A matrix-isolation and density functional theory study of the reactions of laser-ablated beryllium, magnesium, and calcium atoms with methane

Article abstract of DOI:10.1021/ja9804870

Beryllium atoms produced by laser ablation have been co-condensed with methane/argon mixtures onto a substrate at 10 K. Infrared spectroscopy has been used to identify a number of organoberyllium products, viz. CH₃BeH, CH₃BeCH₃, CH₃Be, H₂CBeH, and HCBeH. Assignments of the infrared absorption bands are made on the basis of ²H and ¹³C substitution and by comparison with frequencies supplied by DFT calculations. In the reaction of magnesium or calcium atoms with methane, the only insertion product that could be identified was the monomethyl metal hydride, CH₃MH (M = Mg or Ca).

Full text of DOI:10.1021/ja9804870

J. Am. Chem. Soc. 1998, 120, 6097-6104
6097
A Matrix-Isolation and Density Functional Theory Study of the
Reactions of Laser-Ablated Beryllium, Magnesium, and Calcium
Atoms with Methane
Tim M. Greene,*,† Dominick V. Lanzisera, Lester Andrews,* and Anthony J. Downs†
Contribution from the Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22901
ReceiVed February 12, 1998
Abstract: Beryllium atoms produced by laser ablation have been co-condensed with methane/argon mixtures
onto a substrate at 10 K. Infrared spectroscopy has been used to identify a number of organoberyllium products,
viz. CH3BeH, CH3BeCH3, CH3Be, H2CBeH, and HCBeH. Assignments of the infrared absorption bands are
made on the basis of 2H and 13C substitution and by comparison with frequencies supplied by DFT calculations.
In the reaction of magnesium or calcium atoms with methane, the only insertion product that could be identified
was the monomethyl metal hydride, CH3MH (M ) Mg or Ca).
Introduction
We have extended these matrix-isolation experiments to
detemine the response of methane to laser-ablated magnesium
and calcium atoms. Methylmagnesium hydride may be prepared
in tetrahydrofuran solution by mixing dimethylmagnesium and
magnesium hydride but, like the beryllium diethyl ether
analogue, the molecule is a dimer by virtue of possessing two
bridging hydrogens.12 The monomeric species has been pre-
pared by photoexcitation of magnesium atoms trapped in pure
methane matrices,13 while matrix-isolation studies of a range
of binary magnesium hydrides have also been reported.14
Studies of the reaction of magnesium atoms with methane in
The first organoberyllium compound was reported in 1860
when Cahours treated the metal with ethyl iodide in a sealed
tube giving a solid product later identified as EtBeI.1 Although
details of routes aimed at the preparation of organoberyllium
hydrides, RBeH, have been reported more recently, only in a
few cases have solvent-free compounds been isolated; they are
typically subject to oligomerization and polymerization pro-
cesses.2 Thus, methylberyllium hydride appears to be formed
either by the reaction of [Me2AlH]n with an excess of [Me2-
Be]n or by the pyrolysis of [MeBeBut]n, but is an intactable
material which has been characterized only in the form of
coordination complexes with donor ligands such as Et2O or
Me3N.2-4 The diethyl ether complex may be synthesized by
the reaction of dimethylberyllium, beryllium bromide, and
lithium hydride in diethyl ether; on the evidence of cryoscopic
measurements, it is dimeric in benzene solution and the molecule
[MeBeH‚OEt2]2 is believed to contain a BeH2Be bridging unit.4
1
the gas phase indicate that magnesium excited to the P state
reacts with methane on essentially every collision to produce
MgH and CH3.15,16 There is no report in the literature of the
synthesis of CH3CaH, although phenylcalcium hydride has been
prepared.17 This has been achieved by co-condensing thermally
evaporated metal atoms with benzene on a surface held at 77
K and then permitting the mixture to warm to room temperature.
Experimental Section
To examine the monomeric species, we have employed the
technique of matrix isolation to trap the product formed when
a high-energy beryllium atom inserts into a C-H bond of a
methane molecule. Such a stratagem has proved successful in
our previous study of the species CH3MH (M ) Zn, Cd, or
Hg).5 For the more refractory atoms, laser ablation has proven
extremely effective to examine the reaction between boron and
methane6 and to explore reactions of beryllium atoms with
oxygen,7 hydrogen,8 water,9 acetylene,10 or HCN.11
The apparatus for pulsed laser-ablation matrix-isolation spectroscopy
has been described earlier.18-20 Gas samples were prepared using
(6) Hassanzadeh, P.; Hannachi, Y.; Andrews, L. J. Am. Chem. Soc. 1992,
114, 9239-9240; J. Phys. Chem. 1993, 97, 6418-6424.
(7) Thompson, C. A.; Andrews, L. J. Am. Chem. Soc. 1994, 116, 423-
424.
(8) Tague, T. J., Jr.; Andrews, L. J. Am. Chem. Soc. 1993, 115, 12111-
12116.
(9) Thompson, C. A.; Andrews, L. J. Phys. Chem. 1996, 100, 12214-
12221.
† Current address: Inorganic Chemistry Laboratory, University of Oxford,
Oxford, OX1 3QR, U.K.
(10) Thompson, C. A.; Andrews, L. J. Am. Chem. Soc. 1996, 118,
10242-10249.
(1) Gilman, H.; Schulze, F. J. Am. Chem. Soc. 1927, 49, 2904-2908.
(2) (a) Gmelin Handbook of Inorganic Chemistry, 8th ed., Organo-
beryllium Compounds; Springer-Verlag: Berlin, Heidelberg, 1987; Part 1,
Syst. No. 26, pp 93-104. (b) Bell, N. A. In ComprehensiVe Organometallic
Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon:
Oxford, 1982; Vol. 1, pp 142-144. (c) Bell, N. A. In ComprehensiVe
Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G.,
Eds.; Pergamon: Oxford, 1995; Vol. 1, pp 50-52.
(3) Bell, N. A.; Coates, G. E. Proc. Chem. Soc. 1964, 59.
(4) Bell, N. A.; Coates, G. E. J. Chem. Soc. 1965, 692-699. Bell, N.
A.; Coates, G. E. J. Chem. Soc. A 1966, 1069-1073.
(5) Greene, T. M.; Andrews, L.; Downs, A. J. J. Am. Chem. Soc. 1995,
117, 8180-8187.
(11) Lanzisera, D. V.; Andrews, L. J. Am. Chem. Soc. 1997, 119, 6392-
6398.
(12) Ashby, E. C.; Goel, A. B. J. Org. Chem. 1977, 42, 3480-3485.
(13) McCaffrey, J. G.; Parnis, J. M.; Ozin, G. A.; Breckenridge, W. H.
J. Phys. Chem. 1985, 89, 4945-4950.
(14) Tague, T. J., Jr.; Andrews, L. J. Phys. Chem. 1994, 98, 8611-
8616.
(15) Breckenridge, W. H.; Umemoto, H. In Dynamics of the Excited
State; Lawley, K. P., Ed.; Advances in Chemical Physics; Wiley: New
York, 1982; Vol. 50.
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(17) Mochida, K.; Hiraga, Y.; Takeuchi, H.; Ogawa, H. Organometallics
1987, 6, 2293-2297.
S0002-7863(98)00487-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/09/1998

6098 J. Am. Chem. Soc., Vol. 120, No. 24, 1998
Greene et al.
Figure 1. Infrared spectra of argon matrices following the co-deposition of laser-ablated Be atoms with 2% methane/argon mixtures at 10 K: (a)
Be + CH4, (b) Be + 13CH4, and (c) Be + CD4.
methane (Matheson), 13CH4 (MSD Isotopes, 99 atom %), CD4 (MSD
Isotopes, 99 atom %), or CH2D2 (Cambridge Isotope Laboratories, 99
Results
The infrared spectra obtained when beryllium, magnesium,
or calcium atoms are co-condensed with methane will be
reported in turn.
atom %) diluted with argon (99.995%, Air Products) to give 2%
mixtures. Target metals of beryllium (Johnson-Matthey), magnesium
(nMg rod, Fisher; 26Mg, 96%, Oak Ridge National Laboratory), and
calcium (Alfa Inorganics) were employed. A CsI window at 10 ( 1
K was the substrate for co-deposition of the gas mixture, at a rate of
approximately 2 mmol/h, with the metal atoms ablated using a Nd:
YAG laser (1064 nm) focused (10 cm focal length) onto the rotating
metal target. Laser energies ranged from 20 to 50 mJ/pulse. Infrared
spectra were recorded at 0.5 cm-1 resolution and with an accuracy of
(0.1 cm-1 using a Nicolet 750 FTIR equipped with a liquid N2-cooled
MCTB detector. Photolysis of the cold deposit was carried out using
the output from a 1000 W mercury arc passed through a water filter to
absorb infrared radiation.
Beryllium. Figure 1 shows the infrared spectrum obtained
when laser-ablated beryllium atoms were co-condensed with a
2% methane/argon mixture. The intensity and number of the
features in the spectrum bear witness to the high reactivity of
the metal atoms under the conditions of laser ablation and
suggest that several products have been formed. The spectra
also show the presence of trace quantities of H2O, CO2, and
CO,24-26 along with simple hydrocarbon products, C2H2, C2H4,
and C2H6, all produced in low concentrations.27-29 The broad
band at 617.2 cm-1 observed in the experiment employing CH4,
which shifts to 612.6 (13CH4) and 454.0 cm-1 (CD4), indicates
the presence of the methyl radical.30 Bands arising from BeH
and BeH2 may be assigned by comparison with the earlier matrix
study.7 BeH is identified by a single band at 1971.0 cm-1 in
experiments using CH4 or 13CH4; the deuterated diatomic gives
a very weak band at 1477.5 cm-1 when CD4 is used as the
reagent gas. The BeH2 molecule was observed through absorp-
tions at 2159.6 and 696.8 cm-1 in both the CH4 and 13CH4
experiments, while BeD2 gave rise to bands at 1674.4 and 531.7
cm-1 in experiments involving CD4. The frequencies associated
with all of these hydride species are within (0.5 cm-1 of the
Density functional theory (DFT) calculations were performed using
the Gaussian 94 program package with the BP86 functional and the
6-311G* basis sets for each atom.21-23 The pure DFT functional has
been shown to work well for other simple beryllium species, particularly
for predicting observed vibrational frequencies.11
(18) Burkholder, T. R.; Andrews, L. J. Chem. Phys. 1991, 95, 8697-
8709.
(19) Andrews, L.; Burkholder, T. R. J. Phys. Chem. 1991, 95, 8554-
8560.
(20) Hassanzadeh, P.; Andrews, L. J. Phys. Chem. 1992, 96, 9177-
9182.
(21) Gaussian 94, Revision B.1; Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.;
Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.;
Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.;
Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng,
C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E.
S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.;
Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A.;
Gaussian, Inc.: Pittsburgh, PA, 1995.
(22) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639-
5648. Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys.
1980, 72, 650-654.
(23) Becke, A. D. Phys. ReV. A 1988, 38, 3098-3100. Perdew, J. P.
Phys. ReV. B 1986, 33, 8822-8824.
(24) Redington, R. L.; Milligan, D. E. J. Chem. Phys. 1963, 39, 1276-
1284.
(25) DiLella, D. P.; Tevault, D. E. Chem. Phys. Lett. 1986, 126, 38-42.
(26) Nelander, B. J. Phys. Chem. 1985, 89, 827-830.
(27) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J. Phys. Chem. 1982,
86, 3374-3380.
(28) Andrews, L.; Johnson, G. L.; Kelsall, B. J. J. Chem. Phys. 1982,
76, 5767-5773.
(29) Davis, S. R.; Andrews, L. J. Am. Chem. Soc. 1987, 109, 4768-
4775.
(30) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1967, 47, 5146-5156.

Reactions of Laser-Ablated Be, Mg, and Ca with CH4
J. Am. Chem. Soc., Vol. 120, No. 24, 1998 6099
Table 1. Infrared Absorptions (cm-1) Observed Following the
Co-deposition of Laser-Ablated Beryllium Atoms with Methane/
Argon Mixtures at 10 K
Be/12CH4/Ar
Be/13CH4/Ar
Be/CD4/Ar
p/aa
assignment
2087.3
2071.9
2062.3
2059.4
1218.9
1205.4
1180.6
1178.9
1127.2
1084.1
1082.4
983.7
924.6
851.7
b
2086.4
2071.6
2062.1
2058.9
1206.3
1195.0
1170.4
1168.7
1123.6
1082.4
1080.6
970.7
914.7
843.8
b
1593.5
1578.2
1570.5
1568.0
b
979.3
962.1
vv/V
v/V
HCBeH
CH2BeH
v/V
CH3BeH
}
v/V
v/V
CH3BeCH3
CH3BeH
V/v
CH3Be
?
}
}
1182.2
1154.4
1152.9
935.2
837.1
765.6
748.0
617.8
615.8
580.7
579.8
555.9
541.8
430.2
427.8
vv/V
v/V
CH3BeCH3
vv/V
v/V
V/v
v/V
HCBeH
CH2BeH
CH3Be
CH3BeH
721.3
720.1
704.2
715.8
714.4
700.4
v/V
CH3BeH
}
}
v/V
CH3BeH
682.8
660.0
678.6
654.2
v/V
v/V
?
CH2BeH
534.2
533.0
vv/V
HCBeH
}
}
443.0
435.4
442.9
435.3
b
v/V
CH3BeH
a Effects of photolysis (p)/annealing (a): large increase (vv), small
increase (v), or decrease (V). b Corresponding bands were not observed.
approximately 10%. Finally, bands at 1180.6/1178.9 and 851.7
cm-1 were made distinct by their reduction in intensity following
photolysis and by being the only features in the spectrum to
display growth following annealing of the matrix deposit.
The region of the spectrum 3100-2800 cm-1 associated with
ν(C-H) revealed a number of features, including weak bands
arising from C2H6. These features were also found, however,
in experiments involving other metals31 and so do not have their
origin in a molecule containing beryllium. The absorptions
observed at 1127.2 and 682.8 cm-1 remain unassigned.
The response of the infrared features described here to
photolysis and annealing, along with the corresponding bands
observed when 13CH4 or CD4 were used as the reagent gas, are
listed in Table 1.
Figure 2. Infrared spectra of argon matrix following the co-deposition
of laser-ablated Be atoms with 2% CH4 in argon at 10 K: (a) sample
deposited for 1 h at 10 K, (b) after 15 min photolysis with visible block
filter, and (c) after final Pyrex filter and broad-band photolysis for 60
min each.
values obtained for the molecules isolated in hydrogen-doped
argon matrices.7
The remaining features may then be grouped together by their
varying response to photolysis and annealing. The samples were
irradiated initially using a visible block filter which removed
light in the range of 400 < λ < 700 nm, then a Pyrex filter
(transmitting λ > 290 nm), and finally with broad-band light
(220-1000 nm) from the full arc. Figure 2 contrasts the effect
of visible blocked and subsequent broad-band photolysis.
Thereafter, the deposit was subjected to a number of annealing
cycles from base temperature to ca. 35 K.
For the experiments involving CH4, a set of infrared features
at 2062.3/2059.4, 1205.4, 704.2, and 443.0/435.4 cm-1 may at
once be gathered together. All show a small increase in intensity
of approximately 5% following an initial period of 15 min
photolysis using the visible-block filter. Subsequent photolysis,
as outlined above, continued this trend reaching a 10% growth
as shown in Figure 2c after full-arc photolysis. Annealing the
matrix caused these bands to decrease in intensity. The bands
at 2062.3 and 704.2 cm-1 are the two most intense features in
the spectrum after those attributable to CH4, suggesting that they
and the weaker features of the set are associated with the primary
reaction product. Additional bands at 1218.9, 1084.1/1082.4,
and 721.3/720.1 cm-1 displayed similar behavior but can be
assigned to dimethylberyllium (vide infra).
Magnesium. Experiments analogous to those previously
described were performed using laser-ablated magnesium atoms.
The resulting spectra are shown in Figure 3 for four isotopic
samples. The reduced number and intensity of the new infrared
features are indicative of the less reactive nature of the
magnesium atom under the conditions of our experiments.
Nevertheless, a set of absorptions was observed to increase in
intensity following broad-band photolysis and to decrease when
the matrix was subjected to annealing cycles up to 35 K; these
occur at 1560.3/1552.2, 1127.7, and 547.9/545.4/542.8 cm-1
.
Corresponding features were also observed in the experiments
using 13CH4 and CD4 and when 26Mg was employed as the target
metal; these will be described in the Discussion. Observations
in the higher frequency region, 3100-2800 cm-1, paralleled
those in experiments involving beryllium. The matrix was also
found to contain the trace impurities and various hydrocarbon
molecules observed in the beryllium experiments.
A band at 1421.7 cm-1 was observed in experiments using
CH4 or 13CH4. This shifted to 1419.7 cm-1 when 26Mg was
the metal target and to 1037.4 cm-1 when CD4 was the reagent
gas. It indicates the presence of MgH identified in a previous
Of the remaining infrared features, a set of bands at 2087.3,
983.7, and 534.2 cm-1 was characterized by an increase in
intensity in the order of 50% following initial photolysis, while
another set at 2071.9, 1388.5, and 660.0 cm-1 increased by
(31) Greene, T. M.; Andrews, L.; Downs, A. J. Unpublished results.

6100 J. Am. Chem. Soc., Vol. 120, No. 24, 1998
Greene et al.
spectrum broad features at 1263/1254/1245, 1151, 1098/1093,
and 419/409 cm-1 which are unidentified. Broad-band pho-
tolysis did not bring about any changes in the infrared spectrum,
while annealing altered only the relative intensities of features
split by matrix-site effects. Experiments were also performed
using neat methane in an attempt to increase the yield of any
products formed. Although the spectra of these matrices did
contain a number of new features, they were dominated by the
presence of the hydrocarbons mentioned above, which result
from the short-wavelength photolysis of the methane by the light
emanating from the metal target.
Discussion
The infrared features observed in the experiments that have
been described will be assigned to various organometallic
products. A full justification of this assignment by way of a
comparison with analogous species and with calculated vibra-
tional properties will be presented. The mechanism leading to
the formation of the observed products will then be discussed.
CH3BeH. The group of frequencies associated with the
primary reaction product are assigned to the monomethyl-
beryllium hydride molecule, CH3BeH. Calculations performed
here and by others33 suggest that such a molecule is linear at
the metal and so possesses C3V symmetry. For such a unit, eight
infrared-active fundamentals are to be expected, four nonde-
generate (a1) and four doubly degenerate (e). Table 2 shows
the excellent agreement between the frequencies observed for
CH3BeH and those forecast by DFT/BP86/6-311G* calculations.
The occurrence of a pair of bands where only one fundamental
is expected is the result of matrix splitting;34 that it occurs for
vibrations of both a1 and e symmetry and is not consistent
between the different isotopomers supports this argument, in
preference to the possibility that a splitting of the degenerate e
mode has been brought about by a bent geometry at the metal
center. In addition to the frequencies given in Table 2,
experiments involving CH2D2 led to the identification of features
at 1567.1 (1582.0), 1120.3 (1144.5), and 597.2 (617.9) cm-1
associated with CH2DBeD and at 2061.2 (2089.7), 1002.3
(1016.6), 660.1 (684.8), and 644.0 (667.6) cm-1 associated with
CHD2BeH (the calculated frequencies are given in parentheses).
The descent in symmetry of these molecules from C3V prevents
their inclusion in Table 2.
As noted previously, we were unable to detect any ν(C-H)
bands attributable to CH3BeH either because of their inherently
low intensity or, more probably, because of masking by the
ν(C-H) bands of hydrocarbons present in the matrix deposit.
Thus, the highest frequency that we can assign to CH3BeH is
2062.3/2059.4 cm-1 which is clearly associated with ν(Be-
H). This compares with values of 2119.4 and 2117.7 cm-1 for
ν(Be-H) in HBeCCH10 and HBeOH,9 respectively. The
frequency of the corresponding ν(Be-D) mode gives H/D ratios
of 1.3131/1.3134, which is slightly lower than expected for a
motion primarily involving the hydrogen atom (compare, for
example, the case of CH3MH (M ) Zn, Cd, or Hg) where the
ratio is ca. 1.39). The change reflects the participation of a
much lighter metal atom. Of the fundamentals involving
deformations of the CH3 unit, only that arising from the
symmetric deformation is observed, this being calculated to be
Figure 3. Infrared spectra of argon matrices following the co-
deposition of laser-ablated Mg atoms with 2% methane/argon mixtures
at 10 K: (a) Mg + CH4, (b) 26Mg + CH4, (c) Mg + 13CH4, and (d)
Mg + CD4.
Figure 4. Infrared spectra of an argon matrix following the co-
deposition of laser-ablated Ca atoms with 2% CH4/argon mixture at
10 K.
matrix-isolation study of the reaction of laser-ablated magnesium
atoms with dihydrogen.14 The frequencies observed here are
within (0.4 cm-1 of those reported earlier. A very weak 1571.9
cm-1 band for the dihydride MgH2 was detected. A number
of other weak bands were observed to alter in intensity following
photolysis. A sharp, weak band at 502.2 cm-1 exhibited isotopic
counterparts but no associated bands could be located in these
experiments. Such bands may well represent very small yields
of molecules analogous to those found in the beryllium
experiments (vide infra), but their reduced intensity prevents
the identification of a sufficient number of absorptions to allow
the carriers to be identified with confidence.
Calcium. The infrared spectrum of the matrix deposit
following the co-condensation of laser-ablated calcium atoms
with CH4/Ar gas mixtures is shown in Figure 4. It is at once
apparent that Ca atoms are much less reactive than Be or Mg
atoms under similar conditions. In consequence, the spectrum
displays few new features outside those signaling the presence
of C2H2, C2H4, C2H6, and the CH3 radical. Of these, a band at
1233 cm-1 may be assigned to the diatomic CaH, which has
been observed in earlier studies examining the reaction of
calcium atoms with dihydrogen in argon matrices and very weak
sharp features at 747 and 516 cm-1 due to calcium oxides from
target surface contaminants.32 There then remains in the
(32) Andrews, L.; Tague, T. J., Jr. Unpublished results. Andrews, L.;
Yustein, J.; Thompson, C. A.; Hunt, R. D. J. Phys. Chem. 1994, 98, 6514-
6512.
(33) Hashimoto, K.; Osamura, Y.; Iwata, S. J. Mol. Struct. (THEOCHEM)
1987, 152, 101-117.
(34) Barnes, A. J. In Vibrational Spectroscopy of Trapped Species;
Hallam, H. E., Ed.; Wiley: London, 1973; Chapter 4.

Reactions of Laser-Ablated Be, Mg, and Ca with CH4
J. Am. Chem. Soc., Vol. 120, No. 24, 1998 6101
Table 2. Observed and Calculated Frequencies (cm-1) for CH3BeHa
CH3BeH
calcda
13CH3BeH
calcda
CD3BeD
description of
vibrational mode
obsd
obsd
obsd
calcda
assignment
b
2062.3
2059.4
1205.4
b
2928.5 (2)c
b
2062.1
2058.9
1195.0
b
2924.9 (2)
b
2107.2 (0.1)
1581.5 (141)
ν1 (a1)
ν2 (a1)
ν
sym(C-H)
1570.5
1568.0
979.3
748.0
ν(Be-H)
2090.9 (191)
2090.8 (190)
}
}
}
1237.2 (151)
854.0 (19)
1226.5 (48)
845.1 (20)
993.3 (42)
745.3 (2)
ν3 (a1)
ν4 (a1)
δ
sym(CH3)
ν(Be-C)
b
b
2991.0 (29)
1422.7 (3)
b
b
2980.9 (31)
1420.4 (3)
b
b
2207.7 (7)
1026.9 (5)
ν5 (e)
ν6 (e)
νasym(C-H)
δasym(CH3)
704.2
729.7 (376)
700.4
725.4 (374)
580.7
579.8
602.1 (257)
ν7 (e)
ν8 (e)
F(CH3)
}
443.0
435.4
442.9
435.3
434.9 (65)
434.4 (64)
b
318.4 (31)
δ(C-M-H)
}
}
a Symmetry C3V: C-H ) 1.106 Å, C-Be ) 1.679 Å, Be-H ) 1.344 Å, H-C-H ) 112.6°. b Not observed. c Intensities (km mol-1) are
given in parentheses.
Table 3. Observed and Calculated Frequencies (cm-1) for CH3BeCH3
CH3BeCH3
obsdb
13CH3Be13CH3
CD3BeCD3
obsdb
description of
obsda
calcdc
obsda
calcdc
obsda
calcdc
assignment
ν6 (a2u)
vibrational moded
1218.9
1084.1
1082.4
721.3
1222
1081
1254.7 (167)
1095.4 (59)
1206.3
1082.4
1080.6
715.8
1240.5 (154)
1092.7 (65)
e
994
889.5 (2)
δ
sym(CH3)
asym(C-Be-C)
F(CH3)
1154.4
1152.9
617.8
615.8
1150
1154.6 (204)
ν7 (a2u)
ν
}
}
}
727
}
749.4 (251)
743.2 (246)
}
603
640.7 (206)
ν10 (eu)
}
720.1
714.4
a This work. b Values from gas phase, ref 36. c Intensities (km mol-1) are given in parentheses; calculated bond lengths: C-H ) 1.106 Å, C-Be
) 1.685 Å. d There is an extensive interaction between ν6 and ν7 which is fully discussed in ref 40. e Band obscured by the ν4 mode of CD4.
the more intense. The position of δsym(CH3) at 1205.4 cm-1
may be compared with a value of 1223 cm-1 obtained from
the infrared spectrum of gaseous CH3BeBH4.35 The next
vibrational transition that the calculations suggest we should
The formation of (CH3)2Be by insertion of the metal atom into
the C-C bond of ethane is noteworthy as neither copper38 nor
mercury6 atoms have been observed to react in this way, both
yielding C2H5MH (M ) Cu or Hg). Studies of the reaction of
metal atoms with ethane in the gas phase have led to the
proposal that the stronger C-H bond is more susceptible to
attack than is the weaker C-C bond because of the presence
of a sufficient steric barrier opposing attack of the excited metal
atom on the C-C bond.15 It may well be that the much smaller
Be atom can overcome such a barrier.
CH3Be. A pair of infrared features is assigned in Table 1 to
the radical CH3Be. The DFT-calculated frequencies for this
molecule are given in Table 4 and allow the assignment of the
bands at 1180.6/1178.9 cm-1 to δsym(CH3) and of the band at
851.7 cm-1 to ν(Be-C).
The species CH3M (M ) Li, Na, or K) have also been isolated
in inert matrices.39,40 The frequencies obtained for δsym(CH3)
are 1158, 1092, and 1053 cm-1, a trend which has been
interpreted, at least in part, in terms of an increasing charge
separation between the metal and methyl group. The value we
obtain at ca. 1178 cm-1 indicates a metal-to-carbon bond with
significant covalent character. CH3Be is presumably formed
when the matrix is annealed by the reaction of Be atoms and
CH3 radicals as they diffuse together.
CH2BeH. The set of bands at 2071.9, 1388.5, 660.0, and,
possibly, 924.6 cm-1 increases by 10% following the first period
of photolysis and decays by 20% over the rest of the photolysis
sequence. This family is assigned on the basis of isotopic
changes and DFT calculations to the radical CH2BeH, as
indicated in Table 5. The most intense band, at 660.0 cm-1, is
a CH2 out-of-plane deformation mode as characterized by the
observed 5.8 cm-1 13C and 118.2 cm-1 D shifts (to be compared
encounter approximating to ν(Be-C) is expected near 854 cm-1
.
Although we do indeed see a band at 851.7 cm-1, its behavior
following photolysis and annealing of the deposit implies that
it does not belong to the group of frequencies ascribed to CH3-
BeH. It seems most likely that this feature, which is assigned
later, masks the ν(Be-C) band of CH3BeH and so prevents its
detection. Such a proposal is afforced by experiments involving
CD4 where we observe a band at 748.0 cm-1 attributable to the
ν(Be-C) mode of CD3BeD, along with another feature at 765.6
cm-1 which appears to be the counterpart of the band at 851.7
cm-1 observed in the CH4 experiments. The final feature we
may assign to the CH3BeH molecule lies at 443.0/435.4 being
associated with δ(C-Be-H). The corresponding band for the
perdeuterated isotopomer is not observed; calculations suggest
that it lies below 400 cm-1 and therefore beyond our range of
detection.
CH3BeCH3. As stated earlier, bands at 1218.9, 1084.1/
1082.4, and 721.3/720.1 cm-1 are assigned to dimethylberyl-
lium, (CH3)2Be. This has been investigated in the gas phase,36
with the conclusion that the unsaturated vapor contains only
monomeric species. The infrared spectrum is reported to display
strong features at 1222, 1081, and 727 cm-1 corresponding
closely to the ones we observe for the matrix-isolated molecule.
The assignments for the other isotopomers are made by reference
to our DFT-calculated frequencies for (13CH3)2Be and to the
gas-phase infrared spectrum of (CD3)2Be.36 The bands we
observe are compared with these other data in Table 3. The
same features are also observed in experiments involving the
co-deposition of ethane or CH3Br with laser-ablated Be atoms.37
(38) Ozin, G. A.; Mitchell, S. A.; Garc´ıa-Prieto, J. Angew. Chem., Int.
Ed. Engl. 1982, 21, 211; Angew. Chem. Suppl. 1982, 369-380.
(39) Andrews, L. J. Chem. Phys. 1967, 47, 4834-4842.
(40) Burczyk, K.; Downs, A. J. J. Chem. Soc., Dalton Trans. 1990,
2351-2357.
(35) Cook, T. H.; Morgan, G. L. J. Am. Chem. Soc. 1970, 92, 6487-
6492.
(36) Kovar, R. A.; Morgan, G. L. Inorg. Chem. 1969, 8, 1099-1103.
(37) Lanzisera, D. V.; Andrews, L. Unpublished results.

6102 J. Am. Chem. Soc., Vol. 120, No. 24, 1998
Greene et al.
Table 4. Observed and Calculated Frequencies (cm-1) for CH3Be
CH3Be
13CH3Be
CD3Be
description of
vibrational mode
obsda
calcdc
obsda
calcdc
obsda
b
calcdc
assignment
ν1 (a1)
b
2922.4 (5)c
1205.7 (37)
855.7 (26)
b
2919.0 (5)
1195.4 (34)
847.2 (26)
2100.0 (1)
985.1 (51)
773.8 (5)
ν
sym(C-H)
sym(CH3)
ν(Be-C)
asym(C-H)
asym(CH3)
F(CH3)
1180.6
1178.9
851.7
1170.4
1168.7
843.8
962.1
765.6
ν2 (a1)
δ
}
}
ν3 (a1)
b
b
b
2992.8 (12)
1412.9 (19)
558.8 (4)
b
b
b
2982.6 (16)
1410.3 (18)
555.3 (4)
b
b
b
2210.1 (7)
1021.2 (10)
433.4 (2)
ν4 (e)
ν5 (e)
ν6 (e)
ν
δ
a Symmetry C3V: C-H ) 1.106 Å, C-Be ) 1.688 Å, H-C-Be ) 112.0°. b Not observed. c Intensities (km mol-1) are given in parentheses.
Table 5. Observed and Calculated Frequencies (cm-1) for CH2BeH
CH2BeH
13CH2BeH
CD2BeD
description of
vibrational mode
obsd
calcda
obsd
calcda
obsd
calcda
assignment
b
2071.9
1388.5
b
3005.4 (3)c
2098.8 (178)
1375.3 (18)
924.8 (32)
b
2071.6
1381.0
b
2999.4
2098.7
1369.0
912.7
b
2184.6
1592.3
1046.9
827.3
ν1 (a1)
ν2 (a1)
ν3 (a1)
ν4 (a1)
ν(C-H)
ν(Be-H)
δ(CH2)
1578.2
b
b
ν(Be-C)
b
b
3070.6(10)
692.9 (183)
508.4 (55)
b
b
3059.2
689.1
508.3
b
577.4
b
2274.0
575.1
385.4
ν5 (b1)
ν6 (b1)
ν7 (b1)
ν(C-H)
δ(CH2)
δ(C-M-H)
(577.6)
(576.4)
660.0
b
658.5 (145)
419.1 (32)
654.2
b
652.3
418.6
541.8
b
542.7
305.8
ν8 (b2)
ν9 (b2)
∆(CH2)
∆(C-M-H)
a Symmetry C2V: C-H ) 1.101 Å, C-Be ) 1.656 Å, Be-H ) 1.342 Å, H-C-H ) 125.0°. b Not observed. c Intensities (km mol-1) are
given in parentheses.
Table 6. Observed and Calculated Frequencies (cm-1) for HCBeHa
HCBeH
calcda
3163.5 (0.3)
H13CBeH
calcda
3153.0 (0.1)
DCBeD
calcda
2338.4 (4)
DCBeH
calcda
2339.6 (8)
HCBeD
calcda
3163.0 (0.2)
description of
assignment vibrational mode
obsd
obsd
obsd
obsd
obsd
b
b
b
b
b
ν1 (σ+)
ν2 (σ+)
ν3 (σ+)
ν(C-H)
ν(Be-H)
ν(Be-C)
2087.3 2113.8 (163) 2086.4 2113.7 (163) 1593.5 1608.0 (133) 2084.5 2111.8 (158) 1593.3 1610.8 (136)
983.7 998.3 (53) 970.7 983.1 (51) 935.2 926.5 (32) 961.7 976.5 (50) 947.1 946.7 (33)
534.2 526.4 (358) 533.0 525.1 (354) 430.2
427.8
432.6 (228) 532.7 524.8 (169) 455.3 452.8 (116)
ν4 (π)
ν5 (π)
δ(HCBeH)
δ(HCBeH)
}
b
416.9 (12)
b
414.8 (14)
b
311.1 (1)
b
323.0 (5)
b
386.7 (9)
a Calculated for a linear geometry: H-C ) 1.092 Å, C-Be ) 1.621 Å, Be-H ) 1.339 Å. b Not observed. c Intensities (km mol-1) are given
in parentheses.
with the calculated shifts of 6.2 and 115.8 cm-1, respectively).
The strong Be-H stretching mode shows a small 13C shift (0.3
cm-1), even though the calculated shift is no more than 0.1
cm-1. The CH2 in-plane deformation mode is masked by the
absorption of CH3BeH, but the CD2 counterpart is resolved at
577.4 cm-1. Finally, a weak band at 577.6 cm-1 occurs at a
slightly higher than calculated frequency (508.4 cm-1) but this
mode is difficult to model; a tentative assignment is therefore
appropriate.
HCBeH. Three absorptions with frequencies of 2087.3,
983.7, and 534.2 cm-1 are grouped together by virtue of their
marked growth following photolysis. The band at 2087.3 cm-1
suggests the presence of a Be-H unit while that at 983.7 cm-1
may be thought to arise from a ν(Be-C) mode. Calculations
performed on HCBeH give infrared spectra in excellent agree-
ment with the observed spectrum. The frequencies for HCBeH
and its isotopomers are listed in Table 6. The molecule is
calculated to be linear (3Σ-) in accord with a previous ab initio
study where calculations were also reported for the ground state
of the isomer H2CBe.41 The lowest energy structure (3B1) is
reported to have C2V symmetry. Although it is predicted that
triplet H2CBe lies some 12 kJ mol-1 lower in energy than triplet
HCBeH, a sufficient barrier (210 kJ mol-1) is calculated to exist
between the two molecules to suggest that both isomers are
capable of existing independently. Our findings are at least in
accord with this inasmuch as we have clearly identified the
HCBeH molecule, but not, however, any features for the isomer
H2CBe.
CH3MgH. The set of frequencies observed (Table 7) in
experiments using laser-ablated magnesium atoms may readily
be attributed to the insertion product CH3MgH, by comparison
both with the frequencies and intensity patterns generated by
DFT calculations and with the infrared spectrum reported
previously by McCaffrey et al.13 for CH3MgH isolated in solid
methane. Again we were unsuccessful in assigning any
frequencies to the ν(C-H) modes. As in the work with
beryllium, the bands observed in this region were common to
experiments employing a wide range of other metals, many of
which do not yield monomethyl metal hydride species.31 Our
assigment of the bands at 1560.3/1552.2 cm-1 to ν(Mg-H) and
the perdeuterio shift to 1136.8 cm-1 show excellent agreement
with the calculated values. The discrepancies between the
frequencies for CH3MgH trapped in an argon and in a methane13
matrix reflect the perturbing effects of the different matrix
environments. The H/D ratio of 1.3725/1.3654 is considerably
larger than that found for the same mode in CH3BeH in keeping
with the normal mode that involves principally motion of the
hydrogen atom. Owing to the isolated ν(Mg-H) nature of this
(41) Luke, B. T.; Pople, J. A.; Schleyer, P. v. R. Chem Phys. Lett. 1983,
97, 265-269.

Reactions of Laser-Ablated Be, Mg, and Ca with CH4
J. Am. Chem. Soc., Vol. 120, No. 24, 1998 6103
Table 7. Observed and Calculated Frequencies (cm-1) for CH3MgH
CH3MgH
obsda obsdb
CH326MgH
calcdc
13CH3MgH
obsdb
CD3MgD
calcdc
obsda
obsda
calcdc
obsda obsdb
2036 2101.6 (12)
1136.8 1115 1130.9 (156)
calcdc
assignmentd
ν1 (a1)
e
2896 2928.3 (30)
1524 1563.6 (273)
e
2928.3 (30)
e
2896 2925.2 (30)
1524 1563.6 (273)
e
1560.3
1552.2
1558.3
1550.0
1560.4
1552.3
1561.1 (271)
ν2 (a1)
}
}
1127.7 1122 1159.4 (0.007) 1127.0
1159.4 (0.007) 1119.0
e
1151.8 (0.001)
515.7 (17)
877.1
506.9
e
872
424
892.2 (1)
489.2 (13)
ν3 (a1)
ν4 (a1)
ν5 (e)
ν6 (e)
e
e
e
539
526.2 (18)
537.8
e
e
547.2
544.8
543.0
518.9 (18)
2998.6 (47)
1435.2 (0.4)
534.5
e
e
544.4
542.4
540.5
528
2932 2998.6 (47)
2924 2988.3 (48)
2190 2215.2 (12)
e
1435.2 (0.4)
581.5 (284)
e
1432.4 (0.2)
577.8 (280)
e
e
1038.5 (3)
547.9
545.4
542.8
423.8
422.9
421.1
550
580.5 (280)
546
441
447.6 (195)
ν7 (e)
ν8 (e)
}
}
}
}
353
340
350
340
300
288
e
298.5 (321)
e
297.6 (320)
e
298.5 (322)
}
e
219.2 (164)
}
}
a This work. b Data taken from ref 13. c Intensities (km mol-1) are given in parentheses; symmetry C3V: C-H ) 1.105 Å, C-Mg ) 2.106 Å,
Mg-H ) 1.725 Å, AH-C-H ) 112.3°. d For the approximate description of the vibrational mode, see Table 2. e Not observed; see discussion.
mode, counterparts were slightly shifted (1560.1, 1138.4 cm-1
in an experiment with CH2D2. In the same symmetry class,
the symmetric methyl deformation was found at 1127.7 cm-1
)
Reaction Mechanisms. The signs are that CH3BeH most
likely results from the insertion of an energetically excited
beryllium atom into a C-H bond of methane. The optical
spectrum of Be atoms in a range of different matrices has been
studied.42 In argon the resonance transition 1P1 r 1S0 appears
at 236 nm while the spin-forbidden emission 3P1 f 1S0 occurs
at 465 nm. Laser ablation of the metal target populates both
these states and imparts to the atoms sufficient kinetic energy
to cause them to undergo insertion. Furthermore, the lifetime
of the 3P state is sufficient for a considerable population of the
Be atoms to remain in this excited state and react in the
condensing matrix.43 The results of selective photolysis of the
isolated reagents indicate that a Be atom excited to the 1P1 state
is capable of insertion, as is observed when the matrix is
photolyzed using the visible block filter. However, additional
growth of features attributable to CH3BeH is also observed when
photolysis with λ > 290 nm is employed. The absorption band
.
Assignments of the fundamentals F(CH3) and ν(Mg-C) will
be taken together. Although the previous study of CH3MgH13
reported two features separated by 11 cm-1 which were
identified with these modes, we were able to detect only a split
band at 547.9/545.4/542.8 cm-1. That this band obscures the
presence of the ν(Mg-C) mode is made clear by isotopic
substitution. Experiments involving either 26Mg/CH4 or Mg/
13CH4 gave rise to a less intense band situated to low frequency
of the main feature. This band shifts 3.3 cm-1 between
CH326MgH and 13CH3MgH in excellent agreement with the
calculated shift for ν(Mg-C) of 3.2 cm-1. That the separation
of F(CH3) and ν(Mg-C) is greater for 13CH3MgH than for CH3-
MgH is also supported by the experiments with methane
matrices.13 For the isotopomer CD3MgD, we again observe a
split band at 423.8/422.9/421.1 cm-1 arising from the F(CD3)
motion. In addition, a band at 506.9 cm-1 displays the
appropriate behavior following photolysis and annealing for it
to be assigned to CD3MgD. The ratio of this frequency to that
observed for ν(Mg-C) in CH326MgH is 1.0610 in excellent
agreement with the value of 1.0607 suggested by the calcula-
tions. As such, the band at 506.9 cm-1 is assigned to the ν(Mg-
C) fundamental of CD3MgD. In the previous study of CD3MgD
by McCaffrey et al.,13 a band at 424 cm-1 was assigned to this
mode, but incorrectly on the basis of the evidence of the present
assignment. We note the importance of 26Mg substitution for
making the ν(Mg-C) vibrational assignment.
1
1P1 r S0 for Be atoms is relatively sharp,42 and so it seems
most likely that the formation of CH3BeH in the matrix can
3
also be brought about by exciting Be atoms to their P1 state.
These findings are in accord with our mechanistic studies of
the reaction of Zn, Cd, or Hg atoms with methane in argon
matrices.6
In addition to CH3BeH, we have identified a number of other
products formed when Be atoms react with methane. Our
knowledge of the behavior of these species in response to
photolysis or to annealing allows us to suggest an overall
reaction mechanism, Scheme 1. The first step of the reaction
sequence involves the insertion of an excited Be atom into a
C-H bond of methane, thereby generating CH3BeH in an
excited electronic state. This may in turn simply be quenched
by interaction with the argon matrix to give CH3BeH in its
ground state or may decompose to yield the radicals BeH and
CH3. Furthermore, [CH3BeH]* can decompose by H elimina-
tion to give the radical products CH3Be and CH2BeH or H2
elimination to give HCBeH. The dramatic increase in the
concentration of HCBeH following initial photolysis is the
culmination of the photochemical reaction. The formation of
CH3BeCH3 remains to be incorporated into this scheme; it is
probably formed by the coming together of the radicals CH3Be
and CH3. The CH3 radical is present in the matrix either as a
result of the decomposition of excited CH3BeH or as a result
of the high-energy photolysis of the methane molecule. The
CH3Be radical can also be formed on annealing from the union
Additional experiments involving CH2D2 identified bands at
540.6 and 444.3 cm-1 to be associated with the molecule CH2-
DMgD, while a feature at 474.6 cm-1 is assigned to the
isotopomer CHD2MgH. These frequencies may be identified
with fundamentals involving the methyl rocking mode.
CH3CaH. Of the unidentified features in the Ca and CH4
experiments the band at 1263/1254/1245 cm-1 may tentatively
be assigned to the ν(Ca-H) mode and that at 419/409 cm-1 to
the F(CH3) mode of CH3CaH. Comparison with the spectra of
other CH3MH species (M ) Be or Mg) leads to the expectation
that these two modes are the most intense in infrared absorption.
Although it may be tempting to assign one of the remaining
features at 1151 or 1098/1093 cm-1 to a F(CH3) mode of CH3-
CaH, the relative intensities of these bands compared with that
due to the F(CH3) vibration warn against such an assignment.
For CH3MH (M ) Zn,6 Cd,6 Hg,6 Be, or Mg), these funda-
mentals are always much weaker in infrared absorption than is
the methyl rocking mode. The absorptions at 1151 and 1098/
1093 cm-1 must therefore remain unassigned.
(42) Brom, J. M., Jr.; Hewett, W. D., Jr.; Weltner, W., Jr. J. Chem. Phys.
1975, 62, 3122-3130.
(43) Andrews, L.; Chertihin, G. V.; Thompson, C. A.; Dillon, J.; Byrne,
S.; Bauschlicher, C. W., Jr. J. Phys. Chem. 1996, 100, 10088-10099.

6104 J. Am. Chem. Soc., Vol. 120, No. 24, 1998
Greene et al.
Scheme 1
products MH (M ) Be, Mg, or Ca) and CH3. Any detailed
understanding of the reasons for this reduction in reactivity is
likely then to hinge on a full consideration of the energetics of
the reaction, and the results of the present study invite a full
computational analysis of the insertion process.
Conclusions
A matrix-isolation and density functional theoretical study
of the reactions between laser-ablated beryllium, magnesium,
or calcium atoms and methane has been performed. The
insertion product CH3MH (M ) Be, Mg, or Ca) has been
identified in each case by its infrared spectrum with the aid of
isotopic substitution and also by comparison with the vibrational
properties of other monomethyl metal hydrides. Selective
photolysis studies have been employed in the case of beryllium
to probe the mechanism of the reaction. This reaction yields,
in addition to CH3BeH, a range of other organoberyllium
products, namely, (CH3)2Be, CH3Be, CH2BeH, and HCBeH,
all of which were identified by the infrared spectra of their
different isotopomers and density functional theory calculations
of isotopic frequencies. The identities and structures of the
species observed in our experiments were substantiated by the
results of DFT/BP86 calculations.
of a CH3 radical and Be atom. Previous studies have shown
that excited Be atoms react with dihydrogen to give BeH2 and
BeH,8 molecules also present in these experiments.
The mechanism of the insertion process that yields CH3MgH
has been studied by McCaffrey et al. who have shown that
photolysis at the 3p 1P r 3s 1S Mg atom resonance absorption
results in insertion of the metal into a C-H bond of methane.43
Acknowledgment. The authors thank W. D. Bare, G. P.
Kushto, S. P. Wilson, and M. F. Zhou for performing additional
experiments and calculations. The AFOSR and EPSRC are
acknowledged for financial support and, specifically the latter,
for the award of an Advanced Fellowship to T.M.G. Calculations
were performed on the University of Virginia SP2 machine.
3
Again the laser-ablation process produces an abundance of P
Mg atoms, which survive to react in the condensing matrix.37
It is noteworthy that the reduced tendency of the laser-ablated
metals to form CH3MH by insertion into a C-H bond of
methane in the order Be > Mg > Ca correlates with the
expected decrease in the metal-carbon bond strength. This also
matches an increase in the relative yield of the decomposition
JA9804870