J. Phys. Chem. 1982, 86, 3914-3916
3914
The Bond Length of Cr, D. L. Mlchalopoulos, M. E. GeusIclt S. 0. Hansen,* D. E. Powers,t and R. E. Smalley' Rice Quantum InstlMe and Depamn"e of Chemistry, Rice Unkersity, Houston, Texas 77251 (Received: July 16, 1982; I n Final Form: August 19, 1982)
Chromium clusters were prepared in a supersonic molecular beam by using a pulsed laser vaporization source. Resonant two-photon ionization (RZPI) spectroscopic scans were made with mass-selective detection for the chromium dimer. A single rovibronic band was found near 4600 A and rotationally resolved for the 5oCr52Cr, X 0-0 transition of Crz. 52Cr2,and 52Cr"Cr isotopic species. The band was assigned as the A Measurement of rotational constants confirmed earlier work of Efremov et al. (Opt. Spectrosc. 1974,36,381) where the same band system was observed upon flash photolysis of Cr(CO)@Rotational analysis concludes unequivocally that the bond length (ro)in the u = 0 level of the IZg+ground state of 52Cr2is 1.68 f 0.01 A.
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Introduction In the continuing controversy over the nature of multiple bonding in metals,'I2 the chromium dimer has recently emerged as a key issue. Since the ground state of the chromium atom is the half-filled 3d6 4s' configuration in a high-spin 7S state, one might naively suppose the Cr, ground state to be tightly bound through a single 4sa bond and five d-d bonds involving 3d a, a,and 6 orbitals. With all valence electrons paired in these six bonds, the ground electronic state would then be of lZg+symmetry, and the molecule would be expected to display an exceedingly short bond length as well as a substantial vibrational frequency and dissociation energy. In fact, this sextuple bonding view of Cr2 has received considerable support in a variety of theoretical calculations over the past More recently, though, a number of quite extensive ab initio calculations have suggested that the effective bond order in Cr2 may be considerably less than six7-l0-perhaps as low as one9 or even zero.1° Goodgame and Goddard, for example, recently published a Letter in this journal describing a 6000 configuration MC-SCF calculation of the full ground state and a few excited state surfaces of Cr29a The ground state was found to correspond to a weakly bound (De = 0.3 eV) antiferromagnetically coupled dimer with a 3.0-A bond length and a 110-cm-l vibrational frequency; and much of this binding was only obtained after specifically including van der Waals interactions. Up to now, the experimental data base for Cr, has neither markedly restrained nor guided this theorizing. Mass spectrometric measures of the Cr/Cr2 equilibrium ratio in Knudsen effusion cells have indicated a binding energy of De = 1.56 f 0.3 eV." As Goodgame and Goddard point out: though, this value may have been rendered somewhat too high by failure to consider substantial population of low-lying (high-spin) electronic states and by cooling of the equilibrium vapor through collisions in the not-quite-effusive sampling of the Knudsen cell. The only spectral measurement possibly relevant to the binding in Cr, was a flash photolysis study of Cr(C0)6vapor reported by Efremov, Samoilova, and Gurvich in 1974.12 A transient absorption was observed in the region of 4600 A belonging to some photoproduct of the Cr(CO)6photolysis. Rotational analysis of this spectrum would have indicated a bond length of 1.68A for Cr, (in the u = 0 level of the ground state), but the Russian workers were quite fastidious in pointing out that the observed spectrum might also have been due to CrO, or CrC2. Since the 'Robert A. Welch Predoctoral Fellow. Exxon Postdoctoral Fellow.
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0022-3654/82/2086-39 14$0 1.25/0
1.68-A figure was felt to be rather too short for Cr2-even by those researchers who believed in the sextuple bond5-this early flash photolysis spectrum was widely disregarded. As discussed below, however, mass-selected resonance two-photon photoionization probes of the spectrum of jet-cooled Cr, have now shown beyond any reasonable doubt that the Russians were right: Cr2 was the carrier of the 4600-A spectrum. The bond length in Cr, is, in fact, 1.68 f 0.01 A. Experimental Section A pulsed supersonic beam of chromium clusters was produced by the laser vaporization method described previously for copper.13J4 Briefly, this method consists of using a pulsed Nd:YAG laser (second harmonic) to vaporize a rotating metal target within a pulsed supersonic nozzle. For this Cr, study the target was a chrome-plated steel rod, 0.6 cm diameter (plating depth = 0.013 cm). Vaporization laser pulses of the Q-switched YAG second harmonic (5320 A) were focused to a 0.15-cm diameter spot on the side of the rod. The laser-produced metal plasma expanded into a near-sonic flow of helium carrier gas in a 0.2-cm diameter tube at a point 2-cm upstream of the tube end where the helium + chromium clusters were allowed to freely expand into a vacuum. The carrier gas was pulsed so that the effective helium density above the target rod was equivalent to several atmospheres at the moment the vaporization laser fired. A slow rotation of (1) (a) Cotton, F. A. Acc. Chem. Res. 1973,6, 368, 1978,11, 225. (b)
Cotton, F. A.; Chisholm, M. H. Chem. Eng. News 1982, 60, (26), 40. (2) Troger, W. C.; Gray, H. B. Acc. Chem. Res. 1978, 11, 233. (3) Norman, Jr., J. G.; Kolari, H. J.; Gray, H. B.; Troger, W. C. Inorg.
Chem. 1977,16, 987. (4) Anderson, A. B. J. Chem. Phys. 1976,64, 4046. (5) (a) Bursten, B. E.; Cotton, F. A. Symp. Faraday SOC. 1980,14,180. (b) Bursten, B. E.; Cotton, F. A.; Hall,M. B. J.Am. Chem. SOC. 1980,102, 6348. (6) Klotzbucher, W.; Ozin, G. A. Inorg. Chem. 1977, 16, 984. (7) Wood, C.; Doran, M.; Hillier, I. H.; Guest, M. F. Symp. Faraday SOC. 1980, 14, 159. (8) Harris, J.; Jones,R. 0. J. Chem. Phys. 1979, 70, 830. (9) (a) Goodgame, M. M.; Goddard, W. A. J. Phys. Chem. 1981, 85, 215. (b) Phys. Rev. Lett. 1982, 48, 135. (10) Conga de Mello, P.; Edwards, W. D.; Zemer, M. C. J. Am. Chem. SOC. 1982, 104, 1440. (11) Kant, A.; Straws, B. J. Chem. Phys. 1966, 45, 3161. (12) (a) Efremov, Yu. M.; Samoilova, A. N.; Gurvich, L. V. Opt. Spectrosc. 1974, 36, 381. (b) Efremov, Yu. M.; Samoilova, A. N.; Kozhukhovsky, V. B.; Gurvich, L. V. J.Mol. Spectrosc. 1978, 73, 430. (c) Efremov, Yu. M.; Samoilova, A. N.; Gurvich, L. V. Chem. Phys. Lett. 1976,44, 108. (13) Powers, D. E.; Hansen, S. G.; Geusic, M. E.; Puiu, A. C.; Hopkins, J. B.; Dietz, T. G.; Duncan, M. A.; Langridge-Smith, P. R. R.; Smalley, R. E. J. Phys. Chem. 1982,86, 2556. (14) Hansen, S. G.; Powers, D. E.; Geusic, M. E.; Michalopoulos, D. L.; Smalley, R. E. J. Chem. Phys. To be submitted.
0 1982 American Chemical Society
Letters
The Journal of phvsicel Chemistry, Vol. 86, No. 20, 1982 3915
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RELATIVE FREQUENCY (CM-')
RELATIVE FREQUENCY ( CM-')
Flguro 1. Rotatlonally resotved 0-0 band of the A '2"' X 'Z Q electronic transition of %, in a Supersonic molecular beam. The SpectMn was recorded by monitoring the intensity of mass (mle) 104 photoions produced by resonant two-photon Ionization with a pulsed, presstrstuned dye laser to scan over the spectrum and a simultaneous KrF exciplex laser to ionize the A Z ' : molecules produced when the dye laser was on resonance. Photdon signal was observed only when the two lasers overlapped in tlme, lndlcatlng the A state lifetlme was less than 10 ns. +
the target rod between vaporization laser pulses ensured that no deep holes were drilled in the target. This laser vaporization pulsed supersonic beam source has been found to produce intense cold cluster beams of a wide range of elements including Cu, Ag, Ni, Fe, and Si, as well as the other group 6B metals Mo and W. Spectral results similar to those discussed here for chromium have also now been obtained for beams of molybdenum.16 Full details of the pulsed nozzle construction and techniques for control of the cluster distribution will be found in the paper discussing the high-boiling group 6B metals.15 The supersonic free jet of chromium clusters thus produced was skimmed, and the resulting collimated molecular beam passed through the ionization region of a time-of-flight mass spectrometer (TOF MS).Is Here the absorption spectrum of the Cr, clusters was probed by resonant two-photon ionization (R2PI) with mass-selective detection. For the spectra reported below, the laser which scanned over the absorption band in the vicinity of 4600 8, was a Nd:YAG-pumped, pressure-tuned dye laser (0.1 cm-' bandwidth, 5-11s pulse duration) directed down the molecular beam axis. The electronically excited Cr, species produced by resonant absorption of a photon from this dye laser were then photoionized directly by a KrF exciplex laser beam which crossed the molecular beam at a right angle, centered in the ionization region of the TOF MS. The intensity in each photoion mass channel was recorded by computer for every laser shot so that the R2PI-detected spectrum of Cr2 could be determined simultaneously (but independently) for the various Cr2 isotopic species.
Results and Discussion The supersonic beam version of the 4600-Aabsorption band reported by Efremov et a1.12*is shown in Figure 1. Here the RBPI signal corresponding to the 52-52 isotope of Cr, is plotted vs. the frequency of the scanning dye laser. The well-resolved rotational structure seen here is exactly lZg+ transition of a homothat expected for the lZ,+ nuclear diatomic composed of atoms with zero nuclear spin. In such a lZg+ state, all levels with odd values of the
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(15) Hopkins, J. B.;Morse, M. D.; Langridge-Smith,P. R. R.; Smalley, R. E.J . Chem.Phys. To be submitted. (16) Dietz, T.G.;Duncan, M. A,; Liverman, M. G.; Smalley, R. E. J. Chem. Phys. 1980, 73,4816.
Figure 2. Rotationally resolved 0-0 band of the A 'ZU+ X 'Z Q electronic transitlon of the Isotopically mixed molecule 50Cr52CrIn a supersonic beam.
+
TABLE I: Measured Rotational Line Positions for the A 'E,,++ X ' z S +0-0 Band of the Chromium Dimer position, cm-' transition P(4) P(3) P(2) P(1) R(O) R(1) R(2) R(3) R(4) R(6)
obsda
calcdb
MCr5ZCr - 2.09 -1.63 -1.61 -1.15 -1.14 -0.68 -0.67 0.25 0.27 0.73 0.73 1.19 1.20 1.69 1.66 2.12 2.12 3.01 3.03
- 2.06
R(2) R(4)
5zCr52Cr - 2.81 - 2.79 -1.84 -1.85 -0.91 -0.93 0.45 0.4 5 1.39 1.36 2.27 2.27
P(4) P(3) P(2) R(1) R(2) R(3) R(4)
-1.64 -1.24 -0.75 1.09 1.52 1.95 2.40
P(6) P(4) P( 2)
NO)
52
residual, c m - '
- 0.03 0.02
0.01 0.01 -0.02
0.00 -0.01 0.03
0.00 -0.02 0.02 -0.01 - 0.02 0.00 0.03 0.00
Cr 54 Cr - 1.64 -1.18 -0.73 1.07 1.52 1.97 2.41
0.00 0.06 0.02 0.02
0.00 -0.02 -0.01
All observed line positions were measured relative to the band origin of the 52Cr2molecule. This 0-0 band origin was found t o lie at 21.751.5 f 1.0 c m - ' (76.2 1 cm-' from the 21.827.7-cm-' line of atomic Cr corresponding t o the transition y 5P,(3d44s'4p')+ a 5S(3d54s')which was also observed in this molecular beam experiment). See text and Table I1 for parameters used for this calculation.
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rotational angular momentum, J, are missing, and the It;,+ l.2 spectrum consists simply of P and R branches (AJ = hlk with no Q branch." Further confirmation as to the assignment of this band was obtained by monitoring R2PI signal in the m l e 102 amu channel corresponding to the isotopically mixed molecule 50Cr52Cr. Since this species does not have identical nuclei, there is no symmetry restriction on the existance of rotational levels with odd (or even) J quantum numbers. As shown in Figure 2, this mixed isotopic species does show the expected rotational +
(17) Herzberg, G.'Spectra of Diatomic Molecules"; Van NostrandReinhold: New York, 1950.
3918
TABLE 11: Molecular Constants for the A 'xu+ + 'Z,+ Transition of the Chromium Dimer (s2Cr,)
X
this worka v o 0 , cm-'
w e ' - w e " , cm-'
B o r cm-' , Bo", cm-' A ro,,!A 'e , A '0;;
Letters
The Journal of Wysical Chemistry. Vol. 86, No. 20, 1982
21751 i 1 -39.1 f 4.4 0.2290 i 0.0023 0.2298 ? 0.0028 1.684 ir 0.009 1.681 r 0.010 1.671 i O . O I O c
Efremov et aLb 21751.52 0.2276 r 0.0004 0.2287 i: 0.0002 1.6888 i 0.0015 1.6841 i 0.0008
Indicated error is estimated 95% confidence limit. Calculated from Be" = Bo" + 1 / 2 c y e " Reference 12a. where ae" was estimated from Moskovits' et al. measurement o f w e ' ' = 427.5 cm-' and w e x e " = 15.8 cm'' (ref 20), and the Perkeris relation (ref 1 7 ) . a
lines with even and odd values of J. A similar but weaker spectrum was recorded for the 52CrMCrmolecule. The assignments and measured positions of all observed rotational features are listed in Table I for each isotopic species. The line positions listed in Table I for all isotopic species were fitted simultaneously to the formula V+ = F'(J"* 1) - F"(J'3 where the '+, refers to A J = +1(Rbranch) transitions, y-n to AJ = -1 (P branch) transitions, and
+ 1/pw,' + p2B,'J(J + 1) F"(J) = f/zpw/ + p2B,"J(J + 1)
F'(J) = voo
where P = (p/pi)1'2
and p is the reduced mass of the 62Cr2isotopic species (25.9703m u ) and is the reduced mass of the Cr, isotopic species being measured. The residuals from this leastsquares fit are listed in Table I and the evaluated parameters are presented in Table I1 where they are compared with the earlier results of Efremov et al.12a As testified by the residuals in Table I, the measured data for all isotopes fit the above model excellently well. Since the Cr, molecules in the supersonic beam are rotationally cold (Trot= 4 K), only low J transitions are seen, and extremely accurate rotational constants cannot easily be obtained. Even so, the resolution attained in this supersonic beam measurement is quite adequate to show that the X lZg+(u = 0) bond length of Cr2must lie between the limits of 1.67 and 1.69 A. Table I1 also shows that the agreement with the earlier transient absorption measurementa is excellent. The far more extensive rotational structure seen in the simple bulb experiment permitted Efremov et al. to determine the rotational constants to four significant digits. Although they were unable to resolve individual isotopes, this should not have had a significant effect, and their more precise values should be accepted.
We now know the carrier of their transient absorption was, in fact, Cr . At 1.68 the bare Cr, molecule has the shortest metal-metal bond of any known molecule. Compared to the nearest-neighbor distance in metallic chromium (2.5 A),18 the Cr, bond distance in the bare molecule is shorter by a factor of roughly 2/3. Cotton and Chisholmlb recently pointed out that the 1.83-A chromium-chromium quadruple bond in some Cr24+complexeslg had the shortest homonuclear bond known for any element when the bond length is compared to twice the formal single bond radius. It now seems that the bare Cr, molecule has captured this title from its Cr24+complex brothers. Further evidence for strong bonding in Cr, has recently been reported by Moskovits and co-workers: resonance Raman spectra from Cr, in a cold argon matrix showed the X lZg+ vibrational frequency to be 427.5 cm-l.,O This corresponds to a vibrational force constant which is more than twice that of Cu, which has a single bond. Finally, it should be pointed out that this A lZU+ X lZg+ transition near 4600 A in Cr, is quite similar to the 5300-A A lZU+ X lZg+transition in Mopwhich is generally agreed to have a sextuple bond.15 In both molecules this transition is between states of virtually identical bond lengths and frequencies (only Au = 0 transitions have been observed).,l In both molecules the transition is extremely strong (the peak absorption cross section of the R(2) line in the cm2 in spectrum of Figure 1 was estimated to be a laser power saturation experiment; Moa shows similar saturation behavior).15 The experimental evidence therefore quite strongly indicates Cr, and Mo, have the same electronic structure.
h
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Acknowledgment. We thank M. D. Morse who originally stimulated us to consider the Cr, problem. We also thank M. Moskovits for helpful conversations and communication of his results on the Cr2vibrational frequency in advance of publication. A. Kaldor and D. Cox of Exxon Corporate Research provided much helpful support and encouragement. This research was supported in part by the Department of Energy, Division of Chemical Sciences, and The Robert A. Welch Foundation. (18) Hull, A. W. Phys. Rev. 1919,14, 540. (19) Bino, A,; Cotton, F. A.; Kaim, W. J. Am. Chem. SOC.1979, 101, 2506. (20) Dihlla, D. P.; Lipson, R. H.;Moskovita, M.; Taylor, K. 'Resonance Raman Studies of Metal Dimers and Metal Clusters", Proceedings of the 8th Raman Conference, Bordeaux, France, 1982, and private communication. (21) In the brief survey scans that were made, no evidence for other vibrational bands was observed for this A X transition in Crz. As described in ref 15, Moz under the same nozzle conditions exhibits the 1-1,2-2, and 3-3 sequences but no Av # 0 progressions. In Crz,failure to observe the sequences may be due to facile predissociation in the A state. In fact the 0band of the s2Crz and W P C r isotopes was found to be -l/g weaker than expected when compared to the 0band of the SOCr6zCrmolecule; and 62Cr69Cr was not observed at all. This may have been due either to isotopic dependence on predissociationrate or to +rp variations in the effective photoionization cross section for the KrF ionizing laser.
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