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Observation and Properties of a Hydrogen Bond to Carbon in a Short-Lived Gas-Phase Complex of Methyl Isocyanide and Hydrogen Fluoride.

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493;H. Buschmann, H.-D. Scharf, N . Hoffmann, P. Esser. Angrw. Cliem.
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(Eds.: K. H. Dotz, R. W. Hoffmann), Vieweg. Braunschweig. 1991,p. 33;
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R. R. Eckman, J. Am. Chem. SOC.1989. ill, 2728,and references therein.
[9]See for example: D. Seebacb, R. Imwinkelried, T. Weber in Modern Sinthetir Methods, Vot. 4 (Ed.: R. Scheffold), Springer, Berlin, 1986,p. 125;
M.A. Brook, D. Seebach, Can. J. Chem. 1987,65, 836, and references
[lo] In preliminary experiments the corresponding Arrhenius plots were
determined for the polypropylene production with metallocenei
alumoxane catalysts based on [Cp,TiCI2]/[Cp,TiPh,I; [(C,H,Me),TiCI,]j
In all cas[(C,H,Me),TiPh,l; and [(C,H,tBu),TiCl,]/[C,H,tBu),TiPhz].
es a more complicated picture emerged. Plots of Ig a( 1 - u) ' versus T - I
yields in each case two linear regions with differing slopes. According to
Scharf and PrdCeJUS[d]one can assume that a second level of stereoselection is kinetically significant here. In the proposed reaction (Scheme 2)this
could be the primary step, i.e. the (reversible) addition of the prochiral
olefin to the electrophilic metal center.
structure -N=C of the isocyanide groupL6%
'I suggested that
bonding to carbon rather than nitrogen is favored, but the
solution-phase work did not provide information regarding
geometry. Some recently proposed empirical rules,[*] successful in rationalizing the angular geometries of a wide
range of hydrogen-bonded dimers B ... HX, predict that the
hydrogen bond in, for example, methyl isocyanide- hydrogen fluoride ( I ) should be to the nonbonding (n) electron
pair localized on the C atom rather than to one of the x-electron pairs in the NC triple bond, and that at equilibrium the
H F molecule should lie along the axis of the n electron pair
as envisaged in the conventional model (I). Indeed, in a
corollary to the rules,['] it is proposed that the molecular axis
of H F in dimers B . . - H F may be used as a probe of the
direction of the n electron pair. The rules are based on investigations by rotational spectroscopy which normally
provide["] unequivocal answers to the question of the binding site and yield the angular geometries and further quantitative properties of hydrogen-bonded dimers in effective isolation.
Observation and Properties of a Hydrogen Bond
to Carbon in a Short-Lived, Gas-Phase Complex
of Methyl Isocyanide and Hydrogen Fluoride **
By A . C. Legon,* D. G. Lister, and H . E. Warner
We report here the rotational spectrum of a short-lived
CH,NC...HF complex with C,, symmetry frozen on the
10 ps timescale in a coaxial, pulsed supersonic expansion of
CH,NC/Ar and HF/Ar mixtures. This is the first characterization of a hydrogen bond to an isocyanide group in the gas
Hydrogen bonds can form either by interaction with a
nonbonding (n) electron pair on a terminal atom of a proton
acceptor B or with a x-bonding electron pair on B. Such
bonds involving the x-electron density between carbon
atoms have been characterized extensively, first in solution". through shifts Avo,, in the OH stretching frequencies
of x . . . HO and pseudo-x... HO systems and then in the gas
phase in ethyne ... HC1,[31 ethene ... HC1,[4] and cyclopropane.. .HCIi5' by rotational spectroscopy. Hydrogen
bonds to n electron pairs on terminal carbon atoms are less
common. Isocyanides were first shown to form hydrogen
bonds with suitable donors by infrared and NMR spectroscopy in solution.[6. Consideration of the electronic
[*] Prof. A. C. Legon, Prof. D. G. Lister[+], Dr. H. E. Warnei
Department of Chemistry, University of Exeter
Stocker Road. GB-Exeter E X 4 4 Q D (UK)
[ + ] Permanent address:
Dipartimento di Chimica lndustriale
Casella Postale 29, 1-98166 Sant' Agata di Messina (Italy)
This work was supported by grants from the Science and Engineering
Research Council and MURST.
Verlugsgesellschuft mbH. W-6940 Wrinheim, 1992
A difficulty arises when one attempts to apply these rules
to complexes of isocyanides with acids HX, the obvious
model systems for a hydrogen bond to carbon. Rapid addition reactions of anhydrous hydrogen halides (X = CI, Br, I)
to isocyanides have been reported.[''' The rapid reaction of
CH,NC with H F was demonstrated by monitoring the gas
mixture with infrared spectroscopy,["] and we have confirmed recently that on mixing gaseous CH,NC and H F
there is a virtually instantaneous precipitation on the vessel
walls. The problem of high reactivity can be overcome, however, by using a fast mixing nozzle in conjunction with Fourier-transform microwave ~ p e c t r o s c o p y .The
~ ' ~ ~essence of the
method is to keep the components separate until they expand
into the Fabry-Perot cavity of the spectrometer. Any shortlived complexes of CH,NC and H F formed on mixing are
frozen on the ca. 10 ps timescale in the collisionless phase of
the supersonic expansion. These species can then be probed,
identified, and their properties obtained from the rotational
The complex CH,NC . . . H F was formed by coaxially mixing a pulsed jet of CH,NC/Ar with a continuous jet of HF/
Ar. Frequencies of transitions attributed to the ground-state
rotational spectrum of CH,NC ...H F accurately obeyed the
symmetric-top expression [Eq. (a)]. The observed frev =2B0(J+1)-4D,(J+1)3 -2DJKKZ(J+1)
quencies, the residuals of their least-squares fit by Equation
(a), and the rotational constant B, and the centrifugal distortion constants D, and D,, determined from this expression
are recorded in Table 1 together with the corresponding data
for the isotopomer CH,NC ... DF. Transitions from levels
with K 2 2 had an unobservable intensity because of the low
effective temperature of the expansion. In K = 0 transitions
there was little evidence of hyperfine splitting due to I4N
nuclear quadrupole and H,F nuclear spin-nuclear spin coupling, and only a suggestion of these effects was seen in K = 1
transitions (Fig. 1). Model calculations made by assuming
unchanged I4N nuclear q ~ a d r u p o l e ~
' ~ ~H,F spin-spin
Angew. Chem. Int. Ed. Engl. 31 (1992) No. 2
Fig. 1. Frequency spectrum of the
J = 3 + 2 K = 0 and K = l transitions
of CH, NC. . . HF in the range of
10698.6 to 1069 MHz. This power
spectrum was obtained by averaging
1003 gas pulses over a period of eight
minutes. The data points are separated
by 3.90625 kHz and have been joined
by straight lines. No Doppler doubling
effect is seen when the fast mixing nozzle IS used. Partial resolution of hypertine structure is apparent on the K = 1
transition centerd at 10698.71 MHz
(see text).
constants from the free molecules CH,NC and
H F predict a very complicated hyperfine structure for the
J = 3 +- 2, K = 0 transition, for example, with two-thirds of
the intensity of the many components falling within
- 10 kHz of the unperturbed frequency. This is consistent
with the observed full width at half height of approximately
20 kHz (see Fig. 1). The partial resolution for the K = 1 component seen in the spectrum is also as predicted but is too
complicated to analyse.
The spectrum is characteristic of a symmetric top and
establishes that the complex has C,, symmetry. The magnitude of the change AB, in B, on D/H substitution is consistent only with the order of the nuclei CNC ... HF along the
symmetry axis and establishes that the intermolecular link is
through a hydrogen bond to carbon as in 1. The model
geometry CH,NC-..HF with r(C... F ) = 2.840 8, proposed
below predicts a value ABcalc= 22.5 MHz, in excellent agreement with ABo = 22.6 MHz. The other possible C,, complex, CH,NC ... FH, would require r(C ... F ) = 2.828 8, and
ABcajc= 77.9 MHz, however. Quantitative properties of
CH,NC ... HF are also available from the spectroscopic
constants in Table I .
Table 1 , Observed and calculated rotational transition frequencies and spectroscopic constants of CH,NC ... H(D)F in the ground state.
J+ 1
[MHz] Av [kHz] [a]
B, [MHz][b]
D, [kHz1[bl
D,, [kHz][b]
[a] Av
= vub, - vCalc. [b]
CH,NC ' ' DF
vOb. [MHz]
Ail (kHz] [a]
- 1.7
Standard errors in units of the least significant digit.
First, the distances r(C ... F) = 2.840 A can be obtained
by fitting the rotational constant of either CH,NC ... HF or
CH,NC . ' . DF. A familiar model['61 is used to allow for the
zero-point oscillations of the CH,NC and HF subunits, assuming the same oscillation amplitudes as those of the isomeric complex CH,CN .. . HF['61 and the same geometries
as those of the monomer^.[^^-'^^ The C . . . F distances obtained should be compared with 2.752 A (CH,CN...HF)
and 2.750 8, (CH,CN DF).[16] The difference Ar
between r ( C . . . F ) and r ( N . . . F ) of CH,NC...HF and
CH,CN ... HF is apparently a measure of the difference of
Angew. Chem. Int. Ed. Engl. 31 (1992) No. 2
the van der Waals radii of C and N atoms.["] The accepted
value 1.4 8, for the van der Waals radius of the N atom then
allows the estimate of 1.5 A for the radius of the C atom.
Secondly, the hydrogen-bond stretching force constant k,
can be determined from D, by Equation (b)12'1
= (16 n Z p B 3 / D , ) ( l-B/@H3NC- B/EHF)
where p = mCH3NCrnHF/(mCH3NC
+ inHF).When ground-state
rotational constants IFH3NC,
BHF,and BDFof the monomers
are e m p l ~ y e d [ ' ~ - 'the
~ ~ resulting
values are k, = 19.9(3)
and 20.5(6)Nm-' for CH,NC...HF and CH,NC...DE
virtually the same as for CH,CN ... H(D)F.[16] The
wavenumbers C0 = (271c)-' (kJp)"' for the stretchingmode
cs are correspondingly 158(1) and 158(2) cm-'.
It is possible that the short-lived, hydrogen-bonded C,,
complex CH,NC ... HF characterized in this work is the intermediate first formed in the addition reaction of HF to
Experimental Procedure
Rotational spectra were observed with a Balle-Flygare Fourier-transform microwave spectrometer with a pulsed nozzle.[23.241 The fast mixing nozzle consisted of a pair of coterminal concentric tubes attached to the outlet of a Series
Nine (General Valve Corp.) solenoid valve, as described in detail elsewhere."
A mixture ofroughly 3 YOCH,NC in Ar was pulsed into the outer tube at a rate
of about 3 Hz from a pressure of 2 atm and mixed with a continuous flow of
30% H F (Argot International plc) in Ar at a pressure of 1 atm issuing from the
exit of the inner tube. Methyl isocyanide was prepared by dehydration of
N-methylformamide[2s'and purified by distillation under reduced pressure.
DF was prepared in situ by exchange of H F with D,O on the walls of the
stainless-steel mixing tank.
Received: September 17, 1991 [Z 4916 IE]
German version: Angew. Chem. 1992, 104. 233
CAS Registry numbers:
CH,NC. 593-75-9; HF, 7664-39-3: DF. 14333-26-7
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hydrogen, short, bond, complex, properties, live, gas, phase, methyl, fluoride, observations, carbon, isocyanides
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