18.7.3 Carbon Bonding
Recently, inspired by the identification of halogen, chalcogen and pnictogen
bonding, Mani et al. [78] investigated whether carbon, being a positive centre, can
accept electron density. Indeed, both experimental and theoretical studies agree that
the tetrahedral face of methane can act as a hydrogen bond acceptor. Rotational
spectra of complexes like CH 4 ···HF, CH 4 Á Á Á HCl; CH 4 Á Á Á HCN and CH 4 Á Á Á H 2 O
further confirm this fact [84]. While tetrahedral faces of methane has an electron
rich centre and can act as a hydrogen bond acceptor, substitution of one of its
hydrogens with some electron withdrawing group (such as −F/OH) can make the
opposite face electron deficient. The complex between CH 3 F and H 2 O has a
potential energy minimum with water oxygen pointing towards the tetrahedral face
of CH 3 . Similar interactions are also found for several methanol complexes in
which the electron deficient atom (oxygen) interacts with one of the water’s lone
pair.
Four examples of complexes, which represent minima on the potential energy
surface, are shown in Fig. 18.11a–d. All complexes were optimized at the ω
B97X-D/6-31+G* level of theory. In these complexes, the electron density donors
(O, P, S and F atoms) are oriented towards the CH 3 face of methanol. Through NCI
analysis, the presence of intermolecular interactions are evident in all the complexes. The weak nature of this type of interaction is indicated by the green color of
the NCI-isosurfaces that corresponds to van der Waals interactions.
It should be noted that, in spite of the weakness of this type of interactions, they
are extremely relevant, since these and similar interactions could give an enthalpic
contribution to the so called “hydrophobic interactions” [78].
Fig. 18.11 Carbon bond examples for several electron donors (O, P, S and F): a MeOH–H 2 O,
b MeOH–PH 3 , c MeOH–SH 2 , d MeOH–FH. 3D isosurface was produced using the cut-off values
of s = 0.5 a.u. and À0:05\q\0:05 a.u.
514
C. Narth et al.
Recently, inspired by the identification of halogen, chalcogen and pnictogen
bonding, Mani et al. [78] investigated whether carbon, being a positive centre, can
accept electron density. Indeed, both experimental and theoretical studies agree that
the tetrahedral face of methane can act as a hydrogen bond acceptor. Rotational
spectra of complexes like CH 4 ···HF, CH 4 Á Á Á HCl; CH 4 Á Á Á HCN and CH 4 Á Á Á H 2 O
further confirm this fact [84]. While tetrahedral faces of methane has an electron
rich centre and can act as a hydrogen bond acceptor, substitution of one of its
hydrogens with some electron withdrawing group (such as −F/OH) can make the
opposite face electron deficient. The complex between CH 3 F and H 2 O has a
potential energy minimum with water oxygen pointing towards the tetrahedral face
of CH 3 . Similar interactions are also found for several methanol complexes in
which the electron deficient atom (oxygen) interacts with one of the water’s lone
pair.
Four examples of complexes, which represent minima on the potential energy
surface, are shown in Fig. 18.11a–d. All complexes were optimized at the ω
B97X-D/6-31+G* level of theory. In these complexes, the electron density donors
(O, P, S and F atoms) are oriented towards the CH 3 face of methanol. Through NCI
analysis, the presence of intermolecular interactions are evident in all the complexes. The weak nature of this type of interaction is indicated by the green color of
the NCI-isosurfaces that corresponds to van der Waals interactions.
It should be noted that, in spite of the weakness of this type of interactions, they
are extremely relevant, since these and similar interactions could give an enthalpic
contribution to the so called “hydrophobic interactions” [78].
Fig. 18.11 Carbon bond examples for several electron donors (O, P, S and F): a MeOH–H 2 O,
b MeOH–PH 3 , c MeOH–SH 2 , d MeOH–FH. 3D isosurface was produced using the cut-off values
of s = 0.5 a.u. and À0:05\q\0:05 a.u.
514
C. Narth et al.