Reactions of laser-ablated Be and Mg atoms with C2H2: Infrared spectra and density functional calculations of novel metal-acetylene species

Article abstract of DOI:10.1021/ja9616847

Reactions of laser-ablated Be and Mg atoms with C₂H₂, ¹³C₂H₂, and C₂D₂ upon condensation in excess argon at 10 K give new infrared absorptions that are assigned to the insertion and decomposition products HBeCCH, BeCCH, and MgCCH with the help of density functional theory isotopic frequency calculations. The 2019-cm^-1 C≡C stretching fundamental for BeCCH is between values for BCCH and AlCCH, which are all slightly blue shifted from acetylene itself.

Full text of DOI:10.1021/ja9616847

10242
J. Am. Chem. Soc. 1996, 118, 10242-10249
Reactions of Laser-Ablated Be and Mg Atoms with C H :
2
2
Infrared Spectra and Density Functional Calculations of Novel
Metal-Acetylene Species
Craig A. Thompson and Lester Andrews*
Contribution from the Department of Chemistry, UniVersity of Virginia,
CharlottesVille, Virginia 22901
X
ReceiVed May 20, 1996
Abstract: Reactions of laser-ablated Be and Mg atoms with C2H2, 13C2H2, and C2D2 upon condensation in excess
argon at 10 K give new infrared absorptions that are assigned to the insertion and decomposition products HBeCCH,
-
1
BeCCH, and MgCCH with the help of density functional theory isotopic frequency calculations. The 2019-cm
CtC stretching fundamental for BeCCH is between values for BCCH and AlCCH, which are all slightly blue shifted
from acetylene itself.
Introduction
however, a number of theoretical investigations of the Be-C2H2
1
8-20
interaction have appeared.
Flores and Largo have calcu-
The reaction of metal atoms with small hydrocarbon mol-
ecules is an active field of research.
important for understanding catalysis and chemisorption as well
as for developing new synthetic methods. Recent studies have
shown that thermal aluminum atoms react with acetylene to give
new addition products; however, laser-ablated aluminum and
boron atoms react with acetylene to produce new insertion and
decomposition products, namely AlCCH, HBCCH, and BCCH.
It is of interest to note that thermal Ca and Sr atoms react directly
with acetylene to give CaCCH and SrCCH and laser ablated
lated the most stable structure to be the linear HBeCCH insertion
1
-13
Such studies are
20
product. Studies by Breckenridge and Unemoto on collisional
quenching of electronically excited Mg indicate that Mg (3s
1
3
p, P1) reacts with H2, CH4, C3H6, and C3H8 to produce MgH
2
1
but not with C2H4 and C2H2. In contrast, spectra reported
here show that laser-ablated beryllium and magnesium atoms
react with acetylene to form new product molecules, as well as
BeH and MgH.
7
8,9
10
Experimental Section
Mg reacts with methane in a supersonic expansion to give the
MgCCH molecule.11 However, thermally evaporated Li atoms
The apparatus for pulsed-laser ablation matrix-isolation spectroscopy
2
2,23
has been described earlier.
Reagent grade acetylene (Matheson),
form an addition product, LiC2H2 (C2V),12 while Na atoms form
an acetylene complex that rearranges on photolysis (λ > 5000
Å) to a vinylidene complex, NaCCH2.13
13
13
2 2 2 2
C H (92% C), and C D (98% D, Merck Sharp and Dohme) were
all purified by pumping on condensed samples prior to thawing for
dilution with argon (99.995%, Air Products) to make gas samples.
n
Reactions of laser-ablated beryllium atoms with O2, H2, N2,
Beryllium metal (Johnson-Matthey) and magnesium ( Mg rod; Fisher;
26
and H2O have produced novel chemical species such as OBeO,
Mg, 96%, Oak Ridge National Laboratory) were used. Briefly, Ar:
2 2
C H mixtures ranging from 200:1 to 400:1 were codeposited at 2
ArBeO, HBeH, NNBeNN, and HOBeOH.1
4-17
No experimen-
mmol/h onto a 10 ( 1 K substrate with metal atoms ablated using a
Nd:YAG laser (1064 nm) focused (10 cm focal length) onto a rotating
metal target with laser energies ranging from 40 to 80 mJ/pulse at the
tal reactions of beryllium with acetylene have been reported;
X
Abstract published in AdVance ACS Abstracts, September 15, 1996.
(1) Klabunde, K. J.; Tanaka, Y. J. Am. Chem. Soc. 1983, 105, 3544.
2) Chenier, J. M. B.; Howard, J. A.; Mile, B.; Sutcliffe, R. J. Am. Chem.
-1
target. Infrared spectra were recorded at 0.5 cm resolution and (0.1
(
-1
2
cm accuracy on a Nicolet 750 FTIR using a liquid N cooled MCT
Soc. 1983, 105, 788. Kasai, P. H. J. Am. Chem. Soc. 1983, 105, 6704.
3) Kafafi, Z. H.; Hauge, R. H.; Margrave, J. L. J. Am. Chem. Soc. 1985,
07, 7550.
detector. The samples were subjected to mercury arc photolysis (Philips
75 W) and annealing cycles and more spectra were recorded.
(
1
1
(
(
4) Brown, S. H.; Crabtree, R. H. Chem. Commun. 1987, 970.
5) SelectiVe Hydrocarbon ActiVation; Davies, J. A., Watson, P. L.,
Results
Product bands were grouped based on varying yields in
separate experiments as well as band profiles. Bands are labeled
with their assignments to simplify nomenclature.
Greenberg, A., Liebman, J. F., Eds.; VCH Publishers, Inc.: New York,
1
990.
(6) Greene, T. M.; Andrews, L.; Downs, A. J. J. Am. Chem. Soc. 1995,
1
17, 8180.
(
(
7) Burkholder, T. R.; Andrews, L. Inorg. Chem. 1993, 32, 2491.
8) Chertihin, G. V.; Andrews, L.; Taylor, P. R. J. Am. Chem. Soc. 1994,
Be + C2H2. The codeposition of laser-ablated Be and C2H2
-
1
1
16, 3513.
9) Andrews, L.; Hassanzadeh, P.; Martin, J. M. L.; Taylor, P. J. Phys.
Chem. 1993, 97, 5839.
10) (a) Bopegedera, A. M. R. P.; Brazier, C. R.; Bernath, P. F. Chem.
in excess argon produced a weak new band at 3325.9 cm in
(
(16) Thompson, C. A.; Andrews, L.; Davy, R. D. J. Phys. Chem. 1995,
99, 7913.
(
Phys. Lett. 1987, 136, 97. Bopegedera, A. M. R. P.; Brazier, C. R.; Bernath,
P. F. J. Mol. Spectrosc. 1988, 129, 268. (b) Whitham, C. J.; Soep, B.;
Visticot, J.-P.; Keller, A. J. Chem. Phys. 1990, 93, 991.
(17) Thompson, C. A.; Andrews, L. J. Phys. Chem. In press.
(18) (a) Swope, W. C.; Schaefer, H. F. J. Am. Chem. Soc. 1976, 98,
7962. (b) Witko, M.; Bonacic-Koutecky, V. J. Quantum Chem. 1986, 29,
1535.
(11) (a) Anderson, M. A.; Ziurys, L. M. Astrophys. J. 1995, 439, L25.
(
5
b) Corlett, G. K.; Little, A. M.; Ellis, A. M. Chem. Phys. Letts. 1996, 249,
3. (c) Ellis, A. M. Personal communication.
(19) (a) Sakai, S.; Morokuma, K. J. Phys. Chem. 1987, 91, 3661; (b)
Balaji, V.; Jordan, K. D. J. Phys. Chem. 1988, 92, 3101.
(20) Flores, J. R.; Largo, A. J. Phys. Chem. 1991, 95, 9278.
(21) Breckenridge, W. H.; Unemoto, H. J. Chem. Phys. 1981, 75, 698.
(22) Burkholder, T. R.; Andrews, L. J. Chem. Phys. 1991, 95, 8697.
Andrews, L.; Burkholder, T. R. J. Phys. Chem. 1991, 95, 8554.
(23) Hassanzadeh, P.; Andrews, L. J. Phys. Chem. 1992, 96, 9177.
(
(
12) Manceron, L.; Andrews, L. J. Am. Chem. Soc. 1983, 107, 563.
13) Kasai, P. H.; McLeod, D.; Watanabe, T. J. Am. Chem. Soc. 1977,
9
9, 3521. Kassai, P. H. J. Phys. Chem. 1982, 86, 4092.
(
(
14) Thompson, C. A.; Andrews, L. J. Chem. Phys. 1994, 100, 8689.
15) Tague, T. J., Jr.; Andrews, L. J. Am. Chem. Soc. 1993, 115, 12111.
S0002-7863(96)01684-8 CCC: $12.00 © 1996 American Chemical Society

Reactions of Laser-Ablated Be and Mg Atoms with C2H2
J. Am. Chem. Soc., Vol. 118, No. 42, 1996 10243
Table 1. Absorptions (cm-1) Observed for Be + HCCH Reaction
Products in Solid Argon
12
13
C/ C ratios
1
3
13
calcda
assign
HCCCCH 3325.9
BeH 2159.4
HBeCCH 2119.4
HCCH H C CH DCCD
obsd
3308.9
2159.4
2115.2
1947.8
2593
1674.3
1.005 138
2
1535.0 1.001 986 1.003 02
1873.4 1.003 650 1.036 94
BeCCH
2018.9
BeH
CCH
1970.1
1845.8
1970.1
1785.5
1477.6
1746.9 1.033 772 1.036 736
BeCCH
HBeCCH
855.0b
846.2
829.6
848.9 1.010 399 1.010 012
784.9 1.011 210 1.010 826
838.9c
C
2
H
2
736.7
734.9
542.5 1.002 449 1.003 051
HBeCCH
BeCCH
689.1
675.7
683.3
671.4
1.008 488 1.009 690
553.8 1.006 404 1.009 475
HCCCCH
628.1
624
495.6
(
site)
553.0
550.0
551.5
548.4
451.3 1.002 720 1.003 414
450.0 1.002 918 1.003 414
HBeCCH
a
b
Calculated by DFT, Tables 4 and 5. Mixed 12/13 intermediates
-
1
c
-1
at 851.8 and 849.0 cm
.
Mixed isotopic intermediate at 833.3 cm
.
Figure 1. Infrared spectra from 2135 to 1925 cm-1 for Be + C
H
2
, and (b)
2
reaction products in solid argon at 10 ( 1 K: (a) Be + C
2
H
2
1
3
Be +
C
2
H
2
. Codeposition of Be, 70 mJ/pulse, and Ar/C
2
H
2
) 200/1
samples for 2 h.
addition to the strong acetylene bands at 3301.9 and 3288.9
-1 24
cm . The former band, not present in Ar/C2H2 blank samples,
25
is a reaction product, diacetylene, in laser-ablation experiments.
The spectrum in the region expected for C-C and Be-H
stretching vibrations is shown in Figure 1a for Be + C2H2 in
-1
excess argon. A strong band at 2119.4 cm marked HBeCCH
-1
is accompanied by a sharp band at 2018.9 cm marked BeCCH.
Weak BeH2 and BeH bands were present at 2159.4 and 1970.1
-
1 15
cm . In addition the CCH radical band was observed at
1
845.8 cm-1.26-28 Frequencies for product absorptions are listed
1
4
in Table 1. With the exception of weak beryllium oxide and
nitride16 bands from trace air contaminant, no product absorp-
-
1
tions were observed between 2000 and 900 cm .
The spectrum for the same Be + C2H2 experiment is shown
in Figure 2a in the region expected for Be-C stretching modes
-
1
and CCH bending modes. A sharp band at 866.3 cm is due
14
to the strongest absorption of (BeO)2. Sharp bands at 855.0
-1
and 838.9 cm are marked BeCCH and HBeCCH, respectively.
-
1
Two intense bands at 744.9 and 736.7 cm due to C2H2 dimer
and monomer, respectively, are truncated to show weaker
Figure 2. Infrared spectra from 900 to 650 cm- for Be + C H
1
2
2
-
1
product bands. The sharp band at 696.8 cm is due to the
reaction products in solid argon at 10 ( 1 K: (a) Be + C
2
H
2
, (b) Be
, spectrum expanded ×3 relative to the
absorbance scale given for spectra a and b. Codeposition of Be, 70
mJ/pulse, and Ar/C ) 200/1 samples for 2 h.
bending mode of BeH2.15 A broad band at 689.1 cm and a
-1
13
2 2
, and (c) Be + C D
+
C
H
2 2
-
1
sharper band at 675.7 cm are marked HBeCCH and BeCCH,
respectively. The lower region in Figure 3a reveals a sharp
2 2
H
-
1
doublet at 553.0 and 550.0 cm also marked HBeCCH. A
Be + 13C2H2 codeposition spectrum (Figure 1b) shows a strong
band at 2115.2 cm marked HBe C CH shifted only 4.2 cm
from its C counterpart. In contrast, the sharp band at 1947.8
cm (marked Be C CH) shifted significantly in the
experiments as did the C CH band at 1785.5 cm .
spectrum (Figure 2b) revealed an isotopic counterpart band at
46.2 cm accompanied by weak isotopic intermediate bands
at 851.8 and 849.0 cm marked with isotope bars. Similarly
-
1
25
weak band at 628.1 cm (not shown) is due to diacetylene.
-1
13 13
-1
Photolysis of samples had no effect on product bands.
Annealing to temperatures ranging from 20 to 30 K decreased
band intensities from 10 to 25%; however, all bands decreased
comparable amounts and selective reactions were not observed.
12
-1
13 13
13
C
13 13
-1 26-28
The
1
3
13
Be + C2H2. Experiments with C2H2 were carried out to
-1
8
assist in the identification of product molecules. As expected,
-1
1
3
-1
C2H2 bands at 3284.7 and 3270.7 cm were observed in the
-1
a sharp band at 829.6 cm accompanied by an intermediate
C-H stretching region along with 13C4H2 at 3308.9 cm . The
-1
-1
13 13
band at 833.3 cm is marked HBe C CH. The acetylene
bands are again truncated. BeH2 was observed unshifted along
(
24) Andrews, L.; Johnson, G. K.; Kelsall, B. J. J. Phys. Chem. 1982,
-
1
13 13
8
6, 3374, 3380.
with a broad band at 683.3 cm , marked HBe C CH, and a
(
(
(
(
25) Patten, K. O.; Andrews, L. J. Phys. Chem. 1986, 90, 3910.
26) Jacox, M. E. Chem. Phys. 1975, 7, 424.
27) Shepherd, R. A.; Graham, W. R. M. J. Chem. Phys. 1987, 86, 2600.
28) Jacox, M. E.; Olson, W. B. J. Chem. Phys. 1987, 86, 3134.
-1
13 13
sharp band at 671.4 cm , marked Be C CH. The spectrum
in Figure 3b shows the lowest product absorption with a small
1
3
-1
C shift to 551.5 cm .

10244 J. Am. Chem. Soc., Vol. 118, No. 42, 1996
Thompson and Andrews
Table 2. Absorptions (cm-1) for Mg + HCCH Reaction Products
in Solid Argon
12
13
1
3
13
C/ C ratios
obsd
calcda
1982.7 1911.4 1854.7 1982.9 1.037303 1.037254
HCCH H C CH DCCD HCCH
24
24
24
24
assign
site)
Mg
Mg
Mg
Mg
(
MgCCH 1984.2 1913.0 1856.4 1984.1 1.037219 1.037254
CCH
MgH
1845.8 1785.5 1746.9 1845.8 1.033772 1.036736
1421.9 1421.9
1419.5
C
2
H
2
736.7
734.9
656.2
482.0
542.5 736.7 1.002449 1.003051
660.8 1.007010 1.008659
485.1 482.8 1.020332 1.018639
MgCCH 660.8
MgCCH 491.8
a
Calculated by DFT, Table 8.
Figure 3. Infrared spectra from 570 to 430 cm-1 for Be + C
reaction products in solid argon at 10 ( 1 K: (a) Be + C , (b) Be
. Codeposition of Be, 70 mJ/pulse, and
) 200/1 samples for 2 h.
2 2
H
2 2
H
1
3
+
C
2
2 2 2
H , and (c) Be + C D
2 2
Ar/C H
Be + C2D2. Experiments with deuterated acetylene were also
conducted. Absorptions were generally weaker owing to isotope
intensity effects. As expected, bands for C2D2 were observed
-1
at 2441.7 and 2432.8 cm . The CCD and BeD2 products were
-1
observed at 1746.9, and 1674.3 cm , respectively in agreement
with earlier reports.1
5,26-28
A Be + C2D2 spectrum is shown in Figure 2c. Note the
-1
-1
band at 848.9 cm , marked BeCCD, shifted only 6.1 cm
from its hydrogen counterpart in Figure 2a while the DBeCCD
Figure 4. Infrared spectra from 500 to 460 cm- for Mg + C
1
H
band at 781.9 cm- shifted in excess of 50 cm . Trace C2H2
1
-1
2
2
n
reaction products in solid argon at 10 ( 1 K: (a) Mg + C
2
H
2
, (b)
. Codeposition
2
H ) 200/1 samples for 4 h.
bands were present in the spectrum along with BeH2 at 696.8
n
13
n
26
Mg +
C
2 2
H
, (c) Mg + C
2
D
2
2
2 2
, and (d) Mg + C H
-
1
-1
cm and HCCD at 683.4 cm (note the spectrum is expanded
3 on the absorbance scale). The lowest product absorption
of Mg, 50 mJ/pulse, and Ar/C
×
bands at 1972.1 and 1968.9 cm- are attributed to (C2H2)x. In
addition two weak new bands at 1982.7 and 1984.2 cm were
detected. The radicals CCH and MgH were observed at 1845.8
1
24
-1
in Figure 3c with sites at 451.3 and 450.0 cm shifted in excess
of 100 cm and is marked DBeCCD.
-
1
-
1
Be + CH3C2H. Two experiments were done with beryllium
and propyne under the same conditions employed for acetylene.
Weak bands were observed for BeH2 and (BeO)2 as before. A
trio of new bands was observed at 2038.9, 2033.6, and 2028.0
-1
26-29,31
and 1421.9 cm , respectively.
In addition, a broad band
-1
at 660.8 cm adjacent to CO2, not present in acetylene samples,
shifted with acetylene and magnesium isotopes, and is due to a
product absorption. Diacetylene was again observed at 628.1
cm .
The region expected for Mg-C stretching modes also
contained a strong new absorption; Figure 4a shows a sharp
-
1
cm (A ) 0.03) in addition to sharp, weaker bands at 2159.4
-
1 25
-
1
-1
(
0
BeH2) and 2112.9 cm and a new band at 2019.0 cm (A )
-
1
.01). No 2119-cm absorption was observed.
Mg + C2H2. Similar experiments were carried out reacting
n
-
1
band at 491.8 cm marked MgCCH. Note also the weaker
laser-ablated magnesium atoms with C2H2. Since product yields
in magnesium experiments were lower, longer deposition periods
were required. As with beryllium experiments the upper regions
-1
bands at 487.0 and 482.8 cm , which are of appropriate relative
2
5
26
intensity (8/1/1) to be due to the Mg and Mg isotopes in
natural abundance.
-
1
contained C2H2 bands at 3301.9 and 3288.4 cm and C4H2 at
13
Mg + C2H2. Reactions of Mg with isotopic acetylene were
carried out. With C2H2 a weak band for C3 was observed
at 1961.3 cm . In addition, weak new bands were present
at 1911.4 and 1913.0 cm . The band at 660.8 cm with C2H2
shifted to 656.2 cm with C2H2 samples. Finally, the band
in the lower region in Figure 4b shifted to 482.0 cm (marked
-
1
3
2
325.9 cm . In addition a very weak C3 band was detected at
1
3
13
-
1
-1 29,30
039.0 cm and CCH was observed at 1845.8 cm .
-
1 29
Product absorptions for magnesium-acetylene reactions are
listed in Table 2; no new magnesium-acetylene products were
observed in the C-H stretching region.
Several Ar/C2H2 blank experiments assisted in the identifica-
tion of a weak new band in the C-C stretching region. Sharp
-1
-1
-
1
13
-
1
1
3 13
Mg C CH).
Mg + C2D2. A Mg + C2D2 experiment revealed two weak
absorptions at 1856.4 and 1854.7 cm and a CCD radical at
-
1
(
29) Weltner, W., Jr.; Walsh, P. N.; Angell, C. L. J. Chem. Phys. 1964,
0, 1299.
30) Jacox, M. E.; Milligan, D. E. Chem. Phys. 1974, 4, 45.
4
(
(31) Tague, T. J., Jr.; Andrews, L. J. Phys. Chem. 1994, 98, 8611.

Reactions of Laser-Ablated Be and Mg Atoms with C2H2
Reaction
Products Calculated Using Becke’s Hybrid DFT Method with the
Lee, Yang, Parr Functional (B3LYP) and 6-311G* Basis Sets
J. Am. Chem. Soc., Vol. 118, No. 42, 1996 10245
Table 3. Energies (au) for Be + HCCH and Mg + C
2
H
2
Table 4. Frequencies (cm-1), Bond Lengths (Å), and Bond Angles
(deg) of HBeCCH, HCCH, and BeH Calculated Using B3LYP and
6-311G* Basis Sets
1
atom/molecule
H
symmetry, state
2S
energy (au)
HBeCCH
RBe-H ) 1.324
RC-H ) 1.066
(C , Σ)
∞
V
-0.502 156
-1.176 632
RBe-C ) 1.630 RC-C ) 1.214
H
2
D
∞h, 1Σ
g
3
Be
Be
BeH
CH
CH
P
S
-14.580 757
-14.671 184
-15.264 396
-38.457 060
-38.490 299
-76.626 486
-76.722 749
-77.206 927
-77.281 150
-77.348 605
-91.325 685
-91.462 457
-91.979 737
-91.99 249
1
ω (I, km/mol)
2
C
C
C
C
C
C
C
∞V, Σ
13 13
HBeCCH
3446.1 (16)
σ (Be-H) 2191.0 (314)
σ (C-C) 2102.2 (15)
σ (Be-C) 859.0 (83)
HBe C CH
DBeCCD
4
∞V, Σ
2
σ (C-H)
3427.9 (18)
2184.4 (277)
2033.3 (44)
849.8 (81)
2674.2 (1)
∞V, Σ
2
1639.6 (143)
1980.8 (152)
807.8 (59)
558.1 (2 × 218)
CCH
CCH
CCH
CCH
∞V, Σ
-
1
∞V, Σ
3
2
2
2V, B
2
1
π
π
π
703.2 (2 × 53)
696.6 (2 × 57)
2V, A
∞h, Σ
1
1
587.9 (2 × 218) 585.9 (2 × 213) 486.3 (2 × 164)
HCCH
D
g
4
221.5 (2 × 1)
HCCH
215.7 (2 × 1)
194.1 (2 × 0)
BeCCH
BeCCH
HBeCCH
C
C
C
C
C
C
C
s
, A′
2
∞V, Σ
s
1
3
(D∞h, Σg)
, A′
1
RC-H ) 1.065
RC-C ) 1.199
BeCCH
Be-C
BeCCH
2
2V branched, A
2V ring, B
2V branched, B
1
3
2
H
2
2
2
-92.031 775
-92.041 719
-92.064 345
-92.119 622
-199.990 786
-200.093 099
-200.646 954
-276.835 305
-277.378 882
-277.402 793
-277.453 954
ω (I, km/mol)
3
2
1
13 13
Be-C
2
H
2
2V ring, A
1
HCCH
H C CH
DCCD
1
HBeCCH
Mg
C
∞V, Σ
σ
σ
σ
π
π
g
u
g
3512.8 (0)
3410.0 (69)
2067.8 (0)
3486.5 (0)
3399.8 (69)
2001.4 (0)
2814.7 (0)
2503.7 (37)
1825.5 (0)
3
P
1
Mg
MgH
MgCCH
S
2
C
C
C
C
C
∞V, Σ
2
u
756.1 (2 × 101)
753.8 (2 × 101)
555.1 (2 × 55)
∞V, Σ
1
g
560.8 (0)
552.3 (0)
468.0 (2 × 0)
Mg-C
Mg-C
2
H
H
2
2V ring, A
1
3
2
2
2
2
2V ring, B
2
BeH
RBe-H ) 1.3461
(C , Σ)
〈S 〉 ) 0.7512
∞
V
1
HMgCCH
∞V, Σ
ω (I, km/mol)
-
1
1
746.9 cm . The new band in the magnesium-carbon
BeH
2045.6 (136)
BeD
-
1
stretching region shown in Figure 4c shifted to 485.1 cm
marked MgCCD).
σ
1518.0 (75)
(
26
Mg + C2H2. In contrast to beryllium, another magnesium
isotope, 26Mg, is available to assist in the identification of
product bands. Careful inspection revealed a small shift of the
product in the C-C stretching region to 1982.6 and 1984.1
Møller-Plesset levels of theory (MP2 and MP4).20 In addition,
Hartree-Fock SCF frequency calculations for natural abundance
isotopes of each molecule in general are slightly higher than
the B3LYP frequencies provided here, which are in turn slightly
higher than the observed values. However, frequency and
intensity calculations with isotopic substitution of carbon-13,
deuterium, and magnesium-26 were not reported and these
important calculations are provided here. Several starting
geometries were considered for the optimization routines. Bent
variations of HBeCCH and HMgCCH were considered; how-
ever, these converged to the linear structure.
-1
cm . Similarly, the band in the magnesium-carbon stretching
-
1
26
region, Figure 4d, shifted to 482.8 cm (marked MgCCH),
and is in agreement with the lower weak band observed with
natural isotopic magnesium.
Quantum Chemical Calculations. A number of possible
products were considered for the beryllium-acetylene and
magnesium-acetylene systems. All calculations presented here
were conducted on a Cray C90 Supercomputer at the San Diego
Supercomputer Center using Gaussian 94.32 Geometry opti-
mizations and frequency calculations used Becke’s Three
Parameter Hybrid Density Functional Method with the Lee Yang
Parr functional (collectively abbreviated B3LYP)33 and 6-311G*
basis sets for all atoms. Calculations on open-shell systems
The stability orders for the beryllium-acetylene and mag-
nesium-acetylene systems were the same as those reported by
2
0
Flores and Largo. For both systems the most stable ground
1
+
state species was HMCCH ( Σ ). The stability order for the
1
+
beryllium system using B3LYP is as follows: HBeCCH ( Σ )
2
1
3
3
were unrestricted. Values for 〈S 〉 are provided and indicate
> cyclic BeC2H2 ( A1) > BeCCH2 ( B2) ∼ cyclic BeC2H2 ( B2)
1 3
negligible artificial spin contamination in these calculations.
Table 3 lists the optimized energies for several possible
products. Calculations on different electronic states were
performed to ensure that the ground electronic state was
identified. The relative stability of beryllium-acetylene and
magnesium-acetylene compounds has been studied extensively
by Flores and Largo using complete second- and fourth-order
> BeCCH2 ( A1) >HBeCCH ( A′). Comparison of calculated
frequencies provided in Tables 4-6 demonstrates that although
linear BeCCH and HBeCCH have similar absorptions, the cyclic
and vinylidene Be-C2H2 species are significantly different.
Several theoretical investigations on MgC2H2 species are also
in the literature; MP2 and MP4 studies reported by Flores and
Largo again resemble results presented here, as do SCF and CI
3
4
19b
calculations by Chaquin et al. and Balaja and Jordan.
linear MgCCH radical was also investigated and isotopic
frequencies are listed in Table 8.
Figure 5 shows B3LYP optimized structures for ground state
beryllium-acetylene and magnesium-acetylene species. Com-
parisons of structures demonstrate that, as expected, Mg-C
bonds are consistently longer than Be-C bonds in analogous
The
(
32) 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 92 and Gaussian 94,
ReVision B.1.; Gaussian, Inc.: Pittsburgh PA, 1995.
(
33) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. Becke,
A. D. J. Chem. Phys. 1993, 98, 5648.
(34) Chaquin, P.; Sevin, A.; Papakondylis, A. Chem. Phys. 1990, 143,
39.

10246 J. Am. Chem. Soc., Vol. 118, No. 42, 1996
Thompson and Andrews
Table 5. Frequencies (cm- ), Bond Lengths (Å), and Bond Angles
1
-
(
deg) of BeCCH, CCH, and CCH Calculated Using B3LYP and
6
-311G* Basis Sets
2
2
BeCCH
(C , Σ)
〈S 〉 ) 0.7508
∞
V
RBe-C ) 1.635
RC-C ) 1.215
RC- ) 1.067
ω (I, km/mol)
1
3
13
BeCCH
Be C CH
BeCCD
σ (C-H)
σ (C-C)
σ(Be-C)
π
3442.8 (16)
2108.1 (115)
877.7 (123)
3424.6 (18)
2033.0 (107)
869.0 (120)
2670.3 (0)
1969.0 (120)
869.7 (117)
561.9 (2 × 21)
228.7 (2 × 5)
708.5 (2 × 43)
701.7 (2 × 44)
π
240.7 (2 × 5)
234.0 (2 × 4)
2
2
CCH
RC-C ) 1.201
(C , Σ)
RC-H ) 1.066
〈S 〉 ) 0.7503
∞
V
ω (I, km/mol)
1
3
13
Figure 5. Bond lengths (Å) and angles (deg) for stable isomers of
possible beryllium-acetylene and magnesium-acetylene reaction
products calculated at the B3LYP level with 6-311G* basis sets.
CCH
C CH
CCD
σ (C-H)
σ (C-C)
π
3451.0 (45)
2088.38 (6)
229.1 (2 × 4)
3432.3 (45)
2014.4 (5)
226.8 (2 × 4)
(C∞V, Σ)
RC-H ) 1.0732
2682.2 (15)
1939.0 (11)
182.2 (2 × 4)
Table 7. Frequencies (cm-1), Bond Lengths (Å), and Bond Angles
(deg) of HMgCCH and MgH Calculated Using B3LYP and
-
1
CCH
6
-311G* Basis Sets
RC-C ) 1.2042
1
HMgCCH
(C , Σ)
∞V
ω (I, km/mol)
RMg-H ) 1.693
RC-H ) 1.067
RMg-C ) 2.030
RC-C ) 1.217
1
3
13
CCH
C CH
CCD
σ(C-H)
σ(C-C)
π
3323.4 (14)
3306.6 (16)
1829.9 (33)
508.7 (61)
2554.9 (0)
1781.4 (30)
408.1 (31)
ω (I, km/mol)
1898.2 (136)
513.9 (2 × 51)
13
13
HMgCCH
HMg C CH
DMgCCD
σ (C-H)
σ (C-C)
3434.3 (15)
2066.9 (34)
3416.3 (16)
1992.7 (31)
1667.0 (212)
665.4 (2 × 63)
504.9 (62)
2658.1 (2)
Table 6. Frequencies (cm-1), Bond Lengths (Å), and Bond Angles
deg) of Cyclic Be-C
and 6-311G* Basis Sets
1930.6 (40)
1205.0 (132)
525.5 (2 × 25)
502.4 (58)
(
2
H
2
and BeCCH
2
Calculated Using B3LYP
σ(Mg-H) 1667.1 (212)
π
671.1 (2 × 62)
σ(Mg-C) 514.4 (65)
1
2
Be-C H2
(C , A )
2V
1
π
π
369.0 (2 × 304) 367.6 (2 × 300) 289.8 (2 × 193)
RBeC ) 1.556
RCC ) 1.388
∠ CBeC ) 53.0
ω (I, km/mol)
RCH ) 1.088
144.9 (2 × 1)
140.6 (2 × 1)
ω (I, km/mol)
129.8 (2 × 0)
∠
BeCH ) 170.5
26
26
13 13
26
H MgCCH
H Mg C CH
D MgCCD
1
3
Be-C
2
H
2
Be- C
2
H
2
2 2
BeC D
σ (C-H)
σ (C-C)
3434.3 (15)
2066.8 (33)
3416.3 (16)
1992.6 (31)
1664.4 (208)
665.4 (2 × 63)
495.1 (60)
3416.3 (16)
1992.6 (31)
1664.4 (208)
665.4 (2 × 63)
495.1 (60)
A
1
3148.8 (37)
3127.7 (32)
1407.4 (0)
1169.0 (5)
1063.1 (104)
1023.2 (0)
923.5 (54)
906.1 (22)
707.6 (129)
3137.3 (36)
3117.4 (33)
1366.3 (0)
1147.5 (4)
1044.2 (98)
1013.8 (0)
912.2 (54)
702.9 (126)
905.1 (24)
2347.2 (18)
2312.1 (9)
1362.2 (0)
1061.4 (111)
1051.3 (25)
804.8 (0)
784.0 (33)
647.3 (9)
557.0 (93)
B
2
σ(Mg-H) 1664.4 (209)
A
1
π
671.1 (2 × 62)
B
2
σ(Mg-C) 504.9 (63)
A
A
1
2
π
π
367.1 (2 × 301) 365.8 (2 × 298) 365.8 (2 × 298)
144.8 (2 × 1)
140.5 (2 × 1)
140.5 (2 × 1)
B
2
2
2
MgH
(C , Σ)
〈S 〉 ) 0.7549
A
1
∞V
B
1
RMg-H ) 1.7528
3
2
BeCCH2
(C , B )
2V 2
〈S 〉 ) 2.009
ω (I, km/mol)
MgD
RBe-C ) 1.491
RC-C ) 1.321
RC-H ) 1.095
26MgH
1436.7 (241)
MgH
∠
CCH ) 122.9
σ
1439.0 (241)
1038.2 (126)
ω (I, km/mol)
1
3
13
BeCCH
2
Be C CH
2
BeCCD
2
Discussion
B
2
3090.4 (7)
3039.5 (15)
1749.3 (37)
1410.8 (18)
928.6 (0)
919.7 (46)
860.1 (88)
256.0 (1)
2295.3 (5)
2295.3 (5)
2222.0 (12)
1713.7 (37)
1057.9 (36)
824.1 (64)
750.1 (2)
736.0 (24)
249.6 (1)
225.7 (4)
A
A
A
B
B
A
B
B
1
1
1
2222.0 (12)
1713.7 (37)
1057.9 (36)
824.1 (64)
750.1 (2)
736.0 (25)
249.6 (1)
Product band assignments and reaction mechanisms will be
presented.
HBeCCH. The diagnostic band for the HBeCCH group is
2
-1
the site-split doublet of bands at 553.0 and 550.0 cm ; this
1
1
3
band shows large deuterium and small C isotope shifts and
appears in the region expected for the HBeC bending mode.
The insertion product, HBeCCH, is a likely candidate since
previous studies with laser-ablated beryllium suggest that
1
1
2
246.1 (6)
225.7 (4)
1
4,16,35
metastable Be atoms undergo insertion reactions.
In fact
the observed bands are in excellent agreement with the
species. Bond lengths and angles are also provided in Tables
-8 along with isotopic frequency calculations, which are
discussed with product assignments in the following section.
4
(35) Andrews, L.; Chertihin, G.; Thompson, C. A.; Dillon, J.; Byrne,
S.; Bauschlicher, C. W., Jr. J. Phys. Chem. 1996, 100, 10088.

Reactions of Laser-Ablated Be and Mg Atoms with C2H2
J. Am. Chem. Soc., Vol. 118, No. 42, 1996 10247
Table 8. Frequencies (cm-1), Bond Lengths (Å), and Bond Angles
BeC2H2 structural isomer and the only one observed in these
experiments.
BeCCH. Several bands in the product spectra were adjacent
to HBeCCH absorptions implying a similar chemical species
with similar vibrational modes. Note the band marked BeCCH
(
deg) of MgCCH Calculated Using B3LYP and 6-311G* Basis
Sets
2
2
MgCCH
(C , Σ)
〈S 〉 ) 0.751
∞
V
RMgC ) 2.057
RC-C ) 1.218
RC-H ) 1.067
-
1
-1
at 855.0 cm is 16.1 cm above the Be-C stretch of HBeCCH
ω (I, km/mol)
-
1
13
(
Figure 2). The band shifted 8.8 cm to the red upon
substitution; however, it only shifted 6.1 cm with D substitu-
C
1
3
13
MgCCH
Mg C CH
MgCCD
-1
-
1
σ(C-H)
3432.6 (15)
2054.8 (23)
664.0 (2 × 48)
486.4 (87)
3414.7 (15)
1981.0 (21)
658.3 (2 × 48)
477.5 (84)
2654.5 (3)
1920.4(31)
tion compared to 54.0 cm for the Be-C stretch of HBeCCH,
indicating that the molecule has a similar Be-C stretching mode
with much less hydrogen interaction. These shifts are in
excellent agreement with isotopic frequency calculations for
linear BeCCH presented in Table 5; the Be-C stretching mode
σ(C-C)
π
520.2 (2 × 24)
σ(Mg-C)
480.9 (85)
π
136.1 (2 × 13)
131.7 (2 × 12)
129.3 (2 × 13)
13
13
-1
12 12
ω (I, km/mol)
of Be C C is calculated to red shift 8.7 cm from Be C CH
-
1
26MgCCH
26
13 13
26MgCCD
and the observed shift is 8.8 cm ! A comparison can also be
made of the relative Be-C stretching frequencies of HBeCCH
and BeCCH. As shown in Tables 4 and 5, BeCCH is calculated
to have a Be-C stretching mode 18.7 cm higher than
HBeCCH, in agreement with the observed difference of 15.1
Mg C CH
σ(C-H)
σ (C-C)
π
σ(Mg-C)
π
3432.6 (15)
2054.7 (23)
664.0 (2 × 48)
476.7 (84)
3414.6 (15)
2654.5 (3)
1980.9 (2 × 21)
658.3 (2 × 48)
467.7 (81)
131.4 (2 × 12)
1920.3 (31)
520.1 (2 × 24)
471.2 (82)
129.0 (2 × 12)
-
1
135.8 (2 × 13)
-1
cm .
The molecule BeCCH is calculated to have a CCH bending
calculated absorptions for HBeCCH presented in Table 4; other
molecules lacking the HBeC subunit are not expected to have
-1
mode at 708.5 cm , slightly higher than the analogous mode
-1
of HBeCCH. The observed band at 675.7 cm is actually red-
-
1
bands in the 550-cm region. The molecule HBeCCH also
has an intense Be-H stretching mode blue-shifted about 150
-1
shifted 13.4 cm ; however, this discrepancy is attributed to
the more complex description for the bending mode of the
-
1
cm from the calculated BeH fundamental. Note that the
BeCCH radical. The C-C stretching mode of BeCCH was
-
1
HBeCCH band in Figure 2a is blue-shifted 149 cm from the
-1
observed at 2018.9 cm compared with the calculated fre-
15
BeH fundamental observed by Tague and Andrews at 1970.1
-1
quency of 2108.1 cm . Using the C-C stretching fundamental
-1
cm ; BeH was also observed in these experiments in low yield.
-1 36
of acetylene from the Raman spectrum (1974 cm ),
a
Calculations also predict the Be-H stretching mode of
comparison of the observed/calculated ratio for the C-C stretch
of acetylene (0.955) can be made to the observed/calculated
ratio for the C-C stretch of HBeCCH (0.958). The almost equal
ratios indicate excellent agreement between the calculated and
observed spectra. Note that although HBeCCH and BeCCH
1
3
13
-1
HBe C CH to be red-shifted 6.6 cm from HBeCCH. The
-
1
observed value of 4.2 cm is in good agreement when
considering the complex description of the mode arising from
mixing with the C-C stretch, as indicated by an increase in
13 13
intensity for the C-C stretch of HBe C CH relative to
-1
are calculated to have C-C stretching modes within 6 cm of
HBeCCH. In addition, the observed deuterium shift of 584.4
each other, the C-C stretching mode of BeCCH is calculated
to be seven times greater intensity. Accordingly the failure to
observe the C-C stretching mode of HBeCCH is attributed to
low intensity. For BeCCH the calculated and observed isotope
-
1
cm is in good agreement with the predicted value of 551.4
-
1
cm considering anharmonicity, which is not accounted for in
the B3LYP harmonic frequency calculations.
Excellent agreement between the calculated and observed
isotopic frequencies for Be-C stretching modes and CCH
bending modes of HBeCCH is also found here. The upper band
marked HBeCCH (Figure 3) corresponds to the Be-C stretching
mode. Note that the 13C counterpart band appeared 9.3 cm
red-shifted in excellent agreement with the BeC stretch of
13
-1
shifts are in very good agreement: C (75.1 cm calculated,
-1
-1
7
1
1.8 cm observed) and deuterium (139.1 cm calculated,
-1
45.5 cm observed).
The observation of the Be-C stretching mode significantly
-1
less coupled to hydrogen, relative to HBeCCH, and general
agreement of the observed and calculated isotopic frequencies
-
1
-1
13
HBeCCH (859.0 cm ), which has a 9.2 cm calculated
C
2
strongly supports identification of the new linear Σ BeCCH
-
1
shift. The observed deuterium shift of 54.0 cm is also in
excellent agreement with the calculated deuterium shift of 51.2
cm . Similarly, the broad 689.1-cm band of Figure 3 agrees
radical, which is isoelectronic to the Mg, Ca, and Sr acetylide
10,11
radicals reported earlier.
Comparison of the C-C stretch
-1
-1
-
-1
37
of CCH (1800 cm in solid neon) reveals, as expected, a
lower C-C frequency for the anion species. Calculations
presented in Table 4 predict the C-C stretching vibration of
BeCCH to be 210 cm to the blue of the C-C stretch of CCH ,
in agreement with the observed difference of 219 cm . The
-
1
well with the CCH bend calculated at 703.2 cm . The band
-
1
13
shifts 5.8 cm with C compared to a calculated shift of 6.6
-1
-1
cm . Upon deuterium substitution the band shifted 128.2 cm
compared to the calculated shift of 145.1 cm .
-1
-
-
1
-1
The excellent agreement between calculated and observed
frequencies for the most intense absorptions strongly supports
assignment of these bands to HBeCCH, a linear molecule
observed here for the first time. Andrews et al. identified the
C-C stretch of the similar HBCCH molecule formed by reaction
of laser-ablated boron with acetylene.9 In addition Burkholder
and Andrews observed HAlCCH from the photolysis of
aluminum-acetylene complexes.7 Both HBCCH and HAlCCH
are expected to be bent and have substantially different M-H
stretching modes as well as HMC bending modes. Unfortu-
nately, the C-C stretching mode for HBeCCH is calculated to
be weak, and this mode is not observed here. Based on energy
calculations the linear HBeCCH molecule is the most stable
analogous BCCH and AlCCH molecules have also been
produced by laser-ablated B and Al reactions with C2H2. The
8,9
11
-1
C-C stretch for BCCH (2039.3 cm ) is slightly to the blue
-1
of the C-C stretch of BeCCH (2018.9 cm ). In contrast, the
-1
C-C stretch of AlCCH (2000.8 cm ) is to red of BeCCH as
is a tentative assignment to the C-C mode of MgCCH (1984.2
-1
cm ), which will be discussed in the following section.
BeCCCH3.The Be atom reaction with propyne followed the
reaction with acetylene. Weak bands were observed for BeH2
(
36) Herzberg, G. Infrared and Raman Spectra of Polyatomic Molecules;
D. Van Nostrand Co. Inc.: New York, 1945.
37) Forney, D.; Jacox, M. E.; Thompson, W. E. J. Mol. Spectrosc. 1992,
153, 680.
(

10248 J. Am. Chem. Soc., Vol. 118, No. 42, 1996
Thompson and Andrews
at 2159.4 cm- and BeCCH at 2019.0 cm . The major product
1
-1
Reaction Mechanisms. Previous experiments involving
laser-ablated beryllium and magnesium have shown atomic Be
-
1
gave 3 site split bands at 2038.9, 2033.6, and 2028.0 cm ,
which is assigned to the methyl-substituted BeCCCH3 radical.
The small blue shift in the CtC fundamental on methyl
substitution follows the trend for propyne. The weak, sharp
112.9-cm band is tentatively assigned to the Be-H stretching
1
4,38
3
and Mg to be reactive species.
Metastable Mg( P1) has a
4
0
lifetime of 2-4 ms and is likely the reactive species in
24
38
3
magnesium experiments. Similarly, Be( P1) is the probable
reactant here since the lifetime of metastable Be is expected to
be longer owing to less spin-orbit coupling. The primary
reaction of beryllium probably forms both major products:
-1
2
mode in HBeCCCH3; note that HBeCCH was absent in propyne
experiments but BeCCH was observed.
MgCCH. Product band intensities in Mg + HCCH experi-
ments suffered from two effects: laser-ablated Mg is generally
less reactive than Be resulting in decreased product yield,
relax
Be* + HCCH f [HBeCCH]*
8 HBeCCH
1
4,38
(∆E ) -63 kcal/mol) (1)
and calculated intensities for the product molecules are less for
MgCCH than for BeCCH. The most distinguishing feature in
dec
-
1
8 H + BeCCH
∆E ) +35 kcal/mol) (2)
the magnesium experiments is the strong band at 491.8 cm
in the Mg-C stretching region (Figure 4). It is important to
(
2
4
25
consider the 8/1/1 triplet for this band using natural Mg, Mg,
2
6
The matrix can relax the energized insertion product or it can
decompose. Using B3LYP calculated energies and starting with
ground state Be, reaction 1 is expected to be exothermic by
approximately 63 kcal/mol, while reaction 2 is endothermic by
and Mg isotopic magnesium (Figure 4a). Agreement of the
Mg band with the enriched Mg reaction product (Figure 4d)
confirms the presence of a single Mg atom in this molecule.
26
26
3
-1
Calculations for B2 Mg-C2H2 predict a small (2.4 cm )
3
-1
about 35 kcal/mol. Metastable Be( P1) is 61 kcal/mol higher
carbon-13 and a large (77 cm ) deuterium shift for the strong
41
-1
39
in energy, and if this energy is used, reactions 1 and 2 become
exothermic by 124 and 26 kcal/mol, respectively. Reaction
energetics calculated here are in agreement ((10 kcal/mol) with
those of Flores and Largo using MP4 calculated energies.20
The presence of BeH, BeH2, CCH, and HCCCCH, the dimer
of the CCH radical, all indicate that some fraction of the
beryllium atoms are in effect abstracting hydrogen from HCCH:
5
06-cm Mg-C2 stretching fundamental which is clearly not
-
1
in accord with the 491.8-cm band observed here. Note that
the observed Mg-C stretching frequency is within the range
calculated for both MgCCH and HMgCCH (Tables 7 and 8).
However, the strongest observable band for HMgCCH is the
-
1
Mg-H stretching mode calculated at 1667.1 cm , but such a
band was not detected in the present experiments indicating that
the observed Mg-C stretching band probably belongs to
MgCCH.
Be* + HCCH f BeH + CCH
(∆E ) +81 kcal/mol)
3)
(
Furthermore, the 491.8-cm-1 band in Figure 4 is in better
agreement with the Mg-C stretching mode calculated for
This reaction is calculated to be endothermic by 81 kcal/mol
using ground-state Be and endothermic by 20 kcal/mol using
Be( P1). The obsevation of BeH and CCH in these experiments
requires still more energy in the reacting Be atoms, and such
excess kinetic energy is available in laser-ablated metal at-
The presence of CCH suggests a second possible
pathway for formation of BeCCH from the simple exothermic
combination reaction 4.
-
1
-1
MgCCH (486.4 cm ) than for HMgCCH (514.4 cm ). The
calculated and observed isotope shifts for C (8.9 cm
13
-1
-1
3
-1
-1
calculated, 9.8 cm observed), D (5.5 cm calculated, 6.7 cm
observed), and Mg (9.7 cm calculated, 9.0 cm-1 observed)
2
6
-1
are in excellent agreement for MgCCH. In contrast, the D shift
14,42
oms.
-1
for DMgCCD is calculated to be larger (12.0 cm ) as expected.
-
1
In addition, the band at 660.8 cm is in agreement with the
-
1
13
CCH bend of MgCCH calculated at 664.0 cm . The
C
-
1
counterpart band shifted 4.6 cm to the red in reasonable
Be + CCH f BeCCH
(∆E ) -103 kcal/mol) (4)
-
1
agreement with the calculated shift of 5.7 cm .
-
1
Finally, the combination of CCH radicals accounts for the
formation of diacetylene:
The observation of a Mg-C stretching mode at 491.8 cm
-1
and a CCH bending mode at 660.8 cm along with the absence
of the strong Mg-H stretching absorption are sufficient to assign
observed bands to MgCCH. Weak matrix site bands at 1982.7
HCC + CCH f HCCCCH
(5)
-1
and 1984.2 cm are tentatively assigned to the C-C stretching
mode of MgCCH; this mode is below the C-C stretch of
AlCCH (2000.8 cm ). This trend agrees with the trend
The insertion product HMgCCH was not found in the
magnesium spectra; however, MgH and CCH were observed.
This suggests that the insertion product is not relaxed but
decomposes straightaway by reaction 6. Then MgCCH can be
formed by the simple exothermic combination reaction 7.
-
1
observed for the C-C stretch of BeCCH, which is red-shifted
-
1
from BCCH by 20.4 cm .
-1
The 491.8-cm matrix value for the Mg-C stretching mode
in MgCCH is in excellent agreement with a preliminary 500 (
dec
Mg* + HCCH f [HMgCCH]* 8 MgH + CCH
-
1
5
cm value deduced from the emission spectrum of the
1
1c
(∆E ) +106 kcal/mol) (6)
gaseous species allowing for a small red shift by the argon
matrix. The trend in M-C stretching frequencies and “di-
atomic” stretching force constants calculated for M-CCH
Mg + CCH f MgCCH
(∆E ) -73 kcal/mol) (7)
-
1
molecules from Be (855 cm , kBe-C ) 2.22 mdyn/Å) and Mg
Using ground state Mg, reaction 6 is calculated to be endo-
thermic by 106 kcal/mol, while reaction 7 is calculated to be
exothermic by 73 kcal/mol. Thermodynamic data for acetylene
-
1
-1
(
0
492 cm , kMg-C ) 1.14 mdyn/Å) to Ca (399 cm , kCa-C )
1
0a
-1
10a
.87 mdyn/Å) and Sr (343 cm , kSr-C ) 0.73 mdyn/Å)
reinforces these assignments. Note that the Mg-C force
constant is slightly higher than Ca-C and Sr-C values but
substantially lower than the “diatomic” Be-C force constant.
(40) Husain, D. J. Chem. Soc., Faraday. Trans. 1989, 85, 85. Furcinitti,
P. S.; Wright, J. J.; Balling, L. C. Phys. ReV. A 1975, 12, 1123
(
41) Paschen, F.; Kruger, P. G. Ann. Phys. 1931, 8, 1005. Herzberg, G.
(
38) Andrews, L.; Yustein, J. T. J. Phys. Chem. 1993, 97, 12700.
Atomic Spectra and Atomic Structure; Dover Publications: New York, 1944.
(42) Wang, H.; Salzberg, A. P.; Weiner, B. R. Appl. Phys. Lett. 1991,
59, 935.
(39) Thompson, C. A. Ph.D. Thesis, University of Virginia, Charlottes-
ville, 1996.

Reactions of Laser-Ablated Be and Mg Atoms with C2H2
J. Am. Chem. Soc., Vol. 118, No. 42, 1996 10249
and MgH suggest that reaction 6 is endothermic by 84 ( 10
kcal/mol, which shows that the DFT energy calculation is too
endothermic. Nevertheless, excess energy is required to promote
reaction 6. Metastable Mg is 63 kcal/mol higher40 than ground
state Mg, and substantial excess kinetic energy is available in
laser-ablated metal atoms as well.3
with thermal atom reactions. Once formed CCH radical can
combine straightaway with Be and Mg atoms. The failure to
observe HMgCCH may be due to the weaker MgH bond as
43
compared to the BeH bond. Although ground state Be is
3
unreactive, metastable ( P1) Be is reactive and once formed,
8,42
the BeH bond is strong enough to be stable.
Laser ablated Be and propyne gave BeCCH and BeCCCH3
Another route for the formation of MgCCH, reaction 8, is
-
1
with the latter ν(CtC) mode blue-shifted 15 ( 5 cm from
the former value as is customary for methyl substituents.
The BeCCH and MgCCH molecules have Be-C and Mg-C
Mg* + HCCH f HMgCCH* f H + MgCCH
(∆E ) +65 kcal/mol) (8)
-1
stretching frequencies at 855 and 492 cm , which characterize
stable alkaline earth metal-carbon bonds. These frequencies
-
1
endothermic by 65 kcal/mol when calculated using ground state
Mg and endothermic by only 2 kcal/mol using P Mg. This
shows that excess metal atom kinetic energy is necessary for
these reactions to proceed and suggests that reaction 8 is the
preferred pathway for the formation of MgCCH.
are, of course, higher than the Ca-C (399 cm ) and Sr-C
3
-1
(343 cm ) values for the gaseous CaCCH and SrCCH
10a
molecules.
The C-C stretching frequencies for BeCCH and
MgCCH are slightly lower than values for BCCH and AlCCH,
respectively, but somewhat higher than values for CCH and
-
CCH .
Conclusions
Density functional theory was used to calculate structures and
isotopic frequencies to provide efficient and valuable support
for the experimental observation and identification of these new
organometallic molecules.
Products of laser ablated beryllium and magnesium reactions
with acetylene included the organometallic molecules HBeCCH,
BeCCH, and MgCCH as well as the BeH and MgH metal
hydrides, the CCH radical, and its dimer diacetylene. The
spectra suggest a mechanism involving the insertion of meta-
stable excited metal atoms into the CH bond of acetylene
followed by subsequent relaxation or decomposition. The
observation of CCH in these reactions highlights the availability
of energy for endothermic processes from laser ablated metal
atoms which often results in the formation of products not seen
Acknowledgment. We appreciate financial support from
NSF Grant CHE 92-55662, computer time at the San Diego
Supercomputer Center, and helpful correspondence with P. v.
R. Schleyer.
JA9616847
(43) Huber, K. P.; Herzberg, G. Constants of Diatomic Molecules; Van
Nostrand Reinhold: New York, 1979.