Hydrogen Bonding to Alkanes: Computational Evidence

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Introduction
According to Pauling, the conventional hydrogen bond “...is
formed only between the most electronegative atoms”;1 it is of
the form D-H· · · A, where D is an electronegative atom such
as O or N and both D and A have at least one lone pair.2 In the
view generally adopted today, hydrogen bonding encompasses
a greater variety of donors and acceptors, for example, any
electron-rich region such as π-bonds3 and alkyl radicals4 can
be a hydrogen bond acceptor.
Despite the diversity, hydrogen bonding can nearly always
be described as an incipient proton transfer.5-7 This suggests
that all molecules capable of participating in Brønsted acid-base
reactions may also form hydrogen bonds and prompted us to
take a closer look at a group of weakly basic molecules, alkanes.
The protonation of alkanes by superacids has been investigated
extensively theoretically and experimentally,8 but that
alkanes could be hydrogen bond acceptors was not described
in these studies. However, Ahlberg9 suggests hydrogen bonding
in adducts of methane and H3F2
+; the interaction between
methane and the more acidic H2F+ results in proton transfer.
Kryachko and Zeegers-Huyskens10 found that an adduct of
methane and the protonated water dimer exhibits what they call
multidihydrogen bonding; the formation of gas-phase adducts
of methane and protonated water was observed by Field and
Beggs11 and by Cao, Sun, and Holmes.12 Legon et al.13 obtained
microwave spectra of hydrogen-bonded adducts of methane and
various donors.
The characteristic properties of hydrogen-bonded adducts are
D-H bond elongation, red shift of the accompanying vibrational
stretching frequency with attendant increase in infrared intensity,
and stabilization relative to the isolated components.2,6 These
properties can conveniently be examined computationally, which
facilitates studies of the intra- and intermolecular interactions
between alkanes and strong organic proton donors; in the present
study, we examine the adducts between protonated alcohols,
aldehydes, carboxylic acids, and water as donors and methane,
ethane, propane, and isobutane as acceptors. The study also
includes the intramolecular interactions in protonated long-chain
alcohols. The proton affinities (PA) of the donors span more
than 100 kJ mol-1, but none of the proton donors are strong
enough to protonate the alkane.
Computational Methods
Structure and harmonic vibrational frequencies of molecules
and hydrogen-bonded adducts were determined with the B3LYP/
6-31+G(d,p) method. The G3//B3LYP and G3(MP2)//B3LYP
composite ab initio methods14 were used to estimate the
thermochemical properties of the species studied; the 298 K
heats of formation were obtained as described by Nicolaides et
al.15 All calculations were performed with the Gaussian 03
package.16
Strong coupling of the O-H and C-H stretching vibrations
to other modes was often encountered; in order to determine
the red shifts of the harmonic stretching frequencies, we have,
where appropriate, employed “virtual isotope labeling” to assess
the vibrational properties in the absence of coupling. This
technique, also used by others,17 involves calculation of the
vibrational properties of suitably deuterium-substituted analogues
in order to avoid coupling to the vibration of interest.
The stabilization of the hydrogen-bonded adducts was
calculated as the difference between the heats of formation
of the adduct and the isolated components and takes into
account the bonding between the adduct components as well
as the deformation of these components that accompanies
adduct formation. It is possible to estimate the deformation
energy of each component as the difference between the
energy of that component in isolation and with the adduct
structure, and hence to account explicitly for the deformation.
However, we choose to express the adduct stabilization as
the negative of the enthalpy of association, which brings it
in line with conventional definitions of bond strength. Proton
affinities were calculated as the difference between the G3//
B3LYP energies of the neutral and the protonated molecule,
adding 5/2RT, 6.2 kJ mol-1.