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Nonreactive Interactions between Ethene and Halogens Detection of a -Donor Complex C2H4BrCl by Rotational Spectroscopy.

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terization data: 'H NMR (300 MHz. CDCI,): 6 = 1.03 (t. J(H.H) = 7 Hz, 6H.
CH,CH,). 1.09 (t, J ( H , H ) = 7 HI. 6H. C H , C f f , ) , 1.67 (s, H,O), 1.90 (s. 6 H .
CH,), 2.09 (s. 6H. CH,), 2.33 (9, J(H,H) =7 Hz, 4 H , CH,CH,), 2.39 (9.
J ( H , H ) = 7 Hz, 4H. CfI,CH,). 2.45 (s, 6 H , CH,). 2.80 (s. 6H, CH,). 6.76 (s.
2 H . nwso-H), 6.95 (s. 2H. meso-H), 7.04 (s, 2 H . pyrrole CH), 7.62 (d. 2H.
pyrrole CH), 8.60 (s. 2 H . pyrrole CH), 9.68 (d, 2H. pyrrole CH), 14.23 (s. 2H.
NH). 14.44(s.2HTNH).14.67(s,2H,NH). 15.21 ( ~ , 2 H , N H ) . 1 5 . 5 8(s,2H,
N H ) : UV'VIS (CH,CI,): i,,, = 644 nm; HRMS (FAB): calculated for
C,,H,,N,,: ni'i 923.5237; found: 923.5252 ( M +I)+
- 4HCI)
(CI-), - 3CHiCI, - 3!2(C,H,,).
M , = 1557.29: crystallized as dark green blocks from methylene chloride layered with Pi-hexane in the triclinic space group Pi (no. 2), with u =14.221(6).
h =15.374(3). c = 20.531(5) A. a =100.03(2). /I =101.71(3). 7 =106.23(3) ,
V = 4091(2) A'. pIricd=1.26gcm-' for Z = 2. F(000) =3642. p(MoKZ)=
3.888 c m - ' . i = 0.71073 A. Data usre collected at -90'C on a Nicolet R3
diffractometer equipped with a Nicolet LT-2 low-temperature device. wscans.
5 1 2 'min-',20,mdx= 45 . Atotalot12144retlectionsuwecollected,ofwhich
10699 were unique. The R for averaging gymmetry-equivalent retlccrions was
equal to 0.034 Data uere corrected for Lorentr and polarization effects, and
for decay but not for absorption. The structure was solved by direct methods
and refined by full-matrix least-squares using SHELXTL-Plus 1121,The hydrogen atoms were calculated in ideal positions. with U,,, =1.2 x Licq of the relevant atom. One molecule of ti-hexane and one of dichloromethane are disordered. A total of 853 parameters were refined to a finill R = 0.0825. wR =
0.100. and GOF = 2.607 using 4x69 I-eflections having F > 40(F).Further
details of the crystal structure investigation are nvailable upon request from the
Director of the Cambridge Crystallographic Data Centre. 1 2 Union Road,
GB-Cambridge CB2 1EZ (UK), on quoting the full journal citation.
G. M . Sheidrick. SHELXTL-PILLS.Siemens Analytical X-ray Instrurnenh
Madison. WI, USA. 1991.
A. K . Burrell. G. Hemmi. V. Lynch. J. L. Sessler. J. A m . Chm7. Soc. 1991, 113.
4690- 4692.
The his-uranyl chelate of 9 has recently been prepared in our laboratory: J. L
Sessler. E. Brucker. S. Weghorn. unpublished results. Characterization data:
UV:VIS (CH,CI,): i,,, [nm] = 445. 682; LRMS (FAB): Calculated for
C,sH,,N,oU20,: ni:: 1570: found 1570: The 'H NMR spectrum is similar to
that of the parent compound 9, including diastereotopic splitting patterns i n
the alkyl region.
M. R. Johnson. D. C. Miller, K . Bush, J. J. Becker, J. A . Ibers, J. Org. Chem.
1992. 57. 4414-4417.
Nonreactive Interactions between Ethene and
Halogens: Detection of a z-Donor Complex
C2H, BrCl by Rotational Spectroscopy **
Hannelore I. Bloemink, Kelvin H i n d s ,
Anthony C. Legon,* and Joanna C. T h o r n
We report the first characterization of a molecular n-donor
complex formed by ethene with a halogen or interhalogen in the
gas phase. The microwave spectrum of C,H,. . . BrCl was observed by employing a Fast-mixing nozzle to keep the chemically
reactive components separate until the point at which they expanded simultaneously into the evacuated Fabry -Pirot cavity
of a Fourier transform (FT) microwave spectrometer. Complexes formed where the gases met had very low internal energies
and underwent collisionless expansion before further progress
along the reaction coordinate could occur. Properties of fi-ozen
species were then determined from an analysis of the rotational
Prof. A. C . Legon, Dr. H. I. Bloemink. K . Hinds. J. C. Thorn
Department of Chemistry
University of Exeter
Stocker Road. Exeter EX4 4QD (UK)
Telefax: Int. code +(392) 263-434
This work was supported by a rexarch grant and a studentship (for K H.)
from the Science and Engineering Research Council and a studentship (for
J. C. T.) from the Ruth King Trust of the University of Exeter
The addition reaction of, for example, molecular bromine to
an alkene under "dark" conditions, that is, in the absence of
radical initiation, and in a polar environment has been thoroughly investigated."] It is assumed to proceed through an intermediate formed when electrophilic Br, first encounters the
nucleophilic alkene.IZ1If the electrophile is BrCI, the addition
reaction is about 400 times
in accordance with the
idea that the polar interhalogen "BrC1'is a better electrophilic halogenating agent than Br,.
Two types of reactive intermediate can be envisaged for a
halogen and a n donor such as ethene. Following the proposal
of Mulliken.r31 we refer to an "outer" (weak) complex of the
type 1, and an "inner" complex 2 in which there is significant
charge transfer. Under dark, polar conditions, one mechanism
p r o p o ~ e d ' ~for
. ~ 'the addition of a halogen to an alkene proceeds from the initial pre-equilibrium complex of type 1 to the
halogenium ion of type 2, which subsequently suffers nucleophilic attack by, for example, CI-. Other, similar mechanisms
are discussed in reference [l]. Although reactive intermediates of
types 1 and 2 have been much invoked, they are difficult to
characterize experimentally. particularly for the prototype
alkene C,H, because of rapid subsequent reaction. It is therefore of interest to isolate any molecular complex formed by
ethene with, for example, BrCl and to determine its properties.
What is the geometry of the complex? What is the extent of
electric charge redistribution when it is formed? Is 1 or 2 the
more appropriate description in the gas phase?
_ .
While rotational (microwave) spectroscopy has proved a powerful method for observing molecular complexes isolated in the
gas phase,l6] it is usually conducted with premixed components
in contact with surfaces, particularly those of metals. Since even
so-called gaseous additions of chlorine and bromine to ethene
are reactions on polar surfaces,['] premature reaction of the premixed gases occurs. The problem of reaction has been overcome
here by carrying out rotational spectroscopy on a jet of the gas
mixture supersonically expanded from a fast-mixing nozzle[']
into the spacious Fabry-Pkrot cavity of a FT microwave spectrometer. The essential feature of the fast-mixing nozzle is that
the reactive components mix only as they expand simultaneously
from a pair of coaxial and coterminal tubes into the evacuated
cavity. Complexes of C,H, and BrCl are formed only at the
interface of the concentric gas flows, in the absence of surfaces.
Moreover, they are in collisionless expansion within a few nozzle diameters (after about 10 ps in our experiment) and are then
frozen at a very low effective temperature until they encounter
a wall of the vacuum chamber.
The ground-state rotational spectrum of (C,H,, BrCI) observed in this way was that of a nearly prolate asymmetric-top
molecule with a nonzero a component of the electric dipole
moment.[*]Each transition carried a nuclear quadrupole hyperfine structure characteristic of the presence of two nuclei of spin
I = 3:, 2 . A detailed analysis of the J = 5 t 4 and 4 + 3 transitions with the Watson A reduction['] led to the ground-state
rotational constants A , , B,, and C,, the centrifugal distortion
constants A , and A,,, the halogen nuclear quadrupole coupling
constants l,,,,(X) and xbh(X)- x,,(X). and the bromine spin-rotation constant Mhb(Br).listed in Table 1 for the isotopomers
(C,H,. '"Br"CI) and (C,H,, slBr"C1). By using BrCl as the
halogen we ensured that any species of type 1 had a significant
electric dipole moment and hence a spectrum of reasonable intensity. The spectroscopic constants of Table 1 allow several
conclusions about the nature of the observed complex to be
force constant, as determined['31 from A , , also suggests a weak
interaction. It is similar in magnitude to those of many hydrogen-bonded c ~ r n p l e x e sand
~ ' ~ in
~ particular that of the isostructural analogue (C,H,, HCI) .llsl Significant charge transfer to
give a complex of the inner type 2 would be expected to lead to
a stronger interaction through the incipient chemical bonds. The
parallelism of the properties of (C,H,, HCI) and (C,H,, BrCI)
makes it likely that electrostatic interpretations[' 6 . l 7 ] of the
properties of the former (especially the angular geometry) also
apply to the latter.
Finally, the rotational constants B, and C, of (C,H,, BrCI)
lead, under the assumption of unchanged component geometries,'18. 19] to 2.979(1) A for the distance of the midpoint of the
C=C bond to the Br atom in 1.
E,uperimental Procedure
Table 1 . Ground-state spectroscopic constants of two isotopomers of (C,H,. BrCI)
(C,H,. '9Br35CI)
(C,H,. *'BrZ5CI)
- 0.21(4)
- 3.4(3)
- 94.72(2)
First, the value of A , for both isotopomers is only slightly
larger than the rotational constant C, = 24824.17(5) M H z of
free ethene.["] This provides strong evidence that the BrCl subunit lies along the c axis of ethene, that is, along the C , axis
perpendicular to the ethene plane, as in 1. The small increase
almost certainly arises from zero-point effects." The difference B, - C, is also consistent with the arrangement in 1 and
the 79Br!81Br isotopic invariance of the rotational constants
establishes that the Br atom lies close to the complex's center of
mass. The planar moments Pb and P, (see Table 1) are almost
unchanged from P,,=16.8665(1) uAz and P,,= 3.49188(2) uA',
respectively, of free ethene.["I Not only does this confirm that
the angular geometry is that of type 1; it also shows that the
dimensions of ethene are at most weakly perturbed by complex
forma ti on.
Second, the assumption of weak interaction is reinforced
when the nuclear quadrupole coupling constants are considered.
For both isotopomers, xaa(Br) and x,,(CI) are little changed
from the corresponding values xo(X) for free BrC1.['21This suggests only small changes in the electric field gradient along the
BrCl axis when the diatomic molecule takes up its equilibrium
position along the ethene c axis. Moreover, the difference
zhb(X)- xJX) is a small fraction of X ~ ( X ) [for
~ ' ~both X = Br
and CI, so demonstrating that the axial symmetry of the BrC1
electric charge distribution is only slightly perturbed. Likewise,
the value k , = 10.5(1) Nm-' for the intermolecular stretching
A n g ~ & vChc~m.
Inr. Ed. Engl. 1994, 33, N o . 14
Received: March 15, 1994 [Z 6762 lE]
German version: Angru.. Chem. 1994, 106, 1577
[a] Numhcrs in parentheses are standard errors in units of the least significant digit.
[b] P, = : ( I , I, I J ; 2 , ,8,y to be permuted over a, b. c . [c] Standard deviation
of the tit.
Rotational spectra of (C,H,. BrCI) were observed with a pulsed-nozzle FT microwave spectrometer [20] fitted with a fast-mixing nozzle [ 7 ] .A mixture of approximately 2 % ethene (Argo International) in argon was pulsed through the outer of the
two concentric tubes of the nozzle at a rate of 2 Hz from a pressure of 3 atm and
mixed with a continuous flow of an equimolar mixture of chlorine and bromine
(Aldrich) issuing from the exit of the inner tube. The halogen mixture flow rate was
Torr in the vacuum chamber of the
adjusted to give a steady pressure of 1 x
ypectrometer. Spectra were observed in the range of frequencies 8-13 G H r . For
each isotopomer approximately 75 hyperfine components associated with six rotational transitions were measured with an estimated accuracy of 2 kHz.
C . K. Ingold, Structure and Mechanism in Organic Chemistry. 2nd ed., G. Bell.
London. 1969. Chapter XIII, pp. 964-982, and references therein.
P. B. D. de la Mare, R. Bolton, Electrophilic Additions to UnsaturatedSy.stem.s,
Monograph4 (Eds.: C . Eaborn, E. D. Hughes). Elsevier, London, 1966, Chapter 6, pp. 71 - 112, and references therein.
R. S. Mulliken, J. Phy,s. Chem. 1952, 56, 801-822.
I. Roberts, G . E. Kimball, J. Am. Chem. Soc. 1937.59, 947-948.
A. J. Downs, C. J. Adams in Comprehensive Inorganic Chemisfry, Vol. 2
(Ed.: A. F. Trotman-Dickenson), Pergamon, New York, 1971. Chapter 26,
pp. 1216.
A. C. Legon, Chem. Soc. Rev. 1990, 19. 197-237.
A. C. Legon, C. A. Rego. J. Chem. Soc. Faraduy lruns. IY90, 86. 19151921.
W. Gordy, R. L. Cook, Microwave Moleculur Speclra [Techniques of Orgunic
Chemistry, Vol. 9 (Ed.: A. Weissberger)], Interscience, New York, 1970.
J. K. G. Watson. J. Chem. Phys. 1968, 48, 4511-4524.
F. Herlemont, M. Lyszyk, J. Lemaire, C. Lambeau. M. de Vleeschouwer, A .
Fdyt, J. Mol. Sprctrosc. 1982, 94, 309-315.
A. C. Legon, P. D. Aldrich, W. H. Flygare, J. Chem. Phys. 1981, 75. 625
A. C. Legon, J. C. Thorn, Chem. Phvs. Lett. 1993, 215, 554-560.
D. J. Millen, Can. J. Chem. 1985, 63, 1477-1479.
A. C. Legon, D. J. Millen, L Am. Chem. Soc. 1987. 109. 3566358
P. D. Aldrich, A. C. Legon, W. H. Flygare. J. Chem. P h ) x 1981, 75, 2126
A. C. Legon. D. J. Millen, Faraday Discuss. Chem. Suc. 1982,73.71-77; Chem.
Soc. Rev. 1981, 16,467-498.
A. D. Buckingham, P. W. Fowler. Can. J. Chem. 1985, 63, 2018-2025.
E. Hirota, Y. Endo, S. Saiko, K. Yoshida, 1. Yamaguchi, K. Machida, J. Mol.
Spectrosc. 1981, 89, 223-231.
The value ro = 2.1388 used for BrCl was calculated from ro = ( h / 8 r ~ ~ p E , ) ' ' ~ ,
where the Eo value came from ref. [lo].
A. C. Legon. Annu. Rev. Phys. Chem. 1983,34, 275-300.
VCH Verlu~sg~sell.~chaft
mbH, 0-69451 Weiuheim, 1994
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spectroscopy, complex, interactions, nonreactive, ethene, halogen, detection, donor, c2h4brcl, rotation
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