The document summarizes a study examining hydrogen bonding interactions in methane-water (CH4-H2O) complexes and analogous complexes using ab initio molecular orbital theory and atoms in molecules theory. Key findings include: 1) The global minimum structure of the CH4-H2O complex has the hydrogen from water pointing toward a tetrahedral plane of methane, indicating hydrogen bonding despite methane lacking a lone pair. 2) Similar minimum structures are found for other CH4-HX (X=F, Cl, SH) complexes, with a bond path connecting the hydrogen of HX to the carbon of methane, revealing a penta-coordinate carbon structure. 3) Both C-H

1. Chemical Physics Letters 467 (2008) 37–40
Contents lists available at ScienceDirect
Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Hydrogen bonding with a hydrogen bond: The methane–water complex and the
penta-coordinate carbon
B. Raghavendra, E. Arunan *
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, Karnataka, India
a r t i c l e i n f o a b s t r a c t
Article history: Ab initio and atoms in molecules (AIM) theoretical studies have been used to show that in a 1:1 complex
Received 25 August 2008 formed between CH4 and H2O, CH4 acting as a hydrogen bond acceptor leads to the global minimum struc-
In final form 4 November 2008 ture. There is a bond path connecting C to the H of H2O revealing a penta-coordinate carbon that could be
Available online 7 November 2008
the precursor to the CHþ . It appears to be a general feature in CH4Á Á ÁHX complexes (X = F, Cl, and SH). It is
5
in agreement with the experimental structure for analogous complexes and also the C2H6Á Á ÁHF/HCN com-
plexes, suggesting that it is also a general feature for hydrocarbon–HX interaction.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction critical point between these two atoms as expected [11]. However,
this work did not consider the global minimum geometry and con-
The title of this manuscript draws from the title of a recently centrated on the C–HÁ Á ÁO hydrogen bond, no wonder influenced by
published article ‘Chemical bonding without chemical bonds’ [1]. the increasing attention devoted to this weak, nevertheless impor-
In this work, the Authors discussed the intra-molecular interac- tant interaction [12]. What is the nature of the interaction or bond-
tions in iron trimethylenemethane complex and pointed out that ing in the experimentally observed structure, which is found to be
contrary to chemical wisdom, a bond path was not found connect- the global minimum theoretically? Similar structures have been
ing the metal atom to the three methylene carbons. The ‘chemical observed experimentally for several other CH4Á Á ÁHX (X = F, Br, Cl,
wisdom’ was that the three methylene carbons were ‘bonded’ to and CN) complexes, in which H of HX is pointing towards the cen-
the Fe atom as a triply coordinating ligand. Atoms in molecules tre of a tetrahedral face of methane, represented here as X–HÁ Á ÁD
[2] theoretical calculations, however, revealed that there were no (triangle symbol represents one of the tetrahedron faces of CH4)
bond paths connecting the three methylene carbons to the Fe [13–17]. Evidently, the interaction is similar in all these complexes
which according to this theory implied that there are no ‘chemical and one can see that all the HX referred above are typical hydrogen
bonds’ between them leading to the title as given above. The cen- bond donors. These spectroscopic studies, however, could not
tral carbon, however, was connected to the Fe through a bond path. establish whether there is a ‘bond’ between C of CH4 and H of HX.
In the current manuscript, we discuss the hydrogen bonding in A structure in which O–H points towards a tetrahedron plane of
CH4Á Á ÁH2O and analogous complexes. Results from microwave CH4 could be interpreted as the result of close packing as the ap-
spectroscopic experiments showed that the structure of this com- proach towards a corner or an edge of the tetrahedron would lead
plex has the H from water pointing towards a tetrahedron plane in to relatively larger intermolecular distances. Moreover, such close
CH4 [3]. This was in agreement with a thorough theoretical study approach can lead to better stabilization, when dispersive (van der
[4], published in the previous year, finding this geometry to be ´
Waals?) forces dominate the interaction. Indeed, Szczçsniak et al.
the global minimum. This work also revealed that a structure hav- concluded that inclusion of dispersion is important in determining
ing C–HÁ Á ÁO hydrogen bonding is a local minimum in the intermo- the global minimum of CH4–H2O complex [4]. However, they
lecular potential energy hyper-surface. pointed out categorically that ‘although the dispersion energy
Hydrogen bonding in CH4Á Á ÁH2O complex has attracted a lot of determines the overall trend in the stabilities, it is important to
theoretical attention [4–11]. Except for Ref. [4], many of the other stress that none of the single components of the interaction energy
papers published before and after, discussed only the C–HÁ Á ÁO is capable of a qualitatively correct description of the anisotropy
interactions in this complex. The AIM theoretical calculations on (in the intermolecular potential)’ (Emphasis and words in paren-
CH4Á Á ÁH2O complex were reported earlier, revealing a bond path thesis ours). Anisotropy (directionality) is generally thought to be
connecting H from CH4 to the O and a characteristic (3,À1) bond the defining feature of hydrogen bonding compared to the more
general ‘van der Waals interactions’ between molecules.
* Corresponding author. Fax: +91 80 2360 0282. The CH4–HX complexes are important as they can give some in-
E-mail address: arunan@ipc.iisc.ernet.in (E. Arunan). sight about the CHþ , which has proven to be a very difficult ion to
5
0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2008.11.009

2. 38 B. Raghavendra, E. Arunan / Chemical Physics Letters 467 (2008) 37–40
and SH) complexes. Interestingly, a bond critical point is observed
in all these complexes connecting the C of CH4 to the H of HX. The
bond path goes from H of HX through the tetrahedron plane
formed by the three H atoms of methane and connects to the cen-
tral C atom. This is similar to what was found in Ref. [1] between
the central C atom and the Fe atom through a plane formed by
three C atoms!
2. Computational details
Fig. 1. (a) Electrostatic potential (ESP) of methane indicating the electron rich Ab initio molecular orbital and AIM theoretical studies have
region in light green colour. It is along the centre of the tetrahedron planes.
been carried out to gain more insight about both C–HÁ Á ÁO and O–
Optimized structures (at MP2/aug-cc-pVTZ level) for H4CÁ Á ÁH2O (b) and H3C–
HÁ Á ÁOH2 (c) along with the bond critical points and bond paths from AIM
HÁ Á ÁD interactions. Geometry optimization has been carried out
calculations. HBCP is the hydrogen bond critical point. (a) ESP, (b) H4CÁ Á ÁH2O, (c) at a reasonably higher level calculation i.e. MP2(FULL)/aug-cc-
H3C–HÁ Á ÁOH2. (For interpretation of the references to colour in this figure legend, pVTZ level using the commercial GAUSSIAN-03 software [32]. Fre-
the reader is referred to the web version of this article.) quency calculations at the same level were done to calculate the
frequency shifts on complex formation as well as to ensure that
the optimized geometries are true minima. Single point energies
study, both experimentally and theoretically. The detailed analysis at CCSD(T)/aug-cc-pVTZ level were calculated with the optimized
of the structure of ðCH5 Þþ is somewhat intriguing and all the evi- geometries in order to get more accurate values of interaction
dence indicate that the global minimum can be viewed as a CHþ 3
energies. Four systems were chosen for our study involving CH4
tripod with a H2 moiety attached to the carbon in an eclipsed Cs interaction with HF, HCl, H2O, and H2S, and both C–HÁ Á ÁX and X–
configuration [18–20]. Both H2O and NH3 have lone pairs which HÁ Á ÁD geometries were considered. The interaction energies were
can not only accept hydrogen bonds but also a proton to form corrected for basis set superposition error using the counterpoise
H3 Oþ and NHþ , respectively. Evidently, CH4 does not have a lone
4
method [33]. Wave function for the optimized geometry is gener-
pair to accept a hydrogen bond or a proton. Our understanding ated using the key words OUPUT = WFN, DENSITY = CURRENT and
about hydrogen bonding has evolved over the last century and it SCF = TIGHT in GAUSSIAN03. The AIM (atoms in molecules) calcula-
has been well established now that any electron rich region can tions were done using the AIM 2000 software [34]. Non-bonded ra-
act as a hydrogen bond acceptor [21,22]. Ref. [21] defined the dii of acceptor atom and interacting ‘H’ atom were calculated using
hydrogen bond as ‘an intermediate range intermolecular interac- MORPHY98 [35,36] program.
tion between an electron-deficient hydrogen and a region of high
electron density’. Ref. [22] defined the hydrogen bond as ‘an attrac- 3. Results and discussion
tive interaction between an electron-deficient hydrogen atom al-
ready bonded to one atom and an electron rich region either Table 1 lists the interaction energy, the H–A bond distance, the
within the same or with another atom/molecule’. Examples for X–H–A bond angle (A is the H-bond acceptor atom), the increase in
the electron rich regions include the traditional lone pairs in H2O bond length of X–H in the complex compared to the monomer and
and NH3 [23], p electrons in C2H4 [24], r electrons in H2 [25], the vibrational frequency shift in X–H mode following complex
and even an unpaired electron in CH3 radical [26,27]. Where is formation for both X–HÁ Á ÁD and C–HÁ Á ÁX minima. All these results
the electron rich region in CH4? Analysis of the molecular electro- are from MP2/aug-cc-pVTZ level calculations. The single point
static potential can help in identifying the site of an electrophilic interaction energies calculated at CCSD(T)/aug-cc-pVTZ are also re-
attack, in this case the approach of H from the donor [28,29]. A look ported in the Table. These are the traditional parameters that were
at the electrostatic potential of CH4 clearly identifies the centre of considered for inferring a hydrogen bond [23,24]. In all four com-
each of the four tetrahedron faces as regions of high electron den- plexes investigated, the X–HÁ Á ÁD geometry has a larger complexa-
sity (see Fig. 1). Clearly, the hydrogen bond donors such as H2O tion energy compared to C–HÁ Á ÁX geometry. The results from the
would have their hydrogen atom approaching this region to form single point CCSD(T) calculations predict the binding energies to
the ‘hydrogen bond’. be uniformly lower than those from MP2 calculations but do not
Atoms in molecules theory has proved to be quite useful in change the general trend or even the quantitative difference be-
understanding bonding [2]. Koch and Popelier have come up with tween the two structures for the same complex. All X–HÁ Á ÁD geom-
a set of criteria to characterize interactions involving hydrogen as etries have stabilization energy more than 4 kJ molÀ1 and the C–
hydrogen bonding or van der Waals interaction and they have been HÁ Á ÁX geometries have it less than 4 kJ molÀ1. The difference be-
extensively used [30,31]. Such an analysis on structures corre- tween the two geometries is higher for CH4Á Á ÁHF/HCl complexes
sponding to both minima i.e. C–HÁ Á ÁO and O–HÁ Á ÁD, would be quite ($6.0 kJ molÀ1) compared to CH4Á Á ÁH2O/H2S complexes
useful and it is reported in this Letter for CH4Á Á ÁHX (X = F, Cl, OH, ($2.0 kJ molÀ1).
Table 1
Interaction energy (E in kJ molÀ1) calculated at MP2/aug-cc-pVTZ level and single point CCSD(T) levels corrected for BSSE. optimized H–A bond distances (RH–A in Å) and bond
angles (XHA in °), change in H–X bond length after complex formation (RH–X in Å) and shift in H–X stretching frequency due to complex formation (in cmÀ1). A is the acceptor
atom.
Complex DE MP2 DE CCSD(T) RH–Aa DXHA DRH–X Dm Complexa DE MP2 DE CCSD(T) RH–Aa DXHA DRH–X Dm
H4CÁ Á ÁHF À8.8 À7.9 2.269 (2.20/2.90) 179.5 0.002 60.2 H3C–HÁ Á ÁFH À2.9 À2.2 2.520 (2.43/2.55) 179.3 À0.001 À11.5
H4CÁ Á ÁHCl À9.6 – 2.388 (2.47/2.90) 178.7 0.002 17.2 H3C–HÁ Á ÁClH À3.8 – 2.930 (2.87/3.00) 179.4 0.000 À5.8
H4CÁ Á ÁH2O À6.3 À5.0 2.491 (2.47/2.90) 176.0 0.000 13.8 H3C–HÁ Á ÁOH2 À4.2 À3.3 2.542 (2.47/2.60) 179.3 À0.001 À8.3
H4CÁ Á ÁH2S À6.7 À3.8 2.603 (2.75/2.90) 177.6 0.001 6.2 H3C–HÁ Á ÁSH2 À5.0 À2.4 3.025 (2.90/3.05) 178.2 0.000 À0.6
a
Numbers in parentheses are the sums of hydrogen bond radii defined for the corresponding complexes (Ref. [22]) followed by the sums of van der Waals radii of H and the
acceptor atom from Ref. [23].

3. B. Raghavendra, E. Arunan / Chemical Physics Letters 467 (2008) 37–40 39
All complexes have ‘linear H bonds’ with X–HÁ Á ÁA angle close to a set of eight criteria to settle this issue. Interestingly, most of these
180°. The values vary from 176.0° for H4CÁ Á ÁH2O to 179.5° for criteria were met by all the eight structures investigated here.
H4CÁ Á ÁHF. The H–A distances in all these complexes, RH–A, listed According to Koch and Popelier, mutual penetration of H and
in Table 1 highlight something important about the hydrogen bond acceptor atoms is the necessary and sufficient condition [30]. Table
distances. These are compared to the sum of van der Waals radii 2 lists the values of DrA and DrH, namely the difference between
[23] of H and A atoms and also the sum of the recently recom- non-bonded and bonded radii of the acceptor and hydrogen atoms.
mended hydrogen bond radii [22] for H and A atoms. The reader These values give a measure of the mutual penetration. The
is referred to Ref. [22] for the details of the hydrogen bond radii bonded radius is the distance from the bonded atom to the bond
and these are defined for strong, medium and weak hydrogen critical point along the bond path. The non-bonded radius is calcu-
bonds. The most striking observation is that the RH–A distances lated as the distance from the atom to the point at which the elec-
are significantly smaller than the sum of van der Waals radii for tron density becomes 0.001 au along the bond path in the isolated
X–HÁ Á ÁD structures but only marginally smaller for the C–HÁ Á ÁX molecule. For comparison, values for H2OÁ Á ÁHX and ArÁ Á ÁHX are in-
structures. These distances are within 0.1 Å from the sum of the cluded in Table 2, the former a typical hydrogen bond for all and
hydrogen bond radii recommended for these atoms. More impor- the latter a hydrogen bond for some [2,40]. It is interesting to note
tantly, for all the four X–HÁ Á ÁD structures, the RH–C is very close that the sum of DrA and DrH is about 0.7–0.9 Å for X–HÁ Á ÁC struc-
to the sum of hydrogen bond radii for H and C and in two cases tures and it is larger than that for the C–HÁ Á ÁX structures, 0.5–
(HCl and H2S) it is less than the sum of hydrogen bond radii. How- 0.8 Å. For comparison, the H2OÁ Á ÁHX complexes have this value
ever, all the four C–HÁ Á ÁX structures have RH–A slightly larger than in the range 1.1–1.4 Å and the ArÁ Á ÁHX complexes have 0.6–0.9 Å.
the sum of hydrogen bond radii of H and A, but within about 0.1 Å, If one looks at DrH alone, it is clear that the H2OÁ Á ÁHX complexes
typical uncertainty in these radii. In other words, the X–HÁ Á ÁD have much higher values (0.6–0.8 Å) and the others have values
structures have shorter than average H–A distances and the C– of 0.3–0.4 Å. In any case, it is clear that X–HÁ Á ÁC has more ‘hydro-
HÁ Á ÁX structures have longer than average H–A distances. All the gen bonding’ nature than what C–HÁ Á ÁX shows, based on this
X–HÁ Á ÁD geometries show red-shift in X–H frequencies and the criterion.
C–HÁ Á ÁX geometries show small blue shift in C–H stretching fre- Koch and Popelier have also given a range for values of electron
quency and these are well established [24,37] for similar hydrogen density and Laplacian of the electron densities at the hydrogen
bonded complexes. Clearly, all the traditional criteria employed for BCP. These parameters are also listed in Table 2. The Laplacian val-
identifying a hydrogen bond suggest that the X–HÁ Á ÁD structures ues reported in this Table are defined as L ¼ À1=4r2 q as given in
qualify better as hydrogen bonds than the C–HÁ Á ÁX structures. Ref. [31]. Electron densities and Laplacian at the hydrogen BCPs are
Let us now turn our attention to the results from AIM theoretical well within the range suggested for H-bonds. Here again, X–HÁ Á ÁC
calculations which are quite revealing. The most important obser- qualifies as a hydrogen bond better than C–HÁ Á ÁX. For example,
vation is that all eight complexes show a (3,À1) bond critical point electron density at the BCP for the complexes of H2OÁ Á ÁHF,
along a bond path corresponding to X–HÁ Á ÁD and C–HÁ Á ÁX interac- H4CÁ Á ÁHF, ArÁ Á ÁHF and H3CHÁ Á ÁFH are 0.0436, 0.0121, 0.0158 and
tions. Interestingly, for the X–HÁ Á ÁD geometry the bond critical 0.0061 au, respectively. All of them fall within the range (0.002–
point (BCP) connects the C from CH4 to H from HX through a bond 0.04 au) quoted by Koch and Popelier for a hydrogen bond. Inter-
path (see Fig. 1) and from now on it will be referred to as X–HÁ Á ÁC estingly, the electron density at BCP for CÁ Á ÁHF geometry is twice
hydrogen bond. According to Bader, the presence of a (3,À1) criti- that for CHÁ Á ÁF geometry and both are smaller than that of ArÁ Á ÁHF.
cal point along the bond path is the necessary and sufficient condi- For H2O and H2S complexes, the electron densities at BCP are very
tion for the existence of a bond [2]. Clearly C is bonded to the H of similar ($0.008 au) for both X–HÁ Á ÁC and C–HÁ Á ÁX geometries. The
HX and CHþ would have been possible with a penta-coordinate C.
5 Laplacian for all the complexes shown in Table 2 are negative, indi-
However, as the proton is approaching the C through the tetrahe- cating that they are all closed shell interactions [31].
dron plane, the H–H interaction starts dominating the intermolec- The other parameters given by Koch and Popelier such as
ular potential surface and CHþ ends up with a structure that has
5 change in atomic volume, DV, change in total atomic energy, DE,
been experimentally and theoretically found. Evidently, in the case change in atomic population, DN and change in atomic first mo-
of H2O and NH3, the proton approaches the central atom away ments, DM, all for the H-bonded hydrogen atom all satisfy the cri-
from the hydrogens and these can form the hydronium and ammo- teria for these interactions to be classified as hydrogen bonds. (See
nium ions readily. Not surprisingly, attempts to look for NH2þ have
5 Supporting materials) However, one exception is that the change in
not been very successful as the second proton is again looking at a atomic population of H, DN, is small but positive for X–HÁ Á ÁC
tetrahedron NHþ [38]. On the other hand the existence of H4 O2þ
4 (0.004–0.009) and X–HÁ Á ÁAr complexes and it is negative for both
has been shown experimentally and theoretically [39]. X–HÁ Á ÁOH2 and C–HÁ Á ÁX complexes. According to Koch and
If one is not satisfied with the criterion by Bader for classifica- Popelier criteria, it should have been negative for hydrogen bonded
tion as hydrogen bonding, Koch and Popelier [30,31] have given complexes. Interestingly, they have pointed out that for FHÁ Á ÁPH3
Table 2
Atoms in molecule theoretical parameters for the eight complexes under investigation and the analogous Ar and H2O complexesa.
Complex qb Lb r0 c
A rAc rAd r0 c
H rHc rHd Complex qb Lb r0 c
A rAc rAd r0 c
H rHc rHd
H4CÁ Á ÁHF 0.01208 À0.01156 2.0 1.5 0.5 1.1 0.8 0.3 H3C–HÁ Á ÁFH 0.00605 À0.00618 1.7 1.5 0.2 1.4 1.1 0.3
H4CÁ Á ÁHCl 0.01066 À0.01015 2.0 1.5 0.5 1.3 0.9 0.4 H3C–HÁ Á ÁClH 0.00591 À0.00483 2.2 1.8 0.4 1.4 1.1 0.3
H4CÁ Á ÁH2O 0.00795 À0.00827 2.0 1.6 0.4 1.2 0.9 0.3 H3C–HÁ Á ÁOH2 0.00723 À0.00669 1.9 1.5 0.4 1.4 1.0 0.4
H4CÁ Á ÁH2S 0.00738 À0.00715 2.0 1.6 0.4 1.4 1.0 0.4 H3C–HÁ Á ÁSH2 0.00624 À0.00432 2.3 1.9 0.4 1.4 1.1 0.3
H2OÁ Á ÁHF 0.04356 À0.02625 1.9 1.2 0.7 1.1 0.5 0.6 ArÁ Á ÁHF 0.01584 À0.01394 2.0 1.5 0.5 1.1 0.7 0.4
H2OÁ Á ÁHCl 0.03503 À0.02388 1.9 1.2 0.7 1.3 0.6 0.7 ArÁ Á ÁHCl 0.00941 À0.00876 2.0 1.6 0.4 1.3 0.9 0.4
H2OÁ Á ÁH2O 0.02581 À0.02109 1.9 1.3 0.6 1.2 0.7 0.5 ArÁ Á ÁH2O 0.00805 À0.00804 2.0 1.6 0.4 1.2 0.9 0.3
H2OÁ Á ÁH2S 0.01694 À0.01633 1.9 1.3 0.6 1.4 0.8 0.6 ArÁ Á ÁH2S 0.00690 À0.00635 2.0 1.7 0.3 1.4 1.1 0.3
a
Results from calculation at MP2(FULL)/aug-cc-pVTZ level.
b
Electron density at HBCP, q, and Laplacian of the electron density at HBCP, L defined as L ¼ À1=4r2 q.
c
Non-bonded radii of the acceptor atom and hydrogen atom, r 0 and r 0 , and bonded radii of acceptor and hydrogen atoms, rA and rH in Å.
A H
d
rA = (r0 À rA ) and rH = (r0 À r H ) which are the penetration parameters in Å.
A A

4. 40 B. Raghavendra, E. Arunan / Chemical Physics Letters 467 (2008) 37–40
complex, which they had classified as a hydrogen bonded complex, References
the DN was found to be small and positive. This was attributed to
possible errors in integration. In any case, according to seven of the [1] L.J. Farrugia, C. Evans, M. Tegel, J. Phys. Chem. A 110 (2006) 7952.
[2] R.F.W. Bader, Atoms in Molecules: A Quantum Theory, Clarendon Press,
eight criteria given by Koch and Popelier [30], and the conventional Oxford, 1990.
wisdom (stabilization energy, H–A distance, elongation of X–H [3] R.D. Suenram, G.T. Fraser, F.J. Lovas, Y. Kawashima, J. Chem. Phys. 101 (1994)
bond and red-shift in the X–H vibrational frequency), X–HÁ Á ÁC 7230.
´ ´
[4] M.M. Szczçsniak, G. Chałasinski, S.M. Cybulski, P. Cieplak, J. Chem. Phys. 98
interaction is attractive hydrogen bonding interactions. It is more (1993) 3078.
so than the C–HÁ Á ÁX interaction that is often discussed with respect [5] J.J. Novoa, B. Tarron, J. Chem. Phys. 95 (1991) 5179.
to the CH4Á Á ÁHX complexes in every case considered in this work, [6] C. Rovira, J.J. Novoa, Chem. Phys. Lett. 279 (1997) 140.
[7] M. Hartman, S.D. Wetmore, L. Radom, J. Phys. Chem. A 105 (2001) 4470.
from the strongly hydrogen bonding HF donor to the weakly
[8] J.J. Novoa, M. Planas, M. Carme Rovira, Chem. Phys. Lett. 251 (1996) 33.
hydrogen bonding H2S donor. The experimental geometry of [9] O.A. Ojo, K. Szalewicz, J. Chem. Phys. 123 (2005) 134311.
C2H6Á Á ÁHF/HCN complexes also have the H from HX pointing to- [10] C. Kozmutza, I. Varga, L. Udvardi, Theochem 666–667 (2003) 95.
wards the tetrahedron plane formed by the CH3 group [41,42] [11] G.L. Sosa, N.M. Peruchena, R.H. Contreras, E.A. Castro, Chemistry preprint
archive volume 2001, Issue 3, March 2001, p. 485 <http://
and, not surprisingly the electrostatic potential for C2H6 has a cor- www.sciencedirect.com/preprintarchive>.
responding minimum [29]. The C2H6Á Á ÁHCN complex has indeed [12] G.R. Desiraju, T. Steiner, The Weak Hydrogen Bond, Oxford University Press,
been characterized as a complex with CH3 group as hydrogen bond Oxford, 1999.
[13] A.C. Legon, B.P. Roberts, A.L. Wallwork, Chem. Phys. Lett. 173 (1990) 107.
acceptor [42]. Hence, it is expected that the conclusions reached [14] M.J. Atkins, A.C. Legon, A.L. Wallwork, Chem. Phys. Lett. 192 (1992) 368.
here would be applicable for interactions in all hydrocarbon–HX [15] A.C. Legon, A.L. Wallwork, J. Chem. Soc. Faraday Trans. 88 (1992) 1.
interactions, in which HX is a hydrogen bond donor. [16] Y. Ohshima, Y. Endo, J. Chem. Phys. 93 (1990) 6256.
[17] S.R. Davis, L. Andrews, J. Chem. Phys. 86 (1987) 3765.
[18] E.T. White, J. Tang, T. Oka, Science 284 (1999) 135.
4. Conclusion [19] X. Huang, A.B. McCoy, J.M. Bowman, L.M. Johnson, C. Savage, F. Dong, D.J.
Nesbitt, Science 311 (2006) 60.
[20] O. Asvany, P.P. Kumar, B. Redlich, I. Hegemann, S. Schlemmer, D. Marx, Science
To conclude, high level ab initio calculations coupled with AIM 309 (2005) 1219.
theoretical calculations have been carried out on CH4Á Á ÁHX (X = F, [21] P.A. Kollman, L.C. Allen, Chem. Rev. 72 (1972) 283.
Cl, OH, and SH) complexes. Two geometries for each complex, [22] B. Raghavendra, P.K. Mandal, E. Arunan, Phys. Chem. Chem. Phys. 8 (2006)
5276.
one in which CH4 is the hydrogen bond donor and another in which [23] L. Pauling, The Nature of the Chemical Bond, Cornell University Press, New
it is the acceptor have been considered. In each of these cases, the York, 1960.
X–HÁ Á ÁC interaction is stronger than the C–HÁ Á ÁX interaction. It has [24] G.C. Pimentel, A.L. McLellan, The Hydrogen Bond, W.H. Freeman and Co., San
Francisco, 1960.
been shown that the former resembles a hydrogen bond more clo- [25] S.J. Grabowski, W.A. Sokalski, J. Leszczynski, Chem. Phys. Lett. 432 (2006) 33.
sely than the commonly considered C–HÁ Á ÁX interactions. In all [26] I. Alkorta, I. Rozas, J.F. Elguero, Ber. Bunsen-Ges. Phys. Chem. 102 (1998) 429.
these cases, the H–A distances are very close to the sum of the [27] B. Raghavendra, E. Arunan, J. Phys. Chem. A 111 (2007) 9699.
[28] P. Politzer, D.G. Truhlar (Eds.), Chemical Applications of Atomic and Molecular
hydrogen bond radii proposed [22] recently. All the experimental Electrostatic Potential, Plenum, New York, 1981.
and theoretical results available so far suggest that this conclusion [29] S.R. Gadre, R.N. Shirsat, Electrostatics of Atoms and Molecules, Universities
would be general and applicable for all hydrocarbon–HX Press (India) Ltd., Hyderabad, 2000.
[30] U. Koch, P.L.A. Popelier, J. Phys. Chem. 99 (1995) 9747.
complexes.
[31] P. Popelier, Atoms in Molecules: An Introduction, Pearson Education, Harlow,
2000.
Acknowledgements [32] M.J. Frisch et al., GAUSSIAN03, Revision C-02, Gaussian, Inc., Wallingford CT 2004.
[33] S.B. Boys, F. Bernardi, Mol. Phys. 19 (1970) 553.
[34] F. Biegler-Konig, R. Schonbohm, D. Derdau, D. Bayles, R.F.W. Bader, AIM 2000
One of the authors (BR) thanks Council for Scientific and Indus- Version 1, Bielfield, Germany, 2000.
trial Research (CSIR), India for the fellowship. Authors acknowl- [35] P.L.A. Popelier, Comput. Phys. Commun. 93 (1996) 212.
edge the Supercomputer Educational Research Centre of the [36] P.L.A. Popelier, Chem. Phys. Lett. 228 (1994) 160.
[37] P. Hobza, Z. Havlas, Chem. Rev. 100 (2000) 4253.
Indian Institute of Science for computational facilities. EA’s re- [38] G.A. Olah, A. Burrichter, G. Rasul, G.K. Suryaprakash, J. Am. Chem. Soc. 119
search on hydrogen bonding has been supported by Department (1997) 4594.
of Science and Technology and CSIR, India, and Indo-French Centre [39] J.C. Bollinger, R. Faure, T. Yvernault, D. Stahl, Chem. Phys. Lett. 140 (1987) 579.
[40] V. Aquilanti, E. Cornicchi, M.M. Teixidor, N. Saendig, F. Pirani, D. Capelletti,
for Promotion of Advanced Research and IUPAC. Angew. Chem. Int. Edn. 44 (2005) 2356.
[41] L.W. Buxton, P.D. Aldrich, J.A. Shea, A.C. Legon, W. Flygare, J. Chem. Phys. 75
(1981) 2681.
Appendix. Supplementary material
[42] A.C. Legon, A.L. Wallwork, H.E. Warner, Chem. Phys. Lett. 191 (1992) 97.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cplett.2008.11.009.