1776
J. Phys. Chem. 1995,99, 1776-1785
Structures, Energetics, and Reactions of Proton-Bound Hydrazine Clusters? Wan Yong Feng? Viktorya Aviyente? Tereza Varnali? and Chava Lifshitz*V*J Department of Physical Chemistry and The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel, and BogaziGi Universitesi, F. E. F. Kimya, Bebek 80815 Istanbul, Turkey Received: July 12, 1994; In Final Form: October 4, 1994@
Proton bound hydrazine containing clusters were studied experimentally by a temperature- and pressurevariable ion source in conjunction with tandem mass spectrometry and computationally by semiempirical (PM3 and AMI) and a b initio (6-31G) methods. The series (N2&),H+ demonstrated a magic number at n = 4, confirmed by the dependences of metastable fractions and average kinetic energy releases on cluster size. The suggested favored structure calculated for the tetramer has an NzH5' core ion and three hydrazine molecules bound to the protonated nitrogen atom, leaving two bare hydrogen atoms on the other nitrogen atom. Binding energies for solvent evaporation from (Nz&),H+ were deduced computationally and by fitting of kinetic energy release distributions and using thermal kinetics in small systems. A series of magic numbers n+m = 5 were observed for mass spectra of (NzH&(HzO),H+ ( n = 1-6, m = 1-4). Two collisionally activated dissociation channels were observed for these mixed clusters-water loss for n+m I5 and hydrazine loss for n+m 2 6-demonstrating a structural transformation at n+m = 5 . Additional cluster series (NzH&N&'+, (NzD~),NzD~H'+, and (NZD&NZD~*+ were observed, which can have or Nalo'+ (X = H or D) as core radical cations.
Introduction The gas phase ion chemistry of hydrazine is of interest since hydrazine and its derivatives are used as fuels in spacecrafts.l Protonated hydrazine, NzH5+, is known to be formed in ion/ molecule reactions' and upon ionization of neutral clusters of hydrazine.2 Its structure has been determined by ab initio methods3 Hydrazine itself has two hydrogen-bonding sites, and its neutral clusters tend to form cyclic configuration^?^^ The structures, energetics, and dynamics of proton-bound hydrazine clusters are unknown, to the best of our knowledge. Protonbound ammonia clusters have, on the other hand, been studied quite thoroughly in recent yeam6-18 The dissociation dynamics and multiphoton ionization mechanism of ammonia clusters has been worked 0 ~ t . ~Protonated ~ 3 ~ ~ammonia clusters (NH3),H+ are well established. In addition to (NH3),Hf, solvated NH5+ formed within ionized clusters of ammonia have been proposed14 and formulated a d 7 (NH3),Hz+. High-energy ion beam experiments furnish information about the structure and reaction dynamics of clusters. Unimolecular and collision-induced decompositions of proton-bound clusters such as (NH3),H+ have been reviewed recently." Reactions are characterized by evaporations of solvent molecule units. Cluster sequences demonstrate very often "magic numbers", i.e. ions of special abundance due to their unique stability. In (NH3),H+ n = 5 is a magic number due to closure of the first solvation shell. Magic number clusters such as (NH3)5H+ are characterized by maxima in kinetic energy releases (KERs) upon solvent evaporation, as a function of cluster ~ i z e . The ~ . ~binding energies to solvent monomer units can be calculated, for clusters of different sizes, from the experimental K E R S . ~Structures ~~~
* To whom correspondence should be addressed. 'This paper is dedicated to Professor William A. Chupka, who has greatly influenced the understanding of ionization and fragmentation of molecules and the general line of research of Chava Lifshitz. The Hebrew University of Jerusalem. BogaziGi Universitesi. II Archie and Marjorie Sherman Professor of Chemistry. @Abstractpublished in Advance ACS Abstracts, January 15, 1995. 0022-365419512099-1776$09.0010
of mixed proton-bound clusters may be deduced through KERs1l8l9and through collision-induced decompositions.1sz0,21 Various levels of theoretical calculations of structures and binding energies have been successfully applied to proton-bound clusters.22 Calculations performed by the AM 1 yielded a hydrogen-bonded cyclic structure for the acetic acid pentamer surrounding a central H30+. Similar structures were obtained for other (CH3COOH),(H20)H+ c1uste1-s.~~ Collisioninduced decomposition studies of mixed proton-bound formic acidwater and acetic acidwater clusters tend to verify these structure^.^^ Carboxylic acids, like hydrazine, have potentially two hydrogen-bonding sites per molecule. We have demonstrated recentlyz5 that proton-bound formic acid clusters demonstrate a unique dimer evaporation. The formic acid molecule demonstrates the dual alcoholketone character in its clustering reactions.26 In the present paper we report on the formation, structure, magic numbers, and unimolecular dissociation energetics of hydrazine and hydrazinelwater cluster ions. The results will be based upon experiments employing a high-pressure ion source and tandem mass spectrometry as well as semiempirical and ab initio calculations.
Experimental Section Measurements were performed on a high-resolution doublefocusing mass spectrometer of reversed geometry, the VG ZAB2F.27928Ions were formed by electron impact in a temperatureand pressure-variable ion drift s o ~ r c e The .~~ typical ~ ~ ~conditions for promoting cluster ion formation have been described p r e v i o u ~ l y .The ~ ~ ~typical ~ pressure and temperature employed in the present study were 0.05-0.11 Torr and 263 K. The metastable fragmentations were studied by mass-analyzed ion kinetic energy spectrometry (MIKES).30331An energy resolution EIAE of -4000 was employed. Metastable ion peak shapes were determined by scanning the electrostatic analyzer (ESA) and using single-ion counting. Ion counting was achieved by a combination of an electron multiplier, amplifierldiscriminator,
0 1995 American Chemical Society
J. Phys. Chem., Vol. 99,No. 6,1995 1777
Proton-Bound Hydrazine Clusters and multichannel analyzere6 The metastable ion peak shapes obtained were mean values of several hundred accumulated scans. This was done in a computer controlled experiment, monitoring the main beam scan and correcting for the drift of the main beam?* The kinetic energy spread in the parent ion beam was subtracted from the width recorded for each of the fragmentation processes. Kinetic energy release distributions (KERDs) were obtained from the f i s t derivatives of the metastable ion peak shape^?^-^^ Collisional activation (CA) spectra35were obtained using air as collision gas at a pressure of 2 x IO-' mbar, as measured by an ion gauge situated near the diffusion pump located between the electric sector and the gas cell; the actual gas pressure was higher by approximately a factor of IO3. A hydrazine/water sample (35% wt in water) was from Aldrich Chemical Co. Inc. NzD4-D20,98 atom % D, was from Cambridge Isotope Laboratories.
20
40
60
80
100
120
140
120
140
160
180
200
d Z
Computational Details Proton-bound hydrazine clusters have been analyzed using the same methodology as some of us used in previous similar theoretical workz2on other clusters. PM3 and AM1 calculations were performed on Nz&, (N2H4)H+, (NzH&H+, (N2H4)3H+, (NzH4)4H+, (NzH4)5Hf, and (NZ&)6H+ in the gas phase to provide information on their stabilities, equilibrium geometries, and structures; the MOPAC program36was used on a DEC-5500 machine for this purpose. All geometries were optimized by minimizing the total energy with respect to all geometric parameters using the standard BFGS (Broyden- Fletcher-Goldfarb-Shano) procedure with the criteria defined by the "PRECISE' option. In each case the positive force constants were characteristic of an equilibrium geometry. The ab initio calculations were carried out for N2& and several proton-bound clusters with the Gaussian 92 program3' running on a Dec/5000-133 workstation using the RHF/ 6-31G method. Polarization functions which may take into account the charge displacement in polar molecules have not been taken into account to diminish the computer time. The binding energies (A&), which are the reverse of stabilization energies for similar systems, are calculated using either the total energies (ET) or the heats of formation (A&) for each optimized structure according to the following equation (Hyd = hydrazine).
AET = ET[H+(HJ'~),-~I f &[(HJ'd)I
- ET[H+(HY~),I
20
40
60
80
100
160
180
200
d Z
Results and Discussion 1. Mass Spectra, Cluster Size Distributions, and Magic Numbers. Typical mass spectra obtained for the hydrazine/ water sample using the ion drift source at several temperatures are displayed in Figure 1. As the temperature is lowered, overall clustering, as well as incorporation of water molecules into the clusters, increases. The two major cluster series observed are (Nz&),H+, n = 1-8, and (N2H4),(H20),Hf, n = 1-6, m = 1-4. Minor series observed include (Nz&)~'+, n = 1-8; (N2&),(N&>'+, and (Nz&),(N&>'+. Independent experiments carried out on a SIFT (selected ion flow tube) in our laboratory have indicated formation of N5H4'+ ions in a complex iodmolecule reaction scheme from hydrazine. The identity of the (N2&),(N&,)'+ series was verified through deuterium labeling, by using a NzDJI20 sample. The intensity ratios of (M[(Nz&)nH+]+ l)/kf[(N~&)~H+] were higher than calculated 1 (where M stands for for 15N isotopic contributions to M mass). These ratios as well as the ones for (M[(N2D4),Df]+2)/ M[(N2D&D+] were temperature and pressure dependent. Par-
+
100
80
I riI
P 60
.3
2
Y
40
20
0
2o
40
60
80
LOO
120
140
160
180
200
d z Figure 1. Mass spectra of hydrazine cluster ions-as a function of temperature. The hydrazine sample contains water. The ion source pressure is 0.05 Torr throughout.
Feng et al.
1778 J. Phys. Chem., Vol. 99, No. 6, 1995 a
TABLE 1: Primary CAD Products of (N2H4)n(H20),H+ (W = water loss, -Hyd = hydrazine loss)
T=293K, P4.02torr
n ~~
1 (Nz&),(HzO)H+ -W (N2H4)n(Hz0)zH+ -W
2
-W (NzH&(Hz0)4H+ -W (N2H&(H20)3Hf
3
-W -W -W
-W -W -Hyd
-Hyd
-Hyd
4 -W
-Hyd -Hyd -Hyd
5
6
-Hyd -Hyd -Hyd -Hyd
-Hyd -Hyd -Hyd -Hyd
lb, Reaction l b is analogous to the well-known reaction of
T=293K, P=O.OStorr
ammonia cluster^,^^-^^ 35
36
31
38
39
40
m/Z
b
T=273K, P4.04torr
T=263K, P=O.OStorr
215
216
217
218
219
220
(NH3),'+
-
(NH,),-,H+ -t NH,'
(2)
Reaction l a is certainly thermochemically preferred over l b for n = 1, since IE(N2H3) < IE(H) and can be estimated to be preferred thermochemically also for the dimer and trimer. Beyond the trimer, it is shut off by solvent NzH4 evaporation. This result contradicts observations for electron impact ionization of (N2H4),.2 The species (NzD&(N2Ds)'+ and some of their H-containing isotopomers were studied by CAD. Solvent evaporation is the prevalent reaction for n > 1. For example, the ion mlz = 220 seen in the mass spectrum of Figure 2 loses N2D4 cleanly, as well as two, three, and four N2D4 molecules, consecutively. It can thus be N2D& solvated by five N2D4 units or N4Dlo" solvated by four N2D4 units. The ions (N~H~),(NzH~)'+ and their deuterated isotopomers may be hydrazine analogues of solvated NH5'+ and ND5'+ formed within ionized ammonia cluster^.^^*^^ We have observed not only the solvated but also the bare N2H6*+and N2D6" ions (see Figure 2a). The existence of N2H& has been reported previously for experiments on chemisorption and field ionization of hydrazine on F't ~urfaces.4~ It is possible that the solvated structure should be formulated as (N2H4),-1 (NJ-Ilo)'+, where NdIlo*+ is the analogue of the Rydberg radical cation N2Hs'+. 17,44
m/Z
Figure 2. (a) Partial mass spectra for an N2D4 sample; the N2Ds+/ N2D6'+ (m/z 38/40) range at different ion source pressures. (b) Partial mass spectra for an N2D4 sample: (top) deuteron-bound dimer region; m/z = 74, (NzD~)zD+; m/z = 76, NDlo'+; (bottom) deuteron-bound hexamer region; m/z = 218, (N2D4)&+; m/z = 220, (NzD4)s(NzDs)'+. The peak at m/z = 219 has contributions from an 15N-containing isotopomer of (NzD4)&+ and from an LH-containingisotopomer of (N2D4)dNzDs)".
L
H
H
H
H
J
Formation of hypervalent ammoniated radicals by neutralized ion beam techniques is ~ e l l - k n o w n .The ~ ~ (N2&),(N2H6>'+ or tial mass spectra are presented in Figure 2. The pressure (N~I&)~-~(N&IIo)'+ cluster series may belong to a class of hypervalent radical cations which may very well be Rydberg dependence for the N z D ~ * + / N ~ D ratio ~ + is demonstrated in Figure 2a. The ion m/z = 76 is identified as (NzD~)(N~D~).+, radical cations stabilized by solvation. Thus, while NzHg'+ may be viewed as the monoammoniated NJ&' radical,44 namely, and mlz = 220 is identified as (N2D&(NzD6*+) (Figure 2b). wf.NH4 the ', ion N&Ilo'+ may be viewed as NzH~+*N~H~'. Pronounced abundances, which in cluster terminology are soThe CAD spectra of N&'+ and NDlo*+ which we obtained called "magic numbers", were observed for (N2&)&I+ and for were rather weak and contained artifact peaks. N2D6" demthe n+m = 5 combination in the series (NzH&(HzO),H+. A onstrates D, 2D, and ND3 losses. N&o*+ demonstrates N2D5 pronounced peak for one of the minor series is (N2&)3(N5H$+ at mlz = 170. and ND3 losses but no N2D4 evaporation, in agreement with a 2. Unimolecular and Collisionally Activated Reactions. N ~ D ~ + * N Ztype D ~ 'structure. An alternative interpretation of this The MIKE spectra of the cluster series (N2H4),Hf demonstrate series involves the ammonia dimer as the core ion, either as such or in the NH4+.NH2' isomeric f ~ r m . ~ ~ , ~ ' evaporation of a single hydrazine unit. Collisionally activated dissociations (CAD) demonstrate consecutive N2H4 losses. The The primary CAD channels for (N2H&(H20),H+, as shown in Table 1, display a regular pattem, with N2& loss taking place MIKE and CAD spectra of the (NzH~),," series demonstrate for n f m 25 and H20 loss for n+m 5 5 . This result correlates H'loss for low n (n13)and N2H4 evaporation for high n. Thus, with the magic character of the n+m = 5 combination in the (N2I&),'+ (ns3)undergoes reaction l a in preference to reaction
J. Phys. Chem., Vol. 99, No. 6,1995 1779
Proton-Bound Hydrazine Clusters 1200
980 C
c (R
6 s n y)
c. C
-Y4
0
720
480
a
.I
2
0
8 c
0
240
3
b. T=293K
-50 5850
800
600
700
500
400
ESA (Voltage)
5880
5870
5880
5890
5900
Energy, eV Figure 4. Metastable ion peak shape for the indicated reaction. The reaction takes place in the second field-free region of the ZAB-2F mass spectrometer. The electrostatic analyzer voltage is scanned, and ion counts are accumulated on the multichannel analyzer. Ion counts are plotted as a function of ion energy (in the laboratory frame). The main beam [(N2&)&If] had an energy of -7800 eV. 50
Figure 3. CAD spectra of (Nz&)4(HzO)H+ at different (indicated)
drift ion source temperatures.
mass spectra. Evidence in favor of cyclic structures in protonated hydrogen-bonded complexes has been presented in the p a ~ t . ~ * - ~Whether O n+m = 5 combinations are cyclic or not has to await ab initio calculations, which we have not yet canied out. The mixed cluster (N2H&(H2O)H+ demonstrates different CAD spectra at low and high ion source temperatures, respectively (Figure 3). This is an indication for isomerization of the proton-bound cluster.51 The clean N2H4 loss at 343 K (Figure 3a) indicates a H30+ core ion solvated by N2H4. The H20 loss at low temperatures may indicate a N2H5+ core ion solvated by NzH4 and H2O or a cyclic structure from which H20 is preferentially lost by virtue of its lower proton affinity. 3. Kinetic Energy Releases. The metastable peak shapes were all pseudo-Gaussian. A typical example is shown in Figure 4 for the reaction
The kinetic energy release distributions (KERDs) obtained were Boltzmann-like, as is seen in Figure 5 for reaction 3. The average kinetic energy releases (KERs), ( E ) , deduced from the distributions are summarized in Table 2 and plotted as a function of cluster size for the series (N2H4),Hf in Figure 6. The maximum around n = 4 correlates rather nicely with the special abundance of (N2H4)4H+ in the mass spectra (Figure 1). The character of this ion must reflect some special stability, as has been observed6s7for (NH3)5H+. We will return to this point below. 4. Metastable Fractions. Decay fractions of metastable cluster ions give the metastable peak intensities, resulting from the unimolecular decompositions, relative to the parent ion cluster intensities. They usually demonstrate a monotonic increase with cluster ~ i z e . ~ This * ~ l is understood within the framework of the evaporative ensemble statistical model due to The evaporative ensemble model assumes that each cluster ion has suffered at least one evaporation before entering the field-free region of the mass spectrometer. The metastable
40
30
20
10
0.00
0.02
0.04
0.06
0.08
0.10
C.M.KINETIC ENERGY, eV Figure 5. Product kinetic energy release distribution for N2& loss from (N2H&H+. (0)experimental; (-) model fit.
rate coefficient window is k % 104-106 s-l. The slower the rise of the microcanonical rate coefficient k(E) with energy E is, the broader the internal energy range 6 E covered by the metastable window. The range of internal energies for each cluster in the ensemble is equal to the vaporization energy, AEvap.In the absence of magic character, AEvapis more or less independent of cluster size. The metastable fraction D = dEIAE,,, on the other hand, increases with increasing cluster size. While it is not at all clear that an evaporative ensemble is produced in the drift ion source employed to create the clusters, there is a clear break at n = 4,which is the magic number for this cluster series. The results for the metastable fractions are shown in Table 2 and plotted in Figure 7. 5. Computational Results. The 3-D drawings of the optimized clusters are given in structures I-XI11 of Figure 8. The bond lengths are given in Table 4. The total energies of the compounds are shown in Table 3. We present and discuss separately the results for the different structures in what follows: N2H4. Meier and Coussens optimized hydrazine with the 3-21G, 6-31G*, and DZP levels of ab initio53calculations. It is interesting to point out that the semiempirical methods locate a
1780 J. Phys. Chem., Vol. 99, No. 6, 1995
Feng et al.
TABLE 2: Kinetic Energy Releases, Binding Energies, and Metastable Fractions for Hydrazine Evaporation from (N2&).H+ n
(4,meV 1
F.K
eV AET,eV (6-31G)
D
2
3
4
7.3 f 0.1 0.57 43.2 1.4 1.23 9.1 x 10-5
17.5 f 0.3 0.57 130.4 0.78 0.82 2.1 x 10-3
18.8 f 0.3 0.57 141.5 0.62 0.82 (VII); 0.67 (VIII) 5.5 x 10-3
I
i Ii
1
7
16.1 f 0.0 0.60 114.0 0.38
15.8 f 0.0 0.62 112.6 0.34
15.8 f 0.6 0.60 118.1 0.33
1.64 x 10-2
3.77 x 10-2
5.7 x 10-2
TABLE 3: Ab Initio Calculated Total Energies (au) and PM3 and AM1 Calculated Heats of Formation (kcaymol) of N2& (I), (Nz&)nH+, and N2RP (XnI) n structure PM3 AM1 6-31G
20
i V
h
5
I
1 2 3 3 3 4 4 5 5 6 6
I
I
l o t
t I
5' 1
2
3
4
5
6
7
+
-
0.06
0.05
0.04
0 0.02
2
3
4
5
6
Iv V VI VI1 VIII
M X XI XII XI11
20.65 191.80 199.02 213.14 210.05 232.08 225.51 226.94 240.90 244.31 258.50 259.76 172.08
13.67 183.97 180.37
-111.123 8394 -111.472 5169 -222.641 671
180.77
-333.795 5083
194.50 183.37
-444.949 688 -444.944 078
193.09
- 112.067 8362
8
Cluster Size, n Figure 6. Average kinetic energy release ( E ) for (N&I&H+ (N2H&,H+ N2I& as a function of cluster size n.
1
I I1 111
7
0
Cluster Slze, n Figure 7. Metastable decay fraction of (NzI&)"H+ as a function of cluster size. global minimum for the C2h molecule, whereas with 6-31G we find the experimentally observed54C2 structure to be more stable by 6.24 kcdmol over the C2h point group. N2H5+. All three methods (AM1, PM3, and 6-31G) predict a regular N-H bond for the proton gained by N2H4, in agreement with Gill and Radom's3 optimized structure for the same species with the 6-3 lG* basis set; also the global minimum has the C, point group with both ab initio methods. The protonated site has a global charge (nitrogen plus hydrogens) of 0.748 units with 6-3 lG, and it is obvious that the hydrogens of the protonated site will be more readily attacked by the next hydrazine. ( N z H ~ ) ~ H +The . structure postulated has two hydrazines around a proton. The proton is closer to one hydrazine than to the other. The intermolecular H-N bond is within the limits
of bond lengths for hydrogen-bonded compound^.^^ We tied a six-membered cyclic structure for this species and started our optimization with it; the optimized structure turned out to be the open structure (111), where the distance between the end atoms is greater than 2 A. According to the 6-31G basis set, although the central hydrogen is more positive than the others, attack of this site will not be favored because of the steric hindrance of the neighbors. The next best place for the third hydrazine would be one of the hydrogens on the protonated fragment; this prediction is justified by the preference of the trimer V over N with PM3. (NzHd)sH+. Three different structures (IV, V, VI) have been optimized for the timer with PM3. Structure VI, in which a proton is symmetrically linked to three hydrazines, is energetically disfavored with respect to the others, in contrast with the theoretical findings on the protonated acetonitrile timer (CH3CN)3H+56 and on the protonted acetone trimer (CH3COCH3)3H+,22,57 in which all the ligands bind to the central proton. We were unable to find on the AM1 potential energy hypersurface an equilibrium structure for VI. Starting with the PM3-computed geometry of VI, the AM1 optimization led to the equilibrium geometry of V, which was also optimized with PM3 and 6-31G. Atomic charges generated with 6-31G are again used to predict the identity of the next isomer (tetramer in this case): In structure V among the H's which have only one bond the most positive one is the one with 0.448 unit of charge, and it is this one to which the next hydrazine will stick to form the most stable tetramer, namely, structure VII. (NZH4)@, ( N ~ H ~ ) J Hand + , (N2H4)ai. Due to the number of atoms present in these structures, only the tetramer has been run with ab initio methods. In AM1 the intermolecular N-H distances are longer than the classical lengths. As commented p r e v i o ~ s l y ,one ~ ~ critical parameter responsible for structural differences calculated by PM3 and AM1 may be the charge transfer from donor to acceptor molecules upon hydrogen bond formation. In the tetramer the computed charge transfer per
J. Phys. Chem., Vol. 99, No. 6,1995 1781
Proton-Bound Hydrazine Clusters
TABLE 4: Bond Lengths for the Compounds I-XI11 Calculated by PM3, AM1 (Values in Parentheses), and 6-31G (Values in Square Brackets) structure I I1 111 Iv V VI VII VIII structure IX X XI XI1 XIII Nl-N2 N2-H7 H7-N8 N8-N11 N1 -H7 N15-Nl7 H5-Nl5 N2-H5 H7-Nl7 Hl-H4 H4-N8
[1.396] (1.378) 1.440
[1.433] (1.390) 1.474
[1.430] (1.393) 1.477 [ 1.0571 (1.056) 1.047 [1.727] (1.908) 1.753 [ 1.4321 (1.366) 1.448
1.466
[1.440] (1.391) 1.473
1.772 1.450 1.039 1.445
[ 1.4271 (1.364) 1.444 [1.027] (1.034) 1.041 [1.427] (1.364) 1.443
(1.386) 1.471
(1.390) 1.466
(1.070) 1.038
(1.037) 1.039
(1.877) 1.785
(2.434) 1.783
(1.379) 1.442
(1.362) 1.451
(1.379) 1.450
1.770 1.042
1.467
1.462
1.467
N2-H7 H7-N8 N8-Nll Nl-H5 H5-N26 N26-N29
1.035 1.800 1.450 1.029 1.818 1.439
1.031 1.822 1.440 1.026 1.842 1.437
1.036 1.792 1.450 1.029 1.820
N2-H4 H4-N23 N23-N21
1.034 1.803 1.440
H3-Nl7 N15-Nl7 N2-H3 H18-N29
1.800 1.450 1.034
N8-Hl8 [1.870] (2.575) 1.775 [ 1.0251 ( 1.034) 1.042 [1.913] (2.425) 1.771
N3-N8 N3-HlO N5-Hl0 N2-N5 N3-H1
H3-Nl7
(1.877) 1.783 (1.363) 1.451 1.785 (2.696) (1.051) 1.038
N21-N23 H4-N23 N2-H3 N1-H5 H5-N20 N20-N23 H1 -N2 N2-N5 N2-H4
(1.362) 1.443
Nl-N2
1.620 1.454
hydrazine molecule is 0.089 with PM3 but only 0.006unit with AM1. Apparently AM1 cannot reproduce classic hydrogen bonds because of its deficiency to transfer charges. Two alternative structures, VII and VIII, respectively, were calculated for the tetramer. In the first, the core ion binds three hydrazine units on the protonated nitrogen atom, leaving two bare hydrogen atoms on the other nitrogen (structure VII). In the second two hydrazine molecules are attached to the protonated nitrogen of the core N2H5+, and the third is bound to the other nitrogen (structure VIII). Two alternative structures, IX and X, respectively, were calculated for the pentamer. In one of these, four hydrazine molecules enter into the first solvation shell of N2HSf, whereas in the other, three enter the Fist solvation shell, while the fourth adds into the second solvation shell, leaving two bare hydrogens of the N*H5+ core ion unattached. Similar calculations were carried out for the hexamer (structures XI and XII.) The structure calculated for N2H6" (structure XIn) is in agreement with expectations from the CAD spectra. Two calculations, at the 3-21Gand 6-31Glevels, respectively, were carried out. In the first all H atoms have 0.399 unit of charge and the N atoms have -0.697 unit, while in the second the H's
(1.678) 1.785
H1-N11 N11 -N4 N4-Hl2
(1.034) 1.038 (1.014) 1.033 (2.758) 1.797 (1.362) 1.449
H12-N23 N2-H4 Nl-H6 H6-N32 N32-N35 H32-H9 N8-H9 N26-N29
(1.362) 1.448 (1.362) 1.447
1.805 1.450
1.034 1.799 1.449
1.819 1.449 1.030
1.800 1.441 1.035
[2.170] 1.851
(2.397) 1.800 (1.015) 1.003 (1.389) 1.467 (1.034) 1.037 (1.034) 1.037 (1.364) 1.441 (1.056) 1.040 (1.868) 1.771 (1.364) 1.443 (1.014) 1.023 (2.697) 1.854 1.034 1.026 1.840 1.438
1.442 2.570 1.005 1.440
(1.030) 1.039
have 0.417 unit and the N atoms have -0.750 unit of charge. In 3-21Gall HNN angles are 104.23', while in 6-31Gall HNN angles are 103.06'. The rather long N-N distance (3-21G, 2.103A; 6-3lG,2.179 A) agrees with the idea of an H3"H3'+ type structure. The structure resembles that of the dication H3NNH32+ calculated by Gill and R a d ~ m with , ~ a somewhat shorter N-N distance, and may be viewed as the halfneutralized dication. Different dimer cation structures were calculated by Tomada and K i m ~ r a . ~ ~ . ~ ~ The suggested NdHlo'+ structure was optimized at the 6-31G level. However, the adjacent two N2H5 moieties were found to be 3.13 A apart. This ion seems very intriguing, and calculations on a higher level may be required. 6. Binding Energies. Binding energies were determined for the proton-bound clusters from the experimental KERDS and from the semiempirical and ab initio calculations. The results are included in Table 2 and Figure 9 and discussed below. The unimolecular decompositions of proton-bound hydrazine clusters may be viewed as evaporations from small particles. This process has been treated theoretically by It has been proposed that the average kinetic energy with which a monomeric unit leaves the surface of an aggregate can measure
Feng et al.
1782 J. Phys. Chem., Vol. 99, No. 6, 1995
9 0.431 -0.727
0.383y
0.481
/
-0.589
0370
0375
I1
I
b I11
0363
-0 .674 0.356
p VI
d VI1
VI11
J. Phys. Chem., Vol. 99, No. 6, 1995 1783
Proton-Bound Hydrazine Clusters
IX
X Q
Q
XI1
XI
XI11 Figure 8. 3-D drawings of the computed equilibrium geometries for the compounds I-XIII. Bold numbers give the calculated partial charges on the atoms(6-31G).
the temperature of the transition state, p. This idea was developed further by treating the full KERD. It allows one to extract the vaporization energies (Le. binding energies) of the clusters from the KERDs. In the model-free
approach the KERD is written in the form
P(E) 25 E1exp(-€/@)
0 II5 1
(4) where E is the kinetic energy, kB is Boltzmann’s constant, p is
Feng et al.
1784 J. Phys. Chem., Vol. 99, No. 6,1995
of the ammonia clusters there is a particularly sharp drop in the binding energy between n = 5 and 6, since the sixth ammonia molecule enters the second solvation sphere around Beyond n = 6 the binding energy slowly levels off to -200 meV. While we claim that n = 4 is a magic number for proton-bound hydrazine clusters, the drop in binding energy between n = 4 and 5 is not dramatic, but beyond n = 5 the binding energy does level off. Binding energies calculated for the isomeric pentamers IX and X using PM3 are 5.26 and 3.28 kcaymol, respectively. These calculations suggest that the fourth hydrazine molecule enters preferentially into the first solvation shell of N2H5+, but it can only do it by adding onto the nonprotonated nitrogen atom and is thus less strongly bound than the other three hydrazine molecules. This may be the reason for the "magic" character of n = 4. The result that (N2&)4H+ is a magic number is unexpected on the basis of results from the group of Garvey and cow o r k e r ~ who , ~ ~ reported a fully solvated protonated hydrazine cluster, i.e. ( N H ~ ) ~ ( N ~ H s While ) + . the mass spectra may in themselves not be proof that a partially solvated (N2&)3(NzH5)+ is particularly stable, the KERs which go through a maximum for this cluster and the binding energies calculated so far do point to the magic character of the tetramer quite convincingly. It would be interesting to carry out experiments for higher n values using a r e f l e c t r ~ n . ~On . ~ ~the theoretical side, while structures W-XII are minima on their respective hypersurfaces, further work with ab initio calculations would be necessary to compare theoretical binding energies with experiment.
m+.
s p
1.0 -
aa
C
W
13
3
0.0
1
2
3
4
5
6
7
8
Cluster Size, n Figure 9. Binding (evaporation) energy for (Nz€L+)"H+as a function of cluster size: (0)a b initio 6-31G computations; (0)values derived from experimental KERDs by the procedure by Mots.
the transition state temperature, and 1 is a parameter. The KERDs for all of the reactions studied could be fitted by expression 4. An example of the quality of the fit is shown in Figure 5. The parameters and 1were extracted from the fits, as previously explainedm and are included in Table 2. Once is extracted from the "3,Tb may be calculated from
Conclusion where Tb is the isokinetic temperature to which a heat bath must be set to yield a thermal rate constant k(Tb) equal to the microcanonical rate coefficient, k(E), characteristic for the cluster decomposition; y is the universal Gspann parameter, y = 23.5 & and C is the cluster heat capacity in units of kB minus 1. The cluster vaporization energy mvap is calculated from Trouton's ru1e,52,59-61
The values of hEvap extracted from the experimental KERDs are included in Table 2. Theoretical binding energies calculated by the 6-31G ab initio method are included in Table 2 for comparison. Experimental and calculated binding energies are plotted as a function of cluster size in Figure 9. The major uncertainty in the procedure for extracting binding energies from the KERDs lies in the value assigned to the heat capacity for the cluster of size n, C,,. In the calculations, a value C,, = 6(n - 1) (in units of k ~ was ) adopted following Castleman and cow o r k e r ~ .This ~ value takes into account only the intercluster modes. It gives C,, = 12 for the trimer. It could be checked, in view of the fact that the ab initio calculations provided vibrational frequencies for the trimer. A trial and error procedure involving eq 5 and statistical mechanics equations to derive the heat capacity from the cluster vibrational frequencies at Tb gave c,, = 13.5 for the trimer. Furthermore, c is c,, - 1. The latter value gave a binding energy of 0.78 eV. The PM3 and AM1 computational procedures gave lower binding energies for the trimer. A general decrease in the binding energy with increasing cluster size is observed (Table 2 and Figure 9), in agreement with observations for other proton-bound cluster systems, including the proton-bound ammonia cluster^?.^ 1,42,62 In the case
While neutral hydrazine clusters have been demonstrated2to form cyclic structures, proton-bound hydrazine clusters form open chain motifs. Apparently, the protonation site on one of the nitrogens controls the clustering process. The clusters studied resemble in many more ways ammonia clusters, rather than clusters having two hydrogen-bonding sites such as those of formic acid.25 By analogy with NH5'+(NH&, hypervalent radical cation clusters found in the ammonia system,14we have observed N2H6'+(&&),, clusters for hydrazine. These may possess an ammonia dimer core ion. CAD studies on mixed hydrazine/water proton-bound clusters demonstrate a structural change at n+m = 5. The ion (N2&)4H+ is a magic number in the series (Nz&),H+. This shows up as an abundance maximum in mass spectra, as a maximum in kinetic energy releases upon solvent evaporations for different cluster sizes n, and as a break in the plot of metastable fractions vs n.
Acknowledgment. This research was funded by the James Franck Research Center. The authors would like to thank M. and T. Peres for technical assistance. We also thank the BogaziGi University Research Funds for the support given to the project 94B0544. References and Notes (1) Gardner, J. A.; Dressler, R. A.; Salter, R. H.; Murad, E. J. Phys. Chem. 1992, 96, 4210. (2) Buck, U . ; Hobein, M.; Krohne, R.; Linnartz, H. Z. Phys. D 1991, 20, 181. (3) Gill, P. M. W.; Radom, L. J. Am. Chem. SOC.1989, I l l , 4613. (4) Buck, U.; Gu, X. J.; Hobein,M.; Lauenstein, Ch. Chem. Phys. Lett. 1989, 163, 455. ( 5 ) Buck, U.; Gu, X. J.; Hobein, M.; Lauenstein, Ch.; Rudolph, A. J. Chem. SOC.Faraday Trans. 1990, 86, 1923. (6) Lifshitz, C.; Louage, F. J. Phys. Chem. 1989, 93, 5633. (7) Lifshitz, C.; Louage, F. In?.J. Mass Spectrom. Ion Processes 1990, 101, 101. ( 8 ) Wei, S.; Tzeng, W. B.; Castleman, A. W., Jr. J. Chem. Phys. 1990, 92, 332.
Proton-Bound Hydrazine Clusters (9) Wei, S.; Tzeng, W. B.; Castleman, A. W., Jr. J. Chem. Phys. 1990, 93, 2506. (10) Wei, S.; Kilgore, K.; Tzeng, W. B.; Castleman, A. W., Jr. J. Phys. Chem. 1991, 95, 8306. (11) Lifshitz, C. In Current Topics in Ion Chemistry and Physics: Clusters: Baer, T., Ng.C. Y., Powis, I., Eds.; John Wiley & Sons: New York, 1993; pp 121-164. (12) Shinohara, H.; Nishi, N. Chem. Phys. Lett 1987, 141, 292. (13) Misaizu, F.; Houston, P. L.; Nishi, N.; Shinohara, H.; Kondow, T.; Kinoshita, M. J. Phys. Chem. 1989, 93, 7041. (14) Garvey, J. F.; Bemstein, R. B. Chem. Phys. Lett. 1988, 143, 13. (15) Coolbaugh, M. T.; Peifer, W. R.; Garvey,. J. F. Chem. Phys. Lett. 1989, 156, 19. (16) Peifer. W. R.: Coolbaueh. M. T.: Garvev. J. F. J. Phvs. Chem. 1989. 93,'4700. (17) Peifer, W. R.; Coolbuagh, M. T.; Garvey, J. F. J. Chem. Phys. 1989, 91, 6684. (18) Wei, S.; Purnell, J.; Buzza, S. A.; Stanley, R. J.; Castleman, A. W., Jr. J. Chem. Phys. 1992, 97, 9480. Wei, S.; Purnell, J.; Buzza, S. A.; Castleman, A. W., Jr. J. Chem. Phys. 1993, 99, 755. (19) Wei, S.; Tzeng, W. B.; Castleman, A. W., Jr. J. Phys. Chem. 1990, 94, 6927. (20) Aviyente, V.; Iraqi, M.; Peres, T.; Lifshitz, C. J. Am. SOC. Mass Spectrom. 1991, 8, 113. (21) Wei, S.; Tzeng, W. B.; Keesee, R. G.; Castleman, A. W., Jr. J. Am. Chem. Soc. 1991, 113, 1960. (22) Aviyente, V.; Vamali, T. J. Mol. Struct. (THEOCHEM) 1992,227, 285; Ibid. 1993, 299, 191; A Semi Empirical Study of Protonated Ammonia-Triethylamine Clusters. J. Mol. Struct., in press. (23) Tsuchiya, M.; Teshima, S.; Kaneko, T.; Harano, T. J. Chem. SOC. Jpn. 1993, 6, 687. (24) Teshima, S.; Kaneko, T.; Yokoyama, T.; Tsuchiya, M. 12th International Mass Spectrometry Conference, Amsterdam. (25) Feng, W. Y.; Lifshitz, C. J. Phys. Chem. 1994, 98, 6075. (26) Feng, W. Y.; Lifshitz, C. J . Phys. Chem. 1994, 98, 3658. (27) Morgan, R. P.; Beynon, J. H.; Bateman, R. H.; Green, B. N. Int. J. Mass Spectrom. Ion Phys. 1978, 28, 171. (28) Kirchner, N. J.; Bowers, M. T. J. Phys. Chem. 1987, 91, 2573. (29) van Koppen, P. A. M.; Kemper, P. R.; Illies, A. J.; Bowers, M. T. Int. J. Mass Spectrom. Ion Processes 1983, 54, 263. (30) Iraqi, M.; Lifshitz, C. Int. J. Mass Spectrom. Ion Processes 1989, 88, 45. (31) Cooks, R. G.; Beynon, J. H.; Caprioli, R. M.; Lester, G. R. Metastable Ions; Elsevier: Amsterdam, 1973. (32) Holmes, J. L.; Osbome, A. D. Int. J. Mass Spectrom. Ion Phys. 1977, 23, 89. (33) Lifshitz, C.; Tzidony, E. Int. J. Mass Spectrom. Ion Phys. 1981, 39, 181. (34) Jarrold, M. F.; Wagner-Redeker, W.; Illies, A. J.; Kirchner, N. J.; Bowers, M. T. Int. J. Mass Spectrom. Ion Processes 1984, 58, 63. (35) McLafferty, F.W.; TureEek, F.Interpretation of Mass Spectra, 4th ed.; University Science Books: Mill Valley, CA, 1993. (36) MOPAC, Stewart, J. J. P.; Qwntum Chemistry Program Exchange, No. 455, 1990. L
J. Phys. Chem., Vol. 99, No. 6,I995 1785 (37) Frish, M. J.; Trucks, G. W.; Head-Gordon, G. W. M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, F. S.; Gomperts, R.; Andrews, J. L.; Raghavachaxi, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. GAUSSIAN 92; Gaussian Inc.: Pittsburgh, PA, 1992. (38) Ceyer, S. T.; Tiedemann, P. W.; Mahan, B. H.; Lee, Y. T. J. Chem. Phys. 1979, 70, 14. (39) Futrell, J. H.; Stephan, K.; Mkk, T. D. J. Chem. Phys. 1982, 76, 5893. (40) Shinohara, H. J. Chem. Phys. 1983, 79, 1732. (41) Echt, 0.; Morgan, S.; Dao, P. D.; Stanley, R. J.; Castleman, A. W., Jr. Ber. Bunsen-Ges, Phys. Chem. 1984, 88, 217. (42) Castleman, A. W., Jr.; Wei, S. Ann. Rev. Phys. Chem. in press. (43) Block, J. Z. Phys. Chem. Neue Folge 1972, 82, 1. (44) Kassab, E.; Fouquet, J.; Evleth, E. M. Chem. Phys. Lett. 1988,153, 522. (45) Jeon, SA.;Raksit, A. B.; Gellene, G. I.; Porter, R. F. J. Am. Chem. Soc. 1985, 107, 4129. Gellene, G. I.; Porter, R. F. Int. J. Mass Spectrom. Ion Processes 1985, 64, 55. Griffiths, W. J.; Harris, F. M.; Beynon, J. H. Int. J. Mass Spectrom. Ion Processes 1987, 77,233. Hudgins, D. M.; Porter, R. F. Int. J. Mass Spectrom. Ion Processes 1994, 130, 49. (46) Tomoda, S . ; Kimura, K. Chem. Phys. Lett. 1985, 121, 159. (47) Tomada, S. Chem. Phys. 1986, 110, 431. (48) Wei, S.; Tzeng, W. B.; Castleman, A. W., Jr. Chem. Phys. Lett. 1991, 178,411. (49) Wei, S.; Tzeng, W. B.; Castleman, A. W., Jr. Z. Phys. D. 1991, 20, 47. (50) Daly, G. M.; Gao, J.; El-Shall, M. S . Chem. Phys. Lett. 1993,206, 500. (51) Lifshitz, C.; Iraqi, M. In The Structure of Small Molecules and Ions; Vager, Z . , Naaman, R., Eds.; Plenum: New York, 1988; pp 251260. (52) Klots, C. E. J. Chem. Phys. 1985, 83, 5854; Z.Phys. D. 1987, 5, 83; J. Phys. Chem. 1988, 92, 5864; Z. Phys. D. 1991, 20, 105. (53) Meier, R. J.; Coussens, B. J . Mol. Struct. (THEOCHEM) 1990, 209, 303. (54) Lipkowitz, K. B.; Boyd, D. B. Reviews in Computational Chemistry VCH Publishers, Inc.: New York, 1990. (55) Del Bene, J. E. J. Comput. Chem. 1987, 8, 674. (56) Deakyne, C.; Meot-Ner, M.; Campbell, C. L.; Hughes, M. G.; Murphy, S . P. J. Chem. Phys. 1986, 84, 4958. (57) Yamabe, S.; Minato, T.; Hirao, K. Can. J. Chem. 1983, 61, 2827. (58) Jurema, M. W.; Shields, G. C. J. Comput. Chem. 1993, 14, 89. (59) Klots, C. E. Z. Phys. D 1991, 21, 335; J. Chem. Phys. 1993, 98, 1110. (60) Lifshitz, C.; Sander, P.; Griitzmacher, H.-Fr.; Sun,J.; Weiske, T.; Schwarz, H. J. Phys. Chem. 1993, 97, 6592. (61) Klots, C. E. Int. J. Mass Spectrom. Ion Processes 1990, 100, 457. (62) Castleman, A. W., Jr. Int. J. Quantum Chem. 1991, 25, 527. JP94 1750W
