1048 CHINESE JOURNAL OF CHEMISTRY 2001, 19, 1048-1052

Study on New Typical Ionic Clusters K + ( KN03) and NO; ( mo3) m

SHI, Lei(5S) LIU, Zhi-Qiang(#f&%) SONG, Feng-Ri(%R%) LIU, Shu-Ying"($Jl$2) Changchm Institute of Applied Chmuitry, Chinese Academy of sciences, Changchun , ]din 130022 , Chinu

Tbe sample solution dKNQis ejected into the gas phase and the ionic dusters of K+(KNQ), and NQ'(KNQ), are formed and observed by electrospray ionization mass spec- trometry (DW). The full inas spectra of both the positive ion and the negative ion show that the differenax between each peak nearby are about 10l(m/z), which c c m q m d to themolee~lar weight OPKNQ. me general formula ofthe ionic dusters can be arsigWa 8 s K+(KNQ),, and N S - (KNW,.

Keywords mmv

Cluster, KNQ , electrospray ionization mess spec-

Introduction

It does not genedy occur for h e clusters of atoms with ionic bonds in nature except at the initial stages of crystal p w t h . ' ~n those cases, this type of clusters is rather unstable and easy to react further with other species ( ions or clusters) to form much more stable nanosized particles due to its unsaturation of the com- pounds with the ionic bonds. Of course, it is not &y to prepare such ionic clusters, and determine the structures and the physico-chemical propexties of the clusters with the ionic bonds. The matrix technique is first used to prepare the ionic clusters in various stages of wga-

tion, which can get the ionic clusters in term of monomer LiF, h e r (LiF)', polymer (LiF)k in LiF system' by argon matrix. The argon matrix serves as the solid-like solution to stop the w t i n g of LiF clusters. The ionic clusters h r n argon matrix possess the same number of Li atom and F atom and their structures can be determined by IR technique. But with secondary ion mass spec-

hpscopy ( SIMS)~ as the advanced technique, not ody the ionic clusters ( LiF) ' can be obtained, but also cluster ions with ionic bond such as Li ' ( LiF) and F- (LiF),. Compared with argon matrix technique, SIMS technique need extreme conditions of which a very high vacuum (1.33 x Pa) and an ion bombardment gun to sputtering the surface of the crystal. It is mainly ap- plied to determine the surface of the single crystal and will get the information of its surface structure.

Since Honda et ul . 4 investigated in detail the clus- ter ions by SIMS technique, many researchers have tried to find out the structures of the cluster ions and the mechanism of their formations. The ionic cluster in deed plays an important role in crystal growth, in catalysis, and in the understanding of amorphous structures. Fur- themore, the cluster is still thought to be a bridge be- tween the macrocosm and the microcosm. Up to now, the mechanism of the ionic cluster foxmation in the sput- tering process still r e ~ n s obscure. Some investigators think that the ionic cluster may be formed directly h m the surface of the samples by an argon ion bombard- ed.s*6 But much evidence shows that the formation of the ionic clusters belongs to be in gas-phase ionic resction in the initial bombardment.' At least, there is the extent of molecular rearrangement of the ionic cluster in gas- phase. Because the lifetime of the ionic clusters is very short by SIMS technique, the determination of their structures and physico-chemical properties of the ionic clusters will be very limited. When using an argon bom- barded such as CsF on crystal, not only (CsF)k ' , but also Cs' (CsF), and F( CsF), can be formed. But in crystals with complex anion like KClO,, only cluster ions

* E-mail: mslab@ciac.jl.cn Received April 9 , 2001 ; revised June 25, 2001 ; accepted July 31, 2001.

Vol. 19No. 11 2001 Chinese Journal of Chemistry 1049

such as Cs+ (CsCl),, and Cl-(CsCl),,, can be found. It is impossible to get such cluster ions as Cs ' ( CsC104) and C104-( CsC104) ,,,. ' he mass spectrum of the ionic clusters is extremely disorder due to the C104- break, which shows that there is a significant amount of molecu- lar m g e m e n t during this process. Similar results are also obtained for complex salts NaBF4 and KPF6.'

In this paper, we will in the first time report the applicability of the electrospray ion mass spectroscopy for determination of the ionic clusters of K' ( KNQ ) , and N&-(KNQ), in gas phase from the liquid sample.

Experimental

All the positive ion and the negative ion ESVMS spectra were acquired using a Finnigan MAT LLQTM ion trap mass spectrometer equiped with a Finnigan electro- spray source and capable of analyzing ions up to m / z 2ooO. 'Ihe concentrations of the sample range from 10-' to lo4 moVL. The samples were introduced via a sy- ringe pump at a flow rate of 0 .3 cJ-/min, ?he capillary voltage was set to 4.0 kV and the capillary temperature to 200T. Nitrogen was used as sheath gas at a pressure

o 70

20

20(

n=3 n=6 7

n=7

of 6.895 x 1 6 Pa.

Results and discussion

EIS spectra of KN4

"he full-scan mass spectrum of the positive ion of KNQ aqueous solution is shown in Fig. 1. It is found that there are many main peaks such as m / z 140, 241, 342, 443, etc . The differences between each neighbor- ing peak m / z are all about 101, which correspond to the molecular weight of KNQ. By comparing with the various m / z , it can be assigned that the peaks at m / z 140, 241, 342, 443, etc . are in correspondence with K' (ma), K'(KN@)z, K + (KNQ),, K' ( K N @ ) 4

etc . , respectively. 'Ihe results above can be expressed by a general formula K' ( KNa ), . In the meanwhile, several weak peaks can be observed beside the main peaks. By calculating the isotopic effects of 39K (93.08%) and " K (6 .91%), the distribution of the peak strength of each p u p is completely consistent with the calculated data. 'Ihe results further support that the assignments on K + ( KN& ) ,, species am likely correct.

11-10 n=l l n-8 n=9

947'4 1048.31149.1

6

646.5

I 646 648 650 I &6 6 8 650

n-13 n-14 13s3.0 1453.9

1000 1200 1400 1600 1800 2000 t d Z

Fig. 1 Electrospray positive ion mass spectrum of KA'Q.

1050 Ionic cluster SHI et al.

70 1 nr4

60 1

50 2

40-

30 :

466.0 E 9

m=5

In order to get more information on the ionic cluster of Kt ( KNQ ) ,, , multi-stage mass spectmmetry is em- ployed. 'The M V M S spectrum of Kt ( K N Q ) 6 ion at m/z 645 is shown in Fig. 2. 'The m / t 645 ion pro-

770.7

duces fragment ions at m / z 544, 443, 342, 241 (Fig. 2a) and fragment ion at m / z 544 also produces frag- ment ion at m / e 443, 342, 241 (Fig. 2b) . All the differences between the neighboring peaks m / z are sti l l about 101.

l00j

903 I

80 -1' u

0 .s 40

543.5 1

341.8

1003

90'

80 :

.- $ 40:

3 303 - .

20, 3404

101 240.8

0 200 250 300 350 400 I

d z d z

F&- 2 MWdS spec- ofthe m / z 645 ion (a) and MS3 spectrum of the m / z 544 ion ( b ) .

A series of NQ- ( KNQ ) ,,, are found in elecmpray negative ion mass spectrum ( Fig. 3 ) , which is very similar to the positive spectrum of KN4 aqueous solu-

tion. It should be pointed that there is almost no peak for the m < 4 in the negative ion mass spectrum.

m=9 97?8

' i

1000

l o o 1 90

n a

m-7 769.0

I

19

. . . . . . 1200 1400 I600 Ii00 2600

d z

Electnspray negative ion mass spectrum of KNO.,. Flg. 3

Vol. 19 No. 11 2001 Chinese Journal of Chemistry 1051

u 704 n=l 139.9

.- >

The peak strength of the cluster ions of KNQ is ob- viously reduced with the decrease of the concentration of KNQ . Just only several peaks at lower m / z of the ionic clusters can be found when the concentration of KNQ is decreased to 0.001 moYL. There is nearly any peak of the ionic clusters when the concentration of KNQ is de- creased to O.ooO1 moVL.

n=3 34 I .7

n=6 644.6

The ionic cluster of K' (KNQ) ,, in Fig. 1 should possess the same type of chemical bond as the classical ionic cluster compound Na' ( NaC1) ,, , but their struc- tures are completely different from those of Na' (NaCl),, due to the special structure of N Q - . It predicts the diffi- culty in deducing the structure of Kt (KN@),, due to its complexity in space structure.

It is found that there are several lower -ation

1 40: d

30:

203

to!

01 1

n=2 240.1 100

80

n=4 442.6 n=5

543.6 ,,=7 646.6 n=8

145:4 846,3n+, 947.1p10 ~ l l ,,=12 F13

11-16 n=17

: " . I

stage positive ions such as K' (KNQ) , K' ( K N 4 ) 2

and K' ( K N Q ) 3 , but there are not any lower W g a - tion stage negative ions such as NQ-( KNQ ) , N Q - - ( K N 4 ) z and N Q - ( K N Q ) 3 . With an increase of aggre-

gation stage n , the peak strength will be decreased. lo When the strength of some peaks is very stronger to the neighboring peak, its aggregation n is called the magic number. The existence of the magic number for ionic clusters hints that this ty-pe of cluster ions is not in straight linear structure. From Fig. 1, it can be seen that the peak strength of K'(KN03)3 and K' ( K N 0 3 ) 6

is the strongest in the series of K' (KNQ) ,, . When the energy of collision induced dissociation (CID) is in- creased to loo%, the distribution of the peak strength in Fig. 4 is similar to the result in Fig. 1. It shows that the magic number is 6 in this series of K' ( KNQ) ,, , which means that K' ( K N 0 3 ) 6 ( n = 6 ) possesses a special structure due to its lower energy.

In conclusion," the ionic clusters with ion bond can be obtained in mild condition by using ESIMS tech- nique, compared with SIMS and Matrix techniques. The ionic clusters from ESIMS technique process high stabili- ty, which provides a possibility to determine their physi- co-chemical properties. The principle of the electrospmy ionization is that, the sample solution first enters the ESI needle, to which a high voltage is applied, then the ESI needle sprays it into a fine mist of droplets which are

electrically charged at their surface. The density at the surface of the droplets increases as solvent evaporates from the droplets. So the droplets repeatedly divide into very small highly charged droplets, from which sample ions such as the culster ions K' (KNQ),, are ejected in- to the gas phase, then enter the MS detector to be ana- lyzed.

The results show that this sort of ionic clusters is very stable in gas phase and ionic are formed by an in-

1052 Ionic cluster SHI et al .

teraction between K+ and N Q - ions in liquid phase, but not in gas phase, which forms a series of positive ions K + (KN@),, and negative ions N@--(KN@),.

A series of ionic clusters with the complex anions is obtained by the advantage of ESIMS technique. ?he cluster ions with the complex anion possess the variety in the space structure compare with the typical ionic cluster Na+ ( NaCl) ". We can get further information on the structure of the cluster. It will be si&icant to find out the structure of ionic clusters and the mechanism of their fomtions due to it will contribute to illustrating the pm- cess of crystal nucleation and supplying some new evi- dence for the chemical bondmg theory.

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(ED104093 LI, L.T.; LING, J.)