Microwave spectrum and geometry of the methyl isocyanide-argon complex

Article abstract of DOI:10.1039/a606816b

The rotational spectrum of a complex formed between methyl isocyanide and argon has been observed using pulsed-nozzle, Fourier-transform microwave spectroscopy. The spectrum and geometry of the complex are very similar to those found for CH₃CN...Ar (R. S. Ford et al., J. Chem. Phys., 1991, 94, 5306). The complex is T-shaped, with the argon atom located 3.64 A from the centre of mass of the methyl isocyanide molecule. There appears to be almost free internal rotation of the methyl isocyanide molecule about its symmetry axis in the complex.

Full text of DOI:10.1039/a606816b

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Microwave spectrum and geometry of the methyl isocyanide–argon
complex
Susana Blanco,¤ David G. Lister,” A. C. Legon and Christopher A. Rego°
Department of Chemistry, University of Exeter, Stocker Road, Exeter, UK EX4 4QD
The rotational spectrum of a complex formed between methyl isocyanide and argon has been observed using pulsed-nozzle,
Fourier-transform microwave spectroscopy. The spectrum and geometry of the complex are very similar to those found for
CH CNÉ É ÉAr (R. S. Ford et al., J. Chem. Phys., 1991, 94, 5306). The complex is T-shaped, with the argon atom located 3.64 A
Ž
3
from the centre of mass of the methyl isocyanide molecule. There appears to be almost free internal rotation of the methyl
isocyanide molecule about its symmetry axis in the complex.
Hydrogen-bonded
complexes
of
methyl
cyanide
Spectra were observed using a pulsed-nozzle, Fourier-
(
CH CNÉ É ÉHX) and of methyl isocyanide (CH NCÉ É ÉHX)
transform microwave spectrometer11 of the type Ðrst
described by Balle and Flygare.12 Gas mixtures were made by
admitting the vapour from liquid methyl isocyanide into an
evacuated stainless steel tank and then adding argon to give a
total pressure of ca. 1 atm. Frequencies of resolved 14N
nuclear quadrupole hyperÐne components were measured
with an estimated accuracy of ca. 2 kHz, but the accuracy is
somewhat less for some unresolved doublets.
3
3
have very similar properties.1h8 Table 1 shows that the NÉ É ÉX
distance in a methyl cyanide complex is slightly shorter than
the CÉ É ÉX distance in the corresponding methyl isocyanide
complex. The table also shows that the strength of the hydro-
gen bond, as measured by the intermolecular stretching force
constant (k ), is almost the same for a given HX for the two
types of complex. During the studies of the microwave spectra
p
of the hydrogen-bonded methyl isocyanide complexes2,4,6,8
some lines were found that were attributed to the methyl
Analysis of the spectrum
The search for, and the analysis of, the spectrum of
isocyanideÈargon dimer (CH NCÉ É ÉAr). The closely related
3
methyl cyanideÈargon dimer (CH CNÉ É ÉAr) has been studied
3
CH NCÉ É ÉAr was based on its expected similarity to
by Ford et al.9 They found this complex to be T-shaped with
3
CH CNÉ É ÉAr.9 The nearly free internal rotation of CH CN
the argon atom located 3.65 AŽ from the centre of mass of the
3
3
in CH CNÉ É ÉAr gives rise to a Ðrst excited state (E or m \ 1)
methyl cyanide molecule. They also found that there is nearly
free internal rotation of the methyl cyanide molecule about its
symmetry axis in the complex. The present work describes an
3
about 3 cm~1 above the ground state (A or m \ 0). Because of
nuclear spin e†ects, the A and E states are nearly equally
populated even at the low temperature (\5 K) of the super-
sonic jet.9 The A state can be Ðtted using semirigid rotor
theory while the E state has very large Coriolis perturbations.
investigation of the microwave spectrum of CH NCÉ É ÉAr
3
undertaken to examine whether the similarity, previously
alluded to, between the methyl cyanide and methyl isocyanide
complexes1 also extends to their more weakly bound ana-
logues CH NCÉ É ÉAr and CH CNÉ É ÉAr.
The Ðrst transitions of CH NCÉ É ÉAr to be assigned were those
3
in the J
ÈJ
Q-branch series of the A state. The lines
1
, J~1 0, J
3
3
showed small splittings due to 14N nuclear quadrupole coup-
ling. A search for the 1 È0 , R-branch transition yielded
1
1
00
Experimental
Methyl isocyanide was prepared by the dehydration of N-
methylformamide using the method of Casanova et al.10
two lines with very similar intensities and the expected charac-
teristic nuclear quadrupole coupling patterns (Table 2). The
hyperÐne structure of a 1 È0 transition depends only on
11 00
the 14N nuclear quadrupole coupling constant s . This con-
bb
stant was therefore derived from the hyperÐne splittings and
¤
Permanent address: Departamento de Quimica-Fisica, Uni-
has the value given in Table 2. The assignment of 11
È0
1
00
versidad de Valladolid, Prado de la Magdalena, s/n, 47005 Valladolid,
Spain.
”
versita di Messina, Casella Postale 29, I-98166 SantÏAgata di Messina,
Italy.
°
transitions to the A and E states was made by Ðtting each in
turn together with the A-state Q-branch series and predicting
Permanent address: Dipartimento di Chimica Industriale, Uni-
other lines. The 1 È0 transition was the only unambiguous
11 00
assignment that could be made for the E-state.
Present address: Department of Chemistry, Manchester Metro-
The Ðnal Ðt of the A-state spectrum was made using
politan University, Chester Street, Manchester, UK M1 5GD.
WatsonÏs
A reduced semirigid rotor Hamiltonian,13 as
Table 1 Comparison of the properties of some complexes of CH CN and CH NC
3
3
CH CN
CH NC
3
3
partner molecule
R(NÉ É ÉX) or R(CÉ É ÉAr)/A
Ž
k /N m~1
R(CÉ É ÉX) or R(NÉ É ÉAr)/A
Ž
k /N m~1
p
p
HF
2.75a
3.30c
3.27e
3.43g
19.2a
10.7c
9.51e
4.70g
1.92i
2.84b
3.40d
3.43f
3.60h
19.9b
11.5d
9.3f
4.78h
1.96j
HCl
HCN
HCCH
Ar
3.63i or 3.68
3.64j or 3.67
a Ref. 1. b Ref. 2. c Ref. 3. d Ref. 4. e Ref. 5. f Ref. 6. g Ref. 7. h Ref. 8. i Ref. 9. j This work.
J. Chem. Soc., Faraday T rans., 1997, 93(7), 1287È1290
1287

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Table 2 Observed frequencies of the 1 È0 transition in the A and
Table 4 Rotational, centrifugal distortion and 14N nuclear quadru-
11
00
E states of CH NCÉ É ÉAr, the 14N nuclear quadrupole coupling con-
pole coupling constants and their standard errors for CH NCÉ É ÉAr
3
3
stants (s ) and unperturbed line frequencies (l )
bb
0
spectroscopic constant
value
A state
l/MHz
E state
A/MHz
B/MHz
C/MHz
10182.547(2)
1889.567(2)
1584.271(1)
F@ ^ FA
*l/kHz
l/MHz
*l/kHz
1
2
1
^ 0
^ 1
^ 1
11 766.1462
11 766.3582
11 766.5022
0.7
[1.1
0.4
11 762.2989
11 762.5142
11 762.6563
[0.5
0.6
[0.1
D /kHz
13.46(5)
195.3(3)
2.322(6)
J
D
/kHz
JK
d /kHz
J
s /MHz
0.475 (4)
11 766.3830 (10)
0.476 (4)
11 762.5374 (5)
bb
s /MHz
[0.244(5)
0.6942(8)
0.469(4)
[0.225(4)
l /MHz
aa
0
(s [ s )/MHz
bb
cc
s /MHz
bb
s /MHz
cc
employed in the computer program of Pickett.14 The residuals
from the Ðnal cycle of the Ðt are given in Table 3 while the
resulting rotational, centrifugal and nuclear quadrupole coup-
ling constants are set out in Table 4. For some members of the
the standard errors quoted in Table 4. Fixing D to [210
K
kHz, as in CH CNÉ É ÉAr,9 changes A by [0.2 MHz while the
3
J
ÈJ
Q-branch series, the nuclear quadrupole hyper-
changes to B and C are smaller than their standard errors.
1
, J~1 0, J
Ðne structure was not completely resolved and some of the
observed frequencies were treated as the mean of the fre-
quencies of two hyperÐne components. Only transitions
involving energy levels with K \ 0 or 1 have been observed
The e†ect of presetting d to zero was investigated by making
K
Ðts with d constrained to ^100 kHz. This changes B by
K
^0.2 MHz, C by <0.2 MHz, D by ^1 kHz and D by ^6
J
JK
kHz. The neglect of this constant, like that of D , has no sig-
niÐcant e†ect on the residuals.
~
1
K
and it has therefore been necessary to constrain the centrifugal
distortion constants D and d to zero. The uncertainties in
K
K
the rotational constants are accordingly somewhat larger than
Geometry of the complex
The geometry of complexes involving a molecule and a rare
gas atom is usually deÐned in terms of R, the distance from
the rare gas atom to the centre of mass of the molecule, and h,
the angle between R and one of the principal inertial axes
of the molecule [Fig. 1(a)]. E†ective values of R and h may be
obtained from the vibrational ground-state moments of inertia
of the complex and molecule if we assume that the structure of
the molecule is not changed on forming the complex and if we
neglect vibrational contributions to the moments of inertia. In
complexes with nearly free internal rotation the latter may be
Table 3 Observed frequencies and residuals for CH NCÉ É ÉAr
3
J@
^ JAK~1K`1
F@ ^ FA
0 ^ 1
l/MHz
*l/kHz
K~1K`1
111 ^ 000
11766.1462
11766.5022
11766.3582
[1.3
2.8
[0.4
1
2
^ 1
^ 1
110 ^ 101
1 ^ 0
8597.6979
8598.0478
8597.8735
8597.9461
8597.8058
8597.8735
0.1
[2.0
[7.5
[2.4
[1.9
[1.7
0
1
2
1
2
^ 1
^ 1
^ 1
^ 2
^ 2
of some importance. If CH NCÉ É ÉAr were rigid, the planar
3
moment15
21
2 ^ 101
1 ^ 0
14933.6566
14933.9593
14933.8118
[4.3
2.7
0.8
P \ 1(I ] I [ I )
2 a
(1)
c
b
c
2
3
^ 1
^ 2
would have a value of ca. 1.60 u AŽ 2. The experimental value of
[
0.95 u A
Ž
2 is very similar to those in CH CNÉ É ÉAr ([1.06 u
211 ^ 202
1 ^ 1
8910.6489
8910.4243
8910.7666
8910.5305
8910.6889
8910.4500
8910.6124
[1.6
8.5
0.3
3
AŽ
2),9 CH ClÉ É ÉAr ([0.79 u A
Ž
2)16 and CH CCHÉ É ÉAr ([1.06
2
1
2
3
2
3
^ 1
^ 2
^ 2
^ 2
^ 3
^ 3
3
3
u A
Ž
2)17 and provides further evidence that as in these three
[1.2
complexes nearly free internal rotation is responsible for the
di†erence from 1.60 u A
6.4
Ž
2.
[7.2
4.4
The C rotational constant is likely to be a†ected least by
the internal rotation and hence R was calculated from
313 ^ 202
2 ^ 1
17950.8650
17951.0619
17950.9281
[2.6
3.0
[0.4
3
4
^ 2
^ 3
I \ ICH3NC ] kR2
(2)
c
b
where k is the reduced mass given by k \ mCH3NCmAr/(mCH3NC
312 ^ 303
2 ^ 2
9394.9840
9394.8381
9394.9510
5.7
0.9
9.2
] mAr). The resulting value R \ 3.6434 A
Ž is slightly smaller
).9 KraitchmanÏs
3
4
^ 3
^ 4
than that in CH CNÉ É ÉAr (3.6505
AŽ
3
equations18 can be used to calculate the coordinates of the Ar
413 ^ 404
3 ^ 3
10068.7593
10068.6152
10068.7309
[5.0
3.8
[2.1
atom (xAr, zAr) in the principal inertial axis system of CH NC
4
5
^ 4
^ 5
Fig. 1(a)], and in the present case they may be written as:3
[
50
5 ^ 414
4 ^ 3
10180.4975
10180.3566
10180.4524
10.9
[0.7
C
C
*I
D
D
*
k
I
(xAr)2 \
b
a
1 ]
a
(3)
(4)
5
6
^ 4
^ 5
(ICH3NC [ ICH3NC)
[10.1
a b
514 ^ 505
5 ^ 5
10954.5402
[1.5
*I
k
*I
b
H
(zAr)2 \
1 ]
4
6
^ 4
^ 6
10954.6798
[11.2
(ICH3NC [ ICH3NC)
b a
where *I \ I [ ICH3NC and *I \ I [ ICH3NC. R and h may
615 ^ 606
6 ^ 6
12078.5902
2078.7314
12.5
a
a
a
b
b
b
5
7
^ 5
^ 7
H
then be obtained from
R2 \ (xAr)2 ] (zAr)2
1
[7.0
(5)
1288
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

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Table 5 Summary of the calculation of R and h for CH NCÉ É ÉAra
3
and CH CNÉ É ÉArb
3
structural parameter
CH NCÉ É ÉAra
CH CNÉ É ÉArb
3
3
R/A
o xAr o/A
o zAr o/A
R/A
h/degrees
o xAr o/A
o zAr o/A
R/A
h/degrees
a/degreesf
h/degrees
a/degreesg
h/degrees
Ž
c
3.6434
3.5887
0.3818
3.6089
83.9 or 96.1
3.6229
0.3862
3.6505
3.5934
0.3872
3.6142
83.8 or 96.2
3.6293
0.3902
Ž
d
Ž
Ž
Ž
e
Ž
Ž
3.6434
3.6504
83.9 or 96.1
82.0 (13) or 98.0 (13)
83.4 (11) or 96.6 (11)
81.9 (14) or 98.1 (14)
83.3 (11) or 96.7 (11)
83.8 or 96.2
84.5 or 95.5
85.5 or 94.5
84.7 or 95.3
85.7 or 94.3
a Calculated with BCH3NC \ 10052.8884 MHz20 and A \ 157869 MHz
taken from the structure of CH NC.21 b Calculated with ACH3CN \
3
1
58099.2 MHz22 and BCH3CN \ 9198.8993 MHz.23 c Calculated from
eqn. (2). d Calculated from eqn. (3)È(6). e Calculated from eqn. (3)È(7).
f Calculated from eqn. (8) and (10). g Calculated from eqn. (9) and
(
10).
the 14N nuclear quadrupole coupling constants may be
written as
s \ 1sCH3NC(3 cos2 a [ 1)
aa 2 zz
(8)
(9)
s \ 1sCH3NC(3 sin2 a [ 1)
bb 2 zz
where sCH3NC is the coupling constant of methyl isocyanide
zz
[
0.4894(4) MHz]19 and a is the angle of between the CNC
axis of CH NC and the a-inertial axis of the complex [Fig.
3
1
(b)]. The angles h and a are related by
kR2 sin 2h
tan 2a \
(10)
ICH3NC [ ICH3NC ] kR2 cos 2h
Fig. 1 (a) To show the parameters R and h for CH NCÉ É ÉAr and the
b
a
3
(
x, y, z) principal inertial axis system for CH NC. The molecule is
3
When s from Table 4 is substituted into eqn. (8) a small
shown projected in the (x, z) plane. (b) To show the principal inertial
aa
negative value of cos2 a is obtained. Changing s by one stan-
axis system (a, b, c) of CH NCÉ É ÉAr and the angle of rotation (a)
aa
dard error to [0.241 MHz gives a \ 85.7¡. The value of s
bb
3
between the a, b, c and x, y, z axis systems. For clarity the Ðgure
shows the acute value for h.
from Table 4 used in eqn. (9) gives a \ 80.2¡. Table 5 shows
that the values of a calculated for CH CNÉ É ÉAr from eqn. (8)
3
and (9) are much closer to each other. This indicates that the
and
di†erence found for CH NCÉ É ÉAr is due to errors in the
nuclear quadrupole coupling constants rather than the e†ects
of vibrational averaging. The 14N nuclear quadrupole coup-
ling constant of methyl isocyanide is almost an order of mag-
3
cos h \ zAr/R
(6)
Because the signs of the coordinates are not determined from
nitude smaller than that in methyl cyanide. The value of s
eqn. (3) and (4), acute and obtuse values of h result from eqn.
bb
determined from the hyperÐne splittings of the 1 È0 tran-
(
6) and it is not possible to choose between them without data
1
1
00
sition of CH 14NCÉ É ÉAr (Table 2) gives a \ 82.0¡. If s is
cc
assumed to be [1sCH3NC, then s \ [0.230 MHz and this
zz aa
gives a \ 81.9¡. These values of a calculated from the s
from isotopic substitution.
Table 5 summarizes the results of this calculation of R and
h applied to each of CH NCÉ É ÉAr and CH CNÉ É ÉAr. In both
cases, the value of R is slightly smaller than that calculated
from eqn. (2). Because the internal rotation is almost parallel
to the b inertial axis of the complex the B rotational constant
is more severely a†ected by this motion than the A rotational
3
2
gg
3
3
(
g \ a, b or c) determined via the 1 È0 transition have been
11
00
preferred in the calculation of the h values given in Table 5.
Discussion
constant16 and a better estimate of I is likely to be
The force constant for the stretching of the distance R, k , has
been estimated from the formula24
b
p
I \ I [ I ] ICH3NC
(7)
b
c
a
a
k \ (16n4k2R2/h D )[(B2 ] C2)2 ] 2(B4 ] C4)]
(11)
The use of eqn. (7) results in a value of R calculated from eqn.
p
J
(
3)È(6) that is exactly the same as that calculated from eqn. (2).
to be 1.96 N m~1 and is very similar to that in CH CNÉ É ÉAr9
3
Table 5 shows that the use of eqn. (7) changes zAr more than
xAr but that h is almost unchanged. The use of eqn. (7) implies
a contribution from the internal rotation of [35 MHz to the
B rotational constant. A contribution of ^50 MHz to the A
rotational constant would give a change of ca. ^1¡ in h. It
would therefore appear that the inertial data are leading to a
reasonably reliable estimate of h.
The 14N nuclear quadrupole coupling constants may also
be used to derive h. If the electric Ðeld gradient tensor at the
nitrogen nucleus is assumed to be unchanged on complex for-
mation and e†ects of vibrational averaging may be neglected,
(1.92 N m~1). The force constants in CH ClÉ É ÉAr16 (1.6 N
3
m~1) and CH CCHÉ É ÉAr17 (1.63 N m~1) are slightly smaller
3
than those of the complexes of Ar with the much more polar
CH NC and CH CN molecules. The force constants are
3
3
about half to two-thirds of the value of k found for complexes
p
of Ar with aromatic molecules.25 Table 1 shows that k is
p
almost an order of magnitude smaller than k for the strongest
p
hydrogen-bonded complexes of CH NC or CH CN but only
3
3
a factor of two smaller than that for the weakest complexes.
The analyses of the rotational spectra of CH NCÉ É ÉAr,
3
CH CNÉ É ÉAr9 and CH CCHÉ É ÉAr17 show that the complexes
3
3
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
1289

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Table 6 Comparison of some properties of the complexes
CH NCÉ É ÉAr, CH CNÉ É ÉAr and CH CCHÉ É ÉAr
2
3
4
5
A. C. Legon, D. G. Lister and H. E. Warner, Angew. Chem., Int.
Ed. Engl., 1992, 31, 202.
3
3
3
A. C. Legon, D. J. Millen and H. M. North, J. Phys. Chem., 1987,
91, 5210.
parameter
CH NCÉ É ÉAr CH CNÉ É ÉAra CH CCHÉ É ÉArb
3
3
3
A. C. Legon, D. G. Lister and H. E. Warner, J. Am. Chem. Soc.,
1992, 114, 8177.
N. W. Howard and A. C. Legon, J. Chem. Soc., Faraday T rans. 2,
1987, 83, 991.
A. C. Legon and J. C. Thorn, J. Mol. Struct., 1992, 270, 449.
N. W. Howard and A. C. Legon, J. Chem. Phys., 1986, 85, 6898.
A. C. Legon, D. G. Lister and C. A. Rego, Chem. Phys. L ett.,
R/A
h/degrees
R(NÉ É ÉAr)/A
or R(CÉ É ÉAr)/A
Ž
3.643
83.3 or 96.7
3.63 or 3.67
3.651
85.5 or 94.5
3.64 or 3.67
3.733
97.4
3.76
Ž
Ž
6
7
8
R(HÉ É ÉAr)/A
Ž
3.27 or 2.83
1.96
3.24 or 2.95
1.92
2.92
1.63
k /N m~1
p
1
992, 189, 221.
a Ref. 9. b Ref. 17.
9
0
1
R. S. Ford, R. D. Suenram, G. T. Fraser, F. J. Lovas and K. R.
Leopold, J. Chem. Phys., 1991, 94, 5306.
J. Casanova, R. E. Shuster and N. D. Werner, J. Chem. Soc.,
1963, 4280.
1
1
have similar T-shaped geometries (Table 6). Only for
A. C. Legon, in Atomic and Molecular Beam Methods, ed. G.
Scoles, Oxford University Press, New York, 1992, vol. 2, ch. 9.
T. J. Balle and W. H. Flygare, Rev. Sci. Instrum., 1981, 52, 33.
J. K. G. Watson, J. Chem. Phys., 1968, 45, 4517.
H. M. Pickett, J. Mol. Spectrosc., 1991, 148, 371.
CH CCHÉ É ÉAr, where the spectra of three isotopomers have
3
been observed, has it been possible to determine h unam-
1
1
1
2
3
4
biguously as 97¡. The obtuse values of h of 97¡ for
CH NCÉ É ÉAr and 95¡ for CH CNÉ É ÉAr are very similar to
3
3
that for CH CCHÉ É ÉAr. There are, however, signiÐcant di†er-
15 W. Gordy and R. L. Cook, Microwave Molecular Spectra, Inter-
3
ences between CH CCHÉ É ÉAr on one hand and CH NCÉ É ÉAr
and CH CNÉ É ÉAr on the other. In CH CCHÉ É ÉAr, R is longer
by ca. 0.1 A
science Publishers, New York, 1970, ch. 13.
3
3
1
6 G. T. Fraser, R. D. Suenram and F. J. Lovas, J. Chem. Phys.,
3
3
1
987, 86, 3107.
Ž
and k is smaller by ca. 0.3 N m~1, while R and k
p
p
17 T. A. Blake, D. F. Eggers, S-H. Tseng, M. Lewerenz, R. P. Swift,
are very similar in CH NCÉ É ÉAr and CH CNÉ É ÉAr. Graphs of
3
3
R. D. Beck, R. O. Watts and F. J. Lovas, J. Chem. Phys., 1993, 98,
R vs. h and k vs. h are nearly linear for the three complexes if
6
031.
p
the acute values of h are taken for CH NCÉ É ÉAr and
18 J. Kraitchman, Am. J. Phys., 1953, 21, 17.
3
CH CNÉ É ÉAr. While the choice between the acute and obtuse
3
19 S. G. Kukolich, J. Chem. Phys., 1972, 57, 869.
2
0
A. Bauer and M. Bogey, C.R. Seances Acad. Sci., Ser. B, 1970,
values of h for CH NCÉ É ÉAr and CH CNÉ É ÉAr cannot be
3
3
2
17, 892.
considered to be settled, it can be concluded that the complex-
es have very similar geometries and binding.
2
2
1
2
L. Halonen and I. M. Mills, J. Mol. Spectrosc., 1978, 73, 494.
M. Le Guennec, G. Wlodarczak, J. Burie and J. Demaison, J.
Mol. Spectrosc., 1992, 154, 305.
We thank the EPSRC for a research grant. A grant in support
of S.B. from the EC Human Capital and Mobility Network
SCAMP (Contract CHRX CT93-0157) is gratefully acknow-
ledged. We thank a referee for some helpful suggestions.
23 J. Demaison, A. Dubrulle, D. Boucher, J. Burie and V. Typke, J.
Mol. Spectrosc., 1979, 76, 1.
2
4
W. G. Read, E. J. Campbell and G. Henderson, J. Chem. Phys.,
1
983, 78, 3501.
R. P. A. Bettens, R. M. Spycher and A. Bauder, Mol. Phys., 1995,
6, 487.
2
5
8
References
1
P. Cope, D. J. Millen and A. C. Legon, J. Chem. Soc., Faraday
T rans. 2, 1986, 82, 1197.
Paper 6/06816B; Received 4th October, 1996
1290
J. Chem. Soc., Faraday T rans., 1997, V ol. 93