YS+(1Σ+,3Φ)與COS氣相反應(yīng)YS++COS→+CO的理論研究

楊 樹 楊曉梅 謝小光

(1云南大學(xué)化學(xué)科學(xué)與工程學(xué)院,昆明650091;2昆明理工大學(xué)理學(xué)院,昆明650093;3云南中醫(yī)學(xué)院,昆明650091)

YS+(1Σ+,3Φ)與COS氣相反應(yīng)YS++COS→+CO的理論研究

楊 樹1,2楊曉梅3謝小光1,*

(1云南大學(xué)化學(xué)科學(xué)與工程學(xué)院,昆明650091;2昆明理工大學(xué)理學(xué)院,昆明650093;3云南中醫(yī)學(xué)院,昆明650091)

采用密度泛函理論B3LYP方法研究了硫化釔離子YS+(1Σ+,3Φ)與硫轉(zhuǎn)移試劑COS在氣相中的反應(yīng): YS++COS→.在單重基態(tài)和三重激發(fā)態(tài)勢能面上都找到了四條反應(yīng)通道.但是除一條反應(yīng)通道之外,其它的反應(yīng)機理和幾何結(jié)構(gòu)變化趨勢在不同的勢能面上有很大不同.實驗中生成所表現(xiàn)出的吸熱特征來自于在基態(tài)反應(yīng)中的三條通道(A,B和C),其活化勢壘分別為28.3、140.5和120.2 kJ·mol-1.計算結(jié)果表明硫轉(zhuǎn)移反應(yīng)沒有雙態(tài)反應(yīng)活性,因此產(chǎn)物在低能量區(qū)的放熱特征是由于基態(tài)反應(yīng)物中還混有殘留的激發(fā)態(tài)YS+.

硫化釔離子;COS;反應(yīng)機理;B3LYP

1 Introduction

Interest in transition-metal sulfides arises from their significance in industrial catalysis and biology.1In industry,transition-metal oxides are used as versatile catalysts in many applications,but their reactivity is too high for some processes,and non-specific product formation occurs.In contrast,transitionmetal sulfides are less reactive and less susceptible to poisoning and can show higher selectivity.2Transition-metal sulfides also play a particular role in biochemistry,and heterometallic sulfur complexes form the active sites in several metalloenzymes.Transition-metal-ion chemistry is an active area for both experimental and theoretical studies.3-18

In recent years,Schwarz?s and Armentrout?s research groups7-11have reported experimental studies of the thermochem-istry and reactivity of MS+(M=Sc,Ti,V,Y,Zr,Nb)in the gas phase using guided-ion-beam mass spectrometry and Fouriertransform ion-cyclotron-resonance mass spectrometry.For reactions of ground-state MS+with COS,three productsM+,and MO+,were observed in the energy-dependent crosssections.The reactions of these cationic transition-metal sulfides,MS+,have similar product cross-section patterns.Formations ofand M+are dominant at the lower and higher reaction energies,respectively,whereas the formation of MO+is less efficient across the whole energy range.The formation ofat lower energy was attributed to the S-transfer reaction: MS++COS→+CO,followed by loss of disulfur(S2),leading to the formation of M+;this is supported by the observation that the M+cross-section has its threshold in the region where the endothermic feature of thechannel starts to decline. Another common feature of these reactions is that there is an obviously exothermic feature of thecross-section at the lowest energy(before the endothermic feature appears,at about 1 eV),though this is less pronounced for M=Sc,Y.This implies that there is a barrier-less reaction path for the formation of,and the subsequent endothermic feature is attributed to the formation ofspecies with different geometric structures or different electronic states.7-11

Recently,we reported theoretical studies of gas-phase reactions of(M=Sc,Ti,V,Nb)with S-transfer reagents:MS++ COS→+CO.19-22Although the reactions of these cationic sulfides have similar patterns of product distributions,their reactivities and the relevant reaction mechanisms may be different as a result of their different valence-electron numbers and different electronic configurations.For example,a four-membered cyclic transition-state was found in the S-transfer reaction of ScS+with COS,16whereas no analogous transition state was found in the reaction of TiS+.20Theoretical studies of these reactions are necessary to confirm the relevant mechanisms, but no theoretical studies on the S-transfer reaction of YS+with COS have yet been reported.Thus,as a supplement to the experimental studies,we present here a theoretical study of the S-transfer reaction of the1Σ+ground-state and the3Φ excitedstate of YS+with COS,using density functional theory(DFT), in order to shed some light on this reaction.Possible reaction mechanisms are proposed and the structures and energetics of the stationary points involved in the potential-energy surfaces are examined and discussed.

2 Calculation methods

All the molecular geometries of the reactants,intermediates, transition states,and products were fully optimized using DFT in its B3LYP formulation.The standard 6-311+G*basis set was used for the non-metal atoms and we used the DZVP basis sets for Y atoms given by Goudbout et al.23Harmonic vibration frequencies were computed at the same level(B3LYP/6-311+ G*),both to characterize the stationary points and to estimate the zero-point vibration contributions to the relative energies. In order to evaluate the reliability of the chosen level of theory, we calculated the bond-dissociation energies of several species involved in this reaction at the B3LYP and CCSD(T)levels with 6-311+G*basis sets and TZVP basis sets.The calculation results and experimental values4,24are listed in Table 1.For the title reaction system,the energy obtained at the B3LYP/6-311+ G*level is considerable.The B3LYP method was chosen because of its reliability and efficiency as a practical tool in transition-metal chemistry.25Recent calculations19-21on transitionmetal compounds affirmed this choice.The intrinsic reaction coordinates(IRCs)were also determined at the B3LYP/6-311+ G*level to characterize the reaction path.Natural bonding orbital(NBO)26calculations were also performed to give additional insights into the bonding properties of the stationary points involved in the reactions.All the calculations were carried out using Gaussian 03 program.273 Results and discussion

In order to understand the reaction of YS+with COS,it is helpful to consider the bonding nature of the YS+molecule.28The1Σ+ground-state of YS+results from perfect pairing of Y+(3D)and S(3P),giving a 1σ22σ21π4valence configuration.The 1σ is a ligand-centered orbital mainly composed of S(3s),but the 3s orbital of the S atom is much lower in energy than the valence orbitals of the Y atom,so their interaction is very weak.The 2σ and 1π orbitals are both bonding.A triple bond (one σ and two π)is formed by two d electrons from Y+and four p electrons from the S atom.The3Φ excited-state of YS+has a 1σ22σ21π31δ1valence configuration and is higher in energy than the1Σ+ground state by 1.80 eV(171.7 kJ·mol-1)according to our calculations.

The fully optimized geometry parameters of the stationary points and a sketch of the potential energy surfaces of the ground-and excited-state surfaces are displayed in Figs.1-4. Table 2 lists the total energies,the relative energies,including zero-point energy,and the imaginary frequencies IMG of the transition states.

Table 1 Theoretical and experimental bond-dissociation energies(in eV)at 0 K

Table 2 Total energies(E)and relative energies(ΔE),including zero-point energies(ZPE)correction,at the B3LYP/DZVPlevel for the stationary points(reactants,intermediates,transition states,and products)involved in the reaction of YS+with COS, and the imaginary frequencies(νIMG)for the transition states

3.1 Reaction on the1Σ+ground-state surface

To describe the mechanism of formation of YS+2from YS++ COS→YS+2+CO on the1Σ+ground-state surface,we have identified three one-step reaction paths(paths A,B,and C)initiated from two cis-trans isomers of the collision complexes and another one-step reaction(path D)initiated from the head-tohead collision of the two S atoms.Paths A,B,and C proceed via a three-membered cyclic transition state(1TS1),a fourmembered cyclic transition state(1TS2),and an open transition state(1TS3),respectively,and yield the same cyclic product1YS+2(cyc).Path D proceeds via a linear transition-state(1TS4) and yields a linear product1YS+2(lin).

When the S atom of COS attacks1YS+,two cis-trans isomers of stabilized intermediates(1IM1 and1IM2)may be formed initially,and their binding energies are 63.5 and 71.9 kJ·mol-1,respectively.They are donor-acceptor complexes formed from the interaction between the lone-pair orbital of the S atom and the empty d orbital of the Y atom.According to the natural bond orbital(NBO)analysis,the donor-acceptor interactions essentially do not affect the original Y－S bond, which is still characteristic of a triple bond,in1IM1/1IM2.This agrees with the finding that the Y－S distances in1IM1 and1IM2(0.2265 nm and 0.2270 nm)are very close to that of the isolated1YS+molecule(0.2254 nm).The isomerization between1IM1 and1IM2 can be attributed to the change in the dihedral angle(θ(CSYS))from 0°to 180°with a transition state (1TSa)located at a dihedral angle of 97.9°(see Fig.1).The differences among the other geometric parameters of1IM1,1IM2, and1TSa are very small.IM1 is only higher in energy than IM2 by 8.4 kJ·mol-1.The isomerization barriers for both directions are 11.1 and 19.5 kJ·mol-1,respectively.

Fig.1 Optimized geometries of equilibrium and transition states at the B3LYP/6-311+G＊level on the singlet surfacebond lengths in nm and angles in degree

Paths A and B are initiated from the same trans isomer,1IM1,and proceed via a three-membered cyclic and a fourmembered cyclic transition states(1TS1 and1TS2),respectively,forming the same intermediate,IM3,which dissociates into the products(cyc)and CO,with a dissociation energy of 63.7 kJ·mol-1.In the three-centered transition state,1TS1,one of the two π bonds of the original Y－S bond has been broken and bonds between Y and the S(－C)atoms have simultaneously formed along with C－S bond breaking,and the two S atoms become close(leading to a σ single bond between the two S atoms formed in1IM3).Then,with the breaking of the C－S bond and formation of the S－S bond,the CO group transfers from the S atom to the Y atom and1IM3 is formed.In the fourcentered transition state,1TS2,as the two S atoms become close and the bond angle θ(OCS)bends,the C atom first interacts with the Y atom,and the C－S bond is slightly longer than that in IM1,by 0.123 nm.Then,with the formation of the S－S bond and strengthening of the interaction between Y and the S(－C)atom,the C－S bond breaks and1IM3 is formed.Paths A and B are similar to the analogous reaction channels of ScS+.19

Fig.2 Schematic diagram of the singlet potential-energy curves for the S-transfer reaction involved in the reaction of YS+with COSThe values are given by B3LYP/DZVP plus ZPE in kJ·mol-1.

Fig.3 Optimized geometries of equilibrium and transition states at the B3LYP/6-311+G＊level on the triplet surfacebond lengths in nm and angles in degree

Initiated from the cis isomer1IM2,and with bending of the bond angle θ(SYS)and lengthening of the S－C bond,path C proceeds via an open-structure transition state,1TS3.A similar process and analogous transition state were found in the reaction of ScS+.19As in the case of ScS+,an intermediate product was not found on the reaction path,and the IRC calculation shows that after overcoming the1TS3 transition state,the reaction directly yields the cyclic product(cyc).The calculated barriers of paths A,B,and C are 28.3,140.5,and 120.2 kJ· mol-1,respectively,at the B3LYP/6-311+G*level plus ZPE, relative to the reactants,which may account for the endothermic feature of thecross-section.

Instead of the S atom attacking the Y atom,path D is initiated by head-to-head collision of the two S atoms and proceeds via a linear transition state,1TS4,and yields the linear product(lin).The IRC calculation shows that1TS4 directly connects the separate reactants and products,and no intermediate was found on the reaction surface.The activation energy of1TS4 is 229.2 kJ·mol-1relative to the reactants,but the probability of end-on collisions is very small and this reaction channel makes no significant contribution to the formation of

The high-energy formation of Y+in the experiment can be attributed to simple collision-induced dissociation of,the product of the S-transfer reaction,with CO as the collision gas. This agrees with the observation that thecross-section decreases with elevation of the reaction energy,whereas the Y+cross-section increases,and the formation of Y+is dominant in the high-energy region.8The calculated reaction heat of yielding of3Y+is 398.1 kJ·mol-1(4.13 eV),in good agreement with the value of(421.64±12.54)kJ·mol-1((4.37±0.13)eV)obtained from experimental thermochemical data for bond-dissociation energies.8

3.2 Reaction on the3Φ excited-state surface

In order to clarify whether two-state reactivity exists in the title reaction,the S-transfer reaction of the3Φ excited-state of YS+was also investigated at the same theoretical level.Four parallel reaction pathways(paths A,B,C,and D)were also found on the triplet surface,and these are associated with two isomeric products,(v)and(cyc).Path A and path B yield the same product,(v),with a V-shaped structure. Path A proceeds via one four-centered transition state(3TS1), and is a one-step-mechanism reaction,whereas path B proceeds via two four-centered transition states(3TS2 and3TS3) with a four-membered cyclic intermediate(3IM4)located between them,and is a two-step-mechanism reaction.The other two reaction paths(C and D)proceed via a four-centered transition state(3TS4)and a three-membered cyclic transition state (3TS5),respectively,and yield the same cyclic product,(cyc).

Fig.4 Schematic diagram of the triplet potential-energy curves for the S-transfer reaction involved in the reaction of YS+with COSThe values are given by B3LYP/DZVP plus ZPE in kJ·mol-1.

Like the reaction on the singlet surface,when the S atom of COS attacks the3Φ excited-state of YS+,two stabilized cistrans isomers,3IM2 and3IM3,may be formed,with binding energies of 60.6 and 56.4 kJ·mol-1,respectively.In addition to the two cis-trans isomers3IM2 and3IM3,another complex,3IM1,has been found on the triplet surface,in which the S atom and the C atom of COS interact simultaneously with the Y atom of YS+.The strong interaction leads to bending of the θ(OCS)bond angle in3IM1 and a large binding energy of 94.1 kJ·mol-1.However,it should be noted that3IM1 can easily be formed from the isomerization of3IM3 via3TSa,with a small barrier of 0.7 kJ·mol-1.The reaction paths A and C are both initiated from3IM1 and yield the3YS+(v)and3YS+(cyc)products,respectively.However,the analogous collision complex was not found on the singlet ground-state surface.

The calculated barriers of the four paths,A,B,C,and D, are-72.3,-14.5,20.2,and 27.5 kJ mol-1,respectively,at the B3LYP/6-311+G*level plus ZPE relative to the excited-state reactants.Compared with the reaction barriers on the groundstate surface,these lower barriers imply that the3Φ excitedstate of YS+is much more reactive for the title reaction.However,by carefully inspecting the differences among the mechanisms on the singlet and triplet surfaces,the two-state reactivity of the title reaction can be excluded for the following reasons.The crucial conditions for efficient crossing-over are the existence of crossing-seam and the spin-orbital coupling.In the crossing-seam region,the geometrical structures of the reaction system in different spin states must be quite similar,and have the same or similar relative energies.Spin-orbital coupling is generally strong enough in transition-metal-containing systems.However,from Fig.1 and Fig.2,one can see that although path D on the excited-state surface and path A on the ground-state surface have similar mechanisms and similar trends in the changes in their geometrical parameters,the other reaction paths are quite different on the different surfaces.This means that only path A on the ground-state surface and path D on the excited-state surface have the desired geometrical conditions to form a crossing-seam.However,the energy conditions of the two reaction paths do not match,because the stationary points of path D on the excited-state surface are located at much higher energies than those of path A on the ground-state surface.So,according to our calculation,the title reaction has no two-state reactivity.

The exothermic feature of thecross-sections observed in the experiment means that at least one barrier-less reaction channel is present in the reactions.Theoretically,three cases can make this possible.One is that there really are one or more barrier-less reaction paths on the ground-state surface(as in the reaction of NbS+12).Second,two-state reactivity may provide a lower energy reaction channel,in which,generally,the transition state on the excited-state surface is located at a lower energy than that of its corresponding transition state on the groundstate surface and is also lower than the ground-state reactants, and then the reaction involves a spin inversion.The third case is that the ground-state reactant MS+is impure,mixing with minor excited-state species that react with COS with no barriers. According to our calculations,however,no barrier-less reaction channel has been found on the ground-state surface,despite the numerous strategies pursued and different methods employed,and the reactions have no two-state reactivity.We therefore suggest that the exothermic feature of thecrosssection observed in the experiment should be attributed to the residual excited-state of YS+mixing in the ground-state reactants,which react with COS with no barriers;for example, path A and path B on the excited-state surface proceed with no barriers to yield theproduct. 4 Conclusions

The title reaction was studied using DFT at the B3LYP/6-311+G*level.The following conclusions have been made.(1) Four parallel reaction paths(A,B,C,and D)have been identified for the S-transfer reactions of the1Σ+ground-state of YS+with COS;the reaction mechanism and reactivity are quite similar to the analogous reactions of the1Σ+ground-state of ScS+, the isovalent system.Paths A,B,and C,which have obvious barriers,make the main contributions to the endothermic feature of thecross-section observed in the experiment.(2) On the triplet excited-state surface,four parallel reaction paths have also been found,but the mechanisms and geometrical change trends are quite different from those of the reactions on the ground-state surface.The calculation results show that the3Φ excited state of YS+is more reactive for the title reaction. (3)Because no barrier-less reaction channel on the groundstate surface and no two-state reactivity have been identified, the exothermic feature of thecross-section is attributed to reactions of the residual excited state of YS+according to our calculations.

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February 7,2012;Revised:May 24,2012;Published on Web:May 24,2012.

Theoretical Study on Gas-Phase Reaction of YS+(1Σ+,3Φ)with COS of YS++COS→+CO

YANG Shu1,2YANG Xiao-Mei3XIE Xiao-Guang1,*
(1School of Chemical Science and Technology,Yunnan University,Kunming 650091,P.R.China;2Faculty of Science,Kunming University of Science and Technology,Kunming 650093,P.R.China;
3Yunnan University of Traditional Chinese Medicine,Kunming 650091,P.R.China)

The gas-phase reaction of YS+(1Σ+,3Φ)with an S-transfer reagent(COS),YS++COS→+ CO,was studied using density functional theory at the B3LYP/6-311+G＊level.Four parallel reaction pathways were identified on both the ground-and excited-state surfaces.The mechanisms and the geometrical change trends on the different surfaces are quite different,except in the case of one reaction channel.The experimentally observed endothermic feature of the formation ofcan be attributed to three reaction paths,A,B,and C,with calculation barriers of 28.3,140.5,and 120.2 kJ·mol-1,respectively, on the ground singlet surface.Our calculation results show that the title reaction has no two-state reactivity and the exothermic feature of thecross-section observed in the experiments is attributed to reaction of the residual excited-state of YS+in the reactants.

Yttrium sulfide cation;COS;Reaction mechanism;B3LYP

10.3866/PKU.WHXB201205241

?Corresponding author.Email:xgxie@ynu.edu.cn;Tel:+86-871-5033769.

The project was supported by the National Natural Science Foundation of China(30930074).

國家自然科學(xué)基金(30930074)資助項目

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