Isoelectronic Analogues of PN: Remarkably Stable Multiply Charged

One of our approaches to study multiply charged species is to select a particularly stable neutral molecule and to examine the charged isoelectronic a...
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638

J . Phys. Chem. 1990, 94, 638-644

Isoelectronic Analogues of PN: Remarkably Stable Multiply Charged Cations Ming Wah Wong and Leo Radom* Research School of Chemistry, Australian National University, Canberra, A.C.T . 2601, Australia (Received: July 18, 1989)

The structures and stabilities of PN and its 27 isoelectronic analogues, CS, SiO, BCI, AIF, BeAr, MgNe, SN+, PO+, CCP, SiF+, BAr', AINe', SOz+,NCl2+,PF2+,CAr2+,%Ne2+,OC13+,SF3+,NAr3+,PNe3+,FCI4+,OAr4+,SNe4+,FA$+, CINe5+, and ArNe6+, have been examined by ab initio molecular orbital theory. The CASSCF/6-31 lG(MC)(d) level was used to determine the ground-state potential energy curves and spectroscopic constants for the 28 diatomic systems. Equilibrium structures were also obtained with the 6-31 lG(MC)(d) basis set at the MP3 and ST4CCD levels, and dissociation energies were determined at the MP4/6-311 + G(MC)(2df) and MP4/6-311 + G(MC)(3d2f) levels. For the neutral and monocation analogues of PN, the calculated equilibrium geometries (at MP3/6-31 lG(MC)(d)) and dissociation energies (at MP4/6-3 1 1 + G(MC)(3d2f)) are in very good agreement with available experimental values. All the dication analogues of PN, namely, SO2+,NC12+,PF2+,CAr2+,and SiNe2+,are predicted to be experimentally observable species. Of these, the SO2+,NC12+, and CAr2+ dications are calculated to be kinetically stable species, with large barriers associated with the exothermic charge-separation reactions, while the PF2+and %NeZ+dications are predicted not only to be kinetically stable but also to be thermodynamically stable species. Despite the inherent strong Coulomb repulsion, the triply charged ions SF3+and PNe3+ display remarkable stability toward extremely exothermic fragmentation reactions and are predicted to be experimentally accessible species in the gas phase. The OC13+and NAr3+trications, on the other hand, although having short equilibrium bond lengths, lie in shallow potential wells. Except for the neon-containing species, all the doubly and triply charged ions are characterized by short equilibrium bond lengths. In the case of SO2+,the calculated S-0 bond length may well be the shortest between any first-row atom and second-row atom. The fragmentation mechanism for the multiply charged diatomics can readily be understood in terms of avoided-crossing models.

Introduction I n recent years, the gas-phase chemistry of multiply charged cations has attracted increasing experimental and theoretical interest.' Various modern techniques, including charge-stripping: double-charge-tran~fer,~ photoion-photoion c~incidence,~ and photoelectron-photoelectron coincidence4bspectroscopy, have been developed to generate and detect such ions. On the other hand, with the advent of more sophisticated programs and powerful computers, ab initio molecular orbital theory' has become an increasingly important tool to characterize the structures and stabilities of these remarkable species. One of our approaches to study multiply charged species is to select a particularly stable neutral molecule and to examine the charged isoelectronic analogues of this molecule. We have recently employed such an approach to study the multiply charged isoelectronic analogues of molecular nitrogen (N2).5 One of the important results from that study is that some of the multiply charged ions are found to display very short bonds and great kinetic dication is predicted to have stabilities. In particular, the 022+ the shortest equilibrium bond length between any two heavy atoms and to have a very large barrier (approximately 300 kJ mol-') inhibiting the exothermic dissociation reaction OZ2+ 0' + 0'. Since second-row atoms are larger and more electropositive than the corresponding first-row atoms, they should be able to better accommodate positive charge. Hence, multiply charged species that contain second-row atoms might represent more accessible targets for stable, experimentally observable species. To investigate this possibility, we have examined PN, the first-row-second-row

-

~

~

~~~

~~~

( I ) For reviews, see: (a) Koch, W.; Maquin, F.; Stahl, D.; Schwarz, H. Chimia 1985, 39, 376. (b) Koch, W.; Schwarz, H. In Structure/Reactioity and Thermochemistry of Ions; Lias, S. G., Ausloos, P. A. Eds.; D. Reidel:

Dordrecht, 1987. (2) (a) Ast, T. Ado. Mass Spectrom. 1980, 8, 5 5 5 . (b) Levsen, K.; Schwarz, H. Mass Spectrom. Reo. 1983, 2, 77. (c) Ast. T. Adu. Mass Spectrom. 1985, 47 I . (3) (a) Fournier, P.; Appell, J.; Fehsenfeld, F. C.; Durup, J. J . Phys. B 1972,. 5 , L58. (b) Appell, J. Collision Spectroscopy; Cooks, R. G., Ed.; Elsevier: Amsterdam, 1978. (4) (a) Tsai. B. P.; Eland, J. H. D. Inr. J . Mass Spectrom. Ion Phys. 1980, 36, 143. (b) Eland, J. H . D.; Price, S. D.; Cheney, J. C.; Lablanquie, P.; Nenner. I.: Fournier. P. G. Philos. Trans. R . SOC.London A 1988. 24. 247. ( 5 ) Wong, M W , W o k s , R H Bouma, W J Radom, L J Chem Phys 1989, 91. 2971

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0022-3654/90/2094-0638$02.50/0

analogue of N2, and its 27 possible isoelectronic analogues: CS, SiO, BCI, AIF, BeAr, MgNe, SN', PO', CCI+, SiF+, BAr+, AINe', SO2+,NC12+,PF2+, CAr2+,SiNe2+,OC13+,SF3+,NAr3+, PNe3+, FCI4+, OAr4+, SNe4+, FA$+, CINeS+, and ArNe6+. Special emphasis is placed on the structures and stabilities of the multiply charged species. Although several of the neutral and singly charged species have been studied in detail in the past,' they have been included here to allow comparisons with multiply charged species and to provide some indication of the likely accuracy of the calculated bond lengths, dissociation energies, and spectroscopic constants.

Methods and Results Ab initio molecular orbital calculations were carried out using the GAUSSIAN 868 and GAMESS~series of programs. The complete active space self-consistent-field (CASSCF) approachlo with the triple-{-valence plus polarization 6-3 1 lG(MC)(d) basis set" was used initially to calculate the ground-state potential energy curves of the 28 diatomic systems. The CASSCF active space comprised a the all of the valence orbitals, namely, 5u-8u and 2 ~ 3 for diatomic systems, 2s and 2p for the first-row atomic systems, and (6) For an interesting discussion of isoelectronic 22-electron molecules,see: Laurenzi, B. J.; Litto, C. J . Chem. Phys. 1983, 78, 6808, and references therein. (7) Note particularly the systematic studies of Peterson and Woods: (a) Peterson, K. A.; Woods, R. C. J . Chem. Phys. 1987.87, 4409. (b) Peterson, K. A.; Woods, R. C. J . Chem. Phys. 1988, 89, 4929. Values referred to in the text from these papers are MP4SDQ for bond lengths and spectroscopic constants and full MP4 for dissociation energies. (8) Frisch, M. J.; BinMey, 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.; Fluder, E. M.; Pople, J. A . GAUSSIAN 86: Carnegie-Mellon University: Pitssburgh, PA 15213. (9) (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. I , 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. ( I O ) Roos, B. 0.;Taylor, P. R.; Siegbahn, P. E. M . Chem. Phys. 1980.48,

157.

( 1 1 ) Wong, M. W.; Gill, P. M . W.; Nobes. R. H.; Radom, L. J . Phys.

Chem. 1988, 92, 4875.

0 1990 American Chemical Society

The Journal of Physical Chemistry, Vol. 94, No. 2, 1990 639

Isoelectronic Analogues of PN 3s and 3p for the second-row atomic systems. Such an active space leads to 328 space- and spin-adapted configurations for the diatomic molecules. The use of the full-valence CASSCF procedure here should lead to a balanced description of the dissociation of these triply 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/6-31 IG(MC)(d) level was also used to evaluate spectroscopic constants (we, wexe,Be, and ae)for all the stable diatomic species. For each system, about 10-15 points were obtained within the range re f 0.3 A. The calculated points were fitted to a polynomial of sixth degree (to better than lo4 hartrees or 0.2 cm-I) in the internal displacement coordinate AR = r re:

TABLE I: Equilibrium Bond Lengths Isoelectronic Analogues species PN

cs

Si0 BCI A1F SN+ PO+ CCI+ SiF+ BAr+ AINe'

so2+

where V is the potential energy function with a minimum (V,) at re. The spectroscopic constants were then derived from the coefficients Cf'")) of the polynomial.I2 For those systems displaying a potential minimum, equilibrium geometries were also determined at the third-order Mder-Plesset perturbation theory (MP3)'3a9blevel and with a version of coupled-cluster theory with double substitutions in which the effect of single and triple substitutions is incorporated via fourth-order perturbation theory (denoted ST4CCD),I4 again with the 631 IG(MC)(d) basis set. Dissociation energies were calculated from fourth-order Maller-Plesset (MP4)13c,dcalculations using the MP3 geometries. Since diffuse s and p functions, multiple sets of d functions, and, ' ~ significant in particular, f functions have been s h o ~ n ~to- have effects on the computed bond dissociation energies of multiply bonded diatomics, the 6-31 1 + G(MC)(2df) basis11J6was chosen initially for these calculations. Further extension of the basis set was accomplished by expanding the first polarization space from two sets to three sets of functions15 and the second polarization space from one set to two sets of functions," resulting in the 6-31 1 G(MC)(3d2f) basis set. Dissociation energies were also calculated, for comparison purposes, at the CASSCF, MP3, and ST4CCD levels by using the 6-31 lG(MC)(d) basis set. Zerepoint vibrational corrections to the directly calculated dissociation energies ( D e ) and dissociation barriers (&*) (yielding Doand Do* values, respectively) were evaluated from HF/6-3 1 lG(MC)(d) vibrational frequencies, the latter being scaled by 0.918 to take account of their overestimation at this level of theory. All of the Mdler-Plesset and coupled-cluster calculations employed the frozen-core approximation. Unless otherwise stated, equilibrium bond lengths (re)and dissociation energies (De and Do)in the text refer to MP3/6-31 lG(MC)(d) and MP4/6-311 G(MC)(3d2f) values, respectively, while for the multiply charged species, dissociation barriers (De* and Do*)correspond to CASSCF/631 IG(MC)(d) values. The ground-state potential curves for all the cations with charge four, five, or six are predicted to be purely repulsive, i..e., they do not have bound ground-state equilibrium structures. This is also true for the neutral systems BeAr and MgNe. For the remaining 22 systems, equilibrium geometries are shown in Table

+

+

( 1 2) Hollas, J. H. High Resolution Spectroscopy; Butterworths: London, 1982. (13) (a) Moiler, C.; Plesset, M. S. Pfiys. Reo. 1934, 46, 618. (b) Pople, J. A.; Binkley. J. S.; Seeger, R. In?. J. Quantum Chem. Symp. 1976, 10, 1. (c) Krishnan, R.; Pople, J. A. Int. J . Quantum Chem. 1978, 14, 91. (d) Krishnan, R.: Frisch, M. J.; Pople, J. A. J. Chem. Phys. 1980, 7 2 , 4244. (14) Raghavachari, K. J . Chem. Pfiys. 1985, 82, 4607. ( I 5) Binkley, J. S.; Frisch, M. J. Int. J . Quantum Chem. Symp. 1983, 17, 331. (16) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J . Chem. Phys. 1984, 80, 3265. (17) For multiple f polarization functions, the standard f functions with exponents a are replaced by two functions with exponents 0.5a and 2a. (18) 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. Symp. 1981, 1 5 , 269.

NC12+ PF2+ CAr2+ SiNe2+ OC13+ SF'+ NAr3+ PNe3+

CASSCF 1.516 1.560 1.528 1.749 1.689 1.460 1.450 1.562 1.556 3.145 3.444 1.413 1.462 1.475 1.776 2.540 1.458 1.448 1.654 1.955

MP3 1.484 1.535 1.503 1.727 1.678 1.428 1.422 1.537 1.544 2.489 3.239 1.372 1.416 1.458 1.738 2.389 1.358 1.398 1.504 1.939

(A) for PN and Its

theory" ST4CCD 1.515 1.558 1.530 1.736 1.685 1.463 1.448 1.555 1.555 2.461 3.200 1.422 1.471 1.479 1.749 2.353 1.467 1.457 1.557 1.940

ST4CCDC

exptb

1.491 1.534 1.506 1.722 1.661 1.439 1.424 1.532 1.531 2.437 3.176 1.398 1.447 1.455 1.725 2.329 1.443 1.433 1.533 1.916

1.491 1.535 1.510 1.716 1.654 1.440 1.419d 1.538' 1.526

"All results obtained with the 6-311G(MC)(d) basis set. bFrom ref 21 unless otherwise noted. 'Empirically corrected by -0.024 A; see text. dFrom ref 31. CFrom ref 32. fFrom ref 33.

I while their total energies and derived dissociation energies are shown in Tables I1 and 111, respectively. Spectroscopic parameters for these systems are given in Table IV. Discussion Comparison with Experiment. In order to assess the likely accuracy of the calculated values of re,De, and other spectroscopic constants in this study, we begin our discussion by examining those systems for which relevant experimental data are available for comparison. Theoretical equilibrium bond lengths calculated at the CASSCF, MP3, and ST4CCD levels all generally agree well with experiment, with the mean absolute deviations being less than 0.03 8, in each case (Table I). The calculated bond lengths at both the CASSCF and ST4CCD levels are found to be consistently slightly too long (with mean absolute deviations of 0.027 and 0.024 A, respectively). The smallest mean error (0.009 A) is that at the MP3/6-31 lG(MC)(d) level, and hence the MP3/6-311G(MC)(d) values are referred to in the text as our best (directly calculated) values. The error at the ST4CCD level is somewhat larger but fairly constant at 0.024 f 0.007 A. Thus, empirical correction of the ST4CCD bond lengths by subtraction of 0.024 8, leads to results in excellent agreement with the experimental values. In recent studies5J9we have found, through comparison with results from full-valence CASSCF CISD calculations, that Mdler-Plesset theory appears to be less satisfactory than ST4CCD theory in determining the equilibrium bond lengths of a number of multiply charged ions that contain multiple bonds. Hence, the corrected ST4CCD/6-3 1 lG(MC)(d) bond lengths (which in many cases are, nevertheless, close to the MP3/63 1 lG(MC)(d) values) are considered to provide our "best" estimates for multiply charged systems. In summary, the MP3 and corrected ST4CCD results are generally in reasonable agreement with one another. The former values are used for comparisons involving the neutral and singly charged species (where it would be inappropriate to use empirically corrected ST4CCD values) and the latter for the multiply charged ions. For the calculated dissociation energies (Table III), the mean absolute errors for CASSCF/6-31 lG(MC)(d), MP3/6-311G(MC)(d), and ST4CCD/6-31 lG(MC)(d) levels are large: 80, 104, and 70 kJ mol-I, respectively. However, significant improvement (mean error of 12 kJ mol-') is found at the MP4/6-

+

(19) Gill, P. M. W.; Wong, M. W.; Nobes, R. H.; Pople, J. A,; Radom, L. To be published.

640

The Journal of P h y s i c a l C h e m i s t r y , Vol. 94, No. 2, 1990

Wong and Radom

TABLE 11: Total Energies' (hartrees) and Zero-Point Vibrational Energies (ZPVE, kJ mol-') of Atomic and Diatomic Systems

energy species

NP

cs

Si0 BCI AIF SN+

PO+ CCI+ Si F+ BAr' AINe'

so2+ NCI2+

PF2+ CAr2+

SiNe2'

oc13+ S F3+ NAr3+

PNe3+ B

B+ C

C+

N N+ N2+ 0 Of 02+

F

F+

Ne Ne+ AI AI'

Si Si+ Si2+ P

P+ P*+ S S+

S+ CI

CI+

cs+ Ar Ar' Ar2+

+

6-31 lG(MC)(d) -395.296 22 -435.443 62 -363.939 26 -484.206 17 -341.551 17 -45 1.753 56 -41 5.434 51 -497.018 09 -388.235 96 -551.10268 -370.230 62 -41 1.454 28 -5 12.886 61 -439.301 93 -563.467 27 -416.561 98 -532.071 50 -494.808 72 -578.805 97 -467.22067

MP3/ 6-31 IG(MC)(d) -395.447 03 -435.59999 -364.091 56 -484.371 85 -341.70478 -45 1.905 20 -415.588 77 -497.178 16 -388.397 I5 -55 1.257 84 -370.435 1 I -471.601 37 -513.03333 -439.464 35 -563.626 84 -416.768 34 -532.198 45 -494.957 51 -578.934 80 -467.38664

ST4CCD/ 6-31 IG(MC)(d) -395.477 65 -435.623 24 -364.1 18 52 -484.385 17 -341.7 19 64 -451.938 09 -415.621 I O -497.200 28 -388.41 4 67 -55 1.270 86 -370.443 95 -47 1.643 14 -513.074 65 -439.489 28 -563.647 96 -416.77842 -532.25967 -495.002 23 -578.98460 -467.402 22

MP4/6-311 + G(MC)(2df)' -395.547 39 -435.688 52 -364.195 12 -484.445 50 -341.791 44 -452.009 I O -41 5.697 86 -497.267 74 -388.488 95 -551.33309 -370.501 36 -471.72728 -513.154 56 -439.575 72 -563.7 16 96 -416.838 35 -532.35040 -495.105 16 -579.080 85 -467.472 96

MP4/6-311 G(MC)(3d2Qa -395.557 59 -435.696 28 -364.208 30 -484.461 42 -341 3 0 6 28 -452.02006 -415.711 10 -497.275 12 -388.505 49 -551.338 9 1 -370.5 13 46 -471.741 25 -513.165 57 -439.592 62 -563.724 50 -416.852 03 -532.36447 -495.122 05 -579.09077 -467.489 98

-24.561 47 -24.293 79 -37.70408 -37.33063 -54.394 78 -53.903 53 -52.859 15 -74.800 22 -74.36005 -73.1 13 66 -99.393 43 -98.81 4 79 -1 28.522 67 -1 27.793 00 -241.894 34 -241.705 17 -288.861 19 -288.593 67 -288.031 14 -340.71 1 41 -340.357 00 -339.667 43 -397.498 72 -397.16490 -396.340 22 -459.473 44 -459.039 88 -458.21647 -526.806 87 -526.26403 -525.293 53

-24.582 8 1 -24.283 20 -37.75985 -37.351 87 -54.488 48 -53.96064 -52.878 96 -74.93087 -74.455 36 -73.17012 -99.562 52 -98.944 53 -128.731 32 -127.958 88 -241.91 2 91 -24 1.702 66 -288.906 45 -288.61797 -288.019 60 -340.79053 -340.4 I3 26 -339.695 90 -397.604 93 -397.25244 -396.400 64 -459.608 47 -458.153 62 -458.307 90 -526.96864 -526.404 86 -525.41 I 2 2

-24.591 66 -24.294 95 -37.766 59 -37.362 38 -54.491 37 -53.967 93 -52.89096 -74.933 98 -74.458 03 -73.177 84 -99.565 80 -98.947 12 -128.73542 -127.961 35 -241.91830 -241.707 17 -288.91 1 84 -288.62409 -288.033 57 -340.794 11 -340.419 37 -339.72079 -397.609 00 -397.255 87 -396.407 05 -459.61 140 -459.15741 -458.31 1 32 -526.969 62 -526.407 40 -525.41487

-24.592 79 -24.290 12 -37.775 69 -37.363 26 -54.51042 -53.976 76 -52.882 23 -74.968 43 -74.477 25 -73.18848 -99.6 13 44 -98.98063 -1 28.794 39 -1 28.006 83 -241.92351 -241.705 87 -288.925 84 -288.627 78 -288.032 18 -340.813 55 -340.427 87 -339.70464 -397.644 26 -397.273 86 -396.41629 -459.661 90 -458.19391 -458.331 98 -527.032 94 -526.459 86 -525.45492

-24.593 30 -24.290 14 -37.777 02 -37.36402 -54.5 12 90 -53.978 56 -52.893 42 -74.973 90 -74.480 01 -73.19075 -99.621 87 -98.985 83 -128.80567 -128.01431 -241.923 86 -241.706 08 -288.926 69 -288.628 50 -288.032 57 -340.8 15 23 -540.429 66 -339.706 17 -397.647 62 -397.276 56 -396.41 8 90 -459.666 24 -459.197 52 -458.33509 -527.037 76 -526.464 28 -525.458 83

CASSCF/

ZPVE 9.5 8.5 8.4 5.1 5 .O 10.4 9.9 7.7 6.5 0.5 0.2 10.9 9.8 7.7 4.2 0.8 10.7 8.7 6.3 2.3 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Using fully optimized geometries, unless otherwise noted. Using MP3/6-3 I IG(MC)(d) geometries.

31 1G(MC)(2df) level. At our best level of theory (MP4/6-311 + G(MC)(3d2f)), the mean error is just 6 kJ mol-I. Similar findings for calculated dissociation energies have been noted in a previous study of the isoelectronic analogues of N,.5 This again shows the importance of using large basis sets, which include diffuse s and p functions and multiple sets of d and f functions, for the adequate description of dissociation energies for multiply bonded systems. For other spectroscopic constants (we, up,, Be, and a,), the overall agreement between calculated (CASSCF/6-3 1 1G(MC)(d)) and experimental values is quite good, the mean absolute deviation being 23 cm-' for we, 0.6 cm-l for wg,, 0.03 cm-' for Be, and 0.0003 cm-' for ae. N e u t r a l Analogues. The PN molecule has been studied recently by multireference CIZ0and MP4' theories with large basis sets. These calculations yielded re values (1.491 and 1.494 A, respec(20) Grein, F.; Kapur, A. J . Mol. Spectrosc. 1983,99, 2 5 .

tively) close to experiment (1.488 A).2i For the dissociation energy of PN, the multireference CI calculations yielded a De (530 kJ mol-') considerably smaller than the experimental value (599 kJ while the MP4 value (597 kJ mol-') a rees well with experiment. The present best results for re (1.484 ) and De (602 kJ mol-]) are in pleasing agreement with experiment. The potential energy curve for carbon monosulfide has been investigated recently by CI,23CEPA?3 and MP4' methods. These yielded equilibrium bond lengths of 1.527, 1.543, and 1.537 A, respectively, compared with the experimental valueZi of 1.535 A and the present best value, also of 1.535 A. The last theoretical study also yielded a De value of 71 5 kJ mol-'. This value and the present best estimate (713 kJ mol-I) are in excellent agreement

1

(21) Herzberg, G.; Huber, K. P. Molecular Spectra and Molecular Structure. IV. Conrtants of Diatomic Molecules; Van Nostrand: Princeton, NJ, 1979. (22) Coquart, B.; Prudhomme, J. C . J . Mol. Spectrosc. 1981, 87, 75. (23) Botschwina, P.; Sebald, P. J . Mol.Spectrosc. 1%5, 110, 1.

The Journal of Physical Chemistry, Vol. 94, No. 2, 1990 641

Isoelectronic Analogues of PN

TABLE 111: Dissociation Energies (kJ mol-') for PN and Its Isoelectronic Analogues

-

theory'

+

CASSCF/ MP3/ ST4CCD/ MP4/6-311 + MP4/6-311 6-31 lG(MC)(d) 6-31 lG(MC)(d) 6-31 lG(MC)(d) G(MC)(2df)' G(MC)(3d2OC 499 44 1 505 587 602 cs-+cts 635 618 650 705 713 Si0 Si + 0 730 668 716 790 808 450 474 BCI B + CI 478 501 530 AIF AI + F 692 602 618 668 684 509 SNt St N 43 1 50 1 590 605 PO+ P+ 0 728 642 703 792 807 CCI+ c+ CI 562 572 595 637 643 SiFt Sit + F 653 569 590 650 670 BAr+ Bt Ar 5 16 17 26 29 AINe' AI' Ne 3 7 4 3 5 sol+ s+t o+ -186 -279 -186 -63 -40 NC12+ N t CIt -149 -213 -133 -42 -28 PF2+ Pt F+ 342 280 322 439 465 CAr2* Ct Art -335 -340 -320 -279 -273 SiNe2+ Si2+ Ne 22 46 25 31 36 OCP+ o++ c12+ -1 326 -1483 -1338 -1205 -1183 SF" S2' F+ -909 -1018 -924 -766 -742 NAr3+ N + Ar2+ -1027 -1 I48 -1045 -92 1 -901 PNe3+ P2+ Ne' -630 -704 -735 -626 -605 dissociation process PN-P+N

--- + -- ++ -- + --- ++ -- ++ --- + + -- ++

exptb 599c 717 804 536 670 615 796 6399 29*

DO theory' MP4/6-311 + G(MC)(3d2f)C*d .exptb 594 59lC 706 710 800 797 525 53 1 680 665 596 606 799 788' 636 6329 664 28 5 -50 -36 458 -276 36 -1 I93 -750 -907 -607

'Calculated using total energies shown in Table 11. From ref 21 unless otherwise noted. 'Using MP3/6-31 lG(MC)(d) geometries. dIncluding (scaled) zero-point vibrational correction. CFrom ref 22. fFrom ref 31. 9From ref 32. *From ref 35. TABLE IV: Spectroscopic Constants'Vb (u,,upxL, E,, and a,) for PN and Its Isoelectronic Analogues we, cm-I w d , , cm-I Be, cm-I 1299 (1337) 7.5 (7.0) 0.76 (0.79) cs I252 ( 1 285) 5.1 (6.5) 0.78 (0.82) 1235 (1241) Si0 6.4 (6.0) 0.71 (0.73) 815 (839) BCI 5.5 (5.1) 0.66 (0.68) AIF 805 (802) 4.9 (4.8) 0.53 (0.55) SN+ 1389 (1415) 9.3 0.81 PO+ ' 1378 (1410) 7.2 0.76 CCl+d 1149 (1178) 7.1 (6.6) 0.77 (0.80) SiFt 1032 (1051) 4.4 (5.0) 0.62 (0.64) BAr+ 78 3.1 0.20 AINe' 43 22.7 0.12 so2+ 1374 10.0 0.79 I253 NC12' 9.7 0.79 PF2+ I I73 4.9 0.66 CAr2' 638 5.9 0.58 %Ne2+ 139 6.3 0.22 933 OCP 30.5 0.72 1098 S F'+ 9.9 0.68 NAr" 565 19.7 0.59 426 PNe" 2.6 0.36

species PN

a,,cm-I 0.0054 (0.0055) 0.0058 (0.0059) 0.0047 (0.0050) 0.0064 (0.0065) 0.0042 (0.0050) 0.0061 0.0052 0.0064 (0.0065) 0.0042 (0.0047) 0.0069 0.0109 0.0070 0.0074 0.0044 0.0069 0.0073 0.01 52 0.0068 0.0160 0.0036

"Calculated at the CASSCF/6-31 IG(MC)(d) level. Experimental values (in parentheses) from ref 21 unless otherwise noted. 'Experimental values from ref 3 1. dExperimental values from ref 32.

with the experimental value2' of 717 kJ mol-]. Due to its importance in astrophysics, the silicon monoxide molecule has been the subject of several experimental and theoretical studies. Recent high-level theoretical investigations include calculations at Cl,24MCSCF-CI,25CEPA,26and MP47 levels. These yielded equilibrium bond lengths of 1.496, 1.515, 1.519, and 1.5 17 A, respectively, compared with the experimental value of 1.510 A. The MP4 calculations7 led to a dissociation energy (799 kJ mol-]) very close to the experimental value2' of 804 kJ mol-'. The present best estimates of re (1.503 A) and De (808 kJ mol-') are also in good agreement with experiment. Very few ab initio calculations on BCI have been reported. Recent MP4 calculations of Peterson and Woods7 yielded an re of 1.7 19 A and a De of 506 kJ mol-', compared with the experimental values2I of 1.716 A and 536 kJ mol-], respectively. Again, (24) Langhoff, S. R.; Arnold, J. 0. J . Chem. Phys. 1979, 70, 852. (25) Werner, H.-J.; Rosmus, P.; Grimm, M. Chem. Phys. 1982, 73, 169. (26) Botschwina, P.; Rosmus, P. J . Chem. Phys. 1985, 82, 1420.

the present theoretical estimates of re (1.727 A) and De (530 kJ mol-]) are in pleasing agreement with experiment. Aluminum fluoride has been the subject of numerous visible, microwave, and matrix infrared spectroscopic studies. High-level ab initio studies reported to date for this molecule include SCEP/CEPA27and MP4' calculations. These yielded an re (1.664 A in both cases) close to experiment (1.654 Our CASSCF, MP3, and ST4CCD values all overestimate the AI-F bond length by 0.02-0.03 A (Table I). For the dissociation energy of AlF, the MP4 study gave a value of 671 kJ mol-I, in excellent agreement with the experimental value (670 kJ mol-1).21 Our best estimate of De (684 kJ mol-'), on the other hand, slightly overestimates (by 14 kJ mol-]) the experimental value. Consistent with a previous single-configuration calculation2* state of BeAr on BeAr, we find no minimum for the lowest at the CASSCF/6-31 lG(MC)(d) level. Similarly, the groundstate potential curve of MgNe is also predicted to be purely repulsive; Le., there is no bound equilibrium structure. Monocation Analogues. CEPA,29CI,30and MP47 calculations have recently been reported for the S N + ion. These yielded equilibrium bond lengths of 1.44, 1.443, and 1.439 A, respectively, agreeing well with the experimental value2] of 1.440 A. The last study also obtained a De (605 kJ mol-]) close to the experimental value of 615 kJ mol-]. The present results lead to values of re (1.428) and De (605 kJ mol-') in satisfying agreement with experiment. The ground state ('Z,+) of PO+ has been recently studied by photoelectron spectroscopy.31 The re and De obtained in these experiments are 1.419 A and 796 kJ mol-', respectively. Recent MP4 calculations of Peterson and Woods7 yielded an re of 1.434 A and a De of 805 kJ mol-', in close agreement with experiment. Again, the present best results for re (1.422 A) and De (807 kJ mol-') are in pleasing agreement with the experimental estimates. The potential function for C C P has been investigated recently both e~perimentally~~ and theoretically? Velocity-modulationlaser spectro~copy~~ yielded an equilibrium bond length of 1 S38 A and (27) Klein, R.; Rosmus, P. Theor. Chim. Acta 1984, 66, 21. (28) (a) Kaufmann, J. J. J . Chem. Phys. 1973.3'8.4880. (b) Rzepa, H. S . J . Mol. Struct.: THEOCHEM 1985, 121, 313. (29) Zirc, C.; Ahlrichs, R. Inorg. Chem. 1984, 23, 26. (30) Karna, S. P.; Grein, F. Chem. Phys. 1986, 109, 35. (31) Dyke, J . M.; Morris, A,; Ridha, A. J . Chem. SOC.,Faraday Trans. 2 1982, 78, 2077. (32) Gruebele, M.; Polak, M.; Blake, G.A,; Saykally, R. J. J . Chem. Phys. 1986, 85, 6276.

642

The Journal of Physical Chemistry, Vol. 94, No. 2, 1990

TABLE V: Total Energies (hartrees), Bond Lengths (rn, A), Dissociation Barriers (De' and Do', kJ mol-'), and Kinetic Energy Releases ( T , eV) Associated with Transition Structures for Fragmentation of Dications and Trications species energy' rTq D.' Do' T so2+ -471.32680 2.259 335 325 5.4 -512.762 13 2.473 327 318 4.9 NCI2+ CAr2+ -563.42574 2.723 109 105 4.6 32 22 14.1 -532.05930 1.730 OC13+ S F3+ -494.73087 2.144 204 197 11.5 2.069 16 10.9 NAr" -578.79767 22 PNe3+ -467.171 57 4.013 129 127 7.9 "Calculated at the CASSCF/6-31 IG(MC)* level

a dissociation energy of 639 kJ mol-'. The MP4 calculations of Peterson and Woods7 gave an re of 1.537 8, and a De of 649 kJ mol-', agreeing well with experiment. The present results for re (1.537 8,) and De(643 kJ mol-') are also in excellent agreement with experiment. The first detection of SiF+ has been achieved very recently by means of microwave s e c t r o ~ c o p y . ~These ~ experiments led to a prediction of 1.527 for the equilibrium bond length of SiF+. Theoretically, the ground-state potential curve of SiF+ has been studied at the CIj4 and MP47 levels. These yielded re values of 1.543 and 1.532 A, res ctively. These values and the best present estimate of re (1.544 ) are all close to the experimental value. The latter theoretical study also yielded a dissociation energy of 661 kJ mol-', consistent with the present MP4 estimate (670 kJ mol-'). The BAr+ ion has been studied experimentally through scattering of B+ by Ar.3S In these experiments, the potential energy curve of BAr+ was derived from the measured cross section and a dissociation energy of 29 kJ mol-' was obtained. Theoretically, the BAr+ ion has been studied by a multireference CI approach.36 This yielded an re of 2.37 8, and a De of 34 kJ mol-', in good agreement with experiment. The present estimate of re (2.489 8,) is consistent with the multireference value, and our MP4 estimate of De (29 kJ mol-') coincides with the experimental value. We are not aware of any previous experimental or theoretical study of the AINe+ ion. We find the ground state ('Z') of AINe+ to be characterized by a very long equilibrium bond distance of 3.239 8, and a small binding energy of just 5 kJ mol-'. This species is best pictured as a weak ion-induced-dipole complex of AI+ and Ne. At the equilibrium bond length of AINe', the positive charge resides almost entirely on the AI atom (the Mulliken charge on AI is +0.99). Dication Analogues. The SO2+ dication has a very short equilibrium bond length of 1.398 8, (corrected ST4CCD)-the shortest among the 20 molecules in Table I. As noted in a recent study,5 the valence-isoelectronicanalogue 0;' is predicted to have the shortest equilibrium bond length between any two heavy atoms. Likewise, the S-0 bond length in SO2+ may correspond to the shortest bond between any first-row atom and second-row atom. In accordance with the short equilibrium bond length, this dication is predicted to have a high vibrational frequency of 1373 cm-' (CASSCF/6-31 IG(MC)(d)). Dissociation of SO2+to S+ + 0' is calculated to be exothermic by 50 kJ mol-' (Do). This reaction is inhibited by a large activation barrier (Do*)of 325 kJ mol-'. Thus, SO2+should be experimentally accessible, for example by charge stripping of the SO+ monocation. The calculated ionization energy for the process SO+ SO2+is 19.2 eV (IEJ. The predicted kinetic energy release ( T ) for the production of S+ + 0' from SO2+is 5.4 eV (Table V). In a similar manner to SO2+,NC12+also displays a very short equilibrium bond length and remarkable kinetic stability. The

1

r-

-

(33) Petrmichl, R. H.; Peterson, K. A,; Woods, R. C . J . Chem. Phys. 1988,

89, 5454.

(34) Karna, S. P.; Grein, F. J . Mol. Specrrosc. 1987, 122, 28. (35) Ding, A.; Karlau, J.; Weise, J.; Kendrick, J.; Kuntz, P. J.; Hillier, J. H.; Guest, M . F. J . Chem. Phys. 1978, 68, 2206. (36) Iwato, S.: Sato, N. J . Chem. Phys. 1985, 82, 2346.

Wong and Radom calculated N-CI bond length in NC12+ is 1.447 8, (corrected ST4CCD). Dissociation of NC12+ is exothermic by just 36 kJ mol-' (Do). The calculated barrier to dissociation ( D o t ) to N+ + Cl+ is 318 kJ mol-'. This dication therefore also represents an attractive target for gas-phase observation. The predicted IE, and T values are 19.2 and 4.9 eV, respectively (Table V). For the PF2+dication, there has been a recent calculation of the P-F bond len th at the Hartree-Fock Our best estimate is 1.455 (corrected ST4CCD). In contrast to SO2+ and NCl2+,PF2+is predicted not only to be kinetically stable but also to be a thermodynamically stable species. Fragmentation reactions giving P+ + F+ and P2+ + F are both calculated to be endothermic (by 458 and 688 kJ mol-', respectively). Hence, this doubly charged ion should be readily accessible in the gas phase and perhaps even in solution. Our predicted Doand we values for PF2+ are 458 kJ mol-' and 1173 cm-I, respectively. The potential energy curve for the '2' ground state of CAr2+ has been studied previously by Hurley's semiempirical procedure3* and recently by multireference CI calculation^.^^ Consistent with these previous studies, we find the ground-state CAr2+ dication to have a sizable dissociation barrier (Do'= 105 kJ mol-') inhibiting the exothermic fragmentation reaction CAr2+ C+ + Ar'. With such a large barrier, CAr2+should be experimentally observable in the gas phase. However, no evidence for any CAr2+ was found in charge-stripping experiments of Jonathan et aL3* We have recently examined the low-lying excited states of CAr+ and CAr2+and rationalized the unexpected experimental results in terms of the CArf monocation being produced in an excited (42-)state.40 As in the case of PF2+,SiNe2+ is calculated to be thermodynamically stable. The lowest dissociation pathway in this case corresponds to SiNe2+ Si2+ Ne. This reaction is calculated to be endothermic by 36 kJ mol-'. The charge-separation reaction SiNe2+ Si+ Ne', on the other hand, is considerably higher in energy, 549 kJ mol-' above the SiNe2+ minimum. This is because the second ionization energy of silicon (1 565 kJ mol-') is substantially lower than the first ionization energy of neon (2078 kJ mol-'). One possible means of generating SiNe2+ is through collision of an Si2+dication with neon. Note that the bond length in this species (2.329 A, corrected ST4CCD) is considerably greater than that in the other dicationic systems examined here. Mulliken population analysis shows that there is virtually no 7r-bonding contribution to the Si-Ne bond and that most of the positive charge resides on the Si atom (Mulliken charge of +1.95 on Si at the equilibrium distance). SiNe2+ is therefore best described as Si2+-Ne in which there is significant electrostatic stabilization resulting from the polarization of Ne by the doubly charged silicon. Trication Analogues. All the triply charged ions considered here possess bound equilibrium structures in their ground states. As for the dication analogues, these trications (except for the neon-containing species) are characterized by short multiple bonds. Compared with the corresponding first-row-first-row analogues, these ions display greater kinetic stability. The OC13+trication is predicted to have a short equilibrium bond. However, the calculated value is sensitive to the level of theory used, values ranging from 1.358 to 1.467 8, (Table I). Our best estimate is 1.443 A (corrected ST4CCD) but is associated with a larger than normal uncertainty. Fragmentation of OCl" to 0' + CI2+is extremely exothermic (by 1193 kJ mol-'). This reaction is impeded by a barrier ( D e * )of just 32 kJ mol-'. Inclusion of the zero-point vibrational correction leads to a prediction of 22 kJ mol-' for Do*. Hence, experimental observation of this ion will not be straightforward. As with OC13+, SF3+ also exhibits a short equilibrium bond length (1.433 A). It is of interest to note the successive decrease

w

-

-

-

+

+

(37) OKeefe, M. J . A m . Chem. SOC.1986, 108, 4341. (38) Jonathan, P.; Boyd, R. K.; Brenton, A. G.; Beynon, J. H. Chem. Phys. 1986, 110, 239. (39) Vincent, M. A,; Hillier, 1. H. J . Chem. Soc., Faraday Tram. 2 1988, 84, 1229. (40) Wong, M. W . ; Radom, L. J . Phys. Chem. 1989, 93, 6303.

Isoelectronic Analogues of PN

The Journal of Physical Chemistry, Vol. 94, No. 2, 1990 643

in S-F bond distance in going from neutral S F to the monocation to the dication and to the trication: 1.638, 1S40, 1.457, and 1.398 A, respectively (MP3/6-31 lG(MC)(d) values). This trend of bond lengths in SF"+ ( n = 0-3) can readily be understood in terms of the successive removal of electrons from the antibonding 3 a orbitals. Fragmentation of SF3+to S2++ F+ is highly exothermic (by 750 kJ mol-'). This reaction, however, is associated with a large activation barrier of 197 kJ mol-'. Thus, SF3+ is predicted to lie in a deep well and should be a promising candidate for gas-phase observation. For example, this ion might be accessible via charge stripping of the SF2+dication. The predicted ionization energy for SF2+is 3 1.9 eV (IEa), and the calculated kinetic energy release for the production of S2++ F+ from SF3+is 1 1.5 eV (Table V). Interestingly, our calculations suggest that this triply charged ion can also be generated from neutral S F via conventional electron-impact mass spectrometric methods. The computed triple-ionization energy for neutral SF is 62.0 eV (IEa). In sharp contrast to other noble-gas-containingcations examined in this paper, NAr3+ displays quite a short equilibrium bond length. The calculated N-Ar distance (1.533 A, corrected ST4CCD) is comparable to those in more conventional triply bonded systems such as C S (1.535 A) and Si0 (1.510 A). In fact, in a recent paper,@we have shown that the noble-gas element argon is capable of forming strong multiple bonds in multiply charged species and that the NAr3+trication provides one such example. Despite its short bond, NAr3+ is predicted to lie in a shallow potential well. Dissociation to N+ + Ar2+ is calculated to have a small barrier (Do*) of 16 kJ mol-,. In a similar manner to the neon-containing dication (SiNe2+), PNe3+ is characterized by a long equilibrium bond length (1.91 6 A, corrected ST4CCD) and very small A overlap population. This trication can be considered as a charge-induced-dipole complex, P3+.-Ne. At the equilbrium distance, the Mulliken charges on P and Ne are +2.78 and +0.22, respectively. Compared with SiNeZ+,PNe3+shows greater electron donation from the Ne atom. This can be attributed to the stronger charge-induced-dipole effect and better bonding interaction of the Ne pz orbital with the vacant pz orbital on P3+. As a consequence, the equilibrium bond length in PNe3+ is shorter than that in SiNe2+ (by 0.413 A). This trication is predicted to be a kinetically stable species with a sizable activation barrier (Do',127 kJ mol-') associated with the highly exothermic fragmentation reaction PNe3+ P2+ Ne+. Thus, by means of appropriate experiments, this triply charged ion should be detectable in the gas phase. Note that the bond length in the dissociating transition structure is rather long (4.013 A, Table V).

-

+

Avoided-Crossing Models for Multiply Charged Cations Recently, Gill and Radom have discussed avoided-crossing (AC)41and avoided-crossing with diabatic coupling and polarization (ACDCP)42models to describe the fragmentation behavior of doubly charged ions. These models provide useful predictions of the transition structure bond length (rn) and the kinetic energy released (T) in fragmentations of dicationic systems. According to these model^,^^,^^ the potential energy curve describing the fragmentation of a diatomic dication AB2+ may be considered as arising from an avoided crossing between an attractive diabatic curve (correlating with A2+ + B) and a repulsive diabatic curve (correlating with A+ + B+) (see Figure 1). The asymptotic energy difference between the two curves (A1) is equal to the difference in abiabatic ionization energies of A+ and B: A, = IE,(A+) - IEa(B)

The simplest form of the avoided-crossing (AC) model neglects the charge-induced-dipole interaction between A2+and B and also (41) (a) Gill, P. M. W.; Radom, L. Chem. Phys. Lett. 1987,136, 294. (b) Radom, L.; Gill, P. M . W.; Wong, M. W. In The Structure of Small Molecules and Ions; Naaman, R., Vager, Z . , Eds.; Plenum: New York, 1989. (42) (a) Gill, P. M. W.; Radom, L. Chem. Phys. Left.1988,147,213. (b) Radom, L.: Gill, P. M . W.; Wong, M. W. Int. J . Quantum. Chem. Symp. 1988, 22, 567.

Energy

\\