Rotational spectroscopy of mixtures of ethyne and iodine monochloride: Isolation and characterisation of the π-type complex C2H2···ICl

Article abstract of DOI:10.1039/a904233d

The ground-state rotational spectrum of a complex formed by ethyne and iodine monochloride was observed by using pulsed-nozzle, Fourier-transform microwave spectroscopy. A fast-mixing nozzle was utilised to avoid chemical reaction of the component gases prior to their supersonic expansion. Rotational constants A₀, B₀ and C₀, quartic centrifugal distortion constants Δ(J), Δ(JK) and δ(J), and halogen nuclear quadrupole coupling constants χ(aa)(X) and {χ(bb)(X) χ(cc)(X)}, where X = Cl or I, were determined for the three isotopomers C₂H₂···I³⁵Cl, C₂H₂···I³⁷Cl and C₂D₂···I³⁵Cl. Detailed interpretation of the rotational constants established that the equilibrium geometry of the complex has a planar, T- shape of C(2v) symmetry in which ethyne acts as the bar of the T. This geometry, with the zero-point distance r(*···I) = 3.115(1) A between the centre of the π-bond of ethyne and the nearest halogen atom I, establishes that the interaction in this complex is between ethyne as a π electron donor and I of ICl as the electron acceptor. The halogen nuclear quadrupole coupling constants χ(aa)(X) were interpreted on the basis of a simple model to show that, on complex formation, fractions δ₁ = 0.026 and δ₂ = 0.056 of an electronic charge are transferred from the ethyne π bond to I and from I to C1, respectively, leading to a net decrease of 0.030e at I. The complex is weakly bound, according to the intermolecular stretching force constant kσ = 12.2(1) Nm^-1 determined for the isotopomers C₂H₂···I³⁵Cl and C₂H₂···I³⁷Cl from Δ(J) values. The opportunity is taken to compare the properties of C₂H₂···ICl established here with those similarly determined for the series C₂H₂···XY, where XY = Cl₂, ClF, or BrCl.

Full text of DOI:10.1039/a904233d

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Rotational spectroscopy of mixtures of ethyne and iodine
monochloride: isolation and characterisation of the p-type complex
C H Æ Æ ÆICl
2
2
J. B. Davey and A. C. Legon*
School of Chemistry, University of Exeter, Stocker Road, Exeter, UK EX4 4QD
Received 26th May 1999, Accepted 2nd July 1999
The ground-state rotational spectrum of a complex formed by ethyne and iodine monochloride was observed
by using pulsed-nozzle, Fourier-transform microwave spectroscopy. A fast-mixing nozzle was utilised to avoid
chemical reaction of the component gases prior to their supersonic expansion. Rotational constants A , B
0
0
and C , quartic centrifugal distortion constants D , D and d , and halogen nuclear quadrupole coupling
JK
constants s (X) and Ms (X) [ s (X)N, where X \ Cl or I, were determined for the three isotopomers
aa bb cc
C H É É ÉI35Cl, C H É É ÉI37Cl and C D É É ÉI35Cl. Detailed interpretation of the rotational constants established
2 2 2 2
that the equilibrium geometry of the complex has a planar, T-shape of C symmetry in which ethyne acts as
0
J
J
2
2
2
v
the bar of the T. This geometry, with the zero-point distance r(\É É ÉI) \ 3.115(1) Ó between the centre of the
p-bond of ethyne and the nearest halogen atom I, establishes that the interaction in this complex is between
ethyne as a p electron donor and I of ICl as the electron acceptor. The halogen nuclear quadrupole coupling
constants s (X) were interpreted on the basis of a simple model to show that, on complex formation, fractions
aa
d \ 0.026 and d \ 0.056 of an electronic charge are transferred from the ethyne p bond to I and from I to Cl,
1
2
respectively, leading to a net decrease of 0.030e at I. The complex is weakly bound, according to the
intermolecular stretching force constant k \ 12.2(1) Nm~1 determined for the isotopomers C H É É ÉI35Cl and
p
2 2
C H É É ÉI37Cl from D values. The opportunity is taken to compare the properties of C H É É ÉICl established
2
2
J
2 2
here with those similarly determined for the series C H É É ÉXY, where XY \ Cl , ClF, or BrCl.
2
2
2
of molecular orbital from which and into which electrons are
donated and accepted, respectively, in forming a complex such
as BÉ É ÉXY. An important class contains complexes formally
described as of the bp.ar type, i.e. complexes in which elec-
trons are donated from a p-bonding orbital of B into a r-
antibonding orbital of XY.
1
. Introduction
We report the ground-state rotational spectra of three iso-
topomers of a complex formed by ethyne with iodine mono-
chloride. This investigation is part of a continuing programme
in which we are using rotational spectroscopy to determine
the properties of several extended series of complexes BÉ É ÉX
Although the Ðrst charge-transfer complex to be investi-
2
or BÉ É ÉXY, where B is a Lewis base and X and XY are
2
gated,7 namely benzeneÉ É ÉI , is of the bp.ar type, the simplest
2
homonuclear and heteronuclear dihalogen molecules, respec-
possible member in which I is the electron donor is
tively. The properties of primary interest are the angular
geometry of the complex, the strength of the interaction
between the components, and the extent of the electric charge
redistribution that accompanies formation of the complex. By
varying B and the dihalogen molecule systematically and
ethyneÉ É ÉICl. However, complexes of ethyne with halogens
have been difficult to observe hitherto because of the facile
chemical reaction that accompanies mixing of the components
under normal conditions. This difficulty can be overcome in
the solid state by isolation of the complex in cryogenic
matrices10 and, more recently, in the gas phase by using a
fast-mixing nozzle11 to accomplish supersonic expansion of
the components to be mixed while avoiding chemical reaction.
observing how the properties of BÉ É ÉX or BÉ É ÉXY change, we
2
aim to draw conclusions about the nature of the interaction.
Another aim of these investigations is to examine whether any
parallelism exists between the properties of these so-called
The latter method has been used to produce ethyneÉ É ÉCl ,
2
“
halogen-bondedÏ complexes BÉ É ÉX or BÉ É ÉXY and the corre-
ethyneÉ É ÉClF and ethyneÉ É ÉBrCl in isolation in a jet within
2
sponding hydrogen-bonded series BÉ É ÉHX. So far, some
the FabryÈPe rot cavity of a pulsed-nozzle, FourierÈtransform
general conclusions have been advanced for the series
microwave spectrometer and thereby to observe their rota-
tional spectra.12h14 The same method is employed here to
characterise the simplest BÉ É ÉICl complex of the bp.ar type,
namely ethyneÉ É ÉICl, which is the subject of this paper.
The earlier studies of the rotational spectra of ArÉ É ÉICl5
and OCÉ É ÉICl6 allowed us to develop a method of inter-
preting the changes in the iodine and chlorine nuclear quadru-
pole coupling on formation of the complex BÉ É ÉICl in terms of
electric charge redistribution from B to I and from I to Cl. We
take advantage of this method here to determine the extent of
both inter- and intra-molecular charge transfer in the proto-
type bp.ar complex of the type BÉÉICl. We also establish the
angular geometry of ethyneÉ É ÉICl unambiguously from an
BÉ É ÉCl /BÉ É ÉHCl,1
BÉ É ÉBrCl/BÉ É ÉHBr2
and
BÉ É ÉClF/
2
BÉ É ÉHCl.3,4 We have recently begun considering the series
BÉ É ÉICl.5,6
Complexes of the type BÉ É ÉICl and BÉ É ÉI , in which an
iodine atom acts as the electron acceptor, featured centrally in
2
early investigations of charge transfer complexes by both
experimental and theoretical methods. Notable among these
was the experimental work of Benesi and Hildebrand7 on
benzene/I in the liquid phase, the X-ray di†raction study of
2
complexes in the solid state by Hassel and RÔmming,8 and the
theoretical analysis by Mulliken and Person,9 who also intro-
duced a detailed classiÐcation of complexes based on the type
Phys. Chem. Chem. Phys., 1999, 1, 3721È3726
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interpretation of the rotational spectra of C H É É ÉI35Cl,
Observed frequencies were Ðtted by a standard, nonlinear
least squares procedure in which the matrix of the Hamilto-
nian H deÐned in eqn. (1) was constructed in the coupled basis
I ] J \ F , F ] I \ F and diagonalised. The Hamiltonian
2
2
C H É É ÉI37Cl and C D É É ÉI35Cl and obtain a measure of the
2
2
2 2
intermolecular binding strength (the intermolecular stretching
force constant k ) from the e†ects of centrifugal distortion.
p
I
1
1
Cl
H \ H [ 1/6Q : +E [ 1/6Q : +E ] I ÉMÉJ
(1)
R
I
I
Cl Cl
I
2. Experimental
proved sufficient to Ðt the observed frequencies with a stan-
dard deviation comparable with the estimated error of mea-
surement (namely, 2 kHz). The Ðrst term in eqn. (1) is the
energy operator for the semi-rigid asymmetric rotor, for which
the Watson A reduction17 in the Ir representation was
employed. Centrifugal distortion e†ects were satisfactorily
accounted for by including only quartic terms in the angular
momentum operators. The second and third terms in eqn. (1),
of the form [1/6Q : +E (X \ I or Cl), described the energy
The ground-state rotational spectrum of the ethyneÉ É ÉICl
complex was observed by means of a pulsed nozzle, Fourier-
transform microwave spectrometer of the BalleÈFlygare
design.15,16 A fast mixing nozzle11 was employed as a means
of avoiding the chemical reaction that would occur between
ethyne and ICl if they were mixed in the normal way in a
stagnation vessel. ICl vapour was Ñowed continuously from
above a sample of solid iodine monochloride (Lancaster
Synthesis), through the central glass capillary (0.3 mm internal
X
X
of interaction of the nuclear electric quadrupole moment Q of
x
halogen X with the electric Ðeld gradient +E at nucleus X. It
x
diameter) of this nozzle, into the evacuated FabryÈPe rot
cavity of the spectrometer. The Ñow rate was adjusted so that
a steady state pressure of ca. 2 ] 10~4 mbar was registered in
the chamber. The solid was contained in a glass tube having a
TeÑon tap and the entire line up to the capillary was con-
structed in glass, thereby ensuring that the ICl did not come
into contact with metal, with which it might react. Simulta-
neously, a mixture composed of ethyne (B.O.C. Gases Ltd.) in
argon, having a partial pressure ratio of 2 : 100, respectively,
and held at a total pressure of 3 bar was pulsed down the
outer (TeÑon) tube of the mixing nozzle at a rate of 2 Hz by
means of a Series 9 solenoid valve (General Valve Corp.).
Because the two concentric tubes of the nozzle are coterminal,
the ICl and ethyne came into contact only as they both
was not necessary to consider the e†ects of centrifugal distor-
tion on the nuclear hyperÐne structure, even though the mag-
nitude of the splitting due to I was large. The Ðnal term in
eqn. (1) takes account of the coupling of I to J via the mag-
I
netic spinÈrotation interaction. The quantity M is the spinÈ
rotation coupling tensor. Inclusion of the corresponding
operator for the Cl spinÈrotation coupling was found unneces-
sary.
The Ðtting was conducted using the program SPFIT
written by H.M. Pickett.18 The spectroscopic constants
obtained in the Ðnal cycle of the Ðt are given in Table 1 for the
three isotopomers C H É É ÉI35Cl, C H É É ÉI37Cl and
2
2
2 2
C D É É ÉI35Cl, together with the standard deviations of the
2
2
expanded into the FabryÈPe rot cavity. Complexes formed at
Ðts. The residuals *l \ l [ l
are included in Table S1.
obs
calc
the interface of the concentric gas Ñows were polarised with
pulses of microwave radiation in the usual way and the free-
induction decay at rotational transition frequencies was col-
lected and processed as described elsewhere.16 Individual
hyperÐne components of transitions had a full-width at half-
height of ca. 16 kHz, which led to an accuracy of frequency
measurement estimated at 2 kHz.
Despite the fact that only K \ 0 and 1 transitions were
~
1
observed for these very nearly prolate asymmetric rotors, we
note that the large rotational constant A is moderately well
0
determined in each case. Of the quartic centrifugal distortion
constants only D , D and d could be determined from the
J
JK
J
data available. The only components of the halogen nuclear
quadrupole coupling tensors s (X) \ [ (eQ/h)L2V /LaLb (a,b
ab
X
A sample of dideuterioethyne, prepared by the action of
to be permuted over a, b and c) released in the Ðt were s (X)
aa
D O (99.8 atom%, Aldrich) on calcium carbide (BDH), was
and Ms (X) [ s (X)N, where X \ I or Cl. Evidently, o†-
2
bb
cc
used to observe the spectrum of C D É É ÉI35Cl.
diagonal elements are either zero or make a negligible contri-
bution to the observed hyperÐne splittings. The components
M (I) and M (I) of the iodine spinÈrotation coupling tensor
2
2
3. Results
aa
bb
were both determined for C H É É ÉI35Cl, for which many
2 2
3.1 Spectroscopic analysis
hyperÐne components were available. In view of the nearly
The observed ground-state rotational spectrum of each iso-
topomer of ethyneÉ É ÉICl investigated was of the type normally
associated with a nearly prolate asymmetric rotor for which
the only non-zero component of the electric dipole moment is
zero-values of Ms (X) [ s (X)N (see Table 1), we set M (I) \
bb
cc
bb
M (I). The smaller number of transitions measured for
cc
C H É É ÉI37Cl and C D É É ÉI35Cl coupled with the small con-
2
2
2 2
tribution of the iodine spinÈrotation e†ect to the observed
k . Each observed a-type transition carried an extensive
hyperÐne frequencies obliged us to Ðx M (I) for these iso-
a
aa
hyperÐne structure that clearly arose from the coupling of the
topomers at the value determined for C H É É ÉI35Cl.
2
2
iodine and chlorine nuclear spin vectors I and I , respec-
I
Cl
tively, to the framework rotational angular momentum J via
the nuclear quadrupole interaction. The observed frequencies
of hyperÐne components in the J \ 4 ^ 3, 5 ^ 4 and 6 ^ 5
sets of transitions are given in Table S1¤ for the isotopomers
C H É É ÉI35Cl, C H É É ÉI37Cl and C D É É ÉI35Cl. Fewer tran-
3
.2 Symmetry and angular geometry of C H Æ Æ ÆICl
2 2
The complex C H É É ÉICl is readily shown to have the planar
2
2
T-shaped geometry of C symmetry shown in Fig. 1 by
means of interpretation of the various observed spectroscopic
constants.
2v
2
2
2 2
2 2
sitions were observed for the last of these species, mainly
First, the fact that the zero-point rotational constant A of
because the transitions involving K \ 1 were reduced in
0
~
1
each
isotopomer
C H É É ÉI35Cl,
C H É É ÉI37Cl
and
intensity relative to the K \ 0 transitions. For the iso-
2
2
2 2
~
1
C D É É ÉI35Cl (see Table 1) is almost identical within experi-
mental error to the rotational constant B of the correspond-
ing isotopomer of free ethyne19,20 (as given in Table 2)
demonstrates that ICl lies along the C axis of the ethyne mol-
topomers C H É É ÉI35Cl and C H É É ÉI37Cl, the K \ 1 tran-
2 2 ~1
2
2
2
2
sitions were considerably stronger than in C D É É ÉI35Cl. This
0
2
2
suggests a nuclear spin statistical weight e†ect that di†ers
between the C H and C D species. Discussion of this e†ect
2
2
2
2 2
ecule. If the geometry of the ethyne subunit were unperturbed
is postponed until Section 3.2.
on complex formation, the geometry displayed in Fig. 1 would
require that the equilibrium rotational constant A of the
¤
Available as supplementary material (SUP 57599, 4pp.) deposited
e
complex were identical with B of the free ethyne subunit.
e
with the British Library. Details are available from the Editorial
Office. For direct electronic access see http://www.rsc.org/suppdata/
cp/1999/3721.
Unfortunately, equilibrium rotational constants are not avail-
able, but the small excess of A of each C H isotopomer over
0
2 2
3722
Phys. Chem. Chem. Phys., 1999, 1, 3721È3726

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Table 1 Ground state spectroscopic constants for C H É É ÉI35Cl, C H É É ÉI37Cl and C D É É ÉI35Cl
2
2
2
2
2
2
Spectroscopic constant
C H É É ÉI35Cl
C H É É ÉI37Cl
C D É É ÉI35Cl
2
2
2
2
2 2
A /MHz
35611(120)
1145.9416(2)
1108.7254(2)
0.443(3)
35316(260)
1118.6947(4)
1083.1981(5)
0.403(6)
25037(100)
1099.1048(4)
1052.3151(4)
0.37(1)
0
B /MHz
0
C /MHz
0
D /kHz
J
D
/kHz
26.6(2)
0.013(2)
25.9(3)
0.011(4)
26.2(4)
0.013a
JK
d /kHz
J
s (I)/MHz
[2969.03(2)
[83.29(4)
[78.99(6)
[0.41(1)
[0.0045(30)
0.0062(2)
0.61(5)
[2969.3(4)
[83.27(7)
[62.26(1)
[0.32(3)
[0.0045a
0.0071(4)
0.49(10)
[2969.02(6)
[86.6(2)
[78.92(1)
[0.30(7)
[0.0045a
0.0055(7)
0.26(8)
aa
Ms (I)[s (I)N/MHz
bb
cc
s (Cl)/MHz
aa
Ms (Cl)[s (Cl)N/MHz
bb
cc
M /MHz
aa
M /MHz
bb
D /uÓ2 b
0
N
114
1.4
65
3.1
45
3.8
p/kHz
a Fixed in Ðt (See text for discussion). b D \ I0 [ I0 [ I0, the ground state inertia defect.
0
c
b
a
the B value of free ethyne almost certainly originates from a
small angular oscillation a of the ethyne subunit in the zero-
point state of the complex of the type illustrated in Fig. 2. If
complex and the CÈC bond would be expected to lengthen.
The planar arrangement of the nuclei implied by the C
0
2v
geometry shown in Fig. 1 requires that the relation Ie \ Ie
c b
the equilibrium angle a is 90¡, it is shown in Section 3.5 that
] Ie exists among the equilibrium moments of inertia, but the
e
av
a
the average value a \ sin~1Ssin2 aT1@2, where angular
latter are unavailable. However, a small positive inertia defect
brackets denote the zero-point average, will be smaller than
D \ I0 [ I0 [ I0 is often taken as an indication of a planar
0
c
b
a
9
0¡ and will lead to A larger than A in the case of the C H
geometry. Values of this quantity are given in Table 1. For the
0
e
2 2
isotopomers. The fact that, on the other hand, A for
C D É É ÉI35Cl is smaller than B for C D is also consistent
with this interpretation, as will be elaborated in Section 3.5.
two isotopomers C H É É ÉI35Cl and C H É É ÉI37Cl, D is iden-
0
2 2
2 2
0
tical within experimental error and is very similar in value to
2
2
0
2 2
those of the corresponding isotopomers of C H É É ÉClF,
2
2
An explanation of the increased value of A of C H É É ÉI35Cl
namely 0.639(2) and 0.634(1) uÓ2, respectively.13 The value of
0
2 2
in terms of a contraction of the C3 C bond of ethyne on forma-
tion of C H É É ÉICl seems unconvincing not only because A
D is somewhat smaller for the C D É É ÉI35Cl.
0
2 2
2
2
0
of C D É É ÉI35Cl is smaller than B of C D but also because
2 2
the p-system of ethyne is acting as the electron donor in the
2
2
0
Fig. 2 DeÐnition of the principal inertial axes
a and b of
ethyneÉ É ÉICl, the distance r and the angles a, b and t used in the
cm
Fig. 1 The equilibrium geometry of ethyneÉ É ÉICl drawn to scale.
determination of the geometry of the complex.
Table 2 Some properties of ethyne, iodine monochloride and the atoms I and Cl
Molecule or atom
B /MHz
r/Óa
s (I)/MHz
s (Cl)/MHz
0
0
0
C H
2
b
c
35273.8(4)
25418.2(3)
3414.36695(7)
3269.92335(9)
È
È
È
È
È
0.50196
0.52414
È
È
È
È
È
È
È
2
2
C D
2
I35Cld
I37Cld
[2927.859(2)
[2927.878(3)
[2292.71
È
[85.887(3)
[67.687(3)
È
Atomic Ie
Atomic 35Cle
[109.74
Atomic 37Cle
È
[86.51
a r is the distance of I from the centre of mass in free ICl. Values were calculated from the r
d Ref. 5. e Ref. 22.
0 bond distances given in ref. 5. b Ref. 19. c Ref. 20.
Phys. Chem. Chem. Phys., 1999, 1, 3721È3726
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A planar equilibrium geometry of the type shown in Fig. 1
mode makes a negligible contribution to the di†erence
requires that a rotation Ca about the a-axis exchanges a pair
between se (X) and the corresponding zero-point quantity
2
gg
of equivalent protons in C H É É ÉICl isotopomers. The usual
s (X), that the e†ects of the ethyne subunit angular oscillation
2
2
gg
arguments based on FermiÈDirac statistics then predict that,
in the vibrational ground state, a-type transitions which orig-
inate in K \ 1 levels have a nuclear spin statistical weight
(a deÐned in Fig. 2) are also negligible, and that the motion of
the ICl subunit is two-dimensionally isotropic, the s (X) are
aa
related to the se (X) for both X \ I and X \ Cl by
~
1
aa
of 3 while those involving K \ 0 levels have a weight of
~
1
s (X) \ 1se (X)S3 cos2 b [ 1T
(2)
unity. Conversely, for C D É É ÉICl, the exchange of the pair of
aa 2 aa
2
2
equivalent D nuclei (I \ 1, bosons) leads to a nuclear spin
The next step is to express the se (X) values in terms of those
statistical weight ratio of 1 : 2 for K \ 1 transitions relative
aa
~
1
s (X)5 of free ICl (see Table 2) and the electric charge redistri-
0
to K \ 0 transitions. Certainly, the intensities of the
~
1
bution when ICl is incorporated into the complex. We assume
(
J ] 1)
^ J
and (J ] 1) ^ J
transitions were
1
, J`1
1, J
1, J
1, J~1
that a fraction d of an electronic charge is transferred from
considerably smaller than that of the (J ] 1)
^ J
tran-
1
0
, J`1
0, J
the p-bond of ethyne to the iodine atom. In addition, the
sition of a given J for the C D É É ÉI35Cl isotopomer. For the
2
2
ethyne electric charge distribution will produce an electric
Ðeld and its various gradients at the X nuclei and the response
of the ICl electronic distribution to these leads to an increased
electric Ðeld gradient (efg) at I and a diminished efg at Cl,
relative to the free ICl molecule. We shall assume that the
latter changes can be described in terms of a polarization of
the ICl electronic distribution equivalent to a transfer of a
C H É É ÉICl isotopomers, on the other hand, the K \ 1
~1
transitions were at least as the strong as the K \ 0 com-
2
2
~
1
ponent. Given the difficulty of measuring relative intensities of
transitions when using pulsed-nozzle FT microwave spectros-
copy and given the unknown extent of rotational cooling of
the higher energy K \ 1 levels in the supersonic expansion,
~
1
the nuclear spin statistical weight ratios required by the postu-
lated angular geometry could not be established quantitat-
ively. The qualitative evidence is consistent with the planar,
fraction d of an electron from I to Cl. Thus, in this model,
2
there are net increases of (d [ d )e at I and d e at Cl.
1
2
2
According to the TownesÈDailey model21 for interpreting
nuclear quadrupole coupling constants, as applied to ICl, the
C
geometry, however.
The signs and magnitudes of the determined nuclear quad-
2v
transfer of a fraction d of an electron from the 5p orbital on
rupole coupling constants s (X) and s (X) [ s (X) for both
z
iodine into the 3p orbital on Cl (z is the internuclear axis)
z
aa
bb
cc
X \ I and Cl (see Table 1) are also consistent with the pro-
leads to changes of d Æ s (I) and [d Æ s (Cl) in the free molecule
posed angular geometry. Thus, the former constant is similar
A
A
coupling constants s (I) and s (Cl), respectively, where s (X)
in both respects to s (X) of free ICl5 (values of which for the I
0
0
A
0
is the coupling constant of the free halogen atom X.22 On the
basis of this model, we can write
and Cl nuclei are included in Table 2) while the latter is close
to zero. The fact that none of the o†-diagonal elements s (X),
ab
s (X) or s (X) was required to give a Ðt of transition fre-
ac
bc
se (I) \ s (I) [ (d [ d )s (I)
aa 2 A
(3)
(4)
quencies that is close to the error of frequency measurement is
also evidence in favour of the preferred angular geometry. A
more detailed discussion of the nuclear quadrupole coupling
constants is postponed until Section 3.3.
0
1
se (Cl) \ s (Cl) [ d . s (Cl)
aa
0
2
A
In formulating eqn. (3), it is assumed that the net change (d
1
[
d )e of the electronic charge on I involves the 5p orbital.
2
z
If b \ cos~1 Scos2 bT1@2 were known, eqns. (3) and (4)
av
3
.3 Charge transfer on formation of C H Æ Æ ÆICl
used in combination with eqn. (2) would a†ord a route to
2
2
both d and d . Unfortunately, we do not have a method of
1
2
Information about the extent of the electric charge redistri-
determining b independently. We can make a reasonable
av
bution on formation of C H É É ÉICl is available from the
estimate of this quantity by referring to a recent analysis5 of
2
2
halogen nuclear quadrupole coupling constants but it is con-
volved with the zero-point angular oscillation of the ICl
subunit. Some approximations, and the use of a simple model
for interpreting nuclear quadrupole coupling constants, are
necessary to extract the intermolecular and intramolecular
charge transfer. These are set out below.
the coupling constants s (X) of ArÉ É ÉICl, however. When
aa
intermolecular charge transfer is assumed negligible in this
weakly bound complex, it is possible to obtain simultaneously
the fraction d of an electronic charge transferred from I to Cl
2
and the angle b , as discussed in ref. 5. The results are d \
av
2
5.4 ] 10~3 and b \ 5.45(1)¡. The nearly zero value of d pro-
av
2
According to the arguments of Section 3.2, ethyneÉ É ÉICl has
vides justiÐcation a posteriori of the assumption that d is neg-
1
a planar T-shaped geometry of C symmetry (see Fig. 1). The
ligible in the ArÉ É ÉICl case.
2
v
changes in the rotational constants accompanying isotopic
Given that ArÉ É ÉICl is much more weakly bound than
ethyneÉ É ÉICl according to the criterion of the intermolecular
substitution of 37Cl for 35Cl in C H É É ÉI35Cl establish that I
2
2
lies closer to the ethyne subunit than does Cl (see Section 3.5).
Each component of the X nuclear quadrupole coupling tensor
stretching force constant k , which has values of 3.20(1) and
p
12.2(1) Nm~1 respectively for these two complexes, it is rea-
in the equilibrium state of the complex is deÐned by se (X) \
sonable to assume that the angular oscillation of the ICl
gg
[
(eQ /h)L2V /Lg2, where g \ a, b or c and Q is the conven-
subunit is attenuated in ethyneÉ É ÉICl relative to ArÉ É ÉICl.
x
x
x
tional electric quadrupole moment of nucleus X. The fact that
ethyne is nonpolar and that the Cl nucleus is remote from the
centre (\) of the ethyne subunit [r(\É É ÉCl) B 5.4 Ó, see Section
Because b is small even in ArÉ É ÉICl, we do not expect a large
av
decrease in b in ethyneÉ É ÉICl. Accordingly, if we assume
av
b \ 4.5(5)¡, we can be reasonably conÐdent that the actual
value will lie in the range.
av
3
.5] suggests that the cylindrical symmetry of the electric
charge distribution at Cl is not likely to be signiÐcantly per-
The values of d [ d and d obtained by using the
1
2
2
turbed on formation of the complex. We thus expect Mse (Cl)
observed s (X) (Table 1), the known s (X) values5 and s (X)
aa
bb
0
A
[
se (Cl)N to be very close to zero in the complex. The zero-
values22 (Table 2) and b \ 4.5(5)¡ in eqns. (2), (3) and (4) are
cc
av
point values of this di†erence (see Table 2) are certainly very
small, an observation suggesting that the zero-point angular
oscillation b of the ICl subunit (see Fig. 2 for deÐnition),
which is the main contribution to the zero-point averaging of
the coupling constants, can be assumed to be two-
dimensionally isotropic in the bc plane.
recorded in Table 3 for each of the three isotopomers
C H É É ÉI35Cl, C H É É ÉI37Cl and C D É É ÉI35Cl. The errors
2
2
2 2
2 2
quoted are those resulting from the assumed range of b , to
av
which the values of d [ d and d are not very sensitive. The
1
2
2
implication of the results in Table 3 is that d \ 0.026, i.e. the
1
intermolecular charge transfer corresponds to ca. 2 to 3% of
In the approximation that the intermolecular stretching
an electron from the ethyne p bond to I. The polarisation of
3724
Phys. Chem. Chem. Phys., 1999, 1, 3721È3726

View Article Online
Table 3 Some determined properties of ethyneÉ É ÉICl
It is not possible from results presented here to determine
any small structural changes in the ethyne and ICl subunits
that attend formation of the complex. In view of the weak
binding, we shall assume that the subunit geometries survive
unchanged. The a-coordinate of the 35Cl atom can be deter-
mined from the changes *I and *I in the zero-point
Isotopomer
Property
C H É É ÉI35Cl
C H É É ÉI37Cl
C D É É ÉI35Cl
2
2
2
2
2 2
d [d a
[0.0300(3)
0.0561(2)
12.12(8)
75(2)
4.5(5)
3.6169(14)
3.1150(14)
[0.0302(3)
0.0560(2)
12.3(2)
75(2)d
4.5(5)
3.6398(14)
3.1157(14)
[0.0300(3)
0.0568(2)
13.4(3)
80(2)
4.5(5)
3.6151(14)
3.1131(14)
1
2
2
b
c
d
moments of inertia on substitution of 35Cl by 37Cl in the
usual way by using KraitchmanÏs equation24 for substitution
of an atom on the symmetry axis of a C planar molecule.
k /Nm~1 b
p
a /degreesc
av
b /degreese
2
v
av
r
/Óf
The appropriate expression is
cm
r(\É É ÉI)/Óg
a2 \ (*I ] *I )/2k ,
(6)
Cl
b
c
s
a d is the fraction of an electronic charge transferred from ethyne to I
1
and d is the fraction of an electronic charge transferred from I to Cl
where k \ M Æ *m/(M ] *m). The result is a \ 2.332 Ó. If I
2
s
Cl
on complex formation. See text for discussion. b Calculated from D
lies closer to the ethyne subunit, as in the geometry shown in
J
using eqn. (5). The errors are those propagated from D and do not
J
Fig. 1, this value of a implies the iodine coordinate a B 0,
reÑect systematic errors in the model. c Calculated from eqn. (7). See
Cl
I
since r \ 2.3236 Ó in free ICl.5 The alternative arrangement
text for discussion. d Assumed value. e See text for method of estima-
tion of b
0
av . f Calculated from eqn. (8). g Calculated from eqn. (9).
in which Cl lies closer to ethyne than I would require a B 4.6
Ó, which is clearly unreasonable, given the predominant con-
I
tribution of I to the mass of the complex.
The model used to determine r and r(\É É ÉI) from the
cm
the ICl subunit corresponds to the transfer of 0.056e from I to
Cl, leading to a net decrease of 0.030e at I.
observed zero-point moments of inertia is shown in Fig. 2. It
assumes that the ethyne and ICl subunits execute the zero-
point angular oscillations a, b and t, as deÐned in Fig. 2, but
with the distance r between the mass centres Ðxed. The
3.4 Strength of the intermolecular binding
cm
motion described by the angle t is assumed to be two-
The extent of both inter- and intramolecular charge redistri-
bution, estimated from the nuclear quadrupole coupling con-
stants in the preceding section, is small and suggests that the
complex ethyneÉ É ÉICl is weakly bound. One criterion of the
binding strength is the intermolecular stretching force con-
dimensionally isotropic. By averaging over the angles a, b and
t, the following relations between the principal moments of
inertia of the complex and those, IC2H2 and IICl, of the com-
b
b
ponents can be established:
stant k , which measures the restoring force for unit inÐnitesi-
mal displacement along the weak bond direction. In the
I \ IC2H2Ssin2 aT ] IIClSsin2 bT
(7)
I ] I \ 2kr2 ] IC2H2S1 ] cos2aT ] IIClS1 ] cos2 bT (8)
p
a
b
b
approximation of rigid, unperturbed subunits and ignoring
potential constants beyond the quadratic, the centrifugal dis-
tortion constant D of a planar complex of C
b
c
cm
b
b
The values of I , I , I , IC2H2 and IICl are available from rota-
2v symmetry,
a
b
c
b
b
J
tional constants in Tables 1 and 2. As discussed in Section 3.3,
such as ethyneÉ É ÉICl, is related to k by23
p
b \ cos~1 Scos2 bT1@2 can be assigned the value 4.5(5)¡. The
av
k \ (8p2k/D ) ]
value of a can be determined from eqn. (7). For the iso-
p
J
av
topomer C H É É ÉI35Cl, for which the rotational constant A
[
(1 [ b)B3 ] (1 [ c)C3 [ 1@4(B [ C)2(B ] C)(2 [ b [ c)]
2 2
0
is best determined, the application of eqn. (7) with b \
av
4
.5(5)¡ leads to a B 75(2)¡. The corresponding angle, simi-
(5)
av
larly determined, for the isotopomer C D É É ÉI35Cl is 80(2)¡.
2
2
In eqn. (5), k \ mC2H2 mICl/(mC2H2 ] mICl), the rotational con-
stants B, BC2H2 and BICl refer to the complex, ethyne19,20 and
ICl,5 respectively, b \ (B/BC2H2) ] (B/BICl), and c has the cor-
responding deÐnition in terms of the rotational constants C.
When the same approach is applied to the isostructural
complex ethyneÉ É ÉClF, the angles a \ 78(4)¡ and 80(4)¡ are
av
obtained13 for C H É É É35ClF and C D É É É35ClF, respectively.
2
2
2 2
A smaller oscillation angle for the C D subunit is reasonable
2
2
The values of k obtained by using eqn. (5) for each of the
in view of the larger reduced mass for the motion.
p
three isotopomers of ethyneÉ É ÉICl investigated are included in
Given the angles a and b , eqn. (8) may be used to deter-
av
av
Table 3. Zero-point spectroscopic constants were used for the
complex (see Table 1) and for the free component molecules
mine r . The results for the three isotopomers C H É É ÉI35Cl,
cm
C H É É ÉI37Cl and C D É É ÉI35Cl are given in Table 3, in
2 2
which the values of a and b are also included. In view of
av av
the larger error in A for C H É É ÉI37Cl, the value of a for
2 2 av
2 2
2
2
(
Table 2). We note that k so determined is isotopically invari-
p
ant between the two isotopomers C H É É ÉI35Cl and
2
2
0
C H É É ÉI37Cl within the error propagated from D . The
this isotopomer was assumed unchanged from C H É É ÉI35Cl.
2
2
J
2
2
slightly larger value for C D É É ÉI35Cl is probably a result of
The best approximation possible to the equilibrium value of
2
2
the difficulties of determining A and D separately in this
the distance r(\É É ÉI) is given by
0
J
case. The reduced number of K \ 1 transitions available for
this isotopomer (for reasons discussed in Section 3.2) means
~
1
r(\É É ÉI) \ r [ r,
(9)
cm
that the errors in the two quantities A and D are probably
0
J
where r is the distance of the I atom from the centre of mass
in the free ICl molecule (see Table 3). Values of r(\É É ÉI) so
determined are given in Table 3.
underestimated.
3.5 The intermolecular distances rcm and r(*Æ Æ ÆI)
It was argued in Sections 3.2 to 3.4 that ethyneÉ É ÉICl has a
planar, T-shaped geometry of the type shown in Fig. 1, that
4. Discussion
the intermolecular binding is weak (according to the k
p
criterion) and that the electric charge redistribution on
Use of a fast-mixing nozzle in a BalleÈFlygare, Fourier-
transform microwave spectrometer has led not only to the iso-
lation of complexes ethyneÉ É ÉICl in a mixture of the two
components which would undergo chemical reaction under
normal conditions but also to their characterisation through
analysis of the rotational spectrum so detected. Interpretation
complex formation corresponds to only a few percent of an
electron. In this section, we determine the distance r
between the centres of mass of ethyne and ICl and the dis-
cm
tance r(\É É ÉI), where \ identiÐes the centre of mass of the
ethyne subunit.
Phys. Chem. Chem. Phys., 1999, 1, 3721È3726
3725

View Article Online
Table 4 Comparison of some properties of the series of complexes
of a transfer of a fraction d of an electronic charge from X to
2
ethyneÉ É ÉXY, where XY \ ClF, Cl , BrCl or ICl
Y, depends mainly on the atom X. Thus, the d values lie in
2
2
the order Cl D ClF \ BrCl \ ICl, which is that expected
2
XY
k /Nm~1 d a
r(\É É ÉX)/Ó Mp(C) ] p(X)N/Ób Dr/Óc
p
2
from polarizabilities. The molar refraction constants [R],
Cl d
2
BrClg,h
ICli
5.6
10.0
9.4
0.020 3.163
0.016 2.873
0.032 3.059
0.056 3.115
3.50
3.50
3.65
3.85
0.34
0.63
0.59
0.74
which are proportional to the atom polarizabilities a via
ClFe,f
[R] \ (4pe )~1(4pN a/3), assigned to the atoms Cl, Br and I
0
A
by Ingold are 5.9, 8.8 and 13.8 cm3 mol~1, respectively.27
12.2
a d is the fraction of an electronic charge transferred from XY when
2
Acknowledgements
We thank the Ruth King Trust of the University of Exeter for
a research studentship for J.B.D. and the EPSRC for the
award of a Senior Research Fellowship to A.C.L.
ethyneÉ É ÉXY is formed. The method used to estimate d for the ClF
2
complex is given in ref. 3. b p(C) and p(X) are van der Waals radii
from ref. 25. The value used for C in C H is half the thickness of the
2
2
benzene ring (1.7 Ó) given in ref. 25. c Dr \ Mp(C) ] p(X)N [ r(\É É ÉX),
where \ is the centre of gravity of the ethyne molecule. d Ref. 12. e Ref.
1
3. f Ref. 3. g Ref. 14. h Ref. 2. i This work.
References
of the spectroscopic constants of three isotopomers of
ethyneÉ É ÉICl has established the angular geometry of the
complex, the intermolecular distance r(\É É ÉI), the strength of
1
2
3
4
5
A. C. Legon, Chem. Phys. L ett., 1995, 237, 291.
A. C. Legon, J. Chem. Soc., Faraday T rans., 1995, 91, 1881.
A. C. Legon, Chem. Phys. L ett., 1997, 279, 55.
A. C. Legon, Chem. Eur. J., 1998, 4, 1890.
the interaction (as measured by k ), and some details of the
p
J. B. Davey, A. C. Legon and E. R. Waclawik, Chem. Phys. L ett.,
electric charge redistribution on association of the component
1
999, 306, 133.
molecules.
6
7
J. B. Davey, A. C. Legon and E. R. Waclawik, Phys. Chem. Chem.
The corresponding sets of properties, similarly deter-
mined,12h14 are now available for the series of complexes
Phys., 1999, 1, 3097.
H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Soc., 1949, 71,
2
703.
ethyneÉ É ÉXY, where XY \ Cl , ClF, BrCl or ICl, and are
2
8
9
O. Hassel and Ch. RÔmming, Quart. Rev. Chem. Soc., 1962, 16, 1.
R. S. Mulliken and W. B. Person, Molecular Complexes, Wiley-
Interscience, New York, 1969 and references cited therein.
compared in Table 4. Each member of the series has the
planar, T-shaped angular geometry of C symmetry of the
2
v
type shown in Fig. 1 and in each case the halogen atom of
10 See, for example, B. S. Ault, J. Phys. Chem., 1986, 90, 2825.
11 A. C. Legon and C. A. Rego, J. Chem. Soc., Faraday T rans., 1990,
86, 1915.
higher atomic number is closest to the p-bond of ethyne. The
distances
r(\É É ÉX)
are
systematically
shorter
by
1
2
H. I. Bloemink, S. A. Cooke, K. Hinds, A. C. Legon and J. C.
Thorn, J. Chem. Soc., Faraday T rans., 1995, 91, 1891.
K. Hinds, J. H. Holloway and A. C. Legon, J. Chem. Soc.,
Faraday T rans., 1996, 92, 1291.
*
r \ Mp(C) ] p(X)N [ r(\É É ÉX) than the sum of the van der
Waals radii of C and X,25 as may be seen from Table 4 which
gives values of p(C) ] p(X) and *r. The value p(C) \ 1.7 Ó,
which is the half-thickness of the benzene ring, suggested by
Pauling25 was adopted. The contraction *r for a given XY in
the series of ethyneÉ É ÉXY complexes given in Table 4 is almost
identical to the corresponding *r in the series OCÉ É ÉXY,
which was reviewed recently.6 The origin of the contraction
lies in the so-called “snub-nosedÏ character of the dihalogen
molecules XY, i.e. the smaller van der Waals radius along the
internuclear axis direction than perpendicular to it and has
been discussed elsewhere.1,2,6
1
3
14 H. I. Bloemink, K. Hinds, A. C. Legon and J. C. Thorn, J. Chem.
Soc., Chem. Commun., 1994, 1229.
1
1
5
6
T. J. Balle and W. H. Flygare, Rev. Sci. Instrum., 1981, 52, 33.
A. C. Legon, in Atomic and Molecular Beam Methods, ed. G.
Scoles, Oxford University Press, 1992, vol. 2, ch. 9.
J. K. G. Watson, J. Chem. Phys., 1968, 48, 4517.
1
1
7
8
H. M. Pickett, J. Mol. Spectrosc., 1991, 148, 371.
19 K. F. Palmer, M. E. Mickelson and K. N. Rao, J. Mol. Spectrosc.,
1979, 37, 325.
2
0
M. Hutanantii, J. Hietanen, R. Antilla and J. Kauppinen, Mol.
Phys., 1979, 37, 905.
It is clear from the k values given in Table 4 that the order
p
21 C. H. Townes and B. P. Dailey, J. Chem. Phys., 1949, 17, 782;
C. H. Townes and A. L. Schawlow, Microwave Spectroscopy,
McGraw Hill, New York, 1955.
22 W. Gordy and R. L. Cook, Microwave Molecular Spectra, in
T echniques of Chemistry, Wiley-Interscience, New York, 1984,
vol. XVIII ch. 14.
of binding strength in the series ethyneÉ É ÉXY is XY \ ICl [
BrCl D ClF [ Cl . The same order prevails in the corre-
2
sponding OCÉ É ÉXY series and was readily rationalised in
terms of the electric moments of the XY subunits.6 In fact, the
ratio k (OCÉ É ÉXY)/k (C H É É ÉXY) has the values 0.65, 0.70,
p
p
2 2
2
3 D. J. Millen, Can. J. Chem., 1985, 63, 1477.
0
.67 and 0.66 for XY \ Cl , ClF, BrCl and ICl. This indicates
2
24 J. Kraitchman, Am. J. Phys., 1953, 21, 17.
that the type of systematic behaviour previously noted among
the hydrogen-bonded complexes BÉ É ÉHX (X \ F, Cl, Br,
CN)26 also occurs in the corresponding “halogen-bondedÏ
series.
2
5
L. Pauling, T he Nature of the Chemical Bond, Cornell University
Press, Ithaca, NY, 3rd edn., 1960, p. 260 †.
26 A. C. Legon and D. J. Millen, J. Am. Chem. Soc., 1987, 109, 356.
27 C. K. Ingold, Structure and Mechanism in Organic Chemistry,
Cornell University Press, Itaca, NY, 1953, p. 121.
Finally, we note that polarisation of the XY subunit by the
ethyne molecule, as determined from the halogen nuclear
quadrupole coupling constants s (X) and expressed in terms
Paper 9/04233D
aa
3726
Phys. Chem. Chem. Phys., 1999, 1, 3721È3726