Heavy Water Reactions with Atomic Transition-Metal and Main-Group Cations: Gas Phase Room-Temperature Kinetics and Periodicities in Reactivity

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1 J. Phys. Chem. A 2007, 111, Hevy Wter Rections with Atomic Trnsition-Metl nd Min-Group Ctions: Gs Phse Room-Temperture Kinetics nd Periodicities in Rectivity Ping Cheng, Gregory K. Koyngi, nd Diethrd K. Bohme* Deprtment of Chemistry, Centre for Reserch in Mss Spectrometry nd Centre for Reserch in Erth nd Spce Science, York UniVersity, Toronto, Ontrio, Cnd, M3J 1P3 ReceiVed: April 4, 2007; In Finl Form: June 20, 2007 Rections of hevy wter, D 2 O, hve been mesured with 46 tomic metl ctions t room temperture in helium bth gs t 0.35 Torr using n inductively coupled plsm/selected ion flow tube tndem mss spectrometer. The tomic ctions were produced t c K in n ICP source nd were llowed to decy rditively nd thermlize by collisions with Ar nd He toms prior to rection. Rte coefficients nd product distributions re reported for the rections of fourth-row tomic ctions from K + to Se +, of fifth-row tomic ctions from Rb + to Te + (excluding Tc + ), nd of sixth-row tomic ctions from Cs + to Bi +. Primry rection chnnels were observed leding to O-tom trnsfer, OD trnsfer, nd D 2 O ddition. O-Atom trnsfer occurs lmost exclusively (g90%) in the rections with most erly trnsition-metl ctions (Sc +,Ti +,V +,Y +,Zr +, Nb +,Mo +,Hf +,T +, nd W + ) nd to minor extent (10%) with one min-group ction (As + ). OD trnsfer is observed to occur only with three ctions (Sr +,B +, nd L + ). Other ctions, including most lte trnsition nd min-group ctions, were observed to rect with D 2 O exclusively nd slowly by D 2 O ddition or not t ll. O-Atom trnsfer proceeds with rte coefficients in the rnge of (As + )to (Y + ) cm 3 molecule -1 s -1 nd with efficiencies below 0.1 nd even below 0.01 for the fourth-row tomic ctions V + (0.0032) nd As + (0.0036). These low efficiencies cn be understood in terms of the chnge in spin required to proceed from the rectnt to the product potentil energy surfces. Higher order rections re lso mesured. The primry products, NbO +, TO +, MoO +, nd WO +, re observed to rect further with D 2 Oby O-tom trnsfer, nd ZrO + nd HfO + rect further through OD group bstrction. Up to five D 2 O molecules were observed to dd sequentilly to selected M + nd MO + s well s MO 2+ ctions nd four to MO 2 D +. Equilibrium mesurements for sequentil D 2 O ddition to M + re lso reported. The periodic vrition in the efficiency (k/k c ) of the first ddition of D 2 O ppers to be similr to the periodic vrition in the stndrd free energy ( G ) of hydrtion. 1. Introduction There is much interest in fundmentl spects of the chemistry of tomic ctions, especilly tomic metl ctions. Bre nd ligted tomic ctions generlly ply n importnt role in physics, chemistry, nd biology. Wter, of course, is prcticlly omnipresent nd so is n obvious moleculr trget for tomic ctions found in nturl environments such s the erth s ionosphere but lso in mn-mde devices such s mss spectrometers designed to hndle tomic ctions in which wter is often present s bckground impurity. In the mss spectrometer developed in our own lbortory, tomic ctions cn be generted lmost universlly in n inductively coupled plsm (ICP) ion source, injected into flow tube flushed with helium gs t room temperture, nd then rected with wter vpor. With the ddition of mss selection of the ions entering nd emerging from the flow tube, the resulting inductively coupled selected ion flow tube (ICP/SIFT) tndem mss spectrometer llows for surveys of trends in the room-temperture chemicl kinetics of rections with tomic ctions cross nd down the periodic tble. For exmple, rections of fourth-, fifth-, nd sixth-row tomic ctions hve recently been surveyed with moleculr oxygen, 1 * Corresponding uthor. E-mil: dkbohme@yorku.c; phone: (416) , ext 66188; fx: (416) both in the bsence nd in the presence of benzene, 2 nd with hexfluorobenzene, 3 N 2 O, 4 CO 2, 5 CS 2, 6 nd CH 3 F. 7 Here, we report the results obtined for rections of tomic ctions with D 2 O(D 2 O ws chosen insted of H 2 O to relx the resolution requirements in the downstrem mss spectrometer nd to distinguish between dded nd bckground wter). The first gs phse rections of tomic metl ctions mesured with D 2 O were those of N + nd K + reported in 1971 by Biondi s group. 8 The metl ctions were produced with hot filment coted with the oxide of the lkli metl, nd rte coefficients for their rections were mesured in drift tube mss spectrometer over n energy rnge from therml energies to 5 ev (men kinetic energy). This work ws followed more thn 15 yers lter by vrious studies of the interctions of wter with other tomic metl ctions, especilly tomic trnsitionmetl ctions The therml energy rection of Ti + with H 2 O hs been investigted with drift tube nd fst-flow rector with He s the buffer gs in the pressure rnge from 0.2 to 0.7 Torr. 12,16 The rections of fourth-row tomic trnsition-metl ctions of hevy wter were studied systemticlly s function of trnsltionl energy with guided ion bem tndem mss spectrometer by Armentrout et l. 10 Experimentl results were obtined for rections with Sc + nd Ti +, 10 Fe +, 17 V +, 18 nd Co In these mesurements, the tomic ctions were produced /jp072661p CCC: $ Americn Chemicl Society Published on Web 08/16/2007

2 8562 J. Phys. Chem. A, Vol. 111, No. 35, 2007 Cheng et l. with vriety of ion sources including surfce ioniztion, electron impct, nd flow tube ion sources, in prt to try to discern the influence of electronic excittion of the tomic ctions on rectivity. The ground-stte rections of Sc +,Ti +, nd V + were shown to be exothermic t therml energy, wheres the rections with Fe + nd Co + exhibited endothermic behvior. These uthors lso investigted the solvtion of metl ctions by wter nd reported sequentil bond energies for M + (D 2 O) n with n ) 1-4 nd M ) N +,Mg +,oral +20 nd trnsitionmetl ctions from Ti + to Cu Mesurements of the energetics of hydrtion of lkli ions lso were reported by Kebrle et l., who used high-pressure ion source (HPIS) mss spectrometry Systemtic theoreticl investigtions using b initio clcultions designed to understnd the rectivity of trnsition-metl ctions with wter hve been reported by Uglde et l. 25 These uthors hve ddressed quntum mechnicl constrints introduced by spin-forbidden crossings of the potentil energy surfces tht re involved in the formtion of oxide ctions in the rections of wter with Sc + nd V +, 26 Cr +, 27 Mn +, 27 Fe +, 27,28 Co +,Ni +, nd Cu Similr surfce crossings hve been predicted very recently for the rection of W + with wter. 29 As fr s we re wre, with the exception of the rection with Hf +, 30 none of the rections of wter with the tomic metl ctions from the fifth nd sixth row of the periodic tble ppers to hve been studied previously. In the mesurements reported here, fourth-period tomic ctions from K + to Se +, fifth-period tomic ctions from Rb + to Te + (excluding Tc + ), nd sixth-period tomic ctions from Cs + to Bi + were generted in one source, n ICP, which hs been found to be suitble for virtully ny element on the periodic tble (with the exception of ions with mss less thn Ar). 31 Higher-order chemistry initited by the primry rections of the tomic metl ctions lso cn be monitored with the ICP/SIFT technique. In the experiments reported here with D 2 O, the primry MO + product ions rected further with D 2 O to form MO + 2 or MO 2 D + or simply hydrte with D 2 O. These observtions of higher-order chemistry lso will be reported. 2. Experimentl Procedures The experimentl results reported here were obtined with the SIFT tndem mss spectrometer described in detil elsewhere. 32,33 This instrument ws modified to ccept ions generted in n ICP torch (ELAN series, PerkinElmer SCIEX) through n tmosphere-vcuum interfce. The ICP ion source nd interfce hve lso been described previously, 31,34 s hs the preprtion of the solutions contining the tomic slt of interest tht were peristlticlly pumped vi nebulizer into the plsm. 5 Atomic ions emerge from the ICP t nominl ion temperture of 5500 K with corresponding Boltzmnn stte distributions. These distributions hve been derived from vilble opticl spectr 35,36 nd reported by us previously for the two electronic spin sttes with the highest popultion t 5500 K. 4 Energy levels s high s 3.7 ev ( cm -1 ) were included in the clcultions. The clcultions show tht excited sttes of the min-group elementl ctions except B + re high in energy nd contribute little (never more thn 10%) to the totl ion popultion t 5500 K. The ground 2 S stte of B + contributes 44% nd the excited 2 D stte 55% t 5500 K. The stte distributions re more vrible for the trnsition-metl ctions. The excited sttes t 5500 K contribute 20% or less towrd the popultions of Cr +,Mn +,Ni +,Cu +,Zn +,Rh +,Pd +,Ag +,Cd +, Re +,Au +, nd Hg + nd 50% or more towrd the popultions of Ti +,Y +,Zr +,Nb +,L +, nd Ir + with Sc +,V +,Fe +,Co +, Mo +, Ru +, Hf +, T +, W +, nd Pt + hving intermedite distributions t 5500 K, with Os + not known. The ions emerging from ICP re injected through differentilly pumped smpling interfce into qudrupole mss spectrometer nd, fter mss nlysis, introduced through n spirtor-like interfce into flowing helium crrier gs t 0.35 ( 0.01 Torr nd 295 ( 2 K. The ions re llowed to rect with dded D 2 O few milliseconds downstrem of the flow tube. After extrction from the ICP, the plsm ions my experience both rditive electronic-stte relxtion nd collisionl electronicstte relxtion. The ltter my occur lredy with rgon s the extrcted plsm cools upon smpling nd then by collisions with He toms in the flow tube (c collisions with He) prior to the rection region, but the ctul extent of electronic relxtion (either rditive or collisionl) is not known nd is difficult to ssess. Almost ll of the electronic sttes of the trnsition-metl ions hve positive prity; electric dipole trnsitions between sttes of the sme prity re forbidden (Lporte rule). 37 This mens tht rditive trnsitions between different sttes in metl ctions cn be chieved only by either mgnetic dipole or electric qudrupole rdition. The probbilities for these trnsitions re very low, nd the resulting rditive lifetimes re of the order of seconds or lrger. The time intervl in the ICP-SIFT experiments between the exit of the ICP source nd the entrnce in the rection region is c. 20 ms, nd therefore, no mjor modifiction of stte distributions cn occur in this time intervl by forbidden rditive decy. Tht hving been sid, there were no indictions of excitedstte effects in our previous mesurements of rections of N 2 O with tomic ctions derived from the sme ICP source operted in the sme mnner, except with Pt +. 4 The mny collisions experienced by the tomic ctions with the quite polrizble Ar toms s they emerge from the ICP nd the c collisions with He toms in the flow tube (the helium buffer gs pressure ws 0.35 ( 0.01 Torr) pper to be sufficient to provide for the thermliztion of the excited sttes nd ensure tht the tomic ions rech trnsltionl temperture equl to the tube temperture of 295 ( 2 K prior to entering the rection region. However, the exct extent of electronic relxtion is uncertin. Clues to the presence of excited electronic sttes of the tomic ions in the rection region cn be found in the product ions observed nd the shpe of the semilogrithmic decy of the recting tomic ion upon the ddition of neutrl rectnt. Curvture will pper in the mesured tomic ion decy when the ground stte nd excited stte rect t different rtes even when they give the sme product ions. An excited-stte effect cnnot be seen when the products nd rection rtes re the sme for both the ground nd the excited sttes, but in this cse, the mesured tomic ion decy defines the ground-stte kinetics. Our growing experience hs shown tht excited sttes cn revel themselves when the ground stte of the tomic ion rects only slowly by termoleculr ddition nd when excited sttes rect rpidly in bimoleculr fshion. Our previous studies indicte tht tht the collisions with Ar nd He ensure tht most tomic ions rech trnsltionl nd internl temperture equl to the tube temperture of 295 ( 2 K prior to entering the rection region. 1-6,38,39 Rectnt nd product ions were smpled still further downstrem with second qudrupole mss spectrometer nd were mesured s function of dded rectnt. The resulting profiles provide informtion bout product ion distributions, rection rte coefficients, nd rection moleculrity. Rte coefficients for the primry bimoleculr ion-molecule rections re determined from the rte of decy of the rectnt ion intensity with

3 Hevy H 2 O Rections with Ctions J. Phys. Chem. A, Vol. 111, No. 35, TABLE 1: O-Atom Affinities (D 0 (M + -O) (kcl mol -1 )) nd Ioniztion Energies (IE(M) (ev)) 40 for Fourth-, Fifth-, nd Sixth-Row Atomic Ctions M + OA(M + ) IE(M) M + OA(M + ) IE(M) M + OA(M + ) IE(M) K Rb Cs C Sr B Sc ( 1.4 b 6.56 Y ( 4.2 c 6.22 L ( 4 d 5.58 Ti ( 1.6 b 6.83 Zr ( 2.5 c 6.63 Hf ( V ( 3.5 b 6.75 Nb ( 2.5 c 6.76 T ( Cr ( 2.8 b 6.77 Mo ( 0.5 c 7.09 W ( 10 f 7.86 Mn ( 3.0 b 7.43 Tc + Re ( 15 g 7.83 Fe ( 1.4 b 7.90 Ru ( 1.2 e 7.36 Os ( 12 h 8.44 Co ( 1.2 b 7.88 Rh ( 1.4 e 7.46 Ir Ni ( 1.2 b 7.64 Pd ( 2.5 e 8.34 Pt + 77, i 75 ( 2 j 8.96 Cu ( 3.5 b 7.72 Ag ( 1.2 e 7.58 Au Zn ( 1.2 b 9.39 Cd Hg G In Tl Ge Sn Pb As ( 2 k 9.81 Sb Bi Se + 92 k 9.75 Te Po + Reference 51. b Reference 52. c Reference 53. d Reference 55. e Reference 54. f Reference 59. g Reference 56. h Reference 57. i Reference 58. j Reference 60. k Reference 4. TABLE 2: OH Affinities of Bre Metl Ctions (D 0 (M + -OH) (kcl mol -1 )) M + D 0(M + -OH) M + D 0(M + -OH) M + D 0(M + -OH ) C Sc Mn + 82 Sr ( 5 b Ti Fe B ( 7 b V Co Zn Cr Ni With few exceptions, the vlues for D 0(M + -OH) were tken from ref 9. b Determined form H f found in ref 51. TABLE 3: H-Atom Affinities (HA(M + )) of Bre Metl Ctions (HA(M + ) ) D 0 (M + -H) (kcl mol -1 )) M + HA(M + ) M + HA(M + ) M + HA(M + ) B b Sc ( 2.2 Y ( 1.9 L ( 2.2 Ti ( 2.6 Zr ( 1.9 Hf b 45.4 ( 2.5 c V ( 1.4 Nb ( 1.7 T ( 1.2 Cr ( 2.2 Mo ( 1.4 W ( 1.4 Mn ( 3.4 Tc d Re b 52.8 ( 1.6 e Fe ( 1.4 Ru ( 1.2 Os b Co ( 1.4 Rh ( 1.0 Ir b 71.9 ( 1.4 f Ni ( 1.9 Pd ( 1.0 Pt ( 1.2 Cu ( 3.1 Ag ( 1.4 Au b 50.0 ( 1.8 g Zn ( 3.1 Cd + 42 b Hg b With few exceptions, the vlues for HA(M + ) were tken from the review of ref 61. b Reference 63. c Reference 64. d Reference 62. e Reference 65. f Reference 66. g Reference 67. n uncertinty estimted to be less thn (30%. 32,33 The uncertinty my be somewht lrger for slow rections for which the mesured decy is significntly less thn n order of mgnitude. Often, slow dduct formtion is observed exclusively, nd this cn introduce curvture into the primry ion decy due to the occurrence of the reverse rection (the extent of curvture will depend on the strength of the dduct bond being formed). Wek dduct bonding cn led to erly curvture in the ion decy nd so prevent the proper definition of the forwrd rte coefficient; only the determintion of lower limit to the rte coefficient is possible under such conditions. The pproch to equilibrium cn be monitored with plot of the TABLE 4: Sequentil Binding Energies for M + (H 2 O) n (n e 4) (kcl mol -1 ) M K b 16.1 b 13.2 b 11.8 b C + 28 c 24 c 21.5 c 18.7 c Sc d 31.4 d Ti ( ( ( ( 1.8 V ( ( ( ( 1.8 Cr ( ( ( ( 1.4 Mn ( ( ( ( 1.2 Fe ( ( ( ( 1.6 Co ( ( ( ( 1.4 Ni ( ( ( ( 1.5 Cu ( ( ( ( 1.0 Zn d 22.2 d Rb e 13.5 e 12.8 e 12.8 e Sr f 30.5 f 25.7 f 22.3 f Cs e 11.8 e 12.0 e 14.7 e Ag ( 1.9 g 30.4 ( 1.9 g 15.1 ( 2.2 g 12.1 ( 2.9 g Au ( 2.3 h 45.0 ( 2.5 h 23.1 ( 4.6 h 20.8 ( 4.6 h Pb f 16.9 f 12.2 f 10.8 f Unless indicted otherwise, the vlues for the binding energies of M + (H 2O) n (n e 4) re tken from ref 21. b Reference 22. c Reference 68. d Reference 69. e Reference 70. f Reference 71. g Reference 72. h Reference 73. rtio of the concentrtion of the dduct ion over tht of the bre ion ginst the flow of the dded rectnt. This rtio will curve upwrd s equilibrium is pproched nd increse linerly with flow when equilibrium is chieved. Linerity in the rtio plot provides mesure of the equilibrium constnt, K, nd so the stndrd free energy chnge, G T, for the ddition rection since G T )-RT ln K. D 2 O ws introduced into the rection region of the SIFT s dilute mixture in helium ( 2.5%) nd ws obtined commercilly with high purity (Aldrich, isotopic purity >99.75%). 3. Results nd Discussion The rections of D 2 O were mesured with 46 tomic metl ctions, including 29 trnsition-metl nd 17 min-group ctions, t helium buffer gs pressure of 0.35 ( 0.01 Torr nd temperture of 295 ( 2 K. Both the primry nd the higherorder chemistry ws monitored. The primry rections exhibited wide rnge in rectivity with mesured rte coefficients from < (for K + )to cm 3 molecule -1 s -1 (for

4 8564 J. Phys. Chem. A, Vol. 111, No. 35, 2007 Cheng et l. TABLE 5: Rte Coefficients (cm 3 molecule -1 s -1 ), Rection Efficiencies (k/k c ), Primry Product Ions nd Their Distributions, nd Higher-Order Product Ions Mesured for Rections of Atomic Ions M + with D 2 O in Helium t 0.35 ( 0.01 Torr nd 295 ( 2 K Ordered According to Row in the Periodic Tble M + k k b c k/k c products PD (%) c product ions primry higher-order K + < < C C + (D 2O) 100 C + (D 2O) 2 Sc ScO ScO + (D 2O) 1-5 Ti TiO TiO + (D 2O) 1-5 V VO + 70 V + (D 2O) V + (D 2O) 2,3 30 VO + (D 2O) 1-3 Cr Cr + (D 2O) 100 Mn + < < Mn + (D 2O) 100 Fe Fe + (D 2O) 100 Fe + (D 2O) 2 Co Co + (D 2O) 100 Co + (D 2O) 2-4 Ni Ni + (D 2O) 100 Ni + (D 2O) 2,3 Cu Cu + (D 2O) 100 Cu + (D 2O) 2 Zn Zn + (D 2O) 100 G + < < Ge + < < As AsO AsO + (D 2O) As + (D 2O) 2-3 As + (D 2O) Se + < < Se + (D 2O) Rb Rb + (D 2O) 100 Sr SrOD + 50 Sr + (D 2O) 50 Y YO YO + (D 2O) 1-5 Zr ZrO ZrO 2D + (D 2O) 0-3 ZrO + (D 2O) 1-4 Nb NbO + 95 NbO 2+ (D 2O) 0-4 NbO + (D 2O) 1-3 Nb + (D 2O) 5 Nb + (D 2O) 2-4 MoO + Mo MoO + 90 Mo + (D 2O) 10 (D 2O) MoO 2+ Mo + (D 2O) 2 Ru Ru + (D 2O) 100 Ru + (D 2O) 2 Rh Rh + (D 2O) 100 Rh + (D 2O) 2-5 Pd Pd + (D 2O) 100 Pd + (D 2O) 2 Ag Ag + (D 2O) 100 Cd + < < In + < < Sn + < < Sb + < < Te + < < Cs Cs + (D 2O) 100 B BOD BOD + (D 2O) L LO + LOD LO + (D 2O) 1-5 LOD + (D 2O) Hf HfO HfO + (D 2O) HfO 2D + ((D 2O) 0-4 T TO TO 2+ (D 2O) 0-4 W WO WO 2+ (D 2O) 0-5 Re + < < Re + (D 2O) 100 Os + < < Ir Ir + (D 2O) 100 Ir + (D 2O) 2-5 Pt Pt + (D 2O) 100 Pt + (D 2O) 2 Au Au + (D 2O) 100 Au + (D 2O) 2 Hg Hg + (D 2O) 100 Hg + (D 2O) 2 Tl + < < Pb + < < Bi Bi + (D 2O) 100 Bi + (D 2O) 2 Mesured rection rte coefficient with n estimted ccurcy of (30%. Rte coefficients with vlues of cm 3 molecule -1 s -1 or less my hve higher uncertinty. b Clculted cpture rte coefficient (see text). c PD ) product distribution expressed s percentge, with n estimted ccurcy of (10%. Y + ). Three rection chnnels were observed, nd these re indicted in rection 1. M + + D 2 O f MO + + D 2 f MOD + + D f M + (D 2 O) (1) (1b) (1c) The bimoleculr chnnels correspond to O-tom trnsfer (chnnel 1) nd OD trnsfer (chnnel 1b). The O-tom ffinity of D 2 to form hevy wter, OA(D 2 ), is kcl mol -1, 40 while the DO-D bond energy of D 2 O is kcl mol Avilble O-tom ffinities (Tble 1) indicte tht only 11 O-tom trnsfer rections re exothermic out of the 46 tht re possible with the tomic ctions investigted. Avilble OD ffinities re listed in Tble 2, nd only B + hs n ffinity for OD, kcl mol -1, tht is higher thn the DO-D bond energy. H-tom ffinities re vilble for most metl ctions nd re listed in Tble 3. All these vlues re much lower thn the DO-D bond energy of D 2 O ) kcl mol -1, 40 which mens tht D-tom

5 Hevy H 2 O Rections with Ctions J. Phys. Chem. A, Vol. 111, No. 35, Figure 1. Periodic vritions observed in the rection efficiencies, k/k c (solid circles), for rections of tomic ctions with D 2O. Mesured rection rte coefficient is represented by k, nd k c is the clculted collision rte coefficient (see text). Also indicted re the observed rection chnnels nd electronic configurtions of tomic ctions. Numbers in prentheses indicte the number of sequentil events observed for the process indicted. The br grphs indicte the rection brnching rtios nd re scled to 1.0 full scle. trnsfer is endothermic with ll of these tomic metl ctions. Electron trnsfer ws not observed to ny of the 46 tomic ctions since their recombintion energies re much smller thn IE(D 2 O) ) ( ev. 40 Chnnel 1c is ssumed to occur in termoleculr rection with helium cting s the third body tht crries wy the surplus energy in the rection. Known sequentil binding energies of M + (H 2 O) n (n e 4) re listed in Tble 4. Among the 46 tomic ctions investigted, seven (Sc +,Ti +, Y +,Zr +,Hf +,T +, nd W + ) exhibit the bimoleculr chnnel 1 exclusively with mesured rte coefficients in the rnge from (for W + )to cm 3 molecule -1 s -1 (for Y + ), one ction (B + ) exhibits chnnel 1b exclusively with rte coefficient of cm 3 molecule -1 s -1, nd 21 ctions exhibit only chnnel 1c with mesured rte coefficients in the rnge from < (for K + )to cm 3 molecule -1 s -1 (for Ir + ). Eleven ctions showed no products t ll in the flow region investigted. Among the remining six metl ctions, four (V +,As +,Nb +, nd Mo + ) were observed to proceed by both chnnel 1 nd chnnel 1c, one (L + )by chnnels 1 nd 1b, nd one (Sr + ) by chnnels 1b nd 1c. No ttempt ws mde to mesure the pressure dependence of chnnel 1c since lrge rnge in pressure ws not experimentlly ccessible. No obvious excited-stte effects seemed present in the dt. The semilogrithmic decys of the tomic ions Ti +, Y +,Zr +,Nb +, nd Ir +, ll of which were clculted to hve excited-stte popultions in the Ar plsm of g50%, did not exhibit obvious curvture (see Figures 2-4). The L + rection did produce minor (20%) chnnel tht could not be unmbiguously ssigned to the L + ( 3 F, 5d 2 ) ground stte s is discussed next (see section 3.5.). Tble 5 summrizes the mesured rte coefficients, rection efficiencies, observed primry products, nd product distributions, s well s the observed higher-order products. The rection efficiency is tken to be equl to the rtio of k/k c, where k is the experimentlly mesured rte coefficient nd k c is the cpture or collision rte coefficient computed using the lgorithm of the modified vrition trnsition-stte clssicl trjectory theory developed by Su nd Chesnvich 41 with R(D 2 O) ) cm 3, 42 nd µ d (D 2 O) ) D. 43 Figure 1 displys the dt in Tble 5 nd shows the overll trends in rection efficiencies nd primry product distributions on periodic tble Fourth-Row Atomic Ions. With the exception of the nonrection of K +,G +, nd Ge +, only O-tom trnsfer nd D 2 O ddition were observed s primry rection chnnels for the fourth-row tomic metl ctions. O-Atom trnsfer occurs with the very erly trnsition-metl ctions Sc + nd Ti +.D 2 O ddition ws observed to compete with O-tom trnsfer to V + nd the min-group ction As +. The other fourth-row metl ctions were observed to proceed only by D 2 O ddition. Secondry nd higher-order D 2 O ddition ws observed with both MO + nd M + (D 2 O) ions s seen in Figure 2, which presents the dt for Sc +,Ti +,Fe +, nd As +. Higher-order D 2 O ddition ccording to rection 2 ws observed for the fourth row ctions M + ) C + (n ) 1), V + (n ) 1), Fe + (n ) 1), Co + (n ) 1-3), Ni + (n ) 1, 2), Cu + (n ) 1), nd As + (n ) 1-2). The Cr +,Mn +,Zn +, nd Se + ctions only dded one D 2 Oin the rnge of D 2 O flow tht ws investigted. Rection 3 ws observed for MO + ) ScO + (n ) 0-4), TiO + (n ) 0-4), VO + (n ) 0), nd AsO + (n ) 0) in the rnge of D 2 O flow tht ws investigted. M + (D 2 O) n + D 2 O f M + (D 2 O) n+1 (2) MO + (D 2 O) n + D 2 O f MO + (D 2 O) n+1 (3) A rte coefficient for the rection of Sc + with D 2 O hs been reported by Armentrout et l. 10 (k ) 1.3 ( cm 3 molecule -1 s -1 ), nd the rections of Ti + with H 2 O nd D 2 O

6 8566 J. Phys. Chem. A, Vol. 111, No. 35, 2007 Cheng et l. Figure 2. Composite of ICP/SIFT results for the rections of the fourth-row metl ctions Sc +,Ti +,Fe +, nd As + with D 2O in helium buffer gs t 0.35 ( 0.01 Torr nd 295 ( 2K. hve been mesured by Cstlemn et l. 12,16 nd Armentrout et l. 10 The Ti + results were somewht controversil with suggestions tht excited sttes might ply role. Our result for Ti + (k ) cm 3 molecule -1 s -1 ) is in quite good greement with Cstlemn s most recent flow rector result obtined with H 2 O in He buffer between 0.2 nd 0.6 Torr (k ) cm 3 molecule -1 s -1 ), but our results for Sc + nd Ti + ( nd cm 3 molecule -1 s -1 ) re n order of mgnitude lrger thn Armentrout s results (k ) 1.3 ( nd 3 ( cm 3 molecule -1 s -1 ) obtined with D 2 O t much lower pressures. There is no cler indiction of role for excited Sc + nd Ti + sttes in our dt shown in Figure 2, nd the presence of excited sttes ppers to hve been ruled out for the low-pressure dt. One reviewer suggests tht the discrepncies in rection rte coefficients my be consequence of the difference in pressure (single vs multiple collisions), which my led to difference in the lifetime of the rection intermedite ginst bck dissocition due to the collisionl quenching of the intermedite (with less bck dissocition t higher pressures). However, this proposl is not supported by the pressure independence of the rte coefficient for the rection of Ti + with H 2 O in He buffer between 0.2 nd 0.6 Torr observed by Cstlemn et l. 12 Also, the vlidity of extrcting room-temperture rte coefficients from crosssectionl mesurements t low energies under single-collision conditions s described by Chen et l. 10 my be questioned. The cross sections mesured under single collision conditions pply to recting ion popultion tht is not truly thermlized nd does not hve Mxwell-Boltzmnn popultion t welldefined temperture; s such, the rte coefficients derived from these cross-sections re t best phenomenologicl. The rection of V + with D 2 O hs been mesured previously by our group using ICP-SIFT (k ) 1.0 ( cm 3 molecule -1 s -1 ) 14 nd by Schwrz et l. using FTICR with H 2 O (k ) 8.3 ( cm 3 molecule -1 s -1 ). 14 These results both re in line with our observtion of slow VO + formtion in the rection with D 2 O(k ) 1.1 ( cm 3 molecule -1 s -1 ). However, experiments by Armentrout s group with flow tube ion source did not show mesurble formtion of VO + from the rection of ground-stte V + ( 5 D) with D 2 O, lthough the experimentl results with surfce ioniztion source pper not to be s definitive bout this nonrectivity. 18 Here, too, pressure effects my contribute to the pprent discrepncy. We lso hve reported previously nonrection of Fe + with H 2 O(k < cm 3 molecule -1 s -1 ), in which Fe + ws produced by electronic impct on ferrocene nd for which the ddition of H 2 O could not be demonstrted. 15 The previous rte coefficient of < cm 3 molecule -1 s -1 is much lower thn tht reported here, k ) cm 3 molecule -1 s -1, lthough this vlue my well be n upper limit (see Figure 2). Also, the formtion of some Fe + (D 2 O) ws observed in the experiments reported here (see Figure 2). The two results cn

7 Hevy H 2 O Rections with Ctions J. Phys. Chem. A, Vol. 111, No. 35, Figure 3. Composite of ICP/SIFT results for the rections of the fifth-row metl ctions Y +,Zr +,Nb +, nd Mo + with D 2O in helium buffer gs t 0.35 ( 0.01 Torr nd 295 ( 2K. be brought in line if the wter dduct in the erlier experiments ws collisionlly dissocited in the smpling process; less dduct formtion would be observed, nd the decy of the Fe + signl would be reduced with the resulting regenertion of Fe +. The difference probbly is too lrge to be ttributed to kinetic isotope effect Fifth-Row Atomic Ions. With the exception of the nonrections with Cd +,In +,Sn +,Sb +, nd Te + nd the OD group trnsfer with Sr +, O-tom trnsfer nd D 2 O ddition lso predominte in the rections of the fifth-row metl ctions with D 2 O. O-Atom trnsfer ws observed exclusively with Y + nd Zr + nd D 2 O ddition with Rb +,Ru +,Rh +,Pd +, nd Ag +. The D 2 O ddition ws observed to compete with O-tom trnsfer to Nb + nd Mo + nd to compete with OD group trnsfer with Sr +. Figure 3 displys the primry nd higher-order chemistry observed with Y +,Zr +,Nb +, nd Mo +. The secondry O-tom trnsfer ccording to rection 4 ws observed for the metl ctions M + ) Nb + nd Mo +, nd OD group trnsfer to metl oxides ccording to rection 5 ws observed for Zr +. MO + + D 2 O f MO D 2 (4) MO + + D 2 O f MO 2 D + + D (5) Secondry nd higher-order D 2 O ddition ws observed with M +,MO +,MO 2 +, nd MO 2 D +. Type 2 rections were seen with M + ) Nb + (n ) 1-3), Mo + (n ) 1), Ru + (n ) 1), Rh + (n ) 1-4), nd Pd + (n ) 1). Type 3 rections were observed with MO + ) YO + (n ) 0-4), NbO + (n ) 0-2), nd MoO + (n ) 0). Rections 6 were observed with NbO 2 + (n ) 0-3). Rections 7 were observed with ZrO 2 D + (n ) 0-2) nd cn be seen in Figure 3. MO 2 + (D 2 O) n + D 2 O f MO 2 + (D 2 O) n+1 (6) MO 2 D + (D 2 O) n + D 2 O f MO 2 D + (D 2 O) n+1 (7) 3.3. Sixth-Row Atomic Ions. With the exception of the nonrections with Os +,Tl +, nd Pb +, nd the OD group trnsfers with B + nd L +, O-tom trnsfer nd D 2 O ddition re still the dominnt rections with the sixth-row metl ctions. O-Atom trnsfer ws observed exclusively with Hf +,T +, nd W + nd D 2 O ddition with Cs +,Re +,Ir +,Pt +,Au +,Hg +, nd Bi +. The O-tom trnsfer ws observed to compete with the OD group trnsfer with L +. Figure 4 displys the primry nd higher-order chemistry with Hf +,T +,W +, nd Ir +. Secondry O-tom trnsfer ccording to rection 4 ws observed with M + ) T + nd W + to form TO 2 + nd WO 2 +, respectively, nd OD group trnsfer to metl oxide ccording to rection 5 ws observed with Hf + to form HfO 2 D +. Sequentil D 2 O ddition ws seen with ll three of these secondry ions with the ddition of up to four D 2 O

8 8568 J. Phys. Chem. A, Vol. 111, No. 35, 2007 Cheng et l. Figure 4. Composite of ICP/SIFT results for the rections of the sixth-row metl ctions Hf +,T +,W +, nd Ir + with D 2O in helium buffer gs t 0.35 ( 0.01 Torr nd 295 ( 2K. molecules to form HfO 2 D + (D 2 O) 4, TO 2 + (D 2 O) 4, nd WO 2+ - (D 2 O) 4.Ir + is seen to dd up to five D 2 O molecules to form Ir + (D 2 O) 5. The fster second ddition of D 2 OtoIr + (D 2 O) is clerly rte limited by its slower production from the ddition of D 2 OtoIr +. Secondry nd higher-order D 2 O ddition is generlly observed with the sixth-row metl species M +,MO +, MOD +, MO 2 +, nd MO 2 D +. Type 2 rections were seen with M + ) Ir + (n ) 1-4), Pt + (n ) 1), Au + (n ) 1), Hg + (n ) 1), nd Bi + (n ) 1). Type 3 rections were seen with MO + ) LO + (n ) 0-4) nd HfO + (n ) 0). Type 6 rections were seen with MO 2 + ) TO 2 + (n ) 0-3) nd WO 2 + (n ) 0-4). Type 7 rections were seen with MO 2 D + ) HfO 2 D + (n ) 0-3). Type 8 rections were seen with MOD + ) BOD + (n ) 0) nd LOD + (n ) 0). MOD + (D 2 O) n + D 2 O f MOD + (D 2 O) n+1 (8) Beuchmp nd Irikur 30 previously investigted the rection of Hf + with H 2 O using FTICR with the ion produced by lser bltion of metl foil, but their results re quite different from ours in tht they show the two rection chnnels of O-tom trnsfer nd OH group trnsfer with product distribution estimted s I[HfO + ]/I[HfOD + ] ) 1.6:1 nd mesured rte coefficient of cm 3 molecule -1 s -1. In comprison, our experiments indicte n exclusive O-tom trnsfer rection chnnel with lrger rte coefficient of cm 3 molecule -1 s -1. If the OH trnsfer chnnel is due to the rection of the low-lying 4 F excited-stte of Hf +, then the chnnel discrepncy my be due to higher proportion of this stte produced in the lser bltion of the Hf foil in the FTICR experiments. There is better greement on the higher-order chemistry s both experiments show further rection of HfO + to form HfO 2 H + (HfO 2 D + ) nd then its hydrte, but our rte coefficient of cm 3 molecule -1 s -1 for the rection of HfO + is gin lrger thn tht of cm 3 molecule -1 s -1 reported by Beuchmp nd Irikur O-Atom Trnsfer (Dehydrogention) Thermodynmics nd Efficiency. According to the O-tom ffinities listed in Tble 1, only metl ctions in groups 3B to 6B hve higher O-tom ffinities thn OA(D 2 ) ) ( 0.1 kcl mol -1, 40 nd this mens tht only these ctions re thermodynmiclly llowed to bstrct n O-tom from D 2 O, viz. to cuse dehydrogention nd relese D 2. Indeed, only these metl ctions were observed to dehydrogente D 2 O. Dehydrogention ws observed with the group 3B (Sc +,Y +, nd L + ), group 4B (Ti +, Zr +, nd Hf + ), group 5B (V +,Nb +, nd T + ), nd group 6B (Mo + nd W + ) trnsition-metl ctions nd the min-group ction As +. Rte coefficients for O-tom trnsfer re in the rnge of (As + )to (Y + )cm 3 molecule -1 s -1. OD trnsfer ws observed to compete with O-tom trnsfer in the rections of L + (20%), nd D 2 O ddition ws observed to compete with O-tom trnsfer with V + (33%), As + (90%), Nb + (5%), nd Mo + (10%).

9 Hevy H 2 O Rections with Ctions J. Phys. Chem. A, Vol. 111, No. 35, Figure 5 displys the dependence of the efficiency of O-tom trnsfer on the known O-tom ffinity of the metl ction. It is interesting to note in Figure 5 tht O-tom trnsfer from D 2 O to Mo + is slightly endothermic nd lmost thermoneutrl, H 298 ) 1.1 ( 0.6 kcl mol -1. Also, given tht O-tom trnsfer generlly predomintes when exothermic or nerly thermoneutrl, we cn ssign n upper limit to OA(Re + ), OA(Re + ) < kcl mol -1, since only D 2 O ddition ws observed with Re +. This limit improves the vlue of OA(Re + ) ) 115 ( 15 kcl mol -1 in Tble 1 to 109 ( 9 kcl mol Spin Control nd Rection Mechnism. Figure 5 shows tht the rections with D 2 O often proceed with efficiencies below 0.1 when O-tom trnsfer is exothermic, even below 0.01 for the fourth-row tomic ctions V + (0.0032) nd As + ( ). Low efficiencies for exothermic O-tom trnsfer re interesting nd, given our previous experience with O-tom trnsfer from N 2 O to tomic ctions, 4 re likely to spek to the role of electronic spin conservtion in the formtion of the monoxide ction. O-Atom trnsfer from D 2 O is not overly exothermic, by less thn 88 kcl mol -1, s compred to O-tom trnsfer from N 2 O tht cn be exothermic by s much s 168 kcl mol -1. We note, first of ll, tht the exothermic O-tom trnsfer rections were observed to be inefficient (<0.1) in our experiments with D 2 O, viz. the rections of Sc + (0.068), Ti + (0.018), V + (0.0032), nd As + ( ) in the fourth row, Nb + (0.013) nd Mo + (0.0090) in the fifth row, nd W + (0.0097) in the sixth row, where efficiencies for the O-tom trnsfer chnnel re given in prentheses. The one or two electronic sttes of these tomic ctions with the highest popultion within the ICP re ll highspin sttes: Sc + ( 3 D, 3 F), Ti + ( 4 F)), V( 5 D, 5 F), Nb + ( 5 D, 5 F), Mo + - ( 6 S, 6 D), nd W + ( 6 D). 4 Their rections with D 2 O by O-tom trnsfer to produce metl-oxide ctions in their ground sttes (ScO + ( 1 Σ), TiO + ( 2 ), VO + ( 3 Σ), NbO + ( 3 Σ), MoO + ( 4 Σ), nd WO + ( 4 Σ)) 4 re ll spin-forbidden (see rections 9-14 for which the mesured rection efficiencies nd rection enthlpies in kclories per mol re lso indicted). Sc + ( 3 D, 3 F) + D 2 O( 1 A 1 ) f ScO + ( 1 Σ) + D 2 ( 1 Σ + g ) (9) Ti + ( 4 F) + D 2 O( 1 A 1 ) f TiO + ( 2 ) + D 2 ( 1 Σ + g ) (10) V + ( 5 D, 5 F) + D 2 O( 1 A 1 ) f VO + ( 3 Σ) + D 2 ( 1 Σ + g ) (11) Nb + ( 5 D, 5 F) + D 2 O( 1 A 1 ) f NbO + ( 3 Σ) + D 2 ( 1 Σ + g ) (12) Mo + ( 6 S, 6 D) + D 2 O( 1 A 1 ) f MoO + ( 4 Σ) + D 2 ( 1 Σ + g ) (13) W + ( 6 D) + D 2 O( 1 A 1 ) f WO + ( 4 Σ) + D 2 ( 1 Σ + g ) (14) Apprently, the rtes of these rections re constrined by the chnge in spin required to proceed from the rectnt to the product potentil energy surfces, s hs been proposed previously by Armentrout et l. 10,18 for the rections of Sc +,Ti +, nd V +. The rection of D 2 O with As + my be specil cse mong the min-group tomic ctions in tht the ground-stte Figure 5. Dependence of the rection efficiency, k/k c, on the O-tom ffinity, OA, of the tomic ction M +. Mesured rection rte coefficient is represented by k, nd k c is the clculted collision rte coefficient. Rections on the right of the dshed line re exothermic for O-tom trnsfer, while those on the left re endothermic. Circles refer to fourthrow ctions, squres to fifth-row ctions, nd tringles to sixth-row ctions. Open symbols designte the observtion of D 2O ddition, while solid symbols designte the observtion of O-tom trnsfer. O-tom trnsfer rection 15 is exothermic, spin-forbidden nd inefficient. As + ( 3 P) + D 2 O( 1 A 1 ) f AsO + ( 1 Σ) + D 2 ( 1 Σ + g ) (15) The ppernce of smll mounts of AsO + t low flows of D 2 O recorded in Figure 2 my well be the result of the spin-llowed rection of excited As + ( 1 D), which hs been estimted to be formed in the Ar plsm t levels of c. 6% of the totl As + ion popultion. 4 Other rections tht re spin-forbidden for the formtion of ground-stte products pper to be sufficiently exothermic to chieve high rection efficiency with the formtion of spinllowed excited products. These include the rections with the high-spin sttes of Y + ( 3 D), Zr + ( 4 F), L + ( 3 F, 3 D), Hf + ( 2 D, 4 F), nd T + ( 5 F, 3 F), for which the monoxide ctions hve ground sttes of YO + ( 1 Σ), ZrO + ( 2 Σ), LO + ( 1 Σ), HfO + ( 2 Σ), nd TO + - ( 3 ). 4 However, collisionl de-excittion to the ground stte (e.g., Y + ( 1 S)) or low-lying stte (e.g., L + ( 1 D)) cn promote spin-llowed O-tom trnsfer nd so cnnot be ruled out. In the cse of Hf +, the ground 2 D stte is lredy sufficiently populted (48%) 11 within the ICP to possibly ccount for the observed O-tom trnsfer efficiency (0.36). The potentil-energy surfce for the exothermic but spinforbidden dehydrogention of D 2 O with ground-stte V + ( 5 D) hs been described previously by Armentrout et l. 18 nd Uglde et l. 26 The rection pth involves the initil formtion of the ion dipole complex V + OD 2, which then isomerizes by insertion to form D-V + -OD, evolves through four-centered trnsition stte into VO + D 2, nd then seprtes into the products VO + ( 3 Σ) + D 2. The overll trnsformtion involves crossing to low-spin triplet stte locted between the intermedite D-V +- OD nd the V + OD 2 complex nd lies bove the initil energy of the rectnts. Armentrout et l. 18 nd Uglde et l. 26 lso hve considered the mechnism for metl-oxide production in the rection of D 2 O with the ground sttes Sc + ( 3 D) nd Ti + ( 4 F). The mechnisms re qulittively similr to tht proposed for the nlogous

10 8570 J. Phys. Chem. A, Vol. 111, No. 35, 2007 Cheng et l. TABLE 6: Mesured Equilibrium Constnts (K) nd ( 1 D, 5d6s), which contributes 11%, nd L + ( 3 D), which Derived Stndrd Free Energy Chnges ( G (kcl mol -1 )) contributes 23% in the ICP plsm t 5500 K. 11 These two sttes for Sequentil D 2 O Addition to Metl Ctions t 0.35 ( 0.01 both hve s 1 orbitls vilble for bonding s is the cse of Sr Torr nd 295 ( 2K + nd B +. 4 K ( G ) The OD trnsfer chnnel ws esily observed by Armentrout M + C + V + Cr + M + (D 2O) g (e-6.7) g (e-6.6) g (e-6.1) M + (D 2O) (-8.6) M + (D 2O) 3 et l. 10,17,18,44 in rections of the fourth-row trnsition-metl ctions with D 2 O t the higher energies vilble in guided ion bem instrument. All the OD trnsfer rections were found to be endothermic t room temperture nd so re expected not Mn + g (e-5.8) to be observed in our ICP/SIFT instrument. Studies of the Fe + g (e-7.3) g (e-10.1) Co + g (e-6.8) g (e-9.5) (-8.0) rections of B +,Sr +, nd L + with D 2 O were not reported. Ni + g (e-7.6) g (e-10.0) (-9.8) 3.6. D 2 O Addition (Hydrtion). Addition of D 2 O ws Cu + g (e-7.6) observed exclusively in the rections of the fourth-row ctions Zn + g (e-7.0) C +,Mn +,Fe +,Co +,Ni +,Cu +,Zn +, nd Se +, the fifth-row As + g (e-8.3) Sr + g (e-5.9) ctions Rb +,Ru +,Rh +,Pd +, nd Ag +, nd the sixth-row ctions Mo + Ru + g (e-6.6) g (e-7.4) Cs +,Re +,Ir +,Pt +,Au +,Hg +, nd Bi +. Beside these, D 2 O ddition ws observed to compete with O-tom trnsfer in the Rh (-7.0) (-9.1) (-8.6) rctions with V +,As +,Nb +, nd Mo + nd to compete with OD Pd + g (e-6.2) g (e-8.2) group trnsfer in the rections with Sr +. All these ddition Ag + g (e-6.8) Cs (-6.2) rections re presumed to occur by the collisionl stbiliztion Ir + g-7.3 (e ) (-10.2) (-8.6) rection 16 t 0.35 Torr of He (however, pressure-dependent Pt + g (e-6.3) studies were not performed). Au + g (e-7.2) Hg + g (e-6.7) Bi + g (e-5.9) M + + D 2 O + He f M + D 2 O + He (16) rection with V + ( 5 D) nd lso involve crossings of different spin sttes. The exothermicities for oxide formtion re in the order Sc + > Ti + > V + s re the efficiencies observed by us. All three rections re spin-forbidden overll nd reltively inefficient, > > , for MO + formtion. VO + formtion is by fr the lest efficient of the three (6 less efficient thn TiO + formtion), is the only one of the three tht competes with D 2 O ddition, nd lso is the only one for which clcultions predict tht the spin crossing lies bove the energy of the high-spin regents. The potentil energy surfce computed for exothermic WO + formtion exhibits low-lying sextet/ qurtet crossing nd subsequent brrier on the qurtet surfce of only 5.1 kcl mol -1, nd this is consistent with the observed efficiency of OD Trnsfer. Primry OD trnsfer ws observed exclusively (100%) only in the rection of B + with D 2 O nd competed with O trnsfer in the rection with L + (20%) nd with D 2 O ddition in the rection with Sr + (50%). The DO-D bond energy of D 2 O is quite high, kcl mol Avilble OD ffinities for tomic metl ctions re listed in Tble 2; they re known for only 12 of the 46 tomic ctions tht were investigted. Of these, only B + ppers to hve n ffinity, D 0 (B + -OD) > D(DO-D), with D 0 (B + -OD) ) 127 ( 7 kcl mol -1. We did observe OD trnsfer in the rection of B + with D 2 O, but with low efficiency of bout , perhps due to very slight endothermicity. The hevy hydroxide ction lso ws observed s product in the rection with Sr + but in this cse in competition with dduct formtion. In this cse, however, we must ttribute the hevy hydroxide formtion to rection with excited stte Sr + ( 2 D, 4d) since D 0 (Sr + -OD) ) 105 ( 5 kcl mol -1 so tht OD trnsfer is endothermic by t lest 10 kcl mol -1 for the rection with ground-stte Sr +. The mximum excited-stte popultion of Sr + should be bout 10%, 11 nd this could esily ccount for the observed 50% occurrence of OD trnsfer. The OD ffinity of L + ppers not to be vilble but should be higher thn D(DO-D) for OD trnsfer to be exothermic with ground-stte L + ( 3 F). In this cse lso, we cnnot exclude the possible role of excited sttes such s L + - D 2 O ddition rections were observed to proceed with reltively smll effective bimoleculr rte coefficients, k e cm 3 molecule -1 s -1, presumbly due the short lifetime of the intermedite complex tht is expected from the wek bonding of the tomic ion with wter nd the smll degrees of freedom vilble for energy prtitioning. No products t ll were observed in the rections of K +,G +,Ge +,Cd +,In +,Sn +, Sb +,Te +,Os +,Tl +, nd Pb + with D 2 O. Two D 2 O molecules were observed to dd sequentilly to the first-row ctions C +,Fe +, nd Cu +, the second-row ctions Mo +,Ru +, nd Pd +, nd the third-row ctions Pt +,Au +,Hg +, nd Bi +.V +,Ni +, nd As + were observed to dd three D 2 O molecules. Co + nd Nb + were observed to dd four D 2 O molecules, nd only Ir + nd Rh + were observed to dd five D 2 O molecules in the D 2 O flow rnge tht ws investigted. In generl, the higher-order rte coefficients for ddition re higher thn the primry ones, nd this cn be ttributed to the incresed number of degrees of freedom in the higher-order rection intermedites, which leds to longer lifetimes nd so higher rtes of collisionl stbiliztion. Furthermore, the second wter molecule often binds s strongly s, or more strongly thn, 9,45,46 the first (see Tble 4), nd this is lso kineticlly fvorble for wter ddition since the lifetime of the intermedite lso increses with incresing binding energy. Equilibrium nlyses were performed on the kinetic results by plotting product/rectnt ion signl rtios s function of rectnt flow. Equilibrium is chieved when such plot chieves linerity, viz. when [M + (D 2 O) n ]/[M + (D 2 O) n-1 ] ) K eq [D 2 O]. The equilibrium nlyses show tht equilibrium ws not chieved with most ctions, nd so most of the reported vlues for K eq re lower limits. Clculted equilibrium constnts, K eq, nd the corresponding chnges in stndrd free energy, G, re presented in Tble 6. The pprent bimoleculr efficiencies for hydrtion re presented in Tble 7. According to the formul G ) H - T S nd tht S for ll the metl ctions clustering with D 2 O is nominlly 20 eu, 22,47 there is constnt difference of bout 6 kcl mol -1 between G nd H t 298 K. For certin ctions, we cn mke sure tht equilibrium hs been chieved, nd the vlues for G in Tble 6 cn gree with the vlues of H in Tble 4. For exmple, for Cs +, the

11 Hevy H 2 O Rections with Ctions J. Phys. Chem. A, Vol. 111, No. 35, TABLE 7: Mesured Rection Rte Coefficients (k (cm 3 molecule -1 s -1 )) nd Rection Efficiencies (k/k c ) for Sequentil D 2 O Addition to M + in Helium t 0.35 ( 0.01 Torr nd 295 ( 2K M + (D 2O) M + (D 2O) 2 M + (D 2O) 3 M + k k/k c k k/k c k k/k c C V Cr Mn + < < Fe Co Ni Cu Zn As Sr Mo Ru Rh Pd Ag Cs Ir Pt Au Hg Bi Effective bimoleculr rte coefficient with n estimted ccurcy of (50%. Figure 6. Vritions in the rection efficiency, k/k c (open circles), nd stndrd free energy chnge, G (solid circles), for single D 2O ddition to bre first-, second-, nd third-row tomic metl ctions t room temperture. vlue of H in our experiment is bout 12.1 kcl mol -1 bsed on S ) 19.4 eu, 22 which is in good greement with the vlue 12.9 kcl mol -1 in Tble 4. Figure 6 explores the periodic vrition in the vlues of the mesured efficiencies nd the stndrd binding free energies observed from our equilibrium mesurements (see Tble 6) for the ddition of D 2 OtoM + t 295 ( 2K. D 2 O ddition ws lso observed with mny of the MO +, MOD +,MO + 2, nd MO 2 D + ctions tht were formed in the rections of the erly trnsition-metl ions. Up to five D 2 O molecules were observed to dd sequentilly to MO + nd MO + 2, four to MO 2 D +, nd one with MOD + (see Tble 5) MO + 2 nd MO 2 D + Formtion. A second O-tom trnsfer, rection 4, ws observed with the erly trnsition-metl oxides NbO +, MoO +,WO +, nd TO +, nd OD group trnsfer, rection 5, ws observed with ZrO + nd HfO +. The rte coefficients mesured for these rections re summrized in Tble 8 long with clculted rection efficiencies. O-Atom trnsfer to TO + is the most efficient secondry O-tom trnsfer, k/k c ) 0.14, nd tht to NbO + is the lest, k/k c ) The OD group trnsfer rections with HfO + nd ZrO + re both quite efficient with k/k c equl to 0.31 nd 0.12, respectively. Avilble O-tom ffinities for tomic metl oxide ctions re listed in Tble 9. Of these, five (those for the erly trnsitionmetl oxide ctions NbO +, MoO +, TO +, nd WO + nd the lte trnsition--metl oxide ction IrO + ) hve OA(MO + ) > OA(D 2 ). We observed O-tom trnsfer with ll four of the erly trnsition-metl oxide ctions but could not investigte this rection with IrO +. This ltter ion ws not generted in the rection of Ir + with D 2 O since only in this interesting cse ws OA(M + ) < OA(D 2 ), while OA(MO + ) > OA(D 2 ). The structure of the MO + 2 ions formed in rection 4 is uncertin. Our multi-collision induced dissocition technique, 48 which involves simply rising the potentil of the smpling nose cone, did not yield bond clevge for MO + 2 nd so did not provide bond connectivity informtion. But, bsed on the elementl composition, three different bonding isomers re possible: the end-on metl dioxygen complex I, MOO +, the side-on complex II, c-mo + 2, nd the inserted metl dioxide complex III, OMO +. Severl of these isomers my even coexist for some of these metls. For exmple, structures I nd III were computed to coexist s well-seprted minim on different spin surfces of CrO A generl guide hs been provided by Schwrz et l., 50 who propose tht structure III is energeticlly