in neutral water clustersand how is this structural change manifested in the OHstretching spectra?While it can provide crucial knowledge onhydrogen-bonding networks,size-s

30、pecific IR spectroscopic studyof confinement-free neutral water clusters in the OH stretchregion has been very difficult because of the lack of proper toolsfor the size-selected probe of neutral clusters in general.Recently,we have developed an intense tunable vacuum UVfree-electron laser(VUV-FEL)fa

31、cility(37),which provides apowerful tool for size-selective soft ionization for neutral clusters.This makes it possible to study the IR predissociation spectroscopyof confinement-free neutral water clusters using the IR-VUVscheme.In this work,we have systematically investigated the size-specific IR

32、spectra of neutral water clusters,(H2O)n(n=36),using this IR-VUV scheme.Quantum-chemical studies were alsocarried out in an effort to understand the structural and spectralchanges in these clusters.The experimental study was carried out using an IR-VUVdouble-resonance spectroscopy apparatus at the V

33、UV-FEL facil-ity(38).In the experiment,neutral water clusters were generated bysupersonic expansions of water vapor seeded in helium using a high-pressure pulsed valve(Even-Lavie valve,EL-72011-HT-HRR)that is capable of producing very cold molecular-beam conditions(39).The extraction plates of refle

34、ctron time-of-flight mass spec-trometer(TOF-MS)were powered by a high-voltage direct current(dc)of 2,950 V.Charged clusters were deflected out of the mo-lecular beam by the dc electric field of the extraction plates.Neutralwater clusters in the beam were then threshold-ionized by theVUV-FEL pulse an

35、d mass-analyzed in the reflectron TOF-MS.Thetunable IR laser pulse from an optical parametric oscillator wasintroduced at about 30 ns prior to the VUV-FEL pulse in the sameVUV-FEL interaction region.When the IR laser frequency wasresonant with a vibrational transition of a selected neutral cluster,v

36、ibrational predissociation caused the depletion of neutral clusterVUV ionization signal.The IR spectra of this size-selected neutralcluster were then recorded by monitoring the depletion of the signalintensity,(H2O)n+,for a specific water cluster as a function of IRwavelength.Although the experiment

37、al temperature is not at 0 K,the rotational temperature should be less than 10 K,while the vi-brational temperature of clusters could be much higher(i.e.,a finitetemperature),because vibrations are not easily cooled as rotation inour equipment.ResultsFig.1 shows the measured IR spectra of(H2O)n(n=36

38、)in theOH stretching vibration region using the experimental methoddescribed above and the observed vibrational transitions arelisted in Table 1.VUV wavelength and beam conditions wereoptimized to maximize the signal of a size-specific water cluster ofinterest with no interference from larger cluste

39、rs(SI Appendix,Fig.S1).IR power dependence of the signal was measured toensure that the predissociation yield is linear with photon flux.Asshown in Fig.1,the IR spectra of each individual cluster(H2O)n(n=36)in the OH stretching region are all different from eachother,indicating that these IR spectra

40、 were measured at optimumexperimental conditions with negligible contribution from largerclusters.The comparison of present VUV-FELbased IR spectraof(H2O)n(n=46)with previous IRmolecular-beam spectra of(H2O)nand IR-UV spectra of benzene-(H2O)nis shown in SIAppendix,Fig.S2.The characteristic features

41、 of OH vibrationalmodes are well resolved in the VUV-FELbased IR spectra of(H2O)n,revealing the remarkable evolution with cluster size.DiscussionThe experimental IR spectra of(H2O)n(n=36)can be clas-sified as three groups of bands(Fig.1 and Table 1),which areassigned by comparison with the previous

42、studies(18,19,25,27,40,41).The donor-free OH stretch(Dfree)appears at 3,722 cm1in the IR spectrum of n=3 and is slightly redshifted from 3,720to 3,715 cm1for n=46,similar to the virtually unchangedpositions with cluster size in benzene-(H2O)n(27).The OHstretch of water with single hydrogen-donor con

43、figuration(single-donor hydrogen-bonded OH stretch,labeled as SDHB)shows quartet bands at 3,398,3,410,3,434,and 3,452 cm1forn=3 and is considerably redshifted for n=46.The most striking observation in the present work is the IRspectrum of the pentamer in the OH stretch region(Fig.1C),which is very d

44、ifferent from those of the trimer and tetramer(Fig.1 A and B).The IR spectrum of(H2O)5shows distincttransitions at 3,494,3,532,3,553,and 3,561 cm1,which arecharacteristic of the OH stretch of water molecule with doublehydrogen-donor configuration(double-donor hydrogen-bondedOH stretch,labeled as DDH

45、B)associated with the formation of amore compact,noncyclic structure(27,40).The observed IRspectrum of(H2O)6continues to exhibit the features of DDHB,which becomes more complex and enormously spreads in the3,0003,700-cm1region.Consistent with the general observation,the OH stretch vibrational freque

46、ncies of double hydrogen-donor30003200340036003800DWavenumber(cm?1)6DDHBC5BPredissociation efficiency4SDHBDfreeAn=3Fig.1.IR spectra of neutral(H2O)nclusters(AD)with n=36 in the OHstretch region.2 of 6|www.pnas.org/cgi/doi/10.1073/pnas.2000601117Zhang et al.Downloaded at EAST CAROLINA UNIVERSITY on J

47、une 15,2020 water lie between those of donor-free water and single hydrogen-donor water(25,27,40).To understand the hydrogen-bonding structures of theseneutral water clusters and their IR spectra,theoretical studieswere carried out for the low-lying isomers using the constrainedbasin-hopping global

48、minimum search(42)and ab initio second-order Mller-Plesset/aug-cc-pVDZ(MP2/AVDZ)method.Rel-ative energies were also calculated using the domain-based localpair natural orbital coupled cluster theory with single,double,and perturbative triple excitations/aug-cc-pVTZ(DLPNO-CCSD(T)/AVTZ)method at the M

49、P2/AVDZ optimized geom-etries(see SI Appendix for computational details).The lowest-energy structures identified for neutral water clusters(H2O)n(n=36)are shown in SI Appendix,Fig.S3.Both calculationsindicate that the most stable structure of the water trimer,tet-ramer,and pentamer are cyclic,whereas that of the hexamer isnoncyclic,consistent with previously well-established thresholdof 3D water structures instigating at hexamer(3436).Vibra-tional frequencies for these neutral water clusters were alsocalculated using harmonic model at the MP2 level.For the n=3and 4 water clusters,the calculat