J. Phys. Chem. 1994, 98, 8295-8301
Gas-Phase Clustering Reactions of (NO-N20)-
02-,
8295
NO-, and 0- with NzO: Isomeric Structures for
Kenzo Hiraoka,' Susumu Fujimaki, and Kazuo Aruga Faculty of Engineering, Yamanashi University, Takeda-4, Kofu 400, Japan Shinichi Yamabe' Department of Chemistry, Nara University of Education, Takabatake-cho, Nara 630, Japan Received: February 16, 1994; In Final Form: June 8, 1994'
The gas-phase clustering reactions of 0 2 - , NO-, and 0- with N2O were studied with a pulsed electron beam high-pressure mass spectrometer. The formation of the covalent bond in the complexes (NO-Nz0)- and (0-N20)- is suggested. The covalent character suggested in (NO-Nz0)- is in a marked contrast to the results from Bowen et al., who observed the largely intact NO- core in NO- (NzO), using photoelectron spectroscopy. The presence of isomeric structures for (NO-Nz0)- was postulated by a b initio calculations. For the cluster ions, 02-(N20),, NO-(N20),, and 0-(N20),, the bond energies show an irregular decrease with n=2 3, 3 4, and 3 4, respectively. This falloff indicates that these cluster ions have the core plus and (O-NZO)-(NZO)~(NZO),~. surrounding molecules 02-(NzO)~(N20),2, (NO-NZO)-(N~O)Z(NZO),,-~, A b initio optimized geometries are discussed in relation to the measured binding energies.
- -
-
Introduction Electron attachment and the following processes in nitrous oxide have been extensively studied. Chantry studied the temperature dependenceof dissociative attachment in N20.l The electron attachment process peaked at 2.25 eV is relatively independent of the temperature, whereas the process characteristic of lower electron energies is highly dependent on the temperature. The strongly temperature sensitive portion of the cross section is due predominantly to excitation of the bending mode of vibration, This portion arises from the dependence of the separation in energy between the electronic ground states of N20 and NzO- on bond angles. Chantry2 and Paulson3 found that N2O- was formed by charge transfer between NO- and N20. Milligan and Jacox4 performed a matrix-isolation study of the interaction of electrons and alkali metal atoms with N20. They assigned the absorption at 1205cm-1 to ~4(bl)of a planar N=NO2anion. This anion is formed by the attack of 0- at the central nitrogen atom of N20. Fehsenfeld and Ferguson studied the reactions of atmospheric negative ions 0-, 0 2 - , 03-, CO3-, and N02- with NzOS. They concluded that atmospheric negative ions do not react efficiently with N20 and hence are neither a significant sink for N2O nor a significant source for NO except for 0- (measured rate constant is 2.2 x 10-10 cm3 s-1 molecule-1 for 0- N 2 0 ) . Smit and Field pointed out the analytical importance of N 2 0 as a reagent gas for the negative chemical ionization mass spectrometry.6 They found that the OH- ions can be readily formed from electron bombardment of N20 and H-containing gases. Knapp et al.' measured the mass spectra of negatively charged N20 clusters produced by electron impact or by electron attachment. They found the magic numbers of n = 1 and 6 for O-(NzO),. Yamamoto et al.8 ionized the neutral clusters of N20 formed in a supersonic nozzle expansion by impact of high-Rydberg krypton atoms. They also observed the magic number of n = 6 for O-(NzO),. Hayakawa et al.9 studied the negative ion formation by thermal electron attachment to N2O at atmospheric pressure. They found the existence of apparent activation energy of 0.21 f 0.04 eV for the following processes: NzO = NzO* (vibrationally excited in bending mode), eth + N20* = N2*-,N20*- N 2 0 = N202-+ N2 (eth: thermal electron).
+
+
Abstract published in Advance ACS Abstracts, July 15, 1994.
N2O- and NO2 are isoelectronic, and N2O- may be expected to be bent by -134O. The energy required to bend the NNO angle of neutral N20 from 180 to 134O has been estimated to be about 1 eV.lo This results in a substantial activation energy to three-body electron attachment, in agreement with the results obtained by Chantryl and H a y a k a ~ a . ~While substantial information is available about the structure and bonding of N20-,5J1-13 little has been known about the nature of the bonding interaction in the dimer anion (N20)2-. Bowen et al. studied the negative photoelectron spectroscopy of NzO- and (N20)2-.14 The (N20)~-spectrum can be interpreted as arising from the photodetachment of an ionic species. This species is best described as a bent N20- solvated by a neutral linear N2O molecule, and the ~ 0 . eV 2 shift between the N2O- and the (N20)2-spectra is viewed as a rough measure of the dissociation energy of N2O-...N20. Coe et al. measured the photoelectron spectra of the gas-phase negative cluster ions NO-(NzO) and NO-(N20)2, using 2.540 eV photons.15 Both spectra exhibit structured photoelectron spectral patterns. They strongly resemble that of free NO- but are shifted to successively lower electron kinetic energies with their individual peaks broadened. Each of these spectra is interpreted in terms of a largely intact NO- core ion solvated and stabilized by NzO. For both NO-(NzO) and NO-(N20)2, the ion-solvent dissociation energies for the loss of single N20 solvent moleculesweredeterminedto be -0.2eV. Rindenetal.16revealed that NO- shows a rich and varied chemistry in addition to the expected electron-transfer reactions. They pointed out that the reactions fall into four main classes: electron transfer, dissociative electron transfer and/or displacement, collisional detachment, and clustering. They observed the cluster ion formation between NO- and a number of the reactant neutrals which possess permanent dipole moments. Morris et al.1' found that, in the reaction of NO- with N20, N20- and NOz- are produced very slowly with rate constants of the order of 10-14 cm3 s-1 molecule-1. The barrier for the reaction was considered to be due to the geometry changes between products and reactants. In contrast, they observed that the isotope exchange reaction 14NO- + 14Nl5NO "NO- + 14Nl4NO proceeds much faster with the rate constant of 1.25 X 10-11 cm3 s-1 molecule-' at 298 K. Paulson3 studied reaction 1 using 1 8 0 and 14N15NO isotopic labels and found nearly complete scrambling in the products.
-
-
0022-365419412098-8295$04.50/0 0 1994 American Chemical Society
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The Journal of Physical Chemistry, Vol. 98, No. 34, 1994
This result led him to suggest the existence of a long-lived [N202]reaction complex.
0-
+ N,O
.-+NO-
+NO
(1)
van Doren et a1.18 have performed a doubly labeled experiment and found that the oxygen exchange reaction 2 occurs at 16% of the ADO (averaged dipole orientation) rate.
-
I8O-+ N2160
N2180
+
l60-
16'-
(2) I"
40
These authors proposed that reaction 1 occurs through an intermediate trans-ONNO- complex in which oxygen atoms are symmetrical to explain facile 0- exchange and Paulson's observation3 of isotope scrambling in the NO- N O product channels. Posey and Johnsonl9 measured the photoelectron spectra of negatively charged ions N202- produced in an electron beam ionized free jet expansion of pure N20. They found that themost likely description of N202-isa chemically bound species. This species is possibly similar in nature to the isoelectronic C03-, i.e., the addition of 0- to the central N atom of N 2 0 to form N=NO2-. Barlow and Bierbaum20 performed the totally labeled experiments of reaction 1 using the tandem flowing afterglowselected ion flow tube, In reactions of 160- 15Nl4Nl60 and 16014N15N160,both of the possible NO- products are formed with equal probability. This result indicates that equilibration is achieved within the reaction complex before dissociation. In the totally labeled reactions of I*O- + I5N14Nl6O and I8O- + 14NI5Nl60,all possible NO- products are observed, which supports the formation of both trigonal and linear N202- intermediates along the reaction path. Morriset al.21 determined the branching ratios and rate constants for reactionsof W-and 1 8 0 - with isotopelabeled N2O. They found that the reaction of '80-with 14N1SN160 produces all four possible NO- isotopic products and the branching ratio depends on temperature. The production of significant quantities of 14N16O- and 15NI8O- suggests the attack by IsO- at the central nitrogen atom in nitrous oxide. By irradiation of argon matrix containing N 2 0 and alkalimetal atoms, Milligan and Jacox obtained evidence for the formation of the N=N02- anion with two inequivalent N atoms and two equivalent oxygen atoms4 Hacaloglu et al. used the chemical ionization discharge of N2O for matrix infrared spectroscopic study of isolated anions.2z They identified two isomers N N 0 2 - and (NO)2- by isotopic substitution. The yield ofNNOZ-was much higher than that of They concluded that the anion "02is the chemically bound species N=NOzwith two equivalent oxygen atoms, in agreement with the result obtained by Milligan and J a ~ o x For . ~ the minor product (NO)z-, it was suggested that the anion electron density is dispersed uniformly over the and that the N-N bond is weak like the neutral dimer (NO)2. The formation of is likely to be due to the 0- attack on the terminal N atom of the N N O molecule. No evidence for photoisomerism between the "02and (NO)2- structural isomers was found. So far, the thermochemical stabilities of the negative cluster ions of N2O have not been measured except for our recent results of the X-(N20), for X = F, C1, Br, and 1-23 In the present study, clustering reactions 3-5 of 01,NO-, and 0- ions with N2O are measured.
+
+
+
4 5
5 0
55
60
6 5
1000/ T ( K )
-
Figure 1. van't Hoff plots for the clustering reaction 02-(N20).+1+ N20 = 02-(N,O),. The equilibrium constants below 147 K could not be measured due to the charging of the cold ion source. Integer numbers 1-6 are values of n.
molecule reactions in NzO. The equilibria of the clustering reactions are measured, and the structures of cluster ions are determined by ab initio MO calculations. Experimental and Computational Methods The experiments used a pulsed electron beam high-pressure mass spectrometer. The general experimental procedures were similar to those described in our previous papera23The major gas NzO was purified by passing it through a dry-ice-acetone-cooled 3A molecular sieve trap. For the measurement of equilibria of reaction 3, 50 mTorr of 0 2 was introduced into the 0.5-3 Torr of N 2 0 major gas through a flow-controlling stainless steel capillary. Under these experimental conditions, the cluster ions 02-(N20), become the major product ions. For the observation of the equilibria of reactions 4 and 5, the reagent N2O gas was passed through glass tubing packed with GASCLEAN (Nikka Seiko Ltd.) at atmospheric pressure in order to eliminate 0 2 in the reagent gas. The reagent gas in the ion source was ionized by a pulsed 2 keV electron beam. The ions produced were sampled through a slit made of razor blades (1 0 Fm x 1 mm) and were mass-analyzed with a quadrupole mass spectrometer (ULVAC, MSQ-400). When the N2O reagent gas was introduced into the ion source, a serious negative charging of the wall of the ion source was observed. This charging became more serious with a decrease of ion source temperature. The charging resulted in the drastic decreaseof ion intensities, and thedeterminationof theequilibrium constants was not possible in the low-temperature region. In our previous we found that the charging effect could be greatly reduced by coating a thin film of colloidal graphite (aquadag) on the surface of the ion source. All the experimental data presented here were obtained using the graphite-coated ion source. Geometries of Oz-(NzO), and O-(NzO), (n = 1 and 2) were fully optimized with the spin-restricted open-shell wave function, ROHF/6-31+G. Those of NO-(N20), ( n = 1-4) were determined with the RHF/6-31+G method. "6-31+Gn is the 6-31G basis set augmented by diffuse sp orbitals. These orbitals were needed so as to describe properly the electronic structure of anion ~pecies.2~ No assumption on geometry optimization was made. Theoretical binding energies were estimated by the second-order Moeller-Plesset perturbation scheme with the 6-3 1+G* basis set on RHF(or ROHF)/6-31+G geometries (MP2/6-31+G*// R(O)HF/6-31+G). All the calculations were carried out using GAUSSIAN 9226 installed at the CONVEX (2-220 computer.
-
Experimental Results
The main objective of the present study is to obtain the more detailed information on the cluster ions by observing the ion-
02-(NzO), System. The results for the experimentally measured equilibrium constants for reaction 3 are displayed in the van't Hoff plots in Figure 1. Due to the charging of the ion source, the ion intensities became too weak to measure below 147 K (1000/T 2 6.8). Thus, the measurements of the equilibrium
Gas-Phase Clustering Reactions of 02-, NO-, and 0-
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8297
TABLE 1: Experimental Thermochemical Data, A l P , , + , (kcal/mol) and ASO,1, (cal/mol K) (Standard State, 1 atm), for Gas-Phase Clustering Reactions 0 2 - (N20),1 + N2O = 0 2 - (NzO), NO- (NZO),+~ + NzO = NO- (NzO). and 0(NzO),+i + N20 = 0- (Nz0)t
1 8.8 f 0.2 2 8.7 f 0.2
27 f 2 27 f 2
3 4 5 6 7
24f 2 24 f 2 24 f 2 24f 3
6.4f0.2 5.7 f 0 . 2 5.3 f 0.2 5.0 f 0.3
[28.9] 5.6 f 0 . 2 P.91 5.1 f 0.2 4.5 f 0.2 4.5 f 0.2 4.4 f 0.2 4.2f 0.3
22f 2
5.4f0.2
19f 2
21 f 2 19f 2 20 f 2 21 f 2 23 f 3
5.4f 0.2 5.2f0.2 5.2 f 0.2 5.1 f 0.2 5.1 f0.2
21 f 2 24f 2 25 f 2 26 f 2 28 f 2
Theoretical binding energies in square brackets are evaluated with MP2/6-3 1 +G*//R(O)HF/6-3 1 +G.
I
IO',
IO3
5.5
50
6.5
60
7.0
7.5
1000/ T ( K )
Figure 3. van't Hoff plots for the clustering reaction NO-(N~O),+I,~ + NzO = NO-(N20),. The lowest temperature measured is just above the condensation point of the Nz0 reagent gas.
io',
I
1 4.5
50
6.0
5.5
6.5
70
7.5
1000 / T ( K )
Figure 4. van't Hoff plots for the clustering reaction O-(N20)n-l,,,+ N20 = O-(NzO),. The lowest temperature measured is just above the condensation point of the N20 reagent gas.
"
1
2
3
4 5 n
6
7
Figure 2. Plots of experimental -AHo,,+ (kcal/mol) for clustering reactions OZ-(NZO),+I+ NzO = 02-(N20)n, O z - ( C 0 ~ ) ~+1 COz = Oz-(COz)n,and 0 z - ( W n - i + Oz = OZ-(Odn.
constants below this temperature could not be performed. In Table 1, the enthalpy and entropy changes obtained from Figure 1 are summarized. In Figure 2, the enthalpy changes for reaction 3 are shown as a function of n together with those of reactions 6 and 7 obtained in our previous ~ o r k . ~ ~ J ~
0;(02)n-I
those of reactions 6 and 7 suggests that the bonding nature in 02-(N20),, with n L 1 is mainly electrostatic. The nearly equal bondenergies for-AHo,lJ withn = 1 and2 suggest thesymmetric structure for 02-(N20)2 in which the 0 2 - ion is sandwiched by two N20 molecules. Except for n = 1, the -AHo,l,,, values for reaction 3 are larger than those for reaction 6 . This is due to the charge delocalization (covalent bond formation) in 02-.-C02 which results in the weaker electrostatic interaction in the subsequent clustering reactions. NO-(N20). and 0-(N20). Systems. When the pure N20 is ionized by a 2 keV electron beam, the major negative ions produced are 0-(N20),, with n L 0, NO-(N20), with n 1 0, NzO-, and NO2- ions. The cluster ions (NzO),- with n 1 2 could not be detected. At all temperatures measured, the temporal profiles of ions 0-,NO-, and N20- showed sudden decreases after the electron pulse. This is due to the rapid consumption of these ions by the following r e a c t i o n ~ . 9 * ~ ~ J ~ e-(hot)
+ 0, = Oc(O,Jn
(7)
The bond energy of 02--C02 (19.0 kcal/m01)~~ is much greater than that of 02---N20 (8.8 kcal/mol) in Figure 2. This is somewhat surprising, considering that CO2 and N2O are isoelectronic molecules. For reaction 6 , irregular decreases in the values of -AHowl,n were observed at n = 1 and 3. This indicates that the bond with covalent character is formed through thecharge transfer (CT) 0 2 COZand the C T complex of 02COOaccommodates two C02 molecules preferably, i.e., 02COO-(CO2)2. For 02-(02),,,z8 a similar trend in the dependence of -AHo,,+, on n as for 02-(C02),,is observed in Figure 2. This also indicates that the covalent bond is formed in the dimer anion ( 0 2 ) 2 - , and this accommodates two 0 2 ligands more preferentially. In contrast, the -AHo,,+,values for reaction 3 with n = 1 and 2 are nearly equal and show an irregular decrease with n = 2 3, i.e., the core formation with n = 2 in the cluster, 02-(N20)2(N20),2. The relatively small value of for reaction 3 compared to
-
-
+ N,O
-
0-
+ N2
+ N,O N20*N20*- + N 2 0 N20; + N2 0- + 2N20 N20; + N,O e-th
-
0-+ N,O -,NO-
-
NO- + 2 N 2 0
N,O;
+ NO
+ N20
(8)
(9) (10) (1 1) (1)
(12)
Due to the rapid disappearance of 0-and NO- ions, the equilibria of reactions 4 and 5 with n = 1 could not be observed. This is the reason why the van't Hoff plots in Figures 3 and 4 start with n = 2. In addition to the rapid decrease of the N2O- ion, no cluster ions (N20), with n 1 2 could be detected. Thus, it was
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Hiraoka et al.
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994
lo3,
I
I
0
05
10
TIME
15
20
25
(ms)
Figure 5. Temporal profiles of NO-(N20)1and NO-(N20)2in 2.16Torr of N2O. Ion source temperature = 164.1K; energy of incident electrons = 2 keV; electron pulse on at 0.15 ms and off at 0.4ms; repetition rate of electron pulse = 400 Hz;integration time for NO-(NzO)l 40 s and for NO-(N20)2 120 s.
not possible to measure the equilibria of N2O- with N2O molecules. In this experiment, it was not clear whether reaction 12 is reversible. Even if this is the case, the equilibrium between NOand N302- may not be observed because the reaction of NO- with N20 proceeds mainly by electron detachment at higher temperatwe31 due to the very small electron affinity of NO (0.024 eV) .32 As described in the Introduction, Coe et al.14 found that the NO-(N20), produced from the electron impact of the neutral cluster (NzO),,has the structure of the core NO- ion solvated with N2O solvent molecules; Le., the extra-negative charge is mainly localized in NO-, and the interaction in the cluster NO-(N20),,is largely electrostatic. From the spectral shifts of the photoelectron spectra, they estimated that the dissociation energy for NO-(N20)1 into NO- and N2O is 5.1 kcal/mol, and the dissociation energy for NO-(N20)2breaking into NO-(N~O)I and N2O is -6.0 kcal/mol. In Figure 3, the van't Hoff plots for reaction 4 are displayed. The enthalpy and entropy changes for this reaction are summarized in Table 1. In the present experimental conditions, the NO-(N~O)I ion was observed as one of the major ions at room temperature. The stable existence of this ion suggests that the NO-(N2O)l ion has the bond energy of at least 10 kcal/mol because the clustering reactions with the enthalpy changes of -AHo L 10 kcal/mol are almost always observed near and above room temperature. If the NO--N20 is bound with -5.1 kcal/ mol, this ion could be only observed below -220 K. Hayakawa et al.9 found that the ion NO-(NzO)1 starts to be observed above room temperature in atmospheric N20, and the intensity of the NO-(N20)1 ion increases with a rise of temperature. They observed the NO-(N20)1 ion up to 500 K! This also suggests that the bond energy of NO--N20 is well over 10 kcal/mol. Because the bond energy of NO-(N20)2 is determined to be 5.6 kcal/mol (Table l ) , there seems to be a large gap in the bond dissociation energies between NO-.-N2O and NO-(N20).-N20. This strongly suggests that a covalent bond is formed in the complex [NO-N20]-. In this respect, the present complex should be represented more appropriately as N3O2- rather than NO-(N@) I . This finding is different from the solid photoelectron evidence obtained by Coe et al.15 and strongly suggests the existence of a second isomeric form, one with a covalent bond. Because the N2O molecule has two positive sites N6+N6+Ob, the nucleophilic attack to N20 by NO- may result in the formation of two structural isomers. We think that what we observed in this experiment is the more stable isomer and what Coe et al. detected is the less stable one. Exactly the same situation can be conceivable for the reaction of 0- with N20 (see the latter section). If the NO-(N20), clusters with n 1 2 are also composed with several isomers with different thermochemical stabilities and the less stable isomer isomerizes to the more stable one, some kinetic behavior may be reflected on the temporal profiles of the cluster ions. Figure 5 shows the temporal profiles of NO-(N20)l and
-
?
1 0 '
'
"
"
1
2
4
3
5
"
6
7
"
0
n
Figure 6. Plots of experimental -AHo,+lsl (kcal/mol) for clustering reactions 0-(N20),+1 + N20 = 0-(N20)n, 0-(C02),,4 + C02 = 0-(C02),, and NO-(N20),( + N2O = NO-(N20),,.
NO-(N20)2obtained at 164.1 K. As far as the temporal profiles are concerned, the apparent equilibrium was established almost right after the electron pulse. It is not clear to what extent the observed NO-(NzO),,ions are contaminated with the less stable cluster ions. In our previous work,'3 anomalous (nonlinear) van't Hoff plots were obtained for the clustering reaction (CO),+1+ CO = (CO),+due to the existence of isomeric cluster ions. In the present experiment, no noticeable anomalous van't Hoff plots are observed, as shown in Figures 3 and 4. This suggests that the contamination of the observed cluster ions with less stable isomeric cluster ions is minor for n 2 2 clusters in the temperature range measured. The covalent bond formation in N3O2- results in the dispersion of the negativechargein thecomplex. This explains the relatively small and nearly n-independent -AHo,,+ values with n 1 2 of NO-(N20),,, as shown in Figure 6 . A small irregular decrease in - W , + l , , , at n = 3 suggests that the core ion N3O2- has two nucleophilic sites which accomniodate two N20 ligands more favorably. As described in the Introduction, it is generally accepted that the 0- ion reacts with N2O to form mainly N=N02- in which two 0 atoms are equivalent. A small but discernible irregular decrease in -AHoPl,,, at n = 3 indicates the formation of the solvation shell structure N202-(N20)2(N20),+3;i.e., the equivalent two 0 atoms in N202- accommodate two N20 molecules preferably. In our previous it was pointed out that the C03- ion has also two equivalent 0 atoms, like the N202- ion. The slow decrease of the bond energies of N202-(N20),+lindicates that the interaction in the cluster is largely electrostatic in nature.
+
Computational Results 02-(Nz0),,. In the previous section, it has been shown that 0 2 is bound to N20 weakly and that 02-(N20)2 is a core in the cluster. Figure 7 shows the geometry of N2O and 02-(N20),, (n = 1 and 2) together with that of O ~ - ( C O ~ )02-(N20)1 I.~~ is calculated to be a planar species with two long O.-N (2.698 and 2.733 A) intermolecular bonds. The covalent bond is not formed, and the 02--N20 attacking force is clearly electrostatic. This weak bonding (-AH0o.l = 8.8 kcal/mol in Table 1) is in sharp contrast with the tight bonding ( - W o . l = 19.0 kcal/molt7) due to the-C covalent-bond formation in02-(C02)1. Twonitrogen atoms in N20 interact almost equivalently to one oxygen atom in 0 2 - electrostatically. The electrostatic vs covalent contrast is reflected in the bond direction, almost along the 0 2 - axis in 02-(N20)1vs along the~p~orbitalextensionofO~-(CO2)~. Thus, the n = 2 core formation of 02-(N20),,is understandable by the almost C2h symmetry of 02-(N20)2 in the bottom of Figure 7. O-(NzO),,. Figure 8 exhibits geometries of O-(N20), (n = 1 and 2). As stated in the Introduction, two geometric isomers of NzOz- (Le., n = 1) are obtained by the symmetry-unrestricted optimization. N=NO2- with the C, symmetry is 4.4 kcal/mo134 more stable than the trans-ONNO- complex. The former anion is computed to have vas = 1165.2 cm-1 with ROHF/6-3t+G,35
Gas-Phase Clustering Reactions of 0,;NO-, and 0
(0.701
(0.40)
rhe Journal of Physical Chemistry. Vol. 98, No. 34. 1994 8299
(4.50) (ani
..
AE
-+
w
4.41 kcallmole
145V)
Figures. Geometries ofO-(NzO). (n = 1 and 2). Pointgroupsattached to them are assigned according to the result of geometry optimizations. AE for n = 1 is the relative energy (positive, less stable) for geometric isomers. vu in the Cb n = I is a calculated harmonic frequency
corresponding to the in-plane antisymmetric stretching vibration mode. Eyre 7. Geometries of NzO, Oz-(NzO). (n = 1 and 2) and O Z - ( C O ~ ) ~ ~ ~ optimized with the ROHF/6-31+G method. Both OZ-(NZO)~ and
W
Oi(C031arecomputcdtobeplanar. Numbersin parenthsesareRHF/
(-0.32)
6-31G Mulliken electronic net charges (pmitivc, cationic). The (to distance of the free 0 2 - is computed to bc 1.342 A. which is comparable to the 1205 cm-l reported by Milligan and Jacox.' It is natural that the C, form is better than the Czr one, becausetheLUMOshapeofNz0 hasalargerlobeon thecentral nitrogen atom than that on the terminal one.
AE
-
n =1 + 24.10 keal/mole
lw.4
g? ! '.
14.31
0
w-type
CZh
,P \ )-
AE=O ion-dipole Complex ( t r i p 1et) in-plane charge-donating FMO k t h e next HOMO) in n 4
0
In view of the largely anionic electronic distribution of two oxygen atoms (-0.517) in the C, n = 1, the cluser is thought to have a symmetric structure, N=N02--Nz0. In fact, t h e n = 2 cluster is found to have a long-range forked bond with the C, symmetry. As in Oz-(NzO)I, clearly the N=NO2--NzO bond arises from the electrostaticattraction. Since the intermolecular bond is quite long, thedirectionality of the coordination is almost negligible. In fact, thedecru\seof-AW,l,withnforO-(NzO). is slowest among the three clusters in Table 1. NW(NP), NO- is isoelectronicwith 0 2 and thereforea triplet (biradical) species. If NO- is bound to N20 weakly, the triplet spin is retained in N O ( N z O ) I . If NO- is linked with N20 with a covalent bond, a singlet NO-(NZO)~ will be formed. Figure 9 presents two isomers, a triplet ion-dipole complex and a singlet w-typecovalentoneforNO-(NzO)I.In theion-dipolecomplex, the triplet species is calculated to be 26.7 kcal/mol more stable than the corresponding closed-shell singlet species. The binding energy for NO- (triplet)..N,O is calculated to be 4.8 kcal/mol with MP2/6-31+G8//ROH€/6-31+G. This value is close to the -5.1 kcal/molsuggested byCoeet aI.l4. Thus,theyobserved
Figure 9. Geometries of NO- and NO-(NZO)Itriplet spsCies optimized
with ROHF/6-31+G. That of NO-(NzO), of w-type is obtained with RHF/6-3l+G. At the bottom of the figure, a contour density map of the in-plane next HOMO of the w-type NO-(NzO)l is drawn. HOMO is a = orbital (out-of-plane) which is not concerned with the NzO
coordination. The next HOMO of NO-(NzO), predicts three forked coordination sites, i-iii.
the ion-dipolecomplexin the photoelectronspectra. On theother hand, we have observed the covalent-bonded species, w-type, as NO-(NzO)l. The triplet geometry-optimized w-type species is 58.3 kcal/mol less stable than the singlet w-type species. The energy stabilization by the cluster formation [NO- (triplet) + NzO NO-(NzO)I (singlet w-type)] is computed to be 28.9
-
Hiraoka et al.
8300 The Journal of Physical Chemistry, Vol. 98, No. 34, 1994
n =2
n=
AE=O (iii)
i
( i ) + (ii)'
W
AE = + 0.23 kcal/mole Figure 10. Two geometric isomers of NO-(N20)2. Notations i and iii are those shown at thecontour density of the next HOMO of NO-(N20)1 in Figure 9. AE is the energy difference relative to that of the best geometry (AE positive, less stable).
kcal/mol. The w-typegeometry has been found to be most stable after the stabilities of the following three isomers are compared.
(i) + iii) Figure 11. Three geometric isomers of NO-(N20)3.
N I /N\N
o\N/N\N/O
, / " \ O
I
1.097A
cis
trans
w-type
(worst)
(middle)
(best)
Iless
steric crowding
=>
The NO-.-N20 covalent bond arises from three canonical resonance structures. BO
0
N
Z
n =4 + (ii) + (iii)
Figure 12. A fully optimized geometry of NO-(NzO)3 involving three
0
forked-bond coordinations. antiparallel with each other. There is a larger energy difference, 2.3 kcal/mol between [i + ii] and [i iii], in Figure 11 than the 0.2 kcal/mol between i and ii in Figure 10. Probably, the ligandligand attraction would be operative for the additional stability of [i + ii] relative to that of [i + iii]. Thus, the [i ii] n = 3 can be a core in the cluster. The core formation has been discussed in terms of the falloff of -AHon-l,n, 5.1 f 0.2 (n = 3) 4.5 f 0.2 kcal/mol (n = 4) in Table 1. Figure 12 shows the fully optimized geometry of NO-(N20)4. The frontier-orbitalcontrolled coordination, [i + ii + iii] overcoming the electrostatic cationicxationic repulsion in Figure 9, is exactly obtained.
+
"-08
O
(i)
The net charges in parentheses of the w-type in Figure 9 reflect the resonance. The central nitrogen atom (N2) of the w-type is negatively charged (-0.31, anionic). It is expected that a forked bond is formed at n = 1 2. That is, the central N atom (+0.40, cationic in the top of Figure 7) of the ligand N20 molecule is bound by both anionic N2 and 0 4 (or 05,-0.54) atoms in the bifurcated mode. This possibility is also shown by arrows i and ii in the density plots of Figure 9 for the charge-transfer (CT) interaction. It is noteworthy that arrow iii may lead to the formation of another forked bond in spite of the fact that N1 and N3are positively charged (+0.20, cationic). That is, the direction iii suffers from the electrostatic Nd+-.Na+ repulsion but leads to the CT stabilization. Figure 10 shows two isomers of NO-(N20)2. These forked geometries are what have been predicted by arrows i and iii in Figure 9. The geometry i is computed to be of the binding energy 5.9 kcal/mol, which is in good agreement with - A H o 1 , 2 = 5.6 0.2 kcal/mol measured here. The geometry iii is 0.2 kcal/mol less stable than i. Figure 11 exhibits three isomers of NO-(N20)3. The "i" + "ii" geometry is most favorable, where two N20 axes are almost
-
*
+
-
Concluding Remarks
In this work, gas-phase clustering reactions of N20 toward three ions, 0 2 - , NO-, and 0-, have been studied. Generally, weakly bound clusters with ca. 5 kcal/mol binding energies have been detected. The characteristic bonding nature is found for each anion series. For 0 ~ - ( N 2 0 ) ~the , n = 2 cluster is an electrostatically bound shell with the 8.7kcal/mol energy. For 0-(N20),, the Ca symmetry covalently bound core ion N=N02is formed. In the core ion, the negative charge is delocalized, leading to the subsequent (n 2 2) small binding energy. For NO-(N20)., the covalent w-type NO-(N20)1 is postulated. This
Gas-Phase Clustering Reactions of 02-,NO-, and 0-
is in marked contrast to the results by Bowen et al., who observed thelargely intact NO-in N0-(N20),. Theexperiments by Bowen et al. were performed under very cold conditions and over very short time scales (Le., supersonic expansions into high vacuum), while our experiments weredone under relatively warm conditions, with many collisions, and over a much longer time scale. It is reasonable to suppose that with plenty of time and relatively energetic collisions, the electrostatic triplet complex N C ( N 2 0 ) 1 has undergone an intracluster ion-molecule reaction to form the covalent N302- molecular anion. It would be interesting to see if such a species would give a photoelectron spectrum revealing the higher electron affinity due to covalent bond formation.
The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8301 (15) Coe,J.V.;Snodgrass,J.T.;Freidhoff,C.B.;McHugh,K.M.;Bowen,
K. H. J. Chem. Phys. 1987,87, 4302.
(16) Rinden, E.; Matti Maricq, M.; Grabowski, J. J. J. Am. Chem. SOC. 1989, 111, 1203. (17) Morris, R. A.; Viggiano, A. A.; Paulson, J. F. J . Chem. Phys. 1990, 92, 2342. (18) van Doren, J. M.; Barlow, S.E.; Depuy, C. H.; Bierbaum, V. M. J . Am. Chem. SOC.1987,109, 4412. (19) Posey, L. A.; Johnson, M. A. J. Chem. Phys. 1988,88, 5383. (20) Barlow, S.E.; Bierbaum, V. M. J . Chem. Phys. 1990, 92, 3442. (21) Morris, R. A.; Viggiano, A. A.; Paulson, J. F. J . Chem. Phys. 1990, 92, 3448. (22) Hacaloglu, J.; Suzer, S.;Andrews, L. J . Phys. Chem. 1990,94,1759. (23) Hiraoka, K.; Aruga, K.; Fujimaki, S.;Yamabe, S.J . Am. SOC.Mass Spectrom. 1993, 4, 58. (24) Hiraoka, K.; Mori, T. J . Chem. Phys. 1989,90, 7143. (25) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. Acknowledgment. The authors thank the Information ProJ. Comput. Chem. 1983, 4,294. cessing Center of Nara University of Education for the allotment (26) Gaussian 92, Revision C. Frisch. M. J.; Trucks,G. M.; Head-Gordon, of CPU time on the CONVEX C-220computer. The present M.; Gill, P.M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, work is supported in part by a Grant-in-Aid for scientific research H.B.;Robb,M.A.;Replogle,E.S.;Gomperts,R.;Andres, J.L.;Raghavachari, K.; Binkley, J. S.;Gonzalez, C.; Martin, R. L.; Fox, D. J.; DeFrees, D. J.; on priority area "Theory of Chemical Reactions" from the Baker, J.; Stewart, J. J. P.; Pople, J. Gaussian, Inc.: Pittsburgh, PA, 1992. Ministry of Education, Science and Culture, Japanese Govern(27) Hiraoka, K.; Yamabe, S . J . Chem. Phys. 1992, 97, 643. ment. and the Morino Foundation for Molecular Science. (28) Hiraoka, K. J . Chem. Phys. 1288,89, 3190. (29) Shimamori, H.; Fessenden, R. W. J . Chem. Phys. 1978, 68, 2757. References and Notes (30) Parkes, D. A. J . Chem. SOC.Faraday Trans. I 1972, 2103. (31) McFarland, M.; Dunkin, D. B.; Fehsenfeld, F. C.; Schmeltekopf, A. (1) Chantry, P. J. J . Chem. Phys. 1969, 51, 3369. L.; Ferguson, E. E. J . Chem. Phys. 1972, 56, 2358. (2) Chantry, P. J. J . Chem. Phys. 1971, 55, 2746. (32) Siegel, M. W.; Celotta, R. J.; Hall, J. L.; Levine, J.; Bennet, R. A. (3) Paulson, J. F. J . Chem. Phys. 1970, 52, 959. Phys. Rev. 1972, A6, 607. (4) Milligan, D. E.; Jacox, M. E. J . Chem. Phys. 1971,55, 3404. (33) Hiraoka, K.; Mori, T.; Yamabe, S.J . Chem. Phys. 1991,94,2697. ( 5 ) Fehsenfeld, F. C.; Ferguson, E. E. J . Chem. Phys. 1976, 64, 1853. (34) This energydifference isobtained with ROHF/6-31+G. The MP2/ (6) Smit, A. L. C.; Field, F. H. J . Am. Chem. SOC.1977, 99, 6471. 6-31+G* or even MP4/6-31+G* single-point energy is found to be unreason(7) Knapp, M.; Kreisle, D.; Echt, 0.;Sattler, K.; Recknagel, E. Surf. able after annihilation of higher spin conformations (the c2h species is better Sci. 1985, 156, 313. than the C, one). The expectation value of the C, geometry is 1.001 (8) Yamamoto, S.;Mitsuke, K.; Misaizu, F.; Kondow, T.; Kuchitsu, K. by the UHF wave function, which should be 0.75. This heavy spin J . Phys. Chem. 1990, 94, 8250. contamination would result in the unreasonable MPn-series energies. (9) Hayakawa,S.; Matsumoto, A.; Yoshioka, M.; Matsuoka, S.;Sugiura, (35) The harmonic frequency computed with the Hartree-Fock wave T. Mass Spectrosc. (Tokyo) 1986, 34, 147. function is usually scaled down by 0.89 so as to compare with infrared data: (10) Ferguson, E. E.; Fehsenfeld, F. C.; Schmeltekopf, A. L. J . Chem. Pople, J. A.; Schlegel, H. B.; Krishnan, R.; DeFrees, D. J.; Binkley, J. S.; Phys. 1967, 47, 3085. Frisch, M. J.; Whiteside, R. A.; Hout, R. F.; Hehre, W. J. In?. J . Quantum (11) Hopper, D. G.; Wahl, A. C.; Wu, R. L. C.; Tiernan, T. 0. J . Chem. Chem. 1981, 15, 269. However, as far as nitrogen oxide compounds are Phys. 1976,65, 5474. (12) Yarkony, D. R. J . Chem. Phys. 1983, 78, 6763. concerned, the frequency of the in-plane antisymmetric vibration can be used without the scaling. For the vibration in the NO2 radical, the RHOF/6(13) Bardsley, J. N. J . Chem. Phys. 1969, 51, 3384. (14) Coe,J.V.;Snodgrass,J.T.;Freidhoff,C.B.;McHugh,K.M.;Bowen,31+G calculation gives 1636.7 cm-I, which is in agreement with the experimental 1621 cm-I. K. H. Chem. Phys. Lett. 1986, 124, 274.
