J. Am. Chem. SOC.1994,116. 8229-8231
8229
Propargylene Rainer Herges’ and Alexander Mebel Contribution from the Institut f i r Organische Chemie. UniuersitBt Erlangen-Niirnberg, Henkestrasse 42, 91 054 Erlangen, Germany Received December 31, 1993’
Abstract: A theoretical investigation has been performed on triplet and singlet propargylene (C3H2, ethynylcarbene) using various levels of ab initio theory (MP2, MP4, QCISD, QCISD(T), and multireference CI). According to the results, and in contradiction to earlier calculations,neither the singlet nor the triplet propargyleneis a carbene. Vibrational frequencies of the 1,3-diradicaloid CZstructure of triplet propargylene are in much better agreement with experiment than those of the C, carbene structure. The hitherto assumed transition state (C,) of the singlet propargylene automerization is a minimum, and the singlet (like the triplet) carbene structure (C,) is not a stationary point on the energy hypersurface at the QCISD(T)/6-3 1G* level. The singlet-triplet gap is much larger than previously assumed, and the rotational lines of triplet propargylene are shifted to lower frequencies compared to earlier calculation.
[H-c=c-qi 1
Introduction There are three CsHz isomers that have been thoroughly investigated by both experiment and theoretical calculations: cyclopropenylidene, propargylene, and vinylidenecarbene. Carbene-like structures have been attributed to the ground states.
Cyclopropenylidene and vinylidenecarbeneare singlet ground states, and propargylene has a triplet ground state. Maier et al. have shown that the three can be interconverted in matrix by photolysis using light of different wavelengths.1.2 The two singlet C3H2 species are abundant in molecular clouds in interstellar space.3-6 Their characteristic rotational lines were identified by radio astronomy. Even though both singletcarbenes can be easily isomerized to propargylene by irradiation, propargylene could not be detected in space so far. There have been numerous attempts to determine the structure of propargylene. Trapping experiments, spectra, and theoretical calculations, however, contradict. The triplet ground state of propargylene has been identified by ESR,’ and a quasilinear or linear geometry was postulated on the basis of the zero-field splitting parameters. IR spectra of propargylenes~gand deuterated derivatives9 were recorded in matrix and were interpreted in favor of a rapidly automerizing carbene-like C, structure. The electronic spectrum measured in the gas phase suggests a linear structure.10 ~
a Abstract
~~
~~
published in Advance ACS Abstracts, August 1, 1994. (1) Maier,G.; Reisenauer,H. P.;Schwab, W.; Cirsky, P.;Has, B. A., Jr.; Schaad, L. J. J. Am. Chem. Soc. 1987,109, 5183. (2) Reisenauer, H. P.; Maier, G.; Riemann, A.; Hoffmann, R. W. Angew. Chem. 1984, 96, 596; Angew. Chem., Int. Ed. Engl. 1984, 23, 641. (3) Thaddeus,P.; Vritilek, J. M.; Gottlieb, C. A. Astrophys. J . 1985,299, L63. (4) Adams, N. G.; Smith, D. Astrophys. J . 1991, 368, L39. (5) Cernicharo, J.; Gottlieb, C. A.; GuClin, M.; Killian, T. C.; Paubert, G.; Thaddeus, P.; Vritilek, J. M. Astrophys. J. 1991, 368, L39. (6) Gottlieb, C. A.; Killian, T. C.; Thaddeus, P.; Botschwina, P.; FHigge, J.; Oswald, M. J . Chem. Phys. 1993, 98, 4478. (7) Bernheim, R. A.; Kempf, R. J.; Gramas, J. V.; Skell, P. S.J . Chem. Phys. 1965, 43, 196. ( 8 ) Kang, C . F. Dissertation, Michigan State University, East Lansing, MI, 1972. (9) Maier, G.; Reisenauer, H. P.; Schwab, W.; Cirsky, P.; Spirko, V.; Has, B. A., Jr.; Schaad, L. J. J . Chem. Phys. 1989, 91,4763. (10) Merer, A. J. Can. J . Phys. 1967, 45, 4103.
H-C=C-CH
\\
~ h v b
7
N2
k
CEC-H
W
\
6.5 1
1
19
Figure 1.
Propargylene was first generated in 1960 by Skell and Klebe by photolysis of diazopropyne.1l Trapping reactions with cisand trans-butene gave cyclopropanes with partial loss of stereospecificity (Figure 1). This has been interpreted as a partial spin relaxation of the generated singlet propargylene to the triplet state, the singlet carbene reacting with retention and the triplet with loss of stereochemistry of the olefin. Trapping reactions of labeled propargylene (tritium-labeled at C1) indicate that both carbon atoms C1 and C3 have equal reactivity in the triplet state.lZ Similar observations have been made in substituted systems:I3 Boyer and Selvarajan generated (phenylethyny1)methylene by photolyzing the corresponding ketene precursor. The trapping products with hydrocarbons, however, are derived from the isomeric (phenylethynyl)carbene.14 Similarly, (phenylethyny1)nitreneobviouslyrearranges t6 (pheny1cyano)carbene before being trapped.l5 Diethynylcarbenes in (1 1) Skell, P. S.;Klebe, J. J. Am. Chem. Soc. 1960,82, 247. (12) Skell, P. S.; Klebe,J. Abstrucrs of Papers, 141st National Meeting of the American Chemical Society, Washington, DC, March 20-29, 1962; American Chemical Society: Washington DC 1962. Clos, G. L. In Topics in Stereochemistry; Eliel, E. L., Allinger, N. L., Eds.; Interscience: New York, 1968; Vol. 3, p 226, loc. cit. (13) See also: Bergman, R. G.; Rajadhyaksha, V. J. J. Am. Chem. Soc. 1970, 92, 2163. (14) Selvarajan, R.; Boyer, J. H. J. Org. Chem. 1971, 36, 1679. (15) Selvarajan, R.; Boyer, J. H.J. Am. Chem. SOC.1969, 91, 6122.
0002-7863/94/1516-8229$04.50/00 1994 American Chemical Society
Table 1. Single Determinant ab Initio Energies (in au) and Number of Imaginary Modes of Seven Different Propargylene Structures sym- multimetry plicity
structure H
C,
00
1
6-31G*
singlet -114.571 87 -144.931 47 -114.96283 0
H H
MP4 WTQ) 6-31G*
MP2/ 6-31G8 0
1
QCISD (TI/ 6-31G8
QCISD/ 6-31G* -114.961 40 c
QCISDb 6-31+G8
QCISW (T)/ 6-31+G**
QCISDb VQ)/ 6-31+G**
ZPE QCISD/ 6-31G8
-1 14.972 86 -114.991 77 -1 14.991 64 0.028 32
0
CL, singlet -1 14.565 18 -1 14.927 75 -1 14.961 50 1
PMP4a WTQ) 6-31G8
1
-1 14.960 09 -1 14.966 19 -1 14.971 74 -1 14.992 16 -1 14.992 00 0.027 55
1
2 I:
H
= *= e
D-h
triplet -1 14.629 67 -1 14.939 29 -1 14.970 99 -1 14.986 50 -1 14.973 04 -1 14.986 51 -1 14.995 87 -115.010 33 4
3
4
4
4
4
Cb
triplet
CB
triplet -1 14.631 14 -1 14.939 56 -1 14.971 50 -1 14.987 62 -1 14.974 22 -114.987 91 -1 14.996 78 -1 15.011 44 2 2 2 2 2
C2
triplet -114.632 29 -114.939 95 -114.972 10 -114.988 37 -114.975 17 -114.98901 0 1 1 0 0
-114.997 68 -115.01247
C.
triplet
-114.99677
-114.631 89 -114.939 85 -114.971 92 -114.988 14 -114.974 78 -114.988 52 -114.997 29 -115.01201 1 2 2 2 1
-115.012 84 0.025 30
4
H\-
5
H
H
+$
6
H 7 a
0.025 31
-
2% at MP3 decreased to
