J . Phys. Chem. 1989, 93, 6303-6308 which adequately reproduces the observed spectrum. Along with the studies of c)clobutylmethanel2 and cyclobutylsilane," the present investigation provides clear evidence of a lowering of the barrier to ring inversion on going from lighter to heavier group IVA elements. Barriers to internal rotation for the GeH, moiety were calculated for both conformers from an analysis of sum and difference bands with the A" GeH3 stretch. These barriers and results obtained for the ring-puckering potential have been com-
6303
pared to the corresponding quantities of similar molecules.
Acknowledgment. We gratefully acknowledge partial financial support of this study on National Science Foundation Grant CHE-83-11279. Also, M.D. acknowledges travel support from the Deutsche Forschungsgemeinschaft, which made this collaborative study possible. Registry No. c-C4H,GeH,, 86280-03-1.
Multiply Bonded Argon-Containing Ions: Structures and Stabilities of XAr"' Cations (X = B, C, N; n = 1-3) Ming Wah Wong and Leo Radom* Research School of Chemistry, Australian National University, Canberra, A.C.T. 2601, Australia (Received: December 6. 1988)
The structures and stabilities of the ground-state XAr"+ diatomic ions (X = B, C, N; n = 1-3) have been examined by ab initio molecular orbital theory. Full potential energy curves for dissociation, including dissociation barriers, have been obtained at the CASSCF/6-31 lG(MC)* level. Equilibrium structures have also been determined at the MP3/6-31 lG(MC)* level and used to derive dissociation energies at the MP4/6-311 + G(MC)(Zdf) level. The XAr+ monocations are characterized by long equilibrium bond lengths (re)with binding energies (0,) increasing significantly on going from BAr+ to CAr+ to NAr'. The present best estimates of re and De for these ions are in very good agreement with experimental values. The XAr2+dications are all found to be kinetically stable species, with considerably shorter X-Ar bonds than the corresponding monocations. BAr2+and CAr2+are calculated to lie in deep potential wells, while a smaller well depth is predicted for NAr2+. Unexpected experimental results for the CAr' and CAr2+ions are discussed in terms of CAr+ being produced in an excited (42-)state. The XAr3+ trications display the shortest X-Ar bonds within the XAr"+ series (XAr', XAr2+,and XAr3+). The calculated equilibrium bond distances for BAr3+,CAr3+,and NAr3+ are 1.681, 1.582, and 1.504 A, respectively. Despite the availability of extreme exothermic (by 893 kJ mol-') fragmentation, the BAr3+trication is predicted to have a dissociation barrier of 60 kJ mol-I. Thus, BAr3+is potentially observable in the gas phase. Smaller dissociation barriers are calculated for CAr3+and NAr". Strong r overlaps, comparable to those of conventional, neutral multiply bonded systems, are found in all the XAr2+dications and XAr3+trications. We conclude that the noble-gas element argon is capable of forming multiple bonds in multiply charged ions. Contrasting comparisons with neon-containing cations CNe"+ are also presented.
Introduction Extensive theoretical research on noble-gas-containing molecular ions1in the past few years has brought with it substantial additions to the list of potentially observable noble-gas compounds. Many singly, doubly, and even more highly charged2cations containing helium, neon, or argon are predicted to be stable species, in sharp contrast to expectations based on the chemical inertness of the neutral noble-gas atoms. The existence of these stable molecular ions has been successfully demonstrated by visible and ultraviolet spectro~copy,~ elastic ~ c a t t e r i n g ,and ~ mass spectrometry5 ex( I ) For leading references, see: (a) Cooper, D. L.; Wilson S. Mol. Phys. 1981, 44, 161. (b) Schleyer, P. v. R. A h . Mass Spectrom. 1985, 287. (c) Koch, W.; Frenking, G.; Gauss, J.; Cremer, D.; Collins, J. R. J. Am. Chem. SOC.1987, 109, 5917. (d) Frenking, G.; Koch, W.; Liebman, J. F. J . Am. Chem. Soc., in press. (e) Wong, M. W.; Btirgi, H. B.; Radom, L. To be
published. (2) (a) Hottokka, M.; Kinstedt, T.; Pyykko, P.;Roos, B. 0. Mol. Phys. 1984.52, 329. (b) Wong, M. W.; Nobes, R. H.; Radom, L. J . Chem. Soc., Chem. Commun. 1987,233. (c) Koch, W.; Frenking, G. J. Chem. Phys. 1987, 86,5617. (d) Wong, M. W.; N o h , R.H.; Radom, L. Rapid Commun. Mass Spectrom. 1987, I , 3. (e) Wong, M. W.; Radom, L. J. Am. Chem. Soc. 1988, 110, 2375. (3) For example, see: (a) Dabrowski, I.; Herzberg, G.; Yoshino, K. J . Mol. Spectrosc. 1978, 73, 183. (b) Dabrowski, I.; Herzberg, G. J. Mol. Spectrosc. 1981, 89, 491. (4) For example, see: (a) Sidis, V. J. Phys. B 1972, 5 , 1517. (b) Ding, A,; Karlau, J.; Weise, J. Chem. Phys. Lett. 1977, 45, 92. (c) Hillier, I. H.; Guest, M. F.; Ding, A.; Karlau, J.; Weise, J. J . Chem. Phys. 1979, 70, 864. (5) For recent papers, see: (a) Tsong, T. T.; Kinkus, T. J. Phys. Scr. 1983, T4, 201. (b) Guilhaus, M.; Brenton, A. G.; Rabrenovic, M.; Beynon, J. H.; Schleyer, P. v. R. J. Chem. SOC.,Chem. Comm. 1985, 210. (c) Young, S. E.; Coggiola, M. J. Int. J . Mass Spectrom. Ion Processes 1986, 74, 137. (d) Jonathan, P.; Brenton, A. G.; Beynon, J. A,; Boyd, R. K. Int. J. Mass Spectrom. Ion Processes 1986, 71, 251. (e) Jonathan, P.; Boyd, R. K.; Brenton, A. G.; Beynon, J. H. Chem. Phys. 1986, 110, 239.
0022-3654/89/2093-6303$01.50/0
TABLE I: Number of Configuration State Functions for the Ground-State XAr"+ Cationsa species state configurationb no. of CSFs '2' 5$6a21$2r4 328 BAr+ BAr2+ '2' 5026027a'2r4 616 BAr3* 5a26u22r4 492 CAr+ 2II 5a26$1a22r43~' 252 CAr2+ IZ+ 5a26$1a22r4 328 CAr" 'Z' 5u26$7u12r4 616 NAr+ 3z5a26a21a22r43~2 96 NAr2+ 211 5a26a27$2m43u1 252 NAr3+ IZ+ 5 ~ ~ 6 ~ ~ 1 328 ~ ~
2 ~ ~
The CASSCF calculations for the XAr* diatomin were performed with C, symmetry. bThe inner-core electrons (1 u22u23$lr44a2)are a
not included.
periments. Despite the extreme Coulomb repulsion, multiply charged cations containing noble-gas atoms often display surprisingly strong bonds. A striking example is given by the dihelium dication (H%2+):b which has the shortest bond (0.703 A) between any atoms.6 Another interesting example is the triply charged ion HeCF3+,which is characterized by very short C-He and C-F bonds and predicted to be more tightly bound than the isoelectronic analogue HCF2+.2d Although many strongly bound noble-gascontaining ions have been found recently, no compounds that contain multiple bonds to neon or argon have yet been reported. In a recent study of the multiply charged analogues of NP,7 we found that several argon-containing cations such as CAr2+ and (6) Yagisawa, H.; Sato, H.; Watanabe, T. Phys. Rev. A 1977, 16, 1352. (7) Wong, M. W.; Radom, L. To be published.
0 1989 American Chemical Society
6304
The Journal of Physical Chemistry, Vol. 93, No. 17, 1989
Wong and Radom
NAr3+ display very short equilibrium bond lengths, significantly TABLE 11: Equilibrium Bond Lengths (A)for X A P Cations shorter than typical single-bond distances between first-row and CASSCF/ MP3/ second-row atoms. To investigate further the possibility of forming species 6-31 IG(MC)* 6-3 1lG(MC)* multiple bonds to argon in multiply charged cations, we have BAr+ 3.145 2.489 carried out a systematic investigation of argon-containing diatomic BArZ+ 1.733 1.744 ions XAr"', where X = B, C, and N and n = 1-3. Furthermore, BAr" 1.662 1.68 1 as part of our ongoing studies of highly charged ~ a t i o n s , * ~ , ~ , ~ , ~ , ~ C A P 2.1 14 2.036 we have sought to identify which of these XA@ multiply charged CAr2+ 1.776 1.738 ions might be stable and experimentally observable in the gas CAr" 1.597 1.582 phase. In this paper, we report our results for the structures and NAr+ 1.905 1.863 NAr2' 1.735 stabilities of the nine XAr"+ diatomic systems. 1.657 NAr"
1.654
1.504
Method and Results Ab initio molecular orbital calculations were carried out using the GAUSSIAN and GAMESS~O series of programs. The complete active space self-consistent field (CASSCF) approach" with the triple-{-valence plus polarization 6-31 lG(MC) basis set12was used initially to calculate the ground-state potential energy curves of the XAr"' ions. The CASSCF active space comprised all of the valence orbitals, namely So-8a and 2a-3r for the diatomic systems, 2s and 2p for the first-row atomic systems, and 3s and 3p for the second-row atomic systems. The numbers of configuration state functions (CSFs) generated from such an active space for the diatomic systems are summarized in Table I. The use of the full-valence CASSCF procedure here should lead to a balanced description of the dissociation of the multiply bonded systems and provide more reliable estimates of the dissociation barriers for the multiply charged species than would be expected from single-configuration starting points. The CASSCF/631 lG(MC)' level was also used to evaluate spectroscopic constants (ae,aexerBe, and a,) for the XArn+ cations. For each system, about 10-15 points were obtained in the range re 0.3 A. The calculated points were fitted to a polynomial of sixth degree (to better than lo4 hartrees or 0.2 cm-') in the internal displacement coordinate AR = r - re
*
where V is the potential energy function with a minimum (V,) at re. The spectroscopic constants were then derived from the coefficients @))of the p01ynomial.l~ In addition to the CASSCF calculations, we have also determined the equilibrium geometries for the nine diatomic systems at the third-order Maller-Plesset perturbation theory (MP3) level,I4 again with the 6-31 lG(MC)* basis set. With the MP3 geometries, more accurate dissociation energies were calculated at the full fourth-order Mdler-Plesset (MP4) levells with the larger 6-31 1 + G(MC)(2df) basis set.12,16Such a level of theory, (8) (a) Radom, L.; Gill, P. M. W.; Wong, M. W.; N o h , R. H. Pure Appl. Chem. 1988, 60, 183. (b) Wong, M. W.; Bouma, W. J.; Nobes, R. H.; Radom, L. J . Chem. Phys., in press. (9) Frisch, M. J.; Binkley, J. S.; Schlegel, H. B.; Raghavachari, K.; Melius, C. F.; Martin, R. L.; Stewart, J. J. P.; Bobrowicz, F. W.; Rohlfing, C. M.; Kahn, L. R.; DeFrees, D. J.; Whiteside, R. A,; Fox, D. J.; Ruder, E. M.; Pople, J. A. GAUSSIAN 86; Carnegie-Mellon University: Pittsburgh, PA, 1986. (10) (a) Guest, M.F.; Kendrick, J.; Pope, S. A. GAMESS Documentation; SERC Daresbury Laboratory: Warrington WA4 4AD, U.K., 1983. (b) Dupuis, M.; Spangler, D.; Wendoloski, J. J., NRCC Sotware Catalog. Vol. 1, Program No. QGOI, 1980. (c) Schmidt, M. W.; Boatz, J. A.; Baldridge, K. K.; Koseki, S.; Gordon, M. S.; Elbert, S. T.; Lamb, B. QCPE Bull. 1987, 7, 115. (1 I ) Roos, B. 0.;Taylor, P. R.; Siegbahn, P. E. M. Chem. Phys. 1980,48, 157. (12) Wong, M. W.; Gill, P. M. W.; Nobes, R. H.; Radom, L. J. Phys. Chem. 1988, 92,4875. (13) Hollas, J. H. High Resolution Spectroscopy; Butterworth: London, 1982. (14) (a) M ~ l l e r C.; , Pleuet, M. S. Phys. Reu. 1934, 46, 618. (b) Pople, J. A.; Binkley, J. S.; Seeger, R. Int. J. Qunntum Chem., Symp. 1976, IO, I . (15) (a) Krishnan, R.; Pople, J. A. Int. J . Quantum Chem. 1978, 14, 91. (b) Krishnan, R.; Frisch, M. J.; Pople, J. A. J. Chem. Phys. 1980, 72, 4244. (16) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J . Chem. Phys. 1984, 80, 3265.
TABLE III: Spectroscopic Constants"Sb(w, w a x , Be, and a,)of the XAr"' Cations species BAr' BAr2+ BAr3+ CAr' CArZ+ CAr" NAr' NAr2+ NAr"
We
WAX,
78 806 905 304 638 847 515 562 565
3.1 5.4 12.6 12.7 5.9 26.1 5.0 14.3 19.7
Be 0.20 0.65 0.71 0.41 0.58 0.72 0.45 0.54 0.59
a,
0.0070 0.0068 0.0089 0.0147 0.0069 0.0 145' 0.0044 0.0104 0.0160
"Calculated at the CASSCF/6-31 IG(MC)* level. centimeters.
In reciprocal
including diffuse s and p functions and multiple sets of d functions and f functions, has been shown7sBbto yield dissociation energies for a variety of multiply bonded systems within 0.15 eV of experimental values. Zero-point vibrational corrections to the directly calculated dissociation energies (De)and dissociation barriers (De*)(yielding Doand Do*values, respectively) were evaluated from HF/6-31 lG(MC)* vibrational frequencies, the latter being scaled by 0.9" to account for their overestimation at this level of theory. All of the Maller-Plesset calculations employed the frozen-core approximation. Unless otherwise stated, equilibrium bond lengths (re)and dissociation energies (Deand Do)in the text G(MC)(2df) refer to MP3/6-311G(MC)* and MP4/6-311 values, respectively, while dissociation barriers (De*and Do*) for the multiply charged species correspond to CASSCF/6-3 1 1G(MC)* values. The calculated equilibrium bond lengths and spectroscopic constants for the XArn+ ions are presented in Tables I1 and 111, respectively, and their total energies and dissociation energies are given in Tables IV and V, respectively. Calculated Mulliken a-overlap populations, charges, and bond orders for the XArn+ and CNe"+ ions and some representative, singly and multiply bonded neutral diatomic systems are presented in Table VI. Finally, results related to transition structures for the fragmentation of the XAr2+ and XAr3+cations are summarized in Table VII.
+
Discussion XAF+ Equilibrium Bond Lengths and the Question of a Bonding. We begin our discussion with some general observations regarding the trends in equilibrium bond lengths for the XAr"+ ions. In addition, we examine the question of whether or not a bonding exists to a significant extent in these diatomic cations. All the singly charged XAr+ ions are calculated to be bound species with long X-Ar bonds. BAr+ has the longest equilibrium bond distance (2.489 A; Table 11) and the smallest binding energy (Do= 26 kJ mol-'; Table V). This ion remains almost pure B+ + Ar even at the equilibrium bond distance. Mulliken population analysis shows that B+ has gained just 0.13 electrons from the Ar atom at the equilibrium bond distance of BAr+ (Table VI). BAr+ is therefore best regarded as a weak charge-induced-dipole (17) Pople, J. A.; Schlegel, H.; Krishnan, R.; DeFrees, D. J.; Binkley, J. S.; Frisch, M. J.; Whiteside, R. A.; Hout, R. F.; Hehre, W. J. Int. J. Quantum Chem., Quantum Chem. Symp. 1981, IS, 269.
Structures and Stabilities of XAr"' Cations
The Journal of Physical Chemistry, Vol. 93, No. 17, 1989 6305
TABLE IV: Total Energies (hartrees) and Zero-Point Vibrational Energies (ZPVE; kJ mol-') of Atomic and Diatomic Systems energy' species B+ B2+ C C+ C2+ N N+ N2+ Ar Art Ar2+
state IS 2s
3P 2P
'S
4s 3P 2P
'S 2P 3P
1Zt
BAr' BAr2+ BAr3+ CAr' CAr2+ CAr3+ NAr' NAr2' NAr3+
2z+
lZ+ 2l-I 1.P
2zt 3 x 2n
IZt
CASSCF/ 6-31 lG(MC)* -24.293 79 -23.374 14 -37.70408 -37.33063 -36.476 54 -54.394 78 -53.903 53 -52.859 15 -526.806 87 -526.264 03 -525.293 53
MP3/ 6-311G(MC)* -24.283 20 -23.373 99 -37.759 85 -37.351 87 -36.462 64 -54.488 48 -53.96064 -52.878 96 -526.968 64 -526.404 86 -525.41 1 22
MP4/ 6-3 1 1 + G(MC)(2dQb -24.290 12 -23.37407 -37.775 69 -37.363 26 -36.471 58 -54.51042 -53.976 76 -52.882 23 -527.032 94 -526.459 86 -525.45492
ZPVE 0 0 0 0 0 0 0 0 0 0 0
-55 1.102 68 -550.408 67 -549.324 27 -564.14941 -563.467 27 -562.28266 -580.757 27 -579.968 95 -578.806 97
-551.25784 -550.533 45 -549.422 95 -564.346 56 -563.626 84 -562.40447 -580.994 92 -580.160 70 -578.934 80
-551.33309 -550.603 47 -549.495 76 -564.430 52 -563.71696 -562.493 71 -58 1.088 94 -580.267 08 -579.080 85
0.5 4.7 5.2 1.4 4.2 4.3 2.5 4.2 6.3
With fully optimized geometries, unless otherwise noted.
With MP3/6-31 lG(MC)* geometries.
TABLE V Dissociation Energies' (kJ mol-') for X A e Species
------
dissociation process
+ + + + +
BArt Bt Ar BAr2' Bt Ar+ BAr3+ B2' Art CArt Ct Ar CAr2' C+ Art CAr" C2' + Ar+ NArt Nt + Ar NAr2+ Nt Ar+ NAr3' Nt Ar2'
+ +
CASSCF/ 6-311G(MC)*
De MP3/ 6-31 lG(MC)*
MP4/6-311 + G(MC)(2dQb
5 -392 -824 31 -334 -1202 123 -521 -1027
16 -406 -934 68 -341 -1216 172 -538 -1 148
26 -385 -888 90 -279 -1 149 208 -445 -92 1
'Calculated from total energies shown in Table 111.
soecies BAr+ CArt NArt BAr2' CAr2+ NAr2+ BAr3+ CAr3+ NAr3'
r-overlaD 0.03 0.03 -0.01 0.22 0.20 0.13 0.33 0.35 0.42
dX)
dY)
0.87 0.67 0.50 1.21 1.16 0.97 1.82 1.59 1.37
0.13 0.33 0.50 0.79 0.84 1.03 1.18 1.41 1.63
bond order 0.21 0.49 0.69 1.15 1.17 1.23 1.54 1.66 2.00
CNe' CNe2+ CNe3+
0.01 0.04 0.06
0.97 1.73 1.52
0.03 0.27 1.48
0.05 0.46 0.74
-0.12 0.13 1.07 -0.12 0.23 0.85
0 0 0 -0.30 0.34 -0.05
0 0 0 0.30 -0.34 0.05
0.91 1.73 2.79 0.85 1.84 2.46
F2 0 2
N2 FCI
so cs
+
26 -389 -893 89 -283 -1153 206 -449 -927
With MP3/6-31 lG(MC)* geometries. cIncluding zero-point vibrational correction.
TABLE VI: Mulliken *-Overlap Populations, Charges, and Bond Orders for Diatomic Svstems XY" atomic charge
Do:
MP4/6-311 G(MC)(2dQb*c
"Corresponding to MP3/6-31 lG(MC)* geometries. Calculated MP3/6-31 lG(MC)* equilibrium bond lengths include 2.341 (CNet), 1.630 (CNe2+), 1.382 (CNe3+), 1.399 (F2), 1.191 (02),1.095 (N2), 1.675 (FCI), 1.505 (SO), and 1.535 (CS) A.
complex. There is significantly greater electron donation from the Ar atom in CAr' (0.33) and NAr' (0.50). The equilibrium bond lengths in the series XAr' decrease with the increasing electronegativity of X. Thus, the interatomic distance in NAr+
TABLE VII: Total Energies (hartrees), Bond Lengths (rm, A), Dissociation Barriers (De*and Do*,kJ mol-'), and Kinetic Energy Releases ( T , eV) Associated with XArZt and X A F Transition Structures species energy' D . ' Dn* T rm BAr2+ CAr2+ NAr2+ BAr" CAr3+ NAr3'
-550.36040 -563.42574 -579.95835 -549.29965 -562.26903 -578.79767
2.489 2.723 2.190 2.212 1.951 2.069
127 109 28 65 36 22
123 105 24 60 32 16
5.4 4.6 5.7 9.2 12.8 10.9
'Calculated at the CASSCF/6-31 lG(MC)* level.
(1.863 A) is shorter than that in CAr+ (2.036 A), which in turn is shorter than that in BAr' (2.489 A). In accordance with the trend in bond lengths and charge density on Ar, the computed are 206, 89, and 26 kJ mol-' for NAr+, CAr+, well depths (Do) and BAr', respectively. All the XAr+ monocations considered here involve virtually no a bonding. This is indicated by the very small a-overlap populations in BAr', CAr', and NAr+ (Table VI). The trend of interatomic distances in the series XAr+ can readily be understood by considering the interactions between the orbitals of the separated fragments (i.e., X+ and Ar). For BAr', the valence orbitals consist of 2s and 2p atomic orbitals for the B+ ion and 3s and 3p for the Ar atom. Since the energy gap between the valence orbitals of B+ and Ar is very large, the main bonding in BAr+, which comes from the p e-p u interaction between the 2p u orbital of B+ (formally vacant) and the 3p u orbital of Ar (formally doubly occupied), is small. On going from BAr' to CAr+ and in turn to NAr', the energy of the 2p u orbital of X+
6306 The Journal of Physical Chemistry, Vol. 93, No. 17, 1989 is progressively lower, leading to more favorable interaction with the 3p CT orbital of Ar, which results in a shorter (but still quite long) X-Ar bond. It should be noted (as shown in Table I) that the additional electrons in CAr+ and NAr' are accommodated in A orbitals so that the 2p u orbital of X' remains formally vacant. The calculated X-Ar lengths for the XAr2+ dications are considerably shorter than those for their monocation counterparts (Table 11). For instance, the B-Ar distance in BArZ+is 0.745 A shorter than that in BAr'. From Table VI, it is clear that all three dicationic species, BArZ+,CAr2+, and NAr2+, show significant A overlaps at their equilibrium bond distances and their bond orders are significantly greater than those of systems that contain no A bonds, e.g., F2 and FCI. The stronger bonding in XAr2+ may be explained in terms of the better matching between the orbitals of X2+ and Ar. The closer proximity of the X and Ar atoms in XAr2+ then allows the two sets of p A orbitals to interact effectively to form H bonds. In spite of the enormous Coulomb repulsion in triply charged species, the XAr3+ trications are found to display even shorter X-Ar bonds than the corresponding XAr2+ dications. The shortening of the X-Ar distance on going from XAr2+ to XAr3+ is about 0.15 A in each case (Table 11). In fact, the XAr3+ trications contain the shortest X-Ar bonds in the XAr"' series (Le., XAr', XAr2+,and XAr3+). This finding seems rather typical for noble-gas-containing multiply charged cations. Similar trends in bond length have been reported for the series CHe,"', CNe"', and SiHe"+.2b-C*e The X-Ar H bonds of XAr3+ are stronger than those of XAr2+. This is reflected in the larger A overlaps and greater bond orders in XAr3+ (Table VI). The A overlaps in XAr3+ are, indeed, comparable to those of more familiar multiply bonded 02,CS, and SO. diatomics, e.g., Nz, One of the most important conclusions of this study of the XAr"' systems is that the noble-gas element argon is capable of forming strong multiple bonds in multiply charged species. What about the lighter noble gas neon? Koch and FrenkingZbhave recently described the structures and stabilities of CNe& cations, and they found that there is virtually no A bonding in CNe2+: the occupied A orbitals are exclusively located on Ne. Although CNe3+is calculated to have a shorter equilibrium bond distance (1.382 A) than CNe2+ (1.630 A) and CNe+ (2.341 A), the H overlap in CNe3+ is still very small (Table VI). These results for CNe"' are thus in marked contrast to the argon analogues (CAP'), which show significant A overlaps for both the CAr2+ and CAr3+ cations. XAr+ Monocations. The ground-state potential curve for the BAr+ ion has been studied previously at both SCF18 and multireference CI I9 levels. The single-configuration calculations'* predicted a bond length of approximately 3 A and a dissociation energy of just 9 kJ mol-' for BAr+. The more recent multireference C1 c a l c ~ l a t i o n s 'led ~ to a substantially shorter B-Ar distance (2.37 A) and a greater binding energy of 34 kJ mol-'. Our present best estimates of re (2.489 A) and De (26 kJ mol-') are consistent with the latter multireference values, while the CASSCF calculations yield results similar to the S C F calculations (Tables I1 and V). The existence of BAr+ has been demonstrated by elastic scattering of B+ by Ai-.]* The potential energy curve for the BAr+ ion has also been derived from the measured elastic cross section, and a dissociation energy of 29 kJ mol-' was obtained. This value is close to our best theoretical estimate (26 kJ mol-') at the MP4/6-311 + G(MC)(Zdf) level. A theoretical investigation of the potential energy curve of CAr' using the POL-CI method has been reported by Hillier et al.4c The equilibrium bond length and dissociation energy obtained in that study are 2.000 A and 114 kJ mol-', respectively. Our present study yields a similar value for re (2.036 A) but a slightly smaller De (90 kJ mol-]). Experimentally, CAr+ has also been detected by elastic scattering experiments." These experiments yielded a bond length of 1.995 A and a dissociation energy of 91 kJ mol-', (18) Ding, A.; Karlau, J.; Weise, J.; Kendrick, J.; Kuntz, P. J.; Hillier, I. H.; Guest, M. F. J . Chem. Phys. 1978, 68, 2206. (19) Iwata, S.: Sato, N . J . Chem. Phys. 1985, 82, 2346.
Wong and Radom both of which are in very good agreement with the present best estimates of re and De. NAr+ is the most stable of all the XAr' ions considered here. It has been generated by reaction of N 2 or NH, with Ar in mass spectrometry experiment^.^^*^^ Using a simple Hess cycle, Liebman and Allen2' estimated an approximate experimental binding energy of 222 kJ mol-' for NAr+. We are not aware of any theoretical study to date of this species. Out present calculations predict an equilibrium bond length of 1.863 A for NAr'. for NAr+ N+ Ar The computed dissociation energy (Do) is 206 kJ mol-', which is quite close to the estimated experimental value. With such a deep well, NAr' should also be accessible through elastic scattering of N+ by Ar. We note that the CASSCF calculations yield longer equilibrium distances than the MP3 values and substantially smaller dissociation energies than the MP4 estimates for all the XAr' ions. In particular, the dissociation energies calculated with the CASSCF method are in rather poor agreement with experiment, in contrast to the MP4 values. Similar findings have been noted in recent theoretical studies of the isoelectronic analogues of N2 and NP.798b It therefore appears that inclusion of dynamical correlation is important in describing quantitatively the dissociation of such species. XAG' Dications. As for most other dications, all the XArZ+ dications investigated here are thermodynamically unstable with respect to the charge-separation reaction XAr2+ X+ + Ar', as indicated by negative De values, Table V. However, sizable barriers are predicted to accompany such exothermic fragmentations and therefore BAr2+, CAr2+, and N A P should be experimentally accessible species in the gas phase. To the best of our knowledge, no experimental or theoretical study has yet been reported for BAr2+. Our calculations predict BAr2+ to be a kinetically stable species, with a large barrier (Dos) of 123 kJ mol-' to dissociation to B+ Ar+, a reaction which is exothermic by 389 kJ mol-'. Thus, BAr2+represents a promising candidate for experimental investigation in the gas phase. The calculated kinetic energy release ( T ) for the production of B+ Ar' from BAr2+ is 5.4 eV (Table V). From the data in Table IV, an adiabatic ionization energy (IE,) of 19.9 eV for the singly charged BAr+ ion is predicted. These values of T and IE, may be of assistance in the future experimental identification of BAr2+. The potential energy curve for the ground state of CAr2+ has been studied previously by Hurley's semiemipirical procedure.% These calculations indicated that CAr2+has an equilibrium bond length of about 1.5 A and a large well depth of about 280 kJ mol-'. Our present calculations yield a longer equilibrium distance (1.738 A) and a considerably smaller well depth (105 kJ mol-]). However, the size of the dissociation barrier for CArZ+calculated here should be sufficient for CAr2+ to be observable experimentally. The theoretically predicted values of T and IE, for CAr2+are 4.6 and 19.4 eV, respectively. Surprisingly, recent attempts by Jonathan et aLsd to observe CAr2+ by charge stripping of CAr+ have not been successful. In searching for a possible explanation of this result, we explored initially the nature of the CAr' monocation precursor in the charge-stripping experiments. An important experimental observation in this regard is that collision-induced dissociation of these CAr' ions leads, unexpectedly, to a predominance of C + Ar' rather than the lower energy fragmentation products C+ + Ar. This is not compatible with the CAr+ ions being produced in their 211 ground state (with leading configuration 5 ~ ~ 6 ~ ~(Table 7 ~ I)). ~ 2Rather, ~ ~ it3suggests ~ ' that the CAr+ ions are in an excited 41;- state (with leading configuration 5 a 2 6 u 2 7 a 1 2 ~ 4 3 ~The 2 ) . 41;- state, although lying 234 kJ mol-' above the 2~ state, is tightly bound," our calculated equilibrium bond length and dissociation energy being 1.765 A and 242 kJ mol-', respectively (CASSCF/6-31 lG(MC)*), and it does indeed
-
+
-
+
+
(20) Munson, M . S. B.; Field, F. H.; Franklin, J. L. J . Chem. Phys. 1962, 37, 1790. (21) Liebman, J . F.; Allen, L. C. Int. J . MussSpectrom. Ion Phys. 1971, 7, 27.
Structures and Stabilities of XAr"' Cations
The Journal of Physical Chemistry, Vol. 93, No. 17, 1989 6307
I t Yy
It
I'
22,2 22.0
Energy
t\
\
1
t
0
:
:
u
~+
C*
Ar
-
1.4 1.8 2.2 2.6 3.0 C-Ar Distance ( A )
Figure 1. Schematic potential energy diagram (calculated at the CASSCF/6-3 1 IG(MC)* level) showing vertical ionization processes for the *I1 ground state and first excited 42-state of the CAr* monocation.
+
dissociate to C Ar+ (Figure 1). If we accept that the CAr+ precursor in the charge-stripping experiments is in the 42-excited state, then a pertinent fact is that single ionization from the 42-state of CAr+ does not lead to the IZ+ ground state of the CAr2+ dication (with leading configuration 5a26027a22a4).Instead, 311(with leading configuration 5a26a27a'2a43a') and 3Z- (with leading configuration 5a26a22r43a2)excited states of CAr2+would be expected to be produced. The 311state lies 274 kJ mol-' above the lZ+ ground state of CAr2+but has a shorter C-Ar bond length of 1.675 A. The 32-state is found to be still higher in energy and purely repulsive. All three states ('Z', 311,and 3Z-) of CAr2+dissociate to the same atomic limit, Le., C+(2P) Ar+(2P). Our calculations predict that vertica1,ionization of the 211 ground state of CAr' would produce the lZ+ ground state of CAr2+ 7 0 kJ mol-I below its dissociation barrier (Figure 1). Charge stripping of the 211 state of CAr+ should thus result in a stable doubly charged CAr2+ ion. On the other hand, vertical ionization of the 42-state of CAr+ is predicted to produce the 311state of CAr2+ only 32 kJ mol-' below its dissociation barrier (of 40 kJ mol-'). Our calculations thus indicate that observation of CAr2+ by charge stripping will be considerably more difficult starting from the 42-state of CAr+ than from the 211 state, and this may be the explanation for the experimental results.5d The failure to observe any CAr2+in the charge-stripping experiment^,^^ however, remains somewhat surprising. In agreement with previous semiempirical calculations,sd the X211ground state of NAr2+ is predicted to have a small barrier (24 kJ mol-') for dissociation to N+ Ar+. Although NAr2+lies in a shallow well, it is predicted to be experimentally accessible through charge stripping. Examination of Figure 2 shows that vertical ionization of the NAr+ monocation produces the NAr2+ dication with some excess energy (17 kJ mol-'). However, the energy of the NAr2+ dication produced in this manner is still below the dissociation barrier. Indeed, NAr2+ has been observed in a recent charge-stripping mass spectrometry e ~ p e r i m e n t .The ~~ measured ionization energy, corresponding to the process NAr+ NAr2+, is 20.7 -+ 2.5 eV, which encompasses our present theoretical estimate of 22.4 eV (IEa, MP4/6-31 l+G(MC)(Zdf))
+
+
-
I
I
I
1.4
1.6
1.8 N-Ar
I
I
I
I
-
2.4 2.0 2.2 Distance ( 8 )
2.6
Figure 2. Schematic potential energy diagram (calculated at the CASSCF/6-3 1 lG(MC)* level) showing adiabatic and vertical ionization processes for the NAr+ monocation.
within the rather large, reported experimental uncertainties. XAr3+ Trications. As mentioned earlier, the XAr3+ ions show clearly the characteristics of multiple bonds. However, these trications are extremely unstable toward dissociation to X2+ Ar+ or X+ + Ar2+,such processes being exothermic by about 1 MJ mol-I. For BAr3+,a barrier (Do*) of 60 kJ mol-I is associated with the highly-exothermic (by 893 kJ mol-') fragmentation BAr3+ B2++ Ar+. This barrier is sufficiently large that BAr3+ should be detectable in the gas phase by means of an appropriate experiment. The predicted kinetic energy release for the production of B2+ Ar+ from BAr3+ is 9.2 eV (Table VII). The NAr3+ trication has the shortest equilibrium bond length of all the XAr"+ ions shown in Table 11. In fact, the calculated N-Ar bond length (1.504 A) is even shorter than the length of some neutral triply bonded first-row-second-row diatomics: for example, 1.535, 1.654, and 1.727 8, for CS, BC1, and AlF, respectively. Despite the short bond and strong a overlap, NAr3+ is predicted to have only a small barrier (16 kJ mol-') to dissociation to N + + Ar2+. Similarly, CAr3+ is calculated to have a short bond (1 S 8 2 A) but lies in a shallow potential well (32 kJ mol-'). Hence, experimental observation of these two ions will not be straightforward. Notice that the calculated dissociation barrier becomes progressively smaller in going from BAr3+ to CAr3+ and in turn to NAr3+. Unlike BAr3+ and CAr3+, the lowest fragmentation products for NAr3+ correspond to N + Ar2+ rather than N2+ Ar+ and the atomic charge of the Ar atom (1 5 3 ) is even greater than that of the N atom (1.37) at the equilibrium distance of NAr3+. These differences are attributed to the fact that the second ionization energy of argon (2638 kJ mol-') is lower than the second ionization energy of nitrogen (2874 kJ mol-').22
+
-
+
+
+
Concluding Remarks Several important points emerge from this study. (1) Our ab initio calculations show that the noble-gas element argon is capable of forming strong multiple bonds in multiply charged species. Strong a overlaps, comparable to those of conventional multiply bonded neutral systems, are found in all XAr2+ dications and XAr3+ trications. (22) The numbers in the text (2638 and 2874 kJ mol-') refer to calculated values. Corresponding experimental second-ionization energies for Ar and N are 2666 and 2856 kJ mol-', respectively: Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 67th ed.; CRC Press: Boca Raton, FL, 1986.
6308
J . Phys. Chem. 1989, 93, 6308-631 I
(2) The XAr' monocations are characterized by long bonds but significant binding energies. The calculated equilibrium bond lengths (MP3/6-31 IG(MC)*) and dissociation energies (MP4/6-311 G(MC)(Zdf)) for these ions are in very good agreement with available experimental data. (3) The trends in stability of the ground-state XAr' monocations (NAr' > CAr' > BAr') can be understood in terms of the interaction between appropriate orbitals of the X+ ions and Ar atom. (4)The XAr2' dications are all predicted to be kinetically stable species with shorter X-Ar bonds than the corresponding monocations. Both BAr2+ and CAr2' are calculated to have large dissociation barriers of over 100 kJ mol-', while a small barrier is associated with fragmentation of NAr2+.
+
(5) The XAr3+trications display the shortest X-Ar bonds in the XAr"' series. Although BAr3' is extremely unstable toward fragmentation to B2' Ar+, this reaction is inhibited by a barrier of 60 kJ mol-'. Thus, BAr3+ is potentially observable in the gas phase. CAr3+and NAr3+are predicted to lie in wells of shallower depth.
+
Acknowledgment. We thank Dr. Gernot Frenking for a preprint of ref Id. We gratefully acknowledge a generous allocation of time on the Fujitsu FACOM VP-100 of the Australian National University Supercomputer Facility. Registry No. BAr+, 12769-61-8; BAr2+, 121705-50-8; BAr", 121705-51-9;CAr', 69866-64-4;CAr2+,117533-60-5;CAr3+,12170554-2; NAr', 71 159-51-8; NAr2+, 121705-52-0;NAr3+, 121705-53-1.
I n Situ Time-Resolved X-ray Absorption Near Edge Structure Study of the Nickel Oxide Electrode J. McBreen,* W. E. O'Grady,+ Department of Applied Science, Brookhaven National Laboratory, Upton, New York I1973
G . Tourillon, E. Dartyge, A. Fontaine, LURE-CNRS, 91405 Orsay, Cedex, France
and K. I. Pandya Laboratory for Inorganic Chemistry and Catalysis, Eindhoven University of Technology, Eindhoven, The Netherlands (Received: December 9, 1988; In Final Form: April 18, 1989)
In situ time-resolved X-ray absorption spectroscopy was coupled with cyclic voltammetry to investigate X-ray absorption near edge structures (XANES) at the Ni K-edge of both a-Ni(OH), and B-Ni(OH), in KOH electrolytes. P-Ni(OH), electrodes with and without Co(OH), additions were also studied. The XANES results for the three different electrodes exhibit (i) a continuous shift of the edge absorption to higher energies during the oxidation process in agreement with the general overall electrochemical reaction Ni2+ Ni3+and (ii) a modification of the edge features that is related to changes in the Ni-0 local environment. Initially, there is a regular octahedral coordination where each Ni atom is surrounded by six 0 atoms in the discharged state. Upon oxidation the material evolves to a distorted octahedral structure. This is associated with a continuous decrease of the white line intensity of the XANES spectra. For a-Ni(OH), these processes are completely reversible when the potential is swept in the cathodic direction. In the oxidized state, some differences were observed in the electrodes with Co(OH), additions. There was a splitting of the XANES maximum and a shift of the edge to higher energy. This is due to a more complete oxidation of Ni(OH),. The cyclic voltammograms for the'@-Ni(OH),electrodes gave two well-defined reduction current peaks, as opposed to one for a-Ni(OH),. After the first reduction peak the electrode contains charged and discharged material. No evidence for a new species was detected.
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Introduction Structural determinations of the reactants and products of the nickel oxide electrode are difficult because of the inherently poor crystallinity of these materials. The problems in applying X-ray diffraction techniques have been reviewed.' Recently, extended X-ray absorption fine structure (EXAFS) has been applied to the nickel oxide electrode. This has included studies of a-Ni(OH), and @-Ni(OH)? and the structural changes that occur on cycling P-Ni(OH)2,3*4 in particular those that occur within the brucite layer lanes. The EXAFS results yielded a Ni-Ni distance of 3.08 for a-Ni(OH), and 3.13 A for P-Ni(OH),. Since the distances could be determined within f0.01 A, it confirmed the contraction of 0.05 8, in the a axis that was reported by Bode5 and disputed by McEwen.' With in situ techniques the changes in the Ni-0 and Ni-Ni distances in P-Ni(OH), could be observed as the electrode was c y ~ l e d . The ~ . ~ Ni-0 data for uncycled and
81
'Present address: Code 6 170, Naval Research Laboratory, Washington, D.C. 20375.
0022-3654/89/2093-6308$01.50/0
discharged material could be fitted to a single coordination shell with a coordination number of 6 and a Ni-0 distance of 2.05 A. In the case of the charged material a two-shell fit was necessary, with two oxygens with a Ni-0 distance of 2.07 A and four at a distance of 1.88 A. This could be due to a distorted octahedral coordination of oxygen around the nickel in the charged state or simply to lack of completion of the charging process. The present study was a detailed X-ray absorption near edge structure (XANES) investigation of the charge/discharge processes in the nickel oxide electrode. This was done by coupling in situ timeresolved X-ray absorption spectroscopy with cyclic voltammetry on both a-Ni(OH), and P-Ni(OH), electrodes. Cyclic voltam(1) McEwen, R. S. J . Phys. Chem. 1971, 75, 1782. (2) Pandya, K. I.; O'Grady, W. E.; Corrigan, D. A,; McBreen, J.; Hoffman, R. W. J . Phys. Chem., in press. (3) McBreen, J.; OGrady, W. E.; Pandya, K. 1.; Hoffman, R. W.; Sayers, D. E. Langmuir 1987, 3, 428. (4) Pandya, K. I.; Hoffman, R. W.; McBreen, J.; OGrady, W. E. J . Electrochem. Soc., in press. (5) Bode, H. Angew. Chem. 1988, 7 3 , 5 5 3 .
0 1989 American Chemical Society
