636
J . Phys. Chem. 1992, 96, 636-644
effective nuclear charge, and results in a u bond which is more polarized toward the metal and a compensatory A backbond. The carbene complexes (Figure 3) show a much smaller change in the resonance structure contributions when compared to the analogous silylene complexes. The two most important contributions even at the extreme right of the first transition series are the covalent (I1 111)) and a-ylide (121 10) or M F E) for the MCH2+complexes. There is some variation in the relative importance of these two configurations, but not nearly as much as in the silicon analogues. Likewise, the 12020) configuration does not rise as far to dominate the description of NiCH2+as it does for NiSiH2+. Thus, it is more accurate to describe the double bond with the M e E resonance structure for the later, electron-rich silylenes than it is for their carbene counterparts. Figures 2a and 3a demonstrate that the contribution from the u-ylide (12110)) remains fairly constant at -37% for MCH2+and -30% for MSiH2+,despite the changes in the metal. The u-ylide possesses the dative u bond of the a-dative/?r-backbond resonance structure and the covalent a bond of the all-covalent resonance structure. So, the u-ylide can make significant contributions to the description of both the metalsilicon and metal-carbon double bonds despite the changes in the electronic environment. For the early carbenes, the 12110) and 11111) make nearly equal contributions and account for roughly 70% of the total ground-state wavefunction. The 12020) configuration makes a contribution of less than 10% to ScCH2+(]A1). Thus, for the early transition metals, the silylenes and carbenes are described to a large extent by the same resonance structures, 12110) and 11111). The 11111) configuration is roughly 10% higher than the 12110) configuration for the early MSiH2+complexes while the situation is reversed (i.e. 12110) greater than 11111)) for the early carbene complexes. Thus, a limiting description of MSiH2+ for the early silylenes would be a triplet silylene low-spin coupled to M+ (as in eq 1b) while the later silylenes correspond to the coordination of singlet silylene to M+ (as in eq la).
Summary This paper deals with the effects of changing the metal on the nature of the transition metalsilicon double bond in the ion-beam complexes MSiH2+. The important conclusions to be gathered
from this research include the following. (1) The MSiH2+complexes have significantly weaker bonds (as estimated by calulation of the MSi force constant) than their MCH2+counterparts. For the homologous series CrEH2+(E = C, Si, Ge, Sn) the silylenecomplex has a calculated force constant greater than those of germylene and stannylene. This suggests that type I silylenes are reasonable synthetic targets. (2) An accurate description of the bonding in these complexes necessitates the inclusion of electron correlation effects. The FORS-MCSCF data lead to the reasonable conclusion that the metalsilicon u bond is better described at the Hartree-Fock level than is the metalsilicon ?r bond. The latter requires a multiconfigurational wavefunction to describe the diradical character. The conclusions drawn by Carter and Goddard12d~e,f for carbene complexes are consistent with this point. (3) The MC/LMO/CI data show that the early transition metal silylenes, e.g. ScSiH2+, are dominated by the 11111) (all-covalent or M=E) and 12110) (a-ylide or M F E) resonance structures, with the 12020) (a-dativelr-backbond or M e E) resonance structure making a small contribution. The late transition metal silylenes, e.g. NiSiH2+, are dominated by the 12020) and to a lesser extent the 12110) configurations. The plot of the MC/LMO/CI data in Figure 2 clearly illustrate how one description of the nature of the transition metalsilicon double bond is transformed into the other.
Acknowledgment. This research has profited greatly from conversation with and helpful suggestions by Jan H. Jensen and Michael W. Schmidt (Department of Chemistry, North Dakota State University). This work was supported in part by grants from the National Science Foundation (CHE89-11911) and the Air Force Office of Scientific Research (90-0052). The calculations described herein were carried out on a DECstation 3100 (funded by a grant from the Air Force Office of Scientific Research), an IBM RS-6000 (funded by a grant from the National Science Foundation), and the North Dakota State University IBM 3090-200E (purchased in part by a joint study agreement with IBM). We thank Drs. Walter Stevens, Morris Krauss, Paul Jasien (NIST), and Harold Basch (Department of Chemistry, Bar Ilan University) for providing us with their ECPs prior to publication.
Reactions and Photochemistry of Chromium and Molybdenum with Molecular Hydrogen at 12 K Z. L. Xiao, R. H.Hauge, and J. L. Margrave* Department of Chemistry and Rice Quantum Institute, Rice University, P.O. Box 1892, Houston, Texas 77251 (Received: July 8, 1991) Cr and Mo have been cocondensed with molecular hydrogen in Kr and Ar matrices at 12 K. Both chromium and molybdenum atoms were found to insert into the H-H bond upon absorption of UV light (320-380 nm) to form MH2 ( M = Mo, Cr). The symmetric and antisymmetric stretching frequencies have been identified for both CrH2 and MoH2 and the bond angles are estimated to be 118 f 5' and 110 f 5 O for CrH2 and MoH2, respectively, from the relative intensities of the antisymmetric and symmetric stretching modes. The stretching force constants of CrH2 and MoH2 are determined to be 1.64 and 1.86 mdyn/A. A geometry of slightly unequivalent M-H bonds for M H D (M = Cr and Mo) is suggested. A molecular hydrogen adduct, CrH2(H2), is found in reactions with excess hydrogen. Photolysis of the matrices which contain appreciable CrH2(H2) with light of 520 nm < X < 580 nm leads to the formation of O H 3 . In the molybdenum reactions, MoH, MoH3, and possibly MoH, were identified as products of UV photolysis as well.
Introduction The study of the &hydrogen complexes, both in solutionsi and in low-temperature matrices: has been an active area, due to their
important roles in homogeneous and heterogeneous hydrogenation catalysis. The most well studied systems have been complexes of WUP VI (Cr, Mo, and W)metals which contain $-HZ together with various ligands, such as PR3,3Cp: and C0,5etc. The q2-H2
(1) Van-Catledge, F. A,; Ittel, S. D.; Jesson, J. P. Organometallics 1985, 4, 18.
(2) (a) Sweany, R. L. J . Am. Chem. Soc. 1985,107, 2374. (b) Sweany, R. L.; Russell, F. N. Organomerallics 1988, 7, 719.
0022-3654/92/2096-636903.00/00 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 637
Reactions of Cr and Mo with Hz ligand exhibits characteristicvibrational frequencies6around 3000 and 600 cm-I. Species M(CO),(H2) (where, M = Cr, Mo, and W) have been recently characterized by Molecular hydrogen stretching frequenciesfor Cr(CO),(H2), Cr(CO)5(HD), and Cr(CO),(D,) were observed at 3030,2725, and 2242 cm-I, respectively, where the complexes were made through UV photolysis of Cr(C0)6 in the presence of H2 and its isotopes in liquid Xe solutions at -70 'C. In recent years, a considerable effort has been directed at understanding and developing means of using transition-metal centers to activate the H-H bond. Reactions of metal atoms with molecular hydrogen have been extensively studied both theoretically* and e~perimentally.~Spectroscopic parameters for the neutral fmt- and second-row transition metal monohydrides have been determined theoretically.10 Both CrH and MoH have been found to have a 6Z+ ground state and have similar d electron populations on the metal centers, i.e., 4.82 and 4.86, respectively. However, CrH (1.72 eV) has a smaller dissociation energy than MoH (2.07 eV). Pettersson et al." have calculated the dissociation energies for CrH+ and MoH+ to be 1.20 and 1.40 eV, respectively, which indicates by comparison to the bond energies of neutral species that an electron was removed from a bonding orbital. The ionic species, CrH2+and MoH,', have been studied by Goddard et al.12 They concluded that second-row transition metals bond to hydrogen through 4d rather than 5s orbitals and that MoH,' has two equally favorable geometries (Re = 1.705 A, 8, = 64.6'; Re = 1.722 A, 8, = 112.3'). The dissociation energy &(HMOH+) was calculated to be 35.1 kcal/mol. More recently, the potential energy surfaces for the reactions of Mo H2have been calculated by Balasubramanian et al.I3 It is found that the excited-state Mo(~D,4d55s1) atom inserts spontaneously into H2 to form the bent molecule, MoH,, which has a 5B2ground state, and D,(HMo-H) was determined to be 76 kcal/mol. The bond angle for ground-state MoH, is found to be 116'. The groundstate Mo(~S)atom has a barrier of 89 kcal/mol for insertion. Experimentally, Elkind and ArmentroutI4 have examined the reactions of Cr' with H,, D,, and HD by guided ion beam tandem mass spectroscopy. They observed that the ground electronic state(%) Cr+ reacted with hydrogen largely via an impulsive mechanism. The bond dissociation energy for CrH+ was found to be 1.37 eV. The direct reactions of the ftrst-row transition-metal atoms with H2 have been extensively studied by the authors and others during recent years;', however, the high-melting second- and third-row transition metals with the exception of Pd16 have not been studied.
+
(3) Kubas, G. J.; Unkefer, C. J.; Swanson, B. I.; Fukushima, E. J . Am. Chem. SOC.1986, 108, 7000. (4) Gcofferoy, G. L.; Bradley, M. G. Inorg. Chem. 1978, 17, 2410. (5) Kubas, G. J.; Ryan, R. R.; Wroleski, D. A. J . Am. Chem. Soc. 1986, 108, 1339. (6) Upmacis, R. K.; Gadd, G. E.; Poliakoff, M.; Simposon, M. B.; Turner, J. J.; Whyman, R.; Simposon, A. F. J . Chem. SOC.,Chem. Commun. 1985, 27. (7) Upmacis, R. K.; Poliakoff, M.; Turner, J. J. J . Am. Chem. Soc. 1986, 108, 3645. (8) (a) Saillard, J. Y.; Hoffmann, R. J . Am. Chem. Soc. 1984,106,2006. (b) Balasubramanian, K.; Wang, J. Z. J. Chem. Phys. 1989, 91, 7761. (c) Balasubramanian, K.; Liao, M. Z. J. Phys. Chem. 1988,92, 361. (d) Kang, S.K.; Albright, T. A.; Eisentein, 0. Inorg. Chem. 1989, 28, 1611. (9) (a) Armentrout, P. B. Gas Phase Inorg. Chem. 1989, 1, and references therein. (b) Wright, R. B.; Bates, J. K.; Gruen, D. M.; Inorg. Chem. 1978, 17, 2275. (10) (a) Langhoff, S. R.; Pettcrsson, G. M.; Bauschlicher, C. W.; Partridge, H. J . Chem. Phys. 1987,86,268. (b) Chong, D.P.; Langhoff, S.R.; Bauschlicher, Jr., C. W.; Walch, S.P.; Partridge, H. J . Chem. Phys. 1986, 85, 2850. (1 1) Pettersson, G. M.; Bauschlicher, C. W.; Langhoff, S.R.; Partridge, H. J. Chem. Phys. 1987,87, 481. (12) Schilling, J. B.; Goddard 111, W. A.; Beauchamp, J. L. J . Phys. Chem. 1987, 91, 4470. (13) Li, J. Q.; Balasubramanian, K. J . Phys. Chem. 1990, 94, 545. (14) Elkind, J. L.; Armentrout, P. B. J . Chem. Phys. 1987, 86, 1868. (15) (a) Park, M. Ph.D. Thesis, Rice University, Houston, TX 77251, 1988. (b) Rubinovitz, R. L.; Nixon, E. R. J . Phys. Chem. 1986, 90, 1940.
The reactions of Mo with molecular hydrogen represent the first example of an unligated second-row transition-metal insertion into the H-H bond. The reactions of transition metals have been a great challenge to theoreticians," because of the presence of several low-lying atomic states, i.e., the d"s2,d"+'s', and d%p states, which may be utilized in bonding. For instance, for the selected first- and second-row transition metals, the de2 state and d"+'s states are even more stable than the dns2 state (for example, Pd with a 4 d ' O ground state). For transition metals, the simplest polyatomic molecules to be treated theoretically are the hydrides, and the simplest reactions are those with hydrogen. Studies of the reactions of transition metals with hydrogen promise to be very important in understanding of the bonding behavior of transition metals. Matrix isolation studies can provide an excellent test for theoretical calculations, by providing information related to molecular geometry, bond force constants, and excited-state reactivity. CrH, CrH,, and CrH3have been previously reported as formed by reactions between chromium and hydrogen atoms where the atomic hydrogen was produced by passing molecular hydrogen through a tungsten cell (at -2600 K). They were identified with both ESR and IR spectroscopy by Weltner et al.19 Infrared asymmetric stretching frequencies for CrH2 and CrD2 were reported at 1591 and 1145 cm-', respectively. The vibration frequencies of CrH and CrD in Ar were observed at 1548 and 1 1 12 cm-I. No symmetric stretching frequency for CrH, was observed and the bond angle for CrH2 was undetermined. In this paper, we describe the reactions of Cr and Mo with molecular hydrogen by use of FT-IR matrix isolation spectroscopy. We have found that both Cr and Mo are photoreactive toward molecular hydrogen. CrH, and CrH3 have been identified in the reactions of Cr with H 2 upon photolysis with UV light in both Ar and Kr matrices. Photolysis with UV light of a matrix with Mo and H2 produces MoH2 In the reactions of Mo and Hb MoH, MoH,, and MoH4 are also tentatively assigned. The measured stretching force constants for MoH were found to be 1.51 and 1.75, and 1.64 and 1.86 mdyn/A for CrH, and MoH2,respactively. Both CrH2 and MoH2 are found to be bent molecules with bond angles of 118' and 1 lo', respectively.
Experimental Section The multisurface matrix isolation apparatus used in this study has been described in detail elsewhere.20 The spectra were obtained with an IBM-98 FT-IR spectrometer by reflecting the focused spectrometer beam off a polished metal surface on which the matrix was formed. Experiments typically involved 30 min of trapping with variable amounts of hydrogen and metal atoms in excess Kr or Ar. Matrix surface temperature was typically 12 K. Spectra were usually taken at 1 an-'resolution with an average of 100 scans. Molybdenum metal was directly vaporized from highly pure (99.97%)wires (0.25 mm, diameter, Aldrich) in the temperature range 2100-2300 'C, measured by using an optical pyrometer. Any surface oxide mating was mechanically removed immediately before putting into vacuum. The sample was degassed by slowly heating up to the desired temperature in a separated chamber. An alumina cell enclosed in a resistively heating tantalum furnace was used to vaporize chromium (99.7%,Alfa) over the range from 1150 to 1300 'C. The sample was degassed by heating to 1200 'C and cooled to 1000 OC before the matrix block was cooled. The rates of deposition of Cr, Mo, and matrix gases were measured with a quartz crystal microbalance mounted in the matrix block. (16) Ozin, G. A.; Prieto, J. G. J . Am. Chem. Soc. 1986, 108, 3089. (17) Novaro. 0. A. In The Challenge of d and f electrons; Salahub, D. R., Zerner, M. C., Eds.; ACS Symposium Series 394; American Chemical Society: Washington, DC,1989; p 107. (18) Moore, C. E. Aromic Energy Leuels; Natl. Bur. Stand. (US.) Circ. 1949,467. (19) Zee,R. J.; DeVore, T. C.; Weltner, Jr., W. J . Chem. Phys. 1979, 71, 205 1 .
(20) Hauge, R. H.; Fredin, L.; Kafafi, 2. H.; Margrave, J. L. Appl. Spectrosc. 1986, 40, 5 8 8 .
638 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 1150
1050
950
Xiao et al. 1700
1500
1300
w -
I
l
h
l
I
I 1150
1050
950 I150 wavenumber ( c ~ ‘ )
1050
950
A I
Figure 1. Summary of IR spectra for the reactions of Cr with D2 at different D2 concentrations. (I) The resultant IR spectra recorded after IO-min UV (320-380 nm) photolysis. (A) [D2] = 4.0 X Torr; (B) [D2] = 6.0 X Torr; (C) [D2] = 8.0 X lO-’Torr; (D) [D2] = 1.0 X 10” Torr; (E) [D2] = 2.0 X lo4 Torr. (11) The resultant IR spectra of the respective matrix surfaces recorded after an additional 10-min 520580-nm-light photolysis. a, CrD,; b, CrD2(D2);c, CrD3.
The matrix materials, Ar (99.998%, Matheson) and Kr (99.999%, Matheson) were used without further purification. H2 (99.9995%, Matheson), D2 (99.999%, Air Products), and HD (98% isotope, Cambridge Isotope Laboratories) were used as purchased. The background pressure was typically 1.0 X Torr. The relative amount of added hydrogen was monitored by the rise of pressure in the chamber when hydrogen was added. Photolysis was performed on the matrix surface after deposition using a 100-W medium-pressure mercury short arc lamp and long-pass Coming filters with a deionized water quartz filter. The photolysis time period was usually 10 min.
-
Results 1. cbro”-Hydrogen. In a freshly trapped matrix containing Cr and H2 in an excess of Kr at 12 K, no indication of spontaneous reactions or formation of a complex Cr(H2) was observed. Small amounts of water impurity indicated that a previously studied21 water complex, Cr(H20),was formed. After the matrix surface was photolyzed with light of X > 520 nm, Cr underwent insertion into H 2 0 to form HCrOH2’ which was indicated by a peak at 1637 cm-I. Further photolysis of the same matrix surface with UV light (320-380 nm), resulted in the observation of a number of peaks around 1600 cm-’ (in the H20bending region). These peaks must arise from H2reactions since they shifted to ca. 1170 cm-’ in a D2 reaction. Because the Cr-H stretching frequencies happened to be in the bending mode region of H20, and H 2 0 may interfere with the proper assignment for the chromium hydride species, the reactions of Cr with D2were extensively studied. The IR spectra for a series of reactions with different D2 concentrations are illustrated in Figure 1. Spectra in Figure 1.I were obtained by in situ photolysis with UV (320-380 nm), where the system pressure caused by D2 went to 2.0 X 10” Torr. Several groups of peaks from 5.0 X (labeled a-f) were seen in different D2 concentration regions. When the concentration of D2 was low, only the group a features were present in the spectra, with two peaks at 1180.5 and 1160.7 m-I (with a shoulder). As the level of D2 increased, group b peaks at 1150.3 and 1169.8 cm-l were observed. This strongly suggested that group a peaks result from reactions of a single D2 molecule and group b from more than one D2 molecule. Group a was thus tentatively assigned to chromium dihydride, CrH2. Since group (21) Kauffman, J. W.; Hauge, R. H.; Margrave, J. L. J . Phys. Chem. 1985,89, 3541.
I700
1500
,
I300
wavenumber (mi’) Figure 2. Portion of IR spectra of products for the photoreactions of Cr with H2: (A) freshly trapped matrix; (B) after 10-min UV (320-380 nm) photolysis; (C) after subsequent 10-min photolysis with 520-580-11111light on the same surface as (B); (D) after subsequent 10-min UV (320-380 nm) photolysis on the same matrix surface. a, CrH,; b, CrH2(H2);c, CrH3.
b has a higher growth rate with increased D2 concentrations compared to group a, we conclude that group b is due to the formation of a complex of CrD2 with a second molecule of D2, CrD2(D2). In this complex, Cr donates electrons to the antibonding orbital of the D-D bond which slightly weakens the Cr-D bonds and decreases the stretching frequencies of CrD2. The peak labeled as c at 1097.1 cm-l was seen to grow as D2 concentration increased in a similar manner to CrD,. In those matrices containing higher levels of D2, the peak (labeled as d) at 1099.2 cm-’ became stronger than peak c. Further photolysis with light of 520-580 nm on the same matrix surface enhanced significantly both c and d peaks, as shown in Figure 1.II. In the meantime, the intensities of the peaks for group a and b decreased. As mentioned above, initial photolysis with light from 520 to 580 nm on a freshly trapped matrix did not generate these peaks, we thus concluded that the increase in intensities arose from the excitation of group a and/or group b species. However, it was noticed that the peak (d) at 1099 cm-l grew faster than the peak (c) at 1097.1 cm-’ as the concentration of D2 increased as indicated by the change of relative intensities, we conclude that peaks c and d belong to different species. Figure 2 shows a summary of the IR spectra of the products which result from the reactions of Cr with H2 in a Kr matrix. Group a peaks occurred at 1640.0 and 1606.4 cm-l and group b peaks at 1621.4 and 1592.4 cm-I. Peaks c and d shifted to 1506.9 and 1510.5 cm-], respectively. Additional photolysis with 320-380-nm light weakened the peaks c and d and enhanced peaks due to group a and b. During this process, the unidentified peak e exhibited different behavior from any other peaks, which suggests that peak e belongs to a new species. Similar experiments were also conducted with different HD concentrations. The spectra recorded after photolysis with UV (320-380 nm) and 520-580-nm light are shown in Figure 3, I and 11, respectively. The CrHD (group a) has two sets of peaks, one set in the Cr-H stretching region at 1627.1 and 1617.0 cm-’ and the other in the Cr-D stretching region at 1174.4 and 1167.4 cm-l. The complex (group b), CrHD(HD), has frequencies at 1607.1 and 1160.7 cm-I. As seen in Figure 3.11, in addition to the groups mentioned above, photolysis with light of 520-580 nm which typically created groups c and d peaks gave three peaks in the Cr-H stretching region at 1510.7, 1507.6, and 1563.5 an-’, and two peaks in the Cr-D stretching region, at 1099.2 and 1124.3 an-’.Also,one notices that, as the concentration of HD increased, the relative intensities of peaks at 1124.1 and 1099.2 cm-’changed
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 639
Reactions of Cr and Mo with H2
TABLE I: Observed Frequencies (cm-') a d Relative Intensities for Chromium Hydrides CrHz
CrDz
Kr
Ar
asym stretching
1606.4
1614.9
sym stretching
1640.0
1651.3
1160.7 (2.17)' 1180.5 (0.81) CrD,
CrH, Cr-H stretching Cr-D stretching
1510.5
CrHD
Kr
1167.2 (0.26) 1189.1 (0.09)
Ar 1617.5 1635.7 1172.0 1185.0 CrHDz
1617.0 1627.1 1167.4 1174.4
Cr-H Cr-D CrH2D
1513.0 1099.1
Kr
Ar
1106.4
1510.7 1124.1
1563.5 1099.1
'The numbers in parentheses provide a relative measurement of the intensities of the two stretching modes. I so0
1700 I
'
I
1300 I
'
I
900
1100 '
I
'
I
'
I
I
Iwo
14m
12M
Iwo laa wavenumber (em-1)
14m
IZM
IMO
Figure 4. Summary of IR spectra for the reactions of Cr + R, after 10-min consecutive photolysis with UV (I) and 520-580 nm (11). (A) R = H,; (B) R = HD; (C) R D2; (D) R = H2 + D2
1700
1500
1300
1100
900
wavenumber (cm-1)
F m e 3. Summary of IR spectra for the reactions of Cr with HD, at different HD concentrations. (I) The resultant IR spectra recorded after 10-min UV (320-380 nm) photolysis. (A) [HD] = 4.0 X lo-' Torr; (B) [HD] = 7.0 X 1W'Torr; (C) [HD] = 1.0 X 10-6 Torr. (11) The resultant IR spectra of the respective matrix surfaces recorded after additional 10-min photolysis of 520-580-nm light. a, C r H D b, CrHD(HD); c, CrHzD, CrHDz.
and that a new peak at 1099.2 cm-I overlaps with the peak at 1097.1 cm-'which was seen in the reactions with lower HD levels. A collection of IR spectra for the products of the reactions of Cr with H2, D2, HD, and a mixture of H2 and D2 is shown in Figure 4. The two peaks at 1563.5 and 1124.1 cm-' are present in both reactions with HD and mixtures of H2 and D2 as shown in Figure 4D which strongly indicates that this molecule involves more than one hydrogen molecule. A reasonable assignment for this species (group c) is chromium trihydride, CrH3, and group d is tentatively assigned to a weak complex of CrH3 with H2. In the experiments with HD or mixtures of H2 and D2, one expects two isotopomers for chromium trihydride, Le., CrH2D and CrHD2, where CrH2D has frequencies closer t o CrH2 and CrHD2 has frequencies adjacent to CrH in the Cr-H stretching region. Without direct observations of the bending modes for CrH3, the geometry of CrH3cannot be assigned unambiguously. However, if only the stretching modes are taken into consideration, a planar structure is proposed for the following reasons. In a planar geometry, CrH3 belongs to the D3* group for which there is only one (uj) IR-active bond stretching mode. Additionally, when one
of the H's is replaced by D, the symmetry is lowered to Czuin which all the normal modes are IR active including two stretchmg modes, u3 and ul,in the Cr-H stretching region for CrH2D. The antisymmetric mode is expected to be very similar to the one infrared active mode of CrH3, and the frequency at 1510.7 cm-' can be assigned to the antisymmetric mode of CrH, moiety of CrH2D. The Cr-H stretching mode in CrD2H,on the other hand, seems likely to be responsible for the peak at 1563.5 cm-'. Alternatively, this peak could be assigned to the symmetric stretching mode of the CrH, group in CrH2D. This mode should, however, be much weaker than the antisymmetric mode. We thus prefer to assign the 1563.5-cm-I peak to the Cr-H stretching mode in CrD2H, which is close to the previously reported vibration frequency of CrH.19 A similar assignment can be made for assignments of the CI-D modes as shown in Table I. The observed peaks can be assigned to stretchings of CrHzD and CrHD2. In the studies with different concentrations of D2, it was noticed that CrD, was not formed in those matrices with low D2 concentrations. Because initial photolysis with light of X > 520 nm did not produce CrH3, we conclude that CrH, and the group b species, Le., the complex, CrH2(H2), are the precursors for chromium trihydride. Likely geometries for CrH2(H2)involve either side-on or end-on bonded dihydrogen: H,
.H
H,
H
'*H ..
H
I
,cr::. a
, Cr- - - -H-H b
The excitation of CrH2(H2)by 520 nm can apparently promote electrons from Cr to the antibonding orbital of H-H causing breakage of the H-H bond and the formation of a third Cr-H bond, according the following reactions:
+
520 nm CrH3 H CrH2(H2)* 320-380 nm As shown in Figure 2, the reaction is also to some degree photoreversible.
640 The Journal of Physical Chemistry, Vol. 96, No. 2, I992 1600
1'1'
lUa
'
I'
'
'
' I I
J
Figure 5. Summary of IR spectra for the reactions of Cr with H2 (with small amount of D2 added, [H2]:[Dz] = lO:l), at different Cr concentrations. All spectra were recorded after 10-min exposure of UV (320-380nm) (I) and 520-580-nm (11) light, where the temperature of the furnace was at (A) 1160 OC; (B) 1190 OC; (C) 1215 OC; (D) 1248 OC; (E) 1275 OC; (F) 1300 OC. 1600
I700
I
Xiao et al.
I200
obsd frcq, c m - I Kr Ar 1640.0, 1606.4 1614.9,1651.3 1621.4,1592.4 1627.1,1617.0, 1635.7,1617.5, 1174.4,1167.4 1185.0,1172.0 1563.5,1099.1, 1097.1 1510.7,1124.1, 1506.9 1510.5, 1506.9 1513.0, 1509.0 1180.5, 1160.7 1167.2, 1189.1 1169.8, 1158.5 1607.1,1160.7 1099.1,1097.1 1106.4,1098.5 1375.0 1o00.1 1328.0 1900
proposed assignt CrH2 CrH 2(H 21 CrHD CrHDz CrH2D CrH, CrD2 CrDz(D2) CrHD(HD) CrD, HCrCrH, H-CrCrD DCrCrD, D-CrCrH unknown 1200
1400
1700
MoH2
d
r\
Moor
I I
1500
I
I
1 B
1700
1600
I500
1150
I050
wavenumber (cm")
+
matrices, after consecutive photolysis with UV,and 520-580-nm light: (A) H2 reactions; (B) D2 reactions.
Figure 7. IR spectra for reactions of Mo R in Kr matrices. All spectra were recorded after 10-min photolysis with W (320-380nm) light: (A) R = H2; (B)R HD, (C) R = D2; (D) R H2 + Dz.
The results of a series of experiments with the same H2level, but different chromium concentrations, are summarized in Figure 5, where the temperature of the furnace was varied from 1160 to 1300 OC. During this experiment, a small amount of D2 was intentionally added to form CrD,, and the intensities for CrD, stretchings were used as an internal relative standard with respect to other peaks. No spontaneous reactions due to atomic Cr or diatomic Cr, with Hz were observed. The previous described groups a, b, c, and d peaks are all seen to show similar variation with respect to the Cr concentration and are all thought to result from reactions of atomic Cr. From this study, peak e appears to contain only one Cr atom. When the concentration of Cr was high, in addition to the above groups, a broad peak (f) at 1375 cm-' was seen, which was amplified by further photolysis with 520 nm < X < 580 nm. The above peak shifted to 10oO cm-'in the reactions with D2. Since these peaks are observed only in reactions with a high concentration of Cr, they are tentatively assigned to a diatomic Cr2reaction product. Because the observed frequency is in the region of terminal Cr-H stretching mode, group f is tentatively assigned to a HCrCrH structure. It might also be thought as a weakly interacting dimer of CrH where the hydrogens are partially bridging. Reactions of Cr with H2 and D2 were also performed in Ar matrices. The results are shown in Figure 6 and are found to be very similar to those in Kr matrices except for slight frequency shifts. As was the case in Kr, two peaks at 1513 and 1509 cm-I
appeared after consecutive photolysis with W (32*380 nm) light and light of 520-580 nm in reactions with H2. The wmponding peaks in the reactionswith D2 were observed at 1106.4 and 1098.5 cm-I. This splitting is due to either complexation or slight change in the geometry for CrH3. The observed frequencies and tentative assignments are summarized in Tables I and 11. 2. M o l y W ~ t ~ ~ H y B r o g e2.1. a . R e l ~ t Ofi MO ~ rsitb H2, D2,and HD. The IR spectrum of a newly deposited Kr matrix containing Mo and molecular hydrogen exhibited two new peaks at 1709.3 and 1743.3 cm-I. The intensities of the above two peaks were enhanced significantly upon photolysis with UV light (320-380 nm). The IR spectra for the products of reactions between Mo and Hz, Dz, and HD after W photolysis are shown in Figure 7. One sea that the above two peaks undergo a large shift to 1227.2 and 1250.6 cm-' upon reaction with D2. Similar experiments with HD gave a doublet in the Mo-H stretching in the Mo-Dstretching region, region, at 1720.0 and 1732.2 cm-'; a doublet at 1243.4 and 1234.7 cm-' was observed. Besidesthe above peaks whichcanbeapsinnedtothe symmetric and antisymmetricstretching modes for MoHz, additional peaks with much smaller intensity at 1674.1 cm-' in H2 reactions and at 1202.2 cm-'in the D2 reaction were also observed. These two peah were also presented in the reactionswith HD at exactly the same frequencies. They were thus assigned to molybdenum monohydride, MoH. The vibrational frequency (we) for the ground-state M o H ( V ) was recently calculated by Langhoff et
Figure 6. IR spectra for the reactions of Cr with H2 and D2 in Ar
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 641
Reactions of Cr and Mo with H2 1700
1 2000
, 1900
I 800
,
1700
25x2
wavenumber (cm')
Figure 8. Results for reactions of Mo with H2in Kr, at different H2 concentrations: (A) [H2] = 4.0 X lo-' Torr; (B) [H2] = 8.0 X lW' Torr; (C) [H2] = 1.0 X lod Torr.
al.Ioa to be. 1662 cm-I, which agrees reasonably well with our experimental result. A series of experiments with different hydrogen concentrations was carried out where the temperature of the Mo wires was kept at -2150 "C.The resulting IR spectra are shown in Figure 8. In the reactions with higher concentration of hydrogen, there appears to be an extra peak at 1849.2 cm-l, which was not present in spectra with low H2 concentrations. In the reaction with D2 the analogous peak is found at 1355.5 cm-*. Because this was only apparently observed in the experiments with high H2 concentrations, we have tentatively assigned these peaks to the stretching modes of molybdenum tetrahydrides, MoH4. In the reactions with HD, two analogue peaks were detected at 1850.8 and 1355.5 cm-' in the Mo-H and Mo-D stretching regions, respectively. 2.2. Pbotochemistry of Mo and H2. The atomic absorption spectrum of Mo is dominated by two strongly allowed transitions.u These transitions (a's z7P0,a7S y7P0)have been observed by several groups to occur in pure Kr matnces at 350 and 300 nm, respectively. After molybdenum was cotrapped with H2 in Kr, the matrix surface was exposed to light of different wavelengths. No apparent changes in the spectra were observed after photolysis with light other than UV light. After the matrix surface was photolyzed with the light of 320-380 nm, the peaks which are assigned to molybdenum dihydride increased dramatically. The intensities after photolysis are about 3 times stronger than those before photolysis. This suggests that a barrier does exist for the insertion of ground-state atomic Mo into the H-H bond, in agreement with recent theoretical predi~ti0ns.I~The presence of molybdenum dihydrides in those spectra taken just after trapping may either result from the reactions of long-lived excited states of molybdenum or from reactions of hydrogen atoms formed at the hot Mo filaments (at -2200 "C). After photolysis with UV (320-380 nm) light, the same matrix surface was then exposed to light of X > 400 nm for 15 min, and the peaks for MoH2were seen to decrease in intensities as shown in Figure 9. After 2-h photolysis with light of A > 400 nm,the dihydride was almost gone. Meanwhile the peak at 1849.0 cm-' which was tentatively assigned to the MoH4 grew significantly. The molybdenum dihydride was reproduced by subsequently photolyzing the matrix surface with UV (320-380 nm) light for 10 min. This suggests that photoexcitation with light of A > 400 nm of MoH2 populates an electronic state which can react with
- -
-
,
I
1900
1800
1700
wavenumber (cm')
Figure 9. Summary of 1R spectra for the reactions and photochemistry of Mo with H2:(A) a freshly trapped matrix, (B) after 10-min photolysis with UV (320-380 nm) light; (C) followed by 15-min photolysis with A > 400 nm;.(D) followed by 10-min photolysis with UV (320-380 nm) light. 1500
1400
1300
1200
I500
1400
1300
1200
-
(22) (a) Green, D. W.; Gruen, D. M.J. Chem. Phys. 1974,60, 1797. (b) Moakwits, M.;Ozin, G. A. Cryochemistry; Wiley: New York, 1976; p 401.
wavenumber (cm-')
Figure 10. Summary of IR spectra for the reactions and photochemistry of Mo with D2:(A) a freshly trapped matrix; (B) after 10-min photolysis with UV (320-380 nm) light; (C) followed by 15-min photolysis with A > 400 nm; (D) followed by 10-min photolysis with UV (320-380 nm) light.
a second molecule of hydrogen to form MoH4. The reaction also seems to be photoreversible as
+
X>400nm
M o H ~ H2 ' 320-380
nm
MoH~
The reverse reaction may be similar to the known photoinduced reductive-elimination of H2 from ($-CSHS)2MoHz.23 The resultant IR spectra of the products for the reactions with H2 and D2 are summarized in Figures 9 and 10, respectively. It is also interesting to note that when a matrix which contains the MoH2 product is irradiated with light of 270-320 nm for 10 min, MoH2 is totally removed and MoH2 cannot be reproduced (23) Geoffroy, G. L.; Bradley, M. G. Inorg. Chem. 1978, 17, 2410.
Xiao et al.
642 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 TABLE III: Observed Freauencies (cm-l) and Relative Intensities for Molybdenum Hydrides MoH2 MOD, Kr Ar Kr Ar asym stretching 1709.3 1727.4 1227.2 1234.0 (7.12)" (0.22) (4.00) (0.50) sym stretching 1743.1 1752.7 1250.6 1257.8 (3.62) (0.13) (2.28) (0.22) MoH, MOD,
Mo-H stretching Mo-D stretching
1680.0
1202.3
MoHD
Kr 1720.0 1732.2 1234.7 1243.4
Mo-H Mo-D
Ar
1732 I745 1239 1252 MoHD~
MoH~D
1676.2 1215.3
MoH
1685.9 1202.8 MOD
1675.4
1202.0
"The numbers in parentheses provide a relative measurement of the intensities of the two stretching modes Zoo0 I
Zoo0
I
1900
1800
I
I
1900
I700
1700 I
I
I800
I
I
1650111220 '
l
"
l
'
1170 I
'
I
17W
wavenumber (cm")
Figure 11. Summary of IR spectra for the reactions and photochemistry of Mo with H2: (A) a freshly trapped matrix, (B) after IO-min photolysis with W (320-380 nm) light; (C) followed by 15-min photolysis with 270 < h < 320 nm; (D) followed by 10-min photolysis with UV (320-380
nm).
by photolysis with 320-380-nm light. It seems likely that the more energetic light (270-320 nm) may excite MoH, to a high-lying dissociative electronic state which gives sufficient energy to the molecular hydrogen to cause it to be ejected out of the matrix cage. Spectra for different sets of photolysis of a matrix of Mo and H2 in an excess of Kr are shown in Figure 11. As shown in Figure 12,after UV photolysis of a matrix of Mo and H2, the peak due to MoH (at 1674.1 cm-I) vanished and a new peak at 1680 cm-l became apparent. In the HD reactions, there were two peaks at 1676.0 and 1685.7cm-' which were also observed in the reaction with a mixture of H2 and D2. They were tentatively assigned to stretching modes of molybdenum trihydride, MoH3, for the same reasons as in the Cr H2 study. The formation of MoH without any photolysis may result from reactions of Mo with H atoms produced by the hot Mo filaments. In the reaction of Cr with H2, the peaks due to CrHJwere much stronger than those assigned to MoH3 in reactions of Mo with H2. This is not unexpected, because Cr favors the +3 oxidation state while Mo favors higher oxidation states such as +4 and +6. Finally we have noted that weak peaks after photolysis of X > 400 nm light are observed, as shown in Figure 9C, in the neighborhood of 1961 cm-' in H2 reactions and a group of peaks around 1412 cm-I in D2 reactions. They appeared at higher frequencies than peaks previously assigned MoH4 This suggests that they may be due to a higher hydride such as molybdenum hexahydride, MoH6. In order to observe the effect of the matrix on the above chemistry, we have also investigated Mo and H2 reactions in an argon matrix. The results appeared to resemble closely those for a krypton matrix. Assignments and observed frequencies in both
+
1650 I220
1700
wavenumber (cm-'
Figure 12. Portion of IR spectra for the reactions of Mo with (A) H,; (B) HD; (C) D2; (D) a mixture of H2 and D2. (I) All spectra for the freshly deposited matrim. (11) All spectra taken after 10-min photolysis with UV (320-380 nm) light.
TABLE N: Reaction Product Absorptio~s(cm-I) for Reactiolrs of Molybdenum with Hydrogen in Kr and Ar obsd freq, cm-I
Kr 1675.4 1709.3, 1743.1 1680.0 1849.2 1961 1850.8, 1355.5 1676.2, 1215.3 1685.9, 1202.8 1720.0, 1732.2, 1234.7, 1243.4 1202.0 1227.2, 1250.6 1202.3 1355.5 1412
Ar 1727.4, 1752.7
1732, 1745, 1239, 1252 1234.0, 1257.8
proposed assignt MoH MoH~ MoH, MoH~ MOH6(?) MoH~D~ MoH~D MoHD~ MoHD MOD MOD, MOD, MOD, MOD6(?)
Ar and Kr matrices are given in Tables I11 and IV.
Discussion Atomic Cr and Mo are members of the same group of the periodic table and both have 'S(dSsl) ground states. The absorption spectra of matrix isolated Cr and Mo have been extensively studiedz2and are characterized by two resonance transitions
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 643
Reactions of Cr and Mo with H2 between 7S -+ 7P states with 'P state split into three components by spin-orbital coupling. Both have two absorption bands in the UV-vis region. Cr has two absorptions at 335 and 386 nm in a Kr matrix and Mo has two transitions at 300 and 350 nm in a Kr matrix. The study of reactions of Cr and Mo with molecular hydrogen provides a comparison of the bonding behavior of firstand second-row transition metals with quite similar electronic properties. Generally, transition metals can utilize several low-lying electronic states, Le., the dns2,dnflsl,and dnsp configurations in bonding. However, both chromium and molybdenum have dSsl ground states, which are considerably more stable than the d4s2 states, because of the satisfied half-filled d shell. One e x p t s that both Cr and Mo prefer to use mainly s electrons to form the metal monohydrides and that both Cr and Mo use s and d electron bonding to form metal dihydrides. This is verified by the d orbital populations in CrH and MoH,l0 which have 4.86 and 4.89 d electrons, respectively. In the molecule, MoH,, the gross Mulliken population14 was calculated to be s0.767po~238d4.644 which indicates that the Mo-H bonds of MoH, do utilize d orbitals in their formation. It is generally thought that the bond angle for a triatomic molecule is a good reflection of the hybridization configuration used in the bond formation. For many metal dihydrides it has been possible to obtain a bond angle measurement from the relative intensities of the symmetric and antisymmetric stretching frequencies, where one assumes that the relative transition moments are a vectorial sum of the dipole moment changes of the individual metal-hydrogen bonds. The relative intensities for CrH2 and MoH, which were obtained from integration of peak areas are listed in Tables I and I1 together with the frequencies. The bond angle is obtained by using the following relati~nship:~~ IB2
mM + 2mH sin2 8
IAl
mM + 2mH cos2 8
- = tan2 8
(1)
where mMand mHare the masses of metal and hydrogen; I& and IA, are the relative intensities of the antisymmetric and symmetric stretching frequencies, and 28 is the apical angle. The bond angle for chromium dihydride was calculated to be 118O in both Ar and Kr matrices. In this case the relative intensities for CrD,, rather than those for the CrH,, have been used since interference due to the H 2 0 bending mode occurs for the CrH, stretching modes. The bond angle for molybdenum dihydride is calculated to be llOo in a Kr matrix by using the intensities of both MoH2 and MOD,. In an Ar matrix, the bond angles for MoH2 and MOD, were calculated to be 108O and 114O, respectively. It is thought that the variation of the bond angle mainly arises from the inaccuracy in measurements of the peak area of weak peaks rather than the effects of matrices. In any case, the measured derivation of the bond angle is within f 5 O . The geometry of CrH, has been studied theoretically using FSGO calculations by Simons et al.2s The bond angle of CrH2 was predicated to be 103.1O which was interpreted in terms of n e g atively charged hydrogens which repel each other and open up the bond angle. This work suggests the bond angle for CrH2 is larger than the calculated value. The bond angle for MoH, was calculated by Balasubramanian et al.12 to be 116' with MoH, in a SB2ground state. This is slightly higher than our reported experimental value. Both CrH2 and MoH, have been shown to be highly bent molecules with MoH, slightly more bent than CrH2. This suggests that d electrons are strongly involved in the bonding for both Cr and Mo and that Cr and Mo both tend to use d electrons for bonding rather than promoting electrons to p orbitals. The slightly smaller bond angle for MoH2 may be explained by the increased tendency to use 4d electrons for second-row transition metals in bonding.I3 Recently, Balasubramanian et a1.26found that Mo-H (24) Ozin, G. A.; McCaffrey, J. G. J. Phys. Chem. 1984, 88, 645, (25) Talaty, E. R.; Fearey, A. J.; Simons, G . Theor. Chim. Acta 1976, 41, 133. (26) Balasubramanian, K.; Li, J. J . Phys. Chem. 1990, 94, 4415.
TABLE V
Comparison between HM and HMD
H-CrD H-MO H-MOD
1689.5 1727.4 1776.8
31.2 26.0 28.4
1.64 1.75 1.86
(M= Cr, Mo)Speeies
0.047
118 f 5
0.042
110 f 5
1.7 1.5 1.6
'From ref 19.
bonds have considerable d character both in MoH and MoH+ species. It is noted that in Figures 4, and 7 that both CrHD and MoHD are observed as doublets rather than the expected single peaks in the M-H and M-D stretching regions. This cannot be explained as resulting from two different matrix sites, since a similar splitting was not observed for CrH, and MoH2. We believe that the doublets are best explained by a slightly derivation from a symmetric structure for CrHD and MoHD. It is not known whether this structure only applies to the MHD species or whether it applies to the MH2 species as well. If such a structure exists, it implies the existence of a double minimum in the metal hydrogen bonds in the Cr and MO triatomic hydrides. This situation has not previously been noted for any other XY2 triatomic species. A similar phenomenon was also noticed in the GeH2:OH2complex by Andrews et al.27 The stretching force constants for the monohydrides of Cr and Mo, have been calculated to be 1.51 and 1.75 mdyn/A, respectively, by using the following equations28 W,DX,D = p2u,HxcH, (2) . . wi = w,i - 2w,lx,l, p = [ p H / p D ] 1 / 2 (3) where wi are the observed frequencies, pi are the reduced masses, and x; are the anharmonicities. The stretching force constants for the dihydrides were calculated from frequencies for the HMD species where the M H and MD bonds are treated as independent uncoupled vibrations. A diatomic approximation was used to correct for anharmonicity. The stretching force constants were found to be 1.64 and 1.86 mdyn/A for CrH2 and MoH,, respectively. The larger stretching force constants for MoH and MoH2than for CrH, can also be explained by the larger involvement of the 4d orbital in Mo. The calculated values for the stretching force constants, bond angles, and the anharmonicities are listed in Table 111. The percent anharmonic character (w,x,/w,) is also given as an indication of how sensitive the nature of the bond is to the internuclear distance. There are several factors that make the bonding of second-row transition metals different from that of first-row transition metals. The radial overlap of the nd orbital with the (n 1)s orbital is larger for the second row than that of first row.loa The 4dn5s2 and 4dn++'5s1 configurations are also very close in energy which results in better s-d hybridization and greater d involvement in the bond of second-row transition-metal hydrides. This is confirmed by the larger stretching force constant of MoH2 and the stronger Mo-H bonds. The coupling between the two M-H bonds in MH2 can be evaluated from the interaction force constant k12(MHz). The ,), coupling force constants for a bent molecule MH2, k 1 2 ( ~ ~can be calculated by using the following equations with a heavy metal center appr~ximation~~
w,D
= pueH,
+
(4)
(27) Withnall, R.; Andrews,
L. J . Phys. Chem. 1990.94, 2351. (28) Herzberg, G.Molecular Structure and Molecular Spectra XI, Infrared and Raman Spectra of Polyaromic Molecules; Van Nostrand New York, 1945; p 228. (29) Reference 27, p 187.
J . Phys. Chem. 1992, 96, 644-650
644
where Xi = 4r2ui2and k12 is the coupling force constant. The results for CrH, and MoH2 are listed in Table V together with the stretching force constants. Conclusions In a summary, the reaction patterns for both chromium and molybdenum atoms with molecular hydrogen in Kr and Ar matrices have been found to be extremely similar presumably due to their similar electronic codigurations. A larger stretching force constant and a smaller bond angle for MoH2 than for CrH2 have
Reaction Potential Surface for B+('S)
been observed. The unexpected splitting in the stretching modes of MHD molecules, which has not been observed for any other unligated metal dihydides, suggests that a small difference exists in the bond distanw for M-H and M-D bonds in the MHD(Cr, Mo) species. It would be of interest to explore this possibility theoretically, since the presence of inequivalent bonds requires that a double minium exists for each bond.
Acknowledgment. This work has been supported by the Robert A. Welch Foundation and by MSNW, Inc.
+ H2* HBH+('Z,+),
BH+(2Z)
+ H(2S)
Jeff Nichols,+Maciej Gutowski, Samuel J. Cole,$and Jack Simons* Chemistry Department, University of Utah, Salt Lake City, Utah 84112 (Received: July 9, 1991)
The reaction of B+(IS) with H2on the ground potential energy surface is examined using ab initio electronic structure methods. In the entrance channel, a weakly bound T-shaped B+-H2 complex.of Cb symmetry is found to lie 422 cm-' below the B+ + H2 reactant energy. Its H-H internuclear distance is only slightly distorted from that of H2; the B-H distance (ca.2.6 A) is much longer than the covalent bond length in BH+ (1.2 A). Further along the reaction coordinate is found a narrow valley characterized by strong B+-to-H2interreactant forcts but very small distortion of the H-H bond length or the H-H vibrational frequency. Further up the floor of this valley, a geometry is reached at which, through second-order Jahn-Teller coupling, the asymmetric motion (of b2 symmetry) develops negative curvature and thus becomes geometrically unstable. From this point of instability, distortion along the asymmetric mode can lead directly to the BH+(2Z) + H products. The energy of this instability point is 22 842 cm-I above B+ + H2and 2021 an-' or ca. 0.25 eV above the thermodynamicreaction threshold for BH+ + H formation, which is predicted to be 20000 an-'. In addition, a geometrically stable linear HBH+(IZ#+) species is found to lie 14712 cm-' below B+ + H2. Its BH bond length ( r B H = 1.187 A) is only slightly shorter than that in BH+ (1.199 A). All of these findings are in reasonable agreement with known experimental data on the reactivity of B+ with H2. However, another (collinear) reaction path has been found that leads to BH+(%) + H without any barrier above the thermodynamic requirement; this path, if operative, is not consistent with experimental finding. A proposal is offered to explain how the path that passes through the point of instability may be of more relevance to the guided-ion beam data than the lower energy collinear path.
I. Introduction (A) W h y Study B+(IS) + H2?The present work was under-
TABLE I: Electronic Statea Euergiea (eV)for Reactants and Prod- M d rrith Respect to Bt('S) + HI(X'E:,+)
taken to provide theoretical interpretation for experimental guided ion beam and other datal on the B+ H2=$ BH+ H reaction. The ion beam experiments give the cross section for (?Z)BH+ production as a function of B+ kinetic energy and show a threshold energy for BH+ production that exceeds the endothermicity of the reaction. The origin of this activation energy was a primary inspiration for this study. As shown in section IV, we conclude that the majority of the collisions occurring a t energies characteristic of the ion-beam experiments proceed through a near-Cz, B+H2 structure which lies ca. 0.25 eV above the BH+ + H products and which reaches the BH+ + H product channel because of instability in its asymmetric b2 distortion mode. What is surprising is that this conclusion is reached even though there exists a collinear path connecting B+ + H2 to BH+ + H that has no activation barrier in excess of the reaction endoergicity. (B) Earlier Related Studies and Characteristics of Similar Potential Surfaces. Earlier theoretical calculations2 on Be( 'S) + H2 =$ HBeH(lZ,) and on3 Mg('S) H2 * HMgH('2,) yielded qualitatively similar ground-state 'A,, CZv- constrained potential energy surfaces. The findings of these studies were expected to guide us in the present work and therefore merit consideration. A two-dimensional contour characterization of the Be H2 ground-state surface is given in Figure 1; the variation of the
reactants" Productsb B+(IS) + H2('Cgt) 0.0 BH'(X2Et) + H(%) 2.6 B+('P) + H#ZSt) 4.6 BH+(A211)+ H('S) 5.1 B(2P) + H2+(2Egt) 8.0 BH(XIZ+)+ H+ 7.1 Bt('P) + H2('Egt) 9.1 BH+(B2Et) + H(?3) 10.0 OBt energies obtained from Atomic Energy Levels. Moore,C . E. Nut. Stand. ReJ Data Ser., Nut. Bur. Stand. (US.)1971, 341V.I. H2+/H2energies obtained from ab initio calculation. bBased on calculated results from the present work.
+
+
~~
energy along the asymmetric distortion mode is not depicted in this figure, although in these earlier works attention was paid to points at which motion along the asymmetric direction was geometrically unstable. The surface of Figure 1 clearly shows a narrow entrance channel (labeled a) along which Be approaches an essentially intact H2 molecule, a region in which H-H motion
+
+
Utah SupercomputerInstitute/IBM Corporation Partnership, Salt Lake Cit , UT 841 12. ?Permanent address: CAChe Group, Tektronix, Inc., P.O. Box 500 M.S. 13-400,Beaverton, OR 97077.
~
~~~~~
(1)(a) Armentrout, P. B. Int. Rev. Phys. Chem. 1990,9,115. Elkind, J. L.;Armentrout, P. B., unpublished results. Ruatta, S. A,; Hanky, L.; Anderson, S.L. J. Chem. Phys. 1989,91,226.(b) Ramus, P.; Klein, R. Thesis by Klein, University of Frankfurt,1984. (c) Schneider, F.;Zfllicke, L.; Polak, R.; Vojtik, J. Chem. Phys. k i t . 1984,105,608.(d) Lm,K.C.; Watkins, H. P.; Cotter, R. J.; Kmki,W. S.J. Chem. Phys. 1974,56,1003. Ruatta, S.A.; Hanky, L.; Anderson, S. L. J . Chem. Phys. 1989,91,226. (2)ONeal, D.;Taylor, H.; Simons, J. J . Phys. Chem. 1984,88, 1510. (3)Adams, N.; Breckenridge, W. H.; Simons, J. Chem. Phys. 1981.56, 327.
(4)Herzberg, G.Atomic Spectra and Atomic Structure; Dover Publications: New York, 1944;p 200.
0022-365419212096-644$03.00/00 1992 American Chemical Society
