Density functional theory (DFT) calculations of the structures, binding energies, vibrational frequencies and infrared intensities of methanol clusters containing two to five molecules have been carried out using the Becke3LYP functional. Thirteen representative H-bonded structures have been studied including cyclic, chain, branched-cyclic and branched-chain hydrogen bond structures. In the methanol trimer, tetramer, and pentamer, the cyclic structure is more stable by 3.5, 8.3, and 3.6 kcal/mol over the next most strongly bound minimum. In the tetramer and pentamer, the second-most stable minimum corresponds to a branched cycle. Chain structures are destabilized from the cyclic minimum by the loss of a hydrogen bond and from a smaller cooperative strengthening of the H-bonds that remain. In all branched structures studied, the formation of the branch H-bond strengthens the “branch-point” methanol's H-bond donation to its neighbor, but weakens its two acceptor H-bonds, leading to largely compensating effe...

Density Functional Theory Calculations of the Structures, Binding Energies, and Infrared Spectra of Methanol Clusters

Molecular beam depletion spectroscopy has been employed to study the dissociation of small methanol clusters upon excitation of the O–H stretch vibration at 2.7 μ. The tunable infrared radiation has been obtained from a Nd:YAG laser pumped optical parametric oscillator. Pure methanol dimer spectra, without contamination from larger clusters, have been measured using the scattering selection technique or working at reduced temperatures. The dimer spectrum features two absorption peaks at 3574.4 and 3684.1 cm−1. The lower frequency peak is redshifted by 106.6 cm−1 from the monomer value and is assigned to the excitation of the proton donor in the hydrogen bonded complex. The proton acceptor peak is blueshifted by only 3.1 cm−1. Methanol trimer dissociation is observed at 3462 cm−1, whereas larger clusters absorb at still lower frequencies. Absolute photodissociation cross sections were measured for both methanol dimer bands. The integrated dissociation cross sections were 5.59(25)×10−21 cm2 /molecule and 7.1(2.2)×10−22 cm2 /molecule for the proton donor and acceptor bands, respectively. The integrated dissociation cross sections are directly related to the O–H line strengths in the dimer. The increased line strength, for the proton donor band relative to the acceptor band by a factor of ∼8, is in accord with previous observations associated with hydrogen bond formation.

Dissociation of small methanol clusters after excitation of the O–H stretch vibration at 2.7 μ

Ab initio molecular orbital theory was used to obtain the structure, binding energy and vibrational frequencies of the cyclic trimer of methanol. Since one of the aims of our study is to provide relevant data on cooperativity, we have also investigated the monomer and the dimer. The calculations were carried out at the HF/6-311++G(2d,2p) level. We have found that cooperative effects in the methanol trimer are sizeable. They are reflected in shorter OO distances, longer donor OH bond lengths, larger energies per hydrogen bond and greater shifts of the donor OH bond stretching frequencies for the trimer than for the dimer. These non-pairwise effects upon trimerization are also mirrored in the topological characteristics of the electronic charge densities of the clusters. Our calculations also predict that these cooperative effects are very similar in methanol and water. However, the enthalpy of trimerization of methanol is predicted to be about 2 kcal mol−1 greater than that of water, as a consequence of the much smaller zero point energy correction in the methanol trimer.

Cooperative effects in the cyclic trimer of methanol. An ab initio molecular orbital study

The potential energy surface of methanol trimer has been studied through the use of high-level ab initio calculations and density functional methods. The geometries have been optimized at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. The harmonic vibrational frequencies were obtained at the latter level. The final energies for the most stable (CH3OH)n (n=1,3) clusters were calculated in the framework of the G2(MP2,SVP) theory. For these and all the other structures the final energies were also obtained using the B3LYP/6-311++G(3df,2p) approach. Three local minima have been located. The global minimum corresponds to a cyclic structure with two methyl groups on one side of the O–O–O plane and the third one on the other side. The bowl conformer, where the three methyl groups are on the same side of the O–O–O plane, is predicted to be only 0.8 kcal/mol less stable than the global minimum. The third local minimum, where one of the monomers behaves as a biacceptor is predicted to lie much higher...

Study of the methanol trimer potential energy surface

The methanol-water dimers and the potential energy surface of the cyclic methanol(water)2 trimer have been studied through the use of high-level ab initio calculations and density functional methods. The geometries have been optimized at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. The harmonic vibrational frequencies were obtained at the latter level. The final energies of the different local minima were calculated in the framework of the G2 and G2(MP2) theories. These values were compared with those obtained using the B3LYP/6-311+G(3df,2p) approach. At all the levels of theory considered the most stable conformer of methanol-water heterodimers corresponds to that in which water behaves as a hydrogen bond donor, in agreement with the most recent experimental evidences [P. A. Stockman et al., J. Chem. Phys. 107, 3782 (1997)]. The energy differences between the different conformers of the cyclic methanol(water)2 trimer are rather small, as well as the energy barriers connecting them. The g...

High level ab initio and density functional theory studies on methanol–water dimers and cyclic methanol(water)2 trimer

The effect of molecular interaction on the O−H stretching force constant of methanol (MeOH) is reported for its associated species. The various electron donors (D) and acceptors (A) considered include organic molecules such as methanol, dimethylether, acetone, acetonitrile, dimethyl formamide, pyridine, and ions such as F-, Cl-, Li+, and H+. The variation in the O−H stretching force constant of MeO−H...D species on interaction with the electron acceptor such as in the species A...O|Me−H...D is explained on the basis of the cooperativity effect. (CE). The effect is discussed in terms of the relationship CE=(ΔF/F)×100, where ΔF is the reduction is force constant of the hydrogen-bonded OH stretching mode of the associated methanol species MeOH...D when the lone pair electrons on oxygen of the methanol molecule are involved in hydrogen bonding with A, and F is the hydrogen-bonded O−H stretching force constant of the species when the lone pair electrons are free. The cooperativity effect (CE) is found to increase with increasing electron acceptor and electron donor capacities of A and D. The calculated force constants are compared with the experimental results.

O ? H stretching force constant in associated methanol species and the cooperativity effect

Stretching force constants for formamide and its seven associated species involving two to four molecules hydrogen-bonded through linear and cyclic configurations and 10 structures containing formamide hydrogen-bonded with one to five water molecules are reported. Since ab initio calculations are rather inconvenient to perform on such big clusters and are time-consuming, CNINDO MO calculations were carried out using the gradient method. The results demonstrate, on the one hand, the feasibility of semiempirical calculations for the evaluation of trends in force constants for big clusters where generally ab initio calculations become much involved and, on the other hand, explain the effect of hydrogen bonding and cooperativity on force constants and vibrational spectra of biologically important systems composed of formamide in the condensed phase and its aqueous solutions. The CO and NH stretching force constants are found to reduce significantly on hydrogen bonding. The reduction in force constant is further enhanced when two cyclic dimers become associated through a linear hydrogen bond. The results indicate justification for the stabilization of the formamide structure with two cyclic dimers hydrogen-bonded together. The reduction in the force constants on hydrogen bonding also reflect the cooperativity contribution. The CO and CN stretching force constants for the structures corresponding to formamide in liquid and aqueous solution phases are in agreement with the experimental vibrational frequencies reported.

Effect of hydrogen bonding and cooperativity on stretching force constants of formamide

The cooperativity effect and the effect of self-association on the stretching force constants of acetonitrile

Ab initio molecular orbital (MO) calculations with the 3-21G and 6-31G basis sets are carried out on a series of complexes of $NH_3 with Li^+, C \equiv N-, LiCN$, and its isomer LiNC. The BSSE-corrected interaction energies, geometrical parameters, internal force constants, and harmonic vibrational frequencies are evaluated for 15 species. Complexes with trifurcated $(C_{3v})$ structures are calculated to be saddle points on the potential energy surfaces and have one imaginary frequency each. Calculated energies, geometrical parameters, internal force constants, and harmonic vibrational frequencies of the various species considered are discussed in terms of the nature of association of LiCN with ammonia. The vibrational frequencies of the relevant complexed species are compared with the experimental frequencies reported earlier for solutions of lithium cyanide in liquid ammonia.

Ab initio molecular orbital calculations on the associated complexes of lithium cyanide with ammonia

Ab initio molecular orbital (MO) calculations with the 3-21G and 6-31G basis sets were performed on a series of ion-molecule and ion pair-molecule complexes for the H2O + LiCN system. Stabilisation energies (with counter-poise corrections), geometrical parameters, internal force constants and harmonic vibrational frequencies were evaluated for 16 structures of interest. Although the interaction energies are smaller, the geometries and relative stabilities of the monohydrated contact ion pair are reminiscent of those computed for the complexes of the individual ions. Thus, interaction of the oxygen lone pair with lithium leads to a highly stabilised C2v structure, while the coordination of water to the cyanide ion involves a slightly non-linear hydrogen bond. Symmetrical bifurcated structures are computed to be saddle points on the potential energy surface, and to have an imaginary frequency for the rocking mode of the water molecule. On optimisation the geometries of the solvent shared ion pair structures (e.g. Li+cdots, three dots, centered OH2cdots, three dots, centered CN−) revealed a proton transfer from the water molecule leading to hydrogen bonded forms such as Li-O-Hcdots, three dots, centered HCN. The variation in the force constants and harmonic frequencies in the various structures considered are discussed in terms of ion-molecular and ion pair-molecule interactions.

M. C. Shivaglal

Papers

O ? H stretching force constant in associated methanol species and the cooperativity effect

Effect of hydrogen bonding and cooperativity on stretching force constants of formamide

The cooperativity effect and the effect of self-association on the stretching force constants of acetonitrile

Ab initio molecular orbital calculations on the associated complexes of lithium cyanide with ammonia

Ab initio molecular orbital calculations on ion-molecule and ion pair-molecule complexes of the water-lithium cyanide system