Microwave Spectrum, Structure, and Hyperfine Constants of Kr–AgCl: Formation of a Weak Kr–Ag Covalent Bond

Journal of Molecular Spectroscopy 206, 33–40 (2001) doi:10.1006/jmsp.2000.8286, available online at http://www.idealibrary.com on

Microwave Spectrum, Structure, and Hyperfine Constants of Kr–AgCl: Formation of a Weak Kr–Ag Covalent Bond Linda M. Reynard, Corey J. Evans, and Michael C. L. Gerry Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1 Received July 31, 2000; in revised form December 1, 2000

The pure rotational spectrum of the complex Kr–AgCl has been measured between 8–15 GHz using a cavity pulsed-jet Fourier transform microwave spectrometer. The complex was found to be linear and relatively rigid, with a Kr–Ag bond length ˚ The Kr–Ag stretching frequency was estimated to be 117 cm−1 . Ab initio calculations performed at the MP2 level of ∼2.641 A. of theory gave the geometry, vibration frequencies, Kr–Ag bond dissociation energy, and orbital populations. The Kr–Ag bond dissociation energy was estimated to be ∼28 kJ mol−1 . The Kr–Ag force constant and dissociation energy are greater than those of Ar–Ag in Ar–AgCl. The chlorine nuclear quadrupole coupling constants show slight changes on complex formation. Ab initio orbital population analysis shows a small shift in σ -electron density from Kr to Ag on complex formation. The combined C 2001 Academic Press experimental and ab initio results are consistent with the presence of a weak Kr–Ag covalent bond. °

ical bond, should be reflected in a nonzero nuclear quadrupole coupling constant. This information could not be obtained for Kr–AuCl because any signals of the 83 Kr isotopomer were below the detection limit of the spectrometer. Although Ag (I = 1/2) has two isotopes as opposed to only one for Au (I = 3/2), the latter produces significant hyperfine structure, and it was hoped that conditions for finding 83 Kr hyperfine structure might be better in 83 Kr–AgCl.

I. INTRODUCTION

Recently, some novel linear triatomic complexes have been studied by Fourier transform microwave (FTMW) spectroscopy, following the discovery of Ar–AgCl (1–4). They consist of a noble gas atom bonded to the metal atom of a coinage metal halide. Specifically, they are Ar–CuX , Ar–AgX , Ar–AuX , and Kr–AuCl, where X is F, Cl, Br. These complexes are unusually rigid and have shorter noble gas–metal bonds than those of “regular” van der Waals complexes, suggesting that there is some degree of chemical bonding a present. The noble gas–metal a bond lengths vary from 2.22 A for Ar–CuF to 2.64 A for Ar– AgBr. Previously, only one other complex of this type, Ar–NaCl, had been studied spectroscopically, but in this case the complex is very flexible and has a long Ar–Na bond, consistent with a van der Waals interaction (5). It was noticed that the argon– coinage metal complexes are isoelectronic with the well-known stable ions [Cl–Cu–Cl]− , [Cl–Ag–Cl]− , and [Cl–Au–Cl]− , so that comparison with these species may give some insight into the nature of the bonding in the complexes. This paper describes the FTMW spectrum of a second Krcontaining complex, Kr–AgCl. This spectrum was sought for several reasons. In the first place Kr–AuCl is more strongly bound than Ar–AuCl, and it was interesting to see whether this trend would carry over to the AgCl complex. Secondly, in Ar– AgCl the Ag atom is very close to the centre of mass, making it difficult to locate accurately. This, combined with the fact that Ar has only one usable isotope, left significant uncertainty in its derived geometry. Both these difficulties are reduced in Kr–AgCl: Ag is farther from the centre of mass, and Kr has several abundant isotopes. Third, 83 Kr has nuclear spin I = 9/2 and thus a nuclear quadrupole moment. Any significant rearrangement of the electron distribution at Kr, say by the formation of a chem-

II. EXPERIMENT

The rotational spectrum of Kr–AgCl was measured between 8 and 15 GHz using the Balle–Flygare-type FTMW spectrometer (6), which has been described in detail elsewhere (7). It contains a cylindrical Fabry–Perot cavity approximately 30 cm in length, with spherical mirrors at each end. The mirrors are 28 cm in diameter, with a radius of curvature of 38.4 cm. One of the mirrors may be moved by a micrometer screw in order to bring the cavity into resonance at the microwave excitation frequency. The microwaves used to excite the sample are generated by a microwave synthesizer referenced to an external Loran frequency standard, accurate to one part in 1010 . The microwaves are transmitted into the cavity through an antenna mounted at the center of the movable mirror. Near the center of the fixed mirror is a pulsed nozzle (General Valve, Series 9), which is used to introduce the gas mixture into the cavity. Because the jet and the direction of propagation of the microwaves are parallel, each line is split to a doublet by the Doppler effect. The complexes were produced using the laser ablation system which has been described in detail earlier (8). A 5-mmdiameter Ag rod (Goodfellow, 99.9% purity) was held directly in front of the pulsed nozzle and ablated with the second harmonic 33

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REYNARD, EVANS, AND GERRY

TABLE 1 Transition Frequencies in MHz for Kr–AgCl

(532 nm) of a Nd : YAG laser. The rod was continuously rotated and translated in order to keep a fresh surface available for irradiation. The backing gas mixture contained 0.05–0.15% Cl2 in pure krypton at a backing gas pressure of 6–7 atm. The strongest lines required ∼500 pulses to yield an adequate signal-to-noise ratio; for the transitions of the less abundant isotopomers approximately 3000 cycles were needed. The linewidths (FWHM) were approximately 10 kHz, giving estimated accuracies of ±1 kHz. III. QUANTUM CHEMICAL CALCULATIONS

Quantum chemical calculations were performed at the second-order Møller–Plesset (MP2) (9) level of theory using the GAUSSIAN 98 suite of programs (10). A relativistic effective core potential (RECP) was used for Ag, leaving 19 valence electrons (4s 2 4 p 6 4d 10 5s 1 ). The RECP for Ag and the optimized (31111s/22111 p/411d) Gaussian basis sets were taken from Andrae et al. (11). The Ag basis set was further augmented with two f functions (α f = 3.1235 and α f = 1.3375) (12), as it has been shown that at least one f -symmetry function should be included to obtain reliable correlation energies for the 4d shell of Ag (13). For Cl the (631111s/52111 p) McLean– Chandler basis set (14) augmented with one d-polarization function (αd = 0.75) (12) was used. For Ar and Kr the cc-pVTZ basis set was used (15). The geometry was fixed to a linear configuration. IV. EXPERIMENTAL RESULTS AND ANALYSIS a

Initially, an estimate of the Kr–Ag bond length of 2.65 A was made through a comparison with the corresponding values and trends in Ar–AgCl, Ar–AuCl, and Kr–AuCl. This, along with the Ag–Cl distance and Cl nuclear quadrupole coupling constant and hyperfine structure for the monomer, was used to predict rotational constants and hyperfine structure for Kr–AgCl. A search for the J = 6–5 transition beginning near 8900 MHz revealed a group of lines with the anticipated 35 Cl quadrupole patterns within 70 MHz of the prediction; it was provisionally assigned to 84 Kr–107 Ag35 Cl. This assignment was confirmed by the spectra of five other isotopomers, with the correct hyperfine patterns and intensities, at frequencies consistent with this assignment. In all, five J 0 –J 00 transitions were measured for each isotopomer; the results are presented in Table 1. The coupling scheme used is J + ICl = F. Figure 1 shows a portion of the hyperfine structure of J = 8−7 of 86 Kr–107 Ag35 Cl (abundance 6.8%). It gives an indication of the signal-to-noise ratio obtained in the experiments; this was somewhat less than that obtained for Ar–AgCl (1). Lines of 83 Kr–AgCl (abundance 4.5%) were unfortunately not found. Presumably the low isotopic abundance of 83 Kr, combined with probable hyperfine structure (which, since I(83 Kr) = 9/2, would consist of at least 10 weak components), put the lines below the detection limit of the spectrometer.

a

Residual: observed frequency—calculated frequency (kHz).

No searches were carried out for the transitions of excited vibrational states: since none had been found for the complexes previously studied, it was anticipated that the same situation would apply here. The measured frequencies were fitted using Pickett’s global least-squares program SPFIT (16) to give ground state values

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35

ROTATIONAL SPECTRUM OF Kr–AgCl

stitution had been made at only one of the atoms. The results are given in Table 3. The equilibrium bond length, re , of AgCl is also included in Table 3 for comparison. The bond lengths vary depending on the isotopomer pair chosen: the differences a between the largesta and smallest bond length are 0.008 A for Kr–Ag and 0.014 A for Ag–Cl. Effects of zero-point vibrations on the geometry are clearly significant, though not unusually so. The averaged r0 values and their uncertainties are also in Table 3. Substitution bond lengths, rs , are given in Table 4. In this method one of the isotopomers is chosen as a basis and isotopic substitutions are made at each atom in turn (18). The distance from the substituted atom to the center of mass of the basis molecule, z, is given by s 1I , [1] |z| = µ where 1I is the change in the moment of inertia upon substitution and µ is the reduced mass of the substitution, defined as µ=

FIG. 1. A portion of the observed spectrum for the J = 8–7 transition of Experimental conditions: 0.1% Cl2 in Kr at backing pressure 6–7 atm, Ag rod, 0.2-µs microwave pulse width, 2000 cycles, 4k transform.

86 Kr–107 Ag35 Cl.

of the rotational constant B0 , the centrifugal distortion constant D J , and the chlorine nuclear quadrupole coupling constant eQq for each isotopomer. The results are in Table 2. V. STRUCTURE OF THE COMPLEX

A. Geometry Because rotational constants (B0 ) have been obtained for several isotopomers, there are sufficient data to calculate both bond lengths in the complex. The ground state effective values, r0 , have been calculated using pairs of isotopomers in which a sub-

M1m , M + 1m

with M representing the mass of the basis molecule and 1m the change in mass upon substitution. For two of the basis molecules enough isotopic substitutions had been made to calculate a complete rs structure. Using the other four basis molecules it was possible to calculate one of the bond lengths using Eq. [1], with the other being calculated using the first moment equation, X [3] m i z i = 0. The rs method has produced bond lengths which are more self-consistent than the r0 values, but still with some scatter. In particular, the bond lengths calculated using the first moment equation differ significantly from those calculated using a the full substitution procedure. Since the Ag atom is ∼0.6 A from the center of mass the latter should produce reliable values (18). Ignoring the bond lengths obtained using the first moment equation leads to dramatic improvement in the consistency of the results.

TABLE 2 Molecular Constants for Six Isotopomers of Kr–AgCl

a b

[2]

Root mean square deviations of the fit. Numbers in parentheses are one standard deviation in units of the last significant figure.

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REYNARD, EVANS, AND GERRY

TABLE 3 ˚ r0 Bond Lengths for Kr–AgCl in A

equation I0 = Im + c(Im )1/2 + d(m 1 m 2 · · · m N /M)1/(2N −2) ,

a Numbers in parentheses are one standard deviation in units of the last significant figure. b Reference 17.

However, the uncertainty introduced into the averaged structure given in Table 4 is well within the range normally expected for rs values. Within one standard deviation of the averages, the rs and r0 values agree. Geometries were also obtained using two least-squares fitting procedures. In one case the experimental moments of inertia, I0 , were fitted to the equation I0 = Irigid (r I ² ) + ²,

[4]

where the constant ² is asssumed to be independent of isotopomer. The resulting geometry is the r I ² geometry of Rudolph (19). Since this assumption is the same as the one made in the rs procedure, the r I ² and rs parameters for a large enough data set should be essentially the same. Table 4 shows this to be the case: the r I ² geometry agrees with the averaged rs value within two standard deviations of the latter. In the final fit a mass-dependent rm(2) structure was obtained. In this case the experimental moments of inertia were fit to the TABLE 4 ˚ rs , r Iε , and rm(2) Bond Lengths for Kr–AgCl in A

[5]

where c and d are constants. This procedure has recently been shown by Watson et al. (20) for triatomic molecules to give bond lengths with small standard deviations, and which in many cases are excellent approximations to the equilibrium distances. The results for Kr–AgCl are given in Table 4 as rm(2) values. These values also agree with the rs values within two standard deviations of the latter. It is uncertain which derived geometry is to be preferred. The results of Ref. (20) suggest that the rm(2) values should be closest to equilibrium (re ) bond lengths, though problems can be encountered when there are no atoms close to the center of mass (as in the case here). However, the rm(2) values are the shortest of essentially all the derived bond lengths, which gives confidence, especially if the anharmonicity in all the stretching potentials is cubic, that they do indeed provide the best approximation to the re values. The Ag–Cl bond is somewhat shorter in the complex than in the AgCl monomer; this could be due to charge rearrangement on formation of the complex. More interestingly, the Kr–Ag a bond length of 2.641 A is relatively short, in contrast to longer bonds for van der Waals complexes, such as Ar–NaCl with an a (5) or Ar–Hg with a bond length Ar–Na bond length of 2.89 A a of 4.05 A (21). Table 5 compares the bond lengths of various noble gas–metal halide complexes. The increase in bond length on substitution of Kr for Ar in the silver complex is comparable to the increasea for the gold complex. In both cases it is smaller than the 0.1-A difference in the van der Waals radii of Ar and Kr (22). The fact that the Ar–Ag and Kr–Ag bonds are short, and very close in length, imply that the noble gas–metal bonds are stronger than “regular” van der Waals bonds. B. Distortion Constant, Vibration Frequency, and Force Constant The centrifugal distortion constant (D J ) for the 84 Kr– Ag35 Cl isotopomer is 0.139 kHz, compared to 0.347 kHz for Ar–107 Ag35 Cl (1). This difference also follows the trend set by the corresponding AuCl complexes, where Kr–AuCl has a smaller distortion constant than Ar–AuCl (3). In addition, the distortion constant of Kr–AgCl is almost two orders of magnitude smaller than that of Ar–Na35 Cl (9.087 kHz) (5), again implying that Kr–AgCl is not a loosely bound van der Waals complex. If the diatomic approximation is used, wherein AgCl is treated as a single unit, the Kr–Ag stretching frequency, ω, can be estimated from the expression (24, Chap. 13) 107

a

This bond length was evaluated using the first moment equation (Eq. 3). Numbers in parentheses are one standard deviation in units of the last significant figure. a c Obtained from a least-squares fit using Eq. 4. ε = 1.48 (23) amu A2 . d Obtained from a least-squares fit using Eq. 5. c = 0.21310 (15) (amu)1/2 a a A, d = −0.36078 (51) (amu)1/2 A2 . e Reference 17. b

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s ω∼

4B03 . DJ

[6]

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ROTATIONAL SPECTRUM OF Kr–AgCl

TABLE 5 Bond Lengths, Force Constants, and Dissociation Energies for Various Complexes

C. Nuclear Quadrupole Coupling Constants

a

Reference 3. Reference 4. c This work. d Force constant calculated from 1G 1/2 of the ground electronic state from reference 23. e Force constant calculated from molecular constants given in reference 5. b

This gives ω = 117 cm−1 , which is much larger than the ground state value for the van der Waals complex Kr–Ag (∼20 cm−1 ) (23) or that of Ar–NaCl (21 cm−1 ) (5). The Ar–Ag stretching frequency in Ar–AgCl is 135 cm−1 (1). However, because of the different reduced masses of the Ar and Kr complexes, the force constant of the bond, k, has been evaluated to provide a more meaningful comparison. It is given by s 1 ω= 2π

k µ

[7]

for a diatomic molecule, where µ is the reduced mass. Table 5 gives the force constants for several complexes of this type, using a diatomic approximation. The value for Kr–Ag in Kr–AgCl is 43.2 N m−1 , compared to 33.5 N m−1 for Ar–Ag in Ar–AgCl. Thus the two complexes have similar force constants, with the Kr–Ag bond slightly more rigid. This follows the trend for Ar– AuCl and Kr–AuCl, where the force constants of the noble–gas metal bonds are 78.5 N m−1 and 93.6 N m−1 , respectively. In all cases the values are substantially greater than those of Kr–Ag in its ground electronic state, and of Ar–Na in Ar–NaCl, which are 0.8 N m−1 and 0.6 N m−1 , respectively. In comparison to “regular” van der Waals complexes, the noble gas–coinage metal halide complexes are unusually rigid. Taken in combination with the short bonds, this suggests that the noble gas–metal bonds are relatively strong and raises the possibility that there may be some degree of chemical bonding present.

If Kr–AgCl is to show any evidence of chemical bonding, there should be charge rearrangement on its formation. This rearrangement should change the electric field gradient at each nucleus and thus, where applicable, change the nuclear quadrupole coupling constants (NQCC). For Kr–AgCl, because Ag has zero quadrupole moment, the greatest change should probably be noticeable for Kr. It was thus unfortunate that lines of 83 Kr–AgCl could not be found. The chlorine NQCC, however, have been determined and are given in Table 2. Table 6 compares the 35 Cl NQCC for copper, silver, and gold complexes, the metal chloride monomers, and the [Cl–M–Cl]− ions. The Cl NQCC change slightly on complex formation for both Ar– and Kr–gold and silver chlorides and for Ar–CuCl, indicating that some charge rearrangement may be occurring. For both the silver and gold complexes the change is slightly greater with Kr than with Ar, suggesting that Kr has the stronger interaction with the metal halide. In addition, the NQCC of the complexes can be compared with those of the isoelectronic [Cl–M–Cl]− ions, which have two metal-chlorine chemical bonds. In the gold and copper complexes, the change in the NQCC on complex formation is 30– 37% of the difference between the MCl monomer and [Cl–M– Cl]− , suggesting a significant rearrangement of the electron distribution, even at Cl. However, for the Ag complexes this change is only ∼10% of the difference between the AgCl monomer and [Cl–Ag–Cl]− . This result correlates with the general trend in the force constants indicating stronger bonds in the gold and copper complexes (Table 5). If the change can nonetheless be attributed to charge rearrangement effects, then this gives evidence of a chemical bond. In order to confirm this hypothesis, the effect of bending of the molecule must be accounted for. In the case of a linear van der Waals complex, some degree of bending is expected because the molecule is not rigidly bound. The NQCC can be used to obtain

35

a

TABLE 6 Cl Nuclear Quadrupole Coupling Constants in MHz

Reference 25. MP2 calculations (Reference 1). c Reference 26. d Reference 3. e Reference 27. f Reference 28. g Reference 2. h Reference 29. b

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REYNARD, EVANS, AND GERRY

a measure of the degree of bending using the expression (30) eQq = [eQqo + 1(eQq)]h(3 cos2 β)/2i,

[8]

TABLE 7 Ab initio Results for Kr–AgCl and Comparison with Experimental Results

where β is the Eckart angle between the AgCl bond and the a-axis and is related to θ, the bending angle of the complex, by hsin2 θi ≈ (1 + δ 2 )hsin2 βi,

[9]

with δ=

I (AgCl) . I (Kr–AgCl)

[10]

eQqo is the NQCC of the monomer, and 1(eQq) is the change in its NQCC due to complex formation. If 1(eQq) is assumed to be zero, θ = 15.4◦ . This is slightly larger than the value of 14.3◦ obtained for Ar–AgCl (1). Though such values are not unreasonable for linear van der Waals complexes, there are nonetheless inconsistencies. In particular, the stretching force constants imply rigid complexes: while in principle bending and stretching modes are independent, it is difficult to imagine a van der Waals complex rigid in one but not in the other. (This is not the case for quasi-linear compounds, such as C3 O2 ). Furthermore, the calculated bending angle is bigger for the apparently more rigidly bound complex, which seems unreasonable. The assumption that 1(eQq) is zero is probably incorrect, suggesting that some charge rearrangement is taking place. D. Ab initio Calculations

a r (2) m

bond lengths: see text and Table 4.

and Ar–Ag+ (30 kJ mol−1 ) (23). Comparison of the Kr–Ag dissociation energy in Kr–AgCl with that of Ar–Na in the van der Waals complex Ar–NaCl (7.9 kJ mol−1 ) (5) adds further weight to the idea that the Kr–Ag bond may be weakly covalent. Table 5 collects the force constants and dissociation energies for the Ng–MX complexes whose FTMW spectra have been measured to date. There is a rough correlation between the dissociation energies and the force constants, as can be seen from the graph in Fig. 2. If the potential energy is modeled by a Morse function, then the force constant, k, can be related approximately to the dissociation energy, De , by (31) k = 2De β 2 ,

[11]

where β is the Morse potential constant. If β is the same for all the molecules under consideration, then the graph of De versus k should be linear. This is roughly the case.

The ab initio calculations gave the equilibrium bond lengths, the vibration frequencies, the dissociation energy of the Kr–Ag bond, the electric field gradient at Kr, and the orbital populations. In addition, the Ar–Ag dissociation energy was calculated for the first time for Ar–AgCl. The calculated Kr–AgCl equilibrium (re ) bond lengths are a a 2.6304 A for Kr–Ag and 2.2701 A for Ag–Cl. Both are close to the rm(2) experimental values (Table 7). The ab initio Kr–Ag stretching frequency of 105.1 cm−1 compares reasonably well with the experimental value of 117 cm−1 , supporting the use of the diatomic approximation in this case (Table 7). The calculated dissociation energies of the noble gas–metal bonds of Kr–AgCl and Ar–AgCl are given in Table 5. The Kr– Ag value in the complex, 28 kJ mol−1 , is rather larger than 14 kJ mol−1 for Ar–Ag in Ar–AgCl. The Kr–Ag dissociation energy in the complex approaches that of the Kr–Ag+ ion complex (35 kJ mol−1 ) (23). Probably this difference arises because the Ag–Cl bond in the complex is not purely ionic but has some covalent character, so that Ag does not have a full +1 charge; increased electron density on the metal acts to shield the nucleus and reduce the dissociation energy. A similar trend in dissociation energies is observed in the case of Ar–AgCl (14 kJ mol−1 ) C 2001 by Academic Press Copyright °

TABLE 8 Molecular Orbital Populations for Kr and AgCl and Kr–AgCl

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ROTATIONAL SPECTRUM OF Kr–AgCl

FIG. 2. Plot of dissociation energy versus force constant for the results in Table 5.

The calculation predicted a field gradient at the Kr nucleus of q(Kr) = 1.1945 a.u. With Q(83 Kr) = 0.253 barn (32), and the conversion factor 234.9647 (33), this gives eQq(83 Kr) ∼70 MHz. This is much larger than values found for typical van der Waals complexes (e.g. for Kr–HF it is 10.23 MHz (34)) and is consistent with a much larger charge rearrangement in Kr–AgCl. Finally, Mulliken orbital populations were calculated. Table 8 compares the results for the complex and for Kr and AgCl alone. Changes in the orbital populations on complex formation indi-

cate the degree of charge rearrangement. There is a slight decrease of about 0.06 in the n pσ electron population of Kr from the atom to the complex. Other changes are noticeable in the Ag orbital population; specifically in the values of n s (increase by 0.09), n pσ (decrease by 0.02), and n dσ (decrease by 0.03). There is little change in the Cl orbital populations on complex formation. The changes in Kr n pσ and in Ag n s and n pσ follow the trends for Ar– and Kr–AuCl (3) and for Ar–CuCl (2), though the magnitudes of the changes are smaller in the present case. However, the Ag n pσ population follows the opposite trend, being less in Kr–AgCl than in the AgCl monomer, in contrast to the Au and Cu analogs. In effect, both Cu and Au show some sp hybridization, which can be expected to give increased orbital overlap and stronger bonds than those of Ag. The overall results indicate a decrease in electron density on Kr of 0.05, implying formation of a weak covalent bond. In Kr–AuCl this decrease is 0.20 (3). The MOLDEN3.4 program was used to make contour plots of the valence orbitals of Kr–AgCl (35). Those for the 10σ and 6π orbitals are shown in Fig. 3. In both there is some small orbital overlap between Kr and Ag, again suggesting that a weak covalent bond has been formed. VI. CONCLUSIONS

FIG. 3. Orbital contour diagrams of Kr–AgCl: (a) 10σ orbital, value of contours 0.025n, n = 1–8; (b) 6π orbital, value of contours 0.025n, n = 1–8.

The rotational spectrum of Kr–AgCl has been measured and the molecular constants and geometry have been determined. The short Kr–Ag bond, the high rigidity of the complex, and the larger force constant of the Kr–Ag bond show that this is not a “regular” van der Waals complex and hint that a weak noble gas–metal chemical bond may be present.

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REYNARD, EVANS, AND GERRY

The nuclear quadrupole coupling constants of Cl were determined; they have changed slightly from the value for the AgCl monomer. An analysis of the bending angle of the complex suggests that this change is due at least in part to charge rearrangement on complex formation. An orbital population analysis from an ab initio calculation indicates a small shift in electron density from Kr to Ag, and orbital contour diagrams show orbital overlap between Kr and Ag. The ab initio results also give a high dissociation energy for the Kr–Ag bond, further indicating that Kr–AgCl is not a “regular” van der Waals complex. The overall picture is consistent with a weak coordinate covalent bond between Kr and AgCl. All the results, both experimental and ab initio, suggest that such bonds are much weaker for the complexes of the silver halides than for either the copper(I) or gold(I) halides. In addition, where they have been studied, the bonds to Kr are somewhat stronger than those to Ar for the halides of a particular metal.

11. 12. 13. 14. 15.

16. 17. 18. 19. 20. 21. 22.

ACKNOWLEDGMENTS This research has been supported by the Natural Sciences and Engineering Research Council of Canada and by the Petroleum Research Fund, administered by the American Chemical Society.

23. 24. 25. 26.

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